ABSTRACT New geological mapping in midwestern Nepal, complemented by thermochronological and geochronological data sets, provides stratigraphic, structural, and kinematic information for this portion of the Himalayan thrust belt. Lithofacies and geochronologic data substantiate five genetic (tectono)stratigraphic packages: the Lesser Himalayan (ca. 1900–1600 Ma), Greater Himalayan (ca. 800–520 Ma), Tethyan Himalayan (Late Ordovician–Cretaceous), Gondwana (Permian–Paleocene), and Cenozoic Foreland Basin (Eocene–Pleistocene) Sequences. Major structures of midwestern Nepal are similar to those documented along strike in the Himalaya and include a frontal imbricate zone, the Main Boundary and Ramgarh thrusts, the synformal Dadeldhura and Jajarkot klippen of Greater Himalayan rocks, and the hybrid antiformal-stack/hinterland-dipping Lesser Himalayan duplex. Total (probably minimum) shortening between the Main Frontal thrust and the South Tibetan detachment is 400–580 km, increasing westward from the Kaligandaki River region. The Main Central and Ramgarh thrusts were active sequentially during the early to middle Miocene; the Lesser Himalayan duplex developed between ca. 11 Ma and 5 Ma; the Main Boundary thrust became active after ca. 5 Ma and remains active in places; and thrusts that cut the Siwalik Group foreland basin deposits in the frontal imbricate belt have been active since ca. 4–2 Ma. The Main Central “thrust” is a broad shear zone that includes the boundary between Lesser and Greater Himalayan Sequences as defined by their protolith characteristics (especially their ages and lithofacies). The shape of the major footwall frontal ramp beneath the Lesser Himalayan duplex is geometrically complex and has evolved progressively over the past ~10 m.y. This study provides the basis for understanding the Himalayan thrust belt and recent seismic activity in terms of critical taper models of orogenic wedges, and it will help to focus future efforts on better documenting crustal shortening in the northern half of the thrust belt.

Abstract We studied the zircon U–Pb ages, Hf isotopes, and whole-rock and mineral chemistry of metagranitoids from the Subansiri region of the Eastern Himalaya to constrain their emplacement age, origin and geodynamic evolution. The investigated metagranitoids have high SiO 2 , Na 2 O + K 2 O, Rb, Zr and low Fe 2 O 3 , Nb, Ga/Al ratios with fractionated rare earth element patterns [(Ce/Yb) N = 6.46–42.15] and strong negative Eu anomalies (Eu/Eu* = 0.16–0.44). They are peraluminous (molar A/CNK = 1.04–1.27) and calc-alkaline in nature, with normative corundum (1.04–3.61) and relatively high FeO t /MgO ratios in biotite ( c. 3.38), indicating their affinity with S-type granites. The time of emplacement of the Subansiri metagranitoids is constrained by zircon U–Pb ages between 516 and 486 Ma. The zircon grains have negative ε Hf ( t ) values ranging from −1.4 to −12.7 and yield crustal Hf model ages from 1.5 to 2.2 Ga, suggesting the occurrence of a major crustal growth event in the Proterozoic and re-melting of the crust during the early Paleozoic. The geochemical data in conjunction with the U–Pb ages and Hf isotope data suggest that the Subansiri metagranitoids were produced by partial melting of older metasedimentary rocks in the Indian passive margin.