The document discusses metamorphism in the Himalayas, specifically addressing controversial issues regarding Himalayan metamorphism and recent research in Nepal. It describes the different types of metamorphism that can occur, including regional, contact, and inverted metamorphism. In the Nepal Himalayas, inverted metamorphism is observed, where higher grade metamorphic rocks overlie lower grade rocks. Possible explanations for this include shear heating along fault zones and heat diffusion from hotter rocks thrust over colder rocks.

1. METAMORPHISM ; CONTROVERSIAL ISSUES
REGARDING HIMALAYAN METAMORPHISM &
RECENT RESEARCHES IN THE NEPAL HIMALAYA.
Submitted By :
Aasish Giri
Roll no : 23
M.Sc 1st Semester, 2078
Submitted To:
Ranjan Kumar Dahal PhD, PostDoc,
Department of Geology, Tribhuvan University,
Kirtipur ,Nepal
Mid Term Assignment of Geology of Nepal (GEO. 513)

2. Brief Description Of Geology of Nepal
• Geologically, Nepal Himalaya is divided into four major tectonic zones. They are :
(1) the Siwalik/Sub-Himalaya (2) the Lesser Himalayan , (3) the Higher/Greater
Himalayan & (4) the Tibetan-Tethys Himalayan zones.
• Himalaya is controlled by three northerly inclined master faults (thrusts) : the Main
Frontal Thrust(MFT), the Main Boundary Thrust(MBT) and Main Central
Thrust(MCT) from south to north, respectively.
• The South Tibetan Detachment System (STDS) is a normal fault in contrast to all
the above mentioned boundary faults of the Himalaya which are thrusts ; played an
important role in development of the Himalaya.

3. Major Boundary Separating Tectonic Zones
Indo-Gangetic Plain (Terai)
----- Himalayan Frontal Thrust (HFT)-------
Sub-Himalaya (Siwalik or Churia Group)
------- Main Boundary Thrust (MBT)-------
Lesser Himalaya
------- Main Central Thrust (MCT)-------
Higher Himalaya
------- South Tibetan Detachment System (STDS)-------
Tibetan-Tethys Himalaya
-
-

4. Geological Cross section across Nepal Himalaya
Fig-1 : Geological cross section across Nepal Himalaya

5. What is Metamorphism ?
• Metamorphism is a mineral assemblage and texture modification process that
occurs when solid rocks undergo physical-chemical changes as a result of events
such as crust movement, magma activity, or thermal fluid change in the earth.
• The most important agents of metamorphism include temperature, pressure, and
fluids.
• The original rock that altered to make a metamorphic rock is called the protolith.

6. Fig 2 showing different types of metamorphism ( contact , dynamic and regional metamorphism).

7. Cause of Metamorphism
• Internal heat of the Earth
• Burial
• Differential stress
• Temperature, pressure, and depth
• The role of fluids
• The role of time
• The realm of diagenesis

8. Types of Metamorphism
(I) Burial metamorphism
• Burial metamorphism occurs when sediments are buried deeply enough that
the heat and pressure cause minerals to begin to recrystallize and new
minerals to grow, but does not leave the rock with a foliated appearance.
• As metamorphic processes go, burial metamorphism takes place at relatively
low temperatures (up to ~300 °C) and pressures (100s of m depth).

9. (II) Regional Metamorphism
• Regional metamorphism refers to large-scale metamorphism, such as
what happens to continental crust along convergent tectonic margins
(where plates collide). The collisions result in the formation of long
mountain ranges.
• Rocks that form from regional metamorphism are likely to be foliated
because of the strong directional pressure of converging plates.
• The Himalaya range is an example of where regional metamorphism is
happening because two continents are colliding.

10. (III) Seafloor (Hydrothermal) Metamorphism
• Hydrothermal metamorphism takes place when hot, volatile solutions percolate
into and react with the protolith, or the original rock.
• The heat of the intrusive igneous body and the hot volatile fluids serves to
catalyze metamorphic reactions in the host rock. The incident fluids enhance ion
mobility in the system and are highly reactive.
• High temperatures and pressures can expel water from hydrated minerals such as
gypsum, talc, chalk, and clay. These sources of fluid are typically associated with
local metamorphism.

11. (IV) Subduction Zone Metamorphism
• At subduction zones, where ocean lithosphere is forced down into the hot mantle,
there is a unique combination of relatively low temperatures and very high
pressures.
• The high pressures are to be expected, given the force of collision between
tectonic plates, and the increasing lithostatic pressure as the subducting slab is
forced deeper and deeper into the mantle.
• The lower temperatures exist because even though the mantle is very hot, ocean
lithosphere is relatively cool, and a poor conductor of heat. That means it will
take a long time to heat up, can be several hundreds of degrees cooler than the
surrounding mantle.

12. (V) Contact Metamorphism
• Contact metamorphism happens when a body of magma intrudes into the upper part
of the crust.
• Heat is important in contact metamorphism, but pressure is not a key factor, so
contact metamorphism produces non-foliated metamorphic rocks such as hornfels,
marble, and quartzite.
• Contact metamorphism can take place over a wide range of temperatures—from
around 300 °C to over 800 °C.
• Any type of magma body can lead to contact metamorphism, from a thin dyke to a
large stock.

