Plate margin transition from oceanic arc-trench to continental system: The Kermadec-New Zealand example

Tectonophysics,

49

87 (1982) 49-64

Elsevier Scientific

Publishing

Company,

Amsterdam-Printed

in The Netherlands

PLATE MARGIN TRANSITION FROM OCEANIC ARC-TRENCH TO CONTINENTAL SYSTEM: THE KERMADEC-NEW ZEALAND EXAMPLE

H.R. KATZ New Zealand (Final

Geological Survey, Lower Hutt (New Zealand)

version

received

May 12, 198 1)

ABSTRACT

Katz, H.R., 1982. Plate margin New

Zealand

Boundaries.

example.

The East Coast

Fold

plate boundary.

This, however,

the ECFB

into segments

segmented

subducting

and rates

Trough). (Pacific

system.

which

thus occupies

The structure tilting

locally.

form elongate

Evolution

system:

The Kermadec-

of the India-Pacific

Plate

is the continuation

of the

in a slightly

structural

side. Overall in-place

that the tectonic

evolution

Locally

of the ECFB.

evidence

subduction

and have different

with respect subsiding

and the continental due to continuing

to plate

trough

This foredeep

plate)

filled

(Hikurangi

Chatham

Rise

motion

of the

structural

point of

sediments

are underlain

complex

of imbricate

to an intercontinental

0 1982 Elsevier Scientific

aprons

because

which

the tectonic

slices. It is concluded and tectonic

erosion

of the discrete

change

shear zone.

Publishing

basins on their sediments

beyond

by subsidence

however,

only

shale. Such diapirs

into adjacent

thrust

with

are observed

by land-derived

sediment

but rather

are complicated,

to the plate boundary,

of compression undercompacted

as undeformed

by accretion

normal

Effects

by widespread,

of a subduction

and mo~hological

in the region of East Cape.

by extension margin.

slope and margin

system

of

by a strongly

sense along the shear zone. As a result, the Hikurangi

mainly

The conditions

fragmentation

deformation,

which terminates

they extend

directions

of strike

by a strongly

From a tectonic,

uplifts caused

is not controlled

margin.

arc-trench

0040-195 l/82/~-~/$02.75

in front

highs which in many cases have supplied

deformation.

the continental

from an oceanic

oblique

of the continental

the continental

Large-scale

may be caused

change

shear zone marked ECFB (Indian

of the ECFB is characterized and down-faulting

Trench.

and structure

regional

in the overall

Trench

New Zealand,

front to the east defines the Indian-Pacific

of which strike in different

sediments

the pap between

being involved

front. There is no compelling along

undeformed

and are often due to diapiric

landward exhibit

of a broad

to the Kermadec

strike

the southwest,

with time to the east-northeast.

view. it is unrelated

Island,

its tectonic

width,

segments

Towards

largely

Pacific plate to the southwest.

regional

to continental

The

with the Kermadec

varying

in the formation

is gradually

is shifting

of the North

pfate, individual

deposited,

plate)

Trough

arc-trench

(Editor),

Structurally

is not continuous

of greatly

of subduction.

has resulted

with rapidly

from oceanic Packham

Belt (ECFB)

arc-trench

motion

G.H.

Teerono~h.~sjcs, 87: 49-44.

Tonga-Kermadec

dips

transition In:

Company

50

The Tonga-Kermadec

arc-trench

ple of a convergent

plate

underneath

arc.

an island

Southwards,

system has been regarded

margin.

the Kermadec

where

Ridge and its fore-arc

Coast Fold Belt of the North

Island

Wood.

