Geology of Himalayan Soils

Abstract

The Himalayan mountain range is sub-divided into four principal tectonic zones, from south to north. These are the Sub-Himalaya, the Lesser Himalaya, the Higher Himalayan Crystallines, and the Tethyan Himalaya. The Sub-Himalaya, also known as the Shiwalik Range (250–800 m high), rises above the Indo-Gangetic Plains along the Main Frontal Thrust. The Higher Himalayas, also known as the Central Crystalline zone, are comprised of ductile deformed metamorphic rocks and mark the axis of orogenic uplift. Mica schist, quartzite, paragneiss, migmatite, and leucogranite bodies characterize this uppermost Himalayan zone. Corresponding mineral assemblages are dominated by biotite to sillimanite, representing greenschist to amphibolite facies deformation. The Higher Himalayan Crystalline rocks have imbricated thrust sheets, with the grade of metamorphism increasing from the Chail Group to the Vaikrita Group. The Chail Group consists of a metamorphosed sequence of greenschist facies including phyllites, phyllitic quartzite, psammitic schists, orthoquartzites, arkose, chlorite schist limestones, and metabasic rocks amphibolites. The Sub-Himalaya range consists predominantly of Tertiary and Quaternary sediments and is bounded to the north by northward dipping Main Boundary Thrust, separating it from the overlying Lesser Himalaya. The Lesser Himalayan Range, in general, is quite rugged and higher than 2500 m, however, in the Kashmir Valley (Northwestern Himalaya), the Pir Panjal Range rises to above 3500 m. The Lesser Himalayas consists of Precambrian and Cambrian sequences of the Damtha, Tejam, Jaunsar–Garhwal, and Mussoorie Group in Garhwal region of the western Himalaya. The Mussoorie Group is represented by a persistent horizon of conglomerate intercalated with graywackes and siltstones at the base, which pass into carbonaceous slates and varicolored limestone. This succession is followed stratigraphically by carbonate-limestone, marl, slate, dolomite horizon in the middle and shale, conglomerate interbedded with phosphatic carbonaceous–pyritous slates, black limestone, and pyritous and felspathic quartzite in the upper portion. The Tethyan Himalayas consist of thick, 10–17 km, marine sediments that were deposited on the continental shelf and slope of the Indian continent. The Shiwalik zone consists of clastic sediments that were produced by the uplift and subsequent erosion of the Himalayas and deposited by rivers. These rocks have been folded and faulted to produce the Shiwalik Hills that are at the foot of the great mountains. Sub-Himalayan rocks have been overthrust by the Lesser Himalayas along the Main Boundary Thrust Fault. It not only shows geological divisionsGeological divisions within the mountain belt, but also structuresStructure and geologic relationships between rock types and structuresStructure.