Mantle Dynamics of Australia-Banda Arc Collision as Inferred from Shear Wave Splitting Analysis of Teleseismic and Local Slab Events

Abstract:

Arc-continent collision is an important factor in continent building, orogensis, and ocean closure, yet the details associated with it are not fully understood. East-Timor and the Nusa Tenggara Timur region of Indonesia provide a unique setting to study a young arc-continent collision (~8 Ma) and incipient orogenesis. The NSF funded Banda Arc project affords a rare opportunity to investigate unconstrained processes such as active continental subduction and slab rupture beneath a regional deployment of broadband seismometers. We use data from 35 broadband sensors to analyze seismic anisotropy through measuring shear wave splitting. These stations span the roughly east-west transition from normal oceanic subduction at the Sunda Arc to collision at the Banda Arc, and cross areas associated with back-arc thrusting, arc volcanism, extinct volcanism and a rapidly exhuming forearc. Thirty of the sensors used in the analysis are temporary stations installed by our research team in 2014 and will remain in the field until 2016 or later. The remaining stations are part of the open-access GFZ GEOFON global seismic network. We present preliminary shear wave splitting results for teleseismic (*KS core phases) and local (direct S phase) arrivals in order to inspect the sub-slab mantle and the supra-slab mantle wedge for anisotropic patterns related to olivine flow fabric. These results can be used to assess regional strain linked to ongoing collision and may elucidate any slab tearing that has resulted from the (partial) subduction of buoyant continental material of Australian affinity. Presently, we observe a trend of primarily trench-parallel sub-slab fast polarization directions and perhaps more complicated anisotropy in the mantle wedge. Relative to the trench, there appears to be more spatial variation in fast axis orientation for direct arrivals than teleseismic phases. We discuss how the interpretation of our initial results provides insight into the mantle dynamics of the Banda collision.