esj60695.dviGeomorphic observations from southwestern terminus of Palghat Gap, south India and their
tectonic implications
1,2,∗, G P Ganapathy 1, Abhilash George
3, S Harisanth
4
1VIT University, Vellore, Tamil Nadu 632 014, India. 2National Institute of Rock Mechanics, KGF, Karnataka 563 117, India.
3Cochin University of Science and Technology, Kochi, Kerala 682 016, India. 4National Centre for Earth Science Studies, Thiruvananthapuram, Kerala 695 031, India.
∗Corresponding author. e-mail: b johnp@yahoo.co.in
The region around Wadakkancheri, Trichur District, Kerala is known for microseismic activity, since 1989. Studies, subsequent to 2nd December 1994 (M=4.3) earthquake, identified a south dipping active fault (Desamangalam Fault) that may have influenced the course of Bharathapuzha River. The ongoing seismicity is concentrated on southeast of Wadakkancheri and the present study concentrated further south of Desamangalam Fault. The present study identifies the northwestern continuity of NW– SE trending Periyar lineament, which appears to have been segmented in the area. To identify the subtle landform modifications induced by ongoing tectonic adjustments, we focused on morphometric analysis. The NW–SE trending lineaments appear to be controlling the sinuosity of smaller rivers in the area, and most of the elongated drainage basins follow the same trend. The anomalies shown in conventional morphometric parameters, used for defining basins, are also closely associated with the NW–SE trending Periyar lineament/s. A number of brittle faults that appear to have been moved are consistent with the present stress regime and these are identified along the NW–SE trending lineaments. The current seismic activities also coincide with the zone of these lineaments as well as at the southeastern end of Periyar lineament. These observations suggest that the NW–SE trending Periyar lineaments/faults may be responding to the present N–S trending compressional stress regime and reflected as the subtle readjustments of the drainage configuration in the area.
1. Introduction
Active fault identification in low-relief, densely populated areas holds special importance as very little work has been done in such areas. However, detection of potential seismic sources in those areas is not an easy task because very few applicable methods exist, especially if the area is character- ized by low seismicity. In such areas, morphomet- ric analysis of drainage network is found to be an
appropriate tool (Cox 1994; John and Rajendran 2008). Several studies have documented the influ- ence of vertical crustal movements on the channel pattern, especially in strike-slip tectonic environ- ments (e.g., Ouchi 1985; Jorgensen 1990; Holbrook and Schumm 1999). Following the global trends, there were also some attempts to delineate the sig- natures of active faulting form drainage anomalies in peninsular India (e.g., Subrahmanya 1996; John and Rajendran 2008; Ramaswamy et al. 2011). The
Keywords. Morphometry; river sinuosity; anomalies; brittle faults; earthquakes.
J. Earth Syst. Sci., DOI 10.1007/s12040-016-0695-9, 125, No. 4, June 2016, pp. 821–839 c© Indian Academy of Sciences 821
822 Yogendra Singh et al.
present study is an additional effort to identify sig- natures of active tectonic elements from an area, which is experiencing mild seismic activity. The drainage networks in Kerala are consid-
ered to be tectonically controlled (Sinha-Roy and Mathai 1979) and majority of the microtremor locations in Kerala show rough parallelism with the drainage courses of some of the major rivers (Rajendran and Rajendran 1996). Our studies are focused in an area where there are reports of microseismic activity since 1989. The studies sub- sequent to 1994 Wadakkancheri earthquake, iden- tified Desamangalam Fault, which moved in the ongoing compressional stress regime and changed the course of E–W flowing Bharathapuzha River (John and Rajendran 2008). The present study carried out investigations fur-
ther south of Desamangalam Fault owing to the oc- currence of infrequent seismic activity. It identified the continuity of NW–SE trending Periyar linea- ment, one of the major faults in Kerala. This lin- eament appears as subparallel northwest trending segments and has influenced the drainage system.
Southeastern end of this lineament is associated with seismicity (figure 1). This paper presents the observations from geomorphic studies carried out in the northwestern end of the lineament and their relation to brittle faults and sporadic seismicity.
2. Tectonic framework of the area
The study area lies between Bharathapuzha and Chalakkudi rivers and northern half of it is a part of the Proterozoic Palghat–Cauvery Shear Zone (PCSZ) (Bhaskar Rao et al. 1996) and is also termed as Palghat Gap, a conspicuous geomorphic break. The west-flowing Bharathapuzha River, tra- verses through the central part of the 30 km wide gap area. The E–W trending structural elements in the gap region (PCSZ) may have been associ- ated with Proterozoic tectonic events defined by a large E–W dextral oblique-slip component (D’Cruz et al. 2000) and is mainly confined to the north- ern side of Bharathapuzha River (figure 2). How- ever, the area south of the main trunk of the
Figure 1. Seismotectonic map of south India adopted from SEISAT (GSI 2000). The study area is shown in rectangle.
Geomorphic observations from SW terminus of Palghat Gap, south India 823
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824 Yogendra Singh et al.
Bharathapuzha River has less influence on these E–W trending structural elements and is domi- nated by NW–SE and NE–SW trending structural elements. Central part of the study area shows NW–SE trending dolerite dykes (GSI 1992). It is identified that NW–SE and NNW–SSE lineaments are zones of emplacement of basic dykes related to Deccan volcanism (Krishnaswami 1981; Nair 1990). The local geology, dominated by a metamor-
phic suite of charnockite and khondalite group of rocks is shown in figure 2 (GSI 1992). The charnoc- kitic suite of rocks (consisting of granular quartz, feldspar and hypersthene) forms the basement in the area, and is characterized by well-developed foliation, at many places, striking NW–SE with a southward dip of 30–50. Quartzo-feldspathic gneiss, pink granite and hornblende biotite gneiss are the other major crystalline rocks in the region (GSI 1992). As per the geological map published by GSI (1992), Warkallai Formation is exposed on the coastal zone between the rivers, Wadakkancheri and Bharathapuzha (figure 2). The higher levels of the crystalline basement are occasionally cov- ered by lateritic regolith and the terraces adjoining the southern bank of the Bharathapuzha River are made up of older alluvium (John 2003). The NW–SE trending Idamalayar lineament
located in the eastern part of the study area is another major structure, which influence the Bharatha- puzha River course at Ottapalam (figure 2). Basic dykes are observed along this lineament, south- east of Wadakkancheri (figure 2). Dykes are also observed north of the Bharathapuzha River along the continuity of Idamalayar lineament extending into Palghat–Cauvery shear zone. The Periyar river lineament, which controls the course of the Periyar River for a longer distance (Rajendran et al. 2009), also enters through the southern part of the study area. Ghosh et al. (2004) considered this as a Precambrian structure and named it as Karur– Kambam–Painavu–Trichur shear zone. This linea- ment is also associated with basic dykes. Near Muppilayam, a swam of dykes are observed and most of them are in NW–SE direction (figure 2).