Trench,

1980). The Kermadec

the Pacific

of New Zealand however,

as a classical

oceanic region

(ECFB:

terminates

plate

continue

exam-

is subducted into the East

Katz, 1974a; Katz and

opposite

the northernmost

part of the East Coast (Katz. 1974a). Southward beyond its termination, flat-lying sediments underlie the abyssal plain. occasionally interrupted by basement seamounts. The eastern boundary of the deformed ECFB, i.e. its tectonic front, is abruptly displaced to about 50 km west of the Kermadec strikes in a different direction. Thus there is structurally

Trench termination and and tectonically a pro-

nounced break and change of character between the Kermadec and the ECFB continental margin (Figs. 1 and 2).

arc-trench

system

Profound changes also occur in geophysical parameters. There is a marked discontinuity in seismicity between Kermadec Ridge and North Island (Eiby. 1977). while the isostatic gravity anomaly of the North Island is sharply displaced to the west

and

“substantially

isolated

from.. . the

Kermadec

system”

(Hatherton

and

Syms, 1975). A well-defined Benioff zone is only seen from the distribution of intermediate depth earthquakes between 100 and 250 km deep (Fig. 2; Adams and Ware, 1977), occurring 200-400 km west of the ECFB tectonic front. Thus subduction of the Pacific plate would have to take the form of a very shallow and low-angle underthrust for a long distance, before sudden bending of the slab downwards at an angle of 50”. Microearthquake studies in the southern portion of the ECFB (Robinson, 1978; Reyners, 1978) indeed indicate a shallow band of relatively intense activity which gently dips to the northwest; this band has been identified with the crust of the subducted ( 1979) has postulated would geodetic

account

Pacific plate. In the northern that the Indian

for the extension

measurements

In the following

in the upper

lithosphere

(Fig. 3) as well as earthquake

chapters

part of the East Coast. Walcott

and Pacific plates are totally decoupled,

new observations

that

was found

which from

mechanisms.

and ideas are communicated,

based

on a study of marine seismic records (Katz and Wood. 1980) which cover the offshore portion of the ECFB and beyond. The structural characteristics along the plate boundary east of the North Island, New Zealand, are outlined in some detail and specific problems of their tectonic significance are discussed. In particular, the questions of accretion or tectonic erosion, and of segmentation of an active continental margin are highlighted. CONTINENTAL

MARGIN

TECTONICS-ACCRETION

VERSUS

TECTONIC

EROSION

Lewis (1980), and Cole and Lewis (1981) have postulated that the continental margin is formed by an accretionary wedge of imbricated thrust slices, much as

Fig.

1. Structural

faults. hatching oceanic

Hikurangi sections

map of submerged

Thin dashed represents basement Trough.

part of East Coast Fold Belt. showing

line is stratigraphic region outcrop. A -A’:

outside Broken

tectonic

boundary front,

lines with

between i.e. Pacific

arrow

head

Plio-Pleistocene plate. Stippled represent

line of section Fig. 6. 72-118, 72-l 19: location

main anticlines. and older

synclines rocks.

areas indicate

submarine

canyons

seamounts feeding

of Mobii Oil Corporation

and

Diagonal of into

seismic

shown in Fig. 5.

Karig and Sharman have suggested in their model ( 1975). However, Katz and Wood (1980) have presented a more detailed analysis of the structure and tectonic evolution in the offshore part of the ECFB, which shows little evidence for imbrication. They have concluded that there is no subduction complex along this

52

1740 I

180° I

178" I

1:60

-36'

-38'

-42'

t

I

Fig. 2. Tectonic

framework

Fold Belt (ECFB)

east of North

and, in the northeast,

within

ECFB;

horizontally

plate).

Note that Chatham

gravity

anomaly;

I Island,

Rise is underlain lines-depth

New Zealand.

axis of Kermadec

hatched-sediments

thin dashed

I

and underlying by continental contours

Heavy line-

Trench;

oceanic crust.

tectonic

thin lines-main basement

Heavy

dashed

of Benioff zone (after Adams

front of East Coast structural

outside

alignments

ECFB (Pacific

line-axis and Ware,

of negative 1977).

continental margin, which to the contrary would be characterized by subsidence of older continental blocks, i.e. tectonic erosion. The characteristic feature of the ECFB and its continental slope is a series of elongate and generally fault-bounded, structural highs or “tectonic protrusions”, which in places expose Miocene and older rocks in their core (Katz and Wood, 1980). They are often associated with diapirism, sometimes forming continuous

53

Fig. 3. Direction units of IO ‘/yr.

