Geophysical imaging unveils the largest pull-apart basin in East Antarctica

West Antarctica hosts one of the largest continental rift systems on Earth, the West Antarctic Rift System (WARS) that forms the lithospheric cradle for the West Antarctic Ice Sheet. The WARS is known to have experienced several stages of extension starting with distributed/wide mode extension in the Cretaceous, followed by narrower mode and variably oblique extension in the Cenozoic, the latter potentially triggered by the onset of oceanic seafloor spreading in the Adare Basin (Davey et al., 2016, GRL). However, the extent and impact of Cenozoic extension and transtension within the Transantarctic Mountains sector of East Antarctica is much less well understood. Here we present results from a new project (REGGAE) that by analysing aeromagnetic, aerogravity and land-gravity and bedrock topography images and models provides key new geophysical constraints on the form, extent and kinematics of the largest Cenozoic pull-apart basin recognised so far in East Antarctica, the Rennick Graben (RG). Potential field imaging reveals the extent of part of a Jurassic tholeiitic Large Igneous Province preserved within the RG and helps delineate the inherited structural architecture of the underlying Ross-age basement in northern Victoria Land, including highly magnetic arc basement in the northern Wilson Terrane and the subglacial extent of a thrust fault belt located between the western flank of the RG and the eastern margin of Wilkes Subglacial Basin (WSB). We show that the RG is a major composite right-lateral pull-part basin that extends from the Oates Coast to the Southern Cross Mountains crustal block and propose that it is kinematically connected with both the western edge of the WARS and the eastern margin of the WSB. More cryptic evidence for an earlier phase of left-lateral strike slip deformation is also emerging from our recent geological field work in the study region and relatively subtle offsets in aeromagnetic anomaly patterns. Our findings suggest that the RG is part of a distributed region of the continental lithosphere in East Antarctica that was preferentially deformed in response to Cenozoic transtensional stresses that likely also facilitated propagation of accelerated oceanic transform faulting in the adjacent oceanic lithosphere located between southeastern Australia and Tasmania

New Views of East Antarctica- from Columbia to Gondwana

East Antarctica is a keystone in the Gondwana, Rodinia and the Columbia supercontinents. Recent aerogeophysical research, augmented by satellite magnetic, gravity and seismological data is unveiling the crustal architecture of the continent. This is helping comprehend the impact of supercontinental processes such as subduction, accretion, rifting and intraplate tectonics on its evolution. A mosaic of Precambrian basement provinces is apparent in interior East Antarctica (Ferraccioli et al., 2011, Nature). A major suture separates the Archean-Neoproterozoic Ruker Province from an inferred Grenvillian-age orogenic Gamburtsev Province with remarkably thick crust (up to 60 km thick) and thick lithosphere (over 200 km thick). The age of the suturing and its linkages with supercontinental assembly is debated with both Rodinia and Gondwana candidates being proposed. Further east, magnetic highs delineate a Paleo to Mesoproterozoic Nimrod-South Pole igneous province (Goodge and Finn, 2010 JGR) that flanks a composite Mawson Continent- including the Gawler Craton of South Australia (Aitken et al., 2014 GRL). An over 1,900 km long magnetic and gravity lineament is imaged along the western flank of the Wilkes Subglacial Basin and is interpreted here as a major Paleoproterozoic suture zone linked to the collision of Laurentia and East Antarctica within Columbia. The proposed suture played a pivotal role helping localise Neoproterozoic Rodinia rifted margin evolution and forming a backstop for the Ross-Delamerian cycle of Gondwana amalgamation. Aeromagnetic and gravity imaging help determine the extent of a Keweenawan-age (ca 1.1 Ga) large igneous province in the Coats Land Block -isotopically tied with the Mid-Continent Rift System of Laurentia (Loewy et al., 2011 Geology). Imprints of Grenvillian magmatic arc accretion link together the Namaqua-Natal and Maud belts in South Africa and Dronning Maud Land within Rodinia. The aeromagnetically distinct Southeast Dronning Maud Land province (Mieth and Jokat, 2014 GR) may represent a separate 1000-900 Ma Oceanic Arc Superterrane (Jacobs et al., 2015 Prec. Res.). New geophysical views of the Shackleton Range suture lend weight to more complex collisional and indentation tectonic models for the Pan-African age assembly of Gondwan

Fault-controlled Ancient Hydrothermal Systems in North Victoria Land, Antarctica

Northern Victoria Land (NVL) at the Pacific end of the Transantarctic Mountains is a key region for understanding the geodynamics of East Antarctica and provides a window to its crustal architecture. The structural frame of NVL derives from polyphase tectonics and is mainly characterised by NW-SE striking lineaments, which transect the NVL crust and the Cenozoic rift basins of the Ross Sea and which are responsible for the reactivation of inherited Paleozoic - Mesozoic discontinuities. We present preliminary data on regional scale metasomatic and hydrothermal alterations linked to different exhumed fault systems in NVL. Main hydrothermal alterations are: (i) Mg-Ca- carbonation and silicification of mafic and ultramafic rocks along faults and brittle-ductile shear zones at the boundary between the Wilson and Bowers terranes; (ii) Mg-Ca-Fe carbonation of metavolcanic rocks accompanied by syntectonic carbonate coatings on fault planes, hydraulic brecciation, and quartz-carbonate veining in the Bowers Terrane and at the Bowers-Robertson Bay terranes boundary; (iii) epidote and chlorite indurated gouge and cataclasite in fault cores in granitoid rocks and epidotization of metabasalt in the Lanterman Range and in the Bowers Terrane and in Ferrar Dolerite. We discuss the fluid-rock interaction during faulting and the relevance of these hydrothermal systems in reconstructing the tectonic history of this sector of the Transantartic Mountain

