GENETIC RELATIONS BETWEEN THE CENTRAL AND SOUTHERN PHILIPPINE TRENCH AND THE SANGIHE TRENCH

We surveyed the junction between the central and southern Philippine T rench and the Sangihe Trench near 6 degrees N using swath bathymetry, gravity, and magnetics. These data, along with seismicity, allow us to discuss the genetic relations between these trenches and the forces a cting on converging plates. Our final model favors the northern extens ion of the Halmahera Arc up to 8 degrees N, with three segments offset left-laterally along NW SE transform faults. Accretion of the norther n segment to Mindanao Island 4 to 5 m.y. ago resulted in the failure w ithin the Philippine Sea Plate east of the arc. Initiation of the Phil ippine Trench between 7 degrees N and 10 degrees N agrees with the max imum recorded depth of the Philippine Trench floor (10,000 m below sea -level) and Philippine Sea slab (200 km). South of 6 degrees N (trench junction), another segment of the are is being subducted beneath the Sangihe margin, while south of 3 degrees N, the southern segment of th e Halmahera Are is still active. The rapid southward shallowing of the trench floor along the southern Philippine Trench, the type of faulti ng affecting both sides of the trench, the lack of significant interpl ate seismicity, and the concentration of the seismicity beneath the Mi angas-Talaud Ridge are interpreted as a slowing down of the subduction along this branch of the Philippine Trench compared with the rest of the subduction zone. The Sangihe deformation front has been recognized up to 7 degrees N but seems active only south of 6 degrees N.