The Motunau Fault and other structures at the southern edge of the Australian-Pacific plate boundary, offshore Marlborough, New Zealand

Tecrono&r,ts.

133

88 (1982) 133-159

Elsevier Scientific

Publishing

THE MOTUNAU

Company,

Amsterdam--Printed

FAULT AND OTHER STRUCTURES

EDGE OF THE AUSTRALIAN-PACIFIC MARLBOROUGH,

R.M. CARTER Department

AT THE SOUTHERN

PLATE BOUNDARY,

OFFSHORE

NEW ZEALAND

* and L. CARTER

of Geology, James Cook Universit~y, TownsviNe (Australw)

New Zealand Oceanographic (Received

in The Netherlands

Institute,

P.O. Box 12346, Wellington (New Zealand)

April 13. 1981; revised version

accepted

February

24. 1982)

ABSTRACT

Carter.

R.M. and Carter,

Australian-Pacific The boundary Alpine

Fault.

L., 1982. The Motunau

plate boundary, between

offshore

the Indo-Australian

then links northeastwards

Fault and other structures Marlborough,

at the southern

New Zealand.

and Pacific

with the Hikurangi

plates

passes

subduction

through

margin

transcurrent

faults. The southern

shear zone is situated

a previously

unrecognized

offshore

of the Motunau

Fault,

Porters

Pass Fault

and lineation

(Cretaceous-Cainozoic) formed

data collected evidenced

that underlie

“Tangaroa”

by complexly

by the presence

of deformed

epicentres.

Within

the Marlborough

plate

blocks,

or “microplates”.

20-60

sediments

Inland

Kaikoura.

are most strongly

within each microplate. pattern

being consistent

and geophysical

boundary

causing

right to the southern microplate.

as faults

further

epicentres

Kaikoura

movement

and other

and folding

strata

continue

and Conway

strike-slip

and faulting,

of the Marlborough

Trough.

where the motion

only reach to intermediate

beneath

margin

0040.

University

of Otago,

I95I /X2/0000-0000/$02.75

Dunedin,

to the plate boundary

New Zealand.

i I982 Elaevier Scientific

Publishing

Company

four

Sequence

fault systems; marine

geologic

southwards

durmg

is occurrmg

to the Motunau

t.he southern

developing.

* Formerly

Kaikoura

subduction

is transferred

depths

delimit

to south. these

the overall structural

fault system is not such a continuous

it is likely that this southern

faults

The available

faults. Today,

as

of earthquake

north

blocks respectively.

folding

undegeologtc

of the Motunau

and by the distribution

on the boundary-faults.

Sequence

marine

of the blocks and their subsidiary

to moderate

of the active

in this area today,

of the Pacific plate may have propagated

activation

and since the Motunau

north,

extension

the Kaikoura

shelf are effectively

records

zone the five major

near the margins

head of the Hikurangi

Since earthquake

Conway

successive

limit of the Marlborough

km wide and up to 220 km long. From

is restricted

with dextral

Faulting

late Pleistocene

data suggest that subduction

the late Neogene. system.

deformation

continental

as the

the Marlborough

1097 and 1116 show that the shelf north

sediments.

Seaward

deformed

the Canterbury

zone. 3.5 kHz profiler

Cruises

deformed

substantial

are the Spenser.

To the south

of the plate boundary

on G.R.V.

fault is underlain

system.

sediments

and lie outside

Fault,

New Zealand

through

shear zone. a group of five major, subparallel at the Motunau

edge of the

Tecfonoph.vsics. 88: 133- 159.

physiographic

fault

part of the features

zone is still actively

134

INTRODUCTION

The New Zealand that between connected

Alpine

Fault marks one of the Earth’s major plate boundaries.

the Indo-Australian

to the Hikurangi

and Pacific plates.

Trough

subduction

The north

complex

end of the fault is

by the Marlborough

shear

zone, comprising several large dextral faults splaying northeast from the main Alpine-Wairau Fault (Fig. 1). Walcott has shown from geodetic data that the major faults of the Marlborough shear zone only accommodate about half of the observed modem strain, and that “at least half the movement.. . may be made up of displacements

on numerous

minor

faults, rotation

ductile flow in the crust” (Walcott, Within the terms of plate-tectonic

MOBIL

GULFREX

of blocks between

1978, p. 153). models, the Alpine

the major faults or by

Fault is considered

today as

1972

1973

42’

Fig. 1. Regional bathymetric

locality

profiles

as in the original

map for northeastern

available

track charts:

South

Island

for the Canterbury-Marlborough an uncertainty

of position

showing

the positions

shelr. (Oil-company

of al least

of the seismic profiles

e 2 km is possible.)

and

are plotted

135

a trench-trench major

Hikurangi seismic,

transform

element

(McKenzie

of compression

Trough,

a filled trench

sedimentologic

and

and Morgan,

developed (Katz,

structural

(Scholz

1969), along which

et al., 1973; Lensen,

1974) is situated features

there is a 1975). The

just east of the volcanic,

of the North

Island

subduction

complex (Cole, 1978; Hatherton, 1980; Lewis, 1973, 1980). The Marlborough shear zone between is less well understood. Though most writers agree that the Marlborough faults are dominantly strike-slip, opinion is divided as to how the faults were initiated, and as to whether all faults of the system are equally active or whether they have developed sequentially to the south in response to changing plate motions (Scholz et al., 1973). The precise way in which subduction changes

to dominantly

strike-slip

motion

on the Hikurangi

in the Marlborough

shear

under

discussion;

Arabasz

and Robinson

(1976) show that deep crustal

define

a dipping

Benioff

zone

the more

Fig. 2. Structural

map of the north

data

(1964).

from Gregg

beneath

Canberburlv-hlarlborough

northerly

faults

shelf and adjacent

margin

zone is also earthquakes

of the system,

areas onland.

