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Bachelor-colloqium TU Bergakademie Freiberg
4th
/5th
April, 2008
Structural models of North Himalayan Gneiss Dome evolution
Daniel Rutte
Institute for Geology, Bernhard-v.-Cotta Str. 2, 09599 Freiberg, Germany
Abstract: Gneiss domes are common thermo-tectonic structures, documented
from all over the world and throughout the geologic past. In the past decade
extensive research work was carried out on the North Himalayan Gneiss
Domes (NHGDs) because of their assumed role in mid-crustal processes also
responsible for Tibetan Plateau formation. I discuss the following models for
NHGD formation: (1) channel flow-extrusion (Hodges 2006); (2) intrusion
triggered extension (Aoya et al. 2005); and (3) thrusting over a ramp (Lee et
al. 2004, 2006). Field, geochronologic and thermochronologic data from
various authors give the possibility to test these models to a certain extent
while especially geophysical data are missing at the moment.
1. Definition of a gneiss dome
There is no generally accepted definition for gneiss domes, but a review of
common features is given below. Gneiss domes are three-dimensional structures
that consist of pre-kinematic high-grade rocks. In the North Himalayan Gneiss
Domes (NHGDs) these high-grade cores can be differentiated into basal
orthogneisses and high-grade metasediments. In most cases younger, syn-
kinematic granitoids intruded the high-grade core during Gneiss Dome formation.
The metamorphic core is rimmed by a tectonic contact. This is a shear zone or
fault. Both thrust and normal movements may occur. The tectonic contact
separates the high-grade core from low-grade volcano-sedimentary rocks, mostly
schists and phyllites. These low-grade metasediments are not necessarily
preserved. In map-view gneiss domes appear as elliptical features. Syn-kinematic
fabrics dip away from the centre of the dome (e.g., Yin 2004, Whitney et al. 2004,
Whittington 2004).
2. North Himalayan Gneiss Domes
2.1 Geological Setting The NHGDs lie within the Tethys Himalayan series, which comprises low-grade
and unmetamorphosed sediments of Ordovician to Quaternary age. The South
Tibetan Detachment system separates the Tethys Himalayan sequence from the
underlying Greater Himalayan sequence (GHS), a crystalline basement nappe of
high-grade metamorphic rocks, thrusted along the Main Central Thrust on top of
Daniel Rutte 2
the Lesser Himalayan sequence (LHS). Tectonic windows within the GHS expose
low-grade sediments of the LHS. (see fig. 1)
Figure 1: Regional tectonic map of the central Himalaya orogen after Burchfiel et al. (1992) and Burg
et al. (1984) showing location of the Gneiss Domes. Abbreviations: STDS, South Tibetan Detachment
System; MCT, Main Central Thrust; MBT, Main Boundary Thrust System; YCS, Yadong cross-
structure; ITSZ, Indus-Tsangpo suture zone, GKT Gyrong-Kangmar thrust system. From Lee et al.
(2006).
2.2 Model of Hodges (2006) - Climate controlled channel-flow extrusion leads to
tectonic extrusion of gneiss domes.
Hodges (2006) proposes a model that explains the interplay of the three important
sets of processes in the Himalayan - Tibetan orogenic system: (i) those related to
plate convergence, (ii) those related to a supposed channel flow of a fluid-like mid
crust, (iii) and those related to the erosion processes at the southern flank of the
orogenic plateau (the Himalayas).
He reconstructed three main phases in the evolution of the Himalayan-Tibetan
orogenic system (fig. 2):
Phase I
During the first phase (Early-Middle Miocene) plate convergence and subduction
lead to accretion of Indian crust into the lower crustal sections of the Himalaya.
Partial melting in the mid-lower crust results in formation of a low viscosity
channel and accounts for formation of part of the southern Tibetan plateau. While
Structural models of North Himalayan Gneiss Dome Evolution
3
the metasedimentary rocks deformed and partially melted in the channel, the
orthogneisses (proposed to form parts of the NHGDs later on) are more competent
and are in general weakly deformed.
