Cenozoic tectonic evolution of the New Zealand plate-boundary zone: A paleomagnetic perspective

Tectonophysics 509 (2011) 135–164

Contents lists available at ScienceDirect

Tectonophysics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t e c t o

Review Article

Cenozoic tectonic evolution of the New Zealand plate-boundary zone: A paleomagnetic perspective Simon Lamb ⁎ Department of Earth Sciences, South Parks Road, Oxford, OX1 3AN, UK

a r t i c l e

i n f o

Article history: Received 17 December 2010 Received in revised form 31 May 2011 Accepted 1 June 2011 Available online 15 June 2011 Keywords: New Zealand plate-boundary zone Paleomagnetism Cenozoic tectonic evolution Tectonic rotations Subduction Hikurangi margin

a b s t r a c t New Zealand straddles the obliquely convergent boundary between the Pacific and Australian plates, with subduction of Pacific plate along the Hikurangi margin. Finite plate reconstructions predict ~800 km of relative plate motion since ~43 Ma, with 80–90° clockwise rotation of the Hikurangi margin since ~20 Ma, at an average rate of 4–4.5°/Myr, and the short-term deformation (b10 kyr) shows that rotation is still active. Paleomagnetic measurements in Cretaceous and Cenozoic sedimentary and volcanic rocks record rotation about vertical axes of crustal blocks along the Hikurangi margin, conforming closely with that indicated by the plate reconstructions. Since ~20 Ma, this was accommodated relative to the Australian plate by along strike gradients of extension and shortening, together with dextral shear on arcuate strike-slip faults. The ends of the rotating part of the Hikurangi margin define hinges. In the south, this is accommodated by dextral shear in the Marlborough Fault Zone, and crustal blocks, 1–50 km across, have rotated 50–130°clockwise, creating the eastern part of the New Zealand Orocline. In the north, the hinge is an onshore arcuate zone of dextral shear, 10–50 km wide, with normal faulting. The Paleogene rotational history is poorly constrained, but the few paleomagnetic observations are consistent with distributed shear of basement terranes in a zone of dextral simple shear, b 200 km wide, running up the western part of the New Zealand, and linking with subduction farther north, creating the western half of the New Zealand Orocline. The overall pattern of rotation shows that the continental lithosphere in the Australian Plate is weak, so that deformation is controlled mainly by boundary forces along the plate interface, and passively follows the change in trend of the subduction zone through time. © 2011 Elsevier B.V. All rights reserved.

Contents 1. 2.

3. 4.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . New Zealand plate boundary zone . . . . . . . . . . . . . 2.1. Plate-boundary zone deformation . . . . . . . . . . 2.2. Cenozoic tectonic evolution of the plate-boundary zone Paleomagnetic analysis . . . . . . . . . . . . . . . . . . . 3.1. Rock magnetism . . . . . . . . . . . . . . . . . . Rotation domains . . . . . . . . . . . . . . . . . . . . . 4.1. Raukumara domain . . . . . . . . . . . . . . . . . 4.1.1. Paleomagnetic data . . . . . . . . . . . . . 4.2. Wairoa domain . . . . . . . . . . . . . . . . . . . 4.2.1. Paleomagnetic data . . . . . . . . . . . . . 4.2.2. Anomalous localities . . . . . . . . . . . . 4.3. Wairarapa domain . . . . . . . . . . . . . . . . . 4.3.1. Paleomagnetic data . . . . . . . . . . . . . 4.4. Marlborough domains . . . . . . . . . . . . . . . . 4.4.1. Paleomagnetic data . . . . . . . . . . . . . 4.4.2. Rotation of basement structural trends . . . 4.5. Western domains . . . . . . . . . . . . . . . . . . 4.5.1. Paleomagnetic data . . . . . . . . . . . . .

⁎ Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand.

0040-1951/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2011.06.005

. . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . .

136 136 136 137 138 141 143 143 144 144 144 145 146 146 147 147 148 150 150

136

S. Lamb / Tectonophysics 509 (2011) 135–164

5.

Active rotation in the New Zealand plate-boundary zone . . . . . . . . . . . . . 5.1. Active rotation of the Wairoa and Wairarapa domains . . . . . . . . . . . 5.2. Active rotation in the Marlborough domains . . . . . . . . . . . . . . . 6. Cenozoic tectonic evolution of the Hikurangi margin . . . . . . . . . . . . . . . 6.1. Block reconstructions . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2. Rotation hinges along the Hikurangi Margin . . . . . . . . . . . . . . . 6.2.1. Northern boundary of the rotating Hikurangi Margin . . . . . . . 6.2.2. Boundary between the Wairoa and Wairarapa domains . . . . . . 6.2.3. Cook Strait and the boundary between the Wairarapa and northern 6.2.4. Southern boundary of the rotating Hikurangi Margin . . . . . . . 6.3. Paleogene evolution of the New Zealand plate-boundary zone . . . . . . . 7. Dynamical controls on rotation . . . . . . . . . . . . . . . . . . . . . . . . . 7.1. Rotation of the trend of the subducted slab . . . . . . . . . . . . . . . . 7.1.1. Dynamical controls on the location of northern hinge . . . . . . . 7.2. Strain and rotation . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2.1. Localised block rotation in dextral shear zones . . . . . . . . . . 7.2.2. Rotation and partitioning of relative plate convergence . . . . . . 7.2.3. Block stability . . . . . . . . . . . . . . . . . . . . . . . . . 8. Discussion and conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction The boundaries to the tectonic plates, where they pass through oceanic lithosphere, generally form narrow zones up to a few tens of kilometres wide. However, relative plate motion through continental lithosphere results in displacement, strain and rotation in a broad zone, up to several thousand kilometres wide (Lamb, 1987, 1994; McKenzie and Jackson, 1983a,b). Understanding this marked difference in lithospheric response is a central problem in geology, with two fundamental aspects: firstly, kinematics, describing the evolution of deformation on the Earth's surface, and secondly, dynamics, understanding the forces that drive this deformation. It has been well understood for many decades, from both experimental work and direct observations of earthquakes, that at depths less than a few tens of kilometres, the crust and mantle are cold enough, and at sufficiently low pressure, that they behave in a brittle manner, whereas at greater depths, rocks fail by ductile flow (Ranalli, 1995). Thus, at time scales that are short compared to the seismic cycle, over years to centuries, the response of the shallow parts of the lithosphere is essentially elastic, and the dynamical problem is the relationship between this shallow elastic deformation and ductile deformation at depth. At time scales much longer than the seismic cycle, the elastic part of the lithosphere fails along faults, breaking up into rigid blocks. In this case, the dynamical problem becomes the relationship between the underlying fluid-like ductile flow and the motion of these blocks. At these longer time scales, and at length scales much greater than the dimensions of the rigid blocks, the important question is whether the lithosphere behaves overall as a fluid, dominated by the dynamics of the ductile flow, or whether the details of the interaction between numerous jostling rigid blocks significantly affects this flow (England and McKenzie, 1982; England and Molnar, 1997a,b; Freymueller et al., 2008; Lamb and Watts, 2010; McCaffrey, 2005; Thatcher, 2009). In the former case, crustal blocks are essentially floating — referred to as the floating block model — on an underlying fluid-like flow (Lamb, 1987, 1994; McKenzie and Jackson, 1983a,b). Paleomagnetism offers a powerful approach to tackling this problem, because it allows one to observe directly rigid body rotations about vertical axes. These rotations are those of individual crustal blocks, and so they place important constraints on the behaviour of the blocks, as well as the nature of the faults at their boundaries and their long term stability. This way, paleomagnetically-determined

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Marlborough domain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . .

151 153 153 154 154 156 156 157 157 158 158 159 159 159 160 160 160 161 161 162 162

rotations provide clues to block dynamics in wide zones of deformation, especially in the context of the motions of bounding plates, helping to distinguish the role of edge and basal forces (Lamb, 1987, 1994). In addition, rotation may result in marked re-orientation of earlier-formed geological features, making sense of their long-term geological evolution. Finally, the wider scale pattern of rotation potentially provides powerful constraints on the overall effect of the deformation, yielding important insights into the long and short-term kinematics and large scale dynamics of continental lithosphere. In this paper, I use paleomagnetic data to analyse lithospheric deformation in the New Zealand plate-boundary zone during the Cenozoic, updating previous syntheses of the tectonic evolution of this region based on less paleomagnetic data (King, 2000; Lamb, 1988; Nicol et al., 2007; Rowan and Roberts, 2008; Walcott, 1987, 1989). 2. New Zealand plate boundary zone New Zealand straddles the obliquely convergent boundary between the Australian and Pacific plates, and the entire width of the plate-boundary zone is exposed on-land in a zone ~250 km wide (Fig. 1). The relative instantaneous motion of the plates can be described by a rotation about a pole located to the south of New Zealand. In this study, the NUVEL-1a instantaneous pole of DeMets et al. (1994) is adopted, with ~38 mm/yr of convergence in a roughly east–west direction (261°) in the central part (176°E and 42°S) of the plate-boundary zone (Fig. 1). 2.1. Plate-boundary zone deformation North of New Zealand, both the Australian and Pacific plates are oceanic lithosphere, with subduction of the Pacific plate along the Tonga–Kermedec subduction zone. Plio-Pleistocene back-arc spreading has resulted in the opening of the Havre Trough, with 80–100 km of back-arc extension (Stern, 1984; Wright, 1993, 1994) — the width of the Havre Trough is remarkably constant as far north as 26°S (~ 1200 km north of New Zealand), indicating translation with negligible rotation of the forearc, relative to the Australian plate, for this segment of the subduction zone. In North Island, New Zealand, the Australian plate margin is a broad zone, up to 250 km wide, of deforming continental crust overlying subduction of the Pacific plate along the Hikurangi margin (Fig. 1b). Back-arc spreading terminates onshore in the Central Volcanic Region (Stern, 1984), and a zone of

S. Lamb / Tectonophysics 509 (2011) 135–164

(a)

180°E

35°S

e pin t Al aul F

gi M Hik ura n

NI SB

Hikurangi Plateau

37 mm/yr

(b)

-2000 m

Crust resting on subducted slab

sio n xte n

ura

70 50 20 30 10

180°E

ngi Thr ust

ckar

Dun Mountain Ophiolite 170°E

Fro nt

ce

38°S

Pacific Plate

Ba

Esk Head subterrane

Pu ys eg ur T

45°S

ren ch

Chatham Rise

Hik

Australian Plate

Stokes Magnetic Anomaly

arg

in

Ha vre

35°S

To KerngaTre mede nch c

Tro ugh

170°E

137

Plate interface contours (km)

42°S 176°S

178°S

Fig. 1. Map showing the boundary between the obliquely converging (37 mm/yr) Pacific and Australian plates running through the New Zealand region. (a) In the north, oceanic Pacific plate is being subducted beneath Australian plate along the Tonga–Kermadec subduction zone with Plio-Pleistocene back-arc spreading in the Havre Trough. Farther south, Pacific plate is thickened oceanic crust of the Hikurangi Plateau (Davy et al., 2007), which is being obliquely subducted along the Hikurangi margin with onshore dextral strike-slip in the North Island Shear Belt (NISB). At the southern end of the subduction zone, oblique convergence is transferred through the dextral strike-slip Marlborough Fault Zone to a zone of continental collision along the Alpine Fault, which links with the Puysegur Trench, where Australian plate oceanic lithosphere is being subducted beneath the southern margin of the New Zealand continent. The orientation of prominent geological features such as the Stokes Magnetic Anomaly (outcropping as the Dun Mountain Ophiolite belt, Hunt, 1978) and Eskhead subterrane define the New Zealand Orocline. (b) Depth contours (km) to the top of the subducted plate along the Hikurangi margin (after Wallace et al., 2009). Grey shaded area shows region of crust in the overlying wedge that is in contact with the subducted plate. From crustal structure compiled by Beanland and Haines, 1998.

dextral strike-slip faults, referred to as the North Island Shear Belt (NISB), separates the back-arc from the forearc (Beanland, 1995). Farther south, in South Island, subduction terminates, and the Pacific plate is continental lithosphere, and the plate-boundary is a zone of dextral transpression (~ 100 km wide) within continental crust, comprising the Marlborough Fault Zone, Southern Alps and Alpine Fault (Walcott, 1998). The Alpine Fault is the single most important fault in New Zealand, with ~450 km of dextral strike-slip offset, and up to 100 km of shortening (Sutherland, 1999; Walcott, 1998). It links up at its southern end with the Puysegur Trench, where Australian Plate oceanic lithosphere is being subducted beneath Pacific Plate, with the opposite polarity to subduction along the Hikurangi Margin (Sutherland et al., 2000). The major structural features of the New Zealand plate-boundary zone are shown in Fig. 1. 2.2. Cenozoic tectonic evolution of the plate-boundary zone Cenozoic plate reconstructions of the New Zealand region require ~ 800 km of displacement across the plate-boundary zone since ~ 43 Ma, with profound shape change in the crust of the overlying plate (Fig. 2, Cande and Stock, 2004; King, 2000; Nicol et al., 2007; Sutherland, 1995, 1999; Walcott, 1984a,b, 1987; Walcott et al., 1981). Stratigraphic and structural evidence, however, suggest that subduction along the Hikurangi Margin began in the early Miocene or latest Oligocene, between 25 and 20 Ma, with the development of folding and thrusting and a marked change in the nature of sedimentation along the east coast of New Zealand (Lamb and Bibby, 1989; Rait et al., 1990). At this time, there was a transition from fine-grained deep

water mudstones and limestones in the Paleogene, to Miocene coarse grained facies, including thick conglomeratic sequences such as the Great Marlborough Conglomerate, as well as regional rapid subsidence in the western part of North Island (Audru and Delteil, 1998; Holt and Stern, 1994; Lamb and Bibby, 1989; Rait et al., 1990; Reay, 1993; Stern and Holt, 1994). If the crust in the New Zealand region has remained contiguous across the plate-boundary zone, then the Cenozoic plate motions require large scale clockwise rotation of both the trend of the trench and subduction zone, together with the crust of the overlying Australian plate (Walcott, 1984a, 1987; Walcott et al., 1981; Walcott and Mumme, 1982). Thus, if one considers a marker line spanning the plate-boundary zone, one can estimate this rotation history directly from the finite plate motions (Fig. 3). One would anticipate about 110° of rotation since ~40 Ma, at the time of the inception of the plateboundary zone in the New Zealand region, or an average rate of ~ 2.8°/ Myr, and ~ 90° of rotation of Negene rotation, since ~ 20 Ma, of the subduction zone itself, or an average rate of 4.5°/Myr (Fig. 3) — the latter takes account 80–100 km of back-arc extension in the southern end of the Havre Trough since ~ 4 Ma (Stern, 1984, 1987; Wright, 1993, 1994). Another important clue to the tectonic evolution of the plateboundary zone comes from the geometry of New Zealand's Mesozoic and older basement. Both structural trends and the orientation of distinctive terranes show a marked swing in trend, defining the New Zealand Orocline, from a more nearly northwest or westnorthwest trend in the Pacific Plate, to northeast–southwest in the plateboundary zone itself, and northwest–southeast in the Northland Peninsula, on the Australian Plate (Fig. 1). Offset of these basement

138

S. Lamb / Tectonophysics 509 (2011) 135–164

Today

Northland Peninsula

4 Ma

?

10 Ma

N

RD

20 Ma

RD

ar gin

RD

RD

Hi ku ra ng iM

100 km

30 Ma

RD

40 Ma

RD Chatham Rise

Dun Mountain Ophiolite Belt

RD = Raukumara Domain

Fig. 2. Plate reconstructions for the Australian and Pacific Plates since 40 Ma, close to the inception of the plate-boundary zone, based on the finite rotation poles of Cande and Stock (2004) for the 4, 10, and 20 Ma reconstructions, and Sutherland (1995, 1999) for 30 and 40 Ma, showing the change in shape of the plate-boundary zone, defined by points marking the limits of intense Cenozoic deformation (green zone), and offset of distinctive basement terranes. Solid red lines follow the Hikurangi subduction zone, with dextral shear to the south (dashed red line); thin black dashed lines denote the northern continuation of subduction. Plate motion has resulted in drastic shape change of a roughly triangular zone at ~ 40 Ma to a thin rectangular zone today. The limits of deformation at any stage in the evolution have been always roughly rectangular. Prior to ~ 20 Ma, deformation was most likely restricted to the western side of plate-boundary zone, as a relatively narrow dextral shear zone extending up through western North Island, linking with ~ ESE-trending subduction zone off the Northland Peninsula. Since 20–25 Ma, subduction has propagated much farther east, with the development and subsequent rotation of the Hikurangi margin. The postulated position of the Raukumara domain is shown (marked RD), marking the western limit of the Paleogene plate-boundary zone, and the northern hinge of the Neogene subducting margin.

3. Paleomagnetic analysis

terranes is significantly less than the total ~ 800 km of Cenozoic displacement across the New Zealand plate-boundary zone, requiring more distributed deformation including rotation about vertical axes and bending of the basement terranes (Figs. 1, 2, Bourne et al., 1998; Molnar et al., 1999; Sutherland, 1999).

4 Ma

Today rgi

n

10 Ma

40 Ma

ura ng i Hik

30 Ma

Ma

20 Ma

100 km

Chatham Rise

Inception of Hikurangi Margin

(b) Clockwise rotation (°)

(a)

Over the past 30 years, numerous paleomagnetic studies of Cenozoic and Cretaceous sedimentary rocks and volcanics have documented mainly clockwise rotations of crustal blocks in the New

120

Hikurangi Margin

100

Plate Boundary Zone

80

10°

60

5-6°/Ma

20 0

Australian Plate wrt Pacific Plate

4-5°/Ma

40

~1°/Ma

0

10

20

30

40

50

Age (Ma)

Fig. 3. (a) Finite plate reconstructions of the Australian Plate in the New Zealand region since 40 Ma, relative to the fixed Pacific Plate, as in Fig. 2. Overall, the Australian Plate has rotated ~ 40° clockwise about a vertical axis since 40 Ma, or ~ 1°/Myr. Lines spanning the plate-boundary zone show much more rapid rotation, with marked increase in rotation rate around ~ 20 Ma. (b) Plots showing rotation against time, relative to the Pacific plate, of two lines spanning the plate-boundary zone. A line linking two points outside the main part of the plate-boundary zone, from just north of Christchurch to near the Hauraki Gulf (blue line), has rotated ~ 100° clockwise since ~ 40 Ma, with ~ 80° since 20 Ma (~4°/Myr). A line more nearly parallel to the Hikurangi margin (red line) has rotated overall about ~ 10° more (~ 110° since 40 Ma, or ~ 90° since 20 Ma), as a consequence of back-arc extension in the northern part of the Hikurangi margin in the last 4 Ma. The marked increase in rotation rate of these marker lines around 20 Ma coincides with stratigraphic evidence for the timing of the inception of subduction along the Hikurangi margin at 20–25 Ma.

