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We analyzed refraction and reflection seismic data covering the Central Lau Spreading Center (CLSC) and the Tonga volcanic arc at 18°S in the Pacific Ocean to investigate tectonic inactivity in actively extending back‐arc systems. Our P‐wave travel time tomography shows a characteristic 6–8 km thick back‐arc crust beneath and around the CLSC and a ∼13 km thick arc‐crust at the eastern end of our profile, which corresponds to the Tonga volcanic arc. Lower crustal velocities increase to 7.5 km/s toward the volcanic arc, marking the transition from back‐arc to arc crust. These high P‐wave velocities can be explained by a high pyroxene content in the lower crust originating from depleted melts. Our seismic reflection data show steep normal faults below the CLSC and volcanic structures closer to the volcanic arc, without a tectonically inactive zone (also knows as the "diffuse plate boundary") in between. Based on our results, we suggest that the Niuafo'ou and Tonga microplates should be treated as one tectonic plate with local zones of intra‐plate deformation that are separated by zones that are tectonically quiet. Plain Language Summary: In a subduction zone one tectonic plate sinks below another. A retreating downgoing plate causes tension in the overriding plate, which leads to thinning behind the volcanic arc: a back‐arc basin is born. We investigate the crustal structure of a seismically inactive region within a back‐arc basin in overall extension. Our aim is to understand why there is no seismic activity in this specific area. The Lau Basin is our case study, because it is a type‐example for back‐arc basin tectonics. We have acquired and interpreted seismic data to analyse the structure of the crust along a profile in the Lau Basin. We observe crustal thickness changes from typical back‐arc crust (6 and 7 km) to arc‐crust (13 km), and volcanic remnants in the crust with higher crustal velocities. Current tectonic activity only occurs at the Central Lau Spreading Center (CLSC), not in between the CLSC and the volcanic arc. We therefore reject the idea of a diffuse plate boundary, and propose that the Niuafo'ou and Tonga microplates should be treated as one tectonic plate that consists of localized zones that deform. In between those actively extending zones, there are areas that do not deform and that are tectonically quiet. Key Points: The crustal structure of the central Lau Basin at 18°S complies with typical back‐arc/arc crustal velocities and thicknessSouth of the FRSC no plate boundary is observed, suggesting one Niuafo'ou‐Tonga plate with zones of intraplate deformationHigh crustal P‐wave velocities result from the crystalisation of hydrous melts (upper crust) and pyroxene‐rich material (lower crust) [ABSTRACT FROM AUTHOR]

The O'Higgins Seamount Group is a cluster of volcanic domes located 120 km west of the central Chilean Trench on the crest of the Juan Fernandez Ridge. This aseismic hot spot track is subducting under South America triggering a belt of intraslab earthquake hypocenters extending about 700 km inland. The Juan Fernandez Ridge marks the southern boundary of a shallow subduction segment. Subduction of oceanic basement relief has been suggested as a cause for the 'flat' slab segments characterizing the Andean trench system. The Juan Fernandez Ridge, however, shows only moderate crustal thickening, inadequate to cause significant buoyancy. In 2001, wide-angle seismic data were collected along two perpendicular profiles crossing the O'Higgins Group. We present tomographic images of the volcanic edifices and adjacent outer rise-trench environment, which indicate a magmatic origin of the seamounts dominated by extrusive processes. High-resolution bathymetric data yield a detailed image of a network of syngenetic structures reactivated in the outer rise setting. A pervasive fault pattern restricted to the hot spot modified lithosphere coincides with anomalous low upper mantle velocities gained from a tomographic inversion of seismic mantle phases. Reduced uppermost mantle velocities are solely found underneath the Juan Fernandez Ridge and may indicate mineral alterations. Enhanced buoyancy due to crustal and upper mantle hydration may contribute an additional mechanism for shallow subduction, which prevails to the north after the southward migration of the Juan Fernandez Ridge. INDEX TERMS: 8010 Structural Geology: Fractures and faults; 8015 Structural Geology: Local crustal structure; 8105 Tectonophysics: Continental margins and sedimentary basins (1212); 8180 Tectonophysics: Tomography; 8414 Volcanology: Eruption mechanisms; KEYWORDS: seismic tomography, mantle hydration, aseismic ridge, central Chilean margin, hot spot emplacement.