13. (VI) Shock Metamorphism
• When extraterrestrial objects hit Earth, the result is a shock wave. Where the object
hits, pressures and temperatures become very high in a fraction of a second.
• A "gentle" impact can hit with 40 GPa and raise temperatures up to 500 °C.
Pressures in the lower mantle start at 24 GPa (GigaPascals), and climb to 136 GPa
at the core-mantle boundary, so the impact is like plunging the rock deep into the
mantle and releasing it again within seconds.
• Two features of shock metamorphism are shocked quartz, and shatter cones.
Shocked quartz refers to quartz crystals that display damage in the form of parallel
lines throughout a crystal. Shatter cones are cone-shaped fractures within the rocks,
also the result of a shock wave . The fractures are nested together like a stack of
ice-cream cones.

14. (VI) Dynamic Metamorphism
• Dynamic metamorphism is the result of very high shear stress, such as occurs along
fault zones.
• Dynamic metamorphism occurs at relatively low temperatures compared to other
types of metamorphism, and consists predominantly of the physical changes that
happen to a rock experiencing shear stress.
• At lower pressures and temperatures, dynamic metamorphism will have the effect of
breaking and grinding rock, creating cataclastic rocks such as fault breccia.

15. (VII) Barrovian Metamorphism
• Barrovian metamorphism takes place during regional metamorphism, caused by
crustal thickening in the roots of an orogenic belt (under mountain chains).
Barrovian zones are especially easy to recognize in pelitic rocks.
• It is a sequence of regional metamorphic mineral reactions recorded by the
successive mineral assemblages seen.
• Barrovian metamorphism is the most commonly encountered. It occurs in
intense tectonic conditions associated with Volcanic Arcs, and Major Mountain
Building.

16. Barrovian Metamorphism (contd.)
• Barrovian metamorphism is widely found across time and space on all
parts of the earth, and produces the most common metamorphic rocks.
• The metamorphic sequences produces the following rocks, in order: shale,
slate, phyllite, schist, and gneiss.

17. Fig 3 : Simplified classification of metamorphic rocks based on the temperature and depth of metamorphism.

18. Formation Of Reverse Metamorphism
• Retrograde/ Reverse ? metamorphism is a change in mineral assemblage and its
composition that occurs during uplift (releasing of pressure) and cooling
(decreasing temperature) to reconstitute a rock, which is a rare process.
• There are two factors that mitigate against complete retrogression of
metamorphic rocks during their return.
a. Efficient removal of the water and carbon dioxide released.
b. Metamorphic reactions do not typically operate in reverse during cooling and
reaction rates are increased by rising temperatures.
• Prograde metamorphism involves the change of mineral assemblages
(paragenesis) with increasing temperature and (usually) pressure conditions.

19. Fig 4 : Retrograde metamorphism during upliftment and Prograde metamorphism during increasing P-T.

20. Retrograde Metamorphism VS Inverse Metamorphism
• In retrograde metamorphism high grade metamorphic rock is converted to low
grade metamorphic rocks like as chlorite appears in the rims or fracture of the
garnet or formation of chlorite in garnet bearing schist due to drop of temperature
and pressure.
• In inverted metamorphism high grade metamorphic rocks overlie the low grade
metamorphic rocks like Kyanite-garnet bearing gneiss overlies the garnet-biotite
bearing schist
• The younger metamorphism resulted in a retrograde metamorphism of the
Himalayan gneisses together with the inverted metamorphism and an anataxis
melting in depth.
• This was caused by shear heating due to the thrust movement along the Main
Central Thrust zone during the Alpine tectonism.

21. Is there Single or poly metamorphism ?
• If a metamorphic event involves only one phase of heating or pressing and then
cooling or depressing, it is called monophase/Single metamorphism. If a
metamorphic process involves two or more climaxes in the changes of
temperature and/or pressure, it is called a polyphase metamorphism.
• Polymetamorphism is repeated episodes of heating and deformation
(metamorphism) acting upon a rock system.
• Hence , Monometamorphism is resulted from one metamorphic event, whereas
polymetamorphism resulted from two or more metamorphic events.

22. Metamorphism in Different zone of Nepal Himalaya
• Siwalik : sedimentary rocks, no metamorphism.
• Lesser Himalaya: Chlorite, biotite and garnet and sometimes staurolite
zone. Greenschist facies, Barrovian type , intermediate P/T type regional
metamorphism.
• Metamorphic grade increases northward towards the MCT.
• Higher Himalaya : Retrogressive metamorphism at the base of the Higher
Himalaya. Kyanite Sillimanite zone. Amphibolite to granulite facies,
Barrovian-type, both intermediate and high low P/T type regional
metamorphism. Regional burial metamorphism T= 600 to 750°C, P= 6 to 11
Kbar.

23. Metamorphism in Different Zone of Nepal Himalaya (contd.)
• Higher Himalaya ( Nappe and Klippe) : Metamorphic grade increases from the
base of the thrust sheet to the top (not inverted). Greenschistfacies, Barrovian-
type. Garnet, biotite, chlorite zones. Regional burial metamorphism.
• Tethys Himalaya : Garnet, biotite, chlorite zones. Greenschistfacies, Barrovian-
type, intermediate P/T type. Metamorphic grade gradually decreases from the
base (grtzone) towards stratgraphically up section and vanishes in the upper
part of the Tethys Himalaya. Regional burial metamorphism.

