Earth and Planetary Science Letters 174 (2000) 397^409
www.elsevier.com/locate/epsl
Evolutionary model of the Himalaya^Tibet system: geopoem
based on new modelling, geological and geophysical data
Alexander I. Chemenda a; *, Jean-Pierre Burg b , Maurice Mattauer c
a
c
Gëosciences Azur, UMR 6526, Universitë de Nice-Sophia Antipolis et CNRS, 250 Rue Albert Einstein, Sophia Antipolis,
06560 Valbonne, France
b
Geologisches Institute, ETH, Zurich, Switzerland
Laboratoire de Gëophysique, Tectonique et Sedimentologie, UMR 5573, Universitë 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. 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 Pëclet 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. ß 2000 Elsevier Science B.V. All rights reserved.
Keywords: plate collision; tectonics; subduction; physical models; Himalayas; Xizang China; exhumation
1. Introduction
* Corresponding author. Tel.: +33-4-92-94-26-61;
Fax: +33-4-92-64-26-10; E-mail: [email protected]
The amount of Indian continental lithosphere
consumed 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
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A.I. Chemenda et al. / Earth and Planetary Science Letters 174 (2000) 397^409
leomagnetic data to be about 1000 km [1,2] and
may even reach 1500 km [3]. Recent tomography
data [4] are consistent with the latter estimates,
revealing high velocity zones that can be interpreted as segments of oceanic lithosphere and
continental lithospheric mantle subducted down
to 1000 and 1700 km, respectively. It is unclear,
however, what is the fate of the buoyant continental crust, which is generally believed to be unable to subduct to these depths. INDEPTH seismic data display the Indian crust underthrust
under the Nepalese Himalayas to about 80 km
depth [5]. Eclogites in the western Himalayas indicate that the Indian crust had already reached a
100 km depth equivalent (25^30 kbar) at the beginning of collision, ca. 50^55 Ma ago [6,7]. Since
then, hundreds of kilometers of the Indian crust
have been subducted along the Himalayas. The
major challenge now is to understand how this
subduction has developed and what is the style
of the associated lithospheric deformation. This
can be done only on a large scale, considering a
long-evolving thermo-mechanical system `layered
lithosphere^asthenosphere'. Such a scale is hardly
accessible to geological analysis. Geophysics provide meaningful structural information about the
deep lithosphere and mantle but only about the
present situation. Experimental and/or numerical
modelling thus could play an important role in
understanding past events. Modellers (e.g. [8^
12]) usually investigate simpli¢ed models that cannot consider fundamental factors such as multiphase (non-stationary) subduction of a large
amount of continental lithosphere. Initial stages
of this process from subduction of the continental
margin to a rapid uplift/exhumation of deeply
subducted crust have been experimentally modelled by Chemenda et al. [13,14]. These stages
scaled to nature should take place over a few to
several million years. The evolution of the Himalaya^Tibet collisional system lasted much longer,
50 to 60 Myr during which a number of other
important events may have happened. What are
these events and what is their mechanism? Considering the time-scale of the Himalayan history,
these questions can be addressed by thermo-mechanical modelling that somehow takes into account changes in the mechanical behaviour of
the lithosphere during subduction. The necessity
of thermo-mechanical modelling drastically complicates the task and dooms any modelling results
to ambiguity. We barely know how the mineralogical composition and rheologic structure of
the subducting continental plate change during
subduction. These changes strongly depend on a
number of ill-understood factors such as the kinetics and spatial distribution of mineralogical reactions [15], changes of the grain size that
strongly a¡ects the rheology, the e¡ect on rheology of £uids and the multi-aggregate nature of
rocks (e.g. [16]) etc. Therefore, modelling can represent only a qualitative approximation, even if
one adopts precise rheologic laws. In this work
we attempt simpli¢ed thermo-mechanical experimental modelling of continental subduction under
di¡erent `reasonable' conditions. The obtained
scenarios of initial and mature continental subduction tested against the geological information
allowed us to propose a ¢rst order physically and
geologically consistent model for the evolution of
the Himalaya^Tibet system.
