Himalayan Paleogene Foreland Basin, its collision induced early volcanic history and failed rift initiation

Abstract

The Himalayan Foreland Basin was formed over a failed rift zone that produced early Paleogene volcanism, which was synchronous to Indo-Asia collision. The rift zone was located over acutely flexed leading edge of the Indian continental crust. The volcanism preceded upliftment of the proto-Himalaya. The Jammu Himalaya is one of the few areas exposing the basal part of the foreland basin comprising a persistent horizon of volcani-sedimentary rhyolitic chert breccia at the base of the Subathu Formation, which unconformably overlies the Precambrian Jammu Limestone. Contemporaneous felsic ash beds occur in the Subathu-Kalka area in the Simla Hills at the base of the Subathu Formation unconformably overlying the Precambrian Simla Group. In Arunachal Pradesh, in the eastern-most Himalaya, early-mid Eocene foraminifera fauna bearing Yinkiong Formation overlies the Abor Volcanics. The age of the latter is disputed: whether it is early Permian or Eocene in age, or it exposes a mixture of both components. Recent U-Pb SHRIMP zircon geochronological study indicates Abor Volcanics being part of the Kerguelen mantle plume trail traversing Southern Tibet, Sylhet, Rajmahal, Bunbury and Kerguelen Plateau. Trace element geochemistry of the Abor Volcanics, the exposed volcanic rocks around Yinkiong and those exposed between Pangin and Rotung indicate rift signature. In the frontal belt, early Eocene shallow marine sediments locally associated with mafic volcanics occur tectonically at the base of the early Neogene foreland sequence. The latter is locally thrust at MBT by the early Permian meta-sediments tectonically intermixed with marine Eocene rocks. Early Eocene sediments and rift related mafic volcanics from Arunachal Pradesh possibly occur at the tectonic base of the Tertiary foreland basin sequence from northeastern Himalaya. The early Paleogene mafic volcanics from the Arunachal Pradesh and coeval acid volcanics occurring at the base of the Paleogene sequence in Jammu-Simla Himalaya might represent cogenetic bimodal volcanism.The Subathu Formation with its coal and carbonaceous sediments is lithologically similar to the coal and mud-rock bearing late Paleocene Patala Formation or the Hangu Formation and early Eocene Ghazij Formation from the adjacent Indus Foreland Basin, Pakistan, which overlie the Mesozoic-Paleocene carbonate flat. Although, the lithofacies and sedimentary tectonic setting of the Patala, Hangu and the Ghazij formations are similar to that of the Subathu Formation, the former units typically lack the presence of syn-sedimentary ash beds or any other volcanic rocks that are characteristically present at the base of the Subathu Formation. Mainly late Paleocene coal bearing Patala and the Hangu formations in Salt Range and North Potwar area, Pakistan, although located close to the Jammu sector of the Himalayan Foreland Basin is free of any volcanic activity. Salt Range and Potwar areas are possibly not parts of the foreland fold-and-thrust belt of the Himalaya, as postulated by some workers. Geodynamically Salt Range and Potwar basins belong to northern parts of the Indus Foreland Basin, because the depositional basin from Potwar extending into the adjacent Kohat area during the Paleocene and the Eocene time probably formed a collision zone between the India-Pakistan plate and a micro-continent prior to the collision of the former with Asia. The micro-continent possibly buffered intensity of collision, thereby, there was no tectono-magmatic activity along this belt as distinct from that of direct collision between India and Asia continent.