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The amalgamation and breakup of the West Gondwana shaped the South American platform. The dynamics during the processes can be reflected by crust anisotropy of the platform, but there are no specialized crustal anisotropic measurements yet. Splitting analysis of Moho-converted shear waves in P -wave receiver functions (Pms) can reveal crustal-scale anisotropy, which is important for understanding the dynamic evolution of the crust and for the interpretation of mantle anisotropy from splitting analysis of core–mantle refracted shear waves (XKS phases). This study measured crustal anisotropy for the old and stable South American platform by Pms splitting analysis. The splitting times vary mainly in the range of 0–0.5 s, with a regional mean of 0.2 s, slightly lower than that observed in tectonically active regions. The detected crustal anisotropy shows distinct characteristics and spatial zoning, providing insights into tectonic processes. (1) Fast polarization directions at stations close to the Transbrasiliano Lineament (TBL) are oriented NNE–SSW, generally consistent with the strike of the TBL but inconsistent with the maximum horizontal compressive stress, implying that they might be formed by dynamic metamorphism during the formation of the TBL. (2) Crustal anisotropy along the passive continental margin in the east and northeast is weak. Still, the fast polarization directions tend to be oriented along the margin, implying the existence of fossil extensional crustal fabrics formed during the continental rifting of West Gondwana. (3) The Paraná Basin, one of the world's largest Large Igneous Provinces (LIP) covered by continental flood basalts, shows distinctively strong anisotropy, with fast polarization directions highly aligned with mantle anisotropy, implying that synchronous crust–mantle deformation occurred in these regions as a result of magmatism during the breakup of West Gondwana. [ABSTRACT FROM AUTHOR]

Plate tectonics plays an essential role in the redistribution of life-essential volatile elements between Earth's interior and surface, whereby our planet has been well tuned to maintain enduring habitability over much of its history. Here we present an overview of deep carbon recycling in the regime of modern plate tectonics, with a special focus on convergent plate margins for assessing global carbon mass balance. The up-to-date flux compilation implies an approximate balance between deep carbon outflux and subduction carbon influx within uncertainty but remarkably limited return of carbon to convecting mantle. If correct, carbon would gradually accumulate in the lithosphere over time by (i) massive subsurface carbon storage occurring primarily in continental lithosphere from convergent margins to continental interior and (ii) persistent surface carbon sinks to seafloors sustained by high-flux deep CO2 emissions to the atmosphere. Further assessment of global carbon mass balance requires updates on fluxes of subduction-driven carbon recycling paths and reduction in uncertainty of deep carbon outflux. From a global plate tectonics point of view, we particularly emphasize that continental reworking is an important mechanism for remobilizing geologically sequestered carbon in continental crust and sub-continental lithospheric mantle. In light of recent advances, future research is suggested to focus on a better understanding of the reservoirs, fluxes, mechanisms, and climatic effects of deep carbon recycling following an integrated methodology of observation, experiment, and numerical modeling, with the aim of decoding the self-regulating Earth system and its habitability from the deep carbon recycling perspective. [ABSTRACT FROM AUTHOR]

This study attempts to interrogate the upper mantle deformation pattern beneath the Kumaon-Garhwal region, located in the western Himalaya, using shear wave splitting (SWS) analysis of core-refracted (XK(K)S) phases recorded at 53 broad-band stations. The fast polarization azimuths (FPAs) revealed by 338 well constrained measurements are dominantly clustered around ENE–WSW, with a few along the NE and E–W directions. The delay times vary from 0.2 to 1.4 s, with an average of 0.6 s that is smaller than that for the Indian shield (∼0.8 s), central and eastern Himalayas. The northern part of the lesser Himalaya shows a slightly smaller delay time compared to the southern part, which is attributed to the weakening of azimuthal anisotropy caused by the dipping of the Indian lithosphere. In order to understand the crustal contribution, its anisotropy is measured by analysing the splitting of Ps conversions from the Moho (Pms), akin to that of the XK(K)S phases. However, reliable results for crustal anisotropy could be obtained only at 10 stations. The average delay time due to crustal anisotropy is 0.47 s, with a variation from 0.2 to 0.9 s. Although the dominant period of Pms is smaller than that of SK(K)S, crustal anisotropy contributing to splitting of the latter phases cannot be ruled out. The orientation of FPAs obtained from Pms phases is found to be parallel or sub-parallel to those from XK(K)S phases, suggesting a similar deformation mechanism in the mid- to lower-crust and upper mantle. On the basis of FPAs derived from XK(K)S measurements, the Kumaon-Garhwal Himalaya (KGH) region can be divided into four subregions. In the western and eastern parts, the FPAs are mostly aligned along NE and ENE–WSW, and NE, respectively. In the central and south-eastern parts, their orientation is along ENE–WSW and NW, respectively. The strong ENE–WSW orientation in the central part could result from a slightly variable anisotropy in the crust to the upper part of the lithosphere or basal topography causing deflection of mantle flow. Also, the NW orientation in the south-eastern part of KGH is associated with a shallow source within the lithosphere. Application of the spatial coherency technique to single-layered anisotropic parameters results in a depth of 220–240 km, implying that the dominant source of anisotropy could lie in the upper mantle. [ABSTRACT FROM AUTHOR]

