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Oceanic transform faults are seismically and tectonically active plate boundaries.sup.1 that leave scars--known as fracture zones--on oceanic plates that can cross entire ocean basins.sup.2. Current descriptions of plate tectonics assume transform faults to be conservative two-dimensional strike-slip boundaries.sup.1,3, at which lithosphere is neither created nor destroyed and along which the lithosphere cools and deepens as a function of the age of the plate.sup.4. However, a recent compilation of high-resolution multibeam bathymetric data from 41 oceanic transform faults and their associated fracture zones that covers all possible spreading rates shows that this assumption is incorrect. Here we show that the seafloor along transform faults is systemically deeper (by up to 1.6 kilometres) than their associated fracture zones, in contrast to expectations based on plate-cooling arguments. Accretion at intersections between oceanic ridges and transform faults seems to be strongly asymmetric: the outside corners of the intersections show shallower relief and more extensive magmatism, whereas the inside corners have deep nodal basins and seem to be magmatically starved. Three-dimensional viscoplastic numerical models show that plastic-shear failure within the deformation zone around the transform fault results in the plate boundary experiencing increasingly oblique shear at increasing depths below the seafloor. This results in extension around the inside corner, which thins the crust and lithosphere at the transform fault and is linked to deepening of the seafloor along the transform fault. Bathymetric data suggest that the thinned transform-fault crust is augmented by a second stage of magmatism as the transform fault intersects the opposing ridge axis. This makes accretion at transform-fault systems a two-stage process, fundamentally different from accretion elsewhere along mid-ocean ridges. Oceanic transform faults are systemically deeper than their associated fracture zones, owing to the plate boundary experiencing increasingly oblique shear at increasing depths below the seafloor., Author(s): Ingo Grevemeyer [sup.1] , Lars H. Rüpke [sup.1] , Jason P. Morgan [sup.2] , Karthik Iyer [sup.1] [sup.3] , Colin W. Devey [sup.1] Author Affiliations: (1) GEOMAR Helmholtz Centre [...]

Arc‐backarc systems are inherently shaped by subduction, representing an essential window into processes acting in the Earth's interior such as the recycling of subducted slabs. Furthermore, they are setting where new crust is formed and are believed to be sites where juvenile continental crust emerges. We present a seismic refraction and wide‐angle velocity model across the Izu arc‐backarc system, and use its characteristic features to constrain geochemically and petrologically different compartments, revealing processes governing crustal formation overlying subduction zones. Our result delineates the Izu arc with a maximum thickness of ∼20 km and the Shikoku Basin with thicknesses of ∼7 to 11 km. In the volcanic arc, the middle crust of the felsic to intermediate tonalitic layer (6.0–6.5 km/s) is remarkably thicker beneath the basalt‐dominated area than in the rhyolite‐dominated area, indicating that basaltic volcanism is indispensable in the transformation process from arc to continental crust. However, rhyolitic volcanism may relate to the juvenile stage of arc evolution or the remelting of middle crust due to the insufficient supply of basaltic magma from the mantle. The mafic restite and cumulates, which used to be part of the arc crustal material, are delaminated and foundered into the mantle, forming extremely low mantle velocities (<7.5 km/s). In the Shikoku Basin, our result supports a fertile mantle source with passive upwelling and normal temperature during the opening process, but the lack of high velocity in the lower crust rules out hydrous melts entrained from the subducting slab or anomalous mantle trapped during subduction zone reconfiguration. Plain Language Summary: As a vital factor in supporting the conditions for the evolution of life and ecosystems, the origin and evolution of the continents are still enigmatic. Volcanic arcs are generally seen as a place for creating continental crust while recycling the incoming subducting slab. In this study, we present a seismic velocity structure model across the Izu arc and Shikoku Basin, offshore south of Japan, to demonstrate the rules contained behind the transformation from arc to continental crust. Our results support that basaltic volcanism in the volcanic arc nurtures the generation of felsic to intermediate rocks, which provides the bulk of the continental crust. During this process, other anti‐continent materials, like mafic rocks, tend to be foundered into the mantle. Therefore, we propose that constant basaltic volcanism is critical in transferring arc crust to continental crust. Key Points: A long seismic refraction and wide‐angle profile presents the seismic structure across the Izu arc and Shikoku BasinThe transformation from arc to continental crust is closely associated with basaltic volcanism from the rear arc to volcanic frontPassive melting of a fertile mantle source under normal temperature governs the opening of the Shikoku Basin [ABSTRACT FROM AUTHOR]

Until now, few offshore seismic studies have acquired simultaneously P‐ and S‐ wave data to derive in detail the seismic structure of the oceanic crust. We present 2‐D Vp and Vs models using wide‐angle seismic data at the Indian basin adjacent to the NinetyEast Ridge. Here, an outcrop basement located at the middle of the seismic line presents uppermost crustal Poisson's ratios (ν) of 0.28–0.29 (Vp ∼ 4.2 km/s and Vs ∼ 2.3 km/s). At the flanks of the outcrop basement, the sediment cover is 200–300 m thick and ν values are similar (0.28–0.3), but Vp and Vs values are higher (4.5–4.8 and 2.4–2.6 km/s, respectively). We interpret the relatively lower Vp and Vs around the basement outcrop in terms of hydrothermal alteration, while at the flanks of the basement outcrop, hydrothermal alteration has most likely ceased by sedimentation and compaction processes. Across the seismic layer 2, the Vp–Vs trend is linear and follows a ν value of 0.28–0.29, however, at the seismic layer 2/3 transition, the Vp–Vs trend abruptly changes following a ν value of 0.25–0.26. These reduced observed ν values at the layer 2/3 transition are lower than those reported by laboratory measurements for gabbro (ν ∼ 0.293) and are interpreted in terms of epidotization at the dike‐gabbro contact and/or crack‐change properties around the lower part of the intrusive sheeted dike section. Plain Language Summary: The Ninety(90°)East(E) Ridge (NER) is a hotspot track formed by the Kerguelen hotspot mantle plume onto the Indo‐Australian plate more than 60 Myr ago. The length and width of the NER is about 5,000 and 200 km, respectively, and it extends from the Bengal bay to the southeast Indian mid ocean ridge. West and east of the NER are sited the Indian and Wharton basin, respectively. Here, we study the structure of the Indian basin at the western flank of the NER where the seafloor is highlighted by the presence of prominent oceanic basement outcrops. Our seismic results suggest that mineral alteration processes occur both in the upper and lower crust, respectively, under a basement outcrop due to the contact of seawater with crustal rocks. Probably, crustal accretion in a setting characterized by the interaction of a hotspot mantle plume with an active spreading center affect the porosity and permeability of the ocean crust around the NER region at large‐scale. Key Points: We obtain 2‐D Vp and Vs models from active seismic data for the Indian oceanic crustThe seismic models suggest hydrothermal alteration near a basement outcropPoisson's ratios change at the layer 2/3 transition from 0.28–0.29 to 0.25–0.26 [ABSTRACT FROM AUTHOR]

