Conjugate Strike Slip Faulting Associated with Seamount Subduction at the Hikurangi Deformation Front, New Zealand

Subducting seamounts may have a profound impact on the structure and morphology of the frontal accretionary wedge. Along the offshore northern Wairarapa section of the obliquely convergent Hikurangi subduction margin, where incoming sediment is about 4 km thick, subduction of the largely buried Bennett Knoll seamount is at an incipient stage. Using high resolution, 30 kHz Kongsberg EM302 multibeam swath bathymetry, TOPAS PS18 sub-bottom profiles, and regional seismic reflection data, we document a peculiar style of coeval thrust and strike-slip faulting in association with seamount collision. The strike-slip faults observed are not deforming the immediate hanging wall overriding the incoming Bennett Knoll seamount, but are conjugate faults up to 30 km in length that straddle the buried flanks of the seamount, offset the frontal thrusts and proto-thrust zone, and extend significantly across the Hikurangi Trough basin floor.

The conjugate strike-slip faults are steeply dipping and have propagated approximately parallel and normal, respectively, to the present day plate convergence vector. They are expressed clearly geomorphically, with evidence for dextral and sinistral displacements on the respective conjugate structures. In comparison with published analog and numerical models and other marine field observations of subducting seamounts elsewhere, the Hikurangi margin conjugate faults appear to be unusual. We discuss the kinematic controls that may influence their formation.