24. Fig 5 : Geological map of the Himalaya

25. Inverse Metamorphism in Nepal Himalaya
• The inverted metamorphism in Nepal is supposed to be a Neo-Himalayan event.
• While moving stratigraphically upwards , the inverted metamorphism generally
attains a maximum temperature and pressure gradient and then quitely rapidly
decline into a normal lithostratigraphic gradient.
• A thick pile of non- or low-grade metamorphosed sediments of the Lower(lesser)
Himalayas shows an inverted metamorphism. The Lower Himalayas appear to
increase in metamorphic grade northward and upward both topographically and
structurally and are followed by the overthrusting Higher Himalayas of higher
grade.

26. Inverse Metamorphism in Nepal Himalaya (contd.)
• In Higher Himalaya , pressure gradual decrease from MCT to upper section of
Higher Himalaya . Steep increase of temperature from upper section of LH across
the MCT and continues up to 5 km, then gradually decrease and remains at around
700℃ (Rai et al. 1998) . This , P-T results show inverse metamorphism.
• In Tethys Himalaya ,
lower section : amphibioliteic diopside +amphibole+K-feldspar+Quartz (lower
amphibiolite facies) ;
Middle part : ilimenite + Chlorite + plagioclase + K-feldspar (green schist facies) ;
Upperpart : Sedimentary rocks. This section view shows inverse metamorphism
clearly in Tethys Himalaya.

27. Fig 6 :a) Geological cross-section and metamorphic zones showing inverted metamorphism (far-eastern Nepal) (modified from Imayama et al.,
2010). (b) Le Fort s model (1975) of inverted metamorphism. The grey area represents partial melting zones that developed via fluid-saturated
melting. Line a-b indicates the present topographical surface. Bt: Biotite; Crd: Cordierite; Grt: Garnet; Kfs: K-feldspar; Ky: Kyanite; Ms:
Muscovite; Sil: Sillimanite; St: Staurolite; HHC: Higher Himalayan Crystallines; LHS: Lesser Himalayan Sediments; MCT: Main Central
Thrust; LMCT: Lower Main Central Thrust; HHD: High Himalaya Discontinuity.

28. Why inverted metamorphism in MCT zone ?
• Hubbard(1996) postulated the cause of well-known metamorphic inversion in a
section of the eastern Nepal Himalaya due to the intense ductile shearing within
the Main Central Thrust (MCT) zone and the overlying Tibetan Slab on the
basis of geological and textural data .
• Development of kyanite as well as other index minerals representing dominant
metamorphism was either pre-or synchronous with, but outlasted the
deformation because of paucity of growth of metamorphic minerals across the
main tectonic fabric (Hubbard 1996, p. 496).
• However , In recent years thermal models have received far more attention than
mechanical models for metamorphic inversion (LeFort 1975; Hubbard et al.
1991, England et al. 1992)

29. Why inverted metamorphism in MCT zone ?(contd.)
• Following the early Eocene collision of the Indian and Asian plates,
intracontinental subduction occurred along the Main Central Thrust (MCT)
zone in the High Himalaya.
• In the Kishtwar–Zanskar Himalaya, the MCT is a 2 km thick shear zone of
high strain, distributed ductile deformation which emplaces the amphibolite
facies High Himalayan Crystalline (HHC) unit south-westwards over the
lower greenschist facies Lesser Himalaya.
• Petrography and garnet zoning profiles indicate that rocks in the lower MCT
zone preserve a prograde assemblage, whereas rocks in the HHC unit show
retrograde equilibration.

30. Fig 7 : Diagrams illustrating three scenarios proposed
for MCT inverted metamorphism.
(a) Downward heat conduction ("hot iron effect" of
LeFort [1975]) over duration of MCT motion.
(b) Postmetamorphic, distributed shearing of
originally flat, upright isograds [e.g., Grujic et al.,
1996] within MCT zone.
(c) Long-distance structural transport through thermal
regime of sigmoidally folded isotherms produced
from protracted continental underthrusting [e.g.,
Royden, 1993; Henry et al., 1997; Kohn, 2014].
With significant displacement, points X and X' will
be juxtaposed across the MCT zone (modified from
Kohn [2014]).

31. Explanations of Inverse metamorphism
• Large recumbent folding of the metamorphic isograds ,
• Shear heating along the MCT,
• Thermal perturbation and local inversion of geothermal gradient associated with
the intrusion of Miocene granites,
• Diffusion of heat from hot rocks (HH) thrust over cold rocks (LH).

32. Hot/Flat iron model for Inverse Metamorphism
• The model of diffusion of heat from the hotter Higher Himalayan
Crystalline to the cooler Lesser Himalaya is widely accepted proposed by
Le Fort(1975)
• This model describes the isograd distribution and a mechanism for
producing the Miocene granites : shear heating along the MCT and/or the
release of volatiles of the Lesser Himalaya through prograde metamorphism
resulting the production of anatexic melting near to the base of Higher
Himalaya (Le Fort 1975,1981).

