Journalof StructuralGeology, Vol. 16, No. I. pp. 21 to 34, 1994 0191-8141/94 $06.00+0.

00
Printed in Great Britain © 1993Pergamon Press Ltd

Changing mechanical response during continental collision:

active examples from the foreland thrust belts of Pakistan

D A N M . DAVIS

Department of Earth and Space Sciences, State University of New York at Stony Brook, Stony Brook,
NY 11794-2100, U.S.A.

and

ROBERT J. LILLIE

Department of Geosciences, Oregon State University, Corvallis, OR 97331-5506, U.S.A.

(Received 11 June 1992; accepted in revisedform 26 February 1993)

Abstract--We have used data from teleseismic, seismic reflection and field geologic studies, along with both
geomechanical and gravity modeling to contrast the tectonics of four active orogenic wedges in Pakistan: the
Kashmir Himalaya, the Salt Range-Potwar Plateau foldbelt, the Sulaiman Range and the Makran accretionary
wedge.
In Makran, oceanic crust is still being subducted, and a thick pile of sediments is being accreted and
underplated. Undercompaction and excess pore pressures can explain the narrow cross-sectional taper and
frontal aseismicity of this wedge. Beneath the Sulaiman wedge, continental crust is just starting to be underthrust.
Indirect evidence suggests that fine-grained carbonate rocks found in abundance deep in the stratigraphic section
may be deforming ductilely at the base of the Sulaiman wedge and provide a zone of ductile detachment. The
collision has proceeded to a much more mature stage in the Salt Range-Potwar Plateau foldbelt and the Kashmir
Himalaya. Isostatic response to underthrusting of continental crust has kept the sedimentary pile quite thin in
both of these wedges, so in that respect the two foldbelts are similar. However, thick Eocambrian salt beneath the
Salt Range and Potwar Plateau permits that foldbelt to be much wider in map view, with a thinner cross-sectional
taper and a mixture of thrust vergence directions. A major normal fault in basement causes the Salt Range to rise
in front of the mildly deformed molasse basin of the southern Potwar Plateau.
Much of the diversity among these mountain belts can be understood in terms of differences in the maturity of
the collision process in each area, the resulting thickness of the sedimentary pile encountered at the deformation
front, and the presence or absence of large contrasts in strength between the various layers of the stratigraphic
section and basement relief.

INTRODUCTION approximately 10 km at the deformation front near Sibi,

and the depth to crystalline basement in much of the
THE collision of the Indian plate with Asia is producing a foldbelt may be asmuch as 20 km (Banks & Warburton
remarkable variety of active fold-and-thrust wedges 1986). The Salt Range-Potwar Plateau foldbelt is under-
within Pakistan. These zones extend from the Kashmir lain by an Eocambrian evaporite. The relationship be-
fold-and-thrust belt southwestward to the Salt Range- tween the distribution of this salt layer and the advance
Potwar Plateau foldbelt, the Sulaiman foldbelt, and the of the deformation front into the foreland has been
Makran accretionary wedge (Figs. 1 and 2). The last appreciated for some time (e.g. Sarwar & De Jong 1979,
three are all very wide (approximately 150,200 and 300 Seeber et al. 1981). Unlike these three broad zones of
km, respectively), as compared to the narrower thrust thin-skinned thrusting, the Kashmir belt is narrow with a
zone in Kashmir (less than 100 km). In this paper, we relatively high cross-sectional taper, a geometry that
compare the structural styles of all four of these thin- suggests strong coupling between the sediment wedge
skinned wedges and we offer some possible mechanical and the underlying autochthon (e.g. Chapple 1978,
explanations for differences among them. Davis et al. 1983).
The Makran wedge is a sediment-rich accretionary The fold-and-thrust wedges of Pakistan exhibit a wide
prism in which high pore-fluid pressures have long been variety of tectonic styles which make them ideal for
known to exist (e.g. Ahmed 1969). The weakened contrast and comparison directed towards improving
resistance to sliding that accompanies high pore fluid understanding of thin-skinned contraction. The conti-
pressures is a likely explanation for the enormous width nental collision is happening today, so many of the
and narrow cross-sectional taper of this wedge (Davis et geodynamic modeling constraints that are inaccessible
al. 1983, Platt 1990). Much of the Sulaiman lobe remains for most other foldbelts, however well studied, are
relatively poorly explored and its tectonics are not well available for the mountain belts of Pakistan. There is
understood. However, deformation there is, undoubt- very good surface exposure, and oil and gas exploration
edly influenced by the extreme thickness of the sedimen- activities have produced a large volume of subsurface
tary section: the depth to the top of Jurassic strata is information. The extreme breadth of the Sulaiman
21
22 D . M . DAvis and R. J. LILLIE

70 ° 80 ° 90 °

TARIM BASIN y

~ ,.~

l ,,.T
30° 417...t, l .

,5o~o
2o,_jIo'L
km f I
Fig. I. Tectonic sketch m a p showing Himalayan collision zone and motion of India relative to Asia (in cm year -~ , after
Jacob & Quittmeyer 1979). T h e thrust belts discussed in this paper are located in the northwestern portion of Pakistan.
A F = Alltyn Tagh Fault, B D = Bangladesh, C F = C h a m a n Fault, C L R = C h a g o s - L a c c a d i v e Ridge ( R e u n i o n Hotspot),
H F = Herat Fault, KF = K a r a k o r a m Fault, M B T = Main Boundary T h r u s t , M C T = Main Central T h r u s t , M K T = Main
Karakoram Thrust, M M T = Main Mantle T h r u s t , M R = Murray Ridge (Kerguelen hotspot), O F Z = O w e n fracture zone,
SL = Sri Lanka, SR/PP = Salt R a n g e - P o t w a r Plateau, S R T = Salt R a n g e T h r u s t , TS = T s a n g p o suture. After Jaswal
(1990).

wedge and the great thickness of the sediments within it 3e). The second class of weak-detachment, contrac-
are reminiscent of the Verkhoyansk foldbelt in Siberia tional wedges includes those fold-and-thrust belts that
(e.g. Churkin 1972, Nalivkin 1973) and the Ouachita overthrust, at least in part, a detachment zone in an
Mountains of the southern United States (e.g. Nelson et evaporitic layer. Because of the ductility and weakness
al. 1982, Lillie et al. 1983). Thus, the availability of data of evaporites, they share several readily observable
on the structures and seismicity of the active Sulaiman characteristics that are different from those typical of
wedge may afford an opportunity to learn indirectly mountain ranges without evaporites (Davis & Engelder
about these inactive foldbelts. Likewise, a study of the 1985). The Salt Range-Potwar Plateau foldbelt is a
Salt Range provides insights on the effects of factors prime example of this type of thin-skinned contractional
such as basement topography and the thickness and wedge (e.g. Lillie et al. 1987), illustrated in Figs. 3(b) &
depth of the salt on the tectonics of foldbelts that are no (c). In particular, the presence of a weak detachment
longer active, such as the Pennsylvania Plateau of the zone in evaporites permits a thrust belt to have an
Appalachians. extremely narrow cross-sectional taper and deformation
In contrast to the foldbelts such as the Kashmir with a marked lack of consistent vergence in its struc-
Himalaya, in which there is believed to be a large shear tures. A speculative third class of wide foldbelt with
traction at the base of the overthrust belt (Fig. 3a), there weak mechanical coupling along its base consists of
are at least two fairly common ways in which a very weak those in which the sedimentary section is sufficiently
detachment layer can be formed in a compressional, thick to allow non-evaporitic sedimentary rocks to be-
thin-skinned wedge. A number of investigators have have in a ductile manner, deforming in a time-
concluded that deformation at the frontal toe and along dependent fashion under modest shear stresses (Fig.
the basal detachment of at least some accretionary 3d). It is reasonable to assume that if any modern
prisms occurs under conditions of elevated pore pres- foldbelt fits this description, it would be the Sulaiman
sures and, presumably, very low shear stresses (e.g. von foldbelt, with its very thick sedimentary column.
Huene & Lee 1982, Moore & Biju-Duval 1984). Results The diversity that exists among the foldbelts of Paki-
of oil-industry drilling and the presence of numerous stan is easily noted in the topographic profiles of these
mud volcanoes suggest that the Makran wedge is a prime mountain Oelts. The relatively steep and narrow prufilc
example of an overpressured accretionary wedge (Fig. of the Kashmir Himalaya (Fig. 4a) resembles that of
 Mechanical responses during collision, Pakistan 23

