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[1] There is a strong coupling between the microstructure and the strength of rocks that is thought to play a key role in the evolution of shear zones and in our ability to interpret the mechanics of natural deformation processes. To investigate the microstructural evolution of calcite-rich rocks, we have performed a series of hydrostatic and deformation experiments on synthetic calcite aggregates at 1023 K and 300 MPa. The mechanical data from our experiments were broadly consistent with a composite flow law for concurrent dislocation and diffusion creep. Recrystallization rates responded to the deformation conditions. When the bulk strain rate was dominated by diffusion creep, calcite grains grew at the same rate as occurs by normal grain growth under isostatic conditions. When the dominant deformation mechanism was dislocation creep, the matrix grain size was reduced at a rate that varied directly with product of stress, strain rate, and the square of grain size. Thus, reduction rate was proportional to mechanical work rate. If the stable grain size achieved during deformation depends on the product of stress and strain rate, rather than stress alone, then that grain size is an indication of the work rate and can be used as a paleowattmeter. Following this line of thought suggests that the gradient of recrystallized grain sizes that is often observed in the highly deformed portions of shear zones would not require a gradient in stress but could also be explained by material softening, resulting in locally elevated strain rates under constant stresses.

Deformation in orogenic belts is frequently accommodated in calcite-rich lithologies, owing to their relatively low strength, particularly compared to quartz-rich rocks. Here, we investigate the coupling between calcite grain size, the presence and mineralogy of second phases, and crystallographic preferred orientation (cPO) in a transect through deformed limestones, perpendicular to the dominant foliation in the inverted limb of the Morcles nappe of the swiss Helvetic Alps. calcite grain size becomes progressively finer as the thrust contact is approached, and there is a concomitant increase in cPO intensity, with the strongest cPO's in the finest-grained, quartz-rich limestones, nearest the thrust contact. to understand the distribution of strain and the extent of strain localization, we compared the paleowattmeter and calcite flow laws from laboratory studies to previously published observations of microstructure at a range of locations, with varying peak metamorphic temperatures, along the Morcles nappe. the strain-rates predicted by extrapolation of these laboratory relationships agree well with the geologic constraints. We then applied the same approach to the samples from the present study. the results indicate that strain became progressively localized towards the thrust contact of the Morcles nappe, leading to an increase in strain rate of >1 order of magnitude in a zone <0.50 m thick. For localization to occur system and/or material softening is necessary. If dislocation activity is positively correlated with cPO, then softening cannot have occurred by a complete transition to diffusion creep in the finest grained samples. rather, softening may have resulted from the formation of cPO, possibly coupled with effects related to the distribution of second phases and the overall geometry of the shear zone. strain localization in the Morcles nappe (Helvetic Alps, switzerland)

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To investigate the role of texture on the brittle deformation of dolomite, 23 triaxial deformation experiments were performed at confining pressures of 25, 50, and 100 MPa, dry, at room temperature, on dolomite from three texturally distinct sample suites. The variations in the mechanical response of these mineralogically and chemically similar dolomites, and the ensuing microstructures, indicate that grain boundary textures promote or inhibit the ability of grains to shear and rotate with respect to one another, whereas the presence of intragranular flaws, such as cleavage, that act as weaknesses, promote intragranular deformation. In samples with porosities greater than c. 7%, inelastic pore collapse controls the transition from brittle faulting to extensive intragranular deformation and cataclastic flow. This porosity is much higher than has been observed at the onset of pore collapse in calcite, as a consequence of the inability of dolomite to deform by crystal plastic processes at room temperature. Combined, these textural features may dictate the transition from brittle faulting to cataclastic flow in brittle rocks in the upper crust.