Temperatures and Fluids on Faults Based on Carbonate Clumped-isotope Thermometry

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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 (20 km) 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 (two independent research teams) or a zone of large-scale dissolution with limited tectonic slip (a third research team). 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 continue to test predictions of the detachment, landslide and dissolution models by fully characterizing the temperature and origin of fluids preserved along the fault in veins, breccias and fault gouge, using 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 that has never been applied to a problem in structural geology, and the proposed work accordingly has the potential for transformative impact. It will represent the first characterization of both the temperatures of crystallization and the fluid sources associated with an upper crustal fault. 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. We will use state-of-the-art geochemical techniques to investigate the timing, depth and temperature of deformation along the detachment, which we expect will falsify one or more of the proposed hypotheses. In addition to the research objectives of this project, it is supporting the training of a graduate student; is contributing to the broadening of participation of underrepresented groups in the earth sciences; is contributing to the development and application of a new isotopic thermometer that has transformative potential for a wide range of disciplines, including rock mechanics, structural geology, seismology, seismic hazards analysis, economic geology, and others.