33. Inverted Metamorphism in Darjelling-Sikkim Himalaya
• Since the discovery of an inverted metamorphic field gradient across the
Darjeeling–Sikkim Himalaya by Mallet (1874) and von Loczy (1878), and
across the Indian Himalaya by Oldham (1883), it has been recognized that
metamorphic grade increases up-structural section towards the north from
the Lesser Himalaya to the Greater Himalaya.
• The metapelitic rocks of the Sikkim Himalayas show an inverted
metamorphic sequence (IMS) of the complete Barrovian zones from
chlorite to sillimanite + K-feldspar, with the higher grade rocks appearing at
progressively higher structural levels.(S. DASGUPTA, J. GANGULY AND
S. NEOGI )

34. Fig 8 :Simplified geological map of Sikkim showing the known locations of the inverted Barrovian zones, different
tectonic domains, foliation attitudes and lithological units.

35. Metamorphism Prior to Collision
• When ocean basins close it is inevitable that the opposing areas of continental crust
will be brought together. This may be a relatively passive process, with little
overthrusting, or may lead to one mass of continental crust being thrust over the
other to produce greatly overthickened crust as demonstrated in the Himalayas.
• In the Himalayan ,the development of suture zone in southern Tibet, where
metamorphic effects produced in earlier orogenies and at continental margins have
become juxtaposed, and the effects of the Eocene collision of the Eurasian and
Indian plates have been superimposed .
• Rapid crustal thickening during continental collision produces high temperatures due
to the large amount of radiogenic elements in the continental crust. The pressures are
moderate because continental crust is usually never thicker than ~75 km (2.5 GPa).
Cooling to a normal cratonal geotherm follows these abnormally high temperatures.
This is the cause of so-called "Barrovian" metamorphism, of which typical minerals
are staurolite and kyanite (disthen). Good examples are known from the Tibet and
the Himalaya.

36. Why Barrovian Metamorphism in Himalaya?
• The Himalayan orogeny, which began with the Collison of India and Eurasia at
the Paleocene/Eocene boundary, provides an excellent natural laboratory for the
study of metamorphic processes.
• Since 55 Ma, the shortening due to convergence produced intra-upper crustal
shear zones that thickened the Indian crust to its present thickness of 70 km (Le
Fort, 1975a; Molnar, 1988; Zhao et al., 1993).
• A main consequence of this thickening is the concentration of radioactive heat-
producing elements (K, U, Th) possibly responsible for the development of
Barrovian metamorphism and associated anataxis within the HHC.
• Due to the compressional and extensional faulting, different tectono-
metamorphic units have been juxtaposed along the Himalayan belt.

37. Eclogite Facies Metamorphism occurrence
• A high P/T metamorphic rock composed of more than 75% vol. garnet (often pyrope or Mg-
rich almandine) and omphacite (Na-Ca-Al-Mg clinopyroxene). Eclogite never contains
plagioclase. Rutile, kyanite, and quartz are typically present.
• Eclogite is the highest pressure metamorphic facies and is usually the result of
advancement from blueschist metamorphic conditions. Eclogite-facies metamorphic
conditions encompass a wide range of pressure–temperature space from deep crustal to
mantle depths, reflecting tectonic setting.
• Eclogite in collision-type orogenic belts records continental collision, and eclogite-
facies pseudotachylite records deep earthquake processes in subduction settings.
• Deeply subducted and rapidly exhumed Indian Plate basement and cover rocks directly
adjacent to the suture zone enclose eclogites of Eocene age
• Garnet geochronology was used to provide the first direct measurement of the timing of
eclogitization in the central Himalaya. Lu–Hf dates from garnet separates in one relict
eclogite from the Arun River Valley in eastern Nepal indicate an age of 20.7 ± 0.4 Ma,
significantly younger than ultra-high pressure eclogites from the western Himalaya,
reflecting either different origins or substantial time lags in tectonics along strike.

38. Fig 9 : Schematic diagram of a subduction zone and locations of different metamorphic facies.
Modified after Stern (2002).

39. Eclogite facies metamorphism in Case of Himalaya
• Ultra-high-pressure eclogites occur in the Kaghan Valley of Pakistan (Pognante
and Spencer 1991; Tonarini et al. 1993) and in the Tso Morari dome of Ladakh,
India (de Sigoyer et al. 1997, 2000).
• These eclogites lie close to the Indus–Tsangpo suture zone (O’Brien et al. 2001) &
are believed to be related to the subduction of the leading edge of the Indian
continental crust during the early stage of the India–Asia collision.
• The eclogites record temperatures of 725 ± 25 °C and pressures of 2.8–3.0 Gpa.
• Eclogites are also found in the Ama Drime massif, located about 30 km east of Mt.
Makalu in the north Arun Valley, in the Kharta region of Tibet (Lombardo and Rolfo
2000), & the Makalu–Everest region of the Arun River valley in east Nepal
(Parkinson and Kohn 2002).
• These eclogites are different from those of Pakistan-India, as they did not reach the
ultra-high-pressure metamorphism; they are strongly overprinted by subsequent
metamorphism, leaving meager evidence for eclogitization.