At the Blanco transform fault system (BTFS) off Oregon, 138 local earthquakes and 84 double‐couple focal mechanisms from ocean‐bottom‐seismometer recordings jointly discussed with bathymetric features reveal a highly segmented transform system without any prominent fracture zone traces longer than 100 km. In the west, seismicity is focused at deep troughs (i.e., the West and East Blanco, and Surveyor Depressions). In the east, the BTFS lacks a characteristic transform valley and instead developed the Blanco Ridge, which is the most seismically active feature, showing strike‐slip and dip‐slip faulting. Sandwiched between the two main segments of the BTFS is the Cascadia Depression, representing a short intra‐transform spreading segment. Seismic slip vectors reveal that stresses at the eastern BTFS are roughly in line with plate motion. In contrast, stresses to the west are clockwise skewed, indicating ongoing reorganization of the OTF system. As we observed no prominent fracture zones at the BTFS, plate tectonic reconstructions suggest that the BTFS developed from non‐transform offsets rather than pre‐existing transform faults during a series of ridge propagation events. Our observations suggest that the BTFS can be divided into two oceanic transform systems. The eastern BTFS is suggested to be a mature transform plate boundary since ∼0.6 Ma. In contrast, the western BTFS is an immature transform system, which is still evolving to accommodate far‐field stress change. The BTFS acts as a natural laboratory to yield processes governing the development of oceanic transform faults. Plain Language Summary: The Blanco transform fault system (BTFS) northwest off the coast of Oregon is seismically very active. We used 1 year of ocean bottom seismometer data collected between September 2012 and October 2013 to locate 138 local earthquakes. The events align perfectly with the morphologic features of the BTFS, dividing the BTFS into five transform segments and two short intra‐transform spreading centers. Furthermore, we observe different seismotectonic behaviors of the western and eastern BTFS based on the along‐strike variation in morphology, magnetization, focal depth distribution, and strain partitioning. Although many segmented oceanic transform systems were formed from a single transform fault in response to rotations in plate motion, the BTFS turns out to be originated from non‐transform offsets between ridge segments, as we observed no prominent fracture zone traces neither in morphology nor gravity field data. A clockwise shift in the Juan de Fuca/Pacific pole of rotation at ∼5 Ma followed by a series of ridge propagation events initiated the formation of the BTFS, integrated each segment of the BTFS by shortening the ridge segments in between. Our observations suggest that the Blanco Ridge and the Gorda transform segment in the eastern BTFS were formed at ∼1.6 and 0.6 Ma, respectively, and ever since, the eastern BTFS became a mature transform boundary. In contrast, seismic slip vectors comparing to plate motion directions reveal that stresses in the western BTFS are systematically skewed, suggesting the immature transform plate boundary is still adjusting to the new stress regime. Key Points: Local seismicity of the Blanco transform fault system (BTFS) reveals along‐strike variations dominated by strike‐slip and oblique dip‐slipThe BTFS developed from non‐transform offsets rather than discrete transform faults in response to plate rotation and ridge propagationThe BTFS consists of a mature plate boundary in the east and an immature system in the west, separated by a central spreading center [ABSTRACT FROM AUTHOR]

[1] Based on a compilation of published and new seismic refraction and multichannel seismic reflection data along the south central Chile margin (33[degrees]-46[degrees]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 Fernandez Ridge and north of the Mocha block) characterized by a relative large FAP (20-40 km wide) and (2) the Chilo6 segment (south of the Mocha block and north of the Nazca-Antarctic-South America plates junction) characterized by a small FAP ([less than or equal to]10 km wide). In addition, the Maule and Chiloe segments correlate with a thin (

The VP/VS ratio is an important property for understanding magmatic and tectonic processes at passive continental margins as it is an indicator of the crustal composition. To classify the dominant lithologies in the Zhongsha Block, South China Sea (SCS), we present a detailed VP/VS crustal model based on the independent tomographic inversion of P wave and S wave data. The average VP/VS in the crust of the Zhongsha Block is ∼1.77, indicating an overall felsic to intermediate composition lacking remnant magmatic intrusive rocks. The VP‐density relationship from gravity modeling suggests that the lower crust of the extended continental domain contains more greenschist and hence may have experienced metamorphism resulting from an elevated geotherm in the Northwest Sub‐basin either during the syn‐spreading or postspreading stage. The variability of the VP/VS ratio in the continental block is larger than that in the oceanic basin, showing distinct crustal properties. Several low VP/VS ratio anomalies (VP/VS < 1.7) were found near tectonic boundaries and are interpreted to either result from felsic metamorphism during an interval of rifting, or during the migration of magma along faults and cracks in the postrift period. VP/VS ratios occurring in concert with high VP anomalies in the continent‐ocean transition zone support a mafic composition of metapelitic granulite, which was either formed by magmatic intrusions or contact with mantle melting that stem from the upwelling of the asthenospheric mantle during the initial break‐up and onset of the seafloor spreading stage in the SCS. Plain Language Summary: The composition of offshore continental crust is a mystery due to the difficulty of sampling rocks from depths of several kilometers below the seafloor. However, this information is crucial to understanding the geological processes that shaped our planet. In this study, we use refraction and wide‐angle seismic data to obtain P wave, and S wave velocity structures, and calculate ratios between the VP and VS across the Zhongsha Block in the South China Sea. Density values derived from gravity modeling are used as an additional constraint. The VP/VS ratio in the continental Zhongsha Block ranges from <1.7 to 1.88, while the oceanic Northwest Sub‐basin yields a more compact ratio of ∼1.81, revealing distinct differences between continental and oceanic crust. Furthermore, our results reveal that the rock type of the continental crust was affected by the magmatism related to the adjacent seafloor spreading activities, leading to a larger degree of alteration of the lower crust from the continental to oceanic domain. Low VP/VS anomalies along the tectonic boundaries of the Zhongsha Block trace historical magma upwelling channels. Detailed analysis of the rock type in passive continental margin provides a comprehensive interpretation of the evolution process from rifting to post‐spreading stage. Key Points: A combined analysis of VP/VS ratios and densities from gravity modeling provides lithological constraints on the Zhongsha BlockThe extended continental crust shows an unusual VP‐density relationship, indicating metamorphic overprinting of the lower crustLow VP/VS ratio anomalies occur within the continental crust and are interpreted as felsic metamorphic rocks and/or felsic dikes [ABSTRACT FROM AUTHOR]

