Dating protracted fault activities: Microstructures, microchemistry and geochronology of the vaikrita thrust, main central thrust zone, garhwal himalaya, nw india

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 muscovite39 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.