2003 Seattle Annual Meeting (November 2–5, 2003)

Paper No. 14
Presentation Time: 11:40 AM


FISHER, G. Reid, STEVENSON, Ian M. and ANDERSON, Peter C., Pacific Geotechnical Engineering, 16055-D Caputo Drive, Morgan Hill, CA 95037, pacificgeo@aol.com

The Berrocal fault (BF) is one of several NW-trending thrusts along which the Santa Cruz Mountains are overriding young sedimentary rocks of the Santa Clara Valley.

We found the easternmost fringe of the BF at the far west end of our 43m-long-trench, where it is a 0.5m-thick zone of pervasive platy fabric in a clayey sand matrix rich in tabular and lenticular clasts derived from the Mesozoic Franciscan Complex. In the trench, the BF has an orientation of 154/34SW; it overrides intact Plio-Pleistocene Santa Clara Fm. sandstone. No polished surfaces or slickenlines were seen. The fault zone thickness is unknown, though outcrops of Franciscan graywacke and shale occur within 25m west of the trench. The 0.75m-thick modern colluvium with very weak soil development truncates this fault zone, leaving the timing of last movement open.

About 23m east of the BF in the trench, a semi-disaggregated mass approximately 10m long and 1.8m thick and composed largely of Franciscan chert overlies gray sandy clay containing angular chert fragments and rootless isoclinal folds. This clay and chert in turn overlie a buried colluvium and Santa Clara Formation bedrock. The clay and chert represent fault gouge and Franciscan chert ejected from the thrust system onto a NE-dipping land surface, then reactivated as a landslide riding on the gouge.

At the eastern end of the trench, a west-dipping normal fault downdrops the landslide deposits against the Santa Clara Fm. Cumulative offset across the fault is less than 1.5m. The 2.0cm-thick fault is oriented 143/54SW (dip variable) and compositionally reflects adjacent units. It lacks platy fabric, comminuted grains, slickenlines, or other signs of extensive shearing, though discontinuous polished surfaces are present. We interpret this fault and scattered clay-lined vertical fractures to reflect coseismic extension (spreading), and not to be structurally rooted.

We were lucky to see these relations within trenchable depth; among other scenarios, it would have been easy to misidentify the coseismically-faulted toe of landslide deposits as the BF.