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_Journal of Structural Geology 32 (2010) 667-684_ Contents lists available at ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com/locate/jsg Faults slip analysis and transpressional tectonics: A study of Paleozoic structures in northern Victoria Land, Antarctica L. Federico*, L. Crispini, G. Capponi University of Genova, Dip.Te.Ris., Corso Europa 26, Genova, Italy Article info Article history: Received 5 October 2009 Received in revised form 29 March 2010 Accepted 1 April 2010 Available online 7 April 2010 Keywords: Faults slip analysis Transpressional tectonics Northern Victoria Land Antarctica Abstract The brittle structures of the Bowers Mountains (northern Victoria Land, Antarctica) have been studied to fully characterise the poorly described pre-Cenozoic brittle tectonics of this area. Field work revealed that the dominant structures are steeply-dipping reverse and strike-slip faults, locally associated with positive flower structures, diffuse veining and hydrothermal alteration. Field observations are combined with different methods of inversion of fault slip data and use of open-source computer programs to calculate resolved stress tensors and PeBeT axes in order to unravel the complex polyphase brittle architecture. The resolved regional stress field is characterized by E-W trending, horizontal s1 and local rotations into NE-SW directions in the Lanterman Range Sledgers Glacier area, due to subsequent block rotation or interference between far-field stress with pre-existing ductile discontinuities. The paleostress regime, unique overall structural framework and association with hydrothermal veining together with strong similarities to deformation related to the Benambran Orogeny in the Lachlan fold belt in SE Australia support a Late Ordovician-Silurian deformational event in northern Victoria Land (Antarctica). © 2010 Elsevier Ltd. All rights reserved. 1. Introduction Antarctica presents many challenges to structural geologists due to its remote location and, in many areas, relatively poor exposure. Rocks now exposed in northern Victoria Land were part of the over 4000 km long paleo-Pacific margin of East Gondwana during the Paleozoic. This margin was the site of protracted convergence, with terrane accretion and collision(s) of arc and microcontinental masses (e.g., Veevers, 2000; Cawood, 2005; Vaughan and Pankhurst, 2008). The Rosse-Delamerian (hereafter Ross) Orogeny (Cambrian-Early Ordovician; e.g., Bradshaw, 1987, 1989; Kleinschmidt and Tessensohn, 1987; Findlay et al., 1991; Ferraccioli et al., 2002) was the most pervasive tectonic event of the area, responsible for regionally dominant deformations. Following that event, an outboard migration of the subduction plane possibly occurred inducing less intense deformations typical of foreland areas during the Devonian-Carboniferous, together with a magmatic pulse (Kleinschmidt and Tessensohn, 1987; Fioretti et al., 1997). Finally, the fragmentation of Gondwana and in particular the separation between Australia and Antarctica was responsible for strike-slip to transtensional deformations during the Cenozoic (e.g., Salvini et al., 1997; Rossetti et al., 2003). The area was, therefore, the site of multiple tectonic events with recurrent reactivation of major faults, in particular the trans-lithospheric faults that represent terrane-bounding structures (e.g., Lanterman Fault and Leap Year Fault). The Cenozoic brittle tectonics of northern Victoria Land has been widely documented in the published literature (Salvini et al., 1997; Salvini and Storti, 1999; Rossetti et al., 2000, 2002, 2003; Storti et al., 2001) whereas older brittle structures related to post-Ross orogenic phases are less well known (Wodzicki and Robert, 1986; Jordan et al., 1984; Capponi et al., 1999). In this study we analyse and interpret the deformation patterns from outcrops located in an area that straddles the Lanterman and Leap Year faults focusing on the brittle structures related to pre-Cenozoic tectonics. A complex fault network is preserved, with dominant steeply-dipping reverse and oblique-slip faults, locally forming positive flower structures. The fault system is associated with quartz-carbonate veining and cuts thus postdating the Ross Orogeny-related ductile deformation structures. The aim of this work is to unravel the Paleozoic post-Ross Orogeny tectonic evolution by characterising the tectonic event responsible for the development of the pre-Cenozoic fault system and related stress state. Regional correlations with formerly adjoining fragments of Gondwana, namely SE Australia, are also discussed. 