3. Methodology
The study area is drained by the rivers, Bharatha- puzha, Vadakkancheripuzha, Karuvannur, and Cha- lakkudipuzha and their tributaries. In order to investigate the tectonic influence on landforms of the study area, initially we delineate lineaments from multispectral Landsat images. Drainage net- work is extracted from topographic maps for morphometric analysis, and it is cross-checked with satellite images. Identification of linea- ments, demarcation of drainage system and their
anomalous pattern are carried out through image enhancement and visual interpretation of Landsat image as well as using toposheets. The special associations of anomalous patterns
of drainages with the lineaments are demarcated to understand the influence of lineaments in the drainage system. River sinuosity is calculated for different stretches along the rivers in the area (figure 3), to constrain any change in slope in the river path. To identify anomalies in landscape and drainage system of the area, we have employed morphometric parameters like bifurcation ratio, drainage density, ruggedness number, elongation ratio and asymmetry factor. The Horton’s classifi- cation of stream order (Horton 1945) is used for the morphometric analysis. The study area is divided into 135 sub-basins having 3rd or 4th order streams and the above-mentioned morphometric parame- ters are calculated for individual basins (figure 4). The area is further divided into six zones to delin- eate relative tectonic signatures of the area, if any. The morphometric data are treated from a statis- tical point of view to determine an average state of the drainage systems to highlight the relative influence of lineaments in each zone. Further, we have conducted ground-based checks
to identify the manifestations of brittle faulting along the lineaments. The structural and lithologic nature of faults is studied to understand its rela- tion to present tectonic regime. Slickensides are plotted as per the procedure suggested by Marshak and Mitra (1988) to identify the movement plane. The earthquake data is further integrated into the study to see their spatial relation with geomorphic anomalies and brittle faults.
4. Drainage network and lineaments set-up of the area
Different stages of tectonic evolutions in Indian craton were documented by Drury and Holt (1980) through analysis of lineament pattern with the help of Landsat imageries. Grady (1971) identi- fied the lineaments trending NNE–SSW to NE– SW as faults that had disturbed the dolerite dykes in south India, which are located outside the study area in the eastern side. Nair (1990) identified five sets of lineaments in the Kerala region, viz., WNW–ESE, NW–SE, NNW–SSE, NNE–SSW, and ENE–WSW. The Bharathapuzha, Vadakkancheripuzha, Ka-
ruvannur and Chalakudipuzha are the major rivers in the area (figure 3). While the Bharathapuzha River is flowing in the northern end of the study area, the Vadakancheripuzha, Karuvannur and Chalakudi rivers are located to the south. The Karuvannur River has two major tributaries, Manali
Geomorphic observations from SW terminus of Palghat Gap, south India 825
Figure 3. Map showing major rivers of the area and lineaments. Note the change in river direction at the locations where lineament c, c1 and d cross the rivers. Stretches measured for sinuosity are numbered and values are given in table 3.
and Karumalipuzha. Unlike the Bharathapuzha and Chalakudipuzha rivers that are debouching into sea, the Vadakkancheripuzha and Karuvannur rivers reach the coast as parallel water bodies. Our study also delineates lineament such as the
NW–SE Idamalayar lineament ‘a’ (in figure 1), which is prominent in the eastern side of the area and it has disturbed the Bharathapuzha course at Ottappalam, and it follows the river Gayatripuzha (figure 3). This river, however, shows more mean- dering flow pattern than the main trunk of the Bharathapuzha in the downstream. Continuation of Idamalayar lineament in the gap overprints on the original E–W trend and appears to have formed later (John 2003). Presence of dolerite dykes along
this lineament, which is dated as 76 Ma (Subrah- maniam 1976), suggest that this lineament may be associated with the initial stages of Deccan volcanism. A second set of NW–SE trending lineaments (‘b’
in figure 3) is subparallel to Idamalayar lineament, which follows Desamangalam Fault, identified in the previous studies. In the southern side of the area, two sets of NW–SE trending lineaments (‘c’ and ‘d’ in figure 3), appear to be extending from Periyar Fault (in figure 1) and are passing across the rivers Chalakkudi, Kurumalipuzha, Manali and Vadakancheripuzha (figure 3). A prominent NE– SW trending lineament ‘e’ is identified passing through Pattikadu and Edakunni (figure 3). The
826 Yogendra Singh et al.
Figure 4. Drainage basins demarcated for morphometric analysis. The six zones, based on the lineaments and topography are also shown. The results are shown in figures 5 and 6.
southwest continuity of this lineament is marked by two subparallel lineaments. However, this linea- ment appears to be abutting against the lineament ‘b’ at northeast.
5. Geomorphic analysis
The area is mostly low relief and all rivers flow with a gentle gradient in most of its length. Though the rivers are flowing through vide valleys, they exhibit sharp turn and meandering. Some of the earlier studies proved that even the smallest changes in the topography affect the sinuosity of low gradi- ent rivers (Holbrook and Schumm 1999; Schumm 2000). The composition of the stream system of a
drainage basin is expressed quantitatively through stream order, drainage density, bifurcation ratio and stream length ratio (Horton 1945). It incor- porates quantitative study of the various compo- nents such as stream segments, basin length, basin parameters, basin area, altitude, volume and slopes of the land. The widely accepted objective of mor- phometric analysis is to find out the drainage char- acteristic to explain the overall evaluation of the basin. Quantitative measurements of drainage net- work are also used as a reconnaissance tool to make inferences about relative tectonic activity (e.g., Cox 1994; Keller and Pinter 1996). The geomorphic indicators, viz., drainage basin
asymmetry and sinuosity of river are generally
Geomorphic observations from SW terminus of Palghat Gap, south India 827
used to detect active tectonics (Keller and Pinter 1996; Ramasamy et al. 2011). Apart from these, the study calculated some conventional geomor- phic parameters that are used in hydrologic analy- sis. The definitions of parameters calculated in the study are given in table 1 and the outcome is shown in figures 5 and 6. The following parameters appear to be significant for understanding the analytical methods, followed by us (table 2).