Figure 2: Conceptual cross-sections illustrating the three phases of channel extrusion at the Himalayan
front. Dark-grey shading designates the down going Indian plate. Fields with light-grey shading,
random-dash patterning, and no shading are Indian crust that has been accreted to the overriding plate.
Unpatterned material corresponds to unmetamorphosed to weakly metamorphosed Tibetan sedimentary
series. Material with random dash patterning includes high-grade metamorphic rocks of extruded
channels. The actively extruding material in each frame has a light-red overlay pattern. Previously
extruded material has no overlay shading. Note the development of a ductile shear zone (dashed heavy
line) at the base of the actively extruding material in the frame for Phase II. The dark-red shading in
Phase III frame indicates partially molten material as imaged in the INDEPTH seismic reflection
experiment (Nelson et al. 1996). Circled A and B in Phase I indicate divergence point of the down
going slab and the tunnelling channel and the proposed lower crust duplex.
Blue bars represent the zone of orographic rainfall, colour intensity indicates intensity of rainfall. Note
that the average rainfall is much lower during Phase II, a predication based on sedimentological
evidence. Green bar indicates a zone of extension over the North Himalayan gneiss domes, with the
gradient representing intensity of extensional strain in upper crustal material. Abbreviations as in
Figure 1. From Hodges (2006).
Intensive erosion along the southern flank of the Himalaya causes channel
extrusion (Greater Himalayan Sequence). The upper boundary of this channel is
formed by the normal-slip Southern Tibetan Detachment System (STF), the lower
Daniel Rutte 4
boundary is formed by the Main Central Thrust (MCT). A “steady-state” condition
is suggested, with equilibrium between erosion, channel-flow extrusion, and the
supply of material through plate convergence.
Phase II
In the Middle-Late Miocene the erosion rates decreased at the Southern Himalayan
Front and dramatically slowed the channel flow-extrusion. This could be related to
a climate change that stopped a wet monsoon, possibly active during Phase I. The
uneroded and cooled material acted as a plug in the channel.
Because channel flow-extrusion was an important factor conducting hot material
and stress/shortening and because that processed had now ceased, the thrust
systems were transferred to the foreland and the Main Boundary Thrust (MBT)
overtook the former role of the channel flow-extrusion in relaxation of the orogen.
While convergence and supply of material into the channel continued, it searched
for a new pathway to extrude. This pathway was possibly created by extensional
denudation caused by orogenic collapse between the Indus-Tsangpo Suture zone
and the South Tibetan fault system. This normal faulting with a detachment
reaching to the mid crust triggered a new extrusion channel for the mid crust as
shown in figures 3(a) and 3(b). The ascending material, consisting of high-grade
metamorphosed Indian metasediments and orthogneisses formed a duplex (thrust
over the mid crustal layer), while the hanging walls evaded in northward and
southward direction, creating normal faults as in figures 3(c) and 3(d).
Figure 3: Conceptual model of
the development by upper crustal
extension. Darker-grey shading
indicates upper crust. Random
dashed pattern indicates channel
material; light-grey overlay
shading designates less active
parts of the channel. Half-arrows
indicate slip on individual faults.
Thin dashed lines represent
mylonite zones. Large freeform
arrows indicate large-scale
kinematics and brightness
indicates kinematic activity. From
Hodges (2006).
Structural models of North Himalayan Gneiss Dome Evolution
5
Phase III
In the late Pliocene the climate changed again and, as in Phase I, erosion and
exhumation rates were high at the Southern Himalayan Front. Channel flow-
extrusion was reactivated in the south and the extrusion in the Northern Himalaya
stopped. Extrusion at the South Tibetan Front (along the Himalayas) may continue
until today.
Implications of Hodges’ model:
1. The orthogneisses in the NHGDs should show pre-Himalayan crystallisation
ages.
2. The NHGDs high-grade core should be bound by normal sense shear zones
showing mainly top-to-the-south movement in the south and top-to-the-north
movement in the north.
3. The orthogneisses that are seen as Indian basement should be in tectonic
contact to the surrounding high-grade metasediments.
