Abstract

On the coast of South Island, New Zealand, the active dextral-reverse Awatere fault has <2 km finite dextral slip and initiated <4 Ma. Fault spacing along ≈2 km of coastal sea cliffs decreases from >22 m to <0.5 m as one approaches the fault from the north. The pattern of fault attitudes and slip directions is independent of scale. A power-law relationship between slip and cumulative frequency of faults has an exponent of ≈0.8 for this linear outcrop sample. Near the Awatere fault, faulting-related strains are large, reflecting partitioning of slip into late-stage faults near the core of the active fault; farther away, they are very small, where most of the small-scale faulting is relict. Strain softening led to localization of slip into (and near) the fault's core and was assisted by suprahydrostatic pore-fluid pressure gradients, but evidence for fault-orthogonal compression and extreme fault zone weakness is lacking. Two styles of gouge on faults both have linear scaling ratios relating gouge thickness to finite slip. Different ratios for each may indicate coseismic and aseismic gouge generation processes acting at crustal depths of 1–2 km. Early tensile cracking of pebbles is interpreted to have occurred in the process zone of the Awatere fault as it propagated upward as a mode III crack. If so, the fault has a process zone width/finite slip ratio of >1.5, consistent with an elastic-plastic model of fault growth. Younger domino-style oblique-normal faults resulted in oblique extension of the Awatere fault zone, with mean σ1 oriented ≈30° clockwise from its strike. Transtension may have been caused by divergence between rotating, crustal-scale fault blocks, the result of a mode II fault tip to the east, which caused a local rotation of stress trajectories. The early faults are relict, and their kinematics is a poor index of slip sense on the mature fault. Close to the Awatere fault, dextral-reverse faults are synthetic to that mature structure. Their disposition indicates incremental compression acting ≈45°–50° clockwise to the strike of the Awatere fault. These youngest faults mirror the slip kinematics of the adjacent mature fault. Their attitude is controlled apparently by local kinematics, not by far-field stress orientations.

First Page Preview

First page PDF preview
You do not currently have access to this article.