Marine Geology 272 (2010) 26–48 Contents lists available at ScienceDirect Marine Geology journal homepage: www.elsevier.com/locate/margeo Tectonic and geological framework for gas hydrates and cold seeps on the Hikurangi subduction margin, New Zealand Philip M. Barnes a,⁎, Geoffroy Lamarche a, Joerg Bialas b, Stuart Henrys c, Ingo Pecher d, Gesa L. Netzeband b, Jens Greinert e,1, Joshu J. Mountjoy a,f, Katherine Pedley f, Gareth Crutchley g a National Institute of Water & Atmospheric Research, P.O. Box 14901, Kilbirnie, Wellington 6041, New Zealand b Leibniz Institute of Marine Sciences, IFM-GEOMAR, Wischhofstr. 1-3, 24148, Kiel, Germany c GNS Science, P.O. Box 30368, Lower Hutt, New Zealand d Institute of Petroleum Engineering, Heriot-Watt University, Edinburgh, EH3 9HP, Scotland, UK e Renard Centre of Marine Geology, University of Gent, Belgium f Department of Geological Sciences, University of Canterbury, Private Bag 4800, Christchurch, New Zealand g Department of Geology, University of Otago, P.O. Box 56, Dunedin 9054, New Zealand article info abstract Article history: The imbricated frontal wedge of the central Hikurangi subduction margin is characteristic of wide (ca. Received 2 July 2008 150 km), poorly drained and over pressured, low taper (∼4°) thrust systems associated with a relatively Received in revised form 18 February 2009 smooth subducting plate, a thick trench sedimentary sequence (∼3–4 km), weak basal décollement, and Accepted 8 March 2009 moderate convergence rate (∼40 mm/yr). New seismic reflection and multibeam bathymetric data are used Available online 31 March 2009 to interpret the regional tectonic structures, and to establish the geological framework for gas hydrates and fluid seeps. We discuss the stratigraphy of the subducting and accreting sequences, characterize Keywords: Hikurangi stratigraphically the location of the interplate décollement, and describe the deformation of the upper subduction plate thrust wedge together with its cover sequence of Miocene to Recent shelf and slope basin sediments. interplate décollement We identify approximately the contact between an inner foundation of deforming Late Cretaceous and thrust wedge Paleogene rocks, in which widespread out-of-sequence thrusting occurs, and a 65–70 km-wide outer wedge thrust faults of late Cenozoic accreted turbidites. Although part of a seamount ridge is presently subducting beneath the accretion deformation front at the widest part of the margin, the morphology of the accretionary wedge indicates that subducting seamounts frontal accretion there has been largely uninhibited for at least 1–2 Myr. This differs from the offshore anticline Hawkes Bay sector of the margin to the north where a substantial seamount with up to 3 km of relief has turbidites gas hydrates been subducted beneath the lower margin, resulting in uplift and complex deformation of the lower slope, – fluid seeps and a narrow (10 20 km) active frontal wedge. Five areas with multiple fluid seep sites, referred to informally as Wairarapa, Uruti Ridge, Omakere Ridge, Rock Garden, and Builders Pencil, typically lie in 700–1200 m water depth on the crests of thrust-faulted, anticlinal ridges along the mid-slope. Uruti Ridge sites also lie in close proximity to the eastern end of a major strike-slip fault. Rock Garden sites lie directly above a subducting seamount. Structural permeability is inferred to be important at all levels of the thrust system. There is a clear relationship between the seeps and major seaward-vergent thrust faults, near the outer edge of the deforming Cretaceous and Paleogene inner foundation rocks. This indicates that thrust faults are primary fluid conduits and that poor permeability of the Cretaceous and Paleogene inner foundation focuses fluid flow to its outer edge. The sources of fluids expelling at active seep sites along the middle slope may include the inner parts of the thrust wedge and subducting sediments below the décollement. Within anticlinal ridges beneath the active seep sites there is a conspicuous break in the bottom simulating reflector (BSR), and commonly a seismically-resolvable shallow fault network through which fluids and gas percolate to the seafloor. No active fluid venting has yet been recognized over the frontal accretionary wedge, but the presence of a widespread BSR, an extensive protothrust zone (N200 km by 20 km) in the Hikurangi Trough, and two unconfirmed sites of possible previous fluid expulsion, suggest that the frontal wedge could be actively dewatering. There are presently no constraints on the relative fluid flux between the frontal wedge and the active mid-slope fluid seeps. © 2009 Elsevier B.V. All rights reserved. ⁎ Corresponding author. Tel.: +64 4 386 0372; fax: +64 4 386 2153. E-mail address: [email protected] (P.M. Barnes). 1 Present address: Royal Netherlands Institute for Sea Research, P.O. Box 59, 1790 AB, Den Burg (Texel), The Netherlands. 