Conceptual models of magma-poor rifting are strongly based on studies of the nature of the basement in the continent-to-ocean transition of the Iberia Abyssal Plain, and suggest that exhumed mantle abuts extended continental crust. Yet, basement has only been sampled at a few sites, and its regional nature and the transition to seafloor spreading inferred from relatively low-resolution geophysical data are inadequately constrained. This uncertainty has led to a debate about the subcontinental or seafloor-spreading origin of exhumed mantle and the rift-related or oceanic nature of magmatic crust causing the magnetic J anomaly. Different interpretations change the locus of break-up by >100 km and lead to debate of the causative processes. We present the tomographic velocity structure along a 360-km-long seismic profile centered at the J anomaly in the Iberia Abyssal Plain. Rather than delineating an excessive outpouring of magma, the J anomaly occurs over subdued basement. Furthermore, its thin crust shows the characteristic layering of oceanic crust and is juxtaposed to exhumed mantle, marking the onset of magma-starved seafloor spreading, which yields the westward limit of an ~160-km-wide continent-ocean transition zone where continental mantle has been unroofed. This zone is profoundly asymmetric with respect to its conjugate margin, suggesting that the majority of mantle exhumation occurs off Iberia. Because the J anomaly is related to the final break-up and emplacement of oceanic crust, it neither represents synrift magmatism nor defines an isochron, and hence it poorly constrains plate tectonic reconstructions. [ABSTRACT FROM AUTHOR]

[1] We combine structural balancing with thermal and strength-envelope analysis of the Cascadia accretionary wedge to determine the influence thermal gradient has on the structure of the prism. BSR-derived heat flow in the Cascadia accretionary margin decreases from 90-110 mW/[m.sup.2] at the deformation front to 45-70 mW/[m.sup.2] in the upper slope. Extension of the thermal gradient to the top of the oceanic crust shows that the base of the prism reaches temperatures between 150-200[degrees]C and 250-300[degrees]C at the deformation front and the base of the upper slope, respectively. This high thermal gradient favors the development of a vertical strain gradient, which is accommodated by heterogeneous deformation of the accretionary prism. This process produces two overlying thrust wedges, a basal duplex and an overlying landward- or seaward-vergent imbricate stack. The thermal structure also influences the deformation distribution and structural style along the shortening direction. Initiation of plastic deformation at the base of the prism below the Cascadia upper slope affects the wedge geometry, changing its taper angle and favoring the development of a midcrustal duplex structure that propagates seaward as a dynamic backstop. Uplift related with this underplating process is accompanied with deep incision of submarine canyons, sliding and normal faulting in the upper slope. Heterogeneous deformation accommodated by the development of transfer faults separating landward-vergent from seaward-vergent domains is also observed along the margin. Landward-vergent areas accommodate 30-40% shortening at the front of the wedge, while in the narrower and thicker seaward-vergent segments shortening occurs mostly by underplating below the upper slope.

Oceanic transform faults (OTFs) are an inherent part of seafloor spreading and plate tectonics, whereas the process controlling their morphology remains enigmatic. Here, we systematically quantify variations in transform morphology and their dependence on spreading rate and age‐offset, based on a compilation of shipborne bathymetric data from 94 OTFs at ultraslow‐ to intermediate‐spreading ridges. In general, the length, width and depth of OTFs scale systematically better with age‐offset rather than spreading rate. This observation supports recent geodynamic models proposing that cross‐transform extension scaling with age‐offset, is a key process of transform dynamics. On the global scale, OTFs with larger age‐offsets tend to have longer, wider, and deeper valleys. However, at small age‐offsets (<5 Myr), scatters in the depth and width of OTFs increase, indicating that small age‐offset OTFs with weak lithospheric strength are easily affected by secondary tectonic processes. Plain Language Summary: In the past 5 decades, studies on oceanic transform faults (OTFs) have revealed significant complexity in their morphology, which calls for detailed quantitative analysis to study the processes controlling the morphology of OTFs. Using the most complete and advanced compilation of bathymetric data from ultraslow‐ to intermediate‐spreading ridges, we parameterized the morphological characteristics of OTFs and extracted length, width and depth for each transform fault from the compiled bathymetric data. Moreover, correlations between these morphological parameters and related tectonic factors (e.g., spreading rate, age‐offset) were investigated in this study. We find that correlations between morphological features and spreading rate are rather weak. Comparison of correlations suggests that age‐offset scales better with the morphological parameters, along with scatters mostly at small age‐offsets, indicating small‐age‐offset OTFs are unstable due to their weak lithospheric strength. Our observation evidences extensional tectonics at OTFs. Key Points: We compiled multibeam bathymetric data of 94 oceanic transform faults (OTFs) to quantify their morphological characteristicsMorphology of OTFs is dominated by age‐offset rather than spreading rateTransform valleys get systematically deeper and wider with increasing age‐offset, implying extensional tectonics at OTFs [ABSTRACT FROM AUTHOR]

The Liguro-Provençal basin was formed as a back-arc basin of the retreating Calabrian–Apennines subduction zone during the Oligocene and Miocene. The resulting rotation of the Corsica–Sardinia block is associated with rifting, shaping the Ligurian Basin. It is still debated whether oceanic or atypical oceanic crust was formed or if the crust is continental and experienced extreme thinning during the opening of the basin. We perform ambient noise tomography, also taking into account teleseismic events, using an amphibious network of seismic stations, including 22 broadband ocean bottom seismometers (OBSs), to investigate the lithospheric structure of the Ligurian Basin. The instruments were installed in the Ligurian Basin for 8 months between June 2017 and February 2018 as part of the AlpArray seismic network. Because of additional noise sources in the ocean, OBS data are rarely used for ambient noise studies. However, we carefully pre-process the data, including corrections for instrument tilt and seafloor compliance and excluding higher modes of the ambient-noise Rayleigh waves. We calculate daily cross-correlation functions for the AlpArray OBS array and surrounding land stations. We also correlate short time windows that include teleseismic earthquakes, allowing us to derive surface wave group velocities for longer periods than using ambient noise only. We obtain group velocity maps by inverting Green's functions derived from the cross-correlation of ambient noise and teleseismic events, respectively. We then used the resulting 3D group velocity information to calculate 1D depth inversions for S-wave velocities. The group velocity and shear-wave velocity results compare well to existing large-scale studies that partly include the study area. In onshore France, we observe a high-velocity area beneath the Argentera Massif, roughly 10 km below sea level. We interpret this as the root of the Argentera Massif. Our results add spatial resolution to known seismic velocities in the Ligurian Basin, thereby augmenting existing seismic profiles. In agreement with existing seismic studies, our shear-wave velocity maps indicate a deepening of the Moho from 12 km at the south-western basin centre to 20–25 km at the Ligurian coast in the north-east and over 30 km at the Provençal coast. The maps also indicate that the south-western and north-eastern Ligurian Basin are structurally separate. The lack of high crustal vP/vS ratios beneath the south-western part of the Ligurian Basin preclude mantle serpentinisation there. [ABSTRACT FROM AUTHOR]

