Origin of the Adventure Subglacial Trench linked to Cenozoic extension in the East Antarctic Craton

TECTO-126876; No of Pages 8 Tectonophysics xxx (2015) xxx–xxx

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Origin of the Adventure Subglacial Trench linked to Cenozoic extension in the East Antarctic Craton P. Cianfarra ⁎, F. Salvini Dipartimento di Scienze, Università Roma Tre, L.go S. L. Murialdo 1, I-00146 Roma, Italy

a r t i c l e

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Article history: Received 14 May 2015 Received in revised form 11 December 2015 Accepted 15 December 2015 Available online xxxx Keywords: Adventure Subglacial Trench East Antarctica Intracratonic deformation Strike-slip corridor Cenozoic tectonics

a b s t r a c t The Antarctic plate occupies a unique geodynamic setting being mostly surrounded by divergent or transform margins. Major intracontinental basins and highlands characterize its bedrock, buried under the 34 Ma East Antarctic Ice Sheet (EAIS). Their formation atop of the cratonic lithosphere in the interior of East Antarctica remains a major open question. Post-Mesozoic intraplate extensional tectonic activity has been proposed for their development and is supported by this work. Here we focus on the Adventure Subglacial Trench (AST) whose origin is poorly constrained and controversial, as currently available geophysical models suggest either extensional or compressional tectonic origin. The AST is an over 250-km-long, 60-km-wide subglacial trough, elongated in the NNW–SSE direction adjacent to the westernmost flank of the Wilkes Subglacial Basin, and is parallel to regional scale alignments of magnetic and gravimetric anomalies. Geophysical campaigns allowed better definition of the AST physiography showing its typical half-graben geometry. The rounded morphology of the western flank of the AST was simulated through tectonic numerical modelling. We consider the subglacial landscape to primarily reflect a preserved relict of the tectonic processes affecting the interior of East Antarctica in the Cenozoic, due to the negligible erosion/deposition capability of the EAIS. The bedrock morphology was replicated through the activity of the listric Adventure Fault, characterized by a basal detachment at the base of the crust (34 km) and a vertical displacement of 2.5 km. This length suggests its regional/crustal importance. The predicted displacement is interpreted either as a newly formed fault or as the partial reactivation of a weaker zone along a major Precambrian crustal-scale tectonic boundary. The extensional regime in the AST is part of a more extensive 800-km long intraplate corridor characterized by nearly along-strike extension in Cenozoic times with a leftlateral transpressional component. This corridor may represent the effect of far-field stresses induced by plate motions. © 2015 Published by Elsevier B.V.

1. Introduction East Antarctica is a Precambrian Craton (EAC) that played a central role in early supercontinents such as Rodinia and Gondwana in Precambrian and Paleozoic times (Torsvik, 2003; Boger, 2011; Dalziel, 2013; Harley et al., 2013; Aitken et al., 2014; Aitken et al., 2015). A large interval of the late Mesozoic and Cenozoic geological history of EAC is dominated by the break-up of Gondwana, its separation from the Australian plate, and its movement towards the present polar location through a poly-phased evolution that included continental rifting, block translations, widespread magmatism and uplift of the Transantarctic Mountains (Stern and ten Brink, 1989; Salvini et al., 1997; Tonarini et al., 1997; Ferraccioli et al., 2001; Fitzgerald, 2002; Rossetti et al., 2003; Jordan et al., 2013; Aitken et al., 2014). Ferraccioli et al. (2011) proposed intraplate Permian–Cretaceous age rifting and transtension associated with the East Antarctic Rift System.

⁎ Corresponding author. Tel.: +39 0657338013; fax: +39 0657338201. E-mail address: [email protected] (P. Cianfarra).

