RECORDS GSI VOL 113 Pt IV, 1984, 58-61

TECTONIC TRENDS IN SURMA BASIN AND POSSIBLE GENESIS

OF THE FOLDED BELT

By

SUJIT DASGUPTA Geological Survey of 1ndia

(With Plate 6.1)

ABSTRACT

Based on the study of aerial photographs and Landsat imagery, a tectonic map of the Surma basin has been

compiled. The broad structural features of the Mio-Pliocene Surma folds, and the regional tectonic setup,

namely the Paleogene Dauki fault, have been discussed. Nature of the Surma folds and the tectonic

framework of the basin show similarity with the Jura folds of the Alpine Orogen. This leads to a

proposition of analogous evolution for the Jura and Surma folds that are epidermal in nature suggesting a

decollement origin for the Surma folds.

The folded belt of Surma Valley, Tripura-Mizo

hills, Chittagong hill tracts and coastal Burma

belongs to a single stratigraphic-tectonic unit,

hereafter described as folds of the ‘Surnia Basin’.

The Miocene Surma basin is now represented by a

series of anticline ridges and synclinal valleys

running for several kilometers in an overall north-

south trending arcuate belt having a convexity

towards the west. The Surma group of rocks are

exposed all along this belt from about 20°N

latitude to 25 °N latitude, running for about 550

km, from Cachar hills in the north to the northern

fringe of Ramri Islands in the south. The large

scale regional folds of the Surma basin are related

to the Indo-Burma orogen forming an outer arc of

foreland folding. Towards the north, the basin is

bordered by the Dauki fault and the Barail range

(North Cachar hills); the eastern contact is

probably of the nature of thrust with Barail

(Oligocene) rocks of Mizo hills. In the north-west

the basin extends upto the West Bengal-

Bangladesh border and is probably limited by the

basement fault running through Patnitola,

Jaipurhat, Palasbari and Pirgacha in northern

Bangladesh (Farah, 1973). Surface folds north of

23°N latitude is limited by 91 °E longitude and

further south, the fold belt extends up to the coast

of Bay of Bengal. Generally, it is agreed that the

folded belt in the west is probably limited by the

Barisal- Chandpur gravity (basement) high

(Murthy et al, 1971).

Nowhere within the basin is crystalline

basement exposed. The nearest exposed

basement is towards the north in the Shillong

Plateau and Mikir hills. From geophysical

evidence, Evans (1964) postulated that the

basement is at a depth of about 13000 meters

close to the Tripura-Bangladesh border.

Further west in Bagura district of Bangladesh,

west of Jamuna river, in the Kuchma drillings,

crystalline basement has been encountered at

shallow depth within 2,200 meters, below the

Gangetic alluvium and underlying Tertiary and

Mesozoic strata (Brown and Dey, 1975).

Structural framework of this area has been

studied by Nandy (1972), Sarkar and Nandy

(1974), Ganju (1975) and Ganguly (1975). All

are in favour of vertical tectonics for the

evolution of this fold belt. Recently aerial

photographs of Tripura-Mizoram area and

Landsat imagery of part of the Surma basin

have been studied. From this study and field

knowledge of the author, a tectonic map of the

basin has been compiled (Plate 6.1) showing

the trend of fold axes and the major transverse

lineaments. Some of the diagnostic structural

features of this folded Neogene sedimentaries

are discussed below:

1. The folding in Surma region forms a broad

arc, convex towards the west following the

curvature of the Indo-Burma Orogen.

2. In coastal Burma and Chittagong hill tracts

the fold axes trend NNW-SSW; the trend

being almost N-S in the Tripura-Mizo hills,

wherefrom the trend changes to become NNE-

SSW to NE-SW in the Cachar valley. Further

north, in the vicinity of the Dauki fault there

are two E-W trending anticlines, the Sylhet

and Chhatak structures.

3. The folded zone is wider in the central part

in comparison to the southern part. The central

part shows a large number of anticline ridges

that are less in the south.

4. Many of the anticlines split to form two

anticlinal ridges with sub-parallel axial trends.

In some cases they rejoin to form elliptical

synclinal valley within them.

5. Structural complexity increases from west

to east i.e. towards the main orogen. In the

east, the outcrop patterns have been made

complicate by thrust and transverse faults,

while in the west, in Bangladesh and in

western Tripura, the folds are open showing

simple outcrop patterns.

6. All along the outer fringe i.e., along the

western and northwestern margin, folds are

embryonic with short axial continuity and

display an en- echelon pattern, showing drag -

like bending of the folds.

7. Most of the folds are doubly plunging with

low regional plunge.

8. Axial culminations and depressions are

common feature, particularly among the

longer folds. 9. Besides the asymmetrical and overturned folds,

a large number of box- shaped anticlines have

been recognised.

