Retrospective on "Sense of Shear" in relation to deciphering Metamorphic Core Complexes

George H. Davis

“Making sense of shear” was an important part of the challenge and opportunity presented by recognition of metamorphic core complexes in the mid-1970s. I have found it useful - admittedly with the inherent limitations of partial recall - to think back 3 decades later on the impediments and challenges experienced in describing and interpreting metamorphic core complexes, and how these hurdles were cleared. The scientific frenzy that accompanied mapping, analysis, and interpretation of metamorphic core complexes both advanced and benefited from meaningful new insights regarding shear zone deformation, fault rocks, deformation mechanisms, and sense-of-shear criteria. Structural geologists and tectonists who carried out the discovery-mapping of metamorphic core complexes in the Western Cordillera of the North America elevated extensional tectonics as a formidable mechanism for producing complex mountain systems, doing so in the face of significant resistance. They contributed to the advancements in part through innovative field work and structural geologic mapping of complex regional systems, but also through bringing colleagues together in field and conference settings to puzzle over unresolved questions, many of which revolved around interpretation of fabrics and timing of deformation. A paradigm shift in managing fault-rock taxonomies and demystifying sense-of-shear determinations based on ‘reading’ fine-scale fabrics in the field came importantly from geoscientists working and/or trained outside of the United States, who collectively pioneered a reformed look at fault rocks, deformation mechanisms, and sense-of-shear criteria. U.S. structural geologists and tectonists working on metamorphic core complexes benefited ‘instantly’ from the cross-talk. The immediacy of impact across the structure-tectonics community is historic as related to research, teaching, and PhD training. Altogether it is a perfect example of the transformative power of grassroots interdisciplinary research across boundaries.