Deep Structures and Tectonics of Burmese Arc 1988

8/8/2019 Deep Structures and Tectonics of Burmese Arc 1988

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Tectonophysics, 149 (1988) 299-322

Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

299

Deep structure and tectonics of the Burmese arc:

constraints from earthquake and gravity data

MANOJ ~UKHOPADHYAY ’ and SUJIT DASGUPTA ’

’ Department of Geophysics, Indian School of Mines, Dhanbad-826004 (India)

’ Publication Division, Geological Survey of India, 29, J. L Nehru Road Calcutta-16 (India)

(Received January 1.1987; revised version accepted October 27,1987)

Abstract

Mukhopadhyay, M. and Das8upta, S., 1988. Deep structure and tectonics of the Burmese arc: constraints from

earthquake and gravity data. Tectonophysics, 149: 299-322.

Active subduction of the Indian plate is currently occurring beneath the Burmese arc along an east dipping Benioff

zone which extends to a depth of about 180 km. The overriding Burma plate has an appearance of an inland se ismic

slab that is deflected downwards in the vicinity of the Benioff zone. A crustal seismic zone some 60-80 km east of the

Benioff zone correlates to backarc activity. A triangular aseismic wedge in the top part of the crust outlines the Central

Belt molasse basin east of the Burmese foldbelt. Fault plane solutions show that the Burmese Benioff zone is

characterized by shallow angle thrusting at its upper edge whereas down-tip tensional events dominate its tower edge.

Most of the backam seismicity is accounted for by the Sagaing transform or by the activity of the Shan scarp normal

fault zone at the margin of the Asian plate. A gravity anomaly pair with amplitude of 175 mGal coincides with the1100~km long Burmese arc lying in a north-south direction. The gravity anomalies along a profile in central Burma and

in adjacent areas of the Bengal basin are interpreted in terms of plate subduction as well as near-surface mass

anomalies. This suggests that sediments below the Central Belt may have an average thickness of the order of 10 km

but may be as thick as 15 km at the subduction zone. The oceanic crust underl~g deeper parts of the Bengal basin

experiences phase transition at about 30 km depth in a Benioff zone environment east of the Burmese foldbelt. Several

thrust planes are present within the folded and deformed Cretaceous-Tertiary sediments of the fold belt; these are

often associated with ophiolites and basic to ultrabasic rocks. A low density zone, at least 60 km wide, underlies the

andesitic volcanic axis in the overriding plate.

Mroduction 1946; Gulatee, 1956). With the advent of plate

A variety of natural resources was perhaps one

reason which attracted the early explorers and

field geologists to Burma during the last century;

this eventually led to the publication of two com-

prehensive accounts of the geology and mineral

wealth of Burma, compiled by Chhibber (1934a,

b) more than fifty years ago. The general pattern

of the gravity field for Burma and its adjoining

Indian territories came to be known quite early

through the work of the Burmah Oil Companyand the Survey of India (Evans and Crompton,

tectonics Burma was again studied with renewed

interest with an emphasis on explaining the

tectonic development of the Burmese arc as a

whole together with its neotectonics (see, among

others, Bnmnschweiler, 1966, 1974; Mitchell and

McKerrow, 1975; Curray et al., 1979). More re-

cently correlative study of surface geology with

aerial photographs and Landsat imagery (Nandy,

1980; Le Dain et al., 1984) has signif icantly in-

creased our knowledge on the tectonic framework

of Burma. It is now commonly accepted, as wasinitially suggested by Chhibber (1934a), that the

0040-1951,‘88,‘SO3.50 0 1988 Elsevier science Publishers B.V.



300

Burmese and Andaman arcs together provide an

important transitional link between the Himalayan

collision zone and the Indonesian arc; the latter is

in direct tectonic continuation of the Western

Pacific arc systems.

Several contrasting views have been put for-

ward concerning the tectonic development and

current stress regime of the Burmese arc. For

instance, Mitchell and McKerrow (1975) ascribed

the evolution of the arc to a process of eastward

subduction of the Indian plate at the Asian con-

tinental margin continuing from at least the Late

Cretaceous to the present affecting the Benioff

zone lying to the west of the Burmese foldbelt (the

Arakan-Yoma mountains). Curray et al. (1979)

have proposed a lenticular plate, the Burma plate,forming a structural province in the area between

the Arakan-Yomas on the west and the Indochina

highlands on the east. This plate has been created

since Middle Miocene time as a result of opening

of the Andaman Sea by at least 460 km (Curray et

al., 1982). Le Dain et al. (1984) suggest that sub-

duction of the Indian plate below the Burmese arc

has stopped recently or occurs aseismically and

the hanging lithospheric slab is being dragged

north by India through the surrounding litho-sphere. We have suggested elsewhere (Mukho-

padhyay, 1984) that the Indian plate actively sub-

ducts below the Burmese arc as shown by an

east-dipping Benioff zone that extends to about

180 km depth below the central lowlands east of

the Arakan-Yomas. However, recently Tapponnier

et al. (1982) propose that active spreading in the

Andaman Sea and lateral motion along the Saga-

ing transform in Burma is a consequence of prop-

agating extrusion tectonics in response to rigid

indentation by India into Asia at the northeast

corner of the Himalaya.

In the present article we consider earthquake

data for the Burmese arc in order to study details

of the subduction zone geometry over various

segments of the 1100 km long arc. Stress distribu-

tion and the pattern of faulting within the Burmese

Benioff zone are studied using a large number of

selected earthquakes. Using the Benioff zone con-

figuration, we next interpret a representative grav-

ity profile across central Burma and its forearcregion covering the Bengal basin to infer the deep

structure of the Burmese arc. For gravity interpre-

tation, we use geologic information and available

seismic data as important constraints. The results

obtained from the analysis of earthquake and

gravity data are then discussed from the viewpoint

of tectonics of the Burmese arc.

Regional geology

Burma comprises three major physiographic di-

visions extending north-south: the Shan plateau

to the east, the Central Belt (CB) including the

basins of the Irrawaddy, Chindwin and Sittang

rivers, and the Burmese Fold Mountain Belt

(FMB) on the west (Fig. 1). The Fold Mountain

Belt includes the Arakan-Yoma mountains and

the Chin, Naga, Manipur, Lushai and Patkai hills.

