Stratigraphy, structure, and tectonic evolution of the Himalayan fold-thrust belt in western Nepal

Regional mapping, stratigraphic study, and Ar-40/Ar-39 geochronology provid e the basis for an incremental restoration of the Himalayan fold-thrust bel t in western Nepal, Tectonostratigraphic zonation developed in other region s of the Himalaya is applicable, with minor modifications, in western Nepal . From south to north the major structural features are (1) the Main Fronta l thrust system, comprising the Main Frontal thrust and two to three thrust sheets of Neogene foreland basin deposits; (2) the Main Boundary thrust sh eet, which consists of Proterozoic to early Miocene, Lesser Himalayan metas edimentary rocks; (3) the Ramgarh thrust sheet, composed of Palcoproterozoi c low-grade metasedimentary rocks; (4) the Dadeldhura thrust sheet, which c onsists of medium-grade metamorphic rocks, Cambrian-Ordovician granite and granitic mylonite, and early Paleozoic Tethyan rocks; (5) the Lesser Himala yan duplex, which is a large composite antiformal stack and hinterland dipp ing duplex; and (6) the Main Central thrust zone, a broad ductile shear zon e. The major structures formed in a general southward progression beginning with the Main Central thrust in late early Miocene time. Eocene-Oligocene thrusting in the Tibetan Himalaya, north of the study area, is inferred fro m the detrital unroofing record. On the basis of Ar-40/Ar-39 cooling ages a nd provenance data from synorogenic sediments, emplacement of the Dadeldhur a thrust sheet took place in early Miocene time. The Ramgarh thrust sheet w as emplaced between similar to 15 and similar to 10 Ma. The Lesser Himalaya n duplex began to grow by similar to 10 Ma, simultaneously folding the nort h limb of the Dadeldhura synform. The Main Boundary thrust became active in latest Miocene-Pliocene time; transport of its hanging wall rocks over an similar to8-km-high footwall ramp folded the south limb of the Dadeldhura s ynform. Thrusts in the Subhimalayan zone became active in Pliocene time. Th e minimum total shortening in this portion of the Himalayan fold-thrust bel t since early Miocene time (excluding the Tibetan zone) is similar to 418-4 93 km, the variation depending on the actual amounts of shortening accommod ated by the Main Central and Dadeldhura thrusts. The rate of shortening ran ges between 19 and 22 mm/yr for this period of time. When previous estimate s of shortening in the Tibetan Himalaya are included, the minimum total amo unt of shortening in the fold-thrust belt amounts to 628-667 kin. This esti mate neglects shortening accommodated by small-scale structures and interna l strain and is therefore likely to fall significantly below the actual amo unt of total shortening.