Development of sediment drifts approaching an active plate margin under the SW Pacific Deep Western Boundary Current

Abstract

The abyssal Pacific Ocean is fed by a 1000 km wide, deep western boundary current (DWBC) that flows northward along the continental margin, east of New Zealand. Between the passive margin of Chatham Rise and the subduction zone of Kermadec Trench, a distance of 1200 km, the DWBC has formed a suite of sediment drifts over a depth range of 2200–5700 m. Airgun and 3.5‐kHz profiles record a variety of drift types that reflect regional variations in bathymetry, sediment supply, and the tectonic/volcanic framework. On Chatham Rise the DWBC has deposited a sinuous, linear body along the south flank (3000 m), an extensive apronlike drift on the north flank (2200–4500 m), and a ridgelike drift about the rise base (4500–5200 m). The flow has also deposited a body of sediment over 400 km long within a moat at the base of the nearby Louisville Seamount Chain. Further downcurrent, the 250 km long Rekohu Drift (3600–4190 m) has developed northward to 39°S. South of this latitude, drifts comprise mainly reworked pelagic/hemipelagic material and sediment transported from distant southerly sources. In contrast, drifts north of 39°S have received a major injection of terrigenous sediment from Hikurangi Channel which runs 1400 km from New Zealand, eastward across the Hikurangi Plateau to disgorge on to the abyssal floor at the plateau edge. En route, turbidity current overspill from the channel has moved north under the influence of the shallow DWBC to contribute to a series of small ridge and patch drifts among the numerous seamounts on the plateau at 3500–4200 m. Off Hikurangi Channel mouth, a large fan has accumulated. The DWBC has extended the fan into a drift running over 250 km along the base of Hikurangi Plateau (5150–5770 m) toward Kermadec Trench. Here drift sediment becomes increasingly disrupted by mass wasting associated with the active subduction in this area. The seismic stratigraphy reveals the drifts to rest mainly on a widespread erosional surface that is interpreted to mark the inception of the DWBC in the region with the late Oligocene opening of the Australian‐Antarctic seaway. Drift construction commenced during the Miocene but was punctuated in the late Miocene by another period of erosion that coincided with increased bottom water production in Antarctica. Deposition resumed in Plio‐Pleistocene times when large quantities of sediment from the rapidly rising landmass of New Zealand were injected into the boundary current. The modern flow continues to affect drift deposition as manifest by an active boundary channel along the foot of Hikurangi Plateau and widespread scour zones and sediment wave fields.

Physical oceanography of the deep seas around New Zealand: a review

“…Observations have shown that, up to nine times a year, intense eddies can displace the isotherms associated with the SAF south from their mean position by as much as 5°of latitude. These eddies appear to extend to the abyssal ocean floor and actively move sediment (Carter & McCave 1994;Carter & Wilkin 1999). Moore et al (1999) suggested that the mean path of the PF is strongly steered by the topographic features under the Southern Ocean.…”

Correlation, dispersal, and preservation of the Kawakawa Tephra and other late Quaternary tephra layers in the Southwest Pacific Ocean

“…Once below wind-affected surface waters, particles would be entrained I orthwards, especially within the DWBC whose depth range is 2200-5500 m (Warren 1973;Carter & McCave 1994). Transport would be greatest where the flow is strongest, which is approximately along the 4000-5500 m isobaths (Fig.…”

“…Here the continental margin is part of the Hikurangi subduction system comprising an accretionary prism of anticlinal ridges and slope basins (shelf and slope), and the 2000-3000 m deep Hikurangi Trough, with a thick fill of terrigenous sediment (Lewis 1980). The trough axis is occupied by Hikurangi Channel, which meanders north to Hawke Bay before swinging east across Hikurangi Plateau to eventually debouch onto the abyssal Pacific floor, where a large fan has developed (Carter & McCave 1994). The plateau itself covers a large triangle of ocean floor (40 000 km 2 ) and has a sediment cover of turbidites overspilt from Hikurangi Channel, hemipelagites and sediment drifts, as well as widespread layers of tephra.…”

“…1 Location of cores together with major sediment depocentres and path of the deep western boundary current (DWBC) from Carter & McCave (1994). TVZ, Taupo Volcanic Zone; TVC, Taupo Volcanic Centre; OVC, Okataina Volcanic Centre; HB, Hawke Bay; BP, Bay of Plenty.…”

Depth distribution of Recent deep‐sea benthic foraminifera east of New Zealand, and their potential for improving paleobathymetric assessments of Neogene microfaunas

“…Upper CPDW, at 1450-2500 m south of the rise and down to 2900 m north of the rise (McCave & Carter 1997), has temperatures of 1.8-3.0°C, salinities of 34.5-34.7%c, and includes an oxygen minimum zone. Lower CPDW is a tongue of more saline (>34.7%o) water that extends down to the bottom, and is the primary watermass of the large Deep Western Boundary Current (DWBC) in this region (Carter & McCave 1994 The main inflow of deep water is via DWBC, which enters this region from the south, passes over the Bounty Fan, across the deep eastern end of Chatham Rise, and northwestwards out across the edge of the Hikurangi Plateau. The major surface current north of the Chatham Rise is the East Cape Current (ECC) that flows south along the east coast of the North Island until it is deflected eastwards along the STF.…”