20 years of geological mapping of the metamorphic core across Central and Eastern Himalayas

Abstract

The largest crystalline unit representing the mid-crust in the Himalayan belt is the Greater Himalayan Sequence (GHS) which stretches all over the 2400 km of length of the belt. The GHS, recognised since the first geological explorations of the Himalayas, has been considered for a long time as a coherent tectonic unit, exhumed by the contemporaneous shearing along the Main Central Thrust and the South Tibetan Detachment System in the time span ~ 25-17 Ma. A multidisciplinary approach, integrating geological mapping, structural analysis, petrology and geochronology allowed to better constraints on its internal architecture characterised by several levels of tectonic-metamorphic discontinuities on the regional scale with a top-to-the-S/SW sense of shear and active since ~ 40 Ma. The GHS is consequently divided in three main tectonic units exhumed progressively from the upper part to the lower one by ductile shear zones, later involving the Lesser Himalayan Sequence. Above the Main Central Thrust a cryptic tectono-metamorphic discontinuity (Higher Himalayan Discontinuity; HHD) has been recognized and mapped in Central-Eastern Himalaya. The mapping of the HHD has been allowed by the use of a multidisciplinary approach involving structural analysis, geochronology and petrography. A new map of Western Nepal is presented. In this framework the popular models of exhumation of the GHS mainly based on the contemporaneous activity of the two bounding shear zones (Main Central Thrust and the South Tibetan Detachment) and considering the GHS as a coherent tectonic unit, should be reconsidered. An in-sequence shearing tectonic model, from the deeper to the upper structural levels, further affected by out-of-sequence-thrusts, is more appropriate to explain the deformation, metamorphism and exhumation of the mid-crust in the Himalayan belt. Geological mapping of the Himalayan belt is very far away to be exhaustively completed. Anyway during the last 20, and particularly during the last few years, it has been notably improved due to a new multidisciplinary approach.

Modeling High‐Resolution Pressure‐Temperature Paths Across the Himalayan Main Central Thrust (Central Nepal): Implications for the Dynamics of Collision

“…Ages of monazite inclusions in garnet and reported 40 Ar/ 39 Ar muscovite ages are used only as a guide to inform the model when the MCT and MCT‐I were active. The timing of MBT activity is suggested to be >11 Ma (see review in Yin, ), but this is not well constrained, and other imbricate structures exist south of the MCT and north of MBT that may also be potential candidates to accommodate a transfer of activity (e.g., Carosi et al, ; DeCelles et al, ; Hauck et al, ; Robinson & Martin, ). We choose the MBT as it is a significant thrust system that appears to have operated at the time indicated.…”

Provenance of Bengal Shelf Sediments: 1. Mineralogy and Geochemistry of Silt

“…The Ganga River drains the northern part of the Precambrian Indian shield with its sedimentary and basaltic (Deccan Traps) covers, and all tectonic units of the Himalayan belt. These include the Paleozoic to Eocene sedimentary succession of the Tethys Himalaya [30,31], mainly amphibolite-facies metamorphic rocks of the Greater Himalaya [32,33], lower-grade metamorphic rocks and sedimentary strata of the Lesser Himalaya [34,35], and foreland-basin siliciclastic rocks of the Sub-Himalaya [36,37]. In addition to the Himalayan belt, the Brahmaputra River drains granitoid batholiths and sedimentary covers of the Lhasa block [38], the Transhimalayan forearc-basin [39] and Yarlung Tsangpo ophiolitic suture [40], and high-grade metamorphic rocks of the eastern Himalayan syntaxis [41].…”

Three‐dimensional vorticity and time‐constrained evolution of the Main Central Thrust zone, Garhwal Himalaya (NW India)

“…The most prominent of these tectonic discontinuities is the MCTz (Heim & Gansser, ; Searle et al, ), a km‐thick zone of intensively sheared rocks (Figure ), which separates the GHS from the underlying Lesser Himalayan Sequence (LHS), the latter consisting of low‐ to medium‐grade metamorphic rocks (Arita, ). The exact localization of the MCTz boundaries together with its temporal and structural evolution are still a matter of debate (Carosi, Montomoli, & Iaccarino, ; Martin, ; Searle et al, for updated reviews). The time of activity of the MCTz ranges from 23 to 15 Ma up to c. 3 Ma in different areas of the belt (Godin, Grujic, Law, & Searle, ; Montomoli, Carosi, & Iaccarino, ).…”