40. Adopted Thermal & Structural Model
Proposed For Inverted Metamorphism
These models fall into four general types :
• Type 1: Anatexis and inverted metamorphism are spatially and temporally
related by thrusting
• Type 2 : Himalayan thrusting is caused by melting
• Type 3 : Himalayan anatexis results from decompression melting during
slip along the STDS
• Type 4: Inverted metamorphism is due to transposition of right-way-up
metamorphic sequences

41. I) Type 1 : Anatexis and inverted metamorphism are spatially and temporally
related by thrusting.
• Type 1 models assume a causal relationship between anatexis within the Greater
Himalayan Crystallines and the inverted metamorphic sequence.
• The general requirement of these models is an extraordinary source of heat to
maintain high temperatures in the GHC to permit melting while Indian
underthrusting cools the hanging wall from below.
• The first modern type 1 model was Le Fort's (1975) proposal that thermal
relaxation following thrusting along the MCT heated the footwall sufficently to
induce dehydration reactions.
• Le Fort (1975) introduced shear heating to this role.
• Harris et al. (1993) noted that the high Rb/Sr ratios of Himalayan leucogranites
relative to their assumed GHC source rocks precludes water saturated melting and
instead favors muscovite dehydration melting (e.g., Thompson, 1982).

42. (II) Type 2 : Himalayan thrusting is caused by melting
• Bird (1978) argued that mantle lithosphere delamination beneath the Himalaya
was a plausible consequence of continental collision. One effect of
delamination would be to uplift the crust immediately above the region where
the mantle lithosphere had been removed. This in turn would create a large
strain at the edge of the delaminated region making it a likely site for formation
of a large-scale thrust.
• As asthenospheric heat diffuses upward, the thrust propagates into the newly
weakened crust.
• Nelson et al. (1996) advocated a reversal of roles for thrusting and anatexis.
They interpreted bright spot anomalies beneath the Yadong±Gulu rift as
indicating that the Tibetan middle (15±20 km) crust is partially molten, and
speculated that the region between the MCT and STDS is the earlier extruded
equivalent.

43. (III) Type 3 : Himalayan anatexis results from decompression melting during slip
along the STDS.
• This model is superficially attractive because of the positive dP/dT of vapor
absent equilibria, particularly for reactions involving muscovite, coupled with the
large vertical displacement inferred across the STDS (Burchfiel etal., 1992).
• It links anataxis directly to decompression melting related to slip on the STDS
rather than to thrusting (e.g., Harris et al.,1993), although the development of the
STDS may ultimately be a response to thickening via the various splays of the
MHT (e.g., Burg et al., 1984).
• Himalayan anatexis occurred by fluid absent reactions rather than via water-
saturated melting. This view has subsequently been widely accepted (e.g., Guillot
and Le Fort, 1995; Searle et al., 1997; Davidson et al.,1997; Harrison et al.,
1998b).

44. (IV) Type 4: Inverted metamorphism is due to transposition of right-way-up
metamorphic sequences
• This model involve ductile folding of pre-existing isograds (Searle and Rex,
1989;Grujic et al., 1996), imbricate thrusting (Arita, 1983;Brunel and Kienast,
1986), and ductile shearing of an existing zone right-way-up metamorphic
sequence(Hubbard, 1996).
• Investigators emphasized the role of retrograde transposition of a normally zoned
metamorphic sequence to create the inverted pattern of isograds developed beneath
the MCT.
• Although each of these mechanisms can explain the creation of an apparent inverted
metamorphic sequence, this category of models has generally been viewed as either
inconsistent with geological constraints (e.g., PeÃcher, 1989) or of limited
importance (e.g., England et al., 1992).

45. Fig 10: Schematic illustrations of the four di€erent models
proposed to explain the relationship of inverted metamorphism
and/or anatexis to
large-scale faulting within the Himalaya.
(a) Type 1: Inverted metamorphism developed within the
footwall of the MCT and anatexis in the
hanging wall are spatially and temporally related by thrusting.
(b) Type 2: Thrusting results from anatexis. The illustration
shows the interpretation of Nelson et al. (1996) of a partially
molten Tibetan middle crust that is being actively extruded
southward.
(c) Type 3: Anatexis results from normal faulting. This fgure
illustrates the change in the depth±temperature conditions
in the footwall of a normal fault.
(d) Type 4:
Apparent inverted metamorphism in the footwall of the MCT is
produced by deformation of two right-way-up metamorphic
sequences.

