Chapter 2

As made evident in the previous chapter, the waters surrounding Myanmar have been

least studied, though -tlitegion is characterised by one of the largest fluvial systems

in the world. The large fresh water discharge from the rivers Ayeyarwady and

Salween cause significant salinity changes in the Andaman Sea. The Andaman Sea is

also characterised by a rather high rate of sedimentation within the Indian Ocean and

thus provides a high-resolution record of sediment history. As the major source of

sediment influx here is the River Ayeyarwady, which is in turn controlled by the

monsoons, the sediments of the Andaman Basin are ideal proxy for palaeomonsoonal

precipitation.

2.1 ANDAMAN SEA:

The Andaman Sea is a semi-enclosed basin southeast of the Bay of Bengal, south of

Myanmar, west of Thailand and east of the Andaman Islands; it is part of the Indian

Ocean. It is roughly 1,200 km long (north-south) and 650 km wide (east-west), with

an area of 7,97,700 km2 . Its average depth is 870 m, and the maximum depth is

3,777 m. At its southeastern reaches, the Andaman Sea (Fig. 2.1 & 2.2) narrows to

form the Straits of Malacca, which separate the Malay Peninsula from the island of

Sumatra. It is connected to the Bay of Bengal by several channels amongst which the

Preparis Channel and the Ten Degree Channel are prominent. At its southeastern

reaches, the Andaman Sea narrows to form the Straits of Malacca, which separate the

Malay Peninsula from the island of Sumatra (Wikipedia).

For the present study, a total area of 1,25,000 km 2 of the continental shelf and slope of

Myanmar was sampled. Emphasis was given to cover the regions being influenced by

the three main Burmese rivers. A few samples were collected off the Rakhine coast in

the Bay of Bengal. Details about the sampling have been discussed in the next

chapter. To be able to interpret the data generated during the present study, it was

important to have a thorough knowledge of this little known region. The same is

discussed in the present chapter.

'Tropic =of -

115 _20

VAN GlION

Preparis Channel

aior off afg-oft

N STUDY AREA

Ten Degree Channel

.tatittairti W)

5

WEAN

Eciuttipt ISLA D

15

10

Study Area 18

2.2 LOCATION & EXTENT OF THE HINTERLAND:

Myanmar (formerly known as Burma) is a Southeast Asian country and very much a

part of the Asian Continent (Fig. 2.1). Located along the eastern flanks of Bay of

Bengal, it shares 1,463 km of its land border with India along the Naga Hills. Its

other neighbours with whom its shares its borders are Bangladesh in the northwest

(193 km), China to the north (2,185 km), Laos to the east (235 km) and Thailand to

the southeast (1,800 km).

Fig. 2.1: Regional setting of the study area (Modified after World Atlas, MSN Encarta)

2.3 COASTLINE:

It has a 1,930 km long uninterrupted coastline which accounts for one third of its total

perimeter. The Bay of Bengal occupies its southwest coastline whilst the Andaman

Sea lies to the south.

Arabian Sea

Myanmar phant Point

Yangon

Mouths of the Ayeyarwady

Gulf of Ma

N 30

Cocas Island

Line 2

2000

30 -

2

20'

15'

10'

INDIA Merg Archi

65'E 70 75' 80" 85 90' 95' 100'

Line 1

Bay of

Bengal

•

Fig. 2.2: The bathymetric map of the study area. Line 1-4 depict the transects along which hydrographic data has been obtained from Levitus Climatology (Levitus &

Boyer, 1994)

2.4 DRAINAGE:

Three major rivers drain Myanmar and join the Andaman Sea along the coastline in

the south (Fig. 2.3).

