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Structure of melange and associated units in theTorlesse accretionary wedge, Tararua Range, NewZealandT. O. H. Orr a b , R. J. Korsch a c & L. A. Foley a ba Department of Geology , Victoria University of Wellington , P.O. Box 600, Wellington,New Zealandb L&T Geological Services , 53 Doynton Parade, Mt Waverley, Victoria, 3149, Australiac Division of Continental Geology , Bureau of Mineral Resources , GPO Box 378, Canberra,ACT, 2601, AustraliaPublished online: 23 Mar 2010.
To cite this article: T. O. H. Orr , R. J. Korsch & L. A. Foley (1991) Structure of melange and associated units in the Torlesseaccretionary wedge, Tararua Range, New Zealand, New Zealand Journal of Geology and Geophysics, 34:1, 61-72, DOI:10.1080/00288306.1991.9514439
To link to this article: http://dx.doi.org/10.1080/00288306.1991.9514439
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New Zealand Journal of Geology and Geophysics, 1991, Vol. 34: 61-720028-8306/91/3401-0061 $2.50/0 © Crown copyright 1991
61
Structure of melange and associated units in the Torlesse accretionary wedge,Tararua Range, New Zealand
T. O. H. ORR*R. J. KORSCH†
L. A. FOLEY*
Department of GeologyVictoria University of WellingtonP.O. Box 600Wellington, New Zealand
*Present address: L&T Geological Services, 53 DoyntonParade, Mt Waverley, Victoria 3149, Australia.
†Present address: Division of Continental Geology, Bureau ofMineral Resources, GPO Box 378, Canberra, ACT 2601,Australia.
Abstract Deformation of the Late Jurassic -Early CretaceousTorlesse Complex in the southeastern Tararua Range ischaracterised by development of melange, several generationsof folds, faults at both a low angle and high angle to bedding,shear foliation, and cleavage. The region has undergone thefollowing deformational sequence: (1) Development of at leasttwo fold generations; (2) Fragmentation and disruption byfaulting. Faults at a low angle and high angle to bedding havedisrupted the sequence, in places producing chaoticallydisrupted units (melange). The Tauherinikau Melangerepresents a probable along-strike northern continuation of theEsk Head Melange from the South Island; (3) Post-melangefolding; (4) Holocene faulting. Overall, the deformation isconsistent with accretion at a convergent plate margin, followedby the present strike-slip dominant regime.
Keywords Tararua Range; accretionary wedge; deformation;melange; folds; shear foliation; faults
INTRODUCTION
The Torlesse Complex in the southeastern Tararua Rangeconsists of a complexly deformed, interbedded succession ofquartzofeldspathic greywacke and dark grey argillite, withminor amounts of associated conglomerate, metabasalt, redand green argillite, chert, limestone, and calcareous siltstone.
Rocks have been grouped into a sedimentary association(greywacke, argillite, conglomerate, calcareous siltstone) anda volcanic or seafloor association (basalt, coloured argillite,chert, limestone) (after Bradshaw 1972; Korsch & Wellman
G90007Received 1 March 1990; accepted 6 August 1990
1988). The sedimentary association represents sedimentsdeposited as turbidites in a mid-fan to outer-fan submarineenvironment, whereas rocks of the volcanic associationrepresent seafloor material related to mid-ocean ridge andintraplate (seamount) settings (Foley et al. 1988). Bothlithological associations have metamorphic mineralassemblages indicative of the prehnite-pumpellyite fades.Rocks of the eastern Tararua Range are in the youngest fossilzone of the Torlesse Complex, that is Late Jurassic - EarlyCretaceous (Foley et al. 1986).
The two associations have incompatible depositionalsettings, indicating that they accumulated at a considerabledistance from one another, and also possibly differed in age byup to tens of millions of years (Foley et al. 1986, 1988).Contacts between the two associations, where observed, areeverywhere discordant, being faulted. This has led workerselsewhere in the Torlesse Complex to suggest that the twoassociations were juxtaposed either stratigraphically prior to,or tectonically during, incorporation into an accretionary wedgeat a convergent plate margin (e.g., Sporli 1978; MacKinnon1983; Korsch & Wellman 1988).