46. Fig 11 : Schematic illustration of tectonic development of the
Himalayan thrust system (vertical exaggeration 6:1). Active faults
are shown as bold black lines while abandoned faults are indicated
with bold gray lines.
(a) Possible 25 Ma distribution of the protoliths of Greater
Himalayan Crystallines (GHC) and Lesser Himalayan
Formations (LHF) with respect to Indian cratonic margin after
Eohimalayan thickening from ca 50±25 Ma.
(b) Thrusting along the Main Himalayan Thrust (MHT) flat and
Main Central Thrust (MCT) decollement from 25±15 Ma.
(c) Thrusting along MHT flat and MBT ramp from 15±8 Ma.
Abandonment of the MCT ramp at 15 Ma causes accretion of
upper LHF rocks to the hanging wall.
(d) Out-of-sequence thrusting in the high Himalaya from 8±6 Ma
involving upper LHF (approximately equivalent to reactivated
MCT thrust ramp).
(e) Activation of MCT-I and further development of MCT Zone
(6±2 Ma) leads to accretion of lower LHF rocks to hanging
wall.
(f) Abandonment of the MCT zone at 2 Ma. Southward transfer
of displacement to MFT ramp/MHT decollement. Present
predicted positions of HHL and North Himalayan Granite
(NHG) source regions are shown by regions shaded white

47. Kinematic model
• Holocene slip rate along the MFT, determined from geomorphic studies at
the front of the range is 21±1.5mm/yr
• Determinations of present day convergence from GPS studies range from 12
to 20mm/yr
• Relatively young cooling ages suggest that the MCT may have been
reactivated as an out of sequence thrust in the Pliocene and may even still
be active.
• kinematic model may then be used to predict the metamorphic grade and
cooling ages of rocks collected at the surface.

48. Fig 12 : Model geometry, kinematic and thermal parameters used in model KTM11

49. Channel Flow Model
• In this model Higher Himalayan Crystalline was channeled by the viscous
fluid produced by the deformation due to shear stress and pressure difference.
• It explains that channel flow is a process (Grujic et al.1996;Daniel et al.2003)
in which viscous fluid moves in a channel between two rigid plates
provocating deformation by sheer stress and pressure gradient in a channel.
• it is still the most successful model to explain and predict the tectonics,
metamorphism and exhumation of high-grade terranes in some orogens.
Moreover, the concept of channel flow has stimulated novel approaches to the
study of both the tectonics and metamorphism of large, hot orogens and the
interaction between tectonic and surface processes.

50. Fig 13 : Channel flow model of the GHC in the Himalaya

51. Recent Researches on Nepal Himalaya
A general shear extrusion of the high-grade metamorphic rocks forming the crystalline core
zone of the Himalayan orogen is consistent with:
(1) the general lack of significantly inverted peak pressure field gradient across the High
Himalayan Crystalline Sequence, despite the intense non-coaxial deformation systematically
observed across this unit along the entire range
(2) kinematic indicators demonstrating a ductile deformation combining simple shear and pure
shear in this metamorphic sequence (Grujic et al. 1996; Grasemann,Fritz & Vannay, 1999);
(3) structural and P–T constraints indicating that the High Himalayan Crystalline Sequence,
that corresponds essentially to the subducted sedimentary cover of the Indian plate, behaved as
a low viscosity rock sequence during exhumation controlled by the Main Central Thrust, as a
consequence of the previous high-grade metamorphism and partial melting at peak conditions
between 600 and750 °C
(4) seismic data indicating that the High Himalayan Crystalline Sequence has the geometry of
a large-scale orogenic wedge bounded by the converging Main Central Thrust and South
Tibetan Detachment System (Hauck et al. 1998)
Source : JEAN-CLAUDE VANNAY* & BERNHARD GRASEMANN

52. Fig 14 : (a) General cross-section for the Southeast Himalaya at ~ 90° E , as constrained by the deep seismic reflection data of the
International Deep Profiling of Tibet and the Himalaya project (INDEPTH, Hauck et al. 1998).
(b) Geological cross-section for the Sutlej Valley, based on the geological map projected focal depth of earthquakes in this part of the
Northwest Himalaya (black circles: Ni & Barazangi, 1984; white circles:Molnar & Lyon-Caen, 1989; stars: Kayal, 1996) and seismic data for
the Main Himalayan Thrust beneath the Sub-Himalaya
(Powers, Lillie & Yeats, 1998).

53. Recent Researches on Nepal Himalaya (Contd.)
• Microstructural evidence for the timing of mineral growth across the zone indicates
the field gradient reflect diachronous peak assemblages and has no implications for
the nature of geotherm at any one time.
• Postmetamolrphikc ductile thrusting is thought to be the major control on the
pattern of isograds in the Kishtwar-Zanskar Himalaya.
• A thermobarometric profile records steep gradients in temperature and pressure
across the base of the MCT zone which result from the syn- to postmetamorphic
juxtaposition of high grade rocks, on their retrograde path , over lower grade rocks
which subsequently attain their peak in the MCT zone.
Source : B. J . STEPHENSON,* D. J . WATERS AND M. P. SEARLE

54. Geothermobarometry
• In order to estimate the P-T conditions of metamorphism, suitable
geothermometers and geobarometers were applied to the relevant assemblages
from the different zones.
• Compositions of the minerals in mutual contact have been used in the
thermobarometric calculations.
• For garnets which reveal normal growth zoning, rim composition has
equilibrated with the adjoining matrix, and reflects the peak conditions of
metamorphism. For inversely zoned garnets near-rim and intermediate
compositions were taken.

55. Geothermobarometry (contd.)
• For inversely zoned garnets near-rim and intermediate compositions were taken.
In some cases it was not possible to obtain the core composition of garnets
because of the presence of numerous inclusions.
• The following reactions have been used to constrain the conditions of
recrystallization of the pelitic rocks of the Sikkim-Darjeeling region:
(a) garnet-biotite,
(b) garnet-plagioclase-AIzSiO,-quartz,
(c) garnet-muscovite-biotite-plagioclase.

