Vol.2, No.7, 667-680 (2010) Natural Science
http://dx.doi.org/10.4236/ns.2010.27083
Development of extensional stresses in the
compressional setting of the Himalayan thrust wedge:
inference from numerical modelling
Ganesh Raj Joshi*, Daigoro Hayashi
Simulation Tectonics Laboratory, University of the Ryukyus, Okinawa, Japan; *Corresponding Author:
Received 16 March 2010; revised 10 May 2010; accepted 15 May 2010.
ABSTRACT
The estimation of contemporary tectonic stress
field and deformation in active fold-and-thrust
belts are imperative in identifying active geo-
dynamics and resulting faulting phenomenon.
In this paper, we focus on contemporary exten-
sional tectonics in the overall compressive set-
ting of the Himalayan orogen. Here we examine
the regional tectonic stress field and upper
crustal deformation in the Himalayan thrust
wedge using a 2D finite element technique, in-
corporating elastic rheology under plain strain
condition. The elastic models demonstrate that
the extensional tectonic stress and related nor-
mal faulting is extensively developed in the
southern front of the Himalaya at shallow
crustal level (< 10 km in depth). Our modelling
shows a good consistency with the geological
field evidences of active faulting, focal mecha-
nism solutions of medium size earthquakes in
the several sectors of the Himalaya. Results
based on numerical simulation, tectonic analy-
sis and taking geological and geophysical data
into account, we interpret that the present-day
extensional tectonic activity is not restricted in
the southern Tibet but distributed in the differ-
ent sectors of the Himalayan fold-and-thrust
belt co-exist with compressional structures.
Modelling results also indicate that the nature,
distribution and orientation of the maximum
compressive stress (1) of the Himalaya are
mainly controlled by the intra crustal Main Hi-
malayan décollement (MHT). The significant
amount of shear stress/strain concentration
along the MHT in the western Nepal predict that
the region is prone to moderate and great future
earthquakes.
Keywords: Extensional Stress Field; Convergent
Displacement; Finite Element Modelling; Himalayan
Wedge
1. INTRODUCTION
The existence of syn-orogenic extension is relatively new
discovery, which was described only since the eighties
and has received quite a lot of attention in the past two
decades. Understanding the mechanisms that produce
extensional deformation in contractional orogenic belts is
a major issue in the study of the plate continental struc-
ture and its dynamics. During the past few decades, ex-
tensional deformation structures have been mapped in
several contractional orogen such as the eastern Alps [1],
Andean Cordillera [2], Southern Apennines [3], Scandi-
navian Caledonides [4], North American Cordillera [5],
and the Himalayan orogen [6-8]. Generally, the normal
faults appear to be late stage or post-orogenic structures,
while documented cases of syn-orogenic normal faulting
are less common, which does not fit readily into the
paradigm of plate tectonics [9].
The Himalayan continental collision is formed as a
result of collision between Indian and Eurasian land-
masses ca 65-40 Ma ago [10]. The dominant structures
of the Himalayan chain show a stack of large south-
vergent thrust sheets emplaced successively from north
to south [11] and bounded by major intra-crustal thrusts.
These intra-continental tectonic features, general seismic
phenomenon, major folds and other deformation struc-
tures indicate the predominantly compressional tectonic
regime of the Himalaya. However, lately, a variety of
extensional expressions and normal faults have been
recognized in the southern Tibet [6,12,13]. Although it is
believed that these extensional structures are generally
restricted only in the southern Tibet [6,12]; however,
recent research has shown that normal faults and other
extensional features are not restricted only within south-
ern Tibet but distributed in the several sectors of the Hi-
malaya [14-19]. These extensional features, however, are

