Cenozoic volcanism of the Capel-Faust Basins, Lord Howe Rise, SW Pacific Ocean

Abstract

New bathymetry, geophysical data and samples were acquired in 2007 during a marine reconnaissance survey using the R.V. Tangaroa in the frontier Capel and Faust basins, Lord Howe Rise (LHR) by Geoscience Australia as part of the Australian Government’s Offshore Energy Security Program. This survey identified a number of volcanic features including cones, flows and sill-related features on the seafloor. Based on analysis of seismic data and swath bathymetry, there are at least two distinct ages of volcanism exposed on the seafloor; Late Miocene–Pliocene cones with a largely unmodified conical shape and Eocene–Oligocene volcanic features. The Middle Miocene Gifford Guyot, part of the Lord Howe seamount chain was included in the survey area. Volcanic features are common on the seafloor of the LHR and in the neighbouring Tasman Basin, with two identified north–south seamount hotspot chains to the west of the Capel-Faust region that have been active from the Miocene to recent. The previously completed ZoNéCo 5 swath-mapping, seismic and sampling survey also identified a few small conical seamounts on the northern LHR, along a roughly north-northwest trend, dated as Early Miocene. The younger Capel-Faust seamounts are aligned with these and the age relationships suggest they could be part of a single seamount chain. The older cones do not show any preferred alignment within the survey area and are too old to be related to the current hotspot activity in the region. Samples are variably altered but trace element abundances of all dredged mafic rocks indicate they are alkalic basalt. Samples from the younger cones can be differentiated from each other and from the Gifford Guyot on a plot of Ti, Sr and Tb/Yb reflecting different source depths or degrees of partial melting. A comparison of pyroxene from the older more altered cones and the younger cones shows that these most likely represent two distinct magmatic events and that the younger cones are similar to the ZoNéCo 5 seamounts.