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Chapter II
GEOLO GY, G EO -TECTO NICS AND EARTHQUAKE
HISTORY OF THE REGION
2.1 Geographical Outline
The
Indo-Burma
(Myanmar)
Orogenic
Belt
has
a fascinating
diversified landscape having valleys, razor-edged high hills, meandering
rivers and flat high lands. An overall picture of the region can be obtained
from the satellite imagery (Figure 2.1). The hill ranges comprising of LusaiPatkai-Naga-Manipur-Chin Hills and Arakan-Yoma form a westerly convex
arcuate mobile belt, which was originated during the early-mid Eocene
period due to the northeastward drift of Indian continent and its collision with
the Shan-Tenasserim block of the Asian Landmass. It occurs in close
juxtaposition along the E-W Noah Dihing Valley of Arunachal Pradesh in
India and Upper Hukuang valley, Myanmar Geographically, this belt with
hilly topography is the boarder region between India and Myanmar and is
bounded by 93° and 98° East longitude, and 20° and 28° North latitudes as
shown in Figure 2 2 It comprises the states of Mizoram, Manipur, Nagaland
and parts of Arunachal Pradesh in Indian part, and the adjoining hilly tracks
of Western Myanmar The important urban centers belongs to this belt and
very near to it are Tezu, Golaghat, Diphu, Kohima, Imphal, Silchar and
Aizwal in Indian side, and Putao, lawa, Lahe, Lanton, Kabia, Haka and Saw
in western Myanmar Being a hilly region, the population density is very
sparse in the region except in the urban centers.
Fig. 2.1 : Satellite imagery of Northeast India including the study reg"
(The study region is demarcated by the rectangular box)
2.2 Geology
The Indo-Burma orogenic belt ranges consist of the Arakan Yorr
Chin Hills and the Naga Hills. Geology of the region is oresente
Figure 2 3 The Arakan-Yoma Segment is narrow (300 km wide,
segment ophiolites are absent but exotic blocks are present i
flysch and in Miocene Strata The Chin Hills are wide (about 700 km wid
where ophiolites are also absent but exotic blocks are present in flvsc
unknown in Miocene Strata On the other hand the Naga Hills are
(about 200 km width) and the rocks mostly contain ophiolites but the
°3
L O C A T IO N M A P O F T H E S T U D Y A R E A
92*CTCrF
96"0TrE
94‘ 0‘CTE
Fig. 2.2 : Location map of the study area
blocks are absent from the flysch
The Indo-Burman ranges consist of roughly parallel, north-'
trending folds implying east-west shortening
with the orientatio
shortening becoming NW-SE in the Naga Hills The western edge Indo-Burman ranges is bounded by thrust faults along the Naga hills 11
24
Fig. 2.3: Geological map of study region
north and folds and thrusts in the Bengal basin (Le Dain et al
1984: Nan
2001) which is considered as the present plate boundary (Mitchel '
This boundary is generally known as the Burma Arc There are
thrust faults sub-parallel to the ranges west of this boundary
northernmost extension, the indo-Burman ranges merge with the west
trending Himalayas in a complex structural zone, called the Eastern
Himalayan syntaxis,-The indo-Burman ranges can be divided into two
parallel belts - (a) the western belt consists of flysch type sediments, largely
of early Eocene age, all folded and thrust and (b) the eastern belt consists
largely of much older rocks (Mitchell, 1993). Uplift of the Indo-Burman
ranges probably began in the late Eocene or early Oligocene (Mitchell,
1993) The folding must have been active until recent times, because PlioPleistocene beds in the Surma Basin (Sylhet trough) are affected (Le Dain et
al., 1984; Nandy, 2001) Within the Burmese basin, the back-arc trough
(east of the magmatic arc) is underlain by mid- to late-Tertiary non-marine
sediments and the fore-arc trough (west of the magmatic arc) is occupied by
a thick Cretaceous to Quaternary marine and non-marine fore-arc basin
sequence (Mitchell, 1993)
The central Burma basin is divided into several sub basins along its
nearly 1100 km length (Bender, 1983). Folding can be observed in many
places within the Neogene’s sediments of the basin; as in the Indo-Burman
ranges, the folds curve from northwest in the south to north-northeast in the
north, but folding is much gentler in the lowlands (Le Dain et al., 1984). In a
study of structural data in the Slam sub basin, part of the larger Burmese
Basin, Pivnik et al. (1998) interpret an echelon pattern of the fold axes and