5.1 River deflections and sinuosity
Under a given set of stable tectonic conditions, alluvial rivers tend to evolve as single meandering channels (Zamolyi et al. 2010). If this behaviour is influenced by tectonic movements, it is expected to be reflected in river channel parameters. Within a given range of channel gradients, the meander- ing pattern changes as vertical tectonic movements influence the valley slope. This process is largely independent of river size, once the fluvial system enters the meandering stage (Zamolyi et al. 2010). River sinuosity is relatively lesser in areas where
the valley is narrow and straight. In the study area,
rivers are flowing in NW–SE, N–S and NE–SW directions. Previous studies identified the influence of NW–SE trending lineaments in the E–W course of Bharathapuzha (John and Rajendran 2008). In the present study, 22 stretches on the rivers, namely, Gayathripuzha, Mangalampuzha, Vadak- kancheripuzha, Manali, Kurumalipuzha, Karuvan- nur and Chalakudi, are analysed for sinuosity (figure 3). The river sinuosity values are ranging from 1.6 to 3.38 (table 3). The rivers show changes in directions of flow and sinuosity values on either side of the NW–SE trending lineaments (figure 3). In the southern side of the study area, the Kuru- malipuzha River shows compressed meandering between the lineaments ‘c’ and ‘d’. The direc- tion of the Manali River course changes to SW when it crosses the lineament ‘c1’ southwest of Pattikadu. The Vadakkancheripuzha River also shows the same deflection when it crosses near Erumapetti (figure 3). The lineament ‘c’ appears to have induced a sharp turn towards southeast in the Kurumalipuzha River near Muppiliyam. The same lineament appears to have induced sharp deflec- tions towards SE in the river courses of Manali and
Table 1. Morphometric parameters used in the study.
Sl. no. Morphometric parameter Formula used Reference
1 Bifurcation ratio (Rb) Rb = Nu/Nu + 1; where Nu = total Schumm (1956)
number of stream segments of order
‘U’, Nu + 1 = number of stream
segments of next higher order.
2 Form factor (Rf) Rf = A/Lb2; where A = area of basin, Horton (1945)
Lb = length of basin.
3 Elongation ratio (Re) Re = (2/Lb)× (A/π)0.5 Schumm (1956)
4 Circularity ratio (Rc) Rc = 4πA/P2; where A = area of the Miller (1953)
basin, P = perimeter of the basin.
5 Stream frequency (Fs) Fs = Nu/A; where Nu = total number of Horton (1945)
streams, A = area of basin.
6 Drainage density (DD) DD = L/A; where L = total length of Horton (1945)
streams of all orders, A = area of basin.
7 Constant of channel C = 1/(DD); where, DD = drainage Schumm (1956)
maintenance (C) density.
8 Basin relief (H) H = Hh− Hl; where Hh = highest Schumm (1956)
elevation in a basin, Hl = lowest
elevation in that basin.
9 Ruggedness number(Rn) Rn = H × DD; where H = basin relief, Schumm (1956)
DD = drainage density.
10 Relief ratio (Rh) H/Lb; where H = basin relief, Schumm (1954)
Lb = basin length.
11 Asymmetry factor (AF) AF = 100 × (Ar/At); where Ar = right Keller and Pinter (1996)
half of area of basin while facing
downstream, At = total area of the
basin.
12 Sinuosity of river (S) S = LC/LV where LC = channel length, Schumm (2000)
LV = valley length.
828 Yogendra Singh et al.
Figure 5. Map showing spatial distribution of elongation ratio as well as anomalies in bifurcation ratio and asymmetry factor in different basins. Horizontal dashed lines represent basins with bifurcation ratio values higher than 3; vertical solid lines represent anomalously high values of basin asymmetry.
Vadakkancheripuzha rivers, southeast of Edakkuni and Kunnamkulam, respectively (figure 3). Those drainage segments showing change in
directions and drastic change in sinuosity val- ues compared to subsequent segments represent anomalies. As mentioned earlier, features, such as change in slope, are generally considered as struc- turally controlled and is a strong evidence for the presence of active faults/neotectonic lineaments. Comparison of the sinuosity distribution along the river courses with geological and geomorphological features that changes in sinuosity values reflect- ing neotectonic features (Zamolyi et al. 2010). In
peninsular India too similar features are attributed to active faults (Ramasamy et al. 2011).
5.2 Bifurcation ratio (Rb)
Horton (1945) considered bifurcation ratio as an index of relief and dissections. Strahler (1951) found that bifurcation ratio shows only slight vari- ation in different zones except where the powerful geological controls dominate. The Rb characteris- tically ranges between 3.0 and 5.0 for the basins in which the geologic structure does not affect the drainage pattern (Strahler 1964). In the present
Geomorphic observations from SW terminus of Palghat Gap, south India 829
Figure 6. Sample range diagram representing the morphometric parameters DD, Rb, Rn, Re and relief ratio for different zones. The bar represents the data range within standard deviation; the horizontal lines at the centre of the bars represent the average values; vertical lines are connecting maximum and minimum.
study, Rb for 135 sub-basins has been calculated based on the formula shown in table 1. The mean value of the Rb between first and second order streams for 135 sub-basins is found to be 3.68 (table 2). The high values of Rb are an indication of terrain rejuvenation due to young tectonic move- ments (Zuchiewicz 1989; Panek 2004). The present study shows that higher Rb is mostly confined to basins located in high relief (figures 5 and 6). How- ever, there are basins in low relief area that show high values of Rb and most of them are located bet- ween the Bharathapuzha and Vadakkancheripuzha rivers, where the lineaments are more segmented (figure 5). This could be due to the recent activity along these NW–SE trending lineaments.
5.3 Drainage density (DD)
The DD expresses the closeness of spacing of chan- nels and it depends on the characteristics such as soil type, rock type, vegetation strength, climate and infiltration strength (Strahler 1957). A low DD is more likely to occur in regions of highly resistant or highly permeable subsoil material under dense vegetative cover and where relief is low. High DD is the resultant of weak or impermeable subsurface material, sparse vegetation and mountainous relief (Strahler 1964). In the present study, the DD is cal- culated based on the formula shown in table 1, and it varies between 0.38 and 4.16 (table 2). The mean value of the DD for the area is 2.34. It is found that higher drainage densities are associated with high relief of the basins and are mainly confined to b and c1 lineaments (figure 6a).
