Tectonic Evolution of the Bhumichula Plateau: A High-Elevation Low-Relief Surface in Western Nepalese Himalaya

Abstract

The Bhumichula plateau is an anomalously high elevation low-relief (HELR) surface at >4000 m in the midst of the western Nepalese Himalayan fold-thrust belt. The plateau is situated in the northeastern part of the Dadeldhura klippe, an erosional outlier of Greater Himalayan high-grade metamorphic rocks surrounded by low-grade, structurally underlying Lesser Himalayan rocks. Using low-temperature thermochronology, we test whether the Bhumichula HELR surface formed at low elevation and was later uplifted, developed in situ at high elevation following tectonic disturbance, or if it is a remnant of an older, more extensive HELR that has been shrinking due to recent tectonically driven incision. Average apatite (U-Th)/He ages are ~6 Ma on both the plateau surface and along the valleys that cut through the surrounding region, whereas apatite fission track ages are ~7-9 Ma on the plateau surface and ~11-14 Ma on the flanks. Thermal modeling suggests that the main period of exhumation and erosion was ~14 Ma for the valley bottom samples in the southwest limb of the klippe and ~7 Ma for Bhumichula plateau in the northeast limb of the klippe, consistent with the timing of slip on the Ramgarh thrust and initial growth of the Lesser Himalayan Duplex (LHD), respectively. Spatial differences in the timing of cooling for the northeast and southwest limbs of the klippe 24 is interpreted to indicate heterogenous exhumation due to structural variation. Slow exhumation since ~7 Ma on the Bhumichula plateau suggests attainment of high elevation by mid-Miocene time. Exhumation and uplift ~7 Ma due to growth of the LHD is therefore likely to be responsible for drainage reorganization and resulting preservation of low-relief at Bhumichula due to a combination of fluvial and glacial processes.