reverse faults to be consistent with models of deformation in fore-arc sliver
terrane bounded by an oblique subduction zone and trench-parallel strikeslip faults (Jarrard, 1996). Two major faults recognized in the Indo-Burma
region are the Kabaw and the Sagaing faults. The Kabaw fault (also called
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the Eastern Boundary Thrust (EBT) by Nandy, 1980) forms a major tectonic
break between the Indo-Burman ranges and the Burmese Basin and, it
continued to the south, joins the West Andaman fault. Hla Maung, (1987)
postulates transcurrent movement on this fault initiated before Miocene and
preceding the right-lateral movement along the Sagaing fault However, no
geologic or geophysical evidence was reported to indicate any transcurrent
movement along the Kabaw fault (Khin Zaw, 1990). The Sagaing Fault forms
a major tectonic break between the Burmese Basin and the Eastern
Highlands of Burma and passes southward into the Andaman Sea rift
system. This transcurrent fault extends for about 1000 km and, on the basis
of geological and geophysical investigations in the Andaman Sea, is
estimated to have accumulated a total right lateral displacement of about
460 km probably since Miocene (Currary et al., 1979). However, the
magnitude of movement on this fault is disputed. Khin Zaw (1990) assumes
a value of 250 km for dextral movement along the Sagaing Fault since postLower Miocene based on investigations in the region along the fault.
According to Khin Zaw (1990), the Sagaing Fault was probably active as
early as the Mesozoic and/or Lower Tertiary with a significant vertical
movement and was reactivated in Upper Tertiary (Mid-Miocene) to relatively
recent time with a distinctive strike-slip component, in a recent study
Bertrand et al. (1998) proposed a velocity of 10-23 mm/yr. along the
Sagaing Fault. Thus the transcurrent movement on the Sagaing Fault
existed through mid-Miocene to Quaternary period contemporaneous with
the eastward subduction and volcanic activity.
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In the magmatic arc, that divides the Burmese Basin into back-arc
and fore-arc troughs, the oldest dated plutons are mid-Cretaceous and the
youngest volcanic rocks are Quaternary (Mitchell, 1993). Late NeogeneQuaternary rocks include the Mount Popa volcano and further north the
extinct stratovolcano of Taungthonton. The Mount Popa volcanic center
divides the basin into fore-arc (west) and back-arc (east) components
(Bender, 1983) The volcanic arc lies approximately above and parallel to
the 160 km contour of the Wadati-Benioff zone delineated by Ni et al. (1989)
Such a configuration between the andesitic volcanoes and the underlying
slab is not unusual.
The Palaeocene rocks in upper Assam, Nagaland and Manipur are
represented by Disang and Barail Groups (Mallet, 1876) To the east, all
along this belt, the Disang Group of rocks are overrided by many isolated
blocks of Cretaceous limestone, dismembered ophiolites, metamorphic
rocks and exotic flysch along many imbricate thrust sheets. The upper unit of
the Disang probably corresponds to the Syllet limestone and Kapili formation
(Evan, 1932).
From the closed examination of geological descriptions of broad ‘S'
shaped Indo-Burma mobile belt extends right from Patkoi range in the north
to the Central Burma in the south (Clegg, 1938; Cotter et al, 1938;
Brunnchweiler, 1966 and Bender, 1983).
The
possible
mechanism
for
the generation
and
subsequent
emplacement of Indo-Burma ophiolites was studied by Nandy, (1981),
Nandy et al (1983, 1986), Mitchel (1981, 1985, 1986), Anon (1986) and
Acharyya et al (1990) Benerji et al. (1980) proposed that the Indo-Burma28
Andaman-Nicober ophiolites are tectonically transported as allochthonous
terrain within the older metamorphic, and similar suggestions put forward by
Bachman et a! (1983)
But no evidence of transcurrent movement is
observed (Dasgupta and Nandy, 1995). Nandy (1980) again opined that the
Indo-Burma ophiolites are not part of the underplated oceanic crust of the
loading edge of the Indian plate below the fore arc trough rather upturned
the outer edge of the fore arc trough with oceanic crust and pre-Jurassic
rocks and thrust it over the outer arc ridge sediments by late Oligocene
along the Eastern Boundary Thrust (EBT).
