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THEME 2 : MODIFICATIONS DU CONTINENT ASIE AU
COURS DE LA COLLISION
1. La collision Inde-Asie : introduction
http://www.ipgp.jussieu.fr/%7Elacassin/
Comment la convergence intra-continentale est absorbée lors d'une telle collision ? Au sein de cette très vaste
question l'on peut distinguer trois séries de questions fondamentales
1) Quelle est la zone de déformation ? Les deux continents se déforment-ils symétriquement? Sinon, pourquoi?
2) La déformation est-elle localisée le long de quelques accidents majeurs ou est-elle répartie de manière plus
homogène sur de nombreuses petites structures? Si la déformation est localisée, à quelle échelle et dans quel(s)
niveau(x) (croûte supérieure, croûte inférieure, manteau supérieur) l'est-elle? Existe-t-il un découplage
fondamental entre les déformations observées dans la croûte supérieure et celles du manteau supérieur?
3) Dans la zone de déformation, par quel mécanisme est absorbé le raccourcissement? Par subduction
continentale? épaississement lithosphérique? expulsion latérale de blocs continentaux? une combinaison de ces
mécanismes? Si oui, en quelle proportion et suivant quelle logique?
Plus de trente ans après l'avènement de la tectonique des plaques, de telles questions pourraient sembler triviales,
mais il n'en est rien en raison de la difficulté d'obtenir des données à une échelle de temps pertinente.
Les études sismologiques et géodésiques permettent de caractériser de plus en plus précisément les déformations
actives. Cependant de telles observations sont limitées à une fenêtre de temps très courte correspondant à moins
de 0.001% de l'histoire d'une zone de collision et l'on peut douter de leur représentativité.
D'un autre côté les structures géologiques ne révèlent qu'un état final obtenu par la succession de plusieurs
phases de déformation qui sont souvent difficiles à individualiser et plus encore à quantifier (quantité de
déformation, âge, durée).
Il est donc difficile de déterminer comment la déformation était absorbée à un moment donné (ou pendant la
durée d'un épisode tectonique soit quelques millions d'années).
3 principes:
(1) collecte de données de terrain.
(2) données à toutes les échelles.
(3) Les déformations ductiles sont étudiées en combinant géologie structurale, pétrographie et géochronologie
pour reconstruire les chemins P-T-t-D (Pression-Temperature-temps-Déformation) des roches mylonitiques.
Au-delà des études régionales (Asie, Alpes) notre but est de comprendre la mécanique de fonctionnement des
grandes zones de cisaillement, et de caractériser les modes de localisation, dans le temps et dans l'espace, de la
déformation de la lithosphère continentale.
En Asie du SE nous avons confirmé l'importance des grands décrochements dans la déformation Tertiaire du
continent Asiatique. Fait unique, nous avons pu estimer directement à partir de mesures géochronologiques la
vitesse du plus grand de ces décrochements: la zone de cisaillement de l'Ailao Shan - Fleuve Rouge
2. Géomorphologie Himalaya-Tibet
http://daac.gsfc.nasa.gov/DAAC_DOCS/geomorphology/GEO_2/GEO_PLATE_T-48.HTML
Plate T-48
The most profound deformational event involving the interaction of lithospheric plates is the collision between
continents. The Indian continent began to impinge on the Eurasian continent in the Eocene. The continuing
northward compressional movement of India has produced intracontinental thrust sheets responsible for the
highest mountains in the world. Mount Everest, the Annapurna Range, and the rest of the Himalayas are still
rising, and the region remains seismically active. The effects of this collision are extraordinarily widespread,
being responsible for the 2500-km long thrust front of the Himalayas (see Figure 2-2) that extends from the
Hindu Kush (Plate T-43) on the west to the Burma Ranges (see Figure T-48.9) on the east and for the creation of
tectonically bonded intramontaine basins (e.g., Kashmir and Katmandu) developed on the back slopes of the
frontal thrusts. In addition, the collision is still producing strike-slip faulting far into western and central China,
as well as along the Red River fault of southwest China and Vietnam (Plate T-53) and normal faulting on the
Lake Baikal (Plate KL-14) and Shaanxi graben (Plate T-52) systems. The collision has led to 60 to 80 km of
crustal thickening beneath the Tibetan Plateau, an area of 600 by + 1000 km, which provides the buoyancy in
part necessary to maintain the plateau at an average of 4 km above sea level (Lyon-Caen and Molnar, 1985).