10. Synclines occurring in the central part, in

eastern Tripura and western Mizoram, have steep

flanks with higher plunge of about 20° forming

‘V’-shaped outcrop patterns.

11. Most of the transverse faults trend either NE-

SW to ENE-WSW or NW-SE. The most

prominent transverse fault, in, between .Aizawl

and Lunglei, Mizoram, is the NW-SE trending

‘Mat river fault’. Evidence for dextral movement

along this fault has been recorded (Benerjee et al,

1976).

12. Longitudinal faults, running parallel to the fold

axis, are of the nature of moderate to high angle

thrust and often swerve following the curvature of

the folded rocks.

13. Areas of axial depressions and culminations

and areas where sudden change in trend of fold

axes occur, are commonly traversed by NE-SW

trending transverse fracture lineaments.

14. Tear and thrust faults with trends parallel to

the Dauki fault have been encountered in Sylhet

and Chhatak anticlines. A number of NE- SW

trending transverse faults have resulted in the

formation of fault blocks in the Chhatak structure.

There are indications of WNW-ESE trending

thrust faults on the northern flank of the Chhatak

structure (Khan and Azad, 1963).

Time tectonic events in areas limiting the Surma

Basin are as follows:

1. The Dauki fault system in the north was

considered by Evans (1964) to be of the nature of

strike-slip fault with shear movement of about 250

km from west to east. Now it is argued that this

lineament displays clear evidence of vertical

movement from Cretaceous onwards. The fault

also changes its character from west to east from

high angle reverse to vertical to monocline

(Chakraborty, 1972)

2. The main phase of evolution of the Indo-Burma

Orogen seems to have taken place during late

Oligocene, giving rise to the Miocene Surma basin

in the west and subsequently the Oligocene Barails

were thrust over the Miocene Surmas.

3. Towards the west the fold movement is

absorbed and limited by the submerged Barisal-

Chandpur basement high.

The above three boundary conditions are

responsible for the development of the Surma

basin and subsequent folding. During Pliocene,

fold movement took place in the Surma basin and

proceeded progressively westwards during the

waning phase of the Indo-Burma orogenic

episode. The Surma sediments were folded under

a stress field directed from the east superimposed

on a pre-existing stress field, i.e., the Paleogene

block movement along the east-west trending

Dauki fault system involving the crystalline

basement. Thus the Surma tectonic event found

some inhomogeneities in the basin floor that may

have guided the Pliocene folds and faults. This is

more pronounced towards the north of the basin

where there are E-W trending faults associated

with the Sylhet and Chhatak structures. Mention

may also be made of the drags and axial bends,

towards the NE along the latitude of Silchar which

probably indicates some sort of shear movement

along the EW trending faults.

Two different aspects of the Surma tectonics viz.,

(a) some observed structural features within the

Surma basin and (b) regional time tectonic set-up

for its evolution, clearly bring out certain

similarities, of the Surma folds with the Jura folds

of the Swiss Alps (Pierce, 1966). This prompts to

suggest similar evolution of these two belts. Both

the fold belts take a similar position—the Surma

folds being associated with the Indo-Burma

orogen and the Jura with Alpine orogen, both

representing belt of foreland folding. Fold

geometry both in plan and section and nature of

thrust and transverse faults are similar in many

respects for both the belts. Major folding phase in

the Surma is Pliocene while folding phase of the

Jura has been dated as late Pontian (Laubscher,

1972). The Black forest crystalline mass and the

Rhine graben, north of Jura mountain takes a

similar position both in space and time with the

Dauki lineament along which block movement has

taken place involving the crystalline basement

along with the overlying sedimentaries. The Rhine

graben tectonics and the block movement along

the Shillong Plateau are both essentially Paleogene

events and are responsible for the inhomogeneities

that gave rise to locus of instabilities for the

development of subsequent Surma and the Jura

folds (Laubscher, 1977). Though it is rather

premature to suggest a tectonic model based on

evidence discussed above, yet from the

astonishing similarities with the Jun folds it is

tempting to suggest a decollement origin for the

Surma folds implying an epidermal nature with

little or no involvement of the basement for the

development of the fold belt. Saline springs have

been reported from many areas within this tectonic

province, particularly associated with zones of

dislocation and steeply dipping beds. Source for

saline springs may be attributed to some hidden

salt bed that acted as a passive surface for the

development of the epidermal folds. Further work

sustained by surface ‘and subsurface data will

throw more light on its origin.

Acknowledgements

Shri Kalyan Sarkar, Geologist (Jr), GSI, and the

author jointly studied the aerial photographs and

satellite imageries of this area.

Acknowledgement is due to S/Shri K.V.

Krishnamurthy, Director and D.R. Nandy,

Geologist (Sr.), GSI for going through the

manuscript and offering suggestion for its

modification.

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