parts of the Andes (e.g. Jordan et al. 1983) and western within the wedges vary with the degree of continental
Taiwan (e.g. Suppe 1980). In contrast, topographic underthrusting. The Makran is an accretionary wedge
slopes are quite subdued in the eastern Potwar Plateau over oceanic crust, with enormous quantities of conti-
(Fig. 4b) and, north of the 800 m of relief in the Salt nentally derived sediments over a thinner pelagic se-
Range, in the central and western Potwar Plateau (Fig. quence. The Sulaiman, a very thick wedge consisting of
4c). The Sulaiman wedge has locally impressive topo- a passive margin sequence overlain by flysch and
graphic relief, particularly near the front of the foldbelt, molasse, is located over the transition zone between
but the regional surface slope farther north and west is continent and ocean (Fig. 5c). In contrast, the Salt
gentle (Fig. 4d). Although the Makran accretionary Range-Potwar Plateau and Kashmir foldbelts include
wedge is over 300 km wide, its rear is only 5-6 km higher much thinner cratonic strata and molasse over continen-
than the deformation front, so the mean surface slope is tal crust. By controlling the thickness and type of sedi-
only a bit over 1° (Fig. 4e). ments available for inclusion in each of these
Lillie (in press) has calculated a sequence of density contractional wedges, the contrast in degrees of collision
models for an idealized continental collision, illustrating has a major effect on the tectonics of these foldbelts, and
both the overall geometric development under Airy may explain some of the fundamental differences among
conditions (e.g. Fig. 5) and the resulting gravity signa- them.
ture of the orogen. In terms of the overall structure and In the following sections of this paper, we describe
thickness of the crust, the thrust belts of Pakistan may be some of the ways in which the Salt Range-Potwar
thought of as being on a continuum, with Makran (Figs• Plateau, Makran and Sulaiman contractional wedges
5a & b) as the immature end-member and the Salt differ from the Kashmir wedge and from each other, and
Range-Kashmir region (Fig. 5d) at the mature end of we will propose mechanical explanations for varied
the spectrum• Both the thickness and nature of the strata lithologies and theological behaviors found within the

70 ° 72 ° 74 ° 76 °
3 8 ° - ~

MKT

36 °

34o'

o 50 ioo 150
I I I I
km
32°F-- UPPER
INDUS
BASIN
61 ° 63 ° 65 °

CHAGAI ARC ' CENTRAL

INDUS BASIN . /
I~. /
28 ° /
/'\. .~ \~
4N ~

26 °
LOWER
MAKRAN INDUS
BASIN

®
24 °

Fig. 2. Generalized tectonic map of Pakistan emphasizing active foreland thrust belts (stippled) and the location of the
Makran accretionary wedge. Note positions of cross-sections (a)-(e) portrayed in Fig. 7(b). CMF = Chukhan Manda Fault,
IB = Islamabad, K = Karachi, KF = Kingri Fault, KFTB = Kirthar foreland fold-and-thrust belt, KMF = Kurram Fault,
K R F = Kirthar Fault, N R = Nagarparkar ridge, O N F = Ornach Nal Fault, P = Peshawar, PF = Pab Fault, Q = Quetta,
S = Sargodha, SFTB = Sulaiman foreland fold-and-thrust belt, SH = Sargodha basement high, SR/RP = Salt R a n g e -
Potwar Plateau, SRT = Salt Range Thrust, ST = Sibi trough. Modified from Jaswal (1990).
!$)1-C
24 D. M. DAVIS and R. J . LILLIE

a)

,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,
,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,
Basement---- " """""""""""""""""""""""'"
"
""" ""
""""
"""""
""""
""""
""""
"""
"", ,","
, ,"
, ,"
,,","
, ,"
, ,"
,',"
,,
""""""""""""""""""""""""'"

b)

Salt ---­

Basement---­

c)

Sa lt - - ,!:;,:,".
" "
ebb ,.;.. >
-!:,!,,::.,:,'::.;' ,':'." "" " """"""" " "" "" """""
"~,, >~,>~,,~ ,~,,~,~,~,~,,~,~,>~,~,~,~,~, ~
""" ' ,," "
,..,, "
, ," "
, ," "
, ,", ,",","," ,","
," , ","
, ","
,,"",",",,"", ,"
, ,"
" ", ,"
", ,"
", ,"
", ,"
", ,"
", ,"
", ,"" "
, ,", ,",',,
",
Basement- - ,,'," " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " '" " " '" " " " " " " ','
" " " " ' " " ' " " ' " '" " ' " '" " """"""""' "

d)

- -- - ---- --- --

e)
overpressured
DeCOliement'--, ~~;;~~~~i!iij';~~~ii!i~=~~~=;;iii!=~;iiiii~;~i~::;~
Oceanlc_-­
Basement

Fig. 3. A series of schematic diagrams illustrating the variety of basal strength conditions believed to exist in the foldbelts of
Pakistan. (a) A foldbelt with a basal detachment in rock, such as shale, that is somewhat weaker than most of the strata
above it. In such cases, including the Kashmir Himalaya, there is typically a substantial cross-sectional taper (usually 8° ±
4°) with predominantly forward vergence in stacked thrust sheets. (b) With a basal detachment in extremely weak rocks,
such as salt, foldbelts like the eastern Potwar Plateau can have a cross-sectional taper as small as about 1°. (c) A foldbelt
similar to that in part (b), except for the additional effect of substantial topography in basement that causes ramping in the
foreland. The central and western Potwar Plateau and the Salt Range form such a foldbelt. (d) A model with weak, ductile
behavior important in rocks throughout the deeper part of the wedge, as opposed to only one limited stratigraphic layer as in
parts (b) and (c) above, may be appropriate for the Sulaiman wedge. (e) A highly overpressured sediment-rich accretionary
wedge, of which the Makran accretionary wedge may be an extreme example, can grow to enormous widths and maintain
tapers of only 1_3°.

