DID LATE DEFORMATION OF THE CHOCOLATE MOUNTAINS ANTICLINORIUM AFFECT DEPOSITION IN THE SOUTHERN BOUSE BASIN AND THE SUBSEQUENT COURSE OF THE LOWER COLORADO RIVER?

The 220-km-long Chocolate Mountains anticlinorium (CMA), cored by the early Paleogene Orocopia Schist subduction complex, extends from the Orocopia Mountains (CA) southeastward through the Chocolate Mountains (parallel to the southern San Andreas fault), then eastward through the Black Mountain-Picacho district area (CA) and Trigo Mountains (AZ) into southwest Arizona. The CMA comprises several subparallel and en-echelon antiforms and synforms. It is defined by the distribution of Orocopia Schist (structurally lowest), Mesozoic continental magmatic arc units, latest Paleogene volcanic rocks, and late Neogene conglomerate interstratified in its upper part with basalt. The first three assemblages display similar moderate to steep dips related to Neogene and possibly older folding along the CMA. The conglomerate is less strongly folded, with upward decreasing dips. Latest folding post-dated eruption of the basalt at 9.4 Ma (Ricketts et al., 2011).

Where the south-flowing lower Colorado River (CR) encounters the CMA 25 km north of Yuma (AZ), the river abruptly swings east for 10 km along a synform within the CMA. The river then passes through a bedrock gap between the Picacho district and the Trigo Mountains. This gap is controlled by the late Neogene east-west Golden Dream fault zone, ½–1 km wide, within Orocopia Schist. The river again turns abruptly southward parallel to the north-south Ferguson Lake fault, resuming its course toward the Gulf of California.

The >6.1–4.8 Ma southern Bouse basin, north of the east-west part of the CMA, may have begun as a restricted marine estuary and was later obstructed sufficiently to maintain a lake fed by initial waters of the CR. If so, we propose (1) that late Neogene deformation within the CMA created a barrier that blocked the estuary inlet, and (2) that the CMA subsequently controlled the local course of the CR. This late deformation within the CMA possibly was the waning stage of continued folding and uplift. More likely, deformation represents latest Neogene dextral faulting and transpressional folding related to the nearby developing San Andreas fault system. After 4.8 Ma the CR, impounded as a large lake above the CMA, carved an outlet and drained along Golden Dream and Ferguson Lake faults, and possibly other structurally weakened zones within the CMA.