Tectonic significance of Cretaceous bivergent extensional shear zones in the Torlesse accretionary wedge, central Otago Schist, New Zealand

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<ul><li><p>This article was downloaded by: [University of Louisville]On: 20 December 2014, At: 01:59Publisher: Taylor &amp; FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: MortimerHouse, 37-41 Mortimer Street, London W1T 3JH, UK</p><p>New Zealand Journal of Geology and GeophysicsPublication details, including instructions for authors and subscription information:http://www.tandfonline.com/loi/tnzg20</p><p>Tectonic significance of Cretaceous bivergentextensional shear zones in the Torlesse accretionarywedge, central Otago Schist, New ZealandH. Deckert a , U. Ring a &amp; N. Mortimer ba Institut fr Geowissenschaften , Johannes GutenbergUniversitt , Mainz, 55099,Germanyb Institute of Geological &amp; Nuclear Sciences , Private Bag 1930, Dunedin, New ZealandPublished online: 23 Mar 2010.</p><p>To cite this article: H. Deckert , U. Ring &amp; N. Mortimer (2002) Tectonic significance of Cretaceous bivergent extensionalshear zones in the Torlesse accretionary wedge, central Otago Schist, New Zealand, New Zealand Journal of Geology andGeophysics, 45:4, 537-547, DOI: 10.1080/00288306.2002.9514990</p><p>To link to this article: http://dx.doi.org/10.1080/00288306.2002.9514990</p><p>PLEASE SCROLL DOWN FOR ARTICLE</p><p>Taylor &amp; Francis makes every effort to ensure the accuracy of all the information (the Content) containedin the publications on our platform. However, Taylor &amp; Francis, our agents, and our licensors make norepresentations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose ofthe Content. Any opinions and views expressed in this publication are the opinions and views of the authors,and are not the views of or endorsed by Taylor &amp; Francis. The accuracy of the Content should not be reliedupon and should be independently verified with primary sources of information. 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Terms &amp; Conditions of access and use can be found at http://www.tandfonline.com/page/terms-and-conditions</p><p>http://www.tandfonline.com/loi/tnzg20http://www.tandfonline.com/action/showCitFormats?doi=10.1080/00288306.2002.9514990http://dx.doi.org/10.1080/00288306.2002.9514990http://www.tandfonline.com/page/terms-and-conditionshttp://www.tandfonline.com/page/terms-and-conditions</p></li><li><p>New Zealand Journal of Geology &amp; Geophysics, 2002, Vol. 45: 537-5470028-8306/02/4504-0537 $7.00/0 The Royal Society of New Zealand 2002</p><p>537</p><p>Tectonic significance of Cretaceous bivergent extensional shear zones in theTorlesse accretionary wedge, central Otago Schist, New Zealand</p><p>H. DECKERT</p><p>U. RINGInstitut fr GeowissenschaftenJohannes Gutenberg-Universitt55099 Mainz, Germany</p><p>N. MORTIMER</p><p>Institute of Geological &amp; Nuclear SciencesPrivate Bag 1930Dunedin, New Zealand</p><p>Abstract We describe two shear zones in the Otago Schistof the Torlesse accretionary wedge, South Island, NewZealand: the north-dipping Rise-and-Shine Shear Zone(RSSZ) and the south-dipping Cromwell Gorge Shear Zone(CGSZ). Kinematic indicators (shear bands and asymmetricfolds) indicate top-north movement for the RSSZ and top-south transport for the CGSZ. Back rotation of the shearzones into their Late Cretaceous orientation andconsideration of the relationship of the shear zones to archingof the Otago Schist show that both shear zones areextensional. Offset of textural zones suggests up to 15 kmof dip-slip displacement on the RSSZ and probably a similaramount of slip for the CGSZ. We speculate that the gold-mineralised RSSZ may be the western continuation of thelate Mesozoic gold-bearing Hyde-Macraes Shear Zone ineastern Otago, forming a c. 100 km long extensional shearzone on the northern flank of the Otago Schist. Ageconstraints suggest that shear zone formation took placebetween 135 and 105 Ma. The shear zones aided the finalexhumation of the deeper parts of the Otago Schist. Wediscuss whether normal shearing is related to syn-orogenicsupercritical tapering of the Torlesse wedge, or due to post-orogenic New Zealand-wide Albian rifting.