Formation and Rifting of backarc crust in the Lau Basin: First results of a recent seismic experiment

Several aspects of the formation and rifting of crust in backarc environments, like the Lau Basin in the SW Pacific, are still under question. A major unresolved question is, at what stage in the structural and thermal evolution of arc rifting does magmatism begin? The Fonualei Rift and Spreading Centre in Northeastern Lau Basin separates the Tonga Volcanic Arc from the Lau Backarc Basin and shows a strong gradient in the opening rate from South to North. The rift represents an ideal study site to investigate the formation and rifting of backarc crust at different stages of its tectonic and magmatic evolution. During the multidisciplinary cruise SO267 of RV Sonne (project ARCHIMEDES I), which sailed from Dec. 11, 2018 to Jan. 26, 2019, coincident seismic refraction and reflection profiles were acquired to image the deep crustal structure covering different sections of the Fonualei Rift and Spreading Centre. A total of 50 OBS from the GEOMAR pool were available for the refraction lines. Instruments were spaced at an average distance of 6 km and recorded arrivals from up 120 km offset. Seismic phases show little to no sediment cover on the seabed. Clear mantle arrivals (PmP and Pn) were recorded by the majority of stations and will allow the assessment of crustal thickness and upper mantle velocities. Additional geophysical data included gravity, magnetics and high-resolution bathymetry as well as Parasound and heatflow data, which were acquired along all profiles and will contribute to the tectonic interpretation of this complex region that is characterized by a number of microplates, active and abandoned spreading centres and transfer zones. This contribution will present an overview of the major scientific questions driving the ARCHIMEDES I project and present first results from the acquired seismic profiles, which aim to reveal the crustal architecture and the opening history of the Lau Basin.