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Physical Modelling |
Department of Geology, Royal Holloway, University of London, Egham, Surrey, TW20 0EX, UK
The generation of salt diapirs is associated with a protracted deformation history in the surrounding overburden, which may be difficult to image seismically due to the highly inclined and fractured nature of sediments adjacent to the salt. To further the understanding of overburden deformation related to diapir development, a series of physical models has been produced incorporating a viscous SGM polymer source layer which is overlain by a granular, glass bead overburden. Model diapirs are initially triggered (to the pillow stage) via a basal indentor, but subsequently develop by a passive downbuilding process. The maximum widths of withdrawal basins and diapir-related upturn are approximately equivalent to one another. Experiments in which the salt analogue is allowed to extend laterally result in listric normal faulting of the overburden, to define graben structures centred on the diapir. Injection of the source layer into the footwall of normal faults results in reactive dispirism on the flanks of the major structure, whilst the adjacent central sections illustrate geometries which could be attributed to both active and passive diapirism. Multiple generations of normal faults produce graben with an inward-stepping sequence of fault propagation towards the central block. Complete evacuation of the source material at the keel of the normal fault simulates localized salt welds and results in extensional displacement being transferred into the hangingwall block. Sand-clay mix layers act as relatively competent horizons and display radial fracture patterns which may also be associated with a central ring fault and circular collapse graben above the diapir. Normal faults defining graben structures converge on the radial fracture pattern overlying the diapir. Circular thrust faults surrounding the diapir are developed when more competent horizons temporarily arrest vertical growth resulting in diapiric ‘ballooning’.
