Field geology and morphometric evidences supporting active tectonics of Amguri-Tuli-Merangkong area...

*Corresponding author E-mail address: [email protected]

Field geology and morphometric evidences supporting active tectonics of Amguri-Tuli-Merangkong area in the

Assam-Nagaland border, India

Devojit Bezbaruah a* Kashyapee Kashyap

a and Manash Protim Boruah

a

a Department of Applied Geology, Dibrugarh University, Dibrugarh 786 004, Assam

____________________________________________________________________________________________________________________

ABSTRACT

In the present study the geological mapping is carried out in the Amguri-Tuli-Merangkong area in Sibsagar district of Assam and Mokokchung

district of Nagaland. The study area encompasses the Belt of Schuppen from Naga Thrust to Disang Thrust. The geomorphic evidences such as

change in sinuosity, avulsion of meander belt, shifting of channel within the meander belt, presence of unpaired terraces, presence of lakes

close to Naga Thrust and the epicentral plot are all indicative of active tectonic deformation of the study area. From the study of geology and

geomorphic evidences of active tectonics the morphotectonic evolutionary model of the area is developed.

Key words: Geological mapping, Belt of Schuppen, Geomorphic evidences _____________________________________________________________________________________________

1. Introduction

Naga Schuppen Belt is a narrow linear belt of imbricate

thrust slices which follow the eastern and Southeastern

boundary of the Assam valley for a distance of three

hundred kilometers along the flank of the Indo-Myanmar

range. It is postulated that this belt consisting of eight or

more over thrusts along which Paleogenes of Indo-

Myanmar mobile belt have moved northwest wards

relative to the buried basement of Assam shelf. The

Schuppen belt is limited in the southeast by Disang-

Haflong thrust, beyond which tightly folded Paleogenes

occur. Genetically, the Schuppen Belt is a typical

Neogene molasse basin, related to intense tectonism and

uplift along the main axis of the N-S trending Indo-

Myanmar tectogene at the close of Oligocene resulting in

the development of a narrow, linear fore-deep to the west

of it. The Paleogene sediments in the Schuppen belt are

enormously thick and belong to different facies as

compared to the Paleogenes in the adjoining Assam

platform. Their current proximity both laterally and

vertically, indicates substantial horizontal movement or

crustal shortening. This is due to the continued eastward

oblique subduction of the Indian plate, causing down-to-

basin normal faults transform into thrust faults.

Concurrently some strike-slip faults also developed in the

basin. The end result is a complex mosaic of fault system,

masking the original trends partly or fully.

The study was carried out in Amguri-Tuli-Merangkong

area, part of which lie in the plains of Assam and

remaining part lie within the Belt of Schuppen (Fig.1).

Disang Thrust is the roof thrust of the ‘Belt of Schuppen’

where as Naga Thrust is the floor thrust.

Available online at www.appliedgeologydu.com

SOUTH EAST ASIAN JOURNAL OF SEDIMENTARY BASIN RESEARCH (ISSN 2320-6829)

Society of Petroleum Geophysicists, Dibrugarh University Chapter Society of Petroleum Geophysicists, India

SEAJSBR 2-3-4(1) (2016) 37-46 Joint Volume

D. Bezbaruah et al. /South East Asian Journal of Sedimentary Basin Research 2-3-4(1) (2016) / Joint volume / 37-46

38

Fig.1. A) Location map of the area, B) Geological map of the area, C) Geological cross section of the area


39

Virtually undisturbed sediments of the Assam shelf

continue beneath the Naga thrust. Detailed geological

mapping has been done to understand the morphotectonic

evolution of the area and to identify the active movement

along different faults. To understand the active tectonics

of the area different geomorphic signatures associated

with these active movements are studied. The often used

term tectonic processes are a grab-bag expression that

encompasses all types of deformation, including the

motion of tectonic plates, slip on individual faults, ductile

deformation and isostatic processes. River planform

patterns may provide insight on the distribution and

nature of deformation. The changes in fluvial planform

patterns serve to pinpoint areas where deformation may

be occurring. Moreover, the type of the planform change,

such as from meandering to straight or from lower to

higher sinuosity, suggests what the nature of the

deformation may be: increasing slope, lateral tilting, and

so on. Several different patterns and orientations of

deformation could affect fluvial patterns. Tilting along an

axis that is parallel to a river course could deflect or

displace a river system. A significant displacement from

the statistical center can be interpreted to indicate

ongoing tilting that is forcing continued river migration.

