DefinePK

Abstract

The central and north-western parts of Neelum valley have been mapped, proposing a new lithostratigraphic division for the region. The area comprises two main tectonic units: the Lesser Himalayan Crystalline Unit (LHC) and the Higher Himalayan Crystalline (HHC), separated from each other by the Main Central Thrust (MCT). The two units are characterized by a similar stratigraphy and share a common low grade metamorphic assemblage. They, however, differ in the intensity of the imprints of the Himalayan metamorphism. The lithostratigraphic units comprise, from base to the top, Naril Group (Precambrian basement), the Kundalshahi Group (pre-Himalayan cover) and Surgun Group (an Upper Paleozoic to Mesozoic Himalayan meta-sedimentary cover comprising paragneisses, micaschists, marbles, amphibolites, calc-schists and impure quartzite). This lithostratigraphic sequence is intruded by a megacrystic granite, similar to that of Mansehra (Early Cambrian) and meta-doleritic dykes, correlated with the Permo-Carboniferous Panjal Trap volcanics. Geochemistry reveals an igneous granitic calc-alkaline origin for the Naril granulitic gneiss, whereas, the sediments of Kundalshahi Group are characterized by a high SiO2 and highly variable trace element contents except REE. In the Surgun Group two types of marbles have been distinguished on the basis of high CaO and low MgO. The amphibolites in the Surgun Group as well as of Himalayan cover share the same igneous protolith and are basaltic in composition. Geothermo-barometric studies indicate polyphase metamorphic evolution for the Ethological units of the Neelum valley in the Tertiary Himalayan event. The p-T-t path suggests a prograde metamorphic evolution up to Upper Cretaceous-Eocene times, followed by a retrograde path, essentially of decompressive type. The final part of the path being accompanied by a decrease in temperature. The I-HC tectonic unit and a southern portion of the HHC unit followed a different P-T trajectory, under greenschist-facies conditions. The 40Ar/39Ar determinations on muscovite and biotite from these low grade rocks yield middle Oligocene cooling ages. Four phases of deformation have been distinguished. Isoclinal overturned to recumbent folds represent the DI and D2 phases; contemporaneous with the regional Himalayan metamorphism, during upper Cretaceous to Miocene. The MCT developed after D2 phase of deformation generating the LHC and HHC nappes and the reversed metamorphic zonation of Neelum valley. The D3 structures comprise of open large megascopic flexural slip buckle folds trending NE-SW. The D4 deformation affects the Hazara Kashmir syntaxis.