Basically, the Indian plate consists of peninsular shield characterized
Pre-cambrian metamorphic rocks with late palaeozoic-mesozoic intracratonic continental Gondwana basin. On the other hand, the Burmese plate
consists of cratonic blocks with Paleozoic- Mesozoic mobile belts. The early
Cenozoic convergence and subduction of the Indian plate along the IndoBurma arc was documented by Mitchel (1981), Morgan (1983), Patnal and
Achache (1984) during the investigation of late Cretaceous/early Eocene
ophiolites and volcamcity (Chhiber, 1934; Brunnchweiler, 1966; Hutchinson,
1975; Acharyya et al; 1990; Bhattacharjee, 1991).
The Indo-Burma (Myanmar) boarder region represents the Outer Arc
Ridge in an arc-trench setup comprising of Eocene-Oligocene flysch and
sub-flysch scraped off from the leading edge of the Indian plate along with
narrow
strips
of
older
Paleozoic-Mesozoic
sediments
patches
of
metamorphic and dismembered ophiolites. The central Burma Basin is
located to the east of Indo-Burma range, where the central Burma volcanic
line, Middle cretaceous tonalities intrude basaltic andesitic and Lower29
Eocene quartz-diorite stocks and andesitic dykes are also reported (Mitchel,
1981). The northern part of the Central belt (combmedly the Eastern trough,
Western trough and the volcanic line in between) contain information on the
episodic volcanic activity which began around the late Cretaceous/early
Eocene, and continued until recently with intermittent activities during the
Oligocene, Miocene and Pleistocene along its southern side (Chhiber, 1934;
Rodolfo, 1969, Curray et al, 1979; Gill, 1981; Saikia et al, 1987). To the
extreme east of the study area is the Highlands of Eastern Burma (ShanMogok), which includes the Eastern Burmese Highlands, Shan plateau, and
peninsular Burma-Thai-Malaysia. Though the detailed study on this area is
not yet known clearly, the volcanic rocks recorded in this area are
serpentimtes, gabbros, amphibolites and basaltic andesite and in some
localities trachytic volcanic
2.3 Geotectonic Settings
The Indo-Burma region is a complex tectonic area where major
tectonic and Subduction events have been identified from late Lower
Cretaceous to Mid-Miocene and Quaternary periods (Mitchell, 1993). This is
a region of transition between the main Himalayan collision belt and the
Andaman arc where the Indian plate is currently subducting under Asia
Structurally, the region consists of the Indo-Burman ranges (an arcuate
mountain belt), the Burmese Basin (also called the Central Lowlands) and
the Eastern Highlands A magmatic arc divides the Burmese Basin into
back-arc and fore-arc troughs. A right-lateral strike-slip fault (the Sagaing
Fault, SF) separates the Eastern Highlands from the Burmese Basin. To the
30
left of the indo-Burma region is the Himalayan collision belt To the north of
the ranges are the northwest trending Mishmi thrust faults
The Indo-Burma Orogenic Belt constitutes a high mountainous region
and extensive lowlands, and separates India from Burma. The northern
extension of the belt is restricted by the southern tip of the Mishimi massif,
where as towards south the belt extends up to the Andaman-Nicober island
of India and Mentawai islands of south-west of Sumatra The Patkoi-NagaMampur-Chin Hills and Arakan Yoma region forms a westerly convex
arcuate belt, which is NW-SE trending at its southern extreme and towards
the east it is convex upto N-S Shan Sagaing fault and Sino-Burma high land.
The significant character of the belt is that 'it represents an inter-continental
convergent zone supported by the occurrence of shallow and intermediate
focused earthquakes (Saikia, et al 1987). The area as a whole exhibits a
well-defined arcuate belt, analogous to the present day arc system (Santo,
1969). This statement is supported by the geologic features and seismic
activities in the region, such as - a gravity minimum, dipping Benioff zone
with an inner volcanic and outer sedimentary arcs above the Benioff zone
The arc is orogemcally, however, much advanced and has passed through
stages of island arc formation (Hess, 1937). The classical view of Wadia
(1973) connects the arc with the Himalayan system via the Assam Syntaxial
bend. Van Bemmelan (1949) related its development to the Shan center,
while Klompe (1957) considered it as an outgrowth of Southeast Asia
Petrushevsky (1971) connected its development to the pacific orogeny
through an apophyse
Braunnshweiler (1974) and Powell, et al., (1973)
opined that the Burma was a part of Eurasia.
