Crustal structure and seismicity associated with seamount subduction: Preliminary results from the Tonga-Kermadec Trench - Louisville Ridge collision zone

The Tonga-Kermadec trench, which separates the subducting Pacific plate from the overthrusting Indo-Australian plate, is intersected at ~26° S by the Louisville Ridge seamount chain. The collision zone is characterized by a 3000 m reduction in trench depth, a 15° anticlockwise rotation of the trench axis, a 20 % reduction in the width of extensional bend faulting on the Pacific plate and a rough, hummocky, forearc on the Indo-Australian plate. These morphological characteristics are accompanied by a 40 % reduction in seismicity compared to regions immediately to the north and south. The influence of subducting seamounts on megathrust processes is not limited to their immediate vicinity and there is evidence of morphological and seismological manifestations in the forearc with wavelengths similar to the wavelengths of the flexural moats and bulges that flank these features. The Louisville Ridge collision zone has been the subject of three marine geophysical surveys conducted onboard R/V Sonne in 2004, 2007 and 2011. Swath bathymetry data were collected throughout all surveys and the collated dataset reveals a pronounced forearc high reaching a depth of 2700 m, located ~80 km and at an azimuth of 305° from Osbourn, the oldest known seamount within the Louisville chain. The bathymetric high correlates with a free-air gravity and magnetic anomaly high (50 mGal and 200 nT peaks respectively) and all three datasets reveal both a flanking depression, approximately one third of the amplitude of the peak, and an outer high. Within the region of reduced seismicity, a preliminary forward velocity model derived from strike-parallel wide-angle seismic data, suggest a north-south reduction in depth to the forearc Moho and a local crustal thickening in the vicinity of the high. We present here a preliminary analysis of new and existing seismic, swath and potential field datasets from the Louisville Ridge collision zone. In addition to the anomalous forearc structure introduced above, we discuss the role that subducting seamounts may play in controlling the seismological and structural segmentation of convergent plate boundaries.