Earth-Science Reviews 59 (2002) 101 – 123
www.elsevier.com/locate/earscirev
Shortening budgets and the role of continental subduction
during the India–Asia collision
M.R.W. Johnson *
Department of Geology and Geophysics, Grant Institute, University of Edinburgh, The King’s Buildings,
West Mains Road, Edinburgh, EH9 3JW, UK
Received 26 April 2001; accepted 27 November 2001
Abstract
The India – Asia collision provides a remarkable example of diffused deformation because the zone of compressive strain
extends far into the Hinterland of the Himalaya. Molnar and Tapponnier’s [Science 189 (1975) 419] concept of India as a rigid
indenter responsible for the diffused deformation has been widely accepted, perhaps uncritically. This review gives a reappraisal
of landmark papers by Dewey et al. [Philos. Trans. R. Soc. London, A 327 (1988) 379; Eclogae Geol. Helv. 82 (1989) 717]
which discussed the shortening budget in SE Asia and the evidence for the amount and the rate of the convergence between
India and Asia taking place after the initial collision of these continents at roughly 55 – 50 Ma. Evidence for the total
convergence comes from magnetic anomaly, palaeomagnetic and volumetric balancing studies. The results are consistent in
showing about 1800 – 2100 km convergence between India and Asia in the western sector, 2475 km in the central and 2750 –
2800 km in the eastern sector. The rate of convergence between India and Asia over the period since initial collision is roughly 5
cm/year. However, the possible long-term rate of convergence within the Himalaya is considerably less, i.e. 1.5 – 2.0 cm/year, a
fraction of the total convergence. This shortfall can be made up by adding to the rate for the Himalaya alone the convergence
rates in other parts of the diffused zone of compressive strain, i.e. Tibet, Tien Shan, Altai. This works well for the postOligocene but not for the earlier post-collisional history when the areal extent of thrust/fold tectonics was much more limited.
Possibly, the apparent shortening deficit before the Miocene was made up by strike – slip faulting but with the notable exception
of the Red River Fault evidence for widespread pre-Miocene strike – slip faults is not convincing. Summation of the amount of
shortening in thrust/fold regimes in the Himalaya and its Hinterland falls short of that required by the total convergence figures
quoted above. The deficit arises firstly because of the likelihood that pre-collisional crustal shortening (of uncertain areal extent)
in Tibet reduces Tibet’s contribution ( > 40% according to Dewey et al. [Philos. Trans. R. Soc. London, A 327 (1988) 379;
Eclogae Geol. Helv. 82 (1989) 717]) to the shortening budget for the Cenezoic. Revised shortening budgets are presented on the
assumption that most of the Lhasa Block (Southern Tibet) was already thickened prior to collision: this thickening is difficult to
quantify. Secondly, balanced cross-sections from various parts of the Himalaya give 60 – 70% shortening, that is only about 500
km, roughly half of the shortening proposed by Dewey et al. The conclusion of Le Pichon et al. [Tectonics 11 (1992) 1085] that
only Indian mid/upper crust is involved in the Himalayan thrust sheets is supported but it needs to be reassessed in the light of
the recognition of Eocene – Oligocene thickening in the Himalaya. This early crustal thickening antedates the sequence of
thrusts which started with the Main Central thrust and therefore the shortening estimates based on balanced cross-sections
probably underestimate the total shortening in the Himalaya. The prospect of substantial underthrusting of India beneath Tibet
minimizes the amount of horizontal shortening of Tibetan lithosphere during the Cenozoic. The conclusion is that either the
*
Fax: +44-131-6683184.
E-mail address: [email protected] (M.R.W. Johnson).
0012-8252/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 1 2 - 8 2 5 2 ( 0 2 ) 0 0 0 7 1 - 5
102
M.R.W. Johnson / Earth-Science Reviews 59 (2002) 101–123
total convergence deduced from geophysical and volumetric work is too large or the deficit must be made up by shortening
mechanisms other than thrust/fold regimes, i.e. strike – slip faulting which serves to move rocks out of the way of India.
D 2002 Elsevier Science B.V. All rights reserved.
Keywords: India – Asia collision; shortening budgets; continental subduction
1. Introduction
The India –Asia collision continues to attract the
attention of earth scientists from many disciplines
concerned with all levels upwards from the deep
structure to the atmosphere above the Himalaya and
its Hinterland (Fig. 1). Models devised for the origin of
crustal thickening in the Himalaya and SE Asia (Tibet,
Tien Shan, etc.; Fig. 2) fall into two main categories
(Fig. 3): (1) those in which the Indian lithosphere is
confined to the south of the Tsangbo suture within the
Himalaya, and (2) those in which the Indian lithosphere—either mantle or crust or both mantle and
crust—extends to the north of the suture so as to lie
under part or all of Tibet. This is an oversimplification
because few would now maintain that there has been no
underthrusting by Indian lithosphere. Rather the controversy centres on how much underthrusting has
occurred. Within each category, there is great diversity
of mechanism. Examples of the first category are:
(a) the indentation model, with India acting as a
rigid indenter, resulting in continental expulsion (Molnar and Tapponnier, 1975) or homogenous thickening
in Tibet and areas to the north (Dewey et al., 1988;
England and Houseman, 1988; Platt and England,
1993);
Fig. 1. Tectonic units of the Himalaya and adjoining regions. 1 – 10: Locations of balanced cross-sections listed in Table 1. MCT—Main Central
thrust. MBT—Main Boundary thrust. MFT—Main Frontal thrust. DR—Dras Volcanics; E—Everest; ITS—Indus Tsangpo Suture; NP—Nanga
Parbat; SP—Spongtang Ophiolite; TM—Tso Morari.
M.R.W. Johnson / Earth-Science Reviews 59 (2002) 101–123
103
Fig. 2. Tectonic map of SE Asia showing the major zones of compression resulting from the India – Asia collision. G—Ganges. I—Indus. S—
Sutlej. AT—Altyn Tagh Fault. HF—Herat Fault. MFT—Himalayan Main Frontal thrust. ITS—Indus—Tsangbo suture. LM—Longmen Shan.
NS—Nan Shan. PT—Pamir thrust. QB—Qaidam Basin. QF—Qinling Fault. RRF—Red River fault. SGS—Shansi Graben system. TB—Tarim
Basin. TFS—Tanlu Fault System.
(b) mantle delamination and roll-back (Willett and
Beaumount, 1994);
(c) migratory subduction of continental lithosphere
(Meyer et al., 1998).
Examples of the second category are (a) underthrusting of India beneath all of Tibet (Argand, 1924;
Powell, 1986; Powell and Conaghan, 1973; Beghoul
et al., 1993; Matte et al., 1997); (b) injection of Indian
crust into Tibetan crust, either in the form of a solid
‘piston’ (Zhou and Morgan, 1995) or as a viscous
fluid (Westerway, 1995). Zhou and Morgan’s hypothesis seemingly begs the question: in order to provide
the ‘weak’ lower crust needed for injection in Tibet
presumably it is first necessary to achieve major
crustal thickening, a feature which they are attempting
to explain! However, the weakening could have been
provided by the injection of magma by Andean-type
granite intrusion, at the base of the Tibetan crust
thereby increasing the temperature there.
An Yin and Harrison (2000) have given a recent
comprehensive review of the geological evolution of
the Himalayan – Tibetan orogen. The present review
considers the evidence for the amount and the rate of
the convergence between India and Asia taking place
after initial collision of these continents at roughly
55– 50 Ma. This topic is still highly controversial and
the principal theme of this review is to reappraise the
landmark papers of Dewey et al. (1988, 1989) in the
light of later developments. Evidence for the total
convergence comes from magnetic anomaly, palae-
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M.R.W. Johnson / Earth-Science Reviews 59 (2002) 101–123
Fig. 3. Models for the Himalaya and adjacent areas of SE Asia (redrawn from Willett and Beaumont, 1994). (A) Underthrusting of India
(Argand, 1924; Powell and Conaghan, 1973). (B) Injection Model (Zhou and Morgan, 1995). (C) Diffused Homogeneous Thickening with India
acting as a rigid indenter (Dewey et al., 1988). (D) Model C followed by the convective removal of lithospheric mantle under Tibet (England
and Houseman, 1988). (E) Subduction of Asian mantle and ‘‘roll back’’ of a south dipping subduction zone (Willett and Beaumont, 1994).
omagnetic and volumetric balancing studies. The
results are consistent in showing about 1800 –2100
km convergence between India and Asia in the western sector, 2475 km in the central and 2750– 2800 km
in the eastern sector. The rate of convergence between
India and Asia over the period since initial collision is
roughly 5 cm/year. However the possible long term
rate of convergence within the Himalaya is considerably less, i.e. 1.5 –2.0 cm/year, a fraction of the total
convergence.
2. The shortening budget
How much shortening has taken place across the
orogen (Besse et al., 1984)? Patriat and Achache (1984)
and Klootwijk et al. (1992) considered that the time
available for the shortening of Indo-Eurasian lithosphere since the completion of contact between the
continents was 55– 45 ma. Patriat and Achache (1984)
suggested that 700 F 300 km of shortening had taken
place over this time-span but this was based on an
M.R.W. Johnson / Earth-Science Reviews 59 (2002) 101–123
imprecise estimation of the latitude of the south margin
of Asia at the time of collision. Despite a great deal of
work, there is no consensus on the amount of shortening
in the Himalaya; 500– 1000 km is the range of estimates
but often at the upper end of this range, e.g. Dewey et al.
(1989) and Veevers et al. (1975) but some, e.g. Westerway (1995) go higher to perhaps as much as 1400 km.
2.1. Shortening estimates based on volumetric analyses
Le Pichon et al. (1992) have given the most thorough estimation of the volumetric evolution of the
Himalaya vis-a-vis Tibet and Asia. In their analysis,
the amount of convergence resulting from the India –
Asia collision is 2100 km in the west and 2800 km in
the east. Dewey et al. (1989) gave 1815 and 2750 km,
respectively. In their calculations, Le Pichon et al.
(1992) allowed for a pre-collision topography of
between 500 and 1000 m, citing evidence that Palaeogene red beds in Tibet indicate an arid climate at low
altitudes. However, if Murphy et al. (1997) are correct
in proposing that Tibet reached an altitude of 3– 4 km
in the Cretaceous prior to the collision (see later) and
if these elevations remained at the time of collision
then the conclusions of Le Pichon et al. (1992) are
greatly affected. The controversy over whether or not
all or part of Tibet was substantially above sea-level is
still unresolved (see later).