Seismic interrogation of the upper mantle from the base of the crust to the top of the mantle transition zone has revealed discontinuities that are variable in space, depth, lateral extent, amplitude and lack a unified explanation for their origin. Improved constraints on the detectability and properties of mantle discontinuities can be obtained with P-to-S receiver function (Ps-RF) where energy scatters from P to S as seismic waves propagate across discontinuities of interest. However, due to the interference of crustal multiples, uppermost mantle discontinuities are more commonly imaged with lower resolution S-to-P receiver function (Sp-RF). In this study, a new method called CRISP-RF (Clean Receiver-function Imaging using SParse Radon Filters) is proposed, which incorporates ideas from compressive sensing and model-based image reconstruction. The central idea involves applying a sparse Radon transform to effectively decompose the Ps-RF into its underlying wavefield contributions, that is direct conversions, multiples, and noise, based on the phase moveout and coherence. A masking filter is then designed and applied to create a multiple-free and denoised Ps-RF. We demonstrate, using synthetic experiment, that our implementation of the Radon transform using a sparsity-promoting regularization outperforms the conventional least-squares methods and can effectively isolate direct Ps conversions. We further apply the CRISP-RF workflow on real data, including single station data on cratons, common-conversion-point stack at continental margins and seismic data from ocean islands. The application of CRISP-RF to global data sets will advance our understanding of the enigmatic origins of the upper mantle discontinuities like the ubiquitous mid-lithospheric discontinuity and the elusive X-discontinuity. [ABSTRACT FROM AUTHOR]

Atlantic Canada encompasses geological evidence of the orogenic and rifting episodes that inspired the development of the theory of plate tectonics and the fundamental concept of the Wilson cycle. To provide a regional crustal-scale view that can complement surface mapping studies and sparse seismological investigations, an onshore–offshore 3-D constrained gravity inversion methodology is proposed involving incorporation of topography and an inversion mesh that is laterally variable in terms of its maximum depth extent. A 3-D density anomaly model for the entirety of Atlantic Canada and its environs is generated, with the inverted density distribution structure and extracted isodensity surfaces showing excellent correspondence with independent and co-located controlled source and passive seismic constraints. The full density model and crustal thicknesses from this work are made freely available so that they may be used for further study, for instance as inputs for deformable plate reconstruction modelling. [ABSTRACT FROM AUTHOR]

The southeastern Korean Peninsula (SeKP) has experienced intense deformation owing to subduction and backarc extension at the eastern continental margin of the Eurasian Plate, leading to the formation of complex tectonic structures. Abnormally high surface heat flux, Cenozoic volcanism, signatures of mantle degassing and hydrothermal alteration, and several active fault systems with extensional sedimentary basins have been identified; however, the major driving forces that promote local seismic events and hydrothermal activities remain enigmatic. Here, we constructed 3-D P -wave velocity of the crust and upper mantle in the SeKP for the first time using a teleseismic traveltime tomography method and an extensive data set obtained from a dense seismic network. Our model revealed three distinct velocity patterns at different depths: (1) in the upper crust (depth ∼0–10 km), a low-velocity anomaly beneath the Cenozoic sedimentary basin exhibiting a prominent lateral velocity contrasts with higher velocities in the Cretaceous sedimentary and plutonic rocks; (2) a N–S trending low-velocity anomaly extending from the lower crust to the uppermost mantle (depth ∼20–35 km) beneath the major active fault systems interpreted as a thermally or mechanically weakened structure that could transfer high surface heat flux and transport mantle-driven gases and (3) a low-velocity anomaly adjacent to the Cenozoic basin in the upper mantle at depths of 35–55 km interpreted as the higher temperature upper mantle. Via a series of geodynamic simulations, we demonstrated that the extensional deformation at the eastern continental margin during the Early to Middle Miocene locally enhanced the temperature of the crust and upper mantle beneath the SeKP. We propose that a hydrothermal system, resulting from the thermally modified lithosphere of the continental margin, has contributed to the enhanced local seismicity and geothermal activities observed in the SeKP region. [ABSTRACT FROM AUTHOR]

About half of the rifted margins purportedly formed by extension with minor magmatism. The conceptual models of those magma-poor systems are greatly influenced by the continent-to-ocean transition structure of the archetypal magma-poor West Iberia Margin. In the past, interpretation of magnetic data of West Iberia has been used to constrain conceptual rifting models, including the structure of the transition from the exhumed mantle domain to the oceanic crust formed at a spreading centre. However, uncertainties on geophysical data were generally not considered leading to overdetailed interpretations. We use synthetic magnetic modelling to show that magnetic data acquired at sea level cannot resolve subhorizontal lithological layering in deep-water continental margins. We then present new magnetic modelling guided by a refined velocity model of the wide-angle seismic IAM-9 profile that shows that the magnetic J-anomaly correlates with oceanic crust that abuts exhumed mantle across a vertical boundary. This well-constrained observation supports that seafloor spreading initiated abruptly, terminating mantle exhumation. Conventional wisdom dictates that the sudden efficient melt extraction relates to a mechanical threshold during lithospheric thinning and concomitant asthenospheric upwelling under which melt can migrate toward the surface. However, our results support that mantle melting creating oceanic crust was probably not driven by gradual lithospheric thinning and asthenospheric upwelling, but by seafloor spreading centre propagation that cut across the lithosphere, creating the abrupt structure. [ABSTRACT FROM AUTHOR]

