Unraveling magnitude, style, kinematics of crustal shortening in the Himalayan fold-thrust belt of eastern Nepal

The Himalayan orogen is a classic fold-thrust belt in a collisional plate tectonic setting. Quantifying magnitudes and determining patterns of crustal shortening in the Himalaya is therefore crucial for understanding deformation in convergent plate margins. However, gaps in our knowledge of various sectors of the Himalaya remain primarily due to lack of studies that combine geologic mapping with detrital zircon geochronology and kinematic history analysis. The Okhaldhunga region in eastern Nepal is one such area. In this study, we present a new geological map, two crustal-scale balanced cross-sections and kinematic restorations to reveal the kinematic history of Himalayan fold-thrust belt of eastern Nepal. New zircon U-Pb ages from fourteen samples and igneous zircon U-Pb ages from four samples show that the Okhaldhunga window primarily exposes the Ramgarh thrust sheet, composed of Paleoproterozoic Lesser Himalayan rocks and a hinterland dipping bumpy-roof duplex comprising Paleo-Mesoproterozoic Lesser Himalayan rocks and middle Paleozoic Gondwana Sequence rocks. 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 Rolwaling-Sagarmatha region. A new minimum shortening estimate of ~500 km for rocks structurally below the South Tibetan detachment indicates eastward decline in shortening rates in the Himalayan fold-thrust belt.