Presently, the Antarctic plate (Fig. 1a) occupies a unique geodynamic setting being almost completely surrounded by divergent or conservative margins, with the exception of the limited subduction zones of the South Sandwich and South Shetland Islands (Hayes, 1991; Lawver and Gahagan, 2003; Cianfarra and Salvini, 2013). According to plate tectonics this setting prevents development of regionally scaled tectonic events in its interiors (Cande and Stock, 2004; Müller et al., 2000; An et al., 2015). Despite the generally accepted expectation of tectonic quiescence, a series of depressions and highlands characterize EAC bedrock (Fretwell et al., 2013). The geodynamic setting of East Antarctica does not necessarily imply the production of internal compressional stresses that depend from the relative velocity between the rifts and the craton with respect to the plate accretion velocity. The presence of several depressions within the EAC suggests that an overall extension might be the dominant stress condition of the craton induced by plate tectonics since the Gondwana fragmentation (Ferraccioli et al., 2011). For two of these depressions, namely the Aurora and Concordia trenches, post-Mesozoic extensional tectonic activity has been proposed (Tabacco et al., 2006; Cianfarra et al., 2009). The 34 Ma East Antarctic Ice Sheet (EAIS; De Conto and

http://dx.doi.org/10.1016/j.tecto.2015.12.011 0040-1951/© 2015 Published by Elsevier B.V.

Please cite this article as: Cianfarra, P., Salvini, F., Origin of the Adventure Subglacial Trench linked to Cenozoic extension in the East Antarctic Craton, Tectonophysics (2015), http://dx.doi.org/10.1016/j.tecto.2015.12.011

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Fig. 1. (a) The Antarctic plate with its surrounding divergent margins and ocean floor age. Rectangle shows the location of (b). (b): Subglacial topography map of the East Antarctic Craton from Bedmap-2 dataset (Fretwell et al., 2013) with the main bedrock physiographic features and proposed faults. Dashed black lines represent the margins of the proposed structural corridor characterized by nearly along-strike extension derived by left-lateral transpression. Black lines are the Cenozoic extensional faults within the corridor. Grey lines are faults from Salvini et al., 1997; Ferraccioli et al., 2011; Aitken et al., 2014. The black square shows the location of Fig. 2. Legend: GSM: Gamburtsev Subglacial Mts; LV: Lake Vostok; NVL: Northern Victoria Land; AT: Aurora Trench; CT: Concordia Trench; BSH: Belgica Subglacial Highlands; AST: Adventure Subglacial Trench; AsST: Astrolabe Subglacial Trench; RSH: Resolution Subglacial Highlands; L90°: Lake 90°; 90°F: Lake 90° Fault; VF: Vostok Fault; ATF: Aurora Trench Fault; CF: Concordia Fault; IAAA: Indo-Australo-Antarctic Suture.

Pollard, 2003) prevents the direct analysis of the subglacial geology and landscape, leaving most of the geologic information derived from geophysical investigations (Ferraccioli et al., 2011; Fretwell et al., 2013; Jordan et al., 2013; Aitken et al., 2014; An et al., 2015). The discovery of Antarctic subglacial lakes (e.g. Kapista et al., 1996; Tabacco et al., 2002; Siegert et al., 2005; Wright and Siegert, 2012) contributed renewing the interest in the EAC subglacial geology that has been investigated by a number of international geophysical campaigns. The new Bedmap2 compilation sheds new light on the subglacial topography of the EAC (Fretwell et al., 2013, Fig. 1, 2) and further detailed the major depressions and mountain ranges that characterize the interior of East Antarctica. These are hard to explain given that East Antarctica is assumed to be a stable Precambrian craton since at least Edicaran–early Cambrian times. The observed tectonic setting of East Antarctica links to the broader tectonic issue of basins and ranges formation within intracratonic regions as observed in other cratonic regions. Despite the growing body of geophysical data available for Antarctica, several unanswered questions still exists on the tectonic origin of some subglacial features. Among them, the nature of the tectonic events responsible for the development of the Adventure Subglacial Trench (AST, Figs. 2 and 3) is enigmatic and controversial, and contrasting models on its tectonic origin have been proposed (Ferraccioli et al., 2001; Studinger et al., 2004). Ferraccioli et al. (2001) suggested an extensional tectonic origin for the AST linked to Meso-Cenozoic intraplate extension with a setting similar to the modern Baikal rift system. On the other hand, Studinger et al. (2004), based on extensive aerogeophysical investigations, suggested a compressional scenario for the Precambrian origin of the AST basin. The aim of this paper is to provide new clues on the geological setting of the AST based on the tectonic modelling of airborne Radio

