faults in cook strait and their bearing on the structure of central new zealand

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Faults in Cook Strait and their bearing onthe structure of central New ZealandLionel Carter a , Keith B. Lewis a & Fred Davey ba New Zealand Oceanographic Institute , Department of Scientific andIndustrial Research , Private Bag,Kilbirnie, Wellington , New Zealandb Geophysics Division , Department of Scientific and IndustrialResearch , P.O. Box 1320, Wellington North , New ZealandPublished online: 02 Feb 2012.

To cite this article: Lionel Carter , Keith B. Lewis & Fred Davey (1988) Faults in Cook Strait and theirbearing on the structure of central New Zealand, New Zealand Journal of Geology and Geophysics, 31:4,431-436, DOI: 10.1080/00288306.1988.10422142

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New Zealand Journal o/Geology and Geophysics. 1988. Vol. 31: 431~46 0028-8306/88/3104--0431 $2.50/0 © Crown copyright 1988

431

Faults in Cook Strait and their bearing on the structure of central New Zealand

LIONEL CAR1ER KEITH B. LEWIS New Zealand Oceanographic Institute Department of Scientific and Industrial Research Private Bag, Kilbirnie Wellington, New Zealand

FRED DAVEY Geophysics Division Department of Scientific and Industrial Research P.O. Box 1320 Wellington North, New Zealand

Abstract Seismic reflection data reveal that the major dextral transcurrent faults of the North and South Islands, with the possible exception of the Wairau Fault, do not link directly across Cook Strait. Off Marlborough, the extension of the Wairau Fault is a series of northeast-trending fractures, one of which runs west of Kapiti Island towards the mouth of the Rangitikei River. En route, the Wairau Fault defines the western margins of the prominent Wairau and Narrows sedimentary basins. The southeastern edge of the Wairau Basin is the AwatereFault which bends sharply to the east to merge with the steep topography of Cook S trait Canyon. The neighbouring Hog Swamp Fault follows a similar trend. The Clarence Fault has not been identified offshore whereas the Kekerengu and Hope Faults can be traced only a few kilometres beyond the coast.

On the Wellington side of Cook Strait, the offshore extensions of the Ohariu and Wellington Faults terminate in the thick sedimentary pile of the Wairau Basin. Further along the coast, the West Wairarapa Fault extends to Cook Strait Canyon where it appears to be offset dextrally, the southerly continuation extending to the Marlborough continental shelf.

Received 16 February 1988. accepted 11 May 1988

Faults from both islands terminate in the central strait along an apparent dextral offset. This is attributed to the clockwise rotation and dislocation of the fault belt in response to differential movement between the Indo-Australian and Pacific plates in late Cenozoic times.

Keywords marine geology; faults; structural geo-logy; Quaternary; deformation; axial tectonic belt; seismic surveys; reflection methods; Cook Strait

OBJECTIVE

The continuity of the great transcurrent faults of the North and South Islands across Cook Strait has long been a point of contention. While numerous attempts have been made to resolve this issue all have lacked adequate marine seismic data to delineate and map structures offshore with confidence. Now that such data are available, this paper seeks to shed new light on the faults and the structural complexities of central New Zealand.

PHYSICAL AND STRUCTURAL SETTING OF COOK STRAIT

Cook Strait is a sigmoidal body of water separating the North and South Islands. The central strait, appropriately called the Narrows, is only 22 km wide and is aligned north-south. It is flanked by a narrow continental shelf which drops off into the 350 m deep Narrows Basin. North of the Narrows, Cook Strait widens and swings to the northwest where it merges with the broad western shelf of central New Zealand (Fig. 1). The southern strait is aligned to the southeast where it leads into the Cook Strait submarine canyon system which, in turn, extends to Hikurangi Trough.

From a structural viewpoint, most of Cook Strait lies within a 70-100 km wide axial tectonic belt-a belt of high shear strain rate that defines the boundary between the Pacific and Indo-Australian plates (Fig. 1 inset). Deformation associated with the differential movement between these plates is largely taken up

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432 New Zealand Journal of Geology and Geophysics, 1988, Vo1. 31

Fig.l Cook Strait with metric bathymetry (Mitchell & Lewis 1980) and locations of onshore faults including the Shepherds Gully Fault-SGF (Lensen 1962; Beck 1964; Kingma 1967; Grapes & Wellman 1986). Inset shows the Indo-AustralianlPacific plate boundary and accompanying Axial Tectonic Belt.

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Carter et al.-Faults in Cook Strait

by compressional and strike-slip structures within the onshore tectonic belt (Walcott 1978).

East of Cook Strait, the Pacific plate subducts obliquely beneath the southern North Island. A Miocene-Recent imbricate thrust, accretionary prism forms the east coast hills and the continental slope ridges down to the partially sediment filled Hikurangi Trough. The prism thins rapidly and changes trend south towards Cook Strait (Lewis & Bennett 1985). The strike-slip component of relative plate motion, west of the accretionary prism, is manifest as major dextral transcurrent faults. These shear a forearc basin and frontal ranges (Cole & Lewis 1981), and intersect the coast of Cook Strait as (from west to east) the Shepherds Gully, Ohariu, Wellington, and West Wairarapa Faults and a group of faults in eastern Wairarapa (Fig. 1).

