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What process triggered the Mediterranean Sea restriction remains debated since the discovery of the Messinian Salinity Crisis (MSC). Recent hypotheses infer that the MSC initiated after the closure of the Atlantic-Mediterranean Betic and Rifean corridors, being modulated through restriction at the Gibraltar Strait. These hypotheses however, do not integrate contemporaneous speciation patterns of the faunal exchange between Iberia and Africa and several geological features like the evaporite distribution. Exchange of terrestrial biota occurred before, during and after the MSC, and speciation models support an exchange path across the East Alborán basin (EAB) located a few hundreds of km east of the Gibraltar Strait. Yet, a structure explaining jointly geological and biological observations has remained undiscovered. We present new seismic data showing the velocity structure of a well-differentiated 14–17-km thick volcanic arc in the EAB. Isostatic considerations support that the arc-crust buoyancy created an archipelago leading to a filter bridge across the EAB. Sub-aerial erosional unconformities and onlap relationships support that the arc was active between ~10–6 Ma. Progressive arc build-up leading to an archipelago and its later subsidence can explain the extended exchange of terrestrial biota between Iberia and Africa (~7–3 Ma), and agrees with patterns of biota speciation and terrestrial fossil distribution before the MSC (10–6.2 Ma). In this scenario, the West Alboran Basin (WAB) could then be the long-postulated open-marine refuge for the Mediterranean taxa that repopulated the Mediterranean after the MSC, connected to the deep restricted Mediterranean basin through a sill at the Alboran volcanic arc archipelago.

The southern Mariana subduction zone, home to the Challenger Deep—the deepest known point on Earth—poses significant challenges for studying the hydration of the subducting plate due to its extreme depth. This study uses S‐wave seismic tomography and Vp/Vs ratios to investigate hydration and serpentinization at the Challenger Deep. We observe a low Vp and Vs layer in the upper mantle with Vp/Vs ratios exceeding 1.8, reaching up to 1.95 at the Moho. These high ratios indicate a strong serpentinized layer (>15 vol%) with significant changes in the mechanical properties of the serpentinized peridotite. Additionally, Vp/Vs ratios in the crust and uppermost mantle increase from the outer rise to the trench axis, demonstrating that bending‐related faulting and hydration intensify as the plate approaches the trench. Our results suggest extensive faulting, hydration, and mantle serpentinization at the Challenger Deep, making this region an extreme example of water cycling in subduction zones. Plain Language Summary The southern Mariana Trench, containing the deepest point on Earth's surface, is where the old Pacific Plate (∼125 Ma) is subducting beneath the Philippine plate. Understanding the processes of bending‐related faulting and hydration of the incoming subducting plate has been challenging due to the limitations of using only P‐wave velocity (Vp), which does not provide detailed lithological information. In this study, we identified valuable converted S‐wave arrivals from the incoming plate, allowing us to determine the S‐wave velocity (Vs) structure and calculate the Vp/Vs ratios. Our results reveal that the low Vp layer in the upper mantle is a strongly serpentinized layer. Compared to other subduction zones, the combination of lower Vp and Vs values with higher Vp/Vs ratios suggests more intense serpentinization within the incoming plate at the southern Mariana subduction zone. This study provides a clearer understanding of mantle hydration processes in extreme subduction environments and highlights how plate characteristics influence serpentinization intensity. Key Points S‐wave tomography and Vp/Vs ratios reveal extensive serpentinization and hydration in the subducting plate of the southern Mariana Trench Vp/Vs ratios in the crust and uppermost mantle increase toward the trench axis, indicating intensified hydration as the plate approaching Challenger Deep is an extreme example of water cycling in subduction zones