2. Mechanical essentials of continental subduction
Previous purely mechanical experiments have
revealed two principal factors de¢ning the subduction regime: (1) the strength of the subducting
continental crust (upper crust, Figs. 1 and 2) the
ratio pull/buoyancy force 6 = Fpl /Fb [13,14].
The pull force Fpl acting on the subducting continental lithosphere is composed of the traction
from both the subducted oceanic lithosphere and
the lithospheric mantle of the continental lithosphere. The latter is proportional to both the
size (thickness and length in two dimensions) of
the subducted segment as well as to the density
contrast between the continental and the surrounding mantle. The buoyancy force Fb is proportional to both the thickness and subduction
depth of the continental crust as well as to the
mantle/crust density contrast. For example, an increase in the 6 ratio can be achieved by reduction
of the crustal thickness. A decreasing 6 ratio can
be brought about, in particular, by lowering the
lithospheric mantle density. However, the major
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399
Fig. 1. Scheme of the modelling. 1, overriding plate; 2, upper continental crust with strong strain weakening; 3, ductile, weak
lower crust; 4 plastic mantle layer; 5, piston; 6, liquid asthenosphere; 7, bath. Ts and Tm are the temperatures at the model surface and in the asthenosphere, respectively (in previous purely mechanical modelling discussed in this paper Ts = Tm ). hf is the
lower crust thickness in the frontal part of the subducting plate.
factor causing strong and rapid drop in 6 is
break-o¡ of the dense subducted slab. After
break-o¡ only the mantle layer of the subducting
continental lithosphere can provide the pull force,
if this layer is denser than the surrounding mantle.
Therefore, the largest pull force is exerted at the
beginning of continental subduction whilst the
earlier subducted oceanic slab is still attached to
the continental lithosphere. Initial continental
subduction should be characterised also by comparatively low crustal thickness of the continental
margin. Thus, the 6 ratio should be maximal at
this stage.
Purely mechanical experiments corresponding
to this situation exhibits the following subduction
pattern (Fig. 2): The continental crust subducts to
a depth equivalent to 200^300 km, following the
oceanic subduction. The upper continental crust
then fails near the base of the overriding plate
(Fig. 2a) and the subducted crustal slice rapidly
raises along and within the interplate zone (Fig.
2b). This buoyancy-driven uplift brings material
from several tens to more than 100 km depth
equivalent to shallower levels. The depth to which
the deeply subducted crust is spontaneously uplifted is proportional to the depth of its initial
subduction, which in turn is proportional to the
strength of the upper crust. Uplift of the crustal
slice is followed by break-o¡ of the previously
subducted oceanic lithosphere with part of the
continental lithospheric mantle (Fig. 2b). Breako¡ reduces the pull force, which results in 1^2 km
equivalent isostatic uplift of the lithosphere in
the subduction zone. Another consequence of
the break-o¡ in the experiments conducted at
constant convergent rate, imposed by the moving
piston (Fig. 1), is an increase in horizontal compression of the lithosphere. Accordingly, the
subduction regime was termed low-compressional
before, and highly compressional after break-o¡
[14]. The geometry and dynamics of subduction
after break-o¡ still depend on the 6 ratio, which
is obviously smaller than before break-o¡. The
subduction angle initially is rather steep, 30³ to
60³ even if the subducting continental lithosphere
is denser than the asthenosphere (Fig. 3a). Then,
the subducted lithosphere bends and rotates upwards to come against the base of the overriding
plate (Fig. 3b). Horizontal subduction/underplating of this lithosphere follows (Fig. 3c). At a stage
that depends on the upper crust strength and on
friction between the underplating crust and the
Fig. 2. Low-compressional regime of continental subduction
(high pull force), Chemenda et al. [14]. Symbols as in Fig. 1.
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count the Pëclet criterion:
VH=U ˆ const
Fig. 3. Very highly-compressional regime of continental subduction (no pull force). Symbols as in Fig. 1.
overriding plate-sole, the upper crust fails in front
of the subduction zone (Fig. 3c). In the conducted
experiments the maximal underplating distance L
was 7^8 cm corresponding to 250^300 km in nature.
