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Thin-skinned N-S extension within the convergent
Himalayan region: gravitational collapse of a
Miocene topographic front
L.H. Royden & B.C. Burchfiel
SUMMARY: Recent work by Burg et al. indicates the presence of E-W striking, gently Ndipping normal faults in the High Himalayas and southern Tibet, that formed during the postcollisional convergence of India and Tibet. These faults extend for at least 600 km along strike.
We interpret them as probable late (?) Miocene extensional features with perhaps several tens of
kilometres downward northerly displacement. A simple elastic model suggests that these normal
faults may have formed during gravitational collapse of the Miocene topographic front between
India and Tibet. In this interpretation, gravitational collapse occurred by southward motion,
relative to India and Tibet, of a wedge of crustal rock bounded above by gently dipping normal
faults and below by thrust faults that probably dip N. N-S extension produced in this way is
probably confined to upper crustal levels only and does not reflect regional extension of the entire
lithosphere. Such faults may be common, but so far mainly unrecognized, features developed
during convergence in many orogenic belts.
The Himalayan mountain belt has been formed by
N-S shortening and crustal thickening during
convergence between the Indian and Asian Plates
after their collision at about 45 Ma (Fig. 1). Recent
work by Burg et al. (1984) indicates the existence
of E-W-striking, gently N-dipping normal faults of
regional extent in and N of the High Himalayas
(Fig. 2). These faults appear to have had down-tothe-N displacement of probable Miocene age and
to be a manifestation of N-S sub-horizontal
extensional strain within the Himalayan crust. They
are coeval with S-directed overthrusting along the
Main Boundary Thrust (MBT) or Main Central
Thrust (MCT) to the S and roughly coeval with Ndirected overthrusting along back-thrusts N of the
normal-fault zone.
The existence of E - W extension within the
Himalayan mountains and the Tibetan plateau was
first recognized by Molnar & Tapponier (1975) and
has been still-better documented by more recent
studies (Tapponnier & Molnar 1977; Armijo et al.
1984). This E - W extension is a young example of
the phenomenon of continental escape (Burke 1982,
unpublished lecture) that occurs as material within
the collision zone escapes laterally (perpendicular
to the direction of convergence) towards a zone of
lower compressional strain (see also McKenzie
1972). The driving mechanism for continental
escape is generally believed to arise from the high
confining pressures beneath topographically high
areas in the collision zone, relative to the lower
confining pressures present at the same depth
relative to sea-level in areas adjacent to the collision
zone (see discussion in Dalmayrac & Molnar 1981,
England & MacKenzie 1982, 1983, and Houseman
et al. 1981).
Coney & Harms (1984) have suggested that the
extensional regime in western North America
developed by spreading in an overthickened crust
following a reduction of intraplate compressional
stress. Other studies have suggested that the same
mechanism may lead to extension parallel to the
earlier direction of shortening, even before the
termination of subduction, giving rise to collapse of
the orogenic belt and possible development of an
extensional regime (Dalmayrac & Molnar 1981).
FIG. 1. Location map showing the central Himalayansouthern Tibet region covered in Fig. 2 and its relation
to Tibet, India and the Indian-southern Tibet suture
(Yarlung-Zangbo suture in black).
From COWARD,M.P., DEWEY,J.F. & HANCOCK,P.L. (eds), 1987, Continental Extensional
Tectonics, Geological Society Special Publication No. 28, lap. 611-619.
611
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612
L.H. Royden & B.C. Burchfiel
FIG. 2. (a) Major tectonic elements of the central Himalayan-southern Tibet region and location of line of section
shown in Fig. 2(b). Lines with closed barbs are thrust faults; lines with open barbs are N-vergent back-thrusts; lines
with black semi-circles are N-dipping normal faults; lines with ticks are N-trending normal faults; and the N belt of
leucogranites is in black. MCT =Main Central Thrust; MBT=Main Boundary Thrust; N = Nyalam; G=Guzuo.
(b) Generalized cross-section through the Qomolangma area emphasizing the position of the N-dipping normal faults
and the S-vergent MCT and MBT and the N-vergent back-thrusts.
