Transverse Strike-Slip Faulting
Using sidescan sonar, seismic reflection profiles, and swath bathymetric data we have mapped a set of WNW-trending left-lateral strike-slip faults that deform the Oregon and Washington submarine forearc. Evidence for left-lateral separation includes offset of accretionary wedge folds, channels, and other surficial features; sigmoidal left bending of accretionary wedge folds, and offset of abyssal plain sedimentary units. Five of these faults cross the plate boundary, extending 5-21 km into the Juan de Fuca plate. Using offset of subsurface piercing points, and offset of approximately dated submarine channels, we calculate slip rates for these five faults of 5.5 to 8.5 mm/yr. Little or no offset of these faults by the basal thrust of the accretionary wedge is observed. Holocene offset of submarine channels and unconsolidated sediments is observed in sidescan records and directly by submersible
The
strike-slip faults are most likely driven by dextral shearing of the
subducting
slab and propagate upward through the overlying accretionary wedge. Tangential hydrodynamic drag caused by
oblique insertion of the slab into the mantle is a possible driving
mechanism. Four sinistral faults
observed in only the upper plate may be remanent traces of previous
basement-driven deformation.
Alternatively, a similar, though
unrelated dextral shear couple driven by interplate coupling may drive
these
faults, and may augment deformation of the upper plate for all the
sinistral
faults.
A model of
overall right-lateral simple shear of the submarine forearc is
consistent with
the observed surface faults, which may be R' or antithetic shears to
the
overall right-shear couple. The major
strike-slip faults define elongate blocks that, because of their
orientation
and sinistral slip direction, must rotate clockwise. We infer
that the deformation of the
submarine forearc (defined to include the lower plate) is highly
strain-partitioned into arc-normal shortening, and arc-parallel
strike-slip and
translation. The high slip rates of the
strike-slip faults, coupled with the lack of offset of these faults as
they
cross the plate boundary, imply that the seaward accretionary wedge is
not
moving at the expected convergence rate relative to the subducting
plate. We conclude that the accretionary wedge is
rotating and translating northward, driven by the tangential component
of Juan
de Fuca - North American plate convergence.
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