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

Bathymetric Control on Borchgrevink and Roi Baudouin Ice Shelves in East Antarctica

The stability of ice shelves and drainage of ice sheets they buttress is largely determined by melting at their atmospheric and oceanic interfaces. Subglacial bathymetry can impact ice shelf stability because it influences the onset and the pattern of warm ocean water incursions into the cavities between them and the seafloor. Bathymetry is further important at pinning points, which significantly retard the flow of ice shelves. This effect can be lost instantaneously if basal and surface melting cause an ice sheet to thin and lift off its pinning points. With all this in mind, we have developed a model of bathymetry beneath the western Roi Baudouin and central and eastern Borchgrevink ice shelves in Dronning Maud Land based on inversion from gravity data and tied to available depth references offshore and subglacial topography inland of the grounding line. The model shows deep glacial troughs beneath the ice shelves and bathymetric sills close to the continental shelf. The central Borchgrevink Ice Shelf overhangs the continental slope by around 50 km, exposing its northern parts to the open ocean and higher ocean temperatures. Continuous troughs traverse the central Borchgrevink and western Roi Baudouin ice shelves at depths greater than the offshore thermocline and thus present a risk of Warm Deep Water intrusions into their cavities under the current and future oceanographic regimes. Differing bathymetric characteristics might explain the ice shelves' contrasting dominant mass loss processes.Plain Language Summary: The rate at which Antarctica's ice sheets flow off the continent is largely stabilized by floating ice shelves that form where they meet the surrounding ocean. Assessing the stability of this interconnected system strongly depends on correctly quantifying ice gain processes, such as snowfall, and ice mass loss processes, such as melting at the bases of the ice shelves. This basal melting strongly depends on the flow of warm ocean water into the cavity between the ice shelf and the seafloor below, which is in turn influenced by the shape of the seabed. Using sparse direct measurements together with small variations in the pull of gravity measured from airplanes, we have generated a model of the formerly unknown topography beneath the Borchgrevink and Roi Baudouin ice shelves in East Antarctica. The modeled seabed shows deep troughs beneath the ice shelves and topographic sills along the continental shelf. Gateways within these sills potentially allow for the intrusion of warm water into the cavities, representing a threat to future ice shelf stability.Key Points: We have generated bathymetric models based on gravity inversion beneath the Roi Baudouin and Borchgrevink ice shelves. Results are similar to ice shelves throughout the entire Dronning Maud Land, which are all crossed by deep troughs and bathymetric sills. Deep gateways leading from the open ocean into ice shelf cavities possibly allow for the intrusion of Warm Deep Water into these cavities.Deutsche Forschungsgemeinschaft (DFG) http://dx.doi.org/10.13039/501100001659Alfred Wegener InstituteHelmholtz Centre for Polar and Marine ResearchFederal Institute for Geosciences and Natural Resources (BGR

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