S. Lamb / Tectonophysics 509 (2011) 135–164

139

Table 1a Published paleomagnetic data for Cenozoic sedimentary rocks used in this study for analysis of vertical axis rotations in the New Zealand plate-boundary zone, from the Raukumara, Wairoa, and Wairarapa domains (see Figs. 4 and 6 for locations). α95f

Age Ma

δAge Ma

Rg

δRg

Refh

14.5 12.6 16.3 12.9 29.6 22.0 41.5 18.5 14.6 21.2 35.6 30.9 21.5 10.4 11.8 34.7

6.3 8.1 5.8 8.9 5.0 6.5 6.3 5.7 5.7 13.6 4.9 4.8 4.6 5.9 4.1 4.0

16.8 20.4 20.4 20 17 17 17 17 18 17 17 17 20 20 20 17

0.8 1.4 1.4 1 1 1 1 1 1 1 1 1 1 1 1 1

14 19 16 23 21 −1 28 9 4 20 8 10 24 23 22 9

8 9 7 8 5 8 7 6 6 14 6 6 6 6 4 5

RR RR RR MLW MLW MLW MLW MLW MLW MLW MLW MLW MLW MLW MLW MLW

− 50 63 − 65 − 53 − 66 − 51

55.5 18.7 35.3 30.7 37.3 22.1

4.5 7.0 3.7 5.5 3.9 6.7

9.9 9.9 14.6 8.8 14.6 14.6

1.1 1.1 1.4 2.3 1.4 1.4

86 27 97 53 116 39

6 13 7 7 8 9

RR RR RR RR RR RR

65 64 54 242 258 63 225 95 250 45 41 22 47 40 41 206 197 79 58 238 100 257

− 49 − 60 − 60 53 65 − 63 50 − 62 67 − 43 − 42 − 52 − 57 − 62 − 61 54 57 − 57 − 54 65 − 63 66

79.6 28.4 80 26.4 35.3 25.1 20.8 14.2 6.4 15.9 5.2 8.1 76.9 174 110 8.4 65.1 31.5 54.1 36.2 63.3 36.4

3.2 7.3 3.5 5.1 5.6 5.4 5.4 15.2 13.9 7.2 9.4 8.9 3.0 2.0 3.2 8.4 3.1 7.7 2.8 5.0 4.7 5.0

6 9.9 13.1 13.5 14.2 16 15 15 12 11 5 4 10 9 8 6 2.3 15.6 7.6 7.6 8.8 8.8

1 1.1 2.1 2.5 1 0.5 0.5 0.5 0.5 0.5 0.05 0.05 0.05 0.05 0.05 0.05 0.1 0.5 1.1 1.1 2.3 2.3

65 64 54 62 78 63 45 95 70 45 41 22 47 40 41 26 17 79 58 58 100 77

4 12 6 7 11 10 7 28 30 8 10 12 4 3 5 12 5 11 4 9 8 10

RR RR RR RR RR WM WM WM WM WM WW WW WW WW WW WW WW RR RR RR RR RR

067/23 051/38 201/30

225 28 71

59 − 53 − 48

21.1 176 64.6

6.6 5.8 3.0

8.8 8.8 14.2

2.3 2.3 1

45 28 71

10 8 4

RR RR RR

229/18 223/41 207/50 250/16 250/16 279/22

240 251 76 30 45 5 176 186

59 36 − 57 − 63 − 63 − 62 52 67

32.8 36.8 31.2 48.4 37.8 15.5 28.5 171

4.5 6.9 4.3 4.7 4.9 5.7 3.8 2.1

8.8 19.6 21.9 9 9 2.5 2.2 2.3

2.3 2.1 1.6 2 2 0.5 0.2 0.2

60 71 76 30 45 5 −4 4

7 7 6 8 9 10 7 5

RR RR RR WCM RLM RLM L B

Location

N/na

Dbis

Ibis

Beddingc

Ddtc

Idtc

Raukumara domain MB MS OR TA2 TA3 TA4 (N) TA4 R TA4 (all) TA6 N TA7 R TA7 N TA7all HK4 N HK4 R HK4 all MA1

/38 /27 /39 19 27 21 12 33 42 5 23 28 44 54 98 35

37.9 298 206.1 208 219 20.1 223 28.9 264 168.9 329.7 332.4 12.8 198.1 14.7 169.1

− 52.4 72 56.2 13.5 29.1 − 51.5 36.8 − 47.4 − 82.9 52.5 − 52 − 52.4 − 23.8 3.5 − 12.9 31.3

027/18 088/54 067/12 346/18 016/28 026/19 026/20 026/21 103/70-41 162/31 162/32 162/33 263/37-26 263/37-27 263/37-28 205/28

14 199 196 203 201 359 208 9 4 200 8 10 24 203 22 189

− 52 43 48 25 36 − 46 40 − 45 − 42 40 − 48 − 47 − 53 32 − 42 44

Coastal Raukumara/Wairoa domains WU (N) WU (R) MP TH TP SB

/19 /24 /44 /23 /37 /22

66.8 163.9 77.2 73.4 49.8 56.9

− 70.2 59.3 − 56 − 44.9 − 59.2 − 11.7

194/23 194/23 307/14 038/19 273/32 353/47

86 207 96 53 116 39

Wairoa domain MK* NP TF NG CH Mk1 Mk2 Mk3 Wk1 Hr1 WW11 WW12 WW7 WW8 WW9 WW10 WW13 PC TC PP NR N NR R

/26 /15 /22 /31 /20 /27 /6 /6 /16 /24 /31 /43 30/102 3/110 19/70 39/100 /31 /13 /48 /24 /16 /24

21.8 16.5 15.3 297.9 313.4 39.2 205.1 51.2 273 27.8 14 347 3.1 349.6 341.3 167.7 207.6 58.7 44.3 216.1 6.5 182.6

− 63.8 − 74.5 − 65.5 47.6 27.8 − 78.6 60.9 − 69 59.1 − 48.4 − 53 − 52.6 − 62.9 − 63.3 − 64.6 55.6 63.6 − 76.6 − 58.6 62.3 − 54.5 44.6

187/30 186/23 199/20 084/43 072/56 169/17 177/17 235/20 048/14 198/17 183/26 183/26 183/26 185/29 191/25 193/25 072/09 183/21 203/10 239/11 240/48 240/48

Wairarapa domain (northern) BH WT TI

/24 /5 /37

264.6 79.2 31.2

59.9 − 50.1 − 64.4

Wairarapa domain (southern) BP FP OK HIN1 HIN2 BIR1 BIR2 MS

/33 /13 /37 /20 /24 /44 /48 2/26

210.6 214.6 339.7 15 23 7

57.8 44.2 − 58.5 − 49 − 53 − 40

/10-15NW

ke

Notes: MLW = Mumme et al., 1989, WW = Wright, 1986 and Wright and Walcott, 1986, WM = Walcott and Mumme, 1985, RLM = Randall, 2007 and also Randall et al. 2011-this volume, L = Lamb, 1988, B = Beanland, 1995. a Number of sites/samples. b In situ (geographic) orientation. c Dip clockwise of strike. d Tilt corrected (stratigraphic) orientation. e Fisher precision parameter. f 95% cone of confidence for mean. g Deviation of mean tilt corrected declination from True North, with error after Demarest, 1983 (0.8 sin− 1(sin(α95)/cos(I)). h RR = Rowan and Roberts, 2008.

140

S. Lamb / Tectonophysics 509 (2011) 135–164

Table 1b Published paleomagnetic data for Cretaceous and Cenozoic volcanic and sedimentary rocks used in this study for analysis of vertical axis rotations in the New Zealand plate-boundary zone, from the Marlborough and Western domains (see Figs. 4 and 11 for locations). Location

N/na

Dbis

Ibis

Marlborough domains BR1 BR2 BR3 BR4 UB1 UB2 UB3 UB4 UB5 UB6 UB7 UB8 WB SV RB NC BS CS SW WV DS CC MOT SS HC3 HC2 BB2 WC1 WC2 WD1 WD2 FC WH WC3 CLA KAI PUH WHP BOU BIG STR BIG* CAV DEE SEY LOT LYF

31/ 32/ 36/ 33/ 27/ 29/ 22/ 25/ 30/ 31/ 31/ 22/ 29/ 33/ 25/ 31/ 14/ 52/ 29/ /42 46/ 12/ 36/ /8 /7 /26 /22 /30 /4 /15 /11 6//26 /25 /9 12//10 10//8 14//13 /59 /18 /38 /12 /50 /36 /20 /11 86/ 54/

207.1 208.4 202.2 208.5 186.7 182 190.7 177.1 188.1 184.3 186 194.3 211.8 214.7 196.6 217.6 173.2 239.9 217.3 186.5 157

44.3 45.1 40.7 46.8 45.5 45.1 53 45.4 45.3 51.5 48.8 48.5 58 70.5 57.1 44.9 29.4 42.7 49.6 48.7 59

118 118 147 309 235 330 62 84 167 257 52 117.9 106.3 7.6 32 9 17 14 016 340 1 26 195 5

54 − 10 21 0 45 76 − 32 − 31 37 34 − 36 − 61.5 − 53.6 − 12.7 − 61 − 21 − 40 − 54 − 44 − 68 − 50 − 72 30 − 40

Western domains WAN TUR RAN RAU TAU BEX BEN PON TAR INA (N) INA (R) INA (all) MAG

26/ 101/ 53/ /30 /22 /8 /26 /14 /120 /11 /10 /21 /39

Beddingc

Ddtc

Idtc

ke

α95f

Age Ma

δAge Ma

Rg

δRg

Refh

283/14 283/14 273/20 291/15 241/14 245/23 233/15 225/21 243/15 233/20 233/20 233/20 321/10 136/15 233/10 313/15 247/40 Fold plunge 353/25 245/15 218/55 020/46 055/20 048/101 034/61 071/95 031/56 292/47 215/20

212 215 212 212 198 202 211 199 200 210 210 219 204 224 212 215 197 175 184 200 279 24 354 122 106 121 315 310 318 349 356 180 274 113 352 22 61 62 30 78 73 37 83 76 26 219 42

58 59 59 62 56 64 61 58 57 64 61 58 67 61 62 60 63 59 62 61 59 − 60 − 62 − 44 − 70 − 69 55 66 57 − 55 − 51 59 60 − 72 − 64 − 58 − 69 − 66 − 57 − 46 − 45 − 49 − 79 − 56 − 72 62 − 57

29.2 48.4 65.6 25.6 56.5 45.7 55.5 39.3 76 60.6 34.2 44.1 58.1 67.1 56.5 61.1 29.6 50.8 34.8 84.6 10.5 56.2 6.7 37 90 26 8 67 133 143 53 40.4 7.7 53.1 34.8 32 34 22.4 7 13.4 28 14 41.4 18.6 36.3 9.9 20.6

4.9 3.7 3.0 5.0 3.7 4.0 4.3 4.7 3.0 3.3 4.5 4.7 3.6 3.1 3.9 3.5 7.4 2.8 4.6 2.4 6.4 5.4 9 11.0 7.0 17.0 12.0 3.2 8.0 3.4 6.3 10.6 11.2 19.2 8.3 9.9 7.2 4.0 14.0 6.9 8.4 5.6 3.5 7.8 7.8 5.1 4.4

4.8 5.7 6.7 7.5 4.8 5.8 5.9 6 6.1 6.2 6.3 7 3.9 3.9 4.2 5.4 8 3.3 4.2 8 18 8 9 20 17 17 30 59 59 4 4 5.5 60 60 54 49 66 6 9 9 9 9 95 53 96 30 30

0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 2 0.3 0.6 1 2 2 3 4 1 1 5 5 5 1 1 0.4 5 5 4 5 2 2 2 2 2 1 5 2 3 4 4

32 35 32 32 18 22 31 19 20 30 30 39 24 44 32 35 17 −5 4 20 99 24 −6 122 106 121 135 138 146 − 11 −4 0 97 117 10 41 84 66 34 82 77 37 77 88 21 51 54

7 6 5 8 5 7 7 7 4 6 7 7 7 5 7 6 13.0 4.3 7.8 4 10 9 14 12 15 55 17 6 11 5 8 17 23 21 17 17 19 9 21 9 10 8 22 14 26 10 11

R R R R R R R R R R R R R R R R R R R R MW W W VL VL VL VL VL VL VL VL LR T T HLM HLM HLM RLM RLM RLM RLM RLM RLM RLM RLM RLM RLM

4.7 9.7 13 192 210 238 54.5 29 15 38 215 36 280

− 55 − 58 − 60 51 50 70 − 60 − 59 − 63 − 67 60 − 64 76

6.8 2.0 2.6 4.5 11.4 5.8 4.8 3.6 1.8 4.5 2.6 2.9 6.3

3 3 2.8 20 25 35 30 35 17 40 40 40 30

0.5 0.5 0.5 3 3 3 3 3 0.5 3 3 3 3

10 3 4 6 14 14 7 6 3 9 4 5 22

T05 T05 T05 MW MW MW MW MW T07 MW MW MW MW

235/24 325/30 300/50 055/55 056/45 250/70 245/15 233/50 228/55 214/47 220/20 225/28 224/50 231/34-44 210/69 230/31

31.8 6.75 72.1 32.6 106 87.3 287 108 12.8

4.7 9.7 13 12 30 58 55 29 15 38 35 36 100

Notes: MW = Mumme and Walcott, 1985, W = Walcott et al., 1981, T = Townsend, 2001, VL = Vickery and Lamb, 1995, HLM = Hall et al., 2004, RLM = Randall, 2007 and also Randall et al. 2011-this volume, T05 = Turner et al., 2005, T07 = Turner et al., 2007. a Number of sites/samples. b In situ (geographic) orientation. c Dip clockwise of strike. d Tilt corrected (stratigraphic) orientation. e Fisher precision parameter. f 95% cone of confidence for mean. g Deviation of mean tilt corrected declination from True North or for Pacific Plate (Marlborough), with error after Demarest, 1983 (0.8 sin− 1(sin(α95)/cos(I)). h R = Roberts, 1990 and Roberts, 1992.

S. Lamb / Tectonophysics 509 (2011) 135–164

is very weak in sedimentary rocks. Thus, Paleogene micritic limestones and Neogene mudstones and siltstones along the east coast of New Zealand typically have magnetic intensities b1 mA/m. Late Cretaceous and Oligocene basaltic volcanics are orders of magnitude stronger, with intensities 25–2650 mA/m (Fig. 5). Secondary magnetisations are common in sedimentary rocks in the New Zealand region, especially Neogene mudstones and siltstones (Mumme and Walcott, 1985; Randall, 2007; Rowan and Roberts, 2005, 2006, 2008; Walcott and Mumme, 1982, Paper 1), leading in some studies to an 80% failure rate in identifying any primary magnetisation at all. A particular problem has proved to be the presence of secondary iron sulphides such as greigite, which have high coercivities and break down at moderate temperatures, tending to disguise any primary magnetisation. However, large scale studies of

Zealand region, with rotations up to ~ 130° (Table 1a and 1b, Fig. 4, Hall et al., 2004; Lamb, 1988; Little and Roberts, 1997; Mumme et al., 1989; Mumme and Walcott, 1985; Randall, 2007; Randall et al., 2011this volume; Roberts, 1990, 1992; Rowan and Roberts, 2005, 2006, 2008; Townsend, 2001; Turner et al., 2005, 2007; Vickery, 1994; Vickery and Lamb, 1995; Walcott et al., 1981; Walcott and Mumme, 1985; Wright, 1986; Wright and Walcott, 1986, hereafter referred to as Paper 1). 3.1. Rock magnetism

Ha Tro vre ug h

The principals of the paleomagnetic technique are described by Butler (1992). In New Zealand, the strength of the natural remanent magnetism (NRM) in rocks varies by several orders of magnitude, and

N

PON

rc -a n ck io Ba tens ex

TA7 MA1 TF

MB Hr1 NG Wk1

TAU RAU

Mt Ruapehu

TA2 TA3 TA4

OR, MS

BEN

BEX

AUSTRALIAN PLATE Western Domains

HK4

C Vo entra Re lcan l gio ic n

NORTH ISLAND

100 km

141

SB TH NP CH MP TP MK* Mk1-2 TC, PP Mk1 NR

Wairoa Domain

TUR

MS

in

Alp

FP

HIN

Wairarapa Fault

WH DEE WC1 CS DSSS SEY HC2 WC2 PUH HC3 WC3 KAI LYF LOT CLA SW

Marlborough Domains

Fig.10

i Th ura ng

BP BIR

Wairarapa Domain

Hik

Isl r th

WB SV CC

UB FC BR BB

MOT

Marlborough Fault Zone

OK

t

lt

au eF

CAV

WT

i ra

MAG WV

INA

St

Nelson

rus

BH

No

TAR

ok

SOUTH ISLAND

Co

NW Nelson

tF ron

ar

TI

an

dS

he

Wanganui Basin

t

Be

lt

WAN

Raukumara Domain

WU

WW13 WW7-12

RAN

Fig. 5

Vertical axis rotations wrt Pacific Plate 0 - 10 Ma 10 - 25 Ma 25 - 95 Ma

PACIFIC PLATE

Neogene zone of rotation

Secondary magnetisation?

Fig. 4. Map of the northern part of the New Zealand plate-boundary zone in the northern part of the South Island and eastern part of the North Island, showing the major structural features and rotation domains (Raukumara, Wairoa, etc.) defined in the text. Arrows show palaeomagnetic declination anomalies at sample localities, colour coded for age (blue for b 10 Ma, red for 10–25 Ma, green for 25–95 Ma). The stippled region shows the inferred parts of the plate-boundary zone that have undergone clockwise rotation in the Neogene.

142

S. Lamb / Tectonophysics 509 (2011) 135–164

Sinistral offset of Cretaceous on Moutohoura Fault

HK4

TA7 OR

ion

Raukumara Domain

nt Fro

ce

xte

Pw/r

st

-ar

Wairoa Domain

iT ng ura

Ba

Au

ck

stra

lian

ront

Pla

te

Dextral slip

Hik

Wairoa Domain

i Thrust F

Pw/r

Hikurang

trike-slip

Plate

ortening/s

Raukumara Domain

Overprint?

4 Ma - Present

Sinistral slip

Australian

10 - 22 Ma Cretaceous Mesozoic

(c)

20 Ma - 4 Ma

2 - 9 Ma

Miocene Highly deformed

Hik

100 km

P‘w/r

Locality Pliodeclination Pleistocene

Mahia Peninsula

WAIROA DOMAIN

(b)

Northern ‘hinge’

ura ng Fro i Thr ust nt

WU TF MB TH NG NP CH TP SB Hr1 MP Wk1 NP Gisborne MK* Mk1-2 TC, PP Mk1 WW13 NR

hru

elt ar B She Islan d Nort h

MA1

MS

WW7-12

Back-arc sh

RAUKUMARA TA2 DOMAIN TA3 TA4

ns

Ce nt ra lV ol ca ni c

Re gi on

Bac exte k-arc nsio n

(a)

Fig. 5. (a) Geological map of the Raukumara and Wairoa domains (after Mazengarb et al., 2001), showing the major faults and the mean paleomagnetic declinations for localities in Miocene mudstone and siltstones (see Table 1a for paleomagnetic data for localities). The boundary between the two domains forms a hinge where there has been relative rotation of 50°–70° since ~ 20 Ma (Fig. 6), however the structures which have accommodated this remain surprisingly obscure, though defining overall an arcuate zone (see text). (b) and (c) Cartoons illustrating possible kinematics for the hinge zone separating the rotating Wairoa domain from both the Australian plate and the non-rotating Raukumara domain (relative to the Australian plate) before and since 4 Ma.

magnetostratigraphy, where 100s to 1000s m of continuous stratigraphy are sampled at close intervals, involving thousands of measurements, have proved a powerful way to identify convincing primary magnetisations, with systematic reversals and, in some cases, progressive tectonic rotation through time (Roberts, 1990, 1992; Turner et al. 2005; Wright 1986; Wright and Walcott, 1986). In addition, combining numerous measurements from different parts of fold structures provides the basis of a fold test, demonstrating a magnetisation acquired prior to tilting. Full details of the measurement techniques and results are given in the various publications listed in Table 1a and 1b. The measurements are considered to be representative of the Earth's average magnetic field close to the time of the formation of the rock samples if they fulfil most of the following criteria. (1) Secular variation has been averaged out by sampling a sufficient stratigraphic thickness at any particular locality. In general, a stratigraphic thickness representative of at least 10 kyr of continuous sedimentation is required. If secular variation is fully captured, then an angular dispersion of ~ 15° for the spread of magnetisations would be expected at the latitudes of New Zealand (Butler, 1992). (2) Samples have been stepwise cleaned, either thermally or with an alternating field, and show a progressive unblocking

and/or stripping off of a secondary overprint, resulting in a convergence to a stable direction of magnetisation with an intensity at least an order of magnitude greater than the detection limit of the magnetometer. Most measurements were made with a cryogenic magnetometer after thermal cleaning up to 500 °C. (3) At least some of the data have a reversed polarity relative to the present day field, which is consistent with the magnetostratigraphy for these rocks. (4) The inclination, after correcting for the tilt of the strata, is closer than before the tilt correction to that expected for the latitude of the sample at the time of its formation, and in general is within 10° of this inclination. In some cases, flatter inclinations for reversed samples are accepted (Raukumara domain) if the declinations are consistent with those for normal samples, which have the expected inclination for the sample latitude after the tilt correction. Most samples come from strata which dip less than 30° and can be shown to be part of gently plunging (b10°) fold structures. (5) In many cases, it is also possible to show that remanence magnetisations, either at a single locality or grouping of localities, have maximum clustering after a tilt correction with 100% unfolding (Tauxe and Watson, 1994). There is also the possibility of well-grouped magnetisation acquired during

S. Lamb / Tectonophysics 509 (2011) 135–164

folding (for example, in Miocene mudstones and siltstones), as found by Rowan and Roberts (2008). Rotation and flattening anomalies are calculated by comparing the direction of mean locality magnetisation, given its age, with that predicted for the Pacific plate (or Australian plate) at that time and location (Cande et al., 1995, 2000; DiVenere et al., 1994). A rotation anomaly is interpreted to be the result of rotation of a rigid crustal block about a vertical axis. The Pacific plate has undergone negligible Neogene rotation, and b15° anticlockwise in the Paleogene, and b5° clockwise since the Late Cretaceous, relative to True North. The Australian plate has rotated ~ 40° clockwise (~1°/Myr) relative to Pacific Plate since the inception of the New Zealand plate-boundary zone at ~ 43 Ma. It is often convenient, when discussing rotations in North Island and northwestern South Island, to talk about rotations relative to the adjacent Australian Plate. But, when discussing rotations in northeastern South Island, in the Marlborough Fault Zone, it is easier to consider rotations relative to the adjacent Pacific Plate, which is essentially the same as True North for the Neogene.

4. Rotation domains A convenient way to analyse the paleomagnetic data is by grouping results into domains, forming regions roughly 50 km × 100–200 km (Fig. 4, Lamb, 1988; Walcott, 1989). These domains are not necessarily single crustal blocks, but rather clusters of blocks which have similar rotation histories or rotation histories which can be conveniently grouped together. Thus, domains are defined by both geographic location and similarity of structural style and tectonic setting.