Onland

136

whereas

Lewis (1980)

argues

that the southern

west of where it swings in towards

portion

the Marlborough

of the Hikurangi coast, is largely

Trough.

a transform

feature. The southern boundary

1979; Sporli, and

margin

of the Marlborough

zone, has traditionally 1980). G.R.V.

1116 (December,

the continental

“Tangaroa”

1980) included

the shelf in this region, uplifted.

Deformation

of the Hope Fault.

to the

1979)

and sampling

These data,

together

profiles (e.g. Mobil, shear zone continues

by growing

abruptly

of the plate (e.g. Suggate.

1097 (August-September,

of seismic profiling

petroleum company with the Marlborough

which is underlain terminates

with the Hope Fault

Cruises

periods

shelf to the southeast

unpublished but freely available show that deformation associated

shear zone, and therefore

been identified

on with

1972), across

folds and is being actively

southeast

along

a previously

unmapped, major fault of the Marlborough shear zone, the Motunau fault system (Fig. 2). We take the opportunity here to describe this feature and its associated structures,

and to comment

THE MOTUNAU

FAULT

on the regional

significance

of these new data.

SYSTEM

The Motunau Fault occurs as a major bathymetric feature crossing the continental slope about half way between Banks and Kaikoura peninsulas (Figs. 2-3). Near latitude 43”s the fault trends NE and is marked by a narrow gutter, up to 200 m deep, separating a flat-topped spur from the upper parts of the continental slope, themselves steep and continuous with the western wall of the gutter (Fig. 3, Line T4). At its eastern end the fault passes into north-trending faults that extend along the continental slope to define the eastern side of Conway Ridge. The infrequency of suitable satellite passes over New Zealand means that there is considerable uncertainty in the location of the available deep seismic lines in this vicinity (? c. 2 km). Nonetheless, we infer that profiles Mobile 72-4 (Fig. 4) and Gulfrex

76 (Fig. 5) cross the Motunau

Fault.

The continental dipping sequence

shelf and upper slope (Figs. 4-5) are underlain by an eastward of Cretaceous-Cainozoic rocks which can be divided into two

seismic sequences

across a mid-Cainozoic

unconformity.

Below the unconformity

lie

complexly folded and faulted strata of the Onekakara Group (Carter, 1977), a marine transgressive sequence that developed across eastern South Island between the Late Cretaceous and Oligocene. Above the unconformity lie less deformed, but still folded and faulted, strata equivalent to the Otakou Group, which represents the eastward propagation of the eastern South Island late Cainozoic continental shelfslope complex, a regressive sedimentary sequence which formed consequent upon uplift along the plate boundary zone further west (e.g. Walcott and Cresswell, 1979). By analogy with nearby onland sequences (e.g. Prebble, 1980) the unconformity is probably of early middle Miocene age, and marks the start of tectonically controlled sedimentation associated with the development of the Mariborough shear zone (Carter and Norris, 1976).

137

‘. Kai koura

Motunau Fault

Fig. 3. Bathymetric

profiles

across

the continental

margin

betwern

the Conway

and Hurunui

Kl~ers iwc

Fig. 1 for location).

This bipartite

sedimentary

sequence

is truncated

to the east by a deep-seated

and

active fault which displaces the sea-floor by about 200 m, and is associated with a number of smaller, shallow faults further east still. The sedimentary sequence to the east of the ~otunau Fault comprises gently deformed sediments with entirely different reflection characteristics to those west of the fault, their regularly reflecting nature suggesting they form part of the late Cainozoic Trough (cf. Katz, 1974; Lewis, 1980).

turbiditic

fill of the Hikurangi

Fig. 4. Line interpretatians

of deep seismic profiles

Mobil 72-3 and 72-4

0

(a) LINE

5

72 -3

10 15 km

5s

4s

33

2s

15

OS

139

20 km

10

Fig. 5. The Motunau

Extension

Fault as seen on deep seismic profile Gulfrex

76. (Ignore

multiple

reflection,

WI)

landwards across the shelf

South of latitude 43’S the Motunau Fault bends rapidly and traverses continental shelf in a more nearly easterly direction. Few deep seismic profiles

the are

available from this region (Fig. l), making it difficult to specify the exact location of the fault. However, the Motunau Fault is inferred to run up the sea-valley seen on line T4 (Figs. 1, 3) and to continue westwards to link with faults on the same trend on the inner shelf (Fig. 2). (An alternative interpretation would be that the inner shelf and slope features Despite

represent

discrete

but broadly

the lack of deep seismic cover, a number

which cross the projected lines cover the region following facts:

extension

cohnear

faults.)

