uIntroduction
uFigure
captions
uNahan
salient
uMethodology
uDiscussion
uPlay
types
uIntroduction
uFigure
captions
uNahan
salient
uMethodology
uDiscussion
uPlay
types
uIntroduction
uFigure
captions
uNahan
salient
uMethodology
uDiscussion
uPlay
types
uIntroduction
uFigure
captions
uNahan
salient
uMethodology
uDiscussion
uPlay
types
uIntroduction
uFigure
captions
uNahan
salient
uMethodology
uDiscussion
uPlay
types
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![](thumbs/02.jpg) |
Figure 2. Simplified geological map of
the Nahan salient. Locations of the Subathu, Morni, and Nahan
sections are also shown. Thrusts: BiT, Bilaspur thrust;
BkT, Bisiankanet thrust; ChT, Chail thrust; CrT, Chur thrust;
ENT, East Nahan thrust; GrT, Giri thrust; HrT, Haripur thrust;
JjT, Jarja thrust; JnT, Jaunsar thrust; JuT, Jutogh thrust; MBT,
Main Boundary Thrust; MFT, Main Frontal Thrust; MjT, Majhauli
thrust; NaT, Nalagarh thrust; PaT, Paonta thrust; PrT, Parara
thrust; RgT, Rajgarh thrust; RnT, Ranon thrust; SgT, Sangrah
thrust; SjT, Surajpur thrust; SrT, Sarauli thrust. The axial
traces have been traced using dip domain data. |
Nahan
Salient
A simplified geological map of the Nahan
salient and the locations of the lines of cross sections are shown in
Figure 2. A simplified stratigraphy of the area is as follows:
![](images/table.jpg)
It is generally considered that the three rock
groups; i.e., LHZ, lower SHZ (i.e., Subathu- Dharamsala), and upper SHZ
(i.e., Siwalik), are completely separated from each other by tectonic
surfaces on the surface as well as in the subsurface. However, the
geological map shows that this is not strictly true. Siwalik rocks occur
within lower SHZ, and lower Tertiary rocks occur within LHZ. In addition
to the two regionally important thrusts, viz., MFT and MBT mentioned
above, a large number of map-scale thrusts are present in the frontal
FTB. A number of large-scale anticlines and synclines are also present.
The surface traces of thrusts and axial traces of folds are
approximately parallel to each other. Statistical analyses of dips of
bedding planes and axial planes of small-scale folds suggest that the
rocks of both the SHZ and LHZ were deformed under a single, continuous
deformation episode. For these reasons, we place the LHZ along with the
SHZ rocks in the cover sequence and consider the crystalline rocks of
the HHCZ as the basement. Therefore, the SHZ and LHZ together constitute
the frontal fold-thrust belt (FTB) in the Himalayas. The deformation
pattern in this belt is characteristic of thin-skinned tectonic setting,
although LHZ is made up of Proterozoic sedimentary sequence.
Methodology
The balanced cross sections have been
constructed essentially using geological map and dip domain data. Two
available seismic reflection profiles from this area are about 40 years
old and are of rather poor quality. We have used these profiles only to
constrain the depth and dip of the detachment.
Both forward and inverse modelling were
carried out during section construction. For each thrust, several
fault-related fold models were constructed using dip-domain data in
order to generate admissible structures (forward modelling).
Several cross sections were then constructed using different
combinations of these models and checked for viability or balance
(inverse modelling). Departing from usual practice, we restored
deformed-state cross sections in discrete steps; at each step we
restored only one fault and ensured that the section remained admissible
and viable at every step of restoration. This allowed us to work out the
admissible geometry and sequence of thrust development and also to
estimate slip on each fault at each stage of fault development.
