TOPOGRAPHIC CONSTRAINTS ON THE SUBDUCTION EROSION OF THE YAP ARC, WESTERN PACIFIC

As a specific geological process, subduction erosion can move the substance of the earth crust and arc away from the forearc wedge and cause the missing of forearc crustal material, which generally occurs at the convergent plate boundaries. The Yap trench is an active subduction zone which is located at the convergence zone of the Caroline Plate and Philippine Sea Plate. According to the new multi-beam and seismic data that acquired by the integrated geophysical survey off the Yap Island, western Pacific, conducted by Institute of Oceanology, Chinese Academy of Sciences, we found evidence of erosion as follows: (1)The Yap trench has unusually short distance to the arc from the trench axis (41 km); (2)The cross-sections across the axis show asymmetry V-shape and the accretionary wedge is absent in the trench; (3)The relief of oceanic basement is severe, because ridges, sea mountains, structural belt of horst and graden exist in the oceanic plateaux of Caroline Plate; (4) The trench wall exhibits a steep slope and the mean forearc slope is 8.69° and the accretionary wedge is missing. We speculate that the mode of subduction erosion is different on both sides of the Yap trench. The subduction erosion of oceanic plateau in the northern Yap trench is mainly resulted from the friction between the structural belt of horst and graben and the overlying plate, and the structural belt of horst and graben initially might be filled with sediment, so there is a zone of basal erosion between the plates rather than direct contact; However, the subduction erosion of oceanic plateau in the southern Yap trench is mainly resulted from friction between the seamounts and overlying plate and the two plates contact directly.