Further west lies the Bengal basin. The eastern-

most tectonic unit is the Shan plateau forming the

eastern highlands which is elevated by about 0.7

km above the Burmese plains (CB) containing the

Tertiary deposits against a prominent scarp/fault,

called the Shan scarp. Burma, west of the Shan

plateau is believed to have formed a part of an-

cient Gondwanaland whose shore line possibly

ran along the western boundary of the Shanplateau and is now marked by this great scarp or

the boundary fault (Chhibber, 1934a, p. 510). The

Mogok metamorphic belt, consisting of undif-

ferentiated igneous and metamorphic rocks with

acid intrusives, runs north-south along the west-

ern edge of the Shan plateau for a distance of

more than 1000 km with an average width of

24-40 km. According to Searle and Haq (1964),

metasediments which range in age from Pre-

cambrian to possibly Cretaceous were migmatized

during the Himalayan orogeny to form the Mogok

metamorphic belt. The Mogok Series consists of

migmatites, gneissic rocks (garnet-biotite gneiss,

biotite gneiss and g~et-Capote-si~manite

gneiss), and calcareous and arenaceous rocks

(marbles and talc-silicate granulites). Associated

intrusive rocks contain alaskitic suite and mafic

rocks (Late Eocene-Early Oligocene in age),

pegmatites and aplites (Middle Miocene in age),

and the Kabaing granite which is the largest and

youngest intrusive (Late Miocene-Pliocene). TO-wards the east, the Mogok belt is covered by



301

d8”

m Quoternary

da*m Tethyan ophiol ite

Crystalline rocks of Himalayam and Burma

Cretoceous-PaleogeneIK-P9 Flysch

m Crystalline rocks of Indian

Shield

m Mesozoic votcanics .-+“‘x Structural trend

[-“ %-j Subrecent forecent volcano # Fault ,Thrust

Fig 1. Generalized tectonic map of the Burmese arc, Shari plateau and adjoining areas including the eastern Himdaya (after Gansser,

1964). The Burmese arc is comprised of the Naga hills, At&an-Yoma ranges and the Central Belt molasse basin. The arc is convex

towards the Indian foreland. EBT-Eastern Boundary Thrust in Burma. EBT--Eastern Boundary Thrust in Burma.

fossiIiferous Paleozoic strata that are frequently zones from east to west (after ~~~schweiler~

cut by outcrops of the Chaung Magyi Series (Fre- 1974): (I) the Pegu-Sagaing Rise, a molasse basin

cambrian micaschists). resting on a mainly Paleozoic floor and grading

The CB and the FM3 conjugately form a broad into (2) the CB malasse basin in which post-Eocene

convex arc towards the Indian foreland. This 1100 molasse rests on flysch, folded neritic Creta-

km long, 250-700 km wide, Tertiary erogenic belt ceous-Eocene and older metamorphics; (3) theis best described in terms of the following seven adjoining Inner Thrust zone contains more de-



302

formed sediments with exposed pre-Alpine meta-

morphics including Cretaceous ophiolites: (4) fur-

ther west is a Cretaceous-Miocene flysch trough

of great thickness resting on abyssal Cretaceous-

Mesozoic strata with ophiolites; (5) the coastal

Ramri-Andaman Ridge has a comparable belt of

Cretaceous-Eocene strata, strongly folded and

thrust which overlies (6) an outer molasse basin

containing Tertiary sediments (Tripura fold belt)

INDEX

issl

PRE-CRETACEOUS ROCKS OF

EASTERN AND WESTERN BURMA

up to 20 km thick which are folded and mildly

thrust; (7) beyond the Indian border is the

Bengal-Surma foredeep and relatively undisturbed

Bengal-Asam peri-cratonic foreland in which only

the Pliocene deposits achieved major thicknesses

(Fig. 2). In the CB (zones 1 and 2) young ande-

sitic and basaltic volcanics occur along the Mt.

Popa-Chindwin-Wuntho volcanic line in south

Burma (Chhibber, 1934a) and continue through

Fig. 2. Prominent geologic and tectonic features of Burma. Data sources are cited in text.



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the Jade mines area in north Burma, whereas

Cretaceous ophiolites together with basic and ul-

trabasic submarine flows are found in zones 3. 4

and 5 (Bender, 1985). The Upper Creta-

ceous-Eocene flysch constituting the FMB is

limited by step faults and thrusts on the east side

and also partly on the west side. Ophiolitic rocks

occur along the eastern thrust (cf. Nandy, 1980)

which has been called the Eastern Boundary Thrust

(EBT). The EBT represents a major tectonic ele-

ment as it defines the contact between the FMB

and the CB throughout Burma; north of about

24” N it adopts a NNE trend south of the steeply

thrust Naga hills (Fig. 2) and then turns to the

northeast only to be cut off by the Mishmi crystal-

line thrust sheet at the eastern Himalayan syn-taxis. The Bengal basin underlies a thick section of

Cretaceous-Tertiary deposits whose total thick-

ness may be as high as 13 km in deeper parts of

the basin. Major tectonic features of the basin

(after Zaher and Rahman, 1980) and its underly-

ing basement configuration including the peri-

cratonic areas of the Shillong plateau (source:

Evans, 1964; Rao, 1973; Choudhury and Datta,

1973) are illustrated in Fig. 3. Note that the “Hinge

zone” (HZ) underlying the basin is taken to meanthe slope feature defined by the Eocene limestone

seismic reflector. Structural dips on this reflector

increases from 2-3” at the shelf edge to 6-12” on

the Bogra slope, flattening out to l-2” in the

basin foredeep (Salt et al., 1986). The HZ outlines

a zone of clear differential thickening and subsi-

dence of the overlying Oligo-Miocene sections,

between the shelf on the northwest and deeper

basin to the southeast. The HZ is at least 500 km

in length between the Dauki fault on the north

and Calcutta on the south; it also extends further

south into the Bay of Bengal. Its width varies from

25 km in the north, 110 km in the central part and

35 km in the south (Matin et al., 1986). Across the

HZ, the Bengal basin basement steeply plunges

from 4 to 10 km or even further (Fig. 3).

