BASIN GEOMETRY AND EVOLUTION ALONG THE MAACAMA FAULT, RUSSIAN RIVER WATERSHED, NORTHERN CALIFORNIA

We examine the role of the Maacama fault (MF) in forming basins within the Russian River watershed using inversion of gravity data controlled by surface geology and borehole constraints. This watershed covers an area of 3900 km² and provides water to communities from Ukiah to Santa Rosa. The geology of the watershed consists of various rock types of widespread Mesozoic Franciscan Complex bedrock overlain by relatively shallow basins filled with Neogene continental sediments and, locally, volcanic rocks. The MF, a major strike-slip fault within the Pacific-North American plate boundary, bisects the northern half of the watershed. More than 450 new gravity measurements were collected in the Ukiah, Hopland, and Cloverdale areas. The Ukiah-Redwood Valleys coincide with an elongate 10-mGal gravity low reflecting thicker basin fill that is bisected by the MF. Another elongate 15-mGal gravity low in Alexander Valley is proximal to the MF and reflects a combination of thicker basin fill and less dense bedrock. The margins of these lows do not everywhere coincide with the active traces of the MF. The Hopland area coincides with a small rhomboid gravity low that is transected by a slight gravity ridge coincident with the MF, whereas gravity variations in the Cloverdale area to the south mainly reflect variations in the density of the Franciscan Complex basement. Because gravity values measured on bedrock vary by as much as 20 mGal throughout the watershed, we used an inversion to separate those variations caused by the basin fill from those caused by bedrock. The Alexander Valley basin is as much as 400 m thick, with its NE margin parallel to the MF. Valley fill beneath Cloverdale and Hopland are no more than 100-150 m thick. The mid-basin ridge coincident with the MF east of Hopland likely reflects contraction in a restraining bend. The basin fill beneath Ukiah is modeled to be as thick as 400 m, assuming that a small isolated basement outcrop that lies within the MF zone and the gravity low is rooted into low-density bedrock, or about 550 m if it is not rooted. In both cases, the thickest fill forms a linear trough on the western side of the valley that coincides with the MF only on its SE margin. This implies that the basin formed along older strands of the MF that may have a different effect on groundwater movement than presently active fault traces.