Seismic reflection imaging of the Juan de Fuca plate from ridge to trench: New constraints on the distribution of faulting and evolution of the crust prior to subduction

摘要

We present prestack time-migrated multichannel seismic images along two cross-plate transects from the Juan de Fuca (JdF) Ridge to the Cascadia deformation front (DF) offshore Oregon and Washington from which we characterize crustal structure, distribution and extent of faults across the plate interior as the crust ages and near the DF in response to subduction bending. Within the plate interior, we observe numerous small offset faults in the sediment section beginning 50-70km from the ridge axis with sparse fault plane reflections confined to the upper crust. Plate bending due to sediment loading and subduction initiates at similar to 120-150km and similar to 65-80km seaward of the DF, respectively, and is accompanied by increase in sediment fault offsets and enhancement of deeper fault plane reflectivity. Most bend faulting deformation occurs within 40km from the DF; on the Oregon transect, bright fault plane reflections that extend through the crust and 6-7km into the mantle are observed. If attributed to serpentinization, similar to 0.12-0.92wt % water within the uppermost 6km of the mantle is estimated. On the Washington transect, bending faults are confined to the sediment section and upper-middle crust. The regional difference in subduction bend-faulting and potential hydration of the JdF plate is inconsistent with the spatial distribution of intermediate-depth intraslab seismicity at Cascadia. A series of distinctive, ridgeward dipping (20 degrees-40 degrees) lower crustal reflections are imaged in similar to 6-8Ma crust along both transects and are interpreted as ductile shear zones formed within the ridge's accretionary zone in response to temporal variations in mantle upwelling, possibly associated with previously recognized plate reorganizations at 8.5Ma and 5.9Ma.