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EXTENDED ABSTRACTS: 23RD HIMALAYAN-KARAKORAM-TIBET WORKSHOP, 2008, INDIA
HIMALAYAN JOURNAL OF SCIENCES | VOL. 5 | ISSUE 7 (SPECIAL ISSUE) | 2008
9
5
One of the most coherent crustal sections in the entire Himalaya,
from the point of view of lack of disruption or repetitions of key
beds due to thrusting, is exposed in Sikkim. Structural studies
across this unique section in Sikkim, covering the so-called Lesser
Himalaya, the Higher Himalaya and the intervening Main Central
The Main Central Thrust Revisited: New Insights from
Sikkim Himalaya
Sudipta Neogi
1*
and V Ravikant
2
1
Geological Survey of India, 27 J.L. Nehru Road, Kolkata – 700 016, INDIA
2
Indian Institute of Technology, Roorkee-247667, INDIA
*
For correspondence, email: sudiptaneogi@hotmail.com
Thrust zone (MCT zone) reveals a remarkable continuity in
structures and metamorphic history. Due to lack of clear evidences
for pervasive ductile shearing and thrusting, the position of the
MCT in Sikkim and therefore, the exact demarcation of the Lesser
and Higher Himalayan Belts has remained controversial with the
EXTENDED ABSTRACTS: 23RD HIMALAYAN-KARAKORAM-TIBET WORKSHOP, 2008, INDIA
HIMALAYAN JOURNAL OF SCIENCES | VOL. 5 | ISSUE 7 (SPECIAL ISSUE) | 2008
96
MCT being placed at different positions by various workers.
Rocks from the entire studied section in Sikkim preserve
imprints of four deformational episodes (D1-D4), with broad
similarity in their deformation patterns and structural elements.
A single penetrative regional planar structure (S2), associated with
F2 folds and sub-parallel to narrow ductile shears, formed during
the dominant D2 deformation episode. A complete and systematic
sequence of inverted progressive Barrovian metamorphic zones is
exposed, showing a regular pattern of variation of P and T (increasing
P and T upsection; Neogi et al. 1998, Dasgupta et al. 2004).
Metamorphism was broadly coeval with the progressive deformation
that produced the regional pervasive S2 fabric in all the domains, and
continuing into the D2-D3 interkinematic period. The consistent
relation of the porphyroblastic phases with the marker fabric in all the
domains suggests a common growth history for the “index minerals”
defining the Barrovian metamorphic zones.
In Sikkim, it has not been possible to map the MCT following
its original definition as a thrust fault or as a lithostratigraphic
boundary between the two distinct geological units, Lesser and
Higher Himalaya, based on structural criteria. What is now seen
is a broad zone of distributed ductile shearing with few localised
discrete zones of ductile deformation, which have accommodated
a major part of the shearing strain (Figure 1). This comes closest
to the definition of the MCT zone, as identified from other parts
of the Himalaya. The confusion in the definition of the MCT as
a significant lithostratigraphic boundary between the Lesser and
Higher Himalaya and the ~15-25 km wide MCT zone as observed
now, can be largely reconciled if it is considered that these two had
formed separately in time. We do not rule out the possibility that the
lithostratigraphic boundary may be a relatively older surface that has
been largely modified by a wide ductile shear zone during the main
phase of Himalayan shortening. This is consistent with the work
of DeCelles et al. (2000), Robinson et al. (2001) and Gehrels et
al. (2003), who suggested that the Higher Himalaya may represent
an exotic terrane that was accreted to the Indian margin at some
time during the Palaeozoic. Without discounting the possibility
that parallel fabrics in the Lesser Himalaya, MCT zone and Higher
Himalaya could be time-transgressive and in spite of the fact that
direct correlation of the deformation events in these domains is not
possible based on structural data alone, the simplest explanation
seems to be that at least a part of the deformation and metamorphic
history experienced by these domains was common and that the
inverted metamorphic sequence was established during a single
tectonothermal episode. This is consistent with the observed
structural integrity, coupled with a smooth P-T profile of increasing
P and T upsection, established through rigorous themobarometry
in earlier studies in Sikkim. Available age data from included
monazite are not in conflict with such an interpretation and do not
rule out a common event affecting the entire section.
A workable model on the Himalayan inverted metamorphic
sequence would have to account for mechanisms in terms of
combinations of heat sources and tectonic processes by which
mineral growth could occur syntectonically with the second
deformation event in each domain but at different times and in
addition yield a profile which shows smooth increase in both P
and T with structural height. The results of the present studies are
evaluated in the light of available monazite age data.
References
Dasgupta S, J Ganguly, and S Neogi. 2004. Journal of Metamorphic Geology
22: 395-412
DeCelles, P.G., Gehrels, G.E., Quade, J., Lareau, B., and Spurlin, M.,
2000. Science 288, 497-499
Gehrels, G.E., DeCelles, P.G., Martin, A., Ohja, T.P., Pinhassi, G. and
Upreti, B.N. 2003. Geological Society of America Today 13 (9): 4-9
Neogi, S., Dasgupta, S., Fukuoka, M., 1998. Journal of Petrology 39: 61-99
Robinson, D.M., DeCelles, P.G., Patchett, P.J., Garzione, C.N., 2001.
Earth and Planetary Science Letters 192: 507-521