The northern margin of the Ligurian Basin shows notable seismicity at the Alpine front, including frequent magnitude 4 events. Seismicity decreases offshore towards the Basin centre and Corsica, revealing a diffuse distribution of low-magnitude earthquakes. We analyse data of the amphibious AlpArray seismic network with focus on the offshore component, the AlpArray ocean bottom seismometer (OBS) network, consisting of 24 broadband OBSs deployed for 8 months, to reveal the seismicity and depth distribution of micro-earthquakes beneath the Ligurian Sea. Two clusters occurred between ∼ 10 km to ∼ 16 km depth below the sea surface, within the lower crust and uppermost mantle. Thrust faulting focal mechanisms indicate compression and an inversion of the Ligurian Basin, which is an abandoned Oligocene–Miocene rift basin. The basin inversion is suggested to be related to the Africa–Europe plate convergence. The locations and focal mechanisms of seismicity suggest reactivation of pre-existing rift-related structures. Slightly different striking directions of presumed rift-related faults in the basin centre compared to faults further east and hence away from the rift basin may reflect the counter-clockwise rotation of the Corsica–Sardinia block. High mantle S-wave velocities and a low Vp/Vs ratio support the hypothesis of strengthening of crust and uppermost mantle during the Oligocene–Miocene rifting-related extension and thinning of continental crust. [ABSTRACT FROM AUTHOR]

Plate tectonics characterize transform faults as conservative plate boundaries where the lithosphere is neither created nor destroyed. In the Atlantic, both transform faults and their inactive traces, fracture zones, are interpreted to be structurally heterogeneous, representing thin, intensely fractured, and hydrothermally altered basaltic crust overlying serpentinized mantle. This view, however, has recently been challenged. Instead, transform zone crust might be magmatically augmented at ridge‐transform intersections before becoming a fracture zone. Here, we present constraints on the structure of oceanic crust from seismic refraction and wide‐angle data obtained along and across the St. Paul fracture zone near 18°W in the equatorial Atlantic Ocean. Most notably, both crust along the fracture zone and away from it shows an almost uniform thickness of 5–6 km, closely resembling normal oceanic crust. Further, a well‐defined upper mantle refraction branch supports a normal mantle velocity of 8 km/s along the fracture zone valley. Therefore, the St. Paul fracture zone reflects magmatically accreted crust instead of the anomalous hydrated lithosphere. Little variation in crustal thickness and velocity structure along a 200 km long section across the fracture zone suggests that distance to a transform fault had negligible impact on crustal accretion. Alternatively, it could also indicate that a second phase of magmatic accretion at the proximal ridge‐transform intersection overprinted features of starved magma supply occurring along the St. Paul transform fault. Plain Language Summary: Transform faults represent plate boundaries where two plates move past each other without producing new or destroying the existing lithosphere. Most of the Atlantic transform faults and their inactive traces, fracture zones, were characterized by fractured and altered, thin‐crust overlying serpentinized mantle rocks. However, recent results reveal that the crust beneath fracture zones may not be as thin and challenge the standard view, introducing a mechanism of secondary magma supply at the intersection between the ridge axis and transform fault. Here, we present results from seismic experiments at the St. Paul fracture zone near 18°W in the equatorial Atlantic. Our results suggest that the subsurface of the St. Paul fracture zone is represented by a nearly uniform crustal thickness of 5–6 km and an upper mantle with a velocity of 8 km/s. Both observations argue for a crust of magmatic origin and the absence of strong alteration of the upper mantle. Collectively, constant crustal thickness and little variation in seismic velocities along the profile crossing the fracture zone suggest that the crustal formation process does not vary as a function of distance from the fracture zone. Alternatively, secondary magma supply at the ridge‐transform intersection could overprint any anomalous formation conditions. Key Points: Seismic structure along the St. Paul fracture zone reflects magmatically accreted oceanic crustOceanic crust across St. Paul shows only small thickness variations, lacking evidence for regional crustal thinning near fracture zonesMagmatic nature of crust supports a mechanism where transform crust is augmented before being turned into a fracture zone [ABSTRACT FROM AUTHOR]

At non‐volcanic passive continental margins, seismic techniques often failed to uniquely define the nature of crustal domains. Here, we overcome this problem by studying the structure and composition of the continent‐ocean transition (COT) in the Southwest Sub‐basin of the South China Sea, using P and S wave seismic tomography and Vp/Vs ratios, providing unique constraints on lithology. Throughout the image domain, we can rule out large areas of exhumed mantle as Vp/Vs ratios are always <1.9 in the shallow basement layer. Instead, the COT is characterized by extended and fragmented continental crust, and possibly mafic aggregation at the bottom of the crust. In concert with observations from multichannel seismic reflection data, seismic velocities and Vp/Vs ratios suggest that the oldest oceanic crust was formed by starved magmatism, causing rugged basement, thin crust, nearly absent lower crust, and moderately serpentinized mantle below. Our results reveal that rifting occurred without un‐roofing continental mantle. Plain Language Summary: Traditional P wave velocity is not able to reveal the detailed rock types in the crust or lithosphere, which are crucial for analyzing the structures and geodynamics in non‐volcanic continental margins. In this study, we choose the South China Sea as a case study area, using an effective method to image the Vp/Vs ratio, unraveling the rock type distribution of basalt, gabbro/diabase, serpentinized peridotite in our model from top to bottom. We analyzed the variation of the crustal nature and composition from the continent‐ocean transition zone (COT) to the oceanic basin, revealing that the COT is characterized by magmatic intruded continental crust, while the oldest oceanic domain shows a thin oceanic crust overlying mantle that experienced hydration. Our study provides an integrated seismic study which may become a paradigmatic case study to investigate the blurred area of the COT. Key Points: Vp/Vs ratio obtained by seismic tomography from OBS data is a sensitive tool to constrain lithologies in oceanic basinsLow Vp/Vs ratios in the South China Sea rule out the occurrence of a wide continent‐ocean transition zone of exhumed mantleA magmatic‐affected continent‐ocean transition zone transferring to a thin oceanic domain was revealed [ABSTRACT FROM AUTHOR]