Seismicity and Benioff zone

Several authors gave prepared seismicity maps

for the Burmese arc and its adjacent areas, utiliz-

ing data for different time periods and from vari-ous agencies to study the correlation of seismicity

to tectonics (cf. Santo, 1969: Fitch, 1970, 1972:

Chandra, 1975, 1978; Verma et al., 1976: Khattri

et al., 1984; Le Dain et al., 1984). It was Santo

(1969) who first drew attention to an inclined

seismic zone underlying Burma. However. a major

difficulty in preparing a reliable seismicity map of

Burma is the poor distribution of recording sta-

tions. This led Le Dain et al. (1984) to apply a

selection procedure on the basis of distribution of

seismic stations and the number and consistency

of reported P- or pP-wave phases for each earth-

quake located by the ISC for preparing the

seismicity map. However, Gupta (1976) has shown

that earthquake detectability levels for the Burmese

seismic zone are extremely poor at present: they

are mb = 5.2 for all earthquakes which are re-

ported for the region, while for mb = 4.5 only 4%

earthquakes are reported. The corresponding fig-

ures for the western U.S. are mb = 4.3, and 100%.

Gupta (1976) also estimates that the error in-

volved in epicenter location for Burmese

earthquakes is about 2 km for an event of mb = 5.0

which increases to about 5 km for m,, = 4.0, al-

though the actual capability of the seismic net-

work could be one order of magnitude worse than

these statistical figures. The status of the seismicnetwork has not presumably improved much over

the past decade. It is therefore obvious that severe

restraint must be exercised in evaluating any ex-

isting seismic map of Burma, in particular, for

geologic correlation and hazard studies.

Keeping in view the limitations of the seismic

network and errors involved in epicentral de-

terminations, we have prepared a seismicity map

of Burma (Fig. 4) using ISC data for the period

1964-1980 with events having m,, > 4.0 M, >, 5.0;

for purposes of the present study this is consid-

ered as the cut off magnitude level. With this we

also include ISS/NOAA earthquake data for the

immediately preceding period 1956-1963 using

only those events of M, 2 5.0. For reasons dis-

cussed earlier, this allows us to deal with a greater

volume of data for purposes of correlation with

regional structures. However, for smaller events

the error involved in epicentral determination is

rather high for the Burmese earthquakes. Gen-

eralized tectonic features of the Burmese arc arealso superposed on Fig. 4.



305

23r

C-22*

21

Fig. 4. Seismoteetonic map of Burma; earthquake data sampling period: 19561980. Notice the different notations used to represent

the data for the periods 1956-1963 and 1964-1980 for reasons discussed in the text. Maximum earthquake concentration is seen for

the Central Belt to the east of tire Eastern Boundary Thrust. The seismicity map is classified into four sectors, I-IV, in accordance

with the change in strike direction of the convex Burmese are. Section AA’ through DD’ taken across the middle of the individual

sectors are illustrated on Fig. 5. Results of 45 focal mechanism solutions are schematically shown on the map; dark and blank areasrefer to compressional and dilatational quadrants respectively. Digits identify the solution number as listed in Table 1. Diverging

half-arrows indicate fateral slip.



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In order to best define the seismicity of Burma,

Fig. 4 is classified into four sectors, of comparable

sizes, having their characteristic mo~holo~c and

tectonic settings. They are: Sector I corresponding

to the Naga Hills area in the extreme north to the

northeastern part of the Burmese arc where the

collision process has just set in (cf. Mitchell and

McKerrow, 1975); Sector II corresponding to the

Chin Hills area where the northeast trend of the

Burmese arc takes an arcuate trend to become

N-S in more southern areas, and where deforma-

tion is believed to be largely controlled by a

basement spur of the Precambrian Shillong-Mikir

massif of the Indian foreland (cf. Evans, 1964;

Fig. 1); Sector III corresponding to the central

portion of the Burmese arc where the arc is widest

and folding is mostly N-S; and Sector IV covering

coastal Burma where the arc orientation changes

to NW-SE. For each of these sectors we present a

representative geologic section on the basis of

available informations including our field studies

data in parts of the FMB, and a seismologic

section using the data given on Fig. 4. These are

discussed below.

Figure 4 demonstrates that Sectors I to III are

seismically very active whereas Sector IV is farless seismic. Four east-west sections, AA’-DD’

(Fig. 5), taken through the midparts of these sec-

tors illustrate the surface geology and the hypo-

central pattern of earthquakes underlying these

section lines. The earthquake events have been

projected onto a vertical plane below the section

lines from both the north and south areas located

within the individual sectors. As can be seen from

Fig. 5 an inclined seismic zone is colon for

Sectors I to III, while for coastal Burma this

pattern is either greatly subdued (a quiet zone) or

not well represented by the present data set. The

earthquake foci range in depth from near surface

to around 180 km. The foci dist~bution in the

inclined seismic zone is used here to define the

boundaries of the subducting Indian plate below

the Burmese arc. The horizontal lower boundary

of the lithosphere is placed immediately below the

lowest foci at a depth of 75 km; when traced

eastward this boundary dips to the cast in the

region below the FMB and plunges to a depth ofabout 180 km in Sectors I and II, to 160 km in

Sector III, but reaches up to about 80-100 km in

Sector IV. The thickness of the Benioff zone is of

the order of 60 km below the CB, having an

average dip of about 45 O. The Burmese volcanic

arc generally corresponds to the deepest part of

the Benioff zone. Further east of the Benioff zone

two shallower seismic zones are located within the

overriding Burma plate in Sectors I to III. Follow-

ing Sacks et al. (1978) and Yamashina et al.