56. Fig 15 : Geothermobarometry Using Zoned Minerals ; P-T diagram showing the generalized counterclockwise P-T-t path
calculations for metapelites of the different types of metamorphism

57. Shear Heating Along MCT zone
• The metamorphic temperature of the Main Central Thrust zone is estimated to
be 500-550°C from the diagnostic mineral assemblage of almandine-chlorite-
muscovite.
• The metamorphism of the Midland metasediments under the Main Central
Thrust zone is of the biotite zone of the greenschist facies (400”-450°C) and
decreases downward in grade to the chlorite zone.
• Le Fort (1975), Graham and England (1976) and Scholz (1980) have
concluded that the shear heating along the Main Central Thrust zone involved
the inverted zonation from the chlorite zone in the Lower Himalayas to the
sillimanite zone in the Higher Himalayas.

58. Review of detrital zircon ages and Sm-Nd
isotopic data from Himalaya
• Detrital zircon ages and Sm-Nd isotopic data from Himalayan Orogen (DeCelles
et al. 2000; Myrow et al. 2003;Gehrels et al. 2006) suggest that the Lesser
Himalayan Metasediments (LHM) might have received material from the
Northern Indian Craton, while the Higher Himalayan Gneisses (HHG) mostly
from the Circum-East Antarctic Orogen (CEAO) including western Australia and
east Antarctica, and partly from the LHM and the Arabian Nubian Shield
(Yoshida and Upreti 2006).
• The original material of the Tibetan Tethys Sedimentary Sequence is considered
to be mostly derived from the HHG and partly from the CEAO and only small
amount on the western area from the Arabian Nubian Shield (Yoshida et al.
2005).
• Assumption had been made that the LHM,HHG, and the TTS are more or less
continuous sequence deposited in more or less similar sedimentary basin at the
northern margin of the North Indian Craton.

59. Restoration of the Main Central Thrust system
• A generalized restoration of the Main Central Thrust system in Nepal is shown in Figure
below. Late Proterozoic rocks extend from the Lesser Himalaya to the Greater Himalayan
Sequence and across to the base of the Tethyan Himalaya (Haimanta Group–Cheka
Formation).
• Unmetamorphosed Nawakot Group sedimentary rocks in the Lesser Himalaya pass north
into the same protolith age rocks which have been metamorphosed to greenschist–upper
amphibolite facies in the Ramgarh thrust sheet (Beyssac et al. 2004), up to kyanite grade in
the Kathmandu thrust sheet (Johnson et al. 2001), and finally into high-grade sillimanite
gneisses in the Dadeldhura and Greater HimalayanSequence thrust sheets in the high
Himalaya.
• In the internal parts of the Greater Himalayan Sequence, the sillimanite gneisses (commonly
referred to as Greater Himalayan Sequence Formation 1; Colchen et al. 1986) are
metamorphosed equivalents of the same late Proterozoic protoliths.
• One major implication of the restoration of the Himalaya is that the Main Central Thrust
follows a flat for a long distance across strike. This flat follows a rheologically weak horizon
along the Neoproterozoic shales. True Indian basement rocks (Archaean–Lower Proterozoic)
are never exposed in the Himalaya.

60. Fig 16: Generalized restored section across the Nepal Himalaya showing the pre-thrusting trajectories of the Main Central Thrust and South Tibetan
Detachment shear zones and faults. The shaded horizon represents the Upper Proterozoic sedimentary rocks of the Lesser Himalaya, and Greater
Himalaya. Within the Greater Himalayan Sequence these include the metamorphosed rocks of the Nawakot Group above the Ramgarh thrust (Main
Central Thrust) and the Bhimpedi Group within the Kathmandu nappe, above the Mahabharat thrust.

61. Mahabharat thrust
• Rocks above the Mahabharat thrust include Proterozoic Bhimpedi Group and
early–middle Palaeozoic Phulchauki Group sedimentary rocks, which are intruded
by Ordovician granites and augen gneisses. Metamorphism reaches kyanite grade
at the base and isograds are right-way-up from kyanite through garnet and biotite
to chlorite grade (Johnson et al. 2001).
• Along the Mahabharat thrust dynamic metamorphism has locally inverted the
thermal gradient with formation of garnet–biotite mylonites and phyllonites.
• The Mahabharat thrust climbs up-section in the transport direction, from being
along the base of the inverted metamorphic sequence at Langtang in the north, to
along the isograd fold hinge at Kathmandu .

62. Fig 17 : Geometry of the Main Central Thrust zone in the Langtang–Kathmandu nappe region of central Nepal showing the
relationship of the Mahabharat and Ramgarh thrusts to the metamorphic isograds. This geometry combines the folded
isograd model of Searle & Rex (1989) with the channel flow model for the Greater Himalayan Sequence (Law et al. 2006;
Searle et al. 2006) and with the Johnson (2005) structural model for the Mahabharat thrust and Kathmandu nappe.

