EXHUMATION HISTORY OF THE BHUMICHULA PLATEAU: A RELICT LOW RELIEF SURFACE IN THE HIMALAYAN THRUST BELT OF NEPAL?

The Greater Himalaya generally has extreme topography with local relief greater than 3000 m. However, in western Nepal, several outlying areas of Greater Himalayan rocks have anomalously low relief at moderate to high elevations. One such area is the Bhumichula plateau, located on the northern limb of the synformal Dadeldhura klippe, an erosional outlier of Greater Himalayan high-grade metamorphic rocks structurally above the Main Central thrust. The plateau extends over ~250 km² with an internal relief of ~300-400 m, and flanking relief of up to ~4000 m. This study aims to examine the enigmatic presence of this low relief surface in the context of regional tectonics. Various studies in the Himalayas have used low long-term erosion rates to interpret low-relief/ high-elevation surfaces as relict transient surfaces that are in the process of equilibrating with new incision rates. We will be exploring this idea by constraining long-term erosion rates of the plateau and its neighboring flanks using zircon-He (ZHe), Apatite-He (AHe) and fission track (AFT) low-temperature thermochronology. Our samples extend upstream onto Bhumichula plateau along steep elevation transects and cover the entire breadth of the Dadeldhura synform, allowing assessment of the role played by structural uplift on the synform flanks, as well as headward fluvial erosion. Preliminary ZHe and AFT ages from the klippe are ~14-11 Ma and ~11 Ma respectively while AFT ages on the Bhumichula surface at elevations >4000 m are 7-9 Ma. We suggest that late Miocene ages on the flanks of the klippe reflect exhumation due to uplift of the Dadeldhura klippe during emplacement of the Main Central Thrust sheet and Ramgarh thrust sheet respectively. On the other hand, late Miocene ages on the plateau surface could reflect exhumation and paleodrainage incision due to formation of the Lesser Himalayan duplex.