South of Cook Strait, the subducting plate is difficult to detect structurally. Imbricate thrusting is less evident than in the North Island, and the major dextral transcurrent faults of the Marlborough Shear Zone, with their associated ranges and transform basins, dominate the structure and geomorphology (Carter & Norris 1976; Prebble 1980). These faults are splinters from the northern end of the Alpine Fault (Fig. 1 inset). From northwest to southeast, the Wairau, Awatere, and Clarence Faults approach the coast of Cook Strait, whereas the Kekerengu and

. Hope Faults reach the coast between Cape Campbell and Kaikoura (Suggate et al. 1978).

All major faults, on both sides of Cook Strait, show evidence of late Quaternary movement, and some have moved during major earthquakes in historic times (Stevens 1974; New Zealand Geological Survey 1983). Dextral strike-slip move-ment has been accompanied generally by vertical offset that may change in throw rapidly along the fault strike (e.g., Freund 1971).

Despite the similarities in transcurrent faulting on either side of the strait, there is a significant difference in trend. North Island faults trend N35°E-N45°E, whereas on the South Island the trend is N55°E-N75°E.

PREVIOUS FAULT INTERPRETATIONS

Since Lyell (1856, 1868) first suggested that fault movements observed in Marlborough could be extended to the fault-bounded ranges of Wellington , most geologists have assumed submarine continuity of the major faults (Fig. 2). Some solutions to the "fault puzzle" relied heavily on the onshore geomorphological trends (McKay 1892; Stevens

433

1974); some interpreted seabed morphology (Fleming & Hutton 1949); and others suggested that Holocene rates of deformation, crustal strain, and offshore locations of earthquake epicentres were the key (Lensen 1958; Bibby 1976). Ghani (1974) was first to attempt tracing faults using the few offshore seismic profiles then available. Later, Katz & Wood (1980) produced a seismically based fault map but did not present the data on which their interpretation was founded.

Of the more recent interpretations, some consistency emerges.

1. The Wairau Fault, often thought of as the direct extension of the Alpine Fault, has been depicted as running along the Narrows Basin and then east ofKap;ti Island (Fleming & Hutton 1949; Suggate 1963; Stevens 1974; Suggateetal.1978).Katz& Wood (1980) show it to continue west of Kapiti Island, with an en echelon branch towards the Manawatu River. Reconstruction of Neogene plate motion, however, implies that the Wairau Fault or its ancestor was bent eastwards offshore towards the line of Cook Strait Canyon (Walcott 1978).

2. The Awatere Fault is usually shown connected to the Wellington Fault (Suggate 1963; Katz 1979), with possible bifurcations to the Ohariu Fault and other smaller faults near Wellington (Lensen 1958; Stevens 1974).

3. The Clarence Fault is most often linked to a western branch of the WestWairarapa Fault with the Kekerengu Fault being linked to either a splinter of the West Wairarapa Fault or to faults in eastern Wairarapa (Lensen 1958; Kingma 1974; Stevens 1974).

4. The extension of the Hope Fault is the most contentious. Lensen (1958), Stevens (1974), and Kingma (1974) all favoured a correlation with the so-called "East WairarapaFault". Suggateet al. (1978) and Lewis (1980) considered the Hope Fault extended offshore east of North Island, whereas Wellman (1956) favoured a West Wairarapa connection. Finally, Katz (1979) projected the "East Wairarapa Fault" to east of the South Island thereby curtailing any northeastward projection of the Hope Fault.

DATA

A largely uncoordinated network of deep ( > 1 s penetration) seismic reflection lines has been

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434 New Zealand Journal of Geology and Geophysics, 1988, Vol. 31

McKAY, 1892

KINGMA, 1974

/ /

/ /

KATZ & WOOD 1979

Fig.2 Previous solutions to the Cook Strait "fault puzzle".

LENSEN, 1958

STEVENS, 1974

-----

GHANI, 1974

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~oll~ct~d by petroleum companies, foreign research InStItutIOns, and the Department of Scientific and Industrial Research (Fig. 3). These lines do not form a data set designed to locate Cook Strait faults having generally been obtained from vessels e~ route to o~er s~ey areas. Consequently, many tracks are ahgned either through the strait or towards Wellington and therefore are subparallel to projected fault traces. Many have inadequate navigation records and the seismic data range from single-channel to ~4-fold stacked data of variable quality. Interpretation IS further hampered by intense "ringing" from the shallow, highly reflective seabed. However some major vertical fault throws are obvious, and some more subtle dislocations can be detected within basinal sequences. Despite the difficulties use of deep seismic reflection data was the principle'method of locating faults.

This ~andom data set was supplemented by 3.5 kHz profiles (New Zealand Oceanographic Institute T.angaroa cruises 1036, 1139 and 1147) and 260 cm; au-gun records (NZOIRapuhia cruise 2001). These profil~s show detailed layering in the top 30-300 m of sediment and were run to (1) define faults in areas wh~re ~eep seismic data were inadequate, and (2) to highlight any late Quaternary fault movement (Fig. 4).

