ACTIVE FAULTING AND UPLIFT OF AN EARLY CENOZOIC ANTIFORMAL STRUCTURE EAST OF THE MAACAMA FAULT, NORTHERN CALIFORNIA COAST RANGES

Large strike slip faults pose a significant seismic hazard in the northern California Coast Ranges, yet fault geometries are difficult to determine due to the heterogenous nature of bedrock and the long-lived, complex deformation history. Here we use two- and three-dimensional microseismicity analysis to identify fault geometry to 10 km depth, and channel steepness, chi, and chi-elevation channel profiles to identify patterns of active uplift along a section of the Maacama fault near Ukiah, California, in the northern Mayacmas Mountains. Geomorphic landscape analysis reveals zones experiencing active uplift that are located along the western side of the northern Mayacmas Mountains, suggested by topographic asymmetry of the range. Active uplift is further supported by elevated channel steepness up to 256.8 across the Maacama fault, chi gradients between adjacent basins of 15.3 to 7.2 m, and chi-elevation channel profiles with slopes between 10 and 60 degrees. Two- and three-dimensional distribution of microseismicity reveals two strands of the Maacama fault zone that dip steeply east, oriented at 334/68 NE and 336/76 NE (strike/dip); the western strand is newly identified and referred to here as the “Ukiah Valley fault”. A static stress model along both faults reveals a moderate to favorable orientation for slip, with a mean slip tendency of 0.47 ± 0.06. Microseismicity analysis also identifies two east-dipping reverse faults oriented at 331/57 NE and 332/54 NE, that core the Mayacmas range east of the Maacama fault. These faults coincide with the position of the early Cenozoic Cow Mountain – Ukiah antiform, and other older bedrock structures including the Mill Creek and Chicken Springs faults. Focal mechanism data suggests oblique slip on the Maacama fault zone and the reverse faults to the east drive uplift of the northern Mayacmas Mountains. These faults delineate a significant seismic hazard. Our study shows that the active faulting involves interactions of multiple strands, including favorably oriented, reactivated segments of older faults, formed during pre-strike-slip unroofing of the northern Coast Ranges.