Fault zone migration by footwall shortcut and recumbent folding along an inverted fault: example from the Mosha Fault, Central Alborz, Northern Iran

The Mosha Fault is a multiply inverted fault in the Central Alborz. Field observations and structural data from this fault show that a footwall shortcut is the major mode of response of this fault to contractional deformation. Although the Mosha Fault is a basement-involved fault, there is no evidence of involvement of basement along its footwall shortcuts, at least in the study area. Footwall shortcuts along this fault vary in size from several hundreds of metres to tens of kilometres, suggesting that a footwall shortcut can be scale independent. It is proposed that footwall shortcuts can also occur as blind thrusts under fault-related folds in the terrains near the major inverted faults. Similar cases also exist in other regions such as Japan. Some large footwall shortcuts may be the causative fault of devastating earthquakes in the active inverted terrains such as the south Central Alborz. Incompetent layers acting as detachments may play an important role in the development of footwall shortcuts. Recumbent folding in the form of a cover nappe in the footwall of the Mosha Fault is another case of southward migration of deformation along the Mosha Fault by which the fault has responded to the Oligo-Miocene compression. This case can be considered as a newly recognized style of deforming structure that occurred along an inverted fault.