Stratal extension in thrust footwalls, Makran accretionary prism; implications for thrust tectonics

Small-scale stratal extension is commonly associated with thrust faults in the Makran accretionary prism of southwest Pakistan. Low-angle normal faults, dipping in the direction of transport relative to the thrust, are common in gouge zones and duplexes, and may also cut the footwall and hanging wall, where they cause stratal extension. These faults are probably Riedel shears. We have studied a broad zone of more complex extensional deformation beneath one bedding-parallel thrust. The zone disrupts earlier compressional structures and affects a significant volume of the footwall. Normal faults, initiating in a rearward sequence, branch off the thrust and cut down into the footwall, merging at a lower stratigraphic level. Part of the thrust displacement was thereby transferred to that level, and fragments of the footwall were detached and transferred southward beneath the hanging wall. The latter remained undeformed, and accommodation structures are confined to the footwall. These effects may have been caused by variations in sliding resistance along the fault. If a thrust transfers part or all of its displacement down section along normal faults in this way, before eventually cutting back up toward the surface, it will detach a rootless fragment of footwall rocks from a position forward of the initial ramp through that horizon. This process can be referred to as "footwall plucking." It can explain rootless basement slices along thrusts, for example, and it has important implications for section restoration techniques and the interpretation of thrust belts.