Plaster modelling of fault development and geometry in contractional settings
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The aim of this study has been to analyse the development of contractional structures using analogue plaster models. Structures similar to natural fault systems are generated using plaster of Paris as the deforming medium and barite as basement. Detailed studies of plaster models are useful in the understanding of sub seismic features related to faults and to understand 4D evolution of contractional faults. Six experiments have been analysed based on photos and videos. Characteristic parameters regarding thrust fault development have been investigated including; main fault activity, fault dip evolution, fault displacement, fault spacing, wedge height and geometry, folds and secondary faults. The analysis revealed that faults initiated at the base of the models and propagated upwards forming a wider monoclinal fault-propagation fold in front, whereas in other cases, faults initiated as sets of conjugated shears. Faults initiated had an average dip between 29° and 35°. In most of the models, the faults developed as in-sequence thrusts where mainly one fault was active at any given time. Initiation point of the first faults varies between 1-7% shortening and was strongly controlled by the plaster properties. The wedge geometry and angle varied from 16°-62° and was controlled by the firmness of plaster were more firm plaster created a higher wedge angle. Through balancing of an experiment, three dominant deformation mechanisms are established; layer parallel shortening (LPS), folding and thrusting. Different domains within the model throughout the experimental period was recorded; the front of the wedge was dominated by fault ramp initiation and thrust propagation, the middle was characterised by rotation, steepening and eventual locking of the faults due to the effect of imbrication, whereas the back of the wedge was dominated by vertical thickening. A trend of stepwise increase in imbricate thrust spacing and a decrease in rate of initiation of imbricate thrusts in the transport direction is evident. The latest and thereby the youngest faults developed larger displacement than the older faults, most likely due to increased mechanical strength of the plaster. Smaller structures such as pop ups, fault splay, antithetic and synthetic minor faults forms in association with main faults.