Determination of piedmont fault in Roorkee area
The Himalaya is an excellent example of an active orogenic system. It is the result of the continent – continent collision between the Indian and Eurasian Plates. Since the collision the Himalayas have grown towards south. The age of the major structures in the Himalaya becomes younger as we move towards south indicating its southward progress of the Himalaya. One of the recent studies suggests that there exists a new deformation front called ‘piedmont fault’ to the south of the Main Frontal Thrust (MFT) that is generally considered to be the southernmost part of the Himalaya. In this study, we revisit the area investigated by the previous workers and investigate at some new morphometric parameters to determine whether or not there exists the piedmont fault. We measured river width, sinuosity, braiding index, and SL index values along the rivers originating from the Mohand range and also, of the Ganga River. The results do not support presence of any structure.
Keywords: river width analysis, braiding index, sinuosity, SL index, longitudinal profiles
The following abbreviations are used throughout the paper.
|STD||South Tibetan Detachment|
|MCT||Main Central Thrust|
|MBT||Main Boundary Thrust|
|HFT||Himalayan Frontal Thrust|
The collision between the Indian and Eurasian plates has led to the development of Himalaya - one of the world’s largest mountain chains. Since its development the Himalayas have grown towards south. The early upliftment or mountain building activity took place along the Main Central Thrust (MCT) and after 10-12 Ma of upliftment along the MCT, a new thrust developed to its south at 10 Ma which is known as the Main Boundary Thrust (MBT). The youngest in the sequence, separating the Sub-Himalaya and the Indo-Gangetic Plains, is the Main Frontal Thrust (MFT; also known as Himalayan Frontal Thrust, i.e., HFT or Himalayan Frontal Fault, i.e., HFF) developed at ~ 2 Ma (Yin, 2007). Since these thrust sheets are wedge shaped, they are well explained by the ‘Wedge Theory’ given by Davis, Suppe and Dahlen (1983), and Dahlen et al., (1984). The wedge model uses the analogy of snowplough, where the plough pushes the piles of snow forward and upward. It suggests that the moment when the pile can be pushed forward with ease depends on the shape of the wedge or taper, and a critical taper is required for the wedge to move forward. In the present case Eurasian plate acts as snowplough and the Himalaya is equivalent to the the wedge in front of the Himalaya. The forward movement is equivalent to breaking of a new thrust. According to this analogy there are chances that a new thrust will break in front of the MFT.
Yeats and Thakur (2009) suggested that a new thrust has already broke in front of the MFT and they extended this fault from the western frontal part of the Himalaya to the central frontal part of the Himalaya. However, this has not received much acceptance from the geological community, therefore, we revisited one of the study areas of Yeats and Thakur (2009) to analyse some morphometric parameter to investigate the presence of the piedmont fault.
River flow pattern, channel geometry and its shape depends on several parameters, such as, discharge, sediment flux, slope, lithology, vegetation, bank stability and grain size. A small change in any of the parameter results in the shape and size of the river and that is the reason why rivers are one of the best geomorphic markers.
It has been demonstrated through various experiments and also observed in nature that if a river encounters any obstruction along its course then the river may change is width, sinuosity, and also their nature (i.e., meandering, straight or braided). The sudden change in any of these morphometric parameter implies that there is some obstruction along the course of the river.
Statement of the Problems
The main statement of the research is to investigate whether or not the Piedmont fault is present. To address this problem, we have used methods like river width analysis, river profile, SL-Index, braiding index, and river sinuosity. The area chosen is around Roorkee as this was one of the places where Yeats and Thakur, (2009) showed the presence of fault, which is debatable.
Objectives of the Research
The main objective of this study is to identify presence of piedmont fault using morphometric parameters.
The outcome of this study can strengthen the views either in presence or absence of the piedmont fault. If the results point towards the presence of the fault then it will have major implications on the stress distribution in the Himalaya.
The Himalaya is a typical example of orogenic systems. Because of the collision of the Indian and Eurasian plates, a positive topography was generated known as “Himalaya”. Geographically, the Himalayan range lies from Namche Barwa in eastern to Nanga Parbat in western. The northern boundary of the Himalaya ranges is between the east flowing Yalu Tsangpo and west flowing Indus River. The southern boundary is the Himalayan Frontal Thrust (HFT) which is situated in the north of the Indo – Gangetic plain.
According to geographic and geologic, the classifications of the Himalayan orogeny are as follows (Gansser, 1964, Le – Fort, 1975)
- Sub – Himalaya - It is also known as Siwaliks, includes the sedimentary deposits coarsening upward.
