Chemenda et al. (2000)

Evolutionary model of the Himalaya^Tibet system: geopoembased on new modelling, geological and geophysical data

Alexander I. Chemenda a;*, Jean-Pierre Burg b, Maurice Mattauer c

a Geosciences Azur, UMR 6526, Universite de Nice-Sophia Antipolis et CNRS, 250 Rue Albert Einstein, Sophia Antipolis,06560 Valbonne, France

b Geologisches Institute, ETH, Zurich, Switzerlandc Laboratoire de Geophysique, Tectonique et Sedimentologie, UMR 5573, Universite Montpellier II, Case 060, Pl. E. Bataillon 34095,

Montpellier, Cedex 05, France

Received 10 August 1999; accepted 19 October 1999

Abstract

A two-dimensional thermo-mechanical laboratory modelling of continental subduction was performed. Thesubducting continental lithosphere includes a strong brittle upper crust, a weak ductile lower crust, and a strong uppermantle. 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 arecomposed of material whose strength is sensitive to and inversely proportional to temperature. Throughout theexperiment the model surface was maintained under relatively low temperature and the model base at highertemperature. The subduction rate satisfied the Peclet criterion. Modelling confirms that the continental crust can bedeeply subducted and shows that slab break-off, delamination and tectonic underplating are fundamental events withdrastic 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. Theprincipal successive stages are: (1) subduction of the Indian continental lithosphere to 200^250 km depth followingsubduction of the Tethys oceanic lithosphere; (2) failure and rapid buoyancy-driven uplift of the subducted continentalcrust from ca. 100 km depth to some depth that varies along the mountain belt (20^30 km on average); (3) break-off ofthe Indian subducted lithospheric mantle with the attached oceanic lithosphere; (4) subduction/underplating of theIndian lithosphere under Asia over a few to several hundred kilometers; (5) delamination, roll-back, and break-off ofthe Indian lithospheric mantle; (6) failure of the Indian crust in front of the mountain belt (formation of the maincentral 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 èrosion-activated'uplift and exhumation. ß 2000 Elsevier Science B.V. All rights reserved.

Keywords: plate collision; tectonics; subduction; physical models; Himalayas; Xizang China; exhumation

1. Introduction

The amount of Indian continental lithosphereconsumed in the Himalayas is estimated from pa-

0012-821X / 00 / $ ^ see front matter ß 2000 Elsevier Science B.V. All rights reserved.PII: S 0 0 1 2 - 8 2 1 X ( 9 9 ) 0 0 2 7 7 - 0

* Corresponding author. Tel. : +33-4-92-94-26-61;Fax: +33-4-92-64-26-10; E-mail: [email protected]

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Earth and Planetary Science Letters 174 (2000) 397^409

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leomagnetic data to be about 1000 km [1,2] andmay even reach 1500 km [3]. Recent tomographydata [4] are consistent with the latter estimates,revealing high velocity zones that can be inter-preted as segments of oceanic lithosphere andcontinental lithospheric mantle subducted downto 1000 and 1700 km, respectively. It is unclear,however, what is the fate of the buoyant conti-nental crust, which is generally believed to be un-able to subduct to these depths. INDEPTH seis-mic data display the Indian crust underthrustunder the Nepalese Himalayas to about 80 kmdepth [5]. Eclogites in the western Himalayas in-dicate that the Indian crust had already reached a100 km depth equivalent (25^30 kbar) at the be-ginning of collision, ca. 50^55 Ma ago [6,7]. Sincethen, hundreds of kilometers of the Indian crusthave been subducted along the Himalayas. Themajor challenge now is to understand how thissubduction has developed and what is the styleof the associated lithospheric deformation. Thiscan be done only on a large scale, considering along-evolving thermo-mechanical system `layeredlithosphereâsthenosphere'. Such a scale is hardlyaccessible to geological analysis. Geophysics pro-vide meaningful structural information about thedeep lithosphere and mantle but only about thepresent situation. Experimental and/or numericalmodelling thus could play an important role inunderstanding past events. Modellers (e.g. [8^12]) usually investigate simpli¢ed models that can-not consider fundamental factors such as multi-phase (non-stationary) subduction of a largeamount of continental lithosphere. Initial stagesof this process from subduction of the continentalmargin to a rapid uplift/exhumation of deeplysubducted crust have been experimentally mod-elled by Chemenda et al. [13,14]. These stagesscaled to nature should take place over a few toseveral million years. The evolution of the Hima-laya^Tibet collisional system lasted much longer,50 to 60 Myr during which a number of otherimportant events may have happened. What arethese events and what is their mechanism? Con-sidering the time-scale of the Himalayan history,these questions can be addressed by thermo-me-chanical modelling that somehow takes into ac-count changes in the mechanical behaviour of