of 3.5 kHz profiles

are available

of the fault across shelf, and a number

to the northwest

of the fault. These profiles

of other

demonstrate

the

(1) The shelf sector of the Motunau fault does not involve appreciable displacement of the sea-floor, at least by comparison with that seen on the continental slope. (2) A dramatic change in tectonic style takes place across the general line of the Motunau fault. To the north, Cainozoic sedimentary rocks are tightly folded, faulted and intruded by igneous rocks; to the south, the same sediments have gentle, homoclinal dips, except immediately adjacent to the fault, where minor drag may occur, and such faults as occur are mainly small-scale features (Fig. 6). (3) North of the fault tectonic activity has continued through the late Pleistocene as shown by the presence of growing folds at the sea-floor: similar features do not occur to the south.

3

2

I

,O

4

-I.OS !Zway)

Fig. 6. a. 3.5 kHz profile and faulted nature

nature

of the inner shelf just north of the Motunau

of the Cainozoic

sediments

Fault.

Note the complexly

(T). and the presence of pinnacles

of probable

folded intrusive

(p); ignore multiple reflection ( NI).

b. Deep seismic profile essentially Canyon

undeformed is due

sediments,

Extension

Mobile nature

to velocity

72-7 from

the mid-shelf

of the Cainozoic pull-down.)

then by thin Kekenoda

of the Motunau

Group

sediments.

region

south

(Apparent

Basement

(A) is overlain

(reflector

B) and regressive

of the Motunau folding

under

by transparent Gtakou

Fault.

Note the

the head of Pegasus Onekakara

Group

Group.

Fault onshore

Satellite imagery (Fig. 7) shows a marked, east-trending lineation at the mouth of the Waipara River, and we infer this to mark the landward continuation of the Motunau Fault (cf. Wellman, 1979, Fig. 1). As seen offshore, there is an abrupt change of style of deformation across this feature. Cainozoic sequences north of the fault are complexly deformed (Gregg, 1964) with faulting, tight folding and evidence for more than one period of post-Miocene deformation (Bradshaw, 1975; Bradshaw and Newman, 1979), whereas similar sequences to the south are flat-lying or gently dipping, The same contrast in tectonic activity persists to the present, as shown by the presence of high-level marine benches of late Pleistocene age on the north side of the fault at the coast (Wilson, 1963; Suggate et al., 1978). Inland, and a little to the southwest, a further lineation occurs at the junction of the Canterbury Plains and the north Canterbury foothills of the Southern Alps.

141

142

thence

passing

along

River valley (Figs. dextral

fault (Gregg,

occur along mapped

the Kowhai

1964) and Recent

the southern

system

across

Pass and

into the Rakar;l

coincides

with an active

fault traces along the same WSW trend also

orientation

may extend

Porters

Pass this lineation

side of the Rakaia

there, the colinear

the fault

River,

1 and 7). At Porters

River. Though

of the main

deep into

Rakaia

the Southern

Alps.

no faults are presently headwater Since

suggests the other

that more

northerly faults of the Marlborough shear zone show diminishing throw towards the Alpine Fault (Freund, 1971) it is likely that the Motunau Fault too will die out somewhere

in the headwaters

of the Rakaia.

Ancillary faults Just inland from the coast, and along the same eastward trend as the inner shelf sector of the Motunau Fault, occurs the long-known trace of the Ashley Fault, an active, dextral feature (Lensen, 1977) (Figs. 2 and 7). This feature may well continue under the Canterbury Plains and link further west with the main Motunau Fault. Another, but larger, splay from the Motunau Fault probably runs along the inner shelf north of Motunau uplifting rock-platform

Point (Fig. 2) demarcating the seaward edge of an actively that carries only minor modern sediment cover (cf. Fig. 11).

Just south of the Conway Trough this platform northeasterly trending fault which we tentatively Motunau Fault.

is bounded to the east by a major extend southwards to link with the

The Pegasus Bay Fault The Pegasus Bay Fault sequence in central Pegasus

occurs as a substantial dislocation of the Cainozoic Bay, about 20 km south of and subparallel to the main PEGASUS FAULT

0 -

2

4

6

a

Fig. 8. Line interpretation Group

a replacement

IO km

LINE

of deep seismic profile

term for Conway

of the profile is on the right.

Group,

by Warren

-----To

I

72-6

Mobil 72-6. (Nomenclature

preoccupied

BAY

after Carter,

and Speden.

1977: Otakou

1977.) The north end

143

A\\

z

Motunau

Fault

(Figs. 2 and 8). A small anticline

which displaces continues

the sea-floor

onland

northeasterly

under

trending

and is therefore

the Canterbury concentration

occurs

to the south of the fault.

an active feature.

The fault probable

Plains, where it is marked of earthquake

epicentres

by a conspicuous just

northwest

c)f

Christchurch city (Fig. 9). The lack of epicentres further southwest, and the fact thnt the fault is not visible on seismic line BP-A (Fig. 1). suggests that the Pegasus 1Ja~ Fault

is a discrete

feature

rather

than a splay off the Motunau

fault syxtcm.

