FRAMEWORK OF THE RIGHT-STEPPED RODGERS CREEK - MAACAMA FAULT SYSTEM, NORTHERN SAN FRANCISCO BAY REGION, CALIFORNIA

Geologic mapping at 1:24,000 scale in the northern San Francisco Bay area, focuses on the dextral N-NW-trending Rodgers Creek, Healdsburg, and Maacama faults, young W-NW-oriented thrusts and NS-oriented normal faults. For several reasons this area is unique for addressing active tectonics. First, a relatively continuous record of late Miocene through late Pleistocene volcanism (Sonoma and Clear Lake Volcanics) and coeval nonmarine and marine sediments allows precision stratigraphy using tephrochronology and (⁴⁰Ar/³⁹Ar) radiometric dating. Secondly, the nonmarine and marine section preserves a nearly continuous 7 to 8 my. record of fluctuation in the position of the paleo-shoreline. Thirdly, the section records the stratigraphic and structural transition at this latitude, from subduction to the present transform setting.

Sedimentologic and kinematic data show that the 12 Ma to 2.5 Ma part of the volcano-sedimentary section predates encroachment of modern right-lateral faulting into the Santa Rosa plain northeast of Petaluma Valley. Other sedimentologic data suggest that syntectonic fluvial deposition in strike-slip basins along the Maacama fault northeast of Geyserville probably began 2 to 2.5 Ma. A younger, seismically active strike-slip basin with thin late Pleistocene-Holocene sediment fill underlies the city of Santa Rosa.

Deep basins filled with Miocene-Quaternary sediment northeast of the Rodgers Creek and Healdsburg faults (Napa-Calistoga, Knights,and Alexander valleys) are bounded by, or parallel with curvilinear folds and dextral-reverse faults that trend N-NW to the SE and W-NW to the NW. These thrust faults offset 2.8 Ma and older volcanics, are truncated by the dominant strike-slip faults, and may be associated with microseismicity having thrust mechanisms. Two other deep basins that lie beneath the Santa Rosa plain SW of the Rodgers Creek-Healdsburg fault zone (Windsor and Cotati basins), probably are segmented remnants of a forearc. These basins are bounded to the W by E-side-down normal faults, to the NE, by the Healdsburg-Rodgers Creek fault zone, and they are separated by a W-NW-oriented thrust with late Pliocene or younger offset. These relationships will provide a context for 3D modeling of crustal structure to address numerous earthquake hazards and water-related issues.