143

The kinematics of deformation at the boundaries between domains poses significant and intriguing structural problems, giving important insight into both the consequences and causes of rotation. In the following sections, the paleomagnetic data for the main rotation domains are described in detail.

4.1. Raukumara domain The Raukumara domain comprises the Raukumara Peninsula on the extreme northeastern corner of North Island, with its geological continuation to the north, offshore (Figs. 4–6, Sutherland et al., 2009). It forms the southern end of the Tonga–Kermedec subduction zone, bounded to the west by thinned continental lithosphere, and oceanic lithosphere, in the Havre trough (Reyners et al., 1999; Wright, 1993, 1994). It is underlain by Mesozoic basement rocks in the west, but overlain by extensively deformed marine Cretaceous to Neogene cover sequences farther east (Fig. 5a, Mazengarb et al., 2001). Cretaceous to early Miocene sequences are folded and cut by reverse faults with a predominantly NW–SE trend, forming part of an extensive NW trending allochthonous terrane which extends much farther west, up into the Northland Peninsula. Overlying Miocene and younger mudstones, siltstones and sandstones, up to 1000 m thick, locally rest with angular unconformity on the deformed Paleogene, but elsewhere are pervasively cut by high to moderate angle normal faults which appear to sole into a low angle basal decollement at the base of the Neogene, and thus the extensional deformation here may be of limited vertical extent (b10 km, Mazengarb et al., 2001). Walcott (1987) has suggested that regional uplift in this area may be a consequence of underplating at the base of the crust where it rests on the subducted Pacific plate, with extension in the Neogene

Northeastern North Island Declination deviation from North (°) clockwise +ve

140

Wairoa syncline

Local fold test Magnetostratigraphy

120

Mahia Peninsula East of Gisborne

100

Wairoa Domain

Raukumara Domain Wanganui (WAN) , western North Island

80

10°

Tarakohe Quarry (TAR) , NW Nelson

Hikurangi Margin

Plate Boundary Zone

60

40

Extension in back arc

*

20

* * *

Raukumara Domain Australian Plate

0 0 -20

5

10

15

20

25

Age (Ma)

Fig. 6. Plot of deviation of mean declination of magnetisation from True North (with 95% confidence limits), for localities in the northeastern part of North Island, in the Raukumara and Wairoa domains, against stratigraphic age of the localities (see Figs. 4, 5a, Table 1a). (a) Localities in the Raukumara domain are in the age range 17–22 Ma, with normal and reverse polarities plotted separately, lying within error on the expected curve for the Australian Plate, with an average clockwise rate of rotation up to the present of ~ 1°/Myr. The Australian Plate curve is best defined by a well constrained ~ 17 Ma magnetisation in Northwestern South Island (locality TAR, Fig. 4, Table 1b, Turner et al., 2007), and Pliocene magnetisations in western North Island (locality WAN, Fig. 4, Table 1b, Turner et al., 2005). Defined this way, the localities in the Raukumara domain show about ~ 10° scatter about the Australian Plate curve, either for individual means of reversed or normal components at localities, or between localities, which most likely represents incomplete removal of the effects of magnetic overprints. (b) Localities from the Wairoa domain (including two from east of Gisborne and two from coastal Wairoa domain) show close correlation with the calculated rotation of the Hikurangi Margin or a line spanning the plate-boundary zone, based on finite plate reconstructions (see also red and blue curves in Fig. 3). In this respect, more weight is placed on a mid to late Miocene magnetostratigraphic study (Wright and Walcott, 1986), with sampling every 5 m over 2000 m of stratigraphy (localities marked with asterisk), and also three mid Miocene localities that individually show maximum clustering at 100% unfolding (localities marked with crosses, Rowan and Roberts, 2008). A grouping of all localities older than 10 Ma also show a maximum clustering at 100% unfolding, providing strong support that all magnetisations are primary (see Fig. 8b).

144

S. Lamb / Tectonophysics 509 (2011) 135–164

15 - 25 Ma Mudstones, Raukumara Domain In situ

Tilt corrected

97 - 121% unfolding τ1 principal component

1.0

0.8

0.6

0.4

0.2

0.0 -20

0

20

40

60

80 100 120 140 160

% unfolding

10 - 16 Ma Mudstones, Wairoa Domain In situ

Tilt corrected

99 - 119% unfolding τ1 principal component

1.0

0.8

0.6

0.4

0.2

0.0 -20

0

20

40

60

80 100 120 140 160

% unfolding Fig. 7. Fold tests for groupings of localities from the Raukumara and Wairoa domains, providing strong evidence that the magnetisation is primary, pre-dating folding and acquired when these sediments were laid down. Plots have been constructed by generating 10 random data points for each locality, but with the same locality mean and α95 as the observed data. (a) All localities from the Raukumura domain (15–25 Ma) showing maximum clustering for 100% unfolding, with a ~ 50% increase in the maximum principal component (τ1) compared to in situ co-ordinates. (b) Localities from the Wairoa domain older than 10 Ma, again showing maximum clustering for 100% unfolding, with a ~15% increase in the maximum principal component (τ1) compared to in situ co-ordinates.

cover forming an expression of regional slumping towards the trench (Lamb, 1988; Thornley, 1996). 4.1.1. Paleomagnetic data Fig. 6 shows declinations for 10 paleomagnetic localities in the Raukumara domain (normal and reverse polarities shown separately), plotted against stratigraphic age of the sampled sediments, suggesting only small rotation (b20°) of crustal blocks relative to the Pacific plate in the last 25 Ma, with declinations close to that expected for the Australian Plate (Table 1a, Mumme et al., 1989; Rowan and Roberts, 2005, 2008; Walcott and Mumme, 1982). A fold test on all localities in the Raukumara domain shows maximum clustering for 100% unfolding at the 95% confidence level (Fig. 7a), providing strong evidence that the magnetisations are primary. However, in detail there is about 10° of scatter: localities with normal polarity are slightly steeper and less rotated (declination and inclination is 011/−47) compared to reversed localities (declination and inclination is 200/39). This difference between reversed and normal polarity localities is most easily explained by incomplete removal of a normal modern day overprint on a primary magnetisation that has undergone a small amount of inclination flattening, which will tend to steepen and rotate normal polarities slightly towards north, and flatten and rotate reversed polarities slightly away from south. The best estimate for rotation of the Raukumara domain since the early Miocene is likely to be the mean of reversed and normal polarities, which suggests a declination of 015° for an average age of 18 Ma, essentially identical to that determined for the Australian Plate for 17 Ma rocks at Tarakohe Quarry in northwestern South Island (Locality TAR in Figs. 4, 6, Table 1b, Turner et al., 2007). In this case, the

Raukumara domain, despite evidence for pervasive normal faulting, is part of the Tonga–Kermadec Ridge and has only been translated during the opening of the Havre Trough. 4.2. Wairoa domain The Wairoa domain is a broad regional syncline trending NNE for more than 100 km, folding Miocene to Pleistocene marine mudstones, siltstones and sandstones (Fig. 5a, Mazengarb et al., 2001), bounded to the west by the northern end of the active North Island Shear Belt, where dextral strike-slip faults swing round from a northeasterly to more northerly trend (Figs. 1, 4 and 5a). Even farther west, there is active back-arc extension in the Central Volcanic Region, at the southern end of the Havre trough — this extension terminates about 200 km farther south near Mt Ruapehu, resulting in a gradient of extension which would be anticipated to result in clockwise rotation of both the Wairoa domain and the northern end of the North Island Shear Belt (Lamb, 1988). The finite deformation in the Wairoa domain is small (dips generally less than 30°), except on its eastern and northern margins, and so the domain can be treated as an essentially rigid block with horizontal dimensions of ~ 100 × 100 km and a vertical thickness of between 10 and 20 km, underlain by the subducted Pacific plate (Fig. 1b, Reyners et al., 1999). The eastern limb of the Wairoa syncline, along the coast in the vicinity of the Mahia Peninsula, is cut by reverse faults with the development of smaller scale fold structures. 4.2.1. Paleomagnetic data Extensive paleomagnetic sampling of both limbs of the syncline, including a detailed magnetostratigraphic study of a 2000 m thick

S. Lamb / Tectonophysics 509 (2011) 135–164

145

Northeastern North Island Anomalous Data (Wairoa Domain) Declination deviation from North (°) clockwise +ve

140

Rotation history for Hikurangi Margin proposed by Rowan and Roberts (2008) MP (N)

120

NR (N)

100

?WU corrected

TP (N)

Hikurangi Margin

WU (N)

80

MK (N)

NR (R)

Plate Boundary Zone

60

?MP + TP corrected

?NR corrected 40

Spread of mean rotations in Wairoa Syncline (Fig. 6)

Extension in back arc

20

Australian Plate

0 0 -20

5

10

15

20

25

Age (Ma)

Fig. 8. Plot of anomalous deviations of mean declination of magnetisation from True North (with 95% confidence limits), for 5 localities in the northeastern part of North Island, from the Raukumara and Wairoa domains against stratigraphic age of the localities (see Figs. 4, 5a, Table 1a). All these localities, except Locality MK* (Table 1a), were considered by Rowan and Roberts (2008) to record Late Miocene or younger magnetic overprints acquired during or after folding, forming the basis of a very different interpretation by these authors of the rotation history of the Wairoa domain, compared to that shown by the vast bulk of the paleomagnetic data (Fig. 6, pale red zone). Thus, Rowan and Roberts (2008) determined ‘corrected’ declinations and age of acquisition, defining ~ 70° of clockwise rotation since ~ 8 Ma, or an average rotation rate of 8–14°/Myr (shaded trajectory). However, a detailed analysis of the structure and magnetic sampling at all these localities suggests that the ‘anomalous’ localities are more likely to be a result of inadequate sampling to average out the effects of secular variation, or are present-day field overprints, as well as the possibility of recording local small block rotation (see text) at the edges of a region of pervasive normal faulting (southeastern part of Raukumara domain), and do not constitute a compelling basis for a model of rapid and young regional rotation of the whole Hikurangi Margin.

sequence of middle to Late Miocene sediments, shows that a substantial clockwise rotation up to 80°, relative to True North or the Pacific plate, has occurred in the last 15 Ma (Table 1a, Figs. 1, 4, 5a, 6, Mumme and Walcott, 1985; Rowan et al., 2005; Rowan and Roberts, 2005, 2006, 2008; Walcott and Mumme, 1982; Wright, 1986; Wright and Walcott, 1986). Undisturbed sedimentation occurred in the Miocene and Pliocene during rotation. Overall, the paleomagnetic data are very coherent, showing a progressive rotation since ~ 15 Ma at about 4–5°/Myr relative to True North or the Pacific plate (Figs. 3, 6), and slightly faster in the last ~ 5 Ma (5–6°/Myr). Thus, 19 out of 21 localities follow within error the trend suggested by plate reconstructions for the Hikurangi margin itself (Fig. 6). If these localities are grouped according to age, then it is possible to carry out a fold test. Thus, for the localities older than 10 Ma, there is maximum clustering of locality magnetisations at 100% unfolding, again strongly supporting the conclusion that these are primary magnetisations (Fig. 7b). 4.2.2. Anomalous localities Five localities in all from the Wairoa domain and coastal regions east of Gisborne (Fig. 5a), near the boundary between the Wairoa and Raukumara domains — referred to here as anomalous localities — suggest rotations in Miocene siltstones and mudstones that are up to ~ 50° larger than indicated by the vast majority of the data for the Wairoa domain (localities MK*, NR, MP, TP, WU, Figs. 5a, 8, Table 1a, Rowan and Roberts, 2008). The anomalous localities form the basis of a very different interpretation by Rowan and Roberts (2008) for the rotation history of the whole Hikurangi margin, in which rotation of the margin is thought to commence in the Late Miocene, around ~ 8 Ma, with ~70° of clockwise rotation subsequently (Fig. 8). This inferred rotation history requires regional rotation rates of 8°–14°/Myr (Rowan and Roberts, 2008), significantly faster than the rotation rate of the

Hikurangi margin deduced from the plate motions (4–5°/Myr, Fig. 8), and requiring Late Miocene shortening in the southern part of the Hikurangi margin at a rate faster than the plate convergence rate. For this reason, it is very difficult to reconcile this tectonic interpretation with the known geological and plate tectonic evolution, as acknowledged by Rowan and Roberts (2008). Rowan and Roberts's (2008) tectonic interpretation is largely based on the presence of normal and reversed components in either single localities, or two to three localities up to a few tens of kilometres apart, that show maximum clustering at less than 100% unfolding, interpreted as secondary magnetisations acquired during or after folding (Rowan and Roberts, 2008). Folding in the Wairoa domain and farther north affects both Miocene and Pliocene sequences, and yet the mean declinations of magnetisation for maximum clustering of the magnetic overprints at the anomalous localities could be taken to infer clockwise rotation in the range of 50–80°, implying a rapid and young rotation history (Fig. 8, Rowan and Roberts, 2008). But other aspects of the structural and paleomagnetic data at these anomalous localities cast doubt on this tectonic interpretation: (1) It is unclear whether secular variation is averaged adequately in magnetic overprints, because there is no constraint on the timescale of acquisition of magnetisation. In addition, some of the overprints identified by Rowan and Roberts (2005b, 2006) are based on very few sampled horizons (for example, at locality NR (Fig. 5a), only 3 sampled horizons for the normal component, and 4 sampled horizons for the reversed locality; at WU, the sampling was over only a 7 metre stratigraphic thickness, with an angular dispersion of only ~11° for the magnetisations) suggesting again that some of the discrepancy (especially for Locality NR) could, in fact, just reflect inadequate sampling to average out secular variation.

146

S. Lamb / Tectonophysics 509 (2011) 135–164

(2) The fold tests applied by Rowan and Roberts (2008) involve only two or three localities which are widely spaced, up to 10s km apart, and ignore the possibility that the magnetisations are not synchronous or the effects of fold plunge or local tectonic rotation. These factors could cause maximum clustering to be at less than 100% unfolding, even if the magnetisations were acquired prior to folding (Tauxe and Watson, 1994). Simple tests show that even if the remagnetisations were exactly synchronous, only a 10° fold plunge and/or 10° relative vertical axis rotation could have a significant effect on the outcome of the fold tests. (3) Some of the anomalous rotations could record local large vertical axis rotation of small blocks, especially in the region east of Gisborne, which is located where there is dextral shear and at the southern edge of a region of pervasive extension (Section 6.2.1). This way, these data may show how relative rotation between the Wairoa and Raukumara domains has been accommodated by local small block rotation, rather than recording the rotation of the whole Hikurangi margin (see Section 6.2.1). (4) Magnetisations for four of the five anomalous localities (i.e. excluding the reversed component at locality NR) are normal polarity (Rowan and Roberts, 2008) and may just reflect complex overprints in the present day magnetic field, unrelated to tectonic rotation. For example, locality MK* has an uncorrected declination and inclination of magnetisation (022°/−64°) identical to the present day field in this part of New Zealand and was only identified in a 16 m thick section; the angular dispersion here for magnetisations is low (~9°), and the mean magnetisation is markedly different from detailed magnetostratigraphic studies involving 1000s m of stratigraphy in exactly the same region (Wright and Walcott, 1986). Similar problems of interpretation are encountered for a few localities in Miocene mudstones and siltstones from the Marlborough domains (see Section 4.4.1), where the origin and significance of possible remagnetisations remain unclear. All these data are clearly in

4.3. Wairarapa domain The regions to the south of the Wairoa domain, and east of the North Island Shear Belt, form a continuous nearly straight NE–NNEtrending fold and thrust belt, referred to as the Wairarapa domain (Fig. 4), which has undergone Neogene and active onshore shortening perpendicular to the structural trend, with about 3–5 km of shortening in the last 1 Ma (Barnes et al., 1998; Barnes and Mercier de Lepinay, 1997; Lamb and Vella, 1987; Lee et al., 2002; Nicol et al., 2002, 2007), though normal faulting, which may be part of a regional high-level slump (Pettinga 1985, Hull 1986), similar to that in the Raukumara domain, occurs at the northern end. Deformation along and to the west of the Wairarapa Fault accommodates a significant proportion of the component of plate motion parallel to the plate-boundary zone on dextral strike-slip faults in the North Island Shear Belt (Fig. 4, Barnes and Mercier de Lepinay, 1997; Beanland, 1995; Lamb and Vella, 1987). The Wairarapa domain itself consists of large back-tilted blocks (5–15 km wide × tens of km long and up to 15 km thick) of Mesozoic basement rocks covered mainly by Cenozoic sequences up to 5 km thick, resting on the subducted Pacific plate (Fig. 1b). 4.3.1. Paleomagnetic data Paleomagnetic sampling in the Wairarapa domain is sparse, confined to 9 widely spaced localities in Miocene and Pliocene mudstones and siltstones, spread over a 200 km length of the Hikurangi margin. Fig. 9 shows a plot of declination against the stratigraphic age of paleomagnetic localities in the Wairarapa domain, revealing a progressive rotation with age, following within error the rotation history for the plate-boundary zone or Hikurangi margin (similar to that for the Wairoa domain) predicted by plate reconstructions (Figs. 3, 9).

Southeastern North Island

140

Declination deviation from North (°) clockwise +ve

conflict with the vast bulk of the paleomagnetic data, which give maximum clustering at 100% unfolding, and so it seems safer to leave open the question of their interpretation, rather than using them as the basis of a radically different tectonic interpretation.

120

Wairarapa Domain Western Domains

Hikurangi Margin

100 Spread of mean rotations in Wairoa syncline (Fig. 6)

80

Plate Boundary Zone

60

40

RAN

20

Australian Plate

TUR 0 0

BIR1

WAN 5

10

15

20

25

Age (Ma) -20 Fig. 9. Plot of deviation of mean declination of magnetisation from True North (with 95% confidence limits), for localities in the southeastern part of North Island, from the Wairarapa domains against stratigraphic age of the localities (see Fig. 4, Table 1a). These show a close correlation with the calculated rotation of a line spanning the plate-boundary zone, based on finite plate reconstructions (blue curve in Fig. 3), indicating that the Wairarapa domain may have rotated clockwise since the early Miocene about 10° less than the Wairoa domain (see Fig. 6, pale red zone), also supported by evidence for ~10° less rotation for a Late Pliocene locality in the Wairarapa domain, compared to a similar aged locality in the Wairoa domain (compare localities BIR and WW13, Table 1a). This difference is most likely because of enhanced rotation in the Wairoa domain in the last 4 Ma, associated with extension in the back-arc region.

S. Lamb / Tectonophysics 509 (2011) 135–164

173°E

174°E

~6 UB

4.2 BR 4.8 4.8 BS BOU

9

WH 60

CAV

Ke ke re ng u

F.

en

ar

Cl

SS 54 20

WC1-2 HC2 HC3 17 Cenozoic stratigraphy

DS WD14 18 WD2 CS 3.3

PUH 66

SEY

95

KAI ~50

LYF 30 54 CLA

290°

SW

4.2

31

0° 18 MOT

Southern Marlborough Domain

Conglomerate Siltstone

30

Volcanics

*

Amuri Limestone

40 50

* 42.5°S

70

Shale

80 90

Volcanics

*

Mesozoic Basement

100 Ma

* Paleomagnetic samples

Average basement structural trend, reference azimuth = 290 - 310° (Hall et al. 2004) - See Fig. 12

173°E

*

Siltst./Mudst.

20

60

Upper Cretaceous

eF

Northern Marlborough Domain

10

42°S

Alluvium

Pliocene

Lower Tertiary

.

F ce

.

F atere

WC3

60

DEE*

95

42°S

30 LOT

Assuming primary magnetisation

BB2 30

5.4 NC

Miocene

FC STR

BIG*9

p Ho

CC 8

~6

9

Aw

SV 3.9

RB

WHP

F. irau Wa Hinge of Little & Roberts (1997)

Tectonic rotation (with age in Ma)

WB 3.9

WV

42.5°S

147

50 km 174°E

Fig. 10. Detailed map of the Marlborough Fault Zone comprising the Marlborough domains, showing the general stratigraphy (see text — grey shade on map represents cover sequences, white is basement), basement structural trends (thin dashed lines), and paleomagnetic localities (with ages in Ma, see Table 1b for locality data). The Marlborough Fault Zone links the southern end of the Hikurangi subduction zone with the Alpine Fault, and consists of 5 major dextral strike-slip faults, with slip rates of 4–25 mm/yr in the last 10 Kyr. The marked change in trend of the major Marlborough faults defines the boundary between the southern and northern Marlborough domains (Lamb, 1988), which also coincides in a change in crustal block size, from 10 km scale in the northern Marlborough domain, and 1–10 km scale in the southern Marlborough domain. Note the prominent swing in the basement structural trend (after Hall et al., 2004) and hinge of Little and Roberts (1997), defining part of the New Zealand Orocline.