The depth of crustal subduction at which buoyancy-driven lithospheric bending and rotation occurs (Fig. 3b) is proportional to 6. When the 6
ratio and the strength of the upper continental
crust are such that crustal failure occurs before
bending of the subducted lithosphere, convergence proceeds as in Fig. 4. Note that failure in
Fig. 4b occurs in front of the subduction zone
(highly compressional regime). For the crust to
fail at depth 6 should be higher (low-compressional regime, Fig. 2). Application of erosion to
the growing relief modi¢es the deformation as
shown in Fig. 4c,d. Break-o¡ of the mantle lithospheric layer (Fig. 4d) results in the horizontal
subduction shown in Fig. 3. We now present thermo-mechanical modelling of this process.
3. Set up of thermo-mechanical modelling
The experimental setting (Fig. 1), model materials, techniques and mechanical similarity criteria
are those used in the experiments described above.
The principal di¡erence is the introduction of
thermo-mechanical elements. Along with purely
mechanical similarity criteria, we take into ac-
…1†
where V is the subduction rate; H is the thickness
of the plate, and U is the thermal di¡usivity of the
lithosphere that controls the heating (thermal
equilibration) rate of the subducted material.
The conductive di¡usivity of the lithosphere is
about U³ = 1036 m2 /s [17], but the actual di¡usivity of the subducted material (crust in particular)
could be higher, due to the advective heat transfer
by £uids. The thermal di¡usivity of materials used
to model lithosphere is Um W8U1038 m2 /s (here
and below superscripts `m' and `o' indicate the
model and original parameters, respectively). Let
us assume that the geological convergence rate is
of the order of 1 cm/yr and the crustal thickness is
of the order of 10 km. Then, adopting a model
crustal thickness Hm = 1 cm, we obtain from criterion (1) that the model convergence rate should
be Vm = 8U1035 m/s. For higher e¡ective di¡usivity of natural lithosphere, the model convergence
rate should be slower. In the experiments presented below we used VW4U1035 m/s.
The realistic thermal regime of the subducting
plate is not su¤cient, however, for thermo-mechanical modelling. Adequate changes in lithospheric rheology and crustal density during subduction are also needed. The e¡ective strength of
the crust should considerably reduce while it descends into the asthenosphere. The reduction factor is indeterminate. In the experiments presented
here we have assumed that it is about 5. The
analogue material strength is very sensitive to
temperature, so that ¢ve times reduction of the
strength is achieved through raising temperature
by 2^3³C. On the other hand, the materials density remains virtually constant. The mechanical
experiments above were conducted under a homogeneous temperature near 40³C with a rheologic
contrast between the lithospheric layers caused by
di¡erent layer compositions. In thermo-mechanical experiments (Fig. 1) the model is subjected to
a linear, stationary temperature gradient with surface temperature of 39³C or 40³C and 42³C under
the lithosphere. The temperature slightly increases
with depth in the `mantle' and is 0.2³C higher at
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401
Fig. 4. Highly-compressional regime of continental subduction (low pull force) after Chemenda et al. [13]. (a), (b) and (e) Successive stages of continental subduction in experiments without `erosion'. (a)^(d) Continental subduction in experiments with erosion. 1, overriding plate; 2, upper crust; 3, lower crust, 4, eroded material (sediments).
the bottom of the bath than at the base of the
horizontal lithospheric layer. During subduction,
upper lithosphere layers are heated and, therefore,
weakening.
4. Results
Twelve experiments have been conducted under
di¡erent conditions. We present three representative tests (see Table 1 for parameter values).
Experiment 1, Fig. 5: As in the purely mechanical experiment (Fig. 3b), the subducted slab
bends upward to the base of the overriding plate
(Fig. 5c) with continental subduction becoming
horizontal (Fig. 5d). The subducting crust fails
in front of the subduction zone, tectonic underplating of the crust then being relayed by crustal
accretion and thickening in front of the subduction zone (Fig. 5d^f). Between stages (e) and (f)
two competing processes have been observed :
separation (delamination) of the mantle layer
from the crust at the front of the subducted crust
and failure in the hinge zone of the mantle layer.
