Kewal K. Sharma

Strontium isotopes and rubidium in the Ganga-Brahmaputra river system: Weathering in the Himalaya, fluxes to the Bay of Bengal and contributions to the evolution of oceanic87Sr/86Sr

The concentrations of Rb and Sr and87Sr/86Sr isotopic ratios have been measured in the dissolved load of the Ganga-Brahmaputra (G-B) river system. The Ganga was sampled extensively from its source at Gangotri (in the Higher Himalaya) to Patna (on the alluvial plains). The Brahmaputra was sampled in its stretch in Assam, in India. The average Sr concentration in the Ganga (at Patna) is 1.2 μmol/l and that in the Brahmaputra (at Goalpara) is 0.73 μmol/l; the mean87Sr/86Sr ratios are 0.7239 and 0.7192, respectively. The87Sr/86Sr in the Ganga source waters (the Alaknanda, the Bhagirathi and their tributaries) range between 0.7300 and 0.7986, considerably higher than the global average runoff value of 0.7119. The high87Sr/86Sr in the Ganga source waters result from the intense weathering of Precambrian granites and gneisses enriched in radiogenic Sr. The Sr isotope systematics of the Ganga waters is dominated by silicate weathering, whereas carbonate weathering plays a significant role in their major ion chemistry. The G-B system transports about 910 million moles of dissolved Sr annually to the Bay of Bengal, with an average87Sr/86Sr of 0.7213. The Sr transported by the G-B system is about 2.7% of the global dissolved Sr flux to the oceans via rivers. Model calculations reveal that the G-B system has contributed significantly to the Sr isotope evolution of seawater during the past ∼ 20 Ma. The flux of Rb through the G-B system is ∼ 24 million moles per year. This is about 5% of the global river supply of Rb to the oceans, nearly twice the contribution of water via the G-B system to the oceans. Our study suggests that the marine geochemistries of Sr and Rb (and possibly U) may have been influenced considerably by the Himalayan orogeny.

Major ion chemistry of the Ganga source waters: Weathering in the high altitude Himalaya

A systematic study of the major ion chemistry of the Ganga source waters—the Bhagirathi, Alaknanda and their tributaries—has been carried out to assess the chemical weathering processes in the high altitude Himalaya. Among major ions, Ca, Mg, HCO3 and SO4 are the most abundant in these river waters. These results suggest that weathering of carbonate rocks by carbonic and sulphuric acids dominates in these drainage basins. On an average, silicate weathering can contribute up to ∼ 30% of the total cations.The concentration of total dissolved salts in the Bhagirathi and the Alaknanda is 104 and 115mg/l, respectively. The chemical denudation rate in the drainage basins of the Bhagirathi and the Alaknanda is, respectively, 110 and 137 tons/km2/yr, significantly higher than that derived for the entire Ganga basin, indicating intense chemical erosion of the Himalaya.

Erosion history of the Tibetan Plateau since the last Interglacial: constraints from the first studies of cosmogenic 10Be from Tibetan bedrock

Abstract The cosmogenic 10Be exposure histories of in situ bedrock surfaces from the Tibetan Plateau indicate low erosion rates of

Crustal growth and two-stage India-Eurasia collision in Ladakh

Abstract The post-Palaeozoic history of crustal growth between India and the Karakoram-Lhasa accreted southern margin of Asia begins with the creation of the oceanic crust during the northward drift of the Karakoram-Lhasa block, away from the northern margin of India, after Permo-Triassic rifting. Data suggest initiation of northward subduction and the development of the island arc close to the southern margin of Asia sometime in Upper Jurassic (160 m.y.). The northward retreat of the overriding Eurasian Plate due to movements along transform faults, which subsequently changed into transcurrent faults, resulted in the opening of the Shyok back-arc basin in the Kohistan-Ladakh region during pre-Upper Cretaceous time. The mature and thickened tholeiitic island arc lying between the subducting Indian Plate and the spreading Shyok back-arc basin suffered compression, deformation, emplacement of ophiolitic melanges and metamorphism during the Upper Cretaceous (80 m.y.). The intrusion of the calc-alkaline magma into the deformed rocks of the island arc and the back-arc basin during the Paleocene-Eocene (60-40 m.y.) was followed by explosive calc-alkaline volcanism (40-25 m.y.) resulting in further crustal growth in this part of the Trans-Himalaya. The collision of the Dras island arc with the approaching margins of the Indian and Eurasian plates took place in two stages. In the first stage it collided with the northern margin of India during the Paleocene along the Indus Suture. The final collision of the island arc-accreted northern margin of India with the Eurasian Plate took place along the Shyok Suture during the Early Oligocene (30 m.y.).

