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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.

We describe a new species of Dravidogecko from the poorly-inventoried Periyar Plateau of Southern Western Ghats, Peninsular India. The new species Dravidogecko beddomei sp. nov. is characterized by: precloacal-femoral pores 46‒52; scales across belly 31‒33; supralabials 9–11; infralabials 8–9; lamellae on digit IV of pes 10–11. Additionally, the new species is divergent (>13%) from its related congeners. Like its congeners, the new species is currently known only from its type locality and its vicinity – the Devar Malai, Kakki Reservoir – and Sivagiri Hills in the Periyar Plateau of Southern Western Ghats. This discovery further increases the list of new reptiles described from this under-studied massif in the Southern Western Ghats.

A family of geophysical lineaments have been identified in ∼15,000 km 2 in central Kerala between 9°30′N to 10°45′N and 76°00′E to 77°30′E, integrating geophysical data with geological and geomorphological features. The characteristics of these lineaments in the magnetic and gravity fields and their derivatives have been analysed. The evolution of the lineaments has been traced to the temporal phases of global evolution of the region. A group of these faults have formed by reactivation of the deep-seated distensional fractures associated with and accompanying dyke emplacements during the episodic breakup of Gondwana at ∼90 and 65 Ma under distinctive mantle thermal regimes. It is possible that reactivation of these distensional faults may have started during the cooling interval of time between the two distensions in the 90 and 65 Ma and post 65 Ma periods and later in the Cenozoic, when the lineaments were enlarged to their present dimension, possibly under the influence of forces that led to the uplift of the western Ghats. These may extend down to the crust-mantle interface. A cluster of younger geophysical lineaments has been generated by reactivation along the weak planes of transformation of the charnockitic rocks of the Precambrian. They seem to have a strike-slip character. They are devoid of any dyke association and were formed on a cold crust. They may be confined to the upper-middle crust. They were generated in the high intensity intra-plate palaeo-stress fields of the triple forces arising from (1) the back-thrust from the Himalayan Collision; (2) the impact of epeirogenic forces and related isostatic uplift of the Western Ghats and (3) the flexural isostatic uplifts due to surface loads of late Mesozoic basaltic lavas and Cenozoic sedimentation in the coastal rifted basins in late Cenozoic, probably in the time span of 20 Ma to the present, when the palaeostress fields were most intense. [ABSTRACT FROM AUTHOR]

Detailed magnetic and gravity surveys in the Periyar Plateau and adjoining coastal tract (central Kerala) have brought to light a family of four major geophysical lineaments. These include the NNW-SSE trending Idamalayar lineament (LM-1) and the Kerala lineament (LM-2); both housing gabbroic and doleritic dykes dated ~90 Ma and geochemically related to the Marion plume and the NW trending Periyar lineament (LM-3) and Todupuzha lineament (LM-4) housing in part dolerite dykes dated 65 Ma and chemically related to the Reunion plume. The lineaments largely cut across the litho-tectonic framework of the Precambrian basement. Integrating geological evolution of the region, these lineaments seem to have been formed by reactivation of the deep-seated distensional fractures, that facilitated magmatic emplacements during the episodic breakup of Gondwanaland under distinctive mantle thermal regimes of the Late Mesozoic. Possibly, later in the Cenozoic, these lineaments were enlarged to their present dimensions during the uplift of Western Ghats. [ABSTRACT FROM AUTHOR]

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]