4. The NHGDs should have cooled upwards in the north and also downwards in
the south depending on the displacement (fig. 3).
Discussion of implications:
1. Zircon cores from the orthogneisses are dated by conventional U-Pb as 566–
507 Ma (Schärer 1986 and Lee et al. 2000). Their rims show ages of 18.6 Ma
(Aoya et al. 2005). A crystallisation age of 566-507 Ma could be interpreted
as Indian (Cadomian) crust, while the 18.6 Ma age suggests a syn-Himalayan
intrusion.
2. At the northern flank of Mabja Dome the high-grade/low-grade contact shows
top to the north movement along a mylonitic foliation (Lee et al. 2004). No
data is available for the southern flank.
At the northern flank of Kangmar dome the high-grade/low-grade contact is
developed in a top-to-the-north shear zone and a top-to-the-south dominated
shear zone in the south. At the southern flank some top-to-the north indicators
do appear (Lee et al 2000).
At the Malashan Dome both, at the southern and northern flank, top-to-the-
north shear sense is dominating, while there are two top-to-the-south
indicators in the south (Aoya et al. 2006).
In summary, the implication of a top-to-the-north in the north and a top-to-
the-south in the south evasion is not completely fulfilled. This aberration of
model and reality could be explained by a non symmetric evasion of the
hanging wall.
3. The referenced work comprises contradicting statements about the contact of
the high-grade metasediments to the basal orthogneiss inside the core. Lee et
al. (2000) suggested a tectonic contact and a pre-Himalayan intrusion age for
the Kangmar orthogneiss of about 508 Ma. Lee et al. (2004) observed an
instrusive contact between the high-grade metasediments and the Mabja basal
orthogneiss. Aoya et al. (2005) also suggested an intrusive contact of the basal
Daniel Rutte 6
orthogneiss at the Malashan dome. The question of the basal orthogneisses
will be discussed in “Model of Aoya et al. 2005 and 2006”.
4. The dominating cooling direction is strongly based on the timing relations
(especially the occurring young post-kinematic intrusion) and the magnitude
of displacement at the thrust in the mid crustal layer (fig. 2 and 3). Because
cooling ages were mainly produced by Lee et al. (2000) and (2006), they will
be discussed in section 2.4. Generally, they do not confirm the implications of
the model of Hodges 2006 and ask for a more complex mechanism.
2.3 Modell of Aoya et al. (2005) and (2006) - Basal orthogneisses in North
Himalayan Gneiss Domes are syntectonic intrusions during the Himalayan
orogeny that triggered north-south extension.
Aoya et al. (2005) presented new geochronologic, thermochronologic, and
structural data, which challenge the model of Lee et al. (2004) and Hodges (2006)
that the basal orthogneisses are derived from Indian basement. The authors
propose a modified model based on their new data:
(i) The rims of the zircons dated by Aoya et al. (2005) have an age of 18.5 to 17.2
Ma suggesting that this was the intrusion/crystallisation age. (ii) This correlates
with 40
Ar-39
Ar cooling ages of muscovite and biotite around 15.7 Ma for the
Malashan granite. [leave out: cooling ages tell little about intrusion]
Microstructural analyses of K-feldspars in the Malashan granite support these
suggestions. The K-feldspar porphyroclasts show top-to-the-south contractional
shear and an asymmetric growth in that direction, which implies that they
crystallized syntectonic to a D1 event. These microstructures are overprinted by an
extensional top-to-the-north D2 event, which caused a steep foliation at a high
angle to s1 (D1).
The younger Cuobu and Paiku granites show only D2 suggesting that the
Malashan granite formation stopped the contractional (D1) and triggered top-to-
the-north (extension) in the area. This cause and effect identification is speculative.
Aoya et al. (2006) confirmed the former idea of the syntectonic intrusion in a
comparison with the Kangmar dome, which shows many similarities to the
Kangmar Dome (Aoya et al. (2006)).
Structural models of North Himalayan Gneiss Dome Evolution
7
Figure 4: Time relations between granitoids and deformation at the Malashan Dome.
