STRUCTURE OF SONOMA VALLEY, CALIFORNIA REVEALED BY GEOLOGIC AND GEOPHYSICAL MAPPING: A FAULT-BEND BASIN WITHIN THE RODGERS CREEK AND BENNETT VALLEY FAULT ZONES?

Sonoma Valley, California is bounded on its southwest margin by the Rodgers Creek and Bennett Valley Faults, part of the distributed San Andreas transform margin north of San Francisco Bay. Geologic mapping in the flanking mountains suggests that Sonoma Valley generally occupies the trough of a NNW-striking synform. Folding predates deposition of an undated rhyolitic flow sequence (most likely 3-5 Ma) that is exposed in the eastern mountains. However, geophysical data suggest a complex structure beneath the valley floor. Inversion of gravity data indicates that Sonoma Valley is underlain by two main basins (1-3 km deep) separated by a shallow bedrock ridge near the town of Glen Ellen. The northern basin (3 x 8 km) is centered near the town of Oakmont and has NW-striking margins that are probably fault-related. The southern basin is larger and consists of a series of N- and NW-striking sub-basins. A prominent, N-striking sub-basin underlies the western part of Sonoma valley. At its southern end the sub-basin appears to be bounded by the Tolay, Rodgers Creek, and Bennett Valley Faults. However, the western side of the sub-basin strikes north for about 10 km and truncates northwest-striking magnetic anomalies that lie west of the sub-basin. The eastern margin of the southern sub-basin is also generally north-striking and features a distinctive 2-km-wide north-striking magnetic anomaly. This basin-margin anomaly does not appear to be offset by strike-slip faults; the anomaly is produced by rocks at ~1-1.5 km depth, likely rocks from the Sonoma Volcanic field. We suggest that the southern sub-basin may have formed as a result of a releasing bend between the Rodgers Creek Fault and an unnamed fault that bounds the southwest margin of the Oakmont basin. Coincidence of complex magnetic anomalies with the gravity-defined basins may suggest the presence of fault-controlled volcanic centers. Thermal anomalies in ground-water also coincide locally with the basin-bounding faults.