The oldest locality in the Wairarapa domain (~ 24 Ma) is nearly 10 Myr older than that in the Wairoa domain (~ 15 Ma, Fig. 6). So it is unclear whether the total finite rotation since the early Miocene (~80° clockwise in the Wairarapa domain) is the same as that in Wairoa domain. Indeed, there is a hint that total Neogene rotation in the Wairarapa domain might be 10°–20° less, if the rotation history in the Wairoa domain, continuing back through time, closely follows that for the Hikurangi margin from plate reconstructions (compared to the plate-boundary rotation trend for the Wairarapa domain), reaching ~ 90° clockwise since ~25 Ma (Figs. 6, 9). Further support for this comes from the fact that rotation in the youngest locality in the Wairarapa domain (locality BIR in Fig. 4, Table 1a, 1° ± 9.8° relative to Australian Plate) is indistinguishable from that of the Australian plate itself, whereas a significantly higher rotation (locality WW13 in Fig. 5a, Table 1a, 13° ± 4.5° relative to Australian Plate) is observed in the Wairoa domain (Figs. 5a, 6, 9).

north. Major dextral strike-slip faults in the Marlborough Fault Zone (MFZ), extending for over 100 km and spaced 10–30 km apart, have strike-slip rates between 4 and 25 mm/a averaged over the last 10 Kyr (Wallace et al., 2007 and references therein), and accommodate 70– 100% of the relative plate motion. The finite offsets on individual faults (except the Wairau Fault) are less than ~ 35 km (Little and Jones, 1998), with a cumulative offset of ~ 60 km (Reay, 1993; Wood et al., 1994). Towards the southwest, the Marlborough faults join the transpressive Alpine Fault, which is the major feature of the southern part of the New Zealand plate-boundary zone. There is a distinct change in trend of the Marlborough faults from ~070° in the southwest to ~055° in the northeast, and Lamb (1988) used this to distinguish the northern and southern Marlborough domains (Fig. 10). Cretaceous to Cenozoic cover sequences show a regional ~25° tilt within individual fault blocks, and are more tightly folded into a regional and faulted anticline (half wavelength ~20 km) at the northern ends of the Clarence and Kekerengu Faults

4.4. Marlborough domains The Marlborough fault system marks the southern end of the Hikurangi subduction system, where the plate-boundary zone passes through continental crust (Figs. 1, 4, 10, 11). Here, the component of plate motion parallel to the plate-boundary zone is greater than the normal component, in contrast to the plate-boundary zone further

4.4.1. Paleomagnetic data Paleomagnetic localities in the Marlborough domains span most of the stratigraphy of the Cretaceous–Cenozoic cover sequences that unconformably overly deformed Mesozoic basement. Hence, there is a much wider spread of ages, as well as lithologies, compared to those in the Raukumara, Wairoa, and Wairarapa domains, including a thick

148

S. Lamb / Tectonophysics 509 (2011) 135–164

Northeastern South Island (Marlborough Domains)

(b)

Elongate blocks in shear zone: decreasing rotation rate 10°/Ma with time

-41° 100km Small block rotation

Inception of Hikurangi Margin

180

Paleomagnetic localities > 17 Ma < 11 Ma

6°/Ma 4°/Ma

Regions

140

Region 2

120° - 140° 120

gin M ar

Region 1

80

2°/Ma

100

te n Pla tralia Aus

20

174°

(c)

~80°

60 40

Average clockwise rotation rate since 20 Ma

173°

ne ngi y Zo ura dar n Hik u o te B Pla

Inset (c)

100

it Southern lim n of rotatio

Rotation (°)

Vertical axis rotation (°) clockwise +ve wrt Pacific Plate

160

-42°

50

1 2

~4°/Ma ~7°/Ma

-43°

175°

Creation of N. Awatere/Clarence equidimensional fault block Secondary magnetisations? ~35° Rotation BIG Region WHP train ear s STR er sh in High stra

0 -50

BIG**

2

r hea er s Region 1 Low

BOU

5 FC

10 15 Age (Ma)

0 0 -20

20

40

60

80

100

120

Age (Ma)

Fig. 11. (a) Plot of paleomagnetically determined rotation (with 95% confidence limits), with respect to the Pacific Plate, for localities in the Marlborough Fault Zone (Marlborough domains) against stratigraphic age of the samples (see Figs. 4, 10, Table 1b). Rotations for localities b20 Ma are defined by the mean locality declinations; for localities N 20 Ma, rotations are corrected for the apparent polar wander path for the Pacific Plate. Since ~ 20 Ma, clockwise rotation of crustal blocks has occurred at an average rate of 5–10°/Myr. In detail, the spatial rotation pattern suggests two main regions, characterized by typical average rotation histories since ~ 20 Ma and labelled Regions 1 and 2 (shown as solid red line for Region 1, and pale red zone for Region 2) and defined in (b). Finite plate reconstructions suggest that the whole Hikurangi margin has rotated at ~ 4.5°/Myr since 20 Ma (Fig. 3). (b) Map of the southern end of the Hikurangi margin, showing the Regions 1 and 2, defined by average clockwise rotation rates since ~ 20 Ma. The higher rotation rates (~7°/Myr) are confined to the extreme northern end of the Marlborough Faults. In addition, the boundary between the northern and southern Marlborough domains (heavy dashed line) marks a change from rotation of small blocks (km-scale) in the south, to larger blocks (10 km-scale) farther north. (c) More detailed plot of rotation history for Regions 1 and 2 during the last 15 Ma, showing evidence for marked changes in the rate of rotation through time. In Region 1, a good fit to the paleomagnetically-observed rotation data suggests a decrease in rotation rate from ~ 6°/Myr in the early–middle Miocene to b 2°/Myr today, though if little weight is given to the uncertain result for Locality FC, a constant rate of ~ 4°/Myr also fits the remaining data. In Region 2, detailed paleomagnetic studies of the Late Miocene stratigraphy show that the rotation rate must have decreased markedly from the early to late Miocene, but with a marked increase in the last 4 Ma. These variations in rotation rate can be easily explained as the response of elongate or equidimensional blocks in a zone of distributed dextral shear, with a higher shear strain in Region 2 compared to Region 1 (see text and Fig. 20c–f).

sequence of Late Cretaceous basaltic lavas, Paleogene micritic limestones and basaltic volcanics, and Late Miocene to Pliocene mudstone, siltstones and sandstones (Table 1b, Figs. 4, 10). The paleomagnetism of the rocks, including new data, are described by Randall et al. (2011-this volume), Part 1 and only the main conclusions are summarised here. All magnetisations are regarded as primary unless indicated otherwise. Fig. 11 shows a plot of amount of rotation against age for all paleomagnetic localities relative to the Pacific Plate. Overall, the data suggest all rotation has occurred in the Neogene, since ~20 Ma, with rotations exceeding 120° clockwise in rocks as young as 17 Ma. In detail, two distinct regions can be defined: Region 1 which comprises the southwestern part of the northern Marlborough Domain, and has an average rotation rate ~ 4°/Myr since 20 Ma, essentially the same as the long term rotation rate of the Hikurangi margin (Fig. 11b); Region 2 at the northern ends of the major Marlborough faults, where total rotations are ~130 clockwise, and long term regional rotation rates are in the range of 6–7°/Myr (Regions 1 and 2 in Fig. 11b). The available paleomagnetic data suggest that the rate of rotation has varied through time, with a decrease in the rate of rotation in Region 1 from ~ 6°/Myr in the early–middle Miocene (20–10 Ma), to 2°/Myr or less since then (Randall et al., 2011-this volume, Part 1). In Region 2, a similar decrease in the rate of rotation is apparent prior to the Pliocene, with 4–8°/Myr between 20 and 10 Ma, 1–2°/Myr rotation rate between ~ 10 and ~4 Ma, and rotation rate increasing to ~7°/Myr since ~4 Ma (Fig. 11a and c, Randall et al., 2011-this volume, Part 1). The southern margin of the zone of rotation, in the southern Marlborough domain, is up to 30 km south of the Hope Fault, defined

by small to negligible declination anomalies for paleomagnetic localities here (localities MOT, CLA, Figs. 10, 11b). During the last ~3 Ma, the eastern margin of rotation in the northern Marlborough domain appears to be just east of the Kekerengu Fault, defined by negligible declination anomalies in Pliocene mudstones at localities WD1,2 and CS (Fig. 10, Roberts, 1992; Vickery and Lamb, 1995). However these localities are structurally complex, with dips up to vertical, and it is possible that the fold plunge is incorrectly accounted for (Table 1b). There is evidence (Randall, 2007, Randall et al., 2011-this volume, Part 1) that three localities in the Miocene mudstones and siltstones (WHP, STR, BIG, Table 1b, Fig. 10) record pervasive remagnetisations, similar to those found in the Wairoa domain and farther north (see Section 4.2.2, Rowan and Roberts, 2005, 2006, 2008). Paper 1 argues for acquisition of the magnetisation for localities BIG and STR during the creation of an overlying angular unconformity at ~ 9 Ma (shown as locality BIG*, Table 1b, Fig. 11c). 4.4.2. Rotation of basement structural trends The important structural feature of the northern part of the South Island is the geometry of the Mesozoic basement, which generally dips steeply (N60°). In detail, there is marked local variability, but when averaged over subregions on a scale up to 10 km (Fig. 12b, Hall et al., 2004; Little and Roberts, 1997), a pronounced 80° ± 10° swing in strike becomes apparent (with subvertical average dips), from an azimuth of 300° ± 10° in the SE to ~ 020° farther NW, described in detail by Hall et al. (2004). The greatest curvature is in the vicinity of the Hope Fault (Fig. 12b). The large-scale basement structure is also

S. Lamb / Tectonophysics 509 (2011) 135–164

149

Reference azimuth = 290°

200

Locality Age 95 - 30 Ma 150

Paleomagnetic Rotation (°)

Reference Azimuth = 310° BB2

20 - 17 Ma

WC2 WC1 SS HC2 WC3

100

DEE PUH

Uncertain basement trend

HC3

DS

(b)

WH 173°E

CAV

N

KAI

y

it Un

SEY

20

40

e

nc

e lar

CLA

310°

re

F.

ate

Aw

Strike ridge C

.

F.

42°S

uF

ng

ere

k Ke

Paleomagnetic Locality

0 290°

.

Average structural measurement

LYF

174°E

au F

Wair

LOT 50

Basement kink

60

80

100

120

140

030°

350°

Kaikoura

070°

Hope F.

110°

Local structural rotation (°) or Azimuth

Average basement trend (reference = 290° - 310°)

50 km

43°S

MOT

Profiles

Fig. 12. (a) Plot of paleomagnetically determined rotation (with 95% confidence limits), with respect to the Pacific Plate, for localities in the Marlborough Fault Zone (Marlborough domains) regardless of age (see Figs. 4, 10, Table 1b), against the local structural trend in Mesozoic basement (from Hall et al., 2004; Little and Roberts, 1997), defining part of the New Zealand Orocline (Fig. 1). Horizontal error bars show where there is marked local variation in basement strike. Rotation of basement structural trends is with respect to the reference WNW (290°) or NW (310°) basement trend observed in Pacific plate rocks south of the Marlborough Fault Zone (Hall et al., 2004; Rattenbury et al., 2006). Localities such as DEE and CAV are from the interior of Marlborough Fault Zone, and paleomagnetically determined rotations up to 120° have been observed in rocks as young as 17 Ma (Fig. 10, Table 1b). Therefore, the close correlation between basement trends and paleomagnetically observed rotations strongly suggests that all of the bending in this part of the New Zealand Orocline is Neogene, occurring since ~ 20 Ma. (b) Detailed map of basement structure, from Hall et al. (2004), showing location of paleomagnetic localities plotted in (a).

north adds a further ~ 45° to the basement bending, coinciding with Region 2 (Figs. 10, 12b, Little and Roberts, 1997), making ~ 130° of total bending in the basement structure (Fig. 10, 12b). Fig. 12a shows

reflected in the trend of the Eskhead subterrane, defining part of the eastern part of the New Zealand Orocline (Section 2.2, Fig. 1a,b, Hall et al., 2004; King, 2000; Sutherland, 1999). A pronounced kink in the

Declination deviation from North (°), clockwise +ve

Western Domains Paleogene and Neogene bending of Ophiolite Belt, near Nelson, South Island?

140 120

MAG Magazine Point, SI

100

Paleogene bending of Ophiolite Belt, western North Island?

80 60 40

BEX

BEN

TAR

Bexley, NI

Bennydale, NI

Tarakohe Quarry, SI

RAN

Australian Plate

TAU

Rangitikei, NI TUR Turakina, NI

RAU

Taumaranui, NI

Raurimi, NI

20

Ponganui, NI

PON

(NW Nelson, South Island and Neogene in western North Island) Inangahua, SI

INA

0 5 -20 Wanganui, NI

10

15

20

25

30

35

40

45

50

Age (Ma)

WAN Fig. 13. Plot of deviation of mean declination of magnetisation from True North (with 95% confidence limits), for localities in the Western domain, from western North Island (NI) and Northwestern South Island (SI), against stratigraphic age of the localities (see Fig. 4, Table 1b). Most localities plot on the expected line for the Australian Plate, similar to that for the Raukumara domain. Three Oligocene localities, from mudstone and siltstones deposited between 30 and 35 Ma (Magazine Point, Bexley and Bennydale) show clockwise rotations 30°–80° more than expected for the Australian Plate (blue line), given their age. Despite this, Miocene (20–25 Ma) mudstones and siltstones in the same area (Raurimu and Tauramanui) show rotations expected for the Australian Plate. Assuming the mean magnetisations for Bennydale and Bexley are primary, averaging out secular variation, then the rotations here must be Paleogene in age, and had ceased by the early Miocene, recording bending of basement terranes such as the Dun Mountain Ophiolite belt during the early stages of the Cenozoic development of the New Zealand plate-boundary zone. The large rotation recorded at Magazine Point, near Nelson, most likely reflects both Neogene and Oligocene bendings, as suggested by reconstructions of the plate-boundary zone (see Fig. 22).

150

S. Lamb / Tectonophysics 509 (2011) 135–164

rotation anomalies for Late Cretaceous to Middle Miocene localities, plotted against the average local deviation in strike of the underlying basement from the reference direction in the south (azimuth of 300° ± 10°) — note the uncertainty in regional basement structural trend/rotation where there are local marked variations. Overall, a 1:1 correlation between the paleomagnetic and structural measurements provides clear evidence that this part of the New Zealand Orocline is the result of Neogene rotation, as inferred by Hall et al. (2004) from paleomagnetic data in the coastal regions (Fig. 12a). The region of large rotations, north of the Hope Fault, has effectively behaved as a zone of distributed dextral shear, ~ 100 km wide (described in Paper 1). This shear zone spans both the southern and northern Marlborough domains, and the kinematics of rotation is controlled by the size, shape and orientation of the crustal blocks (Section 7.2). Thus, the coherent bending of the basement terranes, certainly prior to the development of the present Marlborough Faults in Late Miocene and/or Pliocene (Paper 1), shows that crustal blocks in the southern Marlborough domain must be smaller than the detectable curvature of the basement terrains (1–10 km scale), comprising elongate blocks with margins parallel to stratigraphic boundaries. However, the coherent and continuous Cenozoic cover stratigraphy between the major faults in the northern Marlborough domain show that here large elongate blocks, with dimensions 10–20 km × 50 km, have rotated coherently — in the process, the bounding faults have also undergone a major change in orientation. Rotation of smaller and more equidimensional blocks occurred at the northern ends of the Awatere and Clarence faults, where rotations up to ~ 130° clockwise are paleomagnetically observed. 4.5. Western domains The western side of New Zealand, in both the North Island and northern part of the South Island, north of the Alpine and Wairau Faults, is underlain by Mesozoic and older basement terranes, that swing round from a SE-trend in the Northland Peninsula to a southerly, then SW-trend farther south, defining the western part of the New Zealand Orocline (Figs. 1, 2, Sutherland, 1999). In the same way that the eastern part of the orocline, in the Marlborough domains, is a result of Neogene deformation and rotation (Section 4.4.2), it is tempting to assume bending on the western side must also have occurred on the same timescale. However, paleomagnetic and structural data suggest a more complicated history (Mumme and Walcott, 1985; Sutherland, 1999). 4.5.1. Paleomagnetic data Fig. 13 shows paleomagnetic localities in the western part of the North and South Islands. It is clear that most localities, when the mean declinations are plotted against the stratigraphic age of the localities, lie on the curve predicted for the Australian Plate (Figs. 3, 13). For example, west of Nelson, paleomagnetic measurements (localities INA and TAR, Table 1b, Fig. 4, Mumme and Walcott, 1985; Turner et al., 2007) show that N-trending Palaeozoic belts and Cretaceous granites have not rotated relative to the Australian plate in the last 40 Ma. The mean declination for ~ 17 Ma locality TAR is based on a detailed magnetostratigraphic study (Turner et al., 2007), and ~40 Ma locality INA captures the transition from normal to reversed polarity. This lack of rotation is despite the fact that there has been tens of kilometres of Neogene WNW–NW shortening (Nicol et al., 2007), with a substantial amount in the Plio-Pleistocene. Shortening is actively occurring on NE-trending thrusts and N-trending folds and reverse sinistral strikeslip faults. Two Paleogene localities (Bexley and Bennydale) in 30–35 Ma mudstones and siltstones, show evidence of rotation up to 30° clockwise, broadly consistent with the swing in trend of basement terranes from their SW-trend in the Northland Peninsula (Table 1b,

Figs. 4, 13, Mumme and Walcott, 1985). These basement terranes can be traced into South Island. This continuity in structure clearly distinguishes this region from the northern Marlborough domain, which is structurally disconnected from the North Island across Cook Strait (see Section 6.2.3). Paleomagnetic measurements from Oligocene sediments at Magazine Point, just east of Nelson, indicate ~ 80° clockwise rotation relative to the Australian plate in the last 30 Ma (Table 1b, Figs. 4, 13, Mumme and Walcott, 1985). As yet, the number of studies in these older localities is too few to draw any firm conclusions, and more work is clearly needed to verify these results

(a) 36°S

Displacement wrt Pacific Plate Rotation rate (°/Ma) wrt Australian Plate

38°S

Rauk

Block Poles

Wair

2.84°

Rauk Wang

40°S

AxiR

Hast

Hast

2.8°

AxiR Wang

2.99° 0.44°

Wair 3.99° 42°S Block Model for GPS velocities (Wallace et al. 2004) 172°E

174°E

176°E

178°E

(b) 1.5°±0.6°/Ma 36°S

4.3°±1°/Ma 38°S

40 mm/yr

40°S

42°S Active Faulting 10 Kyrs (Beanland and Haines 1998) 172°E

174°E

176°E

178°E

Fig. 14. Short term velocity models for North Island, New Zealand. (a) Rigid blocks defined by Wallace et al. (2004) to model ten years of GPS measurements between 1991 and 2003. There is a strong component of clockwise vorticity in the GPS velocity field, consistent with both clockwise rotation of the Hikurangi margin and the obliquity of relative plate convergence. Note that in their model, two blocks (Rauk and AxiR) span the entire length of North Island, in marked contrast to the pattern of rotated indicated by the paleomagnetic evidence (see Figs. 16, 17). (b) Velocity field based on smoothed pattern of active faulting, averaged over the last 10 kyr, after Beanland and Haines (1998), indicating more rapid rotation relative to the Australian plate of the Wairoa domain (4.3° ± 1°/Myr), compared to the Wairoa domain (1.5° ± 0.6°/Myr), essentially the same as that shown in Fig. 17, and in good agreement with the rotation from paleomagnetic data.

