Structural and Thermal Analysis of an Upper Crustal Low-angle Normal Fault in the Sevier Orogen, Southern Nevada

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The Miocene Mormon Peak detachment is a low-angle normal fault along the Sevier orogenic front in the Basin and Range province in southern Nevada. It is often cited as one of the best examples in the world of a large-displacement (greater than 20 kilometers) normal fault that both formed and slipped at dips of less than 25 degrees, and therefore its origin is relevant to the problem of how brittle fracture and slip can occur on planes oriented at a high angle to the maximum principal stress. In recent years, the detachment has alternatively been interpreted as a system of catastrophically emplaced gravity slide blocks or a zone of large-scale dissolution with limited tectonic slip. These hypotheses challenge the notion that large-displacement extensional detachments are important tectonic elements in the earth's crust. The objective of this project is to use geological mapping, stratigraphy and state-of-the-art low-temperature thermometric methods to provide critical tests for these competing hypotheses. The initial large-slip hypothesis for the Mormon Peak detachment is based on reconstruction of Cretaceous structural markers, and predicts that the structural relief and slope of the frontal Sevier thrust ramp, now exposed in the footwall of the detachment, has the same structural relief and slope as a large monoclinal structure preserved in the hanging wall. The research team will conduct geological mapping and stratigraphic thickness measurements in the poorly understood hanging wall part of the system for quantitative comparison with the footwall. Predictions of the detachment, landslide and dissolution models will be tested with low-temperature thermometry using two independent approaches: (1) (U-Th)/He thermochronometry on apatite-zircon pairs to constrain the time-temperature history of the detachment footwall, and (2) the new clumped isotope carbonate thermometer to distinguish between predictions of the landslide model (cold footwall vein systems, hot gouge on the fault surface) and detachment model (warm footwall veins and warm gouge). Regardless of the outcome of these and other tests for various structural hypotheses, low-temperature carbonate thermometry of fault rocks and vein systems is a new field, and the proposed work accordingly has the potential for transformative impact.One of the primary obstacles to our understanding of the mechanics of earthquakes and faulting is informally referred to as the stress paradox. The stress paradox is simply that the forces in the earth along fault planes are too low to permit either fracture or continued frictional sliding, yet obviously the faults move and generate earthquakes. There is at present no consensus as to why this is the case, and hence the problem is a major research focus area in geology and geophysics. The stress paradox is most acute for a class of faults known as low-angle normal faults, which accommodate horizontal extension of the earth's crust. This project is designed to test competing hypotheses for the origin of one of the best-exposed low-angle normal faults in the world known as the Mormon Peak detachment, located in southern Nevada. Several research teams have presented evidence they claim demonstrate that the Mormon Peak detachment is not a low-angle normal fault at all, and thereby question the notion that any such faults even exist. The research team will use standard geological techniques and state-of-the-art geochemical techniques to investigate the timing, depth and temperature of deformation along the detachment, which is expected to falsify one or more of the proposed hypotheses.