“diapir

of the principal calculated

walls” along pre-existing

are seemingly

rare, except

1980). Basins

between

(Katz,

axes of compression

from retriangulation

protrusions

1975) which in some places

(Fig. 4 and Katz,

the East Coast; after

numbers

are strain

1931. After Walcott

rates in (197%).

fault planes (Dean et al., 1976). True fold structures

where formed

these

along

data for the period

as drape are filled

above diapirs with

form large foresets

1975; Katz and Wood,

(Katz

and Wood,

Plio-Pleistocene

shed in a westerly

1980) from adjoining

deposits direction

high structures

lying

further east. This feature, however, has not been discussed by Lewis (1980), although it is clearly depicted also in his fig. 5, profile 72-124. Tectonic rotation of blocks and their landward tilting contemporaneous with sedimentation, as postulated by Lewis (1980) and Cole and Lewis (198 1) and thought to be a consequence of easterly upthrust and imbrication of structures, is based on very tenuous evidence. Detailed examination of reprocessed seismic sections (Fig. 4 and Katz, 1981) suggests that block faulting coupled with local diapirism is the predominant tectonic process here. In the central East Coast on land, a strong and regionally important cycle of diastrophism is recognized in the Late Miocene to Early Pliocene (Katz, 1973, 1974b). From all the evidence available, this tectonic phase seems to have had its

3-

-3 0 I

5 I

10 I

km Fig. 4. Sediments

shelf southeast

prograding of Hawke

edge, as well as absence line NZ-57 (vertical

in a landward

direction

Bay. Note discordance

of reverse thrust faulting.

scale in seconds

reflection

(northwest)

below seaward

with older sediment Tracing

sequence

of reprocessed,

tilted edge of continentai and diapir

2400% stacked

betow shetf

section, Gulfrex

time).

effect far to the east offshore also, where some structures were sufficiently uplifted to become subject to erosion. Later movements, however, caused regional tilting and stepwise down-faulting seawards to the east. Similar situations may have occurred in earlier geological times. Cretaceous lithologies near the present-day coast strongly suggest derivation of sediments from the east, i.e. from easterly source areas which at some stage were under erosion but later disappeared by subsidence and down-faulting of the continental margin (Katz and Wood, 1980). Such tectonic erosion along active margins, rather than accretion, was proposed by Katz (197 1) for the Chile margin and has recently been substantiated in Japan, the Marianas, Central America and Peru-Chile (Sisskind, 1977; Scholl et al., 1977; Karig et al., 1978; Von Huene et al., 1978; Hussong, 1979; Von Huene, 1979; Mrozowski, 1980). It appears therefore that extensional forces normal to the plate boundary are more common than formerly thought. Walcott (1978b, 1979) has elaborated on the occurrence of alternating periods of locking and unlocking of the subducted and overlying plates which apparently resulted, along the East Coast of the North Island, in episodic compressional and extensional strain in the upper (Australian-Indian) plate. Based on the structure and tectonic evolution of the ECFB over longer time spans of its geologic history, one could conclude that periods of unlocking, i.e. extensional strain; have far outweighed, in time and geological consequences, those of locking and buildup of compressional strain. However, also a locked situation could cause substantial extension and tectonic erosion in the upper lithosphere, as was suggested for the continental margin in Chile (Katz, 1971). IN-PLACE

DEFORMATION

OF CONTINENTAL

SLOPE

DEPOSITS

On the East Coast continental margin, additional evidence for the lack of accretion is found from an analysis of sediment distribution and structure of the

of xismic

sections

72 - 118

72 -119

marem

across Hikurangi

Trough

southeast

10

I”

_

and transltlonal

_ _ _~

A.

hut

20

30

LO

--

1s sham

iauircd

undisturbed and amxwenri\

underneath

50

a&c

7-

~II XCOII& L~
foredeep.