wedges. Some of the variations are themselves related to western end of the Himalayan mountain belt that
the different degrees of maturity of the collision process stretches across Nepal and northern India (Fig. 1). In the
along the western boundary of the Indian plate. currently active thin-skinned thrust wedge of Kashmir,
sedimentary rocks of mostly Cenozoic age are thrust
over the Indian craton in a series of highly deformed
KASHMIR thrust sheets (Burbank et al. 1986). There is no evidence
of involvement of basement rocks in this thrusting and
The Kashmir Himalaya, located in northeastern Paki­ the structural vergence is almost uniformly southward,
stan and northwestern India, is at the extreme north- towards the foreland. The active fold-and-thrust belt
 Mechanical responses during collision. Pakistan 25

south of the Kashmir Basin is about 60 km wide, but thrust zone has an important effect upon the stress field
within that zone it nonetheless attains an overall height within the allochthon above it (e.g. H a f n e r 1951). Basal
exceeding 3 km (Figs. 2, 4a and 6). The most frontal traction causes the axis of maximum principal compres-
thrusts show Holocene slip, consistent with a generally sive stress to dip towards the foreland at an angle
forward progression of the locus of active thrusting (e.g. comparable in magnitude to the wedge taper; indeed, in
Yeats & Lillie 1991). the limiting case of a cohesionless frictional wedge, the
Simple models for the mechanics of the thin-skinned two are precisely equal (Dahlen 1984). Thus, forward-
mountain belts can explain the overall shape of the vergent thrusts (dipping toward the hinterland) tend to
active foldbelt, the general forward progression of de- have shallower dips than backthrusts and are energeti-
formation with time, and the consistent forward ver- cally favored over backthrusts (Figs. 3a and 6a).
gence of structures within it in terms of the magnitude
and sense of the shear coupling along the foldbelt's basal
d6collement (e.g. Chapple 1978, Davis et al. 1983). If we SALT R A N G E - P O T W A R PLATEAU
assume that the failure of rocks in the upper half of the
crust is governed by friction, then the development of a The Salt R a n g e - P o t w a r fold-and-thrust belt of the
contractional wedge with significant topography is pre- northern Pakistan (Fig. 2) is a currently active foldbelt in
dicted if the d6collement layer is only moderately which recent work (e.g. Burbank & Beck 1989) has
weaker than the rocks above it (Fig. 3a). In Taiwan, a placed tight constraints on the chronology of its develop-
frictional strength difference of only about 20% is suf- ment and in which seismic reflection, drillhole, strati-
ficient to allow sliding at a critical taper of about 9 ° graphic and surface mapping data are available.
(Davis et al. 1983, Dahlen et al. 1984). The Salt Range and Potwar Plateau are located south-
The presence of significant traction along a basal west of the northwestern syntaxis of the Himalayas. The
advance of deformation far into the foreland in this area
has been attributed by many workers to the presence of
NORTH SOUTH the Salt Range Formation evaporites as a weak detach-
V Kashmir Himalaya ment horizon (e.g. Sarwar & De Jong 1979, Seeber et al.
1981, Lillie et al. 1987, Butler et al. 1987). The narrow
a cross-sectional taper in the Potwar Plateau (from less
than 1° to about 3 °) can also be attributed to these
I I L I I I evaporites, which are predominantly salt. Laboratory
150 120 90 60 30 0
measurements (e.g. Carter & Hansen 1983, Chester
1988) indicate that salt is ductile at typical geologic strain
3V Eastern Potwar Plateau rates except in the top few hundred meters below the
surface. At depths of only a few km it can be between
o|.... ~ : . . . . . . . . . . . . . . . =--- - ' . . . . . . . . . . . one and two orders of magnitude weaker than most
150 I
120 90 60I I
30 0 other rocks.
The large strength contrast between moderately
strong rocks and salt means that a wedge can maintain a
3[ Centrals_
__ _~_SaltRange-Potwar Plateau much narrower taper than would otherwise be possible
o ..... ................... ----- (as little as 1° or less, as opposed to typical values in the
I I I I I I range of 6-12 ° in foldbelts with stronger coupling) (e.g.
150 120 90 60 30 0
Chapple 1978, Davis et al. 1983). The limited coupling in
salt also means that the maximum principal stress axis
must be nearly horizontal, so that there should be no
301............iulal_man_L_°
be......=..... strong preference in vergence direction (Fig. 3b) (Davis
d
& Engelder 1985).
I I [ I I I
150 120 90 60 30 0 Above the Salt Range Formation are lower Cambrian
clastic rocks (non-marine and shallow marine). Un-
conformably above them are Permian and Mesozoic
Makran
rocks, including some highly competent sandstones and
carbonates. Paleocene and lower Eocene rocks, largely
shallow marine carbonates, are followed by an uncon-
formity and the incompetent molasse of the Neogene
Rawalpindi and Siawalik Groups (Gee 1989). In the
I I I I I I
150 km 120 90 60 30 0 southern Potwar Plateau and Salt Range, the total
IOX Vertical Exaggeratiotl thickness of the competent section (Cambrian to
Fig. 4. Topographicprofilesacrossthe foldbeltsof Pakistan (a-d after Eocene) is only about 600 m, while the incompetent
Jaum6 1987). (a) Kashmir Himalaya. (b) Eastern Potwar Plateau. (c) Eocambrian and Neogene sections are thicker (about
Central Potwar Plateau and the Salt Range. (d) Southern Sulaiman
wedge. (e) Makran accretionary wedge near longitude 63°E (after 0.5-2 km and 1-5 kin, respectively). Thus, the mechan-
Byrne et al. 1992). ical situation can be thought of as one in which there is a
 26 D . M . DAVIS and R. J. L1LLIE

,Sed:ments (2.5) .__~Water {~.0)

77,77; ,

70
-1500 -t000 -500 0
KILOMETERS

~~,L-,t',t',~-,t
~,?;;L',;b:b
40

b
7O
-~500 -iO00 -5'00
,-2 ,," ~, - 2 , "2<;7, "-< ",, ",, ', ",- "......' .',, " .,,- ) ,," ,,',, " ,),, ",, ",,,,-2,-, -,., ,,,

--/t--I.:/k--/t--ik--i.--ik--/N--/t--1 -- , . - - . --, ,--,., x-- ~--/k./k~lk--I.--/k--/.--It--/~--I~--/

,0 ............................ ..................................... e"

70
-1500 -i000 -500 0

i0

4o

7O
-~soo -~6oo -5oo o
Fig. 5. Two-dimensionaldensity models for stages of ocean basin closure and continent-continent collision.The sections
retain a state of Airy isostatic equilibrium at their bases (70 kin), for the thicknesses and densities (in g cm -3) shown for
crust, mantle, sediments and water. Models have vertical exaggerations of 4:1. (a) Convergent continental margin with
oceanic crust subducting. (b) 550 km before continental crusts collide. (c) 50 km before continental crusts collide. (d) 500
km of continental underthrusting followingcollision. Although the width of ocean crust consumed is unknown, the thrust
belts discussed in this paper are at the general states of crustal structure and sedimentary thicknesses depicted in (a) & (b)
for the Makran accretionarywedge, (c) for the Sulaiman thrust lobe and (d) for the Salt Range-Potwar Plateau and Kashmir
thrust belts. Modifiedfrom Lillie (in press).

