Topographic control on shallow fault structure and strain partitioning near Whataroa, New Zealand demonstrates weak Alpine Fault

It is notoriously difficult to characterise the strength and stress states of major plate boundaries. By taking advantage of the well-constrained stress contribution of topography adjacent to a segmented section of the Alpine Fault, New Zealand, we have identified a mechanical mix that produces the distinct fault segmentation pattern seen in field observations. Slope-generated shear and normal stresses rotate the principal stresses relative to the regional tectonically derived stress state and under certain strength states influence the displacement pattern. Three-dimensional models show that the scale and form of the near-surface partitioning depend on both topographic relief and local fault strength relative to the bedrock. The models suggest the Alpine Fault is weak to moderately weak relative to the bedrock and is a single structure to within c. 500 m of the surface, above which segmentation occurs. Adjacent to the Alpine Fault, the stress state is highly variable. The intermediate principal stress, σ₂, is rotated from tectonically dominated, near-vertical beneath ridges to near-horizontal beneath large valleys. Individual segments along the Alpine Fault dominated by strike-slip faulting, oblique thrusting or thrusting, can be identified by extracting the topographic contribution to the stress state from numerical models.