AbstractsEarth & Environmental Science

The Clearwater metamorphic core complex (CMCC) in northern Idaho is one of several core complexes in the northern Rocky Mountains that were exhumed in the early Eocene and is located in the middle of the dextral Lewis and Clark fault zone (LCFZ). The CMCC has been divided into two lithologically distinct zones: an internal zone that consists of Paleoproterozoic anorthosite and basement schist, and an external zone that consists of Archean to Paleoproterozoic intrusive bodies, basement metasediments, and metasediments of the Mesoproterozoic Belt Supergroup. This thesis is focused on constraining the stratigraphy and structural geology of the eastern external zone, the architecture of Precambrian basement provinces in northern Idaho, and the exhumation history of the CMCC. LA-ICPMS U-Pb detrital and igneous zircon geochronology in this study has confirmed the presence of pre-Belt quartzite, as well as 1870 ± 9 Ma basement orthogneiss in the CMCC. Detrital zircons in the quartzites show a dominant peak near ~1800 Ma, with smaller Neo- and Mesoarchean peaks, and suggest correlation with the pre-Belt Neihart Quartzite. The quartzites lie stratigraphically above basement metasediments that were intruded by the 1870 Ma orthogneiss. Zircon Hf isotopic analyses of the orthogneiss yield intital εHf values between -5.23 and +5.46, suggesting that magma genesis for this body involved both a mantle source and a juvenile crustal source. Hf crustal model ages suggest that this crustal source is Paleoproterozoic to Neoarchean in age. The location of the CMCC in the LCFZ suggests that this structure represents a boundary between two distinct basement terranes along which ~1.87-1.86 Ga magmatism occurred and may be the western extension of the Great Falls tectonic zone. A set of previously unknown east-verging normal faults called the Surveyors Ridge fault zone (SRFZ) was mapped. These faults yield footwall 40Ar/39Ar mica cooling ages of 44 Ma; the easternmost of these faults, the Surveyors fault, yields a hanging wall muscovite cooling age of 78 Ma, and thus delineates the eastern boundary of the CMCC. Faults of the SRFZ were crosscut after 44 Ma by the Collins Creek fault. The Clearwater complex can be divided into at least four zones, each with a distinct structural evolution during exhumation. The internal, western external, and eastern external zones were exhumed between 48-41 Ma along east- and west-verging brittle-ductile normal faults. A fourth zone, the Crescendo Peak block (CPB), which lies between the internal and external zones, yields 40Ar/39Ar cooling ages between 58-54 Ma. The low metamorphic grade of this zone suggests that it was not buried as deeply as the other two zones during Sevier orogenesis. The CPB may represent the remnants of a strike-slip duplex that formed during the initial stages of dextral movement in the LCFZ. The complicated exhumation history of the Clearwater complex may be due to inheritance of pre-existing structural complexity from this structure.