rigid 'beam' (Cambrian to Eocene) sandwiched between cause salt to flow into anticlines, with corresponding
two weak zones, the lower of which is ductile; the entire synclinal pinch-out (asperity) zones. Finite element
system is in turn underlain by an essentially unyielding modeling (Moussouris 1990) suggests that even rela-
crystalline basement. tively small local 'asperities' without salt at the base of
Seismic reflection lines indicate that a considerable the overthrust belt might become local, highly coupled
amount of internal shortening has been accommodated zones capable of influencing the generation of structures
in the eastern (Fig. 6b) as compared to the central and within the foldbelt. The sticking of the thrust wedge at
western (Fig. 6c) Potwar Plateau. Jaum6 & Lillie (1988) salt-free asperities in the eastern Potwar Plateau might
attribute this contrast in deformation to differences in help to explain the counterclockwise rotation of the Salt
basement dip, with the central and western Potwar Range and Potwar Plateau that has been observed
(unlike the eastern) having a basement dip enough to paleomagneticaily (e.g. Opdyke et al. 1982).
obviate the need for a substantial build up of topo-
graphic slope. Thinning of the salt towards the east is
Timing and mechanics o f Salt Range and Potwar
another factor that would tend to build the topographic
Plateau deformation
slope there (Butler et at. 1987). However, the applicable
power-law exponent for salt is quite large, probably near
5 (e.g. Carter & Hansen 1983, Chester 1988), so the The Salt Range rises abruptly above the Jhelum River
shear stress is only very weakly dependent upon strain plain, overriding its own depositional fan and exposing
rate. Thus, the shear traction along the base of the the entire sedimentary section down to and including the
foldbelt should not increase greatly unless the salt be- evaporites (Fig. 6c). Extending about 100 km north of
comes extremely thin or if it 'pinches out' locally. Pen- the Salt Range is an elevated, nearly flat region, the
hock et al. (1989) point out that although reconstruction Potwar Plateau, occupying a position analogous to that
yields an average salt thickness of roughly 500 m in the of the Molasse Basin between the Jura mountains and
eastern Potwar (Fig. 6b), the thickness appears to the Alps. There is very little indication of internal
approach zero beneath some synclines (see Johnson et shortening in the southern part of the western and
al. 1986). The mobility and buoyancy of salt tend to central Potwar Plateau, where most of the advance of
 Mechanical responses during collision, Pakistan 27

NE SW

~ T-;-. • • • • • . . . . . "- . . . . . . . . . . . . . . IO

30 o
km

NW SE

• ~tWAT fk~P~- ' ' . " . . . . . . . -" " " ' " "-- " "~" -

I-- ~ - -- 3E~]]~ . . . . ~ ..~.~....:7- -: . . . . L..~."--,--~.;-, :'- ~: - ""---a . '.'"; " -;,, _ L, =

!,.='~',' ~-.,>,-,.'-','-:,'- :,'.V ;,,.~ ~".-,'.-,~.' :--.-"-.'~P-A.-~'-.".,>~,,7-,,(-i"4,;'-',, ;,'~'-'.', J-;,.'-! '.~'L-','-.,, !.,'.;,-. f,;L~ ,,;'.,', }, ~,'~.',<'~,;~:.' ':~.
~o ~o ,o ' ~b ' ~'o ' ~o ' Wo ' b " z'o ' ,G ' 6
KILOMETERS

t TERTIARY
MOLASSE ~_~
RAWALPINDI $~LT RANGE FORMATION

N S
MBT N P D Z SOAN SYNCLINE
8AUN SALT RANGE

POST'SIW'LI, q TEllrllR'r c.,.m,.A. TOEOCE,,,E t J

s,..L,. ~-o.s,E ~ ,O.-,.,..,.L,,..OE ,. ,~o,,~

V,E.= I:1

N
MARl POP'UP ZONE TADRI KURDAN DANDA PIRKOH BUGTI LOTI SUl

1~0 I I u
100 KILOMETERS 50 0
I NEOGENE t=._ L~=,~I
PALEOGENE ~-'-"] CRETACEOUS @

I JURASSIC ~ PRECAMBRIAN-TRIASSIC ~ CRYSTALLINEBASEMENT

SEISMIC
N COAST FRONT DF S
~ ~ . ~ . --'Lq.i:~"~'~" ...... >-->-~=%-~l~u~..~ M~ ~ . ..~--'-~________~~. .,,,~

.---- - - - - . . . . . ~
__
- - ~ z / J z ~ ...........
Zlds
•
"; '":'
zs e

I I I I I I [ I I I [
50
200kin IO0 O
Fig. 6. Generalized cross-sections of the major Pakistan foreland fold-and-thrust belts and the accretionary wedge
discussed in this paper, See Fig. 2 for locations of the sections. (a) Kashmir (after Burbank et aL 1986). (b) Eastern Potwar
Plateau (after Pennock et al. 1989). (c) Central Salt Range-Potwar Plateau (after Baker et al. 1988). NPDZ = Northern
Pakistan deformed zone; MBT = Main Boundary Thrust. (d) Sulaiman Range (after Jadoon 1991, Jadoon et al. 1992).
(e) Eastern Makran accretionary wedge (after Byrne et al. 1992). DF = Deformation Front.
 28 D. M. DAVIS and R. J. LILLIE

a)

. ~ Molasse
Cambrian-Eocene
Salt
~'~ Base ment
Shale Salt

b)

c) Potwar Salt Range

Plateau

Fig. 7. Finite elementmodel for the central Salt Range-Potwar Plateau (after Moussouris1990).Vertical exaggerationis
about 3:1 and deformationsare further exaggeratedby a factor of 3:1. (a) Startingcase, with north to the left, Shadingand
labels indicatethe different rock types. Basement rocks are covered by salt, the strong Cambrian to Eocene section, and
finally molasse, the thickness of which has relatively little effect upon the modeling. Note the basement offsetin the
foreland. (b) Initialmovement is impeded by the buttress effectof the basement offset, so that the salt layer thickensand
'hydraulically'upliftsthe Potwar Plateau in the south (at right). (c) Once the Salt Range Thrust is 'lubricated' by salt, the
deformation front beginsto advancesouthward.

