Plateau uplift in peninsular India

Peninsular India is characterized by some major plateau regions. These include the Deccan and Karnataka plateaus of the southern peninsular shield which together occupy an area of more than 400,000 km2 and the smaller Chotanagpur and Shillong plateaus in eastern and northeastern India, respectively. Major parts of the Deccan plateau are blanketed by basaltic lavas of the Deccan trap. The Bouguer gravity map of the region presents a number of gravity “highs” and “lows” suggestive of marked uplift and subsidence involving the crust and the seismic soundings indicate trap thickness varying from about 100 m in the eastern and southern margins of the trap territory to more than 1000 m in the western parts of Maharashtra. A major north-south fault zone has been indicated within the traps along the Bombay coast to the west of the Koyna earthquake-affected area. Marginal north-south as well as east-west faults have also been delineated by gravity and seismic surveys in the off-shore areas to the west of the Bombay coast, south of the Cambay graben and to the south of the Kathiawar peninsula. The prominent WSW-ENE trending Narmada-Son lineament to the north of the Deccan plateau comprises the Narmada-Son rift, which is characterized by basic alkaline rocks and carbonatite occurrences in its western extremity and kimberlite pipes over its eastern parts, which are of great significance in the study of plateau uplift in this region. The gravity data clearly bring out the fault system of this rift which is divided into segments. The Karnataka plateau contiguous to the Deccan plateau to its south consists mainly of Precambrian gneisses, granites and charnockites with some major plutonic masses, apparently connected to the deep-seated batholiths which are reflected in the Bouguer gravity map as strong negative anomalies. This plateau region is characterized by distinctive geomorphic features and mild seismicity. The Chotanagpur plateau also shows distinctive geomorphic features, including some prominent scarps with steep gradients and water falls. The Shillong plateau in northeast India occurs as an uplifted horst block in a zone of high seismicity astride the Himalayan range. Repeated geodetic levelling conducted during the past 70 years across this plateau indicated a total uplift of 2.5 cm during this period. The whole of peninsular India is characterized by particularly strong negative gravity anomalies, the cause of which clearly appears to be within the mantle. The plateau uplifts of peninsular India are apparently releated to thermal processes within the mantle.