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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]

The Pacific Plate within the collision zone between the Louisville Ridge and the Tonga-Kermadec Trench was formed at the Osbourn Trough, a paleo spreading center that became inactive during the Cretaceous. In this region, the trench shallows from a depth of 8–11 km to ∼6 km below sea surface, while the outer rise topography is obscured by Louisville seamounts that rise 4–5 km above the adjacent seafloor. We derive 2-D P-wave (V p) and S-wave (V s) velocity-depth models along a wide-angle seismic profile oriented sub-parallel to the trench axis, intersecting the 27.6°S seamount. The seismic profile is located in the down-going Pacific Plate eastwards from the trench axis (∼100 km distant at the south end and ∼ 150 km at the north end), where bending-related faulting is limited or absent. Using the derived P- and S-wave velocity-depth models we calculate the corresponding V p / V s ratio model which shows values of 1.7–1.85 throughout the oceanic crust either side of the Louisville Ridge where it is unaffected by magmatism associated with its formation. This range of observations lies within those documented by laboratory measurements on basalt, diabase, and gabbro. Conversely, in the vicinity of the summit of 27.6°S seamount, the relatively elevated V p / V s (∼1.9) ratio observed can be attributed to water-saturated cracks within the shallow sub-seabed section of the intrusive core. Beneath the seamount the uppermost mantle has a V p ranging from 8.0 to 8.9 km/s. Comparing our P-wave model with a pre-existing model running sub-perpendicularly along the Louisville Ridge axis, we observe an anisotropy of up to ∼6% at a depth of 3–4 km below the Moho. The predominant orientation of the faster axis follows the direction of paleo spreading flow when the plate was formed at the Osbourn Trough. • We obtain a V p/ V s ratio of ∼1.9 near the summit of the 27.6°S seamount. • We find mean V p/ V s ratios of 1.7–1.85 for the oceanic crust adjacent to the Louisville Ridge. • Seismic anisotropy of up to 6% is observed in the upper 15 km of the mantle. [ABSTRACT FROM AUTHOR]

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]

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]

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]

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 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]

Recent publications suggest that most of the fluid flow in the upper oceanic crust is channelized through small volumes of rock and vented into the ocean. This implies that at flanks of generally thinly sedimented mid-ocean ridges, focused discharge at the seafloor should be concentrated most likely at outcrops, high-angle normal faults or seamounts. These vents should be associated with a significant heat flow signature. However, only few observations worldwide support this assumption up to now. On our quest for focused fluid exchange between young oceanic crust and the ocean we surveyed a 720 km long and 40–90 km wide off-axis portion of seafloor intersecting the East Pacific Rise near 14°14′S. A wealth of geophysical methods including high-resolution swath mapping bathymetry, single channel seismics, sediment echo sounding, magnetics and heat flow determinations were used. Heat flow data in the tectonic corridor cover crustal ages of 0.3–9.3 Ma. With respect to the conductive plate cooling model the data show the well-known pattern of low values close to the ridge, associated with vigorous hydrothermal circulation of cold seawater through the young upper crust, and a fast recovery to almost lithospheric conductive cooling values at a surprisingly young crustal age of 9.3 Ma. Although the sediment cover is fairly thin, measurements with a 3.6 m violin bow type heat probe were possible almost everywhere within the investigated area. A detailed survey between two large seamounts at 4.5 Ma revealed localized extremely high values of up to 618 mW/m2 (275% of the expected heat flow) at the foot of the seamount. This is interpreted as a clear indication of focused discharge of hydrothermal fluid. If we, however, relate heat flow normalized by the expected conductive heat loss to the character of igneous basement, heat flow is highest in areas with an almost flat and sedimented basement, and lowest within ∼10–20 km of seamounts and other rough basement relief. We therefore hypothesize that the large number of seamounts covering the ocean floors governs a major amount of convective heat loss of aging oceanic lithosphere. [Copyright &y& Elsevier]

The article present the discussion on the depth of seismic faulting at slow- and ultraslow-spreading ridges. Topic include (Grevemeyer et al., 2019) investigated micro-seismicity of the MidCayman Spreading Center and compiled maximum depths of brittle deformation of ten additional slow- and ultraslow-spreading ridges; and Schlindwein (2020) suggests that the aseismic region at the top of the lithosphere might be controlled by hydration of ultramafic rocks.

