Tectonic Quiescence in Actively Extending Back-Arc Regions

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.