Diana Rattanasith

Years participated in RESESS: 2014, 2013


An Overview

Major: Earth Sciences

Academic Affiliation: University of California - Santa Cruz


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.



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.

2014- Characterization study of monazite from Big Thompson Canyon, northern Colorado with implications for geochronology and timing of Proterozoic tectonic processes

The Precambrian tectonic evolution of Northern Colorado and the Northern Front range is a complicated story of perhaps multiple tectonothermal events, which despite years of study, remains relatively unconstrained. Big Thompson Canyon is a fruitful location to continue to unravel the complex geologic past of the region. The canyon is considered an excellent example of regionally zoned metamorphism that consists of an arcuate pattern of metamorphic isograds that mark east to west, biotite-chlorite to migmatite grade rocks that may be the result of multiple phases of metamorphism and deformation over the hundreds of millions of year timescale. The motivation for this study was to collect and use electron microprobe data on monazites to date the growth of multiple staurolite generations in samples that contain both retrogressed and fresh staurolite in hopes of a correlation to two separate metamorphic events.

Here, we report the results of a preliminary investigation into the possibility of a 1.4 billion year old-overprinting event that may be preserved in the pelitic schists exposed in the Big Thompson Canyon region. These metapelites are generally thought to be ~1.7 billion years old and experienced peak metamorphism around that time. Monazites present in samples collected from Bob Cat Ridge were found and imaged using an electron microprobe and QEMSCAN. Several monazite grains were found in staurolite psuedomorphs, garnet and muscovite porphyroblasts. Resulting U-Th-total Pb data on monazites can be used to quantify the timing of crystallization of the fresh staurolite in the samples and thereby estimate the age of a second possible metamorphic event. The timing of the fresh staurolite formation allowed for inferences to be made about the geochronology and timing of Proterozoic tectonic process within Big Thompson Canyon and the northern Front Range as a whole.