Description: At the Australian-Pacific plate boundary, the northern Lau Basin is one of the fastest opening back-arc basins on earth. The current configuration of micro-plates, plate boundaries and motions within the northern Lau Basin is quite well understood, but in the southern part of the Lau Basin questions remain about the crustal structure. Here, the Central Lau Spreading Center (CLSC) and the southern tip of the Fonualei Rift and Spreading Center (FRSC) define the diffuse southern boundary of the Niuafo’ou microplate. It remains unclear where the southern plate boundary is located and what kind of boundary it is.We present 1) seismic refraction data of a 200-km long, E-W transect acquired in the transition zone from the eastern side of the CLSC to the southern tip of the FRSC and 2) seismic reflection data of four E-W profiles of varying length, acquired in both the southern part of the Niuafo’ou microplate and the transition in between the CLSC and the FRSC. The seismic data acquisition was accompanied by parametric sediment echosounder, gravimetric and magnetic measurements and was complemented by heat flow probes and dredged samples of the seafloor in the vicinity of the profile.Our travel time tomography reveals a pronounced lateral variation in seismic P-wave velocities from west to east, within the 7-8 km thick back-arc crust. Towards the east, the crust gradually thickens to 13 km of arc crust. The reflection seismic data reveals sediment pockets, varying between 300m to 1000m depth, located on both the thinner back-arc crust and thicker arc crust. In the abyssal regions, faults that cross-cut the basement, but do not reach the surface, are observed on all reflection seismic profiles and are considered inactive today. Towards the west of the profiles, faults reach the surface and are considered active. Rock sampling from this area retrieved predominantly massive aphyric basalts from the back-arc crust in the west. Olivine-rich basalts, andesites, and a broad spectrum of volcaniclastic rocks are the most common rock-type collected from the arc crust in the east.The lack of a thinner crust near the southern tip of the FRSC, the presence of inactive faults that cross-cut the basement, and the presence of active faults in the CLSC suggest that the southern plate boundary of the Niuafo’ou microplate accommodated extension in a wide-rift tectonic setting in the past. Today, this extension is accommodated in the CLSC in a narrow extensional tectonic setting.

Description: The Lau Basin is a young back-arc basin steadily forming at the Indo-Australian-Pacific plate boundary, where the Pacific plate is subducting underneath the Australian plate along the Tonga-Kermadec island arc. As a result of the asymmetric roll back of the Pacific plate, the Lau basin’s divergence rates decline southwards hence dictating an asymmetric, V-shaped basin opening. Further, the decentralisation of the extensional motion over 11 distinct spreading centres and zones of active rifting has led to the formation of a composite crust formed of a microplate mosaic. One of these centres of extensional motion, and the subject of this study, are two overlapping spreading centres (OLSC), the southern tip of the eastern axis of the Mangatolu Triple Junction (MTJ-S) and the northern tip of the Fonualei Rift spreading centre (FRSC).In 2018, the research vessel Sonne (cruise SO267) set out to conduct seismic refraction and wide-angle reflection data along a 185 km long transect crossing the Lau Basinat ~16 °S from the Tonga arc in the east, the overlapping spreading centres, FRSC1 and MTJ-S2, and extending as far as a volcanic ridge in the west. Additionally, 2D MCS reflection seismic data as well as magnetic and gravimetric data were acquired.The results of our Monte-Carlo P-wave traveltime tomography show a crust that varies between 4.5-6 km in thickness. Underneath the OLSC the upper crust is 2-2.5 km thick and the lower crust 2-2.5 km thick. The velocity gradients of the upper and lower crust differ significantly from tomographic models of magmatically dominated oceanic ridges. Compared to such magmatically dominated ridges, our final P-wave velocity model displays a decreased velocity gradient in the upper crust and an increased velocity gradientin the lower crust more comparable to tectonically dominated rifts with a sparse magmatic budget.The dominance of crustal stretching in the regional rifting process leads to a tectonical stretching, thus thinning of the crust under the OLSC and thereforeincreasing the lower crust’s velocity gradient. Due to the limited magmatic budget of the area, neither the magnetic anomaly nor the gravity data indicate a magmatically dominated spreading centre. We conclude that extension in the Lau Basin at the OLSC at 16 °S is dominated by extensional processes with little magmatism, which is supported by the distribution of seismic events concentrated at the northern tip of the FRSC.