A new Geological Map of the Greater Himalayan Ranges; mountain building processes

A new geological map (scale: 1:1.5 M) of the Greater Himalayan Ranges has been compiled and will be published in two sheets. The Western Sheet includes the Pamir, Hindu Kush, Karakoram, Kohistan, Ladakh ranges, including SW Tibet, with the Nanga Parbat syntaxis, and the Pakistan and Indian Himalaya. It extends from the Afghanistan-Pakistan border in the west to the India-Nepal border in the east. The Eastern Sheet includes the Nepal, Sikkim, Bhutan, Arunachal Pradesh ranges, southeast Tibet, Namche Barwa syntaxis, the Shillong Plateau and northwest Myanmar (Burma) ranges. The map includes four cross-sections, and a detailed Key. Together with magmatic, metamorphic, thermobarometric, strain measurements, and geochronological and geophysical data, several key geological processes can be deduced from all these geological strands:

The India-Asia collision is marked by the Indus – Yarlung Tsangpo suture zone containing Tethyan oceanic sedimentary and volcanic rocks, and Jurassic and Cretaceous ophiolites. The age of final collision is given by the youngest marine Nummulitic limestones (50.5 Ma), after which continental fluvial conglomerates and lacustrine Indus Group molasse sediments predominate. Structures along the ITSZ are steep and dominantly folded and backthrust to the north. The northern Himalaya (Tethyan Himalaya) consist of strongly folded and thrust shelf sediments of Neoproterozoic – Eocene age, showing south-vergent folds and thrusts in the south and steep north-vergent backthrusts and folds along the north (Great Counter Thrust).

The western (Nanga Parbat) and eastern (Namche Barwa) Himalayan syntaxis both exhume Pliocene-Pleistocene metamorphic-magmatic (U-Pb monazite ages 0.9-0.7 Ma) cordierite+sillimanite leucogranite melts and migmatites, that may represent deep, unexposed levels of the northern Himalaya. The Greater Himalayan Sequence comprises the metamorphic core of the Himalaya and corresponds to the major zone of high topography. Eocene – Miocene metamorphism reaches kyanite grade, and later sillimanite grade. The youngest (~13-11 Ma) and deepest (~1.2 GPa) granulite facies rocks in the Ama Drime range, north of Everest, contain lenses of garnet+omphacite+orthopyroxene granulitised eclogites representing the deepest levels of thickened crust. Himalayan migmatites and leucogranites containing K-feldspar+garnet+tourmaline+muscovite+biotite with late cordierite, occur along the highest structural levels of the Great Himalayan Sequence along the entire chain. U-Pb monazite ages span 35-11 Ma, with the majority around 23-19 Ma. Many of the highest peaks are comprised of these leucogranites (Shivling, Manaslu, Shisha Pangma, Langtang Lirung, Nuptse, base of Everest, Makalu, Kangchenjunga, Kula Kangri etc). Most leucogranites have intruded as sills that can mapped for >70 km across strike in the Rongbuk valley, Tibet.

The metamorphic core of the Himalaya is bounded by a north-dipping low-angle normal fault (South Tibetan Detachment) showing right way-up metamorphic isograds along the top, and a south-vergent ductile shear zone (Main Central Thrust) with inverted metamorphic isograds along the base. The geometry is consistent with the southward extrusion of a partially molten layer of middle crust during the Miocene (Channel Flow). The Lesser Himalaya structure is consistent with the Critical Taper model. The active Main Himalayan thrust is the youngest thrust along which the Indian plate is subducting and uplifting the Himalaya now.