Fault–slip analysis and transpressional tectonics: A study of Paleozoic structures in northern Victoria Land, Antarctica

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Abstract

The brittle structures of the Bowers Mountains (northern Victoria Land, Antarctica) have been studied in order 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 with use of different open-source computer programs, that calculate resolved stress tensors and PBT axes in order to unravel the complex polyphase brittle architecture.

The resolved regional stress field is characterized by EW trending, horizontal σ1 and local rotations into NE–SW directions in the Lanterman Range/Sledgers Glacier area, due to subsequent block rotation or to the interference between the far-field stress with pre-existing ductile discontinuities.

The paleostress regime, unique overall structural framework and association with hydrothermal veining together with the strong similarities to deformation related to the Benambran Orogeny in the Lachlan fold belt in SE Australia supports the occurrence of a Late Ordovician–Silurian deformational event in northern Victoria Land (Antarctica).

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/or microcontinental masses (e.g., Veevers, 2000, Cawood, 2005, Vaughan and Pankhurst, 2008). The Ross–Delamerian (hereafter Ross) Orogeny (Cambrian–Early Ordovician; e.g., Bradshaw, 1987, Bradshaw, 1989, Kleinschmidt and Tessensohn, 1987, Findlay et al., 1991, Ferraccioli et al., 2002) was the most pervasive tectonic event of the area, responsible for the 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, Fig. 1, Fig. 2).

The Cenozoic brittle tectonics of the northern Victoria Land has been widely documented in the published literature (Salvini et al., 1997, Salvini and Storti, 1999, Rossetti et al., 2000, Rossetti et al., 2002, Rossetti et al., 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 and thus postdates 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 the formerly adjoining fragments of Gondwana, namely SE Australia, are also discussed.

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, Laó-Dávila 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 Célérier, 1999 and MIM 5.31 by Yamaji et al., 2005a) and PBT axes (Faultkin v. 4.3.5 by Allmendinger et al., 1994).

Section snippets

Northern Victoria Land

The tectonic architecture of northern Victoria Land (Fig. 1, Fig. 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.,

Field data

We investigated the mesoscale fault pattern in the area of the Bowers Mountains between the Lanterman Fault and Leap Year Fault (Fig. 1, Fig. 2). We collected fault–slip data from 153 striated fault planes at 10 localities (Fig. 3), scattered between 70°45′–71°45′ Lat. S and 162°00′–163°45′ Long. E (Fig. 2).

In the study area the sparse number of outcrops of Devonian–Triassic sediments (Beacon Supergroup) and/or Jurassic rocks (Ferrar Dolerite and Kirkpatrick Basalt) make finding age constrains

Inversion of fault–slip data

Most fault–slip inversion techniques use one or two fundamental assumptions:

  • They impose a geometrical constraint requiring that slip on the fault plane takes place parallel to the direction of maximum resolved shear stress (Wallace, 1951, Bott, 1959, so-called "Wallace–Bott hypothesis"),

  • Alternatively or additionally, they include a frictional constraint that the fault plane must form with an orientation that fulfils the Mohr–Coulomb yield criterion, e.g., the shear-to-normal stress ratio equals

Results

Most of the fault populations are heterogeneous, as illustrated by the different orientations of strain axes calculated by Faultkin and of stress axes calculated from FSA and MIM (Fig. 3). Where possible, we have separated different subsets (whose significance is discussed below), whilst in other cases, if the data set was too small, just one stress tensor was calculated, or the faults don’t fit, they have been grouped in the “Rest” column of Table 1. For each stress tensor we show the misfit

Paleostress inversion method

A good agreement exists between stress calculations and field data. All the reactivated faults identified in the field were either assigned to two different stress tensors or were assigned to one stress tensor and to the “rest” group by the software. In contrast, when single fault planes with a curved geometry have been identified (4 cases), the software was able to fit them into the same stress tensor in just one case. As a consequence, caution needs to be taken performing analysis of

Conclusions

In the present study, fault–slip data were inverted to calculate reduced palaeostress tensors through a stepwise procedure that allowed us to compare results obtained from different methods. We found good agreement between FSA and MIM results and a first-order agreement of principal stress axes and PBT axes calculated by Faultkin. Paleostress inversion methods correctly separated faults of different populations when overprinting relationships had been observed, and in general agree well with

Acknowledgments

Fieldwork benefitted from the excellent support of Helicopter NZ; special thanks to Jeff McClintock and Ricky Park.

The Authors thank Blanka Sperner for kindly providing the software FLUMO together with useful suggestions. Richard Allmendinger, Bernard Célérier and Atsushi Yamaji are acknowledged for kindly providing for free the softwares Faultkin and Stereonet, F.S.A. and M.I.M., respectively.

Careful reviews by F. Storti, R. Holdsworth and an anonymous reviewer are greatly appreciated.

This

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