5.4 Ruggedness number (Rn)
This parameter combines the slope steepness of the basin with its length (Strahler 1964; table 1). Higher the basin relief, higher will be the values of Rn. This condition is expected in tropical climate areas with high rainfall (Schumm 1956). The value of Rn within the study area varies from 0.01 to 2.98 with the mean value of 0.80 (table 2). If we con- sider that 0.5 to 1.5 is the normal range of values of Rn within the study area, 52 basins are falling in this range. However, only 20 basins are falling above this range of values. Like DD, higher values of Rn are associated with high relief of the basins and cannot be considered as anomalous (figure 6).
5.5 Elongation ratio (Re)
The range of Re is, in general, between 0 and 1, and it can be classified as into elongated, less elongated, oval and circular (Schumm 1956). It is observed that values close to 1 are in regions of very low relief and values between 0.6 and 0.8 are associated with regions of high relief and steep ground slope (Strahler 1964). There are also attempts to classify tectonic activity based on the values on Re (Zuchiewicz 2001). In the study area, the Re of the basins ranges between 0.48 and 0.90 (table 2). The study shows that only 29 basins are falling in the values less than 0.5 and for 103 basins the value ranges between 0.5 and 0.7, which indicate the basins are relatively elongated (figure 5). Higher elongation ratio is also observed in areas of low relief (figure 6). The most anomalous
830 Yogendra Singh et al.
Table 2. Drainage basin parameters derived for the study area.
Basin Bifurcation Form Elongation Circularity Stream Drainage Basin Ruggedness Relief Asymmetry
no. ratio f/s factor ratio ratio frequency density Ccm relief number ratio factor
1 6.50 0.39 0.70 0.61 3.95 2.44 0.41 0.302 0.74 0.0935 41.81
2 4.75 0.26 0.58 0.51 4.42 2.72 0.37 0.303 0.83 0.0664 29.13
3 4.50 0.41 0.72 0.72 2.24 1.58 0.63 0.282 0.45 0.0779 57.41
4 2.50 0.56 0.85 0.79 0.60 0.69 1.45 0.32 0.22 0.0657 44.84
5 4.33 0.37 0.69 0.54 3.38 2.48 0.40 0.462 1.15 0.0630 59.86
6 5.00 0.32 0.64 0.54 1.29 1.54 0.65 0.162 0.25 0.0289 59.61
7 2.50 0.44 0.75 0.63 2.24 1.67 0.60 0.118 0.20 0.0413 65.53
8 3.50 0.25 0.57 0.48 0.74 1.07 0.93 0.182 0.19 0.0248 40.96
9 3.50 0.41 0.72 0.67 1.89 1.41 0.71 0.169 0.24 0.0471 35.56
10 3.00 0.63 0.90 0.69 1.26 1.67 0.60 0.175 0.29 0.0433 41.89
11 2.50 0.29 0.61 0.54 0.47 0.69 1.45 0.167 0.11 0.0220 45.30
12 2.00 0.36 0.68 0.54 0.42 0.64 1.56 0.102 0.07 0.0151 44.96
13 3.00 0.37 0.69 0.56 0.48 0.93 1.08 0.175 0.16 0.0247 51.84
14 2.50 0.23 0.54 0.49 1.08 1.00 1.00 0.12 0.12 0.0212 43.15
15 3.50 0.40 0.71 0.63 0.84 1.04 0.96 0.462 0.48 0.0844 59.00
16 4.00 0.41 0.72 0.74 2.19 1.90 0.53 0.28 0.53 0.0797 62.59
17 2.50 0.46 0.77 0.67 1.27 1.02 0.98 0.38 0.39 0.1030 19.96
18 3.00 0.34 0.66 0.52 1.73 1.76 0.57 0.36 0.63 0.0916 59.26
19 2.75 0.54 0.83 0.66 1.06 1.06 0.95 0.105 0.11 0.0198 31.75
20 3.33 0.25 0.57 0.46 0.90 1.13 0.89 0.162 0.18 0.0207 41.67
21 3.00 0.45 0.76 0.64 6.01 2.58 0.39 0.12 0.31 0.0659 39.13
22 2.67 0.60 0.87 0.74 3.68 2.59 0.39 0.1 0.26 0.0427 27.90
23 3.00 0.38 0.70 0.59 5.45 3.26 0.31 0.15 0.49 0.0720 50.36
24 2.00 0.38 0.70 0.61 2.00 1.43 0.70 0.101 0.14 0.0333 42.80
25 2.50 0.43 0.74 0.57 1.30 1.36 0.74 0.101 0.14 0.0268 42.16
26 4.50 0.53 0.82 0.58 0.91 1.24 0.81 0.121 0.15 0.0175 67.79
27 5.33 0.31 0.63 0.46 0.41 0.65 1.53 0.08 0.05 0.0064 53.10
28 3.50 0.53 0.82 0.67 0.47 0.90 1.11 0.068 0.06 0.0107 29.82
29 5.50 0.44 0.75 0.70 1.55 1.40 0.71 0.1 0.14 0.0219 24.45
30 3.00 0.34 0.66 0.58 1.34 1.51 0.66 0.14 0.21 0.0263 58.63
31 5.80 0.45 0.75 0.59 3.74 2.50 0.40 0.18 0.45 0.0392 81.71
32 6.00 0.31 0.63 0.57 3.74 2.78 0.36 0.16 0.44 0.0445 52.47
33 5.50 0.31 0.62 0.60 1.31 1.28 0.78 0.16 0.20 0.0271 61.09
34 4.00 0.27 0.58 0.54 0.56 1.04 0.96 0.14 0.15 0.0163 55.86
35 3.50 0.32 0.64 0.57 4.35 2.48 0.40 0.18 0.45 0.0490 55.66
36 3.50 0.45 0.76 0.69 3.95 2.36 0.42 0.14 0.33 0.0593 42.47
37 4.50 0.59 0.87 0.70 3.51 2.70 0.37 0.12 0.32 0.0498 46.73
38 3.00 0.36 0.67 0.61 5.16 2.89 0.35 0.122 0.35 0.0458 38.46
39 6.00 0.52 0.82 0.68 4.10 2.72 0.37 0.17 0.46 0.0531 84.74
40 2.75 0.41 0.73 0.70 5.17 2.87 0.35 0.28 0.80 0.1024 16.83
41 5.00 0.48 0.78 0.81 4.58 2.92 0.34 0.36 1.05 0.1220 62.77
42 2.25 0.33 0.65 0.65 5.02 2.76 0.36 0.36 1.00 0.1243 55.07
43 4.60 0.44 0.75 0.55 5.85 2.93 0.34 0.36 1.05 0.1037 48.36
44 3.00 0.36 0.68 0.53 7.16 3.05 0.33 0.3 0.91 0.1011 27.85
45 2.00 0.28 0.60 0.57 8.36 2.88 0.35 0.28 0.81 0.1630 29.87
46 4.00 0.39 0.71 0.52 6.57 3.49 0.29 0.385 1.34 0.1550 23.00
47 3.67 0.40 0.72 0.74 3.02 2.48 0.40 0.43 1.07 0.1225 58.53
48 5.00 0.29 0.60 0.59 4.81 2.74 0.37 0.36 0.98 0.1172 43.68
49 3.33 0.39 0.70 0.60 4.80 3.09 0.32 0.422 1.31 0.1533 59.94
50 2.50 0.27 0.58 0.46 7.74 3.03 0.33 0.34 1.03 0.1733 75.00
51 3.00 0.44 0.75 0.79 5.78 3.17 0.32 0.362 1.15 0.1931 66.65
52 3.50 0.26 0.58 0.53 4.47 2.73 0.37 0.38 1.04 0.1304 53.68
53 4.17 0.55 0.84 0.67 3.63 2.60 0.38 0.38 0.99 0.0954 65.19
Geomorphic observations from SW terminus of Palghat Gap, south India 831
Table 2. (Continued.)
