Characterization and Age of North-Verging Back Structures in the Tethyan Sedimentary Sequence, Hidden Valley, Central Nepal Himalaya

Abstract

Throughout the central Nepal Himalaya, the Tethyan sedimentary sequence (TSS) is structurally dominated by a train of north-verging folds, in apparent contradiction to the dominantly south-propagating Himalayan orogen. The absolute age of folding is unknown, although previous structural observations have suggested that folding may predate Miocene movement of the South Tibetan detachment system. Age dating and characterization of this deformation event is critical for determining the timing of coupling and/or decoupling of the upper crust from the mid-crust during the Himalayan orogen, as well as elucidating the early collisional history of the orogen. Structural mapping in Hidden valley, central Nepal, reveals four phases of deformation (D2, D3, D4 and D5) within anchizonal to epizonal metamorphic grade Ordovician to Jurassic TSS rocks. Megascopically, D2 is defined by large, asymmetric north-verging folds; microscopically, D2 is seen as an axial planar continuous to spaced cleavage. Bed length restoration of F2 folds indicates a minimum of 40% shortening and 180% thickening during D2. Muscovite geochemistry, geochronology and microstructural analyses are used to identify two chemically and structurally distinct generations of muscovite in the TSS, one of which is detrital (Al), and one of which is new growth (A2). Structurally, Al is randomly oriented and intermittently rotated into S2 and S4 cleavage planes, while A2 is parallel to S2 axial-planar cleavage and also occurs as growth rims on Al. Conventional 40Ar/39Ar step heating methods indicate that Proterozoic and Paleozoic detrital Al muscovite Ar systematics have been weakly reset. A2 muscovite, dated by the in situ UV laser 40Ar/39Ar spot-fusion method and assuming closed system since metamorphism, is at least as young as 35.5±1.7 Ma, which is interpreted as a maximum age for F2 folding. The UV laser shows great potential for in situ dating of cleavage domains. D2 north-verging folds are Himalayan in age, likely occurring between ~35 and 22 Ma; this is the first maximum age constraint obtained for this deformation. D2 folds are interpreted to have formed in Eocene-Early Oligocene time, near the maximum age limit of this range (~35 Ma), coeval with kyanite-grade metamorphism in the mid-crust. The north-verging folds, which formed in the upper crust, were then subsequently detached/decoupled during midcrustal southward extrusion at 22-18 Ma.