Weaker Than Weakest: On the Strength of Shear Zones

Abstract

AbstractThin, laterally continuous ultramylonites within kilometer‐scale ductile shear zones may control middle to lower crustal strength where deformation is localized. 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 localization 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.