quaternary faulting in the offshore flaxbourne and wairarapa basins, southern cook strait, new...

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Quaternary faulting in the offshore Flaxbourne andWairarapa Basins, southern Cook Strait, New ZealandPhilip M. Barnes a & Jean‐Christophe Audru b c

a National Institute of Water and Atmospheric Research , P.O. Box 14901, Kilbirnie,Wellington, New Zealand E-mail:b UMR Géosciences Azur , Université de Nice ‐ Sophia Antipolis , Valbonne, 06560, Francec Bureau de Recherches Geologiques et Minieres , BP6009, Orleans, 45060, FrancePublished online: 23 Mar 2010.

To cite this article: Philip M. Barnes & Jean‐Christophe Audru (1999) Quaternary faulting in the offshore Flaxbourne andWairarapa Basins, southern Cook Strait, New Zealand, New Zealand Journal of Geology and Geophysics, 42:3, 349-367, DOI:10.1080/00288306.1999.9514851

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New Zealand Journal of Geology & Geophysics, 1999, Vol. 42: 349-3670028-8306/99/4203-0349 $7.00/0 © The Royal Society of New Zealand 1999

349

Quaternary faulting in the offshore Flaxbourne and Wairarapa Basins, southernCook Strait, New Zealand

PHILIP M. BARNES

National Institute of Water and Atmospheric ResearchP.O.Box 14901, KilbirnieWellington, New Zealand

email: [email protected]

JEAN-CHRISTOPHE AUDRUUMR Géosciences AzurUniversité de Nice - Sophia Antipolis06560 Valbonne, France

Present address: Bureau de Recherches Geologiques etMinieres, BP6009, 45060 Orleans, France.

Abstract Marine seismic reflection profiles, bathymetricdata, and seabed samples reveal the stratigraphy andQuaternary structure of the southern Wairarapa andFlaxbourne Basins in southeastern Cook Strait and easternMarlborough. These SW-NE-trending basins began formingbefore the late Miocene (>10 Ma), but their developmenthas been mainly during and since that time and continuestoday within the Pacific-Australia plate boundary zone.Recently active structures deforming and bounding thebasins are recognised by growth strata and deformation ofQuaternary sediments. Observed structural geometriesreflect Pliocene-Recent changes in the kinematics of faultingin central New Zealand. The 12-22 km wide southernWairarapa Basin contains up to c. 2.9 km of strata and isdeforming between offshore segments of the dextral strike-slip Wairarapa Fault and associated Wharekauhau Thrust onthe western margin, and offshore extensions of the AorangiMountains range-front reverse faults on the eastern margin.To the southwest, in the eastern Marlborough Fault System,the 15-20 km wide, 80 km long Flaxbourne Basin contains>4.5 km of strata and is deforming by strike-slip and oblique-slip faults including offshore sections of the Hope andKekerengu Faults. A new set of strike-slip faults, probablyyounger than 1 Ma, strike parallel (c. 080 ± 10°) to thecurrent Pacific-Australian plate motion vector and obliquelyto inherited structural trends. Three of these faults arepossibly separating the Flaxbourne and Wairarapa Basinsin central, southern Cook Strait. Curved traces of the Needlesand Wairarapa Faults on the western margins of the basinsare aligned, and may cut across disrupted Miocene structuresto link part of the eastern Marlborough Fault System withthe North Island Dextral Fault Belt.

G98033Received 9 September 1998; accepted 3 May 1999

Keywords Cook Strait; Marlborough; Wairarapa; plateboundary; tectonic; faulting; structure; Quaternary; seismicreflection; bathymetry; stratigraphy; sedimentary basins

INTRODUCTION

Southern Cook Strait forms the transition between NorthIsland's Hikurangi subduction margin and South Island'scontinental transpression zone, and is an actively deformingregion between the mountains of both islands (Fig. 1) (Lewiset al. 1994). Reconstructions of the New Zealand plateboundary zone show that the strait has been situatedapproximately within this transition area for at least 20 m.y.(Fig. 1) (e.g., Walcott 1978, 1987; Little & Roberts 1997).Throughout most of the Neogene, the transition has beenessentially fixed to the northern end of South Island andwestern end of the Chatham Rise, despite the shape of theplate boundary, the azimuth and velocity of relative platemotion, and the paleogeography changing significantly.

Three major sedimentary basins exist beneath southernCook Strait (Fig. 2B) (Carter et al. 1988; Uruski 1992; Lewiset al. 1994). The Wairau, Flaxbourne (Clarence Basin ofUruski 1992), and southern Wairarapa Basins contain stratawith seismic reflectivity of >2.5 s two-way travel (TWT)(>3 km) thick, and the first two are associated with prominentnegative gravity anomalies (Fig. 2A) (Fenaughty 1987; Rose1991). The structural evolution of the basins should reflectthe tectonic complexities associated with changes in theconfiguration of the plate boundary. The basins are beingactively faulted and folded above the subducted Pacific plate.

The subducted Pacific plate lies 15-25 km beneathsouthern Cook Strait (Fig. 2B) and has been thrust beneathnorthwestern South Island to a depth of at least 200 km(Robinson 1986; Eberhart-Phillips & Reyners 1997). Thesubduction decollement reaches the seabed at the thrust-faulted deformation front southeast of the strait on the innerflank of the 2800 m deep Hikurangi Trough (Lewis &Pettinga 1993; Collot et al. 1996; Barnes et al. 1998).Onshore southwest of Cook Strait are major, strike-slip faultsof the Marlborough Fault System (MFS) trending 055°-075°(Fig. 2). These faults accommodate >80% of the predicted38 mm/yr Pacific-Australian plate motion in northern SouthIsland (Freund 1971;Bibby 1981;Cowan 1990; Van Dissen& Yeats 1991 ; Holt & Haines 1995) and have controlled thesedimentation of Neogene basins (Little & Roberts 1997;Audru & Delteil 1998). Onshore northeast of the strait andwest of the Wairarapa forearc basin are the strike-slip faultsof the North Island Dextral Fault Belt (NIDFB) trending035°-045°, which uplift the axial ranges of southern NorthIsland and accommodate c. 21 mm/yr strike-slip dis-placement (Beanland 1995; Van Dissen & Berryman 1996).

Most of the major active strike-slip faults in the NIDFBand the MFS project offshore beneath Cook Strait and thenortheastern Marlborough continental shelf. The crustal

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350 New Zealand Journal of Geology and Geophysics, 1999, Vol. 42

43 mm/yr

Fig. 1 Present tectonic setting of the plate boundary through New Zealand and simplified reconstructions at 10 Ma and 20 Ma (afterBeanland 1995; Little & Roberts 1997). Abbreviations include: SA, Southern Alps; HM, Hikurangi margin; EHS, Esk Head Subterranc;DM0, Dun Mountain Ophiolite. Crosses are active volcanoes. Shaded area offshore is the approximate region of continental crust asdepicted by the 2000 m isobath. The area in the inset box is the approximate area in Fig. 2.