The aim of this paper is to describe the morphology andgeometry of the mesoscopic structures, account for themacroscopic structural evolution of the Torlesse Complex inthe eastern Tararua Range, and to examine if the structures arecompatible with those documented from other ancient andmodern accretionary wedges. The area examined represents atypical, relatively poorly exposed part of the Torlesse Complexin the North Island that had not been examined structurally inany detail before this study. The structural work iscomplemented by paleontological data (Foley et al. 1986) anddetailed lithological descriptions (Foley et al. 1988).
MESOSCOPIC STRUCTURES
Throughout this paper, we use the term "fault" for a cleardiscontinuity and the term "brittle shear zone" for a zone ofbrittle deformation (after Ramsay 1980). We use the term"shear foliation" to mean penetrative, subparallel toanastomosing surfaces with a scaly polished appearance (afterKorsch 1982; see Dr foliation of Cowan 1978). Within theeastern Tararua Range, the lack of marker beds and facingdirections makes correlation between outcrops difficult (Foleyet al. 1988); structural mapping was accomplished more easilythan stratigraphic mapping (cf. Fig. 1 with Foley et al. 1988,fig. 2). The regional strike of bedding is approximately NNE-NE, with brittle shear zones being very common and developedpredominantly along steeply dipping bedding planes. Highlydeformed brittle shear zones, incorporating all recognised rocktypes, have a matrix consisting of shear foliation strikingroughly parallel to the regional strike of bedding; these arezones of melange. Mesoscopic structures observed in outcropare folds, shear foliation and cleavage, faults, and melangefabric.
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62 New Zealand Journal of Geology and Geophysics, 1991, Vol. 34
::x:>x7^:':':x:x:':"'74' " v r « " '¿ ' 77' x%-x-x^vXjXjXjïjXj^£iii T : ¿ V V Í " ~ ~ ^ > J J ' -
' 47 8 ? » > , _ 6 2 • - " -
Alluvial sands and gravels
• Direction of younging
X Dip »strike of bedding
X Dip «strike of shear foliation
Fig. 1 The study area (for location see also Fig. 11 ) showing structural data and the degree of disruption to the Torlesse Complex. A, B, andC are disruption types, where A represents mostly intact beds, B represents moderate amounts of faulting that have produced a lozenge fabric,and C represents complete disruption of bedding to produce melange. C(i) zones contain fragments of only the sedimentary association,whereas C(ii) zones contain fragments of both the sedimentary and volcanic associations.
Folds
Mesoscopic folds are relatively rare in the field area comparedwith some other Torlesse localities (e.g., Wellington, Korsch1984). Folds have been observed in well-preserved, alternatinggreywacke and argillite successions, and in disrupted units(melange). In shallowly plunging folds, facing evidence, suchas graded bedding, where observed, indicates that both upward-facing and downward-facing folds occur. The lack of continuousoutcrop inhibits determination of the relationships betweenthese folds, but at Titahi Bay, Korsch & Morris (1987) showed
that downward-facing folds had formed in rocks that hadalready been deformed during at least one previous foldingevent. Based on overprinting relationships, orientation, andstyle, there are at least four generations of folds in the studyarea. Within a melange zone (metric grid reference S26D/11301710), a radiolarian-chert block contains an early foldwhich has been refolded. The fold has truncated limbs and theblock is now surrounded by melange matrix; hence the foldsformed prior to incorporation of the block into the melangeand their present orientations do not reflect their originalorientations (see Korsch 1982).
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Orr et al.—Structure of Torlesse wedge, Tararua Ra. 63
B
FAUL
Alluvial sands and gravels
Fig. 1 (Continued)
Folds produced during the first event are usually observedas folds within blocks in melange. They are isoclinal to closewith angular hinges and some thickening in the hinge regionrelative to in the limbs. The second generation folds wereobserved only in blocks within melange in a single outcrop,where the limbs of an isoclinally folded chert have beenrefolded into a gentle fold.
Subsequent to the formation of the melange, fold eventsproduced initially close to open folds and then open to gentlefolds. In places, this has led to the melange being folded (Fig. 2).
