Propagation of blind normal faults to the surface in basaltic sequences: Insights from 2D discrete element modelling

A discrete element model is used to investigate the progressive deformation of a thin basaltic cover overlying a pre-existing, blind, normal fault as it propagates to the surface. The cover materials representing basalt are homogeneous, strong and display elastic-brittle material behaviour. Cover deformation is seen to evolve through a series of distinct stages. Initial displacement on the underlying fault produces a very gentle, monoclinal, flexure. With continued displacement, open fractures develop at the monocline surface and propagate downwards, whilst the deeper fault propagates upwards. Simultaneously, a series of fractures, in the future hanging-wall of the main fault, develop in the upper part of the cover. The monoclinal flexure is then cut by these structures, producing a surficial fault- and fracture-bounded wedge. Finally, a prominent surface fracture and the upward-propagating fault link, cutting the entire cover sequence. This fault is dilatant in the upper c. 100 m of the cover, has a significant surface aperture and forms a prominent fault scarp. Many of the key model results are strikingly similar to those seen in natural settings, and emphasise that the occurrence of dilatant faults, open fractures and cavities/caves in extensional settings is not necessarily restricted to the very shallow section but can extend to several hundred metres depth. Therefore, the results have implications for permeability and fluid flow in such settings. Comparison is also made with a weak cover experiment, using granular materials with no cohesion or tensile strength, similar to the dry sand used in many analogue modelling studies.