Soundings along lines only 200 m apart (New Zealand Hydrographic Office 198<+) offered the opportunity of tracing fault scarps on bathymetric evidenc~. Such a criterion has some validity in areas where lin~ surface features run oblique to the powerful tIdal flows eroding Cook Strait(e.g., Carter 1983). However, lineaments aligned with the flow must be c.onsidered with caution as they can originate by scounng, by faulting, or by erosion along fault-weakened rock. An attempt was also made to delimit fault scarps in sheltered bays using side-scan sonar (e.g., off the active Awatere Fault in Cloudy Bay).

A few seismic refraction profiles provide additional geophysical evidence of large vertical offsets (Officer 1959). Recent compilations of earthquake epicentres (e.g., Hatherton 1970· Robinson 1986) yield a scatter that cannot b~ interpreted readily in terms of fault lineations (cf., Lensen 1958).

RESULTS

Despite the less than perfect data sets, it is possible to identify the following offshore faults namr:d after their apparent connections onshore.

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Wairau Fault

Onshore, the Wairau Fault is a major structural and geomorphic discontinuity between Late Paleozoic schist-greywacke ranges and the Mesozoic greywacke-floored basin of the Wairau Plains with its fill of Neogen~ sediments (Lensen 1962; Beck 1964). Approachmg the coast the Wairau Fault splits into subparallel north ~d south branches which become buried beneath alluvial gravels 16 km from the coast. Whether these branches rejoin is uncertain, but Grapes & Wellman (1986) have mapped a solitary Wairau Fault northeast of the branches and extending to within 800 m of the coast The seismic data suggest that branching continue~ offshore (Fig. 5). A northern branch appears to extend obliquely across the narrow Marlborough shelf, beyond which it defines the northwestern side of the ~arrows Basin. This fault-bounded depression contam.s at leas~ 320 m of sediment and is marked by a negatIve gravity anomaly (Ivory 1986). Further to the northeast, ~etween Narrows Basin and Kapiti Island, the fault IS not well shown in seismic profiles but it may coincide with a steep scarp just west of Fishermans Rock (Fig. 6c). Such a location is consistent wi~ the petrochemistry of a dredge haul (N495) from Flshermans Rock showing affinities to Torlesse rocks of Wellington Peninsula rather than Pelorus rocks of Marlborough (Table 1).

West of Kapiti Island, the northern branch is marked by a narrow zone of intensely deformed strata (Fig. 6a, b) which can be traced to just south of the. Manawatu River where it may (1) die out; (2) swmg landward of our seismic tracks to connect with the onshore Rauoterangi Fault near the M~watu River mouth (Anderton 1981); or (3) be ?ff~et 3 km to t?e west (Fig. 5). This westerly fault IS first detected m profiles immediately northwest of Kapiti Island, and extends northeast to 13 km offshore of. th~ Rangitikei River mouth, the limit of our selsml~ tracks: It ~ppears as two fractures in our high resolutIon seismic profiles (Fig. 6b) which is consistent with the interpretation of Anderton (1981) who used deeper seismic records.

AtCloudy Bay, a series of discrete faults suggests the presence of a southern branch to the seaward extension of the Wairau Fault (Fig. 5). This branch may mark the southeastern side of the Narrows Basin, but its presence cannot be substantiated on seism.ic records be~ause of masking by seismic velOCity pull-down Ind~ced by the relatively steep bathymetry. Most studies extrapolated the Wairau Fault along the eastern side of Kapiti Island where a cataclastic zone has formed in pre-Cretaceous rocks

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DEEP SEISMIC LINES

/' MAGELLAN (1969-70)

/,/ GULFREX (1972)

... /"MOBIL (1972)

,,'OTHER /

.../d ANDERTON (1981)

J74"E

Fig. 3 Locations and sources of deep (>1 s penetration) seismic reflection lines examined for this study_ The hatched area is that covered by Anderton (1981), and annotated profiles are referred to in the text.

o 42 S

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HIGH RESOLUTION

SEISMIC LINES

/,," AIR GUN /

/ 3.5kHz

\ \

" (' \ \ , \ , \ '\ \ ~<5' \

/ I

\ I I

\ \ )

\

\ \

\ \ \ \

a ~ .-\"", , , , , ..... .....

..... , I I

I· I

"

\ , \ / , / ", /"

\ " \/

175°E

2000

Fig.4 Track chart for shallow ( <1 s penetration), high resolution seismic profiles collected on NZOI Tangaroa cruises 1036, 1139, and 1147 and Rapuhia cruise 2001. Annotated profiles are referred to in the text.

437

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/ MAJOR FAULTS

/ MINOR FAULTS

BASINS WITH > 1 s (ca.800m) SEDIMENT

THICKNESS

Fig.5 Location of major and minor faults in Cook Strait and adjacent areas together with the prominent Wairau and Wairarapa Basins.

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Fig.6 A series of high resolution, a air-gun seismic profiles displaying fractures of the modem Wairau Fault (W) extending from Cook Strait to the Rangitikei River mouth. Profiles are located on Fig. 4. Fishermans Rock (FR) has a petrochemical affintiy to grey-wacke of Wellington Peninsula.

c

W;::ST

(Fig. 2; Fleming & Hutton 1949). Certainly, an extrapolation of the suggested southern branch to the Wairau Fault would take it east of Kapiti Island. However, the seismic records fail to show any likely extension, northeast of Mana Island (Fig. 5).