Using area balancing, Le Pichon et al. (1992)
calculated a total area loss of 3.75– 3.95 106 km2
assuming a base level of 500 m prior to collision. This
compares with their estimate of 5.7 106 km2 for the
area loss derived from kinematic considerations. These
authors assumed that Molnar’s (1987) estimated slip
rate on Himalayan thrusts of 18 F 7 mm/year can be
applied over the 45 Ma since collision in order to
predict a total shortening across the Himalaya of
810 F 315 km (‘‘kinematic shortening’’). However,
their volumetric analysis across the Himalaya and SE
Asia revealed a deficit in shortening of between 18 and
30 105 km2. The present crust in the Himalaya plus
the volume of sediment eroded from the Himalaya only
accounts for 106 km2 of shortening and if crust of
normal thickness (i.e. 38.5 km) was involved the
surface loss is only equivalent to 350 km of linear
shortening. They concluded that crust cannot be conserved in the Himalaya. If only 20 –25 km thick crust is
105
preserved in the Himalaya, then shortening increases to
2.5– 2.75 106 km2 equivalent to a linear shortening
of 600 –850 km, much more in accord with kinematic
shortening. Le Pichon et al. (1992) considered that
between one third and one half of continental surface
loss between India and Asia has occurred by lateral
extrusion and/or loss of lower crust to the mantle or by
its transfer northward under Tibet. They pointed out
that Lateral Extrusion was probably easier in the Upper
Eocene and Oligocene before Australia collided with
the Indonesian subduction zone, starting in the Upper
Miocene. These conclusions will be discussed further
later. In proposing that only the upper 20 –25 km of
Indian crust has been conserved in the orogen, Le
Pichon et al. (1992) echoed the views of Butler (1986)
who suggested that Indian lower crust is not preserved
in the Himalayan thrust sheets. Le Pichon et al. (1992)
considered other possible explanations for the apparent
crustal deficit in the Himalaya.
(1) Thinning of the northern margin of India prior
to collision (Dewey et al., 1989). They discount this
because they considered that there are no marine rocks
older that the Cenozoic in the Himalaya.
(2) Eclogitisation of lower crust of India.
Both of these proposals are unlikely because,
firstly, the marine Tethyan sequence in the northern
Himalaya indicates a thinned lithosphere, and secondly, the lack of seismic evidence for deep subduction casts doubt on the eclogitisation possibility.
2.2. Shortening estimates based on balanced crosssections
There is therefore great uncertainty on the question
of the amount of shortening based on palaeomagnetism
or on attempts at volumetric balancing. Fortunately, an
increasing number of shortening estimates for the
Himalaya based on balanced cross-sections is now
available (Table 1). There is a remarkable consistency
in shortening values (60 – 70%) from the western,
central and eastern sectors of the Himalaya. The notable
exception is the 280% shortening estimated for the
northern Indian margin in Ladakh by Corfield and
Searle (2000). Of this value, 200% is assigned to the
late Cretaceous obduction of the Spongtang ophiolite
and is therefore pre-collisional. Himalaya shortening in
the range of 60 – 70% suggests that ‘Greater India’
probably extended for about 800– 1000 km north of
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Table 1
Shortening estimates based on balanced cross sections 1 – 10 shown on Fig. 1
Authors
Area
Shortening
%
1
2
3
4
5
6
Coward et al., 1988a,b
Coward and Butler, 1985
Searle et al., 1988
Corfield and Searle, 2000
Srivastava and Mitra, 1994
DeCelles et al., 2001
Kohistan, MMT – MBT
Pakistan: external zone
Zanskar shelf
Ladakh: ITS – STD
Central
Western Nepal
66%
64%
56%
280%
69 – 72%
68 – 70%
7
Schelling and Arita, 1991
East
600 km
470 km
150 km
>85 km
687 – 754 km
628 – 667 km minimum,
perhaps 750 km if MCT is included
185 – 245 km
8
9
Schelling, 1992
Ratschbacher et al., 1994
10
Hauck et al., 1998
East
West and East:
whole belt
East
the present mountain front. The apparent lack of
marked W– E variation in amounts of shortening is
puzzling bearing in mind that the amount of convergence deduced above for the west and the east differs by
several hundred km. The other point is that the shortening values in Table 1 for the Himalaya seem to
account for between 20% and 25% of the total shortening, far less than that suggested be Dewey et al.
(1989).
The shortening values given in Table 1 are disparate
in that some refer to the whole belt, others only to parts
of it, e.g. the Zanskar shelf (Searle et al., 1988).
Uncertainties may arise if major discontinuities like
the MCT are left out of the calculation and only shortening of its hangingwall or footwall is considered. The
amount of slip on the MCT is still controversial as is its
life span. The minimum displacement is 100 km,
enough to carry the MCT sheet across the Lesser
Himalaya, but lack of reliable piercing points onto its
surface creates problems. Schelling and Arita (1991)
used what they assumed to be footwall hangingwall
cutoffs of the Indian basement-cover contact in order to
propose 140– 210 km slip on the MCT.
However, Parrish and Hodges (1996) considered
that the High Himalayan crystallines at Langtang are
probably derived from Late Proterozoic – Cambrian
sediments and are not Indian basement. If so, then
the MCT shown on Schelling and Arita’s sections runs
above the Indian basement, the consequence being that
slip on the MCT was larger than 210 km, Schelling and
Arita’s upper limit. Hauck et al. (1998) opt for a 200-
210 – 280 km
500 km
326 km since MCT plus 200 km
min slip on MCT equals in total 526 km
Only southern half
of the belt 59 – 65%
58 – 65%
66%
64%
km displacement and this may be reasonable (Fig. 4).
Later, another factor, that of pre-MCT shortening, will
be introduced and if real then it means that the
estimates in Table 1 are much too low.
3. Rates of convergence
Earlier reference was made to estimates of shortening rates across the Himalaya. The oft-quoted rate is
15 –20 mm/year based on the analysis of on-lap in the
foredeep (Lyon-Caen and Molnar, 1985). Fortunately,
their calculations are supported by other workers
using other lines of evidence (Yeats and Thakur,
1998; Table 2). In addition, Avouac et al. (1998)
and Wesnousky et al. (1999) quoted rates of 21 F 1
and 13.8 F 3.6 mm/year, respectively, for slip on the
active Himalayan Main Frontal Thrust. These studies
show that plate motion is much faster than the shortening rate in the Himalaya. An approximation to the 5
cm/year plate convergence rate is achieved by adding
shortening rates in regions north of the Himalaya
(Tibet, Tien Shan, Kun Lun, Altyn Tagh, etc.; see
Dewey et al., 1988, Fig. 9). The problem with this is
that it assumes that distributed shortening by thrust/
zfold tectonics has operated since the time of collision. But we know that this is not so. For example the
Tien Shan is a Neogene (post 24 Ma) orogen (Hendrix
et al., 1994). Early on after the initial collision thrust
shortening was concentrated in the Himalaya and
Southern Tibet. The inferences are either that the
M.R.W. Johnson / Earth-Science Reviews 59 (2002) 101–123
107
Fig. 4. Schematic restoration of the Main Central thrust with an assumed slip of ca. 200 km: (A) shows the present cross-section re-drawn from
the INDEPTH profile; (B) shows the restored section after removing the assumed 100 km slip on the MBT. It is also assumed that the High
Himalayan Crystallines are derived from a Late Proterozoic – Lower Palaeozoic protolith (Parrish and Hodges, 1996) and therefore rest on
Indian basement. Hypothetical cut-offs are shown for Ordovician granites found in the frontal part of the MCT. These granites contain xenoliths
of medium-grade metasediments exhibiting polyphase fabrics which antedate the granites. The xenoliths appear to provide unique evidence that
the Indian crust beneath the Himalayan thrust sheets is not Shield-like and has undergone pre-Ordovician (Pan African?) orogeny. It is assumed
that the MCT extends to the frontal zone. Abbreviations as in (Figs. 1, 2 and 5). V—Vindhyan (Late Proterozoic)—equivalent to the High
Himalayan Crystallines (HHC)? 40 km refers to the probable thickness of the HHC and Tethyan sequence (TS) after pre-Miocene thickening
discussed in the text. a – a1: Location of MCT/MBT junction before and after slip on the MBT. b: location of Late Proterozoic/Basement cut-off
in footwall. This cut-off moves to b1 during slip on the MCT and then to b11 after slip on the MBT. Scale: V = H.
shortening rate in the Himalaya was much higher
before the Miocene, or that strike –slip faulting operated over wide area of SE Asia.
There is a remarkable, perhaps illusory consistency
in the estimates at least for the last 20 Ma and even for
the whole period since initial collision. On the one hand,
this appears to be in accord with the steady 5 cm/year
rate for convergence between India and Asia, but on the
other hand there are problems in assuming this. Firstly,
it would be surprising if the changing rheology of Indian
crust since collision had no affect on the convergence
rate. Secondly, if we take 18 F 7 cm/year as the convergence rate across the Himalaya, following Le Pichon
et al. (1992) then the total convergence since initial
collision at 50 Ma is about 900 F 350 km compared to
only about 500 km based on balance cross-sections.
This suggests either that the postulated long-term convergence rate is wrong or that the balanced sections
underestimate the total shortening across the Himalaya.
If Indian lower crust and lithospheric mantle have
been detached from the mid/upper crust (cf. Owens
and Zandt, 1997) then the difference between the
overall the plate and internal Himalaya convergence
in understandable.
4. Subduction of Indian lithosphere
Dewey et al. (1988) gave several arguments against
the under thrusting of India. For example, they cited
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Table 2
Rates of convergence across the Himalaya. After Yeats and Thakur, (1998); for full references (See also Thakur, 1999) see their paper of
‘‘authors’’
Rate (mm/year)
Method
Area
Authors
Time span (Ma)
15 F 5
13
9 – 14
7
14 F 2
18
18
10
17
17.2
20 F 3
13.8 F 3.6
18 F 7
On lap
Retro deformation
Retro deformation
Retro deformation
Retro deformation
Slip rate
Finite element
Finite element
Seismicity, geodesy
Seismicity, geodesy
Seismicity, geodesy
Dated terraces
On lap
Central/West
Pakistan
Central Plateau, Pakistan
Eastern Plateau, Pakistan
Kangra
Whole belt
Nepal, Assam
Jammu
?
Nepal
Nepal
India-central
India-central
Lyon-Caen and Molnar, 1983, 1985
Leathers, 1987
Baker, 1988
Leathers, 1987
Powers, 1996
Avouac and Tapponnier, 1993
Peltzer and Saucier, 1996
Peltzer and Saucier, 1996
Molnar, 1990
Bilham, 1997
Bilham, 1998
Wesnovsky et al., 1999
Molnar, 1987
20 – 0
50 – 0
50 – 0
50 – 0
50 – 0
50 – 0
50 – 0
50 – 0
0
0
0
0
20 – 0
the preservation of the Mesozoic succession on the
Indian continental margin. However, this neglects the
possibility that the Indian upper/middle crust has been
detached or scraped-off from the lower part of the
Indian lithosphere along the surface later taken over
by the South Tibetan Detachment. Another possibility
is that the Mesozoic margin rocks were involved in a
rebound in the manner proposed by Chemenda et al.
