Investigations of the mechanical properties of transform plate boundaries have dominantly focused on the crustal (seismogenic) component of the plate. Although the lithospheric mantle's role in plate boundary deformation has received much less attention, it represents the potentially strongest part of a plate boundary, and thus the region which will control the overall deformation. Previous studies have also suffered from the application of rheologic laws which may not be appropriate for the high strain rates and deformation at plate boundaries. The evolutionary nature of boundaries like the San Andreas has also normally not been included. This project will apply current best estimates of mantle rheologic laws to the development of transform plate boundaries and evaluate the 3-D stress, strain, and mineralogic (grain size, deformation mechanism) evolution of the region.