Tectonic Significance and Hydrocarbon Trapping Consequences of Sequential Folding Synchronous with San Andreas Faulting, San Joaquin Valley, California

Abstract

Subsurface anticlines along the west side of the San Joaquin Valley grew progressively basinward synchronous with the history of strike-slip on the adjacent San Andreas fault. At the south end of the anticlinorium the first conclusive documentation of en-echelon folding appears in early Miocene rocks, corresponding to the start-up of significant late Tertiary displacements on the San Andreas. Absence of important late Eocene or Oligocene folding here corresponds to a previously established period of negligible strike-slip. A marked increase in the area folded occurred near the end of the Miocene, reflecting a recognized increase in rate of fault displacements. Secondary faulting of the structures apparently lags behind fold inceptions but may have the same general outward growth sequence. At the northwest end of the anticlinorium en-echelon folding commenced at least by early Eocene or possibly Paleocene time, coeval in part with displacements on the proposed early Cenozoic proto-San Andreas system. Incomplete data suggest that folding proceeded both outward from the San Andreas here and longitudinally southeastward parallel with the fault, implying a southeastward propagation of the present San Andreas strand of this system during the latter's probable inception in earliest Tertiary-latest Cretaceous time. Hydrocarbon trap closures change critically in response to the timing sequences. Subsurface folds nearest the San Andreas have grown contemporaneously with deposition of the earliest reservoirs, but some ceased growing while deformation was just commencing basinward. Such traps may be hidden below relatively undisturbed younger basin-margin sedimentary wedges. In other instances and more distant from the San Andreas, younger anticlines are expressed at the surface, and vertical closure increases with depth in response to several periods of flexing. Structures at the periphery of the anticlinorium have the latest inceptions, but closure may diminish downward at levels where the gentle late flexing was insufficient to reverse preexisting basinward dips imposed by earlier basin subsidenc . The observed relations suggest that folds and wrench faults are essentially independent, different structural responses to a common, diffuse coupling movement originating in the deeper crust. Folds are propagated generally outward within an expanding, sometimes amorphous and diffuse, deformational front. Where tightly flexed interior structures become less susceptible to additional growth, renewed flexing may be concentrated at the periphery of the advancing front. The former folds then may undergo further stress release in later stage faulting. Outermost folds, having undergone less cumulative shortening, lack this faulting and generally are larger and broader. Regional strike-slip displacements are contemporaneous with the same deep coupling drive mechanism but remain concentrated o the fold set's established, original inner line of thoroughgoing crustal weakness.