the thrust wedge appears to have been accommodated analogous to the uplift associated with the initial slip
along the Salt Range Thrust (e.g. Lillie et al. 1987). along the Salt Range Thrust reported by Burbank &
Moussouris (1990) generated a highly simplified finite Beck (1989). Within most of the Potwar Plateau, the
element model for deformation in a weak-basal foldbelt model predicts that the weak basal traction causes the
with basement relief, using software developed by maximum principal stress axis to be close to horizontal.
Richardson (1978). The starting case (Fig. 7a) assumes As noted earlier, this should result in relatively sym-
an initial rectangular geometry and four different metric structures without a strongly preferred vergence
materials; basement (very strong), Eocambrian salt direction, like that observed in the eastern Potwar
(weak), the Cambrian-to-Eocene section (strong), and Plateau (Fig. 6b). Only if weak salt is somehow intro-
young molasse (weak). Note that a break in the level of duced along the thrust ramp itself is there significant
basement offsets the layers; this break simulates a down- southward m o v e m e n t along it.
to-the-north normal fault observed on seismic profiles to Lillie et al. (1987) show seismic reflection data that
underlay the Salt Range (Baker et al. 1988). Even this indicate basement relief in the form of a down-to-the-
simple model can reproduce several aspects of the tec- north fault beneath the ramp of the Salt Range Thrust.
tonics of the Salt R a n g e - P o t w a r Plateau foldbelt (Figs. They suggest that this relief is the result of a flexural
7b & c). Initially, the region to the north of the basement normal fault that predates that advance of thin-skinned
offset is elevated into a high plain, like the Potwar thrusting into that area. Because the roughly 1 km throw
Plateau. Next, southward towards the foreland there of the normal fault exceeds the local thickness of the
develops a more highly elevated zone with considerable evaporites, the salt-lubricated translation of the alloch-
relief on its right (south) side, bordered farther to the thonous Potwar Plateau is interrupted at the fault.
south by an u n d e f o r m e d zone analogous to the Jhelum Photoelastic modeling by Wiltschko & Eastman (1983)
Plain. The elevated area, which is located above the and a finite element study by Schedl & Wiltschko (1987)
basement offset, results from slip due to the local stress showed that angular offsets in basement can cause the
concentration caused by the basement relief and is generation of stress concentrations that trigger thrust
 Mechanical responses during collision, Pakistan 29

ramping, including some that may reach all of the way to assuming that the initial, abortive slip on the Salt Range
the surface. Moussouris (1990) obtains a very similar Thrust at about 5 Ma provided a geometry in the fault
result even when the weakness of the salt on the up- zone that was suitable for the initiation of the injection of
thrown (south) side of the normal fault is taken into salt up the fault zone ramp. Given the viscosity of salt at
account. such low temperatures, on the order of 1018-1019 Pa s-1,
To the first order, the Salt Range-Potwar Plateau the migration of the salt should have taken a few million
thrust wedge has undergone a generally forward (south- years, providing a possible explanation for the 3 million
ward) progression of deformation. However, Burbank year hiatus in slip along the Salt Range Thrust (Mous-
(1983), Burbank & Raynolds (1988) and Burbank & souris 1990).
Beck (1989) have identified significant 'out-of-sequence'
thrusting. They report that at least 4 km of shortening
happened in the Salt Range from about 5 to 4.5 Ma, well SULAIMAN FOLDBELT
before the end of the major deformation in the northern
Potwar Plateau. This was followed, after a hiatus of The Sulaiman foldbelt is part of the obliquely conver-
about 3 Ma, by renewed slip of at least 12 km, contem- gent margin at the western edge of the Indian plate
poraneous with shortening along structures at the far (Figs. 1 and 2), which is moving northward with respect
eastern end of the Salt Range. We suggest that the to Asia at about 4 cm year -1 (Minster & Jordan 1978).
temporary pause in uplift in the Salt Range from around One remarkable aspect of the festoon-shaped Sulaiman
4.5 to 2 Ma resulted from the local doubling of the strong lobe is that it includes both E-W-striking thrusts at its
Cambrian-to-Eocene section along the thrust. In this southern boundary and N-S-striking thrusts at its east-
way, the initial uplift in the Salt Range may have served ern edge (Fig. 2), although convergence at the eastern
to strengthen that part of the foldbelt with the greatest front must have a very large oblique component. The
degree of local stress concentration. Lacking the lubri- rocks exposed at the front of the Sulaiman Range, facing
cating effect of the salt in its up-dip regions, the thrust east, and the Sulaiman lobe front, facing south, are all
ramp must initially have been much stronger than it is Tertiary in age. The sedimentary section in those areas is
today, when large volumes of salt fill the fault zone (Fig. extremely thick; seismic reflection lines show that Pre-
6c). This lack of weakening, combined with the strength- cambrian basement is at least 10 km deep at the defor-
ening effect of the build up of topography may explain mation front (Banks & Warburton 1986, Humayon et al.
why shortening in the foldbelt did not continue to be 1991, Jadoon et al. 1992). Behind the deformation front,
concentrated in the region of the basement fault. In- the section is expected to be considerably thicker than
stead, the primary locus of shortening returned to the that, based on cross-section balancing (Banks & War-
northern Potwar Plateau at about 4.5 Ma (Burbank & burton 1986, Jadoon et al. 1992) and gravity modeling
Beck 1989). (Khurshid 1991). A likely explanation for this thick
We suggest that as deformation continued in the sedimentary pile is that it represents a northern continu-
northern Potwar Plateau, salt migrated southward be- ation of the Mesozoic rifted margin sequence on the
neath the southern Potwar Plateau (Gee 1983) over the western edge of the Indian subcontinent (Humayon et
basement normal fault buttress, driven by the gradient al. 1991).
in overburden. In effect, the nearly 3 million year hiatus The great breadth of the zone of foreland thrusting
in thrusting in the Salt Range may simply be the amount and accommodation structures along the lateral flanks in
of time it took the viscous Eocambrian salt to be injected the Sulaiman region might be expected to be related to
far enough up the Salt Range Thrust ramp to lubricate the presence of an extremely weak detachment in salt.
the otherwise hard-to-move part of the fault within the However, there is no evidence for evaporites beneath
strong Cambrian-to-Eocene section. The likely mechan- the Sulaiman foldbelt; the nearest documented occur-
ism for this motion is the hydrostatic pressure gradient rence of evaporites is 200 km east of the deformation
resulting from the increasing depth and overburden front (Humayon et al. 1991). Given the apparently
northward along the d6collement. Having migrated up narrow cross-sectional taper of this foldbelt, it is hard to
the fault zone, the salt would then have made large-scale understand how it can be as much as 200 km wide, with
thrusting energetically feasible. 76 km of shortening in the frontal 129 km (Jadoon et al.
If the salt advanced as though it were an extremely 1992), unless there is a weak detachment layer some-
viscous magma being injected into a dyke (in this case where at depth. Furthermore, the mean topography
the Salt Range Thrust) then it would have been driven (Jaum6 1987, Jadoon et al. 1992), steep in the first few
by a pressure gradient of about 1 kPa m-1. This flow tens of kilometers from the deformation front, flattens
would then be replenished by southward flow of salt out towards the interior (Fig. 4d) in a manner compat-
beneath the southern Potwar Plateau. The replenishing ible with a down-dip transition to a weak detachment
flow beneath the Potwar Plateau was driven by pressure (Davis et al. 1983). This suggests that some of the lower
gradients about an order of magnitude smaller, due to part of the thick sedimentary section within the foldbelt
the shallow dip of the salt layer beneath the plateau. deforms in a ductile manner at relatively low shear
Thus, the rate of salt injection up the Salt Range Thrust stress; however, we suggest that the weak zone is not
must ultimately have been limited by how quickly salt necessarily in evaporites. An additional factor likely to
could migrate southward beneath the Potwar Plateau, cause the front to be relatively elevated is the effect of
30 D . M . DAVIS and R. J. LILLIE

