Analogue models of extensional fault geometries

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Summary The progressive development of extensional fault geometries in a sedimentary cover sequence above a rigid or deforming basement has been experimentally investigated using analogue models. Quartz sand (700 μm) was used as a modelling material. The experiments were recorded using time-delay 16 mm cine photography and 35 mm photography. Final models were impregnated with resin and serially sectioned to investigate the three-dimensional fault geometry. Four series of experiments were carried out: (i) extension above a linear basement dislocation; (ii) extension above a uniformly stretching basement; (iii) extension controlled by a planar fault; and (iv) extension controlled by a listric fault. Extension above a linear basement dislocation produced a single graben structure in which initial high-angle bounding faults were cut by later listric faults. Fault nucleation occurred into the hanging wall of the graben and only minor rotation occurred in the fault blocks within the graben. Extension above a uniformly extending basement produced a variety of fault structures. Heterogeneous nucleation of faults occurred with initial planar geometries giving way to more listric faults. Significant fault-block rotation was observed. In some instances rotation of pre-existing fault planes produced a negative (convex upward) listric-fault geometry. Extension controlled by a planar fault produced a single graben structure in which new faults developed into the graben. Little rotation of fault blocks occurred. Extension controlled by a predetermined listric fault showed the progressive development of a rollover anticline. The crest of the anticline collapsed producing a second-order crestal collapse graben. The nucleation of new faults was consistently in the hanging wall above the major detachment faults. In all of the experiments carried out to date, once a major fault had developed the second-order fault nucleation was consistently in the hanging wall fault block. Footwall collapse and hence fault migration into the footwall was not significant. Listric faults produced considerable rotation of hanging wall blocks. In some instances heterogeneous rotation of preexisting faults generated negative listric-fault shapes. In rollover structures collapse of the crestal region of the fold produced a second-order graben structure.