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Epeirogenic studies in India with reference to recent vertical movements
Tectonophysics, 29 (1975) 505-521 0 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
EPEIROGENIC STUDIES IN INDIA WITH REFERENCE VERTICAL MOVEMENTS
505
TO RECENT
L.N. KAILASAM Geophysics Division, Geological Survey of India, National Committee for Geodynamics (ICG), Calcutta (India) (Revised version accepted April 17, 1975) ABSTRACT Kailasam, L.N., 1975. Epeirogenic studies in India with reference to recent vertical movements. In: N. Pavoni and R. Green (Editors), Recent Crustal Movements. Tectonophysics, 29 (l-4): 505-521. Epeirogenic movements of regional extent have been manifest and recorded in several parts of India from Late Proterozoic to present times through the Tertiary and Recent. The main features of these movements, especially Recent vertical movements, in typical areas in the platform and coastal areas of peninsular India are described and the salient results of the study, qualitative and quantitative, of these zones are outlined. Quantitative measurements of Recent vertical movements have been made largely by geodetic methods and precision levelling in the plateau areas as also in the mobile, orogen zones of the Himalayan and sub-Hi~layan tracts and by sea-level measurements through an extensive network of coastal automatic tide-gauge stations. The annual variations of sea ievel have been studied for several Indian ports. The land uplift or subsidence in relation to mean sea level over the Indian coast has also been studied. The application of geophysical techniques in the identification and delineation of regions of epeirogenic movements has been discussed and illustrated by the results in two type areas, viz., the coastal sedimentary belt of Madras and the vast Deccan Trap territory in western India. In the latter area, blanketed by extensive basaltic lavas over a vast region, the gravity data have brought out a number of major zones of subsidence and uplift which have been corroborated by the results of refraction seismic soundings. One of these major zones of subsidence is in the Koyna region which witnessed a major disastrous earthquake a few years ago. The results of deep seismic sounding along a traverse across the southern part of the Indian peninsula now in progress have also been briefly mentioned. Five important areas have been selected for quantitative measurements of current epeirogenic movements, namely the Shillong Plateau in northeast India, the Chhotanagpur Plateau and Singhbhum region in Bihar, the Aravalli region of Rajasthan, the Narmada-Sone Rift in central India and the Deccan Trap territory including the Koyna earthquake zone. INTRODUCTION
The study of epeirogenic movements of regional extent involves mainly the forces causing non-erogenic vertical movements which apparently are not
506
always clearly understood. This study requires the acquisition of information on the lithospheric materials and processes involved in the subsidence and uplift of major basins and plateaus, both above and below mean sea level, and it has been appropriately suggested that integrated surveys be conducted in sensitive areas for detailed sedimentological, stratigraphic and structural analyses, gravity, magnetic, heat flow and seismic refraction and reflection surveys as also repeated precision geodetic levelling. One of the major objectives of this programme on studies of epeirogenic movements under the Inflations Geodynamics Project is the acquisition of data on duration, wavelength, amplitudes and rates of vertical movements, especially rapid movements. As has been well realized and appropriately stated, vertical movements cannot be studied in isolation from horizontal movements nor can epeirogenic studies be conducted to the exclusion of erogenic movements, especially in the mobile zones of orogens where large vertical movements occur as in the recently active areas such as the ~im~ayas in India. The study of vertical movements of regional extent, therefore, involves both the stable areas such as platform areas as well as the erogenic areas. In this paper, the present knowledge of epeirogenic movements in the Indian sub-continent and work done with special reference to recent movements is outlined and the areas selected for detailed study and measurement of current movements are described, as well as the results of application of geophysical techniques in some type areas. GENERAL
ASPECTS
OF ~PEIROG~NIC
STUDfES
The Inter-Union commission on Geodynamics, through its Working Group 7, has laid down broadly the lines on which studies on epeirogenic movements of large regional extent may be undertaken on a global scale under the current International Geodynamics Project. The study of these movement will have to be made through the older as well as the later parts of the time scale through Pleistocene and Recent periods. It will obviously be necessary to separate out the secular trends to gain precise knowledge of the recent movements, especially on a quantitative basis, Large-scale vertical crustal movements have occurred extensively over the world during the Tertiary period accompanied by worldwide regressions, especially over nonerogenic shelves, warping and uplift of peneplains, deformation of strand lines and other related factors. Recent vertical movements, especiaily those which have occurred over the past hundred years or so, are of particular current interest. The first stage in epeirogenic investigations will involve the identification and rough delineation of sensitive areas of massive regional uplift and subsidence to facilitate which geophysical and geological techniques including suitably selected, extended geotraverses can be employed with advantage, The vertical displacements can be grouped under a number of categories such as cratonic deformation; geosynclinai deformation including uplifts and subsidence in the abyssal plains of the oceans and subsidence
507
of ancient ortho-geosynclines; deformation associated with uplift and rifting, such as the intra-cratonic uplifts and rifts as in the case of the Rheintalgraben and oceanic uplifts and rifting as evidenced by mid-oceanic ridges; morphorogenic subsidence and uplifting; subsidence and uplifting not associated with geotectonic causes such as that due to loading and unloading of water bodies and glacial loading and unloading, as also due to sedimentation and erosion; subsidence of foredeep areas, etc. (Bally, 1972). Studies of vertical measurements may be conducted both on a qualitative and quantitative basis. As absolute quantitative measurements are difficult for the simple reason that we cannot have a firm, reliable sea-level reference, we have to rely largely on measurements of relative vertical displacements. Precise measurements of recent vertical movements in areas identified for epeirogenic movements as indicated earlier will by and large consist of: (1) observations on variation of sea levels as also water level in inland lakes by means of tide and water-level gauges, including tiltmeters for measurements of changes in tilt; (2) repeated precision levelling through geodetic networks; and (3) micro-gravimetric traverses along selected geo-traverses across type areas. Mean sea-level changes, however, are the resultant effect of a variety of causes involving worldwide isostatic and eustatic changes through Phanerozoic times, in addition to changes produced by vertical movements over land and these have necessarily to be resolved. However, continuous measurements of sea and water levels combined with geodetic and precision levelling afford some of the effective means of measuring current vertical movements. EPEIROGENIC
MOVEMENTS
IN INDIA
There is evidence for vertical movements of epeirogenic type in India from Late Proterozoic down to the present times through the Tertiary and Recent. The predominant evidence for post-Tertiary to Recent movements is in the Precambrian terrains and some of the coastal tracts besides the great Himalayan belt. Very late vertical movements have been reported from different parts of the Precambrian shield, from the Singhbhum region in eastern India, the Assam Plateau in northeastern India (Krishnan, 1966; Murthy, 1971) and the Rajasthan Precambrian belt in northwestern India (Heron, 1953), the charnockite terrain of South India (Radhakrishna, 1968) and various parts of the Eastern Ghat belt. Although the Himalayan erogenic movements may now be considered to be nearly over, the post-erogenic movements will apparently continue for quite some time before completely quiescent conditions can be expected to prevail and these vertical movements in the Himalayas are probably still taking place (Krishnan, 1966). There are several instances of Recent tectonism in India in platform areas also with a considerable vertical component in the recorded movements. The vertical movements in the platform areas are quite conceivably of nonerogenic type of ‘deep-seated origin, giving surface geomorphic expressions.
