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Page 91

EXTENDED ABSTRACTS: 23RD HIMALAYAN-KARAKORAM-TIBET WORKSHOP, 2008, INDIA

HIMALAYAN JOURNAL OF SCIENCES | VOL. 5 | ISSUE 7 (SPECIAL ISSUE) | 2008 91

Meso and microstructural analyses performed on the Triassic
flysch belonging to the Tibetan Sedimentary Sequence cropping
out in SE Tibet led to recognize a polyphase deformation history
linked to the evolution of continental collision. The study area
is located SE of Lhasa, from Tsedong and Gyatsa to the southern
Yala Xiangbo dome.

In the southern analized sectors the main deformation event
is a D1 tectonic phase. During D1 metric to chilometric asymmetric
F1 folds developed. The folds, facing to the South and striking
ENE-WSW are associated to an axial plane foliation (S1). S1 is a
low grade foliation and varies from a disjunctive spaced stylolitic
cleavage with no dynamic recrystallization to a fine continous
foliation. In the first case pressure solution is the dominant
deformation mechanism while dynamic recrystallization of illite,
quartz, calcite, oxides is associated to the continuous foliation.

Object lineations, trending NW-SE, are well represented by
strain fringes, mainly composed by quartz, around pyrite crystals.

A later D2 deformation phase is superimposed to D1
structures. The D2 is represented by a faible crenulation cleavage
in the southern portions, but moving towards the northernmost
sectors, it becomes the predominant tectonic phase. Moving
towards the Yarlung Tsampo Suture zone (YTSZ) we detected

Polyphase deformation history of the “Tibetan Sedimentary
Sequence” in the Himalayan chain (South-East Tibet)
Chiara Montomoli1*, Erwin Appel2, Antolin Borja2, István Dunkl3, Rachida El Bay2,
Lin Ding4 and R Gloaguen5

1 University of Pisa, ITALY
2 University of Tübingen, GERMANY
3 University of Göttingen, GERMANY
4 Instititute of Tibetan Plateau Research, CAS, Beijing, CHINA
5 University of Freiberg, Freiberg, GERMANY
* For correspondence, email: [email protected]

the development of E-W trending F2 folds, from decimetric to
decametric in size, verging to the North.

A synkinematic recrystallization of illite-sericite has been
observed along S2 foliation that, in more strained areas, becomes
a fine continuous foliation

The further tectonic evolution is characterized by the
development of brittle-ductile shear zones. The shear zones are
often localized on the inverted limbs of F2 folds.

Kinematic indicators are mainly represented by S-C
structures and point to a top-to-the-north sense of movement.

D1 tectonic phase is linked to continental collision; the
continuation of shortening gave rise to the back-verging D2
tectonic phase with a strain increase in the more internal areas
(moving towards the YTSZ) related also to the development of
Renbu-Zedong Thrust (Yin et al. 2000), bringing the TSS over
the mélange complex (Harrison et al. 2000).

References

Harrison TM, A Yin, M Grove and OM Lovera. 2000. The Zedong Window:
A record of superposed Tertiary convergence in southeastern Tibet.
Journal of Geophysical Research 105 (B8): 19,311-19,320

Yin, A and TM Harrison. 2000. Geologic evolution of the Himalayan-Tibetan
orogen. Annual Reviews in Earth and Planetary Sciences 28: 211-280

