Structural Evolution of a Young Sedimentary Basin (Blythe River Basin) in a Subduction to Strike-Slip Setting, NE South Island, New Zealand.

Plate boundary transitions are regions where two plate boundary types come together. In NE South Island, New Zealand there is a subduction to strike slip transition where the Hikurangi trench terminates against the northeast elements of the Alpine fault system. In this region there are several rhomb-shaped basins, including the Blythe River basin. To better understand the relationship between basin formation and transition zone setting, we mapped the structures of the eastern Blythe River basin. Along the east margin of the basin the SE-dipping reverse Blythe River Fault trends N40E. A strongly asymmetric anticline occurs in the hanging wall, and detailed mapping and inverted exposures of the trace fossil Zoophycos indicate an overturned fold limb with an arcuate north-easterly fold axis. To the west, within the basin, Quaternary deposits are disrupted by southeasterly dipping reverse faults. The west flank of the basin is defined by the back limb of a thrust-propagated asymmetric anticlinal fold. The south margin of the basin is defined by the Stonyhurst fault zone, which includes a series of right stepping en-echelon fault scarps in Quaternary deposits and a fault within the Tertiary deposits. The almost E-W trending orientation and apparent oblique-reverse offset of these active fault segments are consistent with an east-west dextral transpressional fault system. The northern boundary of the basin is marked by an inferred fault, with Lower Tertiary units on the south and Quaternary units on the north. The arrangement of basin bounding structures is consistent with several interpretations. One possibility is that the northern and southern bounding faults represent a left step-over of a dextral system thus creating compressional structures between them. Another possibility is that the region is the product of NW-directed upper crustal contraction which is forming the east and west basin margins. The northern and southern basin bounding structures would be strike-slip transfer faults and could represent reactivated inherited basement faults such as those prevalent east of the plate boundary deformation zone, across the north Chatham Rise. These oblique slip faults would be accommodating the displacement transfer between the contractional structures, and would define structural sub-domains in the North Canterbury fold and thrust belt across which significant variations in the regional structural grain are accommodated. Given this interpretation, these transfer structures would also reflect the importance of inherited structural fabrics in the early stages of deformation accompanying the widening of the plate boundary deformation zone in the Late Quaternary.