A Geochronological Study of Paleogene –Neogene Foreland Basin Sediments Western Nepal Himalaya: Implication of Provenance Analysis

Abstract:

After the collision between the Indian and Asian plates, during early Cenozoic several south propagating thrusts were active and the sediments deposited at the northern tip of the Greater India have been reworked, recycled and metamorphosed, and were subsequently exhumed and transported to the foreland basin. Petrography, detrital zircon (DZ) dating and Lu-Hf isotope analysis, and trace element analysis were conducted from two sections of Nepal Himalaya for the determination of change in provenance, and constrain the possible timing of Indo-Asiacollision.

The U-Pb ages of the DZ grains from Upper Cretaceous to Paleocene Amile Formation are older than early Mesoproterozoic with dominant number of grains showing ƐHf (t) value between 0 to +10. The trace element data shows that the sediments are from passive margin with northward paleoflow direction. These finding conclude that the sediments during this time was sourced from northern margin of Indian Plate that elucidate the possibility of the Ind0-Asia collisionsomewhere between late Paleocene to early Eocene (~58-50 Ma).The U-Pb ages from the marine Eocene Bhainskati Formation dominantly clusters between ~1000 and 500 Ma and the ƐHf (t) values ranges from -10 to +10. The trace element result shows that the sediments were from both the passive and active continental margin. The transition of deposition from marine Bhainskati Formation to continental Dumri Formation is marked by 3-4 m thick Oxisol layer with a ~10-15 Mya deposition gap during the Oligocene. The U-Pb ages of detritus from Early to mid-Miocene Dumri Formation have a wide range of detritus ranging from Archean to Mesozoic age with dominant numbers resembling the age of the Tethys Himalaya (TH) and Upper Lesser Himalaya (ULH). The presence of younger grains of Cenozoic age (~80-50 Ma) put the possibility of the Gangdase arc source deposited directly in Dumri Formation or first deposited in the TH and later transported to it. The ƐHf (t) value ranges between +20 and -40 and the result of trace element shows the sediments are from active continental margin. The sediments are entirely source from the rising Himalaya, possibly due to the activation of Main Central Thrust. The petrographic result of Dumri sandstone falls in “recycled orogeny” field.