Seismic activity Summary maps of crustal seismicity (Arabasz and Robinson, 1976; Hatherton. 1980) demonstrate that seismic activity shows a regional change in pattern across the Motunau fault system, and Evison (1971) located the magnitude 7, 1901 Cheviot earthquake near the line of the fault. North and west of the fault seismic activity is similar to that of the main seismic zone of the Hikurangi-Marlborough margins, with frequent earthquakes and earthquake swarms, including deep focus and high intensity (> 6) events (Fig. 9). South of the fault, seismic activity consists of scattered

shallow,

low intensity

earthquakes.

Within

the zone between

the Motunau

and Hope faults, high intensity and deep earthquakes are largely restricted to an area in the northeast, the southern termination of which corresponds to the termination of the Motunau Hikurangi

Trough

fault against

the northerly

trending

faults at the head of the

(Figs. 2 and 9).

Nature and amount of displacement

on the Motunau fault system

The Porters Pass Fault is of dextral structures adjacent to the whole Motunau

strike-slip type, and the style of minor fault system is consistent with it being a

regional dextral feature. No evidence is available regarding the amount of any strike-slip movement on the Motunau Fault. Assuming that the flat-topped plateau to the southeast of the continental slope sector of the fault (profile T4, Fig. 3) represents the downdropped edge of the continental shelf (cf. Kaneko, 1966), and that it was planed at the earliest in the initial glaciations of the late Neogene, a vertical downthrow of c. 500 m to the east has occurred here in the last 3 m.y. But since the fault apparently crosses the shelf further west with little displacement, it seems likely that the throw varies markedly at different places along the fault. Regional significance The change in tectonic style which takes place across the Motunau fault is clearly a regionally significant feature. It has been noted before by, for example, Suggate (1979) but was asserted to occur across the Hope Fault. In fact it is the Motunau fault system which marks the southern boundary of the Marlborough shear belt,

n

and, thereby,

the southern

The discovery feature,

edge of the plate boundary

of the Pegasus Bay Fault,

is also of considerable

zone in eastern

and its identification

significance.

The probable

onland

South island.

as a recently

active

extension

of this

fault lies only a few kilometres northwest of suburban Christchurch, an area currently assigned a grade 2 earthquake risk rating (Clark et al., 1965). This grading may require

reassessment.

The Marlborough zone of strike-slip faulting encompasses the region between the main Alpine (= Wairau) Fault in the north and the Motunau Fault in the south (Fig. 1). Strain

is almost

certainly

not homogeneous

across this 140 km wide zone

(contrast Walcott, 1979, fig. 4), but is rather concentrated in the vicinity of the five major fault systems (cf. Wellman, 1979, fig. 1). However. Walcott (1978) has demonstrated

that

as much

as half of the deformation

may have been

taken

up

within the regions between the major faults. The presence of extensive areas of Cainozoic covering strata without penetrative deformation shows that this strain cannot have been accomplished by ductile flow, at least at higher crustal levels. Thus, we conclude that the regions between the major faults behaved as quasi-elastic blocks or “microplates” during late Cainozoic deformation (cf. the behaviour of the similar Fiordland microplate at the southern end of the Alpine transform-Norris and Carter, 1980). From north to south these blocks are conveniently termed the Spenser, after

Inland

major

Kaikoura,

Seaward

physiographic

features

Kaikoura that

and Conway

they individually

microplates encompass.

respectively, Each

block

contains at least moderately deformed Cainozoic rocks, some occurrences of which are bounded by substantial faults which may have defined even smaller quasi-elastic blocks during deformation (cf. Freund, 1971). Nonetheless, the position of five master fault systems, seems established. THE CONWAY

and consequently

the existence

of four substantial

microplates,

MICROPLATE

The Conway microplate is bounded by the Motunau and Hope fault systems. Within the block there exists a host of smaller faults and folds, arranged in a systematic pattern consistent with dextral transcurrent movement on the bounding master faults (cf. Bishop. 1968: Wilcox et al., 1973). Though our profiler data are insufficient for detailed mapping of offshore structure. we present a composite structural map depicting the major coastal and offshore features (Fig. 2). Growing folds(Figs.

10 and 11)

Most of the continental shelf on the Conway microplate is devoid of modern sediment cover, apart from thin and local patches of biogenic or muddy sediment

pre7c

t

1-1

Stage

I:

125 ky

Tertiary

i 1 I lw5Yl

3%

Fig.