Wellington Fault

The Wellington, Ohariu, Shepherds Gully, and several faults off the northwestern shores of the

439

EAST

Wellington Peninsula, can be traced across the continental shelf and Terawhiti Sill towards Cloudy Bay (Fig. 1,5). The converging traces of the Ohariu and Wellington Faults onshore continue to converge offshore, and the two fractures appear to merge near Cloudy Bay within sediments of the Wairau Basin (Fig. 5, 7). A series of small intrabasinal faults extends from the anticipated points of merger. Similarly, the Shepherds Gully Fault and other faults also tenninate within this sedimentary basin.

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d

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Table 1 XRF elemental analysis of sample N495 from Fishermans Rock, Cook Strait, together with average analyses for Torlesse andPelorus-W aipaparocks, reported by Roser & Korsch (1988). Discriminant scores for N495 plot within the field for the Torlesse terrane as delineated by Roser (1986) leading to the conclusion that Fishermans Rock is part of the Torlesse Supergroup as exposed on Wellington Peninsula, and that the Wairau Fault must lie west of the Rock. Analyses and interpretation by B. P. Roser.

Anhydrous SampleN495 N495 Torlesse Pelorus-W aipapa

Si02 % 71.45 73.09 72.46 63.12 TiOd 0.49 0.50 0.51 0.86 ~3 14.31 14.64 14.21 16.89 Fe20 3 3.46 3.54 3.64 6.56 MnO 0.03 0.03 0.05 0.11 MgO 1.40 1.43 1.26 2.54 CaO 0.78 0.80 1.49 3.54

~60 4.35 4.45 3.73 4.19 1.40 1.43 2.53 2.02

r§f 0.09 0.09 0.13 0.19 1.80

TOTAL: 99.56 100.00 100.02 100m

Sira,IAlP3 4.99 5.10 3.74 ~ !Nap 0.32 0.68 0.48 FeP3+MgO 4.97 4.90 9.10

Awatere Fault The Awatere Fault becomes less well defined as it approaches the coast (Suggate et al. 1978). Further offshore there is a prominent fracture that marks the southern perimeter of the Wairau Basin (Fig. 8). The

EAST

Fig. 7 The Wairau Fault (W) defining the northern side of the sediment-filledWairau Basin (WB) which is locally disrupted by the Ohariu (0) and Wellington (Wn) Faults. Mobil Line 72-ll7,located on Fig. 3(d-d').

fault is downthrown to the northwest and separates nearly flat lying to gently folded basinal sediments from steeply dipping sedimentary rocks in the south. As it extends across Cloudy Bay, the fault swings eastwards towards the head of Cook Strait Canyon where it can no longer be detected in this region of rugged terrain and intense faulting.

Hog Swamp Fault

Compared to the main Marlborough faults, the Hog Swamp Fault is a relatively small feature. Nevertheless, it has received some attention because it is a site of recent movement (Lensen 1970; Bibby 1976). Offshore, the Hog Swamp Fault runs subparallel to the trace of the Awatere Fault, and it approaches the West Wairarapa Fault although no link-up is detectable.

Clarence Fault Although well established inland, the continuation of the Clarence Fault to the coast is in dispute. Some authors have continued the fault to the shore just north of Cape Campbell (e.g., Bibby 1976) and have inferred its extension out to sea (e.g., Lensen 1958). Others have suggested the fault terminates onshore, some 20 km from the coast (e.g., Berryman 1979; Browne,N.Z. GeologicalSurvey,pers. comm. 1984). The few, poor quality seismic lines just north of Cape Campbell failed to show any major dislocation that could be equated with a northeasterly projection of the Clarence Fault thus supporting the contention of Berryman (1979) and Browne (pers. comm.).

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Carter et al.-Faults in Cook Strait 441

Fig.S Thewell-definedAwatere e Fault (A) delimiting the southern side of the Wairau Basin (WB). The Narrows Basin (NB) is masked by ringing multiples and the feather edge of sediments infilling the Wanganui Basin (W gB) is evident to the north. Mobil Line 72-174, located on Fig. 3(e-e').

West Wairarapa Fault

Onshore, the West Wairarapa Fault has a distinct structural setting, forming the western margin of a major graben containing up to 2500 m of sediment, determined from gravity data (Hicks & Woodward 1978). 7his same setting is recognisable offshore where the fault exhibits at least 1000 m down throw to the southeast thus defining a more or less continuous sedimentary basin (here called the Wairarapa Basin) that extends to the South Island shelf (Fig. 5; Ghani 1974, 1978).

The fault itself runs along the western side of the Wairarapa Canyon, across the eastern end of Nicholson Bank to the vicinity of Cook S trait Canyon. Here, it appears to be dextrally offset 3 km, judging by the presence of a fault in a similar structural setting on the opposite side of the canyon. This offset fault continues to the continental shelf off Cape Campbell where there are insufficient data to trace it any further.

