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Early Miocene thin‐skinned tectonics and wrenchfaulting in the Pongaroa district, Hikurangi margin,North Island, New ZealandJean Delteil a , Hugh E. G. Morgans b , J. Ian Raine b , Brad D. Field b & Huntly N. C.Cutten ba Université de Nice & C.N.R.S , U.R.A. 1279, avenue Albert Einstein, Valbonne, F. 06560,Franceb Institute of Geological & Nuclear Sciences , P.O. Box 30368, Lower Hutt, New ZealandPublished online: 23 Mar 2010.

To cite this article: Jean Delteil , Hugh E. G. Morgans , J. Ian Raine , Brad D. Field & Huntly N. C. Cutten (1996) EarlyMiocene thin‐skinned tectonics and wrench faulting in the Pongaroa district, Hikurangi margin, North Island, New Zealand,New Zealand Journal of Geology and Geophysics, 39:2, 271-282, DOI: 10.1080/00288306.1996.9514711

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Sew Zealand Journal of Geology and Geophysics, 1996, Vol. 39: 271-2820028-8306/96/3902-0271 $2.50/0 © The Royal Society of New Zealand 1996

271

Early Miocene thin-skinned tectonics and wrench faulting in the Pongaroadistrict, Hikurangi margin, North Island, New Zealand

JEAN DELTEILUniversité de Nice & C.N.R.SU.R.A. 1279avenue Albert EinsteinF. 06560 Valbonne, France

HUGH E. G. MORGANSJ. IAN RAINEBRAD D. FIELDHUNTLY N. C. CUTTEN

Institute of Geological & Nuclear SciencesP.O. Box 30368Lower Hutt, New Zealand

Abstract The Pongaroa-Akitio area, Northern Wairarapa,North Island, New Zealand, is part of the exposed East CoastDeformed Belt at the obliquely convergent plate boundaryof the Hikurangi margin. The sedimentary successionincludes an allochthonous unit of Early Cretaceousgreywacke basement resting on latest Cretaceous rocks.Since the unit's basal contact is subparallel to the beddingof the strata it overlies, the allochthon is inferred to be anunrooted gliding nappe similar to allochthonous outliersdescribed in Northland and the Raukumara Peninsula. Thesouthward emplacement of this "Greywacke Nappe" issupported by structural markers in the body of the nappeand is well dated as earliest Miocene by the youngest rocksinvolved, which are earliest Miocene (Waitakian; c. Aqui-tanian), and because Otaian-Altonian (c. Burdigalian) faultspostdate nappe emplacement. This thin-skinned tectonicphase immediately preceded inception of dextral strike-slipfaulting along northeast-trending Otaian-Altonian(Burdigalian) faults. The present 300 km offset of similarallochthonous outliers on both sides of the faults of thewestern coastal ranges results from cumulative dextral strike-slip movement on these faults through the Miocene.

Keywords New Zealand; Hikurangi margin; coastalranges; Wairarapa; tectonics; faults; allochthon; nappes;olistostrome; strike-slip movement; basement; Cretaceous;Whangai Formation; Miocene; Akitio Fault Zone; Adams-Tinui Fault; Mara melange; melange

G94003Received 26 January 1994; accepted 12 February 1996

INTRODUCTION

The obliquely convergent boundary between the overridingAustralian plate to the west and the subducting Pacific plateto the east extends along the East Coast of North Island,New Zealand (Fig. 1). In eastern North Island, deformationassociated with the plate boundary changes from the axialranges in the west, where active dextral strike-slip ispredominant (Kingma 1967; Lensen 1968; Sporli 1980;Cashman et al. 1992) to the offshore accretionary Hikurangiprism in the east, characterised by imbricated thrusts (Lewis1980; Davey et al. 1986; Lewis & Pettinga 1993). Betweenthese two domains are, from west to east, a forearc basin,the coastal ranges, and the continental shelf. The emergedforearc basin and the coastal ranges have been studied bymany workers, including Ridd (1964, 1967), Johnston(1980), Pettinga (1982), Moore & Speden (1984), Neef(1984), Moore (1986, 1988a), Barnes & Korsch (1991),Chanier (1991), Chanier & Ferriere (1991), Cashman et al.(1992), and Kelsey et al. (1995). Nevertheless, the structureof the coastal region is still under debate, and differentstructural models have been proposed. Based on fieldworkin Southern Hawke's Bay (Pettinga 1982), the coastal rangeshave been considered as an emerged part of the Hikurangiwedge (Lewis & Pettinga 1993), being characterised bycontractional deformation such as folds and northwest-dipping thrusts, that trend parallel to the plate boundary. Inopposition to such a regular east-verging simple shear modelof deformation, geological mapping to the southwest, in theOwahanga valley area (Neef 1991), shows a complicatedpattern of faults with a great variety of trends. Neitherkinematics nor timing of the faults can be deduced clearlyfrom this structural pattern. In southern Wairarapa, nappesare described (Chanier 1991) and interpreted as an earlystage of the accretionary wedge development of the coastalranges. Recently, Cashman et al. (1992) documented activestrain distribution from the axial ranges to the coast along a40 km wide transect located south of Napier. The easternpart of the area covered by the latter work records twonortheast-trending active strike-slip deformation zones andtwo subparallel contractional zones in the northwesterncoastal ranges. The results presented in the present paperare based on detailed geological mapping which was carriedout in northern Wairarapa along another, more southerly,transect that extends across the Pongaroa region from thetoe of the Puketoi Ranges in the west, to the Pacific coastbetween the mouths of the Owahanga and Akitio Rivers inthe east (Fig. 1).

The New Zealand Early Miocene stages (and theirapproximate international correlatives) consist of theWaitakian (Aquitanian), and the Otaian and Altonian which,together, comprise the Pareora Series (comparable with theBurdigalian). The terms Middle and Late Miocene refer tothe Southland and Taranaki Series, respectively, in NewZealand and are approximately the same as in theinternational timescale.