The rupture behavior of large oceanic strike‐slip earthquakes remains largely unresolved using seismic signals recorded thousands of kilometers away from the source area. Large submarine earthquakes, however, generate hydroacoustic T‐waves propagating through the ocean over long distances. Here, we show that these T‐waves recorded at regional distances on the Ascension hydrophone array of the International Monitoring System can provide critical information on the earthquake location and rupture behavior. We use recordings from 47 events in oceanic transform faults, ranging in magnitude from 5.6 ≤ Mw ≤ 7.1, to investigate the rupture processes. We find that most strike‐slip earthquakes show unilateral rupture behavior, while a few larger events were more complex. Furthermore, earthquakes in oceanic transforms have longer ruptures than events of the same magnitude in continental faults. We argue that differences in the scaling relation of oceanic and continental strike‐slip earthquakes support a lower rigidity in the oceanic lithosphere caused by hydration. Plain Language Summary Oceanic transform faults are strike‐slip faults where one plate moves past another laterally, with new seafloor created at adjacent mid‐ocean ridge segments on either side of the transform. At the transforms, plate motion generates strong earthquakes, causing seismic waves to propagate for thousands of kilometers. Nevertheless, due to their remoteness the rupture behavior of oceanic earthquakes and their scaling relationship between magnitude and rupture length is poorly constrained and understood. Here, we use hydroacoustic signals, so‐called T‐waves, which are excited by seismic deformation of the seafloor. T‐waves are readily observable at the International Monitoring System hydrophone triplet near Ascension Island, Atlantic Ocean. We present a new relationship between the magnitude of equatorial Atlantic strike‐slip earthquakes and the rupture length from 47 events (MW 5.6–7.1). We found that oceanic earthquakes differ from continental ones, showing longer ruptures for the same magnitude, suggesting that oceanic transform faults are weak. Key Points Hydroacoustic T‐waves from 47 oceanic transform fault earthquakes with Mw 5.6–7.1 were recorded by the IMS Ascension hydrophone array Our results show that hydroacoustic energy provides precise constraints on the rupture of strong earthquakes in oceanic transform faults Ruptures of oceanic strike‐slip earthquakes are longer than for continental events of the same magnitude, indicating lower rigidity

Oceanic crust forms at mid-ocean spreading centres through a combination of magmatic and tectonic processes, with the magmatic processes creating two distinct layers: the upper and the lower crust. While the upper crust is known to form from lava flows and basaltic dykes based on geophysical and drilling results, the formation of the gabbroic lower crust is still debated. Here we perform a full waveform inversion of wide-angle seismic data from relatively young (7–12-Myr-old) crust formed at the slow-spreading Mid-Atlantic Ridge. The seismic velocity model reveals alternating, 400–500 m thick, high- and low-velocity layers with ±200 m s −1 velocity variations, below ~2 km from the oceanic basement. The uppermost low-velocity layer is consistent with hydrothermal alteration, defining the base of extensive hydrothermal circulation near the ridge axis. The underlying layering supports that the lower crust is formed through the intrusion of melt as sills at different depths, which cool and crystallize in situ. The layering extends up to 5–15 km distance along the seismic profile, covering 300,000–800,000 years, suggesting that this form of lower crustal accretion is a stable process. The lower oceanic crust forms through the accretion of injected melt that cools and crystallizes in situ over hundreds of thousands of years, according to seismic data from the slow-spreading equatorial Mid-Atlantic Ridge.

Crustal properties of young oceanic lithosphere have been examined extensively, but the nature of the mantle lithosphere underneath remains elusive. Using a novel wide-angle seismic imaging technique, here we show the presence of two sub-horizontal reflections at ∼11 and ∼14.5 km below the seafloor over the 0.51–2.67 Ma old Juan de Fuca Plate. We find that the observed reflectors originate from 300–600-m-thick layers, with an ∼7–8% drop in P-wave velocity. They could be explained either by the presence of partially molten sills or frozen gabbroic sills. If partially molten, the shallower sill would define the base of a thin lithosphere with the constant thickness (11 km), requiring the presence of a mantle thermal anomaly extending up to 2.67 Ma. In contrast, if these reflections were frozen melt sills, they would imply the presence of thick young oceanic lithosphere (20–25 km), and extremely heterogeneous upper mantle. Applying seismic imaging methods on ocean bottom hydrophone data, the authors here describe a horizontal, flat lithosphere base plus lithosphere-asthenosphere boundary beneath the young (0.51 to 2.67 Ma) Juan de Fuca plate.