Fig. 2 A, Contoured gravity anomalies in the southern North Island and Cook Strait region (Rose 1991 ). Anomalies (in uN/kg, when;10 uN/kg = 1 mgal) are Free Air measured at sea level and Bouguer on land. Abbreviations include: WB, Wairau Basin; FB, Flaxbounu-Basin; CB, Campbell Bank; RR, Rimutaka Range; AM, Aorangi Mountains; ECGH, East Coast Gravity High. B, Simplified basementgeology and late Quaternary faults in central New Zealand, and distribution of major sedimentary basins (stippled) in southern CookStrait. Abbreviations not in A include: CC, Cape Campbell; CSC, Cook Strait Canyon; CF, Carterton Fault; Jordan F., Jordan Thrust;MFS, Marlborough Fault System; MKF, Mokonui Fault; MF; Masterton Fault; NIDFB, North Island Dextral Fault Belt; NC, NicholsoiCanyon; PB, Palliser Bay. Bold contours labelled 15 km, 25 km, and 50 km are depths to the subducted Pacific plate (Ansell &Bannister 1996; Eberhart-Phillips & Reyners 1997). MesozoicTorlesse subterranes (Begg & Mazengarb 1996) include: Rakaia Subtern.n(South Island) and Wellington Belt (North Island) light shading; Esk Head Subterrane (South Island) and Rimutaka Belt (North Islanddashed pattern; Pahau Subterrane (South Island) and Wairarapa Belt (North Island), dark shading. Contours offshore are isobaths inmetres. Broken lines are the axis of Cook Strait Canyon and the Hikurangi Channel.

structure and continuity of the faults beneath the strait,however, have been debated (e.g., Carter et al. 1988). Carteret al. and Lewis et al. (1994) interpreted seismic reflectiondata and regional structural trends bounding the sedimentarybasins, and concluded that none of the faults, with the

possible exception of the Wairau Fault, link directly acrossthe strait (Fig. 2B).

Carter et al. (1988) considered that the offshore ends ofthe strike-slip faults have been rotated clockwise relative totheir onshore segments and terminate within the sedimentary

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Barnes & Audru—Marine seismic study of basin faults, southern Cook Strait 351

basins against a NW-SE-trending crustal-scale structurebetween the Flaxbourne and Wairarapa Basins. This structurewas thought to have developed originally as the northernsection of the ancestral Wairau Fault, which connected theAlpine Fault transform system to the Hikurangi subductionfront during the early - middle Miocene (Fig. 1, 2B) (Walcott1978). It was inferred the structure has since been rotatedclockwise and fragmented as the geometry of the plateboundary changed and as new, more favourably alignedstrike-slip faults developed southeast of the fault incontinental crust of the Pacific plate (Fig. 2) (Walcott 1978;Lamb 1988; Little & Roberts 1997). That the ancestralWairau Fault once cut through what is now Cook Strait isevinced by an apparent c. 140 km dextral offset of Mesozoicmarkers exposed in North and South Islands (Fig. 2B)(Mazengarb et al. 1993). Carter et al. (1988) considered thisstructure to be now masked by deep erosion of thesedimentary cover by three arms of the Cook Strait Canyonsystem.

Late Neogene (<4 Ma) dextral slip on a NW-SE-strikingstructure in southeastern Cook Strait was inferred also byDunkin (1995) to explain the abrupt southern terminationof the positive gravity anomaly associated with AorangiMountains and the East Coast Gravity High (Fig. 2A).Furthermore, Lamb (1988) and Little & Roberts (1997)inferred dextral slip on a NW-SE-striking structure to beaccommodating differences in the rates of vertical-axisclockwise rotation of crustal blocks in northeastern SouthIsland and southern North Island. Paleomagnetic dataindicate that whilst early Pliocene strata on the northeastcoast of Marlborough have been rotated (relative to thePacific plate) by 20-40° at a rate of 5-77m.y. (Little &Roberts 1997), late Pliocene strata in southern NorthIsland have rotated clockwise at less than half the samerate (<27m.y.) (Lamb 1988; Beanland 1995). Little &Roberts (1997) inferred that slip on inland strike-slipfaults of the MFS is transferred seaward across a zone offault termination into rigid body rotation of a largenortheastern Marlborough continental block that has beenthrust eastward over the subducting Pacific plate insouthern Cook Strait.

Strike-slip faults of the eastern MFS, including offshorecomponents of the Hope and Kekerengu Faults, have beenrecognised recently beneath the eastern Marlboroughcontinental shelf (Barnes & Audru 1999). The Hope Faulttraverses the shelf obliquely and is thought to merge withthe transpressional Te Rapa Fault beneath the outer shelf(Fig. 2B). This zone possibly crosses the upper continentalmargin and Cook Strait Canyon east of the Flaxbourne Basinto link with the Palliser-Kaiwhata Fault on the southeasternWairarapa continental shelf. The Kekerengu Fault has beenshown to continue offshore as one or two splays in the centralFlaxbourne Basin.

ObjectivesIn this paper we discuss the stratigraphy, structure, andQuaternary deformation of the central and northernFlaxbourne Basin and the offshore southern Wairarapa Basin(Fig. 2). The primary objectives are: (1) to map the structuresthat have stratigraphic or bathymetric evidence of activityduring the Quaternary; (2) to identify active structures thatwere inherited from earlier phases of deformation; (3) toexamine the structural relationship between the basins, in

particular to identify any potentially active structures thatcould link the NIDFB and forearc Wairarapa Basin with thenortheastern MFS; and (4) to interpret the Quaternaryfaulting in light of present plate motions.

DATA AND METHODS

We utilise a reconnaissance network of archived marineseismic reflection profiles acquired by oil companiesbetween 1969 and 1973 (Fig. 3C) (Magellan Petroleum (NZ)Ltd 1969; Australian Gulf Oil Company 1973; MobilInternational Oil Company 1979), together with high-resolution profiles acquired by government agencies. Theoil industry profiles are 3—12 fold multi-channel datasufficient to image strata at a subsurface depth of up to 4 sTWT, and they reveal the location and structure of numerousfaults within, and bounding, the sedimentary basins (Uruski1992). These profiles, together with single-channel airgunseismic and 3.5 kHz profiles recorded by the former NewZealand Oceanographic Institute (NZOI), Department ofScientific and Industrial Research (presently the NationalInstitute of Water and Atmospheric Research) between 1983and 1986, were utilised by Carter et al. (1988) and Lewis etal. (1994). We have reinterpreted these data together withunpublished additional single-channel airgun and 3.5 kHzprofiles acquired by NZOI on R/V Rapuhia surveys 2022,2034, and 2055 between 1989 and 1992. The single-channelairgun profiles typically reveal sub-bottom penetration of400-600 ms TWT (c. 300-500 m).

