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Dating deformation is difficult, as textures and petrogenesis of deformed rocks are complex. Moreover, geochronometer categories are pursued by communities that often do not communicate. Hygrochronology dates the retrograde metasomatic/metamorphic reactions caused by aqueous fluid circulation events. Thermochronology models time-temperature histories by assuming that mineral ages can be uniquely assigned to a ‘closure temperature T c ’, the only process occurring in rocks being Fick's Law diffusion. Diffusion by definition produces a bell-shaped concentration profile. In contrast, patchy intra-grain isotope concentration profiles denounce aqueous retrogression, whose rate is orders of magnitude faster than diffusion. Petrochronology is based on opposite assumptions, as the mobility of structure-forming major cations is higher than that of radiogenic Pb, Ar, and Sr. Whenever the formation of a mineral occurs at T T c , its apparent age dates its formation. Nanochronology analyses samples at the nanometre-scale. These analyses illuminate atomic-scale processes, e.g. open-system transport of soluble ions along self-sealing networks of nanopores. The key to dating deformation and producing correct, regional-sized (up to hundreds of kilometres) tectonic models is the realization that minerals consist of atoms, whose behaviour is only firmly constrained by nanometre-scale analyses. Thematic collection: This article is part of the Isotopic Dating of Deformation collection available at: https://www.lyellcollection.org/cc/isotopic-dating-of-deformation

Fig. S3: Biotite Mg/Mg+Fe+Mn vs Al diagram of P2 ash layers.

Fig. S4: Biotite Mg/Mg+Fe+Mn vs Mn diagram of P2 ash layers.

Histograms reporting thermometric data obtained with Ti-in-biotite and Ti-in-muscovite geothermometers. (a, c) Data on muscovite along the Sm (muscovite-2) and coronitic around garnet (muscovite-3), respectively. (b, d) Data on biotite along the Sm (biotite-2) and coronitic around garnet (biotite-3), respectively. The legend in (b–d) is the same as in (a).

Fig. S5: Biotite Mg/Mg+Fe+Mn vs K+Na+Ca diagram of P2 ash layers.

Fig. S2: Stratigraphic logs of the studied sites.

Fig. S1: 39Ar–40Ar age spectrum of the biotite separate of the tephra layer LB-T11.