| Summary: | This dissertation uses a finite element technique to explore the role of viscoelastic behavior in a wide range of plate tectonic processes. We consider problems associated with spreading centers, earthquake triggering, and subduction zone dynamics. We simulated the evolution of a slow-spreading center upon cessation of active spreading in order to predict long-term changes in the axial valley morphology. Results suggest that the axial valley created at a slow-spreading center persists because the crust is too strong to deform ductily and because no effective mechanism exists to reverse the topography created by rift-bounding normal faults. These results suggest that the persistence of axial valleys at extinct spreading centers is consistent with a lithospheric stretching model based on dynamic forces for active slow-spreading ridges. In our study of earthquake triggering, results suggest that if a ductile lower crust or upper mantle flows viscously following a thrust event, relaxation may cause a transfer of stress to the upper crust. Under certain conditions this may lead to further increases and a lateral expansion of high Coulomb stresses along the base of the upper crust. Analysis of experimentally determined non-Newtonian flow laws suggests that wet granitic, quartz, and feldspar aggregates may yield a viscosity on the order of 1019 Pa-s. The calculated rate of stress transfer from a viscous lower crust or upper mantle to the upper crust becomes faster with increasing values of the power law exponent and the presence of a regional compressive strain rate. In our study of subduction zone dynamics, we model the density and strength structures that drive the Nazca and South American plates. Results suggest that chemical buoyancy and phase changes associated with a cool subducting slab strongly influence the magnitude of driving forces, and the downgoing slab behaves weaker than the strength that would be expected based solely on temperature. Additionally, results suggest that large stresses are produced on the western margin of South American due to forces associated with asthenospheric cornerflow. These forces may be responsible for the high topography of the South American Cordilleran.
|
|---|