(2000) or De Sigoyer et al. (2000). The objection to
underthrusting because it necessitates the removal or
displacement of Asian lithosphere from under Tibet
(Dewey et al., 1988) is accommodated in Owen and
Zandt’s model by the downturn of Indian lithosphere
beneath the Banggong suture (Fig. 5A).
Other objections to underthrusting are:
(1) Basement is involved in the Himalayan thrust
sheets and therefore is not available for subduction.
This assumes that the deeper crust is present in the
thrust sheets and not subducted (but see Parrish and
Hodges, 1996, who regarded the High Himalayan
crystallines as derived from Late Proterozoic sediments).
(2) The south edge of the Lhasa block has moved
by 18j to the north and has rotated clockwise by
< 30j in the last 45 Ma and therefore little of India’s
northern motion has been accommodated by underthrusting of India.
An important control for models which invoke
subduction of continental lithosphere are the limitations placed by gravitational forces on the subduction
of continental lithosphere. According to Molnar and
Grey (1979) for the crust, the buoyant force per unit
area resisting such subduction is: qmqc/gh, where
qm = mantle density, qc = crust density, g = gravitational acceleration, h = thickness of crust. They suggested that only short lengths of intact crust (perhaps
only < 50 km, possibly f 131 km) can be subducted.
As they made clear, this estimate can be modified or
increased by factors such as delamination of the lower
crust and mantle (Powell, 1986) or eclogitisation of
the lower crust (Dewey et al., 1993). Their basic
assumption was that only a small part of the gravitational body force of the down-going slab is transmitted to the surface. But if this is not so, then a large
amount of subduction of crust is possible. The question arises from this—how do we assess the underthrusting models referred to above in the light of these
strictures? Is the onus on supporters of continental
subduction? Let us note immediately that in low-angle
underthrusting (see Beghoul et al., 1993, for an
example), the only work against gravity is that
involved in making space for the denser Indian lower
crust and mantle beneath the lighter upper crust of
Tibet. Note that the fate of Tibetan lower crust and
mantle is not dealt with by these authors, perhaps it is
‘bull-dozed’ out of the way? Royden (1993)
expressed a quite different view: ‘‘a significant portion of the forces that drive convergence across
advancing plate boundaries is independent of the
buoyancy of the subducted plate and must be due to
the far-field stresses related to global motions of the
large plate’’. In the Himalaya, active subduction has
M.R.W. Johnson / Earth-Science Reviews 59 (2002) 101–123
109
Fig. 5. North – South profiles of the Himalaya and Tibet redrawn after (A) Owens and Zandt (1977) and (B) Chemenda et al. (2000). In (B), note
that Asian lithospheric mantle is being replaced by underplated Indian crust. BS—Banggong suture. JS—Jingsha suture. MBT—Main
Boundary thrust. MCT—Main Central thrust. QB—Qaidam Basin. STD—South Tibetan Detachment. Other abbreviations as on Figs. 1 and 2.
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M.R.W. Johnson / Earth-Science Reviews 59 (2002) 101–123
been transferred from the Tsangbo suture to: first (50 –
24 Ma) to thrusts within the Tethyan sequence, then to
the MCT (25 – 18? Ma), then to the MBT ( < ca. 11
Ma) and finally to the HFT (>0 Ma). All except the
first episode of thrusting call for the subduction of
continental crust. The MCT detaches in mid crust
(Fig. 6), the MBT and MFT detach in upper crust.
5. Geophysical tests for underthrusting of India
5.1. What are the tests?
(1) Molnar (1988) showed that the P-wave and Swave velocities in the uppermost mantle under most of
Tibet are relatively high and typical of those of
Precambrian shields. However, travel times and waveforms of S-waves passing through the uppermost
mantle of much of Tibet require a much lower average
velocity than in shield mantle. As he points out, this
does not necessarily disprove the existence of Indian
mantle there because shield mantle may have been
modified during the collision process. Furthermore, it
is not clear that the northern margin of India was
shield-like. Mohan et al. (1997) reported that shear
waves in the lower crust and upper mantle were lower
in northern Peninsula India and the Himalaya than in
the southern Shield. They interpret this evidence as
showing that the northern part of the Indian lithosphere
has undergone post-Archaean additions or thickening.
Possible evidence of the nature of the Indian crust that
is buried beneath the Himalayan thrust sheets is
provided by the medium grade metamorphic tectonites
that occur as xenoliths in Ordovician granites within
the crystalline nappes of the Lesser Himalaya (Johnson
and Rogers, 1997; Upreti and Le Fort, 1999; Fig. 4B).
If the xenoliths were derived from the footwall of the
basal Himalayan detachment then they show firstly, a
Pre-Ordovician orogeny and secondly, that at least part
of the footwall is not granulite facies Shield. During
the Palaeozoic – Mesozoic deposition of the Tethyan
sequence took place on the northern margin of India
with the implication that the margin underwent thinning over that period.
(2) Although geophysical data cannot detect Indian
lithosphere under Tibet, they are important in discriminating between models simply by showing the thickness of the mantle under Tibet. Willett and Beaumont’s
(1994) ‘roll-back’ model requires that there is no
mantle under southern Tibet. England and Houseman’s
(1988) and Dewey et al.’s (1988) Homogeneous
Thickening Model leading to convective removal or
thinning of the mantle, calls for abnormally thick
Tibetan crust but thin or absent mantle lithosphere.
This is contradicted by Beghoul et al. (1993) who
postulated a mantle ‘‘between 130 and 180 km thick’’
beneath southern Tibet. Their upper limit of mantle
thickness is inconsistent with Molnar’s (1988) conclusion that the Tibetan Low Velocity zone is ‘‘shallower than 250 km’’ but if we take the preferred
lithospheric mantle thickness given by Beghoul et al.
(1993) as 140 km then there is no inconsistency, e.g.
40 + 70 = 210 km = total thickness of Tibetan (southern) lithosphere. This is considerably more than the
150-km thick Tibetan lithosphere proposed by Dewey
et al. (1988). Further support for a thick mantle under
Tibet comes from Holt and Wallace (1990) who
suggested that the upper mantle under southern Tibet
is ca. 100 km thick. This evidence tends to disfavour
models requiring thin mantle under Tibet.
(3) The continuity of lithosphere (certainly mantle
lithosphere) across the Tsangpo suture, involving
underthrusting of Indian mantle and perhaps crust,
has been repeatedly claimed recently (Beghoul et al.,
1993; Nelson et al., 1996; Alsdorf et al., 1998a,b;
Hauck et al., 1998; Owens and Zandt, 1997; Searle,
1999, fig. 14; Sandvol et al., 1997; Wu et al., 1998;
Makovsky et al., 1999; Yuan et al., 1997; Huang et al.,
2000; see Fig. 5A). The INDEPTH team likened their
model to Zhou and Morgan’s (1995) injection model
but there is a major difference because in the latter
only Indian crust is injected. Nelson et al. (1996) do
not assign the continental lithosphere in South Tibet to
India or to Asia. On the other hand, Makovsky et al.
(1996) regarded the Tibetan lithosphere as mainly
Asian but with some Indian lithosphere in its lower
part. An important feature in the conclusions of all the
above is that the Himalayan detachment is depicted as
a gently dipping (15 – 20j) fault which does not
steepen significantly beneath the High Himalaya in
the manner proposed by Molnar (1988). Therefore,
there is little scope for eclogitisation beneath the
orogen (cf. Dewey et al., 1993). However, it is
difficult to discriminate between models requiring a
gently dipping detachment and those calling for
rotation of a steeply dipping subduction zone into a
M.R.W. Johnson / Earth-Science Reviews 59 (2002) 101–123
111
Fig. 6. Schematic but balanced cross sections showing the amount of subduction of Indian continental crust resulting from: (A) placing the MCT
no lower than at mid crustal levels and (B) assuming that the MCT cuts down to the Moho. (B) is redrawn after Lyon-Caen and Molnar (1983).
S1, S2, S3: positions of the northern tip of subducted Indian crust. Z – Y: trace of MCT if it cuts down to the Moho. X: contact of Indian crust
and oceanic crust. M: mantle. Abbreviations as on Figs. 1 and 5. K—Kangmar thrust.
112
M.R.W. Johnson / Earth-Science Reviews 59 (2002) 101–123
gentle dip (e.g. Powell and Conaghan, 1973; Chemenda et al., 2000). The Himalayan arc is simple with a
radius of curvature of 13 – 14j and an arc centre
situated at about longitude 91jE and latitude 42jN,
some 1696 km north of the Himalayan front, i.e. to the
east of the Tien Shan (Bendick and Bilham, 2001).
This suggests (cf. Frank, 1968) the outcrop of a
moderately dipping structure (26 – 28j) which has
not altered over time. This of course does not exclude
the possibility that the detachment steepens down its
dip to the north. Makovsky et al. (1996) have discussed three models in two of which the Himalayan
detachment maintains a dip of no more than 9.5j
northwards across the Indus Tsangpo suture (Fig. 7).
But in the third model, the dip of the subduction zone
increases from 9.5j to about 32j as the suture is
approached from the south. This gives the subduction
zone to a radius of curvature of about 200 km which is
as sharply curved as the most curved subduction
zones in the Pacific. Makovsky et al. (1996) reject
this model and cite evidence from the Pamirs where
deep earthquakes define a subduction zone with a
350-km radius. Hauck et al. (1998) also addressed this
problem (see below) and argued for a ramp in the
detachment. On the other hand, Makovsky et al.
(1999) concluded that the Main Himalayan Thrust is
a planar surface which underthrusts the Indus Tsangpo
suture and reaches the Moho well to north of the
suture. Yuan et al. (1997) also support a planar
detachment which extends well to the north of the
Indus Tsangpo suture. This is a problem that calls for
urgent solution.
6. How much subduction is required?
Table 3 shows the amount of subduction that could
be achieved at various rates and assuming that subduction operated since the time of initial collision.
Rates of convergence used are: overall rate for India –
Asia convergence (top) and upper and lower limits for
internal convergence given by Lyon-Caen and Molnar
(1985). Two thousand five hundred kilometers can be
ruled out because it would involve lithospheric thickening on the scale envisaged by Argand (1924). Also
it fails to explain why the crust is not thickened under
the Tarim basin and, perhaps, why the crustal thickness in Tien Shan is only 56 km.
