Collaborative Research: Calibrating quartz fabric intensity as a function of strain magnitude: a field-based investigation in the Snake Range core complex, Nevada

Washington State University, Pullman WA

Investigators

Abstract

Sixteen large tectonic plates cover the Earth’s surface and move relative to one another at rates of several millimeters to tens of millimeters per year. An important element for understanding the operation of plate tectonics is documenting the magnitude of deformation of rocks that has occurred at and near the boundaries of plates. However, rocks that have been deformed at high temperatures and deep within the crust often do not preserve geologic features that allow measuring the magnitude of deformation. To address this issue, and thereby better characterize the rock deformation processes that operate at plate boundaries, recent research has proposed that measuring the degree (or ‘intensity’) of alignment of the mineral quartz within rock samples can be used to delineate areas of relatively high- or low-magnitude deformation within the crust. This represents a promising new technique for understanding the spatial patterns of deformation. However, the intensity of alignment of quartz has not been directly correlated to the magnitude of deformation. The goal of this project is to generate an equation that relates the intensity of alignment of quartz to the magnitude of deformation. To accomplish this goal, we will document quartz intensity patterns and measure the magnitude of deformation associated with each intensity pattern in quartz-rich rocks in the Snake Range in Nevada. These rocks are ideal for this study because they have yielded well-defined intensity patterns in past studies, they preserve features that allow measuring the magnitude of deformation, and they exhibit a change from low-magnitude deformation in the western part of the range to high-magnitude deformation in the east. By collecting intensity and deformation magnitude measurements across the range, an equation relating these two parameters will be generated, which can then be applied globally to understand deformation magnitudes in any region that contains rocks deformed at high temperatures. This project will provide research projects for graduate and undergraduate students, thereby training the next generation of geoscientists. This project will also contribute to STEM education by presenting results in introductory geology courses that will reach 1,500 college students each year, giving talks at Great Basin National Park and at community colleges in eastern Washington and northern Idaho, and through a field trip for undergraduate students from both participating universities. Understanding how and where strain becomes localized during deformation is fundamental for illuminating the processes that thicken, thin, or accommodate strike-slip shearing within continental crust during tectonism. Researchers have proposed that statistical intensity parameters calculated from quartz crystallographic fabrics have the potential to delineate zones of high strain, by interpreting fabric intensity as a proxy for finite strain magnitude. Several recent studies in the Himalaya have successfully utilized intensity parameters such as cylindricity to elucidate the spatial patterns of relative strain magnitude across major shear zones. However, as these studies were performed within packages of pervasively recrystallized rocks that lack deformed markers from which finite strain can be measured, they cannot quantitatively relate fabric intensity to absolute strain magnitude. We propose that generating a calibration equation that expresses fabric intensity as a function of finite strain magnitude would be a critical step forward for expanding the utility of this new approach. In this project, we propose to perform such a calibration by investigating rocks exposed within a Cenozoic extensional shear zone in the Northern Snake Range metamorphic core complex in Nevada. This field locality is ideal because the shear zone contains a quartzite unit that yields well-developed crystallographic fabrics, preserves micro-, meso- and regional-scale strain markers, and exhibits a dramatic across-strike gradient in finite strain magnitude. We propose to obtain cylindricity values from quartzite samples collected along a 30 km-long across-strike transect, and to use micro- and meso-scale strain analyses to generate a detailed model of 3-dimensional finite strain of this quartzite unit across this transect. Integration of these two datasets will allow calculation of a calibration equation that expresses fabric intensity as a function of finite strain magnitude, which will provide an indispensable new tool with which to approach the kinematic and structural analysis of ductile deformation. The results of this project will have far-reaching implications, as we propose to test the veracity of a technique that can then be applied globally within any contractional, extensional, or strike-slip orogenic system, active or ancient. The calibration between cylindricity and finite strain magnitude can be applied as a benchmark within any orogen that contains quartz-rich tectonites, in order to illuminate the spatial patterns of strain localization within shear zones that exhibit ubiquitous recrystallization. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.