Abstract

The structural features of the Castle Mountain Range spur of the Diablo Range and of the adjacent Cholame Valley segment of the San Andreas fault zone in the Southern Coast Ranges provide evidence of long-continued strike-slip movement within the fault zone and along nearby sub-parallel faults, and of close genetic relationships between the fault and the structures of the range. Four adjoining subparallel structural domains with common northwesterly trend can be recognized on the basis of geomorphology and style of deformation: (1) the San Andreas rift belt of fault scarps and related primary features along straight active fault traces and slightly bent dormant fault traces; (2) the San Andreas fault zone of intricately sliced bedrock including exotic fault slivers and strongly flexed fault surfaces with varying dips; (3) a domain of superimposed folds and faults of different ages showing evidence of strike-slip offset roughly proportional to their ages; and (4) a domain of open folds beside the Great Valley.

The cores of major anticlines in the range expose a tectonic megabreccia of Franciscan rock that is structurally overlain along a folded thrust by an orderly Late Mesozoic sequence at least 10,000 feet thick. Resting unconformably on the pre-Tertiary rocks is a varied marine Tertiary succession more than 5000 feet thick. The youngest strata are early and late orogenic Pliocene-Pleistocene continental deposits about 2500 feet thick. Several unconformities that merge eastward into conformable successions record recurrent Cenozoic deformation whose intensity was greatest near the San Andreas fault.

The folds and faults mapped at the surface are attributed to raft tectonics whereby a passive surficial plate is deformed as it rides coupled to a moving undermass. The hypothetical deep movement plan suggested is a pattern of velocity gradient flow longitudinal to the San Andreas fault with the gradient greatest near the fault and with coordinated transverse constriction at depth.

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