Echo-Sounding (RES) profiles and to understand whether the tectonics result from regional uplift, local events or else we are in the presence of a major structural corridor within the EAC that was affected by Cenozoic reactivation. This was possible since the subglacial landscape primarily reflects a preserved relict of the morphology produced by the tectonic processes affecting the interior of East Antarctica in the Cenozoic (Jamieson et al., 2010; Wilson et al., 2012; Rose et al., 2015). In our modelling efforts we therefore neglected glacial overdeepening effects and flexural responses induced by selective fluvial and glacial erosion within the AST, which may have modified the pre-existing landscape. 2. Geological setting and tectonic numerical modelling of the Adventure Subglacial Trench Geophysical data collected in the last decades shed new light into the understanding of the crustal architecture of the EAC (Ferraccioli et al., 2001; Studinger et al., 2004; Ferraccioli et al., 2009; Ferraccioli et al., 2011; Jordan et al., 2013; Aitken et al., 2014; An et al., 2015). Gravimetric and aeromagnetic modeling were interpreted to propose a tectonic origin for Lake Vostok (Studinger et al., 2003). Numerical modelling of the buried bedrock physiography contributed to define the extensional tectonic style responsible for the formation of subglacial depressions in the Vostok-Dome C area (Tabacco et al., 2006, Cianfarra et al., 2009; Cianfarra and Salvini, 2013), characterized by a large number of subglacial lakes (Siegert et al., 2005; Tabacco et al., 2006; Wright and Siegert, 2012). Understanding of the tectonic origin of the AST (Figs. 1b and 2) with the possible presence of hydrological connection among lakes (Rémy and Legrésy, 2004; Wingham et al., 2006; Wright et al., 2008; Carter et al., 2009; Ferraccioli et al., 2007; Jordan et al., 2010; Pattyn, 2010), is of utmost importance for comprehending the

Please cite this article as: Cianfarra, P., Salvini, F., Origin of the Adventure Subglacial Trench linked to Cenozoic extension in the East Antarctic Craton, Tectonophysics (2015), http://dx.doi.org/10.1016/j.tecto.2015.12.011

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Fig. 2. Bedrock physiography of the subglacial valley flanks at the AST characterized by marked dip asymmetry of the subglacial valley flanks with the western slope consistently steeper than the eastern slope. (a) 3D perspective view from the North of the Adventure Trench with 40:1 vertical exaggeration. (b) Map view of the AST. The black line indicates the trace of the RES profile used for tectonic numerical modeling. (c) Ice sheet surface signature of the buried AST expressed as a 250-km-long lineament on the MODIS satellite image mosaic (Haran et al., 2014). Arrows show the tips of the lineament.

geodynamical setting of the region. Revealing the tectonic style responsible for the formation of the AST contributes to the ongoing debate on the geological setting of the Wilkes Subglacial Basin (Drewry, 1976; ten Brink et al., 1997; Ferraccioli et al., 2001; Studinger et al., 2004; Ferraccioli et al., 2009; Jordan et al., 2013) and its tectonic link with the evolution of the Transantarctic Mountains to the East and the Belgica Subglacial Highlands in the interior of the EAC to the West.

