Wrench-related folding: A case study of Bohai Sea basin, China

Wrench motion is relatively common in basins but often goes undetected because of its subtle structural imprint. Even so wrench-related folding is of significance in petroleum exploration and development. Based on the Riedel shear nomenclature, we apply the model of wrench-related folding to deformation in the Bohai sea basin. The folding related to R-shears, T-tensional structural systems (T-systems) and C-compressive structural systems (C-systems) in a Riedel shear model are discussed. The R-system related folding results from differential movements between the two sides of fault blocks, in map view the fold is oriented (90°-φ/2) (in the initial deformation stage) or more than (90°-φ/2) from the principle displacement zone (PDZ), where φ is the angle of internal friction. The T-system related folding is related to the rollover of the hanging wall, and in map view the fold is initiated at 45° from the PDZ in the initial deformation stage. The C-system related folding is resulted from the compression induced by a wrench action, in map view the fold was oriented 45° from initial deformation in the PDZ. Bohai Sea basin fold orientations fit our models, and the wrench-related folds play important roles in oil and gas accumulation.