Topographic Analysis Linkages among Climate, Erosion and Tectonics

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Triangular interactions among climate, erosion and tectonics happen during the course of formation and development of a mountain range. In this study mountain range of Nyainqentanglha of Himalaya has been focused to assess which element played the vital role in this case. Altitude data of the catchments have been used as the primary key of analysis. Significant concentration of catchment areas near glacier equilibrium line altitudes (ELA) proved the presence of glacial buzzsaw mechanism. Swath analysis confirmed the presence of Teflon peak. Finally web of interrelationship has been explored behind the development of this mountainous range.

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Topographic Analysis Linkages among Climate, Erosion and Tectonics

  1. 1. Topographic Analysis: Linkages among Climate, Erosion and Tectonics Shahadat Hossain Shakil ABSTRACT Triangular interactions among climate, erosion and tectonics happen during the course of formation and development of a mountain range. In this study mountain range of Nyainqentanglha of Himalaya has been focused to assess which element played the vital role in this case. Altitude data of the catchments have been used as the primary key of analysis. Significant concentration of catchment areas near glacier equilibrium line altitudes (ELA) proved the presence of glacial buzzsaw mechanism. Swath analysis confirmed the presence of Teflon peak. Finally web of interrelationship has been explored behind the development of this mountainous range. …… INTRODUCTION Since Dahlen and Suppe (1988) explored that „erosion can affect the tectonics of the region undergoing that erosion‟, researchers have been drived to discover the interactions among not only erosion and tectonics but also climate, which affects erosion and is affected by tectonics (Molnar, 2009). Topography represents the net product of tectonic and surficial processes. Interactions between tectonic and surficial processes are complex and involve coupling with feedback through diverse mechanisms (Fig. 1). Fig.1. Feedback loops within the dynamic system defined by tectonics, climate and erosional surface processes. There are two feedback loops; a direct path (I) whereby tectonics increases erosion rates by increasing elevation, relief and drainage basin areas and an indirect loop (II), whereby increased elevation induces increased erosion rates through changes in climate (Adopted from Willett et al. 2003, p.33) Mountains are created and shaped not only by the movements of the vast tectonic plates that make up Earth‟s exterior but also by climate and erosion (Pinter and Brandon, 2005). In particular, the interactions between tectonic, climatic and erosional processes exert strong control over the shape and maximum height of mountains (Brozović et al., 1997) as well as the amount of time necessary to build or destroy a mountain range (Pinter and Brandon, 2005). Paradoxically, the shaping of mountains seems to depend as much on the destructive forces of erosion as on the constructive power of tectonics (Zeitler et al., 2001). The geology of the Himalaya (stretch over 2400 km) is a record of the most dramatic and visible creations of modern plate tectonic forces (USGS, 1999). This immense mountain range was formed by tectonic forces and sculpted by weathering and erosion. Topographically, the belt has many superlatives: the highest rate of uplift (nearly 10 mm/year at Nanga Parbat), the highest relief (8848 m at Mt. Everest Chomolangma), among the highest erosion rates at 2–12 mm/yr (Burbank et al., 1996), the source of some of the greatest rivers and the highest concentration of glaciers outside of the polar regions. Nyainqentanglha Mountains of Himalaya forming the eastern section of a mountain system in the southern part of the Tibet Autonomous Region of south-western China, is a 700-kilometer (430 mi) long mountain range (Fig. 2). It has an average latitude of 30°30'N and a longitude between 90°E and 97°E. The range is divided into two main parts: the West and East Nyainqentanglha, with a division at Tro La pass near Lhari. The Nyainqentanglha Mountains bound the northwest side of the Yangbajian graben and are parallel to it (Pan and Kidd, 1992). The average elevation of the Nyainqentanglha Mountains is about 6000 m. West Nyainqentanglha includes the four highest peaks in the range, all above 7000m. It lies to the southeast of Namtso Lake. East Nyainqentanglha located in the prefecture of Nagchu, Chamdo and Nyingchi marks the water divide between the Yarlung Tsangpo to the south and the Nak Chu river to the north. The area is of special interest for glacio-climatological research as this [1] region is influenced by both the continental climate of Central Asia and the Indian Monsoon system, and it is situated at the transition zone between temperate and sub-continental glaciers (Bolch et al., 2010). Fig. 2. NQTL Range Location, bounded by Tibetan Plateau-north, Bhutan-south, Himalayan-West, Namcha Barwa-East REGIONAL SETTING Geologic mapping in the eastern Himalayan syntaxis confirmed the three regional tectonic elements outlined by previous geologic workers. The Namche Barwa and Nyainqentanglha crystalline complexes lie below and above the Indus-Yarlung Tsangpo suture (IYS), respectively, and both were parts of the northern Indian plate basement rocks. Uplift and exhumation have been the most recent dominant tectonic processes in the late Cenozoic for the High Himalayan crystalline rocks (Namche Barwa Group) in the core of the Namche Barwa antiform (Quanru et al., 2006). Results of a recent study led by Wang et al. (2013) showed high variation in extent of glaciers and lakes with increased temperature and precipitation in the past 40 years in this area. These variations include glacial retreat, increased water level of inland lakes and increased number of glacier lakes to higher altitudes.