Discussion of Results
The three cross sections from the Nahan
salient are shown in Figure 3. In all the three sections the geometry of
faults and fault-related folds increase in complexity from foreland
towards hinterland. Based on the degree of complexity, the sections can
be segmented into three transverse parts. The area towards the foreland
lying between the MFT and the BiT and occupied dominantly by Siwalik
rocks shows rather simple structural geometry. Widely spaced thrusts
with simple fault-related folds are characteristic features in this
segment. The MFT has flat-ramp-flat trajectory in the Subathu and Morni
sections . The ramp anticlines in the hangingwall of the MFT in these two
sections have the geometry of fault-bend folds. The MFT in the Nahan
section is, however, emergent. Fault-propagation folding and low-angle
breakthrough are present in the hangingwall of the MFT. The faults
immediately NE of the MFT, viz., Nalagarh thrust (NaT, Subathu section),
Bisiankanet thrust (BkT, Morni section), and Paonta thrust (PaT, Nahan
section), have varying geometry of hangingwall structures. The
homoclinal sequence in the hangingwall of the NaT represents a
much-eroded fault-bend fault with out-of-sequence breakthrough along
ramp. The Haripur thrust (HrT) is a diverging splay from the NaT. The
surface dip-strike data have been used to model a blind thrust (BkT) in
the footwall of the PaT. Multibend fault-bend folding is associated with
BkT in both the Morni and Nahan sections . In the area between the BiT
and the MBT, occupied dominantly by the rocks of the Subathu-Dharamsala
Groups, the structural geometry becomes more complex. Here the linked
thrusts trace thrust systems (Bilaspur thrust system) that approximately
define leading imbricate fans (Nahan and Morni sections ) or a buried
hinterland-dipping duplex (Subathu section). The structural setting
towards the hangingwall of the MBT is extremely complex. Low ramp
spacing, folded thrust trajectories, interference of axial surfaces, and
breached horses are the characteristic features in this area. The ‘LHZ
thrust system’ (Figure 3) can be best described as stacked-up horses.
The rocks of the LHZ dominantly occupy this area.
From the foregoing description it is obvious
that the frontal FTB can be partitioned into three segments occupied
dominantly by Siwalik, Subathu-Dharamsala, and LHZ rocks, occurring
successively away from the foreland. These three segments show distinct
structural styles with structural complexity increasing from foreland
towards hinterland. Some of the thrusts and fault-related folds can be
traced in all the three sections , but others cannot be correlated. Some
of the thrusts are emergent in one section but are blind or are absent
in other section(s). Also the geometries of fault-related folds vary
from section to section.
The overall structural evolution of the area
cannot be explained by a simple piggy-back sequence of foreland
propagating thrust system giving rise to imbricate or schuppen structure
(Figure 3). All the three sections have been restored successfully. We
restored the sections in discrete steps. In other words, one fault was
restored at a time and in opposite sequence in which they developed;
i.e., the last fault to develop was restored first. The sequence of
development of the structures was derived through forward and inverse modelling. All the in-sequence thrusts have “correct” trajectories in
the restored sections in the sense that they have gentle to moderate
dips towards the hinterland. Some of the out-of-sequence thrusts,
however, have steep, overturned or zigzag trajectories in the restored
sections . This is permissible because many of the out-of-sequence
thrusts truncate previously formed thrusts. A model best describing the
structural evolution in the Nahan salient is the one in which a foreland
propagating, in-sequence thrusting event was followed by out-of-sequence
thrusting in an approximately break-back style. During the
out-of-sequence movement, some of the ramps that formed during
in-sequence thrusting were repeatedly reactivated, leading to very
complex structural geometries, particularly in the LHZ. The shortening
partitioned between the MFT and Chail thrust is 71% (or 72 km), 59% (or
50 km), and 67% (or 67 km) in Subathu, Morni, and Nahan sections ,
respectively.
Play Types
In an area with such structural complexity, a
thorough understanding of subsurface structural geometry is essential
for hydrocarbon exploration. Geophysical methods of subsurface imaging,
such as seismic reflection, on its own may not be able to resolve such
complex geometry of structures. Rigorous structural modelling
incorporating all available data and using techniques of section
balancing may be able to delineate areas for intensive exploration.
Based on our sections , a number of play types can be identified, where
detailed exploration for hydrocarbons may be undertaken. These include
(Figure 4):
Play type I: Ramp anticlines (fault-bend or
fault-propagation folds)
Play type II: Stratigraphic pinchouts/wedgeouts
Play type III: Roof sequences of buried
duplexes
Play type IV: Subthrust structural highs formed due to breaching of
anticlines or synclines by out-of-sequence thrusts.
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