Lithological control on the geometry of strike-slip faults: insight from ground-penetrating radar (GPR) survey and analogue modelling

Continental strike-slip faults often display complex, along-strike geometries with branches and splays, which play an important role in earthquake rupture processes. We use a digital elevation model and a ground-penetrating radar survey to analyse the Awatere Fault in New Zealand and demonstrate that the number of branch faults and the width of the fault zone increases as the fault passes from bedrock to unconsolidated alluvial sediments. With analogue models of a strike-slip fault, we test whether this observation can be reproduced. Each model incorporates a sand body that represents a basin filled with less consolidated sediments above a basement of corn-starch. Fault branch-points repeatedly formed at the basin-basement boundary in the analogue model, indicating that the geometrical complexity of strike-slip faults can be strongly controlled by lateral changes in the properties of the host material. The change of friction coefficient at the material boundary promotes splays and branch fault nucleation and also fosters fault-bend formation. The thicker the sedimentary cover overlying the basement, the wider the zone of deformation. This implies that the lateral passage of faults from bedrock into unconsolidated material leads to a widening of the deformation zone, which is confirmed by the ground-penetrating radar survey across the Awatere Fault. The results of this study have wide application to active strike-slip faults that run into sedimentary basins, as is, e.g., the case for the Newport-Inglewood Fault in the Los Angeles Basin.