Notes de séance

Discussions sur les articles collision Inde-Asie
Séance du 14/11/14, 14h-17h15
VOLET 1 : Evolution pré-collisionnelle
A2
India-Asia collision and the Cenozoic slowdown of
Copley et
the Indian plate: Implications for the forces
al. (2010)
driving plate motions
Journal of
Geophysical
Research
Geoffrey
DESCLOS
The plate motion of India changed dramatically between 50 and 35 Ma, with the rate of convergence between India and Asia dropping from 15 to4 cm/yr. This
change is coincident with the onset of the India-Asia collision, and with a rearrangement of plate boundaries in the Indian Ocean. On the basis of a simple model
for the forces exerted upon the edges of the plate and the tractions on the base of the plate, we perform force balance calculations for the precollision and
postcollision configurations. We show that the observed Euler poles for the Indian plate are well explained in terms of their locations and magnitudes if (1) the
resistive force induced by mountain building in the Himalaya-Tibet area I 5–61012N/m, (2) the net force exerted upon the Indian plate by subduction zones is
similar in magnitude to the ridge-push force (2.51012N/m),and (3) basal tractions exert a resisting force that is linearly proportional to the plate velocity in the
hot spot reference frame. The third point implies an asthenospheric viscosity of 2–51019Pa s, assuming a thickness of 100–150 km. Synthetic Euler poles show
that crustal thickening in the Tibetan Plateau was the dominant cause of the Cenozoic slowdown of the Indian plate.
NOTES PRISES PENDANT L’EXPOSE ET COMMENTAIRES EN SEANCE APRES L’EXPOSE :
Histoire du “trajet de la plaque” ,“histoire” de la plaque, non
Instructions suivies, sauf TIMING : NON ! Un peu trop de texte.
MOT « Technique » REVIENT SANS CESSE, bizarre… « Technique » éclogites : très marginal dans le papier ! « date of formation of the
eclogites currently exposed in the Himalayas [e.g.,de Sigoyer et al., 2000] suggests that slab break off in Tibet occurred before the
rearrangement of plate boundaries in the Indian Ocean
40° vers l’est pendant le ralentissement: :précise!
Ce qui n’est pas traduit, c’est ce que veulent faire les auteurs : updated model of the kinematics of the Indian plate, and discuss how this
model compares with the chronology of mountain building in Asia; describe the forces acting upon the boundaries and base of the Indian
plate.
Anisotropie sismique: colinéarité en fait
Utilisation données magnétiques: si, en fait! C’est même fondamental même si pas directement visible.
Q: comment les forces sont déterminées? Il faut le préciser.
A3a
Aitchison et al. (2007)*
When and where did India
and Asia collide?
A3b
Garzanti (2008) et
Aitchison et al. (2008)*
Comment & Reply to
comment by Garzanti
Journal of Geophysical
Research
Ronan
BONJEAN
Timing of the collision between India and Asia is the key boundary condition in all models for the evolution of the Himalaya-Tibetan orogenic system. Thus it
profoundly affects the interpretation of the rates of a multitude of associated geological processes ranging from Tibetan Plateau uplift through continental
extrusion across eastern Asia, as well as our understanding of global climate change during the Cenozoic. Although an abrupt slowdown in the rate of
convergence between India and Asia around 55 Ma is widely regarded as indicating the beginning of the collision, most of the effects attributed to this major
tectonic episode do not occur until more than 20 Ma later. Refined estimates of the relative positions of India and Asia indicate that they were not close enough
to one another to have collided at 55 Ma. On the basis of new field evidence from Tibet and a reassessment of published data we suggest that continentcontinent collision began around the Eocene/Oligocene boundary (
34 Ma) and propose an alternative explanation for events at 55 Ma.
NOTES PRISES PENDANT L’EXPOSE ET COMMENTAIRES EN SEANCE APRES L’EXPOSE :
‘il veut’ = ils
Restitution +/- ok du papier mais tu restes trop vague
Justification étude: Pourquoi est-ce important? Pas bien dit, revenir au texte : « affects the interpretation of the rates of a multitude of
associated geological processes”
Plein de données utilisées; mais lesquelles sont cruciales, selon toi, dans la note?
