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Himalayan Geology, Vol. 45 (1), 2024, pp. 123-137, Printed in India

Mio-Pliocene tectonic activity along the Main Boundary Thrust and exhumation of the Amritpur Granite Body, Kumaun region, northwestern Himalaya


1Wadia Institute of Himalayan Geology, 33 GMS Road, Dehradun - 248001, India

2Department of Geophysics, Kurukshetra University, Kurukshetra - 136119, India

3Department of Earth and Planetary Sciences, University of Allahabad, Prayagraj - 211002, India

*Email (Corresponding authors):

Abstract: The Main Boundary Thrust (MBT) is studied along the Baliya-Amritpur-Jamrani road section of the Baliya Nala-Gola River valley including an isolated granite body, associated with MBT hanging wall in the Kumaun region of the NW-Himalaya. In this study, we have reviewed available Fission-Track Thermochronological ages and seismicity data in combination with the new field evidence and geomorphological data to understand the mechanism for the exhumation of the Amritpur Granite Body (AGB) and the tectonic movement history along the MBT. Furthermore, the timing of the tectonic activity along the MBT and its role in the exhumation of the AGB are explored. The study focused on two different locations, where the Amritpur Granite is in direct contact with the Siwalik rocks along the MBT. The available apatite and zircon Fission-Track (AFT/ZFT) ages are range between 11.3 Ma to 14.7 Ma with a mean of 13.4 Ma and 12.4 Ma to 15.4 Ma with a mean of 13.9 Ma respectively. Mean AFT and ZFT ages of the AGB have similar age trends from the MBT to the Salari Thrust, which indicates that the AGB was rapidly uplifted and exhumed along the MBT from the basement at a depth of ca. 8-10 km during the middle Miocene (ca. 14-13 Ma). Similarly, the AFT ages of the sandstone samples from the contact zone of AGB and middle Siwalik (MBT's footwall side) are completely reset and age ranges between 4.4 Ma to 5.5 Ma with a mean of ~5.0 Ma reveal the tectonic reactivation of the MBT during Pliocene-Quaternary period. The morphometric trends suggest that the AGB has a lower erosion susceptibility than the rocks available on the northern side of AGB. Furthermore, an aggregated map and morphometric index revealed that the AGB has aligned along the regional faults (i.e., MBT and Salari Thrust), which also indicate the demarcation of the dendritic drainage pattern of the watershed with low susceptibility to erosion. The scattered pattern of seismicity also indicates the tectonic activity along the MBT almost ceased after the Pliocene-Quaternary period. Based on the morphometry, field evidences and available FT thermochronological age data, we envisage that the AGB was exhumed to the surface from the basement as 'Tectonic Slivers' during the development and reactivation of the MBT between Miocene to Pliocene period.

Keywords: Main Boundary Thrust, Amritpur Granite Body, outer LHMS zone, Kumaun region, Northwest Himalaya

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