Reflection stratigraphy tied to new sediment core samplesand onshore exposures constrains the ages of somesedimentary sequences and unconformities as well as thetiming and rates of vertical deformation on faults. These datafacilitate stratigraphic mapping of submarine exposures, andenable the recent activity on faults and folds deforming andbounding the basins to be recognised by growth strata anddeformation of Quaternary sediments. Nineteen gravity coreshave been used in this study. Although the cores are short(<3 m and typically <1 m), the exposure of differentsedimentary units at the seabed by uplift and erosion enabledthe sampling of a range of stratigraphic targets. To determinebiostratigraphic ages, the calcareous nannoflora of 11samples were analysed. The ages are tied to New Zealandstage designations that have been correlated to theinternational time scale by Edwards (1987) and Edwards etal. (1988).

New bathymetric data recorded by the Royal NewZealand Navy Hydrographic Office (Mitchell 1996) reveallocal structural trends in the vicinity of Cook Strait Canyon(Fig. 3). Where certain faults intersect canyons, narrow-beamecho soundings with line spacing of c. 200 m were contouredto produce isobaths at 10 m intervals to search for evidenceof lateral offset of canyon axes.

Because there remains some uncertainty in the structuraland stratigraphic relationships between the basins, we firstdescribe the Wairarapa Basin beneath Palliser Bay,Wairarapa, and Nicholson Canyons, and the southeasternedge of Nicholson Bank. Second, we describe theFlaxbourne Basin from the eastern Marlborough coastalregion in the southwest toward Cook Strait Canyon. Finally,we discuss the possible structural relationships between thebasins and the implications for the Quaternary deformationkinematics of the region.

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42 S 174 41 30' 174 30'E

Multichannel seismic Single channel air gun or sparker -*• 3 5 kH?

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SOUTHERN WAIRARAPA BASIN

Morphology, gravity anomalies, and stratigraphyBouguer and free-air gravity anomalies across the southernWairarapa region are dominated by positive anomaliesassociated with the Aorangi Mountains (part of the EastCoast Gravity High) and the southern Rimutaka Range, anda negative anomaly associated with the offshore Cook Straitcanyons (Fig. 2A). Modelled residual gravity anomaliesderived from removal of the regional gravity field show that:( I) the 12—22 km wide southern Wairarapa Basin onshore ischaracterised by a prominent negative gravity anomaly andcontains up to 3.2 km of probable Miocene-Recentsedimentary strata adjacent to the NW-dipping reverse strike-slip Wairarapa Fault (Hicks & Woodward 1978); and (2) aseries of SW-NE striking, SE-dipping reverse faultsincluding the Waihora Fault, southern Dry River Fault, andseveral unnamed subsurface structures with cumulativevertical separation of c. 1 km, occur beneath the northwesternedge of the Aorangi Mountains (Fig. 4) (Dunkin 1995).

Marine seismic reflection data and a negative residualgravity anomaly show that the Wairarapa Basin continuesoffshore as a 15 km wide subsurface feature beneath PalliserBay between the southern Rimutaka Range and AorangiMountains (Fig. 2B, 4) (Carter et al. 1988; Uruski 1992).The basin, however, is not evident in the bathymetry (Fig.3A), which shows Palliser Bay to be a gently slopingcontinental shelf deeply incised by Wairarapa and Nicholsoncanyons.

An apparently westward-dipping sedimentary section upto 2.5 s TWT thick (c. 2.9 km assuming an average seismic\elocity of 2.3 km/s; Rollo 1992; Dunkin 1995) occursbeneath Palliser Bay (see Fig. 6, 7) and is exposed on thewestern flank of the Aorangi Mountains, where it consistspredominantly of late Miocene-Pliocene mudstone of thePalliser and Onoke Groups (Dunkin 1995). Approximatecorrelation from onshore exposures into seismic profiles(Fig. 6, profile A) and reflection ties to the late Miocene -early Pliocene seabed core U630 (Fig. 5, see also Fig. 12,profile R) imply that much of the section beneath the bay isTongaporutuan age (c. 10-6 Ma). We infer that the packetof strong reflectors typically occurring at 1-2 s TWT beneaththe bay, including reflector eTt (Fig. 6, 7), represent earlyTongaporutuan conglomerate of the Putangirua Formation(Bates 1967). Discontinuous reflectors between eTt and theacoustic basement may be equivalent to the CretaceousWhatarangi Formation that is exposed in a faulted block onthe western flank of the Aorangi Mountains. Marine 3.5 kHzprofiles show that much of the bay is blanketed by post-lastglacial sediment up to 40 ms TWT (c. 30 m) thick.

.< Fig. 3 A, Contoured bathymetry (in metres) and selected majorstructures with bathymetric expression in southeastern Cook Straitand the eastern Marlborough continental margin. Data from theshelf and Cook Strait Canyon system are derived from narrow-beam echo-soundings (Mitchell 1988,1996), and are merged withSimrad EM 12 Dual multibeam data from the continental slope(Collot et al. 1996). Abbreviations not on Fig. 2 include: WC,Wairarapa Canyon. B, Bathymetric details of an apparent dextraloffset of Cook Strait Canyon (CSC) by the Needles Fault. Lightlyweighted contours are 10 m isobaths in the canyon axis. C,Distribution and type of seismic reflection profiles used in thisstudy.

StructuresWairarapa Fault and Wharekauhau Thrust: basin'swestern margin

The southwestern side of the basin is bounded by the oblique-slip Wharekauhau Thrust segment of the Wairarapa Fault,which extends along the southeastern edge of the RimutakaRange and offshore beneath the western flank of WairarapaCanyon (Fig. 3, 4) (Grapes & Wellman 1993). Onshore theWairarapa Fault has a late Quaternary dextral-slip rate of c.12 ± 4 mm/yr (Beanland 1995) and it last ruptured duringan Ms ~8 earthquake in 1855 with lateral displacement of11 + 2 m (Grapes & Wellman 1993) and maximum coseismicuplift of c. 6 m at Turakirae Head (McSaveney & Hull 1995).In seismic profiles, acoustic basement that can confidentlybe interpreted as Mesozoic Torlesse Terrane on the hangingwall is thrust over the late Miocene-Recent section (Fig. 6,profiles A and B). The vertical separation of acousticbasement across the fault is of the order of 1.7-2.0 s TWT(c. 2.0-2.3 km), which is consistent with the displacementpredicted from gravity modelling on land (Hicks &Woodward 1978). An absence of fanning stratal geometryinto the fault on profile A suggests that the verticaldisplacement postdates the deposition of the Tongaporutuansediments. Considering the late Quaternary uplift rate of c.3 mm/yr at Turakirae Head (McSaveney & Hull 1995), theobserved minimum vertical separation of basementimmediately offshore could have been achieved in 0.6—0.8 m.y.

