THE HISTORY AND MECHANISMS FOR LITHOSPHERIC GEODYNAMIC EVOLUTION OF THE SOUTHWESTERN NORTH AMERICA SINCE LATE EOCENE

The tectonic history of the southwestern North America since 40 Ma involves a complex transition from early shallow to flat-subduction of the east-dipping Farallon slab to its present transtensional environment. Through this evolution, as the boundary conditions evolved, topography and crustal thicknesses were dramatically altered from high elevations of orogenic plateaus and corresponding thick crustal welts to the current Basin and Range system that lies adjacent to the more stable Colorado Plateau and Rocky Mountains.The topographic collapse resulted in significant exhumation of deep crustal rocks. We use reconstructed paleotopography and paleocrustal thicknesses, along with kinematic estimates that incorporate both land-based geological observations and plate motion boundary conditions, to produce a geodynamic model of lithospheric deviatoric stresses and viscosity estimates from 36 Ma to present. Our dynamic model matches the full extension and shear history (metamorphic core complex stretch directions, Miocene fault and dyke orientations) and explains changes in strain localization through time. We show that high topography (high GPE) of Nevadaplano and Mogollon Highlands relative to a lower Colorado Plateau is required to match the extension directions and stress magnitudes in the belt of core complexes in Arizona and Nevada during 36 to 20 Ma. After the Nevadaplano and Mogollon Highlands collapsed, the influence of Pacific-North American plate boundary conditions became increasingly important from 15 Ma to present. Stress effects from smaller-scale mantle convection, such as slab rollback, played only a minor role. To account for the role of melts and fluids on lithospheric viscosity variations in our geodynamic estimates, we develop models of time-dependent upper mantle phase changes and effective water content variations. The temporal and spatial correlation between magmatism and the lowering of lithosphere effective viscosity in our model suggests that slab rollback likely played a critical role in lithosphere weakening through the introduction of heat, melts and fluids.