31
Constraints on the motion of the Burma plate in the Cenozoic-torecent plate reconstruction of Southeast Asia (Lee and Lawver, 1995) are
only indirect. The impact between the Greater India and Southeast Asia
was basically west of the Burma block and the Shillong Plateau, which is a
Precambrian outcrop, marks the eastern limit of the Indian Shield The
right-lateral Sagatng Fault is assumed to accommodate plate motion of the
Burma block with respect to the Sino-Burma-Thailand block. The western
edge of the Burma plate was the site of oceanic subduction until the
Eocene, when initial collision with the northeastern edge of the Indian plate
occurred (Mitchell 1993, Lee and Lawver, 1995). This collision may have
coupled the Indian and Burma plates and accelerated the northward motion
of the Burma plate; during the Miocene, the Burma plate acted as a fore­
arc sliver coupled with the Indian plate, subducting obliquely underneath it,
and moved northward relative to Asia along the Sagaing Fault (Pivnik et a l ,
1998). As the sliver moved north, the Mishmi block served as a buttress,
and the east-west oriented component of convergence between the India
and Burma plates controlled deformation, creating compressional and
transpersonal structures in the fore-arc sliver (Pivnik et a l , 1998).
Movements on the Naga and Disang Thrusts suggest convergence
between the Burma and Indian plates as both moved north and India
rotated clockwise after the India-Asia collision (Mitchell, 1993). Based on
similarities in the age of magmatic arcs in Burma (Myanmar) and Sumatra
and their absence from the intervening Andaman Sea, Mitchell (1993)
inferred that western Myanmar lay 1100 km south of its present position in
the late Cretaceous More recent paleogeographic studies on the tectonics
of Southeast Asia invoke much less tectonic transport and originate the
32
Burma plate at a point south and east of its present position (Lee and
Lawver, 1995) Other studies (Tapponnier and Molnar, 1976; Le Dain et al ,
1984) suggest that the Burma plate was located north of the Indian plate
before the collision of India with Asia, and that extrusion and clockwise
rotation of the Burma plate in the late Tertiary brought it to its current
configuration One result of this rotation was the oblique subduction of the
Indian plate beneath Burma and the mid-Miocene opening of the Andaman
Sea (Curray et a l , 1979, Varga, 1998).
Many workers (eg., Mitchel, 1993; Currary et al., 1979; Bender,
1983; Khm Zaw, 1990) have made plate tectonic interpretations of
geological events within the Burma (Myanmar) region. Most of them agree
that Burma (Myanmar) records the accretion of several plates of Gondwana
affinity onto the then southern boundary of Indo-China (Varga, 1998). The
locus of the subduction zone is understood to have shifted to the west
throughout
the
Mesozoic
and
Cenozoic
(Khin
Zaw,
1990)
while
convergence took place. Mitchell (1993) infers that there have been
reversals in the polarity of subduction in the region, the present eastward
subduction having been initiated in Mid-Miocene. Thus, the geology of the
region has been complicated by reversals of polarity of subduction, which
can be well understood from the fault plane solution of earthquakes. It is
possible that the present margin of subduction is shifting westward as
evidenced by the most recent folds and thrusts observed in the adjacent
Bengal Basin and west of the Indo-Burma arc that we consider as the
present boundary between India and Burma
33
Ggeotectomcally, the Indo-Burma orogenic belt can be divided into
two categories, such as
(a) Subduction related setting,
(b) Collision related setting
(a) Subduction related setting
In the subduction related framework the Indo-Burma ranges,
comprising of the Naga-Patkoi-Chin hills and Arakan-Yoma are the most
conspicuous tectonic element on the eastern margin of the Indian plate.
Further east the Central Burma basin is another important complementary
to the Indo-Burma ranges. The easternmost tectonic unit is the Shan
plateau or eastern Burma highlands, which bear evidence related to
subduction
(i) Indo-Burma ranges
The Indo-Burma Ranges represent the western part of the orogenic
belt The important constituent members, from north to south are. (I) the
Naga-Patkoi
Hills,
(II) the Chin Hills, and (III) the Arakan-Yoma
Longitudinally two tectono-sedimentary zones can be identified within the
Indo-Burma Ranges The inner flyschoid zone with ophiolite and the outer
subflysch-molasse zone (Mitra and Uedekar, 1987). This division can be
followed into the Chin Hills-Arakan Yoma.
