STRUCTURAL CONSTRAINTS OF DEXTRAL OFFSET ON THE EAST SAN FRANCISCO BAY REGION FAULT SYSTEM AND ASSOCIATED VOLCANICS, CALIFORNIA

Dextral slip on the East San Francisco Bay Region Fault System (ESFBRFS) was inferred to be 160-170 km based on correlation of Tertiary volcanic occurrences inferred to be controlled by lithospheric gap migration (McLaughlin, et al., 1970). The distribution of this slip was proposed by Graymer et al. (2002) to be on and east of the Hayward fault. I present three new and two prior analyses of surface and subsurface geologic data and published analyses that constrain slip magnitude and distribution on the ESFBRFS.

• The Stockton Arch, as identified by Miocene sub-crop relationships in the Great Valley, continues west from the Great Valley to the Hayward fault and possibly beyond. The 60 km width and gently-dipping southern boundary of the arch limits its precision as a marker, but it provides context for other observations.

• South of the Las Positas Fault, near-vertical, north-facing Eocene strata intersect the Greenville Fault at high-angle and are offset dextrally only 3-4 km, much less than prior estimates based on the 15 km based on apparent offset of Franciscan mafic bodies in the Diablo Range.

• Juxtaposition of thick Neogene Livermore Basin strata with Franciscan rocks of the Diablo Range across the Las Positas Fault is generated by right step of about 10 km dextral slip from the Calaveras Fault to the Greenville Fault. Northward displacement of the Livermore Basin block is accommodated by shortening within the Southern Diablo Fold Belt and the Diablo Mountain Thrust. This isolated rift and compressional systems implies that motion on the Calaveras Fault is largely dip-slip north of the Las Positas Fault and explains why the Calaveras Fault terminates near Alamo with <5 km slip continuing to the north (Graymer, et al.2002).

• Published mapping limits dextral offset within the Oakland Hills block to <20 km and most likely <10 km (Wagner et al., 2021).

• Published analysis of the distribution and sources of distinctive conglomerate clasts (Graham et al., 1984) imply 7 – 26 km of dextral motion on the Hayward fault.

Summing the various offsets yields a total of 20 – 60 km of total slip on the Hayward fault and those to the east. I conclude that (1) slip on this portion of the ESFBRFS is 20 - 60 km and (2) prior correlations of certain Tertiary volcanics are either incorrect or that 100 – 150 km of slip occurs on faults west of the Hayward fault.