Evolution of fracture networks and connectivity during fault–bend folding: Insights from the Sinon Anticline in the southwestern Hongseong–Imjingang Belt, Korea

Crustal shortening in an elastico-frictional regime is mainly accommodated by contractional fault–fold systems with fracture networks. According to recent research, fracture networks in fold–thrust belts express complex internal strain states in response to thrusting and related folding. Furthermore, their connectivity and fluid flow characteristics likely depend on the structural positions and mechanical stratigraphy that control heterogeneous deformation processes. This study provides characteristics of fold-related fracture networks in the Sinon Anticline, which was formed by fault–bend folding in the southwestern Hongseong–Imjingang Belt, Korea. The fracture networks in the metamorphosed turbidites characterized by interbedded competent metasandstone layers and relatively thin incompetent schist layers have evolved through pre-, syn-, and post-folding fracturing events. Their complexity reflects the spatiotemporal variation in the strain pattern related to early layer-parallel shortening and subsequent fault–bend folding. Based on insights from detailed mapping and topological analysis of the fracture network, we conclude that strain partitioning that occurs during flexural folding results in a superposed tangential longitudinal strain expressed by fractures with a high (hydraulic) connectivity in the hinge zones. Strain partitioning is caused by flexural interlayer slip along incompetent schist layers in the fold limbs. Bed-parallel slip localization zones probably have low porosity and permeability and may act as barriers to fluid migration across beds. We suggest that heterogeneous vertical axis rotation, which occurred as the system's hanging wall slid over the footwall ramp, increased the complexity of fracture networks within the Sinon Anticline. Our findings indicate that the evolution, connectivity, and fluid flow properties of fracture networks can be characterized through careful interpretation of folding mechanisms and related strain states during formation of fault–bend fold systems.