Studies of High-Temperature Deformation Processes in Earth Materials: Experimental Investigations and Theoretical Modeling
Abstract
Experimental and theoretical investigations play a crucial role in advancing our understanding of high-temperature deformation processes, responsible for the plasticity and strength of the middle to lower crustal layers of continental lithospheres, plate boundaries, and subducting oceanic plates of subduction zones. I have conducted triaxial deformation experiments on naturally occurring perthitic single crystal feldspars to determine their plastic anisotropy, effects of solid solution, and exsolution and water on their mechanical properties. Deformation experiments were performed using a Griggs solid-medium apparatus, maintaining temperatures in the range of 800-900 degrees Celsius, strain rates (Epsilon) at 1.6x10^-6 s^-1 , and confining pressures ranging from 0.5 to 1.5 GPa. My findings revealed that feldspar samples compressed in [012] direction show the lowest flow strengths, while those deformed along the [001] direction displayed the highest flow strength. Deformation and strength measured in the [012] direction at different pressures (0.75-1.5 GPa) and temperatures (900 degrees Celsius) demonstrate that water weakening in feldspars due to intracrystalline defect interactions with hydrous defects depends on the water fugacity. Earthquakes occurring at depths beyond 15 km are often attributed to low effective pressures, owing to elevated pore pressures, as hydrous silicates such as serpentine dehydrate. However, recent assessments of intermediate-depth earthquakes have revealed that temperatures and pressures of seismogenic subducting slabs do not always correspond to antigorite serpentine, amphiboles, or talc dehydration. Altered mantle rocks of the upper lithosphere and mantle wedges also contain magnesian carbonates, which are stable to great depths. Carbonates of downgoing slabs are weaker than anhydrous mantle silicates. My theoretical modeling study of magnesian carbonate horizons within altered peridotite demonstrates that intermediate-depth earthquakes can result from strain localization within carbonates and thermal shear instabilities that lead to seismic rupture. Models of shear instabilities in ultramafic mantle rocks, are based on grain size-sensitive creep and changes in rheology with changes in grain size. Such analyses cannot be implemented for magnesian carbonates without a better understanding of the kinetics governing grain growth and refinement. The grain growth kinetics study of pure magnesite and Ca-rich magnesite show nearly parabolic grain growth for short annealing times in pure magnesite, when curved grain boundaries are dominant.
Description
Keywords
High-Temperature Deformation, Alkali Feldspars, Continental Lithosphere, Water Weakening, Water Fugacity Effects, Magnesian Carbonates, Intermediate-depth Earthquakes, Magnesite in Mantle, Grain Growth, Grain Growth Stagnation, Grain Boundary Pinning in Magnesian Carbonates