668 L. Federico et al. Journal of Structural Geology 32 (2010) 667-684 Fig. 1 Schematic geological map of northern Victoria Land with terrane boundaries and its location in the Antarctic continent. Squared area indicates the study area. To achieve this aim, we conducted field work investigating cross-cutting relationships, mesoscale fault geometries, and collecting fault slip data. The inversion of fault slip data is a well-established technique that has increasingly been used to unravel very complex tectonic histories responsible for the formation of heterogeneous fault populations (e.g., Wang and Neubauer, 1998; Lamarche et al., 1999; Matenco and Schmid, 1999; Saintot and Angelier, 2002; Burg et al., 2005; De Paola et al., 2005; Bergerat et al., 2007; La?D?vila and Anderson, 2009; De Vicente et al., 2009; Sippel et al., 2009). Fault slip inversion techniques have a number of critical assumptions and limitations (as discussed, for instance, by Pollard et al., 1993; Dupin et al., 1993; Twiss and Unruh, 1998; Marrett and Peacock, 1999), so we applied several different inversion methods and a variety of open-source computer programs to calculate resolved stress tensors (F.S.A. v.28.5 by C?l?rier, 1999 and MIM 5.31 by Yamaji et al., 2005a) and PeBeT axes (Faultkin v.4.3.5 by Allmendinger et al., 1994). 2. Geological and structural setting 2.1 Northern Victoria Land The tectonic architecture of northern Victoria Land (Figs. 1 and 2) was predominantly established during the Neoproterozoic to early Paleozoic Ross Orogeny. This event led to the accretion of three terranes to the East Antarctic craton: from west to east these are: the Wilson; Bowers; and Robertson Bay terranes (e.g., Bradshaw et al., 1985; Kleinschmidt and Tessensohn, 1987; Ricci et al., 1997; Federico et al., 2006). The Wilson and Bowers terranes are juxtaposed by the Lanterman Fault (e.g., Capponi et al., 1999), whereas the boundary between Bowers and Robertson Bay terranes is marked by the high-strain belt of the Millen Schist (Jordan et al., 1984; GANOVEX Team, 1987; Capponi et al., 2005). The Wilson terrane represents the root of a Cambrian continental magmatic arc, with widespread granitoids of the Granite Harbour Igneous Complex intruding high-to low-grade metasedimentary country rocks (e.g., Borg and Stump, 1987). The Bowers terrane mainly consists of very low-to low-grade Cambrian metasedimentary (Molar Formation) and metavolcanic rocks (Glasgow Volcanics, Weaver et al., 1984; Laird, 1987) of arc back-arc affinity (Rocchi et al., 2003). The Robertson Bay terrane is formed by a thick succession of mainly very low-grade Cambro-Ordovician metaturbidites (Wright et al., 1984). After docking during the Ross Orogeny, all three terranes were intruded by Devonian Carboniferous calcalkaline intrusives (Admiralty Intrusives; Borg et al., 1984), associated with felsic volcanics (Gallipoli Volcanics). This magmatic suite is possibly linked to a poorly known orogenic event known as the Borchgrevink Orogeny (Grindley and Warren, 1964; Capponi et al., 2002). The progressive erosion of the Ross Orogen down to its crystalline core produced a peneplain with exhumation of 10-20 km in many places (Stump, 1995). This surface is overlain by the Beacon Supergroup, a Devonian to Triassic sedimentary sequence largely of terrestrial origin (Barrett, 1981; Collinson, 1991), followed by tholeiitic magmatism of Jurassic age (Ferrar Dolerite and Kirkpatrick Basalt). At present this cover sequence is visible only in the Wilson and Bowers terranes. Lastly, post-Jurassic tectonics linked to the fragmentation of Gondwana and opening of the strongly asymmetric West Antarctic Rift System is responsible for the present day high-elevation of the Transantarctic Mountains located on the asymmetric rift shoulder and reactivation of inherited Paleozoic discontinuities (e.g., Salvini et al., 1997). Cenozoic transtensional tectonics is thought to have triggered the emplacement of alkaline volcanics of the McMurdo Volcanic Group. 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