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272 New Zealand Journal of Geology and Geophysics, 1996, Vol. 39

Fig. 1 Location of the study areain the east coast of the NorrIsland. The east coast consists <elongated blocks (Moore 198:31here referred to as "tectonxstrips". Dark-screened areas apre-Miocene series.

FIELD DATA

Main faultsThe coastal ranges are characterised by a series of majorfaults that trend northeast, parallel to the axis of the belt.Between these faults, Moore (1988a) identified severalblocks with major differences in Cretaceous—Paleogenestratigraphic contents as well as sedimentary facies. Thebounding faults display discontinuous segments withextremities that terminate in Neogene and Quaternarysediments, partitioning the coastal ranges into a series ofelongated units. The following descriptions show thatdeposition of unconformable Neogene—Quaternarysediments can account for the apparent dying out of the faultsegments that is shown on large-scale geological maps(Kingma 1962, 1967). We propose that the coastal rangesare cut by steeply dipping, deep-seated faults. These majorstructures bound a series of elongate, northeast-trendingunits. Considering the size and shape of the units (they canreach > 100 km in length and are commonly <10 km wide—even narrower than those described by Moore) the use ofthe term tectonic strips will here be favoured instead of"blocks".

In the eastern half of the coastal ranges, the Owahanga

River catchment extends from the Pongaroa area in the westto the Pacific coast and exposes much of the Tertiarysuccession. Considering the Tertiary age of development ofthe plate boundary through New Zealand (Sporli 1980; Stock& Molnar 1982; Ballance 1988; Sutherland 1995), theOwahanga River region appears to be in a good position t<record the initiation and development of the deformation irthe coastal ranges. We examine the geometry and kinematicof the bounding faults, identify the lithostratigraphic conten:and internal deformation of the strips, and deduce the timinjof the juxtaposition of tectonic strips within the coaslalranges.

Pongaroa Fault Zone

The main fault of the Pongaroa Fault Zone was firstdescribed by Ridd (1967). The dip of the main fault is closeto vertical as it cuts the topography along a rectilinear tracethat consistently trends N53°, west of Pongaroa (Fig. 2).Although the fault plane itself is never seen, the fault traceis easily mapped as it separates thick Paleocene greyish-blue siltstones of the Whangai Formation in the west fromwhite calcareous mudstones of Oligocene Weber Formationin the east. Northwest of the Owahanga River study area.

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Delteil et al.—Thin-skinned basement tectonics, Hikurangi margin 273

RimuManuhara

Strip

Pongaroa Strip

location of Pig. 3

5 km

Rakauhau Strip Akitio Strip

l Mio • Pliocene sed.sexept Mara melange

I Middle Burdigalian1 Mara melange

I Late Cretaceous to1 Eocene sediments

i p t i a n ,1 greywacke

I Barremian1 greywacke

Fig. 2 Main tectonic features of the Pongaroa-Akitio area. Barremian greywacke (exposed only northwest of the southern Adams-Tinui Fault) is part of the autochthonous basement of the Pongaroa Strip. Aptian greywacke (exposed only southeast of the Adams-Tinui Fault) consists of outliers of an allochthonous unit here referred to as the Greywacke Nappe. Late Cretaceous—Eocene sedimentsconsist mainly of siliceous siltstone of Whangai Formation. The middle Burdigalian of Mara is the Mara melange, at the southeasternfringe of the Pongaroa Strip, on top of the Barremian autochthonous basement; it is overlain by allochthonous Late Cretaceous WhangaiFormation (see text). Miocene and Pliocene sediments (except the Mara melange) are exposed principally northwest of the Adams-Tinui Fault and in the Akitio Strip (contours of main sandstone beds are shown).

the northeastern part of the fault splits into two branchesthat cut Waitakian (Aquitanian) rocks and offset themdextrally. The southwestern extremity of the fault also splaysinto two branches (Fig. 2). One of these branches is in linewith the major fault trace and is buried to the southwest bya sequence of alternating mudstone and sandstone of Otaian—Altonian (Burdigalian) age. The other fault branch trends tothe south, connects with a minor fault southwest of theWaihoki summit and cuts Late Miocene sediments. Althoughno precise measurement of the planes of the two latter faultscan be made in the field, their traces can be precisely locatedwhere they cross the road south of Waihoki summit and thevalley next to it. The fault traces show no deflexion in trendwhere they cross the valley and therefore probably have analmost vertical dip. Displacement along the faults isassociated with tight mesoscopic-scale folding resulting inoverturned Late Miocene beds in the close vicinity of thefault traces. In the area of this southern part of the PongaroaFault Zone, near Mount Waihoki, Ridd (1967) previouslynoticed that the pre-Late Miocene series are different on each

side of the Pongaroa Fault Zone. Not only does a major facieschange occur within the Otaian—Altonian, which is muchmore sandstone rich in the east than in the west, but alsoMiddle Miocene strata, which exist in the east, but areentirely missing in the west. These differences in theMiocene rocks on both sides of the southern splays of thePongaroa Fault Zone strongly suggest a structural controlon Early—Middle Miocene sediment deposition. In terms ofMoore's (1988a) structural subdivision of the coastal ranges,the Pongaroa Fault Zone was not considered a prominentfeature as it is located inside his Pongaroa Block. However,we consider that the Pongaroa Fault Zone separates anorthwestern "Rimu-Manuhara Strip" from a southeastern"Pongaroa Strip".

