CONTINENTAL BASIN DEVELOPMENT IN A SUBDUCTION SETTING, NE SOUTH ISLAND, NEW ZEALAND

The northeastern South Island of New Zealand, inland from the terminating Hikurangi trench, consists of high mountains to the north and actively forming medium-small scale basins to the south. Deformation in the high mountains corresponds with the northern end of the transpressional Alpine fault system. Basin development to the south is likely the result of interplay between this continental fault system and the terminating subduction offshore. To better understand the relationship between basin formation and this transition zone setting, we mapped the structures of the eastern Blythe River basin. On the eastern margin of the basin the SE-dipping John Brown Fault has a reverse sense and an overturned anticline in the hanging wall. 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 northern boundary of the basin is marked by an inferred fault, the Blythe River fault with Lower Tertiary units to the south and Quaternary units to 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. Another possibility is that the region is the product of NW-directed upper crustal contraction 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.