East of Turakirae Head the Wharekauhau Thrust appearsto diverge into two main active splays of the Wairarapa Fault(Fig. 4). Between Nicholson and Cook Strait Canyons theactive splays are evident as discontinuous SW-NE-trendingbathymetric lineaments up to 10 km in length crossing thesoutheastern end of Nicholson Bank (Fig. 3 A, 8). Seismicprofiles show that the southeastern lineament correspondswith a subsurface fault with normal-slip separation (e.g., Fig.6, profile B). Unpublished 10 m interval isobaths on the wallsand in the axis of Nicholson Canyon between the lineamentsand the Wharekauhau Thrust show constriction of the canyonbut no evidence of lateral offset despite the probability ofdextral motion on the fault splays. The absence of a clearoffset may result from the combination of Holocene activityin canyon sedimentation and erosion, a reduction in dextral-slip rate south of the Wharekauhau Thrust, and/ordistribution of oblique slip onto more than one splay.

Central Palliser Bay

The central part of the Wairarapa Basin beneath PalliserBay is dominated by two SW-NE-striking thrust faultsthat each displace reflector eTt with vertical separationof c. 0.2 s TWT (c. 230 m) and fold the upper part of thelate Miocene section (Fig. 6, profile A; Fig. 7, profile C).Stratal geometry across the faults indicates negligiblethrust fault and fold growth during deposition of most ofthe late Miocene strata, although extensional growthfaulting may have occurred on one of the faults prior toreflector eTt (centre of profile A). The thrusts thereforedeveloped predominantly since the late Miocene, anobservation consistent with many other contractionalstructures within the Wairarapa Basin (Cape et al. 1990;Beanland 1995; Lamarche et al. 1995; Nicol et al. 1996).The faults, however, have no bathymetric expression (Fig.3A), and in 3.5 kHz profiles there is no evidence of active

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42° S\

42° 41° 30' 174°30'E\ y

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Quaternary StructuresStrike-slip fault J —

Thrust fault

Normal fault

Anticline

Syncline

Shelf edge

Submarine canyon

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Onshore GeologyQuaternary

Cretaceous & Tertiary

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Fig. 4 Late Quaternary structures in the Flaxbourne and southern Wairarapa Basins (light shading) and surrounding region. Onshore geology from Lensen (1962), Kingma (1967), Prebble(1976), Waters (1988), Vickery (1994), Dunkin (1995), Audru (1996), and Begg & Mazengarb (1996). Offshore structure from this study, Barnes et al. (1998), and Barnes & Audru (1999).Abbreviations include: CC, Cape Campbell; CP, Cape Palliser; DRF, Dry River Fault; KF, Kekerengu Fault; LHF, London Hill Fault; NT, Ngapotiki Thrust; TH, Turakirae Head; WNF,Wellington Fault; WF, Wairarapa Fault; WKT, Wharekauhau Thrust; WS, Whangaimoana Syncline; WHF, Waihora Fault; TMF, Te Munga Fault. Seismic profiles A to R are illustrated in Fig.7-12.

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Fig. 5 Sedimentary units exposed within and on the margins of the Flaxbourne and southern Wairarapa Basins, eastern Marlborough and Palliser Bay continental shelves, constrained byseabed cores and seismic stratigraphy. Offshore structure is simplified from Fig. 4. Abbreviations not in Fig. 4 include: CR, Clarence River mouth.

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••••• • • • '. '• i . " : ' . • • . • • • • ' . • i.

Fig. 6 Interpreted six-folcstacked seismic profiles A and Ifrom the western and centraWairarapa Basin, Palliser BI>Abbreviations include: Bst?basement surface; and Mzprobable Mesozoic basemenrocks. Reflector eTt is of probaliearly Tongaporutuan age (c. 1 CMMa). A shallow unconformity a'0.2-0.4 s TWT depth is oiprobable Pleistocene age. Prof1.locations are shown on Fig. 4. 5

folding of the post-last glacial transgressive erosionsurface (c. 18-12 ka).

Waihora Fault: basin 's eastern margin

The inner eastern margin of the Wairarapa Basin in PalliserBay is bounded by a SW-NE striking, SE-dipping reversefault that we infer to be the offshore extension of the WaihoraFault (Fig. 4; Fig. 7, profile D). Vertical separation ofreflector eTt is c. 1 s TWT (c. 1.2 km), comparable to thevertical separation on reverse faults along the western edgeof the Aorangi Mountains (Dunkin 1995). A structure evidentto the east of the Waihora Fault in profile D may be theoffshore extension of the Dry River Fault (Fig. 7).

FLAXBOURNE BASIN

Morphology and stratigraphy

The SW-NE-trending, 15—20 km wide Flaxbourne Basilextends beneath the eastern Marlborough continental shellfor c. 80 km from the Clarence River mouth to Cook StraitCanyon (Fig. 2B). The central part of the basin coincide-*with an elongate negative gravity anomaly (Fig. 2A) andbathymetric depression lying between the coast and an oute:shelf structural high which includes Campbell Bank at thenortheastern margin of the basin (Fig. 3A, 4,5). On a regionalscale the basin plunges to the northeast with seismicreflectivity from basin strata imaged to at least 1.8 s TW1

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Barnes & Audru—Marine seismic study of basin faults, southern Cook Strait

Fig. 7 Interpreted six-foldstacked seismic profiles C and Dfrom the central and easternWairarapa Basin, Palliser Bay.Abbreviations as in Fig. 6. Dashedreflector at 0.3-0.8 s TWT is ofprobable late Miocene-earlyPliocene age. Profile locations areshown on Fig. 4, 5.

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1Vit

91$m

•HinMA L

11ii

i ? 3hVE =

SS5S2.6 @ 2.3 km/s

m

mma

J

(> 2 km) in the southwest deepening to at least 4 s TWT(>4.5 km based on velocity analyses) in the depocentrebetween Cape Campbell and Campbell Bank (e.g., Fig. 9,10, profiles G, H, and I) (Uraski 1992).

Whereas much of the basin is covered by post-last glacialmud up to c. 45 m thick, the uplifted margins of the basinexpose strata of Miocene-Pleistocene age (Fig. 5). At leastseven deformed Pleistocene unconformities, includingreflectors B-G in profiles I-M (Fig. 10), exist within theupper 0.25 s TWT (c. 200-230 m) of sedimentary section.The exposed Miocene strata represent the Motunau andAwatere Groups outcropping nearby in coastal hills (Audru1996). With the existing seismic data reflector eTt in theWairarapa Basin cannot be tied directly into the FlaxbourneBasin. We infer the reflector to correlate approximately witha strongly reflective unconformity at 1.7-2.5 s TWT onprofiles G and H (Fig. 9). By analogy with the stratigraphyof coastal hills onshore the unexposed 2 s TWT thick (c. 3km) sedimentary section beneath this unconformity,ncluding the high amplitude reflectors between 3 and 4 s

TWT in profiles G and H, probably correlates with late

Cretaceous- Miocene strata of the Coverham, Muzzle, andlower Motunau Groups (Uruski 1992).