Constraining the controlling factors of fault rupture is fundamentally important. Fluids influence earthquake locations and magnitudes, although the exact pathways through the lithosphere are not well-known. Ocean transform faults are ideal for studying faults and fluid pathways given their relative simplicity. We analyse seismicity recorded by the Passive Imaging of the Lithosphere-Asthenosphere Boundary (PI-LAB) experiment, centred around the Chain Fracture Zone. We find earthquakes beneath morphological transpressional features occur deeper than the brittle-ductile transition predicted by simple thermal models, but elsewhere occur shallower. These features are characterised by multiple parallel fault segments and step overs, higher proportions of smaller events, gaps in large historical earthquakes, and seismic velocity structures consistent with hydrothermal alteration. Therefore, broader fault damage zones preferentially facilitate fluid transport. This cools the mantle and reduces the potential for large earthquakes at localized barriers that divide the transform into shorter asperity regions, limiting earthquake magnitudes on the transform. Geophysical data from Chain Transform Fault reveal that broad damage zones preferentially facilitate fluid transport that cools the mantle, increasing earthquake depths. Fluids weaken the fault and segment it, limiting earthquake magnitudes.

The updip limit of seismic rupture during a megathrust earthquake exerts a major control on the size of the resulting tsunami. Offshore Northern Chile, the 2014 Mw 8.1 Iquique earthquake ruptured the plate boundary between 19.5° and 21°S. Rupture terminated under the mid-continental slope and did not propagate updip to the trench. Here, we use state-of-the-art seismic reflection data to investigate the tectonic setting associated with the apparent updip arrest of rupture propagation at 15 km depth during the Iquique earthquake. We document a spatial correspondence between the rupture area and the seismic reflectivity of the plate boundary. North and updip of the rupture area, a coherent, highly reflective plate boundary indicates excess fluid pressure, which may prevent the accumulation of elastic strain. In contrast, the rupture area is characterized by the absence of plate boundary reflectivity, which suggests low fluid pressure that results in stress accumulation and thus controls the extent of earthquake rupture. Generalizing these results, seismic reflection data can provide insights into the physical state of the shallow plate boundary and help to assess the potential for future shallow rupture in the absence of direct measurements of interplate deformation from most outermost forearc slopes. The rupture area of the 2014 Iquique earthquake offshore northern Chile was spatially limited to a region where the plate boundary is non-reflective in seismic images, indicative of low fluid pressure. In contrast, north and updip of the rupture area, a coherent highly reflective plate boundary indicates excess fluid pressure, which may inhibit strain accumulation, while strain release in the non-reflective rupture area occurs during large earthquakes.

Based on a compilation of published and new seismic refraction and multichannel seismic reflection data along the south central Chile margin (33°–46°S), we study the processes of sediment accretion and subduction and their implications on megathrust seismicity. In terms of the frontal accretionary prism (FAP) size, the marine south central Chile fore arc can be divided in two main segments: (1) the Maule segment (south of the Juan Fernández Ridge and north of the Mocha block) characterized by a relative large FAP (20–40 km wide) and (2) the Chiloé segment (south of the Mocha block and north of the Nazca‐Antarctic‐South America plates junction) characterized by a small FAP (≤10 km wide). In addition, the Maule and Chiloé segments correlate with a thin (<1 km thick) and thick (∼1.5 km thick) subduction channel, respectively. The Mocha block lies between ∼37.5° and 40°S and is configured by the Chile trench, Mocha and Valdivia fracture zones. This region separates young (0–25 Ma) oceanic lithosphere in the south from old (30–35 Ma) oceanic lithosphere in the north, and it represents a fundamental tectonic boundary separating two different styles of sediment accretion and subduction, respectively. A process responsible for this segmentation could be related to differences in initial angles of subduction which in turn depend on the amplitude of the down‐deflected oceanic lithosphere under trench sediment loading. On the other hand, a small FAP along the Chiloé segment is coincident with the rupture area of the trans‐Pacific tsunamigenic 1960 earthquake (Mw = 9.5), while a relatively large FAP along the Maule segment is coincident with the rupture area of the 2010 earthquake (Mw = 8.8). Differences in earthquake and tsunami magnitudes between these events can be explained in terms of the FAP size along the Chiloé and Maule segments that control the location of the updip limit of the seismogenic zone. The rupture area of the 1960 event also correlates with a thick subduction channel (Chiloé segment) that may provide enough smoothness at the subduction interface allowing long lateral earthquake rupture propagation.