isostasy in overthrusting an ocean-continent boundary. The back of the Sulaiman foldbelt is characterized by
The front of the foldbelt, having reached the thicker and sediments that are very deep (up to 20 km) because of
hence more buoyant continental crust, may simply be their genesis as part of the thick Mesozoic rifted margin
elevated isostatically, in contrast to the rearward portion prism and subsequent thickening by thrusting (Fig. 5c).
of the wedge that remains over oceanic basement (Fig. In contrast, sediments at the extreme front of this
5c). mountain range, including recently deposited molasse,
The propagation of deformation so far into the fore- are about 10 km thick. Hence, the front of the Sulaiman
land without a large cross-sectional taper may be realis- wedge is much less likely than the deep rearward
tic if rocks deep in the section can shorten by a portions of the foldbelt to show shortening by low-stress,
mechanism that operates at shear stresses well below the time-dependent mechanisms such as pressure solution.
stress levels required in the overlying, friction- The very steep topographic slope at both the eastern and
dominated rocks. Unlike the Salt Range, where the southern fronts of Sulaiman lobe (Fig. 4d), manifested
limestone-rich Cambrian-through-Eocene section is as a culmination wall of a passive roof thrust (Fig. 6d),
only 1 km thick, it is over 7 km thick beneath the supports the idea that basal friction is high where the
deformation front of the Sulaiman wedge, and thickens detachment zone is shallower (Humayon et al. 1991,
to the north and west (Humayon et al. 1991, Jadoon et al. Jadoon et al. 1992).
1992). Although they remain brittle to much greater Ideally, one would like to obtain samples of carbon-
depths than evaporites, fine-grained limestones can ates that have been buried to depths of 15 km or so in the
undergo significant non-brittle strain at temperatures Sulaiman wedge in order to determine whether or not
considerably lower than many other rocks. The shorten- they show any signs of distributed, non-frictional defor-
ing would be roughly subhorizontal and its magnitude mation. Unfortunately, such rocks do not appear to be
would decrease with larger grain sizes and with lower exposed at the surface. Unlike rocks that typically pro-
temperatures at shallower depths. vide good zones of detachment, such as shales, carbon-
Although time-dependent mechanisms are probably ates are expected to be weak only when warm and deep.
less important than friction in foldbelts that are under- Thus, any deep d~collement in Triassic carbonates
going rapid shortening at shallow depths, they may be would not bring the carbonates and rocks directly above
significant in many other cases. For example, when them to the surface. Instead, at moderate depths where
strain rates, temperatures, lithologies and grain sizes the carbonates are no longer weak, detachment should
favor pressure solution, that mechanism may play an step up into shales higher in the section that are weak
important role in the strain history of a foldbelt (e.g. regardless of temperature; this appears to be happening
Engelder & Geiser 1979). In order for a 100 m thick in the passive roof duplex observed along the Sulaiman
section to accommodate 3 cm year -1 of slip, strain rates front (Banks & Warburton 1986, Humayon et al. 1991,
of only 10 -12 s -1 are required. Thicker sections could Jadoon et al. 1992). The overall taper of the Sulaiman
accommodate the slip with proportionally lower strain wedge is only about 3° (Jadoon et al. 1992), sufficiently
rates. In a sufficiently thick stratigraphic section with small so as to suggest the presence of a weak zone of
enough fine-grained limestones, it is quite feasible for detachment. Although deformation in the upper half of
pressure solution to accommodate the necessary strain the crust is probably governed primarily by the frictional
rates. Extrapolation of published laboratory data (e.g. strength of rocks, our working hypothesis is that the
Rutter 1976, 1983, McClay 1977) suggests that with relatively subdued character of the profile in Figs. 4(d)
temperatures of 200-250°C, shear stresses on the order and 6(d) results from this not being true of any of the
of only 1 MPa may lead to pressure solution in fine- rearward portions of the Sulaiman wedge. Instead,
grained, calcite-rich rocks at the required strain rates. A much of the strain at depth may be accommodated by
temperature of 200-250°C at the bottom of the Sulaiman weaker, perhaps macroscopically ductile mechanisms.
wedge (=15-20 km deep) corresponds to a mean ther- Frictional behavior is not necessarily seismic in
mal gradient of only about 10--15°C km -1. Vitrinite character (e.g. Scholz 1988), but macroscopically ductile
reflectance data from nearby wells are reported to be behavior is unlikely to accumulate sufficient elastic
consistent with moderately high thermal gradients (H. strain to permit the generation of significant seismicity.
Jorgen personal communication, 1989). Data from wells The assumption of ductile behavior in the back of the
1.8--4.7 km deep, located mostly in frontal regions of the Sulaiman wedge leads to the prediction that
foldbelt (Khan & Raza 1986, Raza et al. 1989), yield detachment-related seismic events would be lacking
geothermal gradients of 24--34°C. Thermal gradients there. Teleseismically determined earthquake locations
probably do not remain quite so high farther from the contain considerable uncertainties in location, but the
front and at greater depth. However, given published data do seem to indicate that relatively few earthquakes
flow laws for carbonate rocks (e.g. Schmid et al. 1977, occur between the frontal portion of the Sulaiman lobe
1980), it is even possible that some intragranular creep and the highly seismic Chaman transform fault zone.
(in addition to large-scale pressure solution) occurs in These data are at least consistent with the hypothesis
some deeper parts of this foldbelt. Deformation by a that most slip along the base of the rearward (but not the
variety of ductile mechanisms has been recognized frontal) parts of the Sulaiman occurs in a ductile and
observationally in the deeper rocks of the foidbelts for aseismic manner. This contrasts sharply with both the
some time now (e.g. Schmid 1975). overall aseismicity of the Salt Range, which is probably
 Mechanical responses during collision, Pakistan 31