</p><p>Keywords normal fault; accretionary wedge; rifting;Torlesse Terrane; Dunstan Range; Otago Schist; NewZealand</p><p>INTRODUCTION</p><p>The role of syn-orogenic normal faulting in ancient andmodern accretionary wedges related to ocean-continentsubduction remains a controversial topic. The Hikurangisubduction zone in the North Island of New Zealand is anexample where orogen-parallel extensional faults occur inthe forearc (Pettinga 1982; Walcott 1987). However, themagnitude of extension accommodated by the listric normalfaults is only 1-5% (Cashman &amp; Kelsey 1990). Syn-orogenic</p><p>G01024; published 6 December 2002Received 1 August 2001; accepted 8 July 2002</p><p>normal faulting has also been proposed along the so-calledCoast Range fault of the Franciscan subduction complex(Platt 1986; Harms et al. 1992). The tectonic juxtapositionof low-grade rocks above higher grade rocks was used asthe major evidence for normal faulting. But subsequent workby Wheeler &amp; Butler (1994), Ring &amp; Brandon (1994), andRing (1995) showed that this evidence is not conclusive,and Ring &amp; Brandon (1994, 1999) and Bolhar &amp; Ring (2001)showed that the Coast Range fault is an out-of-sequencethrust. Other well-studied accretionary wedges, such as theOlympic subduction complex at the North American westcoast, also show no evidence for normal faulting (Brandonet al. 1998).</p><p>Frontal accretion and erosion both tend to promotehorizontal contraction across an orogen. This case is wellillustrated by the strain calculations in Dahlen &amp; Suppe(1988). Horizontal extension may occur in the rear part ofan accretionary wedge thickened by underplating (Platt1986). But horizontal extension need not necessarily beexpressed by normal faulting in the upper rear part of thewedge; it may also be accommodated by vertical ductilethinning related to a subhorizontal foliation. Thus, a largeamount of vertical ductile thinning tends to suppress normalfaulting. Flow fields that give rise to horizontal extensionin the rear of accretionary wedges develop when the rate offrontal accretion and erosion at the rear of the wedge aresmall, and when the rate of underplating is dominantlycontrolling the flow field in the wedge (Platt 1986, 1993;Brandon &amp; Fletcher 1998).</p><p>Before c. 105 Ma, the New Zealand part of the southGondwana margin was a subduction zone along whichcontinental growth took place by terrane accretion and arc-related magmatism (Mortimer et al. 1999) (Fig. 1A). Theolder Torlesse (Rakaia) Terrane and the Caples Terranecomprise greywacke of Permian-Triassic stratigraphic ageand are thought to have been deformed in a Jurassic-Cretaceous accretionary wedge (MacKinnon 1983; Korsch&amp; Wellman 1988). Albian (c. 105 Ma) graben development(Ballance 1993; Laird 1993) is related to the separation ofNew Zealand from Australia and Antarctica. This post-subduction rifting phase culminated in seafloor spreadingin the Tasman Sea at c. 85 Ma (e.g., Korsch &amp; Wellman1988).</p><p>In this paper we present new field and petrographic datafrom the Rise-and-Shine and Cromwell Gorge Shear Zones(RSSZ, CGSZ) in the Torlesse accretionary wedge of centralOtago. The RSSZ and also the Hyde-Macraes Shear Zoneof eastern Otago (Fig. 1B) have previously been regardedas late subduction-related thrusts (e.g., Teagle et al. 1990;Winsor 1991). Nonetheless, our data show that the RSSZand the CGSZ are net normal-sense shear zones. The age ofshear zone formation is not well constrained, which makesit speculative to relate normal shearing to either late-accretionary destabilisation of the Torlesse wedge duringongoing convergence or to post-accretionary rifting.