Thermal Evolution of the Sisters Shear Zone, Southern New Zealand: Formation of the Great South Basin and Onset of Pacific-Antarctic Spreading

TECTONICS, VOL. 28, TC5015, doi:10.1029/2008TC002368, 2009 Click Here for Full Article Thermal evolution of the Sisters shear zone, southern New Zealand: Formation of the Great South Basin and onset of Pacific-Antarctic spreading Joseph Kula,1,2 Andy J. Tulloch,3 Terry L. Spell,1 Michael L. Wells,1 and Kathleen A. Zanetti1 Received 21 July 2008; revised 14 December 2008; accepted 13 July 2009; published 20 October 2009. [1] The separation of Zealandia from West Antarctica 1989; Lister et al., 1991; Spell et al., 2000]. In contrast, was the final stage in the Cretaceous breakup of the details of continental extension leading to formation of the Gondwana Pacific margin. Continental extension Pacific-Antarctic Ridge and separation of eastern Zealandia resulting in formation of the Great South Basin and from West Antarctica are more cryptic. An improved thinning of the Campbell Plateau leading to understanding of this latter phase of tectonism holds im- development of the Pacific-Antarctic spreading ridge portant implications for the development of Zealandia as a continent because several offshore continental features (e.g., was partially accommodated along the Sisters shear the Great South Basin and the Campbell Plateau) formed zone. This east-northeast striking brittle-ductile contemporaneously with this event. Additionally, detailed structure exposed along the southeast coast of Stewart documentation of the spatial and temporal evolution of a Island, New Zealand, is a greenschist facies extensional continental rift system has implications for understanding shear zone that separates a hanging wall of chloritic, changes in driving mechanisms for extension that may brecciated granites, and undeformed conglomerate occur as the rift progresses. from a footwall of mylonitic Carboniferous and Early [3] Kula et al. [2007] proposed the isolation of Zealandia Cretaceous granites. This complex structure exhibits resulted from two distinct rifting events, with separation bivergent kinematics and can be subdivided into from West Antarctica partially accommodated along the a northern and southern segment. The 40Ar/39Ar Sisters shear zone located on the southeast coast of thermochronology indicates that cooling of the shear Stewart Island in southern New Zealand. This tectonic model was largely based on comparison of 40Ar/39Ar K- zone footwall began at 94 Ma with accelerated feldspar thermochronometry from the Sisters shear zone cooling over the interval 89–82 Ma. Structural and with thermochronometry of other Early Cretaceous shear thermochronological data indicate a spatial and zones in New Zealand and Antarctica. Here, a more com- temporal link between the Sisters shear zone, initial plete field, kinematic, and thermochronometry study of the sedimentation within the offshore Great South Basin, Sisters shear zone is presented. These data are used to extension of the Campbell Plateau, and initiation of the constrain the shear zone architecture, determine the defor- Pacific-Antarctic spreading ridge. Citation: Kula, J., mation mechanisms active (and thus temperature condi- A. J. Tulloch, T. L. Spell, M. L. Wells, and K. A. Zanetti (2009), tions) during extensional shearing of the footwall, and Thermal evolution of the Sisters shear zone, southern New Zealand: demonstrate the Sisters shear zone played a significant role Formation of the Great South Basin and onset of Pacific-Antarctic in the Cretaceous extensional tectonics resulting in forma- spreading, Tectonics, 28, TC5015, doi:10.1029/2008TC002368. tion of the Great South Basin, thinning of the Campbell Plateau, development of the Pacific-Antarctic Ridge, and separation of Zealandia from West Antarctica. 1. Introduction [2] The isolation of Zealandia in the South Pacific was a 2. Stewart Island Geology result of formation of the Tasman Ridge and the Pacific- Antarctic Ridge oceanic spreading systems (Figure 1). Early [4] Stewart Island is located just south of South Island Cretaceous extension leading to metamorphic core complex and represents the southeastern continuation of the Median formation, development of the Tasman Ridge and separation Batholith and Western Province of New Zealand [Allibone of western Zealandia from eastern Australia is well docu- and Tulloch, 1997, 2004] (Figure 2). Mapping, geochemical mented [Tulloch and Kimbrough, 1989; Etheridge et al., and isotopic studies by Allibone and Tulloch [1997, 2004] shows that basement rocks making up Stewart Island are dominantly plutonic and of various granitoid compositions 1Geoscience Department, University of Nevada, Las Vegas, Nevada, and ages. Intrusions range from late Paleozoic through USA. Mesozoic ages marking four pulses of magmatism during 2 Now at Department of