Adams-Tinui Fault

This fault was considered by Moore (1988a) to be thesoutheasternmost major fault of the coastal ranges. InMoore's structural pattern, the Adams-Tinui Fault separatesthe Pongaroa Block to the northwest from the Coastal Block

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274 New Zealand Journal of Geology and Geophysics, 1996, Vol. 39

^RakauhauLate Miocene I ' - i Late Campanian to f

' — J Danian siltstone

Middle Miocene r~X"j Santonian to Early1 ''•'•' Campanian flysch

I Early Miocene — i Turanian to Comflysch 1=^-1 cian flysch

] Mid. Burdigalian CTTTTI AptianMara melange timi greywac|<e

j Aquitanian flysch pITHI] BarremianJ and melange ^^^ greywa '

0 3 km'

Fig. 3 Structures of the Mara area. The Barremian greywacke basement of the Pongaroa Strip, northwest of the Adams-Tinui Fault,is autochthonous, whereas the Aptian greywacke of the Greywacke Nappe in the Rakauhau Strip, southeast of the Adams-Tinui Fault,is allochthonous. The Turonian-Coniacian is mainly flysch of the Glenburn Formation. The Santonian-early Campanian is also mainlyflysch (Glenburn Formation, including "Te Mai Formation", which should be regarded as part of the Glenburn; Crampton pers. comm.1995). The late Campanian-Danian is siltstone of Whangai Formation. The Oligocene is mainly calcareous mudstone of Weber Formation.The Waitakian consists mainly of bedded turbidites, north of Waihoki summit, and olistostromes (including olistoliths of Oligocene) inother exposures. The Burdigalian Mara melange lies unconformably on Barremian greywacke basement and is overlain by anallochthonous Campanian—Danian siltstone unit here referred to as the "Whangai Sheet". The Early Miocene strata (excluding theMara melange) consists mainly of flysch. The Middle Miocene flysch includes two olistoliths of Oligocene rocks west of Cross Hills.The two diagonal straight lines near Pakowhai trig mark the northwestern ends of the sections of Fig. 5.

to the southeast. However, southeast of the Adams-TinuiFault, a major fault zone, here termed the Akitio Fault Zone,has been mapped close to the coastline (Fig. 2). Theexistence of this fault zone leads us to divide Moore's CoastalBlock into a northwestern strip, the "Rakauhau Strip", anda southeastern "Akitio Strip" that extends offshore underthe east coast continental shelf.

In the Owahanga catchment area, the Adams-Tinui Faultexhibits a rectilinear trace south of the Owahanga Rivervalley where it separates Barremian age greywackes to thenorthwest from Aptian greywackes to the southeast. Northof the Owahanga River valley the fault trace curves slightly.The lack of significant bends where it crosses valleysindicates the fault plane has a steep dip. The fault surfaceitself is not seen due to poor exposure but it is associated inplaces with gouge zones up to several tens of metres wide(e.g., where the fault crosses the Owahanga River valley,grid ref. U25/897584*). These gouges imply a significant

amount of movement has taken place on the Adams-TinuiFault. The northern extension of the fault, north of MountCadmus, splays into a horsetail pattern consisting of severalbranches, five of which can be mapped displacingCretaceous—Waitakian rocks. The two westernmost splaysof this horsetail pattern are buried by mid-Altoniansediments. South of the horsetail structure, on thenorthwestern side of the single Adams-Tinui Fault trace,transverse minor faults (a normal fault south of theBenvorlich summit and a reverse fault east of Mount Attila)are cut by the Adams-Tinui Fault (Fig. 2, 3). To the west.the extremities of these minor transverse faults are buriedby the Early to early Middle Miocene series of the PongaroaStrip, whereas to the east, in the Rakauhau Strip, noextensions of the minor faults are seen.

*Grid references are to the metric topographical map seriesNZMS260, 1:50 000 sheets.

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Delteil et al.—Thin-skinned basement tectonics, Hikurangi margin 275

Akitio Fault Zone

Many of the fault traces of the Akitio Fault Zone (Fig. 2)have been previously recognised (e.g., Kingma 1967). Thefault zone is composed of a northern braided system ofsteeply dipping faults which splay out in an acute-angle fanopened to the northeast in map view. Southwestward thefault branches merge into a single fault which follows theshoreline. The easternmost branch of the northern braidedpart of the fault system is in line with the single master faultto the south. The northeastern splays and the master faultcan be considered together as a major tectonic feature whichseparates rocks of very different age. Various Cretaceousrocks occur to the west (in the Rakauhau Strip) between thebranches of the fault zone, whereas a narrow but continuousstrip of Waitakian rocks is exposed east of the fault zone,along the shoreline (in the Akitio Strip). The master faultand its easternmost northern splay dip steeply to thenorthwest, based on their relationships with the topography.The westernmost splays of the braided northern fault systemtrend to the north, crosscut almost the entire width of theRakauhau Strip, and probably connect with the Adams-TinuiFault to the north of the area studied.

Stratigraphic content and internal structure of thefault-bounded stripsFour fault-bounded strips can be distinguished in the studyarea: the Rimu-Manuhara Strip in the northwest (4 km wide);the Pongaroa Strip (13 km wide); the Rakauhau Strip (6 kmwide); and the Akitio Strip (of which only a narrow fringeis exposed along the shoreline).

Rimu-Manuhara StripThe Rimu-Manuhara Strip lies northwest of the PongaroaFault Zone (Fig. 2). It consists of thick siltstone of latestCretaceous age (Whangai Formation) overlain by Early-LateMiocene alternating mudstone and minor sandstone. MiddleMiocene (Southland) rocks have not been identified and wereprobably not deposited in this strip, as the Middle Miocenewedges out towards this strip in the adjacent Pongaroa Strip.

Pongaroa StripMiocene rocks cover most of the surface of the PongaroaStrip but the strip is bounded by exposures of pre-Miocenerocks on both sides, along the bounding faults (Fig. 2).Although all stages of the Miocene from Waitakian toKapitean (Aquitanian-Messinian) occur geographicallythroughout the Pongaroa Strip, they are not regularlydistributed. Early Miocene rocks are present throughout thestrip but Middle Miocene sediments wedge out to thenorthwest (Fig. 3). Also, to the southeast, the Early Miocenesuccession thins and the Middle Miocene successionthickens. Syndepositional alternation of dip direction of thepaleoslope might account for part of these thicknessvariations.

