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A myriad of geomorphic expressions consisting of fascinating physical landscapes and intricately sculpted landforms envelop the upper lithosphere of our planet. The scientific analysis of Earths landscapes and the investigation of processes that mold landforms are the fundamental facets of geomorphology. Geomorphic examination of a region includes the genetic analysis of landform assemblages and their spatial distribution to decipher the origin and establish an understanding of its evolution. Geomorphology is a science of immense significance with regard to processes that carve topography in the past and those that alter them today. In essence, the relationship between land surface forms and processes is a close and intricate one. The landform elucidates boundary condition for processes acting on it.
However, in acting on the surface the processes themselves modify the underlying form – the canvas they act on – and thus, finally, the intensity and the patterns of themselves. Thus, topography is both the result of past geomorphic processes and the stage for present geomorphic processes which reshape it again for future geomorphic processes (Swanson et al., 1988; Dehn et al., 2001; Straumann, 2010).
The Earth constitutes a succession of nested spheres of which the interface between the lithosphere, hydrosphere and atmosphere is by far the most complex and dynamic in nature. This interface houses landscapes that appear static to observations made over short periods of time but are extremely active entities undergoing rapid change in geological terms. The continental topography we witness today is by and large a product of processes and their repercussion of the immediate past. In principle, landscapes have the capability to register recent history of Earth’s surface and its environments.
Landscapes are created, constantly modified and torn asunder through the action of processes driven by external forces arising from the surface of the earth and internal forces from within the planet (Sharp, 1982). These processes combined together cause physical stresses and chemical actions on surface material that change the configuration of the earth’s surface are collectively known as geomorphic processes. The links between these driving forces and the resulting topography are complicated by delays between a forcing factor changing and the topographic expression of that change, leading to a system response time characteristic of the forcing factor and the properties of the system it is acting upon (Whipple and Tucker, 1999; Whipple, 2001; Saville, 2013).
The internal forces also called as endogenetic processes are responsible for continuously elevating relief and constructing parts of the Earth’s surface. External forces known as exogenetic processes operate by wearing down relief created by endogenic processes. Variations in land surface forms and physical topography persist as long as the opposing actions of exogenic and endogenic forces remain active. Processes that shift and uplift or build up portions of the crust known as diastrophism or tectonism and volcanism are considered as endogenic processes. Whereas, geomorphic processes such as weathering, erosion and deposition express exogenic forces.
Tectonism comprises of two vital mechanisms – orogenic and epeirogenic processes. Orogenic processes are responsible for mountain building through crustal folding resulting in severe deformation affecting long and narrow belts of the earth’s crust, while, epeirogenic processes involve uplift and warping of large regions on the surface of the earth. In order to measure amount of geomorphic deformation resulting from tectonic processes, it is necessary to have identifiable features that have been displaced called geomorphic markers (Burbank and Anderson, 2001). The best geomorphic markers are readily recognizable landforms, surfaces, or linear trends.
Exogenic processes derive their energy from the atmosphere and also the gradients created by tectonic factors. The effects of most exogenic geomorphic processes are minor and slow and may be imperceptible in a short time span, but will in the long run affect the rocks severely due to persistent activity. Climate and climatic elements such as temperature and precipitation are among the key driving forces behind exogenic geomorphic processes that strip down surface material by erosion, weathering and mass movement, ultimately, modifying landscapes (Burbank and Anderson, 2001). In order to denude, transport and deposit earth material, exogenic processes have to depend on elements of nature that are set mobile due to gravitational forces and earth surface gradients. These elements, called geomorphic agents, are capable of removing material and moving them over slopes and finally depositing them at lower levels. Running water, glaciers, ground water, waves and currents can be classified as geomorphic agents.
Fluvial networks are dynamic systems susceptible to alterations resulting from successful recurrent disturbances in equilibrium over timescales ranging from seconds to tens of thousands of years. They evolve remarkably, adjusting diversely to the wide range of environments in which they can be observed. Topography inherited by constant interaction of fluvial systems and consequent erosion, envelop extensive reaches of the earth’s surface. Based on controlling factors such as landscape morphology, lithology and surface gradient, fluvial systems can develop patterns pertaining to specific conditions within a drainage basin (Charlton, 2007).
Rivers flow into oceans, although some drain to lakes and inland seas, meanwhile some others dry up completely before reaching the ocean. Every river drains areas of continents called its drainage basin or catchment or watershed. The drainage area may be defined as the area which contributes water and sediments to a particular channel or sets of channels. It is the source area of the precipitation eventually provided to the stream channels by various paths. As such it forms a convenient unit for the consideration of the processes determining the formation of specific landscapes in the various regions of the earth. It provides a limited unit of the earth’s surface within which basic climatic quantities can be measured and characteristic landforms described, and a system within which a balance can be struck in terms of inflow and outflow of moisture and energy.
