3D Discrete Element Modelling of Normal Fault Network Evolution in Multi-Phase Rift Basins

Abstract:

Continental rifts commonly experience multiple phases of rifting with different directions and rates of extension. The relative maturity of the fault network developed during the initial rift phase has been shown in physical analogue experiments to influence the fault population developed during subsequent rift phases. However, the full 3D geometry and evolution of the fault networks is difficult to constrain from such models.

We use a 3D discrete element model to investigate the evolution of normal fault networks in multi-phase rift environments and compare these to 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 is recorded through time. Thus incremental fault displacement and geometry and the evolution of the fault network in 3D can be examined. We investigate how the maturity of an initial normal fault network impacts on the fault network evolution and geometry during a second rift phase with an extension direction at 45 degrees to the first phase.

During the first rift phase conjugate fault sets nucleate and organise themselves by segment growth, interaction and linkage into co-linear fault zones. With increasing extension the fault network develops a preferred dip polarity generating crustal-scale half graben with dip domains developed along strike. The degree of development of this first-phase fault network strongly influences the second phase fault geometry and evolution. Even a small amount of Phase I extension promotes fault orientations in Phase II to strongly reflect the Phase I orientation. An intermediate level of Phase I extension result in complex Phase II fault geometries reflecting reactivation of Phase I faults and new Phase II faults. Sigmoidal planform fault geometries develop, with the network having complex, zig-zag and rhomboidal fault patterns. A large amount of Phase I extension, with a mature fault network, results in Phase II being dominated by, and deformation localised onto, Phase I faults. Some domains dominated by new Phase II faults occur where the fault density of Phase I faults is low. In all models, fault geometry shows clear variation with depth - faults become less segmented and more strongly reflect the initial Phase I fault orientation with increasing depth.