Anshu K. Sinha

Tectonic setting and pre-orogenic sedimentation in the Indus-Tsangpo (Yarlung) suture zone of Ladakh Himalaya, India

Abstract There are three tectonically distinguishable zones along the Indus Tsangpo Suture of the Ladakh Himalaya: The Lamayuru Formation of Triassic-Palaeogene age representing accreted oceanic basin sediments on the passive margin of the Indian subcontinent; Ophiolitic Melange Zone of Jurassic_Late Cretaceous age; and the Nindam Formation of Late Cretaceous-Early Palaeogene age consisting of trench and trench slope sediments. The plate tectonic setting of the Lamayuru Formation and the Nindam Formation could be correlated with the modern analogue for the passive setting (trailing-edge) and the subduction (leading-age) related sedimentary deposits respectively. Pre- and syn-tectonism related to subduction of the Indian plate with the Eurasian plate, produced favourable sites for deposition of sediments. The rocks exposed help elucidate the relationship between the ongoing convergence and sediments being shed into the basins; they record the existence of rock masses removed by erosion from tectonic highlands.

Tectonics and sedimentation in the passive margin, trench, fore-arc and backarc areas of the Indus Suture Zone in Ladakh and Karakorum: a review

AbstractThree distinct turbidite basins are recognised in the Indus-Tsangpo Suture Zone of NW Indian Himalayan region. The broad facies organisation of these turbidite basins suggests that they are attributable to passive and active margins of the Neo-Tethyan oceanic realm. The syn-post-rift deep marine, fine-grained Lamayuru Formation, dated from Triassic to Eocene, represents an accreted oceanic basin of the Indian passive margin. The syn-tectogenic trench and trench-slope volcanogenic turbidites of the Nindam Formation represent an accretionary fore-arc basin of Late Cretaceous-Eocene age. The Nindam Formation is correlated with the Congdu trench sediments of southern Tibet. The syn-orogenic composite turbidites of the Indus Formation represent a residual fore-arc basin that lately evolved towards composite type as a result of the changing geometry of provenance terrains. The Middle-Late Cretaceous to Eocene Indus Formation is correlated with the Xigaze forearc basin of southern Tibet.Adjacent to the I...

Tectonic zonation of the Central Himalaya and the crustal evolution of collision and compressional belts

Abstract The Central Himalayan segment is divided from south to north into sub-parallel structural-facies zones. The Main Boundary Fault (MBF) plays the role of an important tectonic element dividing the Siwalik Molasse along the foothills of the Himalaya against the para-autochthonous and allochthonous tectonic units of the Southern Himalaya. On the basis of recent researches I suggest that the Himalayan region should be divided geologically into two major geologic zones. I propose to call the dividing structural line between them, the Main Axial Zone (MAZ) of the Himalaya, which is situated in the crystalline complex of the Higher Himalaya. This deep seated structure in the Himalaya has played a crucial role in the history of geological development as well as incorporating the root zone of southward-pushed thrust sheets. The Main Central Thrust (MCT) tectonically separates the carbonate para-autochthonous zone of Southern Himalaya from the metamorphosed crystalline “Vaikrita” complex. The Vaikrita Central Crystalline Complex in its turn is separated from the huge pile of the sedimentary Tethyan Complex by the “Tethyan Thrust” (TT). Farther north the “Great Himalaya Suture” (GHS) called the Indus-Tsangpo Suture divides the northern extremity of Himalaya from the Karakoram erogenic belt. The GHS has provided excellent conditions for the study of mantle remains on the oceanic crust with the occurrence of ophiolitic melange which was the result of subduction of a continental type plate and obduction of oceanic material. In recent years a large number of data has been recorded especially as a result of Sino-French and other scientific expeditions in the Tibetan region. Reversed critical wide angle reflection profiles of the crust mantle boundary south of the Great Himalaya Suture (GHS) or the Indus-Tsango (Yarlung-Zangbo) Suture in Tibet reveal a deep 70 km Moho extending north of the Higher Himalaya whilst to the south the Moho is 15 km higher. Thus in the southern region of Tibet the crust gets thinner gradually towards the south, and reduces to about 38 km in the Gangetic plain of India. The 300 km of terrain between the GHS and the Gangetic plain represent the transitional collision and compressional belt. The magnetolluric and explosive seismographic data indicate that the layered structures are developed within the crust transisting to the upper mantle. The Moho discontinuity exhibits step terraces. The low velocity and resistance layer within the crust indicates that the crust is overlapping and superimposed. Thus the intense compression from south to north caused by the Indian plate caused the crust to overlap, shorten, thicken and become isostatically adjusted to give rise to the Tibetan plateau. The successive occurrence of nappe-shear zones led to progressive superimposing and thickening of the crust. It also indicates intra-continental subduction and destruction of the lithosphere after plate collision.

Flysch: a historical perspective and the Himalayas

Abstract A common question asked by researchers working on orogenic belts is: what is flysch? Varied views have been presented over the years. Any thick succession of sandstone, calcarenite or conglomerate alternating with shale or mudstone, and interpreted as deposited mainly by turbidity currents or mass-flow in a deep water environment within a tectonically active orogenic belt is commonly defined as flysch. But the creativity in defining the term has meant that we have lost sight of its original meaning. The term is not sufficiently flexible to cope with current ideas on mountain building. In this paper we shall try to evaluate the term based on research published. We hope that this will assist students of flysch in understanding the tectonic implications of the term in general, in addition to its systematic development in space and time.