Subduction of 1000 km involves underthrusting
under the whole of Tibet—is this consistent with the
evidence of thinner ( f 55 km thick) crust in NE
Tibet? The INDEPTH profile stops ca. 200 km north
of the Tsangbo suture. Beghoul et al. (1993) showed
Indian lithosphere extending 400 – 500 km north of
the suture. This amount of subduction would call for
the lowest slip rate if subduction operated for f 50
Ma. Is it reasonable to assume this? Could continental
subduction have started late or finished early? What
factors would control this? Is it essential that continental subduction operated for whole time? Was it
stopped by buoyancy? Patriat and Achache (1984)
suggested that this happened ca. 44 Ma and Chemenda et al. (1995) envisaged a similar buoyancy
controlled rebound of Indian crust at about 25 Ma.
Chemenda et al. (2000) suggested that the subducted
Indian lithospheric mantle eventually suffered ‘‘break
Fig. 7. Differing views on the possibility that the Himalayan detachment continues northwards from under the Tethyan Himalaya (Kangmar
dome). Hauck et al. (1998) placed a ramp (at ‘A’) in the detachment underneath the Kangmar dome but do not speculate on what happens at the
bottom of the ramp and the northwards across the Indus Tsangpo suture. For the other authors the detachment continues northwards under
southern Tibet which is therefore regarded as an allochthonous block. Abbreviations as in (Figs. 1, 2 and 5). STD—South Tibetan Detachment.
M.R.W. Johnson / Earth-Science Reviews 59 (2002) 101–123
Table 3
Amount of Indian lithosphere subducted since 50 Ma at different
rates
Rate of convergence (cm/year)
Amount of Subduction (km)
5
2
1
2500
1000
500
off’’ at about 45 Ma (Fig. 5B). After this the Indian
lithospheric slice (mainly upper crustal) rotated
upwards so as to underplate Asia, causing isostatic
uplift. The Indian crust was heated and softened
between 45 and 25 Ma. Continental subduction was
polyphasal because over time successive slices of
Indian crust are subducted starting soon after initial
collision and renewed in MCT and eventually MBT
times (Fig. 5B). The scaled diagram in Chemenda et
al. (2000) indicated that ca. 700 km shortening was
accumulated by the successive stacking of subducted
slices of Indian crust. De Sigoyer et al. (2000) showed
that eclogite metamorphism in the Tso Morari unit on
the northern continental margin of India indicates
continental subduction to depths of ca. 70 km at about
55 Ma, similar to the event which was responsible for
the Kaghan eclogites in NW Pakistan. (Tonarino et al.,
1993). Furthermore these authors showed a rapid
exhumation of eclogites to a depth of ca. 30 km by
47 Ma at which time they underwent high temperature
metamorphism. On the face of it, this discovery
appears to rule out long term continental subduction
but it may well be that the early subduction recorded
by De Sigoyer et al. (2000) is just the first of several
subduction episodes as envisaged by Chemenda et al.
(2000). Searle (2001a) doubts the relevance of the
conclusions of De Sigoyer et al. (2000) and regards
the Tso Morari eclogite as pre-collisional. The ‘‘early
thrust’’ shown in Fig. 5B may be a fault along which
the eclogites were uplifted early on. Subsequent
underthrusting of India beneath them resulted in high
temperature metamorphism.
7. Partitioning of compressive strain in the
Himalaya and SE Asia
Dewey et al. (1988, 1989) suggested a partitioning
of the shortening by ‘‘thrust regimes’’ following the
India – Asia collision as follows: in the east/central
113
Himalaya: about 1000 km; in Tibet ca. 50% shortening, that is: 660 km in central Tibet and 960 km in
eastern Tibet. According to them, relatively smallscale wrench fault regimes make up the total convergence (2475 km, Central and 2750 km, East). Two
problems with this budget are:
(1) the shortening in the Himalaya appears be
much less than proposed by Dewey et al. and
(2) the contribution of Tibet to the shortening
budget is under question.
The first of these problems has been dealt with
already but will be discussed further below. The
second problem needs to be considered.
Bulk homogeneous shortening in Tibet of >1000 km
(ca. 48% of the total convergence across the orogen) is
not feasible if Murphy et al. (1997, see also An Yin et
al., 1994) are correct in proposing that all or part of the
Tibetan crust had been shortened and thickened prior to
the India –Asia collision. Therefore the present 65 –70
km thickness of the crust in most of Tibet cannot be
ascribed entirely to shortening during the Cenozoic.
Murphy et al. (1997) gave evidence for a 60%
shortening of (? South) Tibet during the Cretaceous
and before the India – Asia collision. The evidence is
as follows: 40Ar/39Ar and U –Pb datings on igneous
rocks which cut thrusts, and early Cenozoic volcanics
that rest unconformably on folded and thrust Cretaceous rocks. In addition, they questioned the widely
held assumption that all Tibet was submarine at the
end of the Cretaceous and instead proposed elevations
of 3 –4 km at this time in parts of the plateau. This
point was referred to earlier in connection with Le
Pichon et al. (1992). Of the present crustal thickness
of ca. 70 km in southern Tibet they suggested that
only about 30 km was due to thickening in the
Cenozoic achieved either by injection of Indian crust
(Zhou and Morgan, 1995) or by underthrusting of
Indian crust. England and Searle (1986) restricted the
pre-collisional shortening during the Mesozoic in
Tibet to the southern part of the Lhasa Block, therefore perhaps affecting a belt some 150 – 200 km
across. This may minimise the amount of pre-collisional shortening in Tibet but a significant problem
remains. At the Chengdu Himalayan – Karakoram –
Tibet Workshop (Searle, 2001b) further support was
given for pre-collisional thickening, e.g. An Yin et al.
(1994) who reported thrusting and folding of late
Triassic age in the Chang Tang Block (North Tibet),
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M.R.W. Johnson / Earth-Science Reviews 59 (2002) 101–123
and by Wilson who reported late Triassic –Jurassic
shortening and little Cenozoic shortening in the Longmen Shan (eastern Tibet).
8. Crustal thickening: mechanisms, timing and
partitioning—a new consensus?
If there is a problem with previous shortening
budgets then solutions other than those involving
major homogenous thickening of Tibetan lithosphere
must be entertained, e.g. loss of crust downwards or
upwards or sideways. Let us consider underthrusting
of Indian lithosphere first. Fig. 6 shows that volume
balancing requires such underthrusting if the basal
detachment, including the Main Central thrust, takes
place in mid-crustal levels. On this diagram the
displacements on thrusts are uncertain but in accord
with the amounts proposed by Owens and Zandt
(1997; Fig. 5A). Thus underthrusting does not extend
to northern Tibet where crustal thickness is less than
in southern Tibet, i.e. 55 km (Beghoul et al., 1993).
Owens and Zandt (1997) continued the Himalayan
detachment for some 300 –400 km under southern
Tibet with consequent underthrusting of Indian lithosphere but in northern Tibet the crust is pervasively
heated and flowing and is supported by a hot lowdensity mantle. The latter may be squeezed between
advancing Indian lithosphere and older Asian continental lithosphere, see also Huang et al. (2000) who
advocated lateral explosion of the hot mantle lithosphere of northern Tibet.
The polyphase underthrusting suggested by Chemenda et al. (2000) would probably allow more underthrusting, say 700 km. Whereas they appear to suggest
that the full thickness of Indian continental crust has been
subducted, Owens and Zandt (1997) proposed that only
some mid but mainly Indian lower crust was subducted.
8.1. Volumetric implications of underthrusting
There must be a volume balance between Indian
crust which has been subducted and that which has
been scraped off to form the Himalaya. The volume of
the Himalayan thrust sheets is 19 106 or 15 106
km3, using respectively the INDEPTH profile and the
profile of Owens and Zandt (1997) in order to
estimate the dimensions of the Himalayan wedge.
The depth to the detachment beneath the Indus suture
is 52 km on the former (but this involves the northwards projection of the detachment shown on their
profile) and 40 km on the latter. The volume loss by
erosion from the Himalaya may be 19 –20 106 km3
equivalent to the removal of 35– 40 km off the whole
Himalaya (Le Pichon et al., 1992; Johnson, 1994).
Therefore total volume in the Himalaya prior to
erosion was between 34 and 38 103 km3. Parrish
and Hodges (1996) suggested that most of the High
Himalayan crystallines are not Indian basement but
are probably derived by the high-grade metamorphism
of later Proterozoic – Lower Palaeozoic sediments. If
we assume that only mid/upper crust is conserved in
the Himalaya with, say, an original thickness of 20
km, then lengths of restored sections are 680 km,
using Owen and Zandt’s data and 760 km using
INDEPTH data. These lengths are roughly in line
with the 500 km shortening derived from the balanced
cross-sections. Thus by the placing the detachment at
mid-crustal levels and using a different method of
estimating shortening we have arrived at a satisfying
confirmation of the analysis of Le Pichon et al. (1992)
(referred to on p. 6), i.e. only mid/upper crust is
conserved in the Himalaya and the lower crust and
lithospheric mantle is ‘‘lost’’, i.e. subducted beneath
Tibet. The INDEPTH profile Zhao et al. (1993) and
Nelson et al. (1996), showed that the Indian crust
beneath the detachment is about 43 km thick, more
than the 38.5 thick Indian crust south of the Himalaya.
These authors concluded that the entire Indian crust is
subducted beneath the Himalaya but Huang et al.
(2000) suggested that this crust underwent moderate
thickening as it underthrust the Himalaya. This piece
of Indian crust would have been situated far to the
south of the orogen during the pre-30 Ma event and
would not have moved beneath the Himalayan thrust
sheets until long after the Eohimalayan event. On a
convergence rate of 1.5 –2.0 cm/year, it has taken only
10 –15 Ma for the 43-km thick crust to underthrust the
thrust sheets as far north as the Tethyan Himalaya.
The Himalayan thrust sheets are not derived from
their substratum and accretion at the toe of the
Himalayan wedge takes place at a high level within
the Siwalik foredeep.
In order to assess the above we need to consider a
recent advance in understanding the Cenozoic evolution of the Himalaya.
M.R.W. Johnson / Earth-Science Reviews 59 (2002) 101–123
8.2. Pre-MCT shortening
It is now generally accepted prior to slip on the
MCT at about 22 Ma that there was a widespread
regional Barrovian metamorphism up to kyanite grade
in the High Himalaya (Treloar et al., 1989; Hodges,
1998, 2000; Hodges et al., 1994; Searle et al., 1992,
1999; Walker et al., 1999; Stephenson et al., 2000;
Simpson et al., 1999; Godin et al., 2001). The
evidence for the early metamorphism seems to be
stronger in the western Himalaya than in central and
eastern parts where overprinting by the so-called
NeoHimalayan metamorphism may have occurred.
