LATE CENOZOIC VOLCANISM AND BASIN FORMATION WITHIN A TECTONIC SETTING TRANSITIONAL FROM SUBDUCTION TO DEXTRAL STRIKE-SLIP, NORTHERN SAN FRANCISCO BAY REGION, CALIFORNIA

The Hayward Fault continues north of San Pablo Bay as the N35°-40°W trending, active right-stepped Rodgers Creek-Maacama fault system. The principal faults of this system, the Rodgers Creek-Healdsburg and Bennett Valley-Maacama fault zones together with faults bounding depositional basins, form an immature dextral system that is evolving within a complex releasing bend approximately 70 km long x 8 km wide. Several pull-apart basins having somewhat different releasing bend geometries (Santa Rosa, SE Alexander Valley, Little Sulphur Ck) are evolving within the more complex regional-scale structure.

The active N-NW-trending strike-slip fault system disrupts an older curvilinear system of ~N40°W- to N50°W-trending folds and thrusts parallel to the structural grain of the northern Coast Ranges that verge regionally to the NE or SW. This older thrust system is the likely southeastward continuation of the Pliocene and older subduction margin between the partially coupled Gorda and North American plates that extends onshore north of the Mendocino Triple Junction. Although formed earlier than the Rodgers Creek-Maacama fault system, these thrusts are seismically active, clearly cut the youngest (2.5-3.3 Ma) Sonoma Volcanics, accommodate uplift and influence the regional topography.

The distribution of Sonoma Volcanics that erupted in Napa Valley between 5 and 2.8 Ma follows a younging trend toward the NW, seemingly parallel to the subduction-related fault system. Earlier work by Fox on dike orientations suggests W-NW to E-SE extension during volcanism, compatible with present-day strike-slip. So although volcanic venting in the wake of the Mendocino Triple junction was in response to the stress field of the transform margin, volcanism has migrated northwestward along pre-San Andreas structures.

Another W-NW-trending, NE- and SW-vergent imbricate zone of inactive reverse and thrust faults splays from the southern Rodgers Creek Fault and extends to the San Andreas Fault near Jenner. Faults of this system have been proposed to have taken up strike-slip between the volcanics of Burdell Mountain and the coeval Quien Sabe Volcanics near Hollister between ~12 and 3.5 Ma (a proto-Hayward fault system). Most strike-slip along this fault system pre-dates the 6.3 Ma Roblar tuff and may be masked by the younger thusting.