Abstract: Is there More Quaternary Displacement on the Chabot Fault than on the Hayward Fault?

R. W. Graymer, D. L. Jones, E. E. Brabb

Faulted Quaternary gravels in the San Leandro Hills, situated between the Hayward fault and the Chabot fault, contain distinctive clasts that can be matched to bedrock sources only known to exist near Alum Rock, about 47 km to the south. These gravels rest on Jurassic keratophyre (part of the Coast Range ophiolite), which also crops out in the Alum Rock area. Diagnostic lithologies in the gravel consist of finely laminated black chert derived from the Claremont Formation, cobbles of keratophyre, porphyry volcanics reworked from Cretaceous conglomerates, and Franciscan detritus. Similar clast associations are found in the Pleistocene Irvington gravels, which outcrop only south of Niles Canyon. North of Niles Canyon, Quaternary gravels locally contain similar clasts, includ ng pebbles of siliceous shale and porcelainite derived from the Claremont Formation, but they appear to lack clasts of the characteristic black laminated chert that is well developed in the Alum Rock area.

These relations suggest that the San Leandro gravels have been displaced from the south by transcurrent right-lateral movement along the Chabot and related faults. Minimum displacement appears to be about 23 km (the distance to Niles Canyon), implying possible Quaternary movement rates considerably greater than estimates of current rates (~0.4 cm/yr.) on the Hayward fault. Complementing this interpretation is our discovery that identical lithic units north of Hayward occur on both sides of the Hayward fault and exhibit as little as a few kilometers of apparent offset. These findings suggests that the currently creeping Hayward fault is a very young feature and that more Quaternary displacement has occurred on other faults now deemed by some workers to he inactive.

AAPG Search and Discovery Article #90958©1995 AAPG Pacific Section Meeting, San Francisco, California