  2. 2. In contrast According to Kang et al. (2007), an intensification of atmospheric circulation and increase of sea-surface and air temperatures, resulting in intensified moisture availability and moisture transport, have been a major cause for the increase of ice-core accumulation over the Mt. Nyainquentanglha region since 1980s. RATIONALE AND METHODS The aim of this study is to analyze the topography of Nyainqentanglha mountain range and to find out the linkages between erosion, climate and tectonics, and to determine which played the dominant role in the development of mountain range. DEM; seed point of catchment and a glacier ELA dataset of the study area has been collected for analysis. TAS GIS and MATLAB have been used for the investigation and illustration purpose. To determine the catchment size, holes within the DEM has been filled first to model the stream flow uninterruptedly. Then using the FD8Quinn algorithm Specific Contributing Area (SCA) has been determined. Visually 79 major catchments have been identified then with the help of their grid reference a Seed Point file has been created which contains these outlets co-ordinates. Afterwards, watersheds have been delineated with TAS and converted to vector for area calculation. Min, max and avg. elevation and average slope from DEM has been extracted for further evaluation. Swath analysis in MATLAB has been also performed to visualise the physical scenario across the range. Swath analysis simplifies the topographic data for better understanding and observations. This practice is also termed as data reduction technique. To extract and analyze data (i.e. correlation with ELA glacier and elevation) from the complex data structures of DEM (i.e. plan view) and to make insights and decision from it (i.e. following the glacial buzzsaw hypothesis?). During this process a DEM is divided into number of clusters based on visual and statistical symmetry. Then considering variation of the field along or across the range, orientation of Swath is determined. To cover the identical area within one frame as well as considering the length of range, width of the Swath frame is figured out. Each swath frame records the maximum, minimum, and average elevation for each pixel band. To cover near about eight identical zone across the range in terms of catchment size, and to visualize the difference between the northern catchments (1-46) and southern catchments (47-79), swath width has been fixed at 25 km (Fig. 3; Fig. 4). Orientation of the swath frame has been fixed across the range (perpendicular with the longer axis of the range) to visualize the difference between north and south catchments. During this process each swath frame recorded the maximum, minimum, and average elevation for each pixel band (90m by 25 km) perpendicular to the 25–40 km swath length (Fig. 4). Similar type of approach has been adopted by Dortch et al. (2011) to determine the longitudinal topographic variation of the central Ladakh Range and Kühni and Pfiffner (2001) in case of topographic analyses of Swiss Alps mountain belts along cross-sections perpendicular to the main structures of different orogens. Fig. 3 Catchments shape and location; ID: 1-46, northern side catchments; ID: 47-79 southern side catchments Swath Analysis: Glaciers concentrate within the elevation of 5500 – 6000 meter (Fig. 8; Swath 1 – Swath 6). In swath 1 and 2 glaciers distributes evenly within the two regions. But in swath 3-6 glaciers concentrates largely in the southern region (right hand side of the Divide). No glaciers have been found in swath 7-8, comparatively lower altitude region of both zones (Fig. 8). RESULTS Spatial Pattern of Catchment Size: area of northern catchments (1-46) range between 1.65 ~ 86.33 sq.km with an average of 24.55 sq.km. Whereas area of southern catchment lies between 7.44 ~ 250.7 sq.km with an average of 47.81 sq.km (Fig. 5). Southern part of the range contains an exceptional catchment area of 250.7 sq.km which increased the average value of this part, while the normal average is slightly higher than the northern part (Fig. 5). Elevation vs. Catchment Size: In case of smaller catchments, southern parts have lower higher elevation than the northern. But the reverse scenario exists in case of bigger catchments, higher maximum elevation for southern zone (Fig. 