Large continental faults extend for thousands of kilometres to form boundaries between rigid tectonic blocks. These faults are associated with prominent topographic features and can produce large earthquakes. Here we show the first evidence of a major tectonic structure in its initial-stage, the Al-Idrissi Fault System (AIFS), in the Alboran Sea. Combining bathymetric and seismic reflection data, together with seismological analyses of the 2016 M w 6.4 earthquake offshore Morocco – the largest event ever recorded in the area – we unveil a 3D geometry for the AIFS. We report evidence of left-lateral strike-slip displacement, characterise the fault segmentation and demonstrate that AIFS is the source of the 2016 events. The occurrence of the M w 6.4 earthquake together with historical and instrumental events supports that the AIFS is currently growing through propagation and linkage of its segments. Thus, the AIFS provides a unique model of the inception and growth of a young plate boundary fault system. The Al-Idrissi Fault System in the Alboran Sea is a major tectonic structure in its initial stage. By using bathymetric and seismic reflection data, the authors unravel a 3D geometry for the AIFS, which corresponds to a crustal-scale boundary and provides a unique model of the inception and growth of a young plate boundary fault system.

We present the first detailed 2D seismic tomographic image of the trench‐outer rise, fore‐ and back‐arc of the Tonga subduction zone. The study area is located approximately 100 km north of the collision between the Louisville hot spot track and the overriding Indo‐Australian plate where ∼80 Ma old oceanic Pacific plate subducts at the Tonga Trench. In the outer rise region, the upper oceanic plate is pervasively fractured and most likely hydrated as demonstrated by extensional bending‐related faults, anomalously large horst and graben structures, and a reduction of both crustal and mantle velocities. The 2D velocity model presented shows uppermost mantle velocities of ∼7.3 km/s, ∼10% lower than typical for mantle peridotite (∼30% mantle serpentinization). In the model, Tonga arc crust ranges between 7 and 20 km in thickness, and velocities are typical of arc‐type igneous basement with uppermost and lowermost crustal velocities of ∼3.5 and ∼7.1 km/s, respectively. Beneath the inner trench slope, however, the presence of a low velocity zone (4.0–5.5 km/s) suggests that the outer fore‐arc is probably fluid‐saturated, metamorphosed and disaggregated by fracturing as a consequence of frontal and basal erosion. Tectonic erosion has, most likely, been accelerated by the subduction of the Louisville Ridge, causing crustal thinning and subsidence of the outer fore‐arc. Extension in the outer fore‐arc is evidenced by (1) trenchward‐dipping normal faults and (2) the presence of a giant scarp (∼2 km offset and several hundred kilometers long) indicating gravitational collapse of the outermost fore‐arc block. In addition, the contact between the subducting slab and the overriding arc crust is only 20 km wide, and the mantle wedge is characterized by low velocities of ∼7.5 km/s, suggesting upper mantle serpentinization or the presence of melts frozen in the mantle. Key Points Mantle hydration of subducting and upper plates, subduction erosion and arc magmatism