Figure 7 peu lisible : argument géochimique peu convaincant : vu dans le comment ?
Fautes formelles d’écriture : « décalage temporelle », non – beaucoup de fautes de frappes – collisions d’îles intra-océaniques
« coupe où on voit d’où ils sortent leur truc » : roches côté indien – mais où est la preuve de l’arc ?
Pourquoi on ne peut pas ? L’Inde pas à la bonne position, pas MONTRE ! Il fallait. « considerable separation between the supposedly
colliding entities”: montrer comment c’est obtenu, comment sont évaluées les incertitudes; reprendre le problème de la taille de la grande
Inde.
A4
Replumaz et al.
(2010)
Indian continental subduction and slab break-off
during Tertiary
collision
Terra
Nova
Aubéry
LONGEAU
High wave speed seismic anomalies in the transition zone and uppermost lower mantle beneath the India-Asia collision zone, imaged by
body-wave seismic tomography, have been interpreted as subducted fragments of continental material. In this study, we focus on the
prominent anomaly located beneath India between depths of about 450 and 900 km. By combining the location of this anomaly with
palaeogeographical positions of India, we constrain the timing of the subduction event probably related to this anomaly. We infer that a
large portion of the north-western margin of India initiated subduction at 35 ± 5 Ma along a 1500-km-long WNW–ESE striking zone and
ended with a progressive slab break-off process. This break-off started most probably around 25 Ma at the western end of the slab and
propagated eastwards until complete break-off around 15 Ma. This study helps to constrain better the amount of convergence between
India and Asia absorbed by continental subduction.
NOTES PRISES PENDANT L’EXPOSE ET COMMENTAIRES EN SEANCE APRES L’EXPOSE :
En 2010, est-ce que cette approche était nouvelle? « quantitative constraints » - quoi de plus (cf données en 98)
« Ils ont modélisé » : impropre, pas de modèle ! Ils ont dessiné et simplement replacé géométriquement
Ici, effectivement, les données utilisées sont plus faciles à identifier
Discussion sur les temps calculés : comment ils les ont estimés, avec +/- ?
Pourquoi la rupture du slab est nécessaire ?
Incertitude sur la date d’initiation de la subduction
Pb résolution des images mais aussi « smearing », non ?
« we only make use of the geometry of the high velocity seismic anomalies and not of the values of the anomalies
amplitudes.”
Part d’extrusion et de sous-placage : si elle n’était pas négligées, quelles conséquences ?
A7a Van Hinsbergen et al. (2011)*
Restoration of Cenozoic deformation in
Asia and the size of Greater India
Ali et Aitchison (2012) et Van Comment & Reply to comment by Ali et
A7b
Hinsbergen et al. (2012)*
Aitchison
Tectonics
Nicolas
RICHET
A long-standing problem in the geological evolution of the India-Asia collision zone is how and where convergence between India and Asia
was accommodated since collision. Proposed collision ages vary from 65 to 35 Ma, although most data sets are consistent with collision
being underway by 50 Ma. Plate reconstructions show that since 50 Ma, ∼2400–3200 km (west to east) of India-Asia convergence occurred,
much more than the 450–900 km of documented Himalayan shortening. Current models therefore suggest that most post-50 Ma
convergence was accommodated north of the Indus-Yarlung suture zone. We review kinematic data and construct an updated restoration
of Cenozoic Asian deformation to test this assumption. We show that geologic studies have documented 600–750 km of N-S Cenozoic
shortening across, and north of, the Tibetan Plateau. The Pamir-Hindu Kush region accommodated ∼1050 km of N-S convergence.
Geological evidence from Tibet is inconsistent with models that propose 750–1250 km of eastward extrusion of Indochina. Approximately
250 km of
Indochinese extrusion from 30 to 20 Ma of Indochina suggested by SE Asia reconstructions can be reconciled by dextral transpression in
eastern Tibet. We use our reconstruction to calculate the required size of Greater India as a function of collision age. Even with a 35 Ma
collision age, the size of Greater India is 2–3 times larger than Himalayan shortening. For a 50 Ma collision, the size of Greater India from
west to east is∼1350–2600 km, consistent with robust paleomagnetic data from upper Cretaceous-Paleocene Tethyan Himalayan strata.
