Quart. Jour. Geol. Min. Met. Soc. India,

Vol. 55, No. 4, p. 186-194, 1983

TECTONIC EVOLUTION OF TRIPURA-MIZORAM FOLD BELT,

SURMA BASIN, NORTHEAST INDIA

D. R. NANDY

1, SUJIT DASGUPTA

1, KALYAN SARKAR

1 AND ANIRUDDHA GANGULY

2

1. Geological Survey of India, Calcutta-700016

2. Department of Geo1ogical Sciences, Jadavpur University, Calcutta-700032

Abstract

The Tripura-Mizoram area, a part of Neogene Surma basin, comprises a belt of elongated folds

with marked sub-meridian trend and arcuate shape with westward convexity. This fold belt, parallel and

subjacent to the Arakan Yoma subduction/ suture zone, is spectacular in the Indian sub-continent and has

evolved within a compressive stress field generated by the eastward drift of the Indian plate during late

Tertiary. The style of folding and outcrop pattern in the region, show that there was minor rotation of the

stress axes which in turn corroborates rotation of the Indian plate during late Tertiary while it drifted

eastward. Geological mapping aided by photogeological and remote sensing studies reveals that the

deformation of the sedimentary units was first initiated by tangential E-W compressive stress resulting in

shortening, principally by folding and strike faulting and adjustment along conjugate shear fractures with

strike slip movements. The present study also reveals that the hypothesis of vertical tectonics as

postulated by earlier workers for the tectonic evolution of the region is not tenable.

Introduction

The area forms the major part of the

Neogene Surma basin to the west of the

Arakan Yoma subduction-collision zone,

which represents the northward extension of

the Sumatra-Java trench—the eastern

margin of the Indian plate. The basinal

sediments are folded into long arcuate belt

in a series of linear narrow anticlines and

synclines forming a unique foreland fold

belt in the Indian sub-continent like the Jura

mountain and the “valley and ridge

province” of the Appalachian.

Though the area is very significant in

respect of regional tectonic evolution and its

set-up in relation to the development of the

eastern margin of the Indian plate, no

comprehensive work has so far been carried

out to unravel the evolutionary process and

history of this fold belt.

As the area is highly inaccessible it

remained terra incognita till recently. In

order to gather a comprehensive idea of the

geology and structure of the area as a whole,

188 D. R. NANDY, SUJIT DASGUPTA, KALYAN SARKAR AND ANIRUDDHA GANGULY

photogeological map of the States of Tripura

and Mizoram, India, was prepared aided by

the knowledge of field mapping and check

traverses (Banerjee et al, 1979; Das Gupta et

al, 1979 ; Nandy et al, 1972 and 1973)

which is generalised and reproduced in

figure 1. Regional structural elements have

been studied in LANDSAT imagery mosaic

(Fig. 2). With the knowledge of regional

tectonic setting of the Surma Basin (Nandy,

1980, 1981 and 1982) and with the results

obtained from the photogeological and

remote sensing studies an attempt has been

made here to work out and synthesise the

geodynamics of this fold belt.

Geological Setting

The Neogene Surma basin of which the

present area is a part, is limited by (a) the

post Barail unconformity (close of Oli-

gocene), subsequently faulted, to the east,

(b) the E-W Dauki fault and the Disang

Thrust to the north and (c) the Sylhet fault

(Das Gupta, et a!., 1982) and the Barisal-

Chandpur high (Sengupta, 1966), concealed

below the alluvium of Bangladesh, to the

west and northwest. The folded sediments of

the Surma Group continue to the south up to

Ramri Island of Burma. Within this vast

terrain the arcuate fold belt continues for

550 km along strike and about 200 km

across at its widest part.

Syndeformation deposition of the

subflysch-molasse sediments commenced at

the close of Oligocene over the basin-floor

formed of the Barails or coeval rocks.

Stratigraphy

The entire sedimentary column of the

area is constituted of sandstone, siltstone,

shale, mudstone, sand rock, silt and rare

pockets of shell-limestone, which is divided

into four major stratigraphic units based

mostly on lithologic characteristics.

Sequentially they are, (1) Barail (Oligocene;

3000 m) sandstone and shale, (2) Surma

Group (Miocene) consisting mainly of (a)

arenaceous Lower Bhuban Formation (+

9000m, (b) argillaceous Middle Bhuban

Formation (3000m), (c) arenaceous Upper

Bhuban Formation (1100 m) and (d)

argillaceous Bokabili Formation (1000 m),

overlain by (3) Tipam Group (Pliocene; +

1300 m) consisting. of feldspathic sand with

fossil wood and minor silt. The youngest (4)

Dupi Tila Formation (not shown in figure 1)

consisting of mottled clay, fine silt and

laterite occupies the synclinal valleys to the

west and overlies the Tipams over a

pronounced unconformity. Older and older

rocks crop out toward east across the strike.

