The northeast Japan margin: an example of slow accretion rather than tectonic erosion?

Abstract:

Tectonic erosion at convergent plate boundaries is invoked to explain the evolution of nearly half the world’s subduction zones. In northeast Japan, basal tectonic erosion is argued to explain regional Miocene forearc subsidence and the lack of a large, young accretionary prism. However, new analysis of upper and lower plate kinematics in northeast Japan suggest that both of can be explained by alternate mechanisms. New analysis of the timing and kinematics of deformation demonstrate that the onset of Miocene forearc tectonic subsidence was coeval with the initiation of upper plate extension and subsidence associated with the opening of the Sea of Japan, and with a rapid acceleration in local plate convergence. The coincidence of tectonic events across the upper and lower plates suggest they are both a response to lithospheric-scale processes, and that forearc tectonic subsidence represents a response to changes in plate boundary geometry, rather than tectonic erosion. New high resolution seismic profiles and ¹⁰Be geochronology of the outer wedge at the Japan Trench indicate that much of the incoming Pacific plate sediments are actively accreted, and while subducting horst and graben topography severely deforms the frontal prism, it is not an efficient mechanism for frontal tectonic erosion. The presence of a small, Plio-Quaternary accretionary wedge and the eastward younging of basal slope basin sediments argue that the outer forearc crustal structure could be explained as the result of slow, but relatively continuous frontal accretion throughout the Cenozoic. These data suggest that the northeast Japan margin has not experienced large quantities of tectonic erosion, but instead experienced: 1) forearc subsidence caused by plate velocity driven changes in slab geometry, and 2) slow accretion and frontal wedge growth disrupted by subduction of seafloor topography.