DEVELOPMENT OF SEDIMENT DRIFTS APPROACHING AN ACTIVE PLATE MARGIN UNDER THE SW PACIFIC DEEP WESTERN BOUNDARY CURRENT

The abyssal Pacific Ocean is fed by a 1000 km wide, deep western bound ary current (DWBC) that flows northward along the continental margin, east of New Zealand. Between the passive margin of Chatham Rise and th e 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 re flect regional variations in bathymetry, sediment supply, and the tect onic/volcanic framework. On Chatham Rise the DWBC has deposited a sinu ous, linear body along the south flank (3000 m), an extensive apronlik e 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 sed iment over 400 km long within a moat at the base of the nearby Louisvi lle Seamount Chain. Further downcurrent, the 250 km long Rekohu Drift (3600-4190 m) has developed northward to 39 degrees S. South of this l atitude, drifts comprise mainly reworked pelagic/hemipelagic material and sediment transported from distant southerly sources. In contrast, drifts north of 39 degrees S have received a major injection of terrig enous sediment from Hikurangi Channel which runs 1400 km from New Zeal and, eastward across the Hikurangi Plateau to disgorge on to the abyss al floor at the plateau edge. En route, turbidity current overspill fr om 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 n umerous seamounts on the plateau at 3500-4200 m, Off Hikurangi Channel mouth, a large fan has accumulated. The DWBC has extended the fan int o a drift running over 250 km along the base of Hikurangi Plateau (515 0-5770 m) toward Kermadec Trench. Here drift sediment becomes increasi ngly disrupted by mass wasting associated with the active subduction i n this area. The seismic stratigraphy reveals the drifts to rest mainl y on a widespread erosional surface that is interpreted to mark the in ception of the DWBC in the region with the late Oligocene opening of t he Australian-Antarctic seaway. Drift construction commenced during th e Miocene but was punctuated in the late Miocene by another period of erosion that coincided with increased bottom water production in Antar ctica. Deposition resumed in Plio-Pleistocene times when large quantit ies of sediment from the rapidly rising landmass of New Zealand were i njected 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.