Shallow Fault Structure and Strains
Topographic control on shallow fault structure and strain partitioning near Whataroa, New Zealand demonstrates weak Alpine Fault
Abstract: 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. 500m 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.
https://doi.org/10.1080/00288306.2017.1397706
P. Upton, B. R. Song, P. O. Koons, Topographic control on shallow fault structure and strain partitioning near Whataroa, New Zealand demonstrates weak Alpine Fault. New Zealand Journal of Geology and Geophysics. 61, 1–8 (2017).
