Abstract
High-resolution airborne LiDAR and field mapping were used to investigate a 29km-long section of the Hurunui segment of the Hope Fault concealed beneath beech forest. Approximately 20km of the dextral strike-slip principal slip zone (PSZ) was identified as a series of 69 individual fault strands on the LiDAR DEM. Mapping revealed 70 normal, 55 dextral-reverse, and 100 secondary faults, many of which were previously unrecognized. Secondary faults are kinematically linked with the PSZ and comprise a complex surface fault deformation zone (FDZ). A Rose diagram weighted by the lengths of the PSZ strands shows that the Hurunui segment strikes between 070° and 075° and is optimally oriented for dextral strike-slip within the regional stress field. The observed fault zone complexity is thus unlikely to result from large-scale fault mis-orientation with respect to regional stresses. FDZ width measurements from 415 locations reveal a spatially-variable, active FDZ up to ~500m wide with an average width of 200m. FDZ width increases with increased hanging wall topography and increased topographic relief (e.g., adjacent to high topography with deeply incised streams), suggesting that along-strike topographic perturbations to fault geometry and stress states increase fault zone complexity and width. Where adjacent PSZ strands strike between 070° and 075°, the FDZ is ≤150m wide, however, FDZ width increases where the tips of adjacent PSZ segments locally vary in strike by ≥10°. FDZ width and surface fracture density also appear to increase with increasing thickness of alluvial deposits overlying bedrock. Our results indicate that spatial variations in near-fault topography and geology can generate along-strike variability in the morphology of surface ruptures, even in the case of fast-slipping, structurally mature faults where more confined, simplistic ruptures are expected at seismogenic depths.
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