Analyses of joints and associated neo-tectonic deformation band shear zones in the Siwalik Group of southern Surghar-Shinghar Range, Trans-Indus Ranges, Pakistan

Abstract

This study presents field observations on joints and deformation band shear zones (DBSZs) at the southern flank of the Sarkai-Mochi Mar (SMM) anticline, which is the southernmost structure of the Surghar-Shinghar Range. The eastern flank of the anticline is eroded to form steep scarps, exposing the high angle trace of the Surghar Thrust (the Dara Tang fault).

Two joint sets NS/NNE and EW/WNW are observed at the southern flank of the SMM-anticIine, where the strike of the strata is about east west. These joints are restricted to the hard sandstone bands of the Dhok Pathan and Nagri formations. Field observations show that NS/NNE oriented joints are compressional and EW/WNW oriented joints are extensional in geometry. The orthogonal relationship of compressional and extensional joints yields rectangular blocks (^-15x6cm) in the hard sandstone bands. Minor folds with fold axes and axial planes parallel to the compressional joints (NS/NNE) are not uncommon.

The area also contains two sets of strike-slip DBSZs that cut across the sedimentary strata at high angles. The NS steeply oriented DBSZs are strongly banded foliated), with mesoscopic dextral sense of movement. These DBSZs are common and well developed on the south-eastern part of the SMM-anticline. The second set with N50^o-60^oE orientation comprising incipient tabular DBSZs with sinistral movement, cut across the fabric of the NS oriented DBSZs. The two DBSZs form a conjugate geometry. The frequency of these DBSZs is high close to the trace of the Dara Tang fault. Since, the Dhok Pathan Formation is the youngest lithological unit in the area (^-0.8Ma), the presence of both the joints and DBSZs in the upper levels of the formation suggests ongoing neo-tectonics in this region.

Based on field evidences, three deformational phases have been suggested for the formation of joints and shear zones. The compressional and extensional joints with a component of folding are formed simultaneously in the first deformation phase. This is followed by the second deformation phase in the form of NS oriented banded shear zones. The third deformational phase is characterized by the N50^o-60^oE oriented tabular shear zones, which sinisterly cut the NS oriented banded shear zones at an angle of about 50^o-60^o.

The dynamics of the SMM anticline in the context of regional tectonics suggest that the structure experienced compression from west to east during the fold formation. The southern flank of the SMM anticline, where the strike of the beds is east west, is recognized as the plunging hinge area of the fold. Thus, the east-west oriented beds experienced east-west compression and north-south extension in the form of joints and minor folds. We assume that the origin of both compressional and extensional joints in the Mochi Mar and part of the Qabul Khel areas may be related with the folding of SMM anticline. As the north-south oriented Dara Tang fault (Surghar Thrust) cuts across the sedimentary strata passing at the core of the SMM anticline the DBSZs in the area appear to be dynamically related with this fault.