CRISS, R. E., Washington University, Saint Louis, MO; M. L.* DAVISSON, Lawrence Livermore National Lab, Livermore, CA

* Denotes speaker other than senior author.

Abstract: Fluids, Faults and Tectonic Evolution of the California Coast Ranges

Field, geochemical and production data across the central California Coast Ranges document intimate relationships between tectonically-overpressured formation fluids and geologic structures, uplift, and seismicity. The overpressured fluids emerge at high elevations (to 1500') as geochemically distinctive, high-¹⁸O springs and have resulted in numerous blowouts. From west to east, fluid chemistry grades from CO₂-rich, travertine-depositing springs and mud volcanoes, to chloride-rich springs that deposit halite, and finally to overpressured, chloride-rich, subsurface fluids commonly associated with oil or gas. Fluid chemistry parallels the change in the predominant tectonic character from strike-slip dominated in the western Coast Ranges to the fold and thrust structures of the eastern Coast Ranges, and ultimately to the Sacramento Valley.

The above observations are compatible with the eastward propagation of the Coast Range fold and thrust belt. Hydrocarbons and chloride-rich formation fluids have been largely expelled in the older, western parts of the belt, where brittle failure dominates and steeply dipping faults occasionally tap very deep sources Of CO₂-rich fluids. Further east, both brittle failure and aseismic deformation operate, with the overpressured saline springs correlating with fault-bounded aseismic blocks undergoing compression, Quaternary uplift and folding (Melchiorre et al, JGR, 1999). Still further east, along the margins of the Sacramento Valley, overpressured saline fluids emerge along fold axes and thrust faults, and are in places associated with oil and gas deposits. In this zone the seismicity is mostly limited to blind thrust events (e.g., Coalinga). Still further east, seismicity is much diminished and the rocks become progressively less deformed toward the valley axis.

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