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Les nappes de Katmandou et du Gosainkund, Himalaya du Népal central : étude cartographique, structural, métamorphique, géochimique et radiochronologique

Abstract : In central Nepal, a multidisciplinary study has been carried out to characterize and distinguish the crystalline nappes of Kathmandu and Gosainkund from the Midland formations. Two principal deformations are recorded: one ductile, syn-metamorphic, marked by microstuctures (stretching lineation, S-C structures, etc.), another, post-metamorphic, recorded by an anticline, roughly EW -directed, and by NNE-SSW -directed folds. The syn-metamorphic P-T conditions show differences between Kathmandou Crystalline Nappe (900-720 MPa; 700-480 oC) and Gosainkund Crystalline Nappe (890-580 MPa; 750-590°C). They exhibit well preserved inverted metamorphism between the Upper Midland Formations (750 MPa; 560°C) and the Gosainkund Nappe. In central Nepal, the augen gneisses and the "Lesser Himalayan" Cambro-Ordovician granites bear similar petrographic and geochemical characteristics which suggest a common origin. However, the geological setting and age of the Proterozoic Ulleri augen gneiss rule out correlation with these formations. 40Ar/39 Ar analyses of muscovite, indicate cooling ages younging from south to north: 22 to13 Ma in the Kathmandu Nappe, 16 to 5 Ma in the Gosainkund Nappe, and 12 to 6 Ma in the Midland Formation. The principal points summarized by this study are the following : (i) Clear distinction between two nappes marked by their lithostratigraphy and metamorphism, (ii) The ductile movement of MCT in the north of Kathmandu is blocked since approximately 25 Ma, (iii) The late emplacement and late or common post metamorphic history of the two nappes, but earlier cooling history of the Kathmandu nappe, (iv) The present uplift of the Kathmandu region, underlined by the intense microseismosity, concems indifferently the two nappes that form a single tectonic block at present, (v) The combined uflift of the two nappes is due to the displacement on a ramp of major décollement surface.
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Submitted on : Monday, November 14, 2011 - 9:13:26 AM
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  • HAL Id : tel-00640663, version 1

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Santa Man Rai. Les nappes de Katmandou et du Gosainkund, Himalaya du Népal central : étude cartographique, structural, métamorphique, géochimique et radiochronologique. Géochimie. Université Joseph-Fourier - Grenoble I, 1998. Français. ⟨tel-00640663⟩

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