</p><p>Dow</p><p>nloa</p><p>ded </p><p>by [</p><p>Uni</p><p>vers</p><p>ity o</p><p>f L</p><p>ouis</p><p>ville</p><p>] at</p><p> 01:</p><p>59 2</p><p>0 D</p><p>ecem</p><p>ber </p><p>2014</p></li><li><p>538 New Zealand Journal of Geology and Geophysics, 2002, Vol. 45</p><p>HaastSchist</p><p>C Late Jurassic Clent Hills Group</p><p>_| Torlesse</p><p>_| Caples</p><p>J Maitai</p><p>J Brook Street &amp; Murihiku</p><p>j I!! i! 11| Median Batholith /</p><p>Western Province / ^</p><p>Marlboroughz ^ . Schist</p><p>= r y Dunedin</p><p>I01Christchurch</p><p>45 S</p><p>100km</p><p>170 EI</p><p>Index mineralassemblages</p><p>albite+pumpellyite</p><p>albite+chlorite+epidote</p><p>albite+biotite garnet</p><p>oligoclasealbite</p><p>Dunedin</p><p>Fig. 1 A, Simplified geological map of pre-Late Cretaceousbasement, South Island, New Zealand (after Mortimer et al. 1999).Haast Schist Belt is subdivided into Marlborough, Alpine, andOtago Schist. B, Map of the Otago Schist showing metamorphicgrade, Cretaceous normal faults and shear zones, and Albian grabensediments (after Mutch &amp; Wilson 1952; Bishop &amp; Laird 1976;Mortimer 1993,2000). HF, Hawkdun Fault; DF, Dansey Pass Fault;WF, Waihemo Fault; BF, Barewood Fault System; TF, Titri Fault;OMF, Old-Man Fault; HMSZ, Hyde-Macraes Shear Zone; CGSZ,Cromwell Gorge Shear Zone; RSSZ, Rise-and-Shine Shear Zone.</p><p>GEOLOGICAL SETTING</p><p>The terranes of the South Island of New Zealand can bedivided into two major provinces (Fig. 1A). (1) TheWestern Province, consisting of Cambrian-Devonianterranes with local Permian-Trias sic sedimentary cover,are considered to have been part of Gondwana since themid Paleozoic (Mortimer &amp; Campbell 1996). (2) TheEastern Province, consisting of Carboniferous-Cretaceous terranes, is interpreted to have been accretedto the Gondwana margin from the Triassic through theCretaceous (e.g., MacKinnon 1983; Korsch &amp; Wellman1988; Mortimer &amp; Campbell 1996). The two provincesare separated by the Median Batholith (Mortimer et al.1999; Turnbull 2000), a Cordilleran-type magmatic arcthat represents the major locus of Carboniferous to EarlyCretaceous magmatism in New Zealand. Magmatic pulsesin the Median Batholith can be related to subduction ofthe oceanic Pacific plate underneath southern Gondwana(Mortimer et al. 1999).</p><p>The Haast Schist in New Zealand's South Island issubdivided into the Otago, Alpine, and Marlborough Schist(Fig. 1A). The Otago Schist is a moderately high pressuremetamorphic belt (Yardley 1982; Mortimer 2000), whichformed by amalgamation of the Eastern Province Caples andTorlesse Terranes during the "Rangitata I" Orogeny in theEarly-Middle Jurassic (Bradshaw et al. 1981; Adams &amp;Robinson 1993; Little et al. 1999). The Otago Schist providesa view into the deeper part of the Torlesse accretionarywedge and forms a c. 150 km wide, two-sided arch.Metamorphism increases from prehnite-pumpellyite faciesin the non-schistose rocks at the periphery of the arch togreenschist facies with peak metamorphic temperatures andpressures of 350-400C and 8-10 kbar near the culminationaxis in the deepest level of present exposure (Mortimer 2000)(Fig. 1B). Accompanying increasing metamorphism, thereis an increase in deformation towards the centre of the OtagoSchist as defined by the progressive development of foldgenerations and foliation transposition. Hutton &amp; Turner(1936), Bishop (1972), Turnbull (2000), Forsyth (2001), andTurnbull et al. (2001) used meso- and microscale texturalchanges in the schist to define up to six different texturalzones (TZs), which parallel the metamorphic and deform-ational changes across the Otago culmination. Textural zonediscontinuities are the best indicators of postmetamorphicfaults in the otherwise monotonous rocks of the Otago Schist(Craw 1998).