The Wairarapa Basin encloses a sequence of nearly flat to gently folded sediments with a maximum thickness of 2200 m recorded at the edge of the South Island shelf (Fig. 9). Small intrabasinal faults, downthrown towards the basin axis, are common. Profiles also suggest the presence of a small thrust fault(s), with a curved plane dipping to the southeast (Fig. 5). The sedimentary fill rests on a prominent unconformity which Ghani (1974) assigned to the Tongaporutuan (Upper Miocene) through correlation with nearby onshore geology.

Kekereugu Fault A few seismic lines allow us to extrapolate the Kekerengu Fault and adjacent faults to just a few kilometres from shore (Fig. 5). Further to the northeast, in central Cook Strait, a fairly prominent fault, downthrown to the northwest, helps define the southeastern side of the WairarapaBasin. However,

SOUTH

there is no clear evidence of faulting between the two traces.

Eastern Wairarapa faults The ranges forming the eastern side of the Wairarapa Graben are broken by numerous small faults that can be traced only a few kilometres from shore with the exception of the Dry River Fault which extends seaward at least 20 km (Fig. 5). This seems to exemplify the faulting sty Ie of southern Cook Strait, namely, small fractures traceable for a few kilometres interspersed with larger faults traceable over 10--20 km.

Hope Fault Onshore, the Hope Fault is a 240 km long, dextral transcurrent fault with about 20 km lateral movement (Freund 1971). Although commonly regarded as a single feature in regional perspectives of the Marlborough Faults (e.g., Lensen 1958; Stevens 1974; Suggateetal. 1978), the Hope Fault has many en echelon offsets and it branches at both ends into numerous small faults (Freund 1971). On the adjacent continental shelf, we cannot identify a discrete, major fracture that could be considered an offshore extension of the Hope Fault. Instead, there are a few, small, discontinuous faults close to the coast (Fig. 5).

Further offshore, a major fault extends at least 65 km along the shelf edge. Part of the trace is concave and is down thrown to the southeast. It appears to be related to a massive slump on the upper continental slope (Fig. 5). It can be traced to due east of the onshore Hope Fault.

COOK STRAIT FAULTS

Fault continuity It has generally been accepted that the dextral transcurrent faults on both islands were continuous

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WEST

acrossCookStrait(e.g.,Lensen 1958; Stevens 1974; Suggate et al. 1978). This view was based largely on the absence of any evidence to the contrary. There were, however, dissenters to this widely held view. For example, Freund (1971) surmised that the major Marlborough and Wellington transcurrent faults did not link but terminated in or near Cook Strait. Our data tend to support this latter view.

With the possible exception of the Wairau Fault, which may extend across Cook Strait as a system of branching and/or discrete fractures, the remaining major transcurrent faults terminate just offshore or within the central reaches of the strait (Fig. 5). Here the terminations suggest the presence of a WNW-ESE lineament traced by: (1) the southwestern ends of the Wellington and Ohariu Faults where they fade into the WairauBasin; (2) the northeastern end of the Awatere Fault where it merges with Cook Strait Canyon; (3) a prominent offset in the faults bounding the Wairarapa Basin; (4) the southwestern ends of faults east of the basin (Fig. 5); and (5) a significant change in trend of continental slope thrust faults, east of Cook Strait (Lewis & Bennett 1985).

Fault termination may be related to several causes.

1. Strike-slip movement may be dispersed along branch faults emanating from the main fault. Freund (1971) applied this mechanism to the Hope Fault, suggesting that the 20 km of horizontal movement could be taken up in minor branch faults near the coast. Certainly, the Hope Fault and its branching neighbour, the Kekerengu Fault, are poorly represented offshore and cannot be traced beyond the inner continental shelf.

2. Fault continuity may be affected by cross faults. Ghani (1974) suggested that most of the major

EAST

Fig. 9 The West Wairarapa Fault (Wa) on the side of the Wairarapa Basin (WaB), the other side of which is limited by a basement high near Nicholson Canyon (NC). Gulfrex Line 65, located on Fig. 3 (f-r).

Cook Strait faults were intercepted by northwest-southeast-trending faults. However, as these fractures were inferred principally on the basis of tentative seismic correlations, not actual profiles of faults, their presence cannot be fully substantiated.

3. The termination of major faults, from North and South Islands, in central Cook Strait adds some support to the contention that a major structural discontinuity exists along the middle of the strait (e.g., King 1939; Cope & Reed 1967; Walcott 1978). Such a contention is given some credence by a recently documented break in the subducting Pacific plate beneath Cook Strait. Robinson (1986) suggested the break, with its vertical offset of 7 km, could act as a barrier and limit the extent of rupture during major earthquakes.

Fault bending

Further support for a major structural change in Cook Strait is provided by the fault trace configurations. The Awatere Fault, and to a lesser extent the Hog Swamp Fault, swing east to disappear within the marked relief of Cook S trait Canyon, itself an east-southeast lineament which may be an eroded zone of fault-weakened rock. South Island faults, east of Cape Campbell, exhibit little easterly curvature. In contrast, the North Island faults swing slightly westward offshore.