Inner zone is comprised of extensive deposits of flyschoid rocks,
which is locally known as Disang and the Barail formations, deposited in a
eugeo synclinal environment The Disangs are composed of thick, poorly
fossilferous tightly fold and thrust zones of turbidite flysch, pelagic
34
sediments with ophiolites (Evans and Mathur, 1964, Brunnshweiler, 1966)
The flysch sediments are thrust over by the tectonised and dismembered
ophiolitic
members
The
later,
in turn overthrusts by the Naga-
metamorphic Their contact is of tectonic nature in a high angle reverse
faults dipping steeply towards east. The major transverse faults trending
NW-SE to E-W are seem to be wrench fault type bringing major offsets of
lithological setup (Chattupadhya et al, 1983).
The outer belt in the Naga hill segment is characterized by a group
of imbricate thrusts, commonly known as the ‘schuppen belt’, which
extends for about 350 km in a NE-SW direction (Evans and Mathur, 1964)
It is a narrow zone of about 20-25 km in width (Roy, 1986). Two marginal
thrusts bound the zone - the Naga thrust follows the boundary between the
Assam foreland basin and the Naga Hills, and Disang thrust in the
southeast constitutes the uppermost member of the 'schuppen belt’ and
marks the outermost outcrop limit of the Disang formation. Towards south,
contrary to the schuppen structure as seen in the Naga Hills, a frontal
folded belt developed. It is comprised of the folded belt of Surma valley,
Tripura-Mizo hills, Chitagang hill tracts and costal Burma, collectively called
as the Surma basin fold (Dasgupta, 1984). The structural complexity
increases from west to east. The large scale regional fold of the Surma
basin and imbricate thrust belt of Naga Hills developed, through
compression during subduction. The entire episode was associated with
easterly subduction and responsible for the formation of the Indo-Burma
Ranges
35
(ii) The central Burma basin
The central Burma basin constitutes a vast expanse of lowland with
rolling hills from eastern border of the Naga-Patkoi-Chin-Arakan-Yoma
ranges to the base of the Shan plateau (Tainsh, 1950). The eastern
boundary, with the Shan plateau is marked by the Shan boundary Fault
The Myitkina-Mandalay ophioltte belt occurs along this tectonised margin.
The basin is divided into eastern and western troughs by the low fold
mountains of Burmese volcanic arc (Chibber, 1934 and Rodolfo, 1969),
being dominantly andesitic and also containing minor basalt and rhyolites.
The volcanic belt extends southward to the Andaman Sea. The
sedimentary deposits of the western trough are thrust westward over the
eastern side of the Indo-Burma ranges, which consists largely of turbidities
sequences of late Cretaceous to early Tertiary age (Mitchell and Mckerrow,
1975). Mitchell and Garson (1981), however, opined that the eastern belt
consists of recumbently folded upper Triassic turbidities in places showing
green schist facies metamorphism. The western trough of the huge Tertiary
basin of central Burma represents the fore-arc basin within the arc-trench
gap (Dickinson, 1976) or is equivalent to interdeep (Van Bammelan, 1949)
of overlying the subduction complex represented by the Naga-Patkoi, Chin
and Arakan-Yoma ranges extending southward into the Andaman-Nicober
subduction zone (Valdiya, 1984). Brounnschweiler (1966) called the central
Burma basin an inter-arc basin comparable to the Pannonian basin of the
Carpathian arc.
In terms of Menard (1967), it may be called a marginal basin
developed due to back-arc spreading. Mantle diaprism is advocated by
36
Karig (1974) for the formation of marginal basin resulting from frictional
heating between lithospheric plates in the focal zone. The down bending of
subducted plate induced flow on overriding plate to fill the space formerly
occupied by the down bending plate resulting extension of the central
Burma basin In this process, a back-arc thrust belt developed with
antithetic thrusts fault owing opposite polarity to subduction zone. Much of
the Shan scrap zone of Burma belongs to such a back-arc eastward
directed thrust belt (Mitchell and Garson, 1981). The back-arc thrust fault in
Burma is 20 km wide (Mitchell and Garson, 1981).
