Reconciling GPS and Geologic Observations for Long-Term Deformation of the Cascadia Forearc
Mark Brandon, Yale University
There are two competing interpretations for long-term deformation in the Cascadia forearc. The "rotating block" interpretation (e.g., Wang, 1996; Wells et al., 1998; McCaffrey et al., 2000) argues that forearc deformation is dominated by clockwise-rotating blocks, driven by oblique convergence between the Juan de Fuca (JF) and North American (NA) plates. The "wide wedge" interpretation (Brandon et al., 1998; Pazzaglia and Brandon, 2001; Batt et al., 2001) argues that the forearc region, from the trench to the east side of the forearc high (Coast Ranges), is underlain by a growing subduction wedge, which is actively shortening in a margin-normal direction.
GPS data provide a test of these interpretations, but the influence of seismic coupling must be removed to resolve the long-term velocity field. McCaffrey et al. (2013) use the DEFNODE program to invert for a best-fit solution that includes spatially variable seismic coupling, and the rotation and internal deformation within discrete blocks. That solution shows that a block-based model can fit the GPS data, but the model is strongly undetermined, so it seems reasonable to consider other solutions.
I have devised a simpler model that provides a more direct test of the wide-wedge interpretation. The observed GPS velocities are defined relative to NA. Coupling at the plate boundary causes velocities to be perturbed in the local direction of JF relative to NA. Coupling is considered to be caused by both seismic coupling, and by tectonic coupling, due to shortening within the wedge. The modern Euler pole for JF relative to NA is poorly resolved because it is based on a plate circuit through the Gulf of California (PVEL in De Mets et al., 2010). My inverse solution finds a new best-fit solution for this Euler pole using the direction of JF/NA plate motion provided by local coupling, and the prior information from De Mets et al. (2010). The solution fully accounts for the observed velocity field in Washington and Canada, without any need for margin-parallel shortening. The solution removes the influence of coupling in northern California and western Oregon, and reveals a wide shear zone that strikes obliquely across this region. The shear zone is consistent with the present NW motion of California due to extension across the Basin and Range. The rotated paleomagnetics directions are also explained by distributed shearing rather than rotation of discrete blocks.