IMPLICATIONS OF ELASTIC GEOBAROMETRY PRESSURES FROM THE WESTERN US CORDILLERA (Invited Presentation)

Elastic geobarometry reveals pressures substantially higher than conventional thermobarometry and some structural reconstructions in the western US Cordilleran orogenic belt. Here, we highlight our previous work on Cordilleran metamorphic core complexes and compare the results to the known geology. Quartz-in-garnet (QuiG) pressures from the Funeral Mountains, Pequop Mountains, and Wood Hills are inconsistent with those from chemical thermodynamic barometry and, in the Wood Hills and Pequops, some structural depth estimates. QuiG on rocks from the Funeral Mountains and Wood Hills yielded 3–4 kbar higher pressures than estimates from G-minimization pressure-temperature (P-T) paths, pseudosection equilibrium assemblages, and GASP barometry. Similar pressure differences between QuiG and thermodynamic barometers have been noted in other orogenic terranes (e.g., Wolfe et al., 2018). In the Wood Hills, QuiG yielded pressures of quartz entrapment in garnet, interpreted as garnet growth pressures, of approximately 8 kbar, which would require a depth of 24 km. Some garnet pressure estimates yielded pressures as high as 11 kbar, which would require a depth of 33 kbar, greater than can be accounted for based on the known structural geology (Hodges et al., 1992; Chamilleri et al., 1997). QuiG yielded a garnet growth pressure of ~7 kbar in the Pequop Mountains and a geothermal gradient of 40–50ºC/km, requiring deeper depths than can be accounted for based on some estimates of the structural geology (Zuza et al., 2022). Although the causes for these selective differences are not fully understood, they may point to a mechanism by which lithostatic pressure differs from the pressure recorded in quartz inclusions in garnet (the pressure felt by the rock) in some orogenic belts, like the core complexes in the western US. Alternatively, the core complexes may record highly localized structural burial, such as by hinterland underthrusting.