Tectonometamorphic Evolution of the Karnali and Jajarkot External Crystalline Klippen, Western Nepal Himalaya

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Soucy La Roche, Renaud
Keyword
Himalayan Tectonics , Himalayan External Crystalline Klippen , South Tibetan Detachment , Greater Himalayan Sequence , Microstructure , Phase Equilibria Modeling , U-Th/Pb Petrochronology , 40Ar/39Ar Geochronology
Abstract
Field mapping and microstructural characterization of metamorphic rocks exposed in the Karnali and Jajarkot klippen in the western Nepal foreland highlight the presence of a top-to-the-southwest shear zone at the base and a top-to-the-northwest shear zone at the top of the metamorphic rocks, correlative with the Main Central thrust and the South Tibetan detachment, respectively. This structural geometry implies that these metamorphic rocks are the southern continuation of the Greater Himalayan sequence (GHS) exposed in the hinterland to the north, and are key locations to improve our understanding of mid-crustal evolution in continental collisions. New pressure–temperature–time–deformation data demonstrate that GHS rocks in the Karnali and Jajarkot klippen escaped the Miocene high-temperature metamorphic overprint pervasive in the hinterland, and consequently reveal the Eocene-Oligocene stages of the evolution of the metamorphic core. In the Karnali klippe, prograde metamorphism occurred between 40 and 35 Ma and P–T conditions peaked at >0.7–1.0 GPa and >650–700 °C between 35 and 30 Ma. Starting at 30 Ma, GHS rocks underwent cooling, decompression and melt crystallization associated with the onset of shearing along the top-to-the-NE normal-sense South Tibetan detachment. The GHS cooled below ~450 °C progressively from the south flank (20–17 Ma) to the north flank (17–14 Ma) of the Karnali klippe. In the Jajarkot klippe, prograde metamorphism occurred from 45 to 25 Ma, when peak P–T conditions of 0.75–1.2 GPa and 550–600 °C were attained. Peak metamorphism was followed rapidly by cooling below ~450 °C at 25–20 Ma. In marked contrast, GHS rocks now exposed in the hinterland were still buried, hot, and actively deforming while the foreland was cooled and exhumed. The klippen therefore preserve a snapshot of the early (i.e. Eocene-Oligocene) evolution of the Himalayan metamorphic core. Along-strike variations in the tectonometamorphic evolution of the two klippen are interpreted to reflect the geometry of the basal detachment of the orogen and suggest the presence of an east-dipping lateral ramp between the Karnali and Jajarkot klippen during the Oligocene.
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