Tectonic and cooling history of the Bihar Mica Belt, India, as revealed by fission-track analysis

Abstract The fission track ages of four cogenetic minerals garnet, muscovite, biotite and apatite collected from the pegmatites of the Bihar Mica Belt of India, emplaced during Satpura Orogeny (~ 950 m.y.), are found to be 830 m.y., 760 m.y., 595 m.y. and 590 m.y., respectively. The discordance in these ages has been interpreted in terms of the tectonic and cooling history of the mica belt. The average rate of uplift for this region has been calculated as ~ 0.08 mm/year and the cooling of the pegmatite bodies subsequent to their crystallization has been at the rate of 3.2°C/m.y. from 950 m.y. to 830 m.y., 0.9°C/m.y. from 830 m.y. to 760 m.y., 0.6°C/m.y. from 760 m.y. to 600 m.y. The region has been near the earths surface temperature for the last ~ 600 m.y.

Paleo-uplift and cooling rates from various orogenic belts of India, as revealed by radiometric ages—discussion (2)

Abstract The significant discordance of the radiometric (Rb-Sr, Pb-U, K-Ar and fission track) ages from various orogenic cycles of the Dharwar, Satpura, Aravalli and Himalayan orogenic belts in India, coupled with their corresponding blocking temperatures for various radiometric clocks in whole rocks and minerals, has been used to evaluate the cooling and the uplift histories of the respective orogenic belts. The blocking temperatures used in the present study of various Rb-Sr (isotopic homogenization at 600°C, muscovite at 500°C and biotite at 300°C), Pb-U (monazite at 530°C), K-Ar (muscovite at 350°C and biotite at 300°C) and fission-track clock (zircon at 350°C, sphene at 300°C, garnet at 280°C, muscovite at 130°C, hornblende at 120°C and apatite at 100°C for the cooling rate l°C/Ma) have been found suitable to explain the differences in mineral ages by different radiometric techniques. The nature of the cooling curves drawn using the temperature versus age data for various orogenic cycles in India has also been discussed. The cooling and the uplift patterns determined for various orogenic cycles of India, suggest comparatively slow cooling (5.0–0.2°C/Ma) and uplift (180–2 m/Ma) for the Peninsular regions and rapid cooling (25.0–1.0° C/Ma) and fast uplift (800–30 m/Ma) during the Himalayan Orogenic Cycle (Upper Cretaceous—Tertiary) in the Extra-Peninsular region.

Fission-track ages and uranium concentration in garnets from Rajasthan, India

Fission-track ages determined from garnets in pegmatite and garnetiferous amphibolite from Bhilwara and Ajmer districts of Rajasthan range from 900 m.y. to 1,370 m.y. They date the time of formation of the pegmatites (about 1,000 m.y. B.P.) and the metamorphism during F 1 (∼1,400 m.y. B.P.) folding in the area. The uranium content of the garnets varies from 1.76 × 10 −8 g/g to 21.4 × 10 −8 g/g.

Geologic and tectonic evolution of the Himalaya before and after the India-Asia collision

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 Palæoproterozoic 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 Palæoproterozoic 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.

Geological setting of the ophiolites and magmatic arc of the Lohit Himalaya (Arunachal Pradesh), India with special reference to their petrochemistry

Abstract Sela thrust separating the lower and the upper crystallines of western Arunachal Pradesh (i.e. Bomdila and Sela thrust sheets) has been recognised in the Lohit sector. Another thrust separating the Sela group of rocks from the Mishmi complex, marks the suture line between the eastern margin of the Indian plate with the Mishmi arc and has been called as the Lohit suture. The tholeiitic meta-volcanics and the calc-alkaline diorite- granodiorite plutonic complex represent the components of the Mishmi arc and have comparable petrochemistry like the Dras volcanics and the Kargil igneous complex of the Ladakh magmatic arc, respectively. Preliminary fission track ages on sphene-apatite pair suggest thermal history of this area similar to that of the Ladakh region. However, it is extremely necessary to determine the absolute ages of various intrusives and extrusives to constraint the tectonomagmatic models of this region.

Fission-track ages from the pandoh-baggi area, himachal himalaya, and their tectonic interpretations

Abstract Apatite fission-track ages with different cover heights from the Pandoh—Baggi Tunnel, a part of the Beas—Satluj Link Project in Himachal Pradesh (India), have been measured to range from 7 to 27 m.y. The relation between the age and cover height is found to be linear. The in situ apatite track analyses have been used to compute the average rates of uplift and cooling for Mandi granite as 0.06 mm/year and ~5°C/m.y., respectively.