The AST was firstly detected in the '70s (Drewry, 1976) during the pioneering phases of reconnaissance airborne RES by the joint collaboration between the US and Denmark teams (SPRI- NSF-TUD) and named after Adventure, one of the two ships of the British expedition, 1772–75. The AST is a subglacial trough, elongated in the NNW–SSE direction around the 135°E meridian, and lies along the westernmost flank of the Wilkes Subglacial Basin, between the Resolution Highlands

Fig. 3. Across-strike RES profile of the AST showing the characteristic eastern steeper slope that contrasts with the gentler western slope with convex shape. Vertical exaggeration 10:1.

Please cite this article as: Cianfarra, P., Salvini, F., Origin of the Adventure Subglacial Trench linked to Cenozoic extension in the East Antarctic Craton, Tectonophysics (2015), http://dx.doi.org/10.1016/j.tecto.2015.12.011

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to the East and the Belgica Highlands to the West (Figs. 1b and 2). The AST is characterized by a width of about 60 km, a length exceeding 250 km, and a depth with elevations ranging between − 800 and −1400 m a.s.l. that increases to the North and contrasts with the local average elevation of the bedrock around 300 m (Figs. 2 and 3). This regionally sized bedrock physiographic feature, buried beneath about 4 km of ice, is also expressed as a 250-km-long lineament on the ice sheet surface, as evidenced by the MODIS satellite image mosaic (Fig. 2c). The dimension of the AST suggests its crustal importance in the architecture of the EAC (Wise et al., 1985; Cianfarra and Salvini, 2014a, 2014b). Throughout its 250-km-length, the AST is depicted by the Bedmap2 as a trough with the eastern slope consistently steeper than the western slope (Fig. 2a). This is a recurrent feature of some elongated troughs in the EAC such as the abovementioned Aurora and Concordia trenches which are characterized by the relatively gentle dipping and rounded shape of the western valley flanks (Tabacco et al., 2006; Cianfarra et al., 2009). The across strike RES profiles of these trenches were interpreted by Tabacco et al. (2006) and Cianfarra et al. (2009) as the result of the activity of crustal listric faults whose hangingwall displacement produced the observed flexure of the western slope towards the trench axis. Although the tectonic origin of the AST is widely accepted (Drewry, 1976; Steed and Drewry, 1982; Ferraccioli et al., 2001; Studinger et al., 2004; Ferraccioli et al., 2007; Aitken et al., 2014) and this trench is associated and parallel to regional scale alignment of magnetic and gravity anomalies (Ferraccioli et al., 2001; Studinger et al., 2004; Jordan et al., 2013; Aitken et al., 2014), debate is still open on the tectonic regime that brought to the development of this depression. From early geophysical investigations, a rifted crust with sedimentary infill of about 3 km was inferred beneath the AST (Drewry, 1976; Steed and Drewry, 1982). Ferraccioli et al. (2001) based on gravity modelling interpreted the AST as a narrow rift basin with about 25 km thick crust and a huge sedimentary succession of about 10 km. On the other hand, Studinger et al. (2004) proposed a compressional scenario for the origin of this depression based on the similarity of the Bourguer gravity at the AST and at the Appalachians. Studinger et al. (2004) also outlined the existence of a region of deeper magnetic basement coincident with a low in the Bourguer gravity that has been related to the presence of 5-km-thick, 150-km-wide sediment infill located on the eastern shoulder of the trench. The AST basin was inferred therefore by Studinger et al. (2004) as representing a foreland basin linked to compression rather than extension in early Paleozoic time. The offset between the old basin and the present-day asymmetric depression of the AST implies that younger geological events gave rise to the observed topography of the AST. In the framework the PNRA (Programma Nazionale di Ricerche in Antartide, Italian National Antarctic Research Program) geophysical campaigns a RES transect was flown from the Transantarctic Mountains to Dome C, roughly parallel to the 76°S parallel and crossing the AST (Forieri et al., 2007; Cianfarra et al., 2009). Radar data were collected with the INGV-IT radar instrumentation (Tabacco et al., 1999; Zirizzotti et al., 2008), which is characterized by a 3.5 kW power envelope system, an operating frequency of 60 MHz, a vertical resolution of 1280 samples, a sampling frequency of 20 MHz and a variable pulse length (from 0.2 to 1 μs). These specifications provide a nominal vertical resolution of about 2.3 m. Data filtering, analyses and ice thickness calculations were performed following the same procedures as described in Tabacco et al. (2006) and Cianfarra et al. (2009). The bedrock elevation was obtained by subtracting the computed ice thickness from the surface elevation data from Rémy et al. (1999). This processed profile allowed to better define the main bedrock physiographic features of the AST, thus allowing its geological modeling. RES data confirmed the asymmetric transversal profile of the AST in its southern sector (Fig. 3), with the gently rounded shape of the western flank characterized by an average slope smaller than 2° towards the trench axis that becomes nearly flat, at the average height of 260 m,

