Search

The Periyar Plateau in central Kerala is characterized by a strong negative Bouguer gravity field around three major islands of a strongly negative field of the value of -150 mGal. Detailed gravity studies reveal that these islands of strong negative field are of a shallow nature and are not traceable in the 1/4° average gravity grids. Spectral analysis of Bouguer anomaly map and 2D gravity modelling of Periyar Plateau region reveal the average depth of the shallow source layer is as shallow as 700-1000 m, which is correlatable with the layer of lateritic/lithomargic soil cover and the underlying weathered country rock having a density of 2.2 g/cc. Such a strong influence on the regional gravity field is due to the wide contrast in the densities of the lateritic cover and the primary rocks of the basement. These results underline the need to recognize the strong influence of such alteration zones on the regional gravity fields, especially the tropics, a factor not always considered.

Abstract: The study region forms the western part of the Madurai block (southern block) and shares several lithological characteristics of the Proterozoic exhumed South Indian Granulite Terrain (SGT). The crustal structure of the area has been derived from gravity data, constrained partly by aeromagnetic data. The Bouguer anomaly map of the region prepared based on detailed gravity observations shows a number of features (i) the Periyar lineament separates two distinctly different gravity fields, one, a high gravity gradient tending to be positive towards the coast in south west and significant gravity lows ranging from −85 to as low as −150 mGal in the NE covering a large part of the Periyar plateau (ii) within the broad gravity low, three localised circular anomalies of considerable amplitude occur in the region of Munnar granite. A magnetic low region in the central part coincides with the area of retrogressed charnockites and the major lineaments suggestive of a genetic link and considerable downward extent. The crustal models indicate that the upper layer containing exhumed lower crustal rocks (2.76 gm/cc) is almost homogeneous, most part of the gravity field resulting from variations in intracrustal layers of decharnockitised hornblendic gneisses and granite bodies. Below it, a denser layer (2.85 gm/cc) of unknown composition exists with Moho depth ranging from 36 to 41 km. The structure below the region is compared with that of two other segments of the SGT from which it differs markedly. The Wynad plateau forming the western part of the Northern Block of the SGT is characterised by a heterogeneity due to the presence of contrasting crustal blocks on either side of the Bavali shear zone, possibly a westward extension of the Moyar shear zone and presence of high density material in the mid-to-lower crustal portions. The crust below the Kuppam–Palani transect has a distinctive four-layer structure with a mid-crustal low density layer. The differences in crustal structure are consistent with the different tectonic settings of the three regions discussed in the paper. It is suggested that the crustal structure below the Kuppam–Palani transect corridor is not representative of the SGT as a whole, an aspect of great relevance to intra-continental comparisons and trans-continental reconstructions of continent configurations of the Gondwanaland. [Copyright &y& Elsevier]