The subducting ocean lithosphere may carry large amount of chemically bound-water into the deep Earth interior, returning water to the mantle, facilitating melting and hence keeping the mantle mobile, 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, governing the return flux of water into the mantle. Thus, extensional faulting in the trench-outer rise is opening pathways into the lithosphere, promoting hydration of the lithosphere, including alteration of dry peridotite to water rich serpentine. In this paper, we review and summaries evidence for supporting that bending-related faulting is indeed a key process in the global water cycle. Two features are found in a world-wide compilation of tomographic velocity models derived from seismic refraction data, indicating that ocean lithosphere is modified when approaching a deep-sea trench: (i) 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; (ii) along profiles approaching a trench, both crustal and upper mantle velocities decrease systematically with decreasing distant to the trench, highlighting an evolutionary process as velocity reduction is related to alteration and hydration. P-wave velocity anomalies suggest that serpentinization at trenches is a global feature of subducting oceanic plates older than 10-15 Ma. However, the amount of water trapped within the uppermost mantle and carried into subduction zones depends on how serpentines are distributed in the upper mantle. Thus, estimates of bulk upper mantle hydration would be reduced from ~3.5 wt% of water to as low as ~0.3 wt% by assuming that reduction of seismic velocitiy results from serpentinized joints. In addition to observations on the state of hydration of the incoming plate, we will evaluate the fate of subducted water carried into subduction zones and its release in the "subduction factory". [ABSTRACT FROM AUTHOR]

When continents break apart, continental crust and lithosphere is stretched until break-up occurs and seafloor spreading forms new oceanic basins. However, at settings with low magma supply the area between the oceanic and continental domain is a wide transition zone (COT) rather than a sharp boundary. This end-member type of margin has been defined as non-volcanic or magma-poor margin and type-examples are the Iberia Abyssal Plain and the conjugated Newfoundland margin. Yet, at both margins, the structure of the COT and the onset of seafloor spreading are poorly defined with available data and a matter of long debate. For instance, some authors suggested that a prominent magnetic anomaly – called the J-anomaly – is not marking an age-isochron, but is mimicking the onset of oceanic crust and hence seafloor spreading. Others have proposed that the COT is actually ultra-slow seafloor spreading before the J-anomaly. Here, we present results from a new seismic refraction and wide-angle profile shot in September of 2018 aboard the Spanish research vessel Sarmiento de Gamboa in the Iberia Abyssal Plain about 20 km to the south of seismic line IAM9. Thirty ocean-bottom-seismometers and hydrophones spaced at 10-12 km intervals recorded seismic shots fired along the 360 km long profile P3. The line is roughly centred at the J-anomaly, revealing the nature of crust and lithosphere on either side. Seismic travel time data were inverted to yield the P-wave seismic velocity (Vp) structure using a least-square damped tomographic inversion procedure. To assess the nature of basement, we used two features derived from the seismic velocity structure: (i) average Vp within 4 km below basement and (ii) maximum P-wave speed in the same depth range. Results show that the J-anomaly marks indeed a major change in lithology. Thus, to the west, sedimentary cover is in the order of 2 km and average crustal velocity ranges from >6 km/s to 6.7 km/s, with a maximum velocity of <7.2 to 6.5 km/s. To the east, the basement is rugged and occurs at 5-9 km below seafloor. Average Vp is >6.6 to 7.5 km/s and maximum velocity ranges from 7.0 to 8.1 km/s. Therefore, the domain to the east of the J-anomaly seems to be dominated by serpentinized mantle as previously proposed, but the structure is more complex than the image obtained in previous sparser data sets. The domain of serpentinized mantle may include some bodies of magmatic crust and thus could either be interpreted as domain of un-roofed mantle, which had been affected by some magmatic activity as observed in the Tyrrhenian Sea or it may represent ultra-slow spreading lithosphere as observed in the Cayman Trough. The domain west of J may indicate magmatic crust, though it shows notable differences when compared to normal oceanic crust. The new higher resolution data set challenges the concepts built in numerous publications based on lower resolution geophysical images. [ABSTRACT FROM AUTHOR]

