TECTONIC EVOLUTION OF THE MARGALA HILLS AND A PART OF THE SOUTH EASTERN HAZARA RANGES, PAKISTAN

Abstract

The Margala Hills and Hazara Ranges are the south-western continuation of the HazaraKashmir Syntaxis in North Pakistan, constituting an east-northeast trending fold-thrust belt of Jurassic to Eocene platform sediments, partially overlying Late Precambrian basement. The current field investigations have revealed that at least three macro-scale fold-thrust nappes, which are structurally linked to a regional basal detachment located at base-Hazara Slate, fabricate the structural grain of the Margala Hills and southeast Hazara Ranges. From north to south these nappes include the Hazara fold-thrust nappe which is exposed along Hazara Thrust. Below the Hazara nappe lays the Garamthun nappe which is exposed at surface along Garamthun Fault. Southeast of the Garamthun Fault the hanging wall sequence of the Main Boundary Thrust constitutes the third fold-thrust nappe of the area and constitute the foothills of Margala Hills. All these fold-thrust nappes are characterised by east-northeast trending major anticlinal folds detached at the level of Jurassic rocks. The stratigraphic set up of these fold-thrust nappes implies a gradual south younging of structuration of the study area. The lithostratigraphic offset along the mapped faults is very little and does not support large scale horizontal movements with the thrust faulting alone. The large scale fold-thrust belt allochthony is shared by fold nappe mechanism which is also believed to be responsible for the thickening of the sedimentary pile. Mesoscopic and macroscopic folds data indicate the presence of at least three distinct lines of folding including northeast-southwest followed by southeast and north-south trends. The northwest-southeast compression is the prominent grain of the southeast Hazara and Margala Hills. Mesoscopic fault planes can be categorised into three distinct sets. The most prominent is the top to the southeast followed by top to the west-northwest and northeast. The top the southeast set is consistent with the movement direction on the macroscopic scale faults (Hazara Thrust, Garamthun Fault and Main Boundary Thrust) and the pitch data is suggestive of a dominant thrust slip motion in addition to left lateral strike slip adjustments. The second set that is the top to the west-northwest can be considered as the back thrusting phenomenon associated with the top to the southeast major displacement. The third set and a few other less prominent fault planes are generally oriented discordant to set one and two and indicate the transpressional related adjustments of the region. Most of dips in the hanging vii wall of Main Boundary Thrust are found to be overturned to the northwest and the axial planes of the folds mostly dip southeast. This back folding is most prominent and consistent along the hanging wall of Main Boundary Thrust and gradually defuse in the northwest direction. The synthesis of the structural data indicate that in the whole Miocene southward directed major shortening created a southwards propagating, flat-ramp thrust fault, cutting up section. This basal thrust carried the Margala Hills Mesozoic and part of its Murree cover towards the south above thicker Murree molasse and underlying Northern Potwar Deformed Zone Mesozoic nappes. Increased shortening took place by tight folding of the hanging wall. This could be the main reason of folds found in the Margala Hills. Late (?) Pliocene major, crustal or lithospheric scale folding created the Hazara Kashmir Syntaxis and created north-south to northeast-southwest axis folds in the Margala Hills. This event is responsible in the first instance for the east or west verging structures in the whole area, for the northwards or northwestwards overturned folds of the Margala Hills; and for the major crustal thickening and subsequent very important uplift of the whole area. The same event could have created strike slip faulting. Repeated south verging thrusting in Quaternary resulted in the southwards movement of the Salt Range above Pleistocene sediments, and flat, mostly south verging thrust faults cutting the older structures. This last event may have generated deeper ramps in the Northern Potwar Deformed Zone and may have formed folds and uplifts in the Northern Potwar deformed Zone and beneath the frontal part of Margala Hills. Strike slip faulting may accompany this event, together with gravitational collapse-induced normal faulting.