Proc
Indian of
Natn
Sci Acad 75 No.1
pp. 27-40 (2009)
Behaviour
Basement-cover
Decoupling
in Compressional Deformation Regime
27
Behaviour of Basement-cover Decoupling in Compressional Deformation
Regime, Northern Kumaun (Uttarakhand) Himalaya
KS VALDIYA1* and KANCHAN PANDE2
Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore–560 064
Indian Institute of Technology-Bombay, Mumbai–400 076. E-mail: [email protected]
(Received on 16 March 2009; Accepted† on 11 May 2009)
In the compressional regime related to India-Asia convergence, the plane of decoupling of easily yielding sedimentary
pile of the Tethys basin from its rigid basement of the Vaikrita crystalline complex behaved differently at different point
of time in northern Kumaun (Uttarakhand) Himalaya. The terrain-defining Trans-Himadri Detachment Fault (T-HDF)
system comprises a number of normal gravity faults that split both hangingwall and footwall rocks. The detachment
related fault propagation took place during the climactic phase of the Himalayan orogeny in the Early Miocene, spreading
on to larger network of faults along the basement-cover contact. Apart from the contrast in the structural architecture and
attitude between hangingwall and footwall, two critical stratigraphic horizons of the footwall were attenuated and eliminated
by faulting. Brecciation and mylonitization of the footwall rocks and gravity-driven northward collapse of folds in the
hangingwall characterize the T-HDF all along its extent. Occurrence of large plutonic bodies of granite in the hangingwall
sedimentary rocks in the Jadhganga valley made the structural design very complex due to faulting along the contacts of
plutons with the host rocks and folding around the intrusive bodies.
There are three spectacular and tectonically significant development invariably related to the T-HDF in the valleys of
Kali, Eastern Dhauli, Gori and Western Dhauli — (i) formation of very narrow gorges with convex walls and slit canyons
immediately downstream of the fault crossing, (ii) rise of footwall basement rocks higher than the hangingwall sedimentary
cover, and (iii) emplacement of large thick deposits of fluviolacustrine and glaciolacustrine sediments of Late Quaternary
age upstream of the T-HDF, that is, in the hangingwall. Penecontemporaneous deformation structures in these lacustrine
sedimentary succession at some (including basal) levels suggest repeated reactivation of the Trans-Himadri Detachment
Fault system.
Key Words: Detachment Fault; Slit Canyon; Penecontemporaneous Deformation Structure; Palaeolake;
Late Quaternary
Introduction
Following India-Asia convergence the horizontal
shortening of the Himalayan crust manifested itself in
compressional deformation in the manner of southward
thrusting and attendant ductile deformation of rock piles.
A multiplicity of planes of splitting developed as rock
masses advanced southwards in northern part of the
Kumaun (Uttarakhand) Himalaya (Fig. 1). One of the
planes of splitting was the plane of detachment of
sedimentary pile from its basement. The basement
comprised high-grade metamorphic rocks intimately
associated with Cambro-Ordovician and Early Miocene
granites that build the bulk of the Himadri (Great
Himalaya). And the sedimentary cover embraced
Neoproterozoic to Eocene sedimentary succession of the
Tethys terrain. The detachment plane behaved differently
in different sectors of the Himalayan arc, and at
different point of time during the Himalayan
orogeny [2-5,24,25,27,30,31,33,44]. Understandably, its
inclination and attitude vary from sector to sector.
Convergence of India and Asia continents
continuing, the rigid basement complex was squeezed
up as a huge wedge or slab to great height, eventually
forming the Himadri, commonly known as the Great
Himalaya. The loaded superincumbent Tethyan
sedimentary cover on the steepened northern slope of
the Himadri slid down, toppling over northwards and
forming northward-vergent backfolds and backthrusts.
This plane of detachment — which is the terrain-defining
fault separating the domain of Himadri from the Tethys
terrain (Fig. 2) — was first designated as the Malari
Thrust Fault in the valleys of Western Dhauli and Gori
in northern Kumaun Himalaya [2–5] and later renamed
Trans-Himadri Fault [6-8].
This paper describes salient structural characteristics
of and geomorphic development related to the formation
of what is now christened Trans-Himadri Detachment Fault
(T-HDF) in the valleys of Kali, Eastern Dhauli (Darma),
Gori, Western Dhauli and Jadhganga (the major tributary
of the Bhagirathi) in Uttarakhand (Figs. 1 & 2).