We present seismic tomographic results from a unique seismic refraction and wide‐angle survey along a 600 km long flow‐line corridor of oceanic lithosphere ranging in age from 0 to 27 Ma in the equatorial Atlantic Ocean at 2° 43′S. The velocities in the crust near the ridge axis rapidly increase in the first 6 Myr and then change gradually with age. The upper crust (Layer 2) thickness varies between 2 and 2.4 km with an average thickness of 2.2 km and the crustal thickness varies from 5.6 to 6 km along the profile with an average crustal thickness of 5.8 km. At some locations, we observe negative velocity anomalies (∼−0.3 km/s) in the lower crust which could be either due to chemical heterogeneity in gabbroic rocks and/or the effects of fault related deformation zones leading to an increase in porosities up to 1.6% depending on the pore/crack geometry. The existence of a low velocity anomaly beneath the ridge axis suggests the presence of partial melt (∼1.3%) in the lower crust. Upper mantle velocities also remain low (∼7.8 km/s) from ridge axis up to 5 Ma, indicating a high temperature regime associated with mantle melting zone underneath. These results suggest that the evolution of the crust and uppermost mantle at this location occur in the first 10 Ma of its formation and then remains unchanged. Most of the structures in the older crust and upper mantle are fossilized structures and could provide information about past processes at ocean spreading centers. Plain Language Summary: Oceanic crust emplaced along mid‐ocean ridges shows significant variations in the crustal thickness and velocity structures that might be controlled by spreading rate, aging or changes in ridge crest segmentation. Here, we have analyzed seismic refraction data along a 600 km long profile covering 0–27 Ma to study the evolution of crust and upper mantle in the equatorial Atlantic ocean. The seismic velocities in the crust rapidly increase up to 6 Myr and remain fairly constant up to 27 Myr. Crustal thickness varies between 5.6 and 6 km due to the heterogeneous accretion of crust at slow‐spreading ridges. At the ridge‐axis, low seismic velocities in the mid‐to‐lower crust indicate higher temperatures greater than 1,200°C and the presence of partial melt. Our results suggest that most of the evolution in the crust and upper mantle occurs in the first 10 Myr and the structures at older ages beyond 10 Myr could shed light on the history of the past spreading processes and tectonics at mid‐ocean ridges. Key Points: Crustal seismic velocities increase rapidly up to 5–6 Ma with local negative velocity anomalies in older crust attributed to inactive faultsCrustal thickness varies between 5.6 and 6 km along the profile with variations due to heterogeneous accretion at slow‐spreading ridgesLow seismic velocities (<6.5 km/s) in the lower crust beneath the ridge‐axis suggest elevated temperatures and presence of partial melt [ABSTRACT FROM AUTHOR]

The aftershock distribution of the 2014 Mw 8.1 Iquique earthquake offshore northern Chile, identified from a long‐term deployment of ocean bottom seismometers installed eight months after the mainshock, in conjunction with seismic reflection imaging, provides insights into the processes regulating the updip limit of coseismic rupture propagation. Aftershocks updip of the mainshock hypocenter frequently occur in the upper plate and are associated with normal faults identified from seismic reflection data. We propose that aftershock seismicity near the plate boundary documents subduction erosion that removes mass from the base of the wedge and results in normal faulting in the upper plate. The combination of very little or no sediment accretion and subduction erosion over millions of years has resulted in a very weak and aseismic frontal wedge. Our observations thus link the shallow subduction zone seismicity to subduction erosion processes that control the evolution of the overriding plate. Plain Language Summary: To better understand the controls on shallow seismicity and subduction erosion following large subduction earthquakes, we use marine recordings of the Mw 8.1 2014 Iquique earthquake aftershocks and long‐offset multi‐channel seismic data. By comparing the aftershock locations and seismic imaging, we observe that most aftershocks occurred in the upper continental plate and abruptly stopped in the frontal forearc. The amplitude characteristics of upper‐crust reflections indicate a fractured and fluid‐filled outer forearc, which is associated with the absence of aftershocks. Large‐scale faulting, as evidenced by disrupted reflections in the seismic image, can be correlated to upper plate seismicity. We propose that the aftershocks updip of the main earthquake area reflect active subduction erosion processes. Key Points: We investigate structure and seismicity at the updip end of the 2014 Iquique earthquake rupture using amphibious seismic dataSeismicity updip of the 2014 Iquique earthquake occurs over a broad range likely interpreted to be related to the basal erosion processesCoseismic stress changes and aftershocks activate extensional faulting of the upper plate and subduction erosion [ABSTRACT FROM AUTHOR]

The Liguro-Provençal basin was formed as a back-arc basin of the retreating Calabrian-Apennines subduction zone during the Oligocene and Miocene. The resulting rotation of the Corsica-Sardinia block is associated with rifting, shaping the Ligurian Sea. It is still debated whether oceanic or atypical oceanic crust was formed or if the crust is continental and experienced extreme thinning during the opening of the basin. We invert velocity models using an amphibious network of seismic stations, including 22 broadband Ocean Bottom Seismometers (OBS) to investigate the lithospheric structure of the Ligurian sea. The instruments were installed in the Ligurian Sea for eight months between June 2017 and February 2018 as part of the AlpArray seismic network. Because of additional noise sources in the ocean, OBS data are rarely used for ambient noise studies. However, we attentively pre-process the data, including corrections for instrument tilt and seafloor compliance. We took extra care to exclude higher modes of the ambient-noise Rayleigh waves. We calculate daily cross-correlation functions for the LOBSTER array and surrounding land stations. Additionally, we correlate short time windows that include teleseismic earthquakes that allow us to derive surface wave group velocities for longer periods than using ambient noise only. Group velocity maps are obtained by inverting Green's functions derived from the cross-correlation of ambient noise and teleseismic events, respectively. We then used the resulting 3D group velocity information to calculate 1D depth inversions for S-wave velocities. The shear-wave velocity results show a deepening of the Moho from 12 km at the southwestern basin centre to 20-25 km at the Ligurian coast in the northeast and over 30 km at the Provençal coast. We find no hint on mantle serpentinisation and no evidence for an Alpine slab, at least down to depths of 25 km. However, we see a separation of the southwestern and northeastern Ligurian Basin that coincides with the promoted prolongation of the Alpine front. [ABSTRACT FROM AUTHOR]