Finally, failure and break-o¡ of the lithospheric
mantle occurred (Fig. 5f). To facilitate delamination (reduce the coupling between the upper crust
and the mantle), there are at least two possibilities : one is to reduce the strength of the lower
crust by using initially weaker or more temperature sensitive material for this layer. Another
possibility is to use the same material, but to increase 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 and
the lithospheric mantle split apart from the beginning of subduction, the crust wedges along the
base of the overriding plate while the dense mantle layer subducts almost vertically at late stage.
This layer undergoes considerable down-dip
stretching during sinking but deformation does
not localise to cause break-o¡ before the slab-tip
reaches the bottom of the bath, when convergence
is stopped.
These two experiments show that continental
subduction is very sensitive to the rheologic structure of the crust. Small variations in lower crust
thickness change drastically the bulk behaviour. It
seems obvious that tuning this parameter between
the two tested values will result in an intermediate
scenario; i.e. subduction starting with horizontal
tectonic underplating (as in Fig. 5d,e) and followed by delamination and break-o¡. Other parameters also a¡ect the result. For example, if the
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strength of the mantle layer of the underplated/
subducted lithosphere in Expt. 1 (Fig. 5) was
higher, then one would expect backward delamination and peeling of this layer instead of rupture.
Break-o¡ would occur later, when the length of
the sinking mantle lithosphere is long enough to
generate a su¤cient pull force for breaking this
strong layer. Such a process is observed in the
next experiment where the same lithospheric model as in Expt. 1 is tested under lower temperature
and hence with higher strength of the lithospheric
layers (Table 1).
Experiment 3, Fig. 7: The initial stages of this
Fig. 6. Experiment 2: Tectonic underplating of the continental crust delamination-break o¡ (model parameters in Table
1).
Fig. 5. Experiment 1: Tectonic underplating of the continental lithosphere break o¡ (model parameters in Table 1).
experiment are similar to Expt. 1, but the subducting lithospheric mantle separates from the
tectonically underplated crust and peels back to
the subduction front (Fig. 7f). This delamination
is followed by break-o¡ of the mantle layer, which
could not be achieved because the sinking mantle
layer reached the bottom of the box. The continental crust underplated the overriding plate (Fig.
7f) over 10 cm (V350 km in nature) and did
not fail because it is colder and stronger than in
Expt. 1.
One can test many other parameter combinations, but the presented experiments provide
enough information to predict other possible options for lithospheric/crustal behaviour: the principal elements are delamination, roll-back and
break-o¡ of the lithospheric mantle layer. The
distance L of tectonic underplating depends on
the crustal strength. Weakening of the crust with
depth/temperature results in shorter L values. As
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403
we do not know the actual weakening factor we
cannot predict the underplating distance in nature. The peel-back distance of the mantle layer
is particularly controlled by the lower crust properties. This process can propagate to the subduction zone (Fig. 6) or mid way (Fig. 7) or go farther below the continent (we obtained such a
result in experiments not presented in the paper).
To complete the scope of continental subduction scenarios we remind below one more relevant
result from purely mechanical modelling [18]. This
modelling takes into account the existence of a
thin and weak lithosphere in the arc area and
shows subduction of most of the fore arc block
(Fig. 8a). Evidences for this process have been
found in the Urals and the Variscan belt [19],
the Kamchatka [20], Taiwan [18,21] and the Himalayas [22,23]. The fore arc block shields from
the hot mantle the deeply subducted continental
crust (Fig. 8d) which, therefore, is kept relatively
cold and hence strong. We did not yet apply thermo-mechanical modelling technique to arc-continent collision and will consider below that the
subducting continental crust behaves at this stage
as in isothermal experiments (Fig. 2).
5. Evolutionary model of the Himalayas
We use the physically possible subduction scenarios presented above to construct an evolutionary model for the Himalaya^Tibet collisional system (Fig. 9).
