Abstract
AbstractWe 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 50–70 km from the ridge axis with sparse fault plane reflections confined to the upper crust. Plate bending due to sediment loading and subduction initiates at ~120–150 km and ~65–80 km 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 40 km from the DF; on the Oregon transect, bright fault plane reflections that extend through the crust and 6–7 km into the mantle are observed. If attributed to serpentinization, ~0.12–0.92 wt % water within the uppermost 6 km 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°–40°) lower crustal reflections are imaged in ~6–8 Ma 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 8.5 Ma and 5.9 Ma.
Highlights
At subduction zones, water stored and transported with the descending oceanic plate is gradually released at depth, strongly influencing the generation of earthquakes and arc magmatism [e.g., Hacker et al, 2003; Bangs et al, 2004; Hyndman, 2007; Plank et al, 2013]
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
The Oregon transect spans ~390 km from Axial Volcano to the deformation front at ~44.6°N (Figures 2a, 3a, and S3a). Along this transect the oceanic plate is almost fully buried by sediments except for Axial Volcano and Son of Brown Bear Seamount located 35 km east of the Juan de Fuca Ridge (JdFR) axis [Smith et al, 1994]
Summary
Water stored and transported with the descending oceanic plate is gradually released at depth, strongly influencing the generation of earthquakes and arc magmatism [e.g., Hacker et al, 2003; Bangs et al, 2004; Hyndman, 2007; Plank et al, 2013]. Faults in the oceanic plate first develop near the axis of mid-ocean ridges, contributing to the formation of the undulating ridge flank abyssal hill topography [Carbotte and Macdonald, 1994; Macdonald et al, 1996]. At fast- and intermediate-spreading ridges, where most downgoing plates of the circum-Pacific subduction zones were formed, these faults are believed to be confined to the uppermost 2–3 km of the crust and cease growing within a few tens of kilometers from the ridge [e.g., Macdonald et al, 1996; Bohnenstiehl and Carbotte, 2001]. Preexisting abyssal hill normal faults may be reactivated and/or new faults develop where bending stresses exceed the yield strength of the plate [Billen et al, 2007]. Geophysical studies (mainly active source seismic studies) at different subduction zones have shown that these subduction bending-related faults may extend to the HAN ET AL
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