Seamount structure and subduction at the Louisville Ridge – Tonga-Kermadec collision

Abstract

The Louisville Ridge (LRSC) is an ~4000 km-long SW Pacific seamount chain currently being subducted at the Tonga-Kermadec Trench (TKT). Both the trend of the LRSC and the subduction of the Pacific plate are oblique to the trench, resulting in southward migration of the intersection point at a rate of 120-180 mm yr-1, and producing significant along-strike variation in forearc structure and seismicity.
The LRSC-TKT intersection was investigated by a multi-disciplinary geophysical experiment aboard the R/V Sonne in 2011, acquiring multichannel and wide-angle seismic, gravity, and bathymetry data, to better understand the effect of subducting bathymetric features on forearc deformation. As part of this, it is necessary to determine the structure and characteristics of the incoming seamounts, and how they are deformed during subduction. This study is underpinned by an ~725 km-long profile traversing the oldest extant LRSC seamounts, that continues along its projection into the trench and forearc.
LRSC seamounts display a range of internal structures, including shallow, high-velocity (≥6 km s-1) cores. The also sit on crust that is not significantly thickened. At the trench, Osbourn seamount is experiencing bend-induced normal faulting which suggests that each seamount may be disarticulated to a size smaller than the imaging resolution. Observed similarity between the P-wave velocity structure of seamount flank material and ordinary subducting oceanic crust also suggests that distinguishing between these in the trench-forearc region is challenging. Consequently, it is not unequivocally possible to determine, within the confines of the profile locations and model resolution, whether the last LRSC seamount to have subducted, was subducted intact along the continuation of the trend of the extant chain.
Along-margin observations indicate that significant seamount-related forearc deformation is superimposed on pre-existing crustal structures, with the maximum deformation occurring in the wake of the migration of active collision. Observations of forearc morphology at the present-day intersection point support those from seismicity and plate reconstruction, which suggests that this location may also coincide with a westerly rotation in the trend of the chain.