Subduction zones may develop submarine spreading centers that occur on the overriding plate behind the volcanic arc. In these back-arc settings, the subducting slab controls the pattern of mantle advection and may entrain hydrous melts from the volcanic arc or slab into the melting region of the spreading ridge. We recorded seismic data across the Western Mariana Ridge (WMR, northwestern Pacific Ocean), a remnant island arc with back-arc basins on either side. Its margins and both basins show distinctly different crustal structure. Crust to the west of the WMR, in the Parece Vela Basin, is 4-5 km thick, and the lower crust indicates seismic P-wave velocities of 6.5-6.8 km/s. To the east of the WMR, in the Mariana Trough Basin, the crust is ~7 km thick, and the lower crust supports seismic velocities of 7.2-7.4 km/s. This structural diversity is corroborated by seismic data from other back-arc basins, arguing that a chemically diverse and heterogeneous mantle, which may differ from a normal mid-ocean-ridge-type mantle source, controls the amount of melting in back-arc basins. Mantle heterogeneity might not be solely controlled by entrainment of hydrous melt, but also by cold or depleted mantle invading the back-arc while a subduction zone reconfigures. Crust formed in back-arc basins may therefore differ in thickness and velocity structure from normal oceanic crust. [ABSTRACT FROM AUTHOR]

Seismic full waveform inversion (FWI) is a powerful method for estimating quantitative subsurface physical parameters from seismic data. As the FWI is a nonlinear problem, the linearized approach updates model iteratively from an initial model, which can get trapped in local minima. In the presence of a high-velocity contrast, such as at Moho, the reflection coefficient and recorded waveforms from wide-aperture seismic acquisition are extremely nonlinear around critical angles. The problem at the Moho is further complicated by the interference of lower crustal (Pg) and upper mantle (Pn) turning ray arrivals with the critically reflected Moho arrivals (PmP). In order to determine velocity structure near Moho, a nonlinear method should be used. We propose to solve this strong nonlinear FWI problem at Moho using a trans-dimensional Markov chain Monte Carlo (MCMC) method, where the earth model between lower crust and upper mantle is ideally parametrized with a 1-D assumption using a variable number of velocity interfaces. Different from common MCMC methods that require determining the number of unknown as a fixed prior before inversion, trans-dimensional MCMC allows the flexibility for an automatic estimation of both the model complexity (e.g. the number of velocity interfaces) and the velocity–depth structure from the data. We first test the algorithm on synthetic data using four representative Moho models and then apply to an ocean bottom seismometer (OBS) data from the Mid-Atlantic Ocean. A 2-D finite-difference solution of an acoustic wave equation is used for data simulation at each iteration of MCMC search, for taking into account the lateral heterogeneities in the upper crust, which is constrained from traveltime tomography and is kept unchanged during inversion; the 1-D model parametrization near Moho enables an efficient search of the trans-dimensional model space. Inversion results indicate that, with very little prior and the wide-aperture seismograms, the trans-dimensional FWI method is able to infer the posterior distribution of both the number of velocity interfaces and the velocity–depth model for a strong nonlinear problem, making the inversion a complete data-driven process. The distribution of interface matches the velocity discontinuities. We find that the Moho in the study area is a transition zone of 0.7 km, or a sharp boundary with velocities from around 7 km s−1 in the lower crust to 8 km s−1 of the upper mantle; both provide nearly identical waveform match for the field data. The ambiguity comes from the resolution limit of the band-limited seismic data and limited offset range for PmP arrivals. [ABSTRACT FROM AUTHOR]

Oceanic transform faults and fracture zones (FZs) represent major bathymetric features that keep the records of past and present strike‐slip motion along conservative plate boundaries. Although they play an important role in ridge segmentation and evolution of the lithosphere, their structural characteristics, and their variation in space and time, are poorly understood. To address some of the unknowns, we conducted interdisciplinary geophysical studies in the equatorial Atlantic Ocean, the region where some of the most prominent transform discontinuities have been developing. Here we present the results of the data analysis in the vicinity of the Chain FZ, on the South American Plate. The crustal structure across the Chain FZ, at the contact between ∼10 and 24 Ma oceanic lithosphere, is sampled along seismic reflection and refraction profiles. We observe that the crustal thickness within and across the Chain FZ ranges from ∼4.6–5.9 km, which compares with the observations reported for slow‐slipping transform discontinuities globally. We attribute this presence of close to normal oceanic crustal thickness within FZs to the mechanism of lateral dike propagation, previously considered to be valid only in fast‐slipping environments. Furthermore, the combination of our results with other data sets enabled us to extend the observations to morphotectonic characteristics on a regional scale. Our broader view suggests that the formation of the transverse ridge is closely associated with a global plate reorientation that was also responsible for the propagation and for shaping lower‐order Mid‐Atlantic Ridge segmentation around the equator. Key Points: Fracture zones are represented by close to normal crustal thickness (∼5 km) that we attribute to the mechanism of lateral dike propagationMajor reorganization in plate motion represents a predominant factor in building transverse ridge at the Chain Fracture ZoneThe analysis of our seismic data sets and interdisciplinary observations define lower‐order tectonomagmatic segmentation of the ridge axis [ABSTRACT FROM AUTHOR]

In continental settings, seismic failure is generally restricted to crustal depth. Crustal structure is therefore an important proxy to evaluate seismic hazard of continental fault systems. Here we present a seismic velocity model across the Gibraltar Arc System, from the Eurasian Betics Range (South Iberian margin), across offshore East Alboran and Pytheas (African margin) basins, and ending onshore in North Morocco. Our results reveal the nature and configuration of the crust supporting the coexistence of three different crustal domains: the continental crust of the Betics, the continental crust of the Pytheas Basin (south Alboran Basin) and onshore Morocco, and a distinct domain formed of magmatic arc crust under the East Alboran Basin. The magmatic arc under the East Alboran Basin is characterized by a velocity structure containing a relatively high‐velocity lower crust (~7 km/s) bounded at the top and base by reflections. The lateral extension of this crust is mapped integrating a second perpendicular wide‐angle seismic profile along the Eastern Alboran basin, together with basement samples, multibeam bathymetry, and a grid of deep‐penetrating multichannel seismic profiles. The transition between crustal domains is currently unrelated to extensional and magmatic processes that formed the basin. The abrupt transition zones between the different crustal domains support that they are bounded by crustal‐scale active fault systems that reactivate inherited structures. Seismicity in the area is constrained to upper‐middle crust depths, and most earthquakes nucleate outside of the magmatic arc domain. Key Points: New velocity model reveals the lithospheric structure under the Betics (South Iberia), the Alboran Basin, and the North African marginThe East Alboran Basin is floored by magmatic arc crust, while the southern area of the Alboran Basin is floored by continental crustSeismic activity is constrained to the upper‐middle continental crust; crustal domains are likely bounded by active faults [ABSTRACT FROM AUTHOR]

