Origin of horsetail faults in extensional regime using physical experiments for the Linnan Sag

Horsetail faults and rotational faults, which maintain slight differences, are all important identity markers for strike-slip movement. However, it is difficult to explain why horsetail faults also appear in the extensional regime. Using the Linnan Sag as the reference, the physical experiments are conducted to reproduce the formation processes of horsetail faults in the extensional regime. The characteristics of these horsetail faults include: (1) horsetail faults are the product of strain superposition during two phases of non-coaxial extension; (2) pre-existing faults generated in the initial stage may reactivate by the combined elongation of dip-slip motion, followed by oblique-slip faulting at the middle-late stage, thus becoming the principal fault of the horsetail faults; this initial distance is obviously less (∼46%) than its distance during the middle-late stage; (3) a large number of new dip-slip faults appear at the tip of the principal fault in the middle–late stage, spatially connecting with the principal fault to form the horsetail faults; (4) other pre-existing faults that intersect the principal fault are not necessary for forming the horsetail faults, but their presence may make the principal fault prone to segmentation. Therefore, although horsetail faults in the extensional and strike-slip regimes are similar in shape, their formation conditions and fault properties are very different. Understanding the characteristics of the horsetail faults in the extension regime contribute to the further study related to the oil and gas migration and accumulation in the Petroliferous basin.