STRUCTURAL ANALYSIS OF FOLDED PELONA SCHIST ON BLUE RIDGE, SAN GABRIEL MOUNTAINS, CALIFORNIA

Pelona schist of Blue Ridge occurs as a structurally isolated block bounded by the San Andreas and Punchbowl faults. The Pelona schist belongs to a group of schist bodies known as the Pelona-Orocopia-Rand schists that are thought to be derived from trench sediments underplated beneath North America as a result of flat subduction of the Farallon plate during the Laramide orogeny. We report new geological mapping and structural observations from central Blue Ridge between Grassy Hollow and Guffy campgrounds. The predominant lithology is chl-musc-qtz-ab grayschist with lesser amounts of mafic schist (metabasalt), calc-silicate gneiss, and metachert. Mineral assemblages in mafic and rare pelitic components indicate upper greenschist to lower amphibolite grade metamorphism. Foliation patterns and map units within the study area define a broad system of map scale NW-trending open folds. Isoclinal folds are common, but distinctive calc-silicate layers in particular display spectacular S and Z folds at outcrop scale. When enclosing foliated strata are rotated to horizontal, these asymmetric folds show consistent west vergence. We interpret the S and Z folds to record top-to-the-west shear, synchronous with prograde metamorphism that accompanied underplating of the schist. Continuity of the stratigraphy is disrupted by poorly exposed NE-trending cross faults that correspond to rare outcrops with weak striations; kinematic analysis of fault-slip indicators suggests oblique left-lateral thrust displacement. A second set of NW-striking hematite-striated faults records right-lateral displacement parallel to the nearby San Andreas fault. The parent Pelona schist underwent significant dissection and displacement during the development of the San Andreas transform plate boundary. Many workers believe that Blue Ridge and the Orocopia Mountains schist bodies are structurally displaced portions of the Sierra Pelona body. Our data suggest that structures of Blue Ridge are rotated 40+ degrees clockwise relative to Sierra Pelona. Detrital zircon studies are ongoing to test possible correlations with Sierra Pelona and the Orocopia Mountains. Given the complex deformational history and locally excellent and accessible exposures, Blue Ridge is an important site for the detailed study of the Pelona schist.