Tectomc

I ). Shaded areas:

RISE

set Fig

B

Trough

SE

19X0: for location

.’

= --=

CHATHAM

fill of EIikurangl in ij i’erlicai

bediment

of Cook Strait (after Katz and Wood.

Rise north slope. which form basement

1s eradual

and older rocks of Chatham

front on its northwestern

Miocene

Fig. 5. Line drawings

NW

Mobil

STRAIT

Mobil

6-

5-

L

3

A

For location

the North

of section see Fig. I.

lines: trace of Benioff zone underneath

Island.

I 200

I

I 0

Island,

I

oceanic

relationship

100

recognized

and tectonic

shown to join up with seismically

position

(EAST COAST FOLD BELTI

100

through North Island (left) to Chatham Rise. showing

I

300

I

400

Fig. 6. Cross-section

l- 300

km

I

CHATHAM

I 300

RISE

basement

dashed

i

and solid

SE

400

of the Pacific plate east of the North

of East Coast Fold Belt. Heaq

200

km

57

tectonic

front.

Diapirs

offshore

(Katz

and

Lower Tertiary

which

Wood,

are widely

present

1980), originate

undercompacted

in subsurface

from

shale formations

extensive

(Katz,

both

on land

Upper

and

Cretaceous

1975) which obviously

to have

a terrigenous source. Such diapirs also occur far east on the outer slope where they may even break through to the surface, a situation similar to that reported by Shepard

(1973) from an area adjacent

coast of Colombia.

Likewise,

most likely is of continental out to form turbidite-filled

a sediment foredeeps

and Hikurangi Trough, undeformed but clearly the ECFB.

to the Magdalena

River delta off the north

the bulk of other sediments

across the shelf and slope

origin.

Locally, continental

slope deposits

extend farther

apron outside the tectonic front, or may occupy as occur both in the north and southwest (Poverty

large Basin

Katz, 1974a, Katz and Wood, 1980). These foredeeps are have derived their thick sediment load from land, i.e. from

The sediments

decrease

in thickness

seaward

or-in

the case of the

Hikurangi Trough-onlap on to the continental foreland of the Chatham Rise (Fig. 5). In many areas, and particularly so in the central part of the ECFB tectonic front where a gradual

dying-out

of deformation

is well displayed

(Fig. 6. inset). the

sedimentary continuity from deformed ECFB into undeformed abyssal plain is clearly visible. There is no evidence for an accretionary prism of imbricated. ocean-derived sediments here, but what we see is in-place deformation of continental slope deposits which are all land-derived, much as described by Scholl et al. (1977). The occurrence and distribution of gas hydrates on the lower slope (Katz, 1981). and their relationship

with structure,

lends strength

Thus a subduction complex as required continent collision (Karig and Sharman, structural suggests

configuration

and Sharman’s

by the imbricate thrust model of ocean 1975) does not seem to be present. The

seen here, including

that the continental

margin

(1975) model-by

to this interpretation.

the lack of a well-defined

structure

whatever

is little affected-in

underthrusting

slope break, terms of Karig

is occurring.

Indeed,

general picture is rather tectonic front advancing

like that found in many classical erogenic zones, towards a stable foreland progressively involves

foredeeps,

or foreland

basins

deformation

pattern

dence for underthrusting FRA~I~ENTATION

filled

with

of the continental

the orogen’s

slope,

of a lower lithospheric

OF CONTINENTAL

therefore,

own

debris.

provides

little

The

the

where a its own overall

direct

evi-

plate.

