Structure and deformational history of the inner forearc region, Hikurangisubduction margin, New Zealand

Seismic reflection and outcrop data from the onshore Hikurangi forearc reve al the styles and history of deformation for a c. 3000 km(2) region between Dannevirke and Hawke's Bay. The data cover the forearc basin, including it s western and eastern boundaries, and delineate folds and faults in a late Miocene-Recent sedimentary sequence. Five seismic horizons, including the b asement/Neogene cover unconformity (variable age), base Waipipian (3.7 Ma), base Mangapanian (3.2 Ma), base Nukumaruan (2.6 Ma), and base Castlecliffi an (1.6 Ma) were identified using outcrop and well ties. These horizons wer e traced across the study region to provide information on the geometry, sp atial distribution, and timing of structures. To the west, the range-front fault is predominantly reverse and separates uplifted Torlesse basement of the axial ranges from Neogene forearc basin sediments. Structures within th e forearc basin are dominated by north-northeast-striking reverse faults an d associated asymmetric folds which parallel the subduction margin and ofte n have sinuous traces. Faults are planar to depths of at least 1-2 km and t ypically dip at 30-80 degrees NW (most often at 40-70 degrees). Many faults in the basin terminate northwards and fault-normal spacings decrease from 2-8 km in the Dannevirke area to c. 20 km near Hastings. Angular unconformi ties and syntectonic strata constrain the timing of deformation on fault/fo ld pairs. Faults within the forearc basin were active over two main periods , at c. ?3.7-2.5 Ma and c. 1.6 Ma-Recent. Active fault traces are confined to the edges of the forearc basin. In the west, the Mohaka and Ruahine Faul ts have mainly recent right-lateral offsets, but interpretation of a seismi c reflection line which crosses the Mohaka Fault indicates minimal (<300 m) right-lateral displacement over the last 3 m.y., and lateral slip may not significantly predate the late Quaternary. In the east, a zone of reverse f aults (including the Longlands, Poukawa, Tukituki, and Oruawharo Faults) be gan forming at c. 1 Ma and remains active. Margin-normal shortening across the forearc basin and basin-bounding structures ranges up to 5 mm/yr (120-1 80 m/km) in the Dannevirke region but decreases to 1-2 mm/yr(c. 35 m/km) ne ar Hastings. Total shortening within the Australian plate and normal to the central portion of the Hikurangi margin is at least 16 mm/yr (i.e. greater than or equal to c. 50% of the relative plate motion component at latitude 41 degrees S), which is consistent with the plate interface being strongly coupled over much of the last c. 0.3-0.5 m.y.