- Lesser Himalaya – It includes the Proterozoic meta – sedimentary rock.
- Higher Himalaya – It is also known as the Greater Himalaya, includes the crystalline sequence.
- Tethyan Himalaya – It is composed of the marine and fossiliferous strata.
There are several structures are seen of the Himalaya but the main structures are South Tibetan Detachment (STD), Main Central Thrust (MCT), Main Boundary Thrust (MCT) and Himalayan Frontal Thrust (HFT). The STD is the normal fault whereas the MCT, MBT and HFT are the reverse fault.
The STD separates the Tethyan Himalaya and the Greater Himalaya, MCT separates the Greater Himalaya and the Lesser Himalaya, the MBT separates the Lesser Himalaya and the Sub – Himalaya and the Sub – Himalaya is bounded by the HFT. The HFT is considered as the present mountain front of the Himalaya.
Yeats and Thakur suggested a new evidence on the Himalaya. According to them, the Himalaya is migrating parallel to the south of the HFT towards the Indo–Gangetic Plain. They named it Piedmont Fault. The characteristics of the key concept is find out the fault if it is present or not. The only existing theory is the Active faulting south of the Himalayan Front: Establishing a new plate boundary given by Yeats and Thakur. According to them, the slope of the new fault is. But no such type of slope is seen in the field. The main problem of the research is to find out the active Piedmont Fault in the study area. To solve the problem we have to test certain geomorphic indices like river width analysis, braiding index, sinuosity, river profiles, SL Index of the rivers that are flowing through the study area. If any sudden change in geomorphic indices is seen, then there may be a fault in the study area.
In this study we have mainly investigated width of the river, SL index values, sinuosity, and braiding index of around 15 rivers. The data used to measure river width, sinuosity and braiding index is the Google Earth images. We have used to Shuttle Radar Topography Mission (SRTM) Digital Elevation Model (DEM) to prepare the river profiles and calculate the SL index values. The measurements for river width, braiding index and sinuosity are taken from the post - monsoonal images taken between 2017 – 2019.
River Width Analysis
During river width analysis, the width of the temporary bars and active channel width are taken as the actual width of the river and addition of width of the permanent bars with actual width of the river is taken as the total width of the river. (FIG – 3) (Parida et al., 2017).
Total Width = Actual Width + Bars
Braiding index is defined as the ratio between the mid – channel length of all the segments of a reach and the mid – channel length of the widest channel of the reach (Friend and Sinha, 1993)
The formula for braiding index (B) is –
B = Lctot / Lmax
where B = Braiding Index
Lctot = The total sum of the mid channel lengths of all the channels in a reach
Lmax = The mid – channel length of the widest channel through the reach
Sinuosity is defined as the ratio between the valley length and channel length of the river (Friend and Sinha, 1993).
The formula for sinuosity (P) is –
P = Lmax / Lr
where P = Sinuosity
Lmax = Valley Length of the reach
Lr = Channel length of the reach
The Stream – Length gradient index (SL Index) is obtained for all the rivers, and graphs are plotted in Sigma plot by taking SL Index, elevation and Length of all the rivers.
The formula for the SL Index is –
SL = ∆H×L/∆L
where ∆H = difference in elevation between the ends of the reach
∆L = length of the reach
L = Stream length measured from the drainage divide
River profiles are made for all the rivers in ArcGIS. SRTM 90 m DEM of the study area is downloaded from the Earth Explorer and used to identify few basic geomorphic features such as samples, profiles, basin of the rivers of the study area.
RESULTS AND DISCUSSION
Certain geomorphic indices like river width analysis, braiding index and sinuosity of the rivers, river profiles and SL Index of the rivers, are taken into consideration to identify the presence of the Piedmont Fault. The results of each to these analyses is discussed below.
River Width Analysis
The measurements of the width of all the rivers is given in Appendix- I. The results do not show any consistency in the width variation of the rivers. The Ganga river shows total maximum width of over 3 km at ~ 5 km from the mountain front whereas the actual width is ~ 2.5 km at ~ 40-42 km in the 50 km segment that has been analysed (Fig. 6). In general, a noticeable change in the width of the river is observed at ~ 6 km distance from the mountain front. A change is also observed between 8-24 km in the river 6 and between 12-32 km between river 8 (Fig. 6); however, the changes are significant only for the total width of the river and the actual width does not show much variation. Results of individual rivers is given below.
Fig. 6 - Graphs of the river width of various rivers.
From the data, the river width analysis graphs are plotted. In X – axis, the distance of the rivers is taken and in Y – axis, the width of the rivers is taken. The blue line is the total width of the river where as the orange line is the actual width of the river. As the river 4 and 5 have only 2 readings, graphs can’t be plotted.