S. Lamb / Tectonophysics 509 (2011) 135–164

(a)

151

(b)

Australian Plate fixed 25°

N

10°

100 km

35°

Pacific Plate fixed

Fig. 15. Displacement field for the Hikurangi Margin over to past 4 Myr (in a present day reference frame), based on a block reconstruction at 4 Ma by Lamb (1988), constrained by finite plate motion, the displacements on the major faults, and assuming rigid body rotation of crustal blocks consistent with paleomagnetic observations: 25° clockwise for the Wairoa domain, 10° clockwise for the Wairarapa domain, 20–35° clockwise for parts of the northern Marlborough domain, relative to Pacific Plate. (a) Displacements relative to Australian Plate. The marked swing in displacements is very similar to that suggested by the pattern of active faulting (see Fig. 14b), and is an inevitable consequence of back-arc extension in the northern part of the Hikurangi margin, and margin parallel dextral strike-slip with some orthogonal compression farther south. (b) Displacements relative to the Pacific Plate; the swing in displacement to more nearly orthogonal to the trend of the subduction zone suggests partitioning of plate convergence to more nearly plate normal convergence near the trench, and margin parallel dextral shear at the back of the deforming wedge, accommodated by both margin parallel dextral strike-slip and clockwise rigid body rotation of crustal blocks resting on the subducted slab.

and determine their regional significance. But they are consistent with Paleogene bending, prior to ~ 25 Ma, in western North Island, and both Paleogene and Neogene bending in South Island east of Nelson, but no Cenozoic rotation, relative to the Australian Plate, of South Island west of Nelson (Figs. 4, 13, Section 6.3). In North Island, at the southern end of the Central Volcanic Region, three major magnetostratigraphic studies in ~ 2000 m thick sections through Pliocene mudstones (2.6–3.6 Ma) along the Wanganui, Turakina, and Rangitikei Rivers, south of the Central Volcanic Region (Fig. 4, Turner et al., 2005; Wilson and McGuire, 1995) have revealed well-constrained vertical axis rotations relative to the Australian plate that progressively increases towards the east, from essentially zero relative to the Australian Plate (−1 ± 10°) at the Wanganui River, to ~ 5° clockwise about 20 km farther east, at the Turakina River, to ~7° clockwise at the Rangitikei River, about 30 km even farther east. This

marked west to east gradient of increasing rotation is surprising, because of the lack of known deformation structures in this region that could accommodate this rotation — the maximum amount of clockwise rotation here is essentially the same as that observed for the same period in the Wairarapa domain, relative to the Australian plate (see Section 6.2.2). 5. Active rotation in the New Zealand plate-boundary zone The short term deformation in New Zealand is well constrained from both geodetic studies (retriangulation and GPS analysis) over 10–100 years, and fault slip vectors, averaged over 1–10 kyr (Fig. 16). For much of New Zealand, there have been no large earthquakes over the last 100 years, and so geodetic measurements are recording strain in the interseismic period in the elastic domain, certainly since the Raukumara Domain no rotation

30.0

Wairoa Domain

Velocity (mm/yr)

25.0

Single block (blue line), rotating 2.8°/Ma (Wallace et al. 2004)

4.7°/Ma

20.0

Three blocks (red line)

15.0

Wairarapa Domain 10.0

Rotation rate relative to Australian Plate Error bars ±3 mm/yr

5.0

Reduced Chi-Squared = 0.2

2.5°/Ma 0.0 100

200

300

400

500

Distance (km)

-5.0

A

B

South

North

Fig. 16. Rigid body orthogonal velocity profiles (relative to Australian plate) along the Hikurangi margin, running up North Island (dashed line AB in Fig. 17), calculated for different rotation rates of the margin and an origin at point A. The red line shows a hypothetical velocity model for three hinged blocks, consistent with rotation rates in the Wairarapa, Wairoa, and Raukumara domains suggested by paleomagnetic data and active fault kinematics — note that in this model the block hinges are close to the boundaries between domains (see Figs. 14b, 17). Blue line shows velocities for Wallace et al.'s (2004) inferred single ‘Rauk’ block (see Fig. 14a), rotating clockwise at 2.8°/Myr in their model for the GPS velocities. If the true pattern follows the red line, with 1-sigma velocity observational uncertainties of ± 3 mm/yr (for example, similar to uncertainties for GPS measured velocities), then the reduced Chi-squared fit for a single block is 0.2. In other words, at this uncertainty level, it is not possible to distinguish a single block from multiple rotating blocks in the GPS velocity field, and the detailed pattern of tectonic rotation is essentially unresolvable with very short term GPS measurements — the pattern of active fault over 10 Kyr, or paleomagnetic measurements on Pliocene sediments, is a much better guide to the pattern of active rotation.

152

S. Lamb / Tectonophysics 509 (2011) 135–164

the north, to WSW or SW azimuth in the south, suggesting an overall clockwise rotational flow of the New Zealand crust about poles of rotation farther west, relative to the Australian Plate (Fig. 14a, Wallace et al., 2004, 2009). Continuum analyses of retriangulation data over ~100 years (Walcott, 1984a,b), and the pattern of Holocene fault slip (Fig. 14b, Beanland and Haines, 1998), as well as the finite displacements over the past 4 Ma (Fig. 15, after Lamb, 1988), all reveal a similar flow pattern. This

Global Positioning System (GPS) record started in the 1990s. These regions are not rigid on this time scale, but rather show continuous elastic strain, and so without a specific model of long term faulting, rotations about vertical axes of rigid blocks are difficult to observe in the geodetic velocity field. The overall pattern of GPS velocities in the Hikurangi margin, relative to the Australian Plate, shows a marked swing from a SSE or SE azimuth in

Oc

ea

on

Tro ug h

n/C

tin

en

tL

ith

os

ph

er

C Lit ontin ho e sp nta he l re

e

‘Step’ in margin

Wairarapa Domain gio

n

2.5° ± 0.6°/Ma wrt Aus

Re

B

Raukumara Domain 17 ±5 mm/yr

Northern ‘hinge’

ront

Ce

ntr

al

Vo

lca

nic

Wairoa Domain 4.7° ± 2.0°/Ma wrt Aus

Wairoa Domain

No

rth

Isl

an

A

Wairarapa Domain

rang

d

Sh

ea

AUSTRALIAN PLATE

Hiku

rB

elt

i Thr

ust F

100 km

Ha vre

N

Wairarapa Fault 20 ± 5 mm/yr

37 mm/yr

25 ± 5 mm/yr

ine

Alp

Northern Marlborough Domain

lt au

F

Hope

Hilurangi Plateau

Fault

Southern ‘hinge’ Continental Lithosphere

PACIFIC PLATE

0 - 4°/Ma wrt Pac

Fig. 17. Colour coded digital bathymetric map (courtesy of National Institute for Water and Atmosphere, New Zealand, 2010) of the New Zealand region, with shaded relief of onshore North Island and the northern part of South Island, with major active tectonic features. The active faulting (Beanland, 1995) in the Central Volcanic Region (onshore southern extension of Havre Trough) and along the North Island Shear Belt, in North Island, and Marlborough Fault Zone (Marlborough domains) in South Island, can be used to estimate the pole of rotation (red ellipses) and rotation rate of the Wairoa and Wairarapa domains, averaged over the last 10 Kyr. Obliquely shaded region defines margin of potentially ‘strong’ lithosphere with thin continental crust outside rotating zone. Note also the step in the continental margin, west of the Raukumara domain, where lithosphere with thin crust may be buttressing the northern end of the rotating margin (Reyners et al., 1999). Small arrows show azimuths of velocities relative to Australian or Pacific plates, with thin radial lines defining poles of rotation. All these suggest the Wairoa domain is actively rotating at 4.7° ± 2.0°/Myr, and the Wairarapa domain at 2.5° ± 0.6°/Myr, relative to the Australian Plate (see text). Faulting in the Marlborough Fault Zone suggests fault blocks between the major fault blocks in the northern Marlborough domain are rotating at 0–4°/Myr relative to the Pacific Plate. These rotations also imply that the North Island Shear Belt itself, as well as the Marlborough Faults in the northern Marlborough domain, are also actively bending, in the process ‘straightening out’ and reducing their curvature. All these rotation rates are identical within error with those inferred from paleomagnetic measurements, averaged over millions of years. The active boundaries between the rotating domains are shown by blue shaded zones, with the ‘northern’ and ‘southern’ hinges defining the ends of the rotating Hikurangi margin.

S. Lamb / Tectonophysics 509 (2011) 135–164

pattern is an inevitable consequence of Plio-Pleistocene and active backarc extension in the north, and margin parallel dextral shear, with a component of orthogonal shortening, in the south. Wallace et al. (2004) attempted to ‘look through’ the elastic deformation by combining the longer term Holocene pattern of faulting with GPS measurements over a period of about 10 years, between 1991 and 2003, in the context of a specific model of interseismic strain and long term rigid crustal blocks (Fig. 14a). Two of their blocks extend 400–500 km up the entire length of the east coast of North Island (Fig. 14a), without taking account of the rotation domains defined by paleomagnetic data (Figs. 4, 17). This way, they determined both the interseismic slip on the subduction interface, and the best fit pattern of rigid body vertical axis rotation of the blocks — in effect, they constrained an average rotation rate (~2.8°/Myr relative to the Australian plate) for the whole margin (Fig. 16). This result has been taken by many authors as a direct measurement of active tectonic rotation rate along the Hikurangi margin (Nicol et al., 2007; Nicol and Wallace, 2007; Rowan and Roberts, 2008; Stern et al., 2006; Wallace et al., 2009). But, although this block model may approximate the overall GPS observed translation of the crust, it is not the same as one defined by the paleomagnetic domains. It therefore implies a major difference between the short term (b10 kyr) and long term (N1 Myr) detailed pattern of rotation along the Hikurangi margin (Nicol et al., 2007; Rowan and Roberts, 2008). Another problem with the Wallace et al. (2004) block model is that it requires the forearc of the Tonga–Kermadec trench, immediately north of New Zealand, to be also rotating rapidly relative to the Australian plate, whereas the along strike variation in the width of the back-arc basin suggests negligible rotation here during the past 4 Ma and longer (Fig. 17). These discrepancies may merely reflect the fact that the average residual in Wallace et al.'s (2004) best-fit model is ~1 sigma (reduced Chi-squared 1.32), or 2.5–3 mm/yr (Fig. 16). Thus, it is easy to show that if block sizes are comparable to the paleomagnetic domains (125 km × 50 km), then the same large-scale velocity field, with similar GPS site spacings, can be fitted at this residual level for a wide range of block rotations (±2°/Myr), including those suggested by the long term paleomagnetic rotation history (Figs. 14–17). This is best illustrated in Fig. 16, where profiles of velocities along the Hikurangi margin (see dashed line AB in Fig. 17 for location) are compared for a one block (Wallace et al., 2004) and three block model (this study) along the profile — the differences between these two models for this profile are small, implying an average misfit of ~0.5 sigma, or a reduced chi-squared of 0.2. The multiple block model is also suggested by the rate and pattern of active faulting, discussed below. 5.1. Active rotation of the Wairoa and Wairarapa domains Beanland and Haines' (1998) produced a smoothed velocity field, based on an integration of Holocene fault slip rates on the major active structures with an estimate of the rates of crustal thickening from uplift data and the crustal thickness. This indicates that the Wairoa domain is rotating clockwise about a vertical axis at 4.3° ± 1°/Myr, and the Wairarapa domain at 1.5° ± 0.6°/Myr, relative to the Australian plate (Fig. 14b). These rotation rates are, in fact, clear to see in the slip vectors on the major faults in North Island on their own, without having to determine a full velocity field (Fig. 17). Thus, extension at the southern end of the Havre Trough constrains the velocity of the northern end of the Wairoa domain to 17 ± 5 mm/yr with an azimuth of 135° to 160° (Fig. 17, Beanland, 1995; Beanland and Haines, 1998; Walcott, 1984a; Wright, 1993, 1994). In the central part of the North Island Shear Belt, the azimuth of the velocity for the southern end of the Wairoa domain and northern end of the Wairarapa domain is constrained to be 200° to 210°, because NW–SE extension on faults is minimal (Reyners, 2010), but there is a component of thrusting on the major strike-slip faults, which trend ~205° (Fig. 17, Beanland, 1995).

153

The velocity of the southern end of the Wairarapa domain is constrained to be 20 ± 5 mm/yr at 225° to 240°, because there is a component of thrusting on the major strike-slip faults, but strike-slip is dominant on faults trending ~ 225° (Fig. 17, Beanland, 1995). All these data require the Wairoa domain to be actively rotating at 4.7° ± 2°/Myr, and the Wairarapa domain at 2.5° ± 0.6°/Myr, relative to the Australian Plate. I conclude that the short term pattern of velocities along the Hikurangi margin is indistinguishable (Figs. 16, 17), within error (at both 2 and 1 sigma confidence levels), from a rotating block model defined by the paleomagnetic domains (on a scale of 50 × 125 km or smaller): no vertical axis rotation of the Raukumara domain relative to the Australian plate; rapid rotation of the Wairoa domain and adjacent parts of the North Island Shear Belt (4–5°/Myr); slightly lower rotation rates for the Wairarapa domain and adjacent parts of the North Island Shear Belt, decreasing to the south (2–3°/Myr). All these rotation rates also require rotation of the Hikurangi Margin, as well as internal bending of the North Island Shear Belt, in the last 4 Ma (Lamb, 1988). 5.2. Active rotation in the Marlborough domains In the Marlborough domains, the active vertical axis rotation rate of fault blocks between the major faults can be estimated from the residual between the dextral shear accommodated by slip on the faults and that predicted from the relative plate convergence. Thus, in the northern Marlborough domain the major dextral strike-slip faults accommodate 33 ± 7 mm/yr or 70–100% of the predicted ~38 mm/yr component of relative plate convergence parallel to the faults (Wallace et al., 2007 and references therein). If all or some of this discrepancy is accommodated by block rotation between the Wairau and Kekerengu Faults, across a zone ~50 km wide, then blocks are rotating at 0–8°/ Myr clockwise (Fig. 17). This gives an upper bound to the block rotation rate, because some of this discrepancy is certainly accommodated by active deformation in the regions to the north and south, especially offshore. Wallace et al. (2007), from an analysis of GPS velocities in South Island, in the context of the pattern of active faulting and relative plate motion, found a shortfall ~ 4 mm/yr in the total estimated fault slip rates, which they suggested was taken up by undetected block rotation and faulting to the south of the Marlborough Fault Zone. In the Marlborough Fault Zone itself, they defined thin fault blocks, 10– 30 km wide and 200–250 km long, which, given their highly elongate shape, were effectively constrained not to rotate with respect to the Pacific Plate. If the pattern of active faulting here is more complicated, such that there are unrecognised (mainly strike parallel) active faults between the major dextral strike-slip faults, then ~ 4 mm/yr could be accommodated by ~ 4°/Myr of clockwise rotation of the fault blocks in the Marlborough domains. The previous analysis shows that the active rotation rate of blocks in the northern Marlborough domain is most likely in the range 0–4°/Myr clockwise. The ~ 80° paleomagnetically observed clockwise rotation since ~ 20 Ma indicates a long term average rotation rate of ~ 4°/Myr, at the upper end of the plausible range of active rotation rate (Fig. 11a and c). Thus, the rate of active rotation here is consistent with a reduction in the rate of rotation towards the present, as also indicated by the detailed paleomagnetic rotation history (Section 4.4.1, Randall et al., 2011-this volume, Part 1). The paleomagnetically observed ~ 35° clockwise rotation since 4–5 Ma for a block about 35 km wide between the northern ends of the Awatere and Kekerengu Faults, straddling the Clarence Fault (Figs. 10, 11b, 22), implies a rotation rate here of 7–8°/Myr. If 0–4°/Myr of this is of regional significance, then this suggests an enhanced local clockwise rotation rate of 4°–8°/Myr, which would accommodate 2–4 mm/yr of dextral shear on the Clarence Fault, averaged over the past 4 Ma. However, it is unclear whether this

154

S. Lamb / Tectonophysics 509 (2011) 135–164

(a)

(b)

(f )

(c)

(d)

(h)

(e)

(i)

Dextral

(j)

(g) Sinistral

1

2

3

Fig. 18. Kinematic models for patterns of faulting that accommodate relative rotation along the Hikurangi margin. Clockwise rotation of the margin relative to the Australian Plate can be accommodated by: (a) dextral strike-slip on an arcuate fault; (b) dextral strike-slip on a set of arcuate or (c) straight faults; (d) shortening on thrust faults, with displacement increasing to the south; (e) extension on normal faults with extension increasing to the north. Relative rotation along the margin suggests the existence of ‘hinges’, which can be accommodated by: (f) and (g) dextral or sinistral slip on a curved strike-slip fault; (h) an abrupt change from extension to compression along the length of the margin; (i) termination of a zone of dextral shear (set of strike-slip faults or rotation of small blocks in a ‘trellis’ system), with a larger block straddling the shear zone and rotating clockwise. (j) The pattern of faulting in (i) will result in progressive ‘straightening out’ of the major faults, illustrated by stages (1)–(3).

block is actively rotating (Wallace et al., 2007; Randall et al., 2011this volume, Part 1). 6. Cenozoic tectonic evolution of the Hikurangi margin Finite plate reconstructions show that the margins of the deforming zone, which today define a rectangular shape, formed a roughly triangular zone when rotated back to their orientations in the early stages of the Cenozoic evolution of the plate-boundary zone (Fig. 2). This large scale shape change from triangle to rectangle was accommodated by strain, displacement and rotation. In particular, the paleomagnetically-determined rotations require gradients of the amount of deformation relative to the boundaries of the deforming lithosphere, in the stable Australian and Pacific plate. The deformation is of three types (Figs. 18–20): (1) dextral strikeslip in an arcuate fault zone (Fig. 18a); (2) dextral strike-slip on a set of oblique faults, which accommodates rotation of a block straddling the strike-slip zone (Fig. 18b, c); and (3) along-strike gradients of extension or compression, which accommodate relative rotation between the two sides of the zone of extension or compression (Fig. 18d, e). As the fault blocks rotate, the deformation structures at their margins will also rotate, re-orientating earlier formed structures. This way, the early pattern of deformation is obscured by subsequent rotation and strain. 6.1. Block reconstructions An essential step in using the paleomagnetic data to unravel the effects of Cenozoic deformation in New Zealand is to define crustal blocks that have undergone coherent rotation, then attempt to reposition them back through time, constrained by the nature and

amount of deformation at their boundaries. The finite motions of the bounding plates determine the total amount of deformation at any stage. These reconstructions should not be regarded as definitive restorations of the New Zealand region, but rather reconstructions that highlight the structural implications of accommodating the paleomagnetically-observed rotations, rather than attempting to deduce these rotations from other observations. For example, they differ in the amount of rotation from Nicol et al.'s (2007) Neogene reconstructions for North Island, where the primary data comes from structural estimates of fault and fold displacements, and the tectonic rotations remain essentially a prediction. Lamb (1988, 1989) produced a ‘cut-out’ block reconstruction of the northern part of New Zealand plate-boundary at 4 Ma, based on the available paleomagnetic data. This reconstruction can be used to calculate finite displacements of material points since 4 Ma (Fig. 15), which shows a marked swing in displacement vectors relative to the Australian plate, accommodated relative to the Australian plate by extension in the northern part (80–100 km back-arc extension at the southern end of the Havre Trough), and dextral strike-slip and thrusting farther south (dextral shear on the southern end of North Island Shear Belt and in the Marlborough Fault Zone). What is striking here is that the main structures accommodating rotation have themselves rotated, so that the kinematics at 4 Ma is significantly different to that today, provide insight into how the faulting accommodated rotation even further back in time. The Paleogene evolution of the plate-boundary remains poorly understood (see Section 6.3), so I focus here on the subsequent Neogene evolution since the inception of subduction along the Hikurangi margin at 25–20 Ma. Following Lamb's (1988) approach (see also Paper 1), Fig. 19 shows illustrative ‘cut-out’ block reconstructions for three stages in the Neogene evolution of the

S. Lamb / Tectonophysics 509 (2011) 135–164

155

Back-arc extension

(a)

(b)

Today

n

gi

100 km

ar

100 km

M gi

Australian Plate

n ra

Australian Plate

ku

Hi

lt

Pacific Plate

au

F ine

ps

p

Al

4 Ma

n

er

th

o

ot

Pr

(c)

100 km

Overlap - future extension

20 Ma

Paleomagnetic North

Plate

100 km ps

So

n er

h

ut

to

Constrained by paleomagnetic domain data ‘Best’ fit

(d)

10 Ma

Australian

u So

Pacific Plate

Al

Al

Pacific Plate

alian

Austr

Plate

Pacific Plate Eskhead subterrane

o Pr

Dunn Mountain Ophiolite Belt Fig. 19. Illustrative ‘cut-out’ block reconstructions for three stages in the Neogene evolution of the Hikurangi margin ((a) 4 Ma, (b) 10 Ma, (c) 20 Ma). The reconstructions are based around the rotation of several ‘key’ blocks or domains (shown by darker grey shade), for which there are good paleomagnetic constraints on rotation. Other blocks (lighter grey shade) are positioned so as to minimise gaps, given the available structural and stratigraphic constraints about the nature and timing of deformation at their boundaries, including offset of prominent basement terrane boundaries. Arrows show paleomagnetic north for individual blocks. The blocks are fitted within the boundary conditions determined by the finite relative plate displacements (Cande and Stock, 2004). Gaps between the blocks represent crustal shortening, whilst overlaps (pink shading) imply subsequent extension. The individual crustal blocks for which the paleomagnetic rotations apply are partly based on the pattern of Neogene faulting, shown in (a) (after Geological Map of New Zealand, GNS). Overall, displacement and rotation has accommodated 70°–90° clockwise rotation of the trend of the Hikurangi margin. In detail, this has involved the progressive clockwise bending of basement structural trends, reflected in the trend of the Eskhead subterrane, and re-alignment of earlier faults including the northern end of the Alpine Fault. At 20 Ma, faulting is predominantly on a series of thrusts parallel to the subduction margin at this time, with no offset of the basement terranes across the Alpine Fault.

