Influence of Heterogeneities on the Initiation of Shear Zones in the Ductile Regime

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Strain localization into shear zones of variable scales is a common feature in tectonic processes. A number of previous studies focused on the evolution of brittle fault zones in the upper crust, but little is known about the initiation and propagation of ductile shear zones at greater depth. One possible origin of the development of these shear zones is the presence of structural heterogeneities that may locally produce stress concentrations leading to the activation of brittle or plastic softening mechanisms. To examine this scenario, we propose a novel experimental study exploring the processes of high temperature shear zone nucleation and growth, and the associated mechanical and microstructural response. Torsion experiments will be conducted on Carrara marble samples that contain artificially prepared inclusions of different shapes and contrasting rheological behavior. Suggested experimental conditions are 500-1000°C temperature, 300-400 MPa confining pressure, and strain rates between 10-3 s-1 and 10-5 s-1. The local strain and stress distribution in the vicinity of the inclusions will be determined using µm-scale strain markers by adopting a recently developed strain mapping technique. In addition, a detailed microstructural analysis will be performed, including transmission electron microscopy and the determination of preferred crystallographic orientations via electron backscatter diffraction. The experimental results will be compared with microstructures of natural shear zones in calcite rocks of the Morcles nappe (Helvetic Alps). The goal of this proposal is to characterize the processes involved in strain localization from an initial structural heterogeneity in the ductile regime and to formulate constitutive laws that capture key aspects of the mechanisms.