active tectonics south india

Review Article

Remote sensing and active tectonics of South India

S. M. RAMASAMY*

Centre for Remote Sensing, School of Geosciences, Bharathidasan University,

Tiruchirappalli – 620023, Tamil Nadu, India

(Received 30 June 2004; in final form 29 November 2005 )

The Indian Peninsula in general and its southern part in particular has been

thought to be a stable shield area and hence inert to younger earth movements and

seismicities. However, in addition to fast relapsing seismicities, the studies carried

out by earlier workers during the past three decades indicate possible pulsatory

tectonism, at least since the Jurassics. The present study is a newer attempt to

identify, analyse, and spatially amalgamate a large number of anomalies visibly

displayed by the tectonic, fluvial, coastal, and hydrological systems in remote

sensing and ground based datasets/observations, and to finally paint a fair picture

on the active tectonic scenario of South India. The study reveals that the

phenomena, viz. extensive soil erosion, reservoir siltation, sediment dump into the

ocean, preferential migration of rivers, restricted marine regression, shrinkage of

back waters, withdrawal of creeks, fall of groundwater table, etc., indicate two E–

W trending ongoing tectonic (Cymatogenic) archings along Mangalore–Chennai

in the north and Cochin–Ramanathapuram in the south. Intervening these two

arches, a cymatogenic deep along Ponnani–Palghat–Manamelkudi exhibiting

phenomena opposite to the above is observed. In addition, the characteristic

tectonic, geomorphic, and hydrological anomalies observed in 1B satellite FCC

data, as well as in the field, indicate N–S trending extensional, NE–SW sinistral,

and NW–SE dextral strike slip faults. These anomalies and the tectonic features

deduced thereupon, indicate that the southern part of the Indian Peninsula is

tectonically active due to the northerly to north–northeasterly directed compres-

sive force related to post collision tectonics. This active tectonic model visualized

for South India gives a further clue that the whole Indian plate is whirling like a

worm with alternate E–W arching and deepening, along with block and transform

faulting from Cape Comorin in the south to the Himalayas in the north.

1. Introduction

The Indian Peninsular Shield in general and its southern part in particular has always

been thought of as being inert to younger earth movements and related seismicities/

earthquakes. For this reason, geoscientists have not shown much interest in studying

the Neo-active-seismotectonics of the southern part of the Indian Peninsula, mostly

restricting themselves to the western (Kutch) and central (Son-Narmada) parts of

India (Auden 1949, West 1962, Choubey 1970, Biswas and Deshpande 1973,

Kailasam 1975, Ghosh 1976, Pal and Bhimashankaran 1976, Crawford 1978, Dessai

and Peshwa 1978, Sharma 1978, Guha and Padale 1981, Kaila et al. 1981, 1985, Murty

and Mishra 1981, Powar 1981, 1993, Bhagwandas and Patel 1984, Bakliwal and

*Email: [email protected], [email protected]

International Journal of Remote Sensing

Vol. 27, No. 20, 20 October 2006, 4397–4431

International Journal of Remote SensingISSN 0143-1161 print/ISSN 1366-5901 online # 2006 Taylor & Francis

http://www.tandf.co.uk/journalsDOI: 10.1080/01431160500502603

Ramasamy 1987, Merh 1987, Ravishankar 1987, Amalkar 1988, Ramasamy et al.

1991, Gupta 1992, Sareen et al. 1993, Ramasamy 1995a, 1998, and many others).

Though the Southern Indian Peninsular Shield has not been studied in great detail

with regards to faults, especially concerning their tectonic alertness, since 1960, a

number of workers have observed in various parts possible repetitive tectonism since

the Jurassics. Some significant observations are: possible Post-Jurassic tectonic

movements along the Palghat graben (Arogyasamy 1963); varying signatures of

Neotectonism of the Mysore plateau (Radhakrishna 1966); possible repetitive Post-

Jurassic tectonic movements in South India (Vaidyanadhan 1967); a positive

relation between Neotectonism and petroleum occurrences in South India (Ermenko

1968); active tectonic graben along the Salem–Attur valley (Srinivasan 1974); a

striking coincidence of historical seismicity data with NE–SW and ENE–WSW

lineaments/faults/lithological boundaries of South India (Vemban et al. 1977);

tectonic wedging and related drainage reversals in the Dharmapuri region

(Suryanarayana and Prabhakar Rao 1981); possible Neotectonism and the related

clockwise rotational migration of Palar in the Chennai region (Rao 1989); Holocene

transform faults of ENE–WSW orientation along the Kerala coast (Nair and

Subramainan 1989); N–S trending cymatogenic arching and related rejuvenation of

the Cauvery river (Radhakrishna 1992); signatures favouring intra plate deformation

in South India (Subrahmanya 1996); dynamic mobile belts in South India (Chetty

1996); multi various evidences favouring Late Quaternary/Holocene earth movements

in South India (Valdiya 1997, 1998, 2001, Valdiya et al. 2000); and signatures on active

tectonic movements in parts of the Western Ghats (Gunnell and Fleitout 2000), etc.

In recent years, the author of this paper and his co-workers (Ramasamy et al. 1987,

Ramasamy 1991, Ramasamy and Balaji 1993) have carried out interpretation of

satellite images and recorded evidence of possible Neo-active tectonics in parts of

South India, with possible land arching in the Chennai and Ramanathapuram areas.

Subsequently, Subrahmanya (1994) and Ramasamy and Balaji (1995) also observed

evidence of possible regional cymatogenic arching along the Mangalore–Chennai

region. Stimulated by the above preliminary observations, the author has taken up

detailed studies to identify and interpret various tectonic, riverine, and coastal

geomorphic anomalies from satellite based remote sensing data and hydrological

anomalies from field based datasets and, further, to spatially integrate this

information to build up a comprehensive picture of Neo-active tectonics for South

India. This would provide vital baseline data in the context of the fast relapsing

seismicities in the region (figure 1). These various anomalies are conspicuous in density

sliced (in which different spectral ranges were assigned different colours individually in

all four bands) and False Colour Composite outputs (in which Band 2 with 0.52–

0.60mm, Band 3 with 0.63–0.69mm, and Band 4 with 0.79–0.90mm were respectively

exposed under blue, green, and red filters and a combined single image was generated)

of IRS 1B data. This paper presents observations on the various anomalies above and

the resultant model visualized on the active tectonics of South India.

2. Remote sensing and field signatures of topographic highs/lows

2.1 Northern and southern sectors

2.1.1 Topographic profile (figure 1). A N–S trending topographic profile (A–A1)

was drawn between the west of Chennai in the north and Ramanathapuram in the

4398 S. M. Ramasamy

south. The said profile indicates a larger amplitude topographic high (topo-high)

along Mangalore–Chennai in the north (1, figure 1), a topographic low (topo-low)

along the Palghat Gap (Ponnani–Palghat–Manamelkudi) in the central south (3,

figure 1), and a low amplitude topographic high along Cochin–Ramanathapuram

(2, figure 1) in the south. But the topographic profiles drawn in an E–W direction

along Mangalore–Chennai (B–B1) and Ponnani–Palghat–Manamelkudi (C–C1)

show a smooth flat top with steep to moderate slopes at both coastal ends.

2.1.2 Fracture swarms (figure 2). The regional interpretation was carried out to

map the lineaments of the study area using 1:1 million, as well as enlarged formats of

IRS 1B satellite FCC images. The same indicates polymodally oriented lineament

systems (figure 2(a)) in general, but with conspicuous fracture swarms in particular

in an ENE–WSW direction along the Mangalore–Chennai topo-high (3, figure 2(b)),

between Bangalore and Chennai, to a breadth of nearly 60–80 km. It can be seen

that these fractures are intruded by swarms of dolerite dykes.

Figure 1. Topographic profile.

Remote sensing and active tectonics of South India 4399

Similarly, along the southern Cochin–Ramanathapuram topo-high, E–W

trending fracture swarms are interpreted in the Varushanad hill ranges of the

Western Ghats to a breadth of 30–40 km (4, figure 2(c)).

2.1.3 River rejuvenation – soil erosion – reservoir siltation (figure 3). The state of

Tamil Nadu has a wide, low, easterly sloping plain, whereas the slope is steep in the

(a)

(b) (c)

Figure 2. Lineaments and fracture swarms of topographic highs. Key Map showing theMangalore–Chennai topographic high (1), the Cochin–Ramanathapuram topographic high(2) and E–W fracture swarms of South India (3 and 4). (a) Lineament map of South Indiashowing polymodally oriented lineaments. (b) IRS 1B image showing ENE–WSW fractureswarms (3) in between the Bangalore and Chennai region along the northern topographichigh (1). (c) Sketch showing E–W fracture swarms (4) of Varushanad region along thesouthern topographic high (2).