Basin Bifurcation Form Elongation Circularity Stream Drainage Basin Ruggedness Relief Asymmetry
no. ratio f/s factor ratio ratio frequency density Ccm relief number ratio factor
54 4.00 0.37 0.68 0.66 3.83 2.97 0.34 0.475 1.41 0.1227 38.95
55 4.00 0.40 0.72 0.70 4.90 3.07 0.33 0.475 1.46 0.2016 79.67
56 4.00 0.30 0.62 0.45 6.66 3.55 0.28 0.46 1.63 0.1638 28.50
57 4.00 0.30 0.62 0.51 5.70 3.53 0.28 0.47 1.66 0.1539 46.79
58 4.80 0.39 0.71 0.57 5.83 3.09 0.32 0.4 1.24 0.1068 44.01
59 3.25 0.28 0.60 0.50 3.08 2.50 0.40 0.46 1.15 0.0728 60.56
60 4.29 0.25 0.56 0.45 2.71 1.87 0.53 0.38 0.71 0.0503 21.44
61 3.50 0.40 0.71 0.60 3.59 1.62 0.62 0.15 0.24 0.0410 57.44
62 3.20 0.35 0.67 0.64 6.51 3.62 0.28 0.12 0.43 0.0387 69.86
63 3.71 0.19 0.50 0.34 2.55 2.00 0.50 0.152 0.30 0.0175 47.07
64 2.50 0.32 0.64 0.56 0.23 0.62 1.61 0.058 0.04 0.0056 38.59
65 3.50 0.47 0.77 0.55 0.52 0.88 1.14 0.12 0.11 0.0129 51.85
66 4.00 0.35 0.66 0.59 4.55 2.78 0.36 0.13 0.36 0.0356 32.47
67 3.00 0.45 0.76 0.76 2.89 2.34 0.43 0.14 0.33 0.0534 36.51
68 3.00 0.26 0.58 0.43 1.92 1.72 0.58 0.14 0.24 0.0173 65.62
69 4.20 0.23 0.54 0.47 1.96 1.63 0.61 0.29 0.47 0.0376 53.68
70 3.75 0.35 0.67 0.60 3.73 2.69 0.37 0.34 0.91 0.0872 28.70
71 4.50 0.38 0.69 0.57 1.33 1.15 0.87 0.149 0.17 0.0305 12.87
72 4.00 0.44 0.75 0.60 4.09 2.54 0.39 0.184 0.47 0.0745 78.32
73 5.20 0.19 0.49 0.39 2.68 2.22 0.45 0.42 0.93 0.0531 32.09
74 4.50 0.53 0.82 0.64 1.53 1.56 0.64 0.16 0.25 0.0416 73.81
75 2.83 0.32 0.64 0.59 2.96 2.33 0.43 0.433 1.01 0.0864 75.65
76 3.50 0.40 0.71 0.69 3.57 2.72 0.37 0.848 2.31 0.3202 65.33
77 3.33 0.38 0.69 0.65 5.32 3.51 0.28 0.848 2.98 0.3203 31.79
78 4.18 0.27 0.59 0.48 3.91 3.11 0.32 0.82 2.55 0.1089 64.79
79 3.00 0.39 0.71 0.58 5.68 3.38 0.30 0.52 1.76 0.2596 29.04
80 2.00 0.44 0.75 0.73 7.65 3.60 0.28 0.6 2.16 0.3487 64.72
81 3.89 0.47 0.78 0.69 4.02 2.94 0.34 0.78 2.30 0.1570 55.03
82 3.00 0.46 0.76 0.67 4.13 2.77 0.36 0.355 0.98 0.1355 73.99
83 5.50 0.36 0.68 0.74 6.45 3.61 0.28 0.26 0.94 0.1057 46.55
84 5.20 0.31 0.63 0.47 4.18 3.17 0.32 0.815 2.58 0.1636 51.61
85 4.00 0.24 0.55 0.52 5.81 3.19 0.31 0.34 1.09 0.1201 64.81
86 3.67 0.27 0.59 0.49 3.65 2.88 0.35 0.74 2.13 0.1911 53.99
87 3.50 0.59 0.87 0.84 4.92 2.90 0.35 0.614 1.78 0.3308 34.94
88 5.67 0.39 0.71 0.68 5.38 3.40 0.29 0.76 2.59 0.2406 36.01
89 3.00 0.40 0.71 0.59 5.14 3.01 0.33 0.614 1.85 0.2446 71.28
90 6.29 0.29 0.61 0.57 4.50 3.09 0.32 0.74 2.28 0.1160 31.70
91 3.00 0.45 0.76 0.79 6.19 3.13 0.32 0.315 0.98 0.1755 52.80
92 3.71 0.40 0.71 0.48 3.60 2.39 0.42 0.45 1.07 0.0644 40.11
93 4.20 0.37 0.69 0.72 2.67 2.37 0.42 0.637 1.51 0.1217 32.80
94 2.50 0.32 0.64 0.53 4.06 2.26 0.44 0.22 0.50 0.0882 72.65
95 3.50 0.48 0.78 0.68 1.51 1.21 0.83 0.157 0.19 0.0422 72.38
96 3.83 0.37 0.69 0.59 5.85 3.07 0.33 0.157 0.48 0.0409 35.84
97 2.60 0.27 0.59 0.32 1.22 1.29 0.78 0.21 0.27 0.0278 61.70
98 2.50 0.31 0.62 0.54 3.66 2.18 0.46 0.21 0.46 0.0786 60.07
99 3.67 0.36 0.68 0.67 2.66 2.01 0.50 0.2 0.40 0.0508 38.18
100 3.00 0.21 0.52 0.39 2.30 2.09 0.48 0.19 0.40 0.0368 27.67
101 5.33 0.27 0.58 0.59 5.75 3.56 0.28 0.365 1.30 0.0722 45.40
102 3.00 0.53 0.82 0.86 3.54 2.82 0.35 0.321 0.90 0.1464 66.79
103 4.00 0.52 0.82 0.69 4.57 3.09 0.32 0.353 1.09 0.1648 45.63
104 2.50 0.50 0.80 0.77 3.63 3.04 0.33 0.679 2.07 0.3223 80.35
105 2.50 0.41 0.72 0.72 4.89 3.07 0.33 0.243 0.75 0.1210 30.02
106 5.50 0.44 0.75 0.74 3.75 2.69 0.37 0.355 0.96 0.1214 60.96
107 3.50 0.47 0.78 0.77 6.01 3.13 0.32 0.261 0.82 0.1390 19.61
832 Yogendra Singh et al.
Table 2. (Continued.)
Basin Bifurcation Form Elongation Circularity Stream Drainage Basin Ruggedness Relief Asymmetry
no. ratio f/s factor ratio ratio frequency density Ccm relief number ratio factor
108 3.50 0.37 0.68 0.72 5.04 3.19 0.31 0.536 1.71 0.1669 61.20
109 5.00 0.47 0.77 0.77 4.02 2.79 0.36 0.556 1.55 0.1527 60.66
