Structural and Tectonics of Kohat Plateau


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The Kohat folds and thrusts belt is situated in North-Western Pakistan made due to the collision between the India-Eurasia plates. Kohat consists of tight anticlines and broad synclines structures. Previous litrature shows that deformed pattern of this area is due to repeated episodes of deformations. At the surface, doubly plunging anticlines and synclines are observable. Seismic data show that the thrusts fault begin from detachment faults located at the sedimentary-crystalline At the surface, the Shakardarra Faults, the Tola-Bangi-Khel-Fault, the Chorlaki-Fault, and the axial trend of fold change their orientation from EW to NS which shows that the thrust and axial trend of folds are rotated .Strike-slip motion is the prominent deformation of northern segment. 

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The current deformation occurs more concentrates on the splay faults in the northern of the Kalabagh -Fault. We conclude that Shakardarra is sequentially evolved in three episodes of deformation. firstly, the detachment folds formed on Eocene evaporites, originated from the basal detachment in the second phase. In the third phase, early formed folds and faults are rotated along the vertical axis by the influence of Kalabagh strike-slip fault.

In India the oroclinal pattern of the Himalayas is NW-SE , it changes to EW in Pakistan, and moves toward becoming NS along the western outskirt of Pakistan . The Kohat plateau involves Eocene and more youthful sedimentary sediments in a structurally (tectonically) limited basin and is situated in the west of the Potwar fold and Thrust belt .The Kohat Fold and Thrust Belt otherwise called Kohat-Plateau have a place with the Himalayan Fold and Thrust System situated in the Sub Himalayas. 

The Kohat Fold and Thrust-Belt stretches out from the MBT in the north to Surghar go in the south . It is isolated from its partner Potwar Fold and Thrust-Belt by the Indus River and its western limit is set apart by Kurram- Fault . Kohat Fold Thrust-Belt and Potwar-Fold Thrust-Belt are hereditarily identified with Himalayan incited deforming. Notwithstanding, the basic unpredictability is increasingly articulated in Kohat-Fold and Thrust-Belt. In the examination zone, Shakardarra is situated at the limit between Kohat-Fold,Thrust- Belt and Kohat-Fold, Thrust-Belt with a functioning Kalabagh strike-slip flaw in the southern bit.

The arrangement for the most part comprise of limestone and shale. The thickness is 95 meters in the zone of the investigation. Thickness diminishes from west to east. predominant lithology in the lower some portion of development is shale, while in uper and center part limestone is found. The shale is grenisg dark to dim. It is slight delicate bedded, calcareous and fossili-ferrous. The limestone is slim to medium bedded fine grained cracks and more nummulites. 

The basia khel-surdag fault in south and kurma anticline in the north covers southern Kohat-plateau. South vergent folds and thrusting belts in karak trough in the east and bannu basin in the west described the southern Kohat-plateau. Being in the clossed vicinity with the Kala-Bagh-fault in the east and kurram- fault in the west. It has under gone noteworthy measure of strike-slip changes alongside the contractile deformation. The southern Kohat-plateau is under lain by the sedimentary rocks of Eocene age and uncovered the rocks as youthful as the upper siwalik . Structural disfigurement with in the southern Kohat-plateau gave an impression of being of plio-pliestocene to Holocene period.

The Indian lithospheric plate covers the north-western limit of Pakistan. Thin skin tectonic highlights are framed in the north and north-western limit of Indian-plate since Eocene time because of its going underneath the Eurasian-plate . Subsequently thick sedimentary layers as foreland fold overlap and thrust-belt are pushed over the indian craton and offering ascend to the Himalayan mountainranges. The watched features of the plate which is converging tectonically all through the world are foreland- folds and thrust-belts.The thrust- belt and Kohat -fold alongside its front scopes of the North-Western Himalayas, one of them, which had been affected by the Southward movement of deformation happened in late Miocene. 

The Northern limit of the Kohat- plateau set apart by MBT , bringring the profoundly deformed Kohat extend including Mesozoic rocks onto the sediments of Kohat- fold and thrust- belt of Eocene to the Miocene age. Toward the west the left parallel transpressive Kurram -fault is simply uncovering the mesozoic rock of Thal, North Waziristan and Samana with the Eocene-Miocene sediments of this folds and thrust- belt. The south-eastern limit of Kohat -fold and thrust- belt been set apart by Surghar Range where Paleozoic rocks have been pushed southwards over the Punjab foredeep . The undeformed sedimentss of Bannu basin denotes the southern limit of Kohat -fold and thrust -belt.

The territory is contained fold and thrust -belt which are thin -skin structures superimposed by thick- skin structures. The significant piece of the plateau is ruled by compressional structures in any case, the strike-slip blaming is kept toward the southern Kohat plateau. In the east of Shakardarra, the east-west inclining structures change their direction toward the north-south direction.

The Kohat Plateau comprises of an intensely deformed and raised thrust -sheet. Pop-ups, wide synclines, a small fault, and evaporite-cored anticlines record an abnormal state interpretation of a substantial thrust mass along the Eocene evaporites. The nature and kinematics of the Kalabagh strike-slip fault.The results show that the nearness and a genuine degree of the evaporites is the overwhelming component controlling the horizontal structural variety in the NW Himalayan fold-and-thrust -belt of Pakistan. The Kalabagh -Fault Zone goes about as a zone that suits the differential shortening and auxiliary variety along the orogene pattern. 

The Kalabagh -Fault Zone is separated into two portions of various tectomic movement. Its northern section is prevalently of a strike -slip nature with salt diapirism and the upward movement of salt. South to Khairabad, the real fault is a sidelong ramp of the Main Frontal Thrust the Potwar fold and thrust- belt and the Salt range thrust wedge proliferate southward speedier and more distant than the Kohat -fold and thrust -belt and the Surghar thrust wedge because of the greasing up impact of the salt. A parallel structure, the Kalabagh- Fault Zone, framed as an exchange zone to accommodate the diverse deformational styles between two thrust wedges.

The location contain folds and also thrust belts that are e.g thin-skin are imposed by thick-skin structures. The stresses are directed towards NS which results in EW trending regional structures. The detachments control all the tectonics of Kohat. The rocks which are exposed on surface are of Eocene-pliocene era. The structure which is observed in the Kohat ,if reversed and observed it shows folding of the layers and then the faults were developed on the limbs of the folds so its structure represent the fault propagating folds. 

The folding occurs due to the presence of salt, gypsum. As there nature is of elastic so they bend when stresses are applied and also form deatachments. The presence of two decollment levels is the reason behind the structural disharmony between the surface and sub-surface lithic units. The fault in the region with related infrequent back thrusting comprising generally thrusts that are south skirting, representing a local thrust framework with a scope of 250 to 2000 m relocation along them.The shortening of the zone happened was 37%.

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