(iii) Eastern Burma Highland
The eastern Burma highland (Shan plateau) abuts against the
Central Burma basin in the west it is separated from the Central Burma
basin by the Shan scrap (Shan boundary fault) along which occur exotic
bodies of upper Cretaceous ophiolites within Eocene turbidities and blueschist's (Brounnshweiler, 1966). It probably developed as a dextral strike
slip fault, and later converted into a vertical fault (Valdiya, 1984). The early
geological build-up of the Burma high land is connected with the Malay
Peninsula, Thailand and the higher Himalayas (Gansser, 1964; Valdiya,
1984; Acharyya, Roy, and Mitra, 1986). According to Valdiya (1984) the
Shan plateau and its extension of the Temasserim peninsula and
Malayasia are the southerly extension of the Luhit (Mishimi massif)
subprovince at the NE corner of the Himalayas. The Shan plateau
comprises the basement metamorphic in the west overlain to the east by
the synclinally folded succession of Triassic red beds and sediments of
Kalaw syncline and the belt to the east are thrusted by granites of the late
37
Cretaceous to Tertiary Peripheral impact of subduction related processes
could be seen through the development of the back-arc thrust belt and
associated magmatism (Mitchell and Garson, 1981)
(iv) Suture zone
The tectonic development of the Indo-Burma orogen is linked with
the subduction of the Indian plate beneath the Burmese plate leading to the
closure of late Jurassic-Tertiary Ocean. Subsequent attempts to consume
the continental lithosphere are resisted by the buoyancy of the continental
crust and a subduction zone is converted into a suture zone (Dewey and
Burke, 1973) The evidences for the existence of a suture zone along the
Indo-Burma boundary are provided by the disposition of synorogenic flysch
sediments, ophiolitic rocks, trench melanges, radiolarian charts, pelagic
sediments, metamorphic rocks of blue-schist facies and arc type igneous
rocks (Brounnshweiler, 1966, 1974; Agarwal and Kacker, 1979; Curray et
al, 1979; Ghose and Singh, 1981; Roy and Kacker, 1982)
(b) Collision related setting
From structural, sedimentological histories and seismic evidences it
has been observed that collision all along the plate boundary was not
uniform There was a progressive development of the collision from north
to south. The most conspicuous dividing line is 25°N latitude. This happens
to be the line of the Dauki tear fault (Evan and Mathur, 1964) or deep fault
(Saikia and Dutta, 1976). It also formed the dividing line between high and
low lands and differential vertical movements (Evans and Mathur, 1964),
presently defined by very steep gravity gradient (Mukhopadhayay and
Dasgupta, 1988) The segment above this latitude probably collided earlier
than the southern segments. Pascoe (1959) stated that the northeast
promontory was the first point to meet the Eurasian continent. The
development of tightly folded and thrust flysch, the pattern of earthquake
activity, and degree of metamorphism of rocks bear the imprints of the
collision process. During the penultimate stage of collision, thrust faults
were extensively developed in the Naga Hills segment, which crowded
towards the craton culminating in the development of an imbricate thrust
zone called schuppen belt. It is about 20-25 km width and 350 km in length
Two marginal thrusts faults bound the belt. The fault bounding the Assam
valley in the north is known as the Naga thrust and the Dissang thrust
delineate
the
southeast
boundary.
Such
crustal
shortening
under
convergent plate margin can be ascribed to the involvement of continental
foreland under thrusting and attenuation of lithosphere due to the upwelling
of anomalous mantle to the base of the crust. That is why; most miogeosynclines have been intensively shortened and become incorporated
into fold belts (Artyuschkov and Baer, 1986). Low seismic activity and non­
involvement of the basement, narrow zone of deformation testify good
decoupling between the upper and lower plates. Such schuppen structures
have not been recognized in the southern part bellow the 25°N latitude. On
the contrary, south of this latitude, as observed in the Cachar-Tripura-MizoChittanga hill tracts and costal Burma; N-S, NE-SW, NNW-SSE folding
constituting of broad symmetrical anticlines predominate the tectonic
environment (Dasgupta, 1983). The presence of this wide zone of
interplate deformation as evidenced by intermediate depth earthquake
activity (Saikia, Kotoki and Sahu, 1989) probably, indicates no or weak
39
decoupling. As stated above continent- continent collision had occurred
within Naga hill segment resulting in major structural configuration of highrise mountains It has been stated that in the Chin hill and Arakan-Yoma
segments continent-continent collision had not taken place and involved
only subduction of oceanic material covered by a thick turbidite sequence
over a period of about 40 my It is opined that in the southern part of the
Indo-Burma orogeneic, continental crust of the China plate is probably
absent beneath the Arakan-Yoma and western trough (Mitchell and
Mckerrow, 1975). The low height and more width of the hills compared to
the Northern Half could be a reflection of collision.