* Author for Correspondence E-mail: [email protected]
†
The March 2009 Issue of the Journal had got delayed. To avoid delay in Publication this article has been included in the present issue
even though it was accepted after March 2009.
28
KS Valdiya and Kanchan Pande
80o
Nila
5450
NELANG
Mana
Gangotri Glacier
R
swati
Sara
Arwa
7770
Gb
.
KEDARNATH
JOSHIMATH
UKHIMATH
Amlangla
Gb
R.
da
MARTOLI
●
●
RILKOT RALAM
PANCHCHULI
Sh
a
Nandkot
TA
RI
IK
VA
I
SIAR
MUN
7160
(M
C)
80o
KUTI
●
TAKLAKOTI
Gb
●
i R.
20 km
DUGTU
●
5770
Gori R.
30o
●
●
●
BALING
6910
LILAM ●
MUNSIARI ●
Pindar
7200
Gurla
Mandhata
.
Nandadevi
6850
Trishul
Alaknanda R.
Gb
●
gR
nan
MILAM
R.
Ala
k
●
7880
●
auli
iR
.
Dunagiri
CHAMOLI
F
TOPIDHUNGA
Dh
kin
●
.
rR
sa
Lis
nda
7060
● BADCHADHURA
LAPTHALI
E.
Ma
●
NT
ME
CH
TA
DE
RI
AD
IM
SH
AN
TR
●
●
31o
Shalshal
5800
lan
●
6700
Kamet
● GOTING
NITI ●
Gb
GAMSALI ●
● BADARINATH
6940
6180
SELA
BUDHI
li
Ka
E. Dhau
li R.
T-HDF
Gori
6910
31o
Kailas
Mansarovar
SONAM
NAGA F
Rak
sas
Tal
●
●
i R.
Gb
Gb
81o
Basement Complex (Vaikrita Group)
Trans-Himadri Deatchment fault System
Main Central (Vaikrita) Thrust
Munsiari Thrust
Other Thrusts
Martoli Fm
Ralam Fm
Garbyang Fm
pan
Jad
hga
nga
R
.
5910
●
R.
DHARCHULA
GUNJI
Gb ●
● TINKAR
GARBYANG
F
30o
81o
Fig. 1: Sketch map of northern part of the Kumaun (Uttarakhand) Himalaya showing position of the Trans-Himadri Detachment Fault.
Note that the Martoli and Ralam formations are attenuated and eliminated in the Kali valley. The Tethyan sedimentary succession of the
hangingwall is split into imbricating thrust sheets and severed from the Gurla Mandhata dome by a tear fault along the upper reaches of
the Karnali River. (Based on Heim and Gansser, 1939 [1], Valdiya, 1979 [2] and recent work by authors).
T-HDF
T-HDF
Garbyang Fm
Basement complex
Martoli Fm
Basement complex
A
B
T-HDF
Martoli Fm
Basement complex
B
Fig. 2: Trans-Himadri Detachment Fault separates the basement complex of high-grade metamorphic rocks with their granites from the
cover sediments — the Middle Ordovician Garbyang limestone (G) in the Kali valley and the Martoli flysch (M) in the Western Dhauli
and the Jadhganga.
29
Behaviour of Basement-cover Decoupling in Compressional Deformation Regime
Structural Characteristics
the Jadhganga valley a number of faults of lesser
magnitude occur above and below the T-HDF.
The evolution of the T-HDF system in the Kumaun
Himalaya entailed formation of a number of structural
features characterizing the terrain boundary between the
Himadri and Tethys domain.
The 65–70º NNE-dipping Vaikrita rocks of the
basement are characterized by NNE–SSW oriented
strong lineation and northeastward-plunging reclined
folds with prominent axial-plane foliation inclined
downdip. The Tethyan rocks of the hangingwall, in
contrast, exhibit disharmonic folds (Fig. 4) and NNEvergent backfolds locally broken by backthrusts.
Obliterating the bedding plane, the strongly developed
axial-plane joints dip 20–35º ENE/NE/NNE and NNW.
