Physical Sciences

Parsons, Travis; Lippert, Peter; Bartley, John (University of Utah)
Faculty Advisor: Lippert, Peter (University of Utah - College of Mines and Earth Science, Geology & Geophysics); Bartley, John (University of Utah - College of Mines and Earth Science, Geology & Geophysics)

Field observations and geochronological measurements of plutons in Yosemite Valley suggest that plutons grow incrementally as a series of stacked sheets of smaller intrusions (i.e., dikes and sills) (Coleman et al., 2004; Glazner et al., 2004; Bartley et al., 2006). This interpretation is in contrast to the traditional view of pluton emplacement through crystallization of a single, massive magma chamber. Most of the observations supporting incremental pluton emplacement use the relationship between zircon U-Pb dating of pluton sections and estimated granitic magma cooling rates to argue that a single magmatic event would crystallize significantly faster than the geochronologic data permit. Incremental pluton emplacement also predicts specific relationships between the age of intruded sheets of magma and the original orientation of these sheets, such that older sheets are expected to be tilted or deformed more than younger sheets. Here we test this prediction of differential tilting by measuring the paleomagnetic inclination preserved in well-dated and structurally characterized sheets of the Tuolumne Intrusive Suite. Magnetic inclination provides a tilt-meter with respect to the Earth's magnetic field direction at the time of pluton emplacement; the reference inclination assuming an untitled pluton is known from independent data sets. We also present rock magnetic data (temperature-dependent magnetic susceptibility, magnetic remanence characteristics) and results from petrographic investigations to characterize the mineralogy and stability of the magnetization. Our results suggest that the low-titanium magnetite remanence carriers are primary and are not biased by secondary magnetizations. The distribution of magnetic inclinations in our sample set — in which older sheets on the periphery of the pluton are shallower than those in younger, more interior sheets, and with respect to the reference inclination — is consistent with predictions from the incremental pluton emplacement hypothesis.

Paulding, Anna (Utah State University)
Faculty Advisor: Bradbury, Kelly (College of Science, Geosciences Department)

A dramatic increase in seismicity has occurred in the midcontinent region since 2009 (Rubinstein and Mahani, 2015), causing public concern for the stability of infrastructure and buildings. Several studies have directly linked this seismicity to the reactivation of buried fault systems near the Paleozoic sedimentary bedrock-Precambrian crystalline basement contact as a result of high volumes of injection of wastewater produced by the oil and gas industry (Ellsworth, 2013; Keranen et al., 2013).

The reactivation of fault zones due to fluid injection is not only influenced by injection rates but also by the ability of fluids to migrate along or across the contact, which is controlled by the rock properties and geologic setting. To better understand the rock property variations that may occur along the nonconformity interface, we use an outcrop analog site of an exhumed fault near Gunnison, Colorado. My undergraduate research focuses on using a portable handheld X-Ray Fluorescence Unit (pXRF) as a tool to measure compositional variations in outcrop. To directly compare data, a calibration using 16 USGS Concentration Standards as well as 12 analog samples will be used to create a calibration optimized for this specific suite of rocks which informs the accuracy of in-situ field data measurements against laboratory measurements of powdered samples, influencing how future pXRF measurements can be analyzed. Micro-scale variations of major and trace element concentrations reflect alteration and related fluid-rock interactions and may serve as a proxy for fluid migration along or across faulted sections of a nonconformity interface. I propose that calibrated pXRF data and whole rock XRF data is a useful tool for understanding the nature and degree of rock alteration in fault zones and across analog sites nonconformity interface. These data can aid in a more broad understanding of how pXRF data can be used in the field to characterize the nonconformity interface and fault zones.