In the process of subduction and collision the mam control was
perhaps provided by the sediments supplied to the trench and morphology
of the subducting plate. The Naga Hill segment had the first contact with
the opposing colliding continental Burmese plate (Eurasian plate).
Furthermore, the trench at this location received major supply of sediment
from the orogemc highlands, immediately north of it, associated with the
Himalayan suture zone. A longitudinal trench is envisaged from the
longitudinal facies change from continental paralic deposits in the north to
the marine strata in the south (Tainsh, 1950; Maung Thien, 1973). This
leads to sequential collision with progressive development of the suture
belt (Graham et al, 1975).
The
grades
of
metamorphism
(Vidyadharan
et
al
(1986),
Bhattacharya and Venkataramana, 1986) and deformation (Gorshkov,
1970; Mitchell and Mckerrow, 1975) also indicate continental collision in
the Naga hill segment. Towards the Eastern part of the Naga hill the
40
Disang flysch sediment show increase in the intensity deformation. This is
reflected in the better development of slaty cleavage at their contact with
the ophiohte belt and of slight metamorphism. The average altitude of IndoBurma ranges shows general increase northwards, commensurate with
northward increase of tectonic activity (Acharyya, 1986),
A genetic relationship between the descent of subductive oceanic
crust along the Benioff Zone and generation of volcanic rock magmas has
been suggested by many workers (Gill, 1984). A gradual advancement of
the orogenic pressure toward south may be discerned from the
reoccurrence of episodic volcanic activity. The volcanic line parallel to IndoBurma ophiolites belt (100-130 km) to the east, can be a possible tectonic
reference for the growth of the arc system. This has become evident from
segmented nature of the volcanic arc progressively younging towards
south of the orogen (Saikia, Kotoky and Duarah, 1987). The volcanic,
usually form NE-SW ridges and the volcanic rocks are considered
comparable to a seismic ridge, intraplate hotpots, high ridges and scraps,
sea-mounts and ocean island basalt (Venkataramana, Dutta and Acharyya,
1986).
The volcanoes along the central Burma basin occurred when the
continental crustal block had further overridden the oceanic plate It
resulted in the extrusion of more falsie and calc-alkaline magma. The
volcanics with pre-dominantly andestic affinity characterize this stage
(Chibber, 1934 and Rodolfo, 1969).
41
Present day seismicity of the orogen, which coincides with the
geologically defined boundaries, provides evidences in explaining the
overall development of the orogen. It has been observed that the segments
are characterized by different types of rocks and nature of seismic activity.
A comparison of the shallow and intermediate earthquake activity in the
zone suggest the state of crust and upper mental disturbances and also
supports the concept of segmentation of the slab. A latitude wise
distribution of shallow and intermediate depth events depicts the state of
geodynamic condition of the orogen.
It is seen that in the Naga segment the seismic activity is mainly
crustal with reduced upper mental activity. Pronounced manifestation of
upper mental activity through intermediate depth earthquake is discernible
between latitude 21° - 25°. One very significant observation is the bulging
out of earthquake activity into the interior part of Manipur state with a
spillover to Tripura state that this part of the orogenic belt has been
undergoing increasing distress is proved by occurrences of past and recent
major earthquake. The seismic activity below this latitude is significantly
less, tapering towards the lower half of the Arakan-Yoma, Lack of
intermediate earthquake between latitude 15° -2 0 ° N to be reckoned with,
Mukhopadhayay (1984) considered it a transition zone between an island
arc and mountain arc. Mitchell and Mckerrow (1975) opined that beneath
the Arakan-Yoma and western trough continental crust is probably absent
and therefore no contact could occur. However, the low seismicity may be
attributed to a low component of convergence of 1.25 cm/yr (Eguchi uyeda
and Tadashi, 1979) in contrast to approximately 4 cm/yr component of
42
convergence between the Assam valley and the Naga Hills (Curray et ai,
1979); oblique convergence (Mitchell and Garson, 1981) leading to a
gradual conversion of subduction into a transform fault (Kumar, 1981);
collision is presently dominated by strike slip fault in Burma as indicated by
predominant strike slip fault plane solutions (Upadhayay.1982 an Le Dam
et al.,1984), Le Dain et al 1984 suggested that the subducting slab is
undergoing a drag along with the Indian plate over the asthenosphere.
