Faulting-related displacement gradients and strain adjacent to the Awatere strike-slip fault in New Zealand

Three hundred and ten faults, most with slips of < 2m, were observed along 3 km of seacliffs near an active dextral-reverse fault. The density of post-early Pliocene faults increases with proximity to this major fault. Most faults are oblique-normal. Younger dextral-reverse faults have larger displacements and are concentrated in a ∼300 m wide zone bordering the active fault. Distribution of fault attitudes and slip-vector pitches is apparently similar for faults of both sets across a range of slip dimensions (from cm to tens of metres). Fault displacement vs cumulative frequency in this linear sample are related by a power-law exponent of 0.78 over the cm–m range in slip, indicating that the largest faults are the chief contributors to bulk strain. cement gradients are locally homogeneous and increase toward the Awatere fault from very small to very large approaching the trace. A new algebraic technique is used to measure faulting-related strains in these rocks. Strains are prolate (reflecting transtension) except adjacent to the Awatere fault where they are oblate (transpression). Faulting-related strains do not compare with geodetic strain rates except near the main fault, suggesting that most faults are relict, and that ongoing transpressive deformation is elastically released during coseismic slip along the main fault. The relict faults formed in a divergent zone between two rotating blocks. Displacement across this zone was 40–50° relative to strike. The data suggest that this angle is approximately the critical angle at which normal faults will initiate rather than strike-slip faults in such zones.