Deformation style and history of the Eketahuna region, Hikurangi forearc, New Zealand, from shallow seismic reflection data

Abstract

A shallow seismic reflection survey across the Eketahuna region, located at the inboard edge of the obliquely convergent Hikurangi subduction margin, New Zealand, records deformation of a Miocene-Pliocene forearc basin sequence and underlying greywacke basement. A suite of northeast-trending strike-slip and reverse faults deforms the region into northwest-tilted blocks. Southeastverging folding associated with the faults includes open hanging-wall anticlines and footwall synclines, and fault propagation folds above non-emergent fault tips. Secondary faults commonly disrupt synclinal cores. Faults emerge from the greywacke basement at near-vertical (strike-slip faults) or 35-70° (reverse faults) dips. Faults in the L'ketahuna and adjacent Dannevirke and Wairarapa Basins are shown from seismic reflection data to have been active, at least in part, contemporaneously with submarine deposition of Miocene-Pliocene sediments in the forearc basin. In the southern part of the basin, widespread Pleistocene uplift and erosion of greywacke basement caused an influx of coarse detritus into the basin. Still later, emergence of the forearc region occurred. Whereas uplift of the forearc is considered to be due to large scale processes (such as underplating, subduction of thick, buoyant oceanic crust, or an influx of sediments to the Hikurangi trough), widespread and closely spaced disruption of the forearc basin may be due to encroachment of the Hikurangi margin on the continental crust of the Chatham Rise causing increased coupling across the subduction interface.

Quaternary faulting in the offshore Flaxbourne and Wairarapa Basins, southern Cook Strait, New Zealand

“…Stratal geometry across the faults indicates negligible thrust fault and fold growth during deposition of most of the late Miocene strata, although extensional growth faulting may have occurred on one of the faults prior to reflector eTt (centre of profile A). The thrusts therefore developed predominantly since the late Miocene, an observation consistent with many other contractional structures within the Wairarapa Basin (Cape et al 1990;Beanland 1995;Lamarche et al 1995;Nicol et al 1996). The faults, however, have no bathymetric expression (Fig.…”

“…Significant subsidence and sedimentation was occurring in the southern forearc Wairarapa Basin during the late Miocene (Cape et al 1990;Lamarche et al 1995;Beanland 1995), but the vertical separation on major structures imaged in the profiles beneath Palliser Bay is largely post-Miocene age (<5 Ma) and on many of the faults it could be Pleistocene ( Fig. 6,7; profiles A-D).…”

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 ' .…”

“…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.…”

Active faults, paleoseismology, and historical fault rupture in northern Wairarapa, North Island, New Zealand

“…Faults are interpreted to be subvertical, based on straight traces across topography; dips at the surface are both southeast and northwest (Kelsey et al 1995). Lamarche et al (1995) inferred from seismic reflection profiles near Ihuraua (Fig. 4) that the Alfredton Fault is composed of several strands that converge at depth.…”