50 km away from the deepest part of the trough profile (−970 m). On the other hand, the eastern flank of the AST is characterized by an average slope of 7° to the West over a distance shorter than 5 km. The described geometry resembles the typical half graben morphology resulting from the activity of listric, normal faults (Burbank and Anderson, 2001) developed both in extensional geodynamic regimes (e.g. East African Rift system, Chorowicz, 2005) and in strike-slip deformation belts with local extension (e.g. Lake Baikal, Tapponnier and Molnar, 1979; ten Brink and Tayor, 2002). Similar asymmetries have been imaged from radar data in West Antarctica and interpreted as reflecting regions of focused Cenozoic extension (Bingham et al., 2012). The gently rounded morphology of the western flank of the AST was simulated through a numerical model following the methodology used in Tabacco et al. (2006) and Cianfarra et al. (2009). The present day morphology at the AST was considered a reference surface preserved from significant erosional/depositional events by the ice/bedrock contact characterized by negligible erosional/depositional capabilities (Jamieson et al., 2010; Wilson et al., 2012; Rose et al., 2013; Creyts et al., 2014; Rose et al., 2015). The investigated area falls in the dome/ ice divide region characterized by limited ice surface slope gradient and minimal ice velocity (Rignot et al., 2011) that further reduces the transport capabilities of the ice. In our modelling efforts we neglected glacial overdeepening effects and flexural responses induced by selective fluvial and glacial erosion within the AST, which may have modified the pre-existing landscape. The tectonic modelling consists of replicating along the RES profile (with a resolution of 7 m) the development of the present day bedrock morphology by the hangingwall sliding of a normal, listric fault according to the geodynamic setting proposed for the nearby Concordia and Aurora trenches (Tabacco et al., 2006; Cianfarra et al., 2009) as well as for the Lake Vostok (Cianfarra and Salvini, 2013). The simulation was performed with FORCtre software, a Hybrid Cellular Automata (HCA)-derived numerical algorithm particularly suited to replicate the evolution of complex, dip-slip geological structures (Salvini et al., 2001; Salvini and Storti, 2004). The crust at the AST is modelled as a 2D section of layered material with a rigidity comparable to that of the upper crust in an intracratonic setting (Poisson's ratio: 0.25; Young's modulus: 7 × 1010 Pa, e.g. Turcotte and Schubert, 2002), a total thickness of 34 km (according to Ferraccioli et al., 2001; Studinger et al., 2004; Ferraccioli et al., 2009; Jordan et al., 2013), and a length of 240 km. The initial horizontal dimension of the cells is about 1300 m, and the model approximation is of the order of 1 m. The used resolution allows averaging the minor anisotropies in the upper crust. Variations of the reconstructed fault trajectory with depth in the kinematic model will reflect possible larger scale crustal rheology variations and marked by changes in the fault dip. The similar fault setting successfully used to simulate the bedrock morphology at the Aurora Trench (Cianfarra et al., 2009) was applied to the AST. A trial-and-error forward modelling approach was followed and fault geometry and displacement were tuned until the final morphological misfit was minimized within the satisfactory approximation (100 m). The tuning neglected minor topographic variations with wavelengths shorter than 5 km that were considered the result of local scale factors not affecting the crustal structural setting of the investigated area at the regional scale. The bedrock morphology at the western flank of the AST is replicated through the activity of a west-dipping, listric fault, hereafter the Adventure Fault, characterized by an initial dip, at the bedrock surface, of 45° (Fig. 4). This dip remains almost constant until the fault surface reaches the depth of 16 km. Then it gently rotates and its dip reduces to 25° in the next 7 km. At this depth (23 km) the fault dip progressively rotates until it flattens at the basal detachment level placed at the depth of 34 km. The location of the upper fault tip at the bedrock surface is inferred from the position of the morphological scarp at the eastern steeper slope of the AST. The best fit between the top of the hangingwall surface and the bedrock was obtained with a fault vertical displacement of 2.5 km.

