A recent marine geophysical survey in the northern Molucca Sea revealed the structure to be that of a classical active convergent margin. We observe from west to east a volcanic arc (Sangihe), a forearc basin resting on an outer ridge (the Molucca ridge), which serves as a buttress for an accretionary wedge, and a composite downgoing plate (Snellius Ridge and Philippine Sea Basin). Gravity modeling indicates a strong negative anomaly above the wedge, which cannot be explained with reasonable density values. Modeling imposes a basement deepening and a rupture of the 700-km-long subducting lithosphere. This process individualized the lithospheric slab from the Snellius Ridge, which in turn was separated recently from the south Philippine Basin by the incipient Philippine Trench. This induces a deformation of the forearc region with backthrusting of the outer ridge and forearc basin, visible on bathymetry and seismic data. We extrapolate the tectonic emplacement of such oceanic blocks to the Oligocene times in order to explain the origin of the Pujada Miangas outer ridge as a sliver previously incorporated to the margin, and we discuss the possibility of this deformation process being fabric for terrane accretion. ¿ 1999 American Geophysical Union