However, Prince et al. (2001) recorded garnet growth
in the HHC of Garhwal (central Himalaya) between
44 and 22 Ma, together with anatexis. The early
metamorphism, sometimes called EoHimalayan, probably occurred before 30 Ma with the implication that
the Indian crust underwent substantial thickening
between 50 and 30 Ma. Evidence for this event is
the discovery of pre-MCT deformation episodes
which were synchronous with the Barrovian metamorphism, e.g. Stephenson et al. (2000). The mechanism for crustal thickening in the pre-30 Ma event is
not well understood. Vannay and Hodges (1996) in
the Annapurna area described a supra MCT thrust
which was responsible for thickening the crust prior to
Barrovian metamorphism. In the eastern Himalaya,
the Kangmar thrust may have served the same function. It is also possible that homogeneous thickening
occurred without thrusting. The distinction between
the mechanisms is important because if there was no
Himalayan basal detachment in the pre-30 Ma event
then presumably it antedated any subduction of Indian
crust. On the other hand, if there was a basal detachment under most of the thickened crust then Indian
lower crust may not have been involved in the
thickening. As a consequence of thickening pressures
of >9 – 11 kbar are recorded in the rocks at the base of
the MCT sheet indicating an overburden at the time of
thrusting of < 40 km (Hodges, 1998; Stephenson et
al., 2000; Johnson et al., 2001). This is equivalent to
doubling the thickness of the Indian mid/upper crust.
It is unclear how far south this zone of early metamorphism stretched. Johnson et al. (2001) have identified pre MCT compressional fabrics associated with
high grade metamorphism in the Kathmandu area
which is fairly near to the leading edge of the MCT
115
sheet. It is not certain that the early metamorphism at
Kathmandu was synchronous with the pre-30 Ma
metamorphism in the High Himalaya (diachronous
southward moving metamorphism is a possibility) but
it can be deduced that a pre-MCT thickening event
occurred in the entire MCT sheet which is about 250 –
300 km wide (Johnson et al., 2001). So if thickened
Indian lower crust have been subducted under Tibet it
could be about 300 km in length moderately thickened
as it underthrusts the Himalaya.
The shortening estimates shown in Table 1 do not
take account of the early shortening. On a very
conservative estimate the pre-MCT shortening could
be at least 50%, that is about 250 – 300 km linear
shortening. This must be added to the estimates shown
in Table 1, so rather than 500 km shortening the likely
total shortening could be at least 800 km and the
original width of the Himalayan belt about 1100 km.
8.3. Extent of underthrusting
Owens and Zandt (1997) solved the problem of
what happens to south Tibetan mantle as a result of
Indian subduction by locating a subduction zone
under the middle part of Tibet beneath the Banggong
suture, that is about 300 km north of the ITS. The
evidence for this is a layer of high P wave velocity
(7.2 – 7.5 km/s) at 60– 75 km under the Lhasa terrane:
Owens and Zandt correlate this with Indian lower
crust.
However, An Yin and Harrison (2000) suggest that
this layer may be mafic-intermediate magma underplated on to Tibet during plutonic activity. Underthrusting of a similar extent is postulated by others,
e.g. Yuan et al. (1997), Makovsky et al. (1999), also
Huang et al. (2000) who identified a transition in
upper mantle properties at 32jN in central Tibet,
marking the southern limit of inefficient Sn propagation and reduced seismic velocities, as the northern
edge of the Indian continent under Tibet. They
reported a strong E – W anisotropy in the upper mantle
of northern Tibet which may be linked to shear on
strike – slip faults controlling mantle flow out of the
way of the advancing Indian lithosphere (see also
Hoke et al., 2000).
Tomographic images (Van der Voo et al., 1999)
show several pieces of high velocity (P-wave) material under India. The deeper ones, which extend down
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M.R.W. Johnson / Earth-Science Reviews 59 (2002) 101–123
to depths of >2000 km, may consist of Tethyan
oceanic lithosphere while the shallower, and at depths
of less than 1000 km are interpreted as bits of Indian
lithospheric mantle some still attached to India. The
latter dip southwards as a result of the northward
motion of India relative to the mantle, raising the
interesting question of whether this Indian lithosphere
first underplated Tibet and was subsequently peeled
back into its present attitude. This process is not in
accord with the models of Owens and Zandt (1997) or
INDEPTH both of which involve low angle subduction of Indian lithosphere including mantle under
Tibet.
On the INDEPTH profile (Nelson et al., 1996), the
trace of the detachment appears to terminate under the
Kangmar dome in the Himalaya at the southern
margin of a zone of partial melting which continues
northwards under southern Tibet at a depth of 15 –40
km. Hauck et al. (1998) placed a ramp in the detachment under the Kangmar dome, the evidence being ca.
15j north dipping reflections at 10 s on the southern
part of Tibet-3 (Fig. 7). They noted that the Himalayan thrusts sheets above the ramp are ca. 28 km
thick, from which they infer that either the ramp cuts
down to near the base of the Indian crust so that
detachment occurs in the lower crust, or that the crust
was thickened prior to thrusting. From previous discussion here it would seem that the latter is more
likely. It is not specified whether or not the detachment continues under or in the melt zone. Such a
continuation is implicit in the model of Searle (1999)
and D’Andrea et al. (1999) who suggested that 0 Ma
aged melting under Tibet represents the present day
expression of a process which, for the past 20 – 25 Ma
has transported the melts southwards into the High
Himalaya. However, this proposal appears to call for
an extremely slow slip rate on the MCT or related
thrusts, i.e. about 4 mm/year which is much less than
slip rates quoted above. The importance of the possible presence of extensive melts or aqueous fluids in
the Tibetan crust cannot be overemphasised, not least
because the melts under pressure would provide a
buoyancy mechanism which would facilitate uplift.
8.4. Shortening budgets revisited.
In Table 4 shortening values for the Tien Shan are
from Dewey et al. (1989). These are broadly con-
Table 4
Shortening by thrust-fold regimes: shortening of Tibet only 40%
Central
(km)
Tibet, thrust regime
Tien Shan, thrust regime
E Tien Shan and Altai
Himalaya, thrust regime
Total
Deficita
533
390
–
500
1423
2475
East
(km)
660
–
350
500
1510
1423 = 1052 2750
1510 = 1240
a
The difference between convergence of India and Asia
deduced from palaeomagnetic evidence (first number) and shortening in the thrust-fold regimes in SE Asia.
firmed by Chen et al. (1991, 1992) but they gave a
high value for shortening in Altai (300 – 400 km)
which if correct would reduce the deficit shown
above. On the other hand, Avouac et al. (1993) gave
rather low shortening estimates, i.e. 125 –203 km for
the Tien Shan. Shortening values for the Himalaya are
from Table 1 and do not allow for pre-MCT thickening discussed above.
Table 4 gives an illustration (unfortunately it cannot claim any higher authority than that) of the
consequences of a reduced Cenozoic shortening in
Tibet. It incorporates possible values for the Cenozoic
thickening of Tibet based on Murphy et al. (1997). If,
however, England and Searle’s (1986) alternative is
followed (i.e. that only a small part of Tibet was
thickened before collision) then obviously the deficits
are reduced but still large (925 km, Central and 980
km, East).
If allowance is made for pre-MCT shortening then
the deficits reduced, i.e. 752 (Central) and 940 (East).
Table 5 shows the post-collisional shortening
budget for the Himalayan –Tibetan orogen as proposed by An Yin and Harrison (2000).
How does this compare with the shortening budget
shown in Table 4? Firstly, it underestimates Himalayan shortening by 140 or 440 km if allowance is
made for pre-MCT shortening. Taking the latter number as being more probable increases total shortening
to 1940 (Central Tibet) and 1460 km (East Tibet).
Comparing with Table 4, the deficit is considerably
reduced, to 535 km, in Central Tibet but hardly at all
in East Tibet.
The important conclusion to be drawn from the
recent advances is that the shortening of Tibetan crust
may be much less than supposed by Dewey et al.
M.R.W. Johnson / Earth-Science Reviews 59 (2002) 101–123
117
Table 5
Post-collisional shortening budget for the Himalayan – Tibetan orogen
(1988, 1989). Instead of < 1000 km, the Cenozoic
shortening taken up in Tibetan crust may only amount
to 533– 660 km if Murphy et al. (1997) are correct.
Even if these authors are wrong in proposing widespread pre-collisional shortening in the Lhasa Block it
should be noted that Coward et al. (1988a,b) reported
that the thrust shortening in the Lhasa-Golmud sector
of Tibet was insufficient to confirm the budget set out
by Dewey et al. (1988). If so then the deficit would
need to be found by increasing the estimate of shortening in the Himalaya (for example, Vannay and
Hodges, 1996 show the importance of supra-MCT
thrusting) and/or by increasing estimates of the
amount of expulsion of Asian continent to values
approaching those proposed by Molnar and Tapponnier (1975), a solution favoured by Coward et al.
(1988a,b). The wrench fault regimes listed by Dewey
et al. (1988, 1989) are not sufficient to close the
deficit, including them still leaves shortenings of
581 km, Central Himalaya and 548 km, East Himalaya, to be accounted for.
8.5. Implications of underthrusting for thermal
decoupling
The recent work shows that the Tibetan lithosphere
is in part derived by the underthrusting of Indian
lithosphere and, furthermore, Cenozoic shortening in
Tibet is less than supposed. What are the implications
for the hypothesis involving thermal decoupling and
convective removal of the lower part of the lithospheric mantle under Tibet (England and Houseman,
1988; see Fig. 3D)? This model which invokes
homogeneous thickening of Tibetan lithosphere
before thermal decoupling has been influential in the
debate on the timing of uplift of the Tibetan Plateau.