Please cite this article as: Cianfarra, P., Salvini, F., Origin of the Adventure Subglacial Trench linked to Cenozoic extension in the East Antarctic Craton, Tectonophysics (2015), http://dx.doi.org/10.1016/j.tecto.2015.12.011

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Fig. 4. Adventure fault model and comparison with the RES profile across the AST with vertical exaggeration of 10:1 (a); complete modelled section with no vertical exaggeration to visualize the full fault trajectory (b). The modelling is constrained to replicate the evolution of the hangingwall sector kinematics relative to the displacement of the fault footwall side.

3. Discussion The length of the AST enables us to estimate an horizontal length exceeding 200 km for the Adventure Fault. This is compatible with the modelled vertical extent (34 km) and the computed displacement (2.5 km) of the Adventure Fault (Walsh and Watterson, 1988) suggesting its crustal importance. The depth variation of the AST along its strike easily reflects changes in the fault displacement. Considering the northward deepening of the AST floor (see Fig. 2a and b) it is reasonable to hypothesize a small increase of the fault displacement in the same direction of the order of few hundreds of meters. The overall dimension of the Adventure Fault (hundreds of km) and its along-strike vertical displacement (2–3 km) are compatible with the existence of a regional, elongated trough characterized by a half-graben geometry adjacent to the westernmost part of the Wilkes Subglacial Basin. The found offsets produce limited changes in the regional geophysical pattern (namely gravimetry and aeromagnetics) that record the overall, long-lasting tectonic evolution of the region. The modelling was based on the study of the morphology of the bedrock independently from its nature. The possible presence of a thick sedimentary cover within the AST (Drewry, 1976; Steed and Drewry, 1982; Ferraccioli et al., 2001; Studinger et al., 2004) does not influence the modelling and would likely simply indicate that the fault activity analyzed here occurred after the sedimentary basin formation. The presence of a thick sedimentary cover both with the AST and in the adjacent highlands likely relates to older tectonic events along the Adventure Fault, while the modelled displacement relates to its younger reactivation responsible for the development of the present day depression. In the Wilkes Basin region the presence of rift-related grabens was inferred by Masolov et al. (1981) and Kadmina et al. (1983) based on the regional sub-ice morphology and continental scale depth to magnetic basement. Moreover a marked NNW lineament in the magnetic and gravity data, corresponding to the older sedimentary basin along the AST, has been interpreted as a major crustal boundary (Ferraccioli et al., 2001; Studinger et al., 2004; Finn et al., 2006; Ferraccioli et al., 2009). The existence of such structural boundary was also proposed

