CAN STRUCTURAL AND PALEOMAGNETIC DATA BE RECONCILED TO DETERMINE THE ORIGINAL GEOMETRY OF THE CHOCOLATE MOUNTAINS ANTICLINORIUM OF SOUTHERN CALIFORNIA AND SOUTHWESTERN ARIZONA?

The Chocolate Mountains anticlinorium (CMA) is a major geologic feature of southern California and southwestern Arizona, and a key component of numerous paleotectonic reconstructions of the San Andreas fault system. The trend of the CMA is curved; it is debated whether this curvature is primary, or the result of bending of an originally linear trend. Paleomagnetic data suggest that, to the west, an originally linear CMA has undergone increasing vertical-axis clockwise rotation, bending it northward. Lineations within the schist in these same segments of the CMA are thought to document subduction of the schist. These lineations are oriented approximately NE-SW, which was the approximate direction of subduction, and thus were thought to preclude substantial rotation of these parts of the CMA. I propose that lineations formed during subduction were rotated toward alignment with approximately NW-SE-oriented simple shear prior to bending of the CMA. I suggest early Paleogene extension, inferred to have occurred along much of the CMA, as the likely cause of this simple shear, with sinistral slip along the Nacimiento fault zone as a second possibility. In this model, paleomagnetic and structural data are compatible rather than contradictory, and indicate an originally linear geometry for the CMA. Sierra Pelona, at the northwestern end of the San Gabriel Mountains in southern California, is widely thought to be a segment of the CMA offset along the San Andreas fault. Lineations measured at Sierra Pelona presently trend N-S to NE-SW, which would restore to NW-SE to N-S after correcting for clockwise rotation implied by paleomagnetism, thus supporting this model.