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

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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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