PACIFIC NORTH-AMERICA PLATE-TECTONICS OF THE NEOGENE SOUTHWESTERN UNITED-STATES - AN UPDATE

We use updated rotations within the Pacific-Antarctica-Africa-North Am erica plate circuit to calculate Pacific-North America plate reconstru ctions for times since chron 13 (33 Ma). The direction of motion of th e Pacific plate relative to stable North America was fairly steady bet ween chrons 13 and 4, and then changed and moved in a more northerly d irection from chron 4 to the present (8 Ma to the present). No Pliocen e changes in Pacific-North America plate motion are resolvable in thes e data, suggesting that Pliocene changes in deformation style along th e boundary were not driven by changes in plate motion. However, the ch ron 4 change in Pacific-North America plate motion appears to correlat e very closely to a change in direction of extension documented betwee n the Sierra Nevada and the Colorado Plateau. Our best solution for th e displacement with respect to stable North America of a point on the Pacific plate that is now near the Mendocino triple junction is that f rom 30 to 12 Ma the point was displaced along an azimuth of similar to N60 degrees W at rate of similar to 33 mm/yr; from 12 Ma to about 8 M a the azimuth of displacement was about the same as previously, but th e rate was faster (similar to 52 mm/yr); and since 8 Ma the point was displaced along an azimuth of N37 degrees W at a rate of similar to 52 mm/yr. We compare plate-circuit reconstructions of the edge of the Pa cific plate to continental deformation reconstructions of North Americ an tectonic elements across the Basin and Range province and elsewhere in order to evaluate the relationship of this deformation to the plat e motions. The oceanic displacements correspond remarkably well to the continental reconstructions where deformations of the latter have bee n quantified along a path across the Colorado Plateau and central Cali fornia. They also supply strong constraints for the deformation budget s of regions to the north and south, in Cascadia and northern Mexico, respectively. We examine slab-window formation and evolution in a deta iled re-analysis of the spreading geometry of the post-Farallon microp lates, from 28 to 19 Ma. Development of the slab window seems linked t o early Miocene volcanism and deformation in the Mojave Desert, althou gh detailed correlations await clarification of early Miocene reconstr uctions of the Tehachapi Mountains. We then trace the post-20 Ma motio n of the Mendocino slab window edge beneath the Sierran-Great Valley b lock and find that it drifted steadily north, then stalled just north of Sutter Buttes at similar to 4 Ma.