Structural evolution along the inner forearc of the obliquely convergent Hikurangi margin, New Zealand

Abstract

The accretionary margin of the Hikurangi forearc on the southeast coast of the North Island of New Zealand is part of the leading edge of the Australian plate, which is overriding the obliquely converging Pacific plate. We investigate the last 10 m.y. of deformation history of the innermost (western) quarter of the total width of the forearc through analysis of the sedimentologic and structural evolution of the Eketahuna area on the east coast of the North Islm•d. The Eketahuna area is ideal for such a study because emergence of the margin in the Quaternary has exposed a complete late Neogene rock record. This record has allowed us to chronicle the strain history. From 10 Ma to about 2.5 Ma this forearc region was the locus of subsidence and marine deposition. In the latest Pliocene this part of the margin began to shorten through folding and reverse faulting, bringing an end to basin filling. The period of shortening was brief, and by the late Pleistocene, reverse faulting had ceased and was immediately succeeded by dextral strike-slip faulting, in some cases along the same faults. Presently, the dominant strain regime in the inner quarter of the torearc is strike-slip faulting. This structural history illustrates that, over time, the pattern of strain partitioning has changed in the Hikurangi forearc. The switch fi'om crustal shortening to dextral shear along the major faults in this area in the last 1 m.y. may be a response to more than 10 ø of clockwise rotation in the southern Hikurangi forearc in Pliocene and Pleistocene time. This rotation is a consequence of the fact that the accretionary margin is undergoing continuous deformation between the obliquely converging Australian and Pacific plates in this area at the southernmost end of the Kennadec-Hikurangi subduction system. The inboard portion of this young accretionary margin is exceptionally well exposed today, probably in part because of the late Neogene subduction of relatively thick, buoyant crust of the Hikurangi-Chatham plateau. IntroductionThe late Neogene accretionary margin on the east coast of the North Island of New Zealand, called the Hikurangi margin, is an unusually good place to study accretionary history because the inboard (western) part of this accretionary wedge is subaerially exposed. Over the last few decades, geologists Paper number 94TC01506. 0278-7407/95/94TC-01506510.00 have taken advantage of the extensive exposures of Paleogene and Neogene marine strata in this area to document depositional environments, structural history and strain partitioning in a young accretionary margin [Beu et al., 1980; Van der Lingen and Pettinga, 1980; Pettinga, 1982; Lamb and Vella, 1987; Cashman and Kelsey, 1990; Cashman et al., 1992]. At a few locations in this accretionary margin the late Neogene stratigraphic and structural record is unusually well exposed because sites of localized subsidence have become emergent in the Quaternary through uplift. Given a good chronostratigraphy for such sites, it should be possible to chronicle, with a resoluti...

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

“…Rapid along-strike change • in displacement (gradients c. >0.3) are also characteristic of a number of faults within the forearc basin (e.g., Fig. 7 this study; Kelsey et al 1995;Lamarche et al 1995), suggesting that fault interaction is not limited to the region in Fig. I ' .…”

“…There is ample evidence for current strike-slip activity on some faults amounting to a few hundred metres over the lastc. 200 000 yr(e.g.,Beanland&Berryman 1987; Erdman & Kelsey 1992;Beanland 1995;Kelsey et al 1995). In the west, faults with demonstrated right-lateral offsets include Wairarapa, Wellington/Mohaka, and Ruahine Faults (Fig.…”

“…1) commenced during the early Miocene (Ballance 1976;Rait et al 1991) and is presently associated with a relative plate motion vector of c. 38-50 mm/yr, trending at c. 40-70° to the strike of the margin (DeMets et al 1994). Numerous geologic, geodetic, and seafloor spreading studies constrain the evolution of the margin and help us to better understand the way in which oblique subduction of the Pacific plate is expressed in the overriding Australian plate (e.g., Walcott 1978;Pettinga 1982;Cashman et al 1992; Lewis & Pettinga 1993;DeMets et al 1994;Darby & Meertens 1995;Kelsey et al 1995;Lamarche et al 1995). Analysis of geodetic data indicates that the subduction thrust has been locked over the last 100 yr at the southern end of the Hikurangi margin (e.g., Walcott 1978;Bibby 1981;Reilly 1990;Darby & Meertens 1995), and that, over short periods of time, strain in the overriding plate accounts for all of the relative plate motion.…”

Evidence from GPS measurements for contemporary interplate coupling on the southern Hikurangi subduction thrust and for partitioning of strain in the upper plate

“…To do this, we need to make some assumptions about the geometry of the faults at depth. The Wellington fault had up to a 10% component of reverse slip, northwest side up before 14 ka [Ota et al, 1981;Berryman, 1990] and may well be a reactivated reverse fault as are other faults to the east, especially the Wairarapa fault [Kelsey et al, 1995;Beanland, 1995]. These strike-slip faults may therefore have significantly nonvertical dips or have dips decreasing with depth in a listric fashion.…”

Early Miocene thin‐skinned tectonics and wrench faulting in the Pongaroa district, Hikurangi margin, North Island, New Zealand

“…Between these two domains are, from west to east, a forearc basin, the coastal ranges, and the continental shelf. The emerged forearc basin and the coastal ranges have been studied by many workers, including Ridd (1964Ridd ( , 1967, Johnston (1980), Pettinga (1982), Moore & Speden (1984), Neef (1984), Moore (1986Moore ( , 1988a, Barnes & Korsch (1991), Chanier (1991), Chanier & Ferriere (1991), Cashman et al (1992), and Kelsey et al (1995). Nevertheless, the structure of the coastal region is still under debate, and different structural models have been proposed.…”