THE EASTERN NEW ZEALAND OCEANIC SEDIMENTARY SYSTEM (ENZOSS)
The available seismic records suggest that the DWBC has been active along the eastern New Zealand margin since at least the Miocene, and probably since the middle Oligocene (32 Ma) (Carter and McCave, 1994). Starting at ~24 Ma, abundant terrigenous material was shed from rising mountains along the Alpine Fault plate boundary (Vella, 1962; Norris et al., 1978) and fed through the eastern South Island shelf into the Solander, Bounty and Hikurangi channel systems. Sediment supply accelerated at ~6.5 Ma in the late Cenozoic, when collision increased along the plate boundary (Walcott, 1998; cf. Kennett, von der Borch, et al., 1986), and supply to the deep sea was probably enhanced again from the start of major glacial lowstands at ~2.6 Ma onward. Much of this sediment ultimately became entrained in the DWBC drift system, which carries it northward to be eventually subducted into the Kermadec Trench.
Sediment is delivered into the DWBC through two newly described transport conduits, the Bounty (Carter and Carter, 1993) and Hikurangi (Lewis, 1994) channel-fan systems. A third feeder channel, Solander, is poorly known, but extends for 450 km before discharging into the DWBC at Emerald Basin between Macquarie Ridge and the western side of Campbell Plateau (L. Carter et al., 1996; Carter and McCave, 1997; Schuur et al., in press). The Hikurangi Fan has been termed a "fan-drift" by Carter and McCave (1994) because it apparently represents the extreme case of a fan whose thickness and facies pattern are directly remolded by a deep current into the form of a sediment drift. In contrast, the Bounty Fan, located in a bathymetric embayment, has retained its fan morphology and has developed directly across the path of the DWBC, the only evidence of modern drift formation being scour of the northern fan and redeposition of material as a series of small, discrete ridges (Carter and Carter, 1993). Compared to Hikurangi Fan Drift, Bounty Fan has formed in a region where the DWBC is inferred to be slowed (1) by the lack of forcing by the shallower water stream of the ACC, which continues east across the Pacific just south of Bounty Trough; and (2) the loss of the topographic steering, and current acceleration, provided by the steep eastern slope of the Campbell Plateau, which ceases abruptly at Bollons Seamount, again at the southern edge of the Bounty Trough. However, six years of satellite sea-surface temperature data, summarized by Carter et al. (1998), indicates that meanders from the ACC periodically affect the outer Bounty Trough, and the water motions that accompany them may also play a role in current-winnowing on the lower Bounty Fan.
During the later Cenozoic, the two described abyssal fans have been supplied with sediment by turbidites passing through the Bounty and Hikurangi channels, each of which is over 1000 km long. Hikurangi Channel heads in the Kaikoura Canyon, only a few hundred meters from shore, and less than 10 km from the rapidly rising, 2.5-km-high Seaward Kaikoura Mountains (Lewis, 1994). The Hikurangi system is therefore active today, in interglacial times. In contrast, the Solander and Bounty Channels head in a number of canyons that indent the edge of the continental shelf. The Bounty and Solander Systems may therefore be sea-level (i.e., climatically) controlled, with most sediment being fed into them during glacial lowstands, whereas in interglacials the same sediment stream is diverted along the inner shelf, some of it reaching the Hikurangi System via the Kaikoura Canyon (Carter and Herzer, 1979).
Eastern New Zealand is thus the site of a major recycling system, whereby sediment is shed from uplifting mountains along the Australian/Pacific plate boundary and provided to the deep sea via several major submarine channel systems. Once at abyssal depths, the sediment is re-entrained by the ACC and DWBC, and passed north along the edge of the Campbell Plateau, around the tip of the Chatham Rise, westward along the foot of the Hikurangi Plateau, to finally arrive in the Hikurangi-Kermadec trench, where it is subducted, melted and enters the cycle again as juvenile volcanic rock. Remarkably, and largely because of the effects of the plate boundary, the two small islands of New Zealand supply two percent of the world's sediment load to the oceans (Milliman and Syvitski, 1992); it is this sediment load that is then entrained in what L. Carter et al. (1996) have termed the Eastern New Zealand Oceanic Sedimentary System (ENZOSS) (Fig. 6).
Recent publications (Carter and Carter, 1993; Lewis, 1994; Carter and McCave, 1994; L. Carter et al., 1996) have delineated the ENZOSS region, between the Solander Trough and the Kermadec Trench, east of the modern Australian-Pacific plate boundary, as an integrated sediment source-transport-sink area. During the latter half of the Cenozoic, sediment from mountains along the New Zealand plate boundary has been transported through deep-sea channel/fan systems, delivered into the path of the DWBC, entrained northward within this current system and finally consumed by subduction at the same plate boundary after a transport path of up to 3500 km.
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