Development of Microstructure and Creep Strength of Marble

Carbonate rocks (limestone, dolomite, marble) are important constituents of the Earth's crust. Notably, layers of limestone and dolomite are among the folded and faulted units within the fold-and-thrust belts of the major mountain ranges - the Idaho-Wyoming overthrust belt and the Alpine nappes, for example. These belts contain major hydrocarbon reservoirs. The processes by which the constituent minerals deform is essential in the understanding of how these belts form and how the deep crust deforms. This project explores the detailed mechanisms by which carbonate rocks deform through a series of experiments in which the constituent minerals are deformed over a range of strain rates, confining pressures, and temperatures. A new novel micro-scale strain mapping method when coupled with various high-resolution microscopy methods, will allow for unprecedented understanding of the atomic scale mechanisms of carbonate mineral deformation. Results will be compared to naturally deformed carbonate rocks collected from the Alps. The project would advance desired societal outcomes through: (1) development of a globally competitive STEM workforce through postdoctoral fellow training; (2) increased partnerships through international collaboration; and (3) enhanced infrastructure for education through development of Open Courseware materials.

The main goals of this project are to investigate the physics and kinetics of the evolution of microstructure and strength of carbonate rocks during creep, and to identify characteristic elements of microstructure necessary to interpret the mechanical history of naturally deformed rocks. The project, in collaboration with scientists at GFZ German Research Centre for Geosciences and Université Montpellier builds on previous work by this research group and will include testing and observations of the microstructure in samples deformed under conventional triaxial and torsion loading of natural and synthetic marbles at shear strain rates between 10^-3 and 10^-6 per second, confining pressures less than 300 MPa, and temperatures between 500-1000 K. Observations of microstructure will be made using optical microscopes, SEM, TEM, and EBSD to correlate dislocation structure, generation of LPO, dynamic recrystallization, and the evolution of strength. Two novel techniques, micro-scale strain mapping and sequential microanalyses, will be used to understand the kinetics and partitioning of strain amongst the deformation mechanisms. Although lab investigations are important, thorough and fundamental understanding of tectonics will come only by combining and reconciling lab experiments, observations of field- and micro- structure, geophysical investigations, and theoretical and computational treatments. Thus, continued observations of microstructures in naturally deformed marbles are an important part of this project. In collaboration with researchers at Universität of Bern, the team will observe microstructure in mylonites from the Helvetic Nappes, which provides opportunities to investigate the influence of temperature on strain localization, to study the influence of varying quartz and dolomite content on strain localization within the carbonates, and to correlate dislocation microstructure, grain structure, and phase chemistry at locations where deformation conditions are well constrained.