Abstracts Category : Other

Faulting and Deformation at Divergent and Transform Plate Boundaries

by Samuel Howell

Ph.D. University of Hawaii at Manoa 2016. Transform tectonic plate boundaries reflect strike-slip interaction between plates of rigid lithosphere that generate many of the planets most harmful earthquakes. The first study presented focuses on vertical uplift surrounding the San Andreas Fault System. The horizontal interseismic motions of the fault system are largely predictable, but vertical motions arising from tectonic sources remain enigmatic. Careful statistical analysis isolates these tectonic signals from hydrologic and environmental noise, revealing the tectonic fingerprint of far-field flexure due to 300+ years of fault locking and creeping depth variability. At divergent boundaries, mid-ocean ridges accommodate the separation of plates and are responsible for most of Earths volcanism, linking fundamental magmatic depth to faulting and topography observed on the seafloor. The second study presented focuses on the intermediate spreading rate Chile Ridge. Specifically, we investigate abyssal hill creation and evolution and how variations in magmatic extension influence morphology and faulting along the spreading axis. Our findings indicate that changes in magmatic strain are accommodated though variations in fault slip, and that ridge morphology is inherently dependent on this relationship. Further, results suggest a form of magmatic segmentation that is partially decoupled from the geometry of the plate boundary. These along-axis variations and the magmatic segmentation exhibited at the Chile Ridge illustrate that mid-ocean ridge processes are inherently three-dimensional. Thus, the last study presented focuses on 3-D geodynamic numerical models of intermediate spreading rate mid-ocean ridges. We model a single ridge segment subjected to an along-axis gradient in the rate of magma injection. Our results illuminate the effects of lithosphere coupling both along and across the ridge axis on faulting and ridge morphology, and provide an explanation for the presence of smaller fault blocks near long-lived detachments faults. Further, models predict an axial valley to form at a depth that increases with the size of faults: individual faults depress the ridge axis, uplift its flanks, and the integrated effect of multiple faults produces most of the axial valley. These three studies motivate future research into post-seismic relaxation, the mechanics of fault weakening and healing, and magma budget variability along slow spreading ridges.