StructuresThere are numerous faults that displace pre-Quaternarybasin strata but are now buried by Quaternary sediments,appear to be no longer active (e.g., Fig. 9, 10; profiles G,H, and I), and cannot be mapped in plan view with theavailable data. Numerous active faults and folds, however,that are easily mapped from their deformation of theQuaternary sediments, occur within and along the marginsof the basin, and the pattern of relative uplift is partiallyreflected in the distribution of sedimentary units exposedat the seabed (Fig. 4, 5). The Quaternary structures in thesouthwestern and central part of the basin have beendescribed in detail elsewhere (Barnes & Audru 1999) andare briefly summarised here. The southeastern marginconsists of a complex, transpressive structural high thatextends along the outer continental shelf and includes theoffshore extension of the Hope Fault and the entirelymarine Kaikoura Fault in the south (Fig. 2B), a 30 km

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E1 0.2-

Sec

onds

(o to

N

ß?

VE =

N i C

10@

h o l s o n B a n k

Bathymétrielineament,

3 km

1.6 km/s

SFig. 8 3.5 kHz profiles E and Iof bathymetric lineaments crossing the eastern end of NicholsotBank. Profile locations are shov/ion Fig. 4, 5.

N i c h o l s o n B a n k

NWFig. 9 Interpreted six-foldstacked seismic profiles G and Ffrom the northeastern end of theFlaxbourne Basin. Reflector a0.4-1.5 s TWT is of probable lat<Miocene-Pliocene age. Abbreviations as in Fig. 6. Profile loca-tions are shown on Fig. 4, 5.

I '^^^^ m- i ' - 26(S23k ; I

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NWNeedles Fault

i (Projected) F l a x b o u r n e B a s i nCampbell Bank

FaultCampbell

Bank

SE

F l a x b o u r n e B a s i n

0.5

0.1-1NW

Needles Fault

Structural high over S EWharanui Fault-Campbell Bank Fault

| intersection

F l a x b o u r n e B a s i n

Fig. 10 Interpreted seismic profiles from the central Flaxbourne Basin. Profile I is a six-fold stacked section, profile K is a single-:hannel air gun section, and profiles J, L, and M are 3.5 kHz data. Quaternary unconformities labelled B to G are from Barnes & Audru1999). Reflector B is the post-last glacial transgressive marine ravinement surface. On profile K, Pl-Sl is strata of Altonian-Lillburnian

(early-middle Miocene) age, and Mult is the first seabed multiple. Profile locations are shown on Fig. 4, 5.

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360 New Zealand Journal of Geology and Geophysics, 1999, Vol. 4 "

long zone of wrench folds and discontinuous faultsnortheast of the Clarence River mouth, and the Te Rapaand Wharanui Faults east of Kekerengu (Fig. 2B, 4). Twoactive strike-slip faults diverge from the coast where theKekerengu Fault extends offshore into the northwesternside of the basin. One is the 073°-striking Chancet Fault,which obliquely crosses the Flaxbourne Basin to mergewith the northern end of the Wharanui Fault and thesouthern end of the Campbell Bank Fault (e.g., Fig. 10,profiles K, L, and M). The other is the 020-045"-strikingNeedles Fault, which lies c. 5 km from shore and extendspast Cape Campbell subparallel to the northeasternMarlborough coast (Fig. 9, 10).

The northern part of the basin described below isbounded on the northwest by the Needles Fault and onthe northeast by the Campbell Bank Fault, and isinternally deforming by several other Quaternarystructures including the newly described Boo Boo andTako Faults (Fig. 4, 5).

Needles Fault and associated structures: basin's westernmarginThe active Needles Fault extends for at least 55 km fromthe Kekerengu coastal region to Cook Strait Canyon andis characterised by multiple surface traces, the longest ofwhich is c. 24 km (Fig. 4, 5). In the south, off thenortheastern Marlborough coast, the average strike is c.037° and several NW-dipping splays deform strata withinthe east-dipping limb of the Flaxbourne Basin Syncline.Normal-slip separations are predominant within the upper200 m of sedimentary section (e.g., Fig. 10, profiles K,L, and M), but reverse-slip separations occur locally (e.g.,Field et al. 1997; their Enclosure 3, seismic line Explora102, near shot point 2100). Uplifted slivers of early-middle Miocene (Altonian-Lillburnian) strata existbetween splays with bathymetric scarps >20 m high andwith late Quaternary vertical separation rates of up to 1.1± 0.2 mm/yr (Barnes & Audru 1999).

Approximately 15 km northeast of Cape Campbell theactive splay dips southeast and has been a major structurecontrolling the western side of the basin for at least 10m.y., as evinced by thickening growth strata of probablelate Miocene-Pliocene age on the hanging wall of thefault (Fig. 9, profile H). Further northeast near Cook StraitCanyon the active splay steps right, swings in strike to065° (Fig. 4), and dips to the northwest (Fig. 9, profileG; Fig. 11, profile O). Quaternary strata on the hangingwall of this part of the fault are uplifted and gently folded,indicating recent structural inversion (Fig. 9, profiles Hand G).

Several features combine to indicate a component ofstrike-slip displacement on the Needles Fault (Barnes &Audru 1999). First, variations in the sense of throw andfault dip occur along the strike of the fault. Second, bothcontractional and extensional structures developedcontemporaneously along the same tectonic zone. Third,at least four upper crustal micro-earthquakes with strike-slip and oblique-slip focal mechanisms were reportedimmediately west of the fault north of Cape Campbell(Eberhart-Phillips & Reyners 1997). Fourth, where theNeedles Fault enters Cook Strait Canyon immediatelynortheast of profiles G, N, and O (Fig. 9, 11), the canyonaxis exhibits an apparent dextral offset of 500-600 m

(Fig. 5B). This canyon is almost certainly PleistoceneRecent in age (Carter 1992; Lewis et al. 1994), but itsprecise age and the timing of offset are poorly constrainedand therefore so is the slip rate on the northern end of theNeedles Fault. If canyon erosion occurs predominant]vduring glacial periods, and if its present form developedmainly within the last c. 4-5 glacio-eustatic sea levelcycles (c. 400-500 ka), then the lateral-slip rate on theNeedles Fault at this location is at least 1 mm/yr.

Campbell Bank Fault: basin's eastern margin, andstructure of Campbell Bank

The 28 km long Campbell Bank Fault forms the easternmargin of Flaxbourne Basin between the Chancet and BooBoo Faults (Fig. 3,4). The strike of the fault is 060-07(1over its southern 8 km, swinging to 035° northof41°51'SUruski (1992) interpreted >3 km (c. 2-3 s TWT) of throwof acoustic basement (e.g., Fig. 9, profile H), andconsidered the fault to have been an extensional structureduring the deposition of the oldest (?late Cretaceous )basin strata. Our profiles show recent structural inversionwith steeply dipping, upward-diverging, reverse-slipseparation fault splays beneath a Quaternary growth foldalong the southwestern margin of Campbell Bank (e.g..Fig. 10, profiles J and K). We interpret this crustalshortening through the change in fault strike to reflecttranspression, with most, or all, of the strike-slipdisplacement on the Chancet Fault being transferredthrough a restraining bend onto the Campbell Bank FaultTranspression diminishes toward the northeast, wherethickening of Pleistocene strata in profile H (Fig. 9)indicates that extension and subsidence of the basinmargin has continued on the fault during theQuaternary.

