Lithological control on geometric complexity of active continental strike-slip faults – insight from GPR surveys and analogue modelling

Strike-slip faults often display complex, along-strike geometries with branches and splays, which play an important role in earthquake rupture processes. Based on field examples of active faults and analogue models, we show that this complexity can be caused by lateral changes in lithology. We use geomorphic and ground-penetrating radar analysis of the Awatere Fault in the South Island of New Zealand, to demonstrate that the number of branch faults and width of the fault zone increases as the fault passes from bedrock to unconsolidated alluvial sediments. With analogue models, we test whether this observation can be reproduced. The setup replicates strike-slip faulting using two plates translated at a constant rate of 3 cm/h relative to each other. This establishes a velocity discontinuity at the centre of the model that leads to the formation of a strike-slip fault zone in the overlying analogue material. Each model incorporates a lenticular sand body that represents a less consolidated sedimentary basin above basement, which is represented by corn starch. During multiple model runs, fault branch-points formed at the boundary between the two different materials in the analogue model, thus confirming that the geometric complexity of strike-slip faults is strongly controlled by lateral changes in the properties of the host material. Two processes could play a role here: 1) the frictional properties change abruptly at the lithological boundary, which promotes the nucleation of branch faults and, 2) the angle of internal friction of the material changes across the lithological boundary, thus fostering fault-bend formation at this point. Our analogue modelling results also show that the thicker the sedimentary basin on top of the basement, the wider the zone of deformation. This implies that the lateral passage of active 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 the study can be applied to situations in which active strike-slip faults run into sedimentary basins, such as the Newport-Inglewood Fault in the Los Angeles Basin. Based on our analogue models, we postulate that the more diffuse, near-surface en-echelon structure in the northwest of the Newport-Inglewood Fault is a function of the higher sediment-basin thickness, compared to the distinct fault trace that is developed above the shallower basin-fill in the southeast.