The Structure of the Continent-Ocean Transition in the Gulf of Lions From Joint Refraction and Reflection Travel-Time Tomography

20 pages, 12 figures, supporting information https://doi.org/10.1029/2021JB021711.-- Data Availability Statement: The authors do not use any new data. All data come from previously published sources, and are available through Gailler et al. (2009) and Moulin et al. (2015). The bathymetric grid is freely available as a .xyz file from the website of the EMODnet (https://www.emodnet-bathymetry.eu/). This is a contribution of the Barcelona Center for Subsurface Imaging that is a Grup de Recerca 2017 SGR 1662 de la Generalitat de CatalunyaThe Gulf of Lions—Ligurian basin, in the Western Mediterranean, opened from the end of Oligocene to the Miocene as a back-arc basin driven by rollback of the Ionian slab. Geophysical surveys across the margin have explored the crustal structure and the continent-ocean transition (COT), but its location and petrological nature is still a matter of debate. Here, we apply joint refraction and reflection travel-time tomography that combines travel-times from multichannel seismic and wide-angle seismic data to provide improved constraints on the P-wave velocity structure of the sediment cover and the geometry of the top of the basement with respect to previous studies. Similar to earlier works, the velocity model shows three crustal domains, but their geometry and internal velocity differ. We identify a ∼100 km-wide domain of oceanic crust with an anomalously high upper crustal velocity possibly caused by porosity-canceling processes triggered by 7–8 km thick overlying sediment cover. The vertical velocity structure and the thickness of the oceanic crust are similar to back-arcs in the Pacific and the Western Mediterranean. In contrast to previous interpretations, we propose that this oceanic domain is confined between two domains of ultra-thin (4–5 km-thick) continental crust, and that the COT is