G. R. Joshi et al. / Natural Science 2 (2010) 667-680
668
not obviously consistent with the dominant seismicity
and southward thrusting of the Himalaya. Moreover, the
mechanism and the kinematics leading to the formation
of orogen-parallel extensional structures are still little
understood. Therefore, it is important to understand the
cause of ongoing paradoxical extensional tectonic activi-
ties in the overall compressive setting of the Himalaya.
Compared to other techniques, numerical modelling is
a fast, more economical and powerful tool which allows
for various geological structures and deformation phe-
nomenon to be modeled in the full scale. Stress and
strain values can be computed over long time periods
using various constitutive laws [20]. Thus, numerical
models have long been used to develop our understand-
ing of the mechanics of crustal deformation [7,21-24]
but most of these studies have been focus on the north-
ern part of the Himalaya and no one describes about the
extensional tectonic activities in the Himalayan front.
In this paper, we primarily focus on present-day on-
going extensional tectonics evidences in the southern
part of the Himalayan fold-and-thrust belt applying a 2D
finite element model (FEM). The final goal of this kind
of work is to understand the possible mechanism that
responsible for the development of the extensional stress
and normal faulting in the overall compressive setting of
the Himalaya thrust wedge.
2. TECTONIC SETTING
The Himalayas, one of the most seismo-tectonically ac-
tive regions of the world, are generally defined as a 2500
km-long arc of mountains stretching between Namche
Barwa in the east and Nanga Parbat in the west. The
Himalaya can be subdivided into western, central and
eastern sectors on the basis of variations in regional
geomorphology. This study focuses on the central sector
of the Himalayan fold-and-thrust belt, between 76°E and
91°E (Figure 1). From the south of the STDS the geo-
logical structure and tectonic setting of the fold-and-
thrust belt of the Himalayas are mainly characterized by
several prominent south-vergent thrust structures. These
intra-continental structural features are the Main Central
thrust (MCT), Main Boundary thrust (MBT) and the
Himalayan Frontal thrust (HFT) from north to south,
respectively. These north-dipping thrust faults have a
southward transport direction, and are inferred to branch
from the décollement thrust known as the Main Himala-
yan Thrust (MHT), which marks the underthrusting of
the Indian lithosphere beneath the Himalaya and Tibet
[25]. Several geological cross-sections across the Hima-
laya as well as seismic and other geophysical data sug-
gest that a crustal ramp is present on the MHT [26,27].
Numerous studies indicating lateral variations in the
geometry of the MHT décollement from east to west in
the Himalaya [25,28,29]. This lateral disparity of MHT
ramp geometry is the main cause for the abrupt change
in topographic relief, variation of elastic strain/stress and
intense seismic activities of the Himalaya [28,30].
The STDS is an east striking, north dipping system of
the normal faults that extends at least 700 km and
probably through the entire length of the northern front
of the Himalaya orogen, initiated during the Miocene
and contemporaneous with the MCT [5,13,21,22]. The
MCT is the large-scale high strain zone that commonly
occurs along the base of ductile shear zone and inverted
metamorphism sequence, which places Tertiary meta-
morphic rocks of the Great Himalayan sequence over
weakly metamorphosed the Precambrain-Paleozoic rocks
Figure 1. Geological map of the Himalaya showing major structural elements (MBT-Main Boundary
Thrust, MCT- Main Central Thrust) (adopted from Lefort, 1988) and tentative locations of regional
geological cross-sections lines A-A’ and B-B’.

