THE PELONA-OROCOPIA-RAND SCHISTS FOR CALIBRATION OF CRUSTAL SEISMIC ANISOTROPY INTENSITY AND SCALES: NATURAL EXPOSURES, THREE-DIMENSIONAL GEOMETRIES, AND QUANTIFICATION OF INHERENT FABRIC ANISOTROPY

Anisotropy of seismic wave propagation has been identified in the upper mantle associated with preferred orientation of olivine due to mantle flow and in the upper crust related to the presence of aligned cracks. In contrast, anisotropic behavior due to structural layering or inherent foliation of crustal material is not as well studied, particularly at middle and lower crustal levels. An excellent natural laboratory for analysis of the latter is provided by the Pelona-Orocopia-Rand schists (“POR schists”) of southern California and southwestern Arizona. The POR schists are dominated by strongly foliated, medium- to low-grade quartzofeldspathic schists with minor amounts of interlayered metabasite, metachert, and serpentinite. Most workers consider that the schists were derived from trench sediments and oceanic crust plated beneath southwestern North America via low-angle subduction of the Farallon plate during the Late Cretaceous-early Cenozoic. Underplating of the schist led to tectonic collapse and erosional denudation of overlying North American crust. Following flat subduction, the schists may well have comprised a subhorizontal sheet extending from a depth of 10-15 km at the top to 30-35 km at the base. During the middle Cenozoic, further exhumation, and ultimately exposure, of the schists occurred along restricted belts of detachment faulting. Most of the current exposures of the schist define relatively simple antiforms a few to ~10 km across related to detachment faulting and/or San Andreas transpression. Locally, however, foliation orientations are highly irregular owing to multiple generations of higher-order folds. Laboratory measurements of the POR schists indicate anisotropic shear wave velocities as high as 23%. Synthetic seismograms based on the laboratory measurements combined with inferences regarding the regional distribution and orientation of the schists suggest that shear wave splitting within the schists could be significant. Seismic studies in regions of schist outcrop can be used to test this prediction. Excellent exposures of the schists allow for calibration of the seismic anisotropy and opportunities to trace these rocks to depth. This may eventually lead to a means for evaluating competing views as to the subsurface distribution of the schists.