TRANSCONTINENTAL GEOLOGIC AND GEOPHYSICAL CROSS SECTION II: WESTERN SEGMENT

A geologic cross section across the southern United States near latitude 36° provides an up-to-date working model for the structure of the North American plate and its interactions with the upper mantle asthenosphere. The cross section also provides an educational tool that synthesize the 4-D structure and evolution of the continental US portion of the North American continent. The western half extends from the California borderlands to western Oklahome (Longitude 123 to 99W) and extends across the coastal Franciscan accretionary complex, the Salinia forearc microplate, the San Andreas fault, the Cretaceous forearc basin, the Sierra Nevada Mesozoic magmatic arc, the eastern California shear zone of the Basin and Range province, the Sevier back-arc fold and thrust belt, the Colorado Plateau, the Rio Grand rift, the Rocky Mountains, and the western Great Plains. A theme of the cross section is the heterogeneous nature of continental crust and mantle lithosphere which preserves a 2 billion year history of lithospheric assemble (1.8-1.4 Ga), rifting (750-500 Ma), and ocean – continent plate collisions. Crustal thickness varies from 25 to over 50 km in different crutal provinces. Relative to eastern U.S., western US is underlain by overall thinner lithosphere, low velocity upper mantle, and large velocity variations that are interpreted to indicate dynamic mantle flow, for example with lithosphere sinking beneath the Sierra Nevada and asthenosphere upwelling beneath the Basin and Range and in the Rio Grande rift. This cross section depicts the structure and evolution of a continental plate through time; it shows a tectonically active domain in which plate boundary deformation is inducing and interacting with a complex intraplate deformational field in a 1000-km-wide uplifted orogenic plateau that extends as far east as the Great Plains. This wide deforming plate margin domain provides a field laboratory for understanding the processes of lithosphere- asthenosphere interactions that drive tectonism, magmatism, and dynamic topography.