Miocene rocks unconformably overlie latest Cretaceousto Oligocene rocks, which in turn rest on undistorted, light-coloured, thick-bedded, soft greywacke sandstone alternatingwith bluish-grey mudstone that has yielded a Neocomianpalynoflora age. These Neocomian sediments are the oldestrocks in the study area. They crop out on the southeasternside of the strip (Fig. 3) between Madden summit (U25/375560) and Mara settlement (U25/844554).

Deformation of the strip is characterised by folding ofthe full sedimentary cover and by the presence of anallochthonous unit in the east. The folds have north-south-trending axes but do not straddle the bounding faults of thestrips. Near the bounding faults, the fold axes gently curvein a more northeasterly direction, becoming parallel to thefaults (Fig. 2, 3). Folding appears to postdate the depositionof Late Miocene sediments as they are deformed. Never-theless the folds may have been developing throughoutMiocene times, thus accounting for part of the sedimentarywedges that are seen in the Miocene series. These wedgesare well developed in the vicinity of faults (e.g., south ofMount Waihoki, Fig. 3).

In addition to folding, an allochthon is recognised onthe eastern side of the Pongaroa Strip, where there is anarrow, northeast-trending, elongated outcrop of highlydisrupted latest Cretaceous siltstone (Whangai Formation).The disrupted siltstone overlies the Neocomian greywackeswith an intervening early Altonian melange: the Maramelange (U25/847549, Fig. 3). Thus, the Whangai For-mation here is part of an allochthonous unit, referred to asthe "Whangai Sheet". On top of the Whangai Sheet areWaitakian sediments which include olistoliths of Oligocenerocks up to hundreds of metres long and tens of metres thick(Fig. 3). The Waitakian sediments are, in turn, uncon-formably overlain by Pareora sediments. This succession ofrocks indicates a polyphased emplacement of the WhangaiSheet. A first stage of emplacement, consisting of glidingtectonics, is suggested by the presence of Oligocene olisto-liths in the Waitakian sediments, and a second phase, ofgravity sliding, led to the final emplacement of the WhangaiSheet together with the overlying Waitakian olistostrome,and forming the Altonian Mara melange. This polyphasedemplacement is consistent with the highly fractured anddistorted internal structure of the Whangai Sheet.

Rakauhau StripThe Rakauhau Strip is almost completely composed ofCretaceous rocks ranging from late Albian greywacke tolatest Cretaceous siltstone of the Whangai Formation(Fig. 2). The strip has undergone tighter folding than thePongaroa Strip but shows similar characteristics. The generaltrend of the folds is north-south although the fold axesplunge and are gently curved as they approach the northeast-trending bounding faults of the strip; also, the folds are left-stepping in map view. These features are characteristic ofdextral regimes.

The Pakowhai anticline (Fig. 3) can be considered as atypical example of this type of folding. The anticline is coredby Turonian - early Coniacian flysch (Glenburn Formation),which in turn is conformably overlain by Campanian-Daniansiltstone of the Whangai Formation. In map view, the centralpart of the fold axial-trace trends north-south, whereas eachend of the fold is dextrally curved. The curves are associatedwith steeper dips, faulting, and a stretched outer limb of thefold.

The oldest rocks of the Rakauhau Strip (greywacke witha late Albian microflora and microfauna) occur in synclinesadjacent to the Pakowhai Anticline (Fig. 4, 5). There is atectonic contact between the Albian greywacke andunderlying latest Cretaceous Whangai Formation, which canbe traced for >22 km. The contact is sinuous and parallelsthe bedding of the underlying conformable stratigraphic

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276 New Zealand Journal of Geology and Geophysics, 1996, Vol. 39

59.

| J H I 3 Aquitanian flysch 58.

and melange

Late Campanian toDanian siltstone

Santonian to Early 5 7 -

Campanian flysch

Albian greywacke

1.5 km

Fig. 4 Detailed geological mapof part of the southeastern borderof the Rakauhau Strip. The Greywacke Nappe and underlyin;:rocks are folded and cut by faultbranches of the Akitio Fault ZoneA tectonic sliver of Early Miocenestrata is preserved at the base ofthe Greywacke Nappe outlierlocated within the Akitio Faul-

Zone.

N 128

Adams Tinui Pakowhai| Greywacke nappeFault Anticline

AkitioFault Zone

Fig. 5 Sections across theRakauhau Strip north of theOwahanga River valley. TheGreywacke Nappe, emplacedfrom north to south (see text andFig. 8), occurs in the tightly foldedRakauhau Strip. The folds and thenortheast-trending dextral strike-slip faults postdate the emplace-ment of the nappe (locations ofthe sections are indicated onFig. 3, 4).

V.'.'.'.'A Burdigalian flysch

ISaftl Earliest Miocene (Whakataki Form.)

I - ' I Late Campanian to Danian (Whangai Form.)

l::::;:::::l Santonian to Early Campanian (Glenburn Form.)

'• . ) Turanian - Coniacian (Glenburn Form.)

^ | Albian greywacke

y .) Neocomian greywacke

succession in the core of the anticline. This folded tectoniccontact was previously observed around the Pakowhaisummit by Moore & Speden (1984) but its significance wasnot recognised. The Rakauhau Strip comprises an alloch-thonous unit made of Albian greywacke, referred to in thispaper as the "Greywacke Nappe".

The main exposure of the Greywacke Nappe encom-passes the northwestern part of the Rakauhau Strip, buteastern outliers of the nappe are also found between thewestern branches of the northern braided Akitio Fault Zone.

It extends from the Pakowhai River valley in the south upto west of Tau No. 2 summit (U24/026744) in the north(Fig. 2). At both southern and northern ends of the outcropsof Albian greywacke, the nappe rests on top of LateCretaceous-Paleocene/Eocene rocks. There is an outcrop ofmid-Waitakian to mid-Pareora Series that is imbricated inthe basal contact of the nappe (Fig. 4). This basal contact isbroadly parallel to the bedding of underlying younger series,and both the contact and the underlying series are foldedwith the same geometries.