Earth Sciences, Syracuse University, Syracuse, the Carboniferous (345–290 Ma), Early Middle Jurassic New York, USA. 3GNS Science, Dunedin, New Zealand. (170–165 Ma), latest Jurassic to earliest Cretaceous (151– 128 Ma), and Early Cretaceous (128–100 Ma) [Allibone and Copyright 2009 by the American Geophysical Union. Tulloch, 2004]. The spatial distribution and age constraints 0278-7407/09/2008TC002368$12.00 TC5015 1 of 22 TC5015 KULA ET AL.: SISTERS SHEAR ZONE GREAT SOUTH BASIN TC5015 where the southern portion of the island represents the continuation of the high Sr/Y belt (younger thick arc crust) on the continent side and the north represents the low Sr/Y belt (older thin arc crust) on the ocean side with the Escarp- ment Fault roughly marking the boundary between the two [Tulloch et al., 2006, available at http://www.crownminerals. govt.nz/cms/pdf-library/petroleum-conferences-1/2006/ papers/Papers_59.pdf]. [5] Structures present on the island include the Freshwater fault zone, the Escarpment Fault, and the Gutter shear zone, which are all northwest–southeast striking reverse faults that, when oroclinal bending through the Alpine Fault is restored, have orientations and kinematics consistent with accommodating dextral transpressive intra-arc shortening [Allibone and Tulloch, 2004, 2008] and may correlate to Early Cretaceous structures in the Fiordland region [Klepeis et al., 2004]. 3. Sisters Shear Zone [6] The Sisters shear zone strikes northeast along the southeast coast of Stewart Island and consists of variably deformed granitoids that include breccia, protomylonite, mylonite, and ultramylonite [Kula et al., 2007]. Recognition of the shear zone as a significant structure is based on outcrop sites studied along 40 km of the coast (Figures 2, 3, and 4). Tulloch et al. [2006] and Kula et al. [2007] described the Sisters shear zone as consisting of two segments, North and South, based on deformation fabrics, kinematics, and appar- ent left-lateral offset of the NNW boundary of deformation (Figure 2). Figure 3 shows the site locations studied in the southern segment, and Figure 4 shows the site locations studied in the northern segment. The sites discussed herein are a subset of all locations visited during two seasons of fieldwork, which results in an apparently random number- ing scheme. Details of the P-numbered samples are recorded Figure 1. (a) Present-day configuration of New Zealand in the PETLAB database (http://pet.gns.cri.nz). Descriptions and related environs (Zealandia) in the South Pacific [from of the outcrop locations studied and microstructural obser- Sutherland, 1999]. Box around southern South Island refers vations made from oriented samples collected at these sites to the inset map in Figure 2. (b) Rigid plate reconstruction are presented here to develop the shear zone architecture and of the Gondwana margin prior to seafloor spreading along to assess the conditions of deformation. the Tasman and Pacific-Antarctic ridges. Restored orienta- tion of the SSZ is shown. Dark gray region indicates 3.1. Field and Microstructural Observations continental crust submerged beneath less than 2000 m of 3.1.1. Southern Segment water depth [from Mortimer et al., 2005; Kula et al., 2007]. [7] In the southern segment, deformation fabrics are Camp, Campbell Plateau; CR, Chatham Rise; HP, Hikurangi preserved in two granitic units; the 105 Ma Gog and Plateau; W, Wishbone Ridge; Chall, Challenger Plateau; Kaninihi plutons of Allibone and Tulloch [2004]. All SLHR, South Lord Howe Rise; STR, South Tasman Rise; southern segment sites are located within the biotite-K- ET, East Tasman Rise; SNR, South Norfolk Ridge; IB, feldspar-plagioclase-quartz Kaninihi granodiorite except Iselin Bank. sites 11 and 6, which are within the quartz-biotite-K- feldspar-plagioclase Gog granite. The northern boundary of deformation in the southern segment is estimated to be in of plutons indicates that in the Early Cretaceous, magmatism the vicinity of South Arm (Figures 2 and 3) based on a lack migrated southward (paleocontinentward) into the Western of visible deformation in exposures to the north of this inlet. Province contemporaneous with episodes of crustal shorten- Exposures tend to show high-strain zones on the order of ing [Allibone and Tulloch, 2004; Klepeis et al., 2004]. tens of meters thick separated by intervals exhibiting very Additionally, the distribution and chemical signatures of weak deformation fabric to virtually undeformed textures. plutons exposed on Stewart Island supports the paired plu- Several brittle normal faults that cut these fabrics have also tonic belt interpretation of Tulloch and Kimbrough [2003] been observed at some locations (Figure 3). The following 2 of 22 TC5015 KULA ET AL.: SISTERS SHEAR ZONE GREAT SOUTH BASIN TC5015 Figure 2. Simplified

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