the lithosphere during subduction. The necessityof thermo-mechanical modelling drastically com-plicates the task and dooms any modelling resultsto ambiguity. We barely know how the miner-alogical composition and rheologic structure ofthe subducting continental plate change duringsubduction. These changes strongly depend on anumber of ill-understood factors such as the ki-netics and spatial distribution of mineralogical re-actions [15], changes of the grain size thatstrongly a¡ects the rheology, the e¡ect on rheol-ogy of £uids and the multi-aggregate nature ofrocks (e.g. [16]) etc. Therefore, modelling can rep-resent only a qualitative approximation, even ifone adopts precise rheologic laws. In this workwe attempt simpli¢ed thermo-mechanical experi-mental modelling of continental subduction underdi¡erent `reasonable' conditions. The obtainedscenarios of initial and mature continental sub-duction tested against the geological informationallowed us to propose a ¢rst order physically andgeologically consistent model for the evolution ofthe Himalaya^Tibet system.

2. Mechanical essentials of continental subduction

Previous purely mechanical experiments haverevealed two principal factors de¢ning the sub-duction regime: (1) the strength of the subductingcontinental crust (upper crust, Figs. 1 and 2) theratio pull/buoyancy force 6= Fpl/Fb [13,14].

The pull force Fpl acting on the subducting con-tinental lithosphere is composed of the tractionfrom both the subducted oceanic lithosphere andthe lithospheric mantle of the continental litho-sphere. The latter is proportional to both thesize (thickness and length in two dimensions) ofthe subducted segment as well as to the densitycontrast between the continental and the sur-rounding mantle. The buoyancy force Fb is pro-portional to both the thickness and subductiondepth of the continental crust as well as to themantle/crust density contrast. For example, an in-crease in the 6 ratio can be achieved by reductionof the crustal thickness. A decreasing 6 ratio canbe brought about, in particular, by lowering thelithospheric mantle density. However, the major

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factor causing strong and rapid drop in 6 isbreak-o¡ of the dense subducted slab. Afterbreak-o¡ only the mantle layer of the subductingcontinental lithosphere can provide the pull force,if this layer is denser than the surrounding mantle.Therefore, the largest pull force is exerted at thebeginning of continental subduction whilst theearlier subducted oceanic slab is still attached tothe continental lithosphere. Initial continentalsubduction should be characterised also by com-paratively low crustal thickness of the continentalmargin. Thus, the 6 ratio should be maximal atthis stage.

Purely mechanical experiments correspondingto this situation exhibits the following subductionpattern (Fig. 2): The continental crust subducts toa depth equivalent to 200^300 km, following theoceanic subduction. The upper continental crustthen fails near the base of the overriding plate(Fig. 2a) and the subducted crustal slice rapidlyraises along and within the interplate zone (Fig.2b). This buoyancy-driven uplift brings materialfrom several tens to more than 100 km depthequivalent to shallower levels. The depth to whichthe deeply subducted crust is spontaneously up-lifted is proportional to the depth of its initialsubduction, which in turn is proportional to thestrength of the upper crust. Uplift of the crustalslice is followed by break-o¡ of the previouslysubducted oceanic lithosphere with part of thecontinental lithospheric mantle (Fig. 2b). Break-o¡ reduces the pull force, which results in 1^2 kmequivalent isostatic uplift of the lithosphere inthe subduction zone. Another consequence ofthe break-o¡ in the experiments conducted at

