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 Indian basement faults, which bound three orogen-perpendicular palaeotopographic ridges of Precambrian Indian basement south of the Himalaya, extend to the base of the Indian lithosphere and to the northern extent of the Indian lithosphere underneath Tibet. In the eastern Himalaya, the active orogen-perpendicular Yadong–Gulu graben is aligned with an earthquake-generating strike-slip fault in the high Himalaya. We argue that the graben results from crustal necking during reactivation of the underplated basement fault. In the central Himalaya, along-strike diachronous deformation and metamorphism within the Himalayan metamorphic core, as well as lateral ramps in the foreland thrust belt, spatially correspond to the Lucknow and Pokhara lineaments that bound the subsurface Faizabad Ridge in the Indian basement. Analogue centrifuge modelling confirms that offset along such deep-seated basement faults can affect the location, orientation and type of structures developed at various stages of orogenesis and suggests that it is mechanically feasible for strain to propagate through a melt-weakened mid-crust. We suggest that inherited Indian basement faults affect the ramp-flat geometry of the basal Main Himalayan Thrust, partition the Himalayan range into distinct zones, localize east–west extension resulting in the Tibetan graben and, ultimately, contribute to lateral variability in tectonic evolution along the orogen's strike.

Abstract South-vergent channel flow from beneath the Tibetan Plateau may have played an important role in forming the Himalaya. The possibility that Greater Himalayan rocks currently exposed in the Himalayan Fold-Thrust Belt flowed at mid-crustal depths before being exhumed is intriguing, and may suggest a natural link between orogenic processes operating under the Tibetan Plateau and in the fold-thrust belt. Conceptual and numeric models for the Himalayan-Tibetan Orogen currently reported in the literature do an admirable job of replicating many of the observable primary geological features and relationships. Ho wever, detailed observations from Greater Himalayan rocks exposed in the fold-thrust belt’s external klippen, and from Lesser Himalayan rocks in the proximal footwall of the Main Central Thrust, suggest that since Early Miocene time, it may be more appropriate to model the evolution of the fold-thrust belt using the criticaltaper paradigm. This does not exclude the possibility that channel flow and linked extrasion of Greater Himalayan rocks may have occurred, but it places important boundaries on a permissible time frame during which these processes may have operated.

Abstract This review presents an objective account of metamorphic, microstructural and geochronological studies in the Greater Himalayan Sequence (GHS) and adjacent units in Nepal in the light of recent research. The importance of integrated, multidisciplinary studies is highlighted. A personal view is presented of strategies for determining pressure–temperature evolution, and of petrological processes at the micro scale, particularly in relation to departures from equilibrium and the behaviour of partially-melted rock systems. Evidence has accumulated for the existence within the GHS of a High Himalayan Discontinuity, marked by differences in timing of peak metamorphism in the hanging wall and footwall, and changes in P–T gradients and paths. Whether or not this is a single continuous horizon, it forms at each location the lower boundary to a migmatitic zone capable of ductile flow, and separates the GHS into an upper division in which channel flow may have operated in the interval 25–18 Ma, and a lower division characterized by an inverted metamorphic gradient, and by metamorphic ages that decrease downsection and are best explained by sequential accretion of footwall slices between 20 and 6 Ma. An overall model for extrusion of the GHS is still not resolved.

Abstract Out-of-sequence deformation in the Himalaya has been caused mainly by thrusting. Out-of-sequence thrusts, usually north- to NE-dipping foreshear planes, occur inside the Sub-Himalaya (SH), Lesser Himalaya (LH) and Greater Himalayan Crystalline (GHC) sequences. Where absolute dates are available, the youngest slip within the SH occurred near the Janauri Anticline (India) at c. AD 1400–1460. The Munsiari Thrust (India) activated within the LH at c. 1–2 Ma and the Main Central Thrust zone in the Marsyandi valley (Nepal) in the GHC was formed during the Holocene ( c. 0.3 ka). Except for the Riasi Thrust (Kashmir, India), the Paonta Thrust (Himachal Pradesh, India) in the Siwalik and the Tons Thrust (Garhwal region, India) within the Main Central Thrust zone, crustal shortening related to out-of-sequence thrusting in the Himalaya has been insignificant. The major litho-/stratigraphic contacts within the SH and the GHC at some places acted as out-of-sequence thrusts. Out-of-sequence thrusts in the SH have been detected mainly based on geomorphological observations. However, more quantitative geochronological studies have detected out-of-sequence thrusting from c. 22 Ma up to Holocene age in the GHC based on age jumps, especially within the Main Central Thrust zone. Crustal channel flow (specifically for the GHC) and/or the critical taper model with or without erosion can be used to explain the Himalayan out-of-sequence thrusts.