4400 S. M. Ramasamy

area west of the Western Ghats in the states of Kerala and Karnataka. Obviously,

the east flowing rivers have longer and well-developed fluvial histories when

compared to the west flowing rivers, but the overall drainage architecture shows

conspicuous water divides along these two Mangalore–Chennai and Cochin–Ramanathapuram topo-highs with drainages of the northern and southern slopes

respectively flowing northerly and southerly (figure 3(a)). In addition, along these

(a)

(c)

(d )

(e)( f )

(b)

Figure 3. River rejuvenation – soil erosion – reservoir siltation. Key Map showing theMangalore–Chennai topographic high (1), the Cochin–Ramanathapuram topographic high(2) and zones of vertical cutting and sheet erosion by rivers (3 – green dots along the axes ofthe topographic highs). (a) Topographic highs as water divides. (b) IRS 1B density slicedimage showing soil erosion (4, red colour) in the Chittur–Tiruttani region (in betweenBangalore and Chennai) of the northern topographic high (1). (c) IRS 1B image showingsilted water bodies (5) in the Chennai region at the eastern end of the northern topographichigh (1). (d) IRS 1B density sliced image showing soil erosion (6, red colour) in the Vaipparregion of the southern topographic high (2). (e) IRS 1B image showing silted water bodies (7)in the Tiruppuvanam region of the southern topographic high (2). (f) Sketch showing thedistribution of silted water bodies (8) in Tamil Nadu along the eastern ends of the northern (1)and southern (2) topographic highs.


topo-highs, the drainages show appreciable gullying, headward, and also sheet

erosions (3, Key Map, figure 3).

Further, the digitally processed, density sliced IRS band 2 (0.52–0.59 mm) datasets

indicate extensive gully and sheet erosions between Bangalore and Chennai along

the Mangalore–Chennai topo-high (4, figure 3(b)) and, in contrast, the chains of

water bodies found at the eastern end of the topo-high in the Chennai area are

heavily silted. Such silted water bodies could be precisely detected and mapped in

IRS FCC data from the deep red colour of the luxuriant vegetation growth,

which shows higher reflectance in the IR band due to its chlorophyll content

(5, figure 3(c)). Such phenomenon of intensive erosion in the topo-high and the

siltation in the downward water bodies shows that the soil so removed from the

topo-high is dumped in the water bodies.

Again, the similar phenomenon of heavy soil erosion in the Vaigai–Vaippar

system (6, figure 3(d)) of the southern Cochin–Ramanathapuram topo-high and the

extensive siltation in the thousands of water bodies, visibly seen again in red in IRS

FCC (7, figure 3(e)) at the eastern end of the topo-high, indicates that the soil so

removed from the topo-high is deposited in the water bodies located at its eastern

end. In fact, in the state of Tamil Nadu, there are over 34,000 water bodies and

reservoirs, of which more than 10,000 water bodies are located in the coastal

segments of these two topo-highs (8, figure 3( f )). While only these water bodies of

the topo-high region are heavily silted (8, figure 3( f )), the remaining water bodies

spread over other parts of Tamil Nadu are comparatively less silted or not at all

silted.

2.1.4 Sediment dumping into the ocean (figure 4). The blue and green bands of the

electro-magnetic spectrum have the credibility to display the concentration of

suspended sediments in water (Lillesand 1989, Gupta 1991). Taking this as a clue,

the entire coastal zone from Chennai to Ramanathapuram was analysed using

density sliced outputs of such blue–green bands (Bands 1 and 2) of IRS 1B data

(0.45–0.52 and 0.52–0.59 mm). The same indicates the heavy concentration and

dispersion of suspended sediments in and off the river mouths in the sea around the

Chennai region (3, figure 4(a)) and the Ramanathapuram region (4, figure 4(b)),

irrespective of seasons of the satellite data.

In fact, rivers such as the Araniyar, Adyar, and Cooum, which drain the

Mangalore–Chennai topo-high and meet the sea on the Chennai coast are

ephemeral, and the Vaigai and Vaippar rivers, which drain the southern Cochin–

Ramanathapuram topo-high and confluence the sea in the Ramanathapuram

region, are also temporary rivers. But the concentration of suspended sediments at

the mouth of these rivers/streams indicates that these heavily dump the sediments

into the sea, when compared to the other major easterly flowing rivers like the

Ponnaiyar and Cauvery of Tamil Nadu. This shows that the soil, which is

aggressively eroded from these two topo-highs, is being deposited in the thousands

of water bodies in the coastal region and the remaining soil is being dumped into the

sea.

2.1.5 Preferential river migration (figure 5). The IRS 1B FCC images display well-

developed old drainage courses/palaeochannels in the Palar river, which currently

flows easterly and meets the sea in the area south of the northern Mangalore–

Chennai topo-high. These bundles of palaeochannels are seen as linear, curvilinear,

contorted, ribbon-like, and loop-like vegetation bands with a dark grey tone in

4402 S. M. Ramasamy

black-and-white images and a deep red colour in FCC images, again due to the

chlorophyll content of the vegetation growing along these palaeochannels (3,

figure 5(a)). This major palaeochannel system branches off from the Palar river at

Walajapet and ends up as palaeo deltas in the north and south of Chennai. The

occurrence of palaeochannels only to the north of the present course of the Palar

river indicates that river has preferentially migrated towards the south.

Vaidyanadhan (1971) has observed that the palaeochannels found in the

Walajapet–Chennai tract are the remains of the mighty river Cauvery, which once

flowed along Hogenekkal–Chennai, and hence refers to it as the ‘Proto Cauvery’.

Ramasamy et al. (1992) also observed that the Cauvery river has flowed in the

Hogenekkal–Chennai tract from 500,000 years to 3000 years BP (Before Present).

But Narasimhan (1990) has recorded it as the old course of the Palar river and called

it the ‘Proto Palar’. However, the said old course is referred to as Proto Palar in this

present discussion, as the same is visibly branching off from the Palar river, and

moreover, its southerly migration is more significant in the context of the present

study, whether it is the Proto Cauvery or the Proto Palar. The Pennar river, found a

little north of the Mangalore–Chennai topo-high, exhibits a wide floodplain to its

south, suggesting its tendency of northerly migration.

(b) (a)

Figure 4. Sediment dumping into the ocean. Key Map showing the Mangalore–Chennaitopographic high (1), the Cochin–Ramanathapuram topographic high (2) and zones ofsediment dumping (3, 4) into the ocean along these highs. (a) IRS 1B density sliced imageshowing sediment dump and dispersal (3) off the Chennai coast. (b) IRS 1B density slicedimage showing sediment dump and dispersal (4) off the Ramanathapuram coast.


Again, the Vaigai river, flowing east southeasterly in the area north of the

southern topo-high, has its old courses (5, figure 5(b)) only to the south of its present

course (6, figure 5(b)). This again indicates that the Vaigai has preferentiallymigrated towards the north. These observations suggest that in the northern

Mangalore–Chennai topo-high, while the Pennar river shows northerly migration,

the Palar river indicates southerly migration. Similarly, this is also the case with the

Vagai river. That is, these rivers show preferential migration away from the axes of

both topo-highs.

2.1.6 Fluvio–marine interface zone anomalies (figure 6). As stated earlier, because

of the low easterly slope of the study area, the rivers have laid well-developed deltas

all along the east coast of Tamil Nadu in between Chennai in the north and

Ramanathapuram in the south (Key Map, figure 6). Ramasamy (1991) has classified

these deltas into lobate, arcuate, cuspate, digitate, and estuarine deltas on the basis

of detailed interpretation of satellite images.

Amongst these multivariate deltas, the Proto Palar delta in the Chennai region(3, figure 6(a)) and the Vaigai delta in the Ramanathapuram region (6, figure 6(b)),

found respectively at the eastern ends of the Mangalore–Chennai and the

(b) (a)

Figure 5. Preferential river migration. Key Map showing the Mangalore–Chennai topo-graphic high (1), the Cochin–Ramanathapuram topographic high (2) and arrows indicatingthe direction of river migration. (a) IRS 1B FCC image showing the old course (3) and thepresent course (4) of the Palar river. (b) IRS 1B FCC image showing the old courses (5) andthe present course (6) of the Vaigai river.

4404 S. M. Ramasamy

Cochin–Ramanathapuram topo-highs, are conspicuous lobate deltas with thou-

sands of crescent-shaped, concentrically-arranged lobes and interlobal depressions.