6). In both cases southern catchments have lower minimum elevation than northern, resulting higher relief in the southern zone (Fig. 8). In case of increase in catchment area for both zones after certain point elevation decreased, with couple of exceptions for southern zone. A positive but weak trend can be seen for maximum (R2 = 0.352) and average elevation (R2 = 0.297) and catchment size. Whereas a negative but very weak correlation can be seen for minimum elevation (R2 = 0.041). Average Slope vs. Catchment Size: For both smaller and larger catchments, southern zone have higher average slope than the northern zone. Southern zones slope range between 24.81 0 ~ 18.140 (degree), with an average of 22.470. In contrast, northern catchments average slope lies between 21.95 0 ~ 14.090, with an average of 18.960. [2] Glaciers are closely related with the maximum and average elevation line in swath 1-3. But due to the presence of some exceptional peak in the southern zone (Fig. 6), glaciers correlate with the average elevation line in swath 4-6. DISCUSSION Concentration of high proportion of catchment area (Fig. 6) within the range (5500 – 6000) of glacier equilibrium line altitudes (ELA) (Fig. 8) suggesting that operation of a Glacial Buzzsaw denudation mechanism effective in reducing surface topography above the snowline and concentrating it at the snowline (Brozović et al., 1997; Egholm et al., 2009). This result supports the findings of Brozović et al. (1997), Montgomery et al., (2001), Mitchell and Montgomery (2006) and Brocklehurst and Whipple (2002) that glaciated orogens in the Himalayas, the Andes, the Cascade Range, and the Sierra Nevada (USA) hold a striking coincidence of snowline altitudes, glacier equilibrium line altitudes (ELA) and elevations with a high proportion of surface area. On the other hand some anomalies/exceptions exist in the southern zone of the range, by the presence of some exceptional peak (Fig. 8; swath 4-5). This has been has termed as Teflon Peak by Anderson (2005), which cannot be well described through glacial buzzsaw. So the range is better characterized as a non-uniform scooping of the landscape between high hard slippery Teflon peaks (Anderson, 2005).
  3. 3. Fig. 4 Swath profile location and orientation; red box – swath frame, blue points – glacier location Fig. 5 Spatial pattern of catchment size Fig. 6 Elevation vs. catchment size; elevation: minimum (circles), average (squares) and maximum (triangles); marker: filled (northern), hollow (southern); trend lines: min. elevation – green line, avg. elevation – yellow line, max elevation – red line; R2 = square of the correlation coefficient between the regression line and basin data Fig. 7 Average slope vs. catchment size Fig. 8 Swath Profile of the Range; lines: blue – max elevation, red – avg. elevation, green – min elevation; black points – glacier location; [3]
  4. 4. According to Anderson (2005, 2010), during glacial buzzsaw process glaciers erode along the slope which make it more steeper. This proposition can be supported through the result of Fig. 7, stating steeper peaks in the southern zone, where majority of the glaciers exists (Fig. 8; swath 2-6). High relief has been experienced from the swath profile (Fig. 8; swath 1-5) of the range in the southern zone. This can be explained by the proposition of Molnar and England (1990), “glaciations have been assumed to increase average relief mainly by incising valley systems, leaving high elevation peaks and hill slopes almost unaffected, and producing significant isostatically driven peak uplift”. CONCLUSIONS Mountains of Nyainqentanglha range follow the glacial buzzsaw hypothesis. With some exceptions in the southern catchments termed as Teflon peak in geology. Glaciers clusters largely in the southern zone due to higher altitudes. In another sense presence of the glaciers played the crucial role for the development of this higher altitude through more erosion therefore uplift feedback from the inner tectonics. Influence of local climate is also very crucial behind the formation and development of mountains as well as glaciers, which can be said in reverse way also (Molnar and England, 1990; Anders et al., 2010; Dahlen and Suppe, 1988; Egholm et al., 2009; Molnar, 2009; Willett et al., 2003). This study echoes the complex interrelations