BASEMENT-INVOLVED THRUST FAULT IN THE PIONEER METAMORPHIC CORE COMPLEX FOOTWALL: IMPLICATIONS FOR DEEP LEVELS OF THE SEVIER THRUST BELT, NEOPROTEROZOIC STRATIGRAPHY, AND CONTROLS ON EXTENSIONAL FAULT GEOMETRIES
We performed detrital zircon geochronology on three newly defined units. Our new ages combined with previously published data yield three age signatures: (1) only age populations of <~2.4 Ga, (2) populations of ~480-500 Ma, ~1.7 Ga, and minor Archean populations, and (3) populations of ~1.1 Ga, 1.4 Ga, and 1.7 Ga. Our detrital zircon signatures are similar to those found in Neoproterozoic-Ordovician sections to the east and north of the PMCC. The 480-500 Ma population is likely derived from Cambrian-Ordovician plutons on the Lemhi Arch and is common in Ordovician sandstones in the region. The new data suggest that the LPT places Archean basement on Ordovician rocks. Metamorphic constraints indicate that the LPT formed at depths of ~13-16 km. The LPT may be the deeper part of the Copper Basin thrust immediately to the east of the PMCC or connect with the Hawley Ck. thrust farther east in the Lemhi and Beaverhead Ranges. Because Belt Group rocks occur in the hangingwall of the Hawley Ck. thrust, the eastward downstepping of the thrust (from Ordovician to Proterozoic) can be explained by the thrust cutting into the Lemhi Arch to the east. Identification of the basement-involved thrust and restorations of the PMCC hangingwall suggest that the PMCC initiated on a thrust culmination.