0025-3227/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.margeo.2009.03.012 P.M. Barnes et al. / Marine Geology 272 (2010) 26–48 27 1. Introduction Darby et al., 2000), and assumed to be widespread within the accretionary wedge (Barnes and Mercier de Lépinay, 1997). Gas hydrates and cold seeps are characteristics of many active Since 2001, multichannel seismic reflection and high-resolution continental margins. They reflect the migration of fluids and gas multibeam bathymetric data have been acquired from the Hikurangi towards the seabed as a result of tectonic deformation, compaction, Margin by research institutes, oil companies, and the New Zealand porosity reduction, and dewatering of the sedimentary sequence Government as part of hydrocarbon exploration and research (Kvenvolden, 1993). Seafloor sites of methane rich fluid expulsion initiatives. The improved understanding of tectonic structure that may be characterized by the presence of chemosynthetic biological can be gained from these data, combined with the widespread communities, development of carbonate hard grounds, pockmark occurrence of gas hydrates and cold seeps makes this margin an depressions, mud volcanism, and/or hydroacoustic flares (e.g., Kulm excellent example to study the relationship between thrust tectonics and Suess, 1990; Henry et al., 1990; Trehu et al., 1999; Faure et al., and fluid flow. This paper presents a revision and synthesis of the 2006). In subduction margins, particularly those dominated by tectonic morphology, fault structure, and composition of the accretion, high fluid pressures are thought to play an important imbricated thrust wedge between Mahia Peninsula and Cook Strait mechanical role in maintaining thrust wedges (Moore and Byrne, (Fig. 1). We then establish the tectonic and geological framework for 1987; Bryne and Fisher, 1990, Moore and Vrolijk, 1992; Bangs et al., each of five study areas with submarine seep sites, and we interpret 1999, 2004; Saffer and Bekins, 2002), and in the seismogenic cycle by these sites within the context of regional structural processes. Finally producing fluctuations in frictional strength of faults (Sibson, 1992; we discuss the role of the deformation structures as fluid conduits. Dixon and Moore, 2007 and papers therein). Fault zones are commonly interpreted as important conduits for the upward flow of 2. Data used in this study fluid (e.g., Moore et al., 1990, 1995). The 25 Myr old Hikurangi Margin of north eastern New Zealand The continental shelf of the Hikurangi Margin is relatively well epitomizes subduction systems and their tectonic complexity and covered with multichannel seismic reflection (MCS) data, from which variability. Lying at the southern end of the Tonga–Kermadec– detailed accounts of active tectonics (e.g., Barnes et al., 2002; Barnes Hikurangi subduction zone, the margin has formed in response to and Nicol, 2004; Lewis et al., 2004), sequence stratigraphy (Paquet, the westward subduction of the thick and bathymetrically elevated 2008; Paquet et al., 2009), and exploration geology (e.g., Field et al., oceanic Hikurangi Plateau (Pacific Plate) beneath the Australian Plate 1997; Uruski et al., 2004) have been published. The continental slope at about 40–50 mm/yr (Fig. 1). The structural trench, referred to as the however, is less well surveyed. Previous regional interpretations of the Hikurangi Trough, is shallow (c. 3000 m) compared to the deep tectonic structure were based largely on RV L'ATALANTE SIMRAD Kermadec trench (N9000 m), and the plate interface dips at a gentle EM12Dual multibeam bathymetry and Hawaii MR1 sidescan sonar angle of about 3° for at least 100 km beneath the margin, before images, supported by the SP LEE MCS profile, RV L'ATALANTE and pre- steepening beneath the North Island (Davey et al.,1986a; Henrys et al., 1980s oil company low-fold MCS data (Fig. 1D), and widespread single 2006; Barker et al., 2009). The margin exhibits complex tectonic channel seismic data (not shown in Fig. 1D) (Davey et al., 1986a,b; structure and stratigraphic evolution (e.g., Lewis and Pettinga, 1993; Lewis and Pettinga, 1993; Collot et al., 1996; Barnes and Mercier de Collot et al., 1996; Field et al., 1997; Barnes et al., 2002; Barnes and Lépinay, 1997, Lewis et al., 1997, 1999; Barnes et al., 1998a,b). Nicol, 2004; Henrys et al., 2006; Nicol et al., 2007). The northern MCS data acquired since 2001, and available for this study, includes margin off Raukumara Peninsula is characterized by non-accretion (Fig. 1D) (1) the GECO RESOLUTION NIGHT high-fold profile off and tectonic erosion associated with seamount impact scars (Lewis Hawkes Bay (Pecher et al., 2004, 2005; Henrys et al., 2006), (2) the et al., 1997, 1998, 2004; Collot et al., 2001), whereas the central margin large grid of MV MULTIWAVE high-fold MCS data (CM05) from the off the Wairarapa coast is a classical imbricated thrust wedge dominated upper margin off Hawkes Bay and East Coast (Multiwave, 2005; Nicol by accretion (Davey et al., 1986b; Lewis and Pettinga, 1993; Collot et al., et al., 2007), (3) low-fold MCS data acquired off Hawkes Bay and 1996; Barnes and Mercier de Lépinay, 1997; Lewis et al., 1999).