Construction of a composite pressure–temperature path: revealing the synorogenic burial and exhumation history of the Sevier hinterland, USA

Metamorphic pressure–temperature ( P – T ) paths derived from 16 growth‐zoned garnets, nine from this study and seven from a previous study, have been combined to construct a detailed composite path for an area in the hinterland of the Cretaceous to early Tertiary Sevier orogenic belt in southern Idaho and north‐west Utah. Samples are from two Proterozoic units in the footwall of the Basin‐Elba thrust: the schist of Mahogany Peaks in the central Albion Mountains, Idaho, and the schist of Stevens Spring in the Basin Creek area of the Grouse Creek Mountains, Utah, ∼40 km to the south. The simulated portions of the garnets analysed in this study grew from reactions involving the breakdown of chlorite in the upper greenschist to lower amphibolite facies. Multiple garnets were analysed from three samples. Overlapping segments of P – T paths from different garnets in the same sample correlate with respect to slope and garnet Mn concentration. The composite P – T path documents three episodes of sharply increasing pressures separated by two episodes of pressure decrease, all during progressively increasing temperatures. The path is interpreted to represent alternating episodes of synconvergent thrusting and extensional exhumation in the hinterland of the Sevier orogen. Burial was probably caused by the Basin‐Elba fault, the only major thrust exposed in the region. Extensional exhumation may have occurred along the Mahogany Peaks or Emigrant Spring faults, or by extensional reactivation of the Basin‐Elba fault. This method of correlating partial P – T paths to reveal a more complete composite path provides a powerful tool in unveiling orogenic histories in metamorphic terranes, where evidence of major structures responsible for burial and exhumation is commonly obscured by later events.