Significance of brittle deformation in the footwall of the Alpine Fault, New Zealand: Smithy Creek Fault zone

• We analyze a rare exposure of brittle faulting in the Alpine Fault footwall.
• The alteration assemblage is similar to fluids in the modern Alpine Fault system.
• Mineralized fractures have kinematics compatible with modern stresses.
• Deformation of AFZ footwall may accommodate more strain than previously suggested.

The Smithy Creek Fault represents a rare exposure of a brittle fault zone within Australian Plate rocks that constitute the footwall of the Alpine Fault zone in Westland, New Zealand. Outcrop mapping and paleostress analysis of the Smithy Creek Fault were conducted to characterize deformation and mineralization in the footwall of the nearby Alpine Fault, and the timing of these processes relative to the modern tectonic regime. While unfavorably oriented, the dextral oblique Smithy Creek thrust has kinematics compatible with slip in the current stress regime and offsets a basement unconformity beneath Holocene glaciofluvial sediments. A greater than 100 m wide damage zone and more than 8 m wide, extensively fractured fault core are consistent with total displacement on the kilometer scale. Based on our observations we propose that an asymmetric damage zone containing quartz–carbonate–chlorite–epidote veins is focused in the footwall. Damage zone asymmetry likely resulted from the fact that the hanging wall was mostly deformed at greater depth than the footwall, rather than resulting from material contrasts across the fault plane. Kinematic inversions on mineralized fractures within the damage zone suggest veins formed in the current stress regime, from fluids comparable to those now circulating in the footwall. The Smithy Creek Fault zone is therefore a rare exhumed example of the modern footwall hydrothermal system, and of a structure actively accommodating footwall deformation near the Alpine Fault zone. Two significantly less mature, subvertical faults having narrow (20 cm or less) damage zones and similar orientations to nearby strike-slip segments of the Alpine Fault crosscut the mineralized zone at Smithy Creek. We envisage that hydrothermal mineralization strengthened the fault core, causing it to widen as later slip was partitioned into the (now) weaker surrounding damage zone. With progressive alteration, formation of favorably oriented faults became preferable to reactivation of the existing structure.