I I. Interpretative

geological

shelf south of the Conway

Trough.

map of the late Quaternary Positions

of profiles

deposits

and shorelines

of the continental

A-D of Figs. 10 and I2 indicated.

offshore, and a narrow band of littoral sand and gravel along the shoreface (Carter et al., 1982). Most of the shelf comprises eroded platforms in either Tertiary sediment or in relict, late Pleistocene sediment wedges (Figs. 10-12). The predominance of erosion over deposition indicates that the shelf is transversed by active bottom currents, and is consistent with active uplift of the microplate with respect to regions further south (cf. Wellman, 1979). Consistent with such an inference, 3.5 kHz profiles reveal the presence of numerous growing folds that affect the late Quaternary, relict sediments on the continental shelf (Figs. 10 and 11). As Lewis (197 1, 1973) has shown for similar folds off Hawkes Bay, successive stages in fold-growth can be inferred from the presence of successively tilted, unconformity-bounded packets of strata on the flanks of the fold. The recent nature of folding, combined with the active uplift and erosion of the shelf, results in a close correlation between folds and bathymetry, with anticlinal

153

154

folds defining young

ridges. In one case the reflecting

fold (Fig.

growing

folds in Hawkes

deep-seated

underthrusting,

the microplate dextral

lo), suggesting

margins,

strike-slip

horizons

the presence

Bay have axes parallel

along the boundary

in the axis of a

pockets

of gas. Whereas

to the continental

those on the Conway su~esting

break-down

of shallow

microplate

they may have been

slope, caused by

have axes oblique

formed

consequent

to

upon

faults (Fig. 2).

The youngest relict sediments of the north Canterbury shelf comprise a series of drowned shoreline wedges, inferred to date from pauses in the rapid transgression after the peak of the last glaciation, i.e. to be between 20 and c. 6 ky old (Carter et al., in prep.).

Unfortunately

these deposits

have been largely eroded

off the Conway

microplate segment of the shelf, making it impossible to confirm that the growing folds affect post-glacial and recent sediment, though such a proposition seems very likely. Most of the microplate is underlain by an earlier, late Quaternary sediment wedge with, in places, older successively tilted Quaternary sediments beneath it. It is most likely that the upper, most widespread

of these earlier late Quaternary

Fig. 11) dates from the last major interglacial, correlation nearby

strengthened

by the existence

i.e. isotope

of warped

deposits

stage 5 at about

terraces

(W3,

125 ky. a

of this probable

age

onland

(Powers, 1962). As the m~imum dip seen on the flanks of folds in rate of tilting of c. 20 micro-degrees,/ky, IV5 is c. 3”, this gives a maximum compatible with that of the growing folds in Hawkes Bay for which Lewis (1971) recorded rates between 2 and 36 micro-degrees/ky. Young faults One 3.5 kHz profile, located just south of the Conway Trough, shows the seaward edge of sedimentary wedge W5 juxtaposed against older Quaternary or Tertiary strata across a fault zone (Fig. 10). The fault has expression on the sea-floor in the form of a gutter up to 10 m deep, confirming

its relatively

young

age.

Tectonic histoy Bradshaw and Newman (1979) have recently described low-angle thrust faults from coastal North Canterbury. They infer that thrusting predated the main phase of faulting and folding, and attribute it to synsedimentary sliding. Probable low-angle thrusting is seen also on a 3.5 kHz profile near the south end of the Conway Ridge (Fig. IO), where it too predates the tilting of the affected sediments. Despite this, and other similar evidence for a multiphase tectonic history, the geometric relations between different structures and the boundary faults of the Conway microplate (Fig. 2) are closely similar to those predicted for trans~urrent tectonic settings (e.g. Wilcox et al., 1973), and found along other major transcurrent faults, such as those of the San Andreas system (e.g. Yeats, 1973). Indeed, a careful

analysis

of secondary

structures

alone would point to the existence

and nature

Motunau fault system, even were no more direct evidence available. We infer that as transcurrent movement proceeded on the Hope faults,

intra-plate

stress was released

plate. Block faults are commonly showing

an irregular

by block

arranged

though broadly

faulting

within

and Motunau

the Conway

at about 60” to the microplate

alternating

pattern

of upthrown

of the

micro-

boundary,

blocks. Throws

of several kilometres are not uncommon, at least on land, where basement Mesozoic metagreywacke is often juxtaposed against the upper formations of the CretaceousCainozoic Kaikoura Sequence (Gregg, 1964). Folding occurred concomitant with block faulting, with anticlinal drape folds located over horsts, and synclines occupying the intervening grabens. Fold axes parallel the NNE trends of the major block faults in the central part of the microplate, but adjacent to the boundary transcurrent faults they swing into positions more nearly parallel to the boundary fault. typically block

making

an angle of 30” or less with the fault (cf. Bishop.

faults are sigmoidal

in shape,

as seen for the Kaiwara

1968). The larger

and Hundalee

faults,

and possibly for the fault bounding the western side of the Conway Trough. which may link with the Hundalee Fault in the north. Though difficult to prove by field evidence,

geometric

considerations

suggest that high-angle

ment on the main part of a sigmoidal on the bent extremities. PLATE-TECTONIC

reverse or normal

block fault will pass into transcurrent

movemotion

SIC;NIFICANCE

Early plate-tectonic interpretations of the Marlborough faults viewed them as secondary transforms which linked the Hikurangi subduction margin with the main Alpine Fault oblique-slip boundary (Wellman, 197 1; Christoffel, 197 1). Following Freund’s (1971) demonstration that individual faults of the Marlborough system have diminishing throws towards the Alpine Fault, and therefore that they may not actually link with the Alpine Fault, Scholz et al. (1973) suggested that the Awatere, Clarence and Hope transform faults have developed sequentially southwards in response to a changing plate motion vector. Later writers have accepted this interpretation. For instance, Arabasz and Robinson (1976) cite seismic data that the descending oceanic lithosphere beneath the Seaward Kaikoura and Conway microplates has only reached depths of c. 100 km and thereby

