geomorphic and structural evidences of neotectonic activity in the sub-himalayan belt of nahan...

0016-7622/2011-77-2-175/$ 1.00 © GEOL. SOC. INDIA

JOURNAL GEOLOGICAL SOCIETY OF INDIAVol.77, February 2011, pp.175-182

Geomorphic and Structural Evidences of Neotectonic Activityin the Sub-Himalayan Belt of Nahan Salient, NW India

TEJPAL SINGH1,2,3, UMAKANT SHARMA

2, A. K. AWASTHI3, N. S. VIRDI

4 and RAVINDRA KUMAR2

1CSIR-Centre for Mathematical Modelling and Computer Simulation, Bangalore – 560 037.2Department of Geology, Panjab University, Chandigarh – 160 014

3Department of Earth Sciences, Indian Institute of Technology Roorkee, Roorkee – 247 6674Wadia Institute of Himalayan Geology, Dehradun – 248 001

Email: [email protected], [email protected]

Abstract: Neotectonism in the Sub-Himalayan belt is not new. Moreover, the word ‘Sub-Himalaya’ is almost synonymouswith ‘neotectonic activity’. In the present paper, we report some of the most convincing geomorphic and structuralevidences of neotectonic activity from the Sub-Himalayan belt in the Nahan Salient. The geomorphic evidences mainlyinclude the four geomorphic surfaces identified from the transverse topographic profiles drawn parallel to the Himalayanfront. These surfaces are commonly covered with terrace deposits that are tilted as well as faulted at a number of places.A number of faults, directly observable in the field, are normal in nature and they are oriented at high angles to theHimalayan Frontal Thrust (HFT). These faults are similar to the E-W extension in southern Tibet in response to theoblique convergence of India at ~N20oE in the NW Himalaya. They are attributable to the kinematics of neotectoniccompression along the HFT, the frontal ramp-oblique ramp-frontal ramp geometry of the thrust fault and relatedadjustments.

Keywords: Geomorphology, Neotectonics, Nahan Salient, Sub-Himalaya.

displacing the present topographic surface and tilting andfaulting of recent to sub-recent terraces.

STUDY AREA

The study area has been chosen in the Nahan Salient,between the Dehradun and Kangra Reentrants that arelocated to its east and west, respectively (Fig. 1). The threeprincipal areas, i.e. Dehradun and Kangra Reentrants andthe Nahan Salient are geologically comprised of the similarSub-Himalayan sequences (Parkash et al. 1980; Raiverman,2002) bound by the Main Boundary Thrust (MBT) towardsthe north and the Himalayan Frontal Thrust (HFT) towardsthe south (Fig.1). In each of these areas, the Sub-Himalayansequences are believed to override the alluvium of theIndo-Gangetic plains along the HFT (Powers et al. 1998).However, there are significant differences in the geomorphicsetup of these three areas (Nakata, 1972; Delcaillau et al.2006; Singh, 2008; Singh and Jain, 2009) probably owingto the variation in the tectonic setup (Powers et al. 1998)and/or basement geometries (Raiverman, 2002). Thegeomorphic manifestations along the HFT zone in the Nahan

INTRODUCTION

Reports on neotectonism in the Sub-Himalayan belt arenot new (Nakata, 1972; Srivastava and John, 1999; Singhet al. 2001). Moreover, they are so common that the word‘Sub-Himalaya’ in the literature has become almostsynonymous with ‘neotectonic activity’. It is interesting tonote that the manifestations of neotectonic activity have beenmainly investigated along some particular segments of theHFT zone eg. Kangra, Pinjore and Dehradun in the NWHimalaya (Nakata, 1972; Srivastava and John, 1999; Singhet al. 2001; Kumar et al. 2006), thereby partly ignoring someof the key areas such as the Nahan Salient in the NWHimalaya, where relatively less amount of published workis available (Singh and Awasthi, 2010 and referencestherein). This may leave significant gaps in ourunderstanding of the nature of neotectonic activity, especiallywhen we talk of significant differences in the convergencerates and slips rates in the NW Himalaya. In the presentpaper, we take up investigations in the Sub-Himalayan beltof the Nahan Salient and present some evidences ofneotectonic activity. The evidences are mainly in the formof geomorphic surfaces (covered by terraces), active faulting

JOUR.GEOL.SOC.INDIA, VOL.77, FEB. 2011

176 TEJPAL SINGH AND OTHERS

Salient are mainly restricted to the topographic break that ismarked by discontinuous scarps at places and some drainageanomalies (Singh, 2006). However, investigations in the fieldwith some aid from the remote sensing images clearly revealmore convincing evidences that are directly related to theactivity along the HFT in this area.

