200 Chapter-V Litho-structural and Tectonic Control on Drainage and Stream Profiles 201 Chapter V LITHO-STRUCTURAL AND TECTONIC CONTROL ON DRAINAGE AND STREAM PROFILES 5.1. Introduction: The most important finding that has emerged from the analysis of the morphometric and morphotectonic indices as well as the long profile characteristics presented in the previous chapters is that the middle domain of the Kaveri Basin, entrenched between the two prominent knickpoints of Shivasamudram and Hogenakkal Falls, most likely exhibits highest degree of tectonic control as compared to the upper and lower domains as well as the Palar and Ponnaiyar Basins. However, in the realm of tectonic geo morphology, one must also take into account various other identifiable features in the field which can provide additional information on the recent or past tectonic deformation or litho-structural control. These surficial features, often called as geomorphic markers provide a reference frame against which differential or active tectonic deformation is gauged (Burbank and Anderson, 2001). These readily recognizable landforms, surfaces or linear trends usually preserve unique records of various pulses of uplift in an area or region. Some of the most common surficial geomorphic markers which can be used as suitable indicators of tectonic activity and litho- structural control in an area include knickpoints, gorges, linear courses of rivers, outsized channels, barbed drainage patterns, paleo-channels and abandoned drainage courses, sharp bends, fluvial hanging valleys, river terraces, etc (Vaidyanadhan, 1971; Seeber and Gomitz, 1983; Burbank and Anderson, 2001; Kale and Gupta, 2001; Bishop et al., 2005; Ramasamy, 2006a; Wobus, et al., 2006; Crosby et al., 2007; Castillo et al., 2013 and the references therein). Here it is pertinent to mention that the rivers are the most sensitive and dynamic components of a landscape and swiftly respond to any change in the prevailing tectonic, climatic and base level conditions. Although the recovery period of the fluvial system varies, a drainage basin larger in areal extent and spread over diverse lithologies usually takes a long time to achieve a new equilibrium after the cessation of tectonic activity (Burbank and Anderson, 2001) or change in climatic or base level conditions. It is also important to 202 establish the age of these markers in order to ascertain whether these are associated with recent tectonism or are signatures of past phases of uphft. For example, a phase of tectonic upheaval will reflect itself with decrease in width-depth ratio of a river channel and hence the specific stream power is likely to increase. Needless to state this would imply greater erosion by the river and hence the sediment flux will undergo a major change. Similarly, the channel cross-section and consequently the specific stream power and sedimentary budget of the rivers will vary across rivers basins spread over diverse lithologies. Therefore, the fluvial hydraulic parameters such as the unit stream power and sediment budget have often been used as indicators to ascertain whether an area is under active tectonic forcing and/or litho- structural control (Milliman and Meade, 1983; Milliman and Syvitsky, 1992; Harbor, 1998; Burbank and Anderson, 2001; Sklar and Dietrich, 2001; Wohl and Achyuthan, 2002; Finnegan et al., 2005; Goode and Wohl, 2010; Pederson and Tressler, 2012; etc.). Apart from the fluvial evidence of tectonics; there are some geological signatures which have been used by a large number of researchers across the globe to understand the response of a river to lithology and tectonics. Prominent among them include control of lineaments and faults on drainage network (Subrahmanyam et al., 1995; Bhagat and Verma, 2006; Subrahmanyam et al., 2007; etc.), response to gravity anomalies (Braile et al., 1986; Singh et al., 2003; Ramasamy, 2006a); seismic epicentres (Valdiya, 2001; Secanell et al., 2004; Roy, 2006; Sitharam and Anbazhagan, 2007; Tiwari et al., 2009); etc. In this chapter, all the various geomorphic markers and other geological evidence observed in the basins under review are compiled, presented and discussed. 