GRAVITY MODELING OF THE PROFILE KOHAT-SAIDU SHARIF

Abstract

Gravity data collected along N-S profile from Kohat to Saidu Sharif(Swat) crosses Main Boundary thrust and Main Mantle thrust of northern Pakistan has been interpreted in terms of subsurface lithostr cture and basement -topography of the foreland and thrust belt of the Himalaya using, 2-D Talwani modeling technique. Analysis of Bouguer gravity anomaly shows that the northward gravity gradients are due to crustal thickening, while short wave length anomalies are produced by local structural features and lithological changes in the sedimentary wedge. These anomalies are modeled in terms of upper crustal changes including offsets of the basement surfaces by Chars ada Basement Fault (CBP), Vale Rift Zone (VRZ) and Bagh Basement Fault (BBF), associated magmatism and thrusting of high density material over relatively low density mass along Main Boundary Thrust (MBT), Reef Belt Thrust (RBT) and Main Mantle Thrust (MMT). The density modeling interpretation along the profile indicates the variation of crystalline basement depth from 9 kms near Kohat to 20 kms in Thana. The basement that dips generally towards north is divided into two major blocks (Peshawar basin and lower Kohistan) in the region, possibly due to Horst-Graben structuring controlled by high angle basement faults which may be penetrating to Moho depth. The Moho depth varies from 47 kms (south) to 53 kms (north) causing a gradual increase in dip from 1.50 in Kohat to 3° beneath the lower Kohistan upto MMT following the basement. The results of modeling suggest that the BBF and CBP are the reverse and normal faults respectively which penetrate in the basement probably upto Moho, while the vale rift zone (VRZ) is associated with igneous activity of Permo Carboniferous/Oligocene age such as Malakand granite complex (MGC). The RBT interpreted as a high angle thrust fault is also associated with a large to small intrusive bodies. It is also concluded that the Peshawar basin is a rifted-graben structure that is bounded by east-west trending CBF and VRZ. The BBF and VRZ appears to be creating a stair like structure in lower Kohistan. Probably CBF and VRZ are coinciding with the western extension of the Punjal thrust (PT) and Main Central thrust (MCT) respectively, whereas the sense of motion of the PT and MCT in contrast to CBP and VRZ is of thrusting nature and that change could be the consequence of Himalayan orogeny. The Main Boundary thrust is the youngest structural (thrust) feature along which older sediments are being thrusted over younger sediments along a plane which dipping 150 to the north, and flats out at a depth of 6 kms. The Dargai ophiolites generate a moderate gravity anomaly, suggesting a thin ultramafic body which initially was a part of Dir ophiolites complex, and was then fragmented and moved probably under gravity influence about 25 kms south of the present outcrop of the Main Mantle thrust, and was preserved as a klippe structure in Dargai.