These estimates for the size of Greater India far exceed documented shortening in the Himalaya. We conclude that most of Greater India
was consumed by subduction or underthrusting, without leaving a geological record that has been recognized at the surface.
COMMENT: Ali et al.:
Conclusion: For the two reasons outlined above, we contend that the “Greater India”of van Hinsbergen et al.[2011a] is untenable when (i)
key geotectonic features of the eastern Indian Ocean-western Australia region and (ii) accommodation of Cimmerian-terrane blocks are
considered. Any model attempting to delimit the Indian sub-continent’s pre Asia-collision size needs to be compatible with tectonic
configurations that are applicable for earlier times in the block’s history.
NOTES PRISES PENDANT L’EXPOSE ET COMMENTAIRES EN SEANCE APRES L’EXPOSE :
Pb de retraduction par écrit du discours qui est bien par ailleurs
Logiciel GPS ?? Non, GPlates !
MONTRER LA CARTE AVEC LES CONTRAINTES CINEMATIQUES
Un peu longtemps sur la même planche
BUT: “reconstruct Asian deformation with respect to stable Siberia”
Pourquoi une collision plus tardive ne conviendrait pas?
Controverse : SUR QUOI ? Dimension de la Grande Inde incorrecte : pourquoi ? On rejoint la discussion de l’article
précédent. Il faudrait mieux retraduire en quoi Ali et al. Conteste précisément l(interprétation, et pourquoi (« untenable
when (i) key geotectonic features of the eastern Indian Ocean-western Australia region and (ii) accommodation of
Cimmerian-terrane blocks are considered.”)
« Taille de la Grande Inde plus petite avant » : à préciser
Discussion : a permis aux auteurs de modifier leur position ? apparemment non ; à préciser
« Nécessité de recherches approfondies » : un peu vague !
VOLET 2 : structures, cinématique et déformation interne de l'Asie
B1 Avouac et al. (2001) Le cycle sismique en Himalaya
C.R. Acad. Sci. Paris Abdelilah AKODAD
Nous décrivons le cycle sismique en Himalaya en relation avec l’édification de la chaîne. Le modèle proposé est fondé sur les études menées
dans l’Himalaya du Népal, où un accident majeur, le Main Himalayan Thrust(MHT), accommode l’essentiel des quelque 21 mm annuels de
raccourcissement entre l’Inde et le Sud-Tibet. Les données géodésiques montrent que cet accident est actuellement bloqué et s’enracine
dans une zone de cisaillement ductile sub-horizontale située sous le Sud-Tibet. Au front de la Haute Chaîne, une forte microsismicité et une
zone de soulèvement résulte de l’accumulation de contraintes à l’extrémité de cette zone de fluage asismique. La microsismicité absorbe
une fraction négligeable des déformations observées. Ces déformations sont donc élastiques et finissent par être entièrement transférées
sur le MHT lors des séismes himalayens majeurs, tel que celui de magnitude Mw 8,4 qui s’est produit au Népal en 1934, rompant un
segment de l’arc de 250 à 300 km de longueur. En considérant l’arc himalayen dans son ensemble, le moment libéré par les séismes
himalayens majeurs depuis le XIXe siècle représente plus de 70 % du glissement sur le MHT, ce qui laisse la possibilité d’un éventuel
glissement asismique, en phase post- ou pré-sismique.
NOTES PRISES PENDANT L’EXPOSE ET COMMENTAIRES EN SEANCE APRES L’EXPOSE :
Liens entre séismes et déformation de la croûte supérieure (mais aussi plus profonde ?)
Définir le couplage sismique (rapport entre la déformation accommodée par le glissement co-sismique et celle
accommodée par l’ensemble des processus qui contribuent aux déformations crustales
Pourquoi disent-ils « Main Himalayan Thrust fault (MHT) » ?