These depressions have only become surface water bodies later on.

In fact, the Vaigai river shows lobate deltas in three stages, with stage I near

Madurai, stage II near Tiruppuvanam, and stage III from Paramakkudi onwards (4,

5, 6, figure 6(b)), which are located respectively 150, 100, and 50 km west of the

present day Ramanathapuram shoreline. These continental deltas occurring in

different stages coincide with well-defined magnetic lows, indicating that these

depressions acted as basins for sediment accumulation (Ramasamy 1991), whereas

the deltas formed by the other rivers, such as the Palar, Ponnaiyar, Cauvery, and

Tambraparani, are arcuate, cuspate, digitate, and estuarine in their morphologies.

2.1.7 Coastal zone anomalies.

A. Shapes of shorelines and beach ridges (figure 7). The shape of the shorelines in

the Southern part of the Indian Peninsula under discussion are very unique,

with conspicuous convexities in Mangalore and Cochin on the west coast and

Chennai and Ramanathapuram on the east coast. These convexities coincide

(b) (a)

Figure 6. Fluvio marine interface zone anomalies. Key Map showing the Mangalore–Chennai topographic high (1), the Cochin–Ramanathapuram topographic high (2) and deltasalong the topographic highs. (a) IRS 1B FCC image showing the Proto Palar delta (3). (b) IRS1B FCC image showing multi stage deltas in the Vaigai river – stage I at Madurai (4), stage IIat Tiruppuvanam (5), and stage III from Paramakkudi onwards (6).


with either end of the Mangalore–Chennai and Cochin–Ramanathapuram

topo-highs (Key Map, figure7).

A. In addition, the beach ridges, comprising oxidized old beach sands, are

developed to a greater breadth only on these convex coasts. That is, these

beach ridges are found up to 3–4 km west of the present day shoreline in the

Chennai region (3, figure 7(a)), 25–30 km west of the present shoreline in the

Ramanathapuram region (4, figure 7(b)), up to 25–30 km east of the present

coast in the Cochin area (6, figure 7(c)), and 3–4 km east of the present coast

in the Mangalore region.

(a)

(c)

(b)

Figure 7. Shapes of the coast and pattern of marine regression. Key Map showing theMangalore–Chennai topographic high (1) and the Cochin–Ramanathapuram topographichigh (2), with conspicuous convexities along these coasts. (a) IRS 1B FCC image showingbeach ridges (3) at the eastern end of the northern topographic high in the Chennai area.(b) IRS 1B FCC image showing beach ridges (4) at the eastern end of the southerntopographic high in Ramanathapuram (5) and arrows indicating the littoral currents. (c) IRS1B FCC image showing beach ridges (6) at the western end of the southern topographic highin the Cochin area.

4406 S. M. Ramasamy

B. Shrinkage and defunct backwaters and estuaries (figure 8). In the east coast of

Tamil Nadu, a number of backwaters and estuaries are found. The analysis of

the topographic sheet of 1915 AD and the IRS satellite data of 1991 AD

(figure 8(a)) shows that the Pulicat lake located to the north of Chennai has

shrunk significantly during the past 70–80 years.

B. Similarly, the Covalam creek, which is a major estuary found south of

Chennai, again to the eastern end of the Mangalore–Chennai topo-high,

shows considerable reduction in its length by about 30–35% during the past

60–70 years, as seen from the above multi dated datasets. These dried-up

parts of the creek are seen now as dry mudflats and salt pans (5, figure 8(b)).

(a)

(b)(c)

Figure 8. Shrinkage and defunct backwaters/estuaries. Key Map showing the Mangalore–Chennai topographic high (1) and the Cochin–Ramanathapuram topographic high (2).(a) IRS IB FCC image showing the old (3) and present (4) limits of Pulicat lake in the Chennairegion. (b) IRS IB FCC image showing salt pans and dried-up mudflats of the defunctCovalam creek (5) in the Chennai region. (c) IRS IB FCC image showing defunct backwaters(6) on the Ramanathapuram–Tuticorin coast.


B. Similarly, a number of small backwaters are observed along the

Ramanathapuram–Tuticorin coast. These also show different stages of

becoming defunct, with totally dried-up backwaters a short away from the

shore (a, figure 8(c)) and partially dried-up ones close to the shore (b,

figure 8(c)), as evidently seen from the salt resistant vegetation in the former

backwaters and salt flats, salt pans, mudflats, and water in the later

backwaters (6, figure 8(c)). Thus, the backwaters and estuaries/creeks found

along the eastern proximities of these two topo-highs in the Chennai and

Ramanathapuram regions either have become totally defunct or are in the

process of drying up. But at the same time, the backwaters found in other

parts of the Tamil Nadu coast (e.g. the Pondicherry region) do not show any

such changes. Again, the Vembanad lake located along the western end of the

Cochin–Ramanathapuram topo-high in the Cochin area also shows similar

shrinkage.

C. Promontories and offshore bars (figure 9). Detailed interpretation of the

satellite data reveals the occurrence of promontories along the northern coast

(3, figure 9(a)) and a chain of offshore islands along the southern coast of

Ramanathapuram (4, figure 9(b)) at the eastern end of the Cochin–

Ramanathapuram topo-high.

2.1.8 Groundwater anomalies (figure 10). The groundwater fluctuation data were

analysed for the entire state of Tamil Nadu with the help of mean water levels

(a)

(b)

Figure 9. Promontories and offshore bars. Key Map showing the Mangalore–Chennaitopographic high (1) and the Cochin–Ramanathapuram topographic high (2). (a) IRS 1BFCC image showing promontories (3) along the northern Ramanathapuram coast. (b) IRS 1Bimage showing chains of offshore islands (4) along the southern Ramanathapuram coast.

4408 S. M. Ramasamy

collected for thirty years from the study area. The same indicates that there is a

perceptible fall in regional groundwater level by approximately 4 to 8 mts in the

northen Chennai and southern Ramanathapuram–Varushanad areas (3, Key Map,

figure 10). The zones of such water level fall appear to be elliptical, with their axes of

groundwater deep coinciding with these two topo-highs. The finer resolution

analysis of water levels taken from approximately 50–60 wells in parts of the Palar

basin (falling west of Chennai) during the past 30 years indicates that within such

zones of water level fall, the groundwater levels show crenulations with alternately

arranged E–W trending highs and lows (figure 10(a)).

2.2 South central sector (figure 11)

In contrast, in the south central sector, namely along the Ponnani–Palghat–

Manamelkudi topographic low, the anomalies appear to be converse to the above

two topo-highs. Along this topo-low, the satellite data vividly show two E–W

trending major sub parallel lineaments/faults (4, 5, figure 11(a)) separated by 30–

40 km and extending from Ponnani on the west coast of Kerala to Manamelkudi on

the east coast of Tamil Nadu (3, Key Map, figure 1). As this zone forms a

conspicuous topographic break/low in the Western Ghats, it is widely known as

Palghat Gap. In IRS 1B satellite FCC data, the faults bounding the valley floor of

the Palghat–Pollachi region appear to be intensively loaded with moisture, as

revealed by the reddish tone due to the chlorophyll content of the moisture-

nourished vegetation (6, figure 11(a)). In addition, shallow groundwater conditions

are also observed in the area. The Amaravathi river shows sinuous flow and a wider

floodplain within the fault-bounded land segment (7, figure 11(a)) whereas, as soon

as it crosses the northern fault (4, figure 11(a)), the river becomes thin and does not

have much of a floodplain.

In the eastern end of this topo-low, in contrast to the convexities seen on the

Chennai and Ramanathapuram coasts, the coast here is concave (figure 11(b)). The

Vellar river has its old courses (8, figure 11(b)) only to the south of its present course

Figure 10. Pattern of groundwater level variations. Key Map showing the Mangalore–Chennai topographic high (1), the Cochin–Ramanathapuram topographic high (2) and depthof groundwater level (3). (a) Pattern of crenulations in groundwater level (1975–1995).


(9, figure 11(b)), suggesting its northerly migration towards the fault-bounded land

segment (figure 11(b)). The flood discharge pattern in the Vellar river is also peculiar

in that, strikingly, it discharges more water to the water bodies located within the

northern fault-bounded land segment (10, figure 11(b)) and less to the southern ones.

(11, figure 11(b)), as seen from the deep blue tone of the former (10) and light blue

tone of the latter water bodies (11, figure 11(b)). Similarly, the coast in the Ponnani

region on the west coast of Kerala also expresses coarse concavity.