(1978), we designate this as the inland seismic slab

within the Burma plate. The relatively shallow

seismic zone closest to the Benioff zone occurs

below the Chindwin forearc basin; hence, it is

termed the forearc seismicity, whereas the crustal

seismic zone farthest away from the Benioff zone

corresponds to backarc activity. The backarc

seismicity is most intense in the Jade Mines area

in north Burma, although the Shan-Sagaing fault

shows appreciable activity over its full extent. It

further appears that the forearc and backarc

seismic zones are distinguished by an aseismic

zone between them. The inland seismic slab un-

derlying the CB is 50-70 km thick; this provides

an approximate indication of the thickness of the

Burma plate (Fig. 5). A triangular “aseismic belt”

at shallower depths appears to be defined by theupper surfaces of the Benioff zone and the inland

seismic slab in all the four sectors in Burma. The

western margin of the aseismic belt is, however, ill

defined at present because of poor depth control

for seismic events; Fig. 5 sections give only an

approximate idea about its configuration. Note

that the apex of the “aseismic belt” is always

deflected downward-concordant with the dip di-

rection of the Benioff zone. This bending of the

inland seismic slab is presumably a consequence

of downward drag experienced by the overriding

plate near the subduction zone.

Focal mechanism study and stress pat ter n

Here we examine 45 focal mechanism solutions

for both shallow and intermediate depth

earthquakes in Burma for purposes of infering the

stress distribution and faulting pattern. For Sec-

tors I-III there is a total of 43 solutions de-

termined which are more or less evenly distributed

in these sectors, while for Sector IV only two



solutions could be obtained. Of these 37 events

relate to the Benioff zone seismicity, 3 events are

for the forearc seismic zone whereas the remaining

5 events correspond to backarc activity. The solu-

tions have been determined using P-wave (both

short and long periods) and first motion directions

(source: ISC Bulletin) plotted as stereographic

projection of the lower hemisphere of the focal

sphere employing (i, A) curves of Ritsema (1958).

A double couple source mechanism was assumed.

The focal mechanism solutions are illustrated on

Fig. 6 and the solution parameters are given in

Table 1. Their main results are discussed below.

@W27 03 1964 25.82N95.7lE 5.3 ll5Km .

12 07 1964 24.88N 953lE 55 152Km. 29 07 1970 26.02N 95.37E 6.4 68Km.

N

28 04 1969 25.93N 9520E 5.0 68Km. 03 06 1964 25.88N 95.69E 5.4 I21 Km. 23 07 1975 26.58N 96.36E 5.2 22Km.

Fig. 6. Lower hemisphere stereographic projection for 45 fault plane solutions of Burmese earthquakes. Solid circle and triangle

symbols indicate compressional and dilational first motions of P waves, respectively. P and T correspond to the P and T axes. The

solution parameters are listed in Table 1.



3001 1967 26.lON 96.l4E 5.4 39Km. 29 12 1971 25.17N 94.73E 5.6 46Km.

N N

3005 1971 25.20N96.4lE 5.6 40Km. 31 05 1971 2522N 96.5tE 5.2 22Km.

I I 08 1979 24.20i94.93E 5.0 ll3Km . 13 12 1975 23.62N94.27E 5.2 62Km

25 I I 1979 25.21\96.32E5.0 50Km.

2606 1971 24.6ON94.78E 5.0 74Km.

N

27 07 1973 23.27N 94.49E 5.4 60Km.

N N N

04 07 1973 23.60N 94.86E 5.0126Km. 29 05 1979 24.5ON 94.74E 5.2 82Km.

Fig. 6 (continued).



1802 1965 24.97N 94.21E 5.4 45Km. 31 05 1973 243lN 93.52E 5.8 3OKm. 29 05 1970 2396N 94.06E 5. I 49Km.

N N N

20 05 IS80 23.72N 94.20E 4.8 63Km. 13 IO 1977 23.47N 93.33E 5.2 61Km. IS 06 1963 24.97N92.06E 5.9 44Km.

17 IO1969 23.OW97.70E 6.1 i24Km . 23 02 IS78 23.Ofk S4.7OE 5.0 113Km. 22 01 1964 2233i 93.58E 6.3 60Km.

Fig. 6 (continued).



311

15 12 1965 22.OON94.47E5.2 lO9Km. 03 02 1978 23.02N94.70E 5.1 92Km. 12 519772(.66N92.96E5.439Km

N

05 04 1974 21.33N 93.&8E 5.0 47Km. 0807 1975 2t.42N94.62E 5.9 112Km. L3 06 1964 23.OON93.95E 5.2 6OKm.

Y-+---L 8

@U @F-L22 IO 1966 23.04N 94.28E 5.1 72Km. 20 I 960 2_7&&t92E 5.2 30Km. 1410 971 3.&%3.86E5. I 47Km.

IO IO 1971 3.CON95.92E4.96Km. 15 02 1967 2033h93.99E 5.4 55Km.

Fig. fcontinued).



