Weaker than weakest: on the strength of shear zones

Abstract

Thin, laterally continuous ultramylonites within kilometer‐scale ductile shear zones may control mid‐lower crustal strength where deformation is localised. Interconnected phyllosilicate networks are commonly suggested to be the weakest geometry a shear zone can reach, yet fine‐grained polyphase mixtures are commonly found in the cores of high strain zones. We study a continental strike‐slip shear zone which deformed granulite facies quartzofeldspathic migmatitic gneisses at retrograde amphibolite‐ to greenschist facies conditions. A brittle feldspar framework and interconnected phyllosilicate networks control the strength of the lower strain protomylonites and mylonites respectively, whereas the ultramylonites comprise a fine‐grained mixture of the host rock minerals. The localisation of strain in ultramylonites demonstrates how fine‐grained polyphase mixtures can be weaker than, and supersede, interconnected phyllosilicate networks with increasing shear strain. This contradicts the common assumption that interconnected layers of phyllosilicates is the weakest state a shear zone can reach.