Shear foliation and cleavage
Within argillite, pervasive deformation in brittle shearzones has produced a shear foliation which has a scaly
appearance in outcrop. This foliation is subparallel tobedding in alternating greywacke and argillite sequences.Where argillite is the dominant rock type or matrix material,shear foliation is often the dominant mesoscopic structurepresent.
A weakly defined, spaced cleavage was observed inargillite at only two locations. Thus, it is not possible to usecleavage to distinguish between fold generations (cf. Korsch& Morris 1987). The lack of a well-developed cleavage,such as is present in Torlesse rocks farther to the west(Korsch 1984; Rattenbury 1986; Korsch & Morris 1987),indicates that the rocks in the Tararua Range represent ahigher structural level than Torlesse rocks from older fossilzones around Wellington and Otaki.
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64 New Zealand Journal of Geology and Geophysics, 1991, Vol. 34
Fig. 2 Folded melange fabric insubvertical outcrop (WaiohineRiver S26D/10472288). Photo-graph taken looking to thenortheast; field of view 4.3 macross.
Fig. 3 Tracing of a photograph showing thin-bedded sandstoneand argillite being cut at a high angle by extensional and contractionalfaults (Tauherenikau Gorge, S26C/02831310). Subhorizontalexposure; northeast is to the right-hand side. Main contractionalfaults strike to the north.
Faults at a low angle to beddingFaults at a low angle to bedding are common, causing offsetsthat are often subparallel to the bedding, resulting in ananastomosing appearance and wedging out of beds. The faultingmeans that individual beds can seldom be traced more than afew metres. The amount of movement ranges from a fewmillimetres to greater than outcrop length. These faults cut,and are cut by, shear foliation. Juxtaposition of rocks of thesedimentary and volcanic associations often resulted frommovement on low-angle faults.
Faults at a high angle to bedding
Faults that cut bedding at a high angle are also common. Thefault planes are usually very sharp, although in some places anarrow (up to 10 mm) gouge zone of fine clay occurs. Beds oneither side of the fault plane can be slightly rotated duringdisplacements which vary from a few millimetres to greaterthan outcrop length. Some high-angle faults are displaced bylater faulting of the same style, implying that this style occurredin several phases. High-angle contractional faults appear to beearlier than high-angle extensional faults (Fig. 3).
Melange
Where faulting is intense, faults at both high and low angles tobedding have disrupted the bedding and produced lenticularphacoids and, less commonly, rhombic-shaped lozenges (Fig.4B). In the early stages, high-angle faults, often as conjugatesets, offset the greywacke beds which are reduced to angularrhombic blocks (cf. Needham 1987). With increased brittleshear parallel to bedding, these rhombic blocks become lessangular and, in extreme cases, phacoid shaped. A continuumoccurs between coherent beds and outcrops that consist ofrounded greywacke phacoids in a highly deformed argillitematrix.
The degree to which beds are disrupted appears to be afunction of the intensity of faulting, coupled with the originallithology; massive and thick-bedded greywacke beds arerelatively resistant to disruption. In units of extreme disruption,lozenges and phacoids are usually greywacke, but less commonphacoids of the volcanic association occur also.
Units that consist of discrete phacoids or lozenges of eitherlithologic association surrounded by a highly deformed matrixare here referred to as melange. The term "melange" was usedto describe mappable, internally fragmented and mixed, rockbodies that contain a variety of inclusions, commonly in apervasively deformed matrix (Silver & Beutner 1980). Thisconcept was broadened by Cowan (1985) who showed that theterm can be applied to outcrops as well as mappable units.Here we follow Cowan's (1985, p. 452) definition of melangeas "fragments enveloped by a finer-grained matrix ofmuds tone".
Melange is recognised at a number of localities within thearea, and there is a large variety in the shape and compositionof the fragments (Fig. 4A), which range in size from a fewmillimetres to several hundreds of metres (e.g., metabasaltblock 500 X 80 m at S26D/10502320). The fragments,irrespective of their shape, usually have their long axessubparallel to the shear foliation developed in the matrix. Thematrix is highly deformed argillite. Contacts between the matrixand the fragments are usually sharp, well defined, and obviouslyfaulted. Where the fragments have irregular margins, the shearfoliation within the matrix flows and swirls around thefragments.