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Delteil et al.—Thin-skinned basement tectonics, Hikurangi margin 277

Fig. 6A, B Folds with verticalaxes in the Akitio Fault Zone. Thefolds are located in the lowercourse of the Waimata River(U25/05857015), 500 m southeastof the main branch of the AkitioFault Zone. They deform EarlyMiocene flysch. (A) Overview.<B) Closeup.

Because the folds deform both the basal tectonic contactof the Greywacke Nappe and the bedding of underlying stratan the same manner, the folding phase postdates the

emplacement of the Greywacke Nappe. This implies thegeometry of the tectonic contact before folding was that ofa flat subhorizontal surface parallel to the bedding of theconformable stratigraphic succession in the relativelyautochthonous underlying strata. The cores of the nappe-inticlines are now eroded and show a typical geometry oftectonic half windows (e.g., in the Pakowhai Anticline,Fig. 3).

Akitio Strip

Mthough this strip widens northwards, only its westernfringe is exposed along the shoreline from north of the Akitio

River mouth to south of the Owahanga River mouth (Fig. 2).The outcrops, some of which can be reached at low tide, areprincipally composed of thick, regularly bedded turbiditesof the Waitakian Whakataki Formation. The strata commonlydip gently to the northwest except those close to theeasternmost fault branch of the Akitio Fault Zone, wherethe series is deformed by mesoscopic folds with vertical axes(e.g., Waimata River at U24/058701; Fig. 6).

On the shore platform southeast of Owahanga Hill (U25/951542), the Waitakian turbidite sequence contains slumpsoverlain by mass-flow deposits. These include conglomeratebeds ranging from 20 to 100 cm thick, with clast sizesranging from 1 to 20 cm (reworked from Turonian-Oligocene rocks). Farther south, SSW of New Homestead(U25/915511), the coarse deposits grade into an olistostrome

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278 New Zealand Journal of Geology and Geophysics, 1996, Vol. ^

Fig. 7 The Owahanga olistostrome. Along the shoreline (U25/91505110), light-coloured lenses of late Campanian-Daniansiltstone (Whangai Formation) and Paleocene mudstone areincluded between darker beds of the earliest Miocene coarse-grained flysch. The attitude of the bedding of the olistostrome isNO23° 49°W.

(the "Owahanga olistostrome") that comprises olistoliths 1—30 m long and 0.20-3.0 m thick (Fig. 7). The olistoliths aremade of either Campano-Danian, Early Eocene, orOligocene sediments. This series was referred to asOligocene by Moore (1988b), but new sampling of themudstone beds (and avoiding olistoliths) provided mid-Waitakian-Pareora Series ages.

DISCUSSION

Kinematics of the allochthonous unitsWhangai SheetThe Whangai Sheet (see "Pongaroa Strip", above) isrestricted to the eastern fringe of the Pongaroa Strip (Fig. 3).The unit does not extend westward, where the Early—MiddleMiocene series is complete with no intervening klippe oflatest Cretaceous siltstone, and where only two (300 m long,50 m thick) Oligocene olistoliths are found in the MiddleMiocene succession (U25/861578) (Fig. 3). Furthermore, theWhangai Sheet does not extend farther south than thePakowhai River valley, where it wedges out into the Maramelange (U25/810520), and there is no clear evidence of

the presence of the unit to the north of Mount Attila. Tins,the location of the unit appears to be restricted to the vicinityof the eastern bounding fault of the Pongaroa Strip: theAdams-Tinui Fault. The underlying, early Altonian Ma amelange contains a variety of rocks, among which are faei sthat are not found in the Pongaroa Strip (e.g., 50 cm blocksof Glenburn facies) but which are very common in theRakauhau Strip east of the Adams-Tinui Fault. The locat i nof the Whangai Sheet, together with the affinities of thereworked blocks within the underlying Mara melange, favoura proximal eastern origin for both the melange and theWhangai Sheet. It is proposed here that Altonian emplace-ment of the Whangai Sheet could be related to relative upli ftof the Rakauhau Strip to the east. This appears to besupported by the absence of significant deposition duringthat period in the Rakauhau Strip.

The Whangai Sheet comprises highly distorted beds andis overlain unconformably by a regularly bedded Waitakianolistostrome containing Oligocene olistoliths. It is inferre dthat the Whangai Sheet was emplaced as an olistolith im othe Waitakian olistostrome and was later emplaced, alo: gwith the olistostrome, over the Mara melange in the lateAltonian. The allochthonous units above the Mara melam ewere probably emplaced from the Rakauhau Strip (that li sto the east) because the facies found do not occur west ofthe Adams-Tinui Fault.

Greywacke Nappe

South-verging simple shear within the Greywacke Nap ;(see Rakauhau Strip, above) consistently indicates asouthwards direction of emplacement. This can be seen aw yfrom fold limbs, where bedding of the greywackes has notbeen deformed by the late folding phase. In these areas, themean strike of the beds is close to east-west, for example,near the axis of the syncline located east of the PakowhaiAnticline (at U25/945603). Here, the bedding attitude isN085°/77°N and the greywacke is deformed by brittle butrelatively pervasive shearing along northeasterly, gentlydipping surfaces. These shear surfaces, oriented N131° 23CE,offset (to the south) beds in the hanging walls (Fig. 8) andbear 007°-trending slickensides. Other indicators of sheardirection and sense, such as ramps dipping northwards andsheared cannonball concretions, also provide a consistentsense of vergence towards the south. These internaldeformation structures are not seen in the Barremi igreywacke of the Pongaroa Strip northwest of the Adams-Tinui Fault. The structures that indicate a southward directionof emplacement of the Greywacke Nappe appear to bespecific to the nappe.