constant convergent rate, imposed by the movingpiston (Fig. 1), is an increase in horizontal com-pression of the lithosphere. Accordingly, thesubduction regime was termed low-compressionalbefore, and highly compressional after break-o¡[14]. The geometry and dynamics of subductionafter break-o¡ still depend on the 6 ratio, whichis obviously smaller than before break-o¡. Thesubduction angle initially is rather steep, 30³ to60³ even if the subducting continental lithosphereis denser than the asthenosphere (Fig. 3a). Then,the subducted lithosphere bends and rotates up-wards to come against the base of the overridingplate (Fig. 3b). Horizontal subduction/underplat-ing of this lithosphere follows (Fig. 3c). At a stagethat depends on the upper crust strength and onfriction between the underplating crust and the

Fig. 1. Scheme of the modelling. 1, overriding plate; 2, upper continental crust with strong strain weakening; 3, ductile, weaklower crust; 4 plastic mantle layer; 5, piston; 6, liquid asthenosphere; 7, bath. Ts and Tm are the temperatures at the model sur-face and in the asthenosphere, respectively (in previous purely mechanical modelling discussed in this paper Ts = Tm). hf is thelower crust thickness in the frontal part of the subducting plate.

Fig. 2. Low-compressional regime of continental subduction(high pull force), Chemenda et al. [14]. Symbols as in Fig. 1.

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A.I. Chemenda et al. / Earth and Planetary Science Letters 174 (2000) 397^409 399

overriding plate-sole, the upper crust fails in frontof the subduction zone (Fig. 3c). In the conductedexperiments the maximal underplating distance Lwas 7^8 cm corresponding to 250^300 km in na-ture.

The depth of crustal subduction at which buoy-ancy-driven lithospheric bending and rotation oc-curs (Fig. 3b) is proportional to 6. When the 6ratio and the strength of the upper continentalcrust are such that crustal failure occurs beforebending of the subducted lithosphere, conver-gence proceeds as in Fig. 4. Note that failure inFig. 4b occurs in front of the subduction zone(highly compressional regime). For the crust tofail at depth 6 should be higher (low-compres-sional regime, Fig. 2). Application of erosion tothe growing relief modi¢es the deformation asshown in Fig. 4c,d. Break-o¡ of the mantle litho-spheric layer (Fig. 4d) results in the horizontalsubduction shown in Fig. 3. We now present ther-mo-mechanical modelling of this process.

3. Set up of thermo-mechanical modelling

The experimental setting (Fig. 1), model mate-rials, techniques and mechanical similarity criteriaare those used in the experiments described above.The principal di¡erence is the introduction ofthermo-mechanical elements. Along with purelymechanical similarity criteria, we take into ac-

count the Peclet criterion:

VH=U � const �1�

where V is the subduction rate; H is the thicknessof the plate, and U is the thermal di¡usivity of thelithosphere that controls the heating (thermalequilibration) rate of the subducted material.The conductive di¡usivity of the lithosphere isabout U³ = 1036 m2/s [17], but the actual di¡usiv-ity of the subducted material (crust in particular)could be higher, due to the advective heat transferby £uids. The thermal di¡usivity of materials usedto model lithosphere is UmW8U1038 m2/s (hereand below superscripts `m' and ò' indicate themodel and original parameters, respectively). Letus assume that the geological convergence rate isof the order of 1 cm/yr and the crustal thickness isof the order of 10 km. Then, adopting a modelcrustal thickness Hm = 1 cm, we obtain from cri-terion (1) that the model convergence rate shouldbe Vm = 8U1035 m/s. For higher e¡ective di¡usiv-ity of natural lithosphere, the model convergencerate should be slower. In the experiments pre-sented below we used VW4U1035 m/s.