Rapid removal of lower lithospheric mantle results in
rapid uplift. Is this model weakened if the present
lithospheric thickness of Tibet cannot be ascribed to a
single cause? Limited shortening of Tibetan crust in
the Cenozoic may mean that the lithosphere under
Tibet has not been thickened sufficiently for decoupling to occur. Let us assume that the Tibetan lithosphere is between 170 and 210 km thick (Beghoul et
al., 1993). If the lithosphere was originally 100 –125
km thick (‘normal’ for continental lithosphere), then
only the lower figure permits substantial decoupling
after doubling the thickness by homogeneous thickening. In addition the concept of homogeneous thickening of Tibetan lithosphere is clearly not compatible
with subduction of Indian lithosphere along a low
angled thrust, involving crustal thickening and the
replacement of Asian mantle by Indian mantle. Mantle
thickening is not necessarily involved in the subduction and therefore thermal decoupling may not have
occurred. The only exception to this conclusion would
be if very thick Indian shield lithosphere (>200 km
thick?) as involved in the subduction (but see the
comments of P. Molnar reported on p. 10). In this
case, the rather thick mantle under Tibet may have
been diminished by thermal decoupling. Recent
advances reported by Searle (2001b) indicate that
postcollisional ultrapotassic shoshonitic rocks are
scattered all over the Tibetan Plateau in contradiction
to earlier suggestions that igneous rocks were confined to the northern part. In addition in East Tibet
(Wang and Xie and Tan et al.) there appear to have
been two distinct episodes of magmatism, the earlier
at 42 –24 Ma and the later post 16 Ma. In Chang Tang
(northern Tibet) Ma et al. and Li Youguo et al. record
ages from 25 –10 to 60 –44 Ma. If delamination and
thermal decoupling call for rapid events then the
considerable age ranges for magmatism provide no
support. It is imperative that future research seeks to
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M.R.W. Johnson / Earth-Science Reviews 59 (2002) 101–123
resolve the problem of whether all or part of Tibet has
been thickened before collision. An Yin and Harrison
(2000) reported that in the Qiangtang terrane Late
Triassic is the time of the last contractional event thus
supporting the prospect that pre-collisional shortening
is indeed widespread in Tibet.
9. Lateral expulsion
Turning now to the possibility that lateral expulsion has taken place on a much larger scale than was
held by Dewey et al. (1988, 1989). Peltzer and
Tapponnier (1988) presented evidence for large slip
on the Altyn Tagh and Red River faults but the
amount of slip on strike –slip faults in SE Asia is still
controversial. Harrison et al. (1998) gave a full discussion of the evidence including the following estimates for displacement on strike –slip faults in Tibet
and elsewhere (see also, An Yin and Harrison, 2000).
Red River fault—550 F 150 km (left lateral); 35 –
17 Ma—based on the opening of the South China Sea
and finite strain analysis across the fault zone.
Altyn Tagh fault—ca. 500 km (west segment); 280
km (east segment) (left lateral); 15 Ma to present—
based on matching the off-set of late Palaeozoic
magmatic belts in W and E Kunlun Shan and matching the Qilian suture zone with the central Tarim uplift
zone.
Qinling fault—150 km (left lateral)—active in the
Quartenary.
Haiyuan fault— < 15 km (? slip sense)—active
since the Pliocene.
Karakoram fault—>160 km (right lateral)—main
part active since 17 Ma.
Total slip shown by faults listed above—1375 km.
As demonstrated by Harrison et al. (1998), some
authors have given lower estimates for these faults,
e.g. Searle et al.(1998) restricted lateral extrusion to
about 200 km and the Karakoram fault to 150 km
(based on the offset of 21 – 18 Ma Baltoro type
leucogranites) since 18 Ma with no slip prior to that
date. Murphy et al. (2000) accepted the estimated slip
for the Karakoram fault listed above but add that its
eastern part shows only 66 F 5.5 dextral slip since 13
Ma, Xu et al. (see Searle, 2001b) suggested that slip
on the Altyn Tagh fault was 400 km, the slip sense
was sinistral as far back as the Cretaceous which is
surprising given the stated displacement. Gilley et al.
(Searle, 2001b) reported 500– 700 sinistral slip on the
Red River fault since 29 Ma, but contrary to the
prediction of the extrusion hypothesis, the propagation of sinistral slip was from SE to NW between 25
and 17 Ma. The other point to note is that most of the
faults were late Cenozoic faults which overlap in time
with the normal fault regime that is related to gravitational collapse of the Tibetan Plateau (e.g. Dewey et
al., 1988). For this reason, An Yin and Harrison
(2000) suggested that the east – west strike – slip faults
in central Tibet may be transfer faults linking N – S
normal faults.
Dewey et al. (1989) list the following objections to
large scale expulsion:
(1) It does not account for thickening of Tibet and
Tien Shan. This is true but firstly we do not know how
much Cenozoic thickening has occurred in Tibet and
secondly timings of extrusion and thickening may
well be different.
(2) The high plateau of Tibet is north of the Indian
indenter (only 15% is east of the indented margin
therefore any lateral extrusion must have predated
crustal thickening and uplift). Datings of strike – slip
faults listed above suggest the opposite but lateral
extrusion in the early Cenozoic is not excluded.
(3) Palaeomagnetism shows a post-collisional
northwards displacement of Lhasa with respect to
Eurasia of 0>1500 km. The uncertainties here are
increased by the results of Murphy et al. (1997).
(4) In Burma motion on strike – slip faults was right
lateral post-45 Ma contrary to the prediction of the
expulsion hypothesis.
(5) The Tanlu fault situated in eastern China (Fig.
2) was right lateral in the Neogene and in the
Mesozoic showed >250 km left lateral slip. It was
not markedly affected by any major extrusion of
Tibet.
(6) Only small displacements are recorded on the
Altyn Jhan, Kun Lun faults and therefore maximum
extrusion was 200 km. This point has been dealt with
above.
It may be concluded that large-scale strike– slip
faulting is permitted by the available evidence. However, the position and time span of activity on the
Tanlu fault may restrict major strike –slip faulting to
the early Cenozoic. An Yin and Harrison (2000)
considered that the Nan Shan thrust belt which forms
M.R.W. Johnson / Earth-Science Reviews 59 (2002) 101–123
the eastern termination of the Altyn Tagh fault existed
in the Oligocene and maybe in the early Palaeocene. If
the linkage between the thrust belt and the Altyn Tagh
fault is real then this is evidence for major strike – slip
in the early Cenozoic. Major strike – slip faulting at
that time would serve to make up the deficit between
overall plate convergence at 5 cm/year and shortening
restricted to the Himalaya at only >2 cm/year. An
interesting question is the depth to which faulting
occurred—did translation continue below the brittle–
ductile transition or was translation confined to the
upper crustal levels implying the existence of a
detachment in the Tibetan crust below the zone of
faulting. Huang et al. (2000) described evidence for
strike – slip motion operating as deep as the lithospheric mantle of the northern Tibet. Referring to
the list above of known strike – slip faults it is evident
that firm conclusions as to the dating and scale of the
faults concerned cannot be drawn at present.
10. Indentation
Finally a comment on the indentation model first
put forward by Molnar and Tapponnier (1975). If the
Himalayan detachment continues under Tibet as suggested by Owens and Zandt (1997) then the concept
of Indian lithosphere acting as a rigid indenter is
unrealistic. Indian middle/upper crust cannot have
been rigid throughout the Cenozoic—it failed by
thrusting and it was weakened by high-grade metamorphism in this time. Furthermore, if all or part of
the Indian lithosphere has underthrust Asia then the
leading edge of the ‘‘indenter’’ appears to be mechanically fallible. This is consistent with the point mentioned earlier (Section 5.1) concerning the possible
non-Shield character of the Indian crust that is underthrusting the Himalaya. However, if rigidity was
provided by the Indian shield to the south of the
Himalaya then the evidence for indentation into Eurasian lithosphere is persuasive, such as the way that
pre-collisional suture zones north of the Tsangbo
suture appear to be wrapped around India (Dewey et
al., 1988, Fig. 7, 1989). As Dewey et al. stated: ‘it
would be an extraordinary coincidence if an embayment in the Eurasian margin. . .happened to coincide
precisely with the collisional arrival position of India’.
However Fig. 7 of Dewey et al. (1988) could permit a
119
different interpretation, namely that the wrap around
of the old sutures may reflect rotation within the zones
of strike – slip rather than indentation. If so, the India –
Asia convergence has been accommodated by the
zones of compression (thrust/fold tectonics) combined
with lateral extrusion on strike –slip faults.
11. Conclusions
(1) Models requiring major underthrusting of Tibet
by Indian lithosphere are difficult to test. Equally
difficult is discrimination between underthrusting
models that advocate entirely low angled subduction
and those proposing rotation of the underthrust slab
and thus underplating of Tibet. However the volumetric evidence suggests that only mid/upper crust
appears to be conserved in the Himalayan thrust
sheets while Indian lower crust and mantle has been
detached along successive roughly coplanar surfaces,
e.g. the Main Central thrust, etc. This system requires
the ‘‘loss’’ of the lower part of Indian continental crust
either by steep or gentle subduction.
(2) The shortening budgets based on the available
evidence leave a deficit when compared to the overall
convergence of India and Asia. Shortening budgets
for the Himalaya based on balanced cross-sections
may give underestimates, viz. 60 – 70%, because
likely pre-MCT shortening is neglected. However if
pre-collisional shortening has occurred in Tibet then a
deficit remains, even when pre-MCT shortening is
allowed for, i.e. 752 km across Central Himalaya –
Tibet or 940 km across Eastern Himalaya– Tibet. A
modification of estimates of An Yin and Harrison
(2000) gives deficits of 535 km across Central Tibet
and 1290 km across Eastern Tibet. These deficits must
be made up by invoking large-scale strike –slip faulting and lateral extrusion.
Acknowledgements
My special thanks are due to Alastair Robertson
and Mike Searle for encouragement, advice and
information. In addition, I am grateful to Grahame
Oliver for helpful comments and references. I am
grateful to Giuliano Panza for directing me to
geophysical data on the nature of the Indian litho-
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M.R.W. Johnson / Earth-Science Reviews 59 (2002) 101–123
sphere. Of course all errors of fact and interpretation
are my responsibility.
References
Alsdorf, D., Brown, L., Nelson, K.D., Makovsky, Y., Klemperer, S.,
Zhao, W., 1998a. Crustal deformation of the Lhasa terrane,
Tibetan Plateau from Project INDEPTH deep seismic reflection
profiles. Tectonics 17 (4), 501 – 519.
Alsdorf, D., Makovsky, Y., Zhao, W., et al., 1998b. INDEPTH (international deep profiling of Tibet and the Himalaya) multichannel seismic reflection data: description and availability. Journal
of Geophysical Research 103, 26993 – 26999.
An Yin, W., Harrison, T.M., 2000. Geologic evolution of the Himalayan – Tibetan orogen. In: Jeanloz, R., Albee, A.L., Burke, K.C.
(Eds.), Annual Reviews of Earth and Planetary Science, vol. 28,
pp. 211 – 280, California.
An Yin, K.C., Harrison, T.M., Ryerson, F.J., Chen, W., Kidd, W.S.F.,
Copeland, P., 1994. Tertiary structural evolution of the Gangdese
thrust system, southeastern Tibet. Journal of Geophysical Research 99 (B9), 18175 – 18201.
Argand, E., 1924. La tectonique de l’ Asie. International Geological
Congress Report Session 13/1, 170 – 372.