DATA AND METHODS

The primary sources of data used in the present studyare the Survey of India (SOI) topographic maps (1:50,000).These maps have been very handy in the field, especiallywhen they were co-registered with the satellite imageries(Landsat TM) in a GIS environment. The digitizedtopography from the SOI maps were overlain by the satelliteimages to readily obtain a 3D visualization of the field area.

Although this technique is not a new one, it provided veryuseful aid in identifying different geomorphic surfaces andlocating terraces within the Sub-Himalayan belt, away fromthe HFT zone. Further, detailed field investigations inidentified areas brought to light some very useful evidencesof young deformation. However, in a rapidly erodingenvironment it is almost impossible to find any datablematerial thereby no actual temporal constraints on theactivity were possible.

EVIDENCES OF NEOTECTONIC ACTIVITY

The evidences of neotectonic activity cited in this paperare mainly observed directly in the field with some aid fromremote sensing images. They are mainly geomorphic andstructural in nature. The topography, where possible, has

Chandigarh

Hoshiarpur

Kangra

Sunder Nagar Rampur

Simla

Dehradun

Hardwar

HFT

MBT

MBT

MBT

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Indo-Gangetic Plains

MCT Main Central Thrust

MBT Main Boundary Thrust

HFT Himalayan Frontal Thrust

Sub Himalaya

0 20 40 60 Km

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Kangra

Reentra

nt

NahanSalient Dehradun Reentrant

Nahan

74 E 80 E

36 N

32 N

Delhi

100 km

N

Dehradun

Fig.1. Litho-tectonic map of the Himalaya in NW India showing the three major subdivisions of the orogen. See inset for location. Notethat the curved nature of the MBT has given rise to structural ‘reentrants’ and ‘salients’. The three key areas discussed in the textare the Nahan Salient that is bound towards its west by the Kangra reentrant and towards its east by the Dehradun reentrant.


GEOMORPHIC AND STRUCTURAL EVIDENCES OF NEOTECTONIC ACTIVITY IN SUB-HIMALAYA, NW INDIA 177

also been used as a relative tool to measure the temporalaspect of the deformation. However, no absolute datingtechnique has been used due to a number of reasons, mainlybeing the rapid rate of erosion. The geomorphic evidencesare mainly related to the geomorphic surfaces that arecovered by fluvial terraces at a number of places whereasthe structural evidences are in the form of normal faultingobserved in the field.

Geomorphic Evidence: Straight Line Nature of the HFT

The most apparent and important geomorphic evidencecomes from the almost straight line nature of the HimalayanFront in this area (Fig. 2a). In fact, a visual analysis clearlyindicates that this particular segment of the HFT is evenstraighter than the HFT in the adjoining reentrants. The HFTzone is marked by small and discontinuous scarps at a fewplaces, with almost no direct evidence of thrusting alongthe front (Fig. 2b). The tectonic significance of the line ofHFT is apparent from the fact that it forms the contactbetween the two basic geomorphic entities i.e. the hills andthe plains.

Geomorphic Evidence: Topographic Profiles Parallel tothe HFT

Topographic profiles parallel to the tectonic line are one

of the most robust tools to identify geomorphic evidencesrelated to deformation. In our study, two topographicprofiles, parallel to the trend of the Himalaya, which alsoconforms to the strike of the HFT, have been constructedfrom the topographic data. Each of these profiles clearlyshows four geomorphic surfaces (Fig.3). Amongst thefour surfaces, the highest and the oldest is named the‘Dhadwali surface’ the next lower surfaces are ‘Dungasurface’, ‘Bhud surface’ and ‘Kambala surface’ respectively.The ‘Dhadwali surface’ lies about 55-60 m above the presentstream bed. It is highly dissected and commonly present aserosional remnants. The ‘Dunga surface’ lies about 40-45 m above the present stream bed. It is separated from the‘Dhadwali surface’ by steep escarpments. It is relativelyless dissected than the ‘Dhadwali surface’. The ‘Bhud’surface is a relatively more extensive surface lying about15 m from the active stream bed (Fig. 3c). It is separatedfrom the ‘Dunga surface’ by subdued slopes which arehighly dissected and eroded. It is comprised of 2-3 m thickcover of terrace deposits and is cultivated and habitated atmost of the places. The lowest and the youngest is the‘Kambala surface’. It lies about 5m from the presentstream bed. It is covered by a thin veneer of terracedeposits. It is also extensive and relatively least dissected(Fig.3d).

Fig.2. The Himalayan mountain front in the Nahan Salient. (a) It is almost straight as marked by the trace of the HFT. (b) Smalldiscontinuous scarps are present in the field.

30

40’

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o30

30’

o

76 55’o

77 00’o

77 05’o

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76 55’o

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77 15’o

0 10

Km

30

40

’o

30

35

’o

30

30

’o

Morni

Tarlokpur

Kala Amb

HFT

IFT

IFTH

FT

HFT

GhaggarR

.