The structure of large-offset transform faults and fracture zones associated with the Mid-Atlantic Ridge studied before the 1990s exhibits anomalous crustal structures that fall well outside the range typically observed for normal oceanic crust, both in thickness and internal structure. The geological nature of the seismically anomalous crust and how this crust forms, are still a matter of debate. One interpretation is that the crust within North Atlantic fracture zones consists of a thin, intensely fractured, and hydrothermally altered basaltic section overlying ultramafics that are extensively serpentinized in places. (Detrick et al., Rev. Geophys., 1993). For nearly three decades, transform faults and fracture zones garnered little attention. However, in late 2017 we surveyed the St. Paul fracture zone near 18°W during Maria S. Merian cruise MSM69. Fourteen ocean-bottom-seismometers and hydrophones sampled seismic shots fired along a 140 km long seismic profile, running within the ~10 km wide valley of the fracture zone, that separates 40 Ma crust in the south with 70 Ma crust in the north. Seismic refraction and wide-angle data provided both first arrival refracted P-waves and wide-angle reflections. Furthermore, a clear Pn refraction branch with the apparent velocity of ~8 km/s has been observed. Joint refraction and reflection tomographic inversion of P-wave travel times supports a seismic structure, which is heterogeneous, but shares common features with 4-5 km thick normal oceanic crust, supporting a magmatic origin of the crust. Both the observed reflections from the crust-mantle boundary and upper mantle P-wave speed of ~8 km/s provide little evidence for large quantities of serpentinites being distributed through the crust and upper mantle. [ABSTRACT FROM AUTHOR]

• 3D rheology-dependent thermal structure impacts gravity calculations. • Rheology-dependent gravity differences are concentrated along transform faults. • Importance of mantle rheology scales with spreading rate and transform age offset. Marine gravity data can provide information on the distribution of mass anomalies in the oceanic crust and upper mantle. Computing corresponding gravity anomalies, especially so-called 'residual' gravity anomalies that directly reflect variations in the crustal structure, relies on gravity corrections of both seafloor relief and lithospheric thermal structure. The lithospheric thermal gravity correction involves either a plate cooling approximation or a mantle flow model with the latter typically done using simplified assumptions on mantle rheology. However, a detailed study of how differing rheological models affect the computed gravity anomalies is still missing. Here, we systematically examine the differences in residual mantle Bouguer anomalies (RMBA) caused by differing assumptions on mantle rheology for 16 mid-ocean ridge – transform fault systems. Our calculations show that isoviscous models tend to underpredict RMBA values within the transform deformation zone and overpredict them in the far field at older plate ages, when compared to plate cooling and nonlinear viscoplastic models. This discrepancy stems from isoviscous models failing to capture plate-like deformation, as well as their inability to resolve brittle failure and the associated strain localization that leads to warm upwelling beneath the transform fault. By exploring a wide parameter range, we find that the importance of mantle rheology scales with plate tectonic parameters at the mid-ocean ridge – transform fault system such as transform age offset, spreading rate, and transform fault length. These findings suggest that gravity thermal corrections at the intrinsically three-dimensional ridge – transform systems should employ mantle flow models that resolve plate-like deformation and brittle failure. [ABSTRACT FROM AUTHOR]

We present seismic data collected in summer 2018 of the structure of the opening of the central Atlantic in NW Africa, and the west Iberia Mesozoic rift basins and rifted margins. We obtained crustal-scale seismic images collected with a large tuned source and a 6-km-long streamer, and new coincident wide-angle seismic data with ~10-km-spaced OBSs. Our cross-sections were collected in transects in regions where previous work from the 1980 to mid 1990's collected some of the most emblematic geophysical cross-sections. New images of the Galicia Interior Basin (GIB) display prominent thinning at the basin center with an asymmetric tectonic and crustal structure, indicating that extension stopped when the basin was in the latest phase before break up. The basin is characterized by large-scale normal faults that are laterally bounded by transfer fault structures. New Vp models from the Iberia abyssal plain indicate that the crustal structure is more complex than models with lower resolution –widely used in geological interpretations- have inferred. The new data preliminarily supports that the exhumed mantle may contain magmatic bodies and that continental break up may have occurred before mantle exhumation. Comparison of observations from Iberia and Tagus Abyssal Plains display the transition from exhumed mantle to the oceanic crust across the J-anomaly ridge, providing data of on the interaction between magma and tectonics. Seismic images from NW Africa display a rather different structure, with upper Triassic evaporite-diapirs extruding from fault-blocks of highly-extended possibly continental crust, with Early Jurassic (drilled) synrift deposits in tilted blocks. Here, a rather abrupt continental slope is followed by a deep-water basin underlain by perhaps as much as 100 km of highly-extended continental crust. [ABSTRACT FROM AUTHOR]