On the Cusp of a Tear: Arc-Continent Collision in the Banda Arc
Eastward along the Sunda-Banda arc, convergence transitions from subduction of oceanic lithosphere to arc-continent collision. This region of eastern Indonesia and Timor-Leste provides an opportunity for unraveling the processes that occur during collision between a continent and a volcanic arc, and can be viewed as the temporal transition of the process of continental collision along strike. We have collected a range of geological, geodetic, seismic, and geomorphic data to place constraints on the geometry and history of subduction. Utilizing ~4 years of new broadband seismic data we image the structure of the crust through to the mantle. Ambient noise tomography of the crust and uppermost mantle lithosphere shows velocity anomalies along strike and across the arc, related to structure of the incoming Australian plate. The pattern of anomalies at depth resemble the system of salients and embayments that are present offshore western Australia, which formed during rifting of east Gondwana. At deeper upper mantle depths, transition from oceanic subduction to continental collision appears reflected in new teleseismic based tomography images, coinciding with previously identified changes in the geochemistry of the arc volcanics. Results from our body wave tomography show continuity of the subducting slab to depths of at least 300 km, with no evidence for tearing at the scale of >~50 km even in the region of arc-continent collision. Our expanded catalogue of Benioff zone seismicity reveals earthquakes in what was previously thought to be a seismic gap (the Wetar gap). Together, our seismic results suggest that tearing is not as advanced in this region as previously hypothesized, implying sustained subduction of continental lithosphere underneath the Banda arc. We suggest the tectonic evolution of this region is defined by inherited structure of the Gondwana rifted continental margin of the incoming plate. Altogether, we suggest that this region is characterized by subduction of continental lithosphere poised for tearing that has perhaps just initiated, but with no large slab windows. Therefore, the initial template of plate structure controls orogenesis and deep mantle structure.
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