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52

1

2

1

3

6

9

N

7

1

1

201

26

97

7

50

1

2

6

1

0

0

NOW 2

8

1

101

22

99

1

50

2

2

6

1

0

1

NE

2

1

1

111

26

92

6

52

3

3

5

1

0

4

N

0

9

1

201

22

94

6

54

3

2

5

5

0

1

NO

1

1

1

201

28

90

5

53

4

1

1

9

4

3

NW 7

6

2

001

26

98

1

50

1

1

7

3

0

1

N

6

1

2

201

25

97

8

52

0

3

4

2

6

7

N

6

1

N

6

NO

6

N

5

NW

6

NW 7

N

5

N

3

NW 0

N

6

NW 8

NW 7

NW 8

NW 8

NW 8

NO

5

N

6

N

8

NW 7

N

4

N

3

NW 6

D~ 3 3 3 2 2 3 3 2 3 3 2 2 2 1 2 2 3 2 3 2 1



2

101

29

92

4

54

1

51

1

6

2

NW 9

2

301

23

95

3

58

0

41

1

7

2

NW 8

2

201

29

90

4

51

0

21

1

7

2

N

8

2

201

27

92

8

48

0

2

1

1

7

3

N

7

2

1m-1

24

93

6

52

0

15

1

3

2

NW 4

2

101

29

90

4

59

3

1

5

3

0

8

N

8

2

201

29

94

4

52

2

2

6

1

0

1

NW 1

2

201

26

94

9

60

3

2

5

7

0

1

N

1

3

111

25

94

8

54

2

2

6

1

0

0

NW 2

3

111

20

97

1

61

6

2

2

1

0

2

N

7

3

201

20

97

1

50

3

2

1

9

4

3

NW 8

3

201

23

95

6

63

5

2

2

1

2

2

N

7

3

111

20

94

1

52

6

2

2

6

0

1

NW 6

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001

20

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9

51

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8

3

1

5

3

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3

101

26

99

3

54

0

1

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6

2

N

6

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001

23

96

4

50

1

8

2

1

5

3

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3

001

24

96

1

59

2

2

6

5

0

1

N

2

3

101

20

99

6

52

4

2

2

2

4

1

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5

4

211

20

92

7

51

1

2

1

3

6

1

N

6

4

211

27

99

3

52

0

2

1

3

7

1

NW 7

4

111

20

98

4

51

2

1

1

9

5

3

NW 8

4

111

20

99

4

49

2

1

1

9

5

3

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4

101

23

99

5

54

2

1

2

7

5

2

N

5

4

001

21

98

8

52

4

2

3

8

1

3

NW 8

0 3 3 2 1 7 9 8 1 6 1 6 4 3 4 9 1 1 8 5 5 3 7

N

6

2

N

7

2

NW 8

2

NW 8

2

N

5

3

NW 1

1

N

7

2

NW 8

2

N

7

2

N

2

3

N

5

3

N

3

3

NW 2

2

N

6

2

NW 7

2

N

5

3

NW 7

2

NW 8

5

NW 8

3

N

8

1

N

5

3

N

6

3

NW 8

2

N

1

3

l P=PuA=AmuhS=SkD=DpD=Dpd

o

Yw



Stress und fault pattern within the Benioff zone

Figures 5 and 6, and Table 1 demonstrate that

three distinct categories of faulting and stress pat-

tern are evidenced in the subducting Indian plate

They are:

(1) Low angle thrust events at the upper edge

of the Benioff zone as indicated by solutions l-5

in Sector I, 15-18 in Sector II and 28-30 with 38

in Sector III, all of which suggest pure thrusting

along a shallow dipping nodal plane (G 30 “) to-

wards the east to southeast. Solutions 29 and 38

are reinterpreted here from Le Dain et al. (1984).

The eastward unde~~sting of the Indian plate is

evidenced by these earthquakes occurring at shal-

low to intermediate depths. Events 26-28 furtherdemonstrate that thrusting continues westward at

shallower levels along the west margin of the FMB

in areas of the T~pura-C~ttagong hills where

E-W shortening has produced N-S folds of

Plio-Pleistocene age.

(2) Down-dip tensional events at the lower edge

of the Benioff zone are shown by solutions 6-8 in

Sector I, 19 in Sector II, 31-34 in Sector III and

45 in Sector IV. In general these events suggest

normal faulting or normal faulting with a compo-nent of right-lateral slip along steeply dipping

nodal planes oriented subparallel to the strike of

the Burmese arc. Solutions 6, 19, 31 and 33 are for

the events already studied by Le Dain et al. (1984).

Notice that the suggested normal faulting occurs

generally below the zone of thrusting in all the

sectors. Inferred shallow-angle thrusting at the

upper edge of the subducting lithosphere and

down-dip tensional events at its lower edge

strongly suggest that a double seismic zone ulti-

mately may be discovered in Burma when the

capability of the local seismic network increases.

Such a faulting and stress pattern, characteristic of

a double seismic zone, have been described for

several Western Pacific arcs (cf. Kawakatsu, 1985).

(3) Lateral faulting at shallower depths is indi-

cated by solutions for event 11 in Sector I, events

22-26 in Sector II, events 36, 37 and 40 in Sector

III, and by event 44 in Sector IV. Most of these

events or their inferred slip motion correspond to

transverse lineaments/faults transecting theBurmese arc in NW-NE directions. Earthquake

events 22-26, 39 and 40 appear to correlate to

already mapped transverse faults on the basis of

surface geology and airphoto inte~retation (see

Fig. 2). Event 26 occurred at the south end of the

Mat fault, the most extensive transverse fault

crossing the FMB including the Tripura foldbelt

bordering the Bengal basin. Similarly, events 39

and 40 appear to have originated in association

with another major but unnamed transverse fault

oriented NE across the FMB. The sense of shear

motion as inferred from these focal mechanism

solutions is corroborated by surface geology in

several areas (Fig. 5).

Focal mechanism results for forearc and backarc

seismic zones

Solutions for events 9 and 10 in Sector I and 41

in Sector III suggest a strike-slip m~ha~sm where

the slip motion is believed to occur along

steeply-dipping NNE oriented nodal planes. All

three events are of magnitudes greater than 5.0,

are of crustal origin (Table 1) and locate below the

Chindwin forearc basin. Hence they provide sig-

nificant information about the current motion of

the overriding Burma plate by virtue of theirdepth of origin above the subducting slab. Accord-

ing to a conceptual model given by Sacks et al.

(1978) such a forearc seismic region is commonly

under the influence of a tensional regime. How-

ever, in the present case where only three solutions

are available for the forearc seismic region, we are

inclined to believe that the overriding Burma plate

may be in relative motion in respect to the Indian

plate in addition to being deflected downwards by

the subducting plate. The evolution of the

Chindwin forearc basin (the “Western Trough” of

Mitchell and McKerrow, 1975) must then be

ascribed to the complex deformation experienced

by the Burma plate in the vicinity of the subduc-

tion zone.

It is noted above that the backarc and forearc

seismic zones are apparently separated by a seismic

gap. The most prominent tectonic feature of the

backarc is the Sagaing transform; further east the

Shan scarp is located, bordering the CB against

the eastern highlands (Fig. 2 shows only a gen-eralized pattern). Earthquake events 12-14 in Sec-



315



tor I and events 42 and 43 in Sector 111 belong to

the backarc area. For events 12-14, focal mecha-

nism solutions suggest right-lateral shear along a

steeply dipping fault plane striking NNE. This

conforms with the known slip motion for the

Sagaing transform. But for events 42 and 43 which

are located some distance further east, dominant

east-west tensional stress is indicated by their

focal mechanism results. This we interpret as due

to westward normal faulting at the Shan scar-p.