(a)

(b)

Figure 11. Topographic low/deepening. Key Map showing the Mangalore–Chennaitopographic high (1), the Cochin–Ramanathapuram topographic high (2) and thePonnani–Manamelkudi topographic low (3). (a) IRS 1B FCC image showing the northernbounding fault (4) and the southern bounding fault (5) of the topographic low, themoisture loaded valley floor (6) and the sinuous flow and wider floodplain (7) of theAmaravathi river. (b) IRS 1B FCC image showing the eastern end of the topographic lowin the Manamelkudi area bounded by the northern fault (4) controlling the Agniar riverand the southern fault (5) controlling the Vellar river, the old (8) and the present (9)courses of the Vellar river, water bodies with a thick water column (10), and water bodieswith a shallow water column (11), respectively to the north and south of the southernbounding fault.

4410 S. M. Ramasamy

3. Remote sensing – field signatures of fracturing/faulting

The satellite data, especially the raw and digitally processed IRS imagery, show a

system of lineaments/faults with prominent N–S, NE–SW, NW–SE and E–W

orientations. Some of the selected lineaments/faults and their visible tectonic

expressions are dealt with here.

3.1 N–S/NNE–SSW lineaments/faults (figures 12 and 13)

Amongst various lineaments, the following five major lineaments have

marked expressions in satellite images and in the field namely, the Stanley

(b) (a) (e)

(c)

(d )

Figure 12. N–S/NNE–SSW lineaments/faults. Key Map showing the Stanley reservoir–Tevaram (1), Krishnagiri–Cape Comorin (2), Gudiyattam–Cape Comorin (3), Tanjore–Avadaiyarkoil (4), and Kumbakonam–Muttupet (5) lineaments. (a) IRS 1D image showinglineament No. 1 in the Stanley reservoir region. (b) Sketch showing expressions of lineamentNo. 2. (c) IRS 1B FCC image showing lineament No. 3 amidst the Eastern Ghats of the Salemregion. (d) IRS 1B FCC image showing lineament No. 3 in the Trichy region. (e) IRS 1B FCCimage showing lineaments No. 2 and 3 in the Cape Comorin region.


reservoir–Tevaram, Krishnagiri–Cape Comorin, Gudiyattam–Cape Comorin,

Tanjore–Avadaiyarkoil, and Kumbakonam–Muttupet lineaments (1–5, Key Map,

figure 12).

The Stanley reservoir–Tevaram lineament (1, Key Map, figure 12), which extends

for 350 km from the Stanley reservoir in the north to Tevaram in the south,

conspicuously deflects the Cauvery river near the Stanley reservoir by approximately

90u (figure 12(a)). In the south, in the Palghat plains, the lineament controls the

Amaravathi river (7, figure 11(a)) and further south, it controls the Suruliar river in

the Kambam Valley (6, figure 14(b)).

(a) (b)

(c)(d )

Figure 13. Morpho genetic expressions of lineaments/faults 4 and 5. Key Map showing theStanley Reservoir–Tevaram (1), Krishnagiri–Cape Comorin (2), Gudiyattam–Cape Comorin(3), Tanjore–Avadaiyarkoil (4), and Kumbakonam–Muttupet (5) lineaments. (a) IRS 1B FCCimage showing undissected Mio-Pliocene Sandstone in the west (6), dissected Mio-PlioceneSandstone in the centre (7) and delta in the east (8). (b) IRS 1B FCC image showing the oldcourses of the Cauvery river (9) in the south and the present course of the Cauvery river in thenorth (10). (c) IRS 1B FCC image showing the defunct backwater (11), chains of beach ridges(12) and the heavily silted Vedaranniyam backwater (13). (d) IRS 1B density sliced imageshowing the silt-laden Vedaranniyam backwater (14) and the offshore sandbars (15) encirclingthe Vedaranniyam backwater.

4412 S. M. Ramasamy

Nearly 550 km long, the Krishnagiri–Cape Comorin lineament (2, Key Map,

figure 12) expresses chains of morphotectonic anomalies from Krishnagiri in the

north to Cape Comorin in the south. Some of the significant anomalies from north

to south (figure 12(b)) are mud eruption, which occurred during January 1997

(Ramasamy et al. 1998a), drainage reversal along the Thoppur and Vaniyar rivers

(Suryanarayana and Prabhakar Rao 1981), clusters of palaeo scars and landslides in

the Shevroy and Chitteri hills, drainage deflection in the Cauvery river, a wide fault

valley in the Anamalai–Palani hill ranges, drainage deflection in the Tambraparani

river, and conspicuous chopping of the Western Ghats in the Cape Comorin region

(2, figure 12(e)).

The Gudiyattam–Cape Comorin lineament (3, Key Map, figure 12) extends for

530–550 km from Gudiyattam in the north to Cape Comorin in the south in a N–S

(a)

(b)(c)

Figure 14. NE–SW lineaments/faults. Key Map showing the Pondicherry–Kambam faultsystem/graben (1) and other NE–SW lineaments/faults (2). (a) IRS 1B FCC image showingsinistrally shifted Mio-Pliocene sandstone (3) and the wider floodplain of the Vellar river (4)within the fault system (1). (b) IRS 1B FCC image showing the south western extremity of thePondicherry–Kambam fault system (1), defining the Kambam tectonic valley (5) and thewider floodplain of the Suruliar river (6) within the fault system (1). (c) IRS 1B FCC imageshowing sets of NE–SW trending sinistral faults along the West Coast of Kerala andKarnataka (7).


to NNE–SSW direction. It bisects the northern Javadi hills, the central Chitteri–

Kalrayan hills, and the southern Kollimalai–Pachaimalai hills (figure 12(c)). While

the western Chitteri and Kollimalai hills are marginally dissected, the eastern

Kalrayan and Pachaimalai hills are intensively dissected and gullied and do

have widespread colluvial deposits along their foothills. The shallow water table

to the west and the deeper water table to the east of this lineament in the Salem

valley indicates that the lineament acts as a groundwater barrier in the area

(figure 12(c)). Along the eastern rim of the Kollimalai hills, where this lineament

forms a well-defined fault line escarpment, perennial streams are observed. Further

south, in the Trichy area, this lineament has modified the groundwater flow, and

thus displays a conspicuous darker tone in IRS imagery (3, figure 12(d)). In the

south, in the Cape Comorin region, this lineament, in conjunction with lineament

No. 2, abruptly chops off the Western Ghat hill ranges (3, figure 12(e)). Further

down, the lineament has sinistrally shifted the land segment on the Cape Comorin

coast.

On the contrary, the Tanjore–Avadaiyarkoil and Kumbakonam–Muttupet

lineaments (4, 5, Key Map, figures 13 and 13(a)) are seen to have formed three

distinct morphotectonic zones in the area southeast of Trichy in parts of the

Cauvery delta, with the western Vallam undissected Mio-Pliocene sandstone (6,

figure 13(a)), the central Pattukottai–Mannargudi dissected Mio-Pliocene sand-

stone, exhibiting fragmentation of the sandstone into small buttes (7, figure 13(a)),

and the eastern Cauvery delta (8, figure 13(a)). The Ambuliar and Agniyar system of

rivers also shows extensive rejuvenation in the central fault trapped Sandstone block

(7, figure 13(a)). The Cauvery river, found to the north of these Sandstones, shows

extensive palaeochannels to its south (9, figure 13(b)), whereas the present river is

flowing on the northern edge of the delta (10, figure 13(b)), thus, indicating the

preferential northerly migration of the Cauvery river.

Bundles of beach ridges were interpreted (12, figure 13(c)) to a breadth of

50–55 km in the area southeast of fault No. 5 in the Vedaranniyam region. In

addition, in the area to the east of the Kumbakonam–Muttupet lineament (5,

figure 13(c)), a major defunct backwater (11, figure 13(c)) and the heavily silt-

soaked Vedaranniyam backwater (13, figure 13(c)) are found. The density sliced

blue–green bands of IRS satellite imagery (0.45–0.52 and 0.52–0.59 mm) show

that not only is the Vedaranniyam backwater heavily silted (14, figure 13(d))

but also the offshore bars are vibrantly built, encircling the Vedaranniyam

backwater (15, figure 13(d)). Thus, this coastal sector shows a hierarchy of

morphotectonic anomalies with intensive dissection of the central fault bounded

Pattukottai–Mannargudi Mio-Pliocene Sandstone (7, figure 13(a)), extensive

river rejuvenation in the same Sandstone block, preferential northerly migration

of the Cauvery river (figure 13(b)), occurrence of dried-up backwater (11,

figure 13(c)), bundles of beach ridges of approximately 55 km in breadth (12,

figure 13(c)), extensive siltation of the Vedaranniyam backwater, and vibrant

sandbar building activity in the offshore region of the Vedaranniyam coast, etc. (14,

15, figure 13(d)).