The Indian continental margin arrived against
Asia 55 to 60 Ma ago [24], resulting in arc-con-
Fig. 7. Experiment 3: Tectonic underplating of the continental lithosphere: peeling of the lithospheric mantle break o¡
(model parameters in Table 1).
Table 1
Model parameters
Ts
(³C)
Experiment 1 40
Experiment 2 40
Experiment 3 39
Tm
(³C)
csl
(Pa)
cs1
(Pa)
cs2
(Pa)
bl
bc1
bc2
ba
Hl
(g/cm3 ) (g/cm3 ) (g/cm3 ) (g/cm3 ) (cm)
Hc1
(cm)
Hc2
(cm)
hf
V
(cm) (m/s)
42
42
42
13
13
18
38
38
53
0.5
0.5
0.7
1.03
1.03
1.03
8
8
8
2
2
2
0.4
0.8
0.8
0.86
0.86
0.86
0.86
0.86
0.86
1
1
1
12
12
12
4U1035
4U1035
4U1035
Ts and Tm are the temperatures at the surface and the base of the lithospheric model before deformation. csl , cs1 ,cs2 are the
average 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 is
the 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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Fig. 8. Purely mechanical model of continental subduction
during arc^continent collision after Chemenda et al. [18].
tinent collision. In the central Himalayas the arc
was of Andean-type [25], while in the NW Himalayas, the Ladakh and Kohistan arcs were intraoceanic [26,27]. We do not aim to discuss in this
paper the arc continent collision along the Himalayas and simply show in Fig. 9a a possible consequence of this process: the core (mainly mantle)
part of the fore arc subducted into the mantle.
During subduction of the Indian margin under
this block the margin upper layers were partly
scraped o¡ and accreted to the overriding lithosphere at di¡erent depths within the interplate
suture zone. A large part of the continental Indian crust, however, subducted to 200^250 km
depth, following the Tethys oceanic lithosphere
(Fig. 9a).
The crust then failed at several tens to about
100 km depth and rapidly moved upward between
the two plates (Fig. 9b). The upward-moving
crustal slice pushed up previously underplated
crustal/sedimentary material squeezed in the interplate zone and now draping gneiss domes pervasive in the western Himalaya [28] and possibly
represented among culminations of the South Tibetan North Himalayan belt [29]. Rapid uplift
then stopped and was followed by slab break-o¡
(Fig. 9b).
Subduction then switched to a highly compressional mode with tectonic underplating of the
whole Indian lithosphere beneath Asia that underwent a ¢rst isostatic uplift (Fig. 9c,d), apparently
in the Eocene, when oceanic sedimentation ceased
on the Indian lithosphere (e.g. [30]) and residual
basins persisted along the suture. The underthrust
Indian crust was heated and thus softening. The
physical state, the rheology and the mineralogical
composition of this crust are not clear : Part of the
crust, more likely the lower crust, was eclogitised
and became denser than the mantle. The upper
crust has likely been molten. After a few to several hundreds kilometers of tectonic underplating,
the mantle layer (probably together with the lower crust) started to delaminate from the crust and
to peel-back to the subduction zone (Fig. 9d).
This process resulted in a new phase of isostatic
uplift of the overriding plate (Tibet) and was terminated by a second break-o¡ (Fig. 9e).
A second crust failure occurred in front of the
subduction zone/mountain belt (Fig. 9e, highly
compressional subduction regime). The major
thrust fault formed at this time was the main central thrust (MCT). The low-viscosity Indian crust
below Asia, in direct contact with the hot asthenosphere, became less viscous and started to intrude the hot dense Asian lithospheric mantle
(Fig. 9e).
This southward propagating process (Fig. 9f) is
consistent with mantle-derived heat input inferred
to promote late Oligocene-early Miocene crustal
anatexis in the arc region (e.g. [29,31]) and along
the Himalayas [32,33^35]. The new subduction
front was the MCT. Underplating along this fault
further increased the crustal thickness below the
Himalayas and resulted in the formation of high,
intensively eroded relief. Erosion triggered both
the exhumation of the Crystalline Himalayas
and the formation of the South Tibetan normal
fault system [36,37]. Subduction of the Indian
lithosphere was again horizontal and resulted in
further thickening of the crust under Tibet by intensively deforming its very weak (molten ?) lower
part and by adding new portions of the Indian
crust.