Le discours est peu assuré et n’explique pas assez clairement l’approche et les messages essentiels du papier. Il faut
prendre le temps de bien expliquer ce que signifient les figures essentielles (notamment ce qui est déformation mong
terme et déformation intersismique, qui est au cœur du papier.
B8a
Hetzel et al. Peneplain formation in southern Tibet predates the
(2011)
India-Asia collision and plateau uplift
B8b
Tian et al. (2013) et Hetzel et al.
(2013)
Geology
Damien
BOUSSEY
Comment & Reply to comment by Tian
et al.
The uplift history of Tibet is crucial for understanding the geodynamic and paleoclimatologic evolution of Asia; however, it remains
controversial whether Tibet attained its high elevation before or after India collided with Asia ~50 m.y. ago. Here we use thermochronologic
and cosmogenic nuclide data from a large bedrock peneplain in southern Tibet to shed light on the timing of the uplift. The studied
peneplain, which was carved into Cretaceous granitoids and Jurassic metasediments, is located in the northern Lhasa block at an altitude of
~5300 m. Thermal modeling based on (U-Th)/He ages of apatite and zircon, and apatite fission track data, indicate cooling and exhumation
of the granitoids between ca. 70 and ca. 55 Ma, followed by a rapid decline in exhumation rate from ~300 m/m.y. to ~10 m/m.y. between
ca. 55 and ca. 48 Ma. Since then, the peneplain has been a rather stable geomorphic feature, as indicated by low local and catchment-wide
erosion rates of 6–11 m/m.y. and 11–16 m/m.y., respectively, which were derived from cosmogenic 10 Be concentrations in bedrock, grus,
and stream sediment. The prolonged phase of erosion and planation that ended ca. 50 Ma removed 3–6 km of rock from the peneplain
region, likely accomplished by laterally migrating rivers. The lack of equivalent sediments in the northern Lhasa block and the presence of a
regional unconformity in the southern Lhasa block indicate that the rivers delivered this material to the ocean. This implies that erosion and
peneplanation proceeded at low elevation until India’s collision with Asia induced crustal thickening, surface uplift, and long-term
preservation of the peneplain.
COMMENT par Tian et al.: this conclusion can only be reached if (1) coeval regional crustal shortening was subdued, and (2) the drainage
systems in the northern Lhasa terrane were connected to the sea, so as to allow rivers to incise and erode the bedrock laterally over large
distances. Neither of these two conditions is supported by geological evidence. (1) >55% (>230 km).
NOTES PRISES PENDANT L’EXPOSE ET COMMENTAIRES EN SEANCE APRES L’EXPOSE :
“Géodynamisme” non
ème
« 2 methode mise en évidence » non
Problème bien posé. Mais il faut présenter le sujet au-delà de la pénéplénation : « controversial whether Tibet attained its
high elevation before or after India collided with Asia ~50 m.y. ago”
Résultats simples: cooling and exhumation of the granitoids between ca. 70 and ca. 55 Ma PUIS rapid decline in exhumation
rate from ~300 m/m.y. to ~10 m/m.y. between ca. 55 and ca. 48 Ma
3–6 km of rock from the peneplain
the rivers delivered this material to the ocean
“Titan et al.”???? Non, Tian!
Tian propose une autre interprétation des observations faites (Figure 1B) : « Gangdese Arc created a drainage divide
separating the northern Lhasa terrane from the forearc basin and Neotethys Ocean to the south” : clarifier la polémique sur
l’origine de l’exhumation!! Et bien valoriser les causes de ce méga-changement autour de 50 Ma entre les processus
d’exhumation.
