EARTHQUAKE RUPTURE AND BASIN FORMATION IN THE OBLIQUE CONVERGENCE REGIME OF NEW ZEALAND’S SOUTH ISLAND

The geomorphology of the northeast quadrant of New Zealand’s South Island is characterized by fault-bound basins that are the product of the termination of the Alpine Fault as the plate boundary transitions to subduction. These faults include the structures of the Marlborough Fault system but also many other faults further south. Step overs in individual faults have produced small pull-apart basins but also compressional structures typically expressed as small ridges between the linear traces of right-lateral faults. In the southern part of this region of the northeastern South Island, the Canterbury Plains dominate the landscape. In 2010 and 2011 this uniform landscape was disrupted by devastating earthquake and aftershock events. The fault pattern of the displacements bares similarity to fault patterns further north. This likeness may allow detailed study of the existing fault-bound basins to the north to shed light on possible future deformation in the Canterbury Plains. Study of the 2010/2011 earthquake deformation on the Darfield Fault may also provide insight on the style of past deformation in some of the basins of the northeastern South Island. We make a comparison between a basin (Blythe River basin) along the Blythe River adjacent to the east coast and the Darfield Fault ruptures. The Blythe basin is bounded to the south by a strike-slip fault and to the east by a west-dipping thrust fault. This strike-slip fault strikes ENE with small (10-30m) right step overs. Springs emerge in the step over region and we interpret this structure to be right lateral with small dilational zones in the step overs. Just south of the fault trace is a small anticlinal ridge. A NE-striking thrust fault and a larger anticlinal ridge bound the eastern edge of the Blythe River basin. We interpret these structures on the east to be the product of a left step over of the right lateral fault bounding the south of the basin. Similarly, the Darfield-Christchurch fault system is marked by local right step overs creating dilation and left step overs that have created compressional regions and NE-striking thrust faults. We suggest that the fault geometry of the Darfield-Christchurch may represent an analog for the early stages of fault rupture in the Blythe River basin and other basins of the northeastern South Island of New Zealand.