Gcol. Brrll. U~ziv.
Pcshcrwnr, 1990, Vol. 23,pp. 175-186
STRUCTURES IN THE HANGINGWALL OF THE
BOUNDARY THRUST: POST- FOLD1
NORMAL FAULTS FROM THE KOTAL-PASS AREA,
KOHAT RA GE, N. P A N S
h1. ASlF KIIAN, KARAhfAT ALI TURI L% IFTIKIIAR AHMEDABBASSI
NCE and Dcpirtmcn t of Goology, University of Pcsl~awar
ABSTRACT
The Main Bounclaiy Tllrust (MBT) is a regional l i ~ ~ e a i n ein
n t t h e external zone
of the Hilllalayan thrust-fold belt in N. Pakistan, which l~ringst h e Mesozoic-Cenozoic
sllclf secliinellts of the Masgala-Kalachi tta-Kohat Hill ranges to lie tectonically ovcr a
pile of molasse sediinents depositctl in the foreland basin. Disruption along t h e MDT
zone started pi~obablyaround early Miocene tillics as suggcstcd by the involvenlent of
the late Oligocene-early Miocene Murree Fornlation in dcfornmtion (Burl~ailk,1983).A
clivcrse asseinblagc of structures is reported associatecI with this luajos fault structure,
iuCludiugthrust faults in imbricate and duplex systems, backt111.usts and divergent fold
structures. Tllese structures are spread over an approxixllately 20 km wide zone in the
Koliat range, where they form a major pop-up structure (Gliauri et al., in prep.).
Earlier accounts of the MBT and its associated structures are given by Cotter
(1933).Various segments of the MBT zone have been subsequelitly mapped in relatively seater details, resulting in tlle recognition of a large number of fold and thrust
structures associated with it (Gardezi, 1974; Gardezi et al., 1976; Ghauri et al., 1983;
Aklltar et al., 1984; Izatt, 1990).
Recently,we have stalted a programme of reinvestigation of the structures in the
liaxigin~vallof the MBT, concentrating in the part of the Koliat range in the Daya
Adam Khel area (K.A. Turi, work in progress). In the process we have carried out
detailed analyses of the thrust and fold structures which were previously outliiied by
Gardezi et al. (1976) and Ghauri et al. (1983). A detailed account of them will be
published suibsequently. In this paper we present data on a set of out-of-the syricliiie
thrust and norilia1 fault structures from the Kotal-Pass area of the Kohat range (Fig. I ) ,
which have not been previously recognized, and which, in our opinion, represent tlle
youngest phase of the tectonic activity associated with the MBT.
GENERAL STRUCTURAL SETUP
The Koliat range is characterized by a con~plesset of structures. The MBT
together with a major backthrust, deliinits tlie range at its southern and nortllcru
flanks, separatirlg it, respectively, from the plains of Kohat and Peshawar (Fig. 1).
Tllese structures divergently thrust tlie Samana Suk Formation of Jurassic age over
the Oligocene-Miocene Murree Fonnations, on the either side of the range. Within the
range, there is an interesting structural disposition of tlie stratigraphy. The Mesozoic
strata including Samana Suk, Chicliali, Lumsliiwall and Kawagarli Fonnations, are
mainly exposed along tlie northern and soutliern flanks of the range (in the imxliediate
liangiiigwalls of the two faults mentioned above), wli~reasthe Paleocene Lockhart aiid
Patala Formations occupy the central part. In tlie latter, there is a repetition of tlie
Lockliart and Patala Formations due to both tight, cylindrical folding, and imbrication
above a detachment occurring at tlie base of the Lockhait Formation. The repetitioll in
tlle older set of rock formations in the either flanks of the range (for instance that of the
Samana Suk and the Chichali Formations) is probably due to imbrication below a roof
thrust which was apparently located in the Chichali-Lumsliiwall Fomlations. A later:
strong phase of folding has developed complex interference patterns, wliicll have further colltributed to the uplift of the range. The structures described in this paper
postdate all these structures, and include 1) out-of-the-syncline thrust faults, wllicli
record tlie culmination of shortening and uplift in the Kohat range, alicl 2) a set of
Fig. 1. Geological map of thc D i m Adamkhcl-Kotal transcct, Kohat Hill rangc (Aftcr Gardcri et
al., 1976; Ghauri ct :ill.,1983).
normal faults, wllicli mark tlie gravitational adjustments following abnormal uplift
associated with tlie MBT and its associated structures.