In the process of shifting, a meandering river abandons

oxbows and meander bends which remain as visible scars

on the landscape. Under conditions of steady tilt we

might see a suite of meander scars all facing in the same

direction and indicating unidirectional migration of the

river across its floodplain towards a low point. If tilting

was abrupt and caused a major avulsion and relocation of

the meander belt, a different channel pattern would be

expected.

Based on an understanding of the ways in which planform

patterns may respond to tectonic deformation along a

river's course, a rapid reconnaissance of domains of

tectonic activity in the study area has been attempted

using maps and satellite images. Simple observations like

sinuosity of the Jhanzi River, slope of the meander belt of

Jhanzi river and the adjacent topography, shifting and

avulsion of meander belt with respect to its present day

position has been used to delineate deformation regimes in

the area. Moreover, the presence of lakes aligned parallel

to the Naga Thrust also indicates active tectonic

movement in the study area.

2. Geology of the area

Geological mapping is carried out in Amguri –Tuli –

Merangkong area (Fig.1 B). It has been observed that in

the northern part of Amguri town topography is

featureless floodplain which changes to undulatory in the

southern part of Amguri and Haluating area. This area

comprises of older alluvium which is part of the older

flood plain of Jhanzi River. The undulation in the older

floodplain is formed by erosion of the overland flow

which leads to the formation of gullies (Fig.2A) indicating

Fig.2. Field photographs showing A) Gullies, B) Abrupt termination of palaeo channels against low hills(Naga

Thrust ), C) Contact between Tipam Sandstone and

Girujan Clay, D) Mottled clay of Girujan Formation, E) Angular unconformity between Dihing Group and

Girujan Formation, F) Coarse grained pebble bed within

Barail Group, G) Boulder bed overlain by Carbonaceous shale and brown shale within Barail Group, H) Coal seam

and Carbonaceous shale unit within Barail Group, I)

Mesoscopic fold within Barail Group, J)Brecciated zone associated with Strike Slip Fault near Tuli town, K)

Vertical coal seam near Neemna Valley, L) Recumbent

fold within Disang Group near Merangkong.


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upliftment above the present floodplain of Jhanzi River. It

can be inferred that a splay from the Naga Thrust probably

uplifted this area. When this zone is compared with the

regional geological map, it lies close to the Jorhat Fault. It

is probable that the Jorhat Fault is also a thrust fault which

developed due to the compression of the sediments wedge

in front of the Naga Thrust. The trend of this fault is

nearly ENE-WSW.

Near Assam Nagaland border, these gentle undulatory

topography changes abruptly into low hills (Fig. 2B) with

exposure of Tipam Group of rocks, overlain

unconformably by high level river terraces. This

juxtaposition of older alluvium with rocks of Miocene age

and change in topography indicates the presence of the

Naga Thrust in the area.

Tipam Group of rocks in the area comprised of

dominantly grey coloured, medium grained sandstone

with salt and pepper texture along with minor pinkish

shale and mottled clay. Near to the thrust, the attitude of

the bed is very gentle (1500350). But about 5 km of

Naga Thrust, the Tipam Group of rocks are folded into

anticline with its northwestern limb moderately dipping

(3503420) but southeastern limb dipping at a steep angle

(7201300) (Fig. 1C). Near to the crest of the anticline,

contact between Tipam sandstone and Girujan clay is

present (Fig. 2C). The Giruajan clay is grey in colour

exhibiting mottled nature. (Fig.2D).In this area, a small

exposure of Dihing Group is present which lie

unconformably above Tipam Group (Fig.2E). The beds of

the Dihing Group have very gentle disposition

(1501400).

Near Tzudikong town along the Nankamtsu Nala, the

Barail Group of rocks is thrusted over the Dihing Group

by the Tzudikong Thrust. In between Tzudikiong Thrust

and Mile post no. 21, lithology mainly comprises of grey

and brown shale with minor light grey, medium to fine

grained sandstone. From Tzudikong town to Mile post no.