Sediments are characterised by various

types of primary sedimentary structures

indicating shallow marine to deltaic

environment (Sarkar et al., 1977). Scanty

faunal and floral record indicates that the

sediments were deposited in shallow paralic

environment (Ganguly, 1975). Details of the

faunal assemblages of the area are given by

Das Gupta (1982) suggesting Mio-Pliocene

age of these sediments.

Structure

There are 15 major long and arcuate

anticlines and corresponding synclines in the

TECTONIC EVOLUTION OF TRIPURA-MIZORAM FOLD BELT 191

area under consideration, trending NNW-

SSE to N-S to NNE-SSW from south to

north with convexity towards west (Figs. 1

and 2). Many of the anticlines bifurcate to

form two anticlinal ridges with sub-parallel

axial trends. In some cases such split axes

merge again to enclose an elliptical synclinal

valley (a common feature in Chittagong

hills) giving rise to flattened lensoid

structural domes and basins. These

structures might have developed as a result

of the compression axis changing orientation

where ‘a’ kinematic axis of the later evolved

folds was very close to the axial plane and

‘a’ kinematic axis of the earlier formed

folds, viz., type 1 of Ramsay (1962).

However, the point that should be borne in

mind in this context is that in a single

exposure or hand specimen of a fold no

interference pattern could be detected and

that the above comments are based solely on

interpretation of structures and linears of

mesoscopic scale. There are more than one

axial culminations and depressions in each

long structure with regional plunge varying

from 5° to 25°. Width of the synclines

gradually diminishes from west to east

where the folds become appressed. In the

westcentral part of the area the major folds

show varying geometry The Baramura,

Langtarai, Machmara and Hachecktlang

anticlines are flat crested and box shaped,

whereas the other anticlines (Fig. 1) in the

neighbourhood are sharp crested. Though all

the synclinal fold forms in the western and

central part of the area are broad and wide,

the Damchhera syncline east of Bansul

anticline is appressed and steeply plunging

(Fig. 1). The fold belt is narrowed down to

the south and gradually widens to the north.

Faults, whose traces run parallel to

subparallel with the fold axes are moderate

to high angle reverse faults and often swerve

to follow the curvature of the fold axes in

plan. The fault traces are located mainly

along the hinge zone of the fold forms.

There are numerous conjugate faults, mostly

intersecting in the zones of culminations and

depressions.

Structural complexity and also the

intensity of deformation gradually increase

from west to east, i.e., from Bangladesh end

to Burma end. To the east the folds are much

compressed, overturned and isoclinal; the

outcrop patterns have been made

complicated by a number of strike faults,

thrusts and transverse faults; while in the

west folds are open, canoe shaped showing

broad and less intricate outcrop pattern.

The mesoscopic folds of the area are

varied in style. They were preserved mostly

in the thinly laminated multilayers of silt

shale alternations of both Bhuban and

Bokabil formations. The wave lengths of

these folds are less than their amplitudes.

Two types of folds are common, viz, sharp

crested (p1 values of the sharp crested folds

are large and the fold form dies out with

depth) (Fig. 3a) and disharmonic folds. The

disharmonic folds are so complex that they

cannot be represented by simple geometric

model. Parallel (concentric) folds (Fig. 3b)

are also seen in the upper Bhubans.

Overturned, recumbent and box folds (Fig.

3c) are studied in the areas bordering

Tripura and Mizoram.

The longitudinal strike faults which are

common in almost all the anticlinal

structures change their attitude along dip.

192 D. R. NANDY, SUJIT DAS GUPTA, KALYAN SARKAR AND ANRUDDHA GANGULY

Faults which are low angle reverse (Fig. 3d)

at higher tectonic level become vertical at

depth and vertical fault at higher tectonic

level changes to low angle normal fault at

depth, the last probably being due to

compressive stress relaxation causing local

set up of tensile stress field.

For showing the planar orientations of

the oblique faults and fractures Statistically,

as drawn on the LANDSAT imagery, rose

diagram was prepared (Nandy, 1981) which

reveals a bimodal pattern ; the major

population falling between N50°E and

N60°E and minor one falling between

N43°W and N60°W. Therefore, these

conjugate set of fractures have their acute

bisectrex orthogonal to the regional fold

axes.

Many of the NW-SE oblique/transverse

faults, viz. Mat, Tuipui, Saitual and Sateek

faults are sinistral, whereas the NE-S’V

oblique faults, viz., Amarpur, Aizwal and

Kaladan faults are dextral in nature (Figs. 1

and 2).