2.4 Seismic History of indo-Burma region
In order to evaluate the seismicity or seismic risk in a region or area
it is necessary to have a detailed knowledge of the earthquakes that
occurred in the region for a long time interval. Unfortunately this is not
available for the north-east Indian region and its surrounding for earlier
periods. For some countries like China, detailed knowledge about the
important earthquakes are available for about 3000 yrs. or so. But a
systematic account of most of the large earthquakes of this study region is
available only from the middle of the 19th century. Here, a brief description
of some of the historical earthquakes are given in order to get some idea
about the destruction caused by them in this region and to bring home the
fact that the region tend to have seismic risk, where destructive
earthquakes have been found to occur from time to time.
Although, mention has been made of a number of pre-instrumental
historical earthquakes, it is difficult to assign their magnitudes to use these
earthquakes in the present study. However, the description of severity of
the damages caused by these events indicates that there might have been
earthquakes of large magnitudes
43
(a) Some Historical Earthquakes
23rd
March,
1839-Burma
earthquake:
This
was
a
devastating
earthquake, though there is no record of large-scale devastation. The
epicenter of the earthquake lies between Lat.21.9°Nand Long.96.5°N The
body wave magnitudes recorded as 8 mb.
14th August 1932 earthquake: It occurred on 14th August 1932. The
epicenter was located at latitude 26.0°N and longitude 95.5°E and its
magnitude was found 7.0 mb, which was felt over the whole of Northeast
India and its surroundings.
16th August, 1938 earthquake: The epicenter was located at latitude
23.5°N and longitude 94.3 °E. The magnitude of this earthquake was 7 2
mb. It was felt over Assam, Bengal and Burma.
23rd October, 1943 earthquake: The epicenter was located at latitude
26.0 °N and longitude 93.0 °E. The magnitude of this earthquake was 7 2
mb. It was felt Assam, Bengal, Burma, part of Bihar and Orissa.
12th September,
1946 Mandalay earthquake: It was located at
Lat.23.5°N Long 96°E its body wave magnitude was 7.8 mb, but no
devastation is recorded due to sparse population.
15th August, 1950 earthquake: It is generally called the great Assam
earthquake (or Himalayan earthquake). It occurred near India-China border
(at Rima). The epicenter of this event was located at latitude 28.5°N and
longitude 96.5°E. The magnitude of the earthquake was 8.6 mb on Richter
scale with focal depth 25 kms. This was felt over entire Northeast India and
44
its surrounding areas including Myanmar. Though the epicenter of this
earthquake was in India-China-Burma border, the destruction and damage
caused by it were widespread over entire Northeast India. It has a
tremendous impact on the physical landscape as well as on the economy
of the entire North-East Region of India.
21st March, 1954 earthquake: This Manipur-Burma border earthquake
was felt over a large part eastern India and its neighborhood. The epicenter
was by latitude 24.38°N and longitude 91.15°E. The magnitude was
calculated at Pasadena was about 7.1 mb. The depth was about 180 kms.
The fault plane solution shows thrust fault striking N 50°E and dipping to
NW at angle of 60° (Tandon and Mukherjee, 1956).
16th July, 1956-Myingyan earthquake: The epicenter of this earthquake
was 22 5°N and Long 95.7°E.lts magnitude was recorded 7.0 mb. The
devastation of this earthquake is not known due to sparse population.
30th December, 1984 earthquake: This earthquake is known as
Sonaimukh (Cachar). The epicenter was latitude 24.67°N and longitude
93.07°E with depth 35 kms and body wave magnitude 5.5 mb. This
earthquake was preceded by a well-defined precursory seismic swarm and
seismic quiescence. Twenty people were killed and many become
homeless.
6th August 1988, Manipur-Burma earthquake: The epicenter of this
earthquake was latitude 25.14°N and longitude 95.12°E with focal depth 92
kms and magnitude was 6.8 mb. The most extensive damage of life and.
property was observed in the districts of Sibsagar, Lakhimpur & Dibrugarh
45
of Assam Cracks developed in many places in Guwahati City. The four
stories blocks of Reserve bank employees have also developed cracks on
the walls. The railway line at several places in Lumding-Diphu section of
N.E.F, Railways was damaged. The National Highway-37 had been
fissured near Raha. The Nagaland pulp and paper mill at Tuli was
completely damaged. At least 4 persons were killed and 15 others
seriously injured during this earthquake, unprecedented flood occurred in
all parts of Brahmaputra and Barak valleys.
-0O0-