23, gritty sandstone beds, pebble beds and boulder beds

(Fig.2F, Fig 2G) are present. Within this unit

carbonaceous shale and a coal seam of 1.5m thickness

(Fig.2H) and minor sandstone beds are sandwiched. At the

top of the boulder bed a small carbonaceous band is

present (Fig.2G). Mesoscopic folding is observed in

sandstone bands (Fig.2I). The attitude of gentle limb is

90700 and the steep limb dip at 800

2300.The presence

of this coarse grain unit within the Barial Group is an

enigma and need detailed study. This unit is overlain by

sequence of fine to medium grain, light grey colour

sandstone with minor shale band upto Tuli town. Some

small scale faulting is observed in this sequence.

Near Tuli town a crushed and brecciate zone is present

(Fig.2J). In east of this brecciated zone, bed has moderate

disposition 40° towards SW but in the west of this

brecciated zone beds become steeper dipping at 60°

towards SE. This evidence indicates the presence of a

strike slip faut having NW-SE trend. This strike slip fault

displacesall the thrusts and offset the mountain front. Near

Longkongnetangba nala, lithology comprised of mainly

laminated sandstone which exhibit some small scale

faulting. Furthermore, from the geological map it is

observed that there are significant changes in attitude of

the beds in either side of the strike slip fault. Most of the

area of the Tuli valley is covered by the recent deposits of

the Jhanzi River. But in a small exposure in southern part

of the Tuli town fine grained arenitic sandstone is

exposed. The disposition of these beds is very gentle. The

lithology comprise of dominantly shale with very minor

sandstone unit. The lithological dissimilarity and abrupt

change in disposition indicates the presence of a thrust

which is concealed beneath the alluvium of Jhanzi River.

Near Assam Rifle camp (Neemna Valley) the disposition

of the beds changes abruptly. The hard, compact

sandstone beds became nearly vertical with steep

disposition of 8501250.. A vertical coal seam of 1.5m

thickness associated with carbonaceous is also observed

(Fig.2K). There is marked change in the topography in

this area. The steep disposition of beds and change in

topography indicates the presence of a thrust in the area.

The lithology of this unit comprises of grey colour shale

with minor sandstone and siltstone bands which continues

to Disang Thrust (Mile post no. 34).

Towards NW of Disang Thrust lie relatively low hills

comprising of sandstone and shale of Barail Group and in

SE, the hills are steep. The Disang Group of rocks is

thrusted over Barail Group along Disang thrust. The beds

of Disang Group have moderate dip (450 to 550) towards

Southeast. The Disang Group of rocks mainly comprised

of alternate bands of shale and sandstone with minor

siltstone.

In Merangkong, the Disang Group of rocks is intensely

folded. A recumbent fold is observed in road section

(Fig.2L). A highly brecciate zone trending in NE-SW

direction is present near mile post no. 43. This indicate

presence of inter formational thrust.

3. Evidences of active tectonics

3.1. Sinuosity of the Jhanzi River

The sinuosity of the Jhanzi River was calculated (Table 1)

from the Google Earth Image (2015) of the area and also

from the topo sheets (1974-75). For the purpose of


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calculation of sinuosity, the river is divided into four

segments as –

The sinuosity of the Jhanzi River has changed from 1.81

to 1.94 in 40 years in the alluvial plain of the Brahmaputra

River. The alluviums are easily eroded by the rivers and

hence the river attains a more sinuous pattern in such

regions. The insignificant change of sinuosity suggests

that there is no significant change in discharge, sediment

load and slope of the area. Therefore, this area is not

subjected to deformation in recent time. The sinuosity of

the river in the area between the Jorhat Fault and the Naga

Thrust is much higher than that of the lower reach of the

Jhanzi River from the Jorhat Fault to the Brahmaputra

River. It is indicative of active tectonic deformation of the

terrain by the Jorhat Fault which uplifted the older

alluvium above the newer alluvium, thereby changing the

slope of the region, forcing the river to adjust to the new

gradient of the area by increasing the sinuosity. The rate

of deformation in the area is probably low which can be

inferred based on the insignificant change of sinuosity of

the river during last 40 years. The area between Naga

thrust and the Tzudikong Thrust is complexly deformed

by the Naga Thrust and the Strike Slip Fault. Hence, it

disturbed the river course over time forcing it to follow a

more sinuous pattern in order to adjust its course to new

equilibrium. The sinuosity of the river has remained

almost same over the last 40 years in the area between the

Tzudikong Thrust and the Disang Thrust in spite of active

tectonics operating in this area. This is probably due to

presence of comparatively hard and indurated rocks of the

Barail Group which are more resistant to erosion by the

river water.

