LONG TERM DISPLACEMENT ACROSS THE BARTLETT SPRINGS FAULT ZONE: IMPLICATIONS FOR EVOLUTION OF THE EAST SAN FRANCISCO BAY FAULT SYSTEM AND STRUCTURAL HOOKS IN THE NORTHERN COAST RANGES, CALIFORNIA

The Bartlett Springs Fault Zone (BSFZ), a major right-lateral fault zone in the northeastern California Coast Ranges, extends southward from north of Arcata to Lake Berryessa, California and steps southwestward from there, to the Green Valley and other active faults of the East San Francisco Bay (ESFB) Fault System. The BSFZ clearly displays evidence of late Cenozoic surface displacement, seismicity and fault creep. As such, it represents a revised northeast boundary of the “Humboldt Plate” of Herd (1978) and is an integral part of long-term displacement partitioned to the subduction margin from the San Andreas Fault via the ESFB Fault System. Long-term displacement along the BSFZ likely contributes to a part of the 170 km of cumulative slip taken up across the ESFB Fault System since 12 Ma.

Accommodation of some or all ESFB Fault System displacement north of the Mendocino Triple Junction since the Middle to Late Miocene, implies that the BSFZ part of the fault system may have initiated long before passage of the Mendocino Triple Junction and propagation of the San Andreas Fault to latitudes north of Lake Berryessa (< 3 Ma). The present creeping BSFZ thus may be superposed on a long-lived oblique subduction zone-related plate boundary fault. Other map relations show that the BSFZ is aligned along and contributes to dextral transposition of the southwest limb of the Wilbur Springs antiform, one of several dextral oroclinal bends (structural hooks) in Jurassic –Cretaceous rocks of the Coast Range Ophiolite and Great Valley Sequence along the southwest side of Sacramento Valley. These structural hooks probably formed in the early Tertiary. Unique Lower Cretaceous fossil cold seep deposits and ophiolitic mélange are right-laterally transposed a minimum of 47-50 km along the BSFZ in this area, leading us to propose a post-12 Ma slip rate of about 4 mm/yr. Pliocene volcanics and Mesozoic basement south of Lake Berryessa, however, are offset only about 20 km across the Green Valley Fault, which seemingly requires about 30 km of BSFZ offset to be accommodated by other faults of the ESFB Fault System, such as the Carneros and West Napa Faults, or other faults of the subduction margin.