Aspects and origins of fractured dip-domain boundaries in folded carbonate rocks

We present comparative field studies in folded areas (Southern France, Moroccan Western Atlas and Abruzzo, Italy) giving new insights into fracture distribution within folded rocks of the shallow brittle crust. We show that the curvature in folds formed in brittle mechanical units is usually accommodated by multiple “dip-domain boundaries” (appearing as curvature discontinuities at fold scale) corresponding to relatively narrow and dense fracture zones, striking parallel or slightly oblique to the fold axis. They separate “dip-domains” where curvature is absent or moderate. It is shown that the dip-domain boundaries (which are obvious in the case of kink folds or box-fold anticlines) are currently present as multiple subtle hinges even when the curvature appears continuous at first sight. The nature of dip-domain boundaries is studied: they often cut through the whole thickness of the mechanical units. Their internal structure varies, and a non-exhaustive typology is proposed. For each type, an interpretative kinematic scenario shows how the dip-domain boundaries could initiate and develop. We suggest two kinds of origins: (1) they could correspond to the reactivation of inherited, along-strike fracture zones (opening-mode fracture concentrations such as big joints, fracture corridors, inherited faults, etc.); (2) they could be created as mechanical instabilities during the fold formation (syn-folding origin), in particular through small reverse faults. In both cases, early zones of weakness localize the dip-domain boundaries, and control the increase in curvature in association with increasing fracture density within the boundary. Because they represent well-defined vertically and axially persistent sub-seismic fracture zones generally limited to the thickness of the folded unit, dip-domain boundaries could enhance the axial permeability of folded and fractured reservoirs.