In their model a contractional D1 takes place during the Himalayan orogeny and
switches to an extensional D2 environment due to the emplacement of the first
granitoids (fig. 4). The thermal input leads to a positive feedback with further
extension and emplacement of Granites (Cuobu granite, Paiku granite).
Discussion of the model by Aoya et al. (2005) and (2006)
A contact metamorphism around the centre of the NHGDs is proven. But the
source of this thermal overprint can not only be explained with the the basal
orthogneises, but with a by Lee et al. (2006) proposed intrusion below.
The question appears, if the data delivered by Aoya et al. (2005, 2006), especially
the microstructures, cannot also be achieved due to partial melting (migmatization)
and a re-crystallization during the deformation? Without availible Th/U ratios it is
not possible to proof that the 18..5 to 17.2 Ma zircon rims indicate a
crystallisation age. Rim growth could occur during a deformational event too. The
question which percentage of the malashan granite must have been molten during
intrusion to explain the data of Aoya et al. (2005, 2006) arises?
2.4 Model of Lee et al. (2004 and 2006) - crustal flow driven lifting over mid-
crustal antiform and thrusting over GKT ramp for exhumation of an antiform.
The model of Lee comprises different ideas concerning the evolution of the Mabja
Dome. It is strongly bound to the hypothesis of channel flow.
Figure 5 illustrates the chronology of events.
Phase I:
Channel flow was already active in the Early Miocene and transported high-grade
orthogneisses toward the south as explained in section 2.2. These Indian block
were metamorphosed already before the Himalayan orogeny and/or in the
Daniel Rutte 8
accretion prism at depth (D1). While caught within the shear zone of the STDS
they were subhorizontally lengthened (4 to 10 times) in a top-to-the-north sense
(D2). The increasing strength of D2 towards the basal orthogneiss suggests that it
was situated in the central high strain part of the shear zone, while the hanging
high-grade rocks were not. Migmatized basal orthogneisses interfingered with the
upper high-grade schists.
Figure 5: Modell of the Evolution of the Northern Himalayan Gneiss Domes,. Abbreviations as in fig.
1. From Lee et al. (2006).
The emplacement of a leucocratic dike swarm (23.1±0.8 Ma) at the Mabja Dome
suggests (partial) melting of lower structural sections. This dike swarm is pre-
/syntectonic to D2.
Structural models of North Himalayan Gneiss Dome Evolution
9
Phase II:
In the Early Miocene, the Gyrong-Kangmar thrust (GKT) formed as an additional
compensator of N-S-shortening because of slowed channel-flow extrusion. Two
ideas could explain this slowdown: (i) It may be an effect of the development of a
ramp at the Main Himalayan Thrust (MHT) that formed an anti-formal thrust
duplex. This duplex may have plugged the channel and slowed down channel
flow-extrusion to the south. (ii) An increased friction along the MHT due to a
change in erosion rates, as proposed by Hodges (2006), slowed down the channel
flow-extrusion.
Both processes would lead to a plugging of the mid crustal channel and caused an
effort for bypassing the plug. This emerging bypass was triggered by thrusting
along the GKT.
The GKT developed from the former STDS and transported parts of the high-
strain shear zone rocks upward with the hanging wall.
Because the decompression was fast and nearly isotherrmal (in the inner part of the
dome) granitoids were able to form at least at the Mabja Dome in the Late
Miocene. The post D2 Paiku and Kuobu granite show ages of arround 14.0 to 14.6
Ma. These ages impose a minimum age of D2.
The thrusting along the GKT lead to underthrusting of cold crust. 40
Ar/39
Ar
cooling ages suggest that downward cooling dominated in the Mabja Dome at
least up to a temperature of about 370°C at 13 (upper part) to 17 Ma (middle part).
At the Kangmar Dome, the cooling ages range from 11 to 16 Ma, also showing a
cooling downward. The decrease of ages in the depth could be explained by
hypothetical granites at depth. These decreasing ages lack at the Kangmar Dome
which probably exposes higher structural levels.