The seaward margin of Campbell Bank is underlai nby two SW-NE striking, NW-dipping thrust faults (Fig. 41.The shortening of Pliocene-Recent strata by these thrust>and the southwestern part of the Campbell Bank Faulthas resulted in the uplift and the exposure of early-lat ePliocene strata on the southern crest of the bank (Fig. 5 i

Boo Boo Fault

The active Boo Boo Fault is at least 18 km long and cutsobliquely across the northern part of the Flaxbourne Basinwith a strike of 070-095° (Fig. 4). The fault is down-thrown to the north, creating a bathymetric depression inthe northern part of the basin (Fig. 3), and is inferred tobe a steeply dipping reverse structure, based onasymmetric folding of Quaternary strata on the southernside of the fault (Fig. 11, profiles O, P, and Q). The verticalseparation of the post-last glacial transgressive erosionsurface across the fault is 52 ± 10 m (profile P), indicatinga vertical separation rate of up to 2.9 + 0.5 mm/yr. Thisrate is a maximum as some relief on the fault may haveexisted before 18 ka and survived wave planation duringthe latest marine transgression.

Existing profile coverage is insufficent to resolve thestructural relationship between Boo Boo and CampbellBank Faults. The eastern end of the Boo Boo Fault mayeither terminate against the Campbell Bank Fault ortruncate the latter and continue across the northern endof Campbell Bank. We have no direct evidence for dextralslip on the Boo Boo Fault, although such motion would

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Fig. 11 Interpreted seismicprofiles from the northernFlaxbourne Basin. Profiles N andP are 3.5 kHz data, profile O is 12-fold stacked section, and ProfileQ is a line drawing interpretationof a sparker profile. Reflector Bis the post-last glacial trans-gressive marine ravinementsurface. Reflector eTt is ofprobable early Tongaporutuan age(c. 10-8 Ma).

N N0.2-,

r 0-3-1

0.4-

Needles Fault

W^í

3 km

VE = 10 @ 1.6ktn/s

Needles Fault

NFF I a X b O U r n e I

P^MESMMMMi

Tako Fault

rBoo Boo Fault

Bas i s ir "1

Boo Boo Fault

sw

be compatible with that inferred on the Chancet and TakoFaults with which it parallels (Fig. 4).

Tako FaultThe Tako Fault strikes obliquely across the northernFlaxbourne Basin, lying 5 km north of and parallel to theBoo Boo Fault (Fig. 4, 5). The fault is at least 10 km long,dips to the south, and displays normal-slip separation andgrowth faulting in strata of probable Pliocene-Pleistoceneage (Fig. 9,11 ; profiles H and O). High resolution details ofthe fault tip in profile P (Fig. 11) indicate an apparent recentstructural inversion on two shallow splays about 350 m apart.We consider this inversion to be consistent with strike-slipdisplacement, with motion being in and out of the plane ofthe seismic profile. The fault displaces late Pleistocene strata

but there is negligible vertical separation or folding of thelast transgressive erosional unconformity B (18 ka) and noapparent offset or folding of the mobile Holocene sedimentcover.

DISCUSSION

Miocene-Pliocene basin development and plateboundary structuresThe Flaxbourne and southern Wairarapa Basins recordNeogene changes in Pacific-Australian plate motion andplate boundary geometry. The azimuth and velocity of platemotion changed throughout the Neogene as the Euler polemigrated southeastward away from New Zealand (Walcott1978; Sutherland 1995). In Cook Strait, the velocity of plate

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A. 15 Ma

Fig. 12 Schematic reconstructions of major tectonic features in the vicinity of the present Flaxbourne and Wairarapa Basins (stippledpattern) at: A, 15 Ma and B, 5 Ma, adopted and modified from Little & Roberts (1997), and C, major structural developmentssince c. 2 Ma. Abbreviations include: AF, Awatere Fault; AM, Aorangi Mountains; A-W. F, Alpine-Wairau Fault; BBF, Boo Boo Fault;CBF, Campbell Bank Fault; CF, Clarence Fault; CHF, Chancet Fault; EHS, Esk Head Subterrane; FCT, Flags Creek Thrust; FB, FlaxboumeBasin; FF, Fidget Fault; HF, Hope Fault; JT, Jordan Thrust; K.F, Kekerengu Fault; LHF, London Hill Fault; KKF, Kaikoura Fault; NF,Needles Fault; OF, Ohariu Fault; PDF, principal deformation front; PF, Pahaua Fault; PKF, Palliser-Kaiwhata Fault; PPAFZ, PortersPass-Amberley Fault Zone; RM, Rimutaka Range; TF, Tako Fault; TRF, Te Rapa Fault; WB, Wairau Basin (stippled pattern); Wf,Wairarapa Fault; WNF, Wellington Fault; WPB, Wairarapa Basin.

motion increased from c. 29 mm/yr at 20 Ma to c. 38 mm/yr today (Fig. 1). The shape of the plate boundary haschanged largely because the southern end of the Hikurangisubduction zone remained pinned to the continental ChathamRise whilst the Hikurangi forearc swung eastward over thesubducting Pacific plate (Walcott 1978). The forearc rotatedclockwise by up to 50° from a WNW trend in the earlyMiocene to its present northeast azimuth.

Plate motion in the central South Island resulted in nearlypure strike-slip faulting during the early-middle Miocene(c. 25-12 Ma), and was increasingly convergent after 12Ma (Sutherland 1995). At c. 6.4 Ma a significant westwardmigration in the position of the Euler pole reflects an increasein plate convergence orthogonal to the trend of the plateboundary (from <2 mm/yr to >10 mm/yr shortening) andsince then the rate of convergence has increased to the

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Fig. 13 Interpreted 12-foldstacked seismic profile (R) acrossa prominent bathymetric linea-ment south of Cape Palliser.Reflector eTt as in Fig. 6. Profilelocation is shown on Fig. 4, 5.U630 is a dated sediment core.

U630(I. Miocene-e. Pliocene)

present day (Walcott 1998). As the Hikurangi margin rotatedand the shape of the plate boundary and azimuth of platemotion changed, the loci of strike-slip and thrust faultingconnecting the proto-Alpine Fault with the Hikurangi marginhave not remained constant (Fig. 1, 12) (Lamb 1988; Lamb& Bibby 1989; Lewis et al. 1994; Audru et al. 1997; Little& Roberts 1997; Little & Jones 1998). By the end of theMiocene the northern end of the Wairau Fault in Cook Straitprobably had already offset Mesozoic terranes (Fig. 2), beenrotated significantly, become fragmented, and ceased to bea major active through-going dextral fault (Fig. 12B). Bythat time widespread strike-slip faults were active throughouteastern Marlborough, controlling the development ofsedimentary basins, and presumably some extended intowhat is now central and southeastern Cook Strait (Audru1996; Audru & Delteil 1998).

