Hydrodynamics and overpressure: Mechanisms in the Sacramento basin, California

Subsurface pore fluid pressures exceeding 70 percent of lithostatic pressure are observed in large areas of the Sacramento basin and adjacent Coast Ranges, California Basin- or crustal-scale processes are required to explain the regional distribution of overpressures. Several plausible mechanisms of overpressuring are (1) depositional compaction by the Miocene and younger strata recently deposited, (2) aquathermal expansion under high heat dow conditions, and (3) horizontal or lateral tectonic compression. The latter mechanism is suggested by observed deformation patterns in the Sacramento basin. Throughout the geologic history, east-west compressive stresses by eastward subduction of the Juan de Fuca plate, subsequent tectonic wedging, right-lateral tectonism (primarily in southern part of basin), and east-west extension in the Basin and Range together created a primarily compressive stress regime. Such a stress regime has likely contributed to the cause and maintenance of overpressures observed in the region. We developed two-dimensional numerical modeling experiments to test these hypotheses. Results of the experiments suggest that aquathermal expansion is negligible as an overpressure mechanism. Sedimentary compaction is a possible mechanism but only for locally high sedimentation rates or for unusually low permeability and/or unusually high compressibility under lower sedimentation rates. Finally, model simulation results suggest that tectonic compression is a plausible, ongoing mechanism creating and maintaining high fluid pressures in the Sacramento basin. We speculate that other processes such as chemical diagenesis may be contributing to overpressures, but probably only on a local scale and of secondary importance.