Dilational normal faults

At low differential effective stress and with minimum principal effective stress near zero or tensile, rocks fail in several modes and with variable failure angles. Under these conditions mechanical stratigraphy exerts a significant influence on initial dip of normal faults. Less competent layers fail in shear mode along fractures that approximate the failure angle predicted by a standard rock mechanics analysis. Deformation of more competent layers, which is driven in part by interaction with the more rapidly deforming incompetent layers, produces hybrid mode failure in which failure angles are smaller than in shear mode. Analyses of small-displacement (<1 m displacement) normal faults cutting Cretaceous carbonate strata in west Texas (USA) indicate that fault geometries resulting from this effect commonly display steep segments where the fault traverses more competent beds. Displacement on these faults has caused dilation of steeper segments. Analogous steep dilational fault segments host ore deposits in the North Pennine Orefield, England. Dilatant segments along faults within carbonates of the Cretaceous age Edwards Group near San Antonio, Texas (USA) have been enlarged by groundwater flow, and are important permeability and shallow groundwater infiltration pathways.