infer the presence

there of a discrete

segment

of Pacific plate that only

commenced downtuming about 2 m.y. ago. Rynn and Scholz (1978) further support such a model, noting that the abundance of seismic activity south of the Hope Fault suggests that “the Arthurs Pass region is a developing shear zone, since no throughgoing faults have been observed geologically in the region”. These authors further suggest that “the southern boundary of the shear zone is indicated by the model to be about 50 km south of and parallel to the Hope Fault”, which is precisely the location of the Motunau fault system of this paper.

Arabasz

transcurrent

their

citing

history,

fault-angle tion

( 1973) have also argued,

and Robinson

Marlborough

as evidence

depressions.

along

glomerates

along

the thick

Miocene

Such an interpretation

strike-slip

margins

readily

transtensional

Reading,

1980). and

transform

or transduction

parts

setting

is possible,

explain

the

facies evidence

that infill

in

theta

but models of sedimenta-

(Kingma.

for the Marlborough

thr:

block faults earlier

accumulation

of thick

con-

1958; Ballance

and

is in fact consistent

with ;i

region since the inception

of

sedimentation there in the early Miocene (cf. Carter and evidence in support of the southward-propagation model of

Scholz et al. (1973) comes from the fact that the Motunau developed suggesting

( 1963). that

Lensen

conglomerates

of the fault

the sedimentary

tectonically influenced Norris, 1976). Further

after

faults were active solely as normal

fault system is not as well

as a single major dislocation as the more northerly Marlborough faults. its more recent origin. Further south still the Pegasus Bay Fault may have

originated most recently of all. and may develop in Motunau fault system, or else become a microplate Careful sedimentary and stratigraphic studies southward-propagation model further, since the

future into either a splay of the boundary fault in its own right, are clearly needed to test the available regional stratigraphic

evidence (Suggate et al.. 1978) is not suitable for revealing detailed sedimentary trends, or basinal evolution. It is particularly critical to establish the timing of the development of the many small, tectonically controlled basins in the region (cf. Carter et al., 1982). Lewis (1980) has shown that the Hikurangi margin is characterized by underthrust anticlinal ridges, upslope from which develop slope-basins with thick sedimentary basins

fills of hemiterrigenous are now emergent

mudstone in eastern

1980) whilst those on the trench

and

North

redeposited Island

(Van

strata.

The westernmost

der Lingen

slope off the east coast remain

and Pettinga.

active today (Lewis.

1973, 1980). The eastern ends of seismic profiles Mobil 72-3 and Gulfrex 76 show the typical Hikurangi margin style of deformation along an uplifted anticlinal ridge at depths of c. 2000 m on the west wall of the head of the Hikurangi Trough (Figs. 4a and 5). To the west the Conway deep-seated

normal

the Motunau

Trough

faults which, further

strike-slip

and Ridge are apparently

south along the continental

fault. Thus, these profiles

the Benioff zone, at approximately

probably

the foot of the continental

controlled

by

slope, pass into

cross the surface trace of slope east of Conway

Ridge, with underthrusting along the western wall of the head of the Hikurangi Trough passing into dextral transform motion along the Motunau Fault (Fig. 2). Such an interpretation is in accord with regional plate motions (e.g. Walcott. 1978, fig. l), and with the seismic evidence that deep crustal and subcrustal earthquakes are restricted to that portion of the Conway microplate lying north and west of the major bend in the Motunau Fault. Therefore the combined evidence suggests that the Hikurangi Trough remains primarily a subduction feature right to its southern end, where the motion is transferred to the southern margin of the plate boundary zone, presently

marked

by the developing

Motunau

fault system.

Since the pole of

157

rotation

between

southwards

the Indo-Australian

during

the Motunau

and Pacific plates

the late Cainozoic

(Walcott,

has migrated

progressively

1978, fig. l), the rate of strain

fault system should be gradually

increasing

along

with time.

CONCLUSIONS

(1) The southern edge of the plate boundary zone between the Indo-Australian and Pacific plates lies along the Motunau Fault, the southernmost dextral strike-slip fault system of the Marlborough shear zone, and coincides with a marked regional change in seismicity. (2) The Marlborough transform fault system boundary

along

successively

shear linking

the Hikurangi

southwards,

being the most recently (3) Sedimentary

zone developed the oblique-slip

in the early-middle Miocene as a Alpine Fault with the subduction

margin.

of the shear

Faults

with the Motunau

fault

system

zone have developed

and

Pegasus

Bay Fault

formed.

sequences

north

of the Motunau

Fault

are complexly

deformed

compared with those that lie further south outside the plate boundary zone. The change in style of deformation is abrupt and sediments only a few km south of the Motunau fault system are effectively flat-lying, (4) The change in tectonic style from the transcurrent regime of the Marlborough shear zone to the underthrust regime of the Hikurangi subduction zone occurs along the foot of the Marlborough continental slope. The Hikurangi Trough is therefore subduction feature even over its more westerly trending southern portions.

a

ACKNOWLEDGEMENTS

We acknowledge Agreement

the financial

and of the University

support

of the NZ-USA

of Otago research

Scientific

committee.

