Current Research in EEPS: Dr. Christine Regalla, Boston University
The northeast Japan margin: an example of slow accretion rather than tectonic erosion?
Nearly half of the world’s subduction zones have been classified as “non-accretionary” and are thought to experience net forearc mass loss over geologic time scales. In Northeast Japan, a type example a “non accretionary” or “erosive” margin, tectonic erosion along the plate interface at depth and near the trench has been argued to explain regional Miocene forearc subsidence, and the lack of a large, young accretionary prism. However, our analysis of upper plate deformation, plate kinematics, and mass fluxes at the trench suggest that the parameters previously thought to require tectonic erosion can be explained by alternate mechanisms. First, we find 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. Second, new analyses of 10Be geochronology and high-resolution seismic profiles in the outer wedge at the Japan Trench indicate tectonic erosion at the trench has been minimal over the Plio-Quaternary. Instead, these data indicate that much of the incoming Pacific plate sediments are actively accreted and internally deformed during horst and graben subduction. Third, 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. Collectively, 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.