Page 92

EXTENDED ABSTRACTS: 23RD HIMALAYAN-KARAKORAM-TIBET WORKSHOP, 2008, INDIA

HIMALAYAN JOURNAL OF SCIENCES | VOL. 5 | ISSUE 7 (SPECIAL ISSUE) | 200892

The absolute role of climate/erosion in extrusion of mountains
has been debated for a long time. Field-studies, micro-structural
observations and few analytical trials confirm that the extrusion
of the Higher Himalayan Shear Zone, Sutlej section took place
initially by simple shearing and was followed by combined simple
shear and channel flow in a shifting mode (Mukherjee 2007). The
extrusion mechanism of the HHSZ is studied with 10 analogue
models of channel flow initiating from a horizontal channel and
extrusion through a linked inclined channel. The inclined channel
is the model HHSZ and is of parallel, gently diverging-up and
strongly diverging-up geometries in different considerations. In
these experiments, Polydimethylsiloxane (PDMS), a transparent
Newtonian viscous polymer is used as the model material and
geometric- and dynamic similarities are maintained with the
prototype. Six flow zones are deciphered in the two channels in
the mature stage of extrusion of the PDMS. Parabolic profiles are
produced at the middle of both the inclined- and the horizontal
channel during a Poiseuille and Jeffery Hamel flows. The part of
the PDMS originally inside the horizontal channel starts moving
through the inclined channel at a faster rate than the part of the
PDMS initially residing in the inclined channel. This in effect
gives rise to thrust movement of the former part of the PDMS.
The thrust plane originates at the corner joining the inclined and
the horizontal channel and rotates while coming closer to the
free surface. The tectonic insights gained from these experiments
are (i) secondary ductile thrusting took place in the HHSZ as
a delayed response to its channel flow mode of extrusion, (ii)
the genesis of the thrust seems to be related to the change in
the direction of extrusive flow– from horizontal to inclined up;
(iii) as these models were performed without any erosion of the
extruded PDMS, erosion induced by climate did not trigger nor
was a deciding factor in the genesis of the secondary thrust and
concomitant extrusion of the HHSZ. However, erosion induced
by a disparity in precipitation on the HHSZ might have augmented
the extrusion process i.e. it had a passive role. In Sutlej section
of the HHSZ, the simulated thrust can be correlated with the
Chaura Thrust with the recorded activation at least 13 Ma after
the ongoing extrusion of the HHSZ by channel flow mechanism
around 18 Ma.

How significant is erosion in extrusion- insights from analogue
and analytical models
Soumyajit Mukherjee1*, Hemin A Koyi2, Christopher J Talbot2 and AK Jain3

1 Engineering Mechanics Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bangalore-560064, INDIA
2 Hans Ramberg Tectonic Laboratory, Uppsala University, 752 36 Uppsala, SWEDEN
3 Department of Earth Sciences, Indian Institute of Technology Roorkee, INDIA
* For correspondence, email: [email protected]

Salt diapirs are spectacular natural structures and are
products of an interaction between the extrusive force that build
and erosion that tends to degrade them. While in humid climate,
their erosion rate can reach few mm per year, in arid climates it
goes down to one tenth to that of the former rate (Bruthans et al.
2008). Salt diapirism has been modeled as a product of the density
difference between the salt and the surrounding limestone country
rock in the islands of Hormuz and Namakdan in the Persian Gulf
(Mukherjee et al. 2007). Most of the 200 or so diapirs of Hormuz
salt in the Zagros mountains of Iran extrude majestic mountains
of salt that rise 400 m above their strong vents in limestones. Even
if a difference in density between the salt and the country rocks
are in the order of 10-1 gm.cm-3, such a low magnitude persistent
over tens of thousands of years can give rise to extrusion of the salt
for kilometers that can have dynamic viscosity as high as 1021 Pa s
(Mukherjee et al. 2007). Without taking into account their erosion
rates, diapirism was modeled as being triggered and maintained
by the density contrast. This indicates that erosion is not a crucial
controlling factor for the extrusion. Erosion is one out of many
components that can, however, modify the pressure gradient
controlling the extrusion process. The other components are (i)
the up-building extruded mass of salt that imparts temporally
increasing weight downwards, and (ii) gravitational spreading that
allows the extruded salt to flow away (Weinberger et al. 2006).

References

Bruthans J, N Asadi, M Filippi, Z Vilhelm. and M Zare. 2008. A study
of erosion rates on salt diapir surfaces in the Zagros Mountains, SE
Iran. Environmental Geology 53: DOI 10.1007/s00254-007-0734-6

Mukherjee S. 2007. Geodynamics, Deformation and Mathematical Analysis of
Metamorphic Belts, NW Himalaya. Unpublished Ph.D. thesis. Indian
Institute of Technology Roorkee: 267p

Mukherjee S, CJ Talbot, HA Koyi. 2007. Estimation of viscosity of natural
salts of the Hormuz and Namakdan salt diapirs in the Persian Gulf.
Annual Transaction of Nordic Rheology Society 15: 189-196

Weinberger R, V Lyakhovsky, G Baer and ZB Begin. 2006. Mechanical
modeling and InSAR measurements of Mount Sedom uplift, Dead
Sea basin: Implications for effective viscosity of rock salt. Geochemistry
Geophysics and Geosystems 14: DOI: 10.1029/2005GC001185

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