For Ganga River, 26 readings are taken. After covering a distance of 6 km from the mountain front, the total width of Ganga River suddenly increases if compared to its actual width. It may suggest that there is some tectonic activity in that particular area. Similarly at a distance of 18 km and 30 km, there is a sudden change in the total width if compared to the actual width, which also signifies presence of some tectonic disturbances there.
For Rivers 1, 2, 3 and 7, there is no sudden change visible between the actual width and the total width which may not be related to tectonic activity.
For river 6, after covering a distance of 12 km and 18 km, there is a sudden change in the total width if compared to the actual width and at 20 km, the total width curve falls down which signifies presence of tectonic disturbances there.
For river 8, after covering a distance of 16 km, there is an immediate change in the total width and after covering a distance of 18 km, the total width of the graph suddenly falls down. It leads to the tectonic activity there. Again after covering distance of 26 km and 28 km, the total width graph rises and falls which relates tectonic activities.
In case of river 9, there are zigzag curves seen for the total width and the actual width which may not be related to any tectonic disturbances.
For river 10, after covering a distance of 10 km, there is a sudden change in the total width if compared to the actual width of the river which results any tectonic activities there.
For rivers 11, 12 and 13, there is a sudden change in the total width if compared to the actual width of the river in the upstream only. After that the total width and the actual width of the rivers remain same which may or may not show any tectonic disturbances there.
In case of river 14, after covering a distance of 12 km and 18 km, there is a sudden change in the total width of the river if compared to the actual width of the river which results the tectonic activities present there.
The braiding index values for the Ganga river is highest and varies between 1.5-5 (Fig. 7). On the other hand, braiding index of other rivers vary between 0.3-1.5. The results does not show any pattern of variation (Fig. 7). Results of individual river is discussed below.
Fig - 7 - Braiding Index of Different Rivers
For Ganga River, the braiding index varies in descending manner as the river flows from the upstream to downstream which doesn’t indicate any tectonic activities.
For Rivers 1, 2, 3, 7 and 9 the braiding index graphs show only a line which doesn’t show any tectonic disturbances in the rivers.
For River 6, after covering a distance of 6 km, there is a sudden change in the braiding index and after covering a distance of 10 km, the line simply falls down which indicates tectonic activities there.
For Rivers 8 and 11, the braiding index graphs show only a zigzag pattern which doesn’t indicate tectonic disturbances throughout the rivers.
For River 10, after covering a distance of 4 km, the graphs falls down and again rises at 6 km which signifies tectonic activities there.
For River 12, the braiding index is high at the upstream and the graph falls down as the river flows towards the downstream which doesn’t show any tectonic disturbances there.
For River 13, after covering a distance of 2 km, the graph falls down and at 4 km, the graph rises and at 8 km again falls down which show a tectonic disturbances there.
For River 14, the graph doesn’t show any evidences towards the tectonic activities.
In general, the sinuosity of some of the reaches of the Rivers 6 and 8 are high (Fig. 8). Like braiding index the sinuosity values also do not show any pattern. The values change through out the river segments (Fig. 8). Results of individual rivers is discussed below.
Fig - 8 - Sinuosity of Different Rivers
For Ganga River, there is no change in sinuosity which doesn’t show any tectonic disturbances.
For Rivers 1, 2, 3, 5, 7 and 9, there is only one reading taken and for river 14, only 2 readings are taken for sinuosity as they flow for a short distance and doesn’t show any evidences towards tectonic activities.
For Rivers 6, 10, 11, 12 and 13, the sinuosity graph possesses only a line and doesn’t show any sudden change in sinuosity which leads no tectonic disturbances are there.
For River 8, after covering a distance of 4 km, the sinuosity increases and at 6 km, the graph falls down which signify presence of some tectonic activities there.
RIVER PROFILE AND SL INDEX
River profiles and SL index analysis helps us to identify the knick points along the river. It is noticed that there occurs several knick points along most of the rivers (Fig. 9). However, river 7 showed maximum number of knick points (Fig. 9). In smaller streams the knickpoints occur within 10 km from the mountain front where as some of the relatively larger streams show knick points between 20-30 km and after 40 km (Fig. 9). The Ganga River, for which the analyses was carried out till its confluence with Yamuna at Allahabad, the major knick points occur at ~ 38 and ~ 124 kms (Fig. 9). The results of individual rivers is discussed below.