Conjugate pairs of joints have complicated the structural
architecture of the hangingwall as seen in the Chhialekh–
Garbyang section in the Kali valley and between Burphu
and Milam in the Gori valley.
Recent studies demonstrate that the T-HDF
represents a system of gravity faults which split both
the hangingwall and footwall rocks (Fig. 3). In the Kali
valley the Budhi Schist unit of the Vaikrita (basement)
complex is split by a fault into two slabs, one steeply
dipping and another gently inclined [9]. This fault is
traceable along the Patang gorge, filled with stupendous
mass of debris stretching downstream upto the Kali.
Budhi is located on the terrace of this debris fan. Not
only is this terrace tilted 10º at a place, but dissected
into two parts, the northern part being some 120 m above
the southern patch. Choked with colluvial debris and
morainic material, the Malchhi stream in the Eastern
Dhauli (Darma) valley, has cut a gorge along the fault
below the T-HDF traceable through Baling.
Excepting the Kali River section, in all valleys the
high-grade metamorphic rocks of the basement
encompass large and small bodies of leucogranite
intimately associated with migmatites. In the Jadhganga
and Eastern Dhauli valleys mylonitization is pronounced
in porphyritic granite, the intensity of cataclasis
increasing progressively towards the shear zone of the
T-HDF. Locally broken augen of feldspar occur in the
chaotic matrix of practically milled or brecciated rocks
as seen in the Gori valley between Mapong and Martoli.
A NNW–SSE striking tear fault (Fig. 1), partly
coinciding with the main fault, dextrally offsets the THDF system in the Gori valley between Mapong and
Martoli. Failure of the mountain slopes has given rise to
huge debris fans, one of which is more than 5 km wide
at the toe and about 200 m in elevation. There are quite
many shear zones in the hangingwall between Bilju and
Milam. The Kosa Gad in the valley of Western Dhauli,
characterized by enormous volume of colluvial-glacial
debris and nearly vertical scarp slope more than 1000 m
high above the stream bed, marks the fault that seems to
branch off from the T-HDF as clearly seen in the Gunti
stream at Malari. Similar situation is witnessed in the
Amritganga ravine at Gamsali upstream of Malari. In
In the proximity of the T-HDF, granites and
migmatites show very prominent, commonly open master
joints hading 70–65º NE/ENE/ESE (Fig. 5). In the
Chhialekh area in the Kali valley, these joints control
the straight courses of ravines and gullies. The formation
of steep to near vertical scarp faces and pinnacled
summits of the mountain rampart of the hangingwall in
the Western Dhauli are attributed to these joints.
S25oW
F?
T-HDF Chhialekh
4100m
o
N25 E
Gunji
Garbyang
Kali R.
T
Kuti R.
T
PatangGad
Budhi
T
3100
4100m
3100
Kali R.
2100
(A)
Pindari Fm
(Vaikrita)
S
Budhi Schist
Lake Deposit
Nagling
T
2100
Garbyang Fm
T-HDF
Malchhig
Shaila Fm
2 km.
Baling
Dugtu
N
3100m
3000m
(B)
2000
Lake Deposit
2000
E. Dhauli R.
2 km.
o
S25 E
Burphu
Martoli Gad
T-HDF
Bilju
Milum
4000m
4000m
3000
2000
N20oW
F
3000
Gori R.
Budhi
Pindari
Marto
2000
Lake Deposit
2 km.
(C)
Colluvial-morainic debris fan
Gravity fault
Joint
T - THRUST
Fig. 3: Diagrammatic cross-sections depicting salient structural features related to the Trans-Himadri Detachment Fault, and formation of
lakes upstream of the fault crossing. (A) Chhialekh near Garbyang, Kali valley, (B) Baling, Eastern Dhauli valley, (C) Burphu, Gori valley
30
KS Valdiya and Kanchan Pande
T-HDF
Martoli Fm
Basement complex
A
T-HDF
Martoli Fm
Basement complex
B
T-HDF
Martoli Fm
C
Basement complex
Fig. 4: (A) Disharmonic folding in the hangingwall rocks — the Martoli flysch — 1.5 km south of Niti in the Western Dhauli valley, (B)
Subsidiary fault in the Martoli flysch intruded by granites at Nelang on the right bank of the Jadhganga, (C) Complex folding in the
Martoli greywacks and phyllites at Nelang on the left bank of the Jadhganga valley
31
Behaviour of Basement-cover Decoupling in Compressional Deformation Regime
Geomorphic Development
The most spectacular feature of considerable tectonic
importance related to reactivation of the T–HDF is the
very narrow gorge formed in the granite- and migmatitedominant complex of every footwall. The deep defiles
— slit canyons — and narrow gorges formed
immediately downstream of the fault crossing (Fig. 7).
