Imprints Of Transpressionalo Deformation On The Southern Kohat ...

Geol. Bull. Univ. Peshawar, Vol. 33, pp. 87-95, 2000FAYAZ ALI', SAJJAD AHMAD', IRSHAD AMMAD* & S. HA MI DULL AH^'~epartment of Geology, University of Peshawar, Pakistan'National Centre of Excellence in Geology, University of Peshawar, PakistanABSTRACT: A suite of evaporite deposits, shelf sediments and mollase sequence ofEarly Cenozoic age from Karak area marks a part of southern Ghat Plateau.Structurally the area is interpreted as east-west trending positive flower structure,which is exposed for about 20 km along strike with widths varying from 3-5laterally. It is bounded by the north dipping Karak Fault in the south and south dippingNan Panos Fault in the north and internally subdivided by the south dipping BandaKunghara Fault. All these faults trend east-west and have moderate suvace dips. Aright lateral component of deformation along the Karak-Nari Panos structure isindicated by: 1) the north-east directed trend of the Daggar Syncline in the footwall ofthe Nari Panos Fault, 2) the local scale thrusts in the core of this syncline and 3) eastnortheast trending right stepping folds located south-east of Banda Charpara in thefootwall of Banda Kunghara Fault. The idea of transpressional deformation is alsosupported by the seismic data, which shows that all the major fault structures becomesteep, converging at deeper level and the presence of horizontal to sub-horizontal slickson different fault suqaces. The region has experienced an early phase of compressionaldeformation overprinted by later phase of transpressional deformation of Plio-Pleistocene age.INTRODUCTIONThe Kohat plateau lies in the north-westernindentor of the Indian Plate of a majororoclinal deflection with structural trend fromEW to NS within the Himalayan chain andassociated mountain ranges (Fig. 1). Theplateau represents a typical foreland fold andthrust belt of the Himalayan collision zone,about 100 krn south of the main foothills ofHimalayan range. The plateau was the maindepocentre of the Himalayan shed duringEarly Miocene time (Wells, 1984; Pivnik1992), now uplifted and deformed,preserving clues about the south progressionof Himalayan deformation.Previous structural interpretations for thetectonic evolution of the region implies thqpropagation of foreland fold and thrust beltalong blind and partially emergent thrustfaults from an upper detachment at the baseof Eocene sequence underlain by passive roofduplex geometry within Mesozoic rocks(Abbasi & McElory, 1991; McDougal &Hussain, 199 1). An alternate interpretationwas proposed by Pivnik et al. (1993, 1995,& 1996), suggesting transpressionaldeformation instead of compressionaldeformation. According to them the KohatPlateau is charectarized by narrow anticlinalhills, which are cored by high angle reversefaults converging to a single strike-slip zone

AfghanistanRangeSarnodha1. Tectonic map of north Pakistan (After Kazmi & Rana, 1982).88

at depth. They believed that virtually everyanticlinal structure within the plateau is apressure ridge representing the surfaceexpression of a positive flower structure atthe depth.The Karak-Nari Panos area is believed tobe very important as far as the understandingof tectonic evolution of Kohat Plateau isconcerned. It is located at the southern edgeof Kohat Plateau and is bounded toward westby the left lateral Kurrwn Fault (and rightlateral Kalabagh Faulit in the caste). It isbelieved that the opposing motion along theseregional , structural discontinuities isresponsible for the complex structural styleof the Kohat Plateau. The study area has wellpreserved imprints of the transpressionaldeformation superimposed upon earliercontractile structures.GEOLOGICAL SETTINGFour tectonic features delineate thetectonomorphic terrains of north Pakistani.e., 1) the Main Karakururn Thrust (MKT)that separates the southern Eurasian Platefrom Kohistan arc (Fig. 1, 2) the MainMantle Trust (MMT) separates Kohistan arcfrom the northern Indian plate, 3) the MainBoundary Thrust (MBT) separates the HillRanges and Hazara Himalayas in the northfrom the deformed foreland basin in thesouth, and 4) the Salt Range Thrust, SurgharRange Thrust and the Khisor Range Thrustforming the southern boundary of thedeformed foreland basin in Pakistan alongwhich Mesozoic rocks are thrust over the undeformedPunjab foreland basin.The Kohat fold and thrust belt is thewestern most deformed part of the Himalayanforeland basin which rims the entireHimalayas with an east-west trend fromGanges delta in the east up to Kohat area inthe north Pakistan and changes to north-southorientation in the. vicinity of B~MUdepression stretching up to Karachi. TheMain Boundary Thrust lies north of theKohat Plateau whereas the left lateral KurramFault flanks its western periphery and towardsoutheast it is flanked by the right lateralKalabagh Fault (Fig. 1).The Kohat Plateau is underlain by acomplex assemblage of shale, limestone,gypsum and sandstone of Eocene age and isunconformably overlain by the thickalternating sequence of sandstone, shale andconglomerate of the mollase sediments.In the study area the oldest rocks belongsto Eocene age which include Bahader KhelSalt and Jatta Gypsum at the base followedupward by shelf sediments of Kuldana andKohat formations. These units areunconformebly overlain by mollase sedimentsof Miocene to Pliocene age, which includeRawalpindi and Siwalik groups (Fig.2).STRUCTURAL GEOLOGYThe area is dominated by regional scale foldsand thrust faults trending east-west (Fig.3).The folds are open, asymmetric and haveshallow plunge angles. Various local scalefolds oriented NNE are mapped as well.These bear an oblique relationship with theregional scale folds and faults.A geological cross-section along lineA-B of Figure 4 has been constructed and thestructures are projected by combining surfacedata with the subsurface data. The subsurfacedata was acquired from OGDCL of Pakistan(line #. .906-NSK-47). On a north to southtraverse along the geological cross-sectionthree major faults i.e., Nari Panos, BandaKunghara and Karak faults control thestructure of the area (Fig.4). At the outcropthe Nari Panos Fault moderatly dipssouthward and brings Bahader Khel Salt and

Kamlial FormationBakhaderkhel Salt100AAAAAAAFig. 2.Stratigraphic column of the study area.

