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The Evolution and Interaction of Normal Faults in Multi-Phase Rifting: A Numerical Modelling Approach


Continental rifts commonly undergo multiple phases of rifting, with variations in both the rate and orientation of extension through time. Physical analogue experiments have demonstrated that the fault network developed during the initial rift phase influences subsequent fault populations. However, the full 3D geometry, evolution and interaction of fault networks are difficult to constrain from such models. A 3D discrete element model is employed to compare the evolution of normal fault networks in multi-phase rift environments with networks developed during a single rift phase. Faults are defined as an accumulation of broken bonds in the brittle layer, and their location, throw and interaction are recorded through time. Thus incremental fault displacement and geometry and the 4D evolution of the fault network can be examined. We investigate how the maturity of an initial normal fault network impacts fault network evolution and geometry during a second rift phase. We examine how strongly the presence of Phase I structures controls the initiation and localization of subsequent structures by setting secondary extension directions at 30, 45 and 60 degrees to the initial phase, and varying the length of Phase I relative to Phase II. Extension in the initial rift phase results in conjugate fault sets that nucleate and organize themselves by segment growth, interaction and linkage into co-linear fault zones. Increased extension leads to a preferred dip polarity and crustal-scale half grabens. The degree of development of this first-phase fault network strongly influences the second phase fault geometry and evolution. A small amount of Phase I extension promotes fault orientations in Phase II that are initially controlled by the orientation of Phase I before Phase II dominates. An intermediate level of Phase I extension results in complex Phase II fault geometries where reactivation of Phase I faults is common and new faults form to accommodate displacement on earlier faults. Sigmoidal planform fault geometries develop, with complex, zig-zag and rhomboidal fault patterns. A mature initial fault network results in Phase II being dominated by, and deformation localized onto, Phase I faults. Domains dominated by new Phase II faults occur where the density of Phase I faults is low. In all models, fault geometry shows clear variation with depth - faults become less segmented and are better represented by the Phase I orientation at deeper structural levels.