Grain size evolution and strain localization in deformed marbles

Abstract

In the lithosphere, strain is frequently accommodated along localized shear zones, the rheology of which are inextricably linked to their microstructural characteristics. Shear zones in orogenic belts frequently form in calcite-rich lithologies due to the relatively low strength of these rocks, especially when compared to quartz- and feldspar-rich lithologies. This dissertation addresses the kinetics of microstructural evolution during deformation of calcite-rich rocks, and the coupling between the rheological properties of deforming rocks and such microstructural characteristics as grain size and crystallographic preferred orientation (CPO) by combining laboratory studies with field based observations. In Chapter 2, a scaling relationship for recrystallized grain size is derived based on a balance between the rate that mechanical work is done during deformation and the rate that this energy can be dissipated. This suggests that recrystallized grain size reflects the product of stress and strain rate rather than stress alone. When this scaling relationship is applied to measured calcite grain sizes from the Morcles nappe, in the Swiss Helveitc Alps, where the microstructure is unaffected by second phases, the geologically estimated regional strain rates are successfully reproduced, and when it is applied to samples collected along a transect perpendicular to the thrust contact, the results suggest strain became progressively localized (Chapter 3). This calculation is consistent with the increased CPO intensity which is observed in the finest grained mylonites closest to the thrust contact. In Chapter 4, laboratory deformation experiments are used gain insight into the kinetics of microstructural evolution in calcite-rich rocks. The grain size evolution rates measured in experiments correlate well with the product of the measured stresses and strain rates, consistent with the scaling relationship presented in Chapter 2. These experiments also suggest that, with increasing strain, strain rates will increase at constant stress, and there is a correlation between this weakening and the formation and intensification of CPO, consistent with the observations in the Morcles nappe. Rocks are seldom composed of a single mineral phase.

(cont.) The influence of rigid second phases on the strength, CPO, and grain size of deforming marbles is investigated through a combination of hydrostatic, compression, and torsion experiments performed in the laboratory, which are presented in Chapters 5&6. Both the volume fraction and geometry of rigid second phases have a dramatic effect on the mechanical strength, the CPO, and the grain size. In samples deformed to high strains in torsion, where the recrystallized grain size resulting from deformation is finer than the grain size due to pinning of grain boundaries by the rigid inclusions, the scaling relationship derived in Chapter 2 successfully predicts the measured recrystallized grain size. Thus, this scaling relationship is consistent with both grain size evolution rates and stabilized grain sizes measured in the laboratory, and with field constraints on the strain rates in the Morcles nappe.