508
These are also manifest in a large measure in the gravity anomaly maps of peninsular India. The manifest geologic evidence for these Recent vertical movements includes sea-level changes as a result of crustal movements over the continent and oceans, resulting in marine transgression and regressions. Uplifted and warped erosion surfaces can also be seen as downwarped inland basins associated with block faulting filled with continental sediments, coastal terraces with marine sediments, river terraces and water falls. This and other evidence in the Precambrian shield may perhaps be attributed to a continuing rise of the continental lithospheric block since the close of the Cretaceous, as has been indicated by Holmes (1955) and Hallam (1953). Some typical examples of changes along the coast, both emergence and sub-emergence, in the Recent may be cited. Parts of the Kathiawar coast, the Rann of Kutch and the eastern coast of South India show distinctive evidence of emergence. Raised beaches comprising Pleistocene and Recent grits and clays are seen in the Tirunelveli and Ramnad districts in the extreme south of peninsular India. There is evidence that several places which were on the sea some centuries ago are now situated several kilometers inland, such as Coringa near the mouth of the Godavari, Kaveripatnam in the Cauveri delta and Korkai on the Tirunelveli coast which were sea ports a couple of thousand years ago. Submerged forests have been noticed in the Valinokkam Bay on the Tirunelveli coast and off the Princes Dock in Bombay. Large parts of the Gulf of Mannar and Palk Strait which constitute shallow seas are presumed to have been submerged only during Recent geologic times. The former town of Mahabalipuram near Madras was similarly submerged several metres below the sea. The Arakan coast and the Andaman and Nicobar Island groups are known to have undergone submergence in the Pleistocene and Recent times (Krishnan, 1960). Vaidhyanathan (1964) has described Recent vertical movements in the Srisailam gorge in the Krishna valley in Andhra Pradesh, with a vertical depth of over 250 metres and cutting into the Srisailam quartzites. He has also mentioned evidence of recent vertical movements in the form of a raised river terrace near Katarapalle in the Cuddapah basin. Measurements
of Recent
vertical movements
So far the quantitative measurements of Recent vertical movements undertaken in India comprise measurements of sea level and geodetic triangulation and precision levelling conducted by the Geodetic and Research Branch of the Survey of India. Mean sea-level measurements are recorded by a network of automatic coastal tide gauges and these observations had been initiated in 1806 and continued since then with a progressively increasing network of tide-gauge stations. Several of these tidal stations have been connected by transcontinental lines of spirit levelling with highest precision. The annual variations of the sea level in Indian waters have been studied for various ports such as Cochin, Visakhapatnam, Madras, Port Blair in the
Andamans, Colombo in Sri Lanka etc., (Chugh, 1973). Chugh reports that the secular variations of the sea level at Visakhapatnam, Madras, Port Blair as also at Aden, Karachi and Bombay show a general rise and are generally of the order of 0.1 m per 100 years. The amplitudes and phase lags worked out for the 19-yearly tide after removing the secular variation are observed to be different from zero, being,thus not in accord with equilibrium tide. The variations in amplitudes have also been observed to be large. The land uplift or subsidence in relation to mean sea level over the Indian coast line has also been studied. It has been observed that generally no measurable uplift or subsidence has been indicated except local subsidence at Cochin harbour of 0.1 m from 1890 to 1954 and at Pamban pass of 0.1 m from 1880 to 1955. The stability of the coastal region of Calcutta has also been under study (Chugh, 1973). Geodetic triangulation chains cover the entire country, form rectangles of roughly 150-200 km by 250-600 km and these operations have been conducted since 1802. Geodetic triangulation for crustal movements has been carried out at a number of places in the nonerogenic zones for the study of crustal movements. Geodetic triangulation studies of earthquake movements in the Assam area since the great Assam Earthquake of 1897 to Recent times suggest a possible movement southward by 2 or 3 m of the central area north of Shillong relative to the undisturbed country 65 km to the east or west. Similarly, the revisionary triangulation for Koyna Dam has shown a more or less easterly movement of land varying from roughly 1 m on the coast to about 4.5 m at Koyna Dam. Geodetic levelling by the Survey of India has been in vogue since 1875 and the lines of these levels embrace the chain of tidal observations established at suitable sites along the coast of India and Burma, the mean sea-level data furnished by these tidal observations serving as a basis for a network of lines of levelling of precision over the whole country. Besides these, precision to high-precision levelling lines have been established in a number of areas over the entire country extending from the Himalayas southwards. The results of such precision levelling at the Pamban Pass, Cochin and near Calcutta have already been alluded to. The changes in levels in the Koyna region after the December 1967 earthquake are of the order of 2-3 cm of subsidence (Chugh, 1973). Application basins
of geophysical
techniques
in the study of zones of uplifts and
The scope of application and usefulness of geophysical exploration niques are illustrated in two type areas in the Indian peninsula.
tech-
Coastal sedimentary belt of Tamil Nadu The peninsular shield of South India is flanked on its eastern coast by sedimentary beds of Cretaceous and Tertiary age laid down by marine Cenomanian transgressions with isolated occurrences of Upper Gondwana (Middle
w
.--.___ ~---__.-__
,;.B
--?y---------r~-
71’;
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8
l.____
‘43’
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----T----o00
I
BOUGUER GRAVITY COASTAL
AREAS
ANOMALY
MAP
OF
THE
,
OF SOUTH ARCOT, TRICHINOPOLY
AND TANJORE
DISTRICTS,
MADRAS.
+
*
+ * ‘
(
:
0
**
“0
.
’
J.‘UIIIN
+’
*
SCI\LE
GEOLOGICAL
+
INDEX
A Fig. 1A. Bouguer tricts. Madras.
gravity
anomaly
map of South
Arcot,
Trichinopoly
and Tanjore
dis-
511
Jurassic) beds between the Archaean and the Cretaceous formations. Systematic geophysical investigations conducted by the Geological Survey of India over 20,000 sq. km of this coastal tract helped in bringing out the bottom configuration and structural features of this tract, indicating three separate basins in the Tamil Nadu coast south of Madras, separated by buried ridges of the Precambrian crystallines and a thickness of the order of 3000 m in the deepest parts of the basins, besides indicating important structural features over the basement having potentialities for oil (Kailasam, 1968). The Bouguer gravity map of this coastal sedimentary belt is presented in Fig. 1A indicating also the main geologic features. In the northernmost South Arcot basin of this belt, the Cuddalore (MioPliocene) formations, consisting of ferrugenious sandstones, grits or clays, are exposed in the central parts flanked by Cretaceous shales, sandstones and limestones on the west and the Middle Jurassic Gondwana rocks on the western fringe of the basin. The Cuddalore formations enclose the wellknown lignite beds of Neyveli. Eocene limestones have been encountered at lower levels in bore holes subsequently drilled in the area. A typical east-west gravity-cum-magnetic profile along a geotraverse across the South Arcot basin from Vriddhachalam on the west to the eastern sea coast and a reflection seismic time-depth section along this traverse are reproduced in Figs. 1B and 1C. A pronounced flexure or faulting of the crystalline basement is indicated on the western margin by the gravity-cummagnetic profile as well as the seismic section. Near the sea coast, however, the gravity data show anomalously high values, apparently due to a combination of effects, partly sub-basement and partly due to a thick section of Eocene limestones of high density, while the magnetic data and the seismic section show a progressive deepening of the crystalline basement towards the sea coast. A pronounced feature brought out by the seismic section is the marked unconformity between the Cuddalore and Eocene formations with a
GRAVITY
400
20,
200
IO’
AND
MAGNETIC
PROFILES
‘.._ / 1----f= 0
-- JggaetiZ_Pr>rire
(ve t,col --_r___--
force:
0,
WEST 0
"4 VRIDDHACHALAM
10
Fig. 1B. Gravity-cum-magnetic
15
79 30’ Ol 2o
profile, Vriddhachalam-Sea
3o 25
35MILES EAST
coast.
