Abstract
The Salmon River Suture Zone (SRSZ) in west-central Idaho represents a boundary between the North American craton and accreted oceanic terranes. Although it is well established as a fundamental feature of the Mesozoic western U.S. Cordillera, the detailed pressure-temperature (P-T) evolution of rocks on the cratonic side of the suture remains poorly constrained. In order to constrain peak pressure-temperature (P-T) conditions and interpret crustal burial and exhumation histories across various tectonometamorphic domains of the SRSZ, this study presents new thermobarometric data from metamorphic and intrusive rocks exposed near Elk City and Dixie, Idaho. Mineral chemistry was determined for assemblages bearing garnet, hornblende, biotite, and plagioclase using electron probe microanalysis (EPMA). Several geothermobarometers, such as garnet-biotite (Holdaway, 2000), garnet-hornblende-plagioclase (Kohn and Spear, 1990; Dale et al., 2000), and hornblende-plagioclase calibrations (Anderson and Smith, 1995), were applied using these data. Estimated burial depths of approximately 17-31 km and geothermal gradients of approximately 21-43 ˚C/km correspond to peak P-T conditions from 8 of the 21 samples, which range from 610-742 ˚C and 4.5-8.9 kbar. West of Elk City, Cretaceous tonalites and orthogneisses typically have higher-grade metamorphic conditions, whereas metasedimentary schists of the Elk City Metamorphic Sequence farther east have lower-grade conditions.
The distribution of syn- to post-tectonic plutonic rocks and major structural boundaries (e.g., Pollock Mountain and Heavens Gate thrust faults) are correlated with these spatial differences in metamorphic grade. This pattern indicates that while eastern structural domains record more modest burial and cooling, western structural domains experienced greater crustal thickening and longer times at depth. These findings are consistent with a model of diachronous exhumation and tectonic stacking across the suture zone that has been thermally altered by plutons. These discoveries improve our knowledge of the tectonothermal evolution of the SRSZ and advance our understanding of metamorphic processes in accretionary orogens.