Seismic imaging reveals crust and upper mantle morphology of seamounts: Understanding the emplacement and evolution of the Louisville Ridge

Abstract:

Geophysical studies of submarine magmatic features have identified diversity in their crustal and upper mantle structure resulting in two end-members: the first showing magmatic underplating of high density material at the pre-existing Moho; the second characterised by less dense material being vertically intruded to shallow depths. The age of the underlying seafloor at the time of emplacement is thought to control the internal structure, as the vulnerability of the crust to magmatism may depend on thickness and temperature. The Louisville Ridge Seamount Chain (LRSC) intersects the Tonga-Kermadec subduction zone at ~26˚S. At this point the LRSC is located proximal to an extinct oceanic spreading center, with the underlying seafloor suggested to be ~10 Ma older than the seamounts.

Multichannel seismic (MCS) reflection and wide-angle (WA) refraction data were acquired along a series of profiles over the LRSC. The bathymetry makes seismic phase identification challenging due to a high degree of lateral variability and scattering. A forward ray-tracing approach, permitting testing of phase identifications, is applied to the refracted first arrivals to develop a crustal and upper mantle velocity model. Constraint on the positioning of upper crustal layers is provided by comparison of model boundaries with reflectors in the MCS data, and identification and modeling of secondary phases, e.g. Moho reflections.

We present velocity models used to establish the nature of the crustal and sub-crustal dynamic support of the LRSC, and whether it displays any along-ridge variation. Profiles acquired perpendicular to the LRSC enable us to investigate the three-dimensionality of structures. In particular, we seek to determine whether the seamount interior structure and 3-D geometry of the crust-mantle interface provide any insight into the controls on the emplacement of the LRSC, and how this fits within the range that has been suggested by studies at other seamount chains.