Co-operation

Ship time on G.R.V.

“Tangaroa” was kindly made available by the New Zealand Oceanographic Institute, and we thank the officers and crew for their support during Cruises 1097 and 1116. The Geophysics Division of the New Zealand D.S.I.R. made available the seismic data plotted as Fig. 9, and we are grateful to Dr. K.F. Priestley for help and advice with this figure. The draft manuscript benefitted from the critical comments of Drs. K.B. Lewis and J.M.W. Rynn, and Bryan Jamieson assisted in the preparation of figures: we thank these colleagues for their advice and help. REFERENCES

Arabasz, Island, Ballance,

W.J. and

Robinson,

New Zealand. P.B.

Sedimental.,

and

R., 1976. Microseismicity

N.Z.J. Geol. Geophys..

Reading.

H.G.,

Spec. Publ., 4: 265 pp.

1980.

and geologic

structure

in the northern

South

19: 569-601.

Sedimentation

in oblique-slip

mobile

zones

Int.

A.\aoc.

15x

Bishop,

D.G.,

1968. The geometric

faults in New Zealand. Bradshaw.

J.D..

J.D. and Newman,

L., Carter,

marine Carter,

R.M..

Carter,

R.M. and Griggs,

R.M.,

Carter.

South Island,

deforrnamni

rocks in CanterhurL.

New, Zealam!

1982. Sednnentation transform

in the Conway

and Hikurangi

Trough,

subduction

a deep near-shott

plate boundary.

Nc\r

to Dunedin.

1976. Cainozoic

and plate-tectonic

L. and Landis.

In: Norris,

R.J. and others (Editors).

Field

Geol. Svc. N.Z , p, Q-D6

Meeting,

history

of southern

predictions.

C.A.. in prep

Earth

New Zealand:

Submergent

an accord

between

Sci. Lett., 3 1: 85-94.

Planet.

late Quaternary

shoreline\

off eastern

1971. Motion

of the New Zealand

Alpine

fault deduced

from the pattern

of sea-floor

Bull. R. Sot. N.Z., 9: 25-30.

Clark, R.H.. Dibble, zoning.

on Kaikournn

New Zealand.

D.A.,

spreading.

with ma1cv \trlkl~-~irt~

in press

R.J..

observations

Christoffel,

ti.B.,

of the Alpine

for the 22nd Annual

R.M. and Norris,

and their bearing

thrusts in Cenozoic

1977. Tour guide for Queenstown

geological Carter,

J.. 1979. Low-angle

Sedimentology,

Trip Guides

associated

22: 4355442.

basin at the Junction

Zealand.

features

I I: 405-417. , 0: 709-217.

Basin. J R. Sot. N.Z

N.Z.J. Geol. Geophya.. Carter,

of structural

1975. The folds at Castle Hill (Canterbury)

style in the Canterbury Bradshaw,

relationships

N.Z.J. <;eol. Geophys.,

Trans.

R.R.. Fyfe. H.E.. Lensen, G.J. and Suggate, I:

R. Sot. N.Z., General

Cole, J.W.. 1978. Andesites

R.P.. 1965. Tectonic

and earthquake

risk

I 13- 126

of the Tongariro

Volcanic

Centre,

North

Island. J. Volcanol.

Geotherm.

Res..

3: 121-153. Evison. F., 1971. Seismicity Freund, Gregg,

D.R.,

Hatherton,

a strike-slip

1964. Sheet 18 Hurunui

T., 1980. Shallow

Kaneko.

Trans.

H.R..

1974. Margins

Geology Kingma.

in New Zealand associated

R. Sot. N.Z.. Geol.. 4( Margins.

J.T.. 1958. Possible

in the Eastern

Springer,

1956-75.

New Zealand.

Pacific.

structures.

(1st ed.), Geological

G.J.,

studies in New Zealand.

Lensen,

G.J., 1977. Late Quaternary

Hawkes

1971. Growth

Lewis, K.B., 1973. Erosion sea level. N.Z.J.

McKenzie, Mobil,

and deposition

Geological

New Zealand. Powers,

W.E., W.M.,

Marlborough.

t : 250000. D.S.I.R., Wellington 29: 54 t - 55I.

I : 2OtKhlOO (1st ed.). N.Z. Geol. Surx surfaces:

coast and continental

shelf.

on a tilting continental

shelf during

Quaternary

oscillations

of

on the Hikurangi

oblique-subduction

and transform

margin,

Spec. Publ. 4: 171-184.

W.J.. 1969. Evolution

of triple junctions.

along the East Coast, North Rep. No. 5X7. available

Nature,

224: l25- 133.

Island, and the Kermadec-Tonga

on open file at Petroleum

Section.

Island Arc to New Zealand

Survey, Lower Hutt.

