Regional tectonic evolution of the pioneer core complex, South-Central Idaho

The Pioneer core complex (PCC) is one of lesser-studied Cordilleran metamorphic core complexes (MCCs) located north of the Snake River Plain in the Rocky Mountain region of the western U.S.. Based on analyses of zircon geochronology, Ti thermometry and thermodynamic modeling, the tectonic evolution of the PCC followed a clockwise P-T-t-d path with prograde to peak conditions of 4 kbar and 860°C from 52 to 48 Ma, followed by retrograde metamorphism to 2 kbar and 590°C at 43 Ma. The prograde metamorphism (corresponding to D1) of the PCC began near the start of the Sevier orogeny (before 109 Ma) in the Late Cretaceous as the Farallon plate began to subduct eastward beneath North America. The PCC underwent compressional deformation during the Sevier orogeny at 86-84 Ma (D2) when the subducting Farallon plate flattened and deformation verged eastward. The Eocene Challis magmatism and extension in the PCC (52-43 Ma, corresponding to D3) likely resulted from slab rollback of the Farallon plate and asthenospheric upwelling, under a stalled, accreted microplate (Siletzia).