The change across the T-HDF is abrupt — from wide
valleys with gentler slopes lined with terraces of colluvial
and/or fluvio-glacial deposits in the hangingwall to slit
canyon or narrow gorge with convex walls in the
footwall. The rivers break into rapids as they flow
through the narrow passages. The Western Dhauli shows
this development at Malari, some two kilometres east of
Gamsali and southwest Goting (Fig. 7). In the Gori and
Kali valleys the gorges are wholly obliterated or blocked
by stupendous fans of colluvial debris and moraines.
g
d
ng Sy
Ga
Jadha
NAKURCHE
F
Gb
Gb
Gb
ad
G
ar
Ch
F
Nil
NELANG
F
Gb
F
B
Fig. 5: Prominent master joints in the granite in the basement
Vaikrita complex of the footwall in the proximity of the faults of
the T-HDF system south of Malari in the Western Dhauli valley.
T-HDF
Jad
hg
5km
an
ga
R
.
F
DUMKU
an
iG
5957
ad
5837
Chaling Gad
Master joint
SONAM
ap
Dum
T-HD
F
A
31o
15’
5259
CHANGDUM
Kun
g
R.
an
Gb
31o
15’
Master joint
dh
anga
Ja
Jadhg
The granite plutons occur not only in the Vaikrita
basement rocks, but also in the covering sedimentary
succession of the hangingwall. Close to the Jadhganga
basin in the Gangotri area, granite laccoliths were
emplaced at the top of the Vaikrita succession during
the onset of an extensional deformation and along
northern dextral shear zone [10]. The presence of larger
intrusive bodies in the Martoli succession in the
Jadhganga basin (Fig. 6) complicated the tectonics of
the region. The contacts of the plutons with country rocks
served as planes of dislocation, resulting in the
development of numerous faults in the hangingwall as
discernible in the Nelang–Sonam area (Fig. 6). One of
these faults oriented east-west has right laterally offset
the T-HDF by about 100 m at Nelang. The anomalous
northward plunging folds with NNE–SSW oriented axis
such as the Jadhang syncline (Fig. 6) [11] is presumably
an outcome of the effect of dome-shaped pluton of
granite. A major NNW–SSE trending normal fault along
the Chor valley (Fig. 6) is a wrench fault, offsetting and
coinciding with the T-HDF. This fault extends NW across
the India–Tibet border.
ncline
Throughout the study area extending from the
Jadhganga to the Eastern Dhauli, the Tethyan succession
(of the hangingwall) begins with the Martoli Fm overlain
successively by the Ralam and Garbyang formations. In
the Gori valley the greywacke-phyllite alternation of the
Martoli Formation is 500 to 800 m thick and the Ralam
conglomerate and quartzite attain a thickness of nearly
500 m (Figs. 2B, 2C, 3B and 3C). In sharp contrast, 80
km east of the Gori in the Kali valley, biotiteporphyroblastic calc schist of the Vaikrita is succeeded
directly by the Middle Ordovician dolomite and
argillaceous limestone of the Garbyang Formation (Figs.
2A and 3A), the Martoli and the Ralam units altogether
missing. This is a clear case of elimination by normal
faulting.
6209
79o
T-HDF
Basement Conglomerate (Vaikrita)
Martoli Fm
Ralam Conglomerate
Ralam Quartzite
Garbyang Fm
Granite Pluton
Fault
Synclinal axis
F
Fig. 6: Sketch map of a part of the Jadhganga basin in the Nelang–
Sonam area showing anomalous northward plunging syncline and
E–W trending and NNW–SSE oriented wrench faults. (Based on
Bassi and Datta, 1987) [11].