R.J. and Carter,

Prebble,

basin

Tectonophysics,

map of New Zealand.

sedimetation

D.W. and Morgan. Petrol.

and secondary

16: 2X1-301.

Int. Assoc. Sedimentol.,

1972. Seismic profiles

Th?

Wellington.

Geol. Geophys..

Fin. Unpublished Norris,

tectonic

(Editors).

Bull. R. Sot. N.Z., 9: 225-231.

Lewis, K.B., 1980. Quaternary New Zealand.

in south-western

I: 269-274.

Map of N.Z.

rate of folds using tilted wave-planed

Bay, New Zealand.

IO: 19-25

and C.L. Drake

local unconformities,

G.J., 1963. Sheet I6 Kaikoura

Lewis, K.B.,

pp

Wellington.

pp. 5499565.

N.Z.J. Geol. Geophys.,

Lensen,

Misc. Ser. Map 12. D.S.I.R.,

D.S.I.R..

coast and sea-floor

In: C.A. Burke

Lensen,

1975. Earth deformation

J.R. Sot. N.Z..

with a subsidiary

New York, N.Y.

origin of piercement

Geosyncline,

Bull. N.Z. Geol. Sure., 86: 49

Map of N.Z. I : 250000,

II ): 21 I -22X.

of the Southwest

of Continental

Bull. R. Sot. N.Z., 9: 161- 165.

fault in New Zealand.

(1st ed.).. Geological

seismicity

S., 1966. Rising promontories

Japan. Katz.

of the Alpine fault, New Zealand.

R., 1971. The Hope Fault:

R.M., 1980. Offshore

Int. Assoc. Sedimental.,

1962. Terraces

of the Hurunui

1980. Late Cainozoic New Zealand.

sedimentary

basins at the southern

end of the Alpine Fault.

Spec. Publ. 4: 237-265. River. New Zealand.

sedimentation

and tectonics

Int. Assoc. Sedimentol..

N.Z.J.

Geol. Geophys..

of the East Coast

Spec. Publ.. 4: 237-265.

5:

Deformed

t 14-129. Belt. in

Rynn.

J.M.W.

Zealand.

and Scholz,

Scholz, C.H., Rynn.

J.M.W.,

zone and Fiordland Sporli,

C.H.,

1978. Seismotectonics

of the Arthurs

Weed, R.W. and Frohlich,

region,

New Zealand.

K.B., 1980. New Zealand

Cainozoic.

and oblique-slip

Int. Assoc. Sedimental..

C., 1973. Detailed

Island.

New

margins:

tectonic

R.P.. 1979. The Alpine Fault bends and the Marlborough R.P., Stevens,

Printer.

seismicity

of the Alpine

fault

development

up to and during

the

Spec. Publ. 4: 147-170.

Suggate.

G.R. and Te Punga,

M.T.,

Faults.

1978. The Geology

Bull. R. Sot. N.Z..

of New Zealand

18: 67- 77.

(Vol. II). Govt

Wellington.

Van der Lingen, Zealand

G.J. and Pettinga,

J., 1980. The Makara

sector of the Australian-Pacific

Sedimentol., Astron.

obliquely

Basin: a Miocene

convergent

slope basin

plate boundary.

along

the New

Spec. Publ. Int. Assoc.

4: 191-215.

R.I..

Walcott,

South

Geol. Sot. Am. Bull., 84: 3297-3316.

Suggate,

Walcott,

Pass region.

Geol. Sot. Am. Bull.. 89: 1373-1388.

1978. Present

tectonics

and late Cenozoic

evolution

of New Zealand.

Geophys.

J. R.

Sot., 52. 137- 164.

R.I.. 1979. Plate motion

and shear strain rates in the vicinity

of the Southern

Alps. Bull. R. Sot.

N.Z.. 18: 5-12. Walcott,

RI. and Cresswell,

M.M. (Editors),

1979. ‘The Origin

of the Southern

Alps. Bull. R. Sot. N.Z..

18. Warren.

G. and

Cretaceous)

Speden,

1.. 1977. The

and correlative

sequences

Piripauan

and

in the Haumuri

Hanmurian

stratotypes

Bluff district,

(Mata

Series.

south Marlborough.

Upper

N.Z. Geol.

Surv. Bull.. 92: 60 pp. Wellman.

H.W..

Tectonophysics, Wellman,

H.W..

Southern Wilcox,

1971. Reference

lines, fault classification,

transform

systems,

and ocean-floor

spreading.

12: 199-209. 1979. An uplift map for the South Island of New Zealand,

Alps. Bull. R. Sot. N.Z..

R.E.. Harding.T.P.

and a model for uplift of the

18: 13- 20.

and Seely, D.R.. 1973. Basic wrench

tectonics.

Bull. Am. Assoc. Pet Geol., 57:

74-96. Wilson. Yeats.

D.D.. RX,

1963. The geology

of the Waipara

1973. Newport-lnglewood

Geol., 57: 117- 135.

subdivision.

N.Z. Geol. Surv. Bull.. 64. 122 pp.

fault zone. Los Angeles

basin.

California.

Bull. Am. Assoc.

Pet.