110 5.00 0.53 0.82 0.64 5.32 2.98 0.34 0.34 1.01 0.1588 38.24
111 2.00 0.41 0.73 0.75 8.41 4.00 0.25 0.4 1.60 0.2820 77.08
112 3.00 0.42 0.73 0.73 8.18 4.16 0.24 0.4 1.66 0.2060 16.97
113 4.00 0.44 0.75 0.62 3.95 2.68 0.37 0.42 1.13 0.1390 64.84
114 2.50 0.36 0.68 0.60 2.39 1.84 0.54 0.08 0.15 0.0262 83.15
115 2.50 0.21 0.51 0.47 0.77 1.08 0.93 0.1 0.11 0.0141 58.46
116 3.00 0.34 0.66 0.61 5.55 2.54 0.39 0.06 0.15 0.0201 43.37
117 3.33 0.27 0.59 0.54 6.70 3.25 0.31 0.08 0.26 0.0289 49.53
118 3.00 0.41 0.72 0.75 5.15 3.04 0.33 0.28 0.85 0.1355 53.05
119 5.00 0.44 0.75 0.66 5.59 3.11 0.32 0.288 0.90 0.1253 36.53
120 3.50 0.42 0.73 0.58 8.98 3.79 0.26 0.362 1.37 0.2217 83.16
121 3.00 0.46 0.77 0.70 6.31 3.87 0.26 0.342 1.32 0.1624 25.90
122 3.50 0.30 0.62 0.64 5.54 3.66 0.27 0.268 0.98 0.1090 57.21
123 3.00 0.44 0.75 0.71 2.12 2.04 0.49 0.3 0.61 0.0964 67.10
124 3.33 0.35 0.67 0.60 1.54 1.56 0.64 0.395 0.62 0.0558 61.91
125 2.00 0.32 0.64 0.54 6.90 3.37 0.30 0.3 1.01 0.1683 32.53
126 3.33 0.24 0.56 0.50 3.34 2.44 0.41 0.36 0.88 0.0866 76.15
127 4.67 0.51 0.80 0.69 3.88 2.64 0.38 0.197 0.52 0.0650 36.30
128 4.75 0.40 0.72 0.69 6.24 3.47 0.29 0.326 1.13 0.1013 35.21
129 5.33 0.41 0.72 0.58 2.24 1.86 0.54 0.119 0.22 0.0255 60.14
130 4.50 0.29 0.61 0.43 5.03 2.65 0.38 0.101 0.27 0.0352 55.80
131 3.00 0.51 0.80 0.72 4.76 2.24 0.45 0.1 0.22 0.0518 58.58
132 4.00 0.34 0.66 0.60 2.45 1.82 0.55 0.14 0.25 0.0386 46.37
133 3.50 0.18 0.48 0.36 0.56 0.99 1.01 0.368 0.37 0.0374 56.25
134 2.00 0.42 0.73 0.68 0.29 0.65 1.54 0.08 0.05 0.0106 65.15
135 4.00 0.51 0.81 0.59 0.28 0.38 2.60 0.02 0.01 0.0023 53.53
values for basin elongation of the area (<0.5) is observed along basins oriented in NW–SE direc- tion (figure 5). This further confirms the influence of NW–SE lineaments in the drainage system.
5.6 Asymmetry factor (AF)
For a stream network flowing in a stable setting and uniform lithology, the AF (table 1) should be equal to 50. In all other cases, there will be a change in value deviating on either side of 50 (Keller and Pinter 1996). This factor is used to detect the tectonic tilt of the basin areas (Hare and Gardner 1985). The value for AF is calculated for sub-basins, and the value ranges from 12.87 to 84.74 and the mean is 50.26 (table 2). If we con- sider that 40–60 is the normal range of values of AF for the study area, only 54 basins are falling in the normal range and 81 basins are showing higher AF (either <40 or >60). The higher and lower val- ues are considered as anomalies and may suggest geological or structural influence. The basins show- ing higher anomalies in symmetry in the present study area are mostly located between lineaments
b and c1, as well as between c and d (figure 5) and may be indicating tectonic adjustment along these lineaments.
6. Field observations on faults
Initially brittle faults were identified along Desamangalam Fault in which multiple deforma- tions were identified (John and Rajendran 2005). In the present study, lineaments are mostly checked wherever fresh surface of charnockite bed rock is exposed through quarrying. In the study area, the charnockite rocks exhibit occasional NW–SE trending foliations. Thin covers of lateritic soil are present at many places in these exposures. Signa- ture of the NW–SE trending lineament is traced as faults at three locations, viz., Erumapetti, Tayyur and Mannuthi (figure 8). The brittle deformation exposures are associated with damage zone, gouge formation and slickensides. Three types of litholo- gies can also be distinguished in each of these fault exposures namely: (1) protolith or host rock, (2) damage zone, and (3) a fault core (e.g., Caine
Geomorphic observations from SW terminus of Palghat Gap, south India 833
Table 3. Sinuosity index measured along different stretches of rivers in the study area.
