Anomalous high fluid potentials exist within the miogeosynclinal Great Valley and eugeosynclinal Franciscan sequences of Jurassic-Cretaceous age within the Coast Ranges and at depth on the west side of the Central Valley, California. These rocks are dominantly mudstones with low fluid transmissibilities.

Certain problems exist as to the probable regional distribution of these high fluid potentials. Low fluid potential areas such as The Geysers geothermal district are present in the Franciscan of northern California within a region generally characterized by high fluid potentials. The low potential areas are attributed to fracture zones with channel-type flow whose transmissive characteristics exceed those of intergranular flow. It is concluded that the Franciscan of northern California probably is characterized regionally by near-lithostatic fluid pressures at depth, but fracture zones with both low (i.e., near-hydrostatic) and high (i.e., near-lithostatic) fluid potentials probably exist at various depths from the surface. The Geysers dry-steam occurrence is envisioned as a fracture one with low fluid potentials by virtue of a decrease in transmissive characteristics of a fracture system with depth, in a local region of high heat flow, possibly caused by the existence at depth of a magma chamber.

An abundance of direct fluid-pressure measurements within the Great Valley section of the Sacramento Valley demonstrates the existence of high fluid potentials. The only direct fluid-pressure measurement that has been made within the Great Valley section in the central or southern San Joaquin Valley indicates high fluid potentials. The regional chemistry of the lower Tertiary waters of the San Joaquin Valley (membrane effluent type) suggests that these waters have been extruded from a widely distributed series of mudstones and other rocks that are undergoing compaction. The presumed source for this widespread compacting sequence is the underlying Great Valley sediments with their postulated high fluid potentials.

It is concluded that the anomalous high fluid potentials of Tertiary rocks within folds on the west side of the San Joaquin Valley reflect indirectly the presence at depth of high fluid potentials in the underlying Great Valley section. The origin of the folds is attributed to dynamic tectonic compression caused by current deep-seated linear diapirism of Great Valley mudstones and related rocks that possess near-perfect plastic properties by virtue of their near-lithostatic fluid pressures. The closed gravity minimum over the south end of South Dome-Lost Hills anticline is postulated as being the result of a diapir of serpentine or similar material.

It is postulated that a fault zone, named herein the "West Side" fault, probably exists at depth along the west side of the Central Valley. This buried fault is envisioned as having an intermittent near-surface expression in the form of faults such as the Midland fault, or long linear folds such as the Kettleman folds. Diapirism along this fault is presumed to be responsible for these folds.

Subsidence along the West Side fault is postulated as having occurred contemporaneously with deposition of the Great Valley sequence and thus provided a local trough in which the thick (maximum 60,000 ft) Great Valley section was deposited. The depositional barrier between the Franciscan and Great Valley sequences is postulated as a zone of serpentinite-ultrabasic rocks that intruded intermittently to form a sediment trap on the continental slope throughout Jurassic-Cretaceous geosynclinal deposition.

The final conclusion reached is that an extensive geographic zone is present in which the pore-fluid pressures of the thick Franciscan and Great Valley geosynclinal sediments reach near-lithostatic values. This zone is 400-500 mi long and 25-80 mi wide; it is bounded on the west by the San Andreas fault and the granitic Salinas block, on the east by the buried West Side fault and the granitic Sierran-Klamath block, on the south by the granitic San Emigdio-Sierran block; the northern boundary is interpreted as being the northern termination of the San Andreas fault in the Cape Mendocino region. Structural deformation of this zone by diapirism and thrusting is facilitated by the lithic plasticity caused by high fluid pressures. Known diapirism and thrusting and possible diapiric folding suggest a late Cenozoic age for the development of the high fluid potentials.

The origin of the anomalous fluid pressures adjacent to the San Andreas fault is attributed to compression between the granitic Sierran-Klamath and Salinas blocks resulting from late Cenozoic extension of the central Great Basin in Nevada and Utah. The San Andreas is a transform fault which separates the independent stress field of the Pacific plate (Salinas block) that is moving northwestward relative to the North American plate (Sierran-Klamath block and the Great Valley-Franciscan sediments). The Sierran-Klamath block also is moving westward or southwestward by continued late Cenozoic central Great Basin extension; this westerly motion is terminated by compression of

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the rocks on both sides of the San Andreas. This compression has the greatest effect within the Franciscan and Great Valley shale mass just east of the fault; the effect is greatly reduced within the granitic basement and overlying sediments of the Salinas block west of the fault but has been responsible for folding of the sedimentary veneer. The high fluid potentials are caused by the squeezing of this belt of highly compressible shales east of the San Andreas in a vise whose jaws are formed of relatively incompressible granite; these anomalous fluid potentials are envisioned as being late Cenozoic phenomena dynamically active today.

Diapirism and diapiric folding instead of thrusting have been the preferred modes of late Cenozoic structural deformation within this high fluid potential belt. The dominance of diapirism is attributed to the limited crustal shortening related to the development of this compressive field, as opposed to the dominance of the shearing stresses related to plate movements on both sides of the San Andreas fault. Diapirsm and more limited thrust faulting related to the current generation of high fluid potentials may develop in the future.

Among the possible consequences of the existence of this postulated extensive zone of near-lithostatic fluid pressures are the shallow-focus earthquakes and extensive aftershocks along the San Andreas fault. The near-continuous fault creep along the San Andreas and related Calaveras and Hayward faults also may be a result of these postulated high pore-fluid pressures adjacent to these faults.

An important implication of this paper is the demonstration that fluid pressures within rocks can serve as extremely sensitive and unique strain gauges for the detection of local or regional structural movements.