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

Abstract

Seismic reflection and outcrop data from the onshore Hikurangi forearc reveal 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 its western and eastern boundaries, and delineate folds and faults in a late Miocene-Recent sedimentary sequence. Five seismic horizons, including the basement/Neogene cover unconformity (variable age), base Waipipian (3.7 Ma), base Mangapanian (3.2 Ma), base Nukumaruan (2.6 Ma), and base Castlecliffian (1.6 Ma) were identified using outcrop and well ties. These horizons were traced across the study region to provide information on the geometry, spatial distribution, and timing of structures. To the west, the rangefront fault is predominantly reverse and separates uplifted Torlesse basement of the axial ranges from Neogene forearc basin sediments. Structures within the forearc basin are dominated by north-northeast-striking reverse faults and associated asymmetric folds which parallel the subduction margin and often have sinuous traces. Faults are planar to depths of at least 1-2 km and typically dip at 30-80°NW (most often at 40-70°). 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 unconformities and syntectonic strata constrain the timing of deformation on fault/fold pairs. Faults within the forearc basin were active over two main periods, at c. ?3.1-2.5 Maandc. 1.6 Ma-Recent. Active fault traces are confined to the edges of the forearc basin. In the west, the Mohaka and Ruahine Faults have mainly recent right-lateral offsets, but interpretation of a seismic 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 faults (including the Longlands, Poukawa, Tukituki, and Oruawharo Faults) began forming ate. 1 Ma and remains active. Margin-normal shortening across the forearc basin and basin-bounding structures ranges up to 5 mm/yr (120- *

Fire and slice: palaeogeography for biogeography at New Zealand's North Island/South Island juncture

“…Although some continue to ignore modern geological mapping and fault analysis, it is now well recognized that total strike-slip displacement in the North Island fault system in the Plio-Pleistocene has been minimal ( B20 km; e.g. Beanland et al 1998;Berryman et al 2002;Bland & Kamp 2006;Nicol et al 2007;Bland et al 2008). An important corollary of this is inference of the timing of formation and extent of past seaways in the region.…”

“…Leslie 1971a, b;Johnston & Francis 1996;Johnston & Langdale 2000) or consider the detailed geological investigations from fault and tectonic analysis (e.g. Cashman et al 1992;Beanland et al 1998;Wallace et al 2004;Nicol et al 2007).…”

“…However, more recent studies have demonstrated that there is little geological evidence for large-scale (20 km) strike-slip displacement in North Island during the late Neogene, and that extensive strike-slip faulting and tectonic rotation (608Á908) is not required to reconstruct the tectonic framework of central New Zealand (e.g. Beanland et al 1998;Wallace et al 2004;Nicol et al 2007). Clockwise rotations of c. 458Á 508 take up much of the margin-parallel (i.e.…”

Three‐stage massive gravitational collapse of the emergent imbricate frontal wedge, Hikurangi Subduction Zone, New Zealand

“…The extensional deformation described in this paper has been ongoing for at least 1-2 m.y., and also co-exists with ongoing thrust faulting in adjacent regions (e.g., Pettinga 1980Pettinga , 1982Beanland et al 1998;Barnes et al 2002). It is entirely possible that the extensional deformation described in this paper represents the continuation of a prolonged episode of extension affecting the frontal wedge of the forearc region, and that the spatial loci of extension has progressively migrated westward across the wedge as the underplated material is progressively taken westward along the decollement interface of the subduction zone.…”

“…Similarly, the region to the west of the plateau and occupied by the Elsthorpe Anticline, a late Pliocene to early Pleistocene thrust-propagated fold, is now also undergoing northwestsoutheast-directed extension. In a more comprehensive regional study, Cashman et al (1992) (see also Beanland et al 1998) documented Quaternary strain partitioning across the entire subaerially exposed portion of the forearc region. They identified northeast-southwest-trending domains of extension, contraction, and strike-slip.…”