THE TOLAY FAULT ZONE: A REINTERPRETATION OF A MISUNDERSTOOD STRUCTURE

The Tolay Fault Zone (TFZ) is a northwest-trending structure that has long been considered to be a northward extension of the Hayward Fault. It has been depicted on maps and in other publications as a dextral fault with many kilometers of displacement. Recent geologic mapping, tephrochronologic correlations, and radiometric dating have demonstrated little if any strike-slip displacement along the TFZ. Geologic mapping has shown the TFZ to be a zone of disparate faults with a complex history. Extensive excavations during construction of the Infineon Raceway at Sears Point revealed the TFZ to be a 600 m wide zone of imbricate thrust faults that could be characterized as a schuppen structure with slivers of Franciscan rock interleaved with sheared siltstone and gravel of the Miocene Petaluma Formation. A few km to the north along Tolay Creek, Franciscan rocks and Tolay Volcanics (8-10 Ma) are thrust over the Petaluma Formation along east-verging, west dipping thrust faults. Similar relationships have been mapped at Meacham Hill north of Petaluma and at Taylor Mountain near Santa Rosa. The regionally extensive Roblar tuff (6.26 Ma) was sampled at several localities on both sides of the TFZ. The Roblar has been folded, locally overturned, and cut by the thrust faults of the TFZ indicating the deformation is ≤ 6.26 Ma. Palinspastic reconstructions based on the distribution of the Roblar tuff indicate that total displacement along the San Andreas Fault System is 308±18 km with at least 23 km attributed to the Rodgers Creek-Hayward faults. Correlation of the Tolay Volcanics with the Berkeley Hills volcanics suggests the total displacement along the Rodgers Creek-Hayward could be 40 to 45 km. A distinctive rhyodacite breccia near Sears Point has been correlated with a similar unit at Taylor Mountain, 28 km to the north, which corroborates the post-Roblar displacement along the Rodgers Creek-Hayward indicated by the tephra-based reconstruction. A fold and thrust belt similar to the one near Sears Point, occurs along the west side of Meacham Hill. It has been suggested this belt was displaced about 10 km northward along a cryptic fault beneath the Petaluma Valley, called the Petaluma Valley Fault. Faults with significant dextral displacement in this area are the Rodgers Creek, the Petaluma Valley and a proto-Hayward fault that must be to the west of the Petaluma Valley Fault.