Strain partitioning during flexural-slip folding

In upper-crustal structures, folding by flexural slip leads to the partitioning and ordering of strains, which are commonly expressed by means of faults, fractures and joints. Flexural slip occurs along only some of the numerous layer-to-layer contacts within a sedimentary rock sequence, and these active bedding-plane slip surfaces partition the succession into mechanical units, each of which, although composed of many rock layers, deforms as though it were a single beam. Thus, in upper-crustal flexural-slip folds, the strain domains associated with bending are sharply deliminted by the active slip horizons and also are related to structural position within the beam. Progressive folding leads to the development of additional active slip surfaces and, therefore, to an increased number of mechanical units which each develop bending strains. As a result of this sequence of events, there is a hierarchical overprinting of strains, which leads to considerable complexity in fracture fabrics; nevertheless, order in the fracture patterns is retained. Most petroleum reservoirs appear to be affected by fracture networks created during flexure, and their fluid flow characteristics are probably related to a fracture-dominated, flexural deformation state which is disposed relative to the layering, is inhomogeneous, and is not distributed stochastically throughout the whole rock volume.