of tectonics, erosion and climate during the development of a mountain range, which is driving researchers of modern times in the domain of geology to unravel the hidden layers of this relationship. Brocklehurst, S.H. and Whipple, K.X. (2002). Glacial Erosion and Relief Production in the Eastern Sierra Nevada, California. Geomorphology, 42(1–2), pp.1–24. Brozović, N., Burbank, D.W. and Meigs, A.J. (1997). Climatic Limits on Landscape Development in the Northwestern Himalaya. Science, 276(5312), pp.571–574. Burbank, D.W. et al. (1996). Bedrock Incision, Rock Uplift and Threshold Hillslopes in the Northwestern Himalayas. Nature, 379(6565), pp.505– 510. Dahlen, F.A. and Suppe, J. (1988). Mechanics, Growth, and Erosion of Mountain Belts. Geological Society of America Special Papers, 218, pp.161– 178. Dortch, J.M. et al. (2011). Asymmetrical Erosion and Morphological Development of the Central Ladakh Range, Northern India. Geomorphology, 135(1–2), pp.167–180. Egholm, D.L. et al. (2009). Glacial Effects Limiting Mountain Height. Nature, 460(7257), pp.884–887. Kang, S. et al. (2007). Annual Accumulation in the Mt. Nyainqentanglha Ice Core, Southern Tibetan Plateau, China: Relationships To Atmospheric Circulation over Asia. Arctic, Antarctic, and Alpine Research, 39(4), pp.663–670. REFERENCES Kühni, A. and Pfiffner, O.A. (2001). The Relief of the Swiss Alps and Adjacent Areas and Its Relation to Lithology and Structure: Topographic Analysis from a 250-m DEM. Geomorphology, 41(4), pp.285–307. Anders, A.M., Mitchell, S.G. and Tomkin, J.H. (2010). Cirques, Peaks, and Precipitation Patterns in the Swiss Alps: Connections among Climate, Glacial Erosion, and Topography. Geology, 38(3), pp.239–242. Mitchell, S.G. and Montgomery, D.R. (2006). Influence of a Glacial Buzzsaw on the Height and Morphology of the Cascade Range in Central Washington State, USA. Quaternary Research, 65(1), pp.96–107. Anderson, R.S. (2005). Teflon Peaks: The Evolution of High Local Relief in Glaciated Mountain Ranges. AGU Fall Meeting Abstracts, -1, p.04. Molnar, P. (2009). The State of Interactions Among Tectonics, Erosion, and Climate: A Polemic. GSA Today, 19(7), pp.44–45. Bolch, T. et al. (2010). A Glacier Inventory for the Western Nyainqentanglha Range and the Nam Co Basin, Tibet, and Glacier Changes 1976– 2009. The Cryosphere, 4(3), pp.419– 433. Molnar, P. and England, P. (1990). Late Cenozoic Uplift of Mountain Ranges and Global Climate Change: Chicken or Egg? Nature, 346(6279), pp.29–34. [4] Montgomery, D.R., Balco, G. and Willett, S.D. (2001). Climate, Tectonics and the Morphology of the Andes. Geology, 29(7), p.579. Pan, Y. and Kidd, W.S.F. (1992). Nyainqentanglha Shear Zone: A Late Miocene Extensional Detachment in the Southern Tibetan Plateau. Geology, 20(9), p.775. Pinter, N. and Brandon, M.T. (2005). How Erosion Builds Mountains. Scientific American, 15, pp.74–81. Quanru, G. et al. (2006). The Eastern Himalayan Syntaxis: Major Tectonic Domains, Ophiolitic Mélanges and Geologic Evolution. Journal of Asian Earth Sciences, 27(3), pp.265–285. USGS. (1999). The Himalayas: Two Continents Collide. U.S. Geological Survey. [online]. Available from: http://pubs.usgs.gov/gip/dynamic/himala ya.html [Accessed January 5, 2014]. Wang, X. et al. (2013). Glacier and Glacial Lake Changes and their Relationship in the Context of Climate Change, Central Tibetan Plateau 19722010. Global and Planetary Change, 111, pp.246–257. Willett, S. et al. (2003). Dynamic Interactions between Tectonics, Climate, and Earth Surface Processes. In D. D. Pollard, ed. New Departures in Structural Geology and Tectonics. Denver Colorado: Tectonics Program , Earth Sciences Division, and Nationa l Science Foundation (GEO/EAR), pp. 32–40. [online]. Available from: http://pangea.stanford.edu/~dpollard/NS F/new_departures.pdf [Accessed January 5, 2014]. Zeitler, P.K. et al. (2001). Erosion, Himalayan Geodynamics, and the Geomorphology of Metamorphism. GSA Today, 11(1), pp.4–9. ACKNOWLEDGEMENT Author expresses his gratitude towards Dr Jason Dortch, Lecturer in Physical Geography, SEED, University of Manchester for his continuous guidance during the course of this study and specially for the MATLAB script. Author is also thankful to Emma Shuttleworth (GTA) for her assistance during the surgery works.

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