2.4.1 River Ayeyarwady (former Irrawaddy)-

With a length of 2,170 km, is the principal river of Myanmar. It rises from the glaciers

of the high and remote mountains of northern Myanmar and flows through western

Study Area 20

Myanmar, draining the eastern slope of the country's western mountain chain. The

Ayeyarwaddy River bisects the country from north to south and empties into the

Andaman Sea after forming a remarkable

nine-armed delta. With its base width of

nearly 200 km, the Ayeyarwady Delta is one

of the world's major rice-growing areas. The

flow in the Ayeyarwady is at its lowest in

February and March, while there is a sharp

rise in level in April-May as a result of

melting snow in the upper catchment, and a

Bay further steep rise in May-June with the onset

of of the monsoon. The maximum flow occurs Bengal Thailand in July or August. Near the head of the

Delta, a mean low and flood discharge of

2,300 m3/s and 32,600 m3/s have been

determined (ENCYCLOPEDIA

Fig. 2.3: Major rivers in Myanmar BRITTANICA, 2000). The Ayeyarwady

River discharges > 430 km 3 of fresh water and > 260 million ton of sediment

annually, of which more than 80% is during the southwest monsoon (Rudolfo, 1969).

2.4.2 River Salween-

With a total length of 2820 km the Salween is the world's 26 th longest river and its

basin covers a total area of 32,000 km 2 (Longcharoen, 2003). The Salween River

originates in the eastern Tibetan highlands, part of the Himalayan mountain range,

flows through China's Yunnan province into Burma before entering Thailand. After

entering Myanmar it forms the border with Thailand for about 110 km and continues

through eastern Myanmar to empty into the Andaman Sea. Although the catchment

area of Salween is limited and sheltered from seasonal rains, its water volume

fluctuates considerably from season to season (ENCYCLOPEDIA BRITTANICA,

2000; FAO, 2000). It is the longest undammed river in the mainland Southeast Asia.

2.4.3 River Sittang-

It is located in the south of the country between the Ayeyarwady and Salween Rivers.

The Pegu Yoma range separates its basin from that of the Ayeyarwady. The river

Andaman Sea

Study Area 21

originates at the edge of the Shan Plateau southeast of Mandalay, and flows

southward to the Gulf of Martaban. Its length is 420 km and its mean annual

discharge is around 50 km3 per year. The annual sediment discharge from the Salween

is about 100 million ton (Meade, 1996).

The Ayeyarwaddy is the 5 th largest river in terms of suspended sediment discharge

and together with the Salween and the Sittang annually deposits 350 million tons of

sediment into the Northern Andaman Sea (Ramaswamy et al, 2004). The Ayeyarwady

and Salween catchments adjoin each other, debouching into the Indian Ocean over a

length scale similar to the deltas of the Ganges-Brahmaputra or the Amazon.

Therefore, the Ayeyarwady and Salween rivers could be considered a single point

source contributing to the global ocean. The implied

organic carbon yield from the catchments is 8.4-12.9

t/km2/yr, which is clearly amongst the highest in the

world among rivers of similarly large size. (Bird et al,

2008).

2.5 THE AYEYARWADY DELTA:

The most dominant feature of the country is the

Ayeyarwady River system, the surrounding valleys and

the river's massive delta in the south. The most densely

populated part of the country is the valley of the

Ayeyarwady River which, with its vast delta, is one of

the main rice-growing regions of the world. The delta Fig. 2.4: The extent of the delta on land

system of the Ayeyarwady River extends in a great

alluvial fan from the limit of tidal influence near Myanaung (18°15'N) and extends to

the Bay of Bengal and Andaman Sea, 290 km to the south. It lies between latitudes

15° 40' and 18° 30' N approximately and between longitudes 94° 15' and 96° 15' E

and has an area of 35,135 km2 . Figure 2.4 gives an idea about the proportion of land

area of the country occupied by the delta. Its altitude above MSL varies between 0-5

m. This alluvial plain is bounded to the west by the southern Arakan Yoma range and

to the east by the Pegu Yoma.

Drainage of the Ayeyarwady River is directly into the Andaman Sea through

nine major river mouths, the Bassein, Thetkethaung, Ywe, Pyamalaw, Ayeyarwady,

Study Area 22

Bogale, Pyapon, China Bakir and Rangoon. These rivers carry a heavy silt load, and

their waters are very turbid. The delta is actively accreting seawards, and as a result

the sea is very shallow for some distance out to sea. Water depths are less than 5.5 m

across the whole coastline fronting the delta and up to 28 km offshore in the east. The

present rate of advance of the delta is estimated at 5-6 km per 100 years, equivalent to

about 1,000 ha per year. Several small islands, some of which are visible only at low

tide, have developed offshore.

