Microstructural, Metamorphic and Experimental Constraints on Differential Stress and Temperature in the Middle Crust

Author: Kidder, Steven Brooks

Year: 2012

Degree: Dissertation (Ph.D.)

Advisor: Avouac, Jean-Philippe

Committee Members: Stock, Joann M.; Saleeby, Jason B.; Wernicke, Brian P.; Eiler, John M.; Avouac, Jean-Philippe

Option: Geology

DOI: 10.7907/R9RS-1A13

Abstract

Because shear stress drives plate tectonics and causes earthquakes, important objectives in the Earth Sciences include quantifying stress magnitudes and variability in space and time, and developing and improving tools to do so. This thesis addresses both objectives. In the first chapter I demonstrate that the Titanium-in-quartz thermobarometer ("TitaniQ") can be used to accurately record deformation temperatures under greenschist facies conditions. In the second chapter, an experimental study, I show that the relationship between recrystallized grain size and flow stress (the “recrystallized grain size paleopiezometer”) can be used to determine the stress history of dynamically recrystallized quartz under non steady state conditions. In the third chapter I apply the paleopiezometer in Taiwan’s Hsüehshan range and compare results to independent constraints (e.g. critical taper theory and potential energy considerations). This analysis demonstrates: 1) the piezometer is accurate to within a factor of two or better under conditions at the brittle-ductile transition; 2) piezometric results are consistent with recent flow laws for quartz; 3) the activation energy of naturally deformed quartzite is >133 kJ/mol, consistent with experimental determinations; and 4) Peak differential stress in the Hsüehshan range was ~210 MPa at temperature ~300°C. Our results indicate hydrostatic fluid pressure and a low friction coefficient of ~0.38 within the Taiwan wedge. Integrated crustal strength in Taiwan is 1.5-2.1*1012 N/m, consistent with the force needed to support the topography of the range. The final chapter investigates stress levels on the Vincent thrust in the San Gabriel Mountains, California by constructing a numerical model of the initiation of flat slab subduction. A model inversion demonstrates that previously hypothesized high stresses are not required to explain inverted metamorphism along the fault.

Files