Figure 6: 40Ar/39Ar (~370°C) cooling ages of Mabja Dome show an increase from the top to the middle
part and then a decreasing age. From Lee et al. 2006
Phase III:
During further thrusting the domal structure developed. Problematic is the
mechanism which formed the domal structure itself.
Final Doming
Daniel Rutte 10
The 40
Ar/39
Ar (~370°C at ~13.0 Ma) cooling isochrones are sub-parallel to the
metamorphic isolines and S2, and are folded. The low-temperature step potassium
feldspar 40
Ar/39
Ar ages ~200°C arround 11Ma and the apatite fission-track ages
~115°C at ~11.0 Ma are not folded. That means that the doming must have
occured between 13.0 Ma and 11 Ma at temperatures 370 - 200°C (probably above
300°C because of ductile deformation of quartz).
Lee et al. (2004) picks up two possibilities for the late doming: (i) Buoyancy-
driven diapirism that could be explained with the proposed intrusions in the depth
or (ii) thrusting over a ramp along the GKT.
Discussion of Lee et al.
Lee et al. (2006) explained the exhumation of the mid-crustal rocks very detailed
by using structural, geochronologic and thermochronologic data. There are no
conflicts with the data of Lee et al. (2000, 2004) and Aoya et al. (2005 and 2006),
except the documented tectonic contact of basal orthogneiss and high-grade
metasediments at Kangmar dome in Lee et al. (2000).
The late doming could be explained with the ramp along the GKT (fig. 7).
Conclusion:
The model of Lee et al. (2006) fits the best to publicated explains more [??] to the
model of Hodges and fits together with the suggestions of Aoya et al. (2005 and
2006).
In contrast to Hodges Lee et al. can explain all documented details and does not
need an extensional setting during the Himalayan orogeny in a stable collision
zone.
The origin of the basal orthogneisses remains an open question. Further
investigation should proove if the youger rim ages reflect a youger crystallisation
or metamorphic event. U/Th ratios are missing in the work of Aoya et al. 2005.
Maybe the basal orthogneisses contain a hint on how much partial melt the mid
crustal channel contains. This poses the question how a partial melt as a crystall
pulp and a completely molten rock could be differentiated in use of geological
methods.
Figure 7: Proposed
mechanism for creation of a
domal shape in a cold
(370°C to 200°C) stockpile.
Doming through thrusting
over a ramp. “ramp-
propagation-fold”. Based on
Lee et al. 2006.
Structural models of North Himalayan Gneiss Dome Evolution
11
Doming must have occurred at a temperature within between of 370°C and 200°C
(Lee et al. 2006). Assuming that ductile rock deformation ceases at about 300°C,
the spectrum gets even smaller. Also the time window within 0.5 Ma (from 13.0
Ma to 12.5) is quite small (Lee et al. 2006).
Why should diapirism appear in this phase? If a significant density difference
existed it must have lead to diapirism before and afterwards. Diapirism is a very
slow process and should not abruptly play an important role for a 0.5 Ma time
span. This leads to the conclusion that diapirism cannot be a significant factor in
the NHGD formation.
The most equitable so far proposed reason for the doming seems to be the ramp
along the GKT. The formation of this ramp could have different reasons. The
thrusting over a ramp could even explain the tilt of about 5° of the Kangmar dome
that was not related to this process in Lee et al. 2000.
The assumption that the ramp could be an effect of the mid crustal antiformal
duplex that spikes in the upper crust would bring it into consensus with the rest of
the Lee et al. model.
The question how a anti-formal duplex in the mid crust could have formed remains
highly speculative: Its formation could be related to a slab break off around 12 Ma.
In dependece on data for the Himalayan orogeny (e.g. Hou et al. 2004, Chemenada
1999 and DeCelles et al. 2002) that predict such a slab break-off I suppose that a
“back flip” in the lower crustal sections (tectonic underplating) of the orogen could
have resulted in a not homogenous indention from below.
This work has benefited from discussions and constructive reviewing by
Konstanze Stübner and Lothar Ratschbacher. Thank you.
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Daniel Rutte 12
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