An indirect conscquclm of tllc sout ll-vrq.$ng, t r t t~, - r t a t ho-sy~lrli~n*
I l ~ i w iftlg in
the soutliern limb of !he 1C0 ti11 ~ j w l i l ~ tIMS
!
IIC'CII
: ~ I l wIIOIIIII~S
t
rlispltwmwt of
war-llorizontal I'aleoccnc strilfa 1111 t q)t L tic;~r-vei~t
i r d h I w ~ x f ~s~~crcssiclit.
ic
Gllurwi ct
a]. (1983) mappctl, just t o t hc s o ~11~ of
t I IIC I(r, t:tl I':iss, twtr ltrlr~vh:l),~~ccl,
so~~llwurtl
directed projcctioi~of tlic I'al~~ocwc!
I,ockll;u-t illid 13:ltaltt Ihm~:itim~
disco~*d:~~ltIy
overlyillg the rest of tllc strutiglulrhy i l l ihis p:lrt o f t l i v I<uh:iI. Ikul f li'ig, 2#5 ) . f ~ ' i l t b l a ~
is a marked diffcrcncc in t ? cltiil.utlt*
~
~f iwtltling ill fl~csc?
tccttttlir i d ~ u t prtrjcctio~~s
u
uf
the Palcoccne strata m
R t l i ~~1nd~r1yI11g
M U S ~SX
I I~
C C~
C SC
S ~ ~111
I I ,11111 ~CWIIICI.,f llc 1)cdclillg is either horizontd or tlips imlg sli;rllr,wly tow;trds 1kort111r:irdy C S C C C ~ :3fi01,
~ ~ I ~
wllile in the underlying M c a m i c succcssioj~,1 1 1 dips
~
are l4a1*c1yless st3ecprth
thrill WU.
This type of relationship, in our opinion, is nowhere near to the normal stratigraphic
type as shown by Ghauri et al. (1983).
The tectonic relationship of the fault-bounded, allocthonous blocks of t h e
Paleocene strata on top of the Mesozoic succession is illustrated in Figs. 4, 5 & 6.
Commonly they are located over a shallowly south- dipping fault plane at the base of the
Paleocene succession. The strata in the hangingwall of these faults is either horizontal
or dips shallowly towards north, suggesting that the basal fault cuts downward in the
stratigraphy as it propagated southward. There appears to be little effect of this fault
structure on the attitude of the underlying Mesozoic succession, which is veitical to
highly steep, and is remarkably continuous in its E-W strike.
An aspect which is probably worth mentioning here is the existence of "window"
type structures in between and on the either side of the southward projections of the
tectonic blocks of the Paleocene strata. Apparently these "windows" are a result of
topographic effect, as they coincide and are roughly parallel to the N-S running valleys
on the sides of the two ridges which are occupied by the tectonic blocks of the Paleocene
strata. There is a considerable difference in the altitude of the two tectonic blocks, the
eastern one lies almost near to the valley floor, while the western one being approximately 200-300 In high near to the top of the ridge. Since the two tectonic blocks
occupying the acljacent ridges are similar in all respects, e.g., stratigraphy, attitude of
their basal fault planes, and that of the bedding planes within these blocks, it is
probable that originally the two blocks were continuous and were displaced along a
single fault. The difference in the altitude of the fault planes a t the bases of the two
tectonic blocks may be due to an originally curvilinear nature of the basal fault plane,
with an eastward tilt which resulted in the much lower position of the eastern block
relative to that of the western.
DISCUSSION
Whereas the recognition of faults in the Kotal Pass area has been a relatively
simple matter of observation, owing mainly to dramatically good exposures and well
established stratigraphy (Gardezi et al., 1976; Ghauri et al., 1983), the assessment
about their nature is not that s t r a i g h t f o i ~ a ~The
d , shallowly noitli-dipping fault (Fig.