2.6 CLIMATE:

Myanmar has a monsoonal climate, with an average annual rainfall of about 1,500-

2,000 mm in the north increasing to 2,500 mm in the southeast and 3,500 mm in the

southwest. Over 90% of the rain falls between mid May and mid November. During

the monsoon season, the maximum and minimum temperatures in the coastal zone are

about 37°C and 22°C, respectively. The seas may be very rough, and there are often

strong winds from the south and southwest. The period from mid October to mid

February is generally dry and cool. Temperatures rise after February, and April and

early May are characterized by hot, variable weather with pre-monsoon squalls.

(Website of the ASEAN Regional Centre for Biodiversity Conservation)

2.7 COASTAL GEOMORPHOLOGY:

The coastline of Myanmar can be divided into 4 major physiographic divisions-

2.7.1 The Rakhine coast-

Fringing the Bay of Bengal to its east, the Rakhine coast is characterized by the

Rakhine Yoma (Arakan Mountains) range in the west. Between the Bay of Bengal

and the hills of the Arakan Yoma is Rakhine State, a narrow coastal plain. The coast

drops steeply towards the Bay of Bengal and in interrupted by occasional shoals,

rocks, cliffs, submerged islands and topographic highs, all of which follow the trend

of the Andaman-Nicobar chain of Islands, which can be traced up to the Naga Hills in

Northeastern India.

2.7.2 The Ayeyarwady continental shelf-

The shelf region off the Ayeyarwaddy Delta has complex geological setting in the

Andaman Basin (Curray et al., 1979). The shelf width is about 170 km off the

Study Area 23

Ayeyarwady River mouths and increases to more than 250 km in the center of the

Gulf of Martaban (Fig. 2.2). Bathymetric data acquired during the present study

shows that the shelf break is at 110 m isobath (Fig. 2.2). Beyond the shelf break the

depth increases rapidly to approximately 2000 m, except in the bathymetric low. The

seafloor within the bathymetric low is riddled with erosion channels and "V-shaped"

notches (Rao et al., 2005).

2.7.3 The Gulf of Martaban-

An arm of the Andaman Sea, it lies on the east of the Ayayerawady delta, indenting

South Myanmar and receiving the waters of the Sittang and Salween rivers (Fig. 2.2).

A complex system of N—S trending dextral strike slip faults runs through the Gulf of

Maratban and the Ayeyaerwady shelf; the most prominent of these is the Sagaing

Fault System that extends southwards and joins the Central Andaman Rift (Curray et

al., 1979; Kamesh Raju et al., 2004). A N—S trending 120 km wide bathymetric low is

present between the above fault systems and the Malay continental margin. The

Martaban Canyon lies within this bathymetric low and appears to be controlled by the

N—S trending fault systems (discussed in section 2.10). Seafloor in the Gulf of

Martaban and adjacent inner shelf is generally smooth whereas the outer shelf has a

rough surface with relief of 2-20 m and has topographic features such as pinnacles,

highs and valleys, buried channels and scarps (Rao et al., 2005).

2.7.4 Islands-

The Mergui Archipelago (also Myeik Archipelago) is an archipelago in far

southern Myanmar (Burma) (Fig. 2.2). It consists of more than 800 islands, varying in

size from very small to hundreds of square kilometres, all lying in the Andaman Sea

off the western shore of the Malay Peninsula near its landward (northern) end where it

joins the rest of Indo-china. Geologically, the islands are characterized mainly by

limestone and granite. They are as a general rule covered with thick tropical growth,

including rainforests and their shorelines are punctuated by beaches, rocky headlands,

and in some places, mangrove swamps. Offshore are extensive reefs. Some islands

have huge boulders, soft corals and sea fans. Most of them are completely

uninhabited. The archipelago's isolation is such that much of it has not even yet been

thoroughly explored (Wikipedia).