MARCiIN

The situation is complicated by the distinct separation into segments of very different style and orientation, and the greatly varying width of the fold belt (Figs. 1 and 6). It is narrow and relatively tightly compressed in the north and southwest (only 45 km wide in Cook Strait south of Wellington), but very wide and generally only feebly compressed in the central portion (170 km wide in cross section of Fig. 6, only 120 km distant from Cook Strait). At this locality one could argue that the folds on the lower slope near the tectonic

_’ I/

\

Tectonic front of Coast Fold Belt

East

Fig. 7. Surface trace of boundary of Indian plate (tectonic front of East Coast Fold Belt, dashed line) northeast of New Zealand. Rate and direction of movement of the Pacific with respect to Indian plates are shown by arrows (from Walcott, 1978a, after Minster et al., 1974). Lambert azimuthal equal area projection with map centre at 40% and 15CPE. Bathymetric contours: MOOm.

59

front and

(inset

Fig. 6) were formed

southwest

trend,

the general

i.e. tectonic

by gravity

sliding.

lack of parallelism

However,

between

front (Fig. 1; Katz and Wood,

both to the northeast

slope structures

and

1980) is a good indication

slope

against

the acting of gravitational body forces (Seely, 1977). The en echelon pattern of slope structures further west which obliquely plunge and flatten southwestwards until ultimately

passing,

along strike, into the undeformed

Trough, strongly suggests that regionally nearly parallel to the axis of the Hikurangi

sediment

fill of the Hikurangi

there is oblique shearing in a direction foredeep trough. This is consistent with

the present-day motion of the Pacific with respect to Indian plates, as calculated by Minster et al. (1974 and Fig. 7). Incidentally, the structural configuration here displayed

(Fig. 5) and the relationship

between

the tectonic

front

and undeformed

foredeep, is another example to suggest-as was discussed in the previous chapterthat there is no accretion to the ECFB through imbrication of offscraped oceanicderived

sediments.

Because of this arrangement of plunging en echelon structures the tectonic front is located further and further offshore when followed to the east-northeast of Cook Strait (Figs. 1 and 2). But at 41”s

and 178:“E

a sharp kink occurs,

beyond

which

the tectonic front continues as a virtually straight and well-developed feature in a direction nearly due north. In front of it the Poverty Basin foredeep is developed (Katz, 1974a; Katz and Wood, 1980). Again at about 384 “C a sharp break occurs in the course and continuation of the tectonic front, and a change also in the structure of the fold belt to the west of it. From 38”s to the north-northeast the Kermadec Trench takes over, the axis of which now defines the tectonic front. This large-scale fragmentation of ECFB and its tectonic front cannot explained geometry

in terms of an underlying subducting plate and behaviour. However, lateral segmentation

easily

be

of more or less uniform of descending plates has

been suggested from other regions (Carr et al., 1973). while Reyners (1978) found a major discontinuity striking down the dip of the subducted plate in the area of his study on the East Coast (Fig. 1, A-A’).

From

focal mechanisms

of earthquakes

and

microearthquakes on this discontinuity, he inferred that it represents a near vertical, sinistral strike-slip fault, perhaps related to contortions of the subducting plate. If so, this could possibly coincide with, and explain tectonic front mentioned above.

the sharp kink in the direction

of the

Obviously, much more detailed investigations are required in the area of this kink, in order to understand its significance and tectonic relationship. But in general, if one wants to explain the complex of a descending Pacific plate, this is decidedly more pronounced and only include changes of strike but

structure of the ECFB in terms of, and as a result latter would need to be fragmented in a way that significant from one segment to the other, i.e. not also of dip and rate of subduction.

