Beniest, Anouk, Dannowski, Anke , Schnabel, M. and Kopp, Heidrun and the SO267 Scientist Party (2024) Tectonic Quiescence in Actively Extending Back‐Arc Regions. Journal of Geophysical Research: Solid Earth, 129 (9). Art.Nr.: e2024JB029236. DOI 10.1029/2024JB029236.

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Abstract

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.

Key Points

The crustal structure of the central Lau Basin at 18°S complies with typical back-arc/arc crustal velocities and thickness

South of the FRSC no plate boundary is observed, suggesting one Niuafo'ou-Tonga plate with zones of intraplate deformation

High crustal P-wave velocities result from the crystalisation of hydrous melts (upper crust) and pyroxene-rich material (lower crust)

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.

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