VOLET 3 : Evolution syn- et post-orogénique (rôle des sutures, signification des
failles et du métamorphisme, modèles)
C1
Evolutionary model of the Himalaya-Tibet
Chemenda et
system: geopoem based on new modelling,
al. (2000)
geological and geophysical data
Earth and
Planetary Science
Letters
Marie
GUILCHER
A two-dimensional thermo-mechanical laboratory modelling of continental subduction was performed. The subducting continental
lithosphere includes a strong brittle upper crust, a weak ductile lower crust, and a strong upper mantle. The lithosphere is underlain by a low
viscosity asthenosphere. Subduction is produced by a piston (push force) and the pull force from the mantle lithospheric layer, which is
denser than the asthenosphere. The lithospheric layers are composed of material whose strength is sensitive to and inversely proportional
to temperature. Throughout the experiment the model surface was maintained under relatively low temperature and the model base at
higher
temperature. The subduction rate satisfied the Péclet criterion. Modelling confirms that the continental crust can be deeply subducted and
shows that slab break-off, delamination and tectonic underplating are fundamental events with drastic consequences on the subsequent
evolution of the convergent system. Combining these results with previous, purely mechanical modelling, we elaborate a new evolutionary
model for the Himalaya-Tibet convergent system. The principal successive stages are: (1) subduction of the Indian continental lithosphere to
200^250 km depth following subduction of the Tethys oceanic lithosphere; (2) failure and rapid buoyancy-driven uplift of the subducted
continental crust from ca. 100 km depth to some depth that varies along the mountain belt (20-30 km on average) ; (3) break-off of
the Indian subducted lithospheric mantle with the attached oceanic lithosphere; (4) subduction/underplating of the Indian lithosphere under
Asia over a few to several hundred kilometers; (5) delamination, roll-back, and break-off of the Indian lithospheric mantle; (6) failure of the
Indian crust in front of the mountain belt (formation of the main central thrust) and underthrusting of the next portion of Indian lithosphere
beneath Tibet for a few hundred kilometers. At the beginning of stage (6), the crustal slice corresponding to the Crystalline Himalayas
undergoes `erosion-activated' uplift and exhumation.
NOTES PRISES PENDANT L’EXPOSE ET COMMENTAIRES EN SEANCE APRES L’EXPOSE :
Situation géologique actuelle”: en fait, de 1999 (papier publié en 2000): un peu daté
Quel matériau utilisé ?
A quoi est dû le sous-placage ?
Avis sur le modèle ? Comp. Données/modèles ?
Discussion sur la grande ou petite Inde ?
Le traitement du sujet est resté très en retrait. Il faut insister sur le schématisme de la modélisation, mais aussi sur les sauts
conceptuels proposés, même s’ils sont contestables. On aborde aujourd’hui de manière plus réaliste et précise ces
processus par la modélisation numérique –(voir exemples en cours)
- Chemenda00: ‘continental crust can be deeply subducted and shows that slab break-off, delamination and tectonic
underplating are fundamental events with drastic consequences on the subsequent evolution of the convergent system.”
Autre système possible? Réalisme du modèle ?
C4
Capitanio et al.
(2010)
India–Asia convergence driven by the subduction of the
Greater Indian continent
Nature
Marine
PAUL
The most spectacular example of a plate convergence event on Earth is the motion of the Indian plate towards Eurasia at speeds in excess of
18 cm yr1(ref. 1), and the subsequent collision. Continental buoyancy usually stalls subduction shortly after collision, as is seen in most
sections of the Alpine–Himalayan chain. However, in the Indian section of this chain, plate velocities were merely reduced by a factor of
about three when the Indian continental margin impinged on the Eurasian trench about 50 million years ago. Plate convergence,
accompanied by Eurasian indentation, persisted throughout the Cenozoic era, suggesting that the driving forces of convergence did not
vanish on continental collision. Here we estimate the density of the Greater Indian continent, after its upper crust is scraped off at the
Himalayan front, and find that the continental plate is readily subductable. Using numerical models, we show that subduction of such a
dense continent reduces convergence by a factor similar to that observed. In addition, an imbalance between ridge push and slab pull
can develop and cause trench advance and indentation. We conclude that the subduction of the dense Indian continental slab provides a
significant driving force for the current India– Asia convergence and explains the documented evolution of plate velocities following
continental collision.
Le message
“We postulate here that the subducting Indian lithosphere, imaged in the upper mantle, has the negative buoyancy needed to sustain
subduction regardless of its attachment to Tethys lithosphere.”-> pas besoin de “external forcing”
NOTES PRISES PENDANT L’EXPOSE ET COMMENTAIRES EN SEANCE APRES L’EXPOSE :
Intro OK, bien posée, retraduisant la problématique.