A typical example of such melange units that occur in thesoutheastern Tararua Range is the metabasalt and associatedmelange at Campsite Creek (S26C/02831310). The outcropconsists of metabasalt slivers with juxtaposed units of intensely
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Orr et al.—Structure of Torlesse wedge, Tararua Ra. 65
Fig. 4 A, Melange fabric—frag-ments are predominantly grey-wacke surrounded by pervasivelydeformed argillite (Waiohine River,S26D/11331716). Compass onsubhorizontal exposure for scale;northeast is to the right-handside. B, Melange fabric withlozenges and phacoids of sandstonein pervasively deformed blackargillite (Tauherenikau Gorge,S26C/02731323). Hammer onsubhorizontal exposure for scale;east is to the right-hand side.
faulted and deformed sedimentary and volcanic rocks. Theunits show a dominant northeast strike; where observed,contacts are faulted and dip steeply to the southeast. Theoutcrop is divided into 10 mappable units (Fig. 5) which havebeen described in detail by Orr (1984).
Mesoscopic deformation eventsMelange units are important in attempting to define the relativetiming of deformational events within Torlesse rocks of theTararua Range because mesoscopic structures observed atother localities can be correlated with structures that occurwithin melange.
Pre-melange deformation
Deformation that occurred prior to the formation of the melangeis recorded within the now discrete fragments surrounded by
melange matrix. At least two generations of folds, representingshortening events, are observed in the inclusions (describedabove).
Melange formation
During formation of melange, faulting, at both a low angle andat a high angle to bedding, has occurred to such an extent thatchaotically disrupted units were produced. Shear foliationdeveloped in response to the faulting. Formation of melangeand development of shear foliation, and subsequent deformationby numerous episodes of faulting, is considered to beprogressive rather than in discrete episodes. The type ofdeformation which formed the melange can be inferred fromthe shape of the fragments. The rhombic shape of mostfragments is due to fracturing and faulting, probably duringextension (cf. Needham 1987), whereas other fragments, the
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66 New Zealand Journal of Geology and Geophysics, 1991, Vol. 34
TauherenlkauRiver
p̂ j) Metabasite
• Argilllte
Q3 Coloured arglllite
^ \ Fault contact with dip
**. Inferred or gradational contact
Phacioid E Unit reference
Bedding -*- Shear foliation
Younglng .A. Phaclodorientation
Fig. 5 Sketch map of metabasite and melange at the confluence ofCampsite Creek andTauherenikauRiver(S26C/02831310). Variationin phacoid size and shape is diagrammatical. Location is shown as"detailed study area" on Fig. 1.
ellipsoids, have been produced during a flattening deformation(Flinn 1962). Measurements from a number of ellipsoidalfragments fall within the region defined by the parameters ofFlinn where (kk< 1 : a flattening type of strain. As the ellipsoidsare predominantly aligned parallel to the shear foliation in thematrix, they have probably been shortened normal to, andextended parallel to, the shear foliation.
In the Tararua Range, features such as the disruption andmixing of rocks by faulting suggest that the melange hasformed by deformation that is tectonic in origin, rather thanforming due to sedimentary processes, as has been proposedfor some melanges (e.g., Gucwa 1975). The melange hereis very similar to other tectonic melanges such as in theFranciscan Complex, described by Aalto (1981) and Korsch(1982).
Post-melange deformation
Deformation after formation of the melange resulted infragments, phacoids, and shear foliation, that make up thefabric of the melange, being folded into close to openfolds; two post-melange fold events are recognised (seebelow).
MACROSCOPIC STRUCTURES
The monotonous nature of the greywacke and argillite beds,and the paucity of facing evidence, coupled with the lack ofmarker beds in Torlesse rocks of the Tararua Range, hinder thedelineation and definition of macroscopic structures. Beddingstrikes predominantly to the northeast with steep dips. Well-preserved bedding occurs only in a few localities, but showsthat facing reversals do occur, and macroscopic folds can beinferred. Although it has not been possible to map macroscopicfolds across the entire area, the degree of disruption has beenmapped (see Fig. 1). Outcrops were classified into three arbitarydivisions which represent stages in a continuum of deformation.