Straight Curved Sinuosity
1 Vadakkancheripuzha 6.8 17 2.5
2 Vadakkancheripuzha 4.4 11 2.5
3 Vadakkancheripuzha 3.3 7 2.121
4 Vadakkancheripuzha 1.05 2.99 2.85
5 Vadakkancheripuzha 1.05 1.90 1.81
6 Vadakkancheripuzha 3.5 10 2.857
7 Manalipuzha 5.9 14 2.372
8 Manalipuzha 3.6 8 2.222
9 Manalipuzha 3.9 10 2.564
10 Karumalipuzha 3.7 11 2.972
11 Karumalipuzha 3.25 11 3.384
12 Karumalipuzha 3.96 12 3.030
13 Karumalipuzha 2.41 8 3.319
14 Karuvannurpuzha 2.47 7 2.834
15 Karuvannurpuzha 3.21 8 2.492
16 Karuvannurpuzha 3.13 6 1.916
17 Chalakkudi river 7.2 15 2.083
18 Chalakkudi river 6.4 13 2.031
19 Chalakkudi river 5.53 18 3.254
20 Chalakkudi river 4.73 11 2.325
21 Gayathripuzha 10.6 25 2.358
22 Mangalampuzha 7.45 20 2.6845
et al. 1996; John and Rajendran 2009). Protolith is the undeformed host rock (charnockite) surround- ing the fault rock and the damaged zone. The damage zone consists of a network of fault-related subsidiary structures that bound the fault core. The fault related subsidiary structures in the dam- aged zone include small offsets, veins, fractures and cleavages (Bruhn et al. 1994). Fault core is the por- tion of a fault zone where much of the displacement is accommodated (Caine et al. 1996). In the follow- ing section, we will present the dominant features at each of the sites. Erumapetti: The NW–SE trending south dip- ping fault is traced in a 30 m vertical section (figure 7a) and about 100 m along strike direction in the adjacent quarry. The fault shows varying thickness of damage zone in the vertical section. The thickness of the damage zone is maximum at the bottom (deeper level) and is associated with green-coloured gouge. A number of joints are observed perpendicular to the fault mainly at the top portion. No major subparallel fractures are observed away from the damage zone. Coulomb shears, indicative of slip movement is also observed in the damage zone (figure 7b). Variation in thick- ness of damage zone is also observed along strike length at the top (figure 7c). Thickness of the fault core also increases towards upper levels.
Tayyur: The lineament ‘c1’ is traced as a brittle fault in another 25 m high quarry face near Tayyur. Here the brittle faulting is marked by closely spaced fractures and gouge (figure 7d). Width of the damage zone is very high compared to that of Erumapetti but the thickness of fault core is less. The damage zone is dominated by fault parallel fractures (figure 7d). Green coloured gouge along with well rounded fine-grained quartz are found along the slip plane (figure 7e). Though water is percolating along the fault, no leaching is observed along any of the fractures within the fault zone. Mannuti: Another prominent NW–SE trending southwest dipping fault is observed in a charnockite quarry near Mannuti (figure 7f). This fault is lying within the zone of ‘c1’. The width of the damage zone is ∼1.5 m. Water seepage is observed along the fault zone and vegetation has grown along it (figure 7g). Rock fragments are also observed within the fault zone, while at the lower portion closely spaced joints are observed parallel to the fault plane within the damage zone. About 300 m fur- ther northwest, this fault is again traced in the road cutting along the strike direction 200 m southeast. At the top end of the fault, hanging wall side is more intact than footwall side (figure 7h). The brittle deformations identified also show
development of slickensides. The fault movements
834 Yogendra Singh et al.
Figure 7. (a) The NW–SE trending brittle fault identified near Erumapetti. The quarry face is about 30 m high; the fault zone showing varying thickness of damage zone from top to bottom. Joints and fractures perpendicular to the fault dominate the upper portion of footwall block; in the hanging wall block such joints appear to be turning towards the fault direction near the slip plane. The area marked in rectangle is shown in (b). (b) The style of damage observed in the bottom portion of the fault mapped, gouge observed on either side of the damaged mass; closely spaced fractures are coulomb joints developed due to the movement. (c) Continuation of the fault in (a) about 100 m northwest of the previous location; note the thick gouge zone near the surface facilitate the growth of vegetation. (d) Brittle faulting observed in a charnockite quarry near Tayyoor; note the closely spaced joints parallel observed in the fault zone; the tallest person in the photo is 170 cm high. (e) View of the slip plane in the fault zone shown in (d); two types of gouge observed, shown in green and white. Close look at the gouge of the area marked in rectangle is shown in (f). (f) Close view of white coloured gouge shown in (e), indicating rounded fine-grained quartz. One rupee coin is kept for scale. (g) View of the NW–SE trending south dipping fault observed near Mannuthi; the central part of the quarry advanced towards NW direction. Close-up view of the fault marked in rectangle is shown in (h). (h) Close view of fault core and damage zone marked in (g); note the vegetation grown along the fault core. (i) Tip of the fault zone near Mannuthi; note the relatively intact hanging wall block siting over a highly crushed and altered footwall.
Geomorphic observations from SW terminus of Palghat Gap, south India 835
Figure 7. (Continued.)
are dominated with reverse and strike slip move- ment. However, the amount of slip could not be measured at these places due to the lack of marker beds across these faults. The movement directions across the faults are evaluated from slickensides by using the technique suggested by Marshak and Mitra (1988), in which movement planes are cal- culated by plotting fault planes and slickensides. The results show that the movement plane con- structed on slip surfaces indicate oblique movement (figure 8). In all cases, the southwestern blocks are moving towards north or northeast (figure 8).
7. Seismicity of the area
Among the historic and recent earthquakes of the region, 1900 Coimbatore earthquake is signifi- cant in south India (Basu 1964). A large number of tremors have been reported in the vicinity of Palghat Gap in the recent past (Rajendran and Rajendran 1996). The biggest of them (M 4.3) is located at 10.75N and 76.25E (Rajendran and Rajendran 1995), and its isoseismal elongation is spatially coincide with the Desamangalam Fault
(John 2003). This earthquake made widespread minor damages to houses around the epicentral area (intensity V), mostly confined to south of Bharathapuzha. Most of the aftershocks detected by IMD network (IMD 1995) are also located close to the above said structure (John 2003). Subse- quent mild activity was reported further south of the 1994 event (Rajendran et al. 2009) along the branches of Periyar lineament (figure 8). On the southeastern terminus of Periyar linea-
ment, an earthquake of M 4.5 was reported on 09.06.1988 (figure 1), which was followed by several aftershocks (Singh et al. 1989). This earthquake was felt in about 50 km radius, and caused damage to buildings in the epicentral area and was assigned a maximum intensity VI. Mishra et al. (1989) based on gravity studies, identified a shallow structure corresponding to Periyar Fault in that area. The composite fault plane solution suggests its asso- ciation with Periyar Fault (Rastogi et al. 1995).
