Abstract

Flows of serpentinite extend downslope more than 1.5 km from the northeastern margin of a large source body of serpentinite exposed on Joaquin Ridge in western Fresno County, California. The parent body, oval in plan, consists of almost completely serpentinized peridotite and dunite in fault contact with surrounding Franciscan metamorphic rocks and upturned strata of the Upper Cretaceous Panoche Group.

The serpentinite flows have an external form which strikingly resembles the forms of debris-flow deposits and solidified lava flows. On the basis of topographic position and surface morphology, flows of several ages can be recognized. Remnants of older flows cap ridges or are perched above valley floors, whereas younger flows respect modern topography and fill present-day canyons which extend into the main serpentinite body. The flows consist of rounded blocks of serpentinized peridotite and dunite enclosed in a matrix of flaky, intensely sheared serpentine. Shear surfaces locally define a weak foliation that diverges around large, un-sheared residual blocks. These internal features are strikingly similar to those encountered in the parent serpentinite body. The lower parts of flows of all ages unconformably overlie sandstones and shales of the Panoche Group.

The geometric form of the Joaquin Ridge flows approximates the theoretical form of a rigid-plastic substance in critical equilibrium. Calculations involving the flow dimensions and the density of serpentine indicate that the sheared serpentinite comprising the flows had a shear strength on the order of 106 dynes cm−2 (1 bar) at atmospheric confining pressure and normal surface temperature. This relatively low value of strength contrasts significantly with much higher values determined experimentally by Raleigh and Paterson (1965) on solid, unsheared serpentinite. The low values indicate that tabular masses of sheared serpentinite found along thrusts in the California Coast Ranges and Klamath Mountains could have been tectonically emplaced by plastic flow at modest stresses and temperatures that were much lower than hitherto suspected.

Surface flows similar to those on Joaquin Ridge may have contributed to the formation of some enigmatic “sedimentary” or “detrital” serpentinites of Cretaceous and Miocene age found in the Coast Ranges.

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