5.2. Methodology: In order to determine whether there is any control of lithology, structure and/or tectonics on the drainage and channel morphology of the rivers under consideration, the lineaments in the Kaveri, Palar and Ponnaiyar River Basins have been mapped from SRTM- DEM and its derivatives. A buffer of 500 m was created around the DEM-derived lineaments and all the streams that were falling in this zone were extracted and their lengths were measured. These lengths were compared with the total lengths of all the streams in the three 203 basins. Then the percentage of lineament control on the streams in the three river basins was obtained. As seismicity is often associated with the prevailing tectonic conditions in an area, records of seismic activity in the last two centuries were mapped from Valdiya (2001), Sitharam and Anbazhagan (2007) and Chopra et al., (2008). This map was overlain on the lineament and fault map of the studied basins in order to ascertain whether any association exists between them. The best geomorphic markers of tectonics are the readily recognizable landforms, surfaces or drainage trends. Under such circumstances, the exercise of identifying and defining the geometry of such markers has often been considered relevant in the realm of tectonic geo morphology (Burbank and Anderson, 2001). These surficial geomorphic markers may be visible in the field as well as on the satellite imageries or maps. Therefore, remote sensing studies as well as field investigations in the Kaveri, Palar and Ponnaiyar Basins were carried out in order to synthesize the information regarding all the geomorphic markers and morpho-tectonic features in addition to the drainage basin and network indices obtained from SRTM-DEM analysis (discussed in the previous chapters). Visual interpretation of satellite imageries and digital elevation models was carried out in order to identify various surficial markers such as knickpoints, gorges, drainage diversions, linear trends of streams, barbed drainage pattern, etc. Detailed maps of the river basins under consideration showing the various drainage anomalies were prepared in ArcGIS. Evidence of the presence or absence of tectonic uplift in an area is also reflected by the relationship between elevation and Bouguer Gravity Anomalies. Therefore, these data were also considered and analyzed. The Bouguer Gravity Anomaly Map was taken from Ramasamy (2006a). An attempt was made to understand the relationship between the gravity anomalies and elevation. Finally an attempt has been made to interpret the results obtained from above studies and provide an explanation for the presence of numerous anomalies in the area under review. Field work was carried out in the Kaveri Basin (Talakaveri to Srirangam) in order to search for depositional records and also to identify major geomorphic anomalies. Sedimentary deposits were located at several places along the Kaveri Channel and some of its tributaries. The older deposits occur at Siddapur, close to the source of Kaveri River, 204 upstream of Shivasamudram Falls in the Kollegal-Narsipura-Talakad area, in the Biligundulu area near Hogenakkal Falls, over the Tamil Nadu Plains and the delta deposits close to the mouth of the Kaveri River. Some of these deposits have been dated by earlier workers from late Holocene to late Pleistocene (Valdiya and Rajagopalan, 2000; Singh and Rajamani, 2001; Kale at al, 2010; Alappat, et al., 2010; Kale et al., 2014). The deposits occurring in the Kollegal-Narsipura-Talakad area (Valdiya and Rajagopalan, 2000) and at Biligundulu are significant from the point of view of tectonic history of the river. Therefore, they are discussed here, particularly the latter deposits, because they occur in the gorge section. Sediment samples were collected from these sites for textural and geochemical analysis as well as for luminescence dating. The Palar River presents an excellent example of an outsized channel on the Tamil Nadu Plains with the channel of the river being too wide as compared to the valley size and the upstream catchment area. In order to ascertain the channel morphological characteristics of the river, cross-sectional surveys were conducted at eleven sites on the river and data regarding channel cross-sections and channel geometry (width and depth) were generated using Electronic Distance Measurer (EDM) or measuring tape (Honnavar, 2008). Regression equation was estimated to evaluate the relationship between bankfull width and upstream drainage area for the Palar River. This was fiirther compared with the relationships of other rivers of the world in different climatic conditions given by Wolman and Gerson (1978). It is generally expected that the areas under active tectonic movement are characterized by higher specific stream powers and enhanced erosion. Therefore, the hydrological parameters of unit stream power, sediment load and the rate of denudation were also taken into consideration for the rivers under review and were compared with other Indian and world rivers. Other types of evidence were also studied to understand and establish the ongoing tectonic activity in the study area. This includes the tidal record and delta plan morphology. In order to ascertain whether the Kaveri Delta is prograding, contours were generated for the delta region from SRTM-DEM data and the contour pattern