G. R. Joshi et al. / Natural Science 2 (2010) 667-680
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669
of the Lesser Himalaya [31]. The MBT, a regional-scale
sinuous, steeply north-dipping active thrust zone, sepa-
rates folded and faulted Miocene and younger molasse
sediments of the Sub-Himalaya from Precambrian rocks
of the Lesser Himalaya [27]. Deformation of the MBT
began before 10 Ma [32]. However, several active fault
systems are commonly associated with the MBT system,
implying significant reactivation along many of its seg-
ments [15,33,34]. The MBT shows a down-to-the-north
displacement in the central and western sector of the
Himalaya [14,15]. The HFT is the southernmost, young-
est, non-continuous, NW-SE striking and recently the
most active imbricate thrust system in the Himalaya
[14,29]. The HFT is a series of thrust faults that separate
the Tertiary assemblage of the Sub-Himalaya from Qua-
ternary sediments of the Ganga foreland basin.
3. EVIDENCE FOR AN EXTENSIONAL
REGIME AND NORMAL FAULTING
3.1. Geometry and Pattern of Active Normal
Faulting
In the overall compressional tectonic regime of the Hi-
malaya, numerous contradictory orogen parallel and
perpendicular microscopic to mesoscopic evidences of
normal faults are well distributed. There are two main
types of normal faults that developed in the Himalayan
orogen; the first are the east-west striking, shallowly
north-dipping, normal faults of the STDS and Zanskar
shear zone which developed during the Early Miocene
[8], while the second are relatively active and younger
age normal faults which mainly restricted south of the
MCT and associated with the mega thrust systems in the
Himalayan [14,15,19,33]. In this section, we focus par-
ticularly on the evidence for syn-orogenic normal fault-
ing and extensional tectonic structures and their signifi-
cance are discussed in order to explain ongoing exten-
sional tectonic activity at the overall compressive setting
of the Himalaya.
Several normal faults on mesoscopic scale are obser-
ved in the Panjal Thrust Zone in the Dalhousie area of
western Himachal [16]. Similarly, on the basis of field
mapping and shear sense criteria Thakur et al. (1995)
found that the boundary between the southern margin of
the Higher Himalaya Crystalline (HHC) of Zanskar and
the Chamba syncline sequence is a normal fault. The
NW-SE and NNW-SSE oriented Karcham normal fault
(KNF) (Figure 2) and the regional scale, steeply south-
dipping Bhadarwas normal fault are found in the central
Hi machal. Regional scale normal faults are also ob-
served in the Pinjor Thrust Zone in the Lesser Himalaya
[16]. In the Kala Amb area, the NE-SW to E-W trending
Figure 2. Geological map of the NW-Himalaya showing main faulting regime deduced from the
fault-slip analysis adopted from the Vanney et al., 2004.

G. R. Joshi et al. / Natural Science 2 (2010) 667-680
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Singhauli Active fault (SAF) shows normal faulting [35].
Near the southern termination of the SAF, another set of
WNW-ESE trending south-side-up normal faults that dip
steeply toward the northeast with well-defined fault
scarps are observed [19]. In the Garhwal Himalaya, up
to 40 km long, E-W to NW-SE trending, northward dip-
ping Donga and Asan normal faults are well developed
parallel to the MBT, while the NE-SW trending Kalsi
normal fault is perpendicular to the MBT [33]. Similarly,
the Surkhet-Ghorahi fault emerges as a single, continu-
ous, steeply-dipping normal fault extends about 90 km
long parallel to the MBT system in western Nepal, [15]
(Figure 3).
3.2. Geomorphologic Features
Geomorphic and morphotectonic analysis of the land-
form further provide insights into style and pattern of
ongoing tectonic deformation. The Himalayan fold-and-
thrust belt is characterized by several landforms pro-
duced by active tectonics along the mega thrust systems.
The field observations in the different parts of centre
Himalaya reveal that advancement of large scale linea-
ments, tilting and shifting of river and piedmont, subsi-
dence of land and older rock sequences, uplift of river
terraces and overriding of younger (Holocene) sediments
by active thrusts and normal faults noticeably indicate
the co-existence of compressional as well as extensional
regimes in the Himalaya [19,36,37]. Moreover, the num-
ber of places just adjacent to the MBT and HFT, which
defines the zone of convergent between Himalaya and
Indo-gangetic plains, evidence of active normal faulting
occurs [36]. These faulting have produced several to-
pographic and geomorphic features such as active fault
traces, lateral offsets of streams, offset of quaternary
terraces, linear valleys running along faults and narrow,
deep gorges in the Himalayan front [19,36,37], and sug-
gest that the overall compressive setting of the Himalaya
front contradictory exhibit extensional tectonic activities
in the several parts.
4. FINITE ELEMENT MODELLING
In this paper, a 2D finite element (FE) technique has
been used to simulate the present-day tectonic stress
field and deformation of the Himalayan orogenic wedge
using a software package developed by Hayashi [38].
The important part of the mathematical formulations
about the software is provided and successfully applied
in previous studies [12,38-40].
4.1. Fault Analysis
It is well-known fact that elastic deformations, even if
small, govern the initiation of fault in nature [21]. Fault-
ing observed in nature is in very good agreement with the
simulated stress distribution for elastic models [41].
Therefore, elastic models have been considered in study-
ing the development of extensional deformation in the
overall compressive setting of the Himalaya. The models
assume elastic rheology for the brittle upper crust of the
Himalaya. Brittle failure is determined by the Mohr-
Coulomb failure criterion, which is based on a linear rela-
tion between the shear stress (τ) and the normal stress (

n)
which can be express by following equation.
tan
failure n
c




 (1)
where, c is the cohesive strength and  is the angle of
internal friction. Failure occurs when the Mohr circle
first touches the failure envelope. This occurs when the
radius of the Mohr circle, (