32
KS Valdiya and Kanchan Pande
A
B
C
D
Fig. 7: Slit canyons and very narrow gorges with convex walls in the footwall immediately downstream of the crossing of the TransHimadri Detachment Fault. (A) Gori valley, downstream of Rilkot, (B) Western Dhauli, south of Malari, (C) Western Dhauli, west of
Malari, (D) Western Dhauli about 2 km east of Gamsali (It is 15–25 m wide defile)
Behaviour of Basement-cover Decoupling in Compressional Deformation Regime
Another geomorphic feature that carries significant
implication is the NW–SE to WNW–ESE oriented
escarpment on the southerly face of the hangingwall,
exposing Tethyan rocks – mostly Martoli flysch injected
by granite. The foot of these scarps are characterized by
large and small cones of debris derived partly from the
shattered and brecciated rocks of the fault zone and partly
from the moraines in the hangingwall. The debris cones
and fans could have caused damming of rivers, reducing
their gradient and allowing aggradation. However, the
primary cause of river ponding must have been uplift of
the footwall along reactivated T-HDF which not only
caused blockage following uplift of downstream block
but also triggered massive landslides. The mountain
ranges of the footwall rise more than 1500–1600 m
higher than the hangingwall rocks in the Eastern Dhauli
implying uplift of the downstream blocks. Satellite
imagery of the Gori valley shows that in contrast to
spread and splaying out of drainage lines in the
hangingwall, the streams and gullies take straight lines
in the footwall domain [12], corroborating the surmise
that the footwall rocks rose up with respect to the
hangingwall sedimentary pile.
River Blockage and Palaeolakes
In the Kali valley Garbyang (30º6’N:80º50’E) is situated
on a terrace of a lake deposit (Figs. 8A and 9). Repeated
slumping triggered by undercutting of the river gave rise
to a number of irregular terraces. The 100- to 160-m
thick deposit comprises dominant silt-clay alternations
(Fig. 12A) with numerous lenses or intercalations of
pebble and gravel. The fluviolacustrine deposit is capped
by 3–4 m thick fluvial gravel, and the basal 5-m thick
horizon is characterized by penecontemporaneous
deformation structures such as convolutional folds and
flame structure. The origin of the lake — that stretched
more than 11 km from the foot of Chhialekh to Gunji
(Figs. 3A and 9) — is attributed to damming of the Kali
by moraines that descended from the Api and Nampa
glaciers in Nepal and building nearly 400 m high
Chhialekh dam [1, 13]. The colluvial debris dam of
considerable height and dimension and resulting
blockade that lasted more than 20 ka imply that it was
the work of tectonic movements on the T-HD. The
alternation of silt and clay and mud has been interpreted
as varvite [1,13,14,15]. The occurrence of lenses and
beds of sands and sandy gravels at various levels in the
mud sequence indicates influence of fluvial regime
throughout the life of the lake.
Thermoluminiscence dating of fine silt collected
from varve and rhythmite yielded ages of 20±3 Ka,
18±3 Ka and 13±2 Ka, suggesting that the
penecontemporaneous deformation occurred sometime
in the temporal interval 20 to 17 Ka and to 13 Ka [16]
33
— subsequent to ponding of the river due to movements
on the T-HDF. In our opinion it was the uplift of the
footwall along the T-HF that was responsible for the
ponding of the Kali and formation of the Garbyang lake
more than twenty thousand years ago [17,18].
In the Eastern Dhauli valley enormous volume of
moraine as well as colluvial debris brought by Machhi
ravine (that has carved out its course along the T-HDF)
built a 500 m high dam. Behind this dam was formed
the Baling Lake, which stretched 6 km upto Dugtu (Fig.
3B). The thick deposit consisting mainly of gravely,
pebbly and granular silt, with intercalation of clay and
mud in the distal part, is mantled by muddy and clayey
patches exhibiting bog-like conditions.
The Gori valley in the 7 km reach between Martoli
and Bilju is lined by three terraces of fluvial sediments,
predominantly gravels (Fig. 3C). The oldest (highest)
terrace at Burphu (30º22’N:80º11’E) embodies 20-25
m thick clay and clay-silt alternation over a stretch of
nearly one kilometre, representing a lacustrine regime
(Fig. 8B). A tributary stream Shalang Gad was also
ponded upstream of the fault crossing. The lacustrine
sediment is characterized by contorted bedding, flame
structure (Fig. 12B), and normal faults typical of
seismites [20]. It may be emphasized that layers with
deformation structures alternate with undeformed beds.
