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Evolution of "UR", an incipent continent, to seychelles, a microcontienent: from northwestern indian shield

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K.K. Sharma and Ritesh Purohit

Government Post Graduate College, Sirohi, Rajasthan, India

Northwestern Indian shield has unique position in continental evolution and global tectonics since the dawn of cratonisation on the Earth. The oldest continental element from this part of the earth is reported from Udaisagar granites dated 3.3 Ga (Roy and Kroner, 1996). This proto-continent was presumably part of the oldest continent “UR”. The Mewar Gneiss is the oldest Supracrustal occurring of Archaean age in the western Indian shield. Several Proterozoic mobile belts viz. Aravalli, Delhi and Sirohi orogenic mobile belts have been reported from this part. Choudhary et al (1984) reported widespread granitic plutonism in the Aravalli-Delhi belt around 850 Ma. This plutonism indicates cratonisation of the Northwest Indian part and the Arabian Peninsula. In the beginning of the Neoproterozoic Gabbro-Diorite intrusions are recorded at Ranakpur (Volpe and MacDougall, 1984). The magmatic event is related to the Rodinian tectonics. The Erinpura Granites in Sirohi region is basement for Sirohi orogeny. Subsequent to this wide-spread magmatism is reported from western Rajasthan, Tusham in Haryana and Nubian Arabian shield. This thermal event began at around 750 Ma with the wide spread anorogenic mafic volcanics followed by the felsic volcanics named as Malani Group (Roy and Jakhar, 2002). Along the western Indian shield present day Seychelles Island was located along the Saurashtran coastline. The magmatic orogeny seems to be basement organization of the Seychelles, which was then the part of the NW Indian shield accreted along the western margin of the present Saurashtra region. The Seychelles Island occupied the basement granite of 630-700 Ma. Similar plutonism is observed in the terminal phase of Malani Magmatism. The Pan-African event dated at 550 Ma, arose due to deep seated crustal changes, was a strong thermal event. These dates are reported from western Rajasthan, western Dharwar, Madagascar and Seychelles. It is presumed that the Permocarboniferous glaciation Bap-boulder type deposits event extended up to the Seychelles Island. Mesozoic Era is marked with the rift formation in the western Rajasthan, Kutch, extended upto Seychelles, Madagascar, Mozambique in southwest Africa and Tasmania in southwest Australia. The end of Mesozoic is characterized by the mass extinction on Earth. Around 180 Ma volcanic events occurred at Rajmahal and Seychelles. The Deccan volcanics and the break of Seychelles and Madagascar from India were caused presumably due to one: the Bolides impact in the Kutch region around 66-65 Ma near the KT boundary. This is evidenced by the high iridium anomalies; Mass extinction of the species and development of natural fullerenes. Two: During the same time interval the northwestern Indian and the adjoining part traversed over a hotspot resulted due to the Reunion Plume. The available data indicates that the Deccan volcanism took place in two phases; first due to the bolides impact and second due to the Reunion plume outburst. This was the time when Arabian Sea developed along a MOR. Several transcurrent faults across the Arabian MOR, one of the transcurrent developed in the Gulf of Kutch. Three parallel transcurrent faults divided the western Kutch into Kutch islands and Saurashtra. At the outburst center of the reunion plume a triple junction had developed from where one arm is extended along the Sonata lineament, second towards the Cambay-Barmer-Sanchor Graben and the third one towards Maldives–Laccadive. The Cambay-Sanchor rift has developed on the continental part whereas the Cambay-Laccadive arm developed along the oceanic part. This tectonic structure has been named as Raval line (By Sreedhar and Raval, 1990) from Laccadive to Karakorum passing through Cambay graben. The early MOR later developed into the Carlsberg Ridge which has continuously shifted the Seychelles south-westward. It follows the Raval line further towards south and has been named as Central Indian Ridge. It is almost parallel to the 90 East Ridge.
During Mesozoic tectonics caused cratonic basins development in northwestern Indian shield. Consequently the Aravalli craton was pushed eastwardly and compressive regime prevailed during that time. As a result of this Precambrian shear zones reactivated and Aravalli Mountain got uplifted. Several peneplained plateau like features along the Aravalli belt are observed (Heron, 1953). The high seismicity along the Raval line exhibits onshore and offshore earthquake epicenters. This indicates recent neo-tectonic activity in the region. The Laccadive and the Maldives islands are volcanic islands, the Raval line is an incipient MOR, which will cause separation of Kutch-Rajasthan, southern Pakistan from the Indian shield in few million years. Recently teleseismic imaging of western Rajasthan, Cambay and the west coast indicates a low velocity zone, which implies thermally induced impacts of the Reunion plume and a probable persisting rifting phenomenon.

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