Upper plate deformation induced by subduction of a volcanic arc: the Snellius Plateau (Molucca Sea, Indonesia and Mindanao, Philippines)

Abstract

The northern Molucca Sea shows the incipient subduction of a composite oceanic and arc volcanic block, the Snellius–Halmahera block (SHB). Multi-beam, reflectivity, seismic and gravity data obtained during the MODEC marine survey showed that the SHB disappears beneath the accretionary wedge and the outer ridge of the Sangihe Arc. To the north in Mindanao Island, ongoing convergence generated shortening of the forearc basin and the backthrusting of the SHB; meanwhile a classical system of paired subduction (Philippine Trench) and strike-slip fault (Philippine Fault) was installed. The transition from lithospheric subduction to crustal overthrusting is located where the Philippine Trench sensu stricto begins, and also coincides with the off-shore extension of the Philippine Fault. We observe a reversal of the thrusts from an eastward vergence in the Molucca Sea to a westward vergence in Mindanao Island. This reversal takes place at the latitude where the forearc area emerges by uplift, and the downgoing crust (SHB) deepens, resulting in a strong gravity low centered above the accretionary wedge. The Philippine Fault initiated in a place where the crust was sliced off by a transfer zone which marked the northern termination of the Molucca Sea, and drags northward a sliver of the previously accreted SHB. The northward drifting of this sliver created an extension, which, however, cannot account for the gravity low. We propose that the shortening and the uplift of the upper plate were induced by the buoyancy of the subducted unit (SHB), and triggered the thrust reversal.