Luminiscence dating of quartz grains in the lake
sediments revealed that tectonic movements occurred
in the interval 16–11 Ka.
In the Western Dhauli no palaeolake deposits are
recognizable between Malari and Niti, despite the river
entering slit canyons at the points of fault crossing and
the fluvial deposits lining the valley. However, at Goting
(30º5’N:80º50’E) a 15–25 m thick lacustrine sequence
consists of ash grey and brownish red clays with
intercalations of morainic material, the ash grey clay
forming the top of the succession (Fig. 10). It is overlain
by 8–10 m thick gravel cover. The silt-clay alternation
has been described as varvite (with dropstone) [21,19].
The penecontemporaneous deformation features (Fig.
12B), including flame structure and convolutional ball
seem to have been formed due to tectonic activity on
the T-HDF. The tectonic movements might have been
accompanied by earthquakes. Some of these features can
therefore be described as seismites. Radiocarbon date
of the organic matter in the 12-m section yielded dates
of 40 Ka and 20 Ka, suggesting the life span of the lake
[19]. Magnetic susceptibility study shows six major
peaks at 39 Ka, 33 Ka, 29 Ka, 26 Ka, 24 Ka and 23 Ka.
Regional Perspective
Across the Kali River the T-HDF system extends
eastwards in Nepal (Fig. 13). Known as the Annapurna
34
KS Valdiya and Kanchan Pande
Fig. 8: (A) Garbyang Lake is represented by the nearly 160 m thick deposits of silt-clay alternation with profuse intercalations and lenses
of fluvial gravel, (B) Deposits in the Burphu Lake rest on a ground moraine (slightly darker in shade) (Photos: Courtesy Dr Navin Juyal).
Detachment in the Kali Gandaki basin, this normal
ductile fault is associated with gravity-driven northvergent large folds in the hangingwall [22], and
characterized by eastward extension of the hangingwall
terrane [23,24]. In the Sagarmatha (Everest) massif in
northeastern Nepal where the Great Himalayan
crystalline complex is split by a number of gently dipping
thrusts, the uppermost 5–15ºN dipping plane, and
traceable north to southern Tibet is known as the South
Tibet Detachment [25,26] or as the Qomolongma
Detachment [27,28]. In Bhutan the DontoLa Detachment
[29] represents the eastern extremity of the T-HDF.
Northwest of the Jadhganga valley (Figs. 1 and 13)
the T-HDF has been described as the Tethys Thrust
[30,31] and further northwest as the Zanskar Shear Zone
[32,33]. In the 2.25 to 6.75 km wide Zanskar Shear Zone
the displacement took place along closely spaced
infinitesimal shear planes, which dip 30–40º NE (Herren,
1987). There was earlier top-SW sense of overthrust
shearing, later superposed by NE–SW oriented extension
Behaviour of Basement-cover Decoupling in Compressional Deformation Regime
35
Fig. 9: Extent of the Garbyang Lake of the past in the Kali valley. The column shows the lithology of the palaeolake. Note the
penecontemporaneous deformation features occur in the lower part. (Based on Heim and Gansser, 1939 [1]; Chamyal and Juyal,
2005 [15]).
Fig. 10: Diagrammatic sketches show the location and sediment succession in the Goting palaeolake upstream of blockage in the
Western Dhauli valley. (The location of Goting palaeolake is after Pant et al. (1998) [19].
36
KS Valdiya and Kanchan Pande
Fig. 11: Deposits of the Goting palaeolake in the Western Dhauli valley. Upper: Behind the debris dam lie lacustrine sediments at the far
end, Lower: Relicts of the Goting lake sediments. (Photos: Courtesy Dr Navin Juyal).
Fig. 12: (A) Rhythmite of the Garbyang palaeolake in the Kali valley, showing alternation of silt and clays, (B) Penecontemporaneous
deformation features in the Goting sediment. Note that deformation features are confined to certain units (beds) which alternate with
undeformed layers.(Photos: Courtesy Dr Navin Juyal)
37
Behaviour of Basement-cover Decoupling in Compressional Deformation Regime
Himadri (Great Himalayan) Basement Complex
dh
Main Central Thrust
u
Srinagar
Z
ITS TT
ITS
Z
F
li
T
Ra
200 km
Ga
pti
k
HFT
ALA
Y
STAD Sagarmatha
S
GLD
MBT
HFT
Kathmandu
Trans-Himadri
Detachment F.