3.2. Avulsion of meander belt and channel shifting

In the study area, avulsion of the channel of the Jhanzi

River has taken place that has completely relocated the

meander belt (Fig.3).

The palaeochannels of the Jhanzi River, traced from the

Google Earth image shows complete relocation of the

river channel over course of time. The presence of the

highlands in between the palaeo channels of the Jhanzi

River indicates abrupt tilting of region which forced the

river channel to avulse and follow a new course to adjust

to the changing topography.

Gradual upliftment and tilting of the area due to the

deformational forces caused by the Naga Thrust and the

Jorhat Fault resulted in major avulsion of the river channel

which is evident from the meander scars observed in the

Google image. It shows that the area is still deformed by

the active tectonic movements of the Naga Thrust and the

Jorhat Fault.

Within the meander belt of the present channel of the

Jhanzi River it has shifted its course a number of times

towards SW direction until it reached its present course.

Generally the shifting of river is rarely observed in hilly

terrains. But in the study area, it is interesting to observe

that the shifting of the river has taken place a number of

times in the zone between the Naga Thrust and the

Tzudikong Thrust. This suggests active tectonic

movement in the area. The upliftment of the Tipam Group

of rocks by the Naga Thrust disturbed the original course

of the Jhanzi River. Hence the river was forced to migrate

from its course a number of times during thrusting. It is

also observed that the shifting of the river is unidirectional

i.e. towards the SW direction. In the process of shifting,

the meandering river has left a suite of visible meander

scars on the landscape. The meander scars are all facing in

the same direction indicating a generally unidirectional

migration of the river. This unidirectional shifting of

Jhanzi River is possibly because of the tilting caused by

the strike slip fault present in this area.

3.3. Presence of lakes along Naga thrust

A number of irregular water bodies are present in the

southern part of the mountain front close to the Naga

thrust. These lakes are formed due to accumulation of

water in the valley of first, second and third order stream

present earlier in these areas (Fig.4). These accumulations

of water are due to the damming effect of those valleys

Fig.3. Avulsion of Jhanzi River and shifting of meander

belt within the present channel of Jhanzi River

Table 1

Temporal variability of sinuosity of the Jhanzi River in

different segments over a period of forty years


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due to upliftment of the area along the Naga thrust. The

presence of these water bodies indicates the active

tectonic deformation of the area.

3.4. Different levels of terrace deposits

Terrace deposits were observed in different parts of the

road section during field survey in the Tipam and the

Barail Group of rocks. Near the Melak bridge, four

different levels of terrace deposits were observed. (Fig.

5A). The elevation of these terrace levels To is 113m, T1

is 114 m , T2 117 m and T3 125 m respectively. This

difference in elevation of the terrace deposits suggests that

the area has been uplifted due to the tectonic movements

caused by the Naga thrust. The unpaired terrace suggests

that upliftment and tilting over time owing to which the

terraces have attained different elevations.

In the Barail Group, terrace deposits are found to be

overlying them .(Fig.5B) The elevation of the terrace

deposits in the area is found to be 163 m while that of the

river bed in the nearest area is 123 m. It can be inferred

that due to movement along Tzudikiong Thrust the area

has been uplifted which raised the terrace deposits by 40m

from the river bed during Quaternary Period. Hence it can

be inferred that the area is undergoing gradual changes

due to the active tectonic movements in and around the

thrusts.

