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
[email protected]
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
F ig u re
2 . G eo lo g ic a l m a p o f th e a re a (g eo lo g y a d o p te
d fr o m
G S I 1 9 9 2 ).
F o ld ed
p a rt
o f P a lg h a t– C a u v er y sh ea r zo n e.
F o li a ti o n s fr o m
th e p re se n t st u d y a re
a ls o m a rk ed
; d o tt ed
ts id en
ti fi ed
in th e p re se n t st u d y.
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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/NLD (Gebruik deze instellingen om Adobe PDF-documenten te maken
voor kwaliteitsafdrukken op desktopprinters en proofers. De
gemaakte PDF-documenten kunnen worden geopend met Acrobat en Adobe
Reader 5.0 en hoger.) /NOR
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>> /Namespace [ (Adobe) (Common) (1.0) ] /OtherNamespaces [
<< /AsReaderSpreads false /CropImagesToFrames true