Range front structural style: An example from Surghar Range, North Pakistan

Abstract

The Surghar Range is the easternmost extension of the Trans-Indus Salt ranges of North Pakistan that appears as an arcuate mountain belt. The range follows in general an east-west structural trend while bordering the southern Kohat Plateau and switches to north-south trend along the eastern flank of the Bannu Basin. Being an active range front it preserves variety of structural styles in the outcropping and non-outcropping rocks. The east-west trending segment of the range owes its evolution to frontal ramping from basal décollement thrusting within the Paleozoic-Mesozoic rocks and emerges as a low angle frontal thrust named as the Surghar Thrust. The hanging wall of Surghar Thrust is moderately to tightly folded with Surghar Anticline being the most prominent fault bend fold structure. The structural geometries along the frontal thrust fault suggest that it is weakly emergent in the west and changes its character to strongly emergent to tip-stick thrust front progressively towards east. This part of the range dominantly reveals a south verging structural system impinging upon the Punjab Foreland in the south.

In contrast the north-south trending segment of the Surghar Range has evolved as an easterly tapering tectonic wedge of Paleozoic-Paleocene rocks inserted underneath the Eocene-Pliocene rocks. The tectonic wedge comprises north-south trending Makarwal Anticline that has developed above a ramp from basal décollement within Paleozoic-Mesozoic rocks and forms the base of the wedge. The ramp beneath the Makarwal Anticline instead of emerging out at erosional surface gets flattened at an approximate depth of 800 meters to form a shallow level flat. The eastern flank of the Makarwal Anticline is steeply east dipping along the range front and is thrust over by a west verging active back thrust namely the Makarwal Active Back Thrust that forms the upper part of the tectonic wedge. The Makarwal Active Back Thrust is linked with the east verging basal fault in the subsurface at the hanging wall cut off of the Jurassic rocks located at the tip line of the basal fault. The Makarwal Active Back Thrust along with another east vergent active back thrust splays from the forelimb of a buried anticline underneath the Makarwal Anticline forming the top of a second tectonic wedge. The term active back thrust is applied to the west verging faults because their hanging and foot wall strata is actively deforming in response to wedge insertion.