2013


Diana Rattanasith

Diana Rattanasith


Years participated in RESESS:
2013


An Overview

Major: Earth Sciences
Academic Affiliation: University of California - Santa Cruz
Research Mentors: Kevin Mahan
Writing Mentor: Fran Boler


Biography

Born and raised in the Bay Area of California, Diana grew up enjoying visits to the California Academy of Sciences. Her curiosity and love for nature led her to study Earth science at the University of California, Santa Cruz. As a RESESS intern this summer, she is one step closer to becoming a researcher in hard rock geology. By studying centimeter scaled shear zones within exhumed deep crustal rocks from Gallatin Canyon, Montana, she has realized that structural geology is her passion.


Abstract

Studying the processes that are responsible for deep crustal seismic anisotropy is important for ultimately understanding the lower crust’s dynamic structure. An excellent opportunity to study these processes occurs in Gallatin Canyon, SW Montana, where metagabbroic rocks commonly contain centimeter-scale shear zones that developed under upper amphibolite to granulite-facies conditions. The motivation for this study was to analyze a transition in deformation state in order to understand the development and evolution of seismic anisotropy in the deep crust. This study employed field and petrographic observations, as well as quantitative modal and electron backscatter diffraction (EBSD) analysis in order to document seismic anisotropy development across strain gradients in the shear zones. The shear zones display a transition from the undeformed metamorphic protolith to a highly deformed mylonite described by four stages. The undeformed host rock contains igneous texture with random orientations for feldspar, pyroxene and incipient metamorphic garnet and hornblende reaction rims. This texture grades into a proto-mylonitic foliation that is oblique to the shear plane. The mylonitic fabric has core-mantle structures of partially recrystallized plagioclase and an anastomosing foliation. Well-ordered alternating bands rich in combinations of hornblende, plagioclase, garnet and quartz define a gneissic layering parallel to the shear plane in the ultra-mylonite. Within the bands, a shape-and crystallographically- defined steady-state foliation is developed that is 20-35 degrees oblique to the shear plane layering.

Modal and textural analyses suggest that pyroxene and plagioclase (+ minor quartz) were the primary minerals in the undeformed metagabbroic protolith but their abundance gradually decreased as deformation, hydration and accompanying metamorphic reactions progressed. Hornblende mode increased significantly from the proto-mylonite to the ultra-mylonite stage, 45% to 54%, respectively. Preliminary results of the EBSD data collected from the ultra-mylonitic fabrics and seismic anisotropy calculations suggest that the anisotropy is controlled by the steady-state foliation in hornblende, which is oblique to the shear plane. The obliquity of the steady-state foliation in hornblende could be significant for recent seismic studies showing that for sub-horizontal shear zones, receiver functions appear to have preferential sensitivity to the dipping component of potentially composite shear zones similar those in this study.


Presentation