Description
The Transverse Ranges are part of a physiographic province of southern California that trends east-west. The study area lies in the western section of this province and presently contains nine locations of thermal water discharge. Cretaceous and Tertiary marine sedimentary rocks underlie most of the study area, although the easternmost portion contains exposed Cretaceous granitics. Structurally, faults and fold axes follow the east-west trend of the province. The numerous faults which are present within the area dominantly display high angle reverse offsets. One major fault of the area, the Santa Ynez fault, can be traced through the entire study area. Moderate to intensive folding throughout the area has left much of the strata dipping 30° to 60°. The thermal springs discharge at temperatures between 35°C and 89°C. Chemically, two types of thermal water are present: alkali bicarbonate and alkali chloride. The alkali chloride waters result from the incorporation of minor amounts (<2.5%) of connate water. For nearly all of the thermal waters, the observed tritium concentrations are compatible with mixtures of small fractions (<3.0%) of "young" water and thermal water older than sixty years. Physical, chemical, and isotopic data strongly support a cool ground water component of about 25% for the main Matilija Creek hot spring. Available chemical, temperature, and flow rate data from before 1915 compared with recent data suggest no major changes have occurred in the hydrothermal systems during the past eighty years. The most prominent spring deposit is a 1-meter-thick travertine deposit at Gaviota hot springs, although calcite, gypsum, thernadite, and potash alum deposits are present at some of the other thermal springs. Various models of chemical geothermometry have been considered for each of the thermal springs. Estimation of subsurface reservoir temperatures using Na-K-Ca and silica geothermometry yielded values ranging from 46°C to 137°C. Utilizing heat flow data, temperature profiles, and calculated reservoir temperatures, the depths of circulation were estimated to be between 2 km and 3 km for most of the springs. The close relationship between tectonic features and the location of the thermal springs suggests that these features control the upward migration of the thermal water. Contacts between rock units and local topography also control the sites of the thermal water discharge. Possible paths of circulation (both upward and downward) for the waters include both structural and stratigraphic aquifers.
