Range Front Structural Geometries and Décollement related Tectonics: An example from Sothern Surghar Range, Trans-Indus Ranges, NW Pakistan

Structural reassessment is described for the eastern and southeastern flank of the north-south oriented segment of Surghar Range, representing the sub-Himalayan frontal structural signatures of northwest Pakistan. Acquisition of outcrop data and structural mapping in the area revealed that the Surghar Range is delimited by a prominent frontal thrust fault which is laterally extended from Kurrum River of Dera Tang in the south to KaIabagh Fault System in the east. Structural geometries describe the presence of an east to southeast vergent thin-skinned fold-and-thrust system developed in the Paleozoic-Mesozoic sequence overlying by a thick overstrain of the Cenozoic rocks. The north-south oriented segment of the range displays tectonics structures developed in response to the coordination of fold-andthrust system. These variant tectonic structures, probably controlled by the east-west directed compressional regime. The study area has been mapped right of Dera Tang in the south to Malla Khel in the north. This segment is comprised of approximately NW-SE oriented thrust fault laterally extended along the range front exploiting different horizons for exhumation at surface. Three different kinds of structures have been developed from south to north as force folding including fault propagation and fault bend folds and transverse strike-slip faults to frontal thrust in the vicinity of Sirkai. A number of anticlinal structures have been mapped in this segment where Siwalik rocks are well exposed at both limbs but the same rocks observed omitted from the fore limbs of some other anticlines. Low to moderate uplifting and strikeslip tectonics are the significant structural style of the southern north-south segment while high uplifting, concentric overturned folding and thrust faulting are the dominant structural mechanism of the northern terminus of this segment. The strike-slip faults observed terminated against the Surghar Thrust in the vicinity of Sirkia. Maturity in tectonic progression, structural growth, maximum crustal telescoping and unearthing of older rock sequences observed from south to north in the mapped area. Chronologically the tectonic phase of transpressional regime observed younger or concurrent as proportional to compressional regime in the region. In general the faulting phase of both regimes is not older than Plio-Pleistocene to sub-Holocene.