512
SEISMIC-CUM-GEOLOGIC VRIDDHACHALAM
Fi
c 2.8 3.0
1
’
(INFERRED) SECTION -PORT0 NOVr). ,
/
___L.L_..--_I_L
Fig. 1C. Seismic-cum-geologic
(inferred)
I section,
Vriddhachalam-
-Porte-Novo
(sea coast).
synclinal base for the Cuddalore formations with a maximum thickness of 650 m in the central parts underlain by Eocene beds having consistently eastward dip, suggesting a possible coastal uplift in these parts during postEocene times. Indications of uplift and subsidence with fault patterns in these coastal parts, which have been subjected to a number of earth tremours in recent years, have been reported by various authors. Gubin (1968) has suggested Quaternary and Tertiary movements in these parts which are apparently still continuing. Grady (1971) has interpreted a number of lineaments as faults due to epeirogenic disturbances in these parts of peninsular India where a number of bodies of alkalic rocks, syenite, ultramafic and carbonatites occur, as in the Koratti area near Tirupattur. Deccan Trap areas of western India The Deccan traps cover an area of more than 600,000 sq. km in western and central India and consist of a series of basaltic lava flows presumed to have been erupted to the surface in the Cretaceous-Tertiary period, blanketing all preexisting rocks from Precambrian to Cretaceous in many parts. The geologically estimated total thickness of the traps ranges from a few metres on the eastern margins to about 1900 m over the Bombay coast, thickness of the individual flows varying from 2 to 100 m. The rock varies from basalt to dolerite and intruding dolerite dykes can be seen at many places. The eastern margin is flanked by Gondwana rocks (Permo-Carboniferous to Jurassic) while the southern side is flanked by rocks of the Kaladgi basin of Cuddapah age and Bhima basin of Vindhyan age (Late Precambrian). The Bouguer gravity map reproduced in Fig. 2A has brought out some
513
prominent gravity ‘highs’ and ‘10~s’. The gravity ‘highs’ in the Nasik and Sangola areas indicate zones of marked uplift while the gravity ‘low’ to the north of Nasik ‘high’ as also the ‘lows’ in the Kurudvadi area, Kaladgi basin and the Koyna region appear to be zones of marked subsidence (Kailasam et al., 1972). The seismic depths computed to the base of the traps by deep refraction seismic soundings indicated in Fig. 2B, generally corroborate this inference from the gravity data, relatively larger thickness of trap being indicated in the zones of subsidence and smaller thickness in the zones of uplift. On the western coast, strongly positive gravity anomalies with a steep gradient bring out the well-known Panvel flexure near Bombay and suggest a deep north-south fault extending all the way to Ratnagiri from the southern proximity of Bombay, parallel to the great off-shore West Coast Fault. A pronounced gravity ‘low’ is indicated in the Karad-Koyna region which experienced a devastating earthquake of magnitude 6.5 on December 10, 1967 with epicentres located all over the neighbourhood with a local intensity of VIII, followed in subsequent years by minor shocks. Chugh (1973) has reported, on the basis of repeated precision levelling and triangulation, that there has been a subsidence of 3 cm in the Koyna region after this earthquake and a possible southeastward horizontal movement of 15 cm in the isoseismal region of highest intensity. The gravity data and the computed seismic depths to the base of traps, as presented in Fig. 2B, indicate possible faults to the west of Koyna in the Pophli area and between Rampur and Guhagar (Fig. 2C) on the coast (Kailasam et al., 1969) in addition to the north-south shear plane brought out by detailed gravity and magnetic surveys near the western bank of the Koyna river, thereby indicating that the tectonic sag in the Koyna area is characterized by a system of faults and shears on its western flank partly at least contributing to the seismicity of this zone. A number of thermal springs also occur in this region (Fig. 2C). The gravity and seismic results thus appear to indicate that the repeated cycles of Deccan volcanic activity have given rise to zones of subsidence with interior basins and uplifts in this negative platform of the Deccan syneclise, lending credence to epeirogenic movements supported by the currently observed vertical movements in the Koyna area. Deep seismic sounding profiles A deep seismic sounding profile from Kavali on the east coast to Udipi on the west coast across the southern part of the peninsular shield and passing through the southern part of the Cuddapah basin (Fig. 3), has been undertaken jointly by a Russian crew and the National Geophysical Research Institute with the personnel of the Geological Survey of India, Oil and Natural Gas Commission and the Seismological Unit of the India Meteorological Department participating under the Indo-Soviet Scientific Collaboration Programme. So far a stretch of roughly 450 km has been shot with the Rus-
‘; 3
Fig. 2A. Ebuguer
t
1:
gravity
i
map of the Deccan
V
Trap areas of Maharashtra
and parts of Mysore
and Andhra.
‘J
DECCAN
TRAP NANDEDe
1,
E
lw
GWIR
3GO
C
E
ZOkES
OF SUBSIDENCE,
3-w
THtRD
ZONES
OF
+%
TREND
THIRD
UPLIFT,
ORDER
STRUCTURES.
260
N
ORDER
FAULTS
FAULTS
PRIMARY
.
t.sI
SOUt@lNt$ HIGH
INFERRED 90:
CDWJECTURIL
FAULTS.
features
of the Deccan
flD
OF THE
INTERFACE
SURFACE,
Fig. 2B, Tectonic results.
PGlRT VtiLOClTY
m
REC
FROM i IN
Trap area as deduced
SERIES
KALADGI
SERIES
SCALE
TRAP? IIIGH
EN1
BHIMA
t fEWtAflVLLY BASE
DEPTH
LOCITY
pyJ
LINES
SGl$HiC 396:R.L.OF
DtSCDNTlNUItY DEEP-SEATED
GEQLOG~CAL FORMATlONS
D
VE-
0 20406DKILOMEfRES
GRDUNDW
MEtRESt
0
I6
32 MILES
from gravity and seismic
517
SCALE 6.4KM
6,4
(1
Contour
IZfKM
lnterval:5Milligals 78,45’
ARABIAN
INDEX -
Grovity
-a52
-+-
contour
R egionoi
t40
X -
Earthquake
Grovity
-105m.gals. Traverse
0 m
Epicentres.
Seismic
6i’O,,, (So,-530,.
Fig.
%I!.
=
soundings
Thermal
springs.
Detailed
gravity
Traverse
at Koyna Dam
---
Approximate limits of Western Ghat section. Computed depth from ground surface to the possible trap base in metree. R.L. of the possible trap base in metres.