Type A: Beds are for the most part intact, although minoramounts of faulting, both at a low angle and at a high angle tobedding, may occur. Most facing evidence comes from rocksof this type.
Type B: Moderate amounts of faulting, at both low anglesand high angles to bedding, cause disruption of the morecompetent sandstone beds. Faulting commonly produces arhombic or lozenge fabric mostly due to extension (cf. Pettinga1982; Needham 1987). Facing generally cannot be determinedbecause greywacke-argillite contacts are faulted and argilliteunits are pervasively deformed.
Type C: Complete disruption of bedding has occurred, withbedding only apparent from alignment of the phacoids. Thistype, often seen interleaved with type B, is subdivided intotype C(i) which contains fragments of only the sedimentaryassociation, and type C(ii) which contains fragments of boththe volcanic and sedimentary associations. In several outcropsalong strike, transition from type B to type C(i) was observed.The units shown in Fig. 1 are defined by the disruption typethat dominates the geology of that area, that is, lesser amountsof other types may be present also.
Macroscopic geometric analysisIn determining the deformation on the macroscopic scale, thestudy area was divided into 24 structurally homogeneous,geographically continuous domains (Fig. 6). Some areas havenot been assigned to a structural domain due to the lack of datacaused by exceptionally poor exposure.
Because shear foliation is parallel or subparallel to bedding,both sets of data have been plotted, but not differentiated, onthe same net (Fig. 7). Within the majority of individual domains,the distribution of poles to bedding (So) fall on (poorly defined)great circle girdles (Fig. 7), indicating that So has been cylin-drically folded, and that the beds were approximately planarwithin each domain prior to this deformational event Theorientations of the great circle girdles for the domains, however,are variable, and on a synoptic net (Fig. 8A) the poles to thesegirdles (the n axes) define a great circle girdle with an orientationof 030/SE/85. Hence the most probable explanation is that therocks have experienced at least two fold events, although theexistence of sheath folds is a possibility. (For a single foldevent, the n axis for all domains would theoretically be thesame.) The pattern displayed on the synoptic net (Fig. 8A)may result from either of two alternatives: (1) An early foldsystem that has since been refolded. In this case, the orientationsof TCSO within a domain reflect the orientation of early fold F t .The different orientations of Ft for each domain are due toreorientation of F! during refolding by F2. Thus, the synopticnet of n axes from each domain would define a plane in which
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Orr et al.—Structure of Torlesse wedge, Tararua Ra. 67
Fig. 6 S tudy area showing the 24structural domains. See Fig. 7 forplots of data for each domain.
the fold axes of F! lie, although the plane would not be relatedto the axial surface of either fold event; (2) Folding of planarsegments of a previously nonplanar form surface. Theorientations of JISO within a domain defines the fold axis forF2. In this case, each domain represents a segment which wasoriginally planar prior to F2, but with a different orientation toother domains. In this alternative, the synoptic great circlegirdles of n axes would define the axial surface (AS2) of thesecond fold event.
To test these alternatives, the geometry of mesoscopicfolds can be used, because they should reflect the geometry offolds produced on the macroscopic scale.
Mesoscopic foldsAlthough no overprinting relationships could be determined,axial surfaces of mesoscopic folds from S26D/10472288 showtwo main orientations (Fig. 8B).
Open to close folds have northeast-southwest-striking axialsurfaces with plunges to the northeast and southwest. In contrast,open to gentle folds have east-west-striking axial surfaces andplunge steeply to the west. It is unlikely that the gentle foldswith east-west axial surfaces were produced during an earlyfold event and then remained unaffected during a laterdeformation which produced open to close folds. Thus thegentle folds are considered to represent a later generation thanthose with northeast-striking axial surfaces. Fold axes ofmesoscopic folds that are post-melange in age can be assignedto either the northeast or east trending folds, based on style andorientation (Fig. 8C).