These bends and offsets of faults in Cook Strait support current concepts regarding the Neogene evolution of New Zealand whereby the eastern North Island is undergoing a clockwise rotation in response to increasingly oblique subduction (e.g., Walcott 1978, 1984). Such rotation bent the main system of

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faults clockwise. However, our observations depart from the hypothesis in that the modem Wairau Fault swings slightly to the west to follow a northeasterly course (Fig. 5). This discrepency could be due to different offshore traces for an ancestral and modem Wairau Fault. An ancestral fault may have bent eastwards. This contention is supported by an easterl y swing of the Wairau Basin, which the fault helps define (Fig. 5). By comparison, the modem Wairau Fault re-established a northeasterly trend and is sufficiently young and westward to be little influenced by the ongoing clockwise rotation of the North Island.

Deformation between major faults

Walcott (1978) pointed out that the total amount of late Quaternary offset on the four main Marlborough faults is about 2-3 times less than the relative plate motion for the same period. He suggests the excess plate motion may be taken up by deformation of the rocks between the major faults, through dissipation along minor faults, by ductile flow in the crust, and by rotation of intrafault blocks.

In Cook Strait, the seismic data indicate fairly pervasive deformation, of which three basic styles are recognised.

(1) Bendingisevidentaseastwardcurvingtraces of South Island faults and westward curving traces of North Island faults.

(2) Small scale faulting occurs within zones like the Wairau Basin where small faults may have dispersed movement associated with the Wellington and Ohariu Faults.

(3) Gentle folding along northeast-southwest axes is widespread in sediments within fault-bounded basins.

Quaternary deformation

All the major faults onshore have undergone late Quaternary movement (see summaries by Suggate et al. 1978; New Zealand Geological Survey 1983). Such movements have been measured against various reference features, in particular, faulted terraces of last glacial age or younger (e.g., Lensen 1964; Freund 1971).

In Cook Strait, an obvious reference is the late Quaternary sediment cover which incorporates two seismic units (Fig. 109-g'). Unit A consists of at least 37 m of nearly flat lying sediments displaying

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numerous continuous internal reflectors. It is most pervasive in northern Cook Strait where cores (NZOI Stations Q824, Q839, Q840) show it to consist mainly of grey, micaceous mud interpreted to have accumulated during the last glacial period when the western entrance to Cook Strait was closed by a landbridge (e.g., Lewis & Eade 1974; Carter 1983).

Unit B represents Holocene sediment wedges which are located mainly within Cloudy and Palliser Bays where up to 15 m of mud and sand have prograded over the continental shelf to near the shelf break (e.g., Carter 1983).

Conclusive morphologic evidence of faulting in unit B sediments is not apparent even in seismic profiles and side-scan sonographs run directly off historically active faults (e.g., Awatere and Wairau Faults in Cloudy Bay). Such a paucity of evidence is not surprising given the presumed low competency of the unconsolidated Holocene sand and mud in the bay and the active wave regime. Elsewhere in Cook Strait, powerful tides with near-bottom speeds in excess of 1 mls (Black 1986) are actively eroding unit A and B sediments so that any fault-induced morphology is unlikely to be preserved. However, outside the zone of erosion, rare examples of faulted unit A sediments are evident. Off Cape Campbell, for example, nearly flat lying shelf sediments of probable late Pleistocene age are downfaulted 6.3 m to the southeast (Fig. 109-g'). The resultant scarp is protected by a 12 m thick cover of Holocene muddy sand which itself was probably faulted, judging by a truncated internal reflector.

Fault movement may have induced other forms of deformation in the surficial sediment cover. For example, west of Kapiti Island, late Quaternary or slightly older sediments directly above the Wairau Fault system have undergone rotational slumping (Fig. lOh-h'). Sediments either side of the fault are undeformed. Slumps and other mass movement structures are also well developed in submarine canyons located near active faults but such an association is inconclusive as to whether there exists a direct cause and effect relationship (e.g., slumps in Wairarapa and Nicholson Canyons, proximal to the West Wairarapa Fault).

Late Quaternary deformation has probably occurred elsewhere in Cook Strait but the absence of reference beds renders the dating of such deformation inconclusive. Seaward of the Wellington Fault, for instance, the greywacke basement and sediment cover of unknown age are disrupted both at depth and at the surface to produce zones of rough seafloor topography (Fig. IOi-i').

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g

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Fig. 10 3.5 kHz high-resolution seismic records of various styles of ?late Quaternary defonnation that include: (g-g') simple faulting of ?late Pleistocene sediments (seismic unit A) without obvious disruption of the surficial sand cover (seismic unit B) indicating faulting either predated the cover or active sedimentary processes have removed any fault expression on the seafloor; (h-h') a slump block (5) above the Wairau Fault as located by deep seismic profiles; (i-i') rough topography and disrupted strata of assumed Pliocene-Pleistocene age (P) on the continental shelf immediately off the onshore Wellington Fault, extrapolation of which would position it in the exposed grey wacke sector (G). Profiles are located on Fig. 4.

