GEOMORPHIC AND STRUCTURAL ANALYSIS OF THE VERONA-WILLIAMS-PLEASANTON FAULT ZONE AND IMPLICATIONS FOR SEISMIC HAZARD, EASTERN SAN FRANCISCO BAY AREA, CALIFORNIA

Folds and thrust faults adjacent to and beneath the Livermore Valley accommodate Quaternary crustal shortening between major faults of the eastern San Andreas fault system. The Verona and Williams faults are NE-dipping thrust or reverse faults that have uplifted the Pliocene-Pleistocene Livermore gravels along the western and southern margins of the valley. The Williams fault extends ~13 km northwest from the Mt. Lewis seismic trend to the sinistral Las Positas fault, which forms the southern margin of the valley. A 3-km left step along the Las Positas fault separates the surface traces of the Verona and Williams faults. The Verona fault extends ~8 km northwest from the stepover to southwestern Livermore Valley. The Las Positas fault may extend to the base of the seismogenic crust separating the Verona and Williams faults into two independent structures. Alternatively, the Verona and Williams faults may merge downdip into a common thrust fault plane, with the Las Positas fault confined to the hanging wall as a tear fault. The Verona and Williams faults exhibit geomorphic evidence for late Quaternary fault rupture propagating to or very near the ground surface. The Williams fault tightly folds and overturns the Livermore gravels, and appears to form scarps that impound late Quaternary alluvium and cross Holocene landslide deposits. Many Holocene(?) alluvial fans exhibit distinct convex longitudinal profiles across the fault trace suggesting active folding above the Verona fault. The geomorphic position of a stream-terrace remnant suggests that >7 m of tectonic uplift is possible across the Verona fault during the late Quaternary. Surficial geologic mapping and geomorphic analysis of the ancestral Arroyo Valle drainage system reveals numerous paleochannels that generally decrease in elevation (age) to the northwest, and provide useful isochronous markers delineating a subtle tectonic uplift in western Livermore Valley. This newly recognized uplift has lateral dimensions comparable to those of the Verona homocline, but generally is centered on or near the inferred trace of the Pleasanton fault. The paleochannels record tectonic uplift of the newly recognized “Pleasanton” anticline (or homocline) and reveal approximately 5 km of north-northwest fold propagation during the late Quaternary.