Geologic and tectonic evolution of the Himalaya before and after the India-Asia collision KEWAL K SHARMA Wadia Institute of Himalayan Geology, Gen. Mahadev Singh Road, Dehra Dun 248 001, India e-mail: [email protected] The geology and tectonics of the Himalaya has been reviewed in the light of new data and recent studies by the author. The data suggest that the Lesser Himalayan Gneissic Basement (LHGB) represents the northern extension of the Bundelkhand craton, Northern Indian shield and the large scale granite magmatism in the LHGB towards the end of the Palmoproterozoic Wangtu Orogeny, stabilized the early crust in this region between 2-1.9 Ga. The region witnessed rapid uplift and development of the Lesser Himalayan rift basin, wherein the cyclic sedimentation continued during the Palaeoproterozoic and Mesoproterozoic. The Tethys basin with the Vaikrita rocks at its base is suggested to have developed as a younger rift basin (~ 900 Ma ago) to the north of the Lesser Himalayan basin, floored by the LHGB. The southward shifting of the Lesser Himalayan basin marked by the deposition of Jaunsar-Simla and Blaini- Krol-Tal cycles in a confined basin, the changes in the sedimentation pattern in the Tethys basin during late Precambrian-Cambrian, deformation and the large scale granite activity (~ 500 • 50 Ma), suggests a strong possibility of late Precambrian-Cambrian Kinnar Kailas Orogeny in the Himalaya. From the records of the oceanic crust of the Neo-Tethys basin, subduction, arc growth and collision, well documented from the Indus-Tsangpo suture zone north of the Tethys basin, it is evident that the Himalayan region has been growing gradually since Proterozoic, with a northward shift of the depocentre induced by N-S directed alternating compression and extension. During the Himalayan collision scenario, the 10-12km thick unconsolidated sedimentary pile of the Tethys basin (TSS), trapped between the subducting continental crust of the Indian plate and the southward thrusting of the oceanic crust of the Neo-Tethys and the arc components of the Indus-Tangpo collision zone, got considerably thickened through large scale folding and intra-formational thrusting, and moved southward as the Kashmir Thrust Sheet along the Panjal Thrust. This brought about early phase (M1) Barrovian type metamorphism of underlying Vaikrita rocks. With the continued northward push of the Indian Plate, the Vaikrita rocks suffered maximum compression, deformation and remobilization, and exhumed rapidly as the Higher Himalayan Crystallines (HHC) during Oligo-Miocene, inducing gravity gliding of its Tethyan sedimentary cover. Further, it is the continental crust of the LHGB that is suggested to have underthrust the Himalaya and southern Tibet, its cover rocks stacked as thrust slices formed the Himalayan mountain and its decollement surface reflected as the Main Himalayan Thrust (MHT), in the INDEPTH profile. 1. Introduction (1975, 1989). With developments in the concept of plate tectonics, the Himalaya believed to be a classic "Geology of the Himalayas" by Gansser (1964) was example of the continent-continent collision related the first regional attempt to synthesize the knowledge mountain belt (Dewey and Bird 1970), the attention of gained in the past one century on the world's highest many researchers was focused to understand the colli- and the youngest mountain, followed by Le Fort sion mountain building processes. Large geochemical, Keywords. Collision tectonics; crustal growth; Himalayan evolution. Proc. Indian Acad. Sci. (Earth Planet. Sci.), 107, No. 4, December 1998, pp. 265-282 Printed in India 265 266 Kewal K Sharma geochronological, isotopic and geophysical data base The Sub or Outer Himalaya forms low altitude hills generated in the past two decades provides a good limited between the Main Frontal Thrust (MFT) in understanding of the geology of the Himalayan the south and the Main Boundary Thrust (MBT) in mountain and a better constrained model of its the north. It preserves the record of the post-collision geodynamic evolution. sediments produced by weathering and erosion of the Despite the fact that many gaps in the under- debris of the rising Himalayan front, carried and standing of the Himalayan geology and its tectonic deposited by rivers in the fore-deep basin called the evolution, have been filled, there are still many grey Murree-Siwalik Basin. areas in the understanding of the Himalaya. Some of The Lesser or Lower Himalaya, limited between the these are: MBT in the south and the Main Central Thrust- Vaikrita (MCT-V) in the north, comprises records of (i) The stratigraphy of the Lesser Himalayan sedi- marine sediments of Proterozoic to Cambrian age and mentary units and their response to changes in some sedimentary