In this project, researchers will make a detailed comparison of the dislocation and grain microstructure of natural samples (collected from the Morcles Nappe, a thrust fault in the Swiss Alps) with those in samples deformed in laboratories at MIT and at the Helmholtz-Zentrum Potsdam Deutsches GeoForschungsZentrum GFZ in Potsdam, Germany. The first suite of Carrara marble samples will be deformed in conventional triaxial compression at strain rates of 0.001 per second to 0.000001 per second, confining pressures between 100 and 300 Megapascals, and temperatures between 300 to 700 Kelvin, conditions that span the transition from cataclastic failure to dislocation creep. The second set of experiments includes synthetic and natural marbles deformed by dislocation flow, but extends to conditions at which diffusion creep also occurs (temperatures between 700 and 1000 Kelvin, grain sizes between 5 and 50 micrometers, strain rates and pressures as above). Experiments include simple shear, torsion, and conventional triaxial loading. For deformation at laboratory strain rates, confining pressures less than 300 Megapascals, and temperature less than 700 Kelvin, dislocation interactions at twin boundaries may cause hardening that eventually lead to brittle failure. Above 700 Kelvin, dislocation cell size may be an important state variable in the constitutive law. Detailed observations of this microstructure and its evolution will be made using optical microscopes, scanning and transmission electron microscopes, and Electron Backscatter Diffraction to correlate structure with the production of lattice preferred orientation, dynamic recrystallization, and the evolution of strength. Similar microstructure observations will be made for suites of natural calc-mylonites collected along traverses perpendicular to the Morcles thrust (Swiss Helvetic Nappes) at several different locations along the dip of the fault. Previous field studies provide detailed information of microstructure, geothermometry, and geochemistry that can be used to constrain the large-scale mechanical history of this. This setting provides two distinct opportunities: 1) to investigate the influence of temperature on the localization of strain into the main thrust plane of the Morcles Nappe, and 2) to investigate the influence of varying quartz and dolomite content on strain localization within the carbonates.
Localization of natural rock deformation is ubiquitous, at all scales of observation, under a broad range of natural temperatures and pressures. In collisional mountain belts, shear zones and faults are often found within carbonates, and the extreme localization implies that the mechanical properties of those rocks are critical in determining the strength of the large scale rock mass. Understanding the mechanical properties of rocks deforming under natural conditions is a difficult problem because natural processes occur at extreme pressure and temperatures and at very slow strain rates. To back-calculate the mechanical behavior of rocks under conditions that are not accessible to direct experimentation requires careful experimentation, a thorough knowledge of the kinetics of the processes involved, observations of the microstructure of naturally deformed rocks, and comparison of the natural and experimental microstructures. The ultimate goal is to understand natural tectonic processes including mountain building, faulting, earthquake mechanics, and the overall mechanical budget of the crustal deformation cycle.
