Tanya Atwater Recent Publications



Plate Tectonic History of Southern California with Emphasis on the Western Transverse Ranges and Northern Channel Islands



The Channel Islands occupy the southern edge of the Transverse Ranges block and thus the islands share and illuminate the history of this block. In the Mesozoic and early Cenozoic, the Transverse Ranges block was oriented north-south, so that the Channel Islands probably lay near San Diego. The block occupied the forearc region of a subduction zone, collecting continental shelf sediments equivalent to those in the Great Valley belt farther north. In the mid-Cenozoic, a spreading center approached the trench in this region, contributing to the uplifts recorded by the Sespe Formation terrestrial rocks. Between 27 and 18 Ma, the Pacific plate made contact with North America and continental pieces began to break off and join the Pacific plate, gradually establishing the modern San Andreas plate boundary.
    In the early stage of plate boundary evolution (Miocene, 18-12 [-5?] Ma), the northern plate boundary lay within and inboard of the Salinian block, but then bent south-westward through the Southern California Borderland in a transtensional geometry. This transtensional phase is responsible for the extension and reconfiguration of the continental rim, including the onset of rotation and left lateral shearing of the Transverse Ranges block and deposition of San Onofre-type breccias, eruption of Conejo-age volcanics, and deposition of marine basin sediments including the Monterey and Sisquoc Formations.
    In a late stage (Plio-Pleistocene, 5-0 Ma), Baja California joined the Pacific plate and began to obliquely ram into southern California. This transpressional phase is responsible for ongoing folding, uplift, tilting, and faulting of the modern Channel Islands.



Pacific-North America Plate Tectonics of the Neogene Southwestern United States - An Update



We use updated rotations within the Pacific-Antarctica-Africa-North America plate circuit to calculate Pacific-North America plate reconstructions for times since chron 13 (33 Ma). We find that the direction of motion of the Pacific plate relative to stable North America was fairly steady between chrons 13 and 4, and then changed and moved in a more northerly direction from chron 4 to the present (8 Ma to the present). No Pliocene changes in Pacific- North America plate motion are resolvable in these data, suggesting that Pliocene changes in deformation style along the boundary were not driven by changes in plate motion. However, the chron 4 change in Pacific-North America plate motion appears to correlate very closely to a change in direction of extension documented between the Sierra Nevada and the Colorado Plateau. Our best solution for the displacement with respect to stable North America of a point on the Pacific plate that is now near the Mendocino triple junction, is that from 30 to 12 Ma the point was displaced along an azimuth of about N60°W, with a rate of about 33 mm/yr; from 12 Ma to about 8 Ma the azimuth of displacement was about the same as previously, but the rate was faster (~52 mm/yr); and since 8 Ma the point was displaced along an azimuth of N37°W with a rate of about 52 mm/yr.
    We compare plate circuit reconstructions of the edge of the Pacific plate to continental deformation reconstructions of North American tectonic elements across the Basin and Range province and elsewhere in order to evaluate the relationship of this deformation to the plate motions. The oceanic displacements correspond remarkably well with the continental reconstructions where deformations of the latter have been quantified along a path across the Colorado Plateau and central California. They also supply strong constraints for the deformation budgets of regions to the north and south, in Cascadia and northern Mexico.
    We examine slab window formation and evolution in a detailed re-analysis of the spreading geometry of the post-Farallon microplates, 28-19 Ma. We find that the development of the slab window seems linked to early Miocene volcanism and deformation in the Mojave Desert, although detailed correlations await clarification of early Miocene reconstructions of the Tehachapi mountains. We then trace the post-20 Ma motion of the Mendocino slab window edge beneath the Sierran-Great Valley block and find that it drifted steadily north then stalled just north of Sutter Buttes about 4 Ma.

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