Avouac, J.-P., Tapponnier, P., Bai, M., You, H., Wang, G.,
1993. Active thrusting and folding along the northeastern
Tien Shan and rotation of Tarim relative to Dzungaric –
Kazakhstan. Journal of Geophysical Sciences 98, 6755 –
6804.
Avouac, J.P., Lave, J., Cattin, R., Pandey, M.R., Tandukar, R.P.,
1998. Active mountain building in the Himalaya of Nepal.
AGU 79, Fall meeting suppl. F204.
Beghoul, N., Barazangi, M., Isacks, B.L., 1993. Lithospheric structure of Tibet and Western North America. Mechanisms of uplift
and a comparative study. Journal of Geophysical Research 98,
1997 – 2016.
Besse, J., Courtillot, V., Westphal, M., Zhou, Y.X., 1984. Palaeomagnetic estimates of crustal shortening in the Himalayan thrusts
and Zangbo suture. Nature 311, 621 – 626.
Bendick, R., Bilham, R., 2001. How perfect is the Himalayan arc?
Geology 29, 791 – 794.
Butler, R.W.H., 1986. Thrust tectonics, deep structure and crustal
subductions in the Alps and Himalaya. Journal of the Geological
Society of London 143, 857 – 873.
Chemenda, A.I., Mattauer, M., Malavielle, J., Bokum, A.N., 1995.
A mechanism for syn-collisional rock exhumation and associated normal faulting: Results from physical modelling. Earth
and Planetary Science Letters 132, 225 – 232.
Chemenda, A.I., Burg, J.-P., Mattauer, M., 2000. Evolutionary model of the Himalaya – Tibet system: geopoem based on new modelling, geological and geophysical data. Earth and Planetary
Science Letters 174, 397 – 409.
Chen, Y.C., Cogne, J.-P., Courtillot, V., Avouac, J.-P., Tapponnier, P.,
Wang, G., Bai, M., You, H., Li, M., Wei, C., Buffetant, E., 1991.
Palaeomagnetic study of Mesozoic continental sediments along
northern Tien Shan (China) and heterogeneous strain in Central
Asia. Journal of Geophysical Research 96, 4065 – 4082.
Chen, Y.C., Cogne, J.P., Courtillot, V., 1992. New Cretaceous palaeomagnetic poles from Tarim Basin, NW China. Earth and Planetary Sciences Letters 114, 17 – 38.
Corfield, R.I., Searle, M.P., 2000. Crustal shorteneing estimates
across the north Indian continental margin, Ladakh, NW India.
In: Khan, M.A., Treloar, P.J., Searle, M.P., Jan, M.Q. (Eds.),
Tectonics of the Nanga Parbat syntaxis and the western Himalaya. Geological Society of London Special Publications, vol.
170, pp. 395 – 410.
Coward, M.P., Butler, R.W.H., 1985. Thrust Tectonics and the deep
structure of the Pakistan Himalaya. Geology 13, 415 – 420.
Coward, M.P., Butler, R.W.H., Chambers, A.F., Graham, R.H., Izatt,
C.N., Khan, M.A., Knipe, R.J., Prior, D.J., Treloar, P.J., Williams, M.P., 1988a. Folding and imbrication of the Indian crust
during Himalayan collision. In: Shackleton, R.M., Dewey, J.F.,
Windley, B.F. (Eds.), Tectonic Evolution of the Himalayas and
Tibet. Philosophical Transactions of the Royal Society, London,
A 326, pp. 89 – 116.
Coward, M.P., Kidd, W.S.F., Pan, Y., Shackleton, R.M., Zhang, H.,
1988b. The structure of the 1985 Tibet Geotraverse, Lhasa to
Golmud. Philosphical Transactions of the Royal Society, London, A 327, 307 – 336.
D’Andrea, J., Harrison, T.M., Grove, M., Zhou, X., 1999. The
thermal and physical state of the south Tibetan middle crust
between 20 – 8 Ma: U – Th – Pb and Nd isotopic evidence from
the Nyainqentanglha massif. Himalayan – Karakoram – Tibet
Workshop, Kloster, 29 – 30.
DeCelles, P.G., Robinson, D.M., Quade, J., Ojha, T.P., Garzione,
C.N., Copeland, P., Upreti, B.N., 2001. Stratigraphy, structure
and tectonic evolution of the Himalayan fold-thrust belt in western Nepal. Tectonics 20, 487 – 509.
De Sigoyer, J.de., Chavagnac, V., Blichert-Toft, J., Villa, I.M.,
Luais, B., Guillot, S., Cosca, M., Mascle, G., 2000. Dating
the Indian continental subduction and collisional thickening in
the northwest Himalaya: Multichronology of the Tso Morari
eclogites. Geology 28, 487 – 490.
Dewey, J.F., Shackleton, R.M., Chang, C., Sun, Y., 1988. The tectonic evolution of the Tibetan plateau. Philosophical Transactions of the Royal Society, London, A 327, 379 – 413.
Dewey, J.F., Cande, S., Pitman, W.C., 1989. Tectonic evolution of
the India – Eurasia collision zone. Eclogae Geologica Helvetiae
82, 717 – 734.
Dewey, J.F., Ryan, P.D., Anderson, T.B., 1993. Orogenic uplift and
collapse, crustal thickness, fabrics and metamorphic phase
changes: the role of eclogies. In: Pritchard, H.M., et al.,
(Eds.), Magmatic Processes and Plate Tectonics. Geological Society of London. Special Publication, vol. 76, pp. 325 – 343.
England, P.C., Houseman, G., 1988. The mechanics of the Tibetan
plateau. Philosophical Transactions of the Royal Society, London, A 326, 301 – 319.
England, P.C., Searl, M.P., 1986. The Cretaceous – Tertiary deformation of the Lhasa Block and its implications for crustal thickening in Tibet. Tectonics 5, 1 – 14.
Frank, F.C., 1968. Curvature of Island Arcs. Nature 220, 363.
Godin, L., Parrish, R.R., Brown, R.L., Hodges, K.V., 2001. Crustal
M.R.W. Johnson / Earth-Science Reviews 59 (2002) 101–123
thickening leading to exhumation of the Himalayan Metamorphic core of central Nepal: insights from U – Pb geochronology
and 40Ar/39Ar thermochronology. Tectonics 20, 729 – 741.
Harrison, T.M., An Yin, Ryerson, F.J., 1998. Orographic evolution
of the Himalaya and Tibetan plateau. In: Crowley, T.J., Burke,
K.C. (Eds.), Tectonic Boundary Conditions for Climate Reconstruction. Oxford Univ. Press, Oxford, pp. 39 – 72.
Hauck, M.L., Nelson, K.D., Brown, L.D., Zhao, W., Ross, A.R.,
1998. Crustal structure of the Himalayan orogen at 90j east
longitude from Project INDEPTH deep reflection profiles. Tectonics 17 (4), 481 – 500.
Hendrix, M.S., Dumitru, T.A., Graham, S.A., 1994. Late-Oligocene – Early Miocene unroofing in the Chinese Tien Shan: and
early effect of Indian – Asian collision. Geology 22, 487 – 490.
Hodges, K.V., 1998. The Thermodynamics of Himalayan orogenesis. In: Treloar, P., O’Brien, P.J. (Eds.), What Drives Metamorphism and Metamorphic Reactions? Geological Society of
London Special Publication, vol. 38, pp. 7 – 22.
Hodges, K.V., 2000. Tectonics of the Himalaya and southern Tibet
from two perspectives. Bulletin the Geological Society of America 112, 324 – 350.
Hodges, K.V., Hames, W.E., Olszewski, W.J., Burchfiel, B.C., Royden, L.H., Chen, Z., 1994. Thermobarometric and 40Ar/39Ar
geochronological contraints on Eohimalayan metamorphism in
the Dinggye area, southern Tibet. Contributions to Mineralogy
and Petrology 117, 151 – 163.
Hoke, L., Lamb, S., Hilton, D.R., Poreda, R.J., 2000. Southern limit
of mantle-derived helium emissions in Tibetan lithosphere: implications for lithospheric structure. Earth and Science Planetary
Letters 180, 297 – 308.
Holt, W.E., Wallace, T.C., 1990. Crustal thickness and upper mantle
velocities in the Tibetan Plateau region from the inversion of Pnl
waveforms: evidence for a thick mantle lid beneath S. Tibet.
Journal of Geophysical Research 95, 12499 – 12525.
Huang, W.C., Ni, J.F., Tilman, F. et al., 2000. Seismic polarisation
anisotropy beneath the central Tibetan Plateau. Journal of Geophysical Research 105, 27979 – 27989.
Johnson, M.R.W., 1994. Volume balance of erosion loss and sediment deposition related to Himalayan uplift. Journal of the Geological Society London 151, 217 – 220.
Johnson, M.R.W., Rogers, G.R., 1997. Rb – Sr ages of micas from
the Kathmandu complex, Central Nepalese Himalaya : implications for the evolution of the Main Central thrust. Journal of the
Geological Society, London 154, 863 – 869.
Johnson, M.R.W., Oliver, G.J.H., Parrish, R.R., Johnson, S.P.,
2001. Syn-thrusting metamorphism, cooling and erosion of
the Himalayan Kathmandu Complex, Nepal. Tectonics 20,
394 – 415.
Klootwijk, C.T., Gee, J.S., Pierce, J.W., Smith, G.M., Mcfadden,
P.L., 1992. An early India – Asia contact:Paleomagnetic constraints from Nineyeast Ridge ODP Leg 121. Geology 20,
395 – 398.
Le Pichon, X., Fournier, M., Jolivet, J., 1992. Kinematics, topography, shortening and extrusion in the India – Asia collision.
Tectonics 11, 1085 – 1098.
Lyon-Caen, H., Molnar, P., 1983. Constraints on the structure of the
Himalaya from the analysis of gravity anomalies and a flexural
121
model of the Himalaya. Journal of Geophysical Research 88,
8171 – 8191.
Lyon-Caen, H., Molnar, P., 1985. Gravity anomalies, flexure on the
Indian plate and the structure, support and evolution of the
Himalaya and Ganga Basin. Tectonics 4, 513 – 538.
Makovsky, Y., Klemperer, S.L., Liyan, H., Deyuan, L., 1996. Structural elements of the southern Tethyan Himalayan crust from
wide-angle seismic data. Tectonics 15, 997 – 1005.
Makovsky, Y., Klemperer, S.L., Ratschbacher, L., Alsdorf, D.,
1999. Midcrustal reflector on INDEPTH wide-angle profiles :
an ophiolitic slab beneath the India – Asia suture in southern
Tibet? Tectonics 18, 793 – 808.
Matte, P., Mattauer, M., Olivet, J.M., Griot, D.A., 1997. Continental
subductions beneath Tibet and the Himalayan orogeny: a review. Terra Nova 9 (5/6), 264 – 270.