6 t Mang River

Myanmar Bay of

Bengal ,e Yangon Rive ,e

Clay Andaman Sea

Gulf of Martaban

Study Area 24

Coco's Island is a group of three small islands in the Bay of Bengal located

about 50 km northeast of the Indian Andaman Islands and 300 km south of mainland

Myanmar. Geographically, they are a part of the Andaman Islands archipelago and

separated from the North Andaman Island (India) by the 20 km wide Coco channel

(Wikipedia).

A few other islands are located in the Bay of Bengal along the Rakhine coast.

The Cheduba Island with an area of approximately 500 km 2 and the Ramree Island are

amongst the better known amongst these (Wikipedia)

2.8 SEDIMENT DISTRIBUTION ON THE AYEYARWADY SHELF:

Fig. 2.5: Textural variation in the sediments of the Ayeyarwady continental shelf (After Rao et al., 2005)

Study Area 25

Based on grain size variations three distinct areas of sediment texture have been

delineated on the Ayeyarwady shelf (i) near shore muds (ii) outer shelf relict sands

and (iii) mixed sediments in the Martaban (Fig. 2.5) (Rao et al., 2005). The outer shelf

has been known to be a zone of non-deposition and starved of modern fine grained

sediments. The relict sands cover an area of about 50,000 km 2 suggesting probable

deposition during the Holocene transgression (Rudolfo, 1969). Most of the sediments

discharged by the Ayeyarwaddy are displaced eastwards by the prevailing westerly

currents into the Gulf of Martaban. The Gulf of Martaban acts as a sediment trap. The

Martaban Canyon is a conduit for terrigenous sediments reaching deep Andaman Sea.

The Gulf is characterised by modern muds and mixed sediments. At the center of the

Gulf the mud belt is as wide as 250 km and ranks amongst the largest modern mud

belts of the world oceans (Rao et al., 2005).

2.9 COASTAL HYDROGRAPHY:

Depth profiles of the annual mean temperature, salinity and oxygen concentrations in

the study area were obtained along two latitudes (14 °N & 15.5°N, i.e. Line 1 & Line 2

in Fig. 2.2) and two longitudes (94.5 °E and 97°E, i.e. Line 3 & Line 4 in Fig. 2.2),

from Levitus Climatology (1994). These profiles have been illustrated as Fig. 2.6-2.9

and discussed below:

2.9.1 Temperature-

The annual mean surface temperature in the study area is —28 °C and drops gradually

to —27°C up to a depth of 40 m, after which the temperature gradient is very high. The

temperature drops to about 22 °C at a depth of 100 m. Again beyond the depth of 200

m the gradient becomes gentle. The temperature is about 10 °C at 400 m depth and

drops to about 5 °C at 1000 m (Levitus Climatology, 1994).

2.9.2 Salinity-

Closer to the coastline along the 15.5 °N latitude the surface salinity varies between

31.4 psu west of the shelf to 30.2 psu in the Gulf of Martaban. The water column is

well stratified in terms of salinity and a horizontal is attained at a shallow depth of

—15 m. However, the salinity increases rapidly up to a depth of 30 m, after which the

waters are thickly stratified. At about 100 m the salinity drops to 34.5 psu and to a

maximum of 35 psu at about 200 m and remains same up to a depth of

30°N_

20°

10° -

' •=r1r1.+LP---rm., 130°E 120°E 150° E

28 5

-

- 21.5

- 195

_

23.5

255

245

22.5

2a5

18.5

17.5

16.5

155

14.5

115

- 12.5

11.5

27.5

- 26.5

105

00

E _c 200

0

300

400

92.5°E 93.5° 94.5° 95.5° 96.5° 97.5° 98.5°

Study Area 26

Temperature (°C)

¶4$

351

349

34.7

34.5

343

34.1

33.9

337

33.5

33.3

33 1

32.9

32.7

32.5

32.3

32.1

31.9

31.7

31.5

31.3

31.1

309

100 -

E _c 200 - Ct.

