Morphology and tectonics of the Yap Trench

We conducted swath bathymetry and gravity surveys the whole-length of the Y ap Trench, lying on the southeastern boundary of the Philippine Sea Plate. These surveys provided a detailed morphology and substantial insight into t he tectonics of this area subsequent the Caroline Ridge colliding with this trench. Horst and graben structures and other indications of normal faulti ng were observed in the sea-ward trench seafloor, suggesting bending of the subducting oceanic plate. Major two slope breaks were commonly observed in the arc-ward trench slope. The origin of these slope breaks is thought to be thrust faults and lithological boundaries. No flat lying layered sedimen ts were found in the trench axis. These morphological characteristics sugge st that the trench is tectonically active and that subduction is presently occurring. Negative peaks of Bouguer anomalies were observed over the arc-w ard trench slope. This indicates that the crust is thickest beneath the arc -ward trench slope because the crustal layers on the convergent two plates overlap. Bouguer gravity anomalies over the northern portion of the Yap Arc are positive. These gravity signals show that the Yap Arc is uplifted by d ynamic force, even though dense crustal layers underlie the arc. This overl ying high density arc possibly forces the trench to have great water depths of nearly 9000 m. We propose a tectonic evolution of the trench. Subductio n along the Yap Trench has continued with very slow rates of convergence, a lthough the cessation of volcanism at the Yap Arc was contemporaneous with collision of the Caroline Ridge. The Yap Trench migrated westward with resp ect to the Philippine Sea Plate after collision, then consumption of the vo lcanic arc crust occurred, caused by tectonic erosion, and the distance bet ween the arc and the trench consequently narrowed. Lower crustal sections o f the Philippine Sea Plate were exposed on the arc-ward trench slope by ove rthrusting. Intense shearing caused deformation of the accumulated rocks, r esulting in their metamorphism in the Yap Arc.