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Two-Stage Rifting of Zealandia-Australia-Antarctica: Evidence from 40Ar/39Ar Thermochronometry of the Sisters Shear Zone, Stewart Island, New Zealand

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Two-Stage Rifting of Zealandia-Australia-Antarctica: Evidence from 40Ar/39Ar Thermochronometry of the Sisters Shear Zone, Stewart Island, New Zealand Two-stage rifting of Zealandia-Australia-Antarctica: Evidence from 40Ar/39Ar thermochronometry of the Sisters shear zone, Stewart Island, New Zealand Joseph Kula Department of Geoscience, University of Nevada, Las Vegas, Nevada 89154-4010, USA Andy Tulloch GNS Science, Private Bag 1930, Dunedin, New Zealand Terry L. Spell Department of Geoscience, University of Nevada, Las Vegas, Nevada 89154-4010, USA Michael L. Wells ABSTRACT (Fig. 1). At some localities, it is as wide as 5 km The Sisters shear zone is a newly discovered Late Cretaceous detachment fault system (map view); however, the boundaries are not exposed for 40 km along the southeast coast of Stewart Island, southernmost New Zealand. well constrained due to relatively poor exposure. Footwall rocks consist of variably deformed ca. 310 and 105 Ma granites that range from The shear zone occurs within Carboniferous undeformed to protomylonite, mylonite, and ultramylonite. The hanging wall includes non- and Early Cretaceous granitic rocks that exhibit marine conglomerate and brittley deformed granite. K-feldspar thermochronometry of the varying degrees of deformation from essen- footwall indicates moderately rapid cooling (20–30 C°/m.y.) due to tectonic denudation over tially undeformed to protomylonite, mylonite, the interval ca. 89–82 Ma. Return to slow cooling at 82 Ma coincides with the age of the oldest and ultramylonite, with widespread but gener- seafl oor adjacent to the Campbell Plateau and refl ects the mechanical transition from conti- ally minor brittle deformation overprints. Shear nental extension to lithospheric rupture and formation of the Pacifi c-Antarctic Ridge. Our bands, oblique-grain-shape fabrics, sigma- and fi ndings support a two-stage rift model for continental breakup of this part of the Gondwana delta-type feldspar porphyroclasts, and mica margin. Stage one (ca. 101–88 Ma) is the northward propagation of continental extension and fi sh indicate shear sense. the Tasman Ridge as recorded in mylonite dredged from the Ross Sea and the Paparoa core The Sisters shear zone is divided into two complex. Stage two (ca. 89–82 Ma) is extension between the Campbell Plateau and West Ant- segments based on the nature of ductile fab- arctica leading to formation of the Pacifi c-Antarctic Ridge. rics, predominant kinematics, and along-strike offset of the western boundary of ductile fabric Keywords: New Zealand, extension, thermochronology, Gondwana, rifting, Cretaceous. (Fig. 1). The northern segment of the shear zone typically consists of granite mylonite and proto- INTRODUCTION ing to seafl oor spreading is documented using mylonite with foliations dipping 20–30ºSSE Plate reconstructions of Mesozoic Gondwana 40Ar/39Ar thermochronometry, which indicates and top-to-the-southeast shear sense. Footwall place Zealandia (New Zealand and surround- that this event is 5 to 10 m.y. younger than exten- rocks there are locally overprinted by south- ing continental shelf, e.g., Mortimer, 2004) at sion documented in the Ross Sea and western east-dipping brittle normal faults, commonly the Pacifi c margin, adjacent to southeast Aus- New Zealand. Our new results and observa- subparallel to the ~060° strike of the foliation. tralia and West Antarctica (e.g., Sutherland, tions, combined with published thermochronol- In the southern segment, foliations are gener- 1999; Eagles et al., 2004). Much attention has ogy data from western New Zealand and West ally less well developed than in the north, and been directed toward extension between west- Antarctica, reveal a sequence of extensional deformation tends to be localized into 5–50-m- ern Zealandia and eastern Australia leading tectonism that can be best explained by a two- thick high-strain zones including ultramylonite. to opening of the Tasman Sea (Tulloch and stage model for breakup of the Pacifi c margin Ductile kinematic indicators in the southern Kimbrough, 1989; Etheridge et al., 1989; Spell of Gondwana. portion exhibit both top-to-the-northwest and et al., 2000) and rift-related deformation in top-to-the-southeast downdip shear sense, but Marie Byrd Land, West Antarctica, and the SISTERS SHEAR ZONE, STEWART brittle normal faults are consistently top-to-the- adjacent Ross Sea (e.g., Luyendyk et al., 2003; ISLAND southeast. Stretching lineations throughout the Siddoway et al., 2005). These studies have Stewart Island is part of the Median batholith shear zone consistently trend 330/150° ± 15°. outlined the timing and style of extension and and Western Province of New Zealand (Fig. 1). Because of apparent along-strike offset of the breakup between Australia and Zealandia, and The Median batholith represents a magmatic western boundary of ductile fabric and differ- of extension between East and West Antarctica. arc that developed above the