by Rémy and Legrésy (2004) based on the variation of the computed geothermal flux across the AST. These considerations show that the Adventure Fault lies along the crustal boundary between different tectonic/geologic units. We speculate that the Adventure Fault developed along ancient tectonic boundary within the composite Mawson Craton that was likely active during the Precambrian (Aitken et al., 2014; Aitken et al., 2015). This boundary has been related to a fold and thrust belt (e.g. Studinger et al., 2004 and Ferraccioli et al., 2009) and may represent a zone of weakness during successive tectonic events. In the AST the bedrock morphology well preserve the expected topographic expression of a normal listric fault. This testifies for fault activity when erosional process rates were significantly smaller than the tectonic activity rates and thus shielding the asymmetry related to the fault activity from erosion/ deposition processes (Jamieson et al., 2010; Rose et al., 2013; Creyts et al., 2014; Rose et al., 2015; Maggi et al., 2016). This contrasts with the long-lasting peneplanation processes that were active in EAC (and in the once adjacent Australian continent, LeMasurier and Landis, 1996) prior to the ice sheet emplacement (34 Ma) and obliterated any morphological feature related to older tectonic activity (Barrett et al., 1972). In this way the basal ice sheet conditions constituted the most favorable environment where the observed topographic-fault interaction developed. In light of the above considerations, the Adventure Fault either reflects rejuvenation of an older fault system or a newly developed extensional event since the onset of the EAIS or shortly before. This provides a time interval for the Adventure fault extension in Cenozoic time since 40 Ma. The proposed extensional regime in AST correlates well with the similar tectonics interpreted at Vostok, Aurora, and Concordia trenches (Tabacco et al., 2006; Cianfarra et al., 2009; Cianfarra and Salvini, 2013; Cianfarra and Salvini, 2014a). All these proposed extensional structures describe an 800-km-long corridor, elongated in the WSW–ENE direction that lies at high angle with the individual depressions and thus characterized by nearly along-strike extension (Fig. 1). Other tectonic depressions, as the 90° Lake Depression (Bell et al., 2006) where a similar tectonic regime may be expected (90° Lake Fault, Fig. 1), may lie within this elongated corridor. The elongated shape of this corridor suggests that the

Please cite this article as: Cianfarra, P., Salvini, F., Origin of the Adventure Subglacial Trench linked to Cenozoic extension in the East Antarctic Craton, Tectonophysics (2015), http://dx.doi.org/10.1016/j.tecto.2015.12.011

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testified by the angle between the transforms and the Antarctica continent (Fig. 1A; Dubbini et al., 2010) and may well contribute to the strike-slip and extension zone within the EAC where the AST and the other trenches are located. In this case we argue for the presence of intraplate deformation due to far field stresses (e.g. Zoback, 1992; Coblentz and Richardson, 1995; Marshak et al., 2003; Storti et al., 2003; Storti et al., 2007; Yin, 2010; Heidbach et al., 2010). Similar tectonic environments have been documented in other cratons as in the Tien Shan-Baikal region of Eurasia or the Cameroon line in Africa (Tapponnier and Molnar, 1979; Cunningham et al., 1996; ten Brink and Tayor, 2002; Buslov et al., 2003; Yin, 2010; De Castro et al., 2012; De Castro and Bezerra, 2015). Far-field stresses may represent one of the contributing causes to (super)continent fragmentation (Dalziel, 2000). The reactivation of older weaker crustal zones such as the Aurora Fault (Fig. 1B, Aitken et al., 2014, 2015), during and following continental break-up may well provide a prime example of how cratonic continental interiors can respond to such far-field processes. The Adventure Fault may frame within a similar scenario. 4. Conclusions

Fig. 5. Sketch of the kinematic setting for the proposed corridor. The extensional faults at high angle with the corridor results from the internal stress conditions (red arrows) induced by left-lateral transpressional regime (orange arrows). Note the external arrows that are slightly oblique to the corridor to reflect the overall transpressional regime. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

