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Thin, 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.

Plain Language Summary Rocks in Earth's upper crust are brittle, causing tectonic movements on fault planes to occur as earthquakes. With increasing depth, temperature and pressure increase and cause the rock to flow rather than fracture. Based on field and seismological studies, we know that deformation in the deeper crust still occur in localized zones, known as "shear zones," but we do not have a good understanding of the strength of these zones and consequently of the strength of the crust. A long-held view is that shear zones are weakest when made up of sheet-like minerals allowing deformation to occur like a deck of cards pushed from its side. We found, however, that the weakest cores of shear zones can surpass this form of weakness and consist of a fine-grained mixture of several minerals with a range of strengths. We hypothesize that this mixture is weaker than any of its components by themselves.