The Shan plateau belonging to the Asian plate has

been uplifted in respect of the Burmese plains

against the Shan scarp normal fault.

Gravity anomalies

The Bouguer anomalies over Burma and ad-

joining Indian territories (source: Evans and

Crompton, 1946; Gulatee, 1956; Verma and

Mukhopadhyay, 1977; Fig. 7) show two distinct

contour trends reflecting the major structural lin-

eation-an E-W trend over the Shillong plateau

and the Assam Valley north of it forming the

eastern Himalayan foredeep, and a N-S trend

over the Bengal basin paralleling the Indian shield

margin and also over the Burmese arc followingthe Shan scarp geometry. The Bouguer anomalies

range between 0 to -25 mGa1 over most of the

Bengal basin, while the Shiflong massif is outlined

by a higher Bouguer gravity field up to 42 mGa1.

Their margin is marked by the Dauki fault system

that is associated with a very steep gravity gradi-

ent. Several alternating gravity highs and lows, of

relative amplitudes up to 40 mGa1 or more,

dominate the otherwise passive gravity field of the

Bengal basin. These clearly correspond to the

structural highs or depressions underlying the

basin as mapped by seismic, aeromagnetic and

other geophysical surveys. They are: (1) an exten-

sive N-S gravity high at the Indian shield margin

in the area of the Rajmahal hills on the extreme

west which relates to a near-surface layer of high

density (= 3.08 g cm-‘) metamorphics (Mukho-

padhyay et al., 1986); (2) a north-south gravity

low over western parts of the Bengal basin, sub-

parallel to the shield margin gravity high, which

results from a buried Gondwana sediment troughmarked by basement faults (Sengupta, 1966;

Choudhury and Datta. 1973); (3) a gravity high

over the Rangpur saddle that has the form of a

structural high between the Indian shield and the

Shillong massif across the northwestern edge of

the Bengal basin: (4) a region of falling gravity

across the Bogra slope: (5) a NNE-trending grav-

ity high, called the Calcutta-Mymensingh gravity

high, coinciding with an Eocene hinge zone at

depth; (6) a region of NNE and finally E-W

trending zone of low gravity over the Bengal fore-

deep and the Surma basin (cf. Zaher and Rahman,

1980); (7) a gravity high over the Bar&al structural

high; and finally (8) a zone of low gravity across

the Tripura foldbelt as well as over the Hatiya

trough near Chittagong on the Burmese coast.

The gravity field of the Bengal basin steadilydecreases eastward across the Burmese orogen. A

Iinear negative-positive anomaly pair, of am-

plitude in excess of 175 mGa1, runs N-S over a

distance of 1100 km encompassing the CB and the

andesitic volcanic axis (VA) of Burma (only the

central portion of this extensive anomaly zone is

shown in Fig. 7). To the east of the positive

anomaly axis, a narrow but linear negative

anomaly zone is only partly developed over a

strike distance of 700 km, which, however, givesway to a less well-developed zone of “high” grav-

ity near the Shan scarp. The Shan plateau has a

wider extent of smoothly varying negative anoma-

lies spread over the Paleozoic basin in the interior

of the plateau.

Deep structure

The gravity anomalies along a profile I-I’ (Io-

cation shown on Fig. 7), extending from the In-

dian shield to the Shan plateau across the Bengal

basin and Burmese arc, are interpreted here in the

context of a descending lithosphere dipping east

below the CB. For this the regional gravity field

due to the sources in the upper mantle is initially

removed from the observed gravity field. The re-

gional gravity along the profile is obtained from a

GEM 10 model as given by Lerch et al. (1979).

The configuration of the Benioff zone under the

Burmese arc (discussed above for Section BB’

across Sector II) together with the lithosphericboundaries are used for model construction as-



E. -9o- a-o_0 OBSERVED

I t-.-.- COMPUTED

-120

a,/ “I

op o Up0 Km

~J;~~--+-BENGAL n*c lhl -FM R -CR-4 VA 1 I-EASTERN HIGH

_^ I.,_~,

ieo-Km

500 600 7DQ 600 900 loo0

200 I-

Fig. 8. Two-dimensional gravity model along profile II’. Inset shows the computed gravity effect due to the descending Indian

lithosphere below 75 km depth corresponding to the Beniof f zone geometry for seismologic section B B’ illustrated on Fig. 5. The

schematic geology underlying the gravity profile is also shown. Digits refer to density values in g cmA3. B .S.-Bogra stope;

H . Z.--Hinge zone; B. F.-Bengal foredeep; B.H.-Bar&l high; T. F. B.-Tripura foldbelt; F. M. B.-Fold Mountain Belt (Indo-

Burman ranges); C.B.-Central Belt molasse basin of Burma; W.T. and E.T.-western and eastern troughs, respectively, together

constituting the C.B.; V.A.--andesitic volcanic axis of Burma; E.&T.---eastern boundary thrust of the F. M. B . against the C.B.

Geologic index is the same as in Fig. 5.

suming two-dimension~ty (Fig. 8). The gravity

model assumes a dip of about 20” on the Benioff

zone beneath the EBT at the eastern margin of the

Arakan-Yoma, but at depths more than 30 km the

dip steepens to about 45-50” below the CB. The

base of the lithosphere of the Indian plate is

assumed to be 75 km below the Bengal basin. The

density structure for the lithosphere and astheno-sphere in the gravity model is assumed to be

primarily due to temperature perturbations in an

upper mantle of peridotite composition, and fol-

lowing Grow (1973), we assume a higher density

for the subcrustal lithosphere (3.40 g cmS3) than

for the upper asthenosphere (3.35 g cmW3). At

destructive plate margins at depths below 150 km,

the asthenosphere is probably denser than 3.35 g

cm-3 and the descending lithosphere probably

denser than 3.40 g cme3 (Grow, 1973); however,

only the density contrast between the descending

slab and the surrounding asthenosphere is signifi-

cant for calculation of the gravity values rather

than their absolute densities.

