A multimethodical approach to decipher extensional processes in the northern Lau Basin at 16 ºS

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 cru