Much of our understanding of the geometry and kinematics of this widespread deformation is derived from
studies of Landsat mosaics and their integration with available geologic and geophysical data (Molnar and
Tapponnier, 1975, 1978; Ni and York, 1978; Tapponnier and Molnar, 1976, 1977, 1979; Tapponnier et al.,
1982). Space imagery is virtually the only tool available to visually span the regions affected by this vast
deformation. Other Plates in this chapter that include elements of this widespread deformation field are T-43, T44, T-45, T-46, T-49, T-50, T-51, T-52, and T-53, whose locations are plotted on Figure 2-2.
On the facing page is a spectacular hand-held camera photograph taken during Shuttle Mission S-17 from a point
almost directly above Mt. Everest looking northwest to the Hindu Kush (snow-covered peaks near the
midhorizon). (A "boot-shaped" lake (A) is a useful reference in locating several other figures in this Plate.) The
Indo-Gangetic Plains (bluish-green), at less than 100 m average elevation, lie to the left, and the haze-filled
Tarim Basin occupies the right half of the horizon (see index map below Plate). Over much of the Himalayas,
topography strongly mirrors underlying structure. The light- to dark-green transition on the left roughly marks
the edge of the Himalayan deformation front and the approximate location of the Main Boundary Fault (Gansser,
1974). Snow delineates the higher Himalayas (many peaks being greater than 7500 m, with Mt. Everest rising to
8848 m); the southern margin of the snow crudely follows the trace of the Central Thrust of the Himalayas.
3. Rhéologie
http://www.sg.geophys.ethz.ch/geodynamics/klaus/WS_20_00/Lithosphere/Dynamics%20III.htm
Continents are softer than oceans…..one obtains this impression when looking at continental collision zones
where at least one colliding continental plate has given up the paradigm of plate tectonic rigidity and forms
mountain belts of sometimes large proportions. The following topographic map shows an image of the IndiaAsia collision. The scale of deformation is several thousand kilometers.
Figure 1 Topography of the India-Asia collision featuring large-amplitude vertical deformation of the
Asian plate. India does not appear to experience significant deformation.
Such a large amount of deformation is not seen in oceanic plates. Three important differences play a role:
1) Continental lithosphere has a much thicker and highly differentiated buoyant crust.
2) The continental lithosphere has differentiated a significant amount of radiogenic element in its
uppermost crust providing significant heat input into its uppermost layer. This upper crustal heat source
is missing in oceanic lithosphere.
3) The rheology of the differentiated crust is a lot weaker than that of the more primitive oceanic crust.
The most abundant mineral in the continental crust is quartz, which controls its rheology. The lower part of the
crust is governed by feldspar rheology, which is stronger than quartz but not so strong as the olivine of the
mantle. We have already discussed the mantle rheology for oceanic plates the same rheology applies for the
mantle portion of continental plates. The crust differs, however and contains soft quartz and feldspar layers. The
following figure shows the two soft layers. These diagrams are normally shown with depth as the vertical axis.
The varying soft and strong layers give the impression of a Christmas tree when plotting the strength for tectonic
compression (positive) and extension (negative).
Figure 2 Classical (Christmas tree, here in resting position) strength profile of continental lithosphere.
The two lines show the differences between "wet" and "dry" rheologies. Only one strain rate contour is
shown.
The competent mantle section of the continental Christmas tree is similar to the strength profile of the oceanic
lithosphere. The behavior of this strong layer cannot be observed directly. However the soft crustal top can be
seen. It adds buoyancy to continental plates thereby preventing them to subduct back into the mantle. This
property and the requirement of continuity calls for a new style of tectonics that does not exist in oceans. The
India Asia collision is a good example.
