RELATION OF FLAT SUBDUCTION TO MAGMATISM AND DEFORMATION IN WESTERN U.S.A

Through a variety of processes, flat subduction beneath western US (wUS) is largely responsible for the region's deformation, uplift and magmatism since ~100 Ma.

Initial slab shallowing was caused by low pressure in the wUS asthenosphere resulting from the combined effects of rapid subduction and an inhibited asthenospheric flow toward the subduction zone (which was blocked by the cratonic root). The low pressure pulled down the continental interior and pulled the North America craton rapidly to the west. Final slab flattening resulted from oceanic plateau subduction, which ripped beneath southern California.

Flat subduction: (1) compressed the continental interior (creating Laramide uplifts). NE motion of the Colorado Plateau shortened Proterozoic lithosphere in central Colorado and New Mexico; upper mantle seismic images suggest weakening of a 200 km-thick lithosphere in the shortened areas. In contrast, the widely spaced uplifts in Archean lithosphere suggest a lower crustal decoupling above a strong lithosphere (also imaged at ~200 km thick). (2) cooled and hydrated the base of the lithosphere. This largely amagmatic hydration made the lithosphere buoyant and fertile, preparing it for the Cenozoic. (3) underplated sediment beneath southern California and western Arizona, which grew in thickness and daylighted in a trench-parallel swath.

Slab removal is marked by the mid-Tertiary ignimbrite flareup, which propagated NW out of Sonora and SSE out of the Pacific Northwest, converging on southern Nevada. The southern California Transverse Ranges “drip” is the last of this slab removal. Exceptionally voluminous magmatism occurred where the hydrated mantle was fertile (outboard of the Paleozoic hingeline); the lithosphere weakened by this compositionally guided thermal event is now the Basin and Range.

Uplift of the wUS continues as the Farallon slab sinks and as continued magmatism both heats the lithosphere and creates buoyant residuum. Forces driving wUS extension result from (1) motion of the Pacific plate away from North America, (2) southern Cascadia rollback (2) high gravitational potential energy of the elevated interior, and (4) craton root drag (which shields wUS from ridge push compression). The occurrence of deformation at a significant rate is a result of the thermal weakening of this region.