3.2 NE–SW lineaments/faults (figure 14)

A spectrum of NE–SW trending lineaments were interpreted from various raw and

digitally processed IRS datasets in parts of Tamil Nadu, Kerala, and Karnataka.

From these the signatures of two NE–SW trending sub parallel lineaments/fault

4414 S. M. Ramasamy

system separated by 30–40 km (1, figure 14) and extending from Pondicherry in the

northeast to Kambam valley in the southwest (Key Map, figure 14) are explained

here. From the northeast to the southwest, all along their strike length, these

lineaments exhibit varied morphotectonic anomalies. In the Pondicherry area,

Mio-Pliocene Sandstone is sinistrally dragged for over 5–7 km (3, figure 14(a)) and a

little to the southwest, the Vellar river exhibits a restricted floodplain (4,

figure 14(a)) within these lineaments. Further southwest, in the Trichy area, the

Cauvery river splits into two rivers, namely the Cauvery and the Coleroon and, after

flowing for a distance of nearly 20 km, these rivers show a tendency of rejoining,

thus exhibiting a mega-eyed drainage, with the eye length being about 20 km within

these sub parallel lineaments. Again, further southwest, these sub parallel

lineaments form the well-defined tectonic valley in Kambam (figure 14(b)), and

the Suruliar river has developed a wider floodplain (6, figure 14(b)) within this

tectonic valley. On the contrary, the other spectrum of NE–SW to ENE–WSW

lineaments/faults (2, figure 14) have sinistrally shifted the west coast of Kerala and

Karnataka into an enechelon pattern (7, figure 14(c)).

3.3 NW–SE lineaments/faults (figure 15)

Bundles of NW–SE trending sub parallel lineaments were interpreted in the study

area from the IRS satellite FCC data. Amongst these, lineament No. 1 sharply

controls the Pambar river at its northwestern end (9, figure 15(a)) and lineament

No. 2 controls the flow of the Ponnaiyar river in its matured and old stages.

Lineament No. 3 controls the Ponnaiyar river at its northwestern end, whereas at its

southeastern extension it sharply deflects the Vellar river, delimits the

Jayamkondam Mio-Pliocene Sandstone, and also causes conspicuous compressed

meandering in the otherwise northeasterly flowing Coleroon/Cauvery river (10,

figure 15(b)). Further along the coast, it abruptly cuts off the beach ridges

(figure 15(b)).

Lineament No. 4 sharply deflects the Cauvery river southeasterly for a short

distance of 4 to 5 km towards the Stanley reservoir area, whereas in the central

Trichy plains, this lineament and its sympathetic fractures are seen to dextrally shift

the crystalline rocks to an enechelon pattern (11, figure 15(c)). Further southeast in

the coastal sector, these strikingly control the Agniyar–Ambuliar system of

drainages with deep gullying along them (12, figure 15(c)).

Lineament No. 5 extends west of Bangalore in the northwest and up to the east

coast of Tamil Nadu in the southeast. At many places, it deflects the Cauvery river

(13, 14, figure 15(d)).

Lineament No. 6 and the associated sub parallel fractures exhibit clear fault line

escarpments and are further seen to have dextrally dragged and shattered the

Precambrian quartzites of Nagamalai–Pudukottai and further into an enechelon

pattern and further (15, figure 15(e)). Lineaments No. 7 and 8 respectively control

the Vaippar and Tambraparani rivers.

4. Active tectonics of South India and discussions

Signatures so observed in the form of multivariate structural, geomorphological,

and hydrological anomalies, both in satellite images as well as in the field, in

different parts of South India, were assembled together to produce a holistic cartoon

of the active tectonics of South India.


4.1 Cymatogenic arching

The topographic profile drawn in a N–S direction shows two distinct topo-

graphic highs, one along Mangalore–Chennai in the north and the other along

Cochin–Ramanathapuram in the south, with in between complimentary deep along

Ponnani–Manamelkudi in the south centre (figure 1).

(a)

(b)

(c)

(e)

(d )

Figure 15. NW–SE lineaments/faults. Key Map showing NW–SE lineaments/faults (1–8).(a) IRS 1B FCC image showing lineament No. 1 controlling the Pambar river (9). (b) IRS 1BFCC image showing lineament No. 3 causing the ‘Z’-shaped anomalous compressed flow ofthe Coleroon river (10). (c) IRS 1B FCC image showing lineament No. 4 and other relatedlineaments causing dextral shift of Precambrian rocks (11) in the Trichy region and drainagecontrol in the Mio-Pliocene Sandstone (12) of the Mannargudi region. (d) IRS 1B FCC imageshowing lineament No. 5 causing the sharp deflection in the Cauvery river (13) near Mysoreand near erode (14). (e) IRS 1B FCC image showing the sub parallel fractures of lineamentNo. 6 showing a system of dextral slip of beds in the Nagamalai–Pudukottai hills (15).

4416 S. M. Ramasamy

The northern Mangalore–Chennai topo-high is conspicuously marked by swarms

of ENE–WSW to E–W fracture swarms along its crest, with a prolific intrusion of

dykes in the Bangalore–Chennai area (Key Map, figures 2 and 2(b)). The tectonics

of the area has been studied by Grady (1971), Sugavanam et al. (1977), Katz (1978),

Drury (1984), Ahmed et al. (1986), Ramachandran (1987), Srinivasan (1992), and

many others. While there were no major arguments on the above E–W fracture

swarms by the above workers, Ramasamy et al. (1999), in their remote sensing-

based Precambrian tectonic model of South India, observed that the E–W fracture

swarms of the Bangalore–Chennai region do not fit in with Precambrian orogeny.

Whereas, Chakrapani Naidu and Jayakumar (1979) have doubted the Post Tertiary

origin of these dykes filling these fracture swarms. While Ghosh (1976) attributed

the E–W to ENE–WSW fracture swarms of the Saurashtra Peninsula (Western

India) to the E–W aligned Amerli cymatogenic arch of Post Trappean age,

Sychanthavong (1985) and Ramasamy (1995a) have also advocated that these

fracture swarms of the Saurashtra Peninsula are related to Post Trappean

cymatogenic arching connected to the collision of the Indian Plate with the

Eurasian Plate. So, owing to the striking similarities between the Amerli

cymatogenic arch and the Mangalore–Chennai topo-high, with similar dyke-filled

fracture swarms at the crest of the latter too, it can be surmised that the Mangalore–

Chennai topo-high may also be a reflection of tectonic arching. Ramasamy et al.

(1987, 1995), Ramasamy (1989), and Subrahmanya (1994, 1996) have also doubted

possible cymatogenic arching in the Mangalore–Chennai region. Similarly, the

fracture swarms that have been interpreted in the present study in the Varushanad

hills coincide with sub parallel E–W fractures observed in the area by Kumanan

(1998) (figure 2(c)). This, together with a further fall along the Cochin–

Ramanathapuram topo-high, indicate that this southern topo-high must also be a

similar cymatogenic arch.

The northern Mangalore–Chennai and the southern Cochin–Ramanathapuram

topo-highs form conspicuous water divides (figure 3(a)). Subrahmanya (1994)

has also observed similar water divide between Mulki (near Mangalore) and

Chennai. In addition, the present study shows that the drainages cause extensive

gullying and sheet erosion along these two topo-highs, and the soil so removed

(figure 3(b), 3(d )) is dumped into the thousands of water bodies/deltaic lakes

(figure 3(c), 3(e)) found in the eastern ends of these topo-highs in the Chennai and

Ramanathapuram coastal sectors. In fact, out of nearly 30,000 water bodies, only

the water bodies located in the Chennai and Ramanathapuram regions are heavily

silted (figure 3( f )). Further, the analysis of IRS band 1 and 2 data shows heavy

sediment discharge into the ocean by the ephemeral streams draining these topo-

highs in the Chennai and Ramanathapuram regions (figure 4), whereas the major

rivers do not. While restricted gullying was attributed in general to land upliftment

(Thornbury 1985), the gullying in the Western Ghats of Kerala and Karnataka

(Radhakrishna 1993), and in the Bangalore region (Valdiya 1998) were explained to

be the effect of Holocene upliftment. Hence, such chains of anomalies, viz. gullying

and sheet erosion in these topo-highs, restricted siltation of water bodies located in

the coastal zones of these highs, and the heavy sediment discharges selectively by the

streams draining these two topo-highs, lead to the conclusion that these sequential

phenomena are due to ongoing arching along these two topo- highs.