Hikurangi margin (4 Ma, 10 Ma, 20 Ma). The reconstructions are based around the rotation of several ‘key’ blocks or domains (Fig. 19) for which there are good paleomagnetic constraints on rotation. Other blocks are positioned so as to minimise gaps, given the available structural and stratigraphic constraints about the nature and timing of deformation at their boundaries, such as bending and offset of basement terranes or structural trends. The blocks are fitted within the boundary conditions determined by the finite relative plate displacements (Cande and Stock, 2004). Gaps between the blocks represent crustal shortening, whilst overlaps imply subsequent extension. Dextral displacement and shortening or extension on the major faults accommodate 70–90° rotation of the whole margin since 20 Ma, with an average rotation rate of 4–4.5°/Myr relative to the Pacific Plate, as inferred by previous studies (Crampton et al., 2003; Lamb and Bibby, 1989; Townsend, 2001; Walcott, 1987, 1989). The overall evolution is one of the progressive bendings of the basement trends, with localised offset across the Alpine Fault and its northern continuation, concomitant with a swing in the trend of both the subduction zone and Early Miocene thrusts in the overlying wedge (Fig. 19). The kinematics of the Neogene evolution of the Hikurangi margin since 20 Ma are summarised in Fig. 20, based on the block reconstructions in Fig. 19: (1) In the northern part of the plate-boundary zone, Neogene rotation of the margin relative to the Pacific plate is mainly

accommodated by slip along the subduction thrust itself, with ~800 km of subduction. (2) A north–south gradient of shortening (or extension) on the western side of the North Island (Fig. 20) has played an important role in accommodating this rotation relative to the Australian plate, with 80–100 km of extension since 4 Ma (Fig. 19b) at the southern end of the Havre Trough (propagating south into the Central Volcanic Region), and ~ 150 km of shortening since the Early Miocene farther south in the Taranaki region (Fig. 19a) — shortening of this amount is consistent with observed crustal thickening there (Stern et al., 2006). The fan-like extension since ~4 Ma in the Central Volcanic Region can also be observed in a 20–30° clockwise rotation of the volcanic arc, presently located along the eastern margin of the extensional zone (Stern, 1987). (3) It is clear that dextral strike-slip on faults orientated obliquely to the trend of the margin has also played a role in accommodating rotation relative to the Australian plate (Fig. 20). Dextral faulting feeds lithosphere southwards, relative to the Australian plate, allowing the margin to rotate clockwise (Fig. 20). However, this dextral strike-slip is very difficult to detect in the field, especially if the locus of faulting has migrated, because the older faults may cut or subdivide the present-day fault blocks. Reconstructions of the plate-boundary zone that do not take account of this strike-slip faulting tend to minimise tectonic rotations, and are more difficult to reconcile with the paleomagnetic data (cf Nicol et al., 2007).

156

S. Lamb / Tectonophysics 509 (2011) 135–164

(a)

20 Ma - 4 Ma

(b)

1°/Ma wrt Pac

4 Ma - Present 1°/Ma

100 km

wrt Pac ~60 mm/yr

Extension

Australian Plate

Australian Plate ~5.5°/Ma

~4°/Ma wrt Pac

wrt Pac

Pacific Plate

Compression

Compression

Pacific Plate ~3.5°/Ma wrt Pac

Enhanced fault rotation

Shear Shear Zone azimuth = 070° Zone

Rapid rotation of equidimensional block(s) ~7°/Ma

SUBDUCTION DOMINATED

Distributed Shear

Slow rotation of main faults <2°/Ma Rotation of elongate fault blocks (2° - 6°/Ma)

SHEAR DOMINATED

Shear taken up mainly by faulting

Passive marker rotation or rotation of many small elongate blocks = basement structure 2° - 6°/Ma Rotation rate decreasing

(c) 0

Width W W ~ 100 km

(d) with displacement/time

Displacement D

Shear Zone

200 km

~6°/Ma ~80°

120

(e)

~2°/Ma

Rotation of passive marker line

Shear rotation model: -1 φ = 90°- θi + tan (D/W - tan(90-θi))

Width W

Shear Zone θi

0 0

100

200

Rotation of elongate blocks

~80°

φ (°) 60

Shear rotation model: φ = 90°- θi + tan-1(D/W - tan(90-θi))

20 Ma

Approximate timescale

2°/Ma

6°/Ma

0 0

300

Displacement (km) 0

Rotation rate decreasing with displacement/time

120

φ (°) 60 θi

(f)

Displacement D

100

200

300

Displacement (km) 0

20 Ma

Approximate timescale

Fig. 20. Cartoon diagrams summarising the Neogene fault and rotation kinematics along the Hikurangi Margin. (a) Rotation prior to 4 Ma was accommodated relative to the Australian Plate by both a gradient shortening along the back of the margin (model d in Fig. 18), with shortening increasing to the south, and also dextral strike-slip on a set of arcuate faults (model b in Fig. 18). The northern and southern boundaries to the rotating margin define hinges with slip on an arcuate fault in the northern hinge (model g in Fig. 18), and rotation of blocks straddling a zone of dextral shear in the southern hinge (model i in Fig. 18). (b) The kinematics of the margin in the last 4 Ma has been modified by back-arc extension in the northern North Island, resulting in ~ 10° bending of the margin itself, so that the northern rotating part of the margin rotates faster, accommodated by increasing extension towards the north in the back arc (model e in Fig. 18), with rotation in the southern part accommodated more-or-less as prior to 4 Ma (models b and d in Fig. 18). (c)–(f) If rotation at the southern end of the Hikurangi margin (Marlborough domains) is modelled as that of a passive marker (numerous small elongate blocks in the southern Marlborough domain), or highly elongate blocks (several large elongate blocks in the northern Marlborough domain), in a zone of distributed dextral shear (width W and a total displacement D), then the amount of rotation will depend on the ratio of displacement to width (D/W). If the displacement rate is constant, then the rate of rotation will progressively decrease through time, as blocks rotate into parallelism with the margins of the shear zone, as is observed from the paleomagnetic data.

(4) A series of ‘hinges’ are apparent along the Hikurangi margin, especially at the northern and southern ends, each accommodating 10°–90° of relative rotation over the past 25 Myr (Fig. 20a, b, and see Section 6.2). (5) In western South Island, along the Southern Alps, the relative plate motion involves oblique continental collision — the crustal root beneath the Southern Alps requires at least ~50 km of shortening (Stern et al., 2007). The full strike-slip displacement on the Alpine Fault occurred since ~20 Ma, when basement terranes such as the Dun Mountain Ophiolite Belt and Eskhead terrane linked up across the fault. However, this dextral shear also involved bending of these terranes on both sides of the Alpine Fault (Fig. 19c). 6.2. Rotation hinges along the Hikurangi Margin 6.2.1. Northern boundary of the rotating Hikurangi Margin The boundary between the Raukumara and Wairoa domain poses perhaps one of the most intriguing structural problems in New Zealand's geology. Paleomagnetic measurements locate the boundary to a region only about 20 km wide, with up to 70° of relative clockwise in the Neogene (Fig. 5a). However, the deformation structures that have accommodated this remain surprisingly cryptic. One of the reasons for this is that deformation has been syn-sedimentary, spanning the same time period as the deformed sediments them-

selves, from the Early Miocene to the Pleistocene and recent. Thus, if the locus of activity has shifted slightly, earlier parts of the boundary will be masked by younger sedimentary sequences. There are several constraints on the nature of this boundary. Firstly, on land, there is no clear topographic difference across the boundary. The boundary also marks the transition from NW–SE shortening to the south, in the Wairoa domain, to NW–SE extension to the north in the Raukumara domain. However, the regional extent of the normal faults, combined with the fact that they do not have a strong topographic expression, strongly suggests that extension is thin-skinned, with a basal detachment at a depth of a few kilometres (Thornley, 1996). Lamb (1988) proposed that the boundary was essentially an arcuate dextral shear zone, with a centre of rotation about 25–50 km north of the boundary, in the Raukumara domain itself, but masked towards the east by thin-skinned extensional structures, with possibly enhanced rotation of small crustal blocks here, accommodating the required dextral shear across the hinge zone (Figs. 5a, c, 20a, b, and see Section 4.2.2). The central part of the hinge region is constrained to a zone ~10 km wide running roughly east–west, at the southern limit of a major ~NEtrending fold and thrust belt (part of the continuation of the early Miocene Northland Allochthon, Rait et al., 1990). Farther west, the hinge swings round to a more northerly trend, and the most-likely structure is a major strike-slip fault (Moutohora Fault) which has a ~20 km apparent sinistral offset of the contact between Cretaceous

S. Lamb / Tectonophysics 509 (2011) 135–164

and older basement sequences (Fig. 5a). It is possible that the sense of slip has changed in the last 4 Ma, with sinistral prior to 4 Ma (Fig. 5b) and dextral subsequently (Fig. 5c) — this would make sense of the present geometry of the hinge. Farther east, the hinge swings to a more northeasterly trend towards the region of pervasive normal faulting, north and east of Gisborne, and may be a more diffuse zone of dextral shear, small block rotation and normal faulting. During the last 4 Ma, the kinematics of rotation of Wairoa domain, relative to the Australian plate, involved a combination of fan-like extension in the Central Volcanic Region, increasing towards the north from an apex near Mt Ruapehu, and dextral shear on the North Island Shear Belt (Figs. 4, 17–20, Lamb, 1988). Fig. 5a and b illustrates possible kinematic models for the Wairoa domain and the northern hinge. Prior to 4 Ma, dextral strike-slip displacement, together with shortening increasing towards the south, accommodated this rotation relative to the Australian plate (Figs. 5b, 19a).

the Wairarapa domain, suggesting 12° ± 11° of relative rotation (clockwise for the Wairoa relative to the Wairarapa domain) since then. This relative rotation also implies bending of the North Island Shear Belt more-or-less at the latitude of Mt Ruapehu (Figs. 4, 17, 19b, 20b, Lamb, 1988). The longer term history of rotation in the Wairarapa and Wairoa domains suggests total relative rotation may be relatively small, b20°, and much of this may have occurred in the last 4 Ma when the Wairoa domain was rotating most rapidly as a consequence of back-arc spreading in the Central Volcanic Region. However, the paleomagnetic evidence for clockwise rotation of Pliocene sequences south of Mt Ruapehu (localities RAN, TUR, and MS, Figs. 4, 13, Table 1a and 1b) suggest rotation in the Wairarapa domain extended west of the North Island Shear Belt, most likely accommodated by strongly arcuate strike-slip faulting here, which would not be expected to have a marked topographic expression. This requires dextral displacements N10 km, linking with the back-arc extension farther north in the Central Volcanic Region. In addition, the effects of Plio-Pleistocene and active thrusting, increasing to the SW and extending offshore on the southeastern margin of the Wanganui Basin, must have helped to accommodate some of this rotation (Fig. 4).

6.2.3. Cook Strait and the boundary between the Wairarapa and northern Marlborough domain Shallow seismic studies in Cook Strait (data from National Institute for Water and Atmosphere (NIWA) website, New Zealand, 2010, Barnes and Audru, 1999) show that the major faults in the northeastern part of the South Island (Marlborough Fault Zone) cannot be directly linked up with those in the North Island. Instead, the crust is broken up into lozenge-shaped blocks, up to 30 km across, bounded by reverse, strike-slip and normal faults (Fig. 21). Cook Strait also marks a change in the pattern of Holocene uplift, with subsidence in Cook Strait compared to uplift onland.

41° S

STRA

ari

n gto

in ell W

NORTH ISLAND

7-8°/Ma

ult Fa re e t a Aw

ce en ar Cl

pa ra ra ai W

2° ± 4°/Ma

Swing in ault basement au F Wair trend

0 - 4°/Ma

u Fa

t ul Fa

IT

SOUTH ISLAND

lt

Oh

K COO

50 km

uF au

lt

6.2.2. Boundary between the Wairoa and Wairarapa domains The boundary between the Wairoa and Wairarapa domains cuts across the Hikurangi margin as a broad zone, N100 km wide, at the latitude of the southern end of the Central Volcanic Region (Fig. 17). This region is the northern end of a NNE-trending zone of dextral transpression, which extends ~200 km up the southeastern part of the North Island as the northward projection of the Wairarapa Fault. Slip on the Wairarapa Fault is predominantly dextral strike-slip, with a dextral slip rate of ~11 mm/yr (Beanland, 1995), but traced farther northeast, the strike-slip component becomes progressively smaller (Figs. 4, 17, Beanland, 1995). In effect, dextral strike-slip in the south is taken up as clockwise rotation of the Wairoa domain farther north, so that the Wairoa domain is rotating faster than the Wairarapa domain, consistent with the paleomagnetic data (Fig. 20b). Thus, the paleomagnetically-determined mean declination in the Wairoa domain for late Pliocene (~ 2.5 Ma) mudstones and siltstones is 017 ± 4.5°, compared to 005 ± 9.8° for similarly aged sediments in

157

lt

u Fa

t

gu

ul Fa

n re

ust gi Thr n a r u Hik

ke

Ke

Front

42° S

Disrupted former northern end of Alpine Fault?

174° E

175° E

Paleomagnetic rotation rate since ~4 Ma wrt Pac

Fig. 21. Map of major active faults in Cook Strait, linking the major dextral-slip faults in the northern Marlborough domain, in South Island, with the North Island Shear Belt and Wairarapa domain (data from National Institute for Water and Atmosphere (NIWA) website, New Zealand, 2010, Barnes and Audru, 1999). In the northern part of the northern Marlborough domain, and Cook Strait, the fault pattern defines several lozenge-shaped fault blocks, up to 30 km across, bounded by reverse, strike-slip or normal faults. Paleomagnetic evidence for ~ 35° of clockwise rotation (wrt Pacific Plate) since ~ 4 Ma of siltstones in the block (shaded red) at the northern end of the Awatere and Clarence faults in South Island, and 5° ± 9° clockwise rotation for ~ 2.5 Ma siltstones in a block (shaded blue) at the southern end of North Island, east of the Wairarapa Fault, indicate that these blocks have rotated, though at variable rates, forming a zone of dextral shear at the northern end of South island and through Cook Strait, in the process kinking the basement structural trends. Reconstructions show that this region formed the northern end of the Alpine fault in the Miocene (Fig. 20c, d), and the present block structure may be partly inherited both from this early phase, and also subsequent dextral strike-slip faulting which has offset segments of the original Alpine Fault. Blocks may have remained coherent because they are underlain by the ‘cold’ subducted slab.

158

S. Lamb / Tectonophysics 509 (2011) 135–164

The pattern of faulting today in Cook Strait suggests that the blocks here are rotating clockwise, comprising a more distributed zone of dextral shear linking up with the trench. Paleomagnetic data show that a block at the northern ends of the Awatere and Clarence Faults, on the western margin of Cook Strait, has rotated on average 7–8°/Myr since ~ 4 Ma, relative to the Pacific Plate (Fig. 21), whereas the region east of the Wairarapa Fault, on the eastern margin of Cook Strait, has rotated on average 2 ± 4°/Myr since ~ 2.5 Ma, relative to the Pacific Plate. Walcott (1984a) suggested that Cook Strait marks the disrupted and rotated former northern end of the Alpine Fault — some of the east–west faults here may be relict segments of this older fault — and some of the present block structure may also be inherited from this early phase (Fig. 21, Barnes and Audru, 1999; Little and Roberts, 1997). 6.2.4. Southern boundary of the rotating Hikurangi Margin The Marlborough domains form the southern hinge to the rotating Hikurangi Margin (Figs. 4, 10, 17, 19, 20), forming a zone of dextral shear, spanning the transition from subduction to deformation through continental lithosphere (Fig. 20). There are several features of the deformation that point to its role as a hinge. Firstly, the swing in basement trends here can itself be thought of as a hinge. The greatest curvature is in the vicinity of the Hope Fault, where there is ~80° swing in trend from more nearly WNW farther south, to a ~NNE trend farther north (Fig. 12b). This change in trend is mirrored by the paleomagnetically-determined rotations for Paleogene to early Miocene rocks (Fig. 12a), and the amount of rotation is essentially the same as that of the Hikurangi margin itself during the Neogene, effectively forming the southern boundary of rotation (Fig. 11b). However, the evolution of deformation in the Marlborough domains shows that the kinematic link between this southern boundary and the rest of the margin, farther north, is more complicated. Lamb (1988) proposed that the boundary between the northern and southern Marlborough domains can also be thought of as a hinge, in a simple kinematic model where the northern Marlborough domain rotates clockwise as a series of coherent and large elongate blocks (10s km scale), accommodated by more distributed dextral shear, with a series of much smaller rotating blocks (km-scale)

(a)

(b) 25 - 35 Ma

between the major faults, in the southern Marlborough domain (Figs. 18–20, Hall et al., 2004; Lamb, 1988, 1989, 1994). This way, Early–Middle Miocene thrust faults in the northern Marlborough evolved into dextral strike-slip faults, as the major Marlborough faults ‘straightened-out’ with ~ 80° clockwise rotation of their northern segments from an initial ~ NW-trend as thrusts to a more nearly NE-trend today as mainly strike-slip faults (Figs. 18i and j, 20b and c), comparable to the rotation of the Hikurangi margin. However, it is clear that although the magnitude of rotation is similar, the rate of rotation at any time is not (Randall et al., 2011-this volume, Part 1). Thus, the rate of rotation in the central part of the northern Marlborough domain appears to have progressively decreased towards the present from ~ 6°/Myr in the early Miocene to ~ 2°/Myr since the Late Miocene, when the major through going strike-slip Marlborough Faults developed (Fig. 11a and c). Finally, the ~45° kink in the basement structural trend at the northern ends of the Marlborough Faults is associated with rotation up to 140° clockwise and significantly greater than the ~80° rotation of the margin itself (Figs. 10, 11b, hinge of Little and Roberts, 1997), with rapid rotation of more equidimensional crustal blocks (~7°/Myr) in the last 4 Ma (Lamb, 1988; Lamb and Bibby, 1989; Roberts, 1995; Townsend and Little, 1998). This makes sense in terms of localised distributed shear adjacent to the Alpine Fault, especially when it passed through Cook Strait in the early–late Miocene (Figs. 19b and c, 20, Barnes and Audru, 1999; Little and Roberts, 1997). The ~140 km dextral offset here of the Eskhead basement terrane (Fig. 19a) indicates significant faulting between the northern Marlborough domain and Wairarapa domains, which is likely to have acted as a hinge to the more coherent rotation of the Hikurangi margin in North Island (Rowan and Roberts, 2008), especially when there was a discrepancy between the rate of rotation of the margin farther north and that of the major fault blocks in the northern Marlborough domain. 6.3. Paleogene evolution of the New Zealand plate-boundary zone Walcott (1987) suggested that the first 15–20 Ma of relative plate motion in the New Zealand region, since the inception of the plate-

(c) 35 - 45 Ma

Emplacement of Northland Allochthon at ~25 Ma

‘Step’ in margin

RD?

RD? Tara

Strain ellipse

nak

Pro

to H Mar ikurang gin i

i Fa

ult

Stage Pole No rotation in NW Nelson

Stage Pole

No rotation in NW Nelson

Passive marker line Simple Shear Zone

Chatham Rise

Dun Mountain Ophiolite Belt

Passive marker rotation in zone of simple shear

RD = Raukumara Domain Fig. 22. Hypothetical reconstructions of the Paleogene plate-boundary zone in the New Zealand region. The geometry of basement terranes on the western side of the Alpine Fault, combined with paleomagnetic data for negligible Neogene rotation (Fig. 13), point to localised Paleogene bending in a zone of dextral simple shear ~ 100 km wide, bounded to the west by the Taranaki Fault (Stagpoole and Nicol, 2008). (a) Diagram illustrating progressive rotation of a passive marker line, spanning a zone of dextral simple shear. This rotation does not necessarily imply actual rigid body rotation, but is a consequence of strain and displacement (Lamb, 1987). (b) and (c) This way, bending between 40 Ma and 20 Ma could have occurred in a transform zone linking up with subduction farther north along the Northland Peninsula in North Island. The postulated position of the Raukumara domain is shown (marked RD), marking the western limit of the Paleogene plate-boundary zone, and the subsequent northern hinge of the Neogene subducting margin.

S. Lamb / Tectonophysics 509 (2011) 135–164

boundary zone at ~ 43 Ma, was accommodated by deformation on the western side of North Island, and east of the Fiordland block in southernmost South Island. This involved bending of the western side of the New Zealand Orocline (see also King, 2000), unless one assumes a sinistral offset of the basement terranes across the New Zealand plate-boundary zone prior to the Neogene (Sutherland, 1999). However, paleomagnetic observations clearly show that the north trending basement structures in northwest Nelson, South Island, have not rotated relative to the Australian plate since ~40 Ma (Figs. 4, 13, 22, Section 4.5). The simplest scenario is Paleogene bending of the Dun Mountain Ophiolite Belt in a relatively narrow zone of dextral simple shear (b200 km wide) extending from South Island up through western North Island, and linking up with a subduction zone north of the Northland Peninsula (Fig. 22, King, 2000; Mumme and Walcott, 1985; Walcott, 1987). This way, bending could have occurred without deformation in adjacent regions, such as northwest Nelson. This deformation is also consistent with the positions of the 30–40 Ma stage poles for the New Zealand plate-boundary zone (Fig. 22, Smith, 1981). The southern and central segments of the Taranaki Fault most probably formed the westernmost limit of this shear zone since the inception of the plate-boundary zone at ~ 43 Ma (Fig. 22, Stagpoole and Nicol, 2008). However, the eastern limit is less clear. Walcott (1987) and King (2000) assumed the shear zone continued up the western side of North Island, to the west of the Central Volcanic Region and Raukumara Peninsula at that time. Given the subsequent Neogene rotational history, with no rotation of the Raukumara domain relative to the Australian plate, it makes dynamical sense that the Raukumara domain has always remained outside the rotational zone (see Section 7.1.1). Thus, the shear zone may have swung round to a more northeasterly trend, consistent with the position of the stage poles, to the SE of the Raukumara domain into what is today the Wairoa, or even Wairarapa, domains (Figs. 4, 22b and c, Whattam et al., 2004). In which case, the northern hinge to the Neogene rotating margin could have been inherited from the Paleogene deformational history. This kinematic configuration requires a step in the ocean/continent margin, where the transform zone joins with the trench, which was progressively reduced by displacement in the transform part of the plate-boundary zone (Fig. 22b and c). If this scenario is correct, then the transform zone only existed until the latest Oligocene (~25 Ma) when the Northland Allochthon — a melange of oceanic crust — was emplaced along the subducting margin (Fig. 22a, Evans, 1992; Rait et al., 1990; Sutherland et al., 2009). The emplacement of the allochthon marks a significant acceleration in the rate of relative plate convergence (Figs. 2, 3), which was ultimately accommodated by subduction and clockwise rotation much farther east, along what is today the Hikurangi margin (Evans, 1992; Lamb and Bibby, 1989; Rait et al., 1990; Walcott, 1987, 1989). However, the possibility of slow subduction along part of the Hikurangi margin in the Paleogene, with very limited tectonic rotation (Fig. 3b), cannot be ruled out (Stagpoole and Nicol, 2008).