due to the ductility of salt all the way to the deformation the earliest sediments to be accreted at that margin, the
front (Seeber & Armbruster 1979), and the distribution Mashkel forearc basin, located in the Baluchistan
of seismicity at the base of the Makran wedge, which is desert, and the Makran accretionary wedge itself (Farah
confined to the deeper regions 75 km or more from the et al. 1984, Leggett & Platt 1984). The subaerial portion
deformation front (Byrne et al. 1992). The apparent of the wedge is divided into three relatively distinct
seismic activity in the frontal regions of the Sulaiman deformational zones: the Northern, Central and the
wedge is superficially similar to that described by Seeber Coastal Makran Ranges. These zones young toward the
et al. (1981) for the central Himalayas. However, in the trench (southward), ranging in age from Paleogene to
latter case the seismicity appears to extend downdip Miocene (Harms et al. 1984, Leggett & Platt 1984).
beneath all of the thin-skinned wedge, as opposed to Of the total 300 km width of the accretionary wedge,
being mostly limited to the frontal regions of the Sulai- 200 km is exposed subaerially. This contrasts sharply
man. with most accretionary wedges, which are typically ex-
The overthrust wedge of Sulaiman should remain well posed only discontinuously in isolated islands along the
coupled to the basement wherever the basal dEcolle- outer-arc high, such as Nias, Barbados and Kodiak. The
ment is in strong, frictional rocks. Weaker coupling is massive size and exposure of the Makran wedge is
expected along the base of the wedge where the d6colle- probably closely related to its very large sediment
ment is deeper. This leads to the expectation that strong supply. The sediment package at the deformation front
mechanical coupling should cause the frontal part of the is 5-7 km thick. Of this, roughly half is accreted at the
eastern Sulaiman wedge to be dragged northward with front into a series of evenly spaced folds and imbricate
respect to the rest of the wedge, causing the eastern thrusts (White 1979). Field relations and mass balance
Sulaiman to translate northward along the left-lateral arguments both strongly suggest that most of the re-
Kingri Fault (Fig. 2). The basal d6collement in the area mainder of the sediments, underthrust at the front, are
of the Kingri Fault is somewhat over 14 km deep underplated farther to the north (White 1979, Platt et al.
(Humayon et al. 1991). Thus, although the data are at 1985).
present insufficient to draw strong conclusions, the The overall taper of the Makran accretionary wedge is
available data are consistent with the idea that the parts about 4°, with a value somewhat greater offshore and
of the Sulaiman wedge with a detachment deeper than less onshore. Critical taper arguments indicate that
roughly 12-15 km slide in a largely ductile manner, narrow tapers result when pore fluid pressures are high,
perhaps over warm and weak carbonates. particularly if the elevation of fluid pressures is most
pronounced in the zone of basal detachment. Davis et al.
(1983) suggest that the taper implies fluid pressure
THE MAKRAN ACCRETIONARY WEDGE values within a few percent lithostatic. Platt (1990)
suggests that superlithostatic pore pressures may exist
The Makran margin of southwestern Pakistan and and that tensile fracturing may be an important process
southeastern Iran (Fig. 2) is a zone in which northward in the Makran wedge. Speculations about overpressures
subduction of neo-Tethyan crust has been taking place are supported by the presence of mud volcanoes along
since the Early Cretaceous (~eng6r et al. 1988). Located the coast (Snead 1964, Ahmed 1969). The undercom-
to the west of the Chaman fault zone, the Makran is not pacted, relatively porous state of overpressured sedi-
strictly part of the same India-Asia collision as are the ments also means that the transition from loose,
other three thin-skinned wedges discussed here. Rather, unlithified sediment to hard rock is likely to occur
it currently constitutes the boundary between the Eura- deeper in the Makran wedge than in subaerial wedges,
sian and Arabian plates (Fig. 1). The Makran is bounded such as the Kashmir Himalaya.
on the west by the Oman line, a zone of major strike-slip One important implication of this transition is its
motion to the west of which is the salt-dominated Zagros effect upon seismicity. There is no simple one-to-one
mountain belt. relationship between brittle and seismically capable be-
Located to the north of the Makran forearc are two havior, but Zhang et al. (1989, 1993) have shown for
major accreted terranes (Fig. 2). Much of Afghanistan clastic rocks that the transition from non-localized, non-
consists of the Afghan, or Helmand block, and to the dilatant slip to discrete, dilatant slip depends upon both
north the Turan block; farther to the west, in eastern effective confining pressure and porosity. At high por-
Iran, is the Lut block. These blocks are Gondwanan in osities, the overall behavior is brittle-frictional, but slip
origin and are thought to have docked with Asia after does not occur in a manner conducive to seismicity. The
the subduction of a large amount of Paleotethys ocean ability of frictional rocks to support seismic slip has been
crust to their north during the Mesozoic (Tirrul et al. described in terms of the relative magnitudes of dynamic
1983, ~eng6r et al. 1988). and static friction coefficients (e.g. Rice & Ruina 1983).
The Kandahar Arc, south of the Lut and Afghan This explains quite nicely the lack of earthquakes nu-
blocks, is associated with convergence at the Makran cleating in the top few km in many fault zones (e.g.
margin since the Late Cretaceous. During that time, an Meissner & Strehlau 1982). Byrne et al. (1988) explain
exceptionally wide (500 km) forearc has formed. South the aseismicity of accretionary wedges in much the same
of the volcanic arc is Late Cretaceous to Paleogene way, except that undercompaction in accretionary
flysch belt in the Ras Koh Range that probably includes wedges means that the aseismic zone above the tran-
32 D . M . DAVIS and R. J. LILL1E

sition to stick-slip behavior and the release of elastic mantle material allows the extremely thick continental
strain in earthquakes is much deeper, typically about margin sequence to be preserved. The section is highly
15 km. deformed at the front, but the narrowed wedge taper
Byrne et al. (1992) have examined the seismic record and apparently reduced seismicity to the north and west
of the Makran margin and conclude that aseismic con- suggest the large-scale occurrence of some sort of
ditions exist from the deformation front northward thermally-activated ductility (perhaps pressure solu-
about 75 kin, to a point about 15 km deep, roughly tion) in fine-grained carbonates of the passive margin
beneath the shoreline (Fig. 6e). The plate boundary in sequence (Fig. 6d). If so, then the d6collement mech-
eastern Makran ruptures in great thrust earthquakes, anics of the Sulaiman are quite distinctive (Fig. 8).
including a magnitude 8.1 event in 1945. Because of the In northern Pakistan, collision has progressed until
lack of large events that are clearly recorded either the lower thrust plate includes full-thickness crust of the
instrumentally or historically, it is not clear whether or craton (Fig. 5d). As a consequence of isostasy, the
not western Makran undergoes great thrust earth- sedimentary section in the Salt Range-Potwar Plateau
quakes. The location of the deep end of the seismogenic foldbelt and in Kashmir is therefore much thinner than it
zone for plate-boundary events at this and other subduc- is in the Makran and Sulaiman regions. However, the
tion zones is apparently determined by the much deeper, Salt Range-Potwar Plateau foldbelt behaves quite dif-
thermally-controlled transition to intragranular creep. ferently than the Kashmir Himalaya because its thin
Thus, the 15-50 km deep zone of basal thrusting earth- sediments include an evaporite horizon which is ductile
quakes in eastern Makran contrasts markedly with the even at quite shallow depths.
Sulaiman wedge, where most events are located where The tectonics of the Salt Range-Potwar Plateau fold-
the base of the wedge is shallow. belt are dominated by the presence of the Eocambrian
The Makran margin and its accretionary wedge have salt at the base of the sedimentary column and by a
had a long and complex history. However, the anoma- major normal fault in the basement. The salt allows the
lous width and moderately narrow cross-sectional taper
of the wedge are understandable in terms of the large
sediment supply and the resulting overpressures. The Strength =
undercompaction within the wedge due to overpressur-
ing explains the aseismicity of its frontal 75 kin, where halite ductility
the base of the wedge is within 15 km of the surface.
Ultimately, it is the fact that the crust beneath the
Makran is oceanic, and subducting, that explains its lack "0
of buoyancy and its attendant ability to support large 01
quantities of clastic sediments without most of it eroding O~
¢-

away (Figs. 5a & b). The great breadth, narrow taper,

-q
and frontal aseismicity result, in turn, from the hydro- KASHMIR
logic implications of that massive sediment supply. .=10- ] ,,,,,"