Polyphase Brittle Tectonics in North Victoria Land and Gondwana Fragmentation

Antarctica was at a centre position within Gondwana and holds a key position for any plate tectonic reconstruction related to its break-up history. North Victoria Land (NVL) is located at the Pacific end of the Transantarctic Mountains (TAM), which represent the uplifted western shoulder of the West Antarctic Rift System. The basement of the TAM formed during subduction of the Palaeopacific Ocean under E Gondwana during the Ross Orogeny. Major uplift of the TAM commenced around the Eocene-Oligocene boundary following sediment accumulation within the wide \u201cMesozoic Victoria Basin\u201d. This long-lasting geological history led to highly anisotropic crust that is susceptible to repeated reactivation. We present evidence for a polyphase structural evolution of NVL after initial break-up of Gondwana at ca. 180 Ma coeval with the Ferrar volcanic event. Mainly Neogene NW-SE striking dextral strike-slip tectonics with local transtension and transpression controls the present structural architecture of NVL. It may be interpreted as dynamic response to intra-oceanic fracture zones between Australia and Antarctica extending into NVL and the Ross Sea. Dextral strike-slip overprints two older increments: (i) WNW-ESE striking sinistral tectonics of possibly late Mesozoic-early Paleogene age within a transform margin setting between Australia and Antarctica, (ii) ENE-WSW directed extension of NVL continental crust possibly coeval to Adare Trough spreading in Eocene-Oligocene times

High-resolution Aeromagnetic Imaging of the Lanterman Range, North Victoria Land

The Lanterman Range covers the boundary between the Wilson Terrane and Bowers Terrane, i.e. the two western tectonometamorphic units of the Ross-orogenic basement of North Victoria Land, Antarctica. This boundary is formed by a distinct belt of highly deformed mafic and ultramafic rocks with UHP relicts. It is interpreted as the trace of a former suture zone that formed during continuous accretion at the Palaeo-Pacific active continental margin of East Gondwana in the Early Paleozoic. We report on high-resolution magnetic anomaly data over the Lanterman Range. Preliminary results show two distinct and nearly parallel magnetic lineaments. These magnetic lineaments follow the main strike of the Wilson-Bowers terrane boundary. The western of these lineaments correlates with the boundary zone itself. The eastern lineament cannot be attributed to any exposed rocks so far, which show only rather low magnetic susceptibility values. Similar paralleling magnetic structures occur further to the southeast, where they are apparently offset by a possibly post-Jurassic WNW-ESE oriented left-lateral strike-slip zone. We tentatively propose that the origin of the eastern lineament is due to remnants of the Palaeo-Pacific subducted slab hidden under the boundary zone of the Bowers Terrane and the easterly dominantly turbiditic Robertson Bay Terrane. The western may represent remnants of a closed back-arc basin intermittent between the Wilson and Bowers terranes

Synsedimentary Deformation in the Permian Takrouna Fm (North Victoria Land)

(Beacon Supergroup) exposed at an unnamed spur east of Mt Remington north of Boggs Valley, central Rennick Glacier area, North Victoria Land. In the lower part of this section, the formation consists of fluvio-lacustrine successions of pebbly sandstone, climbing-ripple-laminated sand- and siltstone, and carbonaceous silt- and mudstone rich in plant debris (Glossopteris leaves and Vertebraria roots). Sand- and siltstone beds in particular are characterized by prominent synsedimentary brittle-ductile deformation structures at the outcrop scale, e.g., sedimentary dikes, thrust, growth microfaults, shear folds, normal faults, laminated convolute beds, and intense slumping. This succession is overlain by about 10 m of barren sand- and siltstone deposits that lack such evidence for synsedimentary deformation, and are instead folded and tilted by post-sedimentary deformations. Our synsedimentary tectonic structures may testify to an active tectonism during the late Permian in northern Victoria Land and give constraints for the changes in paleoclimatic and tectonic conditions of Gondwana during late Paleozoic-Mesozoic transition