THE HIKURANGI

TROUGH

Bathymetry and seismic sections on the East Coast continental 1974a; Katz and Wood, 1980) have shown that there is no structural cal connection

between

concept

of a “Hikurangi

Trench

(Brodie

and

the

Kermadec

Trench”

Trench

and

of earlier workers,

Hatherton,

1958)

having

the

Hikurangi

continuous

been

margin (Katz, or morphologi-

based

Trough----the

with the Kermadec on

inadequate

data

coverage. The Kermadec Trench finds its termination a short distance south of 38”. where further south and in the line of its axis a number of basement seamounts occur (Fig. 1). A local sedimentary basin or foredeep, of structure and morphology very different from the Kermadec Trench, exists here to the west of those seamounts (Poverty Basin, Katz, 1974a; see also Katz and Wood, 1980, fig. 10); thus it is not in line with the Kermadec abyssal

Trench.

plain with a virtually

Southeast

undeformed

of Hawke sediment

Bay the ECFB

cover of limited

borders

a flat

thickness;

there

is no sign of a trench here (Katz, 1974a, fig. 6, profiles4 and 5; Katz and Wood, 1980, fig. 11, profile A). A thickly filled foredeep again develops in the southwest of the region, i.e. in the narrowing wedge between ECFB and Chatham Rise (Figs. 1 and 5) to which the new name of Hikurangi Trough was applied (Katz, 1974a). This nomenclature and definition was officially adopted by the New Zealand Oceanographic Institute (Eade and Carter, 1975). The recent extension of this name to the entire area bordering the ECFB from south of Cook Strait to east of East Cape (Lewis, 1980) is untenable as it defies any detailed analysis of the actual situation (note in particular that in the north the name “Hikurangi Trough” as applied by Lewis covers the area east of the above-mentioned an area of flat abyssal continental

margin

plain which is entirely

or ECFB tectonic

In his thesis of episodic plates,

Walcott

(1978b)

locking

proposed

line of basement

disconnected

and unrelated

front and the Kermadec and decoupling

that regional

seamounts,

to both the

Trench).

of the subducted

dextral

thus

shear parallel

and overlying to the plate

boundary is continuing and equally effective during periods of compressive as well as extensional strain normal to the plate boundary. However, Minster et al.‘s (1974) vectors of plate motion (Fig. 7) indicate that in our southwestern section only little compression and underthrusting to the regional strike of the

would occur, but mainly shear more or less parallel ECFB and its continental foreland, i.e. Chatham

Rise-and it is worth noting that in this sector there is no oceanic crust of the Pacific plate. Indeed, oceanic crust is restricted to somewhere further northeast. In the cross-section Fig. 6 it occurs only as a narrow wedge between ECFB and the continental foreland of Chatham Rise, while the contact or crustal transition with the latter can only be speculated ‘upon. Probably not much further west, no oceanic crust is intervening between ECFB and Chatham Rise. They approach each other as two continental segments not by subducting one beneath the other, but by moving slightly obliquely along a broad, deep-crustal shear zone thus coming closer and closer together (Figs. 2 and 7; note that the Mercator projection of Fig. 2 does not

61

adequately

represent

been created

7 km in about Chatham rapidly

the proper

the last 4 m.y.

deposited

sediments

(Katz,

While immediately terminates westwards wedge-shaped, south

1979)-fills

receding

which has some

the gap between

ECFB

of thousands

of meters

the receptacle

down through

trough

which has subsided

a number

of submarine

and of

canyons

ECFB (Fig. 1).

the convergence

to Walcott’s

with

subsided

foredeep

north of the Chatham Rise the oceanic crust (Figs. 2 and 6) in a narrowing wedge, the Hikurangi Trough itself is also

reflecting

Rise. But contrary

brought

mobile

The deeply

Hikurangi

Rise. As such it has become

from the tectonically

gated

geometries).

in this shear zone-the

time,

(1978a)

I propose

from a position

to the southwest suggestion

that

originally

sedimentary histories and sedimentological Beu, 1979) indicate that only a few million

and Chatham

that it has progressively

the Hikurangi

extending

of ECFB

Trough

still further

propa-

has actually

to the southwest.