The realistic thermal regime of the subductingplate is not su¤cient, however, for thermo-me-chanical modelling. Adequate changes in litho-spheric rheology and crustal density during sub-duction are also needed. The e¡ective strength ofthe crust should considerably reduce while it de-scends into the asthenosphere. The reduction fac-tor is indeterminate. In the experiments presentedhere we have assumed that it is about 5. Theanalogue material strength is very sensitive totemperature, so that ¢ve times reduction of thestrength is achieved through raising temperatureby 2^3³C. On the other hand, the materials den-sity remains virtually constant. The mechanicalexperiments above were conducted under a homo-geneous temperature near 40³C with a rheologiccontrast between the lithospheric layers caused bydi¡erent layer compositions. In thermo-mechani-cal experiments (Fig. 1) the model is subjected toa linear, stationary temperature gradient with sur-face temperature of 39³C or 40³C and 42³C underthe lithosphere. The temperature slightly increaseswith depth in the `mantle' and is 0.2³C higher at

Fig. 3. Very highly-compressional regime of continental sub-duction (no pull force). Symbols as in Fig. 1.

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the bottom of the bath than at the base of thehorizontal lithospheric layer. During subduction,upper lithosphere layers are heated and, therefore,weakening.

4. Results

Twelve experiments have been conducted underdi¡erent conditions. We present three representa-tive tests (see Table 1 for parameter values).

Experiment 1, Fig. 5: As in the purely mechan-ical experiment (Fig. 3b), the subducted slabbends upward to the base of the overriding plate(Fig. 5c) with continental subduction becominghorizontal (Fig. 5d). The subducting crust failsin front of the subduction zone, tectonic under-plating of the crust then being relayed by crustalaccretion and thickening in front of the subduc-tion zone (Fig. 5d^f). Between stages (e) and (f)two competing processes have been observed:separation (delamination) of the mantle layerfrom the crust at the front of the subducted crustand failure in the hinge zone of the mantle layer.Finally, failure and break-o¡ of the lithosphericmantle occurred (Fig. 5f). To facilitate delamina-tion (reduce the coupling between the upper crust

and the mantle), there are at least two possibil-ities : one is to reduce the strength of the lowercrust by using initially weaker or more temper-ature sensitive material for this layer. Anotherpossibility is to use the same material, but to in-crease the thickness of the lower crust hf (Fig. 1),which has been done for the next experiment.

Experiment 2, Fig. 6 (Table 1): The crust andthe lithospheric mantle split apart from the begin-ning of subduction, the crust wedges along thebase of the overriding plate while the dense man-tle layer subducts almost vertically at late stage.This layer undergoes considerable down-dipstretching during sinking but deformation doesnot localise to cause break-o¡ before the slab-tipreaches the bottom of the bath, when convergenceis stopped.

These two experiments show that continentalsubduction is very sensitive to the rheologic struc-ture of the crust. Small variations in lower crustthickness change drastically the bulk behaviour. Itseems obvious that tuning this parameter betweenthe two tested values will result in an intermediatescenario; i.e. subduction starting with horizontaltectonic underplating (as in Fig. 5d,e) and fol-lowed by delamination and break-o¡. Other pa-rameters also a¡ect the result. For example, if the

Fig. 4. Highly-compressional regime of continental subduction (low pull force) after Chemenda et al. [13]. (a), (b) and (e) Succes-sive stages of continental subduction in experiments without èrosion'. (a)^(d) Continental subduction in experiments with ero-sion. 1, overriding plate; 2, upper crust; 3, lower crust, 4, eroded material (sediments).

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strength of the mantle layer of the underplated/subducted lithosphere in Expt. 1 (Fig. 5) washigher, then one would expect backward delami-nation and peeling of this layer instead of rupture.Break-o¡ would occur later, when the length ofthe sinking mantle lithosphere is long enough togenerate a su¤cient pull force for breaking thisstrong layer. Such a process is observed in thenext experiment where the same lithospheric mod-el as in Expt. 1 is tested under lower temperatureand hence with higher strength of the lithosphericlayers (Table 1).