3) a t the contact between the Lockhait and the Kawagarh Formations was apparently
reverse in its sense of movement,' as it brought necn.:-horizontal strata of the Lockhait
Formation probably from the core of the box-shaped syncline to lie on top of the steeply
northward dipping strata of the Kawagarh Formation in the southeastern limb of the
syncline. This fault owes its initiation probably to tight folding in area which resulted in
south-verging out-of-the-syncline thrusting. Late stage movement along this fault
transported the Paleocene strata southward to lie upon Mesozoic succession as tectonic
Fig. 4. An allocthonous block of Palcoccnc strata rcsting tcctonically on top of stceply dipping
Mesozoic succcssion along a normal fkdt. Within the tectonic block thcrc is an imbricate
thrust which brings the Lockhart Formation t .Ilic on top of a normal ~ o c k h a r t Patala
succcssion.
-
ALLUVIUM
Irp.1
PATALA F rn.
El
LOCKHART Fm.
KAWAGARH Fm.
b i ]
pq-
CHlCHALl LUMSHIWAL Fm.
SAMANASUKFrn.
0
F.C. POST
--- - - -- --
-
UN METALLED ROAD
M E T A L L E D ROAD
NORMAL FAULT
+
THRUST FAULT
SY N C L IN€
25
OVERTURNED ANT.
Fig. 5. Geological map of thc southward projecting lobc-shaped, tectonic blocks of Palccccnc
strata resting tcctonically on top of theMesozoic succcssion {modified after Ghauri, c t al.
1983).
blocks. Our interpretation about the initiation and subsequent movement on these
south-verging fault structures have been poitrayyed in (Fig. 7a,b), together with their
relationship to their host fold structure.
We, however, do not believe in the entirely reverse sense of movement of the
faults at the base of the southward projecting tectonic blocks comprising Paleocene
strata. The shallow, southward dip of these faults, together with a considerable displacement of younger strata on top of the steeply dipping Mesozoic succession would
favour a normal sense of movement on these faults. We interpret that southward,
up-dip movement of the Paleocene strata out from the core of the synclinal fold resulted
in an abnormal uplift of the Kotal Pass area (Fig. 7c). This Ied to initiation of southward
dipping normal faults, along which there was a gravitational collapse of the uplifted
Paleocene strata. Abundance of fault breccia associated with the fault planes suggests a
near-surface level of these late stage normal faults.
Fig. 6. A view of western l'obe-shaped tectonic block qf Paleocene strata, resting on top of stceply
dipping Mesozoic succession along a normal fault. Notc thc repetition of Samana SukChichalli-Lumshiwall sequcncc due to folding and imbrication.
CONCLUSIONS
The Kotal Pass area of the Kohat range shows evidence of latest structures
associated with the MBT zone. The early-stage thrust and fold structures in the
hangingwall of the MBT resulted in the formation of box-shaped folds with tight
overturned limbs. One such syncline at Kotal Pass is characterized by several southverging out-of-the syncline-thrust faults, which resulted in an abnormal uplift of the
Kotal: Pass area. This was compensated by the development of southward dipping
shallow normal faults, along which there was a gravitational collapse of the uplifted
Paleocene strata.
Fig. 7. A schematic rcprcsentation of thc successive stages involvcd in thc dcvclopment of
post-folding fault structures in the Kotal Pass arca of thc Kohat Rangc (not to scale). a)
devclopmcnt of the ovcrturncd limb of thc Kotal syncline, with positions of thc out-ofthc-syncline thrust to bc shown by F1 & F2. b) southward displacement of the shallowdipping strata from the core of thc syncline on top of thc stccp limbs of the syncline. Also
shown is thc position of the fault (F3) which, in thc ncxt stage, marked the gravity collapse
of the abnormally uplifted terrain in thc Kotal Pass arca. c) dcvelopmcnt of south-verging
normal fault, which displaced abnormally uplifted Paleocene strata (out-of-the-syncline
thrust in stagc b), downward on thc southern limb of the syncline. Jss = Jurassic Samana
Suk Formation, Kc = Cretaceous Chichali Fm, K1 = Cretaceous Lurnshiwall Fm, Kkg =
Cretaccous Kawagarh Fm, T1 = Tertialy Lockhart Fm, Tpa = Tcrtiary P a t d a Fm.
Acknowledgements: Field work for this study has been supported by National Centre of
Excellence in Geology, University of Peshawar. Sajjad Ahmed (Lecturere, Geology Dept.,
Univ. of Peshawar) and Mehboob Shah (undergraduate Geology student), provided
useful field assistance. This work is result of an ongoing M. Phil. study in the Kohat
Range (K.A. Turi).
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