Tectonic Evolution of the Himalayan Fold‐Thrust Belt in the Okhaldhunga Region, Eastern Nepal

Abstract New regional mapping and two focused geological maps with two crustal‐scale balanced cross‐sections along the Dudh Kosi and Tama Kosi Rivers in the Okhaldhunga region reveal the structural architecture of the eastern Nepal Himalaya. Our detrital zircon U‐Pb ages for 14 samples and igneous zircon U‐Pb ages for four samples show that the Okhaldhunga window primarily exposes the Ramgarh thrust sheet, which is composed of Paleoproterozoic Lesser Himalayan metasedimentary and igneous rocks. Structurally, the Ramgarh thrust sheet lies in the footwall of the Main Central thrust and is underlain by the Lesser Himalayan duplex (LHD), a hinterland dipping bumpy‐roofed duplex, comprising Paleo‐Mesoproterozoic Lesser Himalayan rocks and upper Paleozoic Gondwana Sequence rocks. The frontal part of the LHD is exposed in the southern sector of the Okhaldhunga window, above a major footwall ramp on the Main Himalayan thrust. Neoproterozoic‐Cambrian Greater Himalayan rocks in the hanging‐wall of the Main Central thrust are exposed toward the south in the synformal Mahabharat Range and in the north in the Sagarmatha‐Rolwaling region. Kinematic restorations for our two transects, combined with published chronometric data, indicate that most of the deformation has been accommodated by Lesser Himalayan rocks since Middle‐Late Miocene. The two balanced cross‐sections indicate minimum shortening of ∼561 and 544 km. Comparison of these estimates with published data for rocks structurally below the South Tibetan detachment system in the Himalaya indicates that heterogeneous post‐collisional crustal shortening has determined the current tectonic configuration of the region.