Meyer, B., Tapponnier, P., Bourjot, L., Metivier, Y., Gaudemer, G.,
Peltzer, G., Shunmin, G., Zhitai, C., 1998. Crustal thickening in
Gansu – Qinghai, lithospheric mantle subduction, and oblique,
strike – slip controlled growth of the Tibetan plateau. Geophysical Journal International 135, 1 – 48.
Mohan, G., Rai, S.S., Panza, G.F., 1997. Shear velocity structure of
the laterally heterogeneous crust and uppermost mantle beneath
the Indian region. Tectonophysics 277, 259 – 270.
Molnar, P., 1987. Inversion of profiles of uplift rates for the geometry of dip – slip faults at depth with examples from the Alps and
the Himalaya. Annales Geophysics 5, 663 – 670.
Molnar, P., 1988. A review of geophysical constraints on the deep
structure of the Tibetan plateau, the Himalaya, and the Karakoram, and their implications. Philosophical Transactions of the
Royal Society, London, A 326, 33 – 88.
Molnar, P., Grey, D., 1979. Subduction of continental lithosphere:
some constraints and uncertainties. Geology 7, 58 – 62.
Molnar, P., Tapponnier, P., 1975. Cenozoic tectonics of Asia: effects
of continental collision. Science 189, 419 – 426.
Murphy, M.A., An Yin, Harrison, T.M., Durr, S.B., Chen, Z., Ryerson, F.J., Kidd, W.S.F., Wang, X., Zhou, X., 1997. Did the IndoAsian collision alone create the Tibetan plateau? Geology 25,
719 – 722.
Murphy, M.A., An Yin, Kapp, P., Harrison, T.M., Ding, L., Guo, J.,
2000. Southward propagation of the Karakoram fault system,
southwest Tibet: timing and magnitude of slip. Geology 28,
451 – 454.
Nelson, K.D., et al., 1996. Partially molten Middle crust beneath
Southern Tibet: synthesis of Project INDEPTH RESULTS. Science 274, 1684 – 1688.
Owens, T.J., Zandt, G., 1997. Implications of crustal property variations for models of Tibetan plateau evolution. Nature 387,
37 – 43.
Parrish, R.R., Hodges, K.V., 1996. Isotopic constraints on the age
and provenance of the Lesser and Greater Himalayan sequences,
Nepalese Himalaya. Bulletin of the Geological Society of America 108, 904 – 911.
Patriat, P., Achache, J., 1984. India – Eurasia collision chronology
has implications for crustal shortening and driving mechanisms
of plates. Nature 311, 615 – 621.
Peltzer, G., Tapponnier, P., 1988. Formation and evolution of
Strike – Slip faults, Rifts and basins during the India – Asia col-
122
M.R.W. Johnson / Earth-Science Reviews 59 (2002) 101–123
lision: an experimental approach. Journal of Geophysical Research 93, 15085 – 15117.
Platt, J., England, P.C., 1993. Convective removal lithosphere beneath Mountain belts: thermal and mechanical consequences.
American Journal of Science 293, 307 – 336.
Powell, C.M., 1986. Continental underplating model for the rise of the
Tibetan plateau. Earth and Planetary Science Letters 81, 79 – 94.
Powell, C.M., Conaghan, P.J., 1973. Plate tectonics and the Himalaya. Earth and Planetary Science Letters 20, 1 – 12.
Prince, C., Harris, N., Vance, D., 2001. Fluid-enhanced melting
during prograde metamorphism. Journal of the Geological Society, London, 158, 233 – 241.
Ratschbacher, L., Frisch, W., Lui, G., Chen, C., 1994. Distributed
deformation in southern Tibet during and after the India – Asia
collision. Journal of Geophysical Research 99 (B10) 19,917 –
19,945.
Royden, L.H., 1993. The tectonic expression slab pull at continental
convergent boundaries. Tectonics 12, 303 – 325.
Sandvol, E., Ni, J., Kind, R., Zhao, W., 1997. Seismic anistropy
beneath the southern Himalayas. Journal of Geophysical Research 102, 17813 – 17823.
Schelling, D., 1992. The tectonostratigraphy and structure of eastern
Nepal Himalaya. Tectonics 11, 925 – 943.
Schelling, D., Arita, K., 1991. Thrust tectonics, crustal shortening—
the structure of the far-eastern Nepal Himalaya. Tectonics 10,
851 – 862.
Searle, M.P., 1999. Extensional and compressional faults in the
Everest-Lhotse massif, Khumbu Himalaya, Nepal. Journal of
the Geological Society of London 156, 227 – 240.
Searle, M.P., 2001a. Comment on de Sigoyer et al. 2000. Geology
29, 191 – 192.
Searle, M.P., 2001b. Conference Report, 15th Himalaya – Karakoram – Tibet Workshop, Chengdu, 2000. Journal of Asian Earth
Sciences 19, 547 – 551.
Searle, M.P., Cooper, D.J.W., Rex, A.J., 1988. Collision tectonics of
the Ladakh-Zanskar Himalaya. In: Shackleton, R.M., Dewey,
J.F., Windley, B.F. (Eds.), Tectonic Evolution of the Himalaya and Tibet. Philosophical Transactions of the Royal Society London, A, vol. 326, pp. 117 – 150.
Searle, M.P., Waters, D.J., Rex, D.C., Wilson, R.N., 1992. Pressure,
temperature and time constraints on Himalayan metamorphism
for eastern Kashmir and western Zanskar. Journal of the Geological Society of London 149, 753 – 773.
Searle, M.P., Weinberg, R.F., Dunlap, W.J., 1998. Transpressional
tectonics along the Karakoram fault, northern Ladakh: constraints in Tibetan extrusion. In: Holdsworth, R.E., Strachan,
R.A., Dewey, J.F. (Eds.), Continental Transpression. Geological Society of London Special Publications, vol. 135, pp.
307 – 326.
Searle, M.P., Waters, D.J., Dransfield, M.W., Stephenson, B.J.,
Walker, J.D., Rex, D.C., 1999. Thermal and mechanical models
for the structural and metamorphic evolution of the Zanskar
High Himalaya. In: MacNiocaill, C.S., Ryan, P.D. (Eds.), Continental Tectonics. Geological Society of London Special Publication, vol. 164, pp. 139 – 156.
Simpson, R.L., Parrish, R.R., Searle, M.P., Waters, D.J., 1999. Two
episodes of monazite crystallisation during metamorphism and
crustal melting in the Everest region of Nepalese Himalaya,
Abstracts. 15th Himalayan – Karakoram – Tibet Workshop,
Chengdu.
Srivastava, P., Mitra, G., 1994. Thrust geometries and deep structure
of the outer a and lesser Himalaya, Kumoan and Garhwal (India): implications for the evolution of the Himalayan fold-thrust
belt. Tectonics 13, 89 – 109.
Stephenson, B.J., Waters, D.J., Searle, M.P., 2000. Inverted metamorphism and the Main Central thrust: field relations and thermobarometric constraints from Kishwar window, NW Indian
Himalaya. Journal of Metamorphic Geology 18, 571 – 590.
Thakur, V.C., 1999. Uplift and convergence along the Himalayan
Frontal Thrust of India. Tectonics 18, 967 – 976.
Tonarino, S., Oberli, M., Meier, F., Spencer, D.A., Pognante, U.,
Ramsay, J.G., 1993. Eocene age of eclogite metamorphism in
Pakistan: implications for India – Asia collision. Terra Nova 5,
13 – 20.
Treloar, P.J., Williams, M.P., Coward, M.P., 1989. Metamorphism
and crustal stacking in the Northern Indian Plate, North Pakistan. Tectonophysics 165, 167 – 184.
Upreti, B.N., Le Fort, P., 1999. Lesser Himalayan cystalline nappes
of Nepal: problem of their origin. In: Macfarlane, A., Sorkhabi,
R.B., Quade, J. (Eds.), Himalaya and Tibet: Mountain Roots to
Mountain Tops. Geological Society of America, Special Paper,
vol. 328, pp. 225 – 238.
Van der Voo, R., Spakman, W., Bijwaard, H., 1999. Tethyan subducted slabs under India. Earth and Planetary Science Letters
171, 7 – 20.
Vannay, J.C., Hodges, K.V., 1996. Tectonometamorphic evolution
of the Himalayan metamorphic core between Annapurna and
Dhaulagiri, central Nepal. Journal of Metamorphic Geology
14, 635 – 656.
Veevers, J.J., Powell, C.M., Johnson, B.D., 1975. Greater India’s
place in Gondwanaland and in Asia. Earth and Planetary Science Letters 27, 383 – 387.
Walker, J.D., Martin, M.W., Bowring, S.A., Searle, M.P., Waters,
D.J., Hodges, K.V., 1999. Metamorphism, melting and extension: age constraints from the high himalayan slab of Southeast Zanskar and Northwest Lahaul. Journal of Geology 107,
473 – 495.
Wesnousky, S.G., Kumar, S., Mohindra, R., Thakur, V.C., 1999.
Uplift and convergence along Himalayan Frontal Thrust of India. Tectonics 18, 967 – 976.
Westerway, R., 1995. Crustal volume balance during the India –
Eurasian collision and altitude of the Tibetan plateau: a working
hypothesis. Journal of Geophysical Research 100 (B8), 15173 –
15192.
Willett, S.D., Beaumont, C., 1994. Subduction of Asian lithospheric
mantle beneath Tibet inferred from models of continental collision. Nature 369, 642 – 645.
Wu, C., Nelson, K.D., Wortman, G., Samson, S.D., Yue, Y., Li, J.,
Kidd, W.S.F., Edwards, M.A., 1998. Yadong cross-structure and
the South Tibetan Detachment in the east central Himalaya
(89 – 90jE). Tectonics 17, 28 – 45.
Yeats, R.S., Thakur, V.C., 1998. Reassessment of earthquake hazard
based on a fault bend model of the Himalayan plate boundary
fault. Current Science 74 (3), 230 – 233, New Delhi.
M.R.W. Johnson / Earth-Science Reviews 59 (2002) 101–123
Yuan, X.H., Ni, J., Kind, R., Mechie, J., Sandvol, E., 1997. Lithospheric and upper mantle structure of southern Tibet from a
seismological passive source experiment. Journal of Geophysical Research 102, 27491 – 27500.
Zhao, W., Nelson, K.D., and The Indepth Team, 1993. Deep seismic
123
reflection evidence for continental underthrusting beneath
southern Tibet. Nature 366, 557 – 559.
Zhou, W.L., Morgan, W.J., 1995. Uplift of the Tibetan plateau.
Tectonics 4, 359 – 369.