The basal contact of the Greywacke Nappe is almostparallel to the bedding of the underlying relative autochthonas exposed both to the southwest and northeast of the nappe.This suggests the Greywacke Nappe is not rooted. It istherefore inferred to be a thin-skinned gliding nappe, and isunlikely to be more than a few hundreds of metres thick.

The youngest rocks involved in the basal contact of theGreywacke Nappe are exposed in two small exposures. Oneis northeast of the Pakowhai summit, where sedimentsunderlying the nappe yielded a microfauna of LatePaleocene—Eocene age. A second is located in the northernpart of the braided Akitio Fault Zone, at Bush Hill (U25/982619), where an outlier of Early Cretaceous greywackeappears to overlie Late Cretaceous sediments cropping outat Kereru summit (U25/977620). Here, the sinuous contact

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Fig. 8 Internal deformation of the Greywacke Nappe east of thePakowhai summit (U25/94505990). The azimuth of the exposuresurface is NO 10° to the left. The rock shows two verticalferruginous beds with trends varying between Nl 16° and N137°.These beds are cut by shear planes that trend N131° and dip 23°southeastward. Southwards offset of the beds of the hanging wallsis supported by N007° trending slickensides (the handle of thehammer indicates the attitude of the movement vector).

between the Greywacke Nappe and the underlying rocks canbe traced for 7 km (Fig. 2, 4). The contact is marked by a10 m thick sliver of pebbly and sandy mudstone (U25/972603). The sliver of mudstone and sandstone containsblocks ranging from 1 to 80 cm across. The matrix isalternating green mudstone and dark green fine sandstone,and contains a mid-Waitakian to mid-Pareora microfauna.Not only does this outcrop provide a maximum age for theemplacement of the Greywacke Nappe, but it also containsupper bathyal foraminifera, consistent with emplacement ofthe nappe into deep water. No precise age has been obtainedfor the folding phase within the Rakauhau Strip. Never-theless, as the folds deform an allochthonoUs unit emplacedin the Early Miocene, folding must be Miocene or younger.It is likely that folding of the Greywacke Nappe in theRakauhau Strip commenced during Miocene times andcontinued after the Late Miocene, as this is the age range offolding in the Pongaroa Strip.

Kinematics of faultingThere are few mesoscopic structures that constrain the senseof displacement on the major northeast-trending faults: one

oblique slickenside striation, with dextral sense of motion,was measured (N032°, 90°, 44°E) along an eastern splay ofthe Adams-Tinui Fault (U24/990736), and folds with verticalaxes are observed close to the eastern branch of the AkitioFault Zone. Nevertheless, the kinematics of these majorstructures can be deduced with some confidence frommacroscopic structural characteristics.

Moore (1988a) noted the important differences in thestratigraphic and lithologic contents of the fault-boundedunits, and our investigations support Moore's observations.This is particularly well expressed across the Adams-TinuiFault. This fault separates a northwestern area, where theCretaceous sequence comprises only Barremian greywackeand a soft pelitic series of bluish shales of latest Cretaceousage, from the complete Cretaceous sequence from Albiangreywacke to latest Cretaceous hard white siltstonecontaining sandstone lenses southeast of the fault. Inaddition, the thick series of the Glenbura Formation is seenonly southeast of the fault, and the Greywacke Nappe occursonly on the eastern side of the fault. These significantdifferences in contents of adjacent strips on opposite sidesof the subvertical Adams-Tinui Fault suggest that the stripshave been juxtaposed by a substantial amount of cumulativelateral movement on the fault; they are more difficult toexplain by pure dip-slip movement.

Two important macroscopic features constrain the large-scale kinematics of faulting. The first is the presence of faultsthat are oblique to the regional northeast structural trend.These faults are oriented close to north-south and connectthe northeast-southwest-trending master faults (Fig. 2) in amanner consistent with an origin as transfer faults (Sylvester1988). The second feature is that the main faults splaynorthwards into numerous diverging branches forminghorsetail patterns. This occurs on the Adams-Tinui Fault,which splits into five branches from Mount Cadmusnorthwards (Fig. 2), and it also occurs at the northern partof the Akitio Fault Zone where the main fault trace branchesnorthwards within a fan of eight subordinate faults. Someof these branching faults merge towards the north, generatingnarrow (hundreds of metres) and elongated (up to 10 km)tectonic lenses featuring a braided fault system. These stylesof fault connection, namely transfer faults, horsetail, andbraided fault pattern enclosing vertical duplexes, arecommon in transcurrent deformed belts (Woodcock & Fisher1986), and they imply a strike-slip tectonic regime has playeda major role in the structural development of the study area.

The above interpretation, of steeply dipping strike-slipfaults, is supported by complementary kinematic data. Twofaults that trend close to northwest-southeast, almosttransverse to the regional structural trend, terminate abruptlyto the southeast at the Adams-Tinui Fault. These faults arethe Benvorlich Fault (normal) and the Mount Attila Fault(reverse; Fig. 3), both of which cut Otaian rocks, controlthe thicknesses of Pareora Series deposits, and are buried inthe northwest by Clifdenian (early Middle Miocene) andAltonian (late Early Miocene), respectively. Thus, movementalong these faults was effective during deposition of at leastpart of the Pareora Series, at about the same time the Adams-Tinui Fault was active. Normal and reverse movements ortransverse faults contemporaneous with activity along theorthogonally trending Adams-Tinui Fault strengthens thepresumption that strike-slip motion occurred along theAdams-Tinui Fault during the Early Miocene period. The:northwestern extension of the transverse faults remains

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unclear as they are buried by the Middle Miocene cover ofthe Pongaroa Strip. This suggests that movement along thesefaults was either progressively absorbed northwestwards or,more likely, that the faults connect to a western, northeast-trending fault concealed under the Middle Miocene coverof the Pongaroa Strip. Although these relationships remainunclear, the relationships between the transverse faults andthe Adams-Tinui Fault are consistent with their beingtranstensional and transpressive relays. The normal andreverse movements that occurred on Benvorlich and theMount Attila Faults could have been caused by variablestrike-slip displacement along the strike of the Adams-TinuiFault segments.