The IRS satellite datasets show the northerly migration of the Pennar river, the

southerly migration of the Palar river in the Chennai region, and the northerly shift


of the Vaigai river in the Ramanathapuram region. These rivers drain along the

axes/slopes of these two topo-highs and migrate away from the axes/crests of the

highs (figure 5). Similar preferential migrations of the rivers tutored by tectonic

arching/upliftment were observed in different parts of India by many (Chamberlin

1894, Yashpal et al. 1980, Amalkar 1988, Bakliwal and Grover 1988, Ramasamy et

al. 1991, Rajawat et al. 2003, Gupta et al. 2004). Hence, such preferential migration

of the Pennar, Palar, and Vaigai rivers can be taken as convincing evidence of the

ongoing arching/upliftment in the Chennai and Ramanathapuram regions.

Subrahmanya (1994, 1996) and Gangadhara Bhat (1995) also noted similar

preferential shifting of streams in the Mulki area near Mangalore but doubted it

was caused by land upliftment.

While most of the easterly flowing rivers of Tamil Nadu have developed arcuate,

cuspate, digitate, and estuarine deltas, only the Proto Palar and Vaigai rivers have

developed distinct lobate deltas with thousands of crescent-shaped, concentrically-

arranged lobes and interlobal depressions (Ramasamy 1991). Davis and Richard

(1987) observed that such lobate deltas indicate the phenomenon of land emergence.

While Babu (1975) has profounded continuous land emergence model for the lobes

of the Krishna delta of AndraPradesh, Ramasamy (1991) has explained the lobate

deltas of Tamil Nadu by the phenomenon of continuous land emergence and its

induced withdrawal of the sea and development of lobe after lobe. In this context,

the coincidence of such unique lobate deltas of Proto Palar at the eastern end of the

Chennai topo-high (figure 6(a)) and the Vaigai lobate delta in the eastern proximity

of the Ramanathapuram topo-high (figure 6(b)) may hence indicate land emergence/

land arching.

The coast of South India shows typical convexities at either end of these topo-

highs at Mangalore and Cochin on the west coast and Chennai and

Ramanathapuram on the east coast (Key Map, figure 7). In addition, the beach

ridges are wider only along the convex coasts of Mangalore, Cochin (figure 7(c)),

Chennai (figure 7(a)) and Ramanathapuram (figure 7(b)), all indicating selective

marine regression along convex coasts only, whereas in other parts of both the east

and west coasts, no such well-developed beach ridges are found. While such bundles

of wider beach ridges were also observed by Gangadhara Bhat (1995) in the

Mangalore area, emerged coral beds of 5000–2000 years BP (Before Present) were

observed in the Ramanathapuram–Rameswaram region by Stroddart and Pillai

(1972). Hence, such convex shapes and the restricted marine regressions lead to the

conclusion that these convexities must be the structural culminations of ongoing

arching, and that such arching might have only selectively pushed the sea away.

However, the cuspate features with nosing effect of the Ramanathapuram coast

(figure 7(b)) may be due to divergent littoral currents that were operative in the area

during the last 3500 or so years (Ramasamy 2003), and this would have later

sharpened the convex Ramanathapuram coast.

Again, the selective shrinkage of the Pulicat backwater (figure 8(a)), the

withdrawal of the Covalam creek (figure 8(b)), both along the Chennai coast, and

the observation that the sea level fell by about 1.5 to 3.22 mm per year based on tide

gauge measurements taken at Mangalore coast by Subrahmanya (1994) all show

that the Chennai and Mangalore coasts, which respectively form the eastern and

western ends of the Mangalore–Chennai topo-high, are emerging coasts. In the same

way, the shrinkage of the Vembanad lake on the Cochin coast (figure 7(c)) and the

different stages of the defunct backwaters on the Ramanathapuram coast

4418 S. M. Ramasamy

(figure 8(c)), which again respectively form the western and eastern ends of the

Cochin and Ramanathapuram topo-high, also signify emerging coasts.

The occurrence of promontories (figure 9(a)), as well as a chain of offshore islands

(figure 9(b)) on the Ramanathapuram coast, again suggest the emerging nature of

this coast. However, the absence of such features along the Chennai coast is

attributed to the openness of the coast and its direct exposure to littoral currents.

The conspicuous fall in water level and the coincidence of the axes of such deep

groundwater with the axes of these two topo-highs (Key Map, figure 10) again

suggest probable ongoing land emergence along these topo-highs.

Thus, the multivariate geomorphic anomalies, viz. the E–W fracture swarms,

water divides, soil erosion – reservoir siltation – sediment dumping into the ocean,

preferential migration of rivers away from the topo-highs, convex coasts along with

restricted marine regression, restricted withdrawal and drying of backwaters and

creeks, fall in groundwater, etc., observed only along these two topo-highs, clearly

indicate that the Mangalore–Chennai and Cochin–Ramanathapuram topo-highs

are the reflection of ongoing E–W tectonic/cymatogenic arching.

Further, phenomena such as the drifting of the Cauvery river from the

Hogenekkal–Walajapet–Chennai tract to the Hogenekkal–Trichy tract

(Ramasamy et al. 1992) during 3000–2300 years BP, the southerly migration of

the present-day Palar river around 1100 years BP (Ramasamy et al. 1992), the

interpretation of the palaeo sea during 5060 years BP 3–4 km west of Chennai

(Ramasamy 2004), the palaeo sea at 3–5 km west of the present shoreline on the

Ramanathapuram coast around 3500 years BP (Ramasamy 2003), and the recently

measured tide gauge data indicating a fall in sea level (1.5 to 3.22 mm per year) on

the Mangalore coast (Subrahmanya 1994), etc., all indicate that land arching is

taking place even now along these topo-highs.

4.2 Cymatogenic deepening

While the above two topo-highs/arches show extensive gullying, sheet erosion, and

preferential migration of rivers away from the axes of the topographic highs, convex

coasts with restricted marine regression, withdrawal of creeks and shrinkage of

backwaters, fall in groundwater level, etc., the Ponnani–Palghat–Manamelkudi

topographic low exhibits converse anomalies, viz. a youthful stage floodplain

and acute sinuosity in the Amaravati river (7, figure 11(a)), preferential migration

of the Vellar river (8, figure 11(b)) towards the axis of the topographic low, a

well-defined concave coast at Manamelkudi (figure 11(b)), the absence of

beach ridges and increased tidal activity, along with the growth of mangroves

during the past 50–60 years on the Manamelkudi coast (figure 11(b)), the rise of

groundwater levels evidenced by high moisture-nourished vegetal cover (6,

figure 11(a)), etc. All these converse anomalies suggest ongoing land subsidence

along this topo-low.

Acute sinuous flow and floodplains in youthful stage, preferential migration of

rivers towards the axes of land subsidence, etched shorelines, and shorelines of tidal

activities, etc., have been demonstrated to be the indicators of land subsidence in

many parts of India, as well as around the world. The youthful stage floodplains on

the tributaries of the Cauvery river in the Thalaicauvery region (southwest of

Bangalore) were explained to be the effect of tectonic subsidence (Radhakrishna

1992). Similarly, the preferential migration of the Ganges and Yamuna rivers


towards each other in the area east of Delhi was observed to be due to ongoing

grabening in between the Yamuna and Ganges rivers (Ramasamy et al. 1991).

Further, phenomena such as Post-Jurassic tectonic movements and tectonic

breaks along the Palghat Gap of the Western Ghats (Arogyasamy 1963), possible

tectonic subsidence along the Palghat Gap and its extension up to the Laccadives

and Maldives along the 9u channel (Jacob and Narayanaswami 1954), geophysical

anomalies indicating possible graben along the Palghat region (Qureshy 1964),

occurrence of a series of peripheral faults in South India and the emergence of the

northern Nilgiris and the southern Palani–Anamalai hills, with complementary

subsidence in the intervening Palghat Gap (Gubin 1969) and evidence of tectonic

subsidence along the Palghat Gap (Rao 1977), etc., also corroborate well with the

present geomorphic anomalies. Hence, all such anomalies found along this topo-

low, converse to the above two topo-highs/arches, suggest ongoing cymatogenic

deepening along the Ponnani–Palghat–Manamelkudi topo-low.

While the anomalies favouring such arching and deepening are well seen in parts

of Tamil Nadu, this is not so in parts of the west coast of Kerala and Karnataka.

