MEGA-LANDSLIDES IN NAPA VALLEY, CALIFORNIA

Napa Valley lies above a broadly defined structural synform, bounded on the west by a complexly faulted doubly plunging antiform that forms the Mayacamas Mountains and flanked on the east by a fold and thrust belt that forms the Vaca Mountains. Superposed on these regional structures, three well-defined landforms located north of the city of Napa and south of St. Helena suggest the presence of several mega-landslides: 1. At least three prominent nearly flat surfaces, at elevations of roughly 160 meters above the valley floor, 290 meters, and 480 meters, can be seen clearly in the Vaca Mountains, particularly viewed from the west. 2. A set of arcuate landforms revealed by high-definition air photos superposed on shaded relief 10-m DEMs suggest a group of nested landslide scarps. The largest of the inferred slides is about 12 km across and spans a distance of about 8 km from crest to toe. 3. The knobs and hills that rise above the flat floor of the Napa Valley may be the toes of the landslides to the east. We suggest that the flat surfaces in the hills are down-dropped stair-steps of an uplifted pediment surface that cut across volcanic layers of the approximately 4 Ma volcanic strata. Uplift, obviously younger than 4 Ma, resulted from a deeply buried, west verging thrust system that ramped up beneath the Vaca Mountains. A westward extend of the thrust system produced a "flat" that lies below the valley itself, and a final ramp with tip point that produced the anticlinal folding in the Mayacamas Mountains. The uplift of the Vaca Mountains exceeded the strength of the rock to support the elevated topography. The structural instability may have been further compromised during low stands of sea level when sediments delivered to the Napa Valley from the surrounding hills were swept from the valley and transported to the lower base level. With these conditions in place, the Vaca Mountains gave way and mega landslides moved down to the west into the valley. The up thrown toes of these slides now form the hills and knobs in the valley, which have been partially covered by sediment deposited since a new high-stand of sea level