MCT Nanda Devi
Kiogarh
MC
Almora
o R.
ITSZ
Subansir
Thimpo
HFT
HIMADRI
MBT
Tsangp
T
ta
Tis
nda
SW
Lhasa
HIM
si
AD
Ka
Ganga
na
Yamu
Badarinath
Kailas
T-H
DF
MC
TE
T
TH
YS
MB
HFT T
MC
Ko
HF MBT
T
Himalayan Frontal Fault
li
Satluj R.
Main Boundary Fault
kho
ab
Tha
Chen
Trans-Himadri Fault System
Sin
lam
Z
ZS
Jhe
Raksas tal
hm
Bra
apu
tra R
.
NE
Indus-Tsangpo Suture
Kailas
0
LESSER HIMALAYA
1000m
TETHYS HIMALAYA
TIBET
0
1000 m
50 km
Fig. 13: Regional extent of the Trans-Himadri Detachment Fault (T-HDF) which has been described as the Tethyan Thrust (TT) in the
eastern Himachal Pradesh [30,31], the Zanskar Shear Zone (ZSH) in western Himachal Pradesh [32,33], the Annapurna Detachment
(AD) in western Nepal [22], South Tibetan Detachment (STD) in eastern Nepal [25,26] and the DontoLa Detachment (DT) in northwestern
Bhutan [29]. The rectangle shows the area studied in the central sector of the Himalayan arc.
along the ZSH within the domain of overall overthrusting
geometry [34,35,36]. In the Zanskar belt the southverging folds are believed to be contemporaneous with
the SW-directed thrusting during the Late Oligocene–
Early Miocene time [37]. The NE-directed backfolds are
the result of gravitation collapse of the thrust stack [38].
In the Chandra valley the formation of the shear zone is
attributed to a later phase of deformation [39]. In the
Kishtwar region in southeastern Kashmir, the
northeasterly dipping normal fault of the T-HDF system
was severely deformed in the interval 22–16 Ma [40]
even as the Haimanta (a” Martoli + Ralam formations
in Kumaun) of the hangingwall was thrust 30 km
southwards along the Chamba Thrust [41,42].
It is evident from the brief account of the easterly
and westerly lateral extent of the T-HDF that after the
compressional deformation that culminated in the
formation and evolution of SW-directed folds and thrusts,
there was dominant dip-slip movement related to normal
gravity faulting in the Kumaun and eastern Himachal
Himalaya and large-scale strike-slip dextral movements
superposed on earlier dip-slip movement in western
Himachal Pradesh and Nepal, thus bringing about
extension and resultant tectonic extrusion of the
hangingwall (Tethyan terrain). The Trans-Himadri
Detachment Fault System thus marks a zone of more
than one phase and kind of deformation.
Origin of Trans-Himadri Fault System
When the movements resulting from India–Asia
convergence were blocked or had slowed down on the
Indus–Tsangpo Suture, it was but natural that the thick
easily-yielding Tethyan sedimentary pile was detached
from its rigid foundation of the Vaikrita complex of highgrade metamorphic rocks and granites (Figs. 13 and 14).
The decoupling-related fault propagation started possibly
quite before the main phase of the Himalayan orogeny
in the Early Miocene time. And it spread on to the larger
fault network along the basement-cover contact, giving
rise to splaying faults associated with the main
detachment [18]. The end result was a system of shear
faults — the Trans-Himadri Detachment Fault System
(T-HDFS).
