ISOTOPIC (SR87/SR86 AND ND143/ND144) EVIDENCE FOR A HIMALAYAN PROVENANCE TO THE THAR DESERT SEDIMENTS AND ITS IMPLICATION

The origin of the Thar Desert, particularly the source of the desert sands, is a much-debated topic of the Quaternary geology of India. Remote sensing, archeological and paleohydrological studies show, that this region was experiencing a humid climate and was drained by many Himalayan rivers in the Late Quaternary times. In the green forest countryside of this region flourished one of the greatest civilizations, i. e., the Indus Valley Civilization (3500-1500 B. C.) which subsequently vanished under a hostile arid climate. In order to understand the nature and source to the desert materials and the cause of desertification, the geochemistry (REE, Nd and Sr isotopes) of the sediments from north and northeastern part of the Thar desert and the floodplains of the present day Himalayan rivers (Ganges, Yamuna, Sutlej and Ghaggar) draining from different catchments of the Himalayas have been studied. We observed that the rivers draining the High Himalayas today have radiogenic sediments (87Sr/86Sr=0.752715-0.789324; 143Nd/144Nd=0.511682-0.511835). However, the desert sediments are less radiogenic (87Sr/86Sr=0.728984-0.735157; 143Nd/144Nd=0.511825-0.511994). Comparing with those of the various distinct lithotectonic units of Himalayan orogen, we infer that whereas the desert materials could have been derived from less radiogenic Sub-Himalayan Tertiary sediments, those of the alluvium were derived from High Himalayan Crystallines. Because the rivers draining the Sub-Himalayan ranges could have lacked perennial water sources (glacial), unlike those of the present day rivers draining through the Higher Himalayas, their flow must have been largely controlled by monsoon rainfall. Any change in the intensity and pattern of monsoon rainfall should have made these rivers initially ephemeral and finally extinct. Present day arid phase must have followed the humid phase, resulting in aeolian reworking of Sub-Himalayan river alluvium and desert formation.