Fig - 9 - SL Index of Different Rivers
For the Ganga River, the length is 138 km in the study area. There are some Knick points are found in the graph at the elevation of 272m, 269m, 267m, 265m, 258m, 233m, 231m, 228m, 216m, 210m and after covering a distance of 6 km, 10 km, 37 km, 40 km, 50 km, 92 km, 96 km, 122 km and 125 km from the mountain front show sudden changes which indicate the presence of tectonic activities there.
For River – 1, the length is 46 km in the study area. Knick points are found at the elevation of 289m, 285m, 278m, 269m, 264m, 253m, 232m and after covering a distance of 5 km, 6 km, 11 km, 12 km, 13 km, 18 km and 43 km from the mountain front show immediate changes which may result any tectonic disturbances there.
For River – 2, the length is 12 km in the study area. Knick points are found at the elevation of 311m, 294m, 287m, 281m and 276m and after covering a distance of 0.8 km, 4 km, 6 km and 10km, there is a sudden change in the river which signify the tectonic activities there.
For River – 3, the length is 8 km in the study area. Knick points are found at the elevation of 331m, 321m, 313m, 302m, 294m, 284m and after covering a distance of 2 km, 2.78 km, 3.54 km, 4.61 km, 6.16 km and 8.17 km from the mountain front show some sudden change which indicate any tectonic disturbances there.
For River – 4, the length is 7 km in the study area. Knick points are found at the elevation of 348m, 347m, 340m, 323m, 321m, 301m, 298m, 294m and after covering a distance of 1.99 km, 2.24 km, 2.48 km, 3.18 km, 3.74 km, 5.31 km, 5.73 and 6.50 km from the mountain front show some immediate changes which indicates some tectonic activities there.
For River – 5, the length is 6.35 km from the mountain front. Knick points are found at the elevation of 356m, 353m, 347m, 339 m, 329m, 321m, 317 m, 315m and 321m and after covering a distance of 0.64 km, 0.97 km, 1.62 km, 2.52 km, 3.54 km, 4.66 km, 4.87 km, 5.05 km and 6.08km from the mountain front, it shows sudden changes which signify some tectonic disturbances there.
For River – 6, the length is 37.61 km from the mountain front. Knick points are found at the elevation of 315m, 289m, 277m, 271m, 262m and after covering a distance of 5.05 km, 10.97 km, 16.67 km, 20.57 km, 24.15 km from the mountain front, it shows immediate changes which may result any tectonic activities there.
For River – 7, the length is 13.59 km in the study area. Knick points are found at the elevation of 368m, 358m, 348m, 342m, 332m, 322m, 314m, 310m and 301m and after covering a distance of 1.28 km, 2.17 km, 3.29 km, 4.18 km, 5.30 km, 6.67 km, 7.52 km, 7.77 km, 9.13 km and 11.33 km, it shows sudden changes which may lead some tectonic disturbances there.
For River – 8, the length is 54 km in the study area. Knick points are found at the elevation of 328m, 308m, 277m and 256m and after covering a distance of 3.06 km, 6.37 km, 24.27 km and 44.04 km, it shows sudden changes which may result some tectonic activities there.
For River – 9, the length is 16.58 km in the study area. Knick points are found at the elevation of 325m, 320m, 298m and 289m and after covering a distance of 2.88 km, 3.82 km, 10.49 km and 15.42 km, it shows immediate change which may indicate some tectonic disturbances there.
For River – 10, the length is 44.79 km in the study area. Knick points are found at the elevation of 316m, 286m and 271m and after covering a distance of 8.26 km, 27.20 km, 44.10 km, it shows immediate changes which may lead some tectonic disturbances there.
For River – 11, the length is 52 km in the study area. Knick points are found at the elevation of 313m, 302m, 290m and 271m and after covering a distance of 8.52km, 15.71km, 24.57km and 42.19 km from the mountain front, it shows some sudden changes which may result some tectonic activities there.
For River – 12, the length is 36.56 km in the study area. Knick points are found at the elevation of 311m, 292m, 285m and 283m and after covering a distance of 9.39 km, 20.12 km, 25.73 km and 27.92 km from the mountain front, it shows immediate changes which may indicate some tectonic disturbances there.
For River – 13, the length is 40.65 km in the study area. Knick points are found at the elevation of 311m, 300m, 293m and 285m and after covering a distance of 8.90 km, 13.63 km, 18.42 km and 25.60 km from the mountain front, it shows sudden change which may result some tectonic activities there.
For River – 14, the length is 44 km in the study area. Knick points are found at the elevation of 322m, 307m and 285m and after covering a distance of 8.42 km, 9.24 km and 20.83 km, it shows immediate change which may lead some tectonic activities there.