Bouguer anomaly map of Koyna area showing locations of gravity and seismic
profiles.
sian DSS Model No. POISK-1-48 KMPV-OV seismic equipment using reflection and refraction seismic techniques. A depth of 5 to 8 km has been indicated to the bottom of the Cuddap~ basin over the initial stretch of the profile so far shot, the depth to the Conrad discontinuity varying from 22 to 25 km and the depth to the Moho varying from 42 to 45 km. The data also indicate a thrust zone over the eastern margin of the Cuddapah basin with evidence of uplifted and down-thrown blocks within the trough. Similar DSS profiles are proposed to be undertaken over the Deccan Trap Region including the Koyna area and in the Him~ayan region and these should help to throw light on the disposition of the crustal blocks which may have a bearing on epeirogenic movements. AREAS SELECTED FOR EPEIROGENIC STUDIES UNDER THE INTERNATIONAL GEODYNAMICS PROJECT
The following five areas have been selected for detailed studies of epeirogenie movements on a quantitative basis under the current International
Geodynamics Project by means of repeated precision levelling together with microgravimetric observations along selected geo-traverses across these areas (Fig. 3).
36’
r
’ r’ ’ ‘\
I ‘~. >.
>
I i
_: /.
jl. .\ i
/.
SKETCH MAP OF INDIA SHOWING AREAS SELECTED FOR QUANTITATIVE STUDIES OF EPEIROGENIC MOVEMENTS. 32’ SCALE
0L_~ 100
_i~L?OKilomatres
,-. \
/
‘i ?. J’
,r
\
B
_
‘.p
1:
Y
A
I
OF
:
BENGAL
‘E-l
Alluvium
ond
Recent
[_jj
Trap
( CretoceouslMesozlc)
m
Gondwonos
m
Vindhyan/Cuddapah(Upp, -Cambrian!
m
Crystallines
(Corboniferous)
(
Pre
Pre-Combrion/Archaean)
T/’ Geotraverse I-P, 33
Fig. 3. Geological sketch epeirogenic movements.
map of India showing
Areas
studiesof
areas selected
selected
epeirogenic
for detailed
for quantitotivc movyments
studies
of
519
Shillong Plateau. Assam, I This Plateau, consisting of the Garo, Khasi and Jaintia Hills, is located in northeastern India. The plateau is believed to have been contiguous with the Rajmahal area in the west until the Mio-Pliocene (Krishnan, 1966) and witnessed plateau basalt volcanism giving rise to the wellknown Sylhet traps erupted along fissures, the northern margin of the Sylhet traps over the plateau being faulted. The subsequent transgression of the sea during Late Cretaceous times caused sedimentation which is seen in the form of alternate conglomerates and sandstones, and again subsequently with poorly bedded arkose formation (Mm-thy, 1971). The southern side of the plateau presents a monoclinal structure which terminates on the south as a fault known as the Dauki fault and it is believed that the plateau was uplifted as well as moved eastward over 250 km along this fault in relation to the East Bengal plains (Krishnan, 1966). The plateau is thus disposed in the form of a horst and vertical movements of the plateau are still apparently going on along the fault planes. Chho tanagpur Plateau, II Area II (Fig. 3) comprising the Chhotanagpur plateau and the Singhbhum-Keonjhar-Mayurbhanj region is characterized by a series of flat to generally undulating planar surfaces at different elevations separated by intervening steep gradients and scarps. Four such planar surfaces with gentle slopes with elevations of 940-1000, 600-700, 230-300 and 130-160 m above sea level, are clearly recognizable, the successive planar surfaces being separated by steep gradients characterized by several waterfalls. Such planar surfaces are observed in the Singhbhum-Keonjhar-Mayurbhanj region also at elevations of 970, 600 and 400, 250 and 160 m, respectively. A series of post-Mesozoic uplifts, with or without faulting, has been postulated to explain these planar faces. It is believed that detectable relative vertical motions between adjacent planar surfaces, as also absolute upward movement of the entire region, are in all probability taking place at the present time. There is also some evidence of rejuvenation along some of the fractures associated with the Copper belt thrust zone of Singhbhum in the Sini-Saraikela region, with rejuvenated streams in V-shaped valleys, raised gravel beds and raised and dissected terraces (Banerji, 1969; Banerji et al., 1970) while the area to the south is seen to retain the features of the earlier geomorphic cycle. Minor tremors have also been frequently felt in the vicinity. The relative vertical movements in this region between Sini and Saraikela are proposed to be measured over a period of years by precision levelling and gravimetric observations to measure these vertical movements and the rate along adjacent planar surfaces and also to see if any fault movement of the crust is involved in’ these movements and whether any regular upward motion is still going on (A.K. Saha, personal communication, 1974).
Aravalli region, III In the Aravalli region, on the two sides of the ‘Delhi-Pre-Delhi’ boundary fault, the topographic features show a greater uparching on the west, sloping down rapidly to the north and south suggesting a continuing movement along the available fault planes. There is also linear zone sub-parallel to the axis along which all the westward flowing streams show cataracts of variable sizes (S.N. Sen, personal communication, 1974). Narmada-Sone
Rift, IV
The Narmada-Sane rift valley is a straight feature extending from Broach on the west coast to Jubbalpore in Madhya Pradesh in the east. Most of this feature passes through the Deccan traps, but near its eastern extremity it is flanked on the south by the Upper Gondwana (Jurassic). Krishnan (1966) mentions that the trough was probably formed during the pre-Gondwana (Hercynian) crustal disturbances followed by Deccan trap inundation. Two suitably selected north-- south traverses are to be conducted across this rift with precision levelling and microgravimetric observations for the determination of possible crustal movements along the fault planes. Deccan trap area and Koyna region, V The measurements that have been in progress in the Koyna earthquake region have already been alluded to. The systematic gravity surveys over the Deccan trap region already described are now being extended into the northern parts. In addition to the measurements of vertical movements in these five selected areas, similar geotraverses are also in progress in the orogen areas of the Himalayan and Sub-Himalayan regions. Besides these, regular sea-level observations over the coastal network of tide-gauge stations are also being continued to study sea-level variations. ACKNOWLEDGEMENTS
The author expresses his sincere thanks to Prof. S.N. Sen, Head of the Department of Geology and Dr. A.K. Saha, Professor of Geology, Presidency College, Calcutta for helpful suggestions and discussions on geological aspects and to the Director-General, Geological Survey of India, for permission to present this paper at the symposium. REFERENCES Bally, A.W., 1972. ICG working Regional Extent.