This is because of the high relief of the Western Ghats and the steep westerly

gradient of the terrain, which disabled the rivers to have their systematic fluvial/

fluvio marine histories. Further, as the west coast is also straight and directly facing

littoral currents, no coastal landforms are well developed. Even so, some significant

anomalies, such as convexities, shrinkage of backwaters, and restricted marine

regression are also well documented along the west coast.

4.3 Extensional block faulting

This study has brought out three sets of lineaments/faults with N–S, NE–SW, and

NW–SE orientations, and amongst which the chains of anomalies suggest

extensional/block faulting morphology to the N–S lineament systems.

The Stanley reservoir–Tevaram lineament and the associated sub parallel

lineaments (1, Key Map, figure 12) show a major deflection in the Cauvery river

(1, figure 12(a)) near the Hogenekkal/Stanley Reservoir area. While Vaidyanadhan

(1971) attributed the southerly deflection and flow of the Cauvery river from its

earlier northeasterly Hogenekkal–Chennai flow to probable tectonic movements,

Ramasamy et al. (1992), on the basis of various dating of the Cauvery river’s

sediments, observed that the otherwise northeasterly flowing river (Hognekkal–

Chennai track) took a right-angled southerly turn and entered the Trichy–Tanjore

plains somewhere around 2300 years ago due to the opening up of the N–S fault

near the Stanley reservoir area (1, Key Map, figure 12). Again, Raiverman (1969)

observed that a major N–S lineament played a vital role in bringing the Cauvery

river towards the Trichy–Tanjore plains. These observations indicate alert tectonism

along the Stanley reservoir–Tevaram lineament/fault.

Lineament/fault No. 2, namely the Krishnagiri–Cape Comorin lineament, shows

varied geomorphic anomalies (figure 12(b)) indicating alert tectonism in the form of

mud eruption, drainage reversals, palaeo scars, the tectonic valley in the Anamalai

hills, etc. (Ramasamy et al. 1998a). Suryanarayana and Prabakara Rao (1981) have

observed drainage reversals along the Thoppur and Vaniyar rivers and attributed

this to tectonic wedging along the lineament zone.

Similarly, the multivariate morphotectonic anomalies vividly exhibited by

the Gudiyattam–Cape Comorin lineament (3, Key Map, figure 12), viz. deep

cutting of the Eastern Ghat hill ranges from the Javadi hills in the north to the

4420 S. M. Ramasamy

Kollimalai–Pachaimalai hills in the south (3, figure 12(c)), intense dissection,

extensive erosion and vast colluvial fill spread in the foothills of the eastern Kalrayan

and Pachaimalai hills, only when compared to their western counterparts, namely the

Chitteri–Kollimalai hills (3, figure 12(c)), its role as a groundwater barrier in the Salem

valley, the trapping of the groundwater flow along this lineament in the Trichy region

(3, figure 12(d)), and the abrupt chopping off of the Western Ghat hill ranges and the

sinistral shift of the coastal beds in the Cape Comorin area (3, figure 12(e)), etc., all

indicate the active tectonism of this lineament/fault. In addition, the geomorphic

disharmony, viz. extensive dissection, gullying, erosion, and colluvial fills in the

eastern Kalrayan and Pachaimalai hills and comparatively less such tectonic and

geomorphic features in their western counterparts, namely the Chitteri–Kollimalai

hills (figure 12(c)), which are occurring to the west of this lineament, suggest probable

block faulting along the Gudiyattam–Cape Comorin lineament. On the contrary, the

chopping off of the Western Ghat hills, along with sinistral shifting of the coastal beds

in the Cape Comorin area (figure 12(e)), suggest that this lineament/fault has both

vertical and transverse movements.

The Tanjore–Avadaiyarkoil and Kumbakonam–Muttupet lineaments/faults (4

and 5, Key Map, figures 12 and 13) exhibit intensive dissection, gullying, and

fragmentation of the central fault trapped Mio-Pliocene Sandstone of the

Pattukottai–Mannargudi area (7, figure 13(a)), in contrast to its western counterpart

(6, figure 13(a)), indicates the upliftment of the central fault entrapped Sandstone.

The well-defined preferential northerly migration of the Cauvery river (figure 13(b))

located to the north of such fault trapped Sandstone block is a further confirmation

of the upliftment of the fault trapped Pattukottai–Mannargudi Sandstone block.

Ramasamy et al. (1992), on the basis of archaeological, radiocarbon, and other

dating of the palaeochannels, observed that such northerly migration of the Cauvery

river in the deltaic region has occurred during the time span of 2100–750 years BP.

Further, Ramasamy et al. (1998b), on the basis of radiocarbon dating, estimated

that the beach ridges observed to a breadth of nearly 55 km to the east of the

Pattukottai–Mannargudi Mio-Pliocene Sandstone block (12, figure 13(c)) might

have been built during the past 5000 years or so, at the rate of 11 m per year, and

attributed such land progradation to the upliftment of the central fault trapped Mio-

Pliocene Sandstone. Again, from the extensive siltation of the Vedaranniyam

backwater (14, figure 13(d)) and the vibrant offshore bar-building activity encircling

the Vedaranniyam backwater, Ramasamy and Ravikumar (2002) observed that the

central Pattukottai–Mannargudi Mio-Pliocene Sandstone block is even now

undergoing upliftment. Further, the swelling up of the Vedaranniyam backwater

in between 1930 and 1993 AD was attributed to the N–S faults observed in satellite

imagery of 1993 AD and the resultant inflow of seawater into the backwater

through these N–S faults (Ramasamy and Ramesh 1999). Ramesh (1999) has

further recorded anomalous centrifugal flow of groundwater in the central fault

trapped Pattukottai–Mannargudi Sandstone block. All these clearly indicate that

the upliftment of the Mio-Pliocene Sandstone is even now taking place along these

two Tanjore–Avadaiyarkoil and Kumbakonam–Muttupet lineaments/block faults

(figure 13).

Radhakrishna (1992) observed that the Cauvery river has phenomenally

rejuvenated in the Sivasamudram area, south of Bangalore (figure 1), because of a

series of N–S faults that have uplifted the Bilgirirangan hill ranges in recent years. In

fact, these N–S faults fall west of the presently interpreted faults No. 1, 2 and 3.


Valdiya (1998) also observed a series of N–S/NNE–SSW trending Holocene block

faults along with dextral and sinistral movements in the Bangalore peneplain, which

have uplifted the peneplain by as much as 300–400 m in many places. Ramakrishnan

(1988) earlier documented a N–S fault to the west of the Closepet granite in the

Bangalore/Mysore (Karnataka) region and he felt that the same has aided the recent

upliftment of the Closepet granite. Singh and Venkatesh Raghavan (1989) observed

that the earthquake occurred on 2nd September, 1998, 30 km due north of the

Trivandram falls, in close proximity to the NNE–SSW lineament. Valdiya et al.

(2000) observed Neotectonic reactivation along the N–S faults in parts of the

Hemavati basin (west of Bangalore, Karnataka), causing the ponding of the rivers/

streams during 14,000–1300 years BP. All the above clearly corroborate and confirm

the active tectonics/block faulting and dextral and sinistral movements along the N–

S faults interpreted in the study area.

4.4 NE–SW wrench faults

The present interpretation of satellite data has brought out a spectrum of NE–SW

trending lineaments/faults (figure 14). Amongst these, the varied anomalies shown

by the two major sub parallel lineaments from Pondicherry in the northeast to the

Kambam valley in the southwest (1, Key Map, figure 14), viz. sinistral dislocation of

Mio-Pliocene Sandstone in the Pondicherry area (3, figure 14(a)), restricted

floodplain in the Vellar river (4, figure 14(a)), eyed drainage in the Trichy area,

well-defined tectonic valley in Kambam, along with youthful stage floodplain in the

Suruliar river, suggest ongoing land subsidence along these two sub parallel

lineaments, in addition to sinistral movements. While Ramasamy and Karthikeyan

(1998) made observations favouring possible Holocene grabening along these

lineaments, Ramasamy and Kumanan (2000) doubted possible land subsidence in

between these two sub parallel lineaments on the basis of the eyed drainage

Trichy area. In addition, there are a number of NE–SW trending spectrum of faults

which show sinistral strike slip movements, and some of these faults have

also sinistrally shifted the coastal beds into an enechelon pattern all along the

Kerala and Karnataka coasts (7, figure 14(c)). Ramasamy (1995b) observed that

some of the NE–SW trending lineaments of Andrapradesh and Tamil Nadu take a

swing in a west southwesterly direction and cause sinistral shifts along the west

coast, extend right up to the Laccadives and Maldives, sinistrally shifting these coral

islands too. These all indicate that the NE–SW spectrum of lineaments interpreted in

the study area are predominantly active sinistral faults with grabening at a number

of places.