The India Asia collision
The following sketches show how the India Asia collision may have evolved with time. The position of India
relative to Eurasia has been derived from rigid plate reconstruction as discussed in the chapters on kinematics
and mechanics.
Figure 3 Subduction of the Tethys ocean was completed about 40 Ma ago. Subsequent deformation must
involve some vertical as well as horizontal mass transfer.
Deformation at such a large scale with thrusting, strike slip and extension can be reproduced by an elasto-plastic
rheology. We will for simplicity only discuss the horizontal (strike slip) solution.

4. Structures internes à la chaîne et au Plateau Tibétain
•
INDEPTH Project : Origine (1996)
http://www.agu.org/sci_soc/EOSsokhabi.html
The Tibetan Plateau and its surrounding mountains (the Himalaya on the south, the Kun Lun on the north, and
the Pamir and Karakoram on the west) comprise the largest, loftiest, and youngest highland on the Earth (Figure
1). All of the world's peaks higher than 7000 m (with the lone exception of Anchomuma in the Bolivian Andes)
as well as the largest concentration of alpine glaciers are found in this vast region that geographers have dubbed
"the Roof of the World."
Figure 1. A simplified map of south-central Asia, showing the Himalaya and Tibet, which have uplifted to form
the highest and largest topography on Earth. As mountains uplift, the eroded rocks are deposited in the foreland
basins in the north of the Indian Peninsula as well as in the marine basins in the Indian Ocean (the Indus and
Bengal fans). The Himalaya and Tibet were formed by the head-on collision of the Indian plate with Asia during
the Cenozoic. Once the "forbidden lands" in the world, the high mountains and plateaus of south-central Asia
have increasingly drawn the attention of scientists. This field of geo-science is now an international endeavor,
and scientists hope it will lead to clues about some of the major tectonic processes shaping the Earth.
The Himalaya-Tibet region is virtually the "water tower" of Asia: it supplies freshwater for more than one-fifth
of the world's population, and it accounts for a quarter of the global sedimentation budget. Since 1985, a series of
international workshops have been held to discuss the latest geoscientific research in the Himalaya-KarakoramTibet region. The eleventh workshop was held in Flagstaff, Ariz., from April 29 to May 1, 1996. It was the first
to be held in North America. Nearly 120 people from 12 different countries attended.
The workshops are a timely response to the growing interest in the Himalaya-Karakoram-Tibet region among the
international geophysical community. The Himalaya and Tibet region is a mountain-plateau system that was
formed when the Indian and Asian continents collided during Cenozoic time. It is a "natural laboratory" for
scientists studying the geophysical and geological processes involved in collisional tectonics as well as the
effects of high mountains and mountain-building processes on climate and the environment.
In the first special session, speakers discussed geodynamic models of the Himalaya and Tibet. Since India
collided with Asia about 55 m.y.a., India has moved nearly 3000 km northward. One challenge for geophysicists
is to understand the mechanisms that have accommodated this "missing continental crust." Continental crust may
have been simply shortened by folding and thrusting, and deformation may have been distributed throughout the
Himalaya as well as all of Tibet (the so-called "basement reactivation" model). On the other hand, southeast
Asia's Indochina block may have been extruded southeastward out of the way of an Indian indenter and thus
opened space for northward Indian motion (the "extrusion or escape tectonics"). Perhaps the Indian crust
subducted beneath the entire Tibetan Plateau (the whole-scale "continental subduction" model). Or has a
combination of these mechanisms accommodated the space and strain of the India-Asia collision?
The challenge has been to quantify these mechanisms and their relative dominance in space and through time.
Linked with these mechanisms are several other fundamental phenomena, which scientists would like to
understand, such as the history and patterns of uplift of the Himalaya and Tibet, the deep structure of continental
crust in the India-Asia collision zone, and the interactions between crust and mantle in a rapidly evolving
orogenic system. Speakers at the workshop reported new data and ideas on these topics. Nevertheless, to paint
clear, detailed pictures of the tectonic evolution and associated processes of the Himalaya and Tibet, more
geophysical, geochemical, petrological, geochronological, and structural data are needed from various sectors of
the Himalaya and Tibet.