Group
7 working
paper on Epeiorogenic
Movements
of
521 Banerji, A.K., 1969. A re-interpretation of the geological history of Singhbhum shear zone. J. Geol. Sot. India, 10: 49-55. Banerji, A.K., Mitra, SK. and Mukhopadhyaya, S.C., 1970. Tectonic sequence in the Sini-Saraikela region, Singhbhum district. Q. J. Geol. Min. Metall. Sot. India., 42: 141-149. Chugh, R.S., 1973. Survey of India. Unpublished status report on work on Geodynamics Project. Grady, C., 1971. Deep main faults in south India. J. Geol. Sot. India., 12: 56-62. Gubin, I.E., 1968. Seismic zoning of Indian peninsula. Bull. Int. Inst. Seismol. Earthquake Eng., 5: 109-139. Hallam, A., 1953. Major epeiorogenic and eustatic changes since the Cretaceous. Am. J. Sci., 261: 397-423. Holmes, A., 1955. Dating the Precambrian of Peninsular India and Ceylon. Proc. Geol. Sot. Can. 1: 81-106. Heron, A.M., 1953. Geology of Central Rajaputana. Mem. Geol. India, 78: 389. Kailasam, L.N., 1968. Some results of geophysical exploration over the Cretaceous-Tertiary formations of South India. Geol. Sot. India Mem., 2: 178-195. Kailasam, L.N., Pant, P.R. and Lahiri, S.M., 1969. Seismic investigations in the earthquake affected areas of Koyna and neighbourhood, Satara district, Maharashtra. Mem. Geol. Surv. India, 100 : 123--126. Kailasam, L.N., Murty, B.G.K. and Chayanulu, A.Y.S.R., 1972. Regional gravity surveys of the Deccan Trap areas of Peninsular India. Curr. Sci. India., 41: 403-407. Krishnan, M.S., 1960. Geology of India and Burma. Higginbothams, Madras. Krishnan, M.S., 1966., Tectonics of India. Bull. Ind. Geophys. Union., 3: l-37. Lee, W.H.K. and Raleigh, C.B., 1969. Fault-plane solution of the Koyna (India) Earthquake. Nature, 223: 172-173. Murthy, M.V.N., 1971., Tectonic history of the Deccan and Shillong Plateaus in relation to their seismicity. Ind. Power and Riv. Val. Dev. Spec. Nr., 31-37. Radhakrishna, B.P., 1968. Geomorphological approach to the charnockite problem. J. Geol. Sot. India., 9: 67-74. Sen, D., 1972. Geomorphology of the Aravalli Range. Geogr. Rev., 34: 262-286. Vaidyanathan, R., 1964., Geomorphology of the Cuddapah Basin. J. Indian Geosci. Assoc., 4: 29-36.
EPEIROGENIC STUDIES IN INDIA WITH REFERENCE VERTICAL MOVEMENTS
505
TO RECENT
L.N. KAILASAM Geophysics Division, Geological Survey of India, National Committee for Geodynamics (ICG), Calcutta (India) (Revised version accepted April 17, 1975) ABSTRACT Kailasam, L.N., 1975. Epeirogenic studies in India with reference to recent vertical movements. In: N. Pavoni and R. Green (Editors), Recent Crustal Movements. Tectonophysics, 29 (l-4): 505-521. Epeirogenic movements of regional extent have been manifest and recorded in several parts of India from Late Proterozoic to present times through the Tertiary and Recent. The main features of these movements, especially Recent vertical movements, in typical areas in the platform and coastal areas of peninsular India are described and the salient results of the study, qualitative and quantitative, of these zones are outlined. Quantitative measurements of Recent vertical movements have been made largely by geodetic methods and precision levelling in the plateau areas as also in the mobile, orogen zones of the Himalayan and sub-Hi~layan tracts and by sea-level measurements through an extensive network of coastal automatic tide-gauge stations. The annual variations of sea ievel have been studied for several Indian ports. The land uplift or subsidence in relation to mean sea level over the Indian coast has also been studied. The application of geophysical techniques in the identification and delineation of regions of epeirogenic movements has been discussed and illustrated by the results in two type areas, viz., the coastal sedimentary belt of Madras and the vast Deccan Trap territory in western India. In the latter area, blanketed by extensive basaltic lavas over a vast region, the gravity data have brought out a number of major zones of subsidence and uplift which have been corroborated by the results of refraction seismic soundings. One of these major zones of subsidence is in the Koyna region which witnessed a major disastrous earthquake a few years ago. The results of deep seismic sounding along a traverse across the southern part of the Indian peninsula now in progress have also been briefly mentioned. Five important areas have been selected for quantitative measurements of current epeirogenic movements, namely the Shillong Plateau in northeast India, the Chhotanagpur Plateau and Singhbhum region in Bihar, the Aravalli region of Rajasthan, the Narmada-Sone Rift in central India and the Deccan Trap territory including the Koyna earthquake zone. INTRODUCTION
The study of epeirogenic movements of regional extent involves mainly the forces causing non-erogenic vertical movements which apparently are not
506
always clearly understood. This study requires the acquisition of information on the lithospheric materials and processes involved in the subsidence and uplift of major basins and plateaus, both above and below mean sea level, and it has been appropriately suggested that integrated surveys be conducted in sensitive areas for detailed sedimentological, stratigraphic and structural analyses, gravity, magnetic, heat flow and seismic refraction and reflection surveys as also repeated precision geodetic levelling. One of the major objectives of this programme on studies of epeirogenic movements under the Inflations Geodynamics Project is the acquisition of data on duration, wavelength, amplitudes and rates of vertical movements, especially rapid movements. As has been well realized and appropriately stated, vertical movements cannot be studied in isolation from horizontal movements nor can epeirogenic studies be conducted to the exclusion of erogenic movements, especially in the mobile zones of orogens where large vertical movements occur as in the recently active areas such as the ~im~ayas in India. The study of vertical movements of regional extent, therefore, involves both the stable areas such as platform areas as well as the erogenic areas. In this paper, the present knowledge of epeirogenic movements in the Indian sub-continent and work done with special reference to recent movements is outlined and the areas selected for detailed study and measurement of current movements are described, as well as the results of application of geophysical techniques in some type areas. GENERAL
ASPECTS
OF ~PEIROG~NIC
STUDfES
The Inter-Union commission on Geodynamics, through its Working Group 7, has laid down broadly the lines on which studies on epeirogenic movements of large regional extent may be undertaken on a global scale under the current International Geodynamics Project. The study of these movement will have to be made through the older as well as the later parts of the time scale through Pleistocene and Recent periods. It will obviously be necessary to separate out the secular trends to gain precise knowledge of the recent movements, especially on a quantitative basis, Large-scale vertical crustal movements have occurred extensively over the world during the Tertiary period accompanied by worldwide regressions, especially over nonerogenic shelves, warping and uplift of peneplains, deformation of strand lines and other related factors. Recent vertical movements, especiaily those which have occurred over the past hundred years or so, are of particular current interest. The first stage in epeirogenic investigations will involve the identification and rough delineation of sensitive areas of massive regional uplift and subsidence to facilitate which geophysical and geological techniques including suitably selected, extended geotraverses can be employed with advantage, The vertical displacements can be grouped under a number of categories such as cratonic deformation; geosynclinai deformation including uplifts and subsidence in the abyssal plains of the oceans and subsidence
507
of ancient ortho-geosynclines; deformation associated with uplift and rifting, such as the intra-cratonic uplifts and rifts as in the case of the Rheintalgraben and oceanic uplifts and rifting as evidenced by mid-oceanic ridges; morphorogenic subsidence and uplifting; subsidence and uplifting not associated with geotectonic causes such as that due to loading and unloading of water bodies and glacial loading and unloading, as also due to sedimentation and erosion; subsidence of foredeep areas, etc. (Bally, 1972). Studies of vertical measurements may be conducted both on a qualitative and quantitative basis. As absolute quantitative measurements are difficult for the simple reason that we cannot have a firm, reliable sea-level reference, we have to rely largely on measurements of relative vertical displacements. Precise measurements of recent vertical movements in areas identified for epeirogenic movements as indicated earlier will by and large consist of: (1) observations on variation of sea levels as also water level in inland lakes by means of tide and water-level gauges, including tiltmeters for measurements of changes in tilt; (2) repeated precision levelling through geodetic networks; and (3) micro-gravimetric traverses along selected geo-traverses across type areas. Mean sea-level changes, however, are the resultant effect of a variety of causes involving worldwide isostatic and eustatic changes through Phanerozoic times, in addition to changes produced by vertical movements over land and these have necessarily to be resolved. However, continuous measurements of sea and water levels combined with geodetic and precision levelling afford some of the effective means of measuring current vertical movements. EPEIROGENIC
MOVEMENTS
IN INDIA
There is evidence for vertical movements of epeirogenic type in India from Late Proterozoic down to the present times through the Tertiary and Recent. The predominant evidence for post-Tertiary to Recent movements is in the Precambrian terrains and some of the coastal tracts besides the great Himalayan belt. Very late vertical movements have been reported from different parts of the Precambrian shield, from the Singhbhum region in eastern India, the Assam Plateau in northeastern India (Krishnan, 1966; Murthy, 1971) and the Rajasthan Precambrian belt in northwestern India (Heron, 1953), the charnockite terrain of South India (Radhakrishna, 1968) and various parts of the Eastern Ghat belt. Although the Himalayan erogenic movements may now be considered to be nearly over, the post-erogenic movements will apparently continue for quite some time before completely quiescent conditions can be expected to prevail and these vertical movements in the Himalayas are probably still taking place (Krishnan, 1966). There are several instances of Recent tectonism in India in platform areas also with a considerable vertical component in the recorded movements. The vertical movements in the platform areas are quite conceivably of nonerogenic type of ‘deep-seated origin, giving surface geomorphic expressions.
