Cessation of Slip on the Pilarcitos Fault and Initiation of the San Francisco Peninsula Segment of the (Modern) San Andreas Fault, California

The Pilarcitos Fault (PF) is widely regarded to be the long-term (3-19 Ma) trace of the San Andreas Fault on the San Francisco Peninsula. The active modern San Andreas Fault (MSAF) in this area, however, records less than 3 m.y. of displacement. Middle-Miocene and younger displacement is partitioned to a single San Andreas Fault where the PF and MSAF merge in the Santa Cruz Mountains south of Palo Alto and Monte Bello Ridge. Aside from such general constraints, evidence for the timing of initiation of the MSAF and for cessation of slip on the PF are poorly constrained. As a consequence, there is a lack of consensus on the long term slip rates for the PF as well as the MSAF. Two independent datasets indicate significantly different offsets along the MSAF since 3 Ma. In one dataset distinctive 3.3 Ma (Blancan) fluvial gravels of the Santa Clara Formation are offset 28-31 km from their clast source east of the MSAF near Loma Prieta, yielding a slip rate of 9-10 mm/yr since 3 Ma. In the other dataset, a linear magnetic anomaly associated with Mesozoic serpentinite and gabbro of the Coast Range Ophiolite is offset 22 km across the MSAF, yielding a slip rate since 3.3 Ma of 7 mm/yr. Both datasets assume cessation of slip on the PF by 3 Ma, but imply different amounts of offset for the MSAF. Both offset estimations also yield lower long term slip rates than the present geodetic rate of 13-19 mm/yr. A closer review of these data reveals stratigraphic relations that indicate long term slip rates of 14-21 mm/yr for the PF and MSAF, which are more in line with geodetic rates. These long-term rates result from recognition that the distinctive Santa Clara Formation strata west of the MSAF, that were deposited in a pull-apart wedge between the PF and MSAF, are also underlain by the 5.4-7.0 Ma marine Purisima Formation and by middle Miocene and older marine strata. Along the east side of the MSAF the northwestern most exposures of the Purisima Formation underlain by Miocene marine strata are 74 km southeast of this pull-apart wedge, in the vicinity of the Sargent oil field northwest of Hollister. This relation requires about 43 km of pre-MSAF slip to have been taken up by a segment of the PF that we propose bounds Miocene strata in the fault wedge between the main PF and MSAF. The slip rate implied for the PF between 5.4 and 3.3 Ma is 20-21 mm/yr; the rate implied for the combined PF + MSAF since 5.4 Ma, is 13.7 mm/yr. These stratigraphic relations require a complex displacement history of incremental slip partitioning from the PF to the MSAF from 5.4 Ma until as late as 1.0-1.6 Ma, based on the long term rates for the PF and for the PF + MSAF. The discrepancy between the apparent offset of the Santa Clara Formation across the MSAF and offset of the magnetic anomaly by the MSAF are attributable to 9 km of Santa Clara Fm displacement that was taken up after 3 Ma by the PF south of the offset magnetic anomaly.