The great circle girdle defined by n axes (Fig. 8A) has asimilar orientation to the northeast-striking axial surfaces ofthe open to close folds, suggesting that the n axis patternreflects alternative 2. Macroscopic fold axes, defined by theintersection of opposite limbs of inferred folds from the lowerMarchant Creek and Waiohine River (Fig. 9), lie along, orclose to, the great circle girdle defined by n axes (Fig. 8A), andwere most likely produced during the same deformation thatproduced the mesoscopic folds with northeast-striking axialsurfaces.
CleavageA weakly defined, spaced cleavage was observed in only twooutcrops, where it is subparallel to bedding, implying that itwas produced during an isoclinal folding event The orientation
(Fig. 8D) is strikingly similar to the axial surfaces of the closeto open folds observed elsewhere, suggesting the possibilitythat the cleavage is related to this folding event.
Faults at a high angle to bedding
Numerous poles to faults at a high angle to bedding wereplotted for individual domains, most of which show a scatter inthe data. In some domains, nevertheless, the poles define agreat circle girdle. This raises the possibility that faults at ahigh angle to bedding (Sf) were originally planar and have beencylindrically folded. If this is so, simultaneous cylindricalfolding of two planar surfaces, So and Sf, may have occurred,and So and Sf should share the same axial surface (Turner &Weiss 1963, p. 130). Great circles defined by JC poles for So
and Sf from individual domains show similar orientations,striking northeast (Fig. 10), suggesting that both So and Sfwere folded by the event that produced northeast axial surfaces.
HOLOCENE DEFORMATION
Many features of the geology and geomorphology of theTararua Range are indicative of Holocene fault activity. Threemajor faults, with associated crush, pug, and smaller faultsoccur, having an overall strike of northeast-southwest, similarto other known active faults in the southern North Island. Thefaults are the Wairarapa, Tauherenikau Valley, and WellingtonFaults (see fig. 2 in Foley et al. 1988). The Wairarapa Fault hasa scarp up to 16 m in height and an associated gouge zoneup to 20 m wide (e.g., at S26/08521187). Detailed aspectsof it have been discussed by Ongley (1943), Lensen (1958),Lensen & Vella (1971), and Grapes et al. (1984); it has hadmajor dextral strike-slip displacements during the Quaternary(Wellman 1969; Suggate et al. 1978). Inferred horizontal andvertical displacement rates of 11 mm/yr and 1.7 mm/yr,respectively (Grapes et al. 1984; Korsch & Wellman 1988) arebased on adopting an 11 000 year age for the WaiohineSurface (P. Vella pers. comm. 1984; see also Vella 1963;Lensen & Vella 1971).
The Tauherenikau Valley Fault has a crush zone up to140 m wide and has several, associated, smaller faults. Onaerial photographs, this fault can be traced well to the north,and Lensen (1958) regarded its trace, from a point near ConeHut towards the north, to form part Of the Wellington Fault. To
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1
n 25
New Zealand Journal of Geology and Geophysics, 1991, Vol. 34
3 ^L 4
n 29
n 9
n 25
n 24 n 30
20
n 14 n 14
n 61 n 10
Fig. 7 Lower hemisphere stereographic projections (Wulff nets) of poles to bedding and shear foliation for the 24 domains shown onFig. 6. Contoured diagrams (domains 13 and 21 ) are plotted on equal-area projections (Schmidt nets), n represents the number of observations,and c represents the contour interval in % per 1% area.
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Orr et al.—Structure of Torlesse wedge, Tararua Ra. 69
Fig. 8 Lower hemisphere Wulffnets. A, Synoptic net showing naxes for each domain. B, Axialsurface and fold axis orientationsfor mesoscopic folds (WaiohineRiver, S26D/10472288). Filledcircles represent fold axes of foldswith east-striking axial surfaces;crosses represent fold axes offolds with northeast-striking axialsurfaces. C, Fold axes of meso-scopic folds from the study area.Filled circles represent fold axes offolds with east-striking axialsurfaces; crosses represent fold axesof folds with northeast-strikingaxial surfaces. D, Orientations ofcleavage, where the dashed greatcircle girdle denotes the trace ofthe average cleavage plane.
the south, the fault passes along the Smith Creek valley wherecrush and pug zones occur. Near the headwaters of SmithCreek, aerial photographs indicate that the fault splits into two;possible correlatives are Reed's (1957) faults 3 and 4 in theRimutaka Railway Tunnel.
