ROLES OF SLAB WINDOW MIGRATION, FAULT GEOMETRY AND TECTONIC WEDGING IN CRUSTAL EVOLUTION EAST OF THE NORTHERN SAN ANDREAS FAULT

Migration of the Mendocino Triple Junction (MTJ) with consequent opening of a slab window in its wake is a widely accepted model to explain many aspects of the Neogene geology and geomorphology of northern California. However, some aspects of triple junction migration remain poorly understood, particularly the relative importance, timing, and extent of compression and extension east of and along the San Andreas Fault.

Large-scale tectonic processes that have been shown to influence the temporal variation in extension and contraction in northern California include clockwise rotation of the southern Oregon coastal block, WNW motion of the Sierra Nevada-Klamath (SNK) block, NW motion of the Pacific Plate, and coupling between the subducting Gorda-Juan de Fuca and North American Plates.

Extensional geologic features that have clearly evolved in the wake of the MTJ, with changing length and geometry of the San Andreas Fault, include right-stepped pull-apart basins within the Rodgers Creek, Healdsburg, and Maacama Fault Zones; the distribution of ~8.0-0.3 Ma volcanism; active and fossil hot springs; hydrothermal alteration and epithermal mineralization. Pull-apart basins that are segmented by W-NW-oriented thrusts or folds, that are right-stepped from older transpressional faults (e.g., Santa Rosa Plain), are responses to migration of the restraining and releasing bend geometry of the northern San Andreas Fault.

Other compressional features distributed along the north side or east of the slab window, however, are related to east vergent tectonic wedging of the Franciscan Complex into the Great Valley and SNK blocks. Initiated during subduction in the earliest Tertiary and still active beneath the Coast Ranges, these features include: SW-vergent thrust and reverse faults along the west side of the Great Valley; W-NW-oriented folds SE of the MTJ area and areas of fluid overpressure over the northern edge of the slab window. The latter includes mud volcanoes and artesian springs that vent CO2, methane and traces of He, with isotope signatures suggesting derivation from the deep crust, and in part, the mantle. The still active wedging may be in-part a consequence of convergence of the Coast Ranges with the SNK block, in addition to magma injection, extension and underplating to the west, above the slab window.