Evaluating Tectonic Models for the Pelona, Orocopia, and Rand schists in the Southwestern USA

Flat slab subduction has been proposed along the southwestern U.S. margin during the Late Cretaceous. The model is partly based on exposures of garnet-bearing Pelona, Orocopia, and Rand schists (PORS), which crop out in isolated mountain ranges extending from the Los Angeles California area eastward >400 km into western Arizona. These chlorite+muscovite+/-biotite+/-garnet schists have distinctive characteristics that include graphitic inclusions in albite porphyroblasts, interlayering with mafic schists of MORB composition and Mn-rich siliceous marbles and cherts, and local blocks of metasomatized mantle peridotite. The flat slab model interprets the PORS association as oceanic sediment underplated to North American crust and subsequently exhumed by Basin and Range extension. Other models of the tectonic development of the western US call this interpretation into question. Ongoing research focuses on using Quartz-in-Garnet elastic geobarometry (QuiG), elemental exchange thermometry (e.g., GArnet-BIotite - GABI), Phase Diagram Sections (PhaDS), and garnet Sm-Nd geochronology to construct new Pressure-Temperature-time (P-T-t) paths for select PORS rocks along a west-east transect to evaluate the tectono-metamorphic history of this assemblage. We report temperature estimates of 603, 627, and 620°C (±25°C GABI) obtained from the San Emigdio, Portal Ridge, and Plomosa mountains, respectively. QuiG pressure estimates of 0.72-0.97 GPa at 627°C were obtained from Portal Ridge. These new results are consistent with PhaDS models that predict <0.9 GPa for equilibrium of quartz + plagioclase + muscovite + biotite + chlorite + magnetite + ilmenite. We interpret the results from Portal Ridge and the San Emigdios, the two westernmost sites in this study, to indicate shallow depths of 28-38 km of overlying crust. These crustal thicknesses are significantly lower than ~70 km estimates for North American crustal thickness during the Late Cretaceous. Future P-T-t paths, including garnet Sm-Nd ages, will provide a critical test for models of accretion to the base of the western Cordilleran continental crust by shallow/flat slab subduction of the Farallon oceanic plate.