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COOK STRAIT-A STRUCTURAL DISCONTINillTY

The concept of a major structural discontinuity extending along central Cook Strait has been with us for many years (e.g., King 1939). To rationalise the thesis that the faults were continuous across the strait it was presumed that the discontinuity predated the faults (Cope & Reed 1967; Stevens 1974). Until now, the main evidence for a discontinuity has been provided by the dislocation of key stratigraphic markers on the two islands (e.g., Walcott 1978). Furthermore, structural provinces either side of the strait contrast in tectonic style, particularly when their offshore margins are included in the comparison. The simply sheared ranges and wide, imbricate thrust, accretionary prism to the north is quite different from the broad zone of transpressional (oblique collision) ridges and basins to the south. The Cook Strait discontinuity has been revived recently in plate tectonic reconstructions for New Zealand during the Neogene. Using geodetic measurements of crustal strain, Walcott (1984) has argued that an ancestral Wairau Fault, which formed a transform plate boundary prior to deformation of the Marlborough Shear Zone, has been bent eastward at Cook Strait with the clockwise rotation of the eastern North Island. This recurved fault now forms a boundary and significant offset between the North Island sector of the Hikurangi margin and the newer, still developing, Marlborough segment. This concept also implies that the segment of the Wairau Fault identified west of Kapiti Island is a new feature. In general,ourdataare in accord with Walcott's (1984) thesis.

Of the major faults reaching Cook Strait, none can be traced continuously from coast to coast with any certainty. A modem extension of the Wairau Fault may extend as a series of offset fractures west of Kapiti Island to link with faults near the mouth of the Rangitikei River. This fault system may delimit the western margin of the Axial Tectonic Belt. The remaining faults can be traced no further than the central strait where they terminate to outline a discontinuity trending approximately WNW-ESE. The offshore extensions of North Island faults are either straight or curved slightly to the west, whereas South Island traces, in particular that of the dextral transcurrent Awatere Fault, curve strongly eastward to merge and perhaps form part of the discontinuity. Thus, it is not surprising that the discontinuity has the attributes of a dextral offset confirmed by its dislocation of the central sector of the Wairarapa Basin.

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ACKNOWLEDGMENTS

We are indebted to many local geologists for their input during various stages of this study; to R. I. Walcott, I. C. Wright, and G. J. Lensen for the benefit of their wisdom recorded on the draft manuscript; and to B. P. Roser, Geology Department of Otago University, for petrochemical analyses in Table 1. Glenis Marsden and Beverley Madsen typed the manuscript.

REFERENCES

Anderton, P. W. 1981: Structure and evolution of the South Wanganui Basin. New Zealand journal of geology and geophysics 24: 39--63.

Beck, A. C. 1964: Sheet 14--Marlborough Sounds. Geological map of New Zealand 1 :250 000. Wellington, Department of Scientific and Industrial Research.

Berryman, K. 1979: Active faulting and derived PHS directions in the South Island, New Zealand. In: Walcott, R.I.; Cresswell, M. M. ed. The origin of the Southern Alps. Bulletin of the Royal Society of New Zealand 18: 29-34.

Bibby, H. M. 1976: Crustal strain across the Marlborough Faults, New Zealand. New Zealand journal of geology and geophysics 19: 407-425.

Black, K. P. 1986: Sediment-transport rates in Cook Strait.N ew Zealand Oceanographic Institute field report 24: 14 p.

Carter, L. 1983: Sediments, shallow seismic stratigraphy, and structure in the vicinity of the submarine power cables, Cook Strait, New Zealand. New Zealand Oceanographic field report 29: 14 p.

Carter, R. M.; Norris, R. 1976: Cainozoic history of southern New Zealand: an accord between geological observations and plate-tectonic predictions. Earth and planetary science letters 31: 85-94.

Cole, J. W.; Lewis, K. B. 1981: Evolution of the Taupo-Hikurangi subduction system. Tectonophysics 72: 1-21.

Cope, R. N.; Reed, J. J. 1967: The Cretaceous paleogeography of the Taranaki-Cook Strait area. Proceedings of the Australian Institute of Mining and Metallurgy 222: 63-72.

Fleming, C. A.; Hutton, C. O. 1949: Notes on the geology ofKapiti Island, Cook Strait,N.Z. Transactions of the Royal Society of New Zealand 77: 456-468.

Freund, R. 1971: The Hope Fault-a strike slip fault in New Zealand. Bulletin of the New Zealand Geological Survey 86: 47 p.

Ghani, M. A. 1974: Late Cenozoic vertical crustal movements in the central part of New Zealand. Unpublished Ph.D. thesis, lodged in Victoria University Library, Wellington.

---1978: Late Cenozoic vertical crustal movements in the southern North Island, New Zealand. New Zealand journal of geology and geophysics 21: 117-125.

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ded

by [

Lau

rent

ian

Uni

vers

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at 1

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Grapes, R. H.; Wellman, H. W. 1986: The north-east end of the Wairau Fault, Marlborough, New Zealand. ]ourMI of the Royal Society of New Zealand 16: 245-250.

Hatherton, T. 1970: Gravity, seismicity, and tectonics of the North Island, New Zealand. New Zealand jourMI of geology and geophysics 13: 126-144.

Hicks, S. R.; Woodward, D. J.1978: Gravity models of the Wairarapa region New Zealand. New Zealand jourMI of geology and geophysics 21: 539-544.

Ivory, 1. M. 1986: Sheet 341-Kapiti. Gravity anomaly map coastal series 1:250000. Wellington, New Zealand. Department of Scientific and Industrial Research.