Experiment 3, Fig. 7: The initial stages of this

experiment are similar to Expt. 1, but the sub-ducting lithospheric mantle separates from thetectonically underplated crust and peels back tothe subduction front (Fig. 7f). This delaminationis followed by break-o¡ of the mantle layer, whichcould not be achieved because the sinking mantlelayer reached the bottom of the box. The conti-nental crust underplated the overriding plate (Fig.7f) over 10 cm (V350 km in nature) and didnot fail because it is colder and stronger than inExpt. 1.

One can test many other parameter combina-tions, but the presented experiments provideenough information to predict other possible op-tions for lithospheric/crustal behaviour: the prin-cipal elements are delamination, roll-back andbreak-o¡ of the lithospheric mantle layer. Thedistance L of tectonic underplating depends onthe crustal strength. Weakening of the crust withdepth/temperature results in shorter L values. As

Fig. 6. Experiment 2: Tectonic underplating of the continen-tal crust delamination-break o¡ (model parameters in Table1).

Fig. 5. Experiment 1: Tectonic underplating of the continen-tal lithosphere break o¡ (model parameters in Table 1).

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we do not know the actual weakening factor wecannot predict the underplating distance in na-ture. The peel-back distance of the mantle layeris particularly controlled by the lower crust prop-erties. This process can propagate to the subduc-tion zone (Fig. 6) or mid way (Fig. 7) or go far-ther below the continent (we obtained such aresult in experiments not presented in the paper).

To complete the scope of continental subduc-tion scenarios we remind below one more relevantresult from purely mechanical modelling [18]. Thismodelling takes into account the existence of athin and weak lithosphere in the arc area andshows subduction of most of the fore arc block(Fig. 8a). Evidences for this process have beenfound in the Urals and the Variscan belt [19],the Kamchatka [20], Taiwan [18,21] and the Hi-malayas [22,23]. The fore arc block shields fromthe hot mantle the deeply subducted continentalcrust (Fig. 8d) which, therefore, is kept relativelycold and hence strong. We did not yet apply ther-mo-mechanical modelling technique to arc-conti-nent collision and will consider below that thesubducting continental crust behaves at this stageas in isothermal experiments (Fig. 2).

5. Evolutionary model of the Himalayas

We use the physically possible subduction sce-narios presented above to construct an evolution-ary model for the Himalaya^Tibet collisional sys-tem (Fig. 9).

The Indian continental margin arrived againstAsia 55 to 60 Ma ago [24], resulting in arc-con-

Fig. 7. Experiment 3: Tectonic underplating of the continen-tal lithosphere: peeling of the lithospheric mantle break o¡(model parameters in Table 1).

Table 1Model parameters

Ts

(³C)Tm

(³C)csl

(Pa)cs1

(Pa)cs2

(Pa)bl

(g/cm3)bc1

(g/cm3)bc2

(g/cm3)ba

(g/cm3)Hl

(cm)Hc1

(cm)Hc2

(cm)hf

(cm)V(m/s)

Experiment 1 40 42 13 38 0.5 1.03 0.86 0.86 1 12 8 2 0.4 4U1035

Experiment 2 40 42 13 38 0.5 1.03 0.86 0.86 1 12 8 2 0.8 4U1035

Experiment 3 39 42 18 53 0.7 1.03 0.86 0.86 1 12 8 2 0.8 4U1035

Ts and Tm are the temperatures at the surface and the base of the lithospheric model before deformation. csl , cs1 ,cs2 are theaverage values of yield limit for the mantle and upper and lower crustal layers of the lithosphere under normal load, respectively.bl, ba, bc1 , bc2 are the densities of the mantle lithospheric layer, asthenosphere, upper and lower crustal layers, respectively; V isthe rate of the plate convergence; Hl, Hc1 , Hc2 are the thickness of the mantle lithospheric layer, upper and lower crustal layers(Hc2 decreases to hf toward the subducting plate front, see Fig. 1).