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405
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. Failure of the subducted crust at depth of ca. 100 km.; (b) Rapid uplift of the subducted crustal slice to a few to several tens of
kilometers depth. Break-o¡ of the Indian mantle with the attached previously subducted oceanic lithosphere; (c) Heating, weakening and uplift of the remaining in the mantle crustal segment of the Indian margin; (d) Underplating of the Indian continental
lithosphere under Asia and initiation of delamination of the Indian lithospheric mantle; (e) Failure of the Indian crust in front
of 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) continental
crust, (b) lithospheric mantle; 3: scraped o¡ and accreted Indian margin; 4: erosion; 5: thrust (a) and normal (b) faults.
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Fig. 10. Position of the lithosphere subducted under the Himalayas: drawings based on the tomographic images by Van der Voo
et al. [4].
The Asian lithospheric mantle (if any) was
completely replaced by the Indian crust (Fig. 9g)
and sank into the asthenosphere. This caused additional uplift of Tibet and Miocene crustal-derived leucogranites in the Himalayan Belt
[33,34]. Both thickening of the Himalayan crust
from below and erosion from above caused progressive exhumation of high-pressure/low-temperature rocks uplifted from ca. 100 km to shallow
levels during earlier stages (Fig. 9b). Exhumation
¢rst occurred in areas where uplift and/or erosion
rates were fastest, apparently at the western termination of the Himalayas, where high-pressure
(HP) rocks are preserved [38^40]. In the tectonically similar North Himalayan domes the material
scraped from the Indian margin and accreted at
shallow depths has been pushed up, but the HP
rocks are not yet exposed, or are not preserved.
The present stage (Fig. 9h) corresponds to the
INDEPTH seismic pro¢le [5]. In this ¢gure we
indicate the MBT as the major active thrust since
ca. 10 Ma [41] merging on the crustal scale with
the MCT. The formation of the MBT does not
correspond to new failure of the whole Indian
crust; it is a splay fault resulting from scraping
o¡ and accretion of the crustal/sedimentary material under and in front of the MCT.
6. Discussion
The amount of convergence in the model since
55 Ma is 1000 to 1500 km, which corresponds to
the available estimates (see [42] for review). Subduction of so much continental lithosphere is multiphasic 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 delamination (Fig. 9h); two rapid uplifts of the subducted crustal slices (Fig. 9b,g).
We do not anticipate that natural subduction
occurred exactly in the same way, synchronously
passing through the same stages over the s 2500
km long Himalayan belt. The scenario proposed
in Fig. 9 applies mostly to the Central Himalayas
and is in agreement with recent tomography evidence for two pieces of high velocity material
under India (Fig. 10a) interpreted as subducted
and detached lithosphere [4]. The deepest portion
could correspond to the Tethys oceanic lithosphere detached some 45 Ma ago (Fig. 9b) and
the shallower one to the Indian lithospheric mantle detached ca. 25 Ma ago (Fig. 9e). A roll-over
geometry of the subducted lithosphere with the
deeper portion overturned and dipping southwards (Fig. 10) can be explained by the fact
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that Asia was apparently shortened NS by about
1000 to 1500 km since continental subduction
started [43,44]. The subduction in front of the
Himalayas has been respectively shifted over this
distance to the north with respect to the mantle
which is supposed to be ¢xed.
The tomography section across the western Himalayas displays only one piece of detached lithosphere (Fig. 10b), which could be identi¢ed as the
Tethys slab. The vertical Indian lithospheric mantle is not detached and is bent to the south at 670
km depth transition zone. It is unclear whether
this layer has ¢rst underplated Asia and then delaminated and peeled-back to its present position
before the forthcoming break-o¡, or the separation between the mantle and crustal layers occurred as in exp. 2 (Fig. 6).