Within convergent orogenic belts, significant
amounts of synchronous extension and compression
in the same direction are not generally observed.
Normal faults that are parallel to, and coeval with,
compressional structures are generally absent or
developed only locally, such as at shallow crustal
levels at the crests of anticlines or associated with
pull-apart structures along strike-slip faults.
The existence of E-W-striking Miocene normal
faults within the N-S-convergent regime of the
Himalayas seems at first glance to represent a
mechanical contradiction, because it implies
significant N-S extension within a region of N-S
shortening and convergence. In this paper we will
show how the development of these normal faults
within the High Himalayas may be genetically
related to the shortening strain and consequent high
topography produced by mountain building in the
Himalayas and the Tibetan plateau.
N-dipping normal faults in the
Himalayas
The gently N-dipping (15-30~ normal fault or fault
zone mapped by Burg et al. (1984) in the High
Himalayas extends for at least 600 km along strike
and separates unmetamorphosed to weakly metamorphosed Ordovician rocks above, from highgrade metamorphic rocks, metamorphosed in
Cenozoic time, below (figs 1 & 2, Wang & Zheng
1975). Various shear-sense indicators (mesoscopic
S-C fabrics, quartz microfabrics and rotated
porphyroblasts and pressure shadows) from a zone
100 to 200 m below the fault contact, consistently
show downward northerly displacement of the
hanging wall (Burg et al. 1984). Rocks of the
hanging wall are generally only mildly deformed,
contain only a spaced cleavage and do not include
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N - S extension in the convergent Himalayan region
the leucogranite dykes and sills that lie immediately
below.
Within the hanging wall of the normal fault,
described above, are other N-dipping faults with
normal displacement. In the area N of Nyalam
(Fig. 2), there are two N-dipping faults that place
younger rocks in the hanging wall against older
rocks in the footwall. We interpret these faults as
having normal displacement. About 25 km farther
N (S of Guzuo), mesoscopic folds, in a N-dipping
section of Jurassic rocks have a down to the N sense
of relative displacement (Fig. 2), even though they
are on the N flank of an anticline and should show
the reverse sense of movement. This suggests that
downward northerly displacement within the High
Himalayas may be distributed across a zone at least
25 km wide (J.P. Burg, pers. comm. 1984).
In the Qomolangma (Everest) area, Wang &
Zheng (1975) have mapped three N-dipping faults,
which they interpreted as S-vergent thrust faults.
These faults have a gentle dip (5-7 ~ and are
present through about 4-6 km of structural thickness and for 40-50 km across strike. One of these
faults separates the Sinian (?) to early Ordovician
sedimentary rocks (the Bei Ao Group), at the base
of the Tibetan sedimentary sequence, from higher
grade metamorphic rocks of the crystalline zone
below (Fig. 2) and is the same fault as that
described by Burg et al. (1984). All three faults,
mapped by Wang & Zheng (1975), consistently
place younger rocks on older rocks and, where
some of the Ordovician rocks are locally marble,
place less-metamorphosed rocks on more highly
metamorphosed rocks. The thicknesses of stratigraphic units between these faults or near their
contacts are highly variable and often thin from N to
S. Early ductile structures in the metamorphic rocks
and the mylonitic rocks of the hanging wall are Svergent, suggesting that they developed during
S-vergent displacement, but the faults are characteristically marked by breccia up to a few metres
thick. Evidence for the thrust sense of displacement
on the faults is inferred from the ductile structures,
not from structures associated with the brittle
deformation that characterizes the faults. Descriptions of these faults are reminiscent of the low-angle
normal faults from the Basin and Range Province of
the western United States (for example see Davis
et aI. 1980).
Thus from the reconnaissance work of Burg et al.
(1984) and Wang & Zheng (1975), it appears that
there is a major normal fault at the base of the
Tibetan sedimentary sequence and that gently
dipping normal faults with downward northerly
displacement developed across a 25-50 km wide
zone (Fig. 2). The fault or fault zone can be traced
for more than 600 km along strike in Tibet and may
extend for a further 200 km to the W into India, as
613
suggested by the mapping of Valdiya & Goel (1983)
in the Kumaum area. Thr basal fault or faults in this
array juxtapose rocks from two different metamorphic environments, which suggests that a
significant portion (10-15 km or more) of the
crustal sequence is absent. From the gentle dip of
these faults, we infer that large displacements (of
probably tens of kilometres) are involved.
