North America Dynamics and Western U.S. Tectonics

The origin of North America (NA) plate stress is resolved by finite element modeling the effects of plate margin, basal and internal loads. On a plate scale, “ridge push,” enhanced by unusually buoyant asthenosphere beneath the North Atlantic, compresses NA against the transform faults of western NA. Extensional stress created by high Cordilleran gravitational potential energy nearly overcomes this compression. Earth's interior couples most strongly to NA at the craton, where ~4 MPa of NE-oriented drag occurs; basal tractions elsewhere are ~10 times lower. Cratonic drag indicates NA is pushed over a relatively stagnant deeper mantle, separated from NA by a thin low-viscosity asthenosphere. Shear stress of 40-100 MPa acts on plate boundary faults. This stress is much larger than earthquake stress drops, but very low compared to lab-based expectations for frictional behavior. Weak western U.S. lithosphere causes deformation to distribute broadly under the influence of various forces. High Cordilleran gravitational potential energy drives Basin and Range extension and California Coast Range compression. Pacific-NA transform shear is superimposed on the continental margin; it entrains the Sierra Nevada block and drives western Great Basin shear. The wide Oregon block, whose west side is pushed northward by the Sierra Nevada block and oblique subduction, rotates within this shear zone. Southern Cascadia rollback makes the space necessary for Basin and Range extension and Sierra Nevada block motion. Western U.S. lithosphere is weak because of high temperatures and tensional stresses. High temperatures are created by recent magmatism, a result of asthenospheric contact with relatively fertile Basin and Range lithosphere that was hydrated during flat-slab Laramide subduction. Small-scale convection now occurs beneath most of western U.S. At a larger scale, recently subducted slab controls mantle flow (tomography suggests a Yellowstone plume ascends through a slab gap, SKS splits suggest toroidal flow beneath the Great Basin).