1 −

3)/2, is equal to the
perpendicular distance from the center of the circle at
( 
1 +

3)/2 to the failure envelope which is given by
13 13
cos sin
22
failure
c
 



 

 
 
(2)
According to Melosh and William (1998), the prox-
Figure 3. Geological map of the Surkhet-Goarahi fault in the western Nepal Himalaya showing main faulting regime
deduced from slickenside analysis adopted from Mugnier et al., 2004.

G. R. Joshi et al. / Natural Science 2 (2010) 667-680
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671
imity to failure (Pf) is the ratio between the calculated-
stress and failure stress can be express by
13
13
2
2
f
f
ailure
P

















(3)
When the ratio reaches Pf = 1, failure occurs, but
when Pf < 1 stress is within the failure envelope rocks
does not fail. The proximity to failure reveals which part
of the model is close to failure or already failed by gen-
erating faults.
4.2. Model Set-Up and Assumptions
We adopted two N-S regional structural cross-sections
through the Himachal Himalaya and western Nepal Hi-
malaya for modelling. The NW-Himalaya model extends
up to 200 km in length, with a maximum thickness of 40
km in the north, and the western Nepal model is 272 km
in length and up to 40 km thick in the northern part
(Figure 4). The major tectonic units and the approximate
locations of the profiles are reported in Figure 1. Each
cross-section is divided into different layers that repre-
sent major structural and lithological units according to
their regional setting in the Himalaya. The convergence
displacement has been applied, instead of stress and
forces because the relative velocity of the Indian plate
with respect to Eurasian plate for the central Himalaya is
well constrained. The basement fault (MHT) is intro-
duced with a prescribed geometry (Figure 4). Studies of
the focal mechanism of the large earthquakes in the Hi-
malaya [43] suggest that the MHT dips gently 4° to 9°
from south to north [30], which is consistent with the
INDEPTH profile [25] and topographic studies of the
Himalaya [44]. For simplicity, we adopted dip angles 7°,
5° and 30° for the northern flat, southern flat and the
MHT ramp, respectively [30]. Here, only the brittle por-
tion of the MHT is prescribed. In our model, the crust up
to 40 km is assumed to behave as an elastic material
because of intense seismicity and generate several faults
suggest the brittle characteristics of upper crust in the
Himalaya.
4.3. Boundary Conditions
The Indian plates moves at N20°E with constant rate of
the convergence displacement relative to Eurasia [45],
and subducts along a shallow dipping detachment known
as the MHT beneath the Himalaya, and it is still con-
tinuing [25]. This continuous horizontal displacement
caused by convergence of Indian plate has been consid-
ered as a prime driving force for the overriding crustal
deformation of the Himalayan orogen. Therefore, simu-
lation of the observed tectonic features of the Himalaya
using appropriate boundary condition could be modeled
by applying a horizontal convergence displacement from
the southern face of the model with present-day shorting
rate of the central Himalaya. Since the rate of displace-
Figure 4. Simplified geological cross-sections and applied boundary conditions adopted for the modeling. (a)
NW-Himalaya cross-section along A-A’ and (b) western Nepal cross-section along B-B’ in Figure 1.