Seismic reflection data show that faulting occurred inthe Flaxbourne and offshore Wairarapa Basins both beforethe development of reflector eTt (i.e., before c. 10 Ma and,on some structures, pre-Tertiary (Uruski 1992)) and widelywithin the late Miocene-Pliocene section above the reflector(e.g., Fig. 6, 9; profiles A, G, and H). Because existing dataare insufficient to map in plan view many of the faults thatdo not displace Quaternary strata, it remains unclear howthe pre-Quaternary offshore structures related precisely tocontemporaneous faulting and vertical axis crustal rotationsonshore within the MFS (Lamb & Bibby 1989; Little &Roberts 1997) and in southeastern Wairarapa. Nevertheless,it is clear from the data that much of the structuraldevelopment of both basins occurred during and since thelate Miocene, when up to 3 km of basin strata accumulated

(Fig. 12B, 12C).The occurrence of strike-slip faults of Miocene age in

southeastern North Island is under debate. It has beencommonly argued that the Hikurangi margin was dominatedby thrust faulting, folding, and tectonic imbricationthroughout the Neogene and that strike-slip faults have notplayed a major role in the tectonic development of easternWairarapa (e.g., Pettinga 1982; Cape et al. 1990; Chanier &Ferriere 1991; Rait et al. 1991; Rait 1997). Beanland (1995)considered that: (1) although the azimuth of Pacific-

Australian plate motion has changed during the Neogene,the relative plate motion remained approximately perpen-dicular to the clockwise-rotating margin until 4-2 Ma, suchthat there was no requirement for an older dextral fault beltin the east coast ranges (Fig. 1); and (2) in contrast to thelong history of strike-slip deformation in the MFS, presentlyactive dextral faulting in the NIDFB commenced only 4-2m.y. ago, reworking a suite of rotated, Miocene-Plioceneage reverse faults in the inner fore-arc of the Hikurangimargin. In contrast, Delteil et al. (1996) and Field et al.(1997) inferred significant dextral displacements on earlyMiocene wrench faults in the central east coast ranges (cf.,Rait 1997).

Because the structural style of Miocene deformation insoutheastern Cook Strait cannot be determined withconfidence, we are unable to constrain the above models ofplate boundary structure. Significant subsidence andsedimentation was occurring in the southern forearcWairarapa Basin during the late Miocene (Cape et al. 1990;Lamarche et al. 1995; Beanland 1995), but the verticalseparation on major structures imaged in the profiles beneathPalliser Bay is largely post-Miocene age (<5 Ma) and onmany of the faults it could be Pleistocene (Fig. 6,7; profilesA-D).

Quaternary changes in fault kinematics, and apparentmisalignment of the marine basinsAs the rate of Pacific-Australian plate convergenceorthogonal to the plate boundary increased during thePliocene-Pleistocene (Sutherland 1995; Walcott 1998) anumber of important changes have occurred in the regionalstructure of the plate boundary zone. Although there has beenat least 150 km of subduction beneath Marlborough sincesubduction commenced in the early Miocene, the subductionthrust beneath Marlborough has either now becomepermanently locked or strong coupling has developedbetween the upper plate and the subducted northern marginof the continental Chatham Rise (Holt & Haines 1995; Collotet al. 1996; Eberhart-Phillips & Reyners 1997; Barnes et al.1998; Reyners 1998). The 055-060° trending strike-slipfaults that developed in Marlborough during the Miocene

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364 New Zealand Journal of Geology and Geophysics, 1999, Vol A2

have become increasingly transpressive (Little & Roberts1997). To the east of Cook Strait, the thrust front at the toeof the Wairarapa margin advanced seaward rapidly andcontinued to rotate clockwise within the last 1 m.y. (Barnes& Mercier de Lepinay 1997). Similarly, new contractionalstructures have developed during the Quaternary at the toeof the eastern Marlborough continental slope advancing thedeformation front toward the southeast (Fig. 12C) (Barneset al. 1998).

As a consequence of these combined changes the loci ofstrike-slip faulting in Marlborough has migrated southwardsand new, more favourably oriented structures havedeveloped, progressively capturing slivers of the Pacificplate. The 070°-trending Hope Fault commenced strike-slipmotion about 1 m.y. ago (Wood et al. 1994) and is presentlythe most active structure within the MFS, with an onshorelate Quaternary dextral slip rate of 20-35 mm/yr accom-modating at least half of the present plate motion (Cowan1990; Van Dissen& Yeats 1991; Knuepfer 1992). Significantactive deformation in North Canterbury south of the HopeFault, including development of the strike-slip Porters Passto Amberley Fault Zone (Fig. 12C), commenced in thePleistocene (Nicol 1991; Cowan 1992; Barnes 1996).

The southward advancement of strike-slip deformationin Marlborough has led to the widening of the eastern MFSand to the elongation and enveloping of the existingFlaxbourne Basin by a new system of strike-slip and oblique-slip coastal faults (Fig. 12C). The Flaxbourne Basin haslengthened by the Pleistocene development of the outer shelfstructural high that includes the Hope, Kaikoura, and Te RapaFaults. Two groups of faults bounding blocks with rhomboidsurface areas on the scale of tens to hundreds of squarekilometres have been active during the Quaternary (Barnes& Audru 1999). The first group of faults strike typicallybetween 023° and 057°, i.e., 22-56° from the Nuvel-1 A platemotion vector (079°), and includes inherited, pre-Pliocene,strike-slip faults on the western and eastern margins ofFlaxbourne Basin (Needles and Campbell Bank Faults)together with dextral oblique-slip thrust faults (e.g., JordanThrust, London Hill Fault, and part of the Te Rapa Fault).The second group consists of steeply dipping strike-slipfaults that trend between 067 and 085° (e.g., Hope, Kaikoura,Chancet, and Boo Boo Faults, and seaward segment of TeRapa Fault). These young (probably <1 Ma) faults havedeveloped subparallel to the azimuth of the present platemotion vector, and they may obliquely cut across the trendof older structures.

