Nature of the Lithosphere beneath the Precambrian cratons of Gondwanaland

Mukherjee, Pousali (2019) Nature of the Lithosphere beneath the Precambrian cratons of Gondwanaland. Masters thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

Investigating the lithospheric character of Precambrian cratons is essential to understand their formation and evolution. The Gondwana was an accretion of several cratons, viz., South America, South Africa, Antarctica, Australia, Madagascar and the Indian subcontinent along with Sri Lanka. The break up of Gondwanaland during the Mesozoic, led to the gradual rifting of these different cratons over geologic time. In this study, we are numerically modelling and imaging the lithosphere to understand the differences in the nature of the crust and upper mantle of the Precambrian Gondwanaland. We have investigated the Pilbara in Australia, the Napier Complex in Antarctica, the Antananarivo craton in Madagascar, the African cratons- Zimbabwe, Kaapvaal and Tanzania, the Proterozoic region of Sri Lanka and the Dharwar craton in South India. To image the Precambrian cratons, body wave tomography and P-wave receiver function have been used. The data consists of teleseismic earthquakes with epicentral distances lying between 30⁰ and 95⁰ and having magnitude more than 5.5. The crustal properties obtained have been used to interpret the the nature of crust, the tectonic activity like orogen, erosion in the area along with their relative rates and also the relative density of the different cratons. It was found that excluding Napier, crustal thickness is lesser in Eoarchean (Pilbara) and gradually increases with relatively high values in Meso (Tanzania ) and Neoarchean (Madagascar craton). The Proterozoic region of Sri Lanka was also chosen because it occupied a unique position in Gondwanaland, and a suture zone got exposed in Sri Lanka in the form of Highland complex during Pan African continental collision between Western and Eastern Gondwanaland. It was found that similar crustal properties of Sri Lanka, Madagascar, Kerala Khondalite Belt , Tanzania provide evidence of Sri Lanka Madagascar Gondwana linkage, and confirm significant position of Sri Lanka in Gondwana. Study of the crust and upper mantle in the Dharwar area revealed that there is a Mid Lithosphere Discontinuity in the Dharwar region in the depth of 80 to 100 km. The Lithosphere Asthenosphere Boundary (LAB) is varying from 150 in the Northern part and deepens to 250 km in Southern part. Thus, the evolutionary model of craton formation supports Durrheim and Mooney’s model with a slight modification- the thickness of the Archean crust is not restricted to 40 km, instead higher values are obtained in Madagascar craton and the Napier complex in Antarctica. In the early Earth, the temperature was very high and the plates were less dense and buoyant, so plate tectonics was probably not that effective. Heat flow in the earth controls the rate of cooling of rocks. It takes some time for the lithosphere to cool down and the plates to become denser. So plate tectonics was dominant in the middle to late Archean, as a result of which the crustal thickness increased.

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