300 -

400 r

92.5°E 93.5° 94.5° 95.5° 96.5°

Salinity (psu) 97.5° 98.5°

50

4.8

4.6

4.4

42

40

3.8

3.6

34

3.2

3.0

28

26

2.4

22

2.0

1.8 1.6

1.0 0.8

06

0.4

02

100

E .c 200 0_ a) a

300

400

98.5°

Dissolved 02 (mill)

Fig. 2.6: Depth profiles of the (a) Annual Mean Temperature (b) Annual Mean Salinity and (c) Annual Mean Dissolved Oxygen concentration, obtained from NOAA/PMEL TMAP FERRET Ver. 5.22; Data Set: levannual.nc , along Latitude 14 °N (interpolated)

Profile corresponds to line 2 in Fig. 2.2

30N-'

20]

10°:

_ -0. ,

_ --

_ - _,--- ,-

- ___---- --------•

1 - OM MN - NM

MI - IN if

- i - I i I

- \ --- .__--.,---------------,__.„-L------

II

._,,-------------2% -.

_ - I 1 I I 1 I i I I

0

20

40

E -C

60

80

100

280

278

27.2

268

26.2

260

256

25.2

248

24.4

24.0

236

23.2

2/8

22.4

22.0

Study Area 27

92.5°E 93 5° 94.5° 95.5° 96.5 °

97.5°

98.5°

Temperature (°C)

- - 2 1

0 iii

- al In

- /11 1.1

_ 11.1 IR -

- 1 _ 1

I

33p

3

- 1.-------------

92 5°E

93.5°

94.5°

95.5°

96.5 °

97.5°

98.5°

Salinity (psu)

_________„

-----

---------

----

-

-

_ _ - _ _ - _ 7 I li

. ._ _ 5,r-------7----- ,: _--------- - ------ 2.*-"''-. ------ - • - - ---

- i_-----

92.5°E 93 5° 94.5° 95.5° 96.5°

97.5°

98.5 °

Dissolved 02 (m1/1)

Fig. 2.7: Depth profiles of the (a) Annual Mean Temperature (b) Annual Mean Salinity and (c) Annual Mean Dissolved Oxygen concentration, obtained from NOAA/PMEL

TMAP FERRET Ver. 5.22; Data Set: levannual.nc, along Latitude 15.5 °N (interpolated) Profile corresponds to line 1 in Fig. 2.2

E 40 -

.c

• 60

_c a. a.)

•

60 -

100

100 -I

40 -

20 -

80 -

0

20

80

0

34.6

34.2

33.8

314

33.0

3/5

32.2

31.8

31,4

31.0

30.2

306

298

4.6

4.4

4.2

4.0

3.8

38

3.4

3.2

3.0

2.8

2.6

2.4

2.2

2.0

1.8

1.6

1.4

1.2

1,0

0.8

0.6

0,4

Study Area 28

29

28

27

26

25

24

23

22

21

20

19

18

17

1 6 15

14

13

12

10

a

6

200 -

E ^ 400 -

.c

O 600 -

800 -

1000 I I ,

11.5°E 12.5° 13.5 ° 14.5° 15.5° 16.5 °

Temperature (°C)

20°E 50 ° 80° 110° 17.5°

0

200 -

E•"" 400 -

_ 0) O 600 -

800 -

3*J

35,0

34.6

34,2

33.8

33.4

33.0

32.6

32.2

31.8

31.4

31.0

30.6

1000 r

11.5°E 12.5° 13.5 ° 14.5° 15.5°

Salinity (psu)

30 2

16.5 ° 17.5°

50

46

4.6

4.4

4,2

4.0

3,8

36

3.4

3,2

30

2.8

2.6

29

2.2

2.0

1.8

1.6

1.4

1.2

1.0

400 -

.c _

O 600 -

800 -

200 -

1000

11.5° E 12.5 ° 13.5° 14.5° 15.5° 16.5° 17.5°

Dissolved 02 (m1/1)

0.4

02

Fig. 2.8: Depth profiles of the (a) Annual Mean Temperature (b) Annual Mean Salinity and (c) Annual Mean Dissolved Oxygen concentration, obtained from NOAA/PMEL

TMAP FERRET Ver. 5.22; Data Set: Ievannual.nc, along Longitude 94.5 °E (interpolated) Profile corresponds to line 3 in Fig. 2.2

_ -- - ..