Estimation de la densité – estime qu’il faut soit amincir la croûte soit ôter une partie de la croûte (la partie
supérieure) pour permettre la subduction qui continue (=DELAMINATION ?)
Pourquoi la plaque cont. arrive en subduction à 20 Ma dans le graphe ?
Poussée de la dorsale ne peut pas être le moteur – ne fait qu’accélérer.
Il y a forcément une modif. du bilan des forces : « aspiration de la plaque océanique » c’est quoi ?
Diminution de 50% ou 66% ?
-
C10
Conclusion : ton idée ? Marge indienne + dense : redire pourquoi ?
Tu lis un peu trop ton texte…
Renforcer le message sur le caractère « rupture » du papier
Hirschmiller et al. What controls the growth of the Himalayan
(2014)
foreland fold-and-thrust belt?
Geology
Ruth
MARTINEZ
We provide empirical evidence for the impact of surface processes on the structure of the present-day foreland fold-and-thrust belt of the
Himalaya. We have reconstructed and analyzed ten balanced cross sections distributed along the entire length of the Himalayan arc.
Here, we focus on the Siwalik Group, which represents the deformed part of the foreland basin and consists of synorogenic Middle Miocene
to Pleistocene sediments that form the youngest and frontal part of the Himalayan orogen. We make two important observations: (1) a
distinct west-to-east increase in strain and strain rate correlates with plate convergence rates, and (2) belt morphology is inversely
correlated with rainfall amount. According to the predictions of the critical taper model, an eastward increase in convergence rate would
induce higher rates of material accretion. Thus, the Himalayan fold-and-thrust belt should widen eastward, yet we have observed the
opposite. However, higher annual rainfall amounts and specific stream power appear to favor a narrower belt. Thus, we suggest that the
morphology of the Himalayan foreland fold-and-thrust belt is controlled primarily by surface processes, in accordance with the critical taper
model.
NOTES PRISES PENDANT L’EXPOSE ET COMMENTAIRES EN SEANCE APRES L’EXPOSE :
Objectif : Comparaison de 8 coupes pour variations latérales. Bien mettre en évidence cette variation E-O et insister sur les
arguments conduisant à relier largeur de chaîne et intensité de l’érosion (pas perceptible dans l’exposé).
Reprendre les grandes conclusions des auteurs:
1. Strain rates correlate well with west-to-east increases in convergence rates according to both long-term plate
velocity data and GPS data, suggesting that Pliocene to Holocene shortening is externally imposed and related to
plate convergence rates.
2. Conversely, the eastward decrease in belt width corresponds to an eastward increase in rainfall rates and
specific stream power.
Tableau: montrer l’orientation W-E!
« vitesse raccourcissement : aucun modèle spatial » ??? Maladroit
Epaisseur du FTB ??? Préciser
CONCLUSION: “morphology of the Himalayan FTB is controlled primarily by erosion, in accordance with the critical
taper model.” Car “lithology, erodibility, and rock mechanical properties are relatively homogeneous throughout
the belt.” Discutable mais au moins à bien faire comprendre.
-
RECOMMANDATIONS FINALES POUR TOUS :
1. CORRIGER LES FAUTES D’ORTHOGRAPHE
2. PRENDRE EN COMPTE LE MESSAGE DES « ABSTRACTS » : si une notion importante
exprimée dans l’abstract n’apparaît pas dans vos 8 planches, il y a un problème
3. TENEZ COMPTE DU FAIT QUE VOS AUDITEURS/LECTEURS N’ONT PAS LU L’ARTICLE
4. ILLUSTRER SUFFISAMMENT MAIS PAS TROP
5. CRITIQUE GENERALE : TROP SOUVENT DES DETAILS INUTILES (s’adapter au temps
disponible : si 1h, on peut dire telle ou telle chose ; si 5 mn, gardez ce qui est essentiel…)
6. RELIRE LE PLAN DES AUTEURS ET VERIFIER QU’ON EN A TRADUIT L’ESSENTIEL

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