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Geologic setting

Regional geology

The eastern margin of the South Island of New Zealand is part of a continental fragment, the New Zealand Plateau, that rifted from Antarctica beginning at ~80 Ma (Anomaly 33). Rifting between the New Zealand Plateau and Antarctica–Australia was active along a mid-ocean-ridge system passing through the southern Tasman Sea and Pacific basins until ~55 Ma (Anomaly 24). Linking of the Indian Ocean and Pacific spreading centers truncated the spreading ridge in the southern Tasman Sea in the late Eocene. Spreading rates on the Indian and Pacific segments of this now-continuous Southern Ocean ridge system resulted in the formation of the modern boundary between the Australian and Pacific plates composed of the Macquarie Ridge, the Alpine Fault, and the Tonga–Kermadec subduction zone (Molnar et al., 1975).

The Canterbury Basin lies at the landward edge of the rifted continental fragment and underlies the present-day onshore Canterbury Plains and offshore continental shelf (Field and Browne, 1989). Banks and Otago peninsulas (Fig. F2) are mid–late Miocene volcanic centers. Basin sediments thin toward these features as well as westward, where they onlap basement rocks onshore that are involved in uplift and faulting linked to the latest Miocene (8–5 Ma) initiation of the current period of mountain building along the Southern Alps (Adams, 1979; Tippett and Kamp, 1993a; Batt et al., 2000).

The plate tectonic history of the New Zealand Plateau is recorded in the stratigraphy of the South Island. The postrift Cretaceous to recent sedimentary history of the Canterbury Basin is composed of a first-order (80 m.y.), tectonically controlled transgressive–regressive cycle (Carter and Norris, 1976; Field and Browne, 1989). The basin formed part of a simple passive margin from the Late Cretaceous to some time in the late Eocene, when convergence between the Australasian and Pacific plates began to influence the region, eventually leading to the formation of the Alpine Fault at ~23 Ma (Wellman, 1971; King, 2000). The Cretaceous–Tertiary sedimentary section can be divided into three principal intervals, the Onekakara, Kekenodon, and Otakou groups (Carter, 1988), during which contrasting large-scale sedimentary processes operated (Fig. F4).

Cretaceous–Paleogene transgression and Oligocene highstand

The postrift transgressive phase (Onekakara Group) produced ramplike seismic geometries and terminated during the late Eocene, when flooding of the landmass was at a maximum (Fleming, 1962). Reduced terrigenous influx during the postrift phase of subsidence and transgression resulted in the deposition of regionally widespread siliceous or calcareous biopelagites (Amuri Limestone) as young as early Oligocene (~33 Ma). This sequence is interrupted by a current-induced unconformity, the Marshall Paraconformity (Carter and Landis, 1972), which occurs at the base of the mid–late Oligocene cross-bedded glauconitic sand (Concord Formation) and calcarenite limestone (Weka Pass Formation) that compose the Kekenodon Group (Fig. F4) (Carter, 1985, 1988). Exploration wells and Leg 181 drilling (ODP Sites 1123 and 1124; Carter et al., 2004c) revealed that the paraconformity as well as the probable equivalents of the Amuri Limestone and Weka Pass Formation exist offshore (Wilding and Sweetman, 1971; Milne et al., 1975; Hawkes and Mound, 1984; Wilson, 1985). The paraconformity, the deepest drilling target of Expedition 317, has also been recognized at drill sites throughout the region east of the Tasmanian gateway and is hypothesized to represent the initiation of thermohaline circulation (Deep Western Boundary Current) and shallower circulation upon the opening of the seaway between Antarctica and Australia (~33.7 Ma) prior to the opening of the Drake Passage (Carter et al., 2004c). New Zealand lay directly in the path of this developing current system. The paraconformity has also been studied in onshore outcrops (e.g., Findlay, 1980; Field and Browne, 1989; Carter and Landis, 1972; Lever, 2007) and dated by biostratigraphy and more recently by strontium isotopes at a South Canterbury outcrop. The hiatus has been dated at 32.4–29 Ma (Fulthorpe et al., 1996). Its deepwater representation may have started to form 1–2 m.y. earlier (Carter et al., 2004c).

Miocene–recent regression

Regression commenced in the late Oligocene or early Miocene in response to an increase in sediment supply provided by the initiation of Alpine Fault movement (Carter and Norris, 1976; Kamp, 1987). The Alpine Fault formed as a dextral strike-slip zone, with 500 km displacement since the earliest Miocene (~23 Ma) (Kamp, 1987). In eastern South Island, this resulted in the deposition of a widespread shelf siltstone (Bluecliffs Formation), starting in the latest Oligocene or earliest Miocene. At abyssal depths in the path of the Deep Western Boundary Current, fine-grained terrigenous–carbonate rhythms at 41 k.y. Milankovitch frequency commenced at almost the same time (~24–23 Ma) (Carter et al., 2004c). This early uplift is not recognized by fission track dating (e.g., Batt et al., 2000) and is distinct from the later pulse of uplift that culminated in the present-day Southern Alps. Uplift of the Southern Alps accelerated at ~8–5 Ma (Tippett and Kamp, 1993a; Batt et al., 2000) or ~10–8 Ma (Carter and Norris, 1976; Norris et al., 1978; Adams, 1979; Tippett and Kamp, 1993b), indicating an increased component of convergence along the fault. Transpression led to an increase in the rate of sediment supply to the offshore Canterbury Basin (Lu et al., 2005). As in New Jersey, this sediment influx was deposited as prograding clinoforms (Otakou Group; Fig. F4). Currents continued to influence deposition. At present, the core of the northward-flowing Southland Current, inboard of the Southland Front (part of the Subtropical Front [STF]), is near the ~300 m isobath (Chiswell, 1996). In deeper water of at least 900 m, a local gyre of the Antarctic Circumpolar Current circulates clockwise within the head of Bounty Trough parallel to the Southland Current (Fig. F2) (Morris et al., 2001). Large sediment drifts within the prograding section (Fig. F4) show that similar currents existed throughout much of the Neogene (Fulthorpe and Carter, 1991; Lu et al., 2003; Carter et al., 2004c). Carter (2007) has applied the term "Canterbury Drifts" to describe the ensemble of shelf-slope sediments that have been deposited under the influence of north-flowing currents since the latest Oligocene.

Subsidence history

Backstripping suggests an increase in tectonic subsidence in the central part of the offshore basin starting at ~8 Ma (Figs. F5, F6) (Browne and Field, 1988) or perhaps as early as 20 Ma (M. Kominz, pers. comm., 2009). This increased subsidence may be a response to increasing convergence at the Alpine Fault (Lu et al., 2005). The transect approach of Expedition 317 will help us to model the presumed two-dimensional subsidence effect of Alpine Fault convergence, allowing us to remove the tectonic portion of subsidence and leave the eustatic signal. Figures F5 and F6 also show evidence for uplift at ~50–35 Ma, which could reflect the late Eocene reorganization of plate boundaries.