3.5. Profiles showing break in slope

The topography associated with active thrust faults is very

complex. Upliftment along a range bounding thrust fault

can create an imposing escarpment, but primary surface-

rupture locations shift when new splays of a propagating

thrust-fault zone encroach into the adjacent basin

(Yielding et al., 1981). The former range-bounding fault

becomes an internal mountain front that is less

tectonically active, or becomes inactive. It is evident from

the presence of the Dihing Group of rocks adjacent to the

Barail Group in the study area. The Dihing Group of rocks

was once deposited as alluvial fan near the former

mountain front after the upliftment of the Barail Group by

the Tzudikong Thrust. After the development of the Naga

Thrust, the Tzudikong Thrust became a part of the internal

mountain front.

The new fault at the edge of rapidly rising hills is the

latest in a series of range bounding faults. Tectonic

deformation ruptures and folds this piedmont foreland,

which is a bedrock block, capped by remnants of

piedmont alluvium. New streams dissect the former

depositional slopes as a consequence of piedmont terrain

being incorporated into ever-broadening mountain range.

When the local components of surface uplift in the study

area are evaluated i.e. faulting, folding, erosion, and

deposition, the total tectonic uplift include broader

wavelength styles of uplift. The undulatory topography

from the Naga Thrust to the Jorhat Fault indicates such

type of upliftment. The sum of local and regional

components of uplift resulted in four structural blocks

rising during the late Oligocene to the Present–the

upliftment of the Disang Group by the Disang Thrust ,the

upliftment of the Barail by the Tzudikong Thrust, the

upliftment of the Tipam Group by the Naga Thrust and

Fig.4. Lakes along the mountain front

Fig.5. A) Unpaired terrace deposits near Melak Bridge, B) Terrace deposit overlying Barail Group


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finally the upliftment of the Older Alluvium by the Jorhat

Fault. These rising blocks are well indicated by the breaks

in the slopes in the profiles obtained from the SRTM

DEM data (Fig.6).

The Tzudikong Thrust was the range-bounding fault of a

rising mountain range during the post Miocene. The slope

breaks in the profiles suggest that it has not become totally

inactive, however much of the slip now occurs on the

splay of the Naga thrusts that terminates as the new range-

bounding fault along the mountain front.

For the purpose of study, profiles were drawn across the

Jorhat Fault and the Naga Thrust and also across the Naga

Thrust and the Disang Thrust. There is distinctive slope

breaks in the study area along the Naga Thrust and the

Jorhat Fault (Fig7A). The average slope of the area

between the Naga Thrust and the Jorhat Fault is 0.012,

which reduces to 0.008 in the floodplain of the

Brahmaputra River beyond the Jorhat Fault (Table 2). The

undulatory topography indicates broad wavelength style

of upliftment in the region caused by the tectonic forces of

the thrusting. The area has been uplifted due to thrusting

and active tectonic movements are still operating in the

region.

There are distinctive breaks in the slope of the profiles

drawn from Naga Thrust to the Disang Thrust in the study

area (Fig.7B). The area has been uplifted due to thrusting

and active tectonic movements are still operating in the

region.

3.6. Slope of meander belt and adjacent topography

Profiles are drawn across the meander belt of the Jhanzi

River and the adjacent topography as AB, CD, EF & GH

(Fig.8) to find the general slope of the area(Table 3) It is

observed that the slope of the meander belt is lesser than

that of the adjacent topography in between Naga Thrust

and Jorhat Fault (AB and CD). This difference in the

slope of both areas indicates that the area that has been

uplifted by the Jorhat Fault and has been undergoing

tilting. If there is no tilting the slope of present day river

profile must be same as that of older flood plain of Jhanzi

River. But for the area between Jorhat Fault and

Brahmaputra River, slope of the Jhanzi River is more or

less same as that of present flood plain (EF and GH). This

indicates that the area is tectonically not active.

Fig.6. Image showing the position where there is break in

slope of topography.

Fig.7. A) Profile drawn from Naga Thrust to Jorhat Fault, B) Profile drawn from Naga Thrust to Disang Thrust

Fig. 8. Segments of the flow of the Jhanzi River along

which average slope is measured.

