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

The Pioneer Metamorphic Core Complex (PMCC) in Idaho (US) is divided into three levels separated by two detachment faults: an upper plate of Paleozoic rocks, a middle plate of Neoproterozoic-Ordovician metasedimentary rocks, and a lower plate of high-grade Belt Group rocks overlying a 2.45-2.67 Ga felsic orthogneiss. This Archean orthogneiss overlies a paragneiss (the lower paragneiss or LPg) that yielded detrital zircons with peaks >2.4-2.5 Ga and a single sample with ages as young as 1.7 Ga, leading to the suggestion that this contact is a Cretaceous thrust fault (Lower Plate Thrust: LPT). We performed detailed mapping of the penetratively strained, isoclinally folded LPg and performed additional detrital zircon geochronology tied to newly defined map units. This new work allows us to 1) investigate the deep-levels of the Cretaceous fold-thrust belt, 2) better understand the regional distribution of Neoproterozoic rocks, and 3) characterize the pre-extensional, 3D structural geometry of the PMCC.

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.