Sedimentary basins of the east antarctic craton from geophysical evidence

Abstract

Ninety-five percent of Antarctica is buried under an ice sheet up to 4.7 km thick. Within interior East Antarctica (~10.2 · 10 6 km 2) recent airborne geophysical observations, principally radio echo sounding, have enabled widespread investigation of ice covered bedrock. Limited seismic refraction profiling, magnetic and gravity investigations combined with the radar studies have provided a generalized picture of sedimentary basins in Antarctica between 180° and 60° E. Two major basinal structures have been detected within East Antarctica—the Wilkes Basin and Aurora Basin complex. The former lies sub-parallel to the Transantarctic Mountains, while the Aurora Basin forms a branching system of basins in central East Antarctica trending northwest towards the Wilkes Land coast. Analyses of macro-scale terrain roughness and bedrock reflection coefficients from radio echo sounding indicate significant differences between basins and their surrounding regions. Small-scale surface irregularities and slowly changing, high reflectivities from radar measurements are interpreted as suggesting the presence of a smoothing cover of sediments. Residual magnetic anomalies (from airborne operations), when combined with topographic data, exhibit low gradients over basins, but steep, fluctuating characteristics over adjacent basement highs. Source-depth calculations from oversnow magnetic determinations across the Wilkes Basin indicate an average thickness for the sedimentary layer of <3 km. This is corroborated by reinterpretation of gravity anomalies, which average ~—30 mGal, over the basin. Sediments appear absent or extremely thin on the flanks of the Wilkes Basin where seismic refraction shooting has detected the near-surface presence of granitic crust. Furthermore an increase in roughness of terrain combined with sudden breaks in slope argue that these basin margins may be fault-controlled and deeply eroded. The distribution and configuration of the depressions is therefore thought to be governed by intra-cratonic fracture patterns possibly related to ancient orogenic sutures. Juxtaposition of basins and flanking basement highs of probable Precambrian and Early Palaeozoic age are reminiscent of basin and swell structures of the African and Australian cratons, with which East Antarctica has had a common geologic history throughout most of the Phanerozoic. Any sediments must pre-date growth of the ice sheet and are hence older than Miocene.