Evidence from P-wave receiver functions for lower mantle plumes and mantle transition zone water beneath West Antarctica
Thursday, 18 December 2014
West Antarctica has experienced abundant Cenozoic volcanism, and it is suspected that the region is influenced by upwelling thermal plumes from the lower mantle; however this has not yet been verified, because seismic tomography results are not well resolved at mantle transition zone (MTZ) depths. We use P-wave receiver functions (PRFs) from the 2007-2013 Antarctic POLENET array to explore the characteristics of the MTZ throughout Marie Byrd Land and the West Antarctic Rift System. We obtained over 8000 high-quality PRFs for earthquakes occurring at 30-90° with Mb>5.5 using a time-domain iterative deconvolution method filtered with a Gaussian-width of 0.5 and 1.0, corresponding to frequencies less than ~0.24 Hz and ~0.48 Hz, respectively. We stack P receiver functions as single-station and by common conversion point and migrate them to depth using the ak135 1-d velocity model. Results suggest that the thickness of the MTZ varies throughout the region with thinning beneath the Ruppert Coast of Marie Byrd Land and beneath the Bentley Subglacial Trench and Whitmore Mountains. We identify the 520’ discontinuity throughout much of West Antarctica; the discontinuity is most prominent beneath the Bentley Subglacial Trench and Whitmore Mountains. Additionally, prominent negative peaks are detected above the transition zone beneath much of West Antarctica and may be evidence for water-induced partial melt above the MTZ. We propose that the MTZ beneath West Antarctica is hotter than average in some regions, possibly due to material upwelling from the lower mantle. Furthermore, we propose that the transition zone is water-rich and that upward migration of hydrated material results in formation of a partial melt layer above the MTZ.