51f /

_____--------*-

I I

1 - I

il _ NV

NW 1111 MI

_ IM NM 11111 MK

_

-

i

IM------------- 2 _.___,___________jr_

___/,,..:_____7 __ ------n

21.6 251 24.6

- 74.• -

-

11.5°E 12.5° 13.5° 14.5° 15.5°

16.5°

17.5°

Temperature (°C)

0

20

E 40

-c O. • 60

80

100

284

28.0

27.6

27.2

28.8

284

26.0

25.6

25,2

24/

24.4

24.0

23.6

23.2

22.8

22.4

22.0

21.8

21.2

2/8

204

70°E 85° 100° 115°

-, 4

14

----au

-- ''--- --.,__, , __ _

Ad a-- _

_ 1 III ■

- III 35.0

- II •

/31 _ M

------.'''.-...'-----. -

531

12.5° 13.5°

14.5° 15.5°

16.5°

17.5°

0 346

344

342

34.0

34.8

33.6

33.4

312

33.0

33.8

32.6

32.4

32.2

32.0

31.8

31.6

31.4

31.2

31.0

30.8

30.6

30.4

30.2

3/0

298

20 -

40 -

O 60-

80 -

100 n

11.5°E

E

Q. a)

_ I_ _---- ----

■

... - sit is se - si

- . is.

--- :1 _ '------

11 _ 1 s9

'<::

.---'.-*-' - I II

I

E .c

100

80

0 4.8

4.6

4.4

4.2

40

3.8

16

3.4

3.2

3.0

28

26

2.4

2.2

26

1.8

16

1.4

1.2

20 -

40 -

0. a)

O 60 -

Study Area 29

Salinity (psu)

11.5°E 12.5° 13.5° 14.5° 15.5°

16.5°

17.5

Dissolved 02 (mill)

Fig. 2.9: Depth profiles of the (a) Annual Mean Temperature (b) Annual Mean Salinity and (c) Annual Mean Dissolved Oxygen concentration, obtained from NOAA/PMEL

TMAP FERRET Ver. 5.22; Data Set: levannual.nc , along Longitude 97°E (interpolated) Profile corresponds to line 4 in Fig. 2.2

93°E 95° g7 98 99°1 93°E 04° 95° 96° 97°

•

9

N

17"

16°

14°

13°

12°

18° R 18°

Bay of Bengal

Are& of NO andenieMation raft* tt •

1'1'0 Susperxbed SCOMetlt mite

7—=- Tidal currant

Sears',Orli •sraintto

Sediment niOcOment

General circulation

Mouths of the Am/anon:1y

Andaman Sea

Myan ar Sittang A.

Yangon R.

1° O

N

15°

13°

7°

6°

4°

2°

4i $li:riPIVW1..tt N.,it 4(16C

Tidal current

SediMOM Wire

Sediment fricoderwt

GOrioreA Circulation

Fig. 2.10: Schematic diagram showing sediment sources, transport pathways and depositional areas on the Ayeyarwady shelf (a) During the SW monsoon, sediment discharge is high and general circulation is towards east. (b) During the NE monsoon, sediment discharge is low and general

circulation is towards west (After Rao et al., 2005)

Study Area 31

600 m. (Levitus Climatology, 1994). At shallower depths the salinities are lower

towards Gulf of Martaban.

2.9.3 Dissolved Oxygen-

The surface waters across the delta shelf seem to be well oxygenated (-5 mill

dissolved 02), especially towards the Gulf of Martaban. The drop in oxygen content is

very gradual and falls below 1 m1/1 beyond 90 m. The dissolved oxygen concentration

drops to 0.2 m1/1 at depths between 140 m and 350 in the water column (Levitus

Climatology, Levitus & Boyer, 1994).

2.9.4 Tides and current circulation pattern-

Tides at the mouth of the Ayeyarwady are semi-diurnal, and have a range of 2.0-2.5 m

along the outer coast. At Yangon, 72 km from the open sea, the tidal range is 3.5-5.1

m. Sea dykes have been constructed in some areas to prevent tidal inundation, and the

Government has recently carried out several polderization schemes in the outer delta.