508
These are also manifest in a large measure in the gravity anomaly maps of peninsular India. The manifest geologic evidence for these Recent vertical movements includes sea-level changes as a result of crustal movements over the continent and oceans, resulting in marine transgression and regressions. Uplifted and warped erosion surfaces can also be seen as downwarped inland basins associated with block faulting filled with continental sediments, coastal terraces with marine sediments, river terraces and water falls. This and other evidence in the Precambrian shield may perhaps be attributed to a continuing rise of the continental lithospheric block since the close of the Cretaceous, as has been indicated by Holmes (1955) and Hallam (1953). Some typical examples of changes along the coast, both emergence and sub-emergence, in the Recent may be cited. Parts of the Kathiawar coast, the Rann of Kutch and the eastern coast of South India show distinctive evidence of emergence. Raised beaches comprising Pleistocene and Recent grits and clays are seen in the Tirunelveli and Ramnad districts in the extreme south of peninsular India. There is evidence that several places which were on the sea some centuries ago are now situated several kilometers inland, such as Coringa near the mouth of the Godavari, Kaveripatnam in the Cauveri delta and Korkai on the Tirunelveli coast which were sea ports a couple of thousand years ago. Submerged forests have been noticed in the Valinokkam Bay on the Tirunelveli coast and off the Princes Dock in Bombay. Large parts of the Gulf of Mannar and Palk Strait which constitute shallow seas are presumed to have been submerged only during Recent geologic times. The former town of Mahabalipuram near Madras was similarly submerged several metres below the sea. The Arakan coast and the Andaman and Nicobar Island groups are known to have undergone submergence in the Pleistocene and Recent times (Krishnan, 1960). Vaidhyanathan (1964) has described Recent vertical movements in the Srisailam gorge in the Krishna valley in Andhra Pradesh, with a vertical depth of over 250 metres and cutting into the Srisailam quartzites. He has also mentioned evidence of recent vertical movements in the form of a raised river terrace near Katarapalle in the Cuddapah basin. Measurements
of Recent
vertical movements
So far the quantitative measurements of Recent vertical movements undertaken in India comprise measurements of sea level and geodetic triangulation and precision levelling conducted by the Geodetic and Research Branch of the Survey of India. Mean sea-level measurements are recorded by a network of automatic coastal tide gauges and these observations had been initiated in 1806 and continued since then with a progressively increasing network of tide-gauge stations. Several of these tidal stations have been connected by transcontinental lines of spirit levelling with highest precision. The annual variations of the sea level in Indian waters have been studied for various ports such as Cochin, Visakhapatnam, Madras, Port Blair in the
Andamans, Colombo in Sri Lanka etc., (Chugh, 1973). Chugh reports that the secular variations of the sea level at Visakhapatnam, Madras, Port Blair as also at Aden, Karachi and Bombay show a general rise and are generally of the order of 0.1 m per 100 years. The amplitudes and phase lags worked out for the 19-yearly tide after removing the secular variation are observed to be different from zero, being,thus not in accord with equilibrium tide. The variations in amplitudes have also been observed to be large. The land uplift or subsidence in relation to mean sea level over the Indian coast line has also been studied. It has been observed that generally no measurable uplift or subsidence has been indicated except local subsidence at Cochin harbour of 0.1 m from 1890 to 1954 and at Pamban pass of 0.1 m from 1880 to 1955. The stability of the coastal region of Calcutta has also been under study (Chugh, 1973). Geodetic triangulation chains cover the entire country, form rectangles of roughly 150-200 km by 250-600 km and these operations have been conducted since 1802. Geodetic triangulation for crustal movements has been carried out at a number of places in the nonerogenic zones for the study of crustal movements. Geodetic triangulation studies of earthquake movements in the Assam area since the great Assam Earthquake of 1897 to Recent times suggest a possible movement southward by 2 or 3 m of the central area north of Shillong relative to the undisturbed country 65 km to the east or west. Similarly, the revisionary triangulation for Koyna Dam has shown a more or less easterly movement of land varying from roughly 1 m on the coast to about 4.5 m at Koyna Dam. Geodetic levelling by the Survey of India has been in vogue since 1875 and the lines of these levels embrace the chain of tidal observations established at suitable sites along the coast of India and Burma, the mean sea-level data furnished by these tidal observations serving as a basis for a network of lines of levelling of precision over the whole country. Besides these, precision to high-precision levelling lines have been established in a number of areas over the entire country extending from the Himalayas southwards. The results of such precision levelling at the Pamban Pass, Cochin and near Calcutta have already been alluded to. The changes in levels in the Koyna region after the December 1967 earthquake are of the order of 2-3 cm of subsidence (Chugh, 1973). Application basins
of geophysical
techniques
in the study of zones of uplifts and
The scope of application and usefulness of geophysical exploration niques are illustrated in two type areas in the Indian peninsula.
tech-
Coastal sedimentary belt of Tamil Nadu The peninsular shield of South India is flanked on its eastern coast by sedimentary beds of Cretaceous and Tertiary age laid down by marine Cenomanian transgressions with isolated occurrences of Upper Gondwana (Middle
w
.--.___ ~---__.-__
,;.B
--?y---------r~-
71’;
o----.-__A-____
8
l.____
‘43’
..q_
----T----o00
I
BOUGUER GRAVITY COASTAL
AREAS
ANOMALY
MAP
OF
THE
,
OF SOUTH ARCOT, TRICHINOPOLY
AND TANJORE
DISTRICTS,
MADRAS.
+
*
+ * ‘
(
:
0
**
“0
.
