Evaluating the distinct identity and the orogen-scale nature of the Munsiari thrust in the Himalaya

Abstract

The Munsiari thrust (MT) was first recognized in northwest India within the Paleoproterozoic Lesser Himalayan rocks, but debate persists over its distinct identity and orogen-scale nature. Some researchers have included the MT within a ∼3–10 km-wide Main Central thrust (MCT) zone, whereas others proposed that, it is the northern and deeper extension of the Ramgarh thrust (RT). This study integrates field mapping, quantitative microstructural analysis, grain-size measurement, and piezometry to evaluate the distinct identity of the MT from North Sikkim, in eastern Himalaya. Our study reveals the presence of a ∼1700 m-thick prominent shear zone showing thrust-sense motion. The mylonites in the ∼300 m-thick core of the shear zone deformed by DRX (dynamic recrystallization)-assisted dislocation creep, and by climb-assisted dislocation creep in the ∼400–1000 m-thick flanking zones. We infer the mapped shear zone represents Sikkim-equivalent of the MT, because it transported Paleoproterozoic Lesser Himalayan crystalline rocks over Paleoproterozoic Lesser Himalayan quartzites and phyllites. The MCT and the RT lie ∼6 km structurally above-and-below the MT respectively. By comparing our findings with published results, we infer that, the MT is an orogen-scale structure, traceable from northwest Himalaya to far-eastern Himalaya, and it facilitated rapid exhumation of the Lesser Himalayan crystalline rocks between mid-Miocene and Pliocene. Therefore, we propose to reserve the appellation of the “Munsiari thrust” for a shear zone that transported Paleoproterozoic Lesser Himalayan crystalline rocks over Lesser Himalayan quartzites and phyllites of similar age, and suggest not to confuse it with the MCT or the RT.