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 lithosphère after plate collision.