MULTICYCLIC HISTORY OF THE NORTHERN INDIA CONTINENTAL-MARGIN (NORTHWESTERN HIMALAYA)

The geologic evolution of northern India is best recorded in the stratigraphic succession of the Zanskar Range (northwestern Himalaya), which represents the most complete transect through this ancient continental margin. Sedimentary history began in the late Proterozoic, and recorded a late Pan-African orogenic event around the Cambrian-Ordovician boundary, when the Gondwana supercontinent was eventually assembled. The following long period of epicontinental deposition in shallow seas linked to palaeo-Tethys lasted until the Early Permian, when a neo-Tethyan rift began to open between paleo-India and the Cimmerian microcontinents. Neo-Tethyan history can be subdivided into two sedimentary megasequences, both recording a major tectonic and magmatic event in the lower part. The first one began with breakup in the Late Permian and lasted until the end of the Jurassic. The second one started in the Early Cretaceous with the final detachment of India from Gondwana and the opening of the Indian Ocean, and ended with the India-Eurasia collision in the Early Eocene. The two megasequences can be in turn subdivided into six transgressive/regressive supersequences bounded by tectonically enhanced unconformities. Basal sandstone units of Early Permian, Late Permian, Norian, Callovian, Early Cretaceous, and Paleocene age are invariably associated with oolitic ironstones or reworked glauco-phosphorites, and mark the transgressive part of each supersequence. Next, condensed nodular carbonates or shales with pelagic fauna are typically overlain by thick shallowing-upward marly units capped by regressive platformal carbonates. The six tectono-eustatic supercycles reflect successive rifting episodes which punctuated the progressive separation of India from the rest of Gondwana, and document the combination of plate/microplate reorganizations and eustatic, climatic, and oceanographic changes in the Tethyan domain. After the onset of collision between India and Asia close to the Paleocene/Eocene boundary, obduction of the remnants of the neo-Tethys ocean floor onto the Indian margin began, and the latter underwent multiphase deformation with fold-thrust shortening followed by heating and extension. After the main metamorphic event, ophiolitic nappes were re-thrusted and finally emplaced with their sedimentary sole on top of the passive-margin succession.