Grady (1971), Ray (1977), and Katz (1978) explained that these NE–SW faults in

general are Precambrian dextral faults. But, in addition to the present observations

and the earlier observations of Ramasamy (1995b), Prabhakar Rao et al. (1985) and

Nair (1987) also observed that the Kerala coast is punctuated by a spectrum of

ENE–WSW trending sinistral faults, which act as an opening to lagoons, caused

submerged coastal Platforms, control the river systems, and also shifted the beach

ridges. Valdiya (2001) observed that the NNW–SSE trending Western Ghats

escarpments have been cut into a system of enechelon escarpments by the ENE–

WSW Holocene faults. Ramasamy and Ramesh (1999) observed that a river island

and the water spread area in the Coleroon river, east of Trichy, seem to have

changed from a rectangular shape to a trapezohedron shape between 1930 AD and

1993 AD, and demonstrated this to be due to sinistral movement of the NE–SW

4422 S. M. Ramasamy

trending Coleroon lineament/fault. All these observations thus confirm the present

interpretation that the NE–SW lineaments/faults of the study area are active.

4.5 NW–SE wrench faults

In the present study, a set of NW–SE lineaments/faults were interpreted (figure 15)

that predominantly control many river systems. In addition to controlling and

deflecting the drainages, these lineaments/faults seem to have also conspicuously

dragged even the drainages with a ‘Z’ shape (10, figure 15(b)), truncate the beach

ridges (figure 15(b)), and appear to dextrally shift the Precambrian gneissic rocks

(11, figure 15(c)) and the quartzites (15, figure 15(e)) into an enechelon pattern, thus

providing definite information indicating recent dextral strike slip movements along

these faults.

Vemban et al. (1977) observed that most of the rivers in Tamil Nadu, viz. the

Palar, Ponnaiyar, Cauvery (in parts), and the Vaigai, are controlled by deep faults,

exhibiting dextral strike slip geometry. Agarwal and Mitra (1991) also identified that

the NW–SE trending faults are young and control the hydrocarbon mobilization in

the Cauvery basin. Offshore geophysical anomalies are also found to favour NW–

SE trending structural weak zones in the Udipi region (Subramaniyan 1987). All

these are very confirmatory evidences for the dextral strike slip movements of NW–

SE trending lineaments/faults interpreted in the present study.

4.6 Post collision tectonics

Thus, the present study has lead to the supposition that the Mangalore–Chennai

and Cochin–Ramanathapuram topo-highs are cymatogenic arches with comple-

mentary ongoing cymatogenic deepening along Ponnani–Palghat–Manamelkudi.

The lineaments/faults interpreted in the present study fall into three major azimuthal

groups, with the N–S group showing evidences for extensional faulting, the NE–SW

group expressing signatures of ongoing sinistral strike slip movements, and the NW–

SE trending faults displaying signatures of dextral strike slip movements both in the

Precambrian crystalline rocks and the younger Mio-Pliocene–Quaternary coastal

beds. The disposition of the cymatogenic arches/deeps and the geometry of these

faults indicate that the greatest principal stress can be visualized in a N–S direction

(Anderson 1951), and the said stress/force may be related to the drifting of the

Indian Plate northerly/north northeasterly. Under this stress geometry, the NE–SW

sinistral faults become Left Lateral Wrench faults, the NW–SE dextral faults

become Right Lateral Wrench faults of the Pleistocene–Holocene period (figure 16).

As the N–S lineaments show clear manifestations of block faulting in Javadi,

Shevroy–Chitteri–Kalrayan, and the Kollimalai–Pachaimalai hills, and also in the

Mio-Pliocene sandstone of the Pattukottai–Mannargudi area and parallel to the

greatest principal stress, this system may be referable to extensional failures. Again,

as the E–W fracture swarms are orthogonal to the greatest principal stress and

further confined to the arches, these could be of Pleistocene–Holocene release

fractures. The GIS-based 3D visualization of gravity data (figure 17) shows E–W

alternate highs and lows, N–S and NE–SW gravity anomalies, while the E–W

anomalies are matching with such arches and deeps, and the other ones coincide

with the N–S and NE–SW faults. However, the NW– SE faults are not reflected in

gravity data. Ramasamy (1995b) observed that the NE–SW sinistral faults are more

active in South India due to the additional increment of such Post Collision sinistral


faults by the rising Carlsberg ridge in the Arabian Sea. Hence, this may be the

reason for more geophysical responses of the NE–SW group of lineaments/faults.

Singh et al. (1996) identified NNE–SSW and NW–SE trending prominent sinistraland dextral lineaments from the drainage anomalies in the Indo-Gangetic plains,

and similarly established that these must be the wrench faults related to the

northerly oriented stress connected to post collision tectonics.

Thus, the present study reveals that the study area is whirling like a worm with

alternatively arranged two arches and an intervening deep and related extensional/

block faults and wrench faults. The various riverine, coastal, and hydrological

Figure 16. Pleistocene tectonic scenario of South India.

4424 S. M. Ramasamy

anomalies clearly show that Southern India is tectonically very active. In addition to

the conspicuous fall in groundwater level in the cymatogenic arches and shallowness

of the cymatogenic deep (Key Map, figure 10), the analysis of finer resolution

groundwater data (figure 10(a)) shows crenulations in water levels with E–W

groundwater ridges and valleys, which may be the reflection of the still prevalent/

ongoing northerly directed compressive force related to the post collision

phenomenon. This active tectonic model also gains support from various other

workers. Vaidyanadhan (1967) observed that the southern part of the Indian

Peninsula has witnessed pulsatory tectonic upheavals since Post Jurassics. The

horsting in Nilgiris and grabening in Salem–Attur and Palghat were pointed out by

Qureshy (1964), Gubin (1969), and Rao (1977). Subramaniyan (1987), on the basis

of an offshore geophysical survey, identified the structural grains with E–W, N–S,

NE–SW, and NW–SE directions in the Mangalore region. Reddy et al. (1988)

brought out a system of E–W trending alternate aero magnetic highs and lows and

doubted for possible crustal movements. Valdiya (1989) observed that the cratonic

crust of the Indian shield is periodically relaxing its stress through crustal

movements. Ranadhir Mukhopadhyay and Khadge (1992) established a major

ENE–WSW trending depression in the Indian Ocean, far south of Cape Comorin

along latitude 9–15u south. This depression is flanked with summits and sea mounts

to its north and hills and peaks to its south. It was observed that these arches and

Figure 17. Bouger gravity anomaly of South India.


deeps were traversed orthogonally by NNW–SSE oriented Late Cretaceous

fractures as per them. Ramasamy (1999) established yet another cymatogenic arch

in the Calicut region with ENE–WSW orientation on the basis of various tectonic

and geomorphic anomalies in the Western Ghats. He also observed folding and

swinging of the NNW–SSE trending F2 Precambrian fold axis and attributed the

same to the northerly-directed Post Collision compression. However, as far as the

ages of these arches, deeps, and faults are concerned, they could be Post Mio-

Pliocene/Quaternary as they show clear faulting in Mio-Pliocene Sandstone. The

expression of these faults with similar morphology in the Precambrian rocks

indicates the reactivation of the old faults or the exclusive formation of new faults in

the Post Mio-Pliocene period.

Such active tectonics with arches, deeps, and faults has direct bearing on intra

plate seismicities in this region, as evidently seen from the coincidence of over 200

seismicity data of more than 3M along these arches, deeps, and faults (figure 16). As

this active tectonic cartoon has been built from fluvial and coastal geomorphic

anomalies, and also hydrological anomalies, it follows that such active tectonics has

control over the riverine, coastal, and hydrological ecosystems. Hence, this not only

warrants detailed studies in the context of seismicities but also in understanding the

environmental systems.

Acknowledgements

The author is grateful to the Seismology Division, Department of Science and

Technology, Government of India, New Delhi, which has granted the research

project ‘SEISTA’ (Seismo Tectonics of Tamil Nadu), and to the Department of

Space, Government of India, which has funded the research project ‘CRUSDE’

(Crustal Deformation Studies of South India), both of which have helped the author

in the study. Shri. J. Saravanavel, Scientist, is acknowledged for his assistance and

Dr C. J. Kumanan, Lecturer, Centre for Remote Sensing for checking the

manuscript.

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