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

The northern Molucca Sea shows the incipient subduction of a composite ocea nic and are volcanic block, the Snellius-Halmahera block (SHB). Multi-beam, reflectivity, seismic and gravity data obtained during the MODEC marine su rvey showed that the SHB disappears beneath the accretionary wedge and the outer ridge of the Sangihe Are. To the north in Mindanao Island, ongoing co nvergence generated shortening of the forearc basin and the backthrusting o f the SHB; meanwhile a classical system of paired subduction (Philippine Tr ench) and strike-slip fault (Philippine Fault) was installed. The transitio n from Lithospheric subduction to crustal overthrusting is located where th e Philippine Trench sensu stricto begins, and also coincides with the off-s hore extension of the Philippine Fault. We observe a reversal of the thrust s from an eastward vergence in the Molucca Sea to a westward vergence in Mi ndanao 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 zo ne which marked the northern termination of the Molucca Sea, and drags nort hward a sliver of the previously accreted SHB. The northward drifting of th is sliver created an extension, which, however, cannot account for the grav ity low. We propose that the shortening and the uplift of the upper plate w ere induced by the buoyancy of the subducted unit (SHB), and triggered the thrust reversal. (C) 1999 Elsevier Science B.V. All rights reserved.