Some of the exciting results constraining the tectonic models come from deep seismic reflection profiles. A
collaborative U.S.-Chinese project called International Deep Profiling of Tibet and the Himalaya (INDEPTH)
carried out seismic reflection studies in southern Tibet over the past decade and discovered among other things, a
major north-dipping detachment fault at midcrustal levels that is called the Main Himalayan Thrust. An
important aspect of the INDEPTH project is the integration of deep structural data with the knowledge of surface
geology of the Himalaya and Tibet. The INDEPTH researchers at the workshop presented their latest results,
which have detected fluids at midcrustal levels beneath the Tibetan Plateau. This stimulated debate on the origin
and nature of these fluids.
The Himalaya are being formed by tectonic forces and sculptured by powerful denudation processes.
Understanding the interplay of these tectonic and geomorphologic processes requires the interaction of
geoscientists from various disciplines. The second special session was on the neotectonics and Quaternary
geology of the Himalaya and Tibet. Topics covered in this session represented some cutting-edge areas of
Himalayan research in neotectonics and Quaternary geology, such as the determination of present-day uplift
rates of the Himalaya (both from geophysical modeling and geodetic measurements); monitoring seismic activity
and its implications for a "Big One," which is overdue in the central parts of the Himalaya. Geomorphic analysis
of mountain terrains by space-based imagery techniques was also discussed, and the role of tectonic activities in
the geomorphic development of river systems was examined. Other researchers focused on determining exposure
rates of rocks by cosmogenic nuclides, results gained by mapping and monitoring the dynamics of valley
glaciers, and studies of loess and late Cenozoic glaciation record in the Himalaya and Tibet. A new development
in Himalayan geophysics has been the application of the Global Positioning System to quantify the ongoing
convergence between India and Asia.
Although the Himalaya-Tibet region is fundamentally a product of crustal shortening, the east-west extensional
grabens in Tibet suggest that the plateau spread due to gravitational force. Furthermore, over the past decade,
discovery of a basement-cover detachment faultthe so-called South Tibetan Detachmentin the Himalaya has
drawn widespread interest among geologists. The South Tibetan Detachment, as mapped in several areas of the
Himalaya, essentially separates the Paleozoic sedimentary formations of the Tibetan Himalaya to the north from
the high-grade metamorphic rocks of the Greater Himalaya to the south. Interestingly, this north-dipping
detachment fault is parallel to the east-west strike of the Himalaya, and the extant geochronological data and
structural analysis indicate temporal and spatial links between the tectonic compression and crustal extension.
Another interesting feature of the South Tibetan Detachment is that this normal fault and shear zone can be
traced on top of high summits, such as Mount Everest, at the border between the Higher Himalaya and the
Tethys Himalaya, indicating gravitational response of the rocks to Himalayan uplift and height. Some
researchers at the workshop also presented detailed maps of small-scale extensional structures within the Higher
Himalayan metamorphic core and normal faults in the foothills of the Himalaya. The existence of these two
contrasting tectonic regimescompressional and extensionalin a single orogenic system that is also quite young
and still active provides crucial clues to the response of continental crust to tectonic stress and uplift. The third
special session discussed Himalaya-Tibetan examples of extensional tectonics in a compressional orogenic
system. This area of research is expected to attract more attention in the coming years.
The afternoon sessions of the workshop covered regional topics: western Himalaya and the Karakoram, SubHimalaya (foreland sediments), the Main Central Thrust zone, the Higher Himalayan Crystalline terrain, and
Tibet and Trans-Himalaya. A total of 101 presentations were made at the workshop: 71 oral and 30 poster.
Forty-nine percent of the presentations were on the western Himalaya, indicative of the scarcity of recent
research done on the eastern parts of the Himalaya and on the Karakoram-Pamir-Hindu Kush mountains.
