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