Katz, H.R.1979: Alpine uplift and subsidence offoredeeps. In: Walcott, R. I.; Cresswell, M. M. ed. The origin of the Southern Alps. Bulletin of the Royal Society of New Zealand 18: 121-130.

Katz, H. R.; Wood, R. A.1980: Submerged margin east of the North Island, New Zealand, and its petroleum potential. UN ESCAP, CCOP/SOPAC technical bulletin 3: 221-235.

King, L. C. 1939: The relation between the major islands ofN ew Zealand. Transactions and proceedings of the Royal Society of New Zealand 69: 544-569.

Kingma, J. T. 1967: Sheet 12-Wellington. Geological map of New Zealand 1:250000. Wellington, Department of Scientific and Industrial Research.

---1974: The geological structure of New Zealand. New York, Wiley-Interscience. 407 p.

Lensen, G. 1. 1958: Note on fault correlations across Cook Strait. New Zealand jourMI of geology and geophysics 1: 263-268.

---1962: Sheet 16-Kaikoura. Geological map of New Zealand 1 :250 000. Wellington, Department of Scientific and Industrial Research.

---1964: The faulted terrace sequence at the Grey River, AwatereValley, South Island, New Zealand. New Zealand jourMI of geology and geophysics 7: 871-876.

---1970: Geological aspects of the Seddon earthquake 1966. In: Adams, R. D. et al. ed. Seddon earthquake, New Zealand, April 1966. Bulletin of the New Zealand Department of Scientific and Industrial Research 199: 35 p.

Lewis, K. B. 1980: Quaternary sedimentation on the Hikurangi oblique-subduction and transform margin, New Zealand. InternatioMI Association ofSedimentologists special paper 4: 171-184.

Lewis, K. B.; Bennett, D. 1. 1985: Structural patterns on the Hikurangi Margin: an interpretation of new seismic data. Pp. 3-25 in: Lewis, K. B. comp . New seismic profiles, cores and dated rocks from the Hikurangi Margin, New Zealand. New Zealand Oceanographic Institutefield report 22: 53 p.

Lewis, K. B.; Eade, J. V. 1974: Sedimentation in the vicinity of the Maui Gasfield. New Zealand Oceanographic Institute summary 6: 8 p.

Lyell, C. 1856: Sur un tremblement de terre iila Nouvelle Zeru.ande de 23 Janvier 1855. Bulletin Societe Geologique FraT/{aise 13: 661-667.

---1868: Principles of geology. London, J. Murray, Vol. 2. 649 p.

McKay, A. 1892: On the geology of Marlborough and south-east Nelson. New Zealand Geological Survey reports on geological explorationI890-91: 21.

Mitchell, J. S.; Lewis, K. B. 1980: Cook Strait bathymetry. 2nd ed. New Zealand Oceanographic Institute coort coastal series 1 :200000.

New Zealand Geological Survey 1983: Late Quaternary tectonic map of New Zealand 1:2000000. 2nd ed. New Zealand Geological Survey miscellaneous series map 12.

New Zealand Hydrographic Office 1984: Cook Strait and approaches to Wellington. Unpublished bathymetric collections. Lodged in New Zealand Hydrographic Office, Takapuna, Auckland.

Officer, C. B. 1959: On some offshore seismic refraction profIles in the Cook Strait, TasmanBayandGolden Bay areas of New Zealand. New Zealandjournal of geology and geophysics 2: 350-354.

Prebble, W. W. 1980: Late Cainozoic sedimentation and tectonics of the East Coast Deformed Belt in central New Zealand. Special publication of the InternatioMI Association of Sedimentologists 4: 217-228.

Robinson, R. 1986: Seismicity, structure and tectonics of the Wellington region, New Zealand. Geophysical jourMI of the Royal Astronomical Society 87: 379-409.

Roser, B. P. 1986: Chemistry, provenance and tectonic setting of New Zealand greywacke terranes (abstract). Geological Society of New Zealand miscellaneous publication 35A: 88.

Roser, B. P.; Korsch, R. 1.1988: Provenance signatures of sandstone. Mudstone suites determined using discriminant function analysis of major element data. Chemical geology 67: 119-139.

Stevens, G. R. 1974: Rugged landscape: the geology of central New Zealand. Wellington, A.H. & A.W. Reed. 286p.

Suggate, R. P. 1963: The Alpine Fault. Transactions of the Royal Society of New Zealand 2: 105-129.

Suggate, R. P.; Stevens, G. R.; Te Punga, M. T. ed. 1978: The geology of New Zealand. Wellington, Government Printer. 2 vol. 820 p.

Walcott, R. I. 1978: Present tectonics and late Cenozoic evolution of New Zealand. Geophysical journal of the Royal Astronomical Society 52: 137-164.

---1984: Reconstructions of the New Zealand region for the Neogene. Palaeogeography, palaeo-climatology and palaeoecology 46: 217-223.

Wellman, H. W. 1956: Structural outline of New Zealand. Bulletin of the New Zealand Department of Scientific and Industrial Research 121.

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faults in cook strait and their bearing on the structure of central new zealand

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