Neotectonic significance of Franciscan Complex bedrock structure adjacent to the Maacama Fault Zone in the Ukiah-Hopland area, northern California

New geologic mapping across the creeping Maacama Fault Zone (MFZ) in the Ukiah-Hopland area has identified active and potentially active bedrock structures in the Central and Coastal belts of the Franciscan Complex. Previous studies have shown that ophiolitic mélange of the Coast Range Ophiolite and lower Great Valley Group in the Hopland and Geyserville areas, located on opposite sides of the MFZ, were formerly contiguous and then were displaced by about 22 km of right-lateral slip along the MFZ since 3.2 Ma. West of the MFZ, the Franciscan Central and Coastal belts are separated from each other by the E-dipping Coastal Belt thrust, which has been deformed by W-NW-oriented E-SE-plunging folds that have been dextrally sheared. The folds are cut by sub-parallel, vertical to low-angle faults, that include the Anderson Valley shear zone, which exhibits a diffuse NE-dipping zone of shallow seismicity.

The Central belt NE of the MFZ represents a structurally higher level of the belt than to the SW and is more coherent and continuous, as indicated by the prominent SE-plunging Ukiah-Cow Mountain antiform (UCMA) of the uplifted Mayacmas Mountains. The SE axial trend and plunge direction of the UCMA are defined by a NE-dipping slab of basalt and chert of the Geysers-Marin Headlands terrane (GMHT) of the Central belt and a prominent associated aeromagnetic anomaly. Exposures of the GMHT and its associated aeromagnetic anomaly extend at least 46 km SE from Redwood Valley along the NE side of the UCMA, and then bend SSW around the plunging nose of the antiform east of Hopland.

The UCMA is in the hanging wall block of the Chicken Springs fault zone, a folded and sinuous NE-dipping intra-Franciscan zone of thrusts. The fault zone is delineated by a narrow belt of serpentinite that dips northeastward beneath the UCMA, and extends NW to the more northly-trending, younger MFZ. Well-documented links of serpentinite to fault creep, together with the orientation and position of the UCMA along the crest of the Mayacmas Mountains, leads us to suggest that transpression has been taken up east of the MFZ by the Chicken Springs and other buried faults in the hanging wall block beneath UCMA and contributed to ongoing uplift of the Mayacmas Mountains.