extension was in part triggered by a left-lateral strike-slip component (Fig. 5) with a tectonic scenario similar to the present day kinematic setting in Svalbard (Cianfarra and Salvini, 2014b). The characteristic high angle between the corridor and the extensional faults suggests that the internal extension relates to an overall transpressional setting. The sketch of Fig. 5 shows the possible kinematic configuration of the corridor. Most of the offset is accomplished within the corridor by the development of the high angle extensional faults leaving undeformed the corridor margins. The external arrows are slightly oblique to the corridor to reflect the overall transpressional regime. The proposed Cenozoic extension may in part have overprinted the older extensional/transtensional Permian to Cretaceous regime recognized by Ferraccioli et al. (2011) in the Lake Vostok, Lake Sovetskaya and Lake Vostok areas as part of the larger East Antarctic Rift System or the older Indo-Australo-Antarctic Suture (Aitken et al., 2014, 2015). Important regional erosional processes have been proposed in EAC in the early stages of EAIS (Jordan et al., 2010; Wilson et al., 2012; Thomson et al., 2013; Rose et al., 2013). This might have triggered diffuse extensional faulting during isostatic re-equilibration processes. On the other hand, the proposed distribution of the extensional structures suggests that their origin relates to stresses acting within the proposed corridor. The presence of such corridor may be the result of the internal deformation of the EAC produced by the interaction among the rift margins that almost surround it with different spreading rates (Salvini et al., 1997; Cande et al., 2000). In Cenozoic times the Southern/Indian Ocean between Australia and the EAC shows higher spreading rate than the Atlantic/Pacific one (McAdoo and Laxon, 1997; Cande et al., 2000; Storti et al., 2007). This difference associates to the clockwise rotation of the Antarctica plate with respect to the surrounding plates as

The results from our study confirm the tectonic origin of the Adventure Subglacial Trench. The comparison between the RESderived bedrock morphology and the numerical modelling supports an extensional origin for the Adventure Subglacial Trench. The Adventure Fault is a crustal listric fault reaching its basal detachment at the depth of 34 km, characterized by a horizontal length exceeding 200 km and displacement of the order of 2–3 km. The exceptional preservation of the morphology produced by the Adventure Fault in particular of the asymmetric valley sides is likely to have been aided by the EAIS, even if mixed cold-based and wet-based conditions may have existed since 34 Ma within the AST. The emplacement of the EAIS followed a long-lasting peneplanation able to almost clear any pre-existing morphological evidence of tectonics. This suggests a relatively recent (post 40 Ma) reactivation/formation of the fault within the onset of the EAIS or shortly before. The presence of a several km thick sedimentary cover in both the AST and its flanks is likely related to much older (Precambrian–Paleozoic?) tectonic processes that affected the interior of East Antarctica. The Adventure Fault corresponds to a 200-km-long tectonic lineament evident in the bedrock physiography, on the ice sheet surface, and in gravity and magnetic data. This regional scale lineament represents a crustal boundary related to the evolution of the Pangea/Gondwana during the Paleozoic and earlier supercontinents or even older. Our modelling results add to the growing body of geophysical evidence suggesting that major tectonic structures in the interior of East Antarctica have been reactivated several times following its Precambrian to early Cambrian assembly. The Adventure Fault is part of an elongated corridor characterized by nearly parallel extension that includes the Vostok, Aurora trench, and Concordia faults. It is about 800 km long and oriented roughly ENE– WSW with a left lateral transpressional component. The Adventure Fault and its associated corridor may represent the effect of far field stresses responsible for intraplate deformation related to different rifting velocities developed around Antarctica in the Cenozoic. Acknowledgements Research was carried out in the framework of the Project CABILA (Caratterizzazione Biogeochim-ica dei Laghi Subglaciali Antartici) of the PNRA—MIUR (Programma Nazionale diRicerche in Antartide—Ministry of Education, University and Research) and financed by the PNRA Consortium (project 2009/A2.02). ENEA (Agenzia Nazionale per le Nuove Tecnologie, L’Energia e lo Sviluppo Economico Sostenibile) provided the logistic support. We thank reviewers for insightful comments that improved the paper.

Please cite this article as: Cianfarra, P., Salvini, F., Origin of the Adventure Subglacial Trench linked to Cenozoic extension in the East Antarctic Craton, Tectonophysics (2015), http://dx.doi.org/10.1016/j.tecto.2015.12.011

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