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tinent collision. In the central Himalayas the arcwas of Andean-type [25], while in the NW Hima-layas, the Ladakh and Kohistan arcs were intra-oceanic [26,27]. We do not aim to discuss in thispaper the arc continent collision along the Hima-layas and simply show in Fig. 9a a possible con-sequence of this process: the core (mainly mantle)part of the fore arc subducted into the mantle.During subduction of the Indian margin underthis block the margin upper layers were partlyscraped o¡ and accreted to the overriding litho-sphere at di¡erent depths within the interplatesuture zone. A large part of the continental Indi-an crust, however, subducted to 200^250 kmdepth, following the Tethys oceanic lithosphere(Fig. 9a).

The crust then failed at several tens to about100 km depth and rapidly moved upward betweenthe two plates (Fig. 9b). The upward-movingcrustal slice pushed up previously underplatedcrustal/sedimentary material squeezed in the inter-plate zone and now draping gneiss domes perva-sive in the western Himalaya [28] and possiblyrepresented among culminations of the South Ti-betan North Himalayan belt [29]. Rapid uplift

then stopped and was followed by slab break-o¡(Fig. 9b).

Subduction then switched to a highly compres-sional mode with tectonic underplating of thewhole Indian lithosphere beneath Asia that under-went a ¢rst isostatic uplift (Fig. 9c,d), apparentlyin the Eocene, when oceanic sedimentation ceasedon the Indian lithosphere (e.g. [30]) and residualbasins persisted along the suture. The underthrustIndian crust was heated and thus softening. Thephysical state, the rheology and the mineralogicalcomposition of this crust are not clear : Part of thecrust, more likely the lower crust, was eclogitisedand became denser than the mantle. The uppercrust has likely been molten. After a few to sev-eral hundreds kilometers of tectonic underplating,the mantle layer (probably together with the low-er crust) started to delaminate from the crust andto peel-back to the subduction zone (Fig. 9d).This process resulted in a new phase of isostaticuplift of the overriding plate (Tibet) and was ter-minated by a second break-o¡ (Fig. 9e).

A second crust failure occurred in front of thesubduction zone/mountain belt (Fig. 9e, highlycompressional subduction regime). The majorthrust fault formed at this time was the main cen-tral thrust (MCT). The low-viscosity Indian crustbelow Asia, in direct contact with the hot asthe-nosphere, became less viscous and started to in-trude the hot dense Asian lithospheric mantle(Fig. 9e).

This southward propagating process (Fig. 9f) isconsistent with mantle-derived heat input inferredto promote late Oligocene-early Miocene crustalanatexis in the arc region (e.g. [29,31]) and alongthe Himalayas [32,33^35]. The new subductionfront was the MCT. Underplating along this faultfurther increased the crustal thickness below theHimalayas and resulted in the formation of high,intensively eroded relief. Erosion triggered boththe exhumation of the Crystalline Himalayasand the formation of the South Tibetan normalfault system [36,37]. Subduction of the Indianlithosphere was again horizontal and resulted infurther thickening of the crust under Tibet by in-tensively deforming its very weak (molten?) lowerpart and by adding new portions of the Indiancrust.

Fig. 8. Purely mechanical model of continental subductionduring arc^continent collision after Chemenda et al. [18].

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Fig. 9. Evolutionary model for the Himalaya^Tibet system. (a) Subduction of the Indian continental crust to V250 km depth,scraping of the sedimentary and upper crustal material from the subducting plate, formation of the huge accretionary prism. Fail-ure of the subducted crust at depth of ca. 100 km.; (b) Rapid uplift of the subducted crustal slice to a few to several tens ofkilometers depth. Break-o¡ of the Indian mantle with the attached previously subducted oceanic lithosphere; (c) Heating, weak-ening and uplift of the remaining in the mantle crustal segment of the Indian margin; (d) Underplating of the Indian continentallithosphere under Asia and initiation of delamination of the Indian lithospheric mantle; (e) Failure of the Indian crust in frontof the orogen and initiation of the MCT; (f) Replacement of the Asian mantle by the underplated Indian crust started at stage(e); (g) Formation of the South Tibet detachment (STD) and exhumation of the metamorphics in the Crystalline Himalayas. (h)Present stage with main boundary thrust (MBT). 1: Indian upper (a) and lower (b) crust; 2: Asian lithosphere: (a) continentalcrust, (b) lithospheric mantle; 3: scraped o¡ and accreted Indian margin; 4: erosion; 5: thrust (a) and normal (b) faults.

