Fault System Evolution Controlled by Weakness Arrangement under Multiphase Non-Coaxial Extension：Analogue Modeling Insights

The evolution of rift-related fault systems is strongly influenced by pre-existing structural weaknesses, yet the role of their spatial arrangement in shaping fault patterns during multiphase non-coaxial extension remains unclear. To address this, we conducted a series of brittle–viscous analogue experiments to examine how left-stepping and right-stepping configurations of parallel weaknesses affect fault propagation, linkage, and orientation under two successive phases of orthogonal and oblique extension. We find that (1) fault orientation is jointly controlled by extension direction, weakness orientation, and the relative positioning of pre-existing weaknesses; (2) left-stepping and right-stepping systems, though geometrically identical, evolve differently under the same boundary conditions—left-stepping configurations develop greater fault linkage, strike diversity, and overall structural complexity; and (3) inherited weaknesses reduce the direct control of extension direction on fault strikes, implying that present-day fault patterns may not simply reflect paleostress orientations. Furthermore, our results suggest that apparent strike variability in multiphase rift systems can arise without local stress rotation, emerging instead from the interaction between regional stress and the spatial arrangement of inherited structures. Mechanistically, left-stepping configurations behave analogously to releasing steps in strike-slip systems, promoting more distributed deformation and strike-slip components, whereas right-stepping systems resemble restraining steps, producing simpler and more localized fault networks. Our findings provide new insights into how pre-existing structural configurations modulate rift fault evolution, highlighting the need to consider structural inheritance when reconstructing tectonic histories of multiphase extensional basins.