Similarities between the Th/U map of the western US crystalline basement and the seismic properties of the underlying lithosphere

Abstract

In order to elucidate the nature and history of the different geological units in the western USA and correlate geochemical signatures with features identified in seismic models developed from USArray, we compiled from the literature Pb isotope compositions of ores, granitic K-feldspars, and felsic plutonic rocks and complemented the database by analyzing Pb isotopes for 82 felsic xenoliths and plutonic rocks from the Colorado Plateau and its immediate vicinity. The raw Pb isotope abundances for the complete data set were converted into model ages T (time of last U/Pb fractionation), 238U/204Pb (μ), and 232Th/238U (κ) with computed representative values for grid cells of 1°×1°. The model ages reflect the old formation ages in the Mojave and Yavapai blocks and the Archean Wyoming craton. Model ages are reset wherever the Moho is relatively shallow, most notably west of the Rocky Mountains, likely as a result of metamorphic reheating ± melting above the Pb closure temperature (∼550–650 °C). The κ values correlate with the uppermost mantle VP/VS ratio. High κ values are found in the Snake River basin and central Colorado. High κ values also form a ridge trending south from northwestern Utah through the Great Basin into the Mojave and Yavapai blocks. This ridge may represent exhumation of deep-seated rocks as a result of regional extension and collapse. Alternatively, it may signal the presence of igneous and metamorphic material associated with a channel of flowing crust originating beneath the Colorado Plateau and spreading north.