CONTINUITY OF THE CHOCOLATE MOUNTAINS ANTICLINORIUM LIMITS NEOGENE FAULT DISPLACEMENT IN SOUTHEAST CALIFORNIA AND SOUTHWEST ARIZONA

The Late Cretaceous (to early Paleogene?) Pelona-Orocopia-Rand-Schist (PORS) low-angle subduction complex is exposed in some 25 tectonic windows through the continental crust of southern California and southwest Arizona. Before disruption by the late Neogene San Andreas fault system, the majority of these exposures were aligned to define the core of the gently arcuate Chocolate Mountains anticlinorium (CMA), a major regional tectonic feature. This structure was ≈ 280 km long, extending from Neversweat Ridge (southwest Arizona) west to the Picacho district and southern Chocolate Mtns (southeast California), thence northwest through the northern Chocolate and Orocopia Mtns, and finally west through Sierra Pelona to Mt. Pinos. The CMA is largely a product of middle Cenozoic crustal extension, but may have originated as or been influenced by a synsubduction welt or some discontinuity within the PORS. In certain areas growth of the CMA continued through middle Neogene (≈ 10 Ma) time.

Whatever its origin, continuity of the middle Cenozoic CMA through southeast California and southwest Arizona strictly limits possible displacement on Neogene strike-slip faults that might cut the CMA. Initial recognition of the CMA, in the 1970s, showed that any Neogene “proto-San Andreas fault” east of the present San Andreas system does not traverse the southeasternmost corner of California or southwest Arizona. Likewise, the east-west-oriented eastern end of the CMA lies athwart and limits major (> 10 km) displacement, at this latitude, across the northwest-southeast-trending Eastern California shear zone. Minor rearrangements or rotations of segments of the CMA are allowed and, indeed, observed. Orientation of lineation in PORS at the west end of the CMA, in the Sierra Pelona and Orocopia Mtns, is the same as that at the east end, in SW Arizona. This consistency argues that the arcuate shape of the CMA is original, not imposed. Offset on small strike-slip faults, without rotation, could have reshaped the western CMA, but we see no evidence for this. Finally, two recently discovered exposures of PORS east and north of the CMA in southwest Arizona, at Cemetery Ridge and the Plomosa Mtns, differ markedly in tectonic setting from those along the CMA, and are not continuations or fault-displaced segments of the anticlinorium.