Even though the assumed density contrast of

0.05 g cmP3 for the dipping lithosphere below the

Burmese arc against its surroundings is relatively

small, the positive gravity contribution of the de-

scending lithosphere (below 75 km) reaches a max-

imum of 45 mGa1 over the CB molasse basin and

the VA (see inset in Fig. 8). This positive gravity

contribution must be compensated by a lower

density zone above the descending slab in order to

fit the observed gravity anomalies over the CB and

VA. Although the gradient, amplitude and width

of the observed anomalies put severe constraints

on the possible configuration of the low density

zone, much uncertainty remains due to the lack of

density ~fo~ation. As a result, config~ations of

the low density zone differing from that shown in

Fig. 8 are possible. The Fig. 8 data was obtained

by inferring crustal thickening under the CB



molasse basin (in particular, under the Western

Trough) and by inferring a low density zone under

the VA. For the VA. we use density values varying

from 2.60 to 2.80 g cm -’ between its higher and

deeper parts, and approximate its shape from the

inferred geometry of the inland seismic slab shown

in Fig. 5 in relation to seismologic section BB’.

The density value of 2.60 g cm--’ for the upper

portion of the volcanic arc is our estimated aver-

age value for the measured rock density of the

volcanic rocks of the VA. It is of interest to note

that in places the core of the VA is occupied by

intrusive granites; field evidence is available (cf.

Chhibber, 1934a), describing rocks of continental

character that were ejected during violent volcanic

eruptions among others as volcanic bombs. Thissuggests low density rocks under the VA whose

adjacent basement, at least towards the east, is

possibly continental in character. According to

Grow (1973) in the Kamchatka, Alaska, Kurile

and probably the Aleutian arc areas, the volcanic

zones (comparable to the VA of the present case)

are associated with high seismic shearwave at-

tenuation, high temperatures and low densities in

the crust and lithosphere. Seismological studies

are, however, needed to confirm the proposed lowdensity zone below the VA in Burma.

Another significant density anomaly in the

mantle is caused by the crustal layer below the

FMB that is carried down with the descending

plate. The Bengal basin, in its deeper parts, is

believed to be underlain by oceanic crust (Burke

and Deway, 1973; Verma and Mukhopadhyay,

1977). Current subduction of the Indian plate

below the Burmese arc also requires that the Ben-

gai basin must be underlain by oceanic crust. The

Indian shield crust therefore must transit into

oceanic crust below the Bengal basin. However,

keeping in view the most recent results obtained

by Brune and Singh (1986) from the observed

dispersion of fundamental mode Rayleigh and

Love Waves across the Bay of Bengal sediments,

we believe that the crust underlying the basin may

not be truly oceanic rather it may be semioceanic

considering the fact that the basin contains huge

sediments and also the fact that the basin reached

its present land-status as early as Late Oligocene(cf. Evans, 1964; Nandy. 1982). Since at the mo-

ment no deep seismic sounding data are available

for the basin, we can only speculate that this

crustal transition possibly occurs across the east-

ern slope of the Eocene hinge zone. Our main

reasons for this assumption are:

(1) The hinge zone is a regionally extensive

curvilinear tectonic feature continuing in a

NNE-SSW to SW direction from the Bengal basin

into the Bay of Bengal further south; in the latter

area it apparently correlates to the margin of the

continental crust for the Mahanadi offshore basin

along the east coast of India (cf. Talukdar, 1982):

seaward of this zone, 130-100 m.y. old oceanic

magnetic anomalies are believed to be present (cf.

Johnson et al., 1976).

(2) Across the hinge zone the sediment thick-ness underlying the Bengal basin rapidly increases

toward its deeper parts (see above). Landsat image

shows (cf. Nandy, 1980) prominent NE-oriented

lineaments over the area; the iineaments are corre-

latable to the Eocene hinge zone at depth, and are

seismically active. The fold pattern and subsi-

dence character also differ across the hinge zone

(see, Morgan and McIntyre, 1959).

(3) A two-stage evolution for the Bengal basin

is envisaged by Salt et al. (1986); an initial stageof creation of a new oceanic basin in

Jurassic-Early Cretaceous in association with a

passive Indian continental margin, while the sec-

ond stage involves crustal subduction, collision

and orogeny in the Indo-Burman ranges to the

north and east of the basin during the Tertiary.

This has resulted in continuous basin destruction

along its eastern margin. In a framework of con-

tinuing oblique subduction, the Bengal basin is

thus classified as a “remnant ocean basin”.

(4) The pattern of gravity signature across the

hinge zone (Fig. 7) has a semblance with that of

continent-ocean transitional areas; a gravity high

on the continental side and a gravity low ocean-

ward outlined by a zone of steep gravity between

them (cf. Dehlinger, 1978, p. 231-232). This is

evidenced by an anomaly variation of more than

70 mGa1 between the Rangpur saddle and the

Bengal foredeep across the hinge zone (Figs. 3 and

7). This change in gravity is even more pro-

nounced in the N-S direction {total variation is inexcess of 105 mGa1) between the Shillong massif



319

and the Surma valley across the Dauki fault; the

fault has an associated sharp gravity gradient.

For gravity modeling along profile I-I’, we

assume that the Indian shield crust underlying the

western part of the Bengal basin is a nearly 30 km

thick two layered continental crust having respec-

tive densities of 2.70 and 2.90 g cme3 in its top

and lower parts. This crust would be in approxi-

mate isostatic equilibrium as required by the low

elevation of the region. It is also assumed that

gravity variation along the profile is caused by

changes in crustal configuration from the shield

margin to deeper parts of the Bengal basin. We

have suggested elsewhere (Verma and Mukho-

padhyay, 1976, 1977) that the Bengal basin is in

approximate isostatic equilibrium as required byits near-zero Free-air and isostatic anomalies. The

Cretaceous and Tertiary sediments forming most

of the sediment sequences of the basin are to-

gether considered to be of average density 2.40 g

cmP3 (cf. Evans and Crompton, 1946; Verma and

Mukhopadhyay, 1977); the sediment thickness un-

derlying the profile I-I’ is taken from Fig. 3. In

the gravity model we also incorporate two rather

localized geologic bodies: (1) a layer of high den-

sity (= 3.08 g cma3) metamorphics located on thesurface at the margin of the Indian shield (after

Mukhopadhyay et al., 1986) and (2) locally

densified Eocene limestone strata at about 4 km

depth near the hinge zone having density = 2.65 g

cmw3 (after Tiwari, 1983) whose dimensions are

known from seismic surveys. The gross sediment

thickness pattern and crustal geometry considered

in the following ~te~retation may, however, rep-

resent an oversimplified version of the actual sub-

surface situation. While some of these assump-

tions are ambiguous, certain important inferences

can be made on the approximate configurations of

the sedimentary and crustal layers below the FMB

and near the subduction zone in Burma which

satisfy the gravity data.

