The geophysical exploration of the Ohura Fault North Wanganui Basin, New Zealand

Abstract

The North Wanganui Basin, New Zealand, contains Cenozoic sub-basins that have undergone Tertiary extension and compression, followed by uplift and exhumation in the Pliocene. At Waimiha, a deep sub-basin is indicated by a distinctive -30 mgal residual gravity anomaly. The anomaly can be seen on the 1:1,000,000 NZ Gravity Anomaly Map and new data presented here show the gradient to be as high as 25 mgal/km - a value similar to those seen on the flanks of the larger caldera structures found in the Taupo Volcanic Zone to the east. The Ohura fault bounds the northwestern extent of the anomaly, and forms a 200 m high, steep (20-30°), southeast-facing scarp, suggesting that the Ohura fault has moved during the Quaternary. To investigate the relationship between the gravity anomaly and the Ohura fault, seismic and gravity techniques were employed. Seismic reflection and refraction measurements were made perpendicular to and across the Ohura fault in the Waimiha valley. These measurements produced a two-dimensional structural section through the sediments showing 500 ±50 m of Quaternary volcanics juxtaposed against the Ohura fault. Strong reverse moveout events are seen on shot records and are modelled to be from a steeply (~60°), southeast dipping structure associated with the subsurface extent of the Ohura fault. A dense spread of gravity measurements were taken in and around the Waimiha region and across the Ohura fault. The anomaly clearly delineates the fault, coinciding with its prominent scarp in the Waimiha valley. Modelling of the residual anomaly suggests that the Waimiha valley contains the thickest section (~4000m) of Tertiary sediment in the North Wanganui Basin. It is suggested here that the Waimiha valley and Ohura fault formed as pull-apart basin during dextral tectonics in the Oligocene. Reactivation of the fault in the Pliocene, and continued movement throughout the Quaternary, has further downthrown the Waimiha sub-basin and protected the Tertiary sequence from erosion during uplift of the central and western North Island. The implications of this study, for the distribution of extension in the central North Island, the seismic hazard of the King Country, and the petroleum potential of the North Wanganui Basin, are discussed.