Taking in conjunction with the conspicuous
schuppen zone in the upper reaches of the Kali and
Darma (Eastern Dhauli) rivers (Fig. 1), the formation
and evolution of the Trans-Himadri Detachment Fault
system seems to have been controlled by the Gurla
Mandhata dome located south of Kailas–Mansarovar
area (Figs. 1 and 14). The Gurla Mandhata represents
the granite-dominated top of the basement complex that
buckled up when India collided with Asia. The Indian
plate being nearly 20% less dense than the mantle, and
therefore comparatively buoyant, there was resistance
38
KS Valdiya and Kanchan Pande
Incipient
MCT
S
Indus-Tsangpo Suture
N
T-HF
0
Tibet
50 km
100 km
NE
S-W
Kailas
Himadri
T-HF
I-TS
MCT
0
0
Tibet
50 km
50 km
SW
MCT
Himadri
(Nanda Devi)
N-verging folds
NE
0
0
1000 m
T-HT
1000 m
Lesser Himalayan
schuppen zone
Fig. 14: Diagrammatic cross-sections explain the stages of the formation of the Trans-Himadri Detachment Fault System in relation to
the formation of other terrane-defining thrusts. Detached from the Tethyan sedimentary cover, the basement complex (Vaikrita) was
squeezed up, forming the high Great Himalayan or Himadri ranges. (Modified after Valdiya, 1988) [7].
to its sliding under the Asian plate. Consequently the
leading edge of the Indian plate buckled up, forming
domal upwarp all along the collision zone [5,6,7,8]. The
domal structures are discernible from Nimaling in
Ladakh to Kangmar in northeastern Nepal.
The crustal-scale resistance that caused the doming
up of the frontal part of the Indian plate must have been
responsible for the development of a pack of imbricating
thrust sheets (schuppen zone) and the T-HDF system in
northeastern Kumaun (Fig. 1). As India continued to
advance northwards, the stress buildup in the Gurla
Mandhata dome manifested itself in the reactivation of
the thrusts of the schuppen zone, more so along the plane
of separating the Tethyan sedimentary cover from the
crystalline basement. The T-HDF developed along this
plane between the rigid basement complex and the
sedimentary cover during the period when large-scale
crustal thickening took place. As the deformation front
propagated southwards, other detachment planes and
shear zones were formed, culminating in the evolution
of the schuppen zone of Main Central Thrust during the
climactic phase of the Himalayan orogeny 21 to 22
million years ago. There was simultaneous extension of
the crustal block coupled with strike-slip displacement
in the eastern part. Most workers believe that the T-HDF
system and Main Central Thrust movements were
contemporaneous, the tectonic movement taking place
over a period 22 Ma to 19 Ma and the detachment
commencing around 21.5 Ma [43].
High stress manifested also in the strike-slip
displacement along wrench faults represented by the
NW–SE oriented Humla Karnali Fault framing the Gurla
Mandhata. Right-lateral movement is apparent in the
Gori valley between Rilkot and Milam (Figs. 1 and 13)
and possibly in the Saraswati valley between Badarinath
and Mana Pass. The squeezing up of the Vaikrita complex
(making the Great Himalaya) must have caused
reactivation of the T-HDF. The reactivation of the THDF system time and time again is evidenced by ponding
of rivers earlier and bursting later of natural dams
and attendant draining out of the lakes.
Penecontemporanceous deformation structures at some
levels of the lacustrine sediments column bear eloquent
testimony to the activeness of the T-HDF system.
A significant development was the right-lateral
displacement of the T-HDF — by an E–W tear fault in
Behaviour of Basement-cover Decoupling in Compressional Deformation Regime
the Jadhganga valley, by a NW–SE oriented wrench fault
in the Chor Gad (a tributary of the Jadhganga (Fig. 6),
and by the NNW–SSE trending fault in the Gori valley
where the two faults coincide (Fig. 1). The tear faults
testify to the tectonic resurgence of the Tethyan terrane
in the post-climactic phase of Himalayan orogeny, as
also witnessed in the Lesser Himalayan and Siwalik
terranes [2,3,4,7].
Acknowledgements
The first author is profoundly grateful to the Indian
National Science Academy, New Delhi for providing
extremely generous financial support for his project
under the INSA Golden Jubilee Research Professorship.
The Jawaharlal Nehru Centre for Advanced Scientific
Research extended all possible facilities and help.
The paper has greatly benefited from critical reviews
by Professor Sandeep Singh (IIT, Roorkee) and Professor
Navin Juyal (PRL, Ahmadabad). The authors thank them
for very valuable suggestions for improvement.
The authors are very grateful to Shri BD Patni and
Dr Rajeev Upadhyay for excellently organizing the field
trips in the valleys of Eastern Dhauli and Jadhganga
respectively.
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