Location of normal faults relative
to other structures
The N-dipping normal faults lie between areas
dominated by structures formed as the result of N-S
shortening. To the S are the N-dipping thrust faults
of the MCT and MBT and to the N are the S-dipping
back-thrusts of the Yarlung-Zangbo area (Fig. 2).
The normal faults also lie near to or within a zone of
N-dipping thrust faults present in or at the base of
the Tibetan sedimentary sequence. The N-dipping
normal faults and all the thrust faults and folds,
observable from the MCT to the southern Lhasa
Block, are cut by N-trending normal faults associated with the active E - W extension in the southern
part of the Tibetan plateau.
Timing of normal faults relative to
other structures
Relative timing constraints suggest that the gently
N-dipping normal faults formed during Miocene
(perhaps late Miocene) or earliest Pliocene time.
They are cut by faults that bound N-trending
grabens, giving an upper age limit of 2-4 Ma,
according to Armijo et al. (1984). The gently Ndipping normal faults cut metamorphic rocks and
leucogranites of the crystalline zone in the footwall
(Fig. 2) producing a localized fabric. This suggests
that the faults may have begun to form at a late stage
in the metamorphic and igneous development of the
footwall crystalline zone. Unfortunately, the
metamorphism and time of leucogranite intrusion is
poorly constrained, but the range in radiometric
ages suggests the occurrence of an event or events
between 30 and 15 Ma (Allegre et al. 1984). These
relations imply a time for fault formation of
between 30 and 15 Ma and 4 Ma (i.e. Miocene to
earliest Pliocene time). Similar constraints can be
placed on N-vergent back-thrusts that are present
100-150 km farther N (Fig. 2). N-dipping thrusts
within Tibet are less-well constrained. Recent work
by Burg & Chen (1984) suggests that these faults
may be latest Cretaceous to early Tertiary in age
and related to a pre-collision event, although some
of their movement could be related to convergent
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614
L.H. Royden & B.C. Burchfiel
events following collision. Our interpretation is that
the N-dipping normal faults are, (i) younger than
the thrust faults of Tibet; (ii) broadly contemporaneous with movement on either or both the MCT
and MBT and the back-thrusts in the YarlungZangbo area and (iii) broadly contemporaneous
with but locally outlasting the regional metamorphism and leucogranite intrusion within the
exposed part of the crystalline zone of the High
Himalayas.
Interpretation
From existing data we interpret the E-W-stril~ing,
gently dipping normal faults described above as
probable Miocene or earliest Pliocene extensional
features, with perhaps several tens of kilometres of
downward northerly displacement. This presents
mechanical problems because in this area of
regional N-S convergence (5 cm yr -l between
India and Siberia), the normal faulting and extension were roughly contemporaneous with S-vergent
thrusting and subduction along the MCT or MBT
and with N-vergent back-thrusting about 100150 km N of the normal-fault zone (Fig. 2).
Burg et al. (1984) interpreted normal faulting as
back-sliding on an older thrust fault. They recognized that locally the horizontal N-S compressive
stress must have been less than the vertical stress
and suggested that this might be due to gravitational
effects in the presence of high topographic relief.
In this paper we present an explanation related to
the stress orientation in the convergent zone of
southern Tibet and northern India.
An idealized cross-section through the High
Himalayas at the time that this normal fault zone
was active, indicates southward motion, relative to
both India and Tibet, of a wedge of crustal rocks
(Fig. 3). This wedge was bounded above by gently
N-dipping normal faults and below by N-dipping
thrust faults. Near the surface, the thrust faults that
bounded the base of the southward-moving wedge
may be identical to the contemporaneous sub-
M ocene.
t op,ogra, pnic
South
~ ' , ~
i
100 km
~
Approximate Vertical
Exaggeration 1 : 4
~,~-,~
-~.
