DEFORMATION OF THE CASCADIA CONVERGENT MARGIN, U.S.A. – FROM MINUTES TO MEGA-ANNUMS

The Juan de Fuca plate subducts beneath the U.S. and Canada along the Cascadia subduction zone (CSZ), a remnant of the Farallon subduction zone now caught between the larger Pacific and North America plates. The CSZ is active and produces great megathrust earthquakes, arc volcanism, and a seismically active upper plate. The GPS velocity field of the Pacific Northwest shows a smooth, regional clockwise rotation of the upper plate that reflects the influence of northward Pacific plate motion and confirms decades of paleomagnetic study. McCaffrey et al. (2007, 2013) have produced block models from the GPS data that solve for rotation and elastic deformation due to locking on block boundaries, and they reveal a locked megathrust and secular clockwise rotation of upper plate blocks, with coastwise migration of the forearc at about 6 mm/yr at Astoria.

These modern block motions have persisted for millions of years, as they match coastwise velocities of 6-8 mm/yr at Astoria predicted by the paleomagnetically based microplate models of Wells et al. (1998, 2001). Inversion of geologic fault slip data from coastal CRB outcrops and Quaternary slip in paleoseismic trenches both indicate N-S shortening of the forearc, consistent with modern earthquake focal mechanisms. These data show the forearc has been moving north for at least 16 m.y. and is being squeezed against slower moving Canada.

Spatial variations in rotation rate from the GPS array and from Cenozoic paleomagnetic data show a westward increase toward the plate boundary (McCaffrey et al., 2007; Wells and McCaffrey, 2013), suggesting a small right lateral shear overprint on the Cascadia convergent margin from Pacific-North America plate motion. The forearc sliver is not bounded by a through going right lateral fault, but distributed right lateral faults are known from potential-field and lidar mapping, with Holocene activity recognized in CA, OR, WA, and Vancouver Is. The fault bounded rotating forearc blocks may also affect megathrust behavior, as major forearc faults appear to provide a pathway for fluid to escape from the over-pressured megathrust, locally reducing the occurrence of episodic tremor at 25 to 40 km depth.