Evolution of the internal structure of fault zones in three-dimensional numerical models of normal faults

• DEM models replicate many characteristics of the internal geometry of faults.
• 3D DEM models demonstrate action of asperity and tip-line bifurcation.
• Main control on the fault zone structure is the initial fault geometry.
• Fault zone structure is confining pressure dependent.

Fault zone internal structure is characterised by heterogeneous distributions of both continuous (drag, lens rotation) and discontinuous (joints, faults) deformation which cannot be easily modelled using continuum numerical methods. Distinct element method (DEM) models, that exhibit bulk rheologies comparable to rock, demonstrate emergent behaviours that make them ideal for modelling both the nucleation and growth of fault zones. The ability to model fault zones numerically allows extant conceptual models for fault zone evolution based on outcrop studies to be tested, and controls on fault zone structure to be analysed. Three-dimensional DEM models of faults zones in mechanically layered sequences demonstrate that internal fault zone structure is predominantly controlled by the geometry of the initial fault. Whether the initial fault is a segmented array or an irregular surface determines the complexity of structure it will develop as displacement increases. Confining pressure at the time of faulting determines the irregularity of the initial fault array and also the efficiency with which irregularities are incorporated into a fault and subsequently comminuted, leading to a relationship whereby brittle faulting at high confining pressure results in less complex internal fault zone structure than at low confining pressure.