Each oral presentation was 20 minutes long, and an hour-long open discussion coordinated by two discussants
was held at the end of each session. This permitted more comments, questions, and discussion of session topics.
Financial support for the workshop came from registration fees and a grant from the Continental Dynamics
Program of the U.S. National Science Foundation. The grant mainly supported travel assistance for several
researchers from China, India, Nepal, and Pakistan and students from Europe. Selected, refereed papers resulting
from the workshop will be edited by the organizers and published as Geological Society of America Special
Paper volume. The 12th workshop will be held in Rome in April 1997, on the occasion of the 100th anniversary
of birth of Ardito Desio, a pioneering geologist whose research focused on the Karakoram mountains.
• INDEPTH (suite)
http://www.cg.nrcan.gc.ca/staff/jones/tibet/tibet.html
Phase I: 1995
Using the GSC's 16 LIMS and UW's 5 LIMS-clones (LRMTs built by Phoenix Geophysics), a long period MT
survey was carried out in April to July along three main transects. For a plot of the sites on the topography of
Asia please press here (Warning: the gif file is 150 Kb in size). For a location map of the sites showing the
Zangbo suture and Lhasa, press here. For a geology map with the MT sites, press here.
Phase II (part of INDEPTH-III): 1998
In July and August 1998 Long Period magnetotelluric (MT) data were collected as part of the INDEPTH-III
survey in Central Tibet. Long period MT data were collected at 26 sites on a 400 km profile extending from Nam
Tso to Longwei Tso with the goal of imaging crustal and mantle structure. Broadband MT data were also
collected to image shallow structure by the China University of Geosciences. In this paper we will present initial
results based on the long period data, since the broadband and long period data have yet to be merged.
Preliminary inversions of the long period MT data show the following features.
(1) In the south of the profile a strong conductor is observed at a depth of 15 km. This is at a similar depth to the
conductor observed beneath the Yangbaijin graben during INDEPTH-II in 1995. This conductor terminates 50
km north of the Bangong suture.
(2) Further north, where the shallow conductor is absent, a deeper conductor is imaged at a depth of 50-60 km.
This conductor continues to the northern end of the profile and is at the same depth as the low velocity zone
imaged by seismic observations. It may represent the asthenosphere beneath a 50-60 km crust.
Deep-level constraints on the present-day structure of the orogen are provided chiefly through seismic reflection
and refraction data:
A geological cross section across the entire Tibetan Plateau shows that the southern margin of the Plateau is
underlain by India and the northern margin of the Plateau is underlain by Asia.
5. Histoire de la convergence
• Vue générale
http://www.geol.ucsb.edu/~hacker/geo102C/lectures/part14.html
The geological history of India-Asia is a sequence of continental collisions in the Triassic, Late Cretaceous and
Tertiary (~50 Ma to Recent):
The India-Asia collision zone is our best example of an active continent-continent collision:
A geological map of the central Himalaya shows the crystalline rocks of the Indian crust (Subhimalayan
sequence), passive margin sediments of the Indian plate (Lesser Himalayan sequence), crystalline rocks of the
Tibetan crust (High Himalayan Crystallines), passive margin sediments of the Tibetan crust (Tethyan Himalaya),
and pre-collisional Gangdise Batholith:
Deep-level constraints on the present-day structure of the orogen are provided chiefly through seismic reflection
and refraction data:
A geological cross section across the entire Tibetan Plateau shows that the southern margin of the Plateau is
underlain by India and the northern margin of the Plateau is underlain by Asia
The present-day high (5 km) elevation of the plateau is believed to be related to large-scale crustal thickening,
combined with delamination of the mantle lithosphere and asthenosphere reaching to unusually shallow levels:
The present-day high (5 km) elevation of the plateau is believed to be related to large-scale crustal thickening,
combined with delamination of the mantle lithosphere and asthenosphere reaching to unusually shallow levels:
The thermal evolution during continental thickening can be modeled with a 1-D "sawtooth" thermal gradient,
and attains high temperatures largely through the extreme thickness of radiogenic material.
