Structures gondwaniennes et himalayennes dans la zone tibétaine du Haut Lahul - Zanskar oriental (Himalaya indien)


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Structures gondwaniennes et himalayennes dans la zone tibétaine du Haut Lahul - Zanskar oriental (Himalaya indien)
Spring Laurent
Université de Lausanne, Institut de Géologie et Paléontologie
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Mémoires de Géologie (Lausanne)
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The study of the sedimentary series of the Tibetan zone in the Higher Lahul-SE Zanskar resulted in the establishment of a synthetic lithostratigraphic column, extending from the Lower Cambrian to the Upper Cretaceous.
New elements were brought in the knowledge of the Paleozoic structuration of the Gondwanian segment corresponding to the investigated region. During the Ordovician or the Silurian, locally extensive conditions favoured the creation of a syn-sedimentary fault, which throw is estimated to 800 m. The pre-rift history of the Tibetan zone al the Upper Paleozoic has been precised by the evidence of al least three different magmatic phases: within plate alkaline basalts presumably Carboniferous, an alkaline granite of mantellic or infracrustal affinity dated 284±1 Ma, and the tholeiitic within plate basalts of the Panjal Traps (Upper Permian). The lapse of time between the beginning of the rifting and the oceanization is of about 80 Ma.
The detailed study of lithostratigraphic sections revealed complete sedimentary cycles, ranging from intertidal to supratidal facies, in the lithologies of the Upper Trias. The evaporitic levels in the top series of the Lilang Group explain the almost systematic presence of lower layers of the Kioto Group (Upper Trias) as base levels of the extemally vergent overthrusts in Ladakh-Zanskar.
Consequently to the continental collision of India and Asia, the terrains of the North-Indian margin have been deformed, then overthrusted towards the SW. Five tectonic units have been individualized in SE Zanskar; they form the allochtonous front of a major tectonic structure, the Nyimaling-Tsarap nappe. During the extemally vergent phases (D1 and D2), the metamorphism gradually developed with the stacking of the nappes towards the SW; the climax was reached at approximatively 40 Ma in the eastern Zanskar. The maximal advance of the tectonic units of the Tibetan zone is situated in the southern part of the studied area. From the centre of the area towards the south, the metamorphism gradually decreases from the amphibolite facies to the lower green schists facies.
In a late period of deformation, the metamorphic isogrades are cut by syn- to post-metamorphic SW vergent movements.
The post-metamorphic deformation phases can be divided into two distinct events: a NE vergent backthrusting phase (D3 + D4) and a late extension (D5 + D6). The mass transfer towards the NE (D3) is first underlined by great isoclinal folds, then by backthrusts. This event is underlined by calcite twinnning and quartz recrystallization. The D3 phase caused the formation of a tectonic dome D4.
The Tibetan zone of eastern Zanskar has been affected by late extensional tectonics. The combined effects of a first generation of low angle normal faults (D5), a high-dipping normal fault (D6), and erosion, are responsible for the denudation which allowed the outcropping of high metamorphic terrains.
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04/09/2023 14:57
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05/09/2023 6:12
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