Strain localization in the Morcles nappe (Helvetic Alps, Switzerland)

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.