A peculiar feature of the Himalaya is the occurrence of a system of low‐angle normal faults and shear zones, the South Tibetan Detachment System (STDS), at the mountain crests. The STDS was active during synconvergent tectonics. We describe the STDS‐related sheared rocks along the Dhauli Ganga valley, in the Garhwal Himalaya (NW India), where the Malari granite, reported as an undeformed igneous body crosscutting the STDS, occurs. A detailed multidisciplinary study, integrating field‐based, microstructural, petrographic, and geochronological analyses, was carried out on rocks along this valley. We demonstrate how the noncoaxial ductile portion of the STDS affected the upper part of the Greater Himalaya Sequence migmatite, which experienced peak pressure (P)‐temperature (T) conditions of 0.9–1.1 GPa and ≥750°C at ≥24 Ma. This migmatite has been reworked structurally upward leading to the formation of high‐T sillimanite‐bearing mylonites. Further upward, medium‐T shearing deformed the Malari granite and leucogranite dykes, forming medium‐T mylonites. Ductile shearing was temporally constrained, based on new in situ monazite datings and previously published Ar‐Ar geochronology, between ~20 and 15 Ma. We demonstrate that a preserved ductile to brittle spatial and temporal transition of the STDS deformation exists, with the brittle features overprinting ductile ones. Our data shed new light on the geological evolution of the STDS in the NW Himalaya with implications for the relationship and relative timing of partial melting, granite emplacement, and deformation along low‐angle normal faults.

The timing of shearing along the Vaikrita Thrust, the upper structural boundary of the Main Central Thrust Zone in the Garhwal Himalaya, was constrained by combined microstructural, microchemical and geochronological investigations. Three different biotite–muscovite growth and recrystallization episodes were observed: a relict mica-1; mica-2 along the main mylonitic foliation; and mica-3 in coronitic structures around garnet during its breakdown. Electron microprobe analyses of biotite showed chloritization and a bimodal composition of biotite-2 in one sample. Muscovite-2 and muscovite-3 differed in composition from each other. Biotite and muscovite ³⁹ Ar– ⁴⁰ Ar age spectra from all samples showed both inter- and intra-sample discrepancies. Biotite step-ages ranged between 8.6 and 16 Ma and muscovite step-ages between 3.6 and 7.8 Ma. These ages cannot be interpreted as ‘cooling ages’ because samples from the same outcrop cooled simultaneously. Instead, the Ar systematics reflect sample-specific recrystallization markers. Intergrown impurities were diagnosed by the Ca/K ratios. The age data of biotite were interpreted as a mixture of true biotite-2 (9.00 ± 0.10 Ma) and two alteration products. The negative Cl/K–age correlation identified a Cl-poor muscovite-2 (>7 Ma) and a Cl-rich, post-deformational, coronitic muscovite-3 grown at ≤5.88 ± 0.03 Ma. The Vaikrita Thrust was active at least from 9 to 6 Ma at c. 600°C; its movement had ended by 6 Ma. Supplementary material: Thermometric and ³⁹ Ar– ⁴⁰ Ar data are available at https://doi.org/10.6084/m9.figshare.c.4069076

Dating structurally complex fault rocks often results in internally inconsistent ages, as several mineral generations are intergrown at scales ≪ 10 µm and are almost always altered to various degrees. First, electron probe microanalysis is necessary to assess both inventory and spatial distribution of minerals and their retrogression or alteration phases. We then used ⁴⁰ Ar/ ³⁹ Ar step-heating combining two independent indicators that allow the discrimination of coexisting mica generations from each other: (1) mica stoichiometry, which is proxied by ³⁹ Ar concentration in combination with ³⁷ Ar/ ³⁹ Ar and ³⁸ Ar/ ³⁹ Ar (Ca/K and Cl/K) ratios; (2) furnace temperature at which the degassing peak accompanying dehydration and structural collapse is observed. As dehydration rates depend on average bond strength in the crystal structure, it is predicted and observed that the temperature of the differential Ar release peak is variable among different minerals. We observe that the Ca/Cl/K signatures of pure micas coincide with the Ar release peak. The Main Central Thrust zone in the Garhwal Himalaya records a protracted history. Foliation of the Vaikrita Thrust formed at 15–8 Ma, followed by static decompression at 7 Ma; foliation of the structurally lower Munsiari Thrust formed around 5 Ma. Our elaborate and time-consuming petrochronological procedure should become routine whenever analysing polydeformed metamorphic rocks. Supplementary material: Complete data for the studied samples are available at https://doi.org/10.6084/m9.figshare.c.5357212 Thematic collection: This article is part of the Isotopic dating of deformation collection available at: https://www.lyellcollection.org/cc/isotopic-dating-of-deformation