Newly identified type of fault system: Circular antithetic extensional faults

We describe a previously undocumented fault type that occurs hundreds of meters below certain kilometer-scale, circular geological structures in sedimentary basins. The faults are circular in plan view and are found at a radius of several kilometers from the host geological structure. The faults dip away from, and are therefore antithetical to, the geological structure around which they are concentric. The faults are extensional with displacements in the order of tens of meters and low displacement-length ratios compared with typical extensional faults. Their kinematics demand concentric contraction (hoop strain), but no evidence for this, such as radial or transfer structures, is seen. Examples are presented from a mud volcano in the Caspian Sea and a crater structure in Saudi Arabia. A generic kinematic model was developed in which the antithetically faulted layer acts as a thick detachment accommodating minor amounts of inward-directed movement of the shallower structural levels. The inward-directed displacement is balanced in the central zone by squeezing of the volcanic pipe system or movement into the crater void. The hoop strain required by the kinematics is assumed to be accommodated by lateral compaction that is below the resolution of seismic reflection data. The only prerequisite for these fault systems appears to be that the host geology contains one or more relatively flat-lying layers that is/are prone to simple shear. The examples from different geological settings demonstrate that circular antithetic extensional fault systems may occur in association with volcanic, crater, and perhaps other structure types in sedimentary basins.