Lithospheric flexure induced by the volcanic loads of the Ross Archipelago, McMurdo Sound, Antarctica

Aitken, Alan RA

Estimates of the effective elastic thickness (Te) of continental lithosphere vary from over lOOkm to under lOkm. What controls this parameter in continental crust is poorly understood. The coincidence of lithospheric thinning, heating and faulting due to West Antarctic rifting, and also heating from a potential mantle plume causes weak lithosphere beneath the McMurdo Sound!McMurdo Ice Shelf area. Thus, the area provides an ideal location to estimate the effect of lithospheric thickness, thermal properties, rheology and composition, brittle and/or ductile failure, and load duration on the Te of continental crust. This study uses spectral computer modelling algorithms to predict the response of elastic and visco-elastic plates to progressive volcanic loading of the lithosphere beneath southern McMurdo Sound since llMa. A digital elevation model of the present day topography of the Ross Archipelago is the input to the flexural models. The resulting flexural response is forward modelled to best fit gravity data collected over the 2001/2002 and 2002/2003 seasons and the best fit model is used to derive a flexural history and synthetic stratigraphy. This synthetic stratigraphy is tested by comparison with the acoustic stratigraphy revealed by seismic lines shot in 2002/2003. The best fitting model is an unbroken elastic plate with Te of 2.5km, although the fit is good forTe between 2km and 3.5km. A viscoelastic plate with an initial Te of 4km may be equally good, and there is evidence for a broken plate beneath Hut Point Peninsula and possibly the rest of Ross Island. The best fit model predicts that the flexure has formed two separate basins, one due to Ross Island, and the other due to the loading of the southern volcanoes, particularly Mts Discovery i and Morning. The boundary between the basins lies just north of Black and White islands, and has been present since the loading of Mt Bird at - 4Ma. The southern basin is up to 2300 ± 200 m deep and has formed due to gradual loading since llMa. The resulting synthetic stratigraphy here is varied and regularly- spaced, with sediments as old as 5Ma within 2000m of mean sea level. This basin is almost completely infilled. The northern basin is -2400±400m deep and has formed rapidly throughout the last 1.5Myr as a result of Mts Terror and Erebus loading. Mt Bird contributes localised accommodation space, but the resulting infill has been depressed to depths of - 2300m. The northern basin is incompletely infilled, probably because of the rapid flexure from Mts Terror and Erebus. Key horizons in the synthetic stratigraphy correspond with prominent on-lap reflectors in the seismic lines. Therefore, flexural basin formation corresponding to loading and subsequent infill of the basins is suggested as the origin for the strata.

xxiv, 152 p. some maps in text. Ill. 30cm.

2003Aitken