PALEOSEISMIC INVESTIGATION OF QUATERNARY-ACTIVE FAULTING IN THE SOUTHWEST MONTEZUMA HILLS, SACRAMENTO – SAN JOAQUIN DELTA REGION, CALIFORNIA

Seismic sources and their associated hazards within the Sacramento-San Joaquin Delta region of California are poorly characterized, in part because fluvial and tidal processes obscure surficial evidence of faulting, and slip rates that combine to no more than ~5 mm/yr of dextral motion across the region. Local topographic highs in the western Delta – rising up to 90 m above the otherwise low-relief landscape – are often bounded by bedrock faults, implying that some structures accommodate vertical displacement, but the age and rate of this deformation is not known.

One of these local topographic highs, the Montezuma Hills, is an elevated area ~15 km from north to south by ~20 km from east to west, which we interpret as uplifted and eroded deltaic/estuarine deposits formed in an environment analogous to the modern Delta. Geomorphic analysis of the Montezuma Hills suggests uplift is asymmetric and concentrated toward the southwest. The Montezuma Hills are broadly bounded to the east by the Midland Fault; the Pittsburg – Kirby Hills Fault lies to the west of the Montezuma Hills but does not define the topographic edge, raising the possibility of another active fault system.

We report results from the Talbert Lane site, where a trench investigation confirms the presence of a fault at a break in slope along the topographic scarp that delineates the southwestern edge of the Montezuma Hills. Trench exposures show apparent east-side-up displacement, consistent with regional geomorphic indicators of uplift. The west-dipping fault juxtaposes sedimentary bedrock to the east against a ~5-m-wide massive colluvial zone, which transitions to gently westward-tilted estuarine strata. The western end of the trench exposes extensive soil development to at least 3 m depth, with evidence of multiple generations of soil formation. Our preliminary interpretations suggest that the observed fault is Quaternary active, but not unequivocally Holocene active. The exposed fault displacement likely represents multiple events, and upwarping of hangingwall strata suggests that broader folding accompanies on-fault displacement. Samples collected for age control, using a combination of luminescence, radiocarbon, and U-series dating techniques, will provide data to further constrain timing of on- and off-fault deformation.