Crustal architecture of the largest pull-apart basin in East Antarctica unveiled

The West Antarctic Rift System (WARS) is known to have experienced distributed/wide mode extension in the Cretaceous, followed by narrow mode and variably oblique extension/transtension in the Cenozoic, the latter potentially linked to the onset of oceanic seaoor spreading within the Adare Basin (Davey et al., 2016, GRL). However, onshore the extent and impact of Cenozoic extension and transtension within the Transantarctic Mountains sector of East Antarctica is currently much less well-constrained from a geophysical perspective. Here we combine aeromagnetic, aerogravity, land-gravity and bedrock topography imaging to help constrain the extent, architecture and kinematics of the largest Cenozoic pull-apart basin recognised so far in East Antarctica, the Rennick Graben (RG). Enhanced potential eld imaging reveals the extent of a Jurassic tholeiitic Large Igneous Province preserved within the RG and the inherited structural architecture of its basement, including remnants of uplifted ca 530-500 Ma arc basement in the northern Wilson Terrane and a ca 490-460 Ma subglacial thrust fault belt separating the Cenozoic western ank of the RG from the eastern margin of Wilkes Subglacial Basin (WSB). The architecture of the RG is best explained in terms of a major composite right-lateral pull-part basin that extends from the Oates Coast to the Southern Cross Mountains block. We propose that Cenozoic strike-slip deformation kinematically connected the RG with both the western edge of the WARS and the eastern margin of the WSB. An earlier phase of leftlateral strike slip deformation is also emerging from recent geological eld work in the study region but only relatively subtle osets in aeromagnetic anomaly patterns are visible in currently available regional datasets. We conclude that the RG is part of a wider distributed region of the continental lithosphere in East Antarctica that was deformed in response to an evolving Cenozoic transtensional tectonic setting that may have also aected enigmatic sub-basins such as the Cook Basins in the adjacent WSB region

Microstructures of epidote-prehnite bearing damaged granitoids (northern Victoria Land, Antarctica): clues for the interaction between faulting and hydrothermal fluids

Exhumed faults in granitoids along the Lanterman Fault-Rennick Graben Fault system (northern Victoria Land, Antarctica) show superposed ductile to brittle deformation and pervasive hydrothermal fluid-rock interaction. These processes triggered multiple brittle slip events producing crosscutting epidote and prehnite-rich fault veins, ultracataclasites and pseudotachylytes of crushing origin. Combined microstructural and minerochemical investigations on fault damage zones show three types of alteration: (i) albitization of K-feldspar and Ca-plagioclase; (ii) crystallization of prehnite and calcite in veins; (iii) alteration of magmatic phases by secondary hydrous minerals (e.g. chlorite, white mica, epidote and prehnite). The fault experienced various episodes of strain weakening and hardening, due to alteration of minerals and precipitation of epidote and prehnite within ultracataclastic intervals, at decreasing temperature conditions (200 < T\ub0C < 450) and varying CO2 fugacity of the fluids. Cyclic crystallization of epidote/prehnite within the fault cores caused cementation and locking of faults, concentration of deformation at weaker horizons and a progressive broadening of the fault zone. Our results indicate that multiple co-seismic slip and syntectonic fluid flow very likely occurred prior to the Cenozoic brittle reactivation of inherited anisotropies in the northern Victoria Land crust along the Lanterman Fault-Rennick Graben Fault system and underlines its high potential for polyphasicity. Previous article in issu

Crustal architecture and tectonic evolution of a major pull-apart basin in East Antarctica

Strike-slip faulting can lead to pull-apart basin formation, which can provide clues into complex multi-stage rifting, including oblique extension. The Rennick Graben (RG) is an example of such pull-apart basins and is located in northern Victoria Land (NVL), East Antarctica. The RG has been interpreted as an extensive left lateral Cretaceous(?) pull-apart basin linked to the Victoria Land Basin, part of the West Antarctic Rift System (WARS). Alternatively, it has been interpreted as a more localised Cenozoic right-lateral basin unconnected to the WARS. Here we present the first results of a new project (REGGAE) that aims to re-investigate the architecture and evolution of the RG by analysing aeromagnetic, aerogravity and land-gravity and bedrock topography data together with new structural and thermochronology constraints. Maximum horizontal gradient of pseudo-gravity, tilt derivative and isostatic residual gravity maps provide tantalising new geophysical views of the RG. The north-eastern shoulder of the RG is clearly controlled by a major inherited fault, the Lanterman Fault that was active during the Ross Orogen and may have been repeatedly reactivated. The USARP Mountains are often depicted as the north-western flank of the RG, but here we hypothesise that this region was originally located within the RG, as part of an early stage of more distributed (late Cretaceous-Paleogene?) extension and inferred left-lateral strike-slip faulting. Strengthening of the lithosphere may have followed, leading to narrower more focussed extension during right-lateral strike-slip faulting. Overall, the geophysical images and the spatial distribution of Jurassic volcanics support the interpretation that the RG extends further south and is kinematically connected with both the western edge of the WARS and the eastern margin of the Wilkes Subglacial Basin. However, 3D gravity inversions demonstrate that the RG does not exhibit such thin crust or lithosphere as observed within the WARS. Aeromagnetic imaging confirms that it was also un-affected by voluminuous Cenozoic magmatism and post-Jurassic sedimentary infill is also lacking in contrast to the Ross Sea Rift basins. Major tectono-thermal segmentation is therefore apparent, as observed in many other rift and pull-apart systems affected by multistage evolution