Cooling During Deformation: An Overlooked Scenario with Implications for the Analysis of Ductily Deformed Rocks

Cuny City College, New York NY

Investigators

Abstract

The spatial and temporal variability of stress magnitudes in Earth’s crust are active and longstanding research areas in several branches of the geosciences, e.g. geodynamics and earthquake mechanics. This study investigates a widely used tool which relates grain size of rock samples to stress levels by conducting a series of experiments in which rocks are cooled as they are deformed, mimicking the cooling of natural rocks as they are carried by faults towards Earth’s surface. The project explores the possibility that the statistics of grain size distributions in such samples can be used to extract useful information about the stress and deformation history of such rocks. The project is advancing desired societal outcomes through: (1) improved understanding of processes involved in earthquakes; (2) full participation of women and underrepresented minorities in science, technology, engineering and mathematics (STEM); and (3) the development of a diverse, globally competitive STEM workforce through training of graduate and undergraduate students from underrepresented groups in the Earth sciences and support of two early career scientists. The recrystallized grain size of rocks deformed in the middle crust is routinely used to constrain differential stress magnitudes and rock rheology. Such “grain size piezometry” requires an assumption of microstructural steady state, however most exposed ductile shear zones experienced deformation while cooling. Cooling causes an increase in rock strength, and as a result, a stable recrystallized grain size is a moving target. Recent rock deformation experiments demonstrate that, at ratios of cooling rate to strain rate equivalent to those of typical natural shear zones, recrystallized grain size is markedly out of equilibrium—recrystallized grain sizes are twice as large as would occur if a steady-state were achieved. This result motivates the primary goals of this proposal: 1) determine robust criteria for recognizing microstructures that are affected by cooling during deformation, and 2) develop a toolkit for constraining stress histories from such rocks. Preliminary work on goal one demonstrates that there are systematic differences in the shapes of grain size distributions of cooling ramp experiments versus experiments at constant temperature. A suite of Griggs Rig, general-shear cooling-ramp experiments over a 500°C range of temperature are conducted. These experiments will--in conjunction with Electron backscatter diffraction (EBSD) scanning electron microscopy data from well-constrained natural samples and archival experiments carried out at constant temperature—explore extrapolation of laboratory results to low temperature geologic conditions, and potential effects of strain, initial grain size, and the presence of porphyroclasts and secondary phases. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.