Mark

anda

R.

A

B

Upper Siwalik strata

Alluvium of the Indo-Gangetic

Himalayan Mountain Front

(a)

(b)



Geomorphic Evidence: Presence of Tilted Terraces

The terraces present in these low lying hills aregeomorphic and sedimentologic expression of form andprocess adjustments in a fluvial system (Schumm et al.1993). They usually occur as broad and fairly flat surfacesthat provide excellent ground for agricultural activitiesand habitation. They are usually bound by the streamchannels, often elongated parallel to their length. They arenot observed beyond the hills and therefore are attributed

to the tectonic uplift along the HFT. Terraces have developedalong stream segments undergoing incision, shifting andlateral erosion of banks and on the non-cliff side of meanderloops.

Although the presence of terrace surfaces in closeproximity to the HFT is itself indicative of neotectonicactivity of the HFT. Tilting of the observed terrace surfacesat a few places most convincingly indicate to recentdeformation mainly related to the HFT (Fig. 4).

Kambala Surface

Present river bedFlow direction

Bhud Surface

Flow direction

(c) (d)

500

450

400

ASW

Dhadwali Surface

Dunga Surface

Bhud Surface

Kambala Surface

500

450

400

BSW

Dunga Surface

Dhadwali Surface

Bhud Surface

Kambala Surface

Fig.3. Transverse topographic profiles A and B show four geomorphic surfaces present in the Nahan Salient. For locations of A and Brefer to Fig.2 and discussion in the text. (c) Bhud surface and (d) Kambala surface as observed in the field are covered by terracedeposits.

(a)

(b)

Fig.4. Terrace deposits within the Siwalik hills are often tilted and/or faulted in response to neotectonic deformation along the HFT. TheSiwalik bedrock dipping about 30o towards NE is covered by terrace material. The terrace surface is also dipping (tilted) in thesame direction by a lesser amount. A fault cuts across the terrace surface offsetting it by about 60cm. The fault present in thefigure is normal in nature.



Structural Evidence: Faulting within Terrace Deposits

The presence of the terraces in the close proximity ofthe HFT in its hanging wall block is a clear manifestation ofthe tectonic uplift along the HFT. Moreover, the terracesare tilted at a few places. At some of these places cross cuttingfaults in the terrace deposits are also present (Fig. 4). Thesedo not leave anything to imagination, rather are the mostconvincing evidence to the recent deformation (Fig.4). It isinteresting to note that the fault shown in Fig.4 is normal innature with strike (NNE-SSW), i.e. at high angle to the HFT.

These faults are commonly sub-vertical (dipping 75o-80o)on either side.

Structural Evidence: Fault Offsetting the PresentTopographic Surface

A number of instances of faulting within the uppermostunits of the Siwaliks have also been observed (Fig.5). All ofthese are normal faults oriented at high angles (N-S andNNE-SSW) to the HFT. The most interesting was a set ofconjugate faults which has disrupted the present topographicsurface (Fig.5a, b). Though we do not have absolute ages

Topography collapse due to faulting

(a)

(b)

(c) (d)

Fig.5. Number of faults directly observed in the field. Note that all the faults are normal in nature and are oriented at high angles to theHFT. In (a) a clear topographic collapse can be seen. The nature of offset has been used to infer a recent event that has not givenenough time for the vegetation to stabilize.



for the faulting event, the topographic scenario seems selfindicative that the faulting is very recent in age, based onthe geomorphic relationship. The top surface that sunk alongwith the faulting event is still overhanging. The faulting hasnot even allowed the vegetation to settle again.

DISCUSSION

The present area of investigation is marked by a sinuoustrace of the Main Boundary Thrust (MBT) and an almoststraight Himalayan Frontal Thrust (HFT) that marks themountain front (Figs. 1, 2). The straight line nature of theHFT conforms to the initial geometry of the thrust fault whichbecomes sinusoidal at a later stage (Dubey and Bhakuni,2004), as in case of the MBT. The sinusoidal trace of theMBT has been geometrically compared to the frontal ramp(NW-SE oriented segment) and oblique ramp (NNW-SSEoriented segment) geometry of the thrust fault (Dubey, 1997;Dubey and Bhakuni, 2004). This frontal ramp-oblique ramp-frontal ramp geometry gives rise to the reentrant-salient-reentrant setup in the Sub-Himalayan belt of NW India(Fig.1).

