Plaster modelling of fault development in extensional regimes
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The full complexity of deformation in natural extensional regimes is complicated, as direct observation in nature is limited in regards to outcrops and structural evolution over time. The aim of this thesis is to analyse the growth and evolution of fault systems in rift settings using physical experiments. Seismic imaging is widely used for exploring the subsurface, however its resolution may simplify small-scale deformation. When sub-seismic structures are not accounted for, the limited understanding of the structural complexity of the subsurface can lead to complications during exploration and production of hydrocarbons. Analogue plaster modelling can provide a valuable insight of both large- and small- scale structures in space and time, as the models show the deformation history from fault initiation to a fully developed extensional basin. Plaster of Paris is well suited for producing and preserving large- and small-scale structures for further analysis. Unilateral extensional experiments show that deformation is dependent on the plaster rheology, basement geometry and extension rate. The models are compared to natural extensional systems. The large fault planes interact in three dimensions, and the fault growth style is dependent on the primary structures. Fault planes form by cutting through an array of sub- planar first generation faults. Their exact growth pattern may vary between the two profile planes of the same model, and their timing may also be different. The first brittle deformation occurs within 10% extension and a through-cutting fault plane forms within 23% extension, followed by secondary deformation. The largest amount of deformation in found to be in the hanging wall as it rotates and shears internally to generate faults with a variety of dip angles. The deformation zone growth and structural complexity are promoted by a changing footwall fault gradient. One of the reoccurring features is antithetic faulting and the formation of horst and graben structures above a low-angle fault. Relatively ductile sections with a small cumulative number of faults accommodate 70-90% of the total horizontal extension, while for relatively brittle sections the larger faults accommodate 43-63%. This distribution supports that the extension of a seismic section with a large number of sub-seismic faults may be underestimated, due to a wider distribution of displacement.