Who: Professor Fenglin Niu and Dr. Julia Riberio, Postdoctoral Research Associate working with with Professor Cin-Ty Lee
What: Niu: “Different deformation mechanism between the SE and NE Tibetan Plateau revealed by receiver function and seismic anisotropy data”
Abstract:The large-scale surface deformation, uplifting and faulting occurring at the SW and NE margins of the Tibetan plateau are generally believed to be caused by the continuous collision between the India and Eurasia since approximately 50 millions years ago. However, the style and amount of the subsurface deformation induced by the collision, especially those inside the lower crust and upper mantle, are still debated. We analyze a large amount of receiver function data recorded by regional seismic networks of the China Earthquake Administration (CEA) to estimate crustal structure and deformation beneath the two margins and their surrounding areas. Beneath the SE margin, we find significant seismic anisotropy with a nearly NS fast polarization direction and a splitting time of 0.5-0.9 s, which are comparable to those estimated from SKS/SKKS data. This observation suggests that crustal anisotropy is the main cause of waveform splitting of the SKS/SKKS phases. The crust beneath the SE Tibetan plateau has a thickness of ~50-70 km and a relatively high Vp/Vs ratio of ~1.79, indicating that mafic lower crustal materials compose a significant portion of the crust beneath the margin. These observations are consistent with a scenario of lower crustal extrusion beneath the SE margin. On the hand, the average Vp/Vs ratio of beneath the NE margin of the plateau is significantly lower, with an average value of only ~1.71. The fast polarization directions align well with surface structure, and follow the directions of the maximum horizontal tensile stress. The lower Vp/Vs ratio together and fast polarization direction suggest that whole crustal shortening might be the dominant mechanism for producing the thick crust beneath NE Tibet.
Riberio: “The fate of sulfur in subduction zones”
Abstract: Julia M. Ribeiro, Cin-Ty A. Lee
Sulfur is thought to be efficiently exchanged from the atmosphere and the oceans into the deep Earth via subduction zones. Much of the sulfur (i.e. S) being degassed at arc volcanoes is believed to be released from dehydrating the subducting slab, specifically the release of recycled seawater sulfate. Experimental studies, however, suggest that sulfur, in the form of sulfate, is so soluble in aqueous fluids that much of the sulfate may actually be released into the upper plate before the slab reaches the arc magmatic front. The shallow release of slab sulfur is supported by the strong enrichment in sulfur and chalcophile (sulfur-like) elements in the forearc serpentinites and the release of sulfate-rich fluids at serpentinite mud volcanoes. The origin of sulfur has, thus, remained elusive; and it is still unclear whether the heavy sulfur isotopic content (δ34S > 0 ‰) of arc lavas is imparted to magma degassing, upper crust and/or seawater assimilation, or slab dehydration. Such observations thus raise the questions of what are the sources of sulfur in subduction zone magmas? Does sulfur seen in arc lavas entirely derive from slab dehydration? Here, we aim to answer these questions by examining δ34S of olivine-hosted melt inclusions from arc and back-arc basalts sampled in the Izu-Bonin-Mariana (IBM) convergent margin. IBM is an end-member example of subduction of cold and old (~ 140 Myr) lithosphere, where most (~ 90%) of the present-day slab-derived aqueous fluids and volatiles are thought to be released beneath the volcanic arc. Therefore, if sulfur derives from slab dehydration, the sulfur isotopic composition of the Izu-Bonin-Mariana arc and back-arc melt inclusions should retain the slab fingerprint.