’
J.‘UIIIN
+’
*
SCI\LE
GEOLOGICAL
+
INDEX
A Fig. 1A. Bouguer tricts. Madras.
gravity
anomaly
map of South
Arcot,
Trichinopoly
and Tanjore
dis-
511
Jurassic) beds between the Archaean and the Cretaceous formations. Systematic geophysical investigations conducted by the Geological Survey of India over 20,000 sq. km of this coastal tract helped in bringing out the bottom configuration and structural features of this tract, indicating three separate basins in the Tamil Nadu coast south of Madras, separated by buried ridges of the Precambrian crystallines and a thickness of the order of 3000 m in the deepest parts of the basins, besides indicating important structural features over the basement having potentialities for oil (Kailasam, 1968). The Bouguer gravity map of this coastal sedimentary belt is presented in Fig. 1A indicating also the main geologic features. In the northernmost South Arcot basin of this belt, the Cuddalore (MioPliocene) formations, consisting of ferrugenious sandstones, grits or clays, are exposed in the central parts flanked by Cretaceous shales, sandstones and limestones on the west and the Middle Jurassic Gondwana rocks on the western fringe of the basin. The Cuddalore formations enclose the wellknown lignite beds of Neyveli. Eocene limestones have been encountered at lower levels in bore holes subsequently drilled in the area. A typical east-west gravity-cum-magnetic profile along a geotraverse across the South Arcot basin from Vriddhachalam on the west to the eastern sea coast and a reflection seismic time-depth section along this traverse are reproduced in Figs. 1B and 1C. A pronounced flexure or faulting of the crystalline basement is indicated on the western margin by the gravity-cummagnetic profile as well as the seismic section. Near the sea coast, however, the gravity data show anomalously high values, apparently due to a combination of effects, partly sub-basement and partly due to a thick section of Eocene limestones of high density, while the magnetic data and the seismic section show a progressive deepening of the crystalline basement towards the sea coast. A pronounced feature brought out by the seismic section is the marked unconformity between the Cuddalore and Eocene formations with a
GRAVITY
400
20,
200
IO’
AND
MAGNETIC
PROFILES
‘.._ / 1----f= 0
-- JggaetiZ_Pr>rire
(ve t,col --_r___--
force:
0,
WEST 0
"4 VRIDDHACHALAM
10
Fig. 1B. Gravity-cum-magnetic
15
79 30’ Ol 2o
profile, Vriddhachalam-Sea
3o 25
35MILES EAST
coast.
512
SEISMIC-CUM-GEOLOGIC VRIDDHACHALAM
Fi
c 2.8 3.0
1
’
(INFERRED) SECTION -PORT0 NOVr). ,
/
___L.L_..--_I_L
Fig. 1C. Seismic-cum-geologic
(inferred)
I section,
Vriddhachalam-
-Porte-Novo
(sea coast).
synclinal base for the Cuddalore formations with a maximum thickness of 650 m in the central parts underlain by Eocene beds having consistently eastward dip, suggesting a possible coastal uplift in these parts during postEocene times. Indications of uplift and subsidence with fault patterns in these coastal parts, which have been subjected to a number of earth tremours in recent years, have been reported by various authors. Gubin (1968) has suggested Quaternary and Tertiary movements in these parts which are apparently still continuing. Grady (1971) has interpreted a number of lineaments as faults due to epeirogenic disturbances in these parts of peninsular India where a number of bodies of alkalic rocks, syenite, ultramafic and carbonatites occur, as in the Koratti area near Tirupattur. Deccan Trap areas of western India The Deccan traps cover an area of more than 600,000 sq. km in western and central India and consist of a series of basaltic lava flows presumed to have been erupted to the surface in the Cretaceous-Tertiary period, blanketing all preexisting rocks from Precambrian to Cretaceous in many parts. The geologically estimated total thickness of the traps ranges from a few metres on the eastern margins to about 1900 m over the Bombay coast, thickness of the individual flows varying from 2 to 100 m. The rock varies from basalt to dolerite and intruding dolerite dykes can be seen at many places. The eastern margin is flanked by Gondwana rocks (Permo-Carboniferous to Jurassic) while the southern side is flanked by rocks of the Kaladgi basin of Cuddapah age and Bhima basin of Vindhyan age (Late Precambrian). The Bouguer gravity map reproduced in Fig. 2A has brought out some
513
prominent gravity ‘highs’ and ‘10~s’. The gravity ‘highs’ in the Nasik and Sangola areas indicate zones of marked uplift while the gravity ‘low’ to the north of Nasik ‘high’ as also the ‘lows’ in the Kurudvadi area, Kaladgi basin and the Koyna region appear to be zones of marked subsidence (Kailasam et al., 1972). The seismic depths computed to the base of the traps by deep refraction seismic soundings indicated in Fig. 2B, generally corroborate this inference from the gravity data, relatively larger thickness of trap being indicated in the zones of subsidence and smaller thickness in the zones of uplift. On the western coast, strongly positive gravity anomalies with a steep gradient bring out the well-known Panvel flexure near Bombay and suggest a deep north-south fault extending all the way to Ratnagiri from the southern proximity of Bombay, parallel to the great off-shore West Coast Fault. A pronounced gravity ‘low’ is indicated in the Karad-Koyna region which experienced a devastating earthquake of magnitude 6.5 on December 10, 1967 with epicentres located all over the neighbourhood with a local intensity of VIII, followed in subsequent years by minor shocks. Chugh (1973) has reported, on the basis of repeated precision levelling and triangulation, that there has been a subsidence of 3 cm in the Koyna region after this earthquake and a possible southeastward horizontal movement of 15 cm in the isoseismal region of highest intensity. The gravity data and the computed seismic depths to the base of traps, as presented in Fig. 2B, indicate possible faults to the west of Koyna in the Pophli area and between Rampur and Guhagar (Fig. 2C) on the coast (Kailasam et al., 1969) in addition to the north-south shear plane brought out by detailed gravity and magnetic surveys near the western bank of the Koyna river, thereby indicating that the tectonic sag in the Koyna area is characterized by a system of faults and shears on its western flank partly at least contributing to the seismicity of this zone. A number of thermal springs also occur in this region (Fig. 2C). The gravity and seismic results thus appear to indicate that the repeated cycles of Deccan volcanic activity have given rise to zones of subsidence with interior basins and uplifts in this negative platform of the Deccan syneclise, lending credence to epeirogenic movements supported by the currently observed vertical movements in the Koyna area. Deep seismic sounding profiles A deep seismic sounding profile from Kavali on the east coast to Udipi on the west coast across the southern part of the peninsular shield and passing through the southern part of the Cuddapah basin (Fig. 3), has been undertaken jointly by a Russian crew and the National Geophysical Research Institute with the personnel of the Geological Survey of India, Oil and Natural Gas Commission and the Seismological Unit of the India Meteorological Department participating under the Indo-Soviet Scientific Collaboration Programme. So far a stretch of roughly 450 km has been shot with the Rus-
‘; 3
Fig. 2A. Ebuguer
t
1:
gravity
i
map of the Deccan
V
Trap areas of Maharashtra
and parts of Mysore
and Andhra.
‘J
DECCAN
TRAP NANDEDe
1,
E
lw
GWIR
3GO
C
E
ZOkES
OF SUBSIDENCE,
3-w
THtRD
ZONES
OF
+%
TREND
THIRD
UPLIFT,
ORDER
STRUCTURES.
260
N
ORDER
FAULTS
FAULTS
PRIMARY
.
t.sI
SOUt@lNt$ HIGH
INFERRED 90:
CDWJECTURIL
FAULTS.
features
of the Deccan
flD
OF THE
INTERFACE
SURFACE,
Fig. 2B, Tectonic results.