G. R. Joshi et al. / Natural Science 2 (2010) 667-680
672
ment is varied in the different part of the Himalaya
[44,46] the uniform horizontal displacement rate of 17
mm/yr for the NW-Himalaya and 20 mm/yr for the wes-
tern Nepal have been adopted. These convergence rates
are well consistent with GPS measurements [44,46] and
long-term geological observations [47,48] along the Hi-
malayan front. Figure 4 shows the geometry and applied
boundary conditions of our FE models based on pre-
sent-day tectonic setting and kinematics of the Himala-
yan thrust wedge. The upper part of the entire model
represents surface topography which is free to deform in
all directions. The northern side of the model only can
move vertically. The lowermost nodal point marked by a
triangle is fixed. Since the lower boundary of the model
is inclined, the convergence displacement is resolved
into both x and y directions (Figure 4). The model is
loaded with gravitational body force (g = 9.8 m/s).
4.4. Model Parameters
The model consists of eight layers, which represent dis-
tinct litho-tectonic sequences of the Himalayan wedge.
Each rock sequence has been assigned with distinct rock
layer properties on the basis of predominant rock types
as shown in Figure 5. The crustal density was obtained
from published gravity-seismic model [47]. Since the
density of the rock layers of the individual tectonic
blocks are known, we obtained seismic P-wave (Vp) and
S-wave (Vs) velocities for the each rock layer from the
velocity models [28,50]. In order to solve elastic equations,
we need to know the independent elastic constants Young’s
modulus of elasticity (E) and Poisson’s ratio (υ) where
Poisson’s ratio is assumed to be constant at 0.25 for indi-
vidual tectonic blocks. E can be calculated by [37,49]:
)1(
)21)(1(
2
v
vv
VE p



(4)
Other input parameters required for modelling include
the angle of internal friction (
 ) and the cohesive stren-
gth (c) which obtained from the handbook of physical
constants [53].
5. MODELLING RESULTS
The series of numerical experiments were conducted for
the two representative structural cross-sections of the Hi-
malaya. Here, we present new evidence for extensional
stresses and normal faulting in the several sectors of the
Himalayan shallow crust. The results presented herein are
based on: 1) the distribution, orientation and the magnitude
of the principal stresses, 2) proximity of failure and 3) the
magnitude of the shear stress and strain.
Figure 5. Physical parameters and material properties of the different rock units used for modelling.

G. R. Joshi et al. / Natural Science 2 (2010) 667-680
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673
5.1. Stress Distribution
Figures 6 and 7 show the spatial distribution of the
simulated stress field developed for the different hori-
zontal convergent displacement conditions in both NW-
Himalaya and western Nepal models. The principal axis
of maximum compressive stress (

1) and and minimum
compressive stress ( 
3) are aligned in the horizontal and
vertical lines, respectively. Notice that if

1 aligned in
the horizontal and

3 aligned vertically represent com-
pressional stress regime, while the orientation of

1 is
vertical and 
3 is horizontal together with the tensional
component (red bar) implying extensional tectonic state
of stress. The overall simulated stress pattern shows two
distinct types of the characteristic stress field developed
in the Himalayan crustal wedge. The most compressive
stress regime found in the northern part, while an exten-
sional stress develops in the southern front of the Hima-
laya at shallow (< 10 in km) crustal level. However,
when increase the convergent displacement up to 500 m
the principal axis of stress are considerably tilted and
finally rotated from the original coordinate axis and the
deformation front shifts progressively toward the south-
ern front of the Himalaya (Figures 6(c) and 7(c)).
It is noteworthy that the change in stress field and
progression of deformation shows the thrust faults have
migrated from hinterland to foreland and the stress field
changes from extensional to a compressional regime. In
general, the predicted stress field on the Himalayan up-
per crust shows almost similar and uniform pattern in
both models, while some discrepancy also observed in
the northern part of Himalaya. In the NW-Himalaya
model (Figure 6), the extensional stress field is re-
stricted more northern part of the Himalaya whereas in
the western Nepal model (Figure 7), extensional stress
regime only developed south from the MCT. The stress
is uniformly distributed throughout the Indian crust.
Nevertheless, magnitudes of the principal stresses have
increases with depth because of topographic loading.
5.2. Maximum Shear Stress Distribution
Figure 8 shows the distribution of maximum shear stress
predicted from our modelling within the Himalayan up-
per crustal wedge.
The computed maximum shear stress (max) revealed
Figure 6. Predicted stress distribution in the NW-Himalaya model applying uniform conver-
gence displacements: (a) at 50 m; (b) at 250 m; (c) at 500 m.

