Geology and Structure of the Southern End of the South Fiji Basin, SW Pacific Ocean

The opening of the South Fiji Basin (SFB) in relation to the Norfolk Basin, and the eruptive activity and migration of the Three Kings Ridge (TKR) are long-standing Cenozoic SW Pacific tectonic problems. Seven research cruises in the past 9 years by New Zealand, France and Australia have collected multichannel seismic, multibeam bathymetry, and dredge samples in the Norfolk to southern SFB region, providing a wealth of new data, most of which has yet to reach the mainstream literature. Results presented here focus on the data gathered close to New Zealand. The Northland Plateau, at the southern end of the SFB, is a large, NW trending terrace that forms the largest feature of the northern New Zealand continental slope. It appears to contain tectonic elements of the Three Kings Ridge and Norfolk Basin as well as the SFB. It is divided longitudinally into an inner sedimentary basin and an outer volcanic rise, generally matched by inner negative magnetic and gravity anomalies, and outer positive ones. On the southeastern side of the TKR, we have found NNW trending normal fault terraces, from which we have dredged Oligocene (32 Ma) basaltic andesite; evidence of a rift margin with the SFB. On the west side of the TKR, N-S trending terraces are also extensional; peridotite, and a spectrum of subduction related rocks including BABB, boninite and shoshonite has been reported from them. Terraces are absent on the Northland Plateau but linear gravity trends that correspond with the terraces flanking the TKR appear to merge with NW gravity trends on the Northland Plateau. Some NW gravity trends are due to the Vening Meinesz transform, which cuts the S end of the TKR and appears to continue southeastwards under the continental slope. Within the extended region, ultramafics are suspected in an (undredged) horst trending towards the northern tip of NZ, where such peridotite crops out, possibly linking the onshore ophiolite to the western TKR suite. Schist and granite, outcropping further southeast along the plateau, have been attributed to a continental margin metamorphic core complex with 10 km of exhumation between 23-21 Ma. The presence of a transform in this region introduces the possibility of strike slip exhumation. Paleodepths from fossils indicate 500 m to more than 1500 m of Early Miocene subsidence in the extended Northland Plateau terrains, and 2500 m of subsidence on the western Three Kings terraces facing the Norfolk Basin. Dredged lavas, exclusively Early Miocene calc-alkaline rocks, including both low-med-K basaltic andesite and shoshonite, link the eruptive history of the outer half of the Northland Plateau to that of the Northland Arc and to the Early Miocene phase of the Three Kings arc. Strong positive magnetic anomaly trends suggest that the Northland Plateau volcanics might be continuous only with the TKR volcanics, but also suggest that SFB magma may have contributed to the plateau. The large negative magnetic anomaly of the inner Northland Plateau is radically different from those of the adjacent Norfolk Basin. Its lateral extent coincides with that of the Northland Allochthon and may represent the parautochthon, which disappears under and is also possibly faulted against the Early Miocene outer plateau volcanics.