18 Crossing Borders: Large-scale tectonic models of the North American Cordillera

This session focusses on the Phanerozoic evolution of the North American Cordillera, soliciting studies that address large-scale tectonic models associated with obliquely convergent and divergent movement along the margin.

Thursday, 23 April 2026
8:00 AM - 12:00 PM
LMH, Isla Carmen

Preliminary Metamorphic Pressure Estimates for the Westernmost Exposures of Rand-related Schists in Southern California

Paper number: 18-8
Program: Technical Sessions
Time: 10:20 AM – 10:40 AM

Authors

Abstract

 

A metasedimentary package of rocks known as the Pelona Orocopia Rand schist (PORS) is thought to represent subducted oceanic sediments accreted to the base of the North American lithosphere during Cretaceous subduction of the Farallon oceanic plate. At that time, crustal thicknesses in the western US Cordillera are estimated at ~ 70 km. Garnet in the PORS suite are thought to have grown during underplating and should therefore record the conditions of subduction-related metamorphism. Two of the oldest PORS exposures in California, with detrital zircon spectra similar to the Rand schist, are found at Portal Ridge, the westernmost in-place exposure immediately east of the San Andreas fault, and the San Emigdio Mountains, west of the San Andreas fault. Preliminary pressure estimates for these sites indicate surprisingly shallow depths of metamorphism. New quartz-in-garnet (QuiG) isomekes yield garnet crystallization pressures of 0.70-0.80 GPa at 550 to 650 °C. QuiG pressures overlap with new pressures determined using phase equilibria P-T models (equilibrium assemblage Qtz + Pl + Ms + Bt + Chl + Mag + Ilm), which suggest that pressures never surpassed 0.85 - 0.90 GPa or depths of 25 to 28 km. Additionally, preliminary results from an exposure west of the San Andreas fault in the San Emigdio Mountains produce similar P-T estimates for a mafic schist, comprising Amp + Qtz + Pl + Bt + Ilm. The equilibrium assemblage field is constrained by the pressure-dependent rutile-ilmenite transition, indicating that pressures never surpassed 0.75 to 0.80 GPa at 575 and 680 °C. Work is ongoing to obtain additional pressure estimates, determine metamorphic temperatures, and directly date garnet to test various models to explain the drivers of the PORS metamorphism, including metamorphism during flat-slab subduction, normal-angle subduction, or collision of an exotic terrane.  

Geological Society of America Abstracts with Programs. Vol. 58, No. 3

doi: 10.1130/abs/2026CD-410820

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