Crustal Structure and Flexural Characteristics of the Louisville Ridge and Tonga-Kermadec Subduction System

Abstract:

Pacific oceanic lithosphere has been subducting along the Tonga-Kermadec Trench for at least 28 Myr. The Louisville Ridge Seamount Chain (LRSC) collides with this subduction system at ~26°S and morphologically separates the Tonga Trench and forearc in the north from the Kermadec Trench and forearc in the south. This collision also results in a shallowing of the trench and a shortening of the forearc, primarily due to a region of enhanced subduction erosion. In addition, the southward migration of the LRSC at 180 mm yr^-1 causes the forearc of the overriding plate north of the collision zone to shallow by ~2 km and exhibit increased faulting while, south of the collision zone, the overriding and underthrusting plates are undergoing processes and exhibiting structures and the morphology expected of normal Pacific plate subduction.

In 2011, a suite of geophysical data was acquired to image crustal and upper mantle structure across the LRSC and Kermadec forearc with the aim of: a) establishing the degree of seamount isostatic compensation and the nature of crustal and sub-crustal support at the northwestern end of the LRSC; b) resolving the flexural state of the overriding and underthrusting plates in regions of normal subduction and at the point of LRSC-collision; and from these c) determining the primary influences on the flexural behaviour and nature of deformation in the underthrusting and overriding plates along the Tonga-Kermadec subduction system.

From the 2011 acquisition, we present swath bathymetry data, multichannel seismic (MCS) reflection profiles, and an initial wide-angle (WA) refraction seismic model across the LRSC and the Kermadec forearc, south of the present-day collision zone, which have been interpreted to reveal the mode and extent of deformation on-going in the forearc prior to and after LRSC collision. We also show how structures on the Kermadec forearc evolve due to normal Pacific plate subduction, and demonstrate how the post-collision lithosphere of the overriding plate recovers after individual seamounts of the LRSC are subducted.