Abstract

Near Parkfield in the southern Diablo Range, the smooth summit of Table Mountain, a narrow ridge 12 miles long, is capped by a subhorizontal sheet of weakly foliate serpentinite breccia composed of massive serpentinized-peridotite blocks set in a slickensided matrix of crushed serpentine. The breccia was extruded by plastic flowage from longitudinal fault fissures now occupied by steeply dipping serpentinite breccia dikes that lie near the crest of a complex anticline whose core is Jurassic-Cretaceous(?) Franciscan tectonic breccia made of exotic blocks set in a cataclasite matrix. The fissure feeders probably tapped a long-dormant mass of Franciscan serpentinite that was mobilized by fault disruption and high lateral stresses during the Pliocene-Pleistocene orogenesis of the Diablo Range. Upon reaching the surface, the breccia buried an uneven ridge crest of hills and saddles beneath a gently arched summit carapace. The breccia then moved downslope in earthflow fashion to form narrow, sloping ramps that filled draws between resistant spurs on the flanks of the ridge. On the lower slopes, the extrusion again coalesced to form an evenly sloping piedmont plate through which rejuvenated streams have cut steep gorges. Extensive landslides with prominent headwall scarps have scarred eroded edges of the extrusive sheet, and a degradational sequence of Quaternary pediment gravel surfaces at the base of Table Mountain also postdates the extrusion. The piedmont plate rests discordantly across a steeply dipping anticline limb of marine Cretaceous Panoche Group and a largely marine Tertiary sequence that includes Pliocene-Pleistocene(?) continental gravels. The Tertiary beds rest unconformably on the Panoche Group, but the Panoche Group lies upon the underlying Franciscan tectonic breccia along a folded thrust. The thrust emplacement followed widespread tectonic brecciation of the Franciscan, but preceded deposition of Eocene sandstones containing Franciscan detritus. The folded thrust is cut locally by younger, steep reverse and strike-slip faults that offset Tertiary strata.

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