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GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L08307, doi:10.1029/2007GL029250, 2007

A constitutive model for layer development in shear zones near the brittle-ductile transition

Laurent G. J. Montési

Department of Marine Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, USA


Abstract

The microstructure of ductile shear zones differs from that of surrounding wall rocks. In particular, compositional layering is a hallmark of shear zones. As layered rocks are weaker than their isotropic protolith when loaded in simple shear, layering may hold the key to explain localization of ductile deformation onto ductile shear zones. I propose here a constitutive model for layer development. A two-level mixing theory allows the strength of the aggregate to be estimated at intermediate degrees of layering. A probabilistic failure model is introduced to control how layers develop in a deforming aggregate. This model captures one of the initial mechanism of phase interconnection identified experimentally by Holyoke and Tullis (2006a, 2006b), fracturing of load bearing grains. This model reproduces the strength evolution of these experiments and can now be applied to tectonic modeling.

Received 15 January 2007; accepted 26 March 2007; published 27 April 2007.

Keywords: Shear zone; foliation; rheology.

Index Terms: 3625 Mineralogy and Petrology: Petrography, microstructures, and textures; 5104 Physical Properties of Rocks: Fracture and flow; 8012 Structural Geology: High strain deformation zones; 8020 Structural Geology: Mechanics, theory, and modeling; 8159 Tectonophysics: Rheology: crust and lithosphere (8031).

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Citation: Montési, L. G. J. (2007), A constitutive model for layer development in shear zones near the brittle-ductile transition, Geophys. Res. Lett., 34, L08307, doi:10.1029/2007GL029250.