Upper mantle seismic anisotropy beneath Antarctica and the Scotia Sea region

Abstract

Recent investigations on shear wave splitting from recordings of permanent and temporary Antarctic seismological stations have lead to a greater understanding of the upper mantle dynamics of the Scotia Sea region and the continental margin in the eastern Weddell Sea in terms of their tectonic evolution. The analysis of shear wave splitting from teleseismic core (SKS, SKKS, PKS) and direct S waves reveals the seismic anisotropy and the strain field of the upper mantle. Similar to the Caribbean, anisotropy structures in the Antarctic Peninsula and Scotia Sea regions are assumed to be influenced by mantle flows in easterly directions around the subducting Nazca plate. In general, anisotropy polarization directions in the Scotia Sea do not contradict this hypothesis, with polarizations oriented nearly E-W and therefore aligning with the suggested mantle flow patterns. Anisotropy strengths decrease from delay times of δt=1.8s (PMSA, Palmer Station) in the west towards the east with delay times of δt=0.3s beneath HOPE (South Georgia) and CAND (Candlemas, South Sandwich Islands). Nevertheless, a lithospheric and therefore fossil origin cannot be ruled out. Only the exceptionally high delay times at PMSA probably originate in part from recent asthenospheric flow around the subduction slab of the former Phoenix Plate beneath the northwestern margin of the Antarctic Peninsula. The continental margin of western Dronning Maud and Coats Land plays a crucial role in understanding the early processes during the break-up of Gondwana. Upper mantle seismic anisotropy with delay times well over δt=1s in this region gives new constraints on ancient deformation processes during break-up and former episodes. Two-layer modelling reveals Archaean anisotropy in the upper layer corresponding well to polarization directions of the South African Kaapvaal Craton. Lower layer anisotropy is assumed to have been created during early Gondwana rifting stages.