</p><p>Exhumation of the Otago Schist either occurred veryslowly and continuously at a rate of c. 0.2 km/m.y. from190 to 110 Ma (Adams et al. 1985; Adams &amp; Robinson 1993)or was punctuated by a phase of more rapid exhumation atrates of 0.6-1.0 km/m.y. after 135 Ma (Little et al. 1999).Deeply buried rocks in the Torlesse wedge reached thesurface at c. 105 Ma during the initial rifting phase. This isindicated by the first appearance of schist fragments in theKyeburn Formation (Korsch &amp; Wellman 1988; Adams &amp;Raine 1988). The Kyeburn Formation, together with theHorse Range Formation and the Henley Breccia, wasdeposited in Albian rift-related graben on the Otago Schist(Fig. 1B); similar graben are found throughout the NewZealand terranes (Ballance 1993; Laird 1993). Rifting ismanifested as half graben in the Eastern Province terranesand magmatism and metamorphic core complex develop-ment in the Western Province (Tulloch &amp; Kimbrough 1989).</p><p>Dow</p><p>nloa</p><p>ded </p><p>by [</p><p>Uni</p><p>vers</p><p>ity o</p><p>f L</p><p>ouis</p><p>ville</p><p>] at</p><p> 01:</p><p>59 2</p><p>0 D</p><p>ecem</p><p>ber </p><p>2014</p></li><li><p>Deckert et al.Cretaceous bivergent extensional shear zones 539</p><p>Fig. 2 Map showing the Otagopeneplain with contours in DunstanMountains (modified after Bishop1994 and Turnbull 2000). Cross-sections show Neogene deform-ation of the peneplain.</p><p>Peneplain underTertiary and Quaternary</p><p>Peneplain destroyedby erosion</p><p>Pisa Range Dunstan RangeB20001000SI</p><p>11 11I 'I</p><p>200'1001</p><p>S.I</p><p>'7Dunstan Range^2000</p><p>- M000</p><p>Csx 10 20km</p><p>This deformation has been referred to as "Rangitata II"Orogeny (Bradshaw et al. 1981) and arguably is the oldestgeological event common to all New Zealand basementterranes.</p><p>Widespread erosion of the Otago Schist and also theAlbian graben sediments resulted in peneplanation of muchof the southern South Island. The Waipounamu ErosionSurface is probably composite and time transgressive(Bishop 1994; LeMasurier &amp; Landis 1996). A prominentunconformity at the base of Haumurian (c. 85 Ma) sedimentsin northern and offshore South Island is thought to be a lateralequivalent of the Waipounamu Erosion Surface (Cramptonet al. 1999), and can be used to date the initial formation ofthe erosion surface between 105 and 85 Ma. Miocene-Recent deformation related to formation of the Alpine Faultfolded the Waipounamu Erosion Surface and gave centraland east Otago its present basin-and-range topography(Cotton 1917; Bishop 1994; Jackson et al. 1996; Turnbull2000; Forsyth 2001) (Fig. 2).</p><p>POSTMETAMORPHIC SHEAR ZONES IN THEOTAGO SCHIST</p><p>Postmetamorphic shear zones in the Otago Schist are ofinterest for two main reasons: (1) as hosts of goldmineralisation (e.g., Winsor 1991); (2) as structures thatmight shed light on the exhumation history of the Torlesseaccretionary wedge. The best studied low-angle shear zonein the schist is the northeast-dipping, gold-mineralised Hyde-Macraes Shear Zone in east Otago (Fig. 1B) (Teagle et al.1990; Winsor 1991; Craw et al. 1999; De Ronde et al. 2000;Forsyth 2001). Kinematic studies of the shear zone haveshown that late- to postmetamorphic thrusting was followedby a period of normal faulting (Teagle et al. 1990; Angus1992). The gold mineralisation in the Hyde-Macraes ShearZone took place between 132 and 158 Ma (Adams &amp; Graham1997).</p><p>The large-scale structure of the Otago Schist in thesouthern Dunstan Range (Fig. 3, 4) is dominated by a</p><p>Dow</p><p>nloa</p><p>ded </p><p>by [</p><p>Uni</p><p>vers</p><p>ity o</p><p>f L</p><p>ouis</p><p>ville</p><p>] at</p><p> 01:</p><p>59 2</p><p>0 D</p><p>ecem</p><p>ber </p><p>2014</p></li><li><p>540 New Zealand Journal of Geology and Geophysic...</p></li></ul>


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