been Local

and fauna1 environments (Lewis, 1976; years ago it extended well into the North

Canterbury hill country (north of Christchurch, Fig. 1). The structural conditions suggest that in that area it has been squeezed out of existence and reduced to its present shape and extent in the course of strong shear along the Hope, Hundalee and other faults further south (Gregg, 1964). If regional tectonism continues in a way similar to the last 4 m.y., I surmise reduced i.e. shifting to the east-northeast. the Chatham

Rise, with respect

that the Hikurangi Trough will be further The continuing oblique shear motion of

to the ECFB

between

them, the Hikurangi

Trough

erogenic

zone of the ECFB, such as is indicated

(Fig. 7) will gradually

thus becoming

increasingly

on its northern

close the gap

incorporated margin

in the

in profiles

A

and B, Fig. 5. By this process of ongoing tectonism, the erogenic belt with its advancing tectonic front will gradually overwhelm its own foredeep, which has developed

between

it and a relatively

stable,

continental

foreland.

While

this is a

common feature in most erogenic belts, the controlling factors in the process are the orientation of the stress field and the type of boundary between the two opposing elements. The deformation of the Hikurangi Trough will accordingly reflect the broadly dextral transcurrent nature of the boundary zone. From all these aspects of position, history, tectonic behaviour and relationships with both the ECFB and the Chatham Trough.

which’is

with an oceanic,

not a continuation trench-type

Rise foreland, of the Kermadec

it is clear that the Hikurangi Trench,

cannot

be compared

plate boundary.

CONCLUSIONS

Walcott (1978b) has suggested that the Pacific and Indian plates are presently locked in the Cook Strait area. According to Reyners (1978), the northeastern boundary of this locked segment could be the discontinuity which he found in the lower plate along his profile area (Figs. 1 and 6). However, near Cook Strait the vectors of plate motion (Fig. 7) only allow for rather a small component of strain

62

normal to the plate boundary, whether this is compressional (locked situation) or extensional (unlocked situation), Thus the main component will always be a shear strain

in this southwestern

tectonic

front,

dominant sharp

that strain

Based on the regional virtually

factor from Cook Strait to about

kink in the trend

speaking, involved.

segment.

I suggest

where

of the tectonic

on either

here also where as a result formed, not as a subduction

strike and structure

of the

to the plate boundary

is the

178”E and 4l”S,

front occurs.

side of the plate

This may have some important

ECFB erogenic

parallel

where the

This is also the area. broadly

boundary

bearing

i.e. to about

only

continental

on the structural

crust

situation.

ii

It is

of the broad shear zone the Hikurangi Trough was trench but as a local, sediment-filled foredeep of the

belt.

The marked fragmentation of the tectonic front and change of structural character of the ECFB along the margin of the North Island, suggest that if there is a subducting

plate beneath

it which does affect the structure

of the overlying

plate.

this subducting plate must be profoundly fragmented also, with direction, dip and rate of subduction changing between the various segments. Even so, it is unexplained why the tectonic front is oblique in opposite senses, in the north and southwest, to the structural trends within the ECFB; and why there is no trench even in the northern sector where the Pacific plate apparently is of oceanic crust extending underneath the ECFB tectonic front, and where subduction is assumed to be the main tectonic process (Walcott, 1978a, b, 1979). And why is there. from 38:‘s towards northeast, a sharp 60 km eastward shift in the tectonic front? From this point northwards the oceanic Kermadec Trench develops, its straight axis now marking the plate boundary and tectonic front of the Indian plate margin. Thus

the North

Island

continental

margin

presents

over

its entire

length

a

structural picture that is very different from the classical type of ocean-continent subduction zones. It can perhaps best be character&d as a transition zone of rather discrete, but diffuse and complex changes, along which the plate boundary passes gradually from an area.of an oceanic arc-trench subduction system to one of an inter-continental

shear zone.

ACKNOWLEDGEMENTS

I am indebted to R.A. Wood for having critically read the manuscript suggested many improvements. I also wish to thank A. Jaegers for draughting illustrations very efficiently when he had only little time available.

and the

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