HOLOCENE ACTIVITY OF THE TRANCOS TRACE, SAN ANDREAS FAULT, PORTOLA VALLEY, SAN MATEO COUNTY, NORTHERN CALIFORNIA

We used approximately 158 meters of trench spanning the mapped Trancos Trace (TT) of the San Andreas fault (SAF) to test recent work suggesting the TT is absent or inactive in the vicinity of, and north of, our site.

In Portola Valley, the SAF consists of a western “Woodside” trace that ruptured in 1906, and an eastern inferred “Trancos” trace of hotly debated location and activity. Since 1998, various trenches have collectively spanned these traces and intervening ground, culminating in our investigation of the mapped TT.

Our trenches re-exposed faulted sediments found in a 1975 Woodward Clyde trench, and inferred by them to be of Eocene age. Immediately to the northwest along fault trend, we exposed a fault zone at least approximately 15 meters wide. From east to west, we encountered Franciscan Complex greenstone and serpentinite faulted against Plio-Pleistocene Santa Clara Formation (SCF), in turn faulted against a younger, flat-lying alluvial sequence. Dipping, laminated alluvium inferred to be upper Pleistocene overlies the SCF. Both the SCF and the laminated alluvium are faulted, with multiple events suggested. The western limit of the fault zone is fairly abrupt, faulting the laminated alluvium against the flat-lying younger alluvium. Overall, the stratigraphic throw is up-to-the-east, with the dominance of right-lateral shearing indicated by slickenlines and mullions plunging about 9 degrees northwest.

Preliminary 14C results confirm Holocene faulting, and soil development indicates the youngest faulted alluvium may be as young as approximately 1,400 years B.P. (G. Borchardt, Soil Tectonics consultant’s report), providing the first definitive evidence for activity of the TT. Recent right-lateral shearing and uplift on the TT is consistent with fault severance of the ancestral headwaters of Corte Madera Creek (CMC). These ancestral headwaters (modern Bozzo Gulch and upper Sausal Creek) originally drained eastward across the site into CMC, depositing the observed alluvium at the site. Shearing and uplift cut this connection and a fan system prograded out into Portola Valley. Subsequent stream capture of Bozzo Gulch and upper Sausal Creek by the modern Sausal Creek is evidenced by the deeply incised channel cut into now-abandoned fan surfaces. Portions of Portola Valley may be underlain by CMC alluvium.