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