A clear structural boundary has not been identifiedbetween the Flaxbourne and southern Wairarapa Basinsbeneath Cook Strait Canyon. Although there is an apparentmisalignment between the basins, there is no evidence toprove that they were ever contiguous (Fig. 4). The easternmargin of the Flaxbourne Basin (Campbell Bank Fault)aligns with the present western margin of the WairarapaBasin (Wairarapa-Wharekauhau Fault). The sense ofapparent dextral offset of the basins is the same as the offsetof Mesozoic terranes outcropping in southern North Islandand northern South Island, which are thought to have beendisplaced by the paleo-Wairau Fault (Fig. 2B) (Walcott 1978,Lewis et al. 1994). The misalignment need not necessarilybe caused by dextral fault offset, but if it is the displacementmust be younger than the activity on the proposed NW-SE-striking paleo-Wairau Fault as the basins are largely youngerfeatures. We have not identified the paleo-Wairau Fault in

existing seismic profiles, although it is possible that 080 -striking active structures such as the Tako and Boo Bi. oFaults, as well as the unnamed fault imaged in profile K.(Fig. 13) and expressed in the bathymetry south of CapePalliser (Fig. 3), could be inherited components of such astructure. These faults could potentially have developed nthe early Miocene and since been displaced from theiroriginal positions and reactivated as dextral faults durirgthe Quaternary. Most previous authors have argued that noneof the active strike-slip faults of the MFS cross Cook Straitto link with structures of the NIDFB (Carter et al. 198 s;Lamb 1988; Lewis et al. 1994; Little & Roberts 199").Whilst our data are not conclusive, we note that the northernend of the Needles Fault appears to offset Cook StraitCanyon and is aligned with bathymetric lineaments that crossthe eastern end of Nicholson Bank. These lineaments in tarnappear to merge with the southern end of the WharekauhauThrust segment of the Wairarapa Fault. One possibleinterpretation is that these discontinuous structures representan active strike-slip tectonic zone developing across the straitand disrupting older crustal blocks. The observed oblique-slip displacement on the Wharekauhau Thrust is consistentwith the sense of slip predicted within a restraining bendbetween onshore and offshore segments of the WairarapaFault (Beanland 1995). Our interpretation (Fig. 12C) differsfrom that proposed by Little & Roberts ( 1997, their Fig. 1 2,inset 0-5 Ma) in which they predict trench-vergent thrustfaulting in the Flaxbourne Basin (e.g., Fig. 10, profile 1),but the two models are not mutually exclusive as theyoungest group of strata in which they record vertical axiscrustal rotations in northeastern Marlborough is 3.9-5.5 in v.old, whereas our model applies mainly to the last c. 1 m y.of deformation.

Our study, together with that of Barnes & Audru ( 1999 ),demonstrates the plan view pattern of Quaternary deform-ation in southern Cook Strait. The available data, however,do not constrain dextral slip rates on the major strike-slipstructures mapped offshore, apart from a poorly constrainedminimum rate of slip on the northern end of the NeedlesFault. Van Dissen & Yeats (1991) assumed on the basis >fsubdued onshore topography that the late Quaternary sliprate on the seaward segment of the Hope Fault is of the orderof 3-7 mm/yr. This displacement has been interpreted to hetransferred through the complex outer shelf structural highbeneath the southeastern margin of the Flaxbourne Basin,onto the transpressive Te Rapa Fault, and beneath the uppercontinental slope south of Cape Palliser (Fig. 12) (Baix.cs& Audru 1999). Where the Kekerengu Fault projects offshoreinto the western Flaxbourne Basin, the coastal slip rate hasbeen estimated at c. 3-10 mm/yr by Prebble (1976), Lamb& Bibby (1989), and Knuepfer (1992), and as high as 13-22 mm/yr by Van Dissen (pers. comm. August 1995). Mostof this displacement is inferred to be partitioned offshorebetween the Chancet Fault and the southern part of theNeedles Fault.

CONCLUSIONS

The southern forearc Wairarapa Basin extends offshorebeneath Palliser Bay, where almost 3 km of mainly lateMiocene-Recent sedimentary strata are deformed betweenthe active Wharekauhau Thrust segment of the WairarapaFault and active reverse faults that continue onshore asrange-front thrusts on the western side of the Aorangi

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Mountains. The oblique-slip Wharekauhau Thrust isinterpreted as a restraining bend transfer between the linearonshore segment of the Wairarapa Fault and an offshoresegment that crosses the eastern end of Nicholson Bank inCook Strait. The thrust offsets basement rocks vertically by2.0-2.3 km, and developed after the late Miocene andpossibly after the middle Quaternary. The southern part ofthe basin that is heavily incised by the Cook Strait Canyonsystem appears to merge with, but is misaligned from, thenorthern end of the Flaxbourne Basin.

The 80 km long, 15-20 km wide, Flaxbourne Basinlies within the eastern, offshore part of the MarlboroughFault System and contains >4.5 km of probable LateCretaceous-Recent strata. Whereas pre-late Miocenefaults have deformed the older basin strata, the majorphase of basin development has been during and sincethe late Miocene, when c. 3 km of late Miocene-Recentsediment accumulated. Strike-slip and oblique-slipQuaternary structures that presently deform and borderthe basin reflect a new pattern of fault kinematics inresponse to changes in the geometry of the plate boundaryand in the azimuth of Pacific-Australia plate motion. Thesouthward migration in the loci of strike-slip faulting inthe Marlborough Fault System during the Pleistocene ledto broadening of strike-slip deformation offshore andsouthward lengthening of the Flaxbourne Basin.

Two groups of Quaternary strike-slip and oblique-slipfaults characterise the northeastern Marlborough coastalregion and envelop the Flaxbourne Basin. The first groupstrikes typically between 023 and 057°, and includesinherited pre-Pliocene structures bounding the westernand eastern margins of the basin, as well as dextraloblique-slip thrust faults. The second group probablydeveloped within the last 1 m.y. and consists of strike-slip faults that trend subparallel to the current Pacific-Australian plate motion vector. These structures includethe newly described Tako and Boo Boo Faults, as well asthe Hope, Kaikoura, and Chancet Faults and the seawardsegment of Te Rapa Fault. A strike-slip fault zone isinterpreted to be developing across southeastern CookStrait, linking offshore segments of the Kekerengu andWairarapa Faults. Additional data are required to constrainlateral slip rates on the faults.

The precise structural relationship between theFlaxbourne and Wairarapa Basins remains unclear. TheTako and Boo Boo Faults, and a parallel, unnamed shelfedge fault south of Cape Palliser may be dextral oblique-slip structures that are offsetting the basins. These activestructures are possibly reactivated fragments of the paleo-Wairau Fault, which has been proposed by other workersto have connected the ancestral Alpine Fault with theHikurangi subduction margin in the early Miocene.

ACKNOWLEDGMENTS

We thank the officers and shipboard scientific parties on R/VRapuhia cruises 2000, 2001, 2019, 2022, 2034 and 2055.Biostratigraphic analyses were performed by Tony Edwards ofStratigraphic Solutions. Graeme Mackay and Pam Oliverassisted with production of figures. Barnes thanks the NewZealand Foundation for Research Science and Technology forfunding of contract CO 1624. Scott N odder and two anonymousreviewers provided constructive criticism of the manuscript.

REFERENCES

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quaternary faulting in the offshore flaxbourne and wairarapa basins, southern cook strait, new...

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