Without the above new data, the gross structure of thestudy area could have been envisaged as a succession offault-bounded zones of folded rocks, which at one place aremainly Neogene, and at another place are mostly LateCretaceous-Paleocene. In that context, the large northeast-trending folds would have been associated with unquestion-able but moderate vertical movements on the boundingfaults. However, the fold axes trend north-south, exceptclose to the bounding faults, where they curve and becomeparallel to the faults. Within individual tectonic strips, thepattern of north—south-trending sets of folds that neverstraddle the northeast-trending bounding faults results in aright-stepping en echelon style, which is consistent withdextral movement on the bounding faults (Dibblee 1977;Odonne & Vialon 1983).

Proposed kinematic evolutionThe thin-skinned Greywacke Nappe, which extends east ofthe Adams-Tinui Fault in the Rakauhau Strip and into partof the branching northern Akitio Fault Zone, is a type ofstructural unit only newly recognised in this part of thecoastal ranges. Nevertheless, farther south in the Flat Pointarea, similar structures have previously been described byChanier (1991) on the same, eastern side of the southernextension of the Adams-Tinui Fault. In the latter area, severalallochthonous units comprising Late Cretaceous—Eocenerocks were stacked and then tightly folded.

The Greywacke Nappe appears to have been emplacedbefore extensive faulting and folding of the coastal ranges.This is supported by the existence of a sliver of EarlyMiocene rocks lodged at the base of the nappe. Thus, thin-skinned tectonics occurred in Early Miocene times justbefore commencement of movement along the northeast-trending strike-slip faults. The exact age of emplacement ofthe nappe in the Rakauhau Strip is not known but this canbe inferred from the other strips contiguous to the RakauhauStrip. West of the Adams-Tinui Fault, the polyphasedemplacement of the Whangai Sheet is deduced from thepresence of the overlying Waitakian olistostrome; it isconsidered that the Whangai Sheet is one large olistolith,among others, emplaced into the Waitakian sedimentary pile.In a second gliding phase, the Waitakian olistostrome,including the Whangai Sheet, was probably emplacedwestwards onto the Mara melange from some part of theRakauhau Strip. The Waitakian olistostrome, found atpresent on the eastern fringe of the Pongaroa Strip, wouldhave been part of the Rakauhau Strip, and its depositioncould have coincided with the emplacement of theGreywacke Nappe. Furthermore, east of the Adams-TinuiFault, the narrow coastal western fringe of the Akitio Stripalso contains a late Waitakian olistostrome containing large

(several tens of metres) olistoliths of Late Cretaceous rocks.Both olistostromes adjacent to the Rakauhau Strip art-inferred to record the initial stage of syndepositionalemplacement of the nappe during Waitakian times. Theseare in good agreement with what is known, in the RakauhauStrip, about the timing of the emplacement of the GreywackeNappe. A similar comparison can be made between theGreywacke Nappe and other allochthonous units in the EastCoast (e.g., Stoneley 1968; Ballance & Sporli 1979; Brothers1983; Kenny 1984; Cassidy & Locke 1987; Herzer & Isaac1992). Kenny (1984) described a similar gravity nappe,composed of Albian—Late Cretaceous rocks emplaced afterLate Oligocene time in the Ihungia catchment (RaukumaraPeninsula), 300 km northwest of the projection of theAdams-Tinui Fault trend. Thus, the Greywacke Nappe isnot an isolated structure at the scale of the entire East CoastDeformed Belt and can be regarded as an example of a morewidespread tectonic feature.

The conspicuous northeast structural trend of the studyarea mainly reflects the dense network of steeply dippingfaults that were active in Pareora Series time. Verticalmovement on the faults is clear but it does not represent themain component of movement, because the fault patterndisplays numerous characteristics of strike-slip faultnetworks such as transfer faults, relay faults, verticalduplexes, and horsetail structures. Strike-slip tectonism isalso supported by steeply plunging (up to vertical)mesoscopic fold axes close to the northeast-trending fault*.Within the tectonic strips delimited by the main faults,variable deposition rates and wedging out of parts of theMiocene succession suggest alternate northwestward andsoutheastward tilting of the basin basement, a featurecommon in other strike-slip provinces (Sylvester 1988).Although mesostructural evidence was observed in the fieldin only a few places, a dextral sense of strike-slip movementalong the northeast-trending faults is indicated byprogressive deformation of the body of the tectonic stripsinto a right-lateral en echelon arrangement of folds withdextrally curved extremities.

Dextral faulting in this part of the coastal ranges can bedated as Early-Middle Miocene. The age of initiation ofstrike-slip deformation is constrained by the fact that thenortheast-trending faults all cut Waitakian rocks, whereassome fault splays do not cut the entire Altonian succession.This can be seen west of Pongaroa, where the main branchof the Pongaroa Fault is sealed by Pareora Series sediments,and north of Mount Cadmus, where northern splays of theAdams-Tinui Fault are buried in mid-Altonian sediments.Strike-slip movement continued in some areas as late as post-Miocene times, based on fault splays cutting the LateMiocene rocks southwest of Waihoki summit. Nevertheless,the movement rate is unlikely to have been steady fromPareora Series times up to the early Pliocene as observationsto the southwest (Chanier 1991) clearly show that the mainbranch of the Adams-Tinui Fault is buried by Late Miocenesediments. Extensions of two of the major fault traces ofthe Adams-Tinui and Akitio Fault Zones away from the studyarea cut Early and Middle Miocene series (Kingma 1962,1967) rocks. This suggests the main phase of movementalong these faults occurred during Early and Middle Miocenetimes.

