Dissertations, Theses, and Capstone Projects

Date of Degree

2-2021

Document Type

Dissertation

Degree Name

Ph.D.

Program

Earth & Environmental Sciences

Advisor

Steven Kidder

Committee Members

Harold Connolly

Kennet Flores

Greg Hirth

Subject Categories

Geology | Geophysics and Seismology | Mineral Physics | Tectonics and Structure

Keywords

Rock Deformation, Ductile Deformation, Griggs Apparatus

Abstract

Because of the long timescale of many geological processes (millions of years), it has been traditionally assumed that the existing forces acting on the rocks at deep crustal levels are in equilibrium (i.e., steady-state). An alternative assumption, however, suggests that rocks in the lower crust and upper mantle are continuously evolving and exchanging energy at various time scales. Under the latter assumption (i.e., non-steady-state), certain geologic systems may never attain equilibrium regardless of the length and the time scale of operating geologic events. These two “endpoint” assumptions suggest there is a wide range of geologically reasonable conditions under which rocks of the lithosphere may deform. A common methodology to explore these conditions is to deform geological materials using analog experiments under geologically plausible conditions. To date, most deformation experiments were carried out at steady-state conditions while experiments exploring the microstructural characteristics of minerals under non-steady conditions are few. This thesis explores the microstructural characteristics of quartz under “unconventional” but geologically plausible conditions by carrying out three sets of experiments (1) deformation under constant stress, (2) deformation under increasing stress, and (3) deformation using cyclic stress pulses. The deformation experiments under constant stress were carried out by using a novel supplemental instrument developed for Griggs apparatus. Experimentally deformed samples under these conditions were then compared with samples deformed under a relatively constant displacement rate. Microstructural analysis was carried out using optical microscopy and Electron Backscatter Diffraction (EBSD).

Deforming-while-cooling experiments aimed to simulate the effect of cooling that almost all shear zone rocks experience while exhuming through the middle- and lower-crust. The effect of cooling was explored on synthetic quartzite samples through imposed cooling rates of 2 °C/h, 4 °C/h, and 10 °C/h from 900 °C to 800 °C. We compared the microstructure of the "cooling-ramp" and “control” samples and we found that the recrystallized grain size did not keep pace with evolving stress during the cooling. This observation can change the estimate of peak stress using grain size piezometry up to 40% for natural rocks that experienced fast cooling rates or slow strain rates.

The stress pulse experiments aimed to simulate the effect of earthquakes on rocks below the fault in the ductile zone of the crust. The magnitude and the period of the laboratory-induced stress pulses on the samples were scaled to simulate the stresses imposed in natural settings on underlying ductile extensions of fault zones by large seismic events. The results of these experiments indicated that the average grain size of the stress pulse samples and the average stresses during the final pulse matched the piezometer relationship. These results suggest that the peak stresses associated with transient seismic events are not reflected in recrystallized grain sizes. In other words, the grain size paleopiezometry can "see-through" the effect of seismicity and reflect the long-term stress history of middle crustal shear zones.

This thesis explores the microstructural characteristics of the samples deformed under non-steady-state conditions. The findings of this study indicate for the first time, that the deformation under non-steady-state is widespread and has a significant effect on stress estimate based on the grain size of natural rocks. The results of this work show that the peak deformation stress for many natural rocks is likely to be much larger than would be estimated based on traditional piezometry. In other words, the previously estimated stresses based on grain size are most likely reflect the minimum deformation stress in the crust, and the maximum stress is still unknown.

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