Monowai is an active submarine volcanic centre in the Kermadec Arc, Southwest Pacific Ocean. Multi-beam data acquired during expedition SO225 aboard R/V SONNE in December 2012 indicates that the topography of the main stratocone has evolved significantly since the last survey in June 2011. Bathymetric measurements of the edifice reveal differences of up to 42 m in seafloor depth and indicate a net volume increase of ∼0.037 km3 across the summit area. Explosive volcanism observed onsite during the SO225 mapping campaign could be linked to a 20h-long swarm of unusually coherent T phase arrivals, suggesting that Monowai is a prime source of broadband seismic noise in the Southwest Pacific region during times of activity. Our findings further document the dynamic nature of volcanic processes at Monowai and have implications for future expedition planning. [ABSTRACT FROM AUTHOR]

The Ligurian Basin is located in the Mediterranean Sea to the north-west of Corsica at the transition from the Western Alpine orogen to the Apennine system and was generated by the south-eastward trench retreat of the Apennines–Calabrian subduction zone. Late-Oligocene-to-Miocene rifting caused continental extension and subsidence, leading to the opening of the basin. Yet it remains unclear if rifting caused continental break-up and seafloor spreading. To reveal its lithospheric architecture, we acquired a 130 km long seismic refraction and wide-angle reflection profile in the Ligurian Basin. The seismic line was recorded in the framework of SPP2017 4D-MB, a Priority Programme of the German Research Foundation (DFG) and the German component of the European AlpArray initiative, and trends in a NE–SW direction at the centre of the Ligurian Basin, roughly parallel to the French coastline. The seismic data were recorded on the newly developed GEOLOG recorder, designed at GEOMAR, and are dominated by sedimentary refractions and show mantle Pn arrivals at offsets of up to 70 km and a very prominent wide-angle Mohorovičić discontinuity (Moho) reflection. The main features share several characteristics (e.g. offset range, continuity) generally associated with continental settings rather than documenting oceanic crust emplaced by seafloor spreading. Seismic tomography results are complemented by gravity data and yield a ∼ 6–8 km thick sedimentary cover and the seismic Moho at 11–13 km depth below the sea surface. Our study reveals that the oceanic domain does not extend as far north as previously assumed. Whether Oligocene–Miocene extension led to extremely thinned continental crust or exhumed subcontinental mantle remains unclear. A low grade of mantle serpentinisation indicates a high rate of syn-rift sedimentation. However, rifting failed before oceanic spreading was initiated, and continental crust thickens towards the NE within the northern Ligurian Basin. [ABSTRACT FROM AUTHOR]

The lithosphere is the outermost rigid layer of the Earth and includes the crust and brittle uppermost mantle. Because the poor seismic coverage of the ocean basins is the mantle structure of young lithosphere below midocean spreading centers poorly constrained, especially along slow spreading ridges. Surface waves radiated by midocean ridge earthquakes are excellent agents to study young lithosphere when being recorded in the vicinity of the ridge crest. Here, we use body and Rayleigh waves from six central Atlantic transform fault earthquakes with magnitude Mw>6 to constrain upper mantle structure away from ocean islands. Earthquakes were recorded by a network of broadband ocean-bottom seismometers deployed at the Mid-Atlantic Ridge (MAR) near 14°45' N. Waveform modeling of vertical-component data at periods of 10-60 s yielded the velocity structure of the uppermost ~100 km of the mantle and hence of the depth interval where lithospheric cooling is most evident. The data support that both S-wave velocity of the lithospheric lid and its thickness increases with age; velocities increase from 4.35 to 4.75 km/s and thickness from 30-50 to 70 km, sampling mantle with an average path age of ~7 and 18 My, respectively. With respect to constraints found previously in the Pacific, lid velocities beneath the MAR are faster than beneath fast-spreading ridges, whereas asthenospheric velocities are similar to the Pacific. The fast velocity of the lid and slow velocity of the inversion zone may indicate effective hydrothermal cooling of the lithosphere. [ABSTRACT FROM AUTHOR]

We present a 2‐D P wave velocity model and a coincident multichannel seismic reflection profile characterizing the structure of the southern Costa Rica margin and incoming Cocos Ridge. The seismic profiles image the ocean and overriding plates from the trench across the entire offshore margin, including the structures involved in the 2002 Osa earthquake. The overriding plate consists of three domains: Domain I displays thin‐skinned deformation of an imbricate thrust system composed of fractured rocks. Domain II shows ~15‐km‐long landward dipping reflection packages and active deformation of the shelf sediment. Domain III is little fractured and appears to be dominated by elastic deformation, overlain by ~2‐km‐thick landward dipping strata. The velocity structure supports the argument that the bulk of the margin is highly consolidated rock. Thick‐skinned tectonics probably causes the uplift of Domains II and III. The oceanic plate shows crustal thickness variations from ~14 km at the trench (Cocos Ridge) to 6–7 km beneath the shelf. We combine (1) interplate geometry and fracturing degree, (2) tectonic stresses and brittle strain, and (3) earthquake locations, to investigate relationships between structure and earthquake generation. The 2002 Osa sequence nucleated at the leading flank of subducting seamounts in the area of highest tectonic overpressure. Both estimated rock fracturing and modeled brittle strain steadily increase from the leading flank of the subducting seamounts to their top, reflecting the progressive damage caused by the seamount. Therefore, the seismicity and structural‐mechanical evolution of the upper plate reflect the downward propagation of the leading edge of seamounts. Key Points: Costa Rica Osa margin has three across‐strike domains based on structural and physical propertiesThe bulk of the margin is consolidated rock, fronted by a 20‐ to 30‐km‐wide accreted prismSubducting seamounts correlate with upper‐plate strain, overpressure, and earthquakes [ABSTRACT FROM AUTHOR]

The depth of earthquakes along mid-ocean ridges is restricted by the relatively thin brittle lithosphere that overlies a hot, upwelling mantle. With decreasing spreading rate, earthquakes may occur deeper in the lithosphere, accommodating strain within a thicker brittle layer. New data from the ultraslow-spreading Mid-Cayman Spreading Center (MCSC) in the Caribbean Sea illustrate that earthquakes occur to 10 km depth below seafloor and, hence, occur deeper than along most other slow-spreading ridges. The MCSC spreads at 15 mm/yr full rate, while a similarly well-studied obliquely opening portion of the Southwest Indian Ridge (SWIR) spreads at an even slower rate of ~8 mm/yr if the obliquity of spreading is considered. The SWIR has previously been proposed to have earthquakes occurring as deep as 32 km, but no shallower than 5 km. These characteristics have been attributed to the combined effect of stable deformation of serpentinized mantle and an extremely deep thermal boundary layer. In the context of our MCSC results, we reanalyze the SWIR data and find a maximum depth of seismicity of 17 km, consistent with compilations of spreading-rate dependence derived from slow- and ultraslow-spreading ridges. Together, the new MCSC data and SWIR reanalysis presented here support the hypothesis that depth-seismicity relationships at mid-ocean ridges are a function of their thermal-mechanical structure as reflected in their spreading rate. [ABSTRACT FROM AUTHOR]

