GEOTHERMOBAROMETRY OF THE EASTERN TRANSVERSE RANGES, SOUTHERN CALIFORNIA

The eastern Transverse Ranges expose a section of continental crust tilted up toward the west. Sedimentary rocks are exposed to the east and become generally older and are replaced by igneous and gneissic rocks westward. Utilizing Al-in-hornblende and plagioclase geothermobarometry, depths and temperatures were calculated from 42 hornblende and plagioclase crystals in 11 samples with hornblende in equilibrium with quartz and plagioclase.

Pressures were first calculated using Al-in-hornblende barometry (Hammarstrom and Zen, 1986; Schmidt, 1992) taking the average pressure for each sample given by individual crystals and converted to depth using average crustal density. Depths ranged from 22.5 to 9.8 km with an average of 18 km. A linear correlation of y = -0.0002x + 152.13 (r-squared = 0.8537) was made between depth (km) and surface UTM E position, yielding a tilt of 15 degrees. Depth was recalibrated taking temperature effects into account (Anderson and Smith, 1995) and adding three samples with pressures already calculated (Mayo et al., 1998). Depths ranged from 19.3 to 6.3 km with an average of 14.5 km. A linear correlation of y = -0.0001x + 97.714 (r-squared = 0.7724), yielding a tilt of 10 degrees. Differences in results are caused by differences in calculation methods. A direct correlation is observed between temperature and rock composition, causing a need for temperature recalibration. However, temperature data taken from individual crystals may be a misleading representation of the total element composition of the bulk rock and adversely affect pressure and temperature calculations.

Tilting occurred between Late Cretaceous and Pliocene time; the older age limit is based on the youngest dated rock in our suite, and the younger constraint is based on the Miocene – Pliocene (?) age of overlying basaltic rocks (Probst et al., 2004). Tilting probably occurred during the Laramide orogeny, as subduction of the Farallon Plate became shallower.