The effect of topographic relief on
stress distribution
In this paper we present a simple two-dimensional
analysis of stress distribution within the lithosphere
beneath the Himalayas arising from the differences
in topographic elevation N and S of the belt. The
lithosphere of the Indian-Tibetan region is treated
as a perfectly elastic half-space, initially under
horizontal compression and with its surface at sealevel (Fig. 4). The effects of the topographically
high areas (Himalayas and Tibet) are approximated
by treating material above sea-level as a vertical
load applied to the surface of the elastic sheet,
so that the assumed load is proportional to the
elevation above sea-level at each point. The surface
of the infinite half-space is assumed to have no shear
stresses. This approximation yields an analytical
solution, (see for example, Scott 1963). More
complicated models require solution by numerical
North
rr~ni
-
duction boundary between India and Tibet, but are
not necessarily so. The nature of the geometry o f
this crustal wedge at depth (shown in Fig. 3) is
totally speculative.
We suggest that normal faulting and southward
displacement of the crustal wedge (Fig. 3) were the
results of gravitational collapse along the Mioceneearliest Pliocene topographic front between the
Indian foreland (present average elevation a few
hundred metres above sea-level) and the Tibetan
plateau (present average elevation about 4-5 km
above sea-level). A simple elastic model, described
below, suggests that this large, abrupt change in
elevation may have produced N-S sub-horizontal
extensional stress (or strain) extending for several
tens of kilometres N of the topographic front and to
10-20 km depth. Sub-horizontal extensional
stresses (or strain), produced in this way, appear to
be restricted mainly to crustal levels and do not
reflect N-S extension of the lithosphere at greater
depths.
~///~
~
:
sea level
FIG. 3. Diagrammatic cross-section
through the Himalayas and southern
Tibet showing; (1) thrust faulting
along the MCT (or MBT); (2)
normal faulting within the High
Himalayas; and (3) back-thrusting
near the Tsangpo suture zone.
Kinematic relationships imply that a
shallow wedge of crustal material
must have moved southward relative
to both India and Tibet. The wedge
is bounded above by N-dipping
normal fault(s) (2), and below by
N-dipping thrust faults, possibly but
not necessarily the MCT or MBT
(1). The geometry shown at depth is
speculative.
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615
N - S extension in the convergent H i m a l a y a n region
•
y=o
,
Y~
~ ~ _
LOAD
ELASTIC PLATE
1
"
E
~.~./I
ELASTIC
LOA D
HALF-S~
C~xx: pgy + ay
GRAVITY
+
O-yy: p g y
FIG. 4. Simple elastic model used to generate the stress trajectories shown in Fig. 5. The Indian-Tibetan lithosphere
is taken to be an infinite elastic sheet with a surface at y = 0 for x ~< -e, y=h for x >/0, and a uniform slope between
x = - e and x=0 (y=-h(x+e)/Q. The resulting stresses are approximated by considering the sum of (1) the stresses
produced by loading a semi-infinite half-space with a vertical load equivalent to the vertical load of all material
above y=0 (load=q=ogh for x >/O, load=ogh(x+e)e for -e ~< x <0), and (2) an elastic sheet with a stress
distribution Oxx=Qgy+ay, ayy=Ogy. There is no shear stress on the surface y=0. Solutions can be found in the
text.
techniques, (see for example, Bott & Dean 1972).
Because the effective elastic thickness of the Indian
Plate beneath Tibet is estimated to be - 8 0 km
(Lyon-Caen & Molnar 1983), this approach should
provide reasonable results sufficiently far above the
base of the elastic plate, to depths of 40-50 km.
Prior to loading of the elastic half-space, the
stress-I-distribution was taken
to be
a yy = ogY, axx =
.
.
ogY
ay " a xy = 0. This choice of initial stress
distribution is somewhat arbitrary, but we reason
that in the Earth, differential stresses are likely to be
smaller near the surface than at depth. The topographic elevation and the associated load, is
assumed to have been zero over the Indian shield
and 5 km (or q = 1.25 kb load) over Tibet and the
increase in topographic elevation (or load) is
assumed to have occurred linearly across a finite
width, e (Table 1).
