Models of the Formation of Extensional Fault Arrays

Observations from extensional domains, such as the northeast Atlantic margin and the Basin and Range, point to the frequent occurrence of an array of normal faults rooting onto a large normal master fault, all dipping in the same direction. The master fault can be considered the dominant feature, controlling the formation of fault blocks in its hanging wall. Such settings inspired us to investigate the formation and evolution of normal fault arrays in a general sense, aimed at determining the existence of a preferred sense of propagation of faults, the role of isostasy and the sensitivity of fault arrays to, for example, rheological layering and fault weakening, among others.We investigate fault arrays by using numerical models, analogue experiments, and interpretations of seismic observations. The numerical experiments use two-dimensional finite element models. The three-dimensional analogue experiments are build of sand and silicone and their internal deformation is visualised in an X-ray scanner. All models are on the scale of the upper crust. Previous studies have already pointed to the difficulty of simulating arrays of parallel faults in models. Our first models are, therefore, simple and generic in character, examining triggers for fault arrays. The models are build of horizontal layers utilising the strength contrast between materials for localising deformation on faults. Our brittle-viscous experiments show: (1) outward propagation of successively younger faults from a central fault zone, (2) formation of fault arrays with faults predominantly dipping towards a central graben and rooting in a viscous layer, and (3) progressive abandonment of older faults in the central domain, which are passively rotated about a horizontal axis into shallower orientations. The development of normal fault arrays seems to be related to the shear stress at the interface of the viscous and brittle layer, caused by downsagging of brittle layers in the main graben and concomitant sideways and upward flow of the underlying viscous material.