R. Sutherland , G.R. Dickens , P. Blum , C. Agnini , L. Alegret , G. Asatryan , J. Bhattacharya , A. Bordenave , L. Chang , J. Collot , M.J. Cramwinckel , E. Dallanave , M.K. Drake , S.J.G. Etienne , M. Giorgioni , M. Gurnis , D.T. Harper , H.-H.M. Huang , A.L. Keller , A.R. Lam , H. Li , H. Matsui , H.E.G. Morgans , C. Newsam , Y.-H. Park , K.M. Pascher , S.F. Pekar , D.E. Penman , S. Saito , W.R. Stratford , T. Westerhold , X. Zhou
2 February 2019
International Ocean Discovery Program (IODP) Site U1507 (26°29.316ʹS, 166°31.716ʹE; 3568 m water depth) is located on the eastern side of New Caledonia Trough, adjacent to Norfolk Ridge. It is distal from previous scientific drilling sites: ~460 km south of New Caledonia, ~620 km north of Deep Sea Drilling Project (DSDP) Site 206, and ~530 km east of DSDP Sites 208 and 588. The site was chosen to determine the timing of deformation and uplift of Norfolk Ridge, to constrain the age of trough formation and sediment fill, and to obtain a record of subduction-related volcanism. Sediment in New Caledonia Trough was drilled in 1971 at Site 206, where deformed Paleocene and Eocene calcareous ooze and chalk lies beneath middle Oligocene to recent ooze that is clay rich in places. Basement rock was not sampled at Site 206. In Taranaki Basin, which is at the southern end of New Caledonia Trough and ~1500 km from Site U1507, Cretaceous rifted basin fill is overlain by a transgressive shelf and coastal plain sedimentary sequence that has been imaged by seismic reflection data and drilled by petroleum wells. The existence of sediments as old as the Cretaceous in the basal strata of southern New Caledonia Trough has led most previous authors to suggest that the trough formed as a Cretaceous rift basin that subsequently underwent postrift subsidence. However, seismic reflection interpretations of a drowned shelf near Taranaki imply major tectonic subsidence (>2 km) of southern New Caledonia Trough since the Eocene. This subsidence, combined with structural and stratigraphic observations, has led to an alternate view that Eocene subduction zone initiation may have substantially modified water depth in New Caledonia Trough and been a primary cause of the first-order topography and basin structure observed today. Site U1507 was chosen to fill a knowledge gap in the northern and central New Caledonia Trough. The northern end of the trough is 1700 km from Site 206, and most tectonic models infer that the northern and central segments of New Caledonia Trough (i.e., north of Site 206) were proximal to where Tonga-Kermadec subduction initiated. Seismic reflection profiles reveal the sedimentary sequence to be relatively thin (suitable for drilling) in the central part of the trough and to contain at least three seismic units that can be related to the regional tectonic history. Site U1507 is located on the basin floor adjacent to Norfolk Ridge. The seabed gently slopes toward the axis of the trough, and swath bathymetry images reveal subtle relief associated with basin floor fans that are derived from incised canyons on the lower part of the Norfolk Ridge slope. Norfolk Ridge has a series of volcanic cones of late Oligocene age lying just west of the ridge crest, and one of them is close to Site U1507. A grid of seismic reflection lines was collected to locate the best site for drilling. The upper seismic unit comprises continuous or semicontinuous, moderate-amplitude reflections mixed with chaotic layers several kilometers across that are interpreted as debris flows. The middle seismic unit comprises semicontinuous, high-amplitude reflections in its upper part and moderate-amplitude, discontinuous reflectors in its lower part that downlap onto the lower unit. The lower unit comprises continuous or semicontinuous, moderate-amplitude reflectors that are locally folded and show fanning relationships. Small faults and apparent volcanic intrusions cut through the lower unit. The technical objective at Site U1507 was to sample all three units to sufficient depth to understand the Paleogene evolution of the New Caledonia Trough.