The resulting stress field is described by:
/
~x.X -~- ~q--- I ~-- -Jr- O1 -t- ~--x
(01 _ 02)
t
---
e
x 2 + 22
gyy= ~q
[ ~7r
X
+ Ol +-g-
-- qY
7r"'~ (01
tIxY =
+ o g y + ay
(01 _ 0 2 ) ]
+ ogy '
- 02)
where
01 =
arctan[(x + e ) / y ] a n d 0 z = a r c t a n ( x / y ) .
Figure 5 shows the calculated stress trajectories
for loading of an elastic half-space, with two
different assumptions of initial stress and with a
5 km increase in elevation over a 50 km interval.
Far from the region of change in topography, o 1
(maximum compressive stress) is nearly horizontal,
but near the topographic slope, the principal
stresses are rotated so that a~ becomes steep.
Figure 5 shows that beneath the northem part of the
topographic slope and the southern part of the
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L.H. Royden & B.C. Burchfiel
616
TABLE 1.
Symbol
Value
Definition
a
0.01, 0.02 kb km-a
Horizontal stress prior to
loading is ogy+ay
g
Qg = 0.25 kb km-i
Gravitational constant
Qg = 0.25 kb km
Crustal density
h
5 km
Topographic elevation of plateau
q
1.25 kb
Load equivalent to topographic
elevation h
f
50 km
Width of zone of linearly
increasing topographic elevation
o,
0" 2
,=-,00
y=0km-
'
)
/
N
02
x.o/"
I
[
N
O0 km
I
_
Y = 40 km -
y=80
km-
(a)
02
0" 2
X= -
,
x_O / x , ,
...l~ll-,i,.-- 01
x = 100 km
i
y:Okm-
y: 40 km_
y=80
_
kin-
(b)
FIG. 5. Stress trajectories as described in the text. In both cases the load q = 1.25 kb (corresponds to roughly 5 km
elevation) and the width of the zone of linearly increasing load is e=50 km. (a) Horizontal stress prior to loading is
axx=0.26y, vertical stress prior to loading is a)y=0.25y, y in km, stress in kb.
(b) Horizontal stress prior to loading is axx=0.27y, vertical stress prior to loading is ayy+O.25y, y in km, stress in
kb. North and South refer to compass directions analogous to the topographic differences between India and Tibet.
Dark lines show orientation of at (maximum compressive stress) and a 2 (minimum compressive stress). Note rotation
of principal stress axes near the southern edge of the high-standing region. This effect becomes more pronounced for
smaller values of a and e and larger values of q (Table 1).
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N-S extension in the convergent Himalayan region
plateau, o~ axes plunge S, with gentle plunges at
depth and progressively steeper plunges near the
surface. Likewise, trz (minimum compressional
stress) axes plunge N in the same area and become
horizontal at the surface. Comparing Figs 3 and 5
shows that beneath the northern part of the
topographic slope and the southern part of the
plateau, the calculated orientation of the principal
stress axes is consistent with the sense of movement
inferred for the southward-moving crustal wedge
from geological field data (Fig. 3). Far away from
the topographic slope, the calculated stress field is
consistent with sub-horizontal compression and
thrust faulting.
The maximum deviatoric stresses calculated
within the area, where the vertical compressive
stresses exceed the horizontal compressive stresses,
are about 800 bars in Fig. 5(a) and 400 bars in
Fig. 5(b). This occurs nearx = 0 in both cases and
at depths of about 25-30 km in Fig. 5(a) and about
10-15 km in Fig. 5(b).
The simple model for stress distribution
presented in this paper is intended only to explain
the apparently incompatible juxtaposition of
contemporaneous thrust faults and E-W-striking,
N-dipping normal faults in southern Tibet. The
stress orientations plotted in Fig. 5 are designed
only to illustrate qualitatively the effects of large
changes in topographic elevation on a regionally
compressive stress regime. They are not intended to
be realistic representations of the stress field
beneath the Himalayas or any other mountain belt.
