Description
Seismic reflection profiles in southeastermost California have been made available to CALCRUST by Exxon U.S.A. for reprocessing and regional geologic analysis. The seismic lines image the collective deformation imposed on the region by Mesozoic thrusting and possible terrane accretion represented by the Chocolate Mountains thrust system, Miocene crustal extension along high-angle normal faults and low-angle detachment faults, and Miocene through Holocene transform faulting related to the San Andreas fault system. These lines show elements of the same crustal structure as imaged on other industry and CALCRUST lines in the Colorado River region, the northwestern Mojave desert, and areas west of the San Andreas fault. The primary conclusions of this study are: first, structural relationships observed in the surface geology are recognizable on a crustal scale on the seismic reflection profiles; second, the components of the Mesozoic Chocolate Mountains thrust system have been segmented by mid-Tertiary extension. Elements of the Chocolate Mountains thrust system are identifiable on the seismic records. The Orocopia Schist has a characteristic reflectivity that may provide a direct way to identify the schist at places on seismic lines; it has a thickness of two to three seconds (TWTT). The Orocopia Schist appears to have a pronounced base defined by tripartite reflections, which are seen here as well as beneath Sierra Pelona and in the northwestern Mojave Desert. The lithologic unit below the Orocopia Schist is more highly reflective than the schist. The Tertiary Mount Barrow Granodiorite intrudes the Orocopia Schist as a laccolith. The reflection at the base of the Mt. Barrow Granodiorite is variably developed and produces the most pronounced reflection within the schist. Locating the Chocolate Mountains thrust at depth is definable only because of the good exposures of the thrust in surrounding ranges. The Chocolate Mountains thrust is not highly reflective, but the difference in reflective packages between the Orocopia Schist and upper-plate crystalline rocks can be recognized. High-angle normal faults form half-grabens that are major Tertiary structural features of the upper crust. Tertiary basins in the Chocolate Mountains region developed as half-grabens in response to penetrative extensional deformation. These half-graben basins sit spatially above pronounced middle-crustal reflections, some of which seem best interpreted as the product of major low-angle crustal simple shear. Basin development appears to be largely a passive response of the tilting of the upper plate above this middle-crustal extensional fabric. This extensional fabric appears to have been produced by anastomosing detachment faults or their related ductile equivalents, and is overprinted on Mesozoic thrust faulting of the Chocolate Mountains thrust system. Early formed high-angle faults are tilted to more gentle dips by progressive deformation and later normal faults. Insight into the evolution of the normal fault geometries is shown by the tectonic contact separating the structural components of the Chocolate Mountains thrust system. This tectonic contact has been folded into antiforms and synforms by a combination of high-angle normal faults and multiple detachment faults. Redbeds are genetically related to the half-grabens and antiforms and synforms. Deformation continued through the deposition of the redbeds and volcanic sequence; the lower units are more strongly deformed than the younger units. Tilting of Tertiary and underlying crystalline rocks is in two opposing directions forming an intervening antiform by a combination of multiple normal faults above a regional detachment fault. Antiforms and synforms are produced parallel and perpendicular to the movement direction by the interaction of multiple faults and the three-dimensional shapes of the fault blocks.