Table 2

Showing the slope of different segments of the study area


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3.7. Earthquake epicentre

The epicenter plot obtained from the USGS website shows

the epicenters of four earthquakes is present in and around

the study area (Fig.9). The recorded magnitudes of these

earthquakes are (Table 4):

The presence of these epicentres depict that the area is

tectonically active. But lack of data of micro earthquake in

this area impaired the demarcation of the active tectonic

features using epicentre plot. In this area both Tzudikong

and Naga Thrust displace soft and friable rock. The Naga

Thrust brings the Tipam Group of rocks above alluvium.

Moreover, these thrusts have shallow detachment plane.

Due to these conditions high stress accumulation is not

possible in these thrusts. Hence it will not give rise to

earthquake of higher magnitude. For studying activeness

of these thrusts micro earthquake network should be

established which is good future scope of study in this

area.

4. Morphotectonic evolution of the area

The footwall shortcuts created by the compression of

upward-steepening reverse faults (Fig.10A).

Almost all faults show some degree of concave-upward

curvature (Coward, 1994). At the topographic surface,

failure is often brittle and the resulting fault dips are

commonly near vertical. Dips decrease toward 60° at 3–4

Table 3

Showing the slope of the meander belt and the adjacent

topography

Fig.9. Earthquake epicentres in and around the study area

Table 4

Showing recorded earthquake epicentres in the study area

Fig.10. Schematic representation of : A) Showing

footwall shortcut, B) Disang and Barail basin in the study area, C) development of Disang Thrust and upliftment of

the Disang Group, D) developent of Tzudikong Thrust

and upliftment of Barail Group, E) development of Tzudikong Thrust and deposition of Tipam and Dihing

Group, F) deposition of Dihing Group as alluvial fan

facies after upliftment of Barail, G) development of Naga

Thrust and Upliftment of Tipam Group, H) development

of Jorhat Fault and upliftment of Older Alluvium, I)

development of the strike slip fault and displacement of the Tzudikong and Naga Thrusts.


45

km depth due to the change in failure mechanism from

hybrid to shear fracturing (Walsh and Watterson, 1988).

Proffet (1977) and Hamblin (1965). For crustal-scale

faults, dips must decrease further toward 45° at ~10 km

depth as the host rock rheology changes from brittle to

plastic (Walsh and Watterson, 1988). Finally, if the fault

is detached at some deeper level, then it must eventually

bend toward horizontal. At higher dips, it is often easier to

create a new, lower-angle fault than to reactivate the old,

steep one. Dip catalogs of seismically active reverse faults

typically show a cut off at 60° (Sibson and Xie, 1998;

Collettini and Sibson, 2001). Upward-steepening faults

are thus susceptible to pure reverse-sense reactivation

only where they dip less than ~60°, i.e., in their lower

extents, at depths >3–4 km beneath the contemporary

topographic surface. The upper, steep portion is difficult

or impossible to reactivate in pure reverse motion which

often leads to the creation of one or more “footwall

shortcuts,” i.e., splays that branch off the old fault and

form new, lower angle routes to the surface, bypassing the

steep segment of the original fault. Thus, it suggests that

ancient faults can be reactivated at depth but form new

paths to the surface. Under compressive stress, weak

sections of existing faults may localize the initial failure.

Bends or other strength heterogeneities in those faults,

however, may necessitate the growth of new, more

favourably oriented segments if slip continues.

The tectonic setup of the study area is complex. It lies in

the “belt of schuppen” zone, bounded by the Naga Thrust

and the Disang Thrust. The Tipam, Dihing, Barail and

Disang Group of rocks are exposed in the study area. The

area is complexly dissected by a number of thrusts and

cross faults.

The Assam Arakan basin initially developed as a passive

margin. As India separated from Gondawana land and

moved towards Eurasian plate, sediments were deposited

in this passive margin basin. As the Indian plate

approaches the Burmese plate and starts subducting

beneath it these sediments of the passive margin basin

were compressed. Due to compression of the different

lithological units from the SE resulted in number of thrust

slices in the area. During Late Oligocene period intense

compression resulted in thrusting of the Disang Group.

