| Abstract |
Study of faults and permeability at Pumpernickel Valley resulted in a structurally-based conceptual model of the hydrothermal system that provided deep drilling targets. Subsurface structural patterns were interpreted from modeling that fit active-seismic, gravity, and surface mapping data. This combination provided useful constraints on the location, orientation, and offset of faults at depth and reduced the uncertainties related to gravity ambiguity and seismic noise. Comparison of the structural pattern with indicators of the shallow distribution of hot water revealed a subtle fault pattern – permeability correlation that suggests attractive Piedmont fault exploration targets buried in basin sediments east of the hot springs. Recent faults mapped at the Pumpernickel Valley geothermal prospect include the thermally-active, NNE-trending Pumpernickel Valley Range Front Fault Zone on the eastern flank of the Sonoma Range and an ENE to NE-trending fault at the range front on the southern flank of Edna Mountain. These faults intersect north of the hot springs. Fault offsets appear to be normal and right-lateral/ oblique respectively (Syzbinski and Shore, 2006). Integrated gravity-seismic interpretation shows that buried Piedmont faults occur under basin sediments SE of the intersection with orientations that proceed from NE to NNE to NE proceeding north, subparallel to the pattern of faults at the range front. The Piedmont faults are roughly 1 and 2 kilometers east and south of the range front. The hydrothermal system occurs in two or more of the NNEtrending range front fault zone segments between areas with NE fault intersections or NE fault trend deviations to the north and south. This pattern suggests that permeability occurs on the NNE faults and tightens in the NE faults consistent with the critical stress-fault orientation and shear-extension fault permeability models (Barton et al, 1995 and Sibson, 1998). The Piedmont faults may also show permeability in a similar pattern. The NNEtrending fault segments provide targets for exploration drilling. |