Highly variable structure along the Banda arc imaged by ambient noise tomography

Abstract:

The tectonic configuration of the Banda region in southeast Asia captures the spatial transition from subduction of Indian Ocean lithosphere to subduction and collision of the Australian continental lithosphere beneath the Timor Sea, which can be considered as the temporal transition of the initiation of continental collision. An ongoing broadband seismic deployment funded by NSF is aimed at better understanding the mantle and lithospheric structure in the region and the relationship of the arc-continent collision to orogenesis. Here, we present results from ambient noise tomography in the region utilizing this temporary deployment of 30 broadband instruments and 38 permanent stations in Indonesia and Australia. Despite having only a year of data from the new experiment and about 2.5 years of data in total, we are already able to measure stable dispersion curves for several paths resulting in good recovery for the Savu Sea, Timor Leste, and the Nusa Tenggara Timur (NTT) region of Indonesia. The resulting initial three dimensional shear velocity model indicates significant lateral heterogeneity along the plate boundary. The model contains clear anomalies associated with the subducting oceanic lithosphere, subducted Australian continental lithosphere, and perhaps delaminated oceanic sediments forming the core of the island of Timor. In this initial 3D model, the subducted lithosphere appears to shallow under the islands of Timor and Sumba relative its position beneath to the Savu Sea where oceanic or transitional lithosphere is being subducted. Additionally, a low velocity anomaly associated with the island of Timor from near the surface to ~8 km depth is observed. Under the island of Sumba, a high velocity anomaly is observed between 10 and 20 km depths. These differences may reflect variation in the stage of subduction along strike, compositional variations, or the influences of subducting seamounts such as the Scott Plateau. Finally, we observe localized low velocity mid-crustal anomalies along the active arc to the north of Timor and Sumba associated with volcanic centers. This suggests these volcanoes may be sampling a common mid crustal magma chamber.