Late Quaternary Movements and Landscape Rejuvenation in Southeastern Karnataka and Adjoining Tamil Nadu in Southern Indian Shield

Late Quaternary movements on N-S trending segmented faults in the Southern Indian Shield have given rise to en echelon linear hills and isolated hillocks that abruptly rise 300 to 400m above the 900 m Bangalore peneplain covered thickly with red soil, and are made up of remarkably fresh Precambrian rocks devoid of soil capping. Tilting of soil-covered regolith on the fault-delimited hill flanks together with extraordinarily severe gully erosion in the confines of narrow fault zones bear testimony to continuing movements on these active faults, which also gave rise to many tectonic lakes following stream ponding. Further south, > 1800 m high Biligirirangan-Mahadeswaramalai Hills represent fault-lined horsts, drained by meandering streams that fall and cascade down their steep scarps. Folding of Recent fluvial gravel, development of spectacular knee bend in an otherwise very straight fault valley, and abrupt pronounced northward shift of the Kaveri River from its original southeasterly course indicate Holocene movements on the faults delimiting the horsts.

The >2500 m high Nilgiri massif amidst the 300-150 m Tirupattur-Coimbatore plain is characterized by gently undulating top with very thick soil cover and lazily flowing streams that drop down precipitous scarps (of 1800 to 2000 m sheer fall) through chasms and gorges. Ponding of streams due to reactivation of faults gave rise to lakes comprising 20,000 to 5000 year-old peat within black clays. Thrusting up of the Nilgiri block on the E-W oriented Moyar Shear Zone (which separates the Southern Granulite Terrane from the Dharwar Craton) is a continuing phenomenon, as evident from spectacular rejuvenation of meandering streams, development of multitudes of deep defiles and waterfalls, presence of unfurrowed straight planar valley slopes, and recurrent seismicity in the Moyar Shear Zone.

The thrust movements on the E-W shear zones have been, and are being transmitted northwards as strike-slip and oblique-slip movements on the N-S trending faults in the Dharwar Craton. Close association of epicentres with the fault zones implies that the strain accumulated in the Peninsular Shield is being relaxed through reactivation of these active faults of Precambrian antiquity.