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Three-Dimensional Evolution of Normal Faults in Two Phase Rifting


Extensional provinces are known to have experienced multiple phases of rifting. The relative maturity of the initial phase of rifting to a subsequent phase has been shown through physical analogues to influence the reactivation of existing faults, the orientation and location of new faults, the evolution of the number-length relationship and strain accommodation. These assess the surface geometry and interaction of the structures but cannot determine the three-dimensional interaction of the faults within the media. Understanding the nature of these interactions, may hold the key to determining the underlying controls on fault activity in rifted margins. A three-dimensional discrete element model has been developed to examine the growth, interaction and activity on faults and associated strain accommodation in a two-phase rift. The model discretises the crust into an assembly of >2 million spherical elements in a two-layer system that employs brittle and ductile interactions between element pairs, dependent on their initial location within the model and gravitational and isostatic forces. Faults are defined as the breaking of bonds between elements in the upper, brittle crust. The location of elements is known throughout and failure between element pairs is monitored, which in turn is used to define fault location, failure time and throw which is then analysed and interpreted. Results from two-phase rifting are presented where the relative maturity of phase I (0-10%) to a constant phase II (15%) is investigated. The obliquity of phase I is held constant to the main (phase II) extension throughout. The smallest length of phase I extension results in a modification to the location of phase II structures. Increased maturity of structures in phase I results in localisation and focus of strain accommodation onto early forming structures which become dominant basin-bounding faults; an increased number of faults accommodating the partitioning of the brittle crust and greater displacement of later phase faults parallel to the phase II extension direction. Fault activity and strain accommodation vary through time, where the complexity of interactions at depth are not readily discernible from surface geometries, and the linkage of along strike antithetic structures within the crust is common.