In the field, shutter ridges attest that the Wellington Faultis Holocene and dextral. It is considered a major dextralstrike-slip fault of regional importance (Lensen 1958) andhas a horizontal slip rate of 4 mm/yr (Korsch & Wellman1988).
Holocene deformation appears to be restricted to mainlydextral strike-slip faults and associated minor faults. Althoughuplift of the Tararua Range has been occurring since at leastthe Neogene, major uplift is mainly a Quaternary event(Wellman 1969; Suggate et al. 1978). An uplift rate of 4 mm/yr for the Rimutaka - Tararua Ranges was determined byWellman (1969).
SUMMARY AND DISCUSSION
A number of deformational events produced the structuresseen in Torlesse rocks of the Tararua Range.
1. The earliest deformational episode, evidence for which isnow preserved in fragments within melange, was one ofshortening and deformation which produced at least twofold events with different axial surface orientations.
2. The second episode, mainly extensional in character, ischaracterised by the fragmentation and disruption ofbedding, producing faults at a low angle to bedding, shearfoliation, and faults at a high angle to bedding. In places,fragmentation has been so complete that chaoticallydisrupted units (melange) have formed.
3. Post-melange deformation has produced at least two sets offolds, the earlier folds having a subveru'cal northeast-strikingaxial surface whereas the latter folds have an east-strikingaxial surface. Geometric analysis of bedding orientationsindicates the likelihood that three generations of foldingoccurred; the northeast-striking folds have probably formedin a previously nonplanar form surface.
4. Holocene deformation by major dextral strike-slip faultsappears to be confined to narrow zones, but associateduplift has affected a much wider region. Currently activeare the Wairarapa, Tauherenikau Valley, and WellingtonFaults, which are part of the present strike-slip regime inNew Zealand.
The model of formation of the Torlesse Complex currentlyfavoured by many workers is that it represents an ancientaccretionary wedge. At a convergent, continental margin plateboundary, seafloor material and trench-fill sediments arescraped off, undergo decoupling from the subducting plate,and are accreted into an accretionary wedge at the inner trenchwall (frontal accretion, Leggett et al. 1985; Mascle et al. 1986).
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70 New Zealand Journal of Geology and Geophysics, 1991, Vol. 34
Fig. 9 Map, cross section, and net of fold axes of macroscopicfolds inferred from facing evidence, lower Marchant Creek. Greatcircle girdle is from Fig. 8A.
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1zone
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' \ i iV4 jy& Rimutaka\ [W Melange\ Featherston
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100 kmI
Fig. 10 Lower hemisphere Wulff net showing great circles whichcontain the 71 poles for bedding (filled circles) and faults (crosses)from individual domains. Numbers refer to domains on Fig. 6.
Fig. 11 Central New Zealand showing the Esk Head Melange inthe northern part of the South Island and possible correlatives in thesouthern part of the North Island. Fossil zones are 3, Late Triassic?(Jorlessia); 4, Late Triassic (Monotis); 5, Late Jurassic — EarlyCretaceous (after MacKinnon 1983 and Foley et al. 1986).
Accretion of progressively younger packets of sediments istypical of presently active subduction systems (e.g., Karig &Sharman 1975; Scholl et al. 1980; von Huene 1984; Brown &Westbrook 1987; Moore & Shipley 1988).
In transferring the sediment pile from the trench orsubducting plate to the accretionary wedge, the sediment isthought to have undergone the following sequence of events:
(1) With initial subduction, the sediments are dewatered andlithified. Folding of the unconsolidated sediment and softsediment deformation may occur.
(2) Consolidated sediment passes through a "master" shearzone, which represents the initial dislocation of the sedimentfrom the subducting plate.
(3) Sediments are faulted, with development of shear foliationin response to underthrusting, as sediment is accreted.