During April–July 2012, another swarm of earthquake events occurred southeast of Wadak- kancheri. The biggest event M=3.8 occurred on 19.07.2012 near Tannikulam, as recorded in Peechi Observatory. Many such local events are plotted
836 Yogendra Singh et al.
Figure 8. Location of the identified brittle faults and earthquakes within the study area. Movement planes of three faults are plotted to show fault plane and movement plane using the procedure suggested by Marshak and Mitra (1988). Note the spatial association of brittle faults and earthquakes with lineaments.
with the help of single seismological observatory located at Peechi. Though the location accuracy may not be sufficient to confidentially associate them to any particular lineament or to assign a trend, the ongoing seismicity may be the result of the tectonic adjustment along NW–SE structures, which are associated with geomorphic anomalies and faulting (figure 8).
8. Discussion
For better understanding of tectonic processes operating in the area, the study area has been
divided into six zones based on topography and lin- eament pattern (figure 5). Lineament C1 separates zone I from zone II, and zone IV from zone V. Lin- eament ‘b’ separates zone III from zone II and IV. The NE–SW trending Pattikadu lineament (e), running across the study area, separates zones I and II from zones IV and V. The zone VI is demar- cated by ‘d’ lineament separating it from zone V. Each morphometric parameter is statistically analyzed for different zones (figure 6). The zones I, III and VI lie in low relief area,
whereas zones II, IV and V lie in high relief area (figure 6). The study indicates that drainage density (DD) and ruggedness number (Rn) are consistent
Geomorphic observations from SW terminus of Palghat Gap, south India 837
with the basin relief (figures 5 and 6) whereas elon- gation ratio (Re) do not have much influence on relief. Higher values of bifurcation ratio (Rb) are observed in the NE part of zone I in the vicinity of NW–SE trending lineament ‘c1’, in NE and SE parts of zone II and in the vicinity of NW–SE trending lineament ‘b’ (southeastern continuity of Desamangalam Fault). Higher values of ‘Rb’ are also observed at SW part of zone III in the vicinity of NW–SE trending lineament b and in the cen- tral part of zone IV. The Rb values are relatively high even though DD is low in zone I (figure 6), which may be an indication of uplift of the block (zone I). The basins in the area are mostly elongated
in shape; however, less elongated basins are also observed in this area randomly. The lineament ‘c1’ across which the Vadakkancheripuzha River changes its course shows anomalously high elon- gated basin in the southwestern end of the area (figure 5). Similarly, the lineament ‘d’ which may be the reason for compressed meandering of the Kurumalipuzha River is also associated with a very elongated basin (figure 5). The courses of rivers in the area are also con-
trolled by the NW–SE trending lineaments. The lineament ‘c’ induces a sharp turn towards SE that affected all the rivers (Kurumalipuzha, Man- ali and Vadakkancheripuzha). The lineament ‘c1’ also influenced the course of the Manali and Vadakkancheripuzha rivers and both of them took SW direction once they cross the lineament. It appears that the meander patterns of smaller rivers are also influenced by the NW–SE trending linea- ments and they induced compressed meandering in the upstream, may be due to the change in slope induced by the lineament. Earlier studies identified that compressed meandering is one of the impor- tant anomalies that can be attributed to active tectonics (Ramasamy et al. 2011). The present observations suggest that the NW–SE trending structural set-up in the area is influencing the drainage network of the area. The NW–SE trending lineaments that associated
with brittle faulting are identified as the continuity of Periyar lineament. At three locations these three faults are developed into visible zones of faulting and damage zones that are dipping SW. Slicken- sides from these fault zones indicate a consistent north-directed movement of respective hanging wall blocks (figure 8). The sporadic seismicity in the region is also concentrating in this area, which may be an indication of tectonic disturbance. In the southeastern end of Periyar Fault too there were incidences of earthquakes (Rajendran et al. 2009).
Peninsular India is suggested to be under com- pressional stress regime due to the continued northward movement of Indian landmass and the
resistive forces from Himalayan collision zone. In this present scenario, strike slip and reverse fault- ing are the dominating mechanisms in peninsular India. Studies by Gowd et al. (1992) identified that NW–SE trending structural weaknesses are one of the favourable directions that can facilitate move- ment in the present tectonic regime. The NW– SE trending lineaments and the north directed movements observed in the area may be indicat- ing that these have resulted from the neotectonic movements.
9. Conclusions
The present study delineates three sets of linea- ments, viz., NW–SE, NNE–SSW and NE–SW lin- eaments in the area. The Periyar lineament enter- ing from south into this area appears branched out into three en-echelon segments. The study indi- cates that the anomalies in morphometric parame- ters like drainage density (DD), ruggedness number (Rn), bifurcation ratio (Rb) and stream frequency are observed in basins lying around segments of Periyar lineament. Like the Desamangalam Fault (which is identified as active fault), Periyar linea- ments seem to exert control on the present drainage system configuration of the area. The drainage deflections and associated change
in meandering pattern are the significant observa- tions for Periyar lineaments. The anomalies in the river sinuosity are mostly confined to these linea- ments. A number of brittle faults that appear to be formed in the present tectonic regime are identified to align or parallel the NW–SE trending linea- ments, defined by the segments of Periyar lineament. This area is also experiencing frequent seismic activity. The present study suggests that the NW–SE trending Periyar lineaments/faults may be respond- ing to the present regional stress regime and are reflected on subtle adjustment of drainage system.
Acknowledgements
We express our sincere gratitude to the SERB divi- sion of the Department of Science and Technology for funding (SR/S4/ES-434/2009) this study. The investigators thank Dr V Venkateswarlu, Direc- tor, NIRM for his encouragement, support and facilities. YS, BJ, GPG thank VIT University for permission to publish this work. Director NCESS is thanked for providing earthquake data of the region. We also thank the reviewers for their crit- ical evaluation and helpful suggestions to improve the manuscript.
838 Yogendra Singh et al.
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MS received 15 September 2015; revised 11 January 2016; accepted 24 January 2016
Corresponding editor: N V Chalapathi Rao
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