7. Dynamical controls on rotation Perhaps the most striking feature of the paleomagnetic data is the coherency of the observed rotation within individual domains, and indeed between domains, indicating a dominant simple pattern of progressive clockwise rotation since the early Miocene (~ 20–25 Ma) at a rate that is essentially the same as that of a marker line spanning the plate-boundary and predicted from finite plate reconstructions. A corollary of this is the stability, throughout the rotation history, of the northern and southern hinges to the rotating margin (Fig. 19). Superposed is ~10° of relative rotation between the Wairoa and Wairarapa domains in the last 4 Ma.

159

7.1. Rotation of the trend of the subducted slab Walcott (1987, 1989) suggested that the dominant control on rotation of the Hikurangi margin is the trend of the underlying subducted slab (Fig. 1b), which is constrained by the relative plate motion. The subducted slab here appears to be anomalous oceanic lithosphere, most likely an oceanic plateau with crust thickened by extensive volcanism (Davy et al., 2007). However, this has not been a barrier to subduction, so that the slab extends to a depth of 250– 600 km beneath New Zealand. However, along the Chatham Rise, the slab is attached to continental crust of the Campbell Plateau (Davy et al., 2007) — here the lithosphere is clearly too buoyant to subduct — and so the southern end of the subducted plate is essentially anchored at the Chatham Rise. Southwest of the Chatham Rise, the plateboundary zone has evolved as a zone of oblique continental collision (Fig. 1). The northern end of the Hikurangi margin marks a transition to subduction of normal Mesozoic oceanic lithosphere, extending north of New Zealand along the Tonga–Kermedec subduction zone (Fig. 1). Here, the Pacific plate has been progressively overridden up to 800 km by the Australian plate since the early Miocene. In addition, there has been back-arc spreading north of New Zealand, behind the Tonga– Kermadec subduction zone — the age of ocean floor here suggests that since ~ 20 Ma, this has resulted in a further 80–100 km eastward translation of the forearc as far north as ~26°S, with negligible rotation about a vertical axis relative to the Australian Plate, and all this occurred in the last 4–5 Ma (Figs. 2, 3, 17, King, 2000; Wright, 1993, 1994). Together, the back-arc spreading and Australian plate motion require a roll-back of the subducted plate, but this decreases markedly southwards along the Hikurangi margin from the northeastern corner of New Zealand to the Chatham Rise. If the overlying plate follows this roll back, then the trend of the trench and subduction zone here must also rotate clockwise, as defined by the rotation of a marker line spanning the plate-boundary zone (Fig. 3). This provides a simple explanation for the steady long term Neogene history of rotation in New Zealand, recorded by the paleomagnetic data, and argues against any abrupt triggering or acceleration of rotation in the Late Miocene by collision between the Australian plate and the Hikurangi plateau on the subducted plate (cf Rowan and Roberts, 2008; Wallace et al., 2004, 2007). The difference in rotation history between the Wairoa, Wairarapa, and northern Marlborough domains represents a second order effect on top of the average long term rotation of the margin. 7.1.1. Dynamical controls on the location of northern hinge The northern hinge of the rotating Hikurangi margin, marking the boundary between the Wairoa and Raukumara domains, has remained fixed in position since ~25 Ma, accommodating ~ 70° of relative rotation. The simplest explanation for this is that the location of this hinge has been determined by an along length contrast in the ease at which the overlying plate can shorten, with either stronger lithosphere, or higher lithospheric gravitational potential energy to the north, where the margin has not rotated relative to the Australian Plate, to weaker lithosphere or lower lithospheric gravitational potential energy farther south along the rotating part of the margin. This make sense in terms of the lithospheric structure here — certainly prior to back-arc spreading and the opening of the Havre Trough in the last ~4 Ma — which marks a north–south transition from old and cold, and hence stronger, lithosphere with oceanic or thinned continental crust, to more normal continental lithosphere (Reyners et al., 1999). The fact that the hinge remained active during back-arc spreading in the Plio-Pleistocene, suggests that the forearc itself also has a north to south strength contrast. Thus, the step-like geometry of the margin on the western side of the Raukumara domain (Fig. 17), suggests that forearc lithosphere with thin crust is essentially acting as a relatively strong buttress, compared to the forearc with thicker crust farther south, preventing rotation, and maintaining the position

160

S. Lamb / Tectonophysics 509 (2011) 135–164

of the northern hinge during opening of the Havre Trough (Fig. 17, Reyners et al., 1999). This geometry may have been inherited from the pre-Cenozoic configuration of the margin, or Paleogene evolution (Fig. 22a, b).

B wrt A

Block wrt A

Plate A

7.2. Strain and rotation

Mainly strike-slip The view described above of the rotation of the Hikurangi margin, essentially driven by the roll-back and change in orientation of the subduction zone, implies that both the nature of the subducted plate and the strength of individual crustal blocks in the overlying plate play only a small role in determining the pattern of rotation. In other words, deformation is similar to that predicted in more general models of lithospheric deformation, such as the floating block model, where the strength of the brittle crust is only a small part of the overall lithospheric strength (Lamb, 1987, 1994; McKenzie and Jackson, 1983a,b). Where the subducted slab lies beneath the crust, crustal blocks cannot be said to be floating on an underlying fluid-like flow, but basal tractions and/or buoyancy contrasts here must drive the overlying deformation, just as if the crust really were floating (Lamb, 1989). The dominant structural trend is parallel to the trend of the trench and subduction zone, defined by fold and fault structures. Therefore, individual crustal blocks are elongate, aligned parallel to the subduction margin. For blocks with an aspect ratio N5, there will be a strong tendency for them to remain parallel to the margin, regardless of the obliquity of subduction or shear along the length of the subduction zone (Lamb, 1987, 1988, 1994). Thus, rotation of these blocks will record rotation of the trend of both the underlying subducted slab and the margin itself. This way, the major thrusts here, which form the edges of the elongate crustal blocks, have remained more-or-less parallel to the trend of the subduction zone throughout their history. Local departure from this behaviour may occur when blocks are more equidimensional, rotating in response to the component of shear along the margin (and see below). 7.2.1. Localised block rotation in dextral shear zones At the ends of the Hikurangi margin, in the vicinity of the rotation hinges (Fig. 20), blocks have undergone localised rotation that is markedly different from that of the margin itself. This rotation is better modelled as a response to distributed shear, with the blocks either embedded in the shear zone or ‘floating’ on an underlying ductile flow (Lamb, 1987, 1994). For example, Hall et al. (2004) and Randall et al., 2011-this volume, Part I (Section 6.2.4) modelled rotation in the Marlborough domains as the response of blocks in a shear zone (Fig. 20), at the southern end of the Hikurangi margin, forming a transform linking subduction with transpression in continental lithosphere. However, deformation within the transform can also be seen as a hinge, accommodating the rotation of the whole Hikurangi margin farther north. The amount of rotation in the shear zone is determined by both the shear strain (displacement to width ratio) in the shear zone, and the aspect ratio and initial orientation of the crustal blocks (Fig. 20c–f, Lamb, 1987, 1994). For high shear strains, local rigid body rotation can exceed that for the whole margin, as is observed in the northern Marlborough domain where paleomagnetically-determined rotations are up to 140° clockwise (Fig. 11). In addition, the rate of rotation will vary with orientation for elongate blocks, either rotating slower or faster than the margin as a whole. This is because the rate of rotation varies from a maximum when the long axes are orientated at right angles to the margins of the shear zone, to zero as they rotate into parallelism with the margins — in the latter case, the amount of rotation approaches a constant value (Lamb, 1987). These effects explain why there might be a decrease in rotation rate towards the present day for the elongate fault blocks in the northern Marlborough domain, as suggested by both the paleomagnetic

Block

Rotating Margin

Block wrt A Block wrt B

Block

Plate B (fixed) Mainly thrusting

Block wrt B

A wrt B Fig. 23. Diagrams illustrating how rotation in the plate-boundary can accommodate partitioning of the deformation into normal compression in the frontal parts and strikeslip at the back of the crustal wedge overlying the subducted slab (after Lamb, 1988). This demonstrates why rotation of the Hikurangi margin will inevitably result in partitioning of plate convergence A crustal block lies within the plate-boundary zone between Plates A and B. The motion of Plate A relative to Plate B (fixed) is described by a translation resulting in deformation with components of dextral shear and compression within the plate-boundary zone (rotation about pole at infinity). The instantaneous motion of the crustal block, relative to Plate B, is described by a clockwise rotation about a pole on the southern margin of the deforming zone, resulting in mainly thrusting between the crustal block and Plate B. The instantaneous motion of the crustal block, relative to Plate A (fixed), is described by a clockwise rotation about pole to the north of the deforming zone, resulting in mainly strike-slip between the crustal block and Plate A. Block instantaneous rotation poles and the relative plate rotation pole are co-linear.

observations and the pattern of short term deformation (Figs. 11a and c, 20c–f, Section 6.2). However, equidimensional blocks will rotate at a constant rate, only determined by the displacement to width ratio of the shear zone (Lamb, 1987, 1994). This explains why the more equidimensional block at the northern ends of the Awatere and Clarence faults, in the northern Marlborough domain, has rotated at ~7°/Myr since 4 Ma, at a faster rate than the margin as a whole (Figs. 19a and b, 20a and b, 11c, Section 6.2.4). Some of the large and rapid rotations in the hinge zone between the Wairoa and Raukumara domains, inferred by Rowan and Roberts (2008), may also be localised small block rotation in a distributed zone of dextral shear, accommodating some of the required dextral displacement across the hinge zone (Fig. 5a–c, Section 4.2.2). 7.2.2. Rotation and partitioning of relative plate convergence The obliquity of relative plate motion today, with the relative plate convergence vector at 30°–45° to the subducting plate margin, suggests a component of dextral shear along its length, accommodated today in part by dextral strike-slip in the North Island shear belt (Fig. 17). Thus, plate convergence in the Hikurangi margin is partially partitioned in a deforming wedge resting on the subducted slab, between a large component of thrusting on the plate interface and frontal parts of the wedge, and a large component of dextral shear at the back of the wedge (Barnes et al., 1998; Barnes and Mercier de Lepinay, 1997; Beanland and Haines, 1998; Bibby, 1981; Lamb and Vella, 1987, 1988; Lamb and Bibby, 1989; Walcott, 1984a,b; Wallace et al., 2004).

S. Lamb / Tectonophysics 509 (2011) 135–164

Lamb (1988) showed how this partitioning could be accommodated by rotation of a crustal block that straddles the plate-boundary zone, illustrated in Fig. 23. For example, back-arc spreading in the northern part of North Island has driven enhanced rotation of the Wairoa domain during the last 4 Ma, helping to accommodate both rotation of the whole margin and the component of motion parallel to this part of the margin, where the North Island Shear Belt is least active (Beanland, 1995; Beanland and Haines, 1998; Wallace et al., 2004). In fact, wherever the margin is rotating, at least some partitioning of the plate convergence will be an inevitable kinematic consequence, and so both margin parallel shear and clockwise rotation of the Hikurangi margin must have been closely linked throughout the Neogene, making it difficult to separate the dynamical controls of each (Fig. 23). 7.2.3. Block stability Crustal blocks in the deforming zone tend to be smaller away from the subduction zone, on a scale of 1–10 km, in contrast to those where the crust is underlain by the subducted slab (Fig. 1b), with dimensions N10 km (Lamb, 1988; Lamb and Bibby, 1989, Paper 1). For example, the pervasive basement rotation in the southern Marlborough domain (Figs. 19, 20) implies crustal blocks on a scale smaller than that of detectable bending (b10 km size), whereas blocks between the major faults in the northern Marlborough domain, or the Wairoa syncline and Wairarapa domain, extend for tens of kilometres. These differences in block size reflect a fundamental contrast in the lithospheric thermal structure, with large blocks where the crust is underlain by the relatively ‘cold’ subducted slab (Figs. 19, 20). In this case, the boundary between the northern and southern Marlborough domains, presently defined by the marked change in orientation of the Marlborough Faults (Fig. 10), may represent a long term contrast in lithospheric strength, so that the southern Marlborough domain has remained largely in ‘warm’ and weak continental lithosphere, away from the effects of subduction and breaking up into small crustal blocks (Figs. 19, 20). In contrast, the northern Marlborough domain has remained relatively ‘cool’, resting on the subducted slab since the early Miocene when subduction initiated in this part of the plateboundary zone (Bibby, 1981; Lamb, 1988; Lamb and Bibby, 1989), breaking up into larger blocks (Fig. 19). In this case, the southern edge of the subducted slab in the Hikurangi margin has exerted a fundamental control on the mode of crustal deformation. 8. Discussion and conclusions Paleomagnetic analysis of vertical axis rotations of crustal blocks in a plate-boundary zone, when combined with knowledge of the finite relative motions of the bounding plates, places powerful constraints on both the tectonic evolution of the plate-boundary zone, and the dynamical controls on crustal deformation. In the obliquely convergent New Zealand plate-boundary zone, finite plate motions show that large scale vertical axis rotations, up to 90°, should be anticipated since the inception of the plate-boundary zone at ~43 Ma, resulting in progressive bending of basement terranes and also rotation of the Hikurangi subduction zone. Paleomagnetic studies on Cretaceous and Cenozoic volcanic and sedimentary rocks reveal a remarkably coherent pattern of tectonic rotation in the Neogene that follows closely the rotation history suggested by the finite plate motions. The pattern and rate of fault slip over the last 10 kyr show that this rotation is active today. Overall, the behaviour of the deforming lithosphere in the New Zealand plate-boundary zone suggests that the strength of individual crustal blocks plays a negligible role in determining the bulk pattern of flow. Rather, where the lithosphere is weak enough to break up into crustal blocks, the motion of these blocks is dominated by the large scale boundary forces of the deforming zone, such as

161

those provided by the relative plate motions and the buoyancy forces or shear tractions of the subducted slab or back-arc region. This behaviour is similar to that predicted in models of lithospheric deformation, such as the floating block model, where the brittle crust passively follows the underlying ductile flow. However, where the crust is resting on the subducted slab, it cannot be said to be floating on an underlying ductile flow. None-the-less, shear tractions at the base of a block, have played a similar role to those in the floating block model. The low thermal gradients in the subduction zone seem to have strengthened the crust sufficiently that it broke up into blocks on a scale of tens of kilometres. The presence of relatively strong oceanic lithosphere at the northern end of the Hikurangi margin may have played an important role in buttressing the subducting margin, preventing rotation here relative to the Australian plate (although there has been translation without rotation as a consequence of PlioPleisticene back-arc spreading farther north) and localising the northern hinge. At the southern end of the subduction zone, and away from the thermal effects of the subducted slab, small crustal blocks (1–10 km scale) have experienced large tectonic rotations in a zone of distributed dextral shear strain, up to 100 km wide. This mode of deformation supports inferences from shear wave splitting studies of a wide zone of distributed ductile deformation at depth in South Island (Molnar et al., 1999; Savage et al., 2007). The Paleogene development of the New Zealand plate-boundary zone is, as yet, poorly constrained, and so I only list below the main paleomagnetic constraints on the Neogene tectonic evolution: (1) Paleomagnetic data define domains along the Hikurangi margin, with dimensions 50 km × 100–200 km. Each domain comprises a cluster of rigid crustal blocks which have similar rotation histories or rotation histories. These show that the eastern parts of the New Zealand plate-boundary zone, comprising the northern Marlborough, Wairarapa and Wairoa domains and extending for ~ 500 km from northeastern South Island to northeastern North Island, have rotated 70°–90° clockwise relative to the Pacific plate (50°–70° clockwise relative to the Australian Plate) since ~20 Ma, at an average rate of 4–4.5°/Myr relative to the Pacific Plate, closely following the rotation history for the margin calculated from finite plate reconstructions. (2) Since 4 Ma, there has been more rapid rotation of the northern part of the Hikurangi margin, with 20–25° of clockwise rotation in the Wairoa domain compared to ~ 10° in the Wairarapa domain. Relative rotation between the Wairoa and Wairarapa domains is accommodated by a marked reduction in dextral strike-slip on the northern extension of the Wairarapa Fault, where it continues up towards the Wairoa domain. It also resulted in bending of the North Island Shear Belt, which forms a zone of dextral shear running up the North Island. (3) The northern and southern limits of the rotating margin define hinges that have remained fixed in position throughout the Neogene, since the inception of subduction. The northern hinge marks the boundary between the Raukumara domain, which has not rotated during the Neogene relative to the Australian Plate, and the Wairoa domain to the south. The hinge forms an arcuate zone of dextral shear, up to 10–50 km wide, which in the last ~ 4 Ma has linked back-arc extension at the southern end of the Havre Trough with the subduction zone. (4) The southern hinge lies in the Marlbourough Fault Zone, in northeastern South Island, which forms a set of dextral strikeslip faults at the southern end of the Hikurangi subduction zone, forming a zone of distributed dextral shear ~ 100 km wide that spans the transition from deforming continental lithosphere to subduction. In detail, there is a pronounced change in trend of the Marlborough Faults from an ~ ENE trend in the

162

S. Lamb / Tectonophysics 509 (2011) 135–164

south, to ~NE trend farther north, defining the boundary between the southern and northern Marlborough domains. Distributed dextral shear in the southern Marlborough domain has accommodated 80°–140° of clockwise block rotation in the northern Marlborough domain, as part of the rotation of the southern end of the Hikurangi margin itself. This has resulted in a progressive ‘straightening-out’ of the Marlborough Faults, as well as 50°–140° bending of basement terranes and the formation of this part of the New Zealand Orocline. Major faults in the northern Marlborough domain have evolved from ~NW trending and margin parallel thrusts in the early Miocene (~20 Ma) to predominantly dextral strike-slip faults with a component of orthogonal convergence today. (5) Crustal blocks in western North Island, and NW Nelson in South Island, have not rotated relative to the Australian plate during the Neogene. These regions define the western limit of Neogene tectonic rotation of the Hikurangi margin. (6) Both geodetic observations (such as GPS measurements) and the pattern and rate of active faulting over the last 10 kyr, show that clockwise vertical axis rotation of the Hikurangi margin is active today, with distribution and rates that are essentially the same as that defined by the paleomagnetic data for the last 4 Ma. An active hinge at the boundary of the non-rotating Raukumara domain, accommodates ~4.7 ± 2°/Myr clockwise rotation relative to the Australian Plate of the Wairoa domain. Farther south, the Wairarapa domain is actively rotating clockwise at a lower rate 2.5° ± 0.6°/Myr relative to the Australian Plate. The faults in the northern Marlborough domain in northeastern South Island, at the southern end of the rotating Hikurangi margin, are actively rotating clockwise at 0–4°/Myr relative to the Pacific Plate, and the rate of rotation has most likely markedly decreased through time towards the present, with a maximum rate (6°–8°/Myr) in the early Neogene. (7) Block ‘cut-out’ reconstructions of the New Zealand plateboundary zone back to 20 Ma show that rotation of the Hikurangi margin, and associated bending of the eastern part of the New Zealand Orocline, is accommodated relative to the Australian Plate by slip on arcuate dextral strike-slip faults, running up North Island, and gradients of shortening increasing to the south. In these reconstructions, all displacement of the Dun Mountain Ophiolite Belt across the Alpine Fault has happened in the last ~ 20 Ma, with associated ~30° clockwise bending of the Ophiolite Belt near the Alpine Fault. In the last 4 Ma, opening up of the Havre Trough and its onshore extension in the Central Volcanic Region in North Island, with extension decreasing to the south, has accommodated ~ 10° additional rotation of the Wairoa domain. Acknowledgements This work was supported by the Natural Environment Research Council, UK and Royal Society of London. An important part of the paleomagnetic database for New Zealand is the result of the painstaking work over a number of years of Sarah Vickery, Lisa Hall, and Karen Randall, who I had the privilege of supervising for their D. Phil theses. I have also greatly benefited from many discussions with Dick Walcott, who saw the big picture long before anybody else. Tim Stern and Martha Savage encouraged me to write this paper, and Jeff Freymueller's careful review greatly improved the manuscript. References Audru, J.C., Delteil, J., 1998. Evidence for early Miocene wrench faulting in the Marlborough fault System, New Zealand: structural implications. Geodin Acta 11 (5), 233–247.

