4D evolution of segmented strike-slip fault systems: applications to NW Europe

Abstract

Scaled, physical sandbox modelling has been used to simulate the progressive deformation of segmented strike-slip fault systems at both releasing (pull-apart basins) and restraining oversteps (‘pop-up’ structures) developed in a weak sedimentary cover above rigid basement. Pull-apart basin evolution is characterized by the initial development of a spindle graben bounded by oblique-extensional faults that cross the ‘basement’ overstep, and subsequent lengthening of the basin as displacement on the master faults increases. The pull-apart basins are bounded by terraced oblique slip extensional sidewall fault systems that link the laterally offset principal displacement zones (PDZ). The sidewall faults show changes in kinematics from dominantly dip-slip extension in their central sections to oblique slip and strike-slip at either end where they merge with the PDZ. The pull-apart basin models are flat-bottomed in the centre of the basin and become asymmetric at either end where the sidewall fault systems join the PDZ. Cross-basin fault systems, which are characteristic of all models, cut the floor of the pull-apart basins and link the offset PDZs. Deformation above restraining offsets is characterized by the formation of rhombic to lozenge-shaped uplifts or ‘pop-ups’ – structures bounded by oblique-slip reverse faults that fan outwards from the PDZ. With increased displacement on the master faults the rhombic-uplifts grow in amplitude. The upper surfaces of the ‘pop-up’ structures are dissected by both synthetic and antithetic strike- and oblique-slip faults. Distinctive asymmetric positive flower structures characterize the lateral margins of the ‘pop-up’ structures.The analogue models are compared with natural examples of strike-slip pull-apart and ‘pop-up’ fault systems from NW Europe. The models show many strong similarities in structural geometry and stratal architectures with many natural examples, although it must be emphasized that similarities between structures of purely extensional origin (multiphase) and those associated with inversion do exist. Therefore, these analogue models of complex strike-slip structures may provide useful templates for seismic interpretation in petroleum basins provided there is a demonstrable link between strike-slip motion on basement structures and deformation in the cover.