LINKAGE BETWEEN UPPER-PLATE CASCADIA FAULTING AND THE EVOLUTION OF THE NORTHERN SAN ANDREAS PLATE BOUNDARY FAULT SYSTEM

The faults that accommodate Pacific - North America plate motion along the San Andreas plate boundary occupy structures that initially formed within the upper plate of the southern Cascadia subduction zone. Although these faults form within a subduction regime, ahead (north) of the Mendocino triple junction (MTJ), many of them form or adjust behavior to tectonics associated with the advancing MTJ. As a result of the northward migration of the MTJ, the Cascadia subduction zone is decreasing in length at a rate of ~ 50 km/Ma. We can correlate present-day fault development north of the MTJ with the San Andreas Faults south of the MTJ in order to discern the signature of this plate boundary evolution. New tomography imagery of this region of northern California provides key crustal constraints on deformation and fault localization, both within Cascadia, in advance (north) of the MTJ, and after (south of) the transition from subduction to translation. Using these tomographic images and analyses of GPS data within the region, we have developed a tectonic model that helps explain the present fault systems north and south of the MTJ. With this model, and the crustal structure constraints provided by the tomography, we can apply similar processes to the development of the newly forming San Andreas plate boundary over the past ~ 10 Ma. We find that the development of active faults in southern Cascadia (i.e. their location and orientation), such as the presently active Grogan and Lost Man faults (which GPS data indicate likely accommodate right-lateral shear in addition to shortening), plays an important role in determining where and how the major plate boundary structures form along the Pacific - North America plate boundary. Key to this evolution of the plate boundary, both for the faults forming north of the MTJ and for their transition to San Andreas faults south of the MTJ is the existence and position of the Great Valley crustal block. This crustal terrane acts as a backstop to Cascadia deformation (both shortening and shear) and constrains the crustal deformation associated with the MTJ. Our results lead us to argue that active, upper-plate faulting in southern Cascadia (i) includes faulting associated with the encroaching MTJ, and (ii) that these subduction regime faults provide structures on which the main San Andreas faults will later form.