• How did it all begin? Investigating the birth of the Himalaya
Alan S. Collins
To understand how mountain chains form why not go to the biggest and the best? The Himalaya is not only
large, it was formed over a considerable period of time by at least two plate collisions. The first involved a
collision between an island arc, known as Kohistan, and Eurasia during the Cretaceous. The second is the better
known bulldozing of India also into Eurasia during the Tertiary. Unravelling the early history of these collisions
is not easy because of subsequent deformation as India has continued to grind north. But on the top of Kohistan
and along the Shyok suture sediments occur that preserve the earliest evidence of the forthcoming deformation
that would culminate in formation of the Himalaya. These sediments illuminate a submarine story of mass
wastage and tectonic instability as Kohistan was dragged into the subduction zone and thus provide a relic of a
long defunct plate margin.
• The rise and fall of Tibet - NATURE - VOL 374 - 2 MARCH 1995
by Mike Searle
The Tibetan plateau is the largest uplifted part of the Earth today, occupying an area of some two million square
kilometres at an average elevation of just over 5,000 metres. Nevertheless, we are not seeing it at its peak; at
some point since its uplift, the plateau has begun to collapse. Now Coleman and Hodges, on page 49 of this
issue1, have set a date of 14 million years ago on thestart of that collapse, and thus a minimum age on the rise of
the plateau. The timing of uplift, maximum elevation and subsequent collapse of the plateau is a question of
great interest. Not only is it needed for understanding tectonic processes and the mechanism of forming thick
continental crust, but it also features prominently in global atmospheric circulation models of past climate.
Whereas most of the plateau is remarkably flat, it is surrounded by some of the highest, most deeply eroded
mountain ranges on the Earth. To the south lies the Himalaya; to the southwest, the Karakoram; north, the Kun
Lun-Altyn Shan-Qilian Shan range; and east, the Longmen Shan. There seems little doubt from the position
of the plateau, immediately north of the Himalaya, that the unusual crustal thickness under Tibet (at 50-70 km
thick, twice the normal value) was a direct consequence of the collision of India with Asia2. The timing of the
India-Asia collision is reasonably well constrained at 54-49 Myr ago - between the uppermost Palaeocene and
Middle Eocene from the dating of the last marine sediments on the Indian plate margin and within the Indus
suture zone3,4. Since the collision, folding and thrusting have shortened Indian plate crust of the Himalaya and
south Tibet by hundreds of kilometres. North of the Indus suture zone, the Asian plate crustof the Tibetan
plateau has been undergoing homogeneous deformation involving north-south shortening, resulting in the double
crustal thickness and 5-km elevation. But fault plane solutions of earthquakes and the active tectonics of the
plateau show that, although India is still moving northwards relative to stable Siberia at a rate of 4-5 cm per year,
normal faulting and east-west extension are dominant within most of Tibet today, whereas thrust faulting and
strike-slip faulting are dominant in the mountain ranges bordering the plateau5. Since the India-Asia
collision, the Tibetan plateau has been growing in elevation and crustal thickness. At some time in the past, the
plateau exceeded its maximum sustainable elevation and began to collapse. The evidence is a system of northsouth normal faults indicating east-west extension (Fig. 1)6. If we can date the movement on
these normal faults, it should therefore give us a minimum age on the maximum elevation of the plateau.
That is what Coleman and Hodges' have done. They present 4oAr/39Ar age data from hydrothermal muscovites
growing along a north-south aligned fault associated with one of the largest graben systems,
the Thakkola graben (Fig. 2). These faults have acted as channels for fluid flow along which hydrothermal
minerals have grown. Their age of about 14 Myr provides, for the first time, a lower boundary
to the onset of east-west extension along the southern part of the plateau, and it is considerably older than
previously thought. The north-south aligned normal faults cut the east-west aligned mylonite fabric associated
with the South Tibetan detachment, the normal fault bounding the top of the High Himalayan slab.
