ACTIVE TRANSTENSIONAL BASIN FORMATION ALONG THE MAACAMA FAULT ZONE, NORTHERN CALIFORNIA COAST RANGE

The Maacama Fault zone (MFZ) within the northern California Coast Range is the dominant onshore fault within the young northern portion of the Pacific-North America transform boundary and therefore offers the opportunity to study the early stages of evolution of a major plate-boundary transform zone. It is also of interest because the details of the seismic hazard and slip budget in this area, and the relationship of individual faults to the better studied faults of the San Francisco Bay area, are poorly understood. Geologic and geomorphic mapping, combined with (U-Th)/He in apatite thermochronometry, suggests that the MFZ comprises a complex set of anastamosing fault segments. Previously identified faults along the northeastern edge of the northern Russian River Valley (NRRV) and along the southwestern edge of the Little Lake Valley (LLV) are creeping rapidly, with theodolite measurements showing 6.5 mm/yr of creep west of Willits, CA and 4.4 mm/yr of creep east of Ukiah, CA (Galehouse, 2000). These rates are the highest measured in northern California, yet they account for less than half of the ~14 mm/yr of right-lateral shear accommodated by the MFZ (Freymueller et al., 1999). Newly identified fault strands, which surround the margins of the LLV and NRRV, as well as cutting through the Franciscan Complex rocks of the surrounding ranges, may accommodate the >7.5 mm/yr of unaccounted-for slip along the MFZ. These faults have strong geomorphic and geologic expressions, including large, steep fault scarps, clay gouge zones, and shear fabrics within unconsolidated gravels. The geometry of these faults suggests that they are kinematically linked within right-stepping zones along the MFZ, and are responsible for active subsidence of the LLV and NRRV. Further evidence of active transtension along the MFZ is provided by (U-Th)/He in apatite thermochronometry, which yields cooling ages of ~10 Ma for rocks along the east side of the LLV and NRRV, but older ages of ~20 Ma for rocks farther to the NE (near Round Valley, where apatite fission track dating yielded late Cretaceous to Paleocene ages; Tagami & Dumitru, 1996). The (U-Th)/He ages apparently reflect local structurally controlled variations in exhumation, although further work is necessary to fully constrain the effects of regional processes (e.g. uplift above a northward migrating slab window; Guzofski & Furlong, 2002).