The Himalayan passive margin from Precambrian to Cretaceous times

Abstract

The Himalaya passive margin should be separated into the Indian Himalaya and the Pakistan Himalaya, divided by the Nanga Parbat-Haramosh basement uplift in the northwestern syntaxes. The Indian passive margin shows a complex history involving splitting of microcontinents off the northern Gondwana margin from early Paleozoic times until the Jurassic. In contrast, the Pakistan Himalaya formed part of the stable Indian shelf until separation from Africa started in the Jurassic. Each margin has a distinct stratigraphic history until the mid-Mesozoic after which they show similar histories until the Tertiary.The Himalaya mountains in India consist of three distinct tectonic units juxtaposed during the Neogene by southward thrusting of the northern Indian Precambrian to Mesozoic passive margin. The Lesser Himalaya on the south forms a late Precambrian passive margin usually directly overlain by late Cretaceous to Tertiary clastics: its stratigraphy is closest to that of the Indian Shield. In contrast, the High Himalaya has a thin Ordovician to Carboniferous shelf sequence deposited after early Ordovician deformation and granite intrusion. Above this, thick Permian and Mesozoic shelf sequences mark the separation of continental blocks off the northern Indian margin and the opening of the Neotethys ocean. The North Himalaya formed the slope and basin of this ocean and consists of reactivated Paleozoic gneiss domes overlain by thick Mesozoic sediments which pass, in the northeastern Himalaya, into an enormously thick sedimentary sequence resting on oceanic crust.Paleomagnetic and structural evidence indicates at least 500 km of southward thrusting along the Main Central Thrust between the Lesser and High Himalaya. This thrusting, together with a further 250 km of erosion at the front of the nappes, has removed the entire inner shelf of the Himalaya passive margin and can explain the startling contrast between the High and Lesser Himalaya stratigraphic sequences. But there are still discrepancies, particularly in the eastern Himalaya, where Precambrian basement is juxtaposed with the North Himalaya Mesozoic slope. These discrepancies can be partly resolved by strike-slip movements roughly parallel to the Himalaya which have removed parts of the northern Indian passive margin. Such faults of the requisite orientation, displacement and age occur in southeast Asia where their cumulative displacements can add up to several thousand kilometres. Similar faults, of undoubted pre-Tertiary age, occur in the Pakistan Himalaya. Here, the passive margin shows a much simpler history of Jurassic to Miocene subsidence.