Tectonic Evolution of Fractures in the Precambrian Manki and Dakhner Formations, Khairabad, Pakistan

Abstract

The Attock Cherat Range that forms the southern boundary of the Peshawar Basin evolved ~ 20 Ma by N-S directed convergent margin tectonism related with Himalayan Orageny along the northern margin of the Indian Plate . This orageny led to the initiation of pervasive conjugate fractures in the Precambrian Manki and Dakhner formations. Systematic tectonic conjugate fractures are arranged in such a way that their acute bisector δ1  is parallel to N-S horizontal bulk shortening. The acute bisector δ1 of the conjugate fracture of the Precambrian Manki Formation is parallel to the acute bisector of the pervasive conjugate fractures in the Precambrian Dakhner Formation. These conjugate fractures collectively indicate N-S shortening and southward thrusting that has affected the Precambrian Manki and Dakhner formations in the Attock Cherat Ranges.

The through going E-W trending fracture set-1 was exclusively observed within the Manki Formation. It is filled with crystallized quartz and aligned parallel to E-W striking beds of the Formation. Fracture set-1 predates conjugate fractures because the later have displaced fracture set-1 across the study area. The presence of well developed plumose marking within the Manki Formation shows brittle extensional fractures and evidently pre-dates all other fracture sets in the region. The quartz crystals in fracture set-1 are fractured, which are later healed by secondary calcite. The presence of well developed curved calcite fibers in conjugate fractures indicates a possible opening direction with smooth boundaries. The calcite crystals, which are not the main ingredient of the wall rocks, initiate from the narrow median suture and gradually grow towards fracture boundaries. Therefore, the conjugate fractures are characterized by antitaxial vein growth.