ORIGIN AND MECHANISMS OF SPRING DISCHARGE AT TECOPA HOT SPRINGS, SOUTHEASTERN CALIFORNIA, USA

Recent bedrock and surficial geologic mapping near Tecopa Hot Springs, SE California, and geochemical and isotopic data from springs and seeps place constraints on the origin of the spring waters and mechanisms of spring discharge. The Tecopa Hills expose a faulted and fractured, largely homoclinal section (mean bedding: 360°, dip 45°E) of Precambrian through Lower Cambrian strata comprising quartzite, shale, and lesser quantities of dolostone. Faulting styles include an older low-angle detachment fault that is displaced by younger N-S trending normal faults and later SW-NE trending vertical faults (mean vector: 70°). Prominent joint orientations include bedding-plane perpendicular sets in quartzite, as well as pervasive SW-NE trending vertical sets (mean vector: 81°). Reconnaissance investigation of similar-age strata in the Resting Springs and Nopah ranges reveals similar joint styles and slightly more NE trends (mean vector 56° for each range).

Hot and warm spring discharge in the Tecopa Hills area occurs mainly on the NE side of the hills and is aligned mainly along SW-NE trending faults, although minor seeps likely emerge from the SW-NE trending joint sets along much of the NE margin of the hills. Thermal spring water geochemistry ranges from type-1, slightly alkaline, Na+Cl+SO₄+HCO₃ water with moderate to high dissolved solids to type-2, pH-neutral to slightly alkaline, Na+Ca+Mg+SO₄+HCO₃ water with moderate dissolved solids. The type-1 waters have a compositional affinity to siliciclastic rocks and derive from a localized reservoir beneath the Tecopa Hot Springs area, whereas the type-2 waters show a compositional affinity to the regional carbonate aquifer and discharge in the southern part of the Tecopa Hills. Both water types have oxygen and hydrogen isotopic compositions compatible with recharge in the Spring Mountains of southern Nevada. Noble gas composition, Na-K geothermometry, and helium isotopic composition (R/R_a = 0.08) of type-1 waters are consistent with deep regional ground-water flow path from the Spring Mountains along with a component of crustal Noble gas flux. The type-2 waters are interpreted to originate from the Spring Mountains along a shallower regional ground-water flow path within the regional carbonate aquifer system.