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New marine geophysical data recorded across the Tonga-Kermadec subduction zone are used to image deformation and seismic velocity structures of the forearc and Pacific Plate where the Louisville Ridge seamount chain subducts. Due to the obliquity of the Louisville Ridge to the trench and the fast 128 mm yr⁻¹ south–southwest migration of the ridge-trench collision zone, post-, current and pre-seamount subduction deformation can be investigated between 23°S and 28°S. We combine our interpretations from the collision zone with previous results from the post- and pre-collision zones to define the along-arc variation in deformation due to seamount subduction. In the pre-collision zone the lower-trench slope is steep, the mid-trench slope has ∼3-km-thick stratified sediments and gravitational collapse of the trench slope is associated with basal erosion by subducting horst and graben structures on the …