The longitudinal strike faults (Fig 3e) a

common feature, have vertical throw of the

order of 700 m to 1700 m, producing many

drag folds at higher level.

Tectonic Evolution

Tectonically the Surma basin of which

Tripura- Mizoram area is a part, is related to

the eastward subduction of the Indian plate

along the Arakan Yoma suture during

Eocene time and the subsequent

development of the Indo-Burman Orogenic

belt (Nandy, 1982). The west side of the

Orogen was marked by the development of

a narrow restricted molasse basin of the

Tipams to the northeast of the Shillong

plateau and an open bell-shaped Surma

basin south of the Shillong plateau with a

regional palaeoslope to the southwest

(Sarkar et a!., 1977). Mention may be made

here that Tipams of upper Assam and

Tipams of Surma basin are not coeval

(Murthy et al., 1976). While the Tipams

were directly deposited over the Barails in

upper Assam, the Surma sediments were

laid down over the Barails or coeval rocks in

the Surma valley, south of the Shillong

plateau—both after a post Barail hiatus, a

widespread feature throughout the entire

area including the Indian ocean at the close

of Oligocene (Davis et al, 1975).

Surma sediments continued to be laid

down in a shallow paralic basin since

Miocene and were folded and faulted owing

to post-Oligocene eastward subduction and

collision of the Indian plate along the

Arakan Yoma suture zone. The crust was

shortened horizontally and thickened

vertically as the plate continued to converge.

The long arcuate linear folds of the

Surma basin occurring sub-paia[lel to the

suture zone, and the conjugate faults with

acute bisectrix orthogonal to the regional

fold axes clearly indicate their origin

through layer-parallel compression

generated by stress system grossly acting E-

W. This conclusion is in striking contrast to

the contention of some workers (Mitra,

1971; Ganguly, 1975; Ganju, 1975 and

Nandy, 1977) who invoked the concept of

vertical tectonics in explaining the

geological evolution of this orogenic belt.

However, the probable deviation of the

stress axis indicated by the interference

TECTONIC EVOLUTION OF TRIPURA-MIZORAM FOLD BELT 193

pattern of folds might lend credence to the

rotation of the indian plate during its

eastward drift in late Tertiary as inferred

from other tectonic considerations.

Increasing intensity of deformation from

west to east and westward convexity of the

fold belt suggest that the tectonic transport

was directed westerly.

The mesoscopic box fold, overturned

and recumbent folds, which conform the

regional folds are also indicative of their

origin in a compressive stress field.

Absence of igneous activity and gradual

increase in complexity of folding and

intensity of deformation from west to east

corroborates to the origin of the folds

through E-W compressive stress as against

through vertical tectonics. Pattern of

Bouguer gravity anomaly also goes in

favour of the above contention.

Recently Mukhopadhyay (1983, in

press) obtained fault plane solution for three

earthquake events from the present area

indicating thrust faulting having

cornpressional pressure axes oriented at 23°

towards 280°, 6° towards 241° and 25°

towards 295° respectively indicating thereby

that the sediments are under E-W stress field

till to date.

Summary and Conclusion

1) The Surma sediments were deposited in

bell-shaped gradually shal lowing basin over

the post Barail (close of Oligocene)

unconformity extending for 550 km in strike

and about 200 km across in front

of the Indo-Burman Orogen (eastern margin

of the Indian plate).

2) The floor of the basin might have been

formed by sub-flysch Barail Group or rocks

coeval to it.

3) The sediments of the basin yielded by

folding and faulting in a compressive stress

field as an upper crustal decollement or

“Supra Structure”; intensity of the

deformation was maximum in the east near

the zone of subduction and collision due to

eastward drift of the Indian plate during

Mio-Pliocene time.

4) Deposition of sediments and folding went

hand in hand till Pleistocene time as marked

by the deposition of the Dupi Tilas over an

unconformity in the western part of the

basin and the polarity of the basin was from

east to west.

5) The deformation of sediments in the basin

was first initiated by layer-parallel

compressive strain and by shortening, then

by brittle failure through long strike faults

and adjustment along the conjugate shear

fractures with strike slip movements and

lastly by forming decollement structures

which glided over the “infra-structurcs” of

the Barails or their equivalents to form the

present orography.

6) The folded belt is under E-W stress field

even to the present day.

Acknowledgement

The first three authors are thankful to A. N.

Sarkar, Geological Survey of India, for

helpful and stimulating discussion and

suggestion for improvement of the text.

194 D. R. NANDY, SUJIT DAS GUPTA, KALYAN SARKAR AND ANIRUDDHA GANGULY

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