STRUCTURE AND GEOCHRONOLOGY OF THE SAWTOOTH METAMORPHIC COMPLEX, IDAHO: IMPLICATIONS FOR THE TECTONICS OF THE CORDILLERAN HINTERLAND IN THE IDAHO BATHOLITH SECTOR

Detailed structural and geochronologic data of meta-sedimentary country rocks in the Idaho batholith region are key for understanding the tectonic evolution of the Cordilleran hinterland in central Idaho. The Sawtooth Metamorphic Complex (SMC) in the northern Sawtooth Range ID, contains intermediate to high-grade paragneisses, and is intruded by at least five generations of magmatic rocks (~416 Ma tonalite, ~140-120 granite, ~95 Ma gabbro, ~71 Ma biotite-tonalite, and Eocene granite/rhyolitic dikes), on the basis of U-Pb zircon ages. U-Pb geochronology of detrital zircons from quartzites and quartzofeldspathic gneisses are consistent with various North American sources (early Neoarchean, late Paleoproterozoic, early Mesoproterozoic, and late Mesoproterozoic). The youngest concordant detrital zircons suggest that the maximum depositional age of the SMC quartzites is ~510 Ma. The ages of euhedral rims (overgrowths) on the detrital zircons in quartzites are consistent and suggest a metamorphic/hydrothermal event at ~86 Ma. Based on geologic mapping data from a traverse across the southern SMC, structures include a D1 event of folding/thrusting and a second deformational event (D2) dominated by shearing and flanking folding. The D1 deformation likely resulted from SWW-NEE shortening related to structural thickening of the Cordilleran hinterland. The D2 deformation was generated by a partitioned transpressional shearing in which a substantial wrench component is accommodated by discrete dextral strike-slip shear zones. Syn- to post kinematic granitic dikes suggest that the D2 also occurred at about 86 Ma. We propose that the D2 SMC shear zone is an inboard equivalent of the Western Idaho Shear Zone as they are kinematically compatible, and were active around the same time. This study also indicates that Cambrian-Ordovician miogeoclinal strata in southern Idaho and Utah extend to the north of the Snake River Plain.