The Andaman Sea experiences the seasonally reversing Asian monsoon (Wyrtki,

1973). Circulation in the Andaman Sea is cyclonic during southwest monsoon (May—

September) and anti-cyclonic during northeast monsoon (December—February). The

Gulf of Martaban is a macro-tidal area with its highest tidal range of nearly 7 m

recorded at the Elephant Point (Indian Tide Tables, 2002). Near the mouths of the

Ayeyarwady, tidal range is between 2 and 4 m and can be classified as meso-tidal.

The tidal currents are strongest during spring tide, reaching as high as 3 m/s in the

Gulf of Martaban (Bay of Bengal Pilot, 1978).

During the SW monsoon, the surface currents flow eastwards and prevent

sediments escaping into the Bay of Bengal (Rodolfo, 1969). The general circulation

reverses during the NE monsoon period (November—January) and the surface currents

flow towards west. These currents may push some of the suspended sediment-tongues

westwards into the Bay of Bengal. Satellite images obtained during November—

December reveal tongues of suspended sediments heading westwards into the Bay of

Bengal. The sediments carried into the Bay of Bengal may move northwards by the

anti-cyclonic circulation of NE monsoon (Shetye and Gouveia, 1998) and probably

reach the shelf region off Rakhine coast westwards into the eastern Bay of Bengal

(Fig. 2.10).

Study Area 32

2.10 TECTONIC SETTING OF THE STUDY AREA:

Situated on one of the most active continental margins of the world, the tectonic

framework of the study area cannot be neglected. This is true especially because it

could have implications while making estimates about palaeoclimatic events such as

sea level fluctuations. The presence of relict sands in on the shelf are signatures

enough of such events in the past.

The tectonic and geological history of the Andaman Sea cannot be separated

from the tectonics and geological histories of Myanmar (Burma) on the north, the

Andaman and Nicobar Islands part of the accretionary prism on the western side of

the Andaman Sea, and Sumatra on the south (Curray, 2005). Running in a rough

north-south line on the seabed of Andaman Sea is the boundary between two tectonic

plates, the Burma plate and the Sunda Plate (Fig, 2.11). These plates (or microplates)

are believed to have formerly been part of the larger Eurasian Plate, but were formed

when transform fault activity intensified as the Indian Plate began its substantive

collision with the Eurasian continent. As a result, a seafloor spreading centre was

created, which began to form the marginal basin which would become the Andaman

Sea, the current stages of which commenced

approximately 3-4 million years ago (Ma). Within

the sea to the east of the main Great Andaman

island group is Barren Island, an active volcano

(the only presently active volcano associated with

the Indian subcontinent). Its volcanic activity is

due to the ongoing subduction of the India Plate

beneath the Andaman island arc, which forces

magma to rise in this location of the Burma Plate.

As on date, The Andaman Sea is an active

backarc basin lying above and behind the Sunda

subduction zone where convergence between the

overriding Southeast Asian plate and the

subducting Australian plate is highly oblique. As

stated before a complex system of N—S trending

dextral strike slip faults runs through the Gulf of

Maratban and the Ayeyaerwady shelf; the most

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Fig. 2.11: Tectonic framework of the study

area (Wikipedia)

Study Area 33

prominent of these is the Sagaing Fault System that extends southwards and joins the

Central Andaman Rift (Curray et al., 1979 and Kamesh Raju et al., 2004). Vigny et al.

(2003) estimate total strike-slip plate motion in Myanmar as 35 mm/yr, with <20

mm/yr along the Sagaing Fault itself. Curray (2005) estimates that N—S motion in the

Central Andaman Basin is 27 mm/yr. Sieh and Natawidjaja (2000) and Genrich et al.

(2000) estimate 25 mm/yr at northwest Sumatra, decreasing to 10-20 mm/yr in

southeast Sumatra. The E—W component of opening of the Central Andaman Basin at

the present time is 12 mm/yr. This compares with the rate of convergence between the

Andaman Islands and mainland eastern India of 15 mm/yr reported by Paul et al.

(2001) from GPS surveys.