SIGNIFICANCE OF NORTHEAST-SOUTHWEST ORIENTATION OF PROGRADE LINEATION IN THE PELONA-OROCOPIA-RAND-SCHIST LOW-ANGLE SUBDUCTION COMPLEX, SOUTHERN CALIFORNIA AND SOUTHWEST ARIZONA

The Late Cretaceous (to early Paleogene?) Pelona-Orocopia-Rand-Schist (PORS) low-angle subduction complex underlies much of southern California and at least part of SW Arizona. Metamorphosed oceanic rocks of the PORS, dominantly quartzofeldspathic schist (metasandstone), are exposed in some 25 tectonic windows through the continental crust of this region. One of the unifying characteristics of PORS exposures is a penetrative prograde lineation, defined by quartz, feldspar, and mica. This lineation formed during subduction and concurrent early exhumation, and is inferred to indicate direction of subduction flow. Our comments regarding the lineation refer to PORS bodies in their relative positions before late Neogene right slip on the San Andreas fault system (SAFS).

Overall, prograde lineation in the PORS is remarkable for its consistency—in areas as widely separated (440 km) as the Rand Mtns and Cemetery Ridge, lineation trends NE-SW to NNE-SSW. The single significant exception is the southern Chocolate Mtns–Peter Kane Mtn domain, where lineation trends N-S to NNW-SSE. This deviation could represent either local nonuniform flow within the subduction complex, or later rotation. If the latter, the apparent CCW sense of rotation is opposite that expected for right shear along the SAFS; but likely results from reshaping of the Chocolate Mtns anticlinorium (CMA) by a zone of E-W to NE-SW sinistral faults, which may link to the SAFS.

In most areas, regional consistency of NE-SW to NNE-SSW lineation in the PORS seriously constrains tectonic reconstruction involving Neogene rotation. For example, significant rotation of the Sierra Pelona and Orocopia Mtns PORS relative to one another or to most other PORS is contraindicated. In some places paleomagnetic or paleocurrent evidence for major rotation of supracrustal rocks conflicts with lineation data indicating little or no rotation. This dichotomy raises the possibility that low-angle faults separating supracrustal rocks and PORS have accommodated significant differential rotation (as suggested by others as well). Finally, lineation at the west end of the CMA, in the Sierra Pelona and Orocopia Mtns, has the same standard NE-SW to NNE-SSW orientation as at the east end, in SW Arizona. This consistency argues that the arcuate shape of the CMA is original, not imposed.