PGlRT VtiLOClTY
m
REC
FROM i IN
Trap area as deduced
SERIES
KALADGI
SERIES
SCALE
TRAP? IIIGH
EN1
BHIMA
t fEWtAflVLLY BASE
DEPTH
LOCITY
pyJ
LINES
SGl$HiC 396:R.L.OF
DtSCDNTlNUItY DEEP-SEATED
GEQLOG~CAL FORMATlONS
D
VE-
0 20406DKILOMEfRES
GRDUNDW
MEtRESt
0
I6
32 MILES
from gravity and seismic
517
SCALE 6.4KM
6,4
(1
Contour
IZfKM
lnterval:5Milligals 78,45’
ARABIAN
INDEX -
Grovity
-a52
-+-
contour
R egionoi
t40
X -
Earthquake
Grovity
-105m.gals. Traverse
0 m
Epicentres.
Seismic
6i’O,,, (So,-530,.
Fig.
%I!.
=
soundings
Thermal
springs.
Detailed
gravity
Traverse
at Koyna Dam
---
Approximate limits of Western Ghat section. Computed depth from ground surface to the possible trap base in metree. R.L. of the possible trap base in metres.
Bouguer anomaly map of Koyna area showing locations of gravity and seismic
profiles.
sian DSS Model No. POISK-1-48 KMPV-OV seismic equipment using reflection and refraction seismic techniques. A depth of 5 to 8 km has been indicated to the bottom of the Cuddap~ basin over the initial stretch of the profile so far shot, the depth to the Conrad discontinuity varying from 22 to 25 km and the depth to the Moho varying from 42 to 45 km. The data also indicate a thrust zone over the eastern margin of the Cuddapah basin with evidence of uplifted and down-thrown blocks within the trough. Similar DSS profiles are proposed to be undertaken over the Deccan Trap Region including the Koyna area and in the Him~ayan region and these should help to throw light on the disposition of the crustal blocks which may have a bearing on epeirogenic movements. AREAS SELECTED FOR EPEIROGENIC STUDIES UNDER THE INTERNATIONAL GEODYNAMICS PROJECT
The following five areas have been selected for detailed studies of epeirogenie movements on a quantitative basis under the current International
Geodynamics Project by means of repeated precision levelling together with microgravimetric observations along selected geo-traverses across these areas (Fig. 3).
36’
r
’ r’ ’ ‘\
I ‘~. >.
>
I i
_: /.
jl. .\ i
/.
SKETCH MAP OF INDIA SHOWING AREAS SELECTED FOR QUANTITATIVE STUDIES OF EPEIROGENIC MOVEMENTS. 32’ SCALE
0L_~ 100
_i~L?OKilomatres
,-. \
/
‘i ?. J’
,r
\
B
_
‘.p
1:
Y
A
I
OF
:
BENGAL
‘E-l
Alluvium
ond
Recent
[_jj
Trap
( CretoceouslMesozlc)
m
Gondwonos
m
Vindhyan/Cuddapah(Upp, -Cambrian!
m
Crystallines
(Corboniferous)
(
Pre
Pre-Combrion/Archaean)
T/’ Geotraverse I-P, 33
Fig. 3. Geological sketch epeirogenic movements.
map of India showing
Areas
studiesof
areas selected
selected
epeirogenic
for detailed
for quantitotivc movyments
studies
of
519
Shillong Plateau. Assam, I This Plateau, consisting of the Garo, Khasi and Jaintia Hills, is located in northeastern India. The plateau is believed to have been contiguous with the Rajmahal area in the west until the Mio-Pliocene (Krishnan, 1966) and witnessed plateau basalt volcanism giving rise to the wellknown Sylhet traps erupted along fissures, the northern margin of the Sylhet traps over the plateau being faulted. The subsequent transgression of the sea during Late Cretaceous times caused sedimentation which is seen in the form of alternate conglomerates and sandstones, and again subsequently with poorly bedded arkose formation (Mm-thy, 1971). The southern side of the plateau presents a monoclinal structure which terminates on the south as a fault known as the Dauki fault and it is believed that the plateau was uplifted as well as moved eastward over 250 km along this fault in relation to the East Bengal plains (Krishnan, 1966). The plateau is thus disposed in the form of a horst and vertical movements of the plateau are still apparently going on along the fault planes. Chho tanagpur Plateau, II Area II (Fig. 3) comprising the Chhotanagpur plateau and the Singhbhum-Keonjhar-Mayurbhanj region is characterized by a series of flat to generally undulating planar surfaces at different elevations separated by intervening steep gradients and scarps. Four such planar surfaces with gentle slopes with elevations of 940-1000, 600-700, 230-300 and 130-160 m above sea level, are clearly recognizable, the successive planar surfaces being separated by steep gradients characterized by several waterfalls. Such planar surfaces are observed in the Singhbhum-Keonjhar-Mayurbhanj region also at elevations of 970, 600 and 400, 250 and 160 m, respectively. A series of post-Mesozoic uplifts, with or without faulting, has been postulated to explain these planar faces. It is believed that detectable relative vertical motions between adjacent planar surfaces, as also absolute upward movement of the entire region, are in all probability taking place at the present time. There is also some evidence of rejuvenation along some of the fractures associated with the Copper belt thrust zone of Singhbhum in the Sini-Saraikela region, with rejuvenated streams in V-shaped valleys, raised gravel beds and raised and dissected terraces (Banerji, 1969; Banerji et al., 1970) while the area to the south is seen to retain the features of the earlier geomorphic cycle. Minor tremors have also been frequently felt in the vicinity. The relative vertical movements in this region between Sini and Saraikela are proposed to be measured over a period of years by precision levelling and gravimetric observations to measure these vertical movements and the rate along adjacent planar surfaces and also to see if any fault movement of the crust is involved in’ these movements and whether any regular upward motion is still going on (A.K. Saha, personal communication, 1974).
Aravalli region, III In the Aravalli region, on the two sides of the ‘Delhi-Pre-Delhi’ boundary fault, the topographic features show a greater uparching on the west, sloping down rapidly to the north and south suggesting a continuing movement along the available fault planes. There is also linear zone sub-parallel to the axis along which all the westward flowing streams show cataracts of variable sizes (S.N. Sen, personal communication, 1974). Narmada-Sone
Rift, IV
The Narmada-Sane rift valley is a straight feature extending from Broach on the west coast to Jubbalpore in Madhya Pradesh in the east. Most of this feature passes through the Deccan traps, but near its eastern extremity it is flanked on the south by the Upper Gondwana (Jurassic). Krishnan (1966) mentions that the trough was probably formed during the pre-Gondwana (Hercynian) crustal disturbances followed by Deccan trap inundation. Two suitably selected north-- south traverses are to be conducted across this rift with precision levelling and microgravimetric observations for the determination of possible crustal movements along the fault planes. Deccan trap area and Koyna region, V The measurements that have been in progress in the Koyna earthquake region have already been alluded to. The systematic gravity surveys over the Deccan trap region already described are now being extended into the northern parts. In addition to the measurements of vertical movements in these five selected areas, similar geotraverses are also in progress in the orogen areas of the Himalayan and Sub-Himalayan regions. Besides these, regular sea-level observations over the coastal network of tide-gauge stations are also being continued to study sea-level variations. ACKNOWLEDGEMENTS
The author expresses his sincere thanks to Prof. S.N. Sen, Head of the Department of Geology and Dr. A.K. Saha, Professor of Geology, Presidency College, Calcutta for helpful suggestions and discussions on geological aspects and to the Director-General, Geological Survey of India, for permission to present this paper at the symposium. REFERENCES Bally, A.W., 1972. ICG working Regional Extent.
Group
7 working
paper on Epeiorogenic
Movements
of
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