COMPARISONS AND SUMMARY

N
We have surveyed, in varying degrees of brevity, the 15"
SUL . . . . . .
tectonics of four major thin-skinned contractional
wedges in Pakistan. All are part of the closure of ocean
and eventual continental collision along the southern
margin of Asia. However, they are at very different
stages of this process (e.g. Fig. 5). In each case, the 0° 2° 4" 6° 8° 10 ° 12"

width and taper of the region of thin-skinned thrusting wedge taper (approx.) D
can be related to the surmised strength of the zone of Fig. 8. Schematic illustration of the major thin-skinned contractional
detachment, based upon the likely lithology and tem- wedges of Pakistan in terms of the pressure and temperature in the
zone of basal detachment (vertical axis) and the strength of coupling
peratures of the strata there (Fig. 8). there (horizontal axis). The straight lines represent the confining-
In Makran, oceanic crust is still being subducted (Figs. pressure-dependent frictional strength of crustal rocks, which in-
5a & b), and a huge supply of clastic sediments is being creases with depth. With elevated pore fluid pressures (as in Makran),
frictional strengths increase more slowly with depth. The shaded
accreted and underplated, probably in an undercom- curves represent ductile strengths, which decrease with increasing
pacted and overpressured state. This has led to the temperature. The ductile strength curves are dashed at depths shal-
formation of a wide but fairly thinly tapered wedge that lower than the brittle--ductile transition, where friction is weaker and is
the dominant deformation mechanism. Stronger coupling along the
is aseismic in its frontal 75 km (Fig. 6el. basal thrust requires larger wedge tapers. The Kashmir Himalaya is
The Sulaiman wedge is undergoing active collision; governed by strong friction, but the Salt Range-Potwar, Sulaiman and
gravity data suggest that the zone of transition from Makran wedges are weak due (respectively) to low-temperature
ductility in salt; moderate temperature weakness of carbonates be-
oceanic to continental crust is just now being under- neath a thick sedimentary pile; and elevated pore-fluid pressures that
thrust (Fig. 5c). Isostatic compensation by shallow greatly reduce effective pressure.
 M e c h a n i c a l r e s p o n s e s d u r i n g collision, P a k i s t a n 33

overall wedge taper to be e x t r e m e l y n a r r o w (Fig. 8), Range; Temporal constraints on thrust wedge development, north-
west Himalayas, Pakistan. In: Tectonics of the Western Himalayas
permits the s o u t h e r n Potwar P l a t e a u to be t r a n s l a t e d (edited by Malinconico, L. L. & Lillie, R. J.). Spee. Pap. geol. Soc.
southward with very little i n t e r n a l d e f o r m a t i o n (Fig. Am. 232,113-128.
6c), and causes deformation to occur o n b a c k w a r d - as Burbank, D. W. & Raynolds, R. G. H. 1988. Stratigraphic keys to the
timing of deformation: an example from the northwestern Hima-
well as forward-vergent structures (Fig. 6b). T h e r e is a layan foredeep. In: New Perspectives in Basin Analysis (edited by
large offset in b a s e m e n t at the f r o n t of the c e n t r a l a n d Paola, C. & Kleinspehn, K.). Springer, New York, 331-351.
western portions of the Potwar P l a t e a u , resulting from Burbank, D. W., Raynolds, R. G. H. & Johnson, G. D. 1986. Late
Cenozoic tectonics and sedimentation in the north-western Hima-
the p r e s e n c e of a flexural n o r m a l fault. This offset layan foredeep: II. Eastern limb of the Northwest Syntaxis and
a p p a r e n t l y acted as a stress c o n c e n t r a t o r a n d triggered regional synthesis, In: Foreland Basins (edited by Allen, P. A &
the r a m p i n g to the surface of the Salt R a n g e T h r u s t , Homewood, P.). Spec. Publ. Int. Ass. Sediment. 8,293-306.
Butler, R. W. H., Coward, M. P., Harwood, G. M. & Knipe, R. J.
exposing the entire section from the E o c a m b r i a n up- 1987. Salt control on thrust geometry, structural style and gravi-
ward (Fig. 6c). T h e timing of s u b s e q u e n t m o t i o n o n the tational collapse along the Himalayan front in the Salt Range of
Salt R a n g e Thrust m a y be closely r e l a t e d to the mi- northern Pakistan. In: Dynamical Geology of Salt and Related
Structures (edited by Lerche, I. & O'Brien, J. J.). Academic Press,
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(Fig. 7). Byrne, D., Davis, D. M. & Sykes, L. R. 1988. Mechanics of the
In the K a s h m i r H i m a l a y a , the collision is seen at its shallow regions of subduction zones and the loci and maximum size
of thrust earthquakes. Tectonics 7, 833-857.
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accreted, o t h e r t h a n molassic m a t e r i a l d e r i v e d from the earthquakes and aseismic slip along the plate boundary of the
H i m a l a y a s themselves, is relatively thin (Fig. 6a). Be- Makran subduction zone. J. geophys. Res. 97,449-478.
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8). T h u s , the f o r e l a n d f o l d - a n d - t h r u s t belt is highly Bull. geol. Soc. Am. 89, 1189-1198.
Chester, F. M. 1988. The brittle-ductile transition in a deformation-
d e f o r m e d , with p r e d o m i n a n t l y f o r w a r d - v e r g e n t struc- mechanism map for halite. Tectonophysics 154,125-136.
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taper that is much larger than those of the o t h e r m a j o r Plate in Asia. Bull. geol. Soc. Am. 83, 1027-1036.
Dahlen, F. A. 1984.Noncohesive critical Couloumb wedges: an exact
m o u n t a i n belts of Pakistan. solution. J. geophys. Res. 89, 10,125-10,133.
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of these differences are explicable in t e r m s of the differ- 1172.
Davis, D. & Engelder, R. 1985. The role of salt in fold & thrust belts.
ent thicknesses a n d lithologies of the s e d i m e n t a r y piles Tectonophysics 119, 67-88.
that the wedges e n c o u n t e r at their d e f o r m a t i o n fronts. Engelder, T. & Geiser, P. 1979. The relationship between pencil
T h e s e differences are r e l a t e d n o t o n l y to the v a r i o u s cleavage and lateral shortening within the Devonian section of the
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ant, to the various stages in the collisional process the tectonics of Pakistan. In: Marine Geology and Oceanographyof
r e p r e s e n t e d by each belt. Arabian Sea and Coastal Pakistan (edited by Haq, B. U. & and
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Acknowledgements--We would like to thank Bob Lawrence, Yiorgos Gee, E. R. 1983. Tectonic problems of the sub-Himalayan region of
Moussouris, lstiaq Jadoon, Monsoor Humayon and Wei-hau Wang, Pakistan. Kashmir J. Geol. 1, 11-29.
all of whom have collaborated with us on aspects of the work described Gee, E. R. 1989. Overview of the geology and structure of the Salt
here and have provided us with many useful insights and suggestions. Range, with observations on related areas of Pakistan. In: Tectonics
We thank M. P. Coward and J. P. Platt for their reviews. The finite of the Western Himalayas (edited by Malinconico, L. L. & Lillie,
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Moussouris from a package given to us by Randy Richardson. This Hafner, W. 1951.Stress distribution and faulting. Bull. geol. Soc. Am.
work was supported by Petroleum Research FoundationStarter Grant 62,373-398.
No. 21054and NSF Grant EAR-89-15942(to D. M. Davis), as well as Harms, J. C., Cappel, H. N. & Francis, D. C. 1984.The Makran coast
by INT86-09914and EAR-88-16962 (to R. J. Lillie). of Pakistan: its stratigraphy and hydrocarbon potential. In: Marine
Geology and Oceanography of Arabian Sea and Coastal Pakistan
(edited by Haq, B. U. & Milliman, J. D.). Van Nostrand Reinhold,
New York, 3-26.
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