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The Asian lithospheric mantle (if any) wascompletely replaced by the Indian crust (Fig. 9g)and sank into the asthenosphere. This caused ad-ditional uplift of Tibet and Miocene crustal-de-rived leucogranites in the Himalayan Belt[33,34]. Both thickening of the Himalayan crustfrom below and erosion from above caused pro-gressive exhumation of high-pressure/low-temper-ature rocks uplifted from ca. 100 km to shallowlevels during earlier stages (Fig. 9b). Exhumation¢rst occurred in areas where uplift and/or erosionrates were fastest, apparently at the western ter-mination of the Himalayas, where high-pressure(HP) rocks are preserved [38^40]. In the tectoni-cally similar North Himalayan domes the materialscraped from the Indian margin and accreted atshallow depths has been pushed up, but the HProcks are not yet exposed, or are not preserved.

The present stage (Fig. 9h) corresponds to theINDEPTH seismic pro¢le [5]. In this ¢gure weindicate the MBT as the major active thrust sinceca. 10 Ma [41] merging on the crustal scale withthe MCT. The formation of the MBT does notcorrespond to new failure of the whole Indiancrust; it is a splay fault resulting from scrapingo¡ and accretion of the crustal/sedimentary mate-rial under and in front of the MCT.

6. Discussion

The amount of convergence in the model since55 Ma is 1000 to 1500 km, which corresponds tothe available estimates (see [42] for review). Sub-duction of so much continental lithosphere is mul-tiphasic and includes the following major events:two break-o¡s (Fig. 9b,e) ; one delamination (Fig.9d,e) and probably the onset of a second delami-nation (Fig. 9h); two rapid uplifts of the sub-ducted crustal slices (Fig. 9b,g).

We do not anticipate that natural subductionoccurred exactly in the same way, synchronouslypassing through the same stages over the s 2500km long Himalayan belt. The scenario proposedin Fig. 9 applies mostly to the Central Himalayasand is in agreement with recent tomography evi-dence for two pieces of high velocity materialunder India (Fig. 10a) interpreted as subductedand detached lithosphere [4]. The deepest portioncould correspond to the Tethys oceanic litho-sphere detached some 45 Ma ago (Fig. 9b) andthe shallower one to the Indian lithospheric man-tle detached ca. 25 Ma ago (Fig. 9e). A roll-overgeometry of the subducted lithosphere with thedeeper portion overturned and dipping south-wards (Fig. 10) can be explained by the fact

Fig. 10. Position of the lithosphere subducted under the Himalayas: drawings based on the tomographic images by Van der Vooet al. [4].

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that Asia was apparently shortened NS by about1000 to 1500 km since continental subductionstarted [43,44]. The subduction in front of theHimalayas has been respectively shifted over thisdistance to the north with respect to the mantlewhich is supposed to be ¢xed.

The tomography section across the western Hi-malayas displays only one piece of detached litho-sphere (Fig. 10b), which could be identi¢ed as theTethys slab. The vertical Indian lithospheric man-tle is not detached and is bent to the south at 670km depth transition zone. It is unclear whetherthis layer has ¢rst underplated Asia and then de-laminated and peeled-back to its present positionbefore the forthcoming break-o¡, or the separa-tion between the mantle and crustal layers oc-curred as in exp. 2 (Fig. 6).

