Abstract

East of the Oakhurst and Fine Gold metamorphic belts in the western Sierra Nevada foothills, saline water is believed to be leaking upward from an eastward-dipping fault which is truncated at depth by a carapace of granitoid rock. Saline water draining from the Sierran block is presumed to have migrated westward through metamorphic basement ultimately to rise along the buried fault, which appears to be the southeastward continuation of the Melones fault. Wells yielding the sodium chloride-rich water lie along a N30°W, 100-km-long trend referred to as the “Foothill lineament,” which lies east of, and subparallel to, the surface expression of the Melones. Estimates of thickness of the granitoid capping in the vicinity of Oakhurst and Fine Gold are obtained by applying the angle of dip of the Melones fault and the horizontal distance between the Melones and the saline lineament. Data from the Fine Gold metamorphic bell appear to be the most reliable because the average angle of dip (62° east) is reasonably consistent throughout the shear units comprising the fault zone. Calculated thickness along the saline lineament near Fine Gold ranges from 9.2 to 10.3 km for eastern and western shear zones and 9.7 km for the median position between shear units. These values show a startling correlation with an estimate of granitoid thickness of 9.8 km in the western foothills on the basis of data obtained from a northeast-southwest refraction seismic transect through the Sierra Nevada by Bateman and Eaton.

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