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Crustal structure provides the key to understand the interplay of magmatism and tectonism, while oceanic crust is constructed at Mid-Ocean Ridges (MORs). At slow spreading rates, magmatic processes dominate central areas of MOR segments, whereas segment ends are highly tectonized. The TAMMAR segment at the Mid-Atlantic Ridge (MAR) between 21°25'N and 22°N is a magmatically active segment. At ~4.5 Ma this segment started to propagate south, causing the termination of the transform fault at 21°40'N. This stopped long-lived detachment faulting and caused the migration of the ridge offset to the south. Here a segment center with a high magmatic budget has replaced a transform fault region with limited magma supply. We present results from seismic refraction profiles that mapped the crustal structure across the ridge crest of the TAMMAR segment. Seismic data yield crustal structure changes at the segment center as a function of melt supply. Seismic Layer 3 underwent profound changes in thickness and became rapidly thicker ~5 Ma. This correlates with the observed “Bull's Eye” gravimetric anomaly in that region. Our observations support a temporal change from thick lithosphere with oceanic core complex formation and transform faulting to thin lithosphere with focused mantle upwelling and segment growth. Temporal changes in crustal construction are connected to variations in the underlying mantle. We propose that there is a link between the neighboring segments at a larger scale within the asthenosphere, to form a long, highly magmatically active macrosegment, here called the TAMMAR-Kane Macrosegment. Seismic data confirm ridge propagation induced by increased melt supply due to increased fertility or upwelling of the asthenosphereSource of melt supply is migrating, ended a transform fault, and possibly stopped OCC formationThe growth direction of the enhanced melt supply region is influenced by the distance to “colder” mantle zones

The West Iberia margin is the focus of intense research since the 1980s, with some of the most exemplary geophysical cross‐sections and drilling expeditions. Those data sets have been used to create conceptual models of rifting used as a template to interpret margins worldwide. We present two collocated ∼350 km long lines of multi‐channel seismic (MCS) streamer data and wide‐angle seismic (WAS) data collected across the Tagus Abyssal Plain (TAP). We use travel‐times of first arrivals identified at WAS and reflected seismic phases identified at both WAS and MCS records to jointly invert for the Pwave velocity (Vp) distribution and the geometry of a sediment unconformity, the top of the basement, and the Moho boundary. The Vp model shows that the TAP basement is more complex than previously inferred, presenting abrupt boundaries between five domains. Domain I under the foot of the slope and Domain III under the abyssal plain display Vp values and gradients of thin continental crust. In between, Domain II displays a steep Vp gradient and high Vp values at shallow depth that support that basement is made of exhumed partly serpentinized mantle. Domain IV and Domain V, further oceanward, have oceanic crust Vp structure. The new results support an unanticipated complex rift history during the initial separation of Iberia and America. We propose a geodynamic scenario characterized by two phases of extension separated by a jump of the locus of extension, caused by the northward propagation of the oceanic spreading center during the J‐anomaly formation, which terminated continental rifting. The underground offshore of Western Iberia has been studied since the early 1980s to understand the mechanism that formed the North Atlantic Ocean around 200 million years ago. Some of the geophysical experiments carried on in this area have been used to created conceptual models that explain the opening of this region. Here, we present two new geophysical data sets to explore the Tagus Abyssal Plain (TAP), located in front of south Portugal, and to understand how the ocean opened up in this region. The integration of these two data sets allows us to create a seismic velocity model which decreases the uncertainty on the final interpretation regarding the origin of the TAP. The model from our study reveals that the TAP structure is more complex than previously proposed. We explain the new configuration by making the hypothesis that the ocean opens in two distinct and independent phases of extension. This is in contrast to the classical theories that states that the opening of this ocean took place progressively, in a unique episode of extension. The first Vp crustal model from joint travel‐time tomography of streamer and ocean bottom seismometer records of the Tagus Abyssal PlainDetermination of the extent of two continental domains, an intervening exhumed mantle domain, and two distinct oceanic basement domainsOpening occurred in two extension episodes: rifting leading to mantle exhumation and a seafloor spreading with the formation of oceanic crust The first Vp crustal model from joint travel‐time tomography of streamer and ocean bottom seismometer records of the Tagus Abyssal Plain Determination of the extent of two continental domains, an intervening exhumed mantle domain, and two distinct oceanic basement domains Opening occurred in two extension episodes: rifting leading to mantle exhumation and a seafloor spreading with the formation of oceanic crust

The 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 <10 km wide on either side of the oceanic crust. The Gulf of Lions and Ligurian Basin formed during the opening of the Mediterranean Sea 30–20 Ma ago. Understanding the geology of the crust along these basins is key to infer the formation processes that governed the opening of the Western Mediterranean, and more generally, the evolution of Earth's crust. Many studies have explored this region with geophysical data but the crustal structure along these basins is yet debated. For the first time, here, we integrate two seismic data sets to explore the crustal structure along the Gulf of Lions and Ligurian Basin. The integration of these two data sets allows us to decrease the uncertainty on the final interpretation regarding the extent and petrological nature of crustal domains. Our results reveal that the oceanic crust is present offshore the Gulf of Lions, but in contrast to earlier studies, it is confined between continental crust. These results redefine the configuration of crustal domains in this region of the Western Mediterranean, showing that the continent‐ocean transition is spatially sharper than previously thought, implying that the process of oceanization was brief in space and time during the opening of this portion of the Mediterranean basins. We interpret a confined lens‐shaped oceanic domain in between highly stretched continental crust challenging previous interpretationsThe oceanic crust shows similarities in vertical velocity structure and thickness with oceanic back‐arcs in the Pacific and MediterraneanOur results imply that the continent‐ocean transition occurs abruptly in less than 10 km at each side of the oceanic domain We interpret a confined lens‐shaped oceanic domain in between highly stretched continental crust challenging previous interpretations The oceanic crust shows similarities in vertical velocity structure and thickness with oceanic back‐arcs in the Pacific and Mediterranean Our results imply that the continent‐ocean transition occurs abruptly in less than 10 km at each side of the oceanic domain