63. Dadeldhura and Ramgarh thrusts
• The Dadeldhura thrust sheet consists of garnet–muscovite–biotite schists,
mylonitic augen gneiss and Cambrian–Ordovician granites.
• The Ramgarh thrust sheet consists of greenschist-facies metasedimentary rocks
of the Kushma and Ranimata Formations.
• The Ramgarh thrust forms the roof thrust to a series of imbricated thrust slices
of unmetamorphosed Lesser Himalayan rocks of Late Archaean,Proterozoic and
Cambrian age.
• The Ramgarh thrust marks the southern limit of Tertiary Himalayan
metamorphism in western Nepal and we prefer to link this with the Main
Central Thrust.

64. Dadeldhura & Ramgarh Thrust (contd.)
• Restored sections show that the Ramgarh, Dadeldhura and Main Central thrust
sheets of DeCelles et al. (2001) all have Proterozoic sedimentary rocks, Ulleri augen
gneiss and Cambrian–Ordovician sedimentary rocks and granites as protoliths.
• In central and eastern Nepal the location of the Ramgarh thrust is almost entirely
interpreted from lithological repetition; a possible fault surface has been observed
onlyin the Tribeni area of eastern Nepal. A more southerly location for the Ramgarh
thrust is supported by pervasive deformation documented by quartz c-axis fabrics
throughout central Nepal(Bouchez & Peacher 1981).
• During the Late Miocene ductile shearing along the Main Central Thrust–Ramgarh
thrust ceased, and thrusting propagated downsection to the Lesser Himalayan brittle
imbricate thrust system.At least 120 km of southward translation has been estimated
across the Ramgarh thrust sheet (Robinson et al. 2006).

65. Fig 18 :Map of the Annapurna–Manaslu Himalaya,
showing the structure of the Greater Himalayan
Sequence and our proposed location of the Main
Central Thrust. Our Main Central Thrust is located
along a high-strain zone further south, south of Gorhka,
and corresponds to the southern limit of Tertiary
metamorphism. The mapped locations of the South
Tibetan Detachment system (STDS) normal faults are
from Searle & Godin (2003).

66. Fig 19 : Simplified, schematic section
across the Annapurna Himalaya showing
key features of the structure, stratigraphy
and mineral isograds, and our proposed
location of the Main Central Thrust in
central Nepal.
Shaded area represents the migmatites and
leucogranites within the partially molten
channel.

67. Fig 20:Simplified, schematic
section across the Manaslu
Himalaya, showing key features and
our proposed location of the Main
Central Thrust. Shaded area
represents the zone of partial
melting with migmatites and
leucogranites (crosses). The
Manaslu leuocogranite is wholly
within the Greater Himalayan
Sequence, following Searle &
Godin (2003), with the South
Tibetan detachment (STD)
wrapping around the upper level of
the granite.

68. Seismic research in Nepal Himalaya
• In Nepal Himalaya maximum strain energy is released in its western and eastern
parts, but the intermediate portion is little active.
• It’s worth mentioning here that western and eastern Nepal experienced great
earthquakes (of magnitude > 7.5) in 1916 and 1934, respectively. The seismic activity
in these areas is associated with the MBF.
• Himalayan structures have resulted from cumulative deformation over several tens of
million years in a way that is similar to active deformation at present, or that the
present tectonics is partly controlled by inherited structures.

69. Seismic research in Nepal Himalaya (contd.)
• Microseismicity reflects stress accumulation during interseismic periods as the
belt of intense microseismic activity at the front of the Higher Himalaya in
Central Nepal coincides with a zone of uplift revealed in leveling data (Jackson
et al., 1992; Pandey et al., 1995; Bilham et al., 1997).
• The ramp beneath the Higher Himalaya behaves as an asperity focusing stress
and strain build up in interseismic periods (Pandey et al., 1995)
• The geodetic signal was found to be consistent with continuous creep at depth
beneath the Higher Himalaya at a rate of 21±3 mm/year, close to the long-term
slip rate on the MFT of 21.5±2 mm/year (Lave and Avouac, 1999).

70. Fig.21 : Geometry of the slip in Himalayan
earthquakes (section from Mencin et al. 2016).
Updip blind thrusts from oil exploration seismic lines
(Bashyal 1998). The square wave indicates locked,
the bold line indicates rupture and the sine wave
indicates aseismic slip. (a) The region of interseismic
strain accumulation is the locus of microseismicity
and occasional moderate earthquakes occurring as
high-level thrusts. (b) Incomplete rupture in the 2015
Gorkha Mw 7.8 earthquake, with subsequent induced
post-seismic creep (4 min–4 years). No slip occurred
on the Main Frontal Thrust (MFT) but triggered
surface slip was recorded on the Main Dun Thrust.
InSAR (interferometric synthetic aperture radar)
imagery shows that this occurred in the form of
decaying creep for 26 km along strike with no slip
below c. 5 km. The slip increased over a few weeks
to ≥5 cm and then ceased (Elliott et al. 2016). (c)
Complete rupture as inferred to occur in great
earthquakes (e.g. in 1505 and 1950 Mw ≥ 8.6).
Occasionally these may activate blind thrusts south
of the MFT.

71. Thank You  !!