Holocene tectonics revealed by Tamil Nadu deltas, India

Overall geological histories of the Quaternary deltas of all over the world have been studied considerably, to understand the evolution, eustatic-isostatic changes, land-ocean interactive phenomehon, resources, environment etc. But tectonic signatures of these deltas have been less studied, though these have significance in understanding the natural resources, environment and evaluation of natural disasters. In this context, as the state of Tamil Nadu is a low easterly gradient plain, all the easterly flowing rivers have developed well evolved geomorphic features related to youthful, mature and old stages. Such old stages of the rivers are marked by the spectacular development of deltas. These deltas of Tamil Nadu have been studied in detail for understanding the tectonics with the help of visibly seen tectonic and geomorphic features in satellite based remote sensing and ground based data sets. The study has revealed that the upper age limit/ages of these deltas are probably of Middle to Late Holocene and the various tectonic and geomorphic anomalies observed in these deltas obviously indicate the tectonic processes/phenomena that prevailed during Middle Holocene-till date. The Proto Cauvery delta of Chennai region is witnessing ongoing NE-SW sinistral, NW-SE dextral and E-W vertical tectonic activities. The streams in Ponnaiyar delta are showing persistent southerly migration due to the probable ongoing land subsidence along NE-SW faults in the southern part of the delta. The Cauvery river which has originally developed a broad bird-foot shaped-arcuate delta, has undergone a phase of anticlockwise rotational migration and stabilized in the northern rim of the delta as Coleroon river during 2300-750 Y.B.P. due to the ongoing tectonic emergence in the south of the delta almost since the last 6000 years or so. The Vellar river, in deltaic regime, shows northerly migration due to E-W grabening in the northern part of the delta. Vaigai and Manimuttar rivers are again migrating northerly in their deltaic regions due to the probable land arching to the south of their present day flow. Thus, the paper discusses the post deltaic/Middle-Late Holocene geological processes of Tamil Nadu deltas which are predominated by tectonic activities.