G. R. Joshi et al. / Natural Science 2 (2010) 667-680
674
Figure 7. Predicted stress distribution in the western Nepal model applying uniform conver-
gence displacements: (a) at 50 m; (b) at 250 m; (c) at 500 m.
Figure 8. Predicted maximum shear stress (in MPa) distribution for (a) NW-Himalaya model and
(b) western Nepal model, at 50 m and 500 m convergent displacement conditions, respectively.

G. R. Joshi et al. / Natural Science 2 (2010) 667-680
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675
Figure 9. Predicted faulting elements in the NW-Himalaya model by imposing. (a) at 50 m; (b) at
250 m and (c) at 500 m convergent displacements.
that the major concentration of max along the Himalayan
basal décollement (MHT) zone. The trajectories of (max)
along the MHT increases from the southernmost part and
reach a maximum to the northern flat of the MHT ramp
in both models. Our model shows that the maximum
shear stress (max) levels of 250 MPa and 300 MPa from
NW-Himalaya model and western Nepal model, respec-
tively (Figure 8). This result suggests that the Himalaya
has continually built up significant amounts of stress
along the MHT décollement.
5.3. Faulting Patterns
We also examined the faulting pattern in the Himalayan
brittle upper crust based on the proximity of failure. The
Mohr-Coulomb failure criterion is used to determine the
faulting conditions. Because in the faulting regimes

1
and 
2 are nearly vertical and horizontal, the types of
faulting can be easily determined using the method of
Anderson [54]. Figures 9 and 10 illustrate the predicted
faulting pattern for both the NW-Himalaya and western
Nepal models. In general, both models demonstrate
similar patterns of fault development under different
horizontal convergence displacement conditions. Nor-
mally, our models clearly show two distinct types of
faulting patterns in the Himalayan wedge where thrust
faults are primarily concentrated in the north part (hin-
terland) and normal faults are extensively developed at
the southern front (foreland) of the Himalaya. Figure 9
shows failure pattern in the NW-Himalaya models from
50 to 500 m convergence displacement conditions. At 50
m horizontal convergent displacement, the modelling
results show that normal faults have predicted within
the Higher Himalaya, central and southernmost Lesser
Himalaya and Siwalik front, while thrust faults are
primarily predicted north of the MCT and in and
around the MHT ramp in the Higher Himalaya. Small
open circle indicates normal and pair of perpendicular
lines represents thrust faults, respectively. Nonetheless,
when the applied displacement gradually increases up
to 500 m, normal faults reduces considerably and
thrust faults were developed in such region and pro-
gressively migrated towards the foreland. Figure 10
illustrates the distribution of predicted failure elements
in the western Nepal model. This model shows no-
ticeably more failure elements compared with the
NW-Himalaya model. In this model, normal faults are
mostly developed in the northern part of the Lesser
Himalaya and the Siwalik area, where the juxtaposi-
tion of normal and thrust faulting along the ramp part
of the MHT décollement is well developed at low
convergent displacement (Figure 10(a)).