It is clear that crustal deformation beneath Tibet is
not elastic and that zones of ductile, plastic and
viscous flow are important in controlling local and
regional deformation, particularly in this area of
young magmatic activity. Other potentially significant features ignored in this analysis are the Ndipping main subduction boundary beneath the
Himalayas and Tibet, crustal anisotropy produced
by N-dipping bedding and foliation in upper-plate
rocks, changes in crustal thickness between India
and Tibet, stress concentrations at the end of preexisting cracks, the role of ductile deformation at
depth, and the effect of near-surface stresses in the
topographically high regions.
We do not mean to imply that a large change in
topographic relief necessarily produces subhorizontal extension, but only that the stresses resuiting from such geometries are conducive to subhorizontal extension as illustrated in Fig. 5. These
calculations are intended only as a qualitative explanation for field observations, not as a predictive or
quantitative tool. For example, if larger values of
horizontal compressional stress are assumed near
the surface, the loading effect of the topographically
high region will be swamped and sub-horizontal
compression will be present almost everywhere.
617
Discussion
A simplified elastic model shows that large changes
in topographic elevation may generate subhorizontal extensional stresses near the edge of a
high-standing plateau that are parallel to the
direction of regional compression. Sub-horizontal
extensional stresses generated in this way appear to
be confined to crustal levels (within several tens of
kilometres of the surface) and probably do not
extend deep into the lithosphere. The orientations of
the principal stress axes near the edge of the highstanding region are consistent with the N-S
extension of the southernmost Tibetan plateau and
High Himalayas at shallow crustal levels; southward motion of a crustal wedge as shown in Fig. 3
is in reasonable agreement with gently N-plunging
T axes (tr2) as shown in Fig. 5. Thus N-S crustal
extension at shallow crustal levels within the High
Himalayas and southern Tibet can be mechanically
consistent with contemporaneous N-S shortening to
the S (along the main subduction boundary) and to
the N (back-thrusting in the Tsangpo area).
Normal faults that form in this stress regime
could dip either N or S. The well-developed Ndipping structural anisotropy in the High Himalayas
may have been responsible for the development of
N-dipping normal faults. In fact most mountain
belts would have similarly orientated anisotropy
which would favour the development of normal
faults synthetic to contemporaneous, major thrust
faults formed within an overall convergent system.
The sub-horizontal extensional stresses produced
in southern Tibet and the High Himalayas can be
interpreted as the direct result of gravity acting on
the high-standing plateau and extensional faulting
can be considered as a type of gravitational collapse
of the Himalayan topographic front. Events leading
to gravitational collapse of the front may include
continued crustal thickening at the leading (southern)
edge of the Tibetan Plateau by underplating of the
crust with material from the Indian crust that is
incompletely subducted (Fig. 3). As the crustal
thickness increases beneath the leading edge of
the Tibetan Plateau, the topographic elevation
increases as well (present average elevation of
the High Himalayas is about 6 km) and provides
increased vertical stress to drive extension. We
suggest that eventually the difference in topographic elevation between the Indian foreland and
the southern edge of the Tibetan Plateau increases to
the point at which the stresses generated can no
longer be supported by the strength of the rocks
within the upper crust, and gravitational collapse
occurs.
Gravitational collapse may also be related to
changes in the coefficient of friction along the
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618
L.H. Royden & B.C. Burchfiel
S-dipping subduction boundary in the Himalayas.
For example, Dahlen (1984) has shown, theoretically, that by decreasing friction (e.g. by increasing
pore-pressure) at the base of an accreting wedge,
deformation within the wedge may change from
sub-horizontal compression to sub-horizontal
extension (figs 12 & 13, Dahlen 1984). Thus a
decrease in the effective stresses along the subduction boundary beneath the High Himalayas may
also have induced N-S extensional failure within
the mountain belt.
Decrease in the effective compressive stresses
within the subduction zone could also be caused by
changes in the partitioning of deformation between
the Himalayan thrust zone and other fault zones
farther N in the Tibetan plateau or northern China.
Thus from the above considerations, the circumstances that trigger normal faulting could be
episodic.