LEGEND-ERNARYp a lALLUVIUMElSOAN FMTTdq- :DHOK PAFAN FMI --KAMLIALZ $ LFMKULDANA FMLi-JATTA GYPSUMF - - 7LA!!-I-6AHADERKHEL SALTCONTACT ANTICLINE ---+SYNCLINE -+DIP & STRIKETHRUST FAULT --PLUNGINGANTICLINEPLUNGINGSY NCLlNETHRUST FAULTINFRREDY'*Fig. 3.Geological map of a part of southern Kohat Plateau, north of Karak.AKF = Andi Karak Fault, BKF =Banda Kunghara Fault, KKF = Karak Fault,MLF=Mitha Khel Fault, NPF =Nari Panos Fault, KK=Karak Town, DR=Daggar,BK=Banda Kungharat and NP=Nari Panost

Jatta Gypsum over the Nagri Formation inthe footwall. Nagri Formation is about 4 Krnthick in the mapped area (Meissner et al.,1974) suggesting a vertical displacement ofabout 3Km associated with Nari-Panos Fault.Along the Banda Kunghara Fault, JattaGypsum is in thrust contact with the ChinjiFormation in the footwall and moderatelydips toward south. The cutoff relationshipalong the fault shows about 3 Km verticaldisplacements. The Karak Fault moderatelydips northwards and brings Jatta Gypsum inthe hanging wall above the Chinji Formationin the footwall. Chinji Formation is aboutone Km thick; underlain by one and halfkilometer thick sequence of Karnlial, Kohatand Kuldana formations. This implies about 3Krn vertical displacements.The seismic data suggests that all themapped faults have moderate dips at thesurface and become steeps with depth(Fig.4). At as approximate depth of 4 kmthese faults appear to be converging towardseach other and become almost vertical. Thearea in between Nari Panos and Karak faultis named as Karak-Nari Panos PositiveFlower Structure. The marginal faultsdiverge upward in opposite direction havingopposing vergence with a central structuraldiscontinuity i .e. Banda Kunghara Fault(Fig.4). The Karak-Nari Panos structure isexposed about 20 Km along strike withwidths ranging from 3-5 Km laterally(Fig. 3).Pivnik and Sercombe (1993), have welldocumented the role of transpressionaldeformation in the Kohat Plateau, however,the present work further enlighten the ideawith the support of currently acquired fieldand seismic data. The following points can beput forwarded in favour of transpressionaldeformation. Slicks data was recorded andplotted on stereographic net. The dominantsets of data suggest horizontal translationalong the faults in addition to dip-slip motion(Fig.5). Surface expression of the strike-slipmotion is displayed by local structures aswell. Two local scale thrust faults aremapped in the core of Dagger Syncline.These faults are oriented N65" E, and makean acute angle of 25" with east west orientedNari Panos Fault (Fig.3). In addition, theorientation of the axis of the Dagger Synclineitself is parallel to these local scale faults andintersects the Nari Panos Fault at an acuteangle of about 35". Furthermore, in thehanging wall of the Nari Panos Fault, severalnortheast oriented local folds are mappedwhich bears oblique relationship with theNari Panos Fault (Fig.3). South of BandaCharpara, in the hanging wall of BandaKunghara Fault, several northeast orientedanticlinal and synclinal pairs are mapped.These also bear an oblique relationship withthe Banada Kunghara Fault. All these localscale folds are believed to be the latermanifestation of .the oblique compressionover printed upon the early-formed east-westtrending contractile structures. The geometricrelationship of the mapped folds and faultswithin the study area is quite comparablewith the possible geometric array ofstructures developed in a right-lateral strikeslip fault zone (Harding, 1974) (Fig. 6). Thusa right-lateral sense of motion is believed tohave occurred along Karak-Nari Panosstructure.CONCLUSIONThe present study leads to the conclusion thatthe Karak area has experienced an earlierepisode of north-south compressionaldeformation overprinted by a north-westoriented oblique compressional deformation.The north-south phase of compressionaldeformation is responsible for the initialarchitecture of the area, which resulted ineast-west oriented large-scale folds andfaults. Whereas the later, north-west oriented

Fig. 4. Geological cross-section along line A-B of Figure 3 (Legend as in Fig. 3).Cr M1- Pr=cretacious, Mesozoic and Paleozoic rocks undifferentiated.Fig. 5.Equal area plot of the slikeline data along different fault surfaces.S

Fig. 6.Geometric relation of folds and faults to right-slip wrench fault combined scl~lernatically withstrain ellipse and principal strain directions, contraction (C) and extension (E). (After Harding,1974).

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