FORE-ARC UPLIFT AND EXTENSION, SOUTHERN HAWKES BAY, NEW-ZEALAND - MIDPLEISTOCENE TO PRESENT

The Maraetotara Plateau‐Cape Kidnappers area in southern Hawke's Bay, New Zealand, lies in the forearc region of the obliquely convergent Hikurangi margin, where the Pacific Plate is presently subducting beneath the leading edge of the Australian Plate. Uplift accompanied by extensional faulting has been the dominant style of deformation in this area since mid‐Pleistocene time. Principal strain directions and strain ratios, determined from mesoscale fault arrays associated with major faults, document the regional extension. In the Cape Kidnappers area, fault arrays record plane strain with northwest horizontal extension and vertical shortening. In the Maraetotara Plateau area, fault arrays record triaxial strain with roughly equal magnitudes of horizontal extension in two principal strain directions, either east‐west and north‐south, or northwest and northeast, and vertical shortening. The morphology, seismicity, and nature and distribution of active faults of the southern Hawke's Bay region suggest that the Maraetotara Plateau‐Cape Kidnappers area is the on‐land expression of an outer‐arc high, which is developed between the accretionary wedge on the east and the forearc basin on the west. The cause of uplift and extension in the Maraetotara Plateau‐Cape Kidnappers area may be either upper plate deformation related to structural evolution of the outer‐arc high and underlying backstop, underplating on the Hikurangi subduction zone, or a combination of these processes. The uplift and horizontal extension occurring in the Maraetotara Plateau‐Cape Kidnappers area contrast sharply with active deformation in adjacent parts of the forearc, reflecting a distinct partitioning of strain in this part of the obliquely convergent plate boundary. Copyright 1990 by the American Geophysical Union.