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A new 3-D seismic P wave velocity model of the Sunda-Banda Arc Transition from local earthquake tomography reveals (i) northward subduction of oceanic lithosphere, associated with the convergence of Australia and Sundaland, as a high-velocity zone extending down to similar to 200 km depth; (ii) two distinct low-velocity zones, one immediately above the slab, which is likely a zone of partial melt, and one in the 0-40 km depth range, which is probably a magma chamber associated with active volcanoes above; and (iii) a northerly dipping high-velocity zone that bisects the two low-velocity anomalies, which we interpret as an underthrust forearc sliver of continental origin. Based on He-3/He-4 measurements from volcanic outflows, it is possible that the magma supply is contaminated by interaction with this forearc sliver as it ascends from the melt region below.

Plain Language Summary The subduction of one tectonic plate beneath another is a fundamental Earth process that is integral to the growth of continents and accommodates the emergence of new crust beneath the oceans. In general, it is the denser oceanic plates which sink into the mantle, while the more buoyant continental plates remain at the surface. In this study, we investigate the Sunda-Banda region in southeast Indonesia, where convergence of the Eurasian and Australian plates has resulted in the collision of a volcanic arc and the Australian continent. By exploiting earthquake signals recorded by a set of stations at the surface, we show that a sliver of continental forearc (region between the subduction trench and volcanic arc) has been thrust beneath the overriding plate due to the northward motion of Australia and now sits above the downgoing oceanic plate. Our new results also suggest that the underthrust forearc material may contribute to the contamination of volcanic products by crustal material of continental origin.