Upper-plate deformation in southern Cascadia in response to the NNW-migration of non-subduction tectonic processes over the past 6 million years

Subduction zone forearc deformation in southern Cascadia is controlled by three dominant tectonic processes:

Cascadia subduction earthquake cycle deformation.

The northern migration of a wave of crustal shortening/uplift subsequently followed by subsidence driven by plate interactions at the Mendocino Triple Junction (MTJ).

A northward push from the NW-migrating Sierra Nevada-Great Valley (SNGV) block on the southern margin of Cascadia.

At present, crustal displacements produced by these three processes combine to produce the GPS displacement field observed in southern Cascadia. The interaction of these three processes produces NE- and NNW-directed shortening within the Franciscan Coast Ranges. Inboard of the Coast Ranges, the Klamath Mountains are moving to the NNW at ~8-12 mm/yr, and deforming at low shortening strain rates, in response to the push from the SNGV block to the south since ca. 6 Ma. However, this is a geologically recent plate boundary configuration. At ~10 Ma, the Klamath Mountains and adjacent terranes were centrally located along the Cascadia margin and were primarily experiencing NE-directed subduction-driven deformation. At that time superposed NE- and NNW-directed shortening was occurring within the Franciscan Coast Ranges near San Francisco, bounded on the east by the SNGV block.

Although geodetic data suggest that the Klamath Mountains are deforming at relatively low strain rates, they form a region of anomalously high topographic relief within the southern Cascadia forearc. We combine new 10Be measurements of erosion rates from 12 watersheds that span regions of high and low topographic relief within the Klamath Mountains with analyses of the topography, plate kinematic history, geodetic data, and seismic tomography to investigate how the Klamath Mountains terrane in the southern Cascadia forearc has responded to the onset of non-subduction-related deformation. We suggest that the anomalously high regions of topographic relief within the Klamath Mountains are a relatively recent feature within the southern Cascadia forearc, reflecting a rejuvenation in uplift associated with the onset of (MTJ and SNGV-driven) NNW-directed tectonics and related mid-crustal flow.