Geometry and Kinematic Evolution of Inversion Structures

Abstract

Positive inversion structures form by the compressional reactivation of preexisting extensional structures. Experimental models and observations of natural structures are used to develop quantitative models for the geometry and kinematic evolution of inversion structures. In this paper, I analyze two main formation mechanisms of inversion structures: (1) fault-propagation folding on planar faults, and (2) fault-bend folding on listric faults. Inversion structures formed by fault-propagation folding occur in the southern North Sea, the Central Montana platform, and the Kangean Basin. During extension, a broad fault-propagation (or drape) fold develops above the master fault, with the fault subsequently breaking through the fold. Synextensional growth units deposited in the hanging wall typically thicken into the basin. Compressional reactivation results in slip reversal on the master and secondary faults, their rotation to shallower dips, and the development of a compressional fault-propagation fold. Inversion structures formed by fault-bend folding on listric faults occur in the Taranaki Basin, and possibly in the southern North Sea. Rollover folding in the hanging wall occurs during extension, possibly accompanied by a small component of fault-propagation folding in the vicinity of the fault tip. Deformation is primarily localized along a system of antithetic faults. Synextensional growth sediments typically thicken into the fault, but also show thinning in the immediate vicinity of the fault. During compression, the extensional fold is first unfolded and then folded into a compressional fault-bend fold. The characteristic variations in bed geometry and thickness provide predictive models for interpreting the subsurface geometries of these two classes of inversion structures in areas with poor seismic data. These models are particularly useful in exploring for structural traps in the complex and relatively unexplored synextensional growth units.