We emphasize that the model proposed here is not
the same as those proposed by van Bemmelen
(1954) or Ramberg (1967), who proposed that
thrust nappes form as secondary consequences of
vertical tectonics. Instead, we propose gravitational
spreading of the upper to middle crust of the High
Himalayas within a primarily convergent regime as
crustal material at mid-crustal levels is displaced
southward. In some respects our analysis is similar
to that of Elliott (1976), but in contrast to Elliott, we
believe that the gravity spreading or gravitational
collapse inferred for the Miocene Himalayas is a
secondary effect superimposed on regional N-S
compression. The driving force for this gravitational collapse is similar to that proposed by
Dalmayrac & Molnar (1981), where differences in
confining pressure at a constant depth (relative to
sea-level) tend to drive material from areas of
higher topography to those of lower topography. In
our interpretation, however, extension is confined
to shallow or mid-crustal levels.
Speculation
The relationships outlined above, between normal
faulting and thrust faulting imply local shallow
decoupling within the lithosphere and may even
suggest decoupling on a more regional scale
beneath Tibet. Decoupling implies that very
different and apparently incompatible structures
can develop contemporaneously at different crustal
(or sub-crustal) levels in the same region. An
ancient situation analagous to that observed in
the Himalayas may help to explain paradoxical
relations in southern Canada between the Columbia
River fault zone, a possible extensional low-angle
normal fault, and the convergent deformation
recorded in the adjacent metamorphic rocks of the
Monashee and Shuswap Complexes (Reed &Brown 1981). Similar examples would be in
the Raft River and Albion Ranges, NW Utah
(Allmendinger & Jordan 1981; Compton 1980), the
faults in the Drauzug of southern Austria (Tollmann
1963), the Rhone-Simplon line in the Alps (Steck
1984), or the normal faults described by Platt et al.
(1983) in the Betic Cordillera of southern Spain.
One of the major questions in thrust fault
mechanics has been the role of gravity in thrust
dynamics. The kinematics and dynamics we
envisage for the Himalayan structural development
would suggest that displacement on the normal
faults that bound the top of the southward-moving
crustal wedge in the Himalayas (Fig. 3) and part of
the displacement on the thrust faults that bound its
base, are the result of gravity acting on a region with
considerable topographic relief. Thus a part of the
thrust-fault displacement that is contemporaneous
with normal faulting may be contributed by gravity.
This component, however, is probably small (tens
of kilometres?), relative to the total amount of
Miocene convergence between India and Siberia
(-IOOO kin).
Normal faulting within the Himalayan belt may
be related to other features. For example, the
northern belt of the Himalayan leucogranites is
parallel to the zone of normal faulting (Fig. 2)
and alignment of the batholiths may have been
controlled by the shallow crustal extension and
thinning. The locus of granitic intrusion could have
been controlled by the site of extension and ascent
of magma into the extensional environment above.
Extension in the shallow part of the lithosphere
may also have produced a local lowering of the
elevation of the highest part of the mountain front,
which in turn may have affected the climate in
southern Tibet by reducing the rain shadow effect
that is so pronounced today. Such an effect might
explain the distribution of organisms, such as the
Pliocene Hipperions in Tibet, which features so
prominently in arguments concerning the age of
uplift of the Tibetan plateau (Zheng et al. 1981).
ACKNOWLEDGMENTS" We would like to thank the
geologists of the French-Chinese Tibetan Mission.for the
opportunity to visit southern Tibet in 1984, and to profit
from the results of their scientific cooperation. L.R. is
grateful to the Kerr-McGee Foundation, and the Kerr
Foundation for their generous support of her research, and
Victor Li for helpful discussion and suggestions. B.C.B.
would like to thank the Schlumberger Corporation for
support of his research on this project.
Downloaded from http://sp.lyellcollection.org/ at Pennsylvania State University on September 13, 2016
N - S extension in the convergent Himalayan region
619
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L.H. ROYDEN* & B.C. BURCHFIEL*, Department of Earth, Atmospheric and Planetary Sciences,
Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
*B.C.B. was responsible for geologic interpretations of the faults described, including geometrical
and timing constraints. L.H.R. was responsible for the kinematic interpretation of these faults
within their convergent setting, and for the development of the physical models.