CONSTRAINTS ON EVOLUTION AND LONG-TERM SLIP RATES FOR EASTERN FAULTS OF THE SAN ANDREAS TRANSFORM BOUNDARY FROM TEPHROCHRONOLOGY, NEW ³⁹AR/⁴⁰AR AGES, GEOLOGIC MAP RELATIONS, GRAVITY AND MAGNETIC DATA

In northern California the structural evolution and long term slip rates of the right-stepping Rodgers Creek-Maacama Fault system during and following Neogene volcanism along the east side of the wide transform plate boundary are constrained by tephrochronology and ³⁹Ar/⁴⁰Ar dating, integrated with geologic mapping and aeromagnetic and gravity data.

Pliocene and younger slip on the Maacama Fault is constrained by offset of a geochemically distinct suite of silicic tephra deposits in the Sonoma Volcanics that erupted at 3.4 - 2.5 Ma from a caldera centered over Mount St. Helena in NW Napa Valley. ³⁹Ar/⁴⁰Ar ages and tephra geochemistry show that a section of the Mount St. Helena eruptive sequence and underlying gravels exposed on the SW side of the Maacama Fault Zone along the Geysers-Healdsburg Road, is offset right-laterally ~17 to 21 km from the Franz Valley area east of the fault zone, yielding a slip rate of ~6.0 ± 0.7 mm/yr since ~3.2 Ma.

Stratigraphically high dispersed tephra in synclinally-folded fluvial deposits in the Santa Rosa area correlate with the 1.2-0.8 Ma Bishop-Glass Mountain eruptive sequence of Long Valley. Flat-lying late Pleistocene and Holocene alluvium that overlies the folded strata was deposited in a pull-apart basin in the right step between the Rodgers Creek and Maacama faults. Slip on the Maacama and Rodgers Creek faults that accompanied formation of the pull-apart basin (~5 km for the Maacama Fault and ~6 km for the Rodgers Creek Fault, as indicated from basin geometry) yields a slip rate of ~5.3 ± 1.1 mm/yr for the Maacama Fault and ~6.3 ± 1.3 mm/yr for the Rodgers Creek Fault since 1.2-0.8 Ma. .

Possible earlier slip at a slower rate on the Rodgers Creek Fault Zone is suggested by a unique sequence of volcanic and sedimentary rocks, including coarse rhyodacite-sourced fault scarp breccia, deposited along a strike-slip fault basin margin (Cotati basin). Together with the axis of a prominent antiform, these rocks are offset ~ 28 km right-laterally along the Rodgers Creek Fault Zone. ³⁹Ar/⁴⁰Ar ages of the rhyodacitic rocks are ~7.3 to 8.2 Ma, and fluvial deposits (Petaluma Formation) west of the fault that overlie the fault scarp breccia include the 6.26 Ma Roblar Tuff. About 22 km of slip on the fault between 1.2 and 6.3 to 7.3 Ma, yields a slip rate of ~4.0 ± 0.4 mm/yr, much lower than rates since 1.2 Ma.