KINEMATIC AND GEOCHRONOLOGIC EVIDENCE BEARING ON PARTITIONING OF DEXTRAL SLIP BETWEEN THE RODGERS CREEK AND MAACAMA FAULTS, NORTHERN CALIFORNIA

Stratigraphic relationships and new geochronologic data from Neogene volcanic rocks in the northern San Francisco Bay area, suggest that active right-lateral strike-slip has been partitioned northeastward since ~3.0 Ma, from a W-NW splaying "proto"-Hayward fault system. The active Rodgers Creek fault is mapped northwestward from its junction with the "proto"-Hayward fault to Santa Rosa, where it right-steps NW across Rincon Valley, to the Healdsburg fault and also ~8 km northeastward, to the south end of the Maacama fault. The kinematic development of this wide extensional stepover is here examined in the context of the evolving San Andreas transform system.

Faults cutting the 2.5 Ma and older volcanic and sedimentary section within the extensional corridor locally exhibit well developed kinematic indicators. A composite of the orientations and slip vectors for 45 fault surfaces define an overall stress field with subhorizontal axes of compression (Pc=S10º E) and extension (Tc=N 84ºE) and a composite intermediate axis 69º from vertical, toward N 30º E.

Slip sense and fault orientations show significant variation, with dextral to sinistral, extension-dominated, NW to N-NE trends along the Healdsburg and Maacama faults, and W-NW to E-NE trending, extension-dominated faulting mixed with contractional faulting northeast of the Maacama fault. Northeast of Franz Valley, the composite P and T axes of slip vectors are rotated counterclockwise from faults in the extensional corridor to the SW, suggesting an eastward transition from extension- to contraction-dominated slip.

Together, the data suggest a kinematic model, in which northeastward slip is transferred from the Rodgers Creek to the Maacama fault zone in a N-NE-oriented corridor of subhorizontal E-W extension. This immature extensional corridor is flanked to the northeast, and interspersed with, upland areas deformed by contraction that largely predated the active extension. The contractional deformation probably formed in a pre-San Andreas, southern Cascadian forearc setting. Alternatively, active contractional deformation might result from convergence on both sides of the axis of active extension.