Geophysical investigation of the Wilkes subglacial basin, East Antarctica

Abstract

New geophysical data collected during the East Antarctic Seismic Traverse of 1993/94 (EAST-93) over the polar plateau of East Antarctica provide improved constraints upon which to base an examination of the Wilkes Subglacial Basin, an extensive sub-ice bedrock depression below the 2-3 km thick East Antarctic ice-cap. The physical scale of the Wilkes Basin in combination with the broad wavelength free air gravity low that coincides with the basin make it unusual on a global scale. Study of the Wilkes Basin and the coinciding gravity low has implications for continental tectonics in general, and possibly for sea level change resulting from melting of the Antarctic ice-cap in the recent past. EAST-93 employed the most modern geophysical techniques available for a major traverse to the interior of East Antarctica. Seismic reflection, gravity, magnetic and radio echo sounding data were collected on the traverse that covered almost 320 km. Use of conventional optical surveying techniques tied to differential GPS allowed height determinations substantially more accurate than the earliest geophysical work over the plateau (±2 m versus ±50 m). Accurate height determination in turn led to accurate calculation of gravity anomalies (±1 mgal versus ±15mgal from the earliest traverses). Previous explanations for the origin of the Wilkes Basin have considered it to be an extensional basin filled with sediment. Isostatic arguments require that sediments in an extensional basin with a size equivalent to that of the Wilkes Basin (about 500 km in width) are compensated. Because of this compensation, the gravity effect of a wide, sediment filled basin is not consistent with the constraints imposed by the wavelength and amplitude of the free air gravity low over the Wilkes Basin. Hence a model is employed here in which the Wilkes Basin is linked on a regional scale to the adjacent Transantarctic Mountains (TAM) and extensional Ross Embayment. In this regional sense, the Wilkes Basin is modelled as a 'hinterland' basin, or flexural outer low, compensating for the flexural uplift of the TAM. The flexural uplift of the TAM occurs as a rift flank uplift induced by thermal conduction into the margin of East Antarctica from the adjacent rift environment of the Ross Embayment, a region characterised by thinned crust and reduced seismic velocities through the lithosphere. The uplift induced by thermal buoyancy is augmented by isostatic adjustments that occur in response to erosional unloading in the TAM, and to lesser extent, footwall block adjustments that result from an inclined normal fault assumed to exist at the boundary between East Antarctica and the Ross Embayment. Modelling the uplift in the TAM is constrained by: the Kukri peneplain, an unconformity in the TAM that dips gently toward the East Antarctic interior and acts as a reference surface; fission track thermochronology which gives a measure of the amount of uplift and erosion in the TAM; and the wavelength of the broad, negative free air gravity low observed over the Wilkes Basin. To model the gravity low, further constraints are provided by the need for gravity models to maintain mechanical equilibrium, and results from seismology that indicate crustal thicknesses and the depth of compensation over which density differences are distributed. A model that matches the constraints imposed by these factors suggests that East Antarctic elastic thickness is Te = 90 ± 10 km, and its flexural rigidity (5.2 ±1.9) x 1024 Nm, values that confirm that away from its margin, East Antarctica is typical of other old cratonic regions. The free air gravity low over the Wilkes Basin is shown by EAST-93 data to have have a magnitude of -70 mgal. Flexural displacement in the Wilkes Basin accounts for -25 mgal of that low. A further -15 mgal comes from gravity edge effects associated with the juxtaposition of the two geologically contrasting regions of East and West Antarctica, in combination with the effects of reduced density under the TAM resulting from thermal conduction from the Ross Embayment. The remaining -30 mgal is attributed to a regional anomaly consistent with satellite derived observations of the global gravity field. The regional anomaly is considered to be caused by effects associated with flow within the mantle. By comparison to long wavelength satellite derived gravity, -13 to -20 mgal of the regional anomaly is assumed to be a result of convective flow in the deep Earth. If estimates of the Antarctic contribution to sea level rise of around 37 m are correct, then the remainder of the regional anomaly (-17 to -10 mgal) is consistent with a delayed rebound of East Antarctic lithosphere after partial melting of the ice-cap.