(4) With subsequent accretion, the sediment is tilted androtated toward the land, with lesser movement on thethrust faults.
(5) Late folding and penetrative faulting occurs (cf. Karig &Sharman 1975; Moore & Wheeler 1978; Byrne 1982;Knipe & Needham 1986; Fisher & Byrne 1987; Moore &Silver 1987; Platt et al. 1988).
The structural style described for Torlesse rocks from theTararua Range, of initial folding and subsequent fragmentation(leading to development of melange) followed by later folding,is similar to that outlined above for accretionary wedgeselsewhere. The juxtaposition of rocks of the sedimentary andvolcanic associations is explained by accretion of trench-filldeposits along with seafloor material in a subduction zone.Thus, the data presented here are compatible with anacccretionary wedge model for the development of the TorlesseComplex.
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Orr et al.—Structure of Torlesse wedge, Tararua Ra. 71
Regional correlations
In the South Island, the Esk Head Melange lies between theLate Triassic (Warepan) Monotis Zone and the Late Jurassic -Early Cretaceous zone, and the melange belt is gradationalinto Torlesse rocks on either side (Bradshaw 1973; Botsford1983). The Monotis fossils occur in seafloor material (limestone)whereas the Late Jurassic - Early Cretaceous age fossils occurwithin turbidites (Botsford 1983). The Esk Head Melange isregarded as a tectonic melange related to a westward dippingsubduction zone during the Early Cretaceous or possibly LateJurassic (Botsford 1983).
Although present-day structures are difficult to correlateacross Cook Strait (Carter et al. 1988), mesoscopic structuresmust once have been continuous, as suggested by the strikingsimilarity of rocks present in the north of the South Islandand the south of the North Island, such as the same Torlessefossil zone rocks and the Late Cretaceous - Tertiarysequences.
In the southern North Island, Torlesse rocks at OtakiForks, 14 km west of the study area, contain Monotis (LateTriassic, Warepan; Grant-Taylor & Waterhouse 1963) andin the eastern part of the study area, Late Jurassic - EarlyCretaceous radiolaria have been recovered from chert (Foleyet al. 1986).
The Western Belt of the study area (see fig. 2 in Foley et al.1988) is very similar to the Esk Head Melange in that withinboth areas there are: ( 1) a high percentage of seafloor material,relative to the "typical" areas of Torlesse rocks; (2) a highpercentage of melange and associated disrupted units; and(3) fine-grained calcareous sediment conformable withinrocks of the sedimentary association. Both areas also liebetween rocks of Late Triassic (Warepan) and Late Jurassic- Early Cretaceous age. On the basis of these similarities,the Western Belt in the southeastern Tararua Range can beconsidered a melange zone and may represent a northwardcontinuation of the Esk Head Melange. Other units in thesouthern North Island, such as the Pohangina Melange(Sporli & Bell 1976) and Torlesse rocks from the ManawatuGorge and Rimutaka Range (pers. obs.) show manysimilarities to rocks of the Tauherenikau melange zone andalso the Esk Head Melange. Thus, these units may representa possible continuation of the Esk Head Melange into thesouthern North Island (Fig. 11). The units show a trend ofdecreasing width northwards: Esk Head Melange (Botsford1983), 10 km wide; melange in the Rimutaka Tunnel (Reed1957) and Rimutaka Road (Rimutaka Melange), 3.1 km wide;melange in the southeastern Tararua Range (TauherenikauMelange, this study), 3-4 km wide; and Pohangina Melange(Sporli & Bell 1976), 1.5 km wide. This trend is also reflectedby the fossil zones of the Torlesse Complex (Korsch & Wellman1988). Structures within Torlesse rocks of the southeasternTararua Range can be related to events during incorporationof material into an accretionary wedge at a convergent platemargin and to Holocene strike-slip faulting. A melange zonein the range is probably a northern lateral equivalent of the EskHead Melange.
ACKNOWLEDGMENTS
We wish to acknowledge the use of departmental facilities while wewere at the Department of Geology, Victoria University of Wellington,and thank C. A. Landis and K. B. Sporli for their comments onversions of this work.
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