A RECORD OF REPEATED THRUSTING AND EXHUMATION IN FOOTWALL ROCKS OF THE BASIN-ELBA FAULT, SOUTHERN IDAHO, SEVIER HINTERLAND

We have determined pressure-temperature (P-T) paths in the footwall of the Basin-Elba fault, Albion Mountains, Idaho, by numerically simulating garnet growth from upper-greenschist facies rocks of two different pelitic compositions. One bulk composition is represented by a horizon near the top of the schist of Stevens Spring (SS) in the Basin Creek area of the Grouse Creek Mountains, Utah, and the second by the schist of Mahogany Peaks (MP) in the central Albion Mountains, approximately 40 km to the north. Garnet growth occurred by different reactions during overlapping P-T conditions. Garnets in the SS grew from the reaction: qtz + bt + chl + plag=grt + ms + H₂O. Garnets in the MP grew from the reaction: qtz + margaritic paragonite + chl=grt + ky + ms + H₂O. Element maps suggest that at least one discontinuity is contained in all three MP garnets analyzed. The discontinuity most likely represents garnet consumption that took place during an excursion to lower temperatures. We used the Gibbs method based on Duhem’s theorem to simulate detailed compositional profiles. Temperatures of garnet growth were determined using garnet-biotite or garnet-chlorite thermometry. In one MP sample, P-T paths obtained from three garnets indicate growth initiating at different times; segments of the different paths overlap and represent comparable spessartine values. Composite results from all models, including five previously published and two new models from the SS and three from the MP, show a maximum pressure increase of 2.7 kbar during a temperature increase from 445 ± 25 °C to 475 ± 25 °C, followed by a pressure decrease of 1.1 kbar coincident with a temperature increase to 533 ± 40 °C. The pressure decrease is interrupted by two episodes of pressure increase. All pressure increases terminate at similar values, suggesting that extension may have been promoted by the attainment of a physical limit to topographic relief. The Late Cretaceous Mahogany Peaks fault, structurally above the MP, has been identified as one possible structure along which synorogenic extension may have taken place. The similarity among P-T paths from the Grouse Creek and Albion Mountains suggests that the Basin-Elba fault could be responsible for at least some of the burial episodes recorded in the footwall P-T paths.