According to the model crustal thickening anduplift of Tibet was heterogeneous and ¢rst propa-gated to the north during tectonic underplating ofthe Indian crust under Asia (Powell and Cona-ghan [45] and Fig. 9c,d). Underplating shouldhave started after a rapid rise of the HP rocksand break-o¡ (Fig. 9b) some 45 Ma ago. Delami-nation and peel-back of the Indian lithosphericmantle (Fig. 9d,e) caused southward propagatinguplift ampli¢ed by the replacement of the Asianlithospheric mantle by the Indian crust (Fig. 9f).This process very schematically presented in Fig.9f is hypothetical. Its possibility and style (mech-anism) should be tested numerically. Rise of theoverriding plate continued due to further tectonicunderplating of the Indian crust (Fig. 9h). Thus,Tibet uplift occurred through the whole Hima-layan collision, but the most important pulsewas Early Miocene (see Fig. 9d^f) as indicatedby geological records (see [22] for review). Ac-cording to the model this period corresponds tothe formation of the MCT (Fig. 9e), which is notreliably dated but it is generally agreed thatthrusting along this fault did not begin beforethe late Oligocene to early Miocene (e.g. [46]).Normal faulting along the STD (Fig. 9g) startedapproximately at the same time or somewhat later[47,48].

The proposed model is two-dimensional andhas an undeformable overriding plate. The modelis thus not designed to explain the Asian tecton-

ics. Nevertheless, it provides ideas about the evo-lution of the e¡ective strength and stress regime inthe Asian lithosphere. The ¢rst conclusion thatstems from modelling is that the strength of thislithosphere decreased from stage Fig. 9e until nowdue to thickening of its weak crustal layer andremoval of the mantle layer (Fig. 9f). The NScompression within the Asian plate evolved asfollows: initial continental subduction (Fig.9a,b) was characterised by low-compressional re-gime and therefore compression of Asia wassmall. After break-o¡ (Fig. 9c), 40^45 Ma agothe regime switched to a highly compressionalmode and, therefore, the Asian plate was sub-jected to forceful compression, but was probablystill too strong to fail. Compression then gradu-ally increased during tectonic underplating of theIndian lithosphere and at 30^35 Ma became su¤-cient to cause major failure within the Asian litho-sphere, leading to the formation of major strikeslip faults (e.g. the Aliao Shan Fault) [49,50].Starting from stage Fig. 9e (20^25 Ma), the Asianlithosphere became considerably weaker while thesubduction regime remained highly compressionaluntil now. The intense deformation and failure ofdi¡erent modes (thrust and strike-slip) of thislithosphere is thus very likely. Indeed, alongwith the mentioned events (uplift of Tibet, forma-tion of MCT and STD etc.) the 20^25 Ma periodis characterised by the initiation of the majorAsian faults such as the Altyn-Tagh and Kun-Lun [9]. This is also the time of lithospheric-scalefailure and initiation of intracontinental subduc-tion in the Pamir, Qilian Shan and Tian-Shan(e.g. [51]).

7. Concluding remarks

The geopoem is consistent to a ¢rst approxima-tion with geological and geophysical data.Although this model looks complex (Fig. 9), itis a very simpli¢ed two-dimensional representa-tion of what may have happened in the Hima-layas. Thermo-mechanical modelling shows towhich extent the available data are limited andthey are far from being su¤cient to reconstructthe geodynamic evolution of a mountain belt over

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a long time. This evolution occurs on the litho-spheric scale and involves complex interactionbetween all lithospheric layers, which cannot beaddressed without geodynamic modelling. Model-ling, however remains limited by the complexityand ambiguity of initial and boundary conditions.A further step in the modelling of continentalsubduction in the Himalayas consists in incor-porating three dimensionality with deformableAsia.

Acknowledgements

We thank R. Van der Voo for providing un-published tomography data, and C. Burch¢el andM. Harrison for the constructive review. Thiswork was supported by 2HIM Himalaya(CNRS/INSU) programme. This is contributionnumber 274 of Geosciences Azur.[RV]

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Chemenda et al. (2000)