Area to the south of Mohand range is investigated for the presence of piedmont fault using certain morphometric analyses. The study area is covered by the alluvium brought down by the Himalayan rivers. Lithology is one parameter that effects the river and change in lithology could force a river change is shape, size and pattern. However, in the study area there is no variability in the lithology and entire area is covered by alluvium. Apart from the lithology, there are other parameters such as slope, discharge, vegetation etc. that can influence the river flow. In the study area, we consider that most of the changes we observe is due to the change in slope as the rivers are not joined by any significant tributary to change their discharge. However, there is one parameter which needs attention - it is the grain size of the sediments. We know that in piedmont zone the area close to the mountain front have larger sediments and their size decreases as we move downstream. This variation in grain size affects the permeability of the underlying strata that in turn effects the rate of infiltration, which ultimately causes change in discharge. This process can help us to explain the variability observed close to the mountain front i.e., between 2-8 km. Moreover, as the piedmont is comprises of amalgamated fans, the slopes on the fans decreases as we move downstream; therefore, change of slope could be another parameter that must have caused change in the river parameters.
The piedmont fault south of the Mohand range has been marked ~ 25 km south of the MFT. There are few rivers that display knick points at around this range - rivers 6, 8, 10, 11, 12, 13 and 14. As first order interpretation, these can be considered as an evidence of presence of a structure (which could well be the piedmont fault). But one of the previous work by Pandey (2012) suggests that these knick points are probably caused as a result of incision by the Ganga River. It should be noted that the Ganga River has incised upto 30 m, and as it is the base level for most of the rivers originating from the Mohand range, these rivers have also incised and it is the reason given by Pandey (2012) for the presence of knick points at these locations.
One of the major knick points in the tributaries (rivers 1, 8 ,10, and 11) is also observed after 40 kms. These are again related to either the confluence with the major river or due to local factors. Amongst all the parameters investigated in this study, the SL index analysis was found to be most useful. The results of this study are inconclusive and more in-depth investigations like seismic profiling of the area is required to indicate the presence or absence of piedmont fault.
CONCLUSION AND RECOMMENDATIONS
This study was carried out to identify the presence or absence of ‘piedmont fault’ to the south of the MFT using morphometric indices. Area around Roorkee was chosen for this study. The morphometric indices include - river width, sinuosity, braiding index, river profiles and SL index. Only SL index provided some evidences of presence of some structure whereas rest of the indices failed to show any strong evidence in favour of the presence of Piedmont fault. Hence, we conclude that further investigations are required to showcase the presence of the structure in the area.
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I would like to take this opportunity to express my profound gratitude and sincere regards to my eminent guide Dr. Vimal Singh, Assistant Professor, Department Of Geology, University of Delhi, Delhi who has been a source of inspiration and guiding light for me throughout the internship. He has not only supported me academically but also morally during this period. His excellent mentorship and constructive criticism has given me complete freedom of approach in pursuing my work. I am in awe of his command on the subject and his unequivocal way of explaining, which hopefully I have inherited for lifetime.
I am highly obliged to Department Of Geology, Centre for advanced studies, University Of Delhi, Delhi for extending various departmental arrangements and facilities required to carry out this piece of work.
I can never thank Mr. Vicky Shankar enough who is also doing his research work in the lab on the Piedmont Fault for sharing his knowledge, experience and his helpful discussions and suggestions that always ensured a more thoughtful approach towards problem solving thus making this learning process more enjoyable.
I express my sincere thanks to all my lab seniors Mr.Parv Kasana, Mrs.Naazima Bashir, Mr.Sukumar Parida, Dr.Ananya Divyadarshini, Mr.Debojyoti Basuroy, Mr.Arkoprabha Sarkar.
Special thanks to Ms.Amreena Zaidi, who is also a summer research fellow with me in the lab.
I would also like to thank and deeply gratitude to Indian Academy of Sciences (IASc), Bengaluru, Indian National Science Academy (INSA), New Delhi and The National Academy Sciences of India (NASI), Prayagraj for supporting financially to complete the research. It is a great initiative by IASc – INSA – NASI for improve knowledge of the students as well as to focus towards the research work.
Appendix - I
Table showing measurements of the river width analysis.
|River Name||DISTANCE(KM)||TOTAL WIDTH(METRE)||ACTUAL WIDTH(METRE)|
Appendix - II
Table showing measurements of the braiding index of the rivers analysed.
|River Name||Serial No||Distance(KM)||Braiding Index|
Appendix - III
Table showing measurements of the sinuosity of the rivers analysed.