According to the model crustal thickening and
uplift of Tibet was heterogeneous and ¢rst propagated to the north during tectonic underplating of
the Indian crust under Asia (Powell and Conaghan [45] and Fig. 9c,d). Underplating should
have started after a rapid rise of the HP rocks
and break-o¡ (Fig. 9b) some 45 Ma ago. Delamination and peel-back of the Indian lithospheric
mantle (Fig. 9d,e) caused southward propagating
uplift ampli¢ed by the replacement of the Asian
lithospheric mantle by the Indian crust (Fig. 9f).
This process very schematically presented in Fig.
9f is hypothetical. Its possibility and style (mechanism) should be tested numerically. Rise of the
overriding plate continued due to further tectonic
underplating of the Indian crust (Fig. 9h). Thus,
Tibet uplift occurred through the whole Himalayan collision, but the most important pulse
was Early Miocene (see Fig. 9d^f) as indicated
by geological records (see [22] for review). According to the model this period corresponds to
the formation of the MCT (Fig. 9e), which is not
reliably dated but it is generally agreed that
thrusting along this fault did not begin before
the late Oligocene to early Miocene (e.g. [46]).
Normal faulting along the STD (Fig. 9g) started
approximately at the same time or somewhat later
[47,48].
The proposed model is two-dimensional and
has an undeformable overriding plate. The model
is thus not designed to explain the Asian tecton-
407
ics. Nevertheless, it provides ideas about the evolution of the e¡ective strength and stress regime in
the Asian lithosphere. The ¢rst conclusion that
stems from modelling is that the strength of this
lithosphere decreased from stage Fig. 9e until now
due to thickening of its weak crustal layer and
removal of the mantle layer (Fig. 9f). The NS
compression within the Asian plate evolved as
follows: initial continental subduction (Fig.
9a,b) was characterised by low-compressional regime and therefore compression of Asia was
small. After break-o¡ (Fig. 9c), 40^45 Ma ago
the regime switched to a highly compressional
mode and, therefore, the Asian plate was subjected to forceful compression, but was probably
still too strong to fail. Compression then gradually increased during tectonic underplating of the
Indian lithosphere and at 30^35 Ma became su¤cient to cause major failure within the Asian lithosphere, leading to the formation of major strike
slip faults (e.g. the Aliao Shan Fault) [49,50].
Starting from stage Fig. 9e (20^25 Ma), the Asian
lithosphere became considerably weaker while the
subduction regime remained highly compressional
until now. The intense deformation and failure of
di¡erent modes (thrust and strike-slip) of this
lithosphere is thus very likely. Indeed, along
with the mentioned events (uplift of Tibet, formation of MCT and STD etc.) the 20^25 Ma period
is characterised by the initiation of the major
Asian faults such as the Altyn-Tagh and KunLun [9]. This is also the time of lithospheric-scale
failure and initiation of intracontinental subduction in the Pamir, Qilian Shan and Tian-Shan
(e.g. [51]).
7. Concluding remarks
The geopoem is consistent to a ¢rst approximation with geological and geophysical data.
Although this model looks complex (Fig. 9), it
is a very simpli¢ed two-dimensional representation of what may have happened in the Himalayas. Thermo-mechanical modelling shows to
which extent the available data are limited and
they are far from being su¤cient to reconstruct
the geodynamic evolution of a mountain belt over
EPSL 5319 21-12-99
408
A.I. Chemenda et al. / Earth and Planetary Science Letters 174 (2000) 397^409
a long time. This evolution occurs on the lithospheric scale and involves complex interaction
between all lithospheric layers, which cannot be
addressed without geodynamic modelling. Modelling, however remains limited by the complexity
and ambiguity of initial and boundary conditions.
A further step in the modelling of continental
subduction in the Himalayas consists in incorporating three dimensionality with deformable
Asia.
Acknowledgements
[8]
[9]
[10]
[11]
[12]
We thank R. Van der Voo for providing unpublished tomography data, and C. Burch¢el and
M. Harrison for the constructive review. This
work was supported by 2HIM Himalaya
(CNRS/INSU) programme. This is contribution
number 274 of Geosciences Azur.[RV]
[13]
[14]
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