DAVISSON, M. LEE, Isotope Sciences Division, Lawrence Livermore National Laboratory, Livermore, CA; ROBERT E. CRISS, Earth and Planetary Sci., Washington University, St. Louis, MO; TIMOTHY P. ROSE, Isotope Sciences Division, Lawrence Livermore National Laboratory, Livermore, CA

Abstract: Evidence for Mixing of Shallow and Deep Sources in Overpressured Saline Fluids

Deformation along the eastern California Coast Ranges forms wedge structures having shallow westward-vergent backthrusts rooted to deep eastward-vergent blind thrusts. Overpressured fluids commonly emerge from high elevation springs in these areas, having a typical basinal fluid character. For example, in the Rumsey Hills, springs are saline (up to 2 wt. percent NaCl) and have isotopically enriched d¹⁸O values (-7.5 to +5.0 ‰; Davisson et al., 1994). d ¹¹B decreases from +39.5 to +21.5 ‰ as boron concentrations increase from 4 to 200 ppm, consistent with boron desorption from clays at ~100<deg>C. Similar temperatures calculated from chemical geothermometers suggest fluid sources of ~4km . ⁸⁷Sr/⁸⁶Sr ratios of dissolved Sr decrease from 0.70702 to 0.70585 with increasing Ca/Na ratios, consistent with active plagioclase albitization at similar depths. However, two ³He/⁴He ratios (Ra = 0.2 and 1.9) measured in natural gases effervescing from springs indicate significant mantle contribution and possible fluid sources >>4km. The saline spring discharge can be related to volume strain, such as pore volume dewatering in undeformed sediments within the wedge. This type of fluid expulsion has been shown in similar areas to occur aseismically within fault-bounded blocks (Melchiorre et al., 1999). However, at depth sediments are likely already dewatered and fluid transport probably occurs along faults rooted to the mantle. Additional evidence is inferred from CO2-rich saline waters emerging as springs from Great Valley rocks ~40 km north of Mt. Diablo. The d¹³C of the CO₂ (-5.0 ± 1.0 ‰) in equilibrium with high dissolved inorganic carbon (up to 8300 ppm) is similar to known mantle values.

AAPG Search and Discovery Article #90920©1999 AAPG Pacific Section Meeting, Monterey, California