Barnes, P.M., Audru, J.-C., 1999. Quaternary faulting in the offshore Flaxboume and Wairarapa Basins, southern Cook Strait, New Zealand. N. Z. J. Geol. Geophys. 42, 349–367. Barnes, P.M., Mercier de Lepinay, B.M., 1997. Rates and mechanics of rapid frontal accretion along the very obliquely convergent southern Hikurangi Margin, New Zealand. J. Geophys. Res. 102, 24,931–24,952. Barnes, P.M., Mercier de Lepinay, B., Collot, J.-Y., Delteil, J., Audru, J.-C., 1998. Strain partitioning in the transition area between oblique subduction and continental collision, Hikurangi Margin, New Zealand. Tectonics 17 (4), 534–557. Beanland, S., 1995. The North Island Dextral Fault Belt, Hikurangi subduction margin, New Zealand. Unpublished PhD thesis, lodged in the Library, Victoria University of Wellington, Wellington, New Zealand. Beanland, S., Haines, J., 1998. The kinematics of active deformation in the North Island, New Zealand, determined from geological strain rates. N. Z. J. Geol. Geophys. 41, 1–323. Bibby, H.M., 1981. Geodetically determined strain across the southern end of the Tonga–Kermadec–Hikurangi subduction zone. Geophys. J. R. Astron. Soc. 66, 513–533. Bourne, S.J., Arnadottir, T., Beavan, J., Darby, D.J., England, P.C., Parsons, B., Walcott, R.I., Wood, P.R., 1998. Crustal deformation of the Marlborough fault zone in the South Island of New Zealand: geodetic constraints over the interval 1982–1994. J. Geophys. Res. 103, 30,147–30,165. Butler, R.F., 1992. Paleomagnetism: Magnetic domains to geologic terrains. Blackwell Scientific Publishing. Electronic edition www.geo.arizona.edu/Paleomag/book/. Cande, S., Stock, J., 2004. Pacific–Antarctic–Australia motion and the formation of the Macquarrie Plate. Geophys. J. Int. 157, 399–414. Cande, S.C., Raymond, C.A., Stock, J., Haxby, W.F., 1995. Geophysics of the Pitman Fracture Zone and Pacific–Antarctic plate motions during the Cenozoic. Science 270, 947–953. Cande, S.C., Stock, J., Dietmar Müller, R., Ishihara, T., 2000. Cenozoic motion between East and West Antarctica. Nature 404, 145–150. Crampton, J., et al., 2003. Palinspastic reconstructions of southeastern Marlborough, New Zealand, for mid-Cretaceous–Eocene times. N. Z. J. Geol. Geophys. 46, 153–175. Davy, B., Hoernle, K., Werner, R., 2007. Hikurangi Plateau: crustal structure, rifted formation, and Gondwana subduction history. Geochem. Geophys. Geosyst. 9 (7), Q07004. doi:10.1029/2007GC001855. Demarest Jr., H., 1983. Error analysis for the determination of tectonic rotation from paleomagnetic data. J. Geophys. Res. 88, 4321–4328. DeMets, C., Gordon, R.G., Argus, D.F., Stein, S., 1994. Effect of recent revisions to the geomagnetic times scale on estimates of current plate motion. Geophys. Res. Lett. 21, 2191–2194. DiVenere, V.J., Kent, D.V., Dalziel, I.W.D., 1994. Mid-Cretaceous palaeomagnetic results from Marie Byrd Land, West Antarctica: a test of post-100 Ma relative motion between East and West Antarctica. J. Geophys. Res. 99, 15,115–15,139. England, P.C., McKenzie, D.P., 1982. A thin viscous sheet model for continental deformation. Geophys. J. R. Astron. Soc. 70, 295–321 (Correction to A thin viscous sheet model for continental deformation, Geophys. J. R. Astron. Soc., 73, 523–532, 1983.). England, P.C., Molnar, P., 1997a. The field of crustal velocity in Asia calculated from Quaternary rates of slip on faults. Geophys. J. Int. 130, 551–582. England, P.C., Molnar, P., 1997b. Active deformation of Asia: from kinematics to dynamics. Science 278, 647–649. Evans, R., 1992. Tertiary tectonic development of Whangaroa district, northeastern Northland, New Zealand. N. Z. J. Geol. Geophys. 35, 549–559. Freymueller, J., Woodard, H., Cohen, S.C., Elliott, J., Larsen, C., Hreinsdottir, S., Zweck, C., 2008. Active deformation processes in Alaska, based on 5 years of GPS measurements. Active Tectonics and Seismic Potential of Alaska Geophysical Monograph Series, 79, pp. 1–42. 0.029/79GM02. Hall, L.S., Lamb, S.H., Mac Niocaill, C., 2004. Cenozoic distributed rotational deformation, South Island, New Zealand. Tectonics 23 (TC2002). doi:10.1029/2002TC001421. Holt, W.E., Stern, T.A., 1994. Subduction, platform subsidence, and foreland thrust loading: the late Tertiary development of Taranaki Basin, New Zealand. Tectonics 13 (5), 1068–1092. Hunt, T., 1978. Stokes magnetic anomaly system. N. Z. J. Geol. Geophys. 21, 595–606. Hull, A. 1986. The Quaternary geology of the Cape Kidnappers region, Hawke's Bay, New Zealand. Unpublished M.Sc. thesis, Victoria University of Wellington, New Zealand. King, P., 2000. Tectonic reconstructions of New Zealand: 40 Ma to present. N. Z. J. Geol. Geophys. 43 (4), 611–638. Lamb, S., 1987. A model for tectonic rotations about a vertical-axis. Earth Planet. Sci. Lett. 84, 75–86. Lamb, S., 1988. Tectonic rotations about vertical axes during the last 4 Ma in part of the New Zealand plate-boundary zone. J. Struct. Geol. 10, 875–893. Lamb, S., 1989. Rotations about vertical axes in part of the New Zealand plateboundary zone: Theory and Observation. In: Kissel, C., Laj, C. (Eds.), Paleomagnetic Rotations and Continental Deformation. Kluwer, Dordrecht, Netherlands, pp. 473–488. Lamb, S., 1994. Behaviour of the brittle crust in wide plate boundary zones. J. Geophys. Res. 99, 4457–4483. Lamb, S., Bibby, H., 1989. The last 25 Ma of rotational deformation in part of the New Zealand plate-boundary zone. J. Struct. Geol. 11, 473–492. Lamb, S., Vella, P., 1987. The last million years of deformation in part of the New Zealand plate-boundary zone. J. Struct. Geol. 9, 877–891. Lamb, S., Watts, A.B., 2010. The origin of mountains — implications for the behaviour of Earth's lithosphere. Current Science, Indian Academy of Sciences, 99, p. 12. Lee, J.M. et al., 2002. Wairarapa Sheet, 1:250,000 QMAP series, Geological and Nuclear Sciences, New Zealand.

S. Lamb / Tectonophysics 509 (2011) 135–164 Little, T.A., Jones, A., 1998. Seven million years of strike-slip and related off-fault deformation, northeaster Marlborough fault system, South Island, New Zealand. Tectonics 17, 285–302. Little, T.A., Roberts, A.P., 1997. Distribution and mechanism of Neogene to present-day vertical-axis rotations, Pacific–Australian plate boundary, South Island, New Zealand. J. Geophys. Res. 102, 20,447–20,468. Mazengarb, C. et al., 2001. Raukumara Sheet, 1:250,000 QMAP series, Geological and Nuclear Sciences, New Zealand. McCaffrey, R., 2005. Block kinematics of the Pacific–North America plate boundary in the southwestern United States from inversion of GPS, seismological, and geologic data. J. Geophys. Res. 110, B07401. doi:10.1029/2004JB003307. McKenzie, D.P., Jackson, J.A., 1983a. The relationship between strain rates, crustal thickening, paleomagnetism, finite strain and fault movements within a deforming zone. Earth Planet. Sci. Lett. 65, 182–202. McKenzie, D.P., Jackson, J.A., 1983b. The relationship between strain rates, crustal thickening, paleomagnetism, finite strain and fault movements within a deforming zone. Earth Planet. Sci. Lett. 65, 182–202. Molnar, P., et al., 1999. Continuous deformation versus faulting through the continental lithosphere of New Zealand. Science 286, 515–519. Mumme, T.C., Walcott, R.I., 1985. Paleomagnetic studies at Geophysics Division 1980– 1983. Geophys. Div. Rep. N.Z. Dept. Sci. Ind. Res, 204. Mumme, T., Lamb, S., Walcott, R., 1989. The Raukumara paleomagnetic domain: constraints on the tectonic rotation of the East Coast, North Island, New Zealand, from paleomagnetic data. N. Z. J. Geol. Geophys. 32, 317–326. Nicol, A., Wallace, L.M., 2007. Temporal stability of deformation rates: comparison of geological and geodetic observations, Hikurangi subduction margin, New Zealand. Earth Planet Sci. Lett. doi:10.1016/j.epsl.2007.03.039. Nicol, A., Van Dissen, R., Vella, P., Alloway, B., Melhuish, A., 2002. Growth of contractional structures during the last 10 Ma, Hikurangi fore-arc, New Zealand. N. Z. J. Geol. Geophys. 45, 365–385. Nicol, A., Mazengarb, C., Chanier, F., Rait, G., Uruski, C., Wallace, L., 2007. Tectonic evolution of the active Hikurangi subduction margin, New Zealand, since the Oligocene. Tectonics 26 (TC4002). doi:10.1029/2006TC002090. Pettinga, J.R., 1985. Seismic evidence of the offshore extension of the KairakauWaimarama regional slump, Hikurangi Margin. In: New Seismic Profiles, Cores, and Dated Rocks from the Hikurangi Margin, New Zealand (edited by Lewis, K. B.). New Zealand Oceanographic Institute Field Report 22, 27–35. Rait, G., Chanier, F., Waters, D.W., 1990. Landward- and seaward-directed thrusting accompanying the onset of subduction beneath New Zealand. Geology 19, 230–233. Ranalli, G., 1995. Rheology of the Earth, 2nd Edition. Chapman & Hall, London. Randall, K., 2007. Cenozoic tectonic rotations in part of the New Zealand plate boundary zone, D.Phil Thesis, Faculty of Physical Sciences, University of Oxford, England, pp 325. Randall, K., Lamb, S., MacNiocaill, C., 2011. Large tectonic rotations in a wide zone of Neogene distributed dextral shear, northeastern South Island, New Zealand 509, 165–180 (this volume). Rattenbury, M., et al., 2006. Kaikoura Sheet, 1:250,000 QMAP series, Geological and Nuclear Sciences, New Zealand. Reay, M.B., 1993. Geology of the Middle Clarence Valley, Geological Map 10, 1:50000 series, Geological and Nuclear Sciences, N.Z. Reyners, M., 2010. Stress and strain from earthquakes at the southern termination of the Taupo volcanic zone, New Zealand. J. Volcanol. Geotherm. Res. 190, 82–88. Reyners, M., Eberhart-Philips, D., Stuart, G., 1999. A three dimensional image of shallow subduction: crustal structure of the Raukumara Peninsula, New Zealand. Geophys. J. Int. 137, 873–890. Roberts, A. P., 1990. Cainozoic paleomagnetism and tectonics of the Marlborough region, South Island, New Zealand, PhD thesis, Victoria University of Wellington, New Zealand. Roberts, A.P., 1992. Paleomagnetic Constraints on the tectonic rotation of the Southern Hikurangi margin, New Zealand. N. Z. J. Geol. Geophys. 35, 311–323. Roberts, A.P., 1995. Tectonic rotation about the termination of a major strike-slip fault, Marlborough fault system, New Zealand. Geophys. Res. Lett. 22, 187–190. Rowan, C., Roberts, A., 2005. Tectonic and geochronological implications of variably timed magnetizations carried by authigenic greigite in marine sediments from New Zealand. Geology 33, 553–556. Rowan, C., Roberts, A., 2006. Magnetite dissolution, diachronous greigite formation, and secondary magnetizations from pyrite oxidation: unravelling complex magnetizations in Neogene marine sediments from New Zealand. Earth Planet. Sci. Lett. 241 (2006), 119–137. Rowan, R., Roberts, A., 2008. Widespread remagnetizations and a new view of Neogene tectonic rotations within the Australia–Pacific plate boundary zone, New Zealand. J. Geophys. Res. 113, B03103. doi:10.1029/2006JB004594. Rowan, C., Roberts, A., Rait, G., 2005. Relocation of the tectonic boundary between the Raukumara and Wairoa Domains (East Coast, North Island, New Zealand): implications for the rotation history of the Hikurangi margin. N. Z. J. Geol. Geophys. 48, 185–196. Savage, M., Duclos, M., Marson­Pidgeon, K., 2007. Seismic anisotropy in the South Island, New Zealand. In: Okaya, D., Stern, T., Davey, F. (Eds.), A Continental Plate Boundary: Tectonics at South Island, New Zealand: American Geophysical Union Geophysical Monograph Series, 175, pp. 95–116. Smith, E., 1981. Calculation of poles of instantaneous rotation from poles of finite rotation. Geophys. J. R. Astron. Soc. 65 223-221. Stagpoole, V., Nicol, A., 2008. Regional structure and kinematic history of a large subduction back thrust: Taranaki Fault, New Zealand. J. Geophys. Res. 113, B01403. doi:10.1029/2007JB005170.

163

Stern, T.A., 1984. A back-arc basin formed within continental lithosphere: the Central Volcanic Region of New Zealand. Tectonophysics 112, 385–409. Stern, T.A., 1987. Asymmetric back-arc spreading, heat flux and structure associated with the Central Volcanic Region of New Zealand. Earth Planet. Sci. Lett. 85, 265–276. Stern, T.A., Holt, W.E., 1994. Platform subsidence behind an active subduction zone. Nature 368, 233–236. Stern, T.A., Stratford, W.R., Salmon, M.L., 2006. Subduction at a continental margin: kinematics and dynamics of the central North Island, New Zealand. Rev. Geophys. 44 (RG4002). doi:10.1029/2005RG000171. Stern, T.A., Okaya, D., Kleffmann, S., Scherwath, M., Henrys, S., Davey, F., 2007. Geophysical exploration and dynamics of the Alpine Fault Zone. In: Okaya, D., Stern, T., Davey, F. (Eds.), A Continental Plate Boundary, Tectonics at South Island, New Zealand, American Geophysical Union: Geophysical Monograph, 175, pp. 207–233. Sutherland, R., 1995. The Australia–Pacific boundary and Cenozoic plate motions in the SW Pacific: some constraints from Geosat data. Tectonics 14, 819–831. Sutherland, R., 1999. Cenozoic bending of New Zealand basement terranes and Alpine Fault displacement: a brief review. N. Z. J. Geol. Geophys. 42, 295–301. Sutherland, R., Davey, F., Beavan, J., 2000. Plate boundary deformation in South Island New Zealand, is related to inherited lithospheric structure. Earth Planet. Sci. Lett. 177, 141–151. Sutherland, R., et al., 2009. Reactivation of tectonics, crustal underplating, and uplift after 60 Myr of passive subsidence, Raukumara Basin, Hikurangi–Kermadec fore arc, New Zealand: implications. Tectonics 28 (TC5017). doi:10.1029/2008TC002356. Tauxe, L., Watson, G., 1994. The fold test: an eigen analysis approach. Earth Planet. Sci. Lett. 122, 331–341. Thatcher, W., 2009. How the continents deform: the evidence from tectonic geodesy. Ann. Rev. Earth Planet. Sci. 17, 237–262. Thornley, S., 1996. Neogene tectonics of Raukumara Peninsula, Northern Hikurangi Margin, New Zealand, unpublished Ph.D thesis, Victoria University of Wellington, New Zealand. Townsend, D., 2001, Neogene evolution of the Pacific–Australia plate boundary zone in NE Marlborough, South Island, New Zealand, unpublished PhD thesis, Victoria University of Wellington. Townsend, D., Little, T.A., 1998. Pliocene–Quaternary deformation and mechanisms of near-surface strain close to the eastern tip of the Clarence Fault, northeast Marlborough, New Zealand. N. Z. J. Geol. Geophys. 41, 401–417. Turner, G., Kamp, P., McIntyre, A., Hayton, S., McGuire, D., Wilson, G., 2005. A coherent middle Pliocene magnetostratigraphy, Wanganui Basin, New Zealand. J. R. Soc. N. Z. 35, 197–227. Turner, G., Michalk, D., Morgans, H., Walbrecker, J., 2007. Early Miocene magnetostratigraphy and a new palaeomagnetic pole position from New Zealand. Earth Planets Space 59, 841–851. Vickery, S., 1994. Cenozoic deformation in a plate boundary zone, Marlborough, New Zealand, D.Phil. Thesis, Univ. Oxford, U.K. Vickery, S., Lamb, S., 1995. Large tectonic rotations since the Early Miocene in a convergent plate-boundary zone, South Island, New Zealand. Earth Planet. Sci. Lett. 136, 43–59. Walcott, R.I., 1984a. Reconstructions of the New Zealand Region for the Neogene. Palaeogeogr. Palaeoclimatol. Palaeoecol. 46, 217–231. Walcott, R.I., 1984b. The kinematics of the plate boundary zone through New Zealand: a comparison of short-and long-term deformations. Geophys. J. R. Astron. Soc. 79, 613–633. Walcott, R.I., 1987. Geodetic strain and the deformational history of the North Island of New Zealand during the Late Cainozoic. Philos. Trans. R. Soc. London, Ser. A 163–181. doi:10.1098/rsta.1987.0009. Walcott, R.I., 1989. Paleomagnetically observed rotations along the Hikurangi margin of New Zealand. In: Kissel, C., Laj, C. (Eds.), Paleomagnetic Rotations and Continental Deformation. Kluwer, Dordrecht, Netherlands, pp. 459–471. Walcott, R.I., 1998. Modes of oblique compression: Late Cenozoic tectonics of the South Island, New Zealand. Rev. Geophys. 36, 1–26. Walcott, R., Mumme, T.C., 1982. Palaeomagnetic study of the Tertiary sedimentary rocks from the East Coast of the North Island, New Zealand. Department of Scientific and Industrial Research, Geophysics Division Report 189. Walcott, R.I., Christoffel, D.A., Mumme, T.C., 1981. Bending within the axial tectonic belt of New Zealand in the Last 9 Myr from paleomagnetic data. Earth Planet. Sci. Lett. 52, 427–434. Wallace, L., McCaffrey, R., Darby, D., 2004. Subduction zone coupling and tectonic block rotations in the North Island, New Zealand. J. Geophys. Res. 109, B12406. doi:10.1029/ 2004JB003241. Wallace, L., Beavan, J., McCaffrey, R., Berryman, K., Denys, P., 2007. Balancing the plate motion budget in the South Island, New Zealand using GPS, geological and seismological data. Geophys. J. Int. 168, 332–352. doi:10.1111/j.1365-246X.2006.03183.x. Wallace, L., et al., 2009. Characterizing the seismogenic zone of a major plate boundary subduction thrust: Hikurangi Margin, New Zealand. Geochem. Geophys. Geosyst. 10 (No 10). doi:10.1029/2009GC002610 Geophysics, Q10006. Whattam, S.A., Malpas, J.G., Ali, J.R., Smith, I.E.M., Lo, Ching-Hua, 2004. Origin of the Northland Ophiolite, northern New Zealand: discussion of new data and reassessment of the model. N. Z. J. Geol. Geophys. 2004 (47), 383–389. Wilson, G.S., McGuire, D.M., 1995. Distributed deformation due to coupling across a subduction thrust: mechanism of young tectonic rotation within the south Wanganui Basin, New Zealand. Geology 23, 645–648. Wood, R.A., Pettinga, J.R., Bannister, S., Lamarche, G., McMorran, T.J., 1994. Structure of Hanmer strike-slip basin, Hope fault, New Zealand. Geol. Soc. Am. Bull. 106, 1459–1473.

164

S. Lamb / Tectonophysics 509 (2011) 135–164

Wright, I., 1986. Paleomagnetic studies of the Late Miocene Mangapoike River section, northern Hawkes Bay, New Zealand, Unpublished PhD Thesis, Victoria University of Wellington, New Zealand. Wright, I., 1993. Pre-spread rifting and heterogeneous volcanism in the southern Havre Trough back-arc basin. Mar. Geol. 113 (179–200), 179.

Wright, I., 1994. Nature and tectonic setting of the southern Kermadec submarine arc volcanoes: an overview. Mar. Geol. 118, 217–236. Wright, I., Walcott, R., 1986. Large tectonic rotation of part of New Zealand in the last 5 Ma. Earth. Planet. Sci. Lett. 80, 348–352.