Further, based on the field investigation carried out inthe Sub-Himalayan belt, a variety of geomorphic and

structural observations are recorded from the Nahan Salient.The most interesting of these observations are the fourgeomorphic surfaces demonstrated by the topographicprofiles parallel to the HFT (Fig.3). These geomorphicsurfaces are readily comparable to the four geomorphicsurfaces reported from the Pinjore Dun (Nakata, 1972) andDehradun (Singh et al. 2001). The presence of these surfaces,spread over a distance of more than 200km laterally alongthe HFT in the three key areas of Kangra Reentrant, NahanSalient and Dehradun Reentrant from west to east in thatorder, suggest to an almost similar (not exactly same)geomorphic response to the neotectonic deformation alongthe HFT. However, the difference is mainly in thepreservation and expanse of these geomorphic surfaces.They are poorly preserved and less extensive in the NahanSalient, probably due to more focused erosion in the drainagebasins by smaller streams. Alternatively, the absence of anymajor drainage system in the Nahan Salient may not haveprovided any extensive areas for the development andpreservation of geomorphic surfaces.

The geomorphic surfaces wherever present are coveredby terrace material at a number of places that are all post-Siwalik, i.e. Upper Pleistocene to Recent, in age (Table 1).Tilting of the terraces is clearly indicative of post-Siwalik

Table 1. Generalized stratigraphy of the area (modified after Raiverman, 2002)

Main Units Formation Lithological characters Age (approx.)

Post-Siwalik Alluvium/Piedmont Fluvial unconsolidated sands, silts and clays of Recentdeposits floodplain deposits

- - - - - - - - - Unconformity - - - - - - - - -

Fluvial deposits poorly sorted and unconsolidated, U. PleistoceneTerrace deposits consists of silts, sands, gravels, clays with variable

proportion of gravels.

- - - - - - - - - Unconformity - - - - - - - - -

Boulder Coarse boulder conglomerate comprised of cobbles, L. PleistoceneConglomerate pebbles, clasts which are either clast or mud supported

with inclusions of sand, grit or clay nodules

Pinjor Sandstones, siltstones and clays comprise bulk of the U. Pliocenelithology. Well bedded bright buff coloured clays andmassive sandstone units

Siwalik Group Tatrot Soft sandstones, clays comprise bulk of the lithology Mid. Pliocenewith some conglomerate. Grey clays are characteristic

- - - - - - - - - Tectonic contact - - - - - - - - -

Sandstones comprise bulk of the lithology, with minor Mid. MioceneNahan shales, clays and few pseudoconglomerates. Siltstones to L. Pliocene

greyish-green to yellow-brown. Well bedded thicksandstone units

- - - - - - - - - Tectonic contact - - - - - - - - -

Pre-Siwalik deposits Subathu Earthy yellow to red shales, olive green splintery shales Eocene(Lower Tertiaries) and greenish-grey sandstone

U.

S I W

A L

I K

L.

S I W

A L

I K



neotectonic deformation that has even led to faulting at anumber of places (Figs. 4, 5). These as well as other faultsobserved in the field are normal (extensional) in nature.Based on the model experiments Dubey and Bhakuni (2004)demonstrated that such extensional faults oriented at highangles to the thrust faults are a result of the kinematics ofcompression and frontal ramp-oblique ramp-frontal rampgeometry of the thrust fault. These normal faults are similarto the orthogonal faults of the Tethyan Himalaya (Dubeyand Bhakuni, 2004) and also the E-W extension in southernTibet that is attributed to the oblique convergence of Indiaat ~N20oE in the NW Himalaya (Armijo et al. 1986; Paul etal. 2001). In the present case, we attribute the observednormal faults to the neotectonic compression along the HFTand related adjustments.

CONCLUSIONS

The present field observations and topographic analysisin the Nahan Salient reveal convincing evidences of theongoing tectonic activity along the HFT. Based on thecomparison with adjoining areas, it is inferred that thetopography in the Sub-Himalayan belt of NW India has

responded in an almost similar way giving rise to fourdistinct geomorphic surfaces in the three key areas ofKangra, Nahan Salient and Dehradun spread over morethan 200 km laterally along the HFT. However, the limiteddevelopment of the geomorphic surfaces in the NahanSalient is attributed to the more focused erosion by smallerstreams and absence of any major drainage in the area. Atthe same time, the presence of sub-vertical normal faultsoriented at high angles to the HFT is interpreted as amanifestation of the kinematics of compression and frontalramp-oblique ramp-frontal ramp geometry of the thrust fault.

Acknowledgements: The present work was initiated bythe financial support of the CSIR in the form of a researchfellowship to the first author. Further financial support wasprovided by the Department of Science and Technology(DST), New Delhi via Fast Track project (Grant no. SR/FTP/ES-52/2006). The authors thank the Scientist-in-charge,C-MMACS, Bangalore; Head, Department of Geology,Panjab University, Chandigarh. Head, Department of EarthSciences, IIT Roorkee for the infrastructural support. Criticalcomments and useful suggestions by the reviewers benefitedto focus and improve the manuscript.

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(Received: 30 November 2009; Revised form accepted: 7 December 2010)

geomorphic and structural evidences of neotectonic activity in the sub-himalayan belt of nahan...

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