Muscovites from the mylonites, dated' at 17.610.3 Myr, provide a minimum age constraint on exhumation of the
High Himalaya in the Annapurna area. Between 17.6 and about 14 Myr, therefore, the stress field in southern
Tibet changed from north-south compression to east-west extension. Why should a single date be so exciting?
The only previous age reported from a normal fault in Tibet is the 40Ar139Ar cooling age of 813 Myr from the
Nyainqentangiha metamorphic core complex7,8, a completely atypical structure in central Tibet that is aligned
northeast-southwest. This has been used to argue that the collapse of the plateau was contemporaneous with
marked climate change in south Asia and strengthening of the Indian monsoon at about 8 Myr (ref. 9). Coleman
and Hodges' new age proves that the plateau must have been high long before the apparent main changes in
vegetation and climate occurred. This older age for the maximum height attained by the plateau is supported by
4oAr/39Ar ages of about 13 Myr from potassic basalts and minor felsic magmas'10 in Tibet, which imply
melting of the lithosphere.
Nevertheless, it seems likely that the uplift of a feature as large and as high as the Tibetan plateau and the 2,500km-long Himalayan mountains would have had a profound effect on global circulation, climate and erosion. The
uplift of Tibet would have disrupted the west-to-east air flow across the Northern Hemisphere, initiated the
monsoon-driven wind systems across India and increased the precipitation along the Himalaya, with higher
chemical erosion rates causing a drawdown of atmospheric carbon dioxide and resulting in global cooling11,12.
The rise of Tibet probably coincided with the initiation of the Indian Ocean monsoon, although the Himalaya
today provides the most spectacular and abrupt northern barrier to the monsoon. The maximum rates of erosion
and exhumation along the High Himalaya in India, Nepal and Bhutan probably occurred between 21 and 19 Myr
ago when the main bounding structures the Main Central Thrust at the base, and the South Tibetan detachment
normal fault at the top of the slab - were active, and most of the tourmaline-bearing leucogranites being
exhumed13.
The surge in geological data from the Himalaya in the past 15 years has resulted in a spectacular leap in our
understanding of collision-related processes. Tibet, however, remains an enigma. Considering the huge size of
the plateau, our knowledge of its geological secrets is still based on few facts and a lot of speculation. Crucial
factors such as the elevation history of the plateau may be unravelled by a detailed and wide-ranging
palaeobotanical study. But the timing of the switch from north-south compression to east-west extension will
only be unravelled by mapping in detail all the graben systems in Tibet and, following the lead given by
Coleman and Hodges, dating the faults.
Mike Searle is in the Department of Earth Sciences, University of Oxford, Oxford OXI 3PR, UK.
• Tectonic evolution of the northwest Himalaya
Peter Treloar, Kingston University
Mountain building in the Himalayan region is the result of Tertiary collision between India and Asia. Collision
started at about 55 Ma ago and continues to the present day. The NW Himalaya are atypical of the rest of the
Himalayan chain due to: firstly, the presence of a Cretaceous island arc trapped between the converging Asian
and Indian Plates; secondly, evidence for early regional metamorphism that occurred within 5 - 10 Ma of
collision; thirdly, absence of the Miocene regional metamorphism and associated leucogranite generation and
emplacement that characterises the main Himalayan chain in India, Nepal and Bhutan; and, fourthly, the
presence of the Northwest Himalayan syntaxes which deform the main Himalayan thrusts.
The Kohistan-Ladakh island arc was initiated offshore of Asia above the subducting Tethyan oceanic plate.
Subsequent to accretion and suturing to the southern margin of Asia at about 90 Ma ago the arc was
magmatically and tectonically thickened and acted as an Andean type margin until being underthrust by the
leading edge of continental India at the beginning of the Himalayan collision.
Metamorphism and deformation within the crystalline internal zones of the Indian Plate was synchronous with
early stages of collision. Ductile deformation and shearing of cover and basement assemblages occurred on the
footwall of the Main Mantle Thrust (MMT) which separates rocks of the Kohistan arc from those of the Indian
Plate. Metamorphic textures show that metamorphic mineral growth was synchronous with deformation.
