SHALLOW RESISTIVITY SURVEYS CHARACTERIZE FAULTS WITHIN THE MAACAMA FAULT ZONE INDICATING A MORE COMPLEX ACTIVE FAULT SYSTEM

Shallow Resistivity surveys of 16 sites within the Maacama Fault Zone characterize fault geometries and locations that indicate a wider, more complex fault system that includes several active pull-apart basins. Faults mapped by previous researchers as active or possibly active on the east side of the Little Lake Valley have been more accurately characterized by shallow resistivity surveys, and more recently have been independently mapped using LIDAR data, increasing the likelihood that these are active faults. Other nearby faults, also identified in resistivity sections, have positions and orientations that suggest they are also active.

In the south end of Little Lake Valley, a resistivity section outlines coarse gravel deposits identified in nearby water wells. These gravels are interpreted to be deposited by a stream channel that was structurally influenced by the normal fault it abuts against. Numerous other resistivity surveys on the west and northeast sides of Little Lake Valley characterize faults or related features that bound the valley and are interpreted as evidence that the valley formed by pull-apart kinematics. Faults bounding the east and west sides of the valley are interpreted to include pre-existing subduction related fault zones that are reactivated as strike-slip faults in the San Andreas Fault System. These faults exploit weak serpentinite and talc zones in the Franciscan formation. Serpentinite and talc outcrop along linear trends on the northeast side of Little Lake Valley and align with topographic lineaments and an abrupt boundary identified in a resistivity section.

Several other resistivity survey sites near Ukiah identify faults that bound the east and west sides of the Northern Russian River Valley. These faults are interpreted in resistivity sections to have a normal component of displacement and to connect with faults determined to be active in surface creep and paleoseismic trench studies. The transfer of slip between faults widely accepted as active, and nearby faults identified in shallow resitivity sections suggests they too are active and that the active fault zone is a broader and more complex fault system. This more complicated system includes the pull-apart fault systems of the Little Lake and Northern Russian River Valleys.