Dating ductile deformation of the Earth’s crust: the critical role of fluids
Abstract
Abstract Absolute dating of ductile deformation is fundamental to developing accurate models of the temporal and kinematic evolution of orogens. However, our limited understanding of how chemical reactions and isotopic (re)equilibration progress during mylonitisation hinders unambiguous interpretations of deformation ages. We test the hypothesis that deformation, chemical reactions and isotopic (re)equilibration are spatially and temporally correlated. We combine high resolution microchemical and microstructural mapping of white mica, biotite and K-feldspar with in-situ 40 Ar- 39 Ar and 87 Rb- 87 Sr dating to quantify isotopic relicts preserved in variably deformed meta-granites across a shear zone in the Gotthard nappe (Swiss Alps). We find that the abundance of isotopic relicts positively correlates with strain accumulation, thereby rejecting our hypothesis. Chemical and isotopic identification of pre-, syn- and post-deformational Alpine mineral generations constrains metamorphism to 21.78 ± 0.78 Ma, the inset and end of mylonitisation to > 18.81 ± 0.68 and 17.12 ± 0.23 Ma, respectively, and retrogression to 13.33 ± 0.29 Ma. These results demonstrate that fluids, rather than strain accumulation or temperature drove isotopic (re)equilibration. Consequently, reliable direct dating of ductile deformation requires establishing a link between fluid-driven mineral (re)equilibration and deformation, rendering chemical mapping an essential tool.
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