Drainage patterns by theory specify shapes or designs the individual streams may form. A distinction can be made between the patterns of the individual tributaries and their spatial relationships to one another. They can be termed as drainage arrangements since they refer more to the spatial relationships of individual streams than to the over-all pattern made by the individual drainage lines. It is generally recognized that drainage patterns reflect the influence of such factors as initial slopes, inequalities in rock hardness, structural controls, recent diastrophism and the recent geologic and geomorphic history of the drainage basin. Because drainage patterns are influenced by so many factors they are extremely helpful in the interpretation of geomorphic features, and study of them represents one of the more practical approaches to an understanding of structural and lithological control of land form evolution. The most commonly encountered drainage patterns are dendritic, trellis, radial, parallel, rectangular and deranged (Figure 1.1). Of these, dendritic patterns are by far the most common. They are characterized by irregular branching of tributary streams in many directions and at almost any angle, although usually at considerably less than a right angle. They develop upon rocks of uniform resistance and imply a notable lack of structural control. Dendritic patterns are most likely to be upon nearly horizontal sedimentary rocks or in areas of massive igneous rocks, but may be seen on folded or complexly metamorphosed rocks, particularly when imposed upon them by superposition. Trellis patterns display a system of sub-parallel streams, usually aligned along the strike of the rock formations or between parallel or nearly parallel topographic features recently deposited by wind or ice. The major streams frequently make nearly right-angled bends to cross or pass between aligned ridges, and the primary tributary streams are usually at right angles to the main stream and are themselves joined at right angles by secondary tributaries whose courses commonly parallel the master stream. Trellis patterns reflect marked structural control of most stream courses, except perhaps the trunk streams. The tributary valleys are usually subsequent strike valleys. A variety of the trellis pattern is the fault trellis pattern, which may be found where a series of parallel faults have brought together alternating bands of strong and weak rock. Radial patterns have streams diverging from a central elevated tract. They develop on domes, volcanic cones, and various other types of isolated conical or sub-conical hills. Parallel patterns are usually found where there are pronounced slopes or structural controls which lead to regular spacing of parallel or near-parallel streams.
Landscape results from the competition of tectonic and geomorphic processes (Burbank and Anderson, 2001). The impact of tectonics on landscape development and geomorphic processes in tectonically active regions can be quantitatively and qualitatively analyzed by studying the drainage system (Jackson et al., 1998; Sung and Chen, 2004; Delcaillau et al., 2006; Ramsey et al., 2008; Bahrami, 2013). The drainage system, ergo, is a key element of the physical topography and analyzing their evolution can provide information on denudational history and tectonic impacts on landscape development (Small, 1978; Vandana, 2013).
The deviation of course of present-day river networks from their common regional patterns, termed as drainage anomalies, are a product of active tectonic deformation (Zernitz, 1932; Howard, 1967; Clark, et al., 2004) and bedrock resistance. Hence, drainage patterns and their corresponding anomalies have potential to archive information regarding regional and local tectonic events (Deffontaines et al., 1992; Simoni et al., 2003; Delcaillau et al., 2006, Ramasamy et al., 2011; Bahrami, 2013). In tectonically active zones, tectonics may directly affect the development of fluvial systems through the modification of stream gradients, tilting of surfaces and/or structural deformation (Maher and Harvey, 2008).
The geometry of river networks is sensitive to surface uplift in actively deforming regions, thus recording the history of tectonically driven crustal motions (Howard, 1967; Seeber and Gornitz, 1983; Oberlander, 1985; Cox, 1994; Jackson and Leeder, 1994; Clark et al., 2004). Innate geomorphological characteristics of motions related to tectonics, reflect features such as linear valleys, ridgelines, steep slopes of uniform aspect and tilt of terrain (Jordan, 2003).
Ancient riverine networks termed as paleochannels or paleo-valleys are remnants of stream channels which are good geomorphological indicators of fault movements related to tectonics, redirecting the flow regime, abandoning ephemeral or perennial systems, that now stands buried or lost (Bates and Jackson, 1987). Rearrangement of the paleo-fluvial flow to present day configuration can be the contribution of recent tectonic deformation events which produces changes in polarity of regional slopes (Bonnet, 2009). As a result of these tectonic events, the vestiges of former floodplains may be exposed or/and uplifted. These former floodplain deposits are termed as fluvial terraces. Some terraces may have little or no alluvium on them and may thus be classed as bedrock terraces in contrast with alluvial terraces, which consist of gravel, sand and finer alluvium. Terrace staircases can be results of regional uplift (Burbank et al., 1996; Pazzaglia and Brandon, 2001; Wegmann and Pazzaglia, 2002; Bridgland and Westaway, 2008; Cunha et al., 2008; Claessens et al., 2009; Viveen, 2012) whereas differential tectonic movements produce unpaired terraces along the stream (Peters and van Balen, 2007; D’Allesandro et al., 2008; Larue, 2008; Martins et al., 2009).
Chapter 1 |
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