The amount of dextral movement along individual strike-slip faults is unknown as no piercing points have beenrecognised. The general regional structure, however, may

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provide a rough estimate of the total displacement. FromNorthland to Raukumara Peninsula and to the Pongaroa area,outliers of allochthonous thin-skinned units were emplacedat the same time, during the Waitakian. In Northland andRaukumara these units are part of a northwest-trendingWaitakian belt characterised by similar rocks, all emplacedas nappes. In the Pongaroa area, this tectonic event occurredbefore the inception of wrench faulting. We propose thatthe southeastern extremity of the Waitakian belt underwentdextral displacement from Pareora Series time onwards,resulting in the present-day location of allochthonous outliersnear Pongaroa.

In order to use the allochthonous outliers as passivemarkers with regard to subsequent strike-slip displacement,the following three points must be considered:(1) the allochthonous units must consist of rocks of similar

ages and facies;(2) the present amount of displacement represents

cumulative movement on the whole set of faultsseparating the allochthonous units;

(3) the present distribution of allochthonous units whichwere emplaced some 23 m.y. ago is partly erosiondependent (unless the units were buried and preserved).

The following comments regarding the GreywackeNappe can be made.1. There is a strong similarity between the Northland

Allochthon, the Ihungia decollement, and the GreywackeNappe. All of them include Albian greywacke, which isconsistent with their being parts of the same Waitakianorogenic belt.

2. The fact that the northeasternmost outlier of the Ihungiaarea is located east of the axial ranges trend but west ofthe northward projection of the coastal ranges masterfaults implies that the offset observed between theIhungia outlier and the Greywacke Nappe is distributedbetween the Adams-Tinui Fault and all the paralleltrending faults exposed or buried westwards as far asthe forearc basin.

3. The southern limit of the Ihungia decollement is atpresent preserved from erosion by younger overthrustedunits (Kenny 1984; Mazengarb et al. 1991). In contrast,although the southern tip of the Greywacke Nappe isexposed in the Pakowhai River valley, it could haveextended farther south. The net cumulative offset of bothallochthonous units could therefore have exceeded thepresent amount of 300 km.The model of wrench faulting described here for this part

of the coastal ranges differs significantly from previousMiocene tectonic models proposed by Pettinga (1982) andLewis & Pettinga (1993) northeast of the Pongaroa district,and by Chanier (1991) to the southwest in southernWairarapa. Pettinga's conclusions obtained from SouthernHawke's Bay were in favour of imbricated southeast-vergingsheets rather than transcurrent movements. Pettingadescribed tectonic contacts with variable dips ranging from20 to 90° in the coastal zone, which are compatible withshallow parts of half positive flower structures, and heacknowledged that wrench faulting before the Late Miocenecould not be discounted. Furthermore, post-Late Miocenecompressional deformation described to the west, on theElsthorpe Anticline (which is in line with the Adams-TinuiFault), is not in conflict with major right-lateral movementduring the Early-Middle Miocene along this fault. The

differences between the two models could therefore be partlyreconciled, assuming the shallow-dipping structures mappedby Pettinga close to the coast represent the contractional,superficial parts of deeper seated transpressional zones. Insouthern Wairarapa, Chanier (1991) described a phase ofnappe emplacement in Early Miocene times, which heconsidered to be associated with the first stage of imbricationof the Hikurangi wedge. Within this framework, the nappeswere envisaged by Chanier to be rooted to the northwest atthe trace of the Adams-Tinui Fault, which he believed tohave developed as a shallow-dipping structure which wasrotated to vertical during subsequent deformation. Thegeometry of the final polyphased structure of the Pongaroadistrict is similar to Chanier's descriptions in southernWairarapa. The difference in interpretation comes from theabundance of structures indicative of wrench faultdeformation and from the ability to date the events and gaina better, three-dimensional picture using stratigraphic datafrom the Miocene succession in the Pongaroa district.

The results presented here have important implicationsfor the development of the deformed belt of eastern NorthIsland.

CONCLUSIONS

The Owahanga River catchment provides fairly good datingof the structural development and successive kinematics ofpart of the Hikurangi margin, North Island. From thepresence of exposures of the whole Miocene sequence andtheir varied relationships with different structures, weinterpret a polyphased Miocene structural development.During Waitakian times, a thin-skinned, unrooted glidingunit of Albian greywacke was emplaced. This GreywackeNappe is considered to be part of a widely distributed featurefrom Northland to Southern Wairarapa. A close relationshipis proposed between the Greywacke Nappe and the Ihungiadecollement described by Kenny (1984) in the RaukumaraPeninsula, as both units comprise similar rocks. Soon afternappe emplacement, during Pareora Series times, majordextral strike-slip deformation occurred. This second phaseof deformation continued, but with decreasing intensitythrough the Miocene, dividing the coastal ranges into severalstrips that are narrower and more numerous than the "blocks"described by Moore (1988). The strike-slip patterns producedby the latter phase of deformation might extend offshorebeneath part of the continental shelf, as a major fault zonetrends close to the shoreline. Using the allochthonous outliersas passive markers, offset by subsequent strike-slip tectonics,right-lateral displacement on part of the northeast-trendingfaults in the coastal ranges may have accommodated up toc. 300 km total displacement—the distance between theIhungia and Owahanga areas. Although recent strike-slipfaulting is described northeast of the study area by Cashmanet al. (1992), our results suggest that, at a large scale, thestrike-slip tectonic system has migrated westward from thecoastal ranges during the Neogene to the axial ranges whereit is active at present.

ACKNOWLEDGMENTS

This paper is based on research undertaken with the support ofthe University of Nice, the French Embassy, and the Institute ofGeological & Nuclear Sciences (formerly DSIR Geology and

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Geophysics). We thank these institutions for funding this work(in part from the FRST PGSF Sedimentary Basins Programme)and R. Herzer for helping to initiate the study. G. Neef and D.Francis provided interesting discussions in the field. We are gratefulto S. Cashman, R. Lynch, P. Barnes, and an anonymous reviewerfor their helpful comments and suggestions. Special thanks go tothe Wardles of Pongaroa for their warm hospitality and to FrancoiseDelteil for her assistance in the field and valued discussions.

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