HOW DID MESOZOIC COMPRESSION INFLUENCE CENOZOIC EXTENSION IN THE EASTERN BASIN AND RANGE? A THERMOCHRONOMETRY STUDY OF THE HOUSE RANGE, UTAH

The distribution of strain accumulation across the Basin and Range is not uniform, and the controls on this spatial variability are debated. Here we present a new example of extensional strain heterogeneity in the House Range, eastern Basin and Range, and explore potential explanations for observed patterns of strain accumulation. Workers have observed significant Miocene (~19 Ma) exhumation of the Snake Range metamorphic core complex and the Canyon Range. The Snake experienced as much as 6 km of unroofing over ~4 myr in the Miocene, while 150 km to the east, the Canyon Range simultaneously exhumed by ~5 km. Here we present a new multi-thermochronometer exhumation study from the House Range, located between the Snake and Canyon Ranges. Using apatite helium, apatite fission-track, and zircon helium techniques on a vertical transect through the Jurassic (169 Ma) Notch Peak pluton, we model a time-temperature history showing exhumation from emplacement at ~6 km depth to within 2 km of the surface by ~90 Ma. The remaining 2 km of unroofing occurred over the Cenozoic, but the pluton was already too shallow to resolve a detailed Cenozoic cooling history using thermochronometry.

To explain such variability in Cenozoic extensional exhumation of these ranges, we explore the pre-extension structure of the region left behind by the Sevier orogeny. We map the post-Sevier crustal structure of the western United States by using a variety of compiled data including thermochronometry, paleogeologic subcrop maps, conodont teeth alteration data, vitrinite reflectance data, and stratigraphic data. Our analysis expands on previous work that identifies the frontal thrust belt of the Sevier as a region of deep pre-Cenozoic exhumation in the eastern Basin and Range (Long, 2012). Our House Range thermochronometric data, combined with regional palinspastic reconstructions of Basin and Range extension, suggest that this region of early exhumation along the thrust front was potentially excluded from later Cenozoic extensional tectonism. These data potentially give insight into regional-scale controls on crustal strength, and how broad zones of deforming continental crust partition strain. Future work in the region will include further low temperature thermochronometry to better map patterns of exhumation in the eastern and central Basin and Range.