records of transgressing shallow sea tectonics within the basin and the surrounding during Permian, Upper Cretaceous and Middle regions, Eocene. The LHS are covered by two thrust sheets of (ii) the status of the 2.5 Ga Rampur-Bhowali vol- low to medium grade metasediments (Jutogh-Almora canics (Bhat and Le Fort 1992; Bhat et a11998) and Chail-Ramgarh). The sediments of the Lesser and associated sediments of the Lesser Hima- Himalayan basin in general provide information in layan basin, terms of tectonic changes in the basin and associated the relationship of the Lesser Himalayan Sedi- (iii) magmatism and its surrounding regions during mentaries (LHS) and the Lesser Himalayan Proterozoic and Phanerozoic. Crystallines (LHC) which cover the LHS as The Great or Higher Himalaya, limited by MCT in thrust sheets, the south and Tethyan Detachment Fault (TDF) in (iv) time and space relationship of the rocks of the the north, is represented by the high grade Precam- lower and upper crystalline thrust sheets of the brian crystallines, Cambro-Ordovician (500 + 50 Ma) LHC, granites/orthogneisses and the Tertiary leucogranites. (v) did the LHS and the LHC rocks, during their The Tethys Himalaya is confined between the TDF long geological history from Archaean?-Palaeo- in the south and the Indus-Tsangpo Suture (ITS) in proterozoic till their collision induced Himala- the north. It comprises Late Precambrian to Phaner- yan deformation and metamorphism, remain as ozoic (Palaeozoic-Mesozoic) marine sediments depos- such without any change? or do they show any ited in the Tethyan Sea that extended up to the Lesser evidence of pre-Himalayan orogeny?, Himalaya during the Vendian-Cambrian Cycle (Shah (vi) status of Vaikrita/Tibetan slab/Higher Hima- 1991). layan Crystallines (HHC) vis-a-vis the Tethyan The Trans-Himalaya or Indus- Tsangpo suture zone sedimentary sequences (TSS) and the LHC, comprises the obducted slices of the oceanic crust of (vii) the limit to which the Archaean-Proterozoic the Neo-Tethys, deep-sea trench sediments, subduc- continental crust of the Indian shield extends tion and collision related island arc magmatic rocks, under the Ganga Alluvium and the Himalaya, the arc derived accretionary prism related sediments and and the post-collision Kargil molassic sediments. (viii) the nature of the continental crust that lies beneath the Himalaya and South Tibet, obser- ved as a reflecting surface in the INDEPTH 3. Geology of the Himalayas profile (Zhao et al 1993). The present paper is an attempt to answer some of Various aspects of the Himalayas can be covered these questions and discuss the current status of the under this heading, however, the author has preferred geologic and the tectonic evolution in the light of to discuss only some important aspects of the geology. new observations and data from the NW Himalaya The Lesser Himalayan Basin sedimentaries (LHS). (Himachal, Garhwal, Kumaun) and Central (Nepal) In view of the general paucity of fossils, difficulty to Himalaya. The NW Himalaya, being a less squeezed assign reliable stratigraphic ages due to sheared con- section than the Central (Nepal) and the NE Himalaya, tacts, scarcity of the dependable isotopic numbers has been the main focus in this paper. capable of resolving the age problems together with the confusion created by multiplicity of names assig- ned to similar lithologies in the adjoining areas, the 2. Divisions of the Himalayas LHS were considered as the most confused sequence of the Himalaya (Gansser 1981). No consensus has yet The Himalayas from south to north are divided into emerged to build a reliable time stratigraphy of the five well-known and generally accepted lithotectonic LHS, and the confusion still exists in recent literature units (figure I). (Shanker et a11989; Valdiya 1995; Virdi 1995). Under Evolution of the Himalaya 267 ~-~ .~,-~ ~.~ .~ ~ .~ ~ '~ ~ ~ ~"~ *dN~ '~.~ ~'~ 268 Kewal K Sharma Figure 2. Time and space relationship of various sedimentary basins developed in the Himalayan region since Proterozoic (pre- Himalayan collision scenario). the circumstances, the author considers the division of represents the oldest unit of the LHS. The quartz the LHS on the basis of sedimentary cycles as a fairly dominated lithologies with a high degree of mineralo- reasonable approach, and recognizes four sedimentary gical and textural maturity and the associated rift cycles (Rampur-Sundernagar cycle, Larji-Shali-Deo- related tholeiitic volcanics (Ahmad and Tarney 1991; ban cycle, Jaunsar-Simla cycle and Blaini-Krol-Tal Bhat and Le Fort 1992) in the lower part of the cycle) given in figure 2. In view of the age controversy sequence suggest that the Lesser Himalayan sedimen- the earliest (Rarnpur-Sundernagar) cycle needs ela- tation began