Calculated pressure-temperature paths show that metamorphism, along a path of increasing pressure, occurred
during active subduction of the leading edge of continental India beneath Kohistan. Peak metamorphism was at
about 47 Ma, with initial cooling back through 500oC by 40 Ma. This latter reflects imbrication of Indian Plate
metamorphic rocks and their accretion to the base of the MMT together with thinning of the over-riding arc
sequence. A second phase of cooling and exhumation at 20 Ma was the result of regional scale extension,
accommodated by N-vergent normal faulting along the MMT and structures immediately above and below it.
Subsequently deformation stepped downward and southward into a fold and thrust belt developed in the noncrystalline external zones. Although thrust displacements may be large (> 100km for the Main Boundary Thrust)
the major structures have sub-horizontal dips permitting little uplift of the crystalline rocks of the internal zones.
The Nanga Parbat and Hazara syntaxes are young (Pliocene and younger) N-trending crustal scale antiformal
structures which deform the earlier W-trending thrust systems. In the Nanga Parbat syntaxis, uplift (at a rate of
ca 7mma-1) and associated unroofing has resulted in decompression melting and the emplacement of leucogranite
sheets as recently as 1 Ma ago. Although a number of regional scale models have been proposed for the
evolution of these structures, they can best be related to interference between the S-vergent W-striking thrusts of
the Pakistan Himalaya and the SW-vergent NW-striking thrusts of the main Himalayan chain.
6. Métamorphisme et tectonique: Continent-Continent Collisions
http://www.geol.ucsb.edu/~hacker/geo102C/lectures/part14.html
Rapid crustal thickening during continental collisions produces high temperatures because of the large
amount of radiogenic elements in continental crust. The pressures are moderate because continental crust
is never thicker than ~75 km (2.5 GPa). These abnormally high temperatures are followed by cooling to a
normal cratonal geotherm. This is the cause of so-called "Barrovian" metamorphism, signature minerals
of which are staurolite + kyanite. Good examples are known from the Tibet and Himalaya.
The present-day high (5 km) elevation of the plateau is believed to be related to large-scale crustal thickening,
combined with delamination of the mantle lithosphere and asthenosphere reaching to unusually shallow levels:
The thermal evolution during continental thickening can be modeled with a 1-D "sawtooth" thermal gradient,
and attains high temperatures largely through the extreme thickness of radiogenic material.
Continental thickening can lead to interesting kinds of melts and residuum that subsequently influence the
deformation and evolution of the orogen:
One example of typical Barrovian continental metamorphism is Kangmar Dome in southern Tibet:
The calculated P-T conditions/paths of various samples indicate a large range of pressures from ~3 to 8 kbar, but
a modest range in temperature from ~400 to 600°C.
This group of P-T paths can be explained as part of what one would expect from the kind of burial and heating
characteristic of Barrovian metamorphism.
Here are the calculated P-T conditions of samples shown in the cross section.
The P-T conditions of the samples constrains a surprisingly complicated tectonic history.
Ultrahigh-Pressure Metamorphism
Small pieces of continental material subducted to as much as 200 km are eventually regurgitated, leading to
ultrahigh P/T metamorphism. These rocks contain extremely unusual crustal minerals, such as coesite, diamond,
and majorite. Some of the most famous sites include the Dabie Shan of China, western Norway, and the Dora
Maira Massif of the Italian Alps.
These unusual rocks are now known from many localities around the world:
Coesite is the most common indicator of ultrahigh-pressure metamorphism:
Coesite converts partially or completely to quartz during exhumation:
UHP rocks in eastern China:
Some of the key mineral equilibria that govern the identification of UHP rocks are shown here:
The pressures of UHP terranes are as high as 6 GPa, indicating subduction to depths of 200 km. Most terranes
have temperatures of <800°C, but some are as hot as 1200°C:
One of the best-known UHP belts is the Dabie-Sulu belt of eastern China:
High- and ultrahigh-pressure rocks crop out in a wedge shape overlain by a supra-lithospheric scale extensional
fault: