Structure of melange and associated units in the Torlesse accretionary wedge, Tararua Range, New Zealand
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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
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
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).
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.
62 New Zealand Journal of Geology and Geophysics, 1991, Vol. 34
::x:>x7^:':':x:x:':"'74' " v r " ' ' 77' x%-x-x^vXjXjXjjXj^iii T : V V " ~ ~ ^ > 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.
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).
Orr et al.Structure of Torlesse wedge, Tararua Ra. 63
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 cleavageWithin 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.
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).
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...