paleosubduc- ences in kinematics and foliation attitudes, we This paper focuses on the outstanding prob- tion zone along the Gondwana Pacifi c margin infer that the north and south segments of the lem of the nature and timing of extension in (Tulloch and Kimbrough, 2003). Major struc- shear zone are separated by a transfer fault (e.g., eastern Zealandia leading to Pacifi c-Antarctic tures on Stewart Island include the northwest- Lister et al., 1986) (Fig. 1). Ridge forma tion and separation of the Campbell striking Freshwater fault zone, Escarpment Microstructures in the deformed granites indi- Plateau from West Antarctica. fault, and Gutter shear zone. These structures cate greenschist facies metamorphic conditions Field observations and 40Ar/39Ar data from are related to pre-breakup convergent margin followed by decreasing temperatures during the Sisters shear zone on Stewart Island, south- tectonism and have been described by Allibone shearing. In thin section, quartz exhibits features ernmost New Zealand, are presented here as and Tulloch (1997, 2004). In contrast, the Sis- of plastic deformation including oblique-grain- evidence for a Late Cretaceous detachment fault ters shear zone, located along the southeast shape fabrics in dynamically recrystallized system that accommodated continental exten- coast and oriented obliquely to these structures, grains (regime 2 of Hirth and Tullis, 1992) sion and thinning of the Campbell Plateau and is here interpreted to represent an extensional and ribbons with patchy to undulose extinc- was kinematically linked to formation of the detachment fault system. tion, whereas feldspars exhibit dominantly Pacifi c-Antarctic Ridge. The timing of exten- The Sisters shear zone is exposed along the brittle deformation. The lack of postdeforma- sion and the transition from continental rift- southeast coastline of Stewart Island for ~40 km tional growth in ~30 µm grains of recrystallized © 2007 The Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGYGeology, May, May 2007; 2007 v. 35; no. 5; p. 411–414; doi: 10.1130/G23432A.1; 4 fi gures; Data Repository item 2007099. 411 40 39 167o E pet/). The Ar/ Ar analyses were conducted at Median Batholith the Nevada Isotope Geochronology Laboratory 10000E 5330000N Western 2100000E 21 2120000E 2130000E 1 9 at University of Nevada–Las Vegas (UNLV); Province ca. 305 Ma data tables and descriptions of analytical meth- Knob pluton 1 8 (?) ods are given in appendices DR1 and DR2 in the Fiordland Alpine FaultEastern GSA Data Repository.1 45o S Province PP 4 9 105 Ma ) Median Stewart (? 4 5 P76106 Footwall Mica Ages 3 8 Batholith Gog pluton Figure 2 5320000N Island 3 5 Muscovite and biotite were collected from 2 0 2 3 PP 2 4 footwall rocks from the Knob pluton in the north- 2 5 5 thern Segment Nor ern segment ~50–100 m below the detachment 2090000E 1 0 1 0 Inferred surface (P76106, Fig. 1). Muscovite yielded a 1 5 1 5 transfer relatively fl at age spectrum with a plateau age 1 4 P67866 5 km 2 9 fault of 93.8 ± 0.4 Ma (uncertainties 2σ), incorporat- 5310000N ing 96% of the gas released (Fig. 3A). Biotite Southern Segment yielded a plateau age of 90.0 ± 0.8 Ma (59% of Paleozoic granite & Hanging-wall Solid-state P62424 the gas released) and an isochron age of 90.6 ± schist breccia mylonitic North Traps 1.2 Ma with a 40Ar/36Ar intercept of 294.5 ± 2.2, Detachment Cretaceous granite fabric (FW) granite 40 fault indicating no excess Ar in the sample. Figure 1. Generalized geologic map of southern Stewart Island (modifi ed from Allibone and Footwall and Hanging-Wall K-Feldspar Tulloch, 2004) showing dominantly plutonic nature (Median batholith—black in inset). Note Three K-feldspar separates were analyzed distribution of ductile fabric, stretching lineation orientation, and inferred transfer fault (see text). Sample locations are labeled with P-numbers (PETLAB database (http://data.gns.cri. using detailed furnace step-heating, including nz/pet/). North Traps are a set of low-lying rock and reefs consisting of undeformed granite. isothermal duplicates, to determine Ar diffusion Box indicates area of Figure 2. Eastings and northings conform to the New Zealand Map Grid kinetics for application of multiple diffusion (NZMG). PP—Port Pegasus. domain (MDD) thermal modeling (Lovera et al., 1989, 1991). Two samples were collected from footwall rocks: P76106, (discussed previously), quartz, preservation of unrecovered quartz rib- The Sisters Islets, a pair of ~200 × 400 m and P67866 from the western side of the southern bons with undulose extinction, and cataclastic islets ~1 km offshore (Fig. 2), are composed segment of the shear zone (Fig. 1). The footwall “crush zone” overprinting collectively indicate of essentially undeformed conglomerate samples yielded maximum ages of 89–90 Ma, cooling during deformation. (Fleming and Watters, 1974) and represent the and sample P76106 exhibited a prominent age A brittle detachment surface oriented hanging wall of the Sisters shear zone. Con- gradient over the initial gas release that was 061/27°S is exposed in a small bay in the glomerate beds on the Sisters strike ~070°, absent in sample P67866 (Fig.
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