G. R. Joshi et al. / Natural Science 2 (2010) 667-680
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6. DISCUSSION
Previous studies illustrate that using an elastic models
are appropriate to obtain the tectonic stress field and
deformation of the brittle upper crust [7,21]. Although
several models have been proposed to explain the exten-
sional state of stress and formation of the normal faults
in the southern Tibet and Himalaya [7,21,22,41], but
none of them explain the development of extensional
stress regime and normal faults in the Himalayan front.
Here, we have presented a series of experiments using
2D finite element elastic models to examine the pre-
sent-day tectonic state of stress and deformation in the
Himalayan upper crustal wedge. For better understand-
ing the basic mechanism of the extensional tectonics of
the overall compressive tectonic setting of the Himala-
yan thrust wedge we have kept the models relatively
simple, and limited the model. We prescribed conver-
gence displacement boundary conditions consistent with
the contemporary plate kinematics of the central Hima-
laya region. We further assumed that the Himalayan up-
per crust behaves elastic in order to model faulting pat-
tern of the region. Some factors are not incorporated in
the current models are potentially important in the con-
vergent tectonic belts. For example, we did not consider
the effect of erosion at the surface of the Himalayan
thrust wedge. However, most of syn-orogenic erosion
process controls the dynamics of the orogenic wedge
[55]. Despite the simplicity of the model our results may
provide useful insight into extensional tectonics in the
overall contractional setting of the Himalaya.
6.1. Present-Day Stress Distribution in the
Central Himalaya
The distribution of the present-day stress field can pro-
vide a significant explanation for the ongoing geody-
namics and tectonic forces acting in the India-Asia colli-
sion zone. Since the convergence rate of Indian plate has
been decreasing ca 40 Ma [56], the Himalayan stress
field would show consequence changes in overall com-
pressive regime of the Himalaya. Nakata et al. [57], ar-
gue that the direction of the horizontal compressive
stress axis has changed due to changes in the direction of
the relative plate motion between the Indian plate and
tectonic sliver. Our modelling results also support this
idea and show two types of stresses developed in the
Himalayan upper crustal wedge. The compressional
stresses regime predicted in the northern part while ex-
tension stress regime predicted in the southern front.
These stresses further retained at all convergence dis-
placement conditions (Figures 6 and 7). Although pre-
dicted extensional stresses developed in the Himalayan
front are obviously not consistent with major seismic
events and dominant southward thrusting of the overall
compressive setting of the Himalaya, but well corre-
sponding with the field observations of active faulting in
the several sector of Himalayan thrust wedge [14-16,
19,34], neotectonic model of Nakata [57], and focal
mechanism of solutions of medium size earthquakes
(Figure 11). Taking advantage from predicted exten-
sional stress regime from our models, it can be inferred
that the recent changes in compressional tectonic regime
locally in the several part of the Himalayan front. Al-
though we strongly believed that the entire orogen is still
in overall compression, the predicted extensional stress
regime and observed normal faults in the Himalayan
frontal part are mainly due to local adjustment of tecton-
ics regime due to the taper angle readjustment and weak
Himalayan décollement. The computed stress fields
Figure 10. Predicted faulting elements in the western Nepal model by imposing different convergent
displace- ment conditions for (a) at 50 m; (b) at 250 m; (c) at 500 m convergent displacements.

G. R. Joshi et al. / Natural Science 2 (2010) 667-680
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677
Figure 11. Focal mechanism of solutions of medium size earthquakes in
NW-Himalaya adopted after Yadav et al. (2009).
show significant deflection and rotation of the maximum
compressive stress ( 
1) from their original horizontal
position to vertical when increase the compressive dis-
placement (Figure 7). The predicated rotation of the
stress trajectories might be the effect of ramp geometry
of the MHT décollement, and consistent with the seismic
study of Pandey et al. [28]. In which they demonstrated
that the MHT ramp acts as a geometrical barrier to
changes in the orientation of the maximum principal
stress in the Himalaya, causing the significant change in
the stress field of the Himalaya. Nonetheless, Mugnier et
al. [15] argued that if the principal stress axis deviates
significantly from the horizontal, and when this devia-
tion exceeds the dip of the vectors normal to back-tilting
thrusts, the normal component of displacement may act
along these faults; therefore, steep north-dipping seg-
ments of the MBT show a normal component of dis-
placement. The rotation of the stress trajectories would
facilitate the formation of imbricated thrust in the mov-
ing thrust sheet. In both models, thrusts are propagated
towards the foreland of the Himalayan wedge as we in-
crease the convergent displacement (Figures 6 and 7).
This is characteristic feature of thrust propagation in
fold-and-thrust belts, and consistence with fold-bend-
fault model of Yeats and Thakur (1998) [51].
6.2. Magnitude of the Stresses in Central
Himalaya
Our models predicate the horizontal stress level of 300-
500 MPa at the depth of 40 km of the Indian lithosphere
which is consistent with other studies [58,59]. The re-
gion with high maximum shear stress (max) induced will
be more active [60]. If we compare both of our models;
the western Nepal model shows considerable amount of
maximum shear stress (300 MPa) accumulated along the
MHT décollement suggests that possible major earth-
quake in the region due to the reactivation of the basal
décollement of this part of the Himalaya. This prediction
is further supported by the concentration of microseis-
mic clusters in and around ramping part of the MHT [28],
consistent with the numerical simulations [30] and GPS
measurements [46].
6.3. Development of Faulting Pattern in the
Central Himalaya
The computed results from our model predicted two
types of characteristic faulting patterns developed in the
Himalayan thrust wedge. In general, thrust faults are
predicted in the northern Himalaya (north of MCT)
while normal faults are predominantly predicted in the
southern front of Himalaya for all 50 m to 500 m con-
vergent displacements (Figures 9 and 10). According to
Mugnier et al. [15] normal faults that are associated with
the Himalayan major thrust structures in the frontal part
of Himalaya (e.g., normal fault along the MBT) is due to
a very poor strength contrast between the basal décolle-
ment and rock in wedge body and high pore fluid pres-