The sedimentary and crustal layers underlying

the Bengal basin are carried down the trench with

gentle dips at shallow level which steepen at depth

following the geometry of the Benioff zone (Fig.

8). Immediately to the east of the Chin hills, the

average sediment thickness is of the order of 13km, but down the trench the sediments may ex-

tend to a depth of about 15 km. These figures are

comparable to geological estimates (see fig. 5 in

Brunnschweiler, 1974). Grow (1973) and Grow

and Bowin (1975) have discussed the effect on

density as the oceanic crust experiences transition

from lower to higher pressure assemblages (basalt

to eclogite) at pressures between 10 to 20 kb

(30-60 km depth) in a Benioff zone environment.

This process is grossly simplified here by a simple

density change of the oceanic crust from 2.9 to 3.4

g cme3 at about 30 km depth. The derived geom-

etry in Fig. 8 shows that both the sedimentary and

deeper crustal layers are depressed into their re-

spective substrata over a distance of 200 km below

the FMB (covering the Tripura foldbelt and the

Chin hills at the location of the gravity profile)towards the immediate east of the Bengal basin

before being carried down the trench. The Barisal

gravity and structural high developed within the

Bengal basin would therefore correspond, both in

location and amplitude, to typical “outer” gravity

and bathymetric highs seaward of trenches as seen

for several western pacific arcs (cf. Watts and

Talwani, 1974). As the Barisal structural high is

buried below a thick sediment cover, its exact

magnitude is not known; according to Matin et al.(1986) its dimension does not exceed that of a

local fold. But the wavelength of its associated

gravity “high’ however suggests otherwise. Our

gravity model infers that a corresponding upward

bulge by about 400-500 m at the base of the

oceanic crust is required to explain the amplitude

of the gravity high.

Focal mechanism solutions for earthquake

events 26-28 originating under the Tripura fold-

belt (see above) suggest E-W compression, where

faulting presumably occurs in a N-S direction.

This is in accordance with the mapped faults and

axial trend of the FMB. Sediments with an aver-

age thickness of 8-10 km are present for at least

50-60 km east of the gravity ~~rnurn below the

forearc basin (the western trough belonging to CB

molasse basin in Fig. 8) east of the FMB; the

basin appears to have an asymmetric shape, its

maximum sediment thickness of 15 km overlies

the area where the Burma plate is deflected down-

wards in the vicinity of the inclined seismic zone.These sediments together with the underlying crust



of the Burma plate are seemingly thrust over the

layers of the descending Indian plate through an

efficient decoupling. Whether the decoupling is

accomplished by a decollement plane or by im-

bricate thrusting at the juncture of the two plates

it difficult to determine on the basis of gravity

data alone (see, for instance, McCaffrey and

Nabelek. 1984 on gravity modeling for Sunda arc).

This is more difficult since the gravity map availa-

ble to us shows only the broad nature of gravity

anomalies; detailed gravity coverage plus seismic

control are essential for this critical region. The

gravity model (Fig. S), however, illustrates that the

Cretaceous flysch and other sediment stratas of

the FMB continue subsurfacially for some dis-

tance eastward below the forearc basin as a resultof underth~sting manifested by the EBT and its

subparallel thrusts/faults. The EBT, over most

parts of the FMB, denotes outcrops of

serpentinized peridotites with associated gabbros,

ophiolites and volcanic rocks of Late Cretace-

ous-Eocene age (Chhibber, 1934a, p. 329-335;

Bender, 1983; Fig. 2). Many of these masses are

definitely dismembered suites or exotic blocks

moved to their present position (cf. Chhibber,

1934a; B~nnschw~iler, 1974: Nandy, 1982). The

gravity model shows a possible mechanism of

overthrusting of the Burma plate over the east-di-

pping Indian plate at the juncture between the

FMB and CB. It is tempting to correlate the EBT

and its associated ophiolites or other volcanic

rocks with this process of westward overthrusting,

but our gravity data control is too meagre to draw

specific conclusions. For the CB molasse basin

east of the EBT, Brunnschweiler (1974) has al-

ready noted that its underlying Paleozoic and

crystalline basement are also strongly folded,

thrusted and metamorphosed. Sediments of the

forearc basin (including the molasse, flysch, shale

and limestone) were mainly folded during mid-

Eocene to Oligocene times but movements also

occurred in mid- to end-Pliocene; the main folds

are inclined 20-40° eastwards and their axial

surfaces parallel the trend of the Burmese orogen.

Along the eastern flank of profile II’, the Shan

plateau crust is considered to be 32 km thick,

corresponding to its elevation. This crust is con-sidered to be the normal continental type that also

extends below the Burmese plains westward though

it may be considerably thinner there. This thin-

ning possibly occurs in a seismically active crustai

zone below the eastern trough of the CB as stated

above. The eastern trough is assumed to contain

only 2 km sediments of density 2.4 g cm ‘” At the

far eastern end of the profile. the discrepancy

between the computed and observed anomalies

may be attributed to the Paleozoic sediments laid

down over the Shan plateau whose effects were

not considered in the present model.

Acknowledgements

We are thankful to Shri DR. Nandy for helpful

discussions. Shri N. Karmakar drafted the figures.We thank Shri L.V. Ramana for assistance in

computer modeling of gravity data.

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Deep Structures and Tectonics of Burmese Arc 1988

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