The Disang Group of rocks was thrusted by the Disang

thrust over the Barail Group of rocks (Fig.10B). As the

Indian plate continues to subduct beneath the Burmese

plate the sediments in front of the steeper section of the

Disang Thrust get compressed. As the upper, steep portion

is difficult or impossible to reactivate in reverse motion

which often leads to the creation of “footwall shortcuts,”

i.e., splays that branch off the old fault and form new,

lower angle routes to the surface, bypassing the steep

segment of the original fault. Thus Tzudikong Thrust

uplifted the Barail Group over the Dihing Group by a foot

wall short cut(Fig.10C). The presence of Dihing close to

Tzudikong Thrust and the lithology of the Dihing i.e.

comprising dominantly of pebble bed with minor sand

lenses suggest alluvial fan facies of this group. (Fig.10D-

E) Therefore, it may be inferred that the upliftment of the

Barail along Tzudikong Thrust lead to development of the

mountain front in this area. The river draining this uplifted

region forms the alluvial fans which were deposited

unconformably above the Tipam Group of rocks which

are folded. The ongoing tectonic deformation had

resulted in more and more southward compression and a

number of thrusting took place in the study area. The

Naga Thrust probably originated as a foot wall cut off

during Late Pleistocene time when the basin was intensely

compressed. With the development of Naga Thrust, the

Tipam Group of rocks was uplifted above the older

alluvium of the Jhanzi River (Fig.10F). The Older

Alluvium was deposited in the Pleistocene period which

suggested that the Naga Thrust originate in the region

during Late Pleistocene. As subduction continues further

compression of the sediments piled in front of the Naga

thrust, results in propagation of a splay thrust (Jorhat

Fault) from the Naga Thrust and uplifted the Older

Alluvium of the Jhanzi River above the present floodplain

(Fig.10G).

The NW-SE trending strike slip fault which offset the

mountain front and the thrusts was developed after the

Naga Thrust. This strike slip fault probably developed due

to the difference in resistance against thrusting by the

lithology on either side of the fault. This strike-slip fault

displaced the Tzudikong Thrust and the Naga Thrust,

which indicate that the age of the strike slip fault is

probably Late Pliestocene to Holocene. Due to this strike

slip fault the mountain front is displaced by a distance of

3-4 kms on either side of the cross fault (Fig.10H).

5. Conclusions

The Tertiary sediments in the study area are deformed as

the Indian plate subduct beneath the Burmese plate. This

orogeny is responsible for the development of number of

thrust sheets. The thrust sheets are propagating towards

the NE direction. This progressive propagation of the

thrust sheet gives rise to the Belt of Schuppen. This NE

propagation of the thrust is still continuing which is

evident from the formation of the Jorhat Fault.

In the present study of the sinuosity of the Jhanzi River

indicates that the area between the Jorhat Fault and the

Tzudikong Thrust it is deformed in the Recent time which

result in increase in sinuosity of Jhanzi River in this area.

The avulsion of the meander belts is indicative of abrupt

episodic tilting in this area. Further, the unidirectional


46

migration of the present Jhanzi River channel towards SW

within the meander belt indicates gradual tilting of the

region in the present time due to deformation induced by

NW-SE trending strike slip fault.

The presence of unpaired terraces and numerous lakes

close to the Naga Thrust indicates the upliftment and

tilting of the region by the thrust. Further, the prominent

break in slope along each thrust sheet is indicative that

when range bounding fault propagates towards the

foreland earlier range bounding faults become less active.

Therefore in the present area sharp break in slope is

observed along thrusts. The presence of four earthquake

epicenters also suggests that the study area is tectonically

active.

From the study of morphotectonic evolution it is observed

that the Assam Arakan basin starts as a passive margin.

As the subduction beneath the Burmese plate occurs the

basin undergoes different modification processes and

ultimately the marine basin is closed. After closing of the

marine basin foredeep basin was formed during Early

Miocene time where Mio-Pliocene sediments were

deposited. The part of the Mio-Pliocene basin later get

uplifted and the foredeep migrate further towards NE

where Quarternary sediments were deposited. The

modification of this foreland is still continuing.

Acknowledgement

The authors acknowledge the partial financial support

received from the DRS-SAP-II program of the

Department of Applied Geology, D.U. to carry out the

field work for the research.

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Field geology and morphometric evidences supporting active tectonics of Amguri-Tuli-Merangkong area...

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