G. R. Joshi et al. / Natural Science 2 (2010) 667-680
678
sure ratios (up to 1). They further argued that the outer
Himalaya extends over the width of several tens of kilo-
meters in the central sector of Himalaya, and are pre-
sumably displaced along a décollement located in pelitic
formations. There is therefore a small contrast of streng-
th between rocks in the décollement levels and displaced
sheets, a condition that favors normal faulting in the re-
gion [15]. However, Nakata et al. [57] argued that the
Lesser Himalayan block is subsiding, causing normal
faulting due to a change in tectonic regime of the Hima-
laya. From computer simulation Shankar et al. [22] pro-
posed that the presence of a weak MHT décollement
below south Tibet is the main cause of extension in the
Himalaya. Our modelling results shows that the Hima-
layan décollement thrust (MHT) have significant role to
the development of extensional state of stress and normal
faulting in the Himalayan front. We believed that the
formation of ramp geometry on the MHT décollement is
the major causes of the development of extensional stress
and normal faulting in the various part of the Himalaya
fold-and-thrust belt. Moreover, we suggest that the local
adjustment of the tectonic activities influenced by taper
angle adjustment further influences to the formation of
the extensional tectonic activities and normal faulting in
the overall contractional setting of the Himalaya.
6.4. Comparison with Field Observations
Since our models were constrained by present-day stru-
ctural sections of the Himalaya, the predicted stress re-
gime and faulting pattern from our models can be di-
rectly compared with field observations. The simulated
faulting pattern appropriately corresponds to the natural
situation of the Himalaya and several faults are com-
puted at analogous locations, which show good agree-
ment with the position of normal faults and the exten-
sional regime across the Himalayan thrust wedge. The
stress fields and faulting pattern predicted from our
model is well consistent with field observations of pre-
vious studies [13-19,22,26,37]. Our modelling results
further support the formation of extension stress field
and normal faulting mainly dominated in the frontal part
of the Himalayan thrust wedge.
In addition, if we compare out numerical experiment
results with the observations of the microseismicity and
medium size earthquake focal mechanism solutions of
the Himalayan front our result shows good consistency
(Figure 11).
7. CONCLUSIONS
In this paper, the 2D finite element modelling was used
to simulate the present-day extensional tectonic state of
stresses and deformation in the overall compressional
setting of the Himalayan brittle upper crust incorporated
an elastic rheology under plain strain condition. Based
on our modelling results, compared with geological field
observations, records of active normal faults and the in
situ stress regime, we conclude that the extensional tec-
tonic regime and normal faulting of the Himalaya is not
only restricted within the southern Tibet but distributed
throughout the Himalayan thrust wedge as observed in
field. Modelling results further indicate that the exten-
sional tectonic deformation in the brittle upper crust of
the Himalayan thrust wedge is still active; although the
overall tectonic setting of the Himalaya is in compres-
sion. Our results show that the MHT décollement plays a
pivotal role in changing the tectonic stress field and
faulting regime in the Himalayan thrust wedge, and ac-
cumulating significant amount of stress/strain along the
ramp part. This continuous process of the stress/strain
accumulation along the MHT décollement triggers in-
tense microseismic activities, and increases the risk of
the future great earthquake in the central Himalayan re-
gion.
8. ACKNOWLEDGEMENTS
The Ministry of Education, Culture, Science and Technology (Mon-
bukagakusho) of Japan is acknowledged for the financial support of the
first author. The authors are thankful to Prof. Laurent Godin, Dawn
Kellett and P. Mandal for their valuable comments and suggestions,
which greatly helped to improve the earlier version of the manuscript.
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