The Ligurian Basin is located in the Mediterranean Sea to the north-west of Corsica at the transition from the western Alpine orogen to the Apennine system and was generated by the south-eastward trench retreat of the Apennines-Calabrian subduction zone. Late Oligocene to Miocene rifting caused continental extension and subsidence, leading to the opening of the basin. Yet, it still remains enigmatic if rifting caused continental break-up and seafloor spreading. To reveal its lithospheric architecture, we acquired a state of the art seismic refraction and wide-angle reflection profile in the Ligurian Basin. The seismic line was recorded in the framework of SPP2017 4D-MB, the German component of the European AlpArray initiative, and trends in a NE-SW direction at the centre of the Ligurian Basin, roughly parallel to the French coastline. The seismic data recorded on the newly developed GEOLOG recorder, designed at GEOMAR, are dominated by sedimentary refractions and show mantle Pn arrivals at offsets of up to 70 km and a very prominent wide-angle Moho reflection. The main features share several characteristics (i.e. offset range, continuity) generally associated with continental settings rather than documenting oceanic crust emplaced by seafloor spreading. Seismic tomography results are augmented by gravity data and yield a 7.5-8 km thick sedimentary cover which is directly underlain by serpentinised mantle material at the south-western end of the profile. The acoustic basement at the north-eastern termination is interpreted to be continental crust, thickening towards the NE. Our study reveals that the oceanic domain does not extend as far north as previously assumed and that extension led to extreme continental thinning and exhumation of sub-continental mantle which eventually became serpentinised. [ABSTRACT FROM AUTHOR]

The subducting oceanic lithosphere may carry a large amount of chemically bound water into the deep Earth interior, returning water to the mantle, facilitating melting, and hence keeping the mantle mobile and, in turn, nurturing plate tectonics. Bending-related faulting in the trench--outer rise region prior to subduction has been recognized to be an important process, promoting the return flux of water into the mantle. Extensional faults in the trench--outer rise are opening pathways into the lithosphere, supporting hydration of the lithosphere, including alteration of dry peridotite to water-rich serpentine. In this paper, we review and summarize recent work suggesting that bend faulting is indeed a key process in the global water cycle, albeit not yet well understood. Two features are found in a worldwide compilation of tomographic velocity models derived from wide-angle seismic data, indicating that oceanic lithosphere is strongly modified when approaching a deep-sea trench: (1) seismic velocities in both the lower crust and upper mantle are significantly reduced compared to the structure found in the vicinity of mid-ocean ridges and in mature crust away from subduction zones; and (2) profiles shot perpendicular to the trench show both crustal and upper mantle velocities decreasing systematically approaching the trench axis, highlighting an evolutionary process because velocity reduction is related to deformation, alteration, and hydration. P-wave velocity anomalies suggest that mantle serpentinization at trenches is a global feature of all subducting oceanic plates older than 10-15 Ma. Yet, the degree of serpentinization in the uppermost mantle is not firmly established, but may range from <4% to as much as 20%, assuming that velocity reduction is solely due to hydration. A case study from the Nicaraguan trench argues that the ratio between P-wave and S-wave velocity (Vp/Vs) is a key parameter in addressing the amount of hydration. In the crust, the Vp/Vs ratio increases from <1.8 away from the trench to >1.9 in the trench, supporting the development of water-filled cracks where bend faulting occurs. In the mantle, the Vp/Vs ratio increases from ~1.75 in the outer rise to values of >1.8 at the trench, indicating the increasing intensity of serpentinization. [ABSTRACT FROM AUTHOR]

Abstract: The structural complexity of back‐arc basins is related to the evolution of the associated subduction system. Here, we present an integrated geophysical and geological study that constrains the 3D spatial variability of magmatic activity along the Tyrrhenian back‐arc basin. We use wide‐angle seismic and gravity data, acquired in 2010 within the MEDOC experiment along a ~300 km‐long NW‐SE transect that extends from SE Sardinia Island to the NW Sicily continental margin, across the Cornaglia Terrace. The geophysical transect is coincident with a seismic reflection line from the Italian CROP experiment that we have re‐processed. The geophysical results, together with available basement dredges, support a basement along the profile fundamentally composed of continental‐type rocks, locally affected by subduction‐related magmatism. The continental nature of this region contrasts with the nature of the basement inferred along two geophysical cross‐sections located to the north of the Cornaglia Terrace in which seismic velocity of the lower crust supports significant magmatic crustal accretion. The comparison of these three cross‐sections supports that the highest magmatic activity occurred in the central and most extended region of the basin, whereas it was less important in the North and practically nonexistent in the South. These observations indicate abrupt variations of magmatism during the basin formation. As in other back‐arcs, the temperature, water content and composition of the mantle might have played an important role in such variation, but they fail to explain the abruptness of it. We propose that the interaction of the overriding continental lithospheres of Adria and Africa with the Apenninic‐Calabrian subduction system caused changes in slab rollback and trench retreat dynamics, which in turn resulted in variations of back‐arc stretching and magmatism. Based on our observations, we suggest that the Cornaglia Terrace formation process might share some similarities with the formation of oceanic crust in the Red Sea. [ABSTRACT FROM AUTHOR]

The Gulf of Cadiz and the passive continental margin of southern Iberia to the west of the Strait of Gibraltar locally accommodate the presently ongoing convergence between Africa and Eurasia by widespread, rather diffusive, seismic activity. Seismicity of the northern Gulf of Cadiz was derived from an amphibious seismological network, including 24 temporary marine offshore stations, besides the permanent networks in Portugal, Spain, and Morocco. During the 6 month of the offshore network operation, in total 86 local earthquakes were located at six or more offshore stations with the majority of earthquakes occurring to the southwest of Iberia and along the Algarve continental margin off southern Iberia. The distribution of events along the Algarve margin mimics features reported for the Atlantic passive continental margins of both South and North America. Focal mechanisms at the Portimão Bank support that seismically active areas are associated with compression. Similar stress patterns are reported for the east coast of South and North America. However, while earthquakes along the American east coast occur at crustal levels, earthquakes in the northern Gulf of Cadiz occur both in the lower crust and upper mantle, with the majority of events rupturing within the mantle, including a number of well-located earthquakes beneath crust forming the continent-ocean transition zone. The large number of earthquakes in the mantle might be caused by the unique geological setting, where deformation occurs in cool lithosphere of Mesozoic age. We suggest that seismicity along the Algarve margin is caused by re-activation of pre-existing margin-parallel faults rather than corresponding to newly formed structures related to a new developing plate boundary. [ABSTRACT FROM AUTHOR]