Evolution and progressive geomorphic manifestation of surface faulting: A comparison of the Wairau and Awatere faults, South Island, New Zealand: COMMENT

Abstract

Zinke et al. (2015) documented a flight of Saxton River terraces deformed by the Awatere fault, South Island, New Zealand. They concluded that the manifestation of off-fault deformation (OFD) is a cumulative effect, and therefore may not be immediately apparent on younger terraces, with implications for slip rate determinations. I argue that they did not adequately consider the detailed structural setting of elements of OFD within the Saxton River releasing bend on the Awatere fault. The surface deformation at Saxton River is probably controlled by the coincidence of terraces with structural zones within the bend, and I contest the authors’ conclusion that off-plane deformation is simply hidden in younger terraces. The influence of structural complexity on the width and geometry of OFD is well established (e.g., Milliner et al., 2015). Zinke et al.’s figure DR2 (in GSA Data Repository 2015341) shows that OFD along the Awetere fault occurs at bends or steps in the fault plane, including at Saxton River where the fault bends right relative to the main trace by 5–10°, ~50 m east of the T2-T3 riser. This gives rise to a graben (T1 tread) and two pressure ridges (T2 tread and bedrock spur) that define and bound an overall 150-m-wide releasing bend (Mason et al., 2006) (Figs. 1A and 1B). The paired pressure-ridge and graben geometry is predicted when the wall rocks on either side of a strike-slip fault approach and interact with a fault bend during an earthquake (Sibson, 1989), and has been reported on the Hope (Cowan, 1990) and Greendale (Duffy et al., 2013) faults in New Zealand, and the San Andreas fault in California (Ben-Zion et al., 2012), among others.