Gas hydrates that occur on many continental margins have received global attention. In reflection seismic imaging, the bottom-simulating reflector (BSR) is a common indicator of gas hydrates. However, it is difficult to identify gas hydrates and quantify their amounts through the BSR alone. For gas-hydrate characterization, it is therefore useful to measure seismic stratigraphic and attenuation attributes. Short-scale patterns of layering that contain information about the amount and mechanism of gas hydrates can be identified through stratigraphic and attenuation attributes. We measure the complete time-variant spectra by using sparse strongest peaks, and the spectral differences at different times through attenuation parameters Q –1 and γ. The traditional Q –1 is associated with the attenuation of the frequency-dependent part of wavefield, and the γ characterizes the frequency-independent attenuation. The measurement approach is straightforward and requires no sophisticated inverse algorithm and is applied to surface seismic data acquired over the Hikurangi and Gondwana margins, eastern New Zealand. High-quality spectral and attenuation images are obtained. Spectral attributes correlate with BSRs and large positive Q –1 and negative γ -values are below and above the BSRs, which are interpreted as being related to free-gas and gas-hydrate accumulations. These results will aid the quantification of gas hydrates and the assessment of their roles as an energy resource, as a potential geological hazard, and in climate change and ocean warming. [ABSTRACT FROM AUTHOR]

The United Arab Emirates (UAE)-Oman mountain belt exposes a large intact ophiolite thrust sheet that was obducted onto a rifted continental margin during the Late Cretaceous. Crustal properties of the mountain belt and the foreland region are important to better understand the mechanism of crustal deformation during the obduction process. In this study, we analysed P -wave receiver functions across the UAE and northern Oman mountain belt to determine the Moho depths, crustal properties and velocity structure beneath 53 broad-band seismic stations. Crustal thickness varies from 30.0 km in the western UAE to 46.6 km in the UAE-Oman mountain belt. The deeper Moho depth beneath the UAE-Oman mountain belt is the outcome of crustal thickening and flexure of the lithosphere during the obduction process. The eastern flank of the UAE-Oman mountain belt has relatively high- Vp / Vs ratios of 1.76–1.89. On the contrary, comparatively low- Vp/Vs ratios (1.61–1.67) are estimated in the western flank. Moreover, relatively low- Vp/Vs ratios (1.61–1.70) are observed in the foreland basin and western UAE. The high- Vp/Vs ratios and thick crust are indication of a thick ophiolite thrust sheet, while low- Vp/Vs ratios suggest that the crust is more felsic/intermediate in nature. The Vs -depth profiles in the mountain belt reveal a low- Vs zone beneath a remarkably high- Vs top layer. There is no such high- Vs top layer in the foreland area and western UAE. The high- Vs layer is correlated to a ∼10–15-km thick sequence of Semail crust and mantle ophiolite. Common Conversion Point imaging across the UAE-Oman mountain belt imaged the Moho boundary, which agrees with H-k stacking results, and intra-crustal discontinuities. [ABSTRACT FROM AUTHOR]

The development of trench-arc-backarc (TABA) systems is uniquely associated with modern-style plate tectonics on Earth. The Qilian orogenic belt in NW China records the evolution history of the Proto-Tethys Ocean at the transition time from the Proterozoic to Phanerozoic. This paper presents systematic studies of petrography, U–Pb chronology and geochemistry on various rocks from a middle-ocean ridge (MOR)-type ophiolite belt, active continental margin and back-arc basin in the Qilian orogenic belt to address the development of a modern-style TABA system. Arc magmas include felsic intrusions with ages of 531 to 477 Ma and felsic-mafic arc volcanic rocks with ages of 506 to 439 Ma, showing distinctive features of typical magmatic rocks formed at an Andean-type continental margin. The back-arc basin is recorded by a 490- to 448-Ma suprasubduction zone (SSZ)-type ophiolite with boninite, and Silurian turbidite flysch formation. We establish a three-stage tectonic history from the initiation of subduction to the formation of a mature Japan-Sea-type back-arc basin at the active continental margin in the Early Paleozoic era. (1) Northward subduction of Proto-Tethys Ocean initiated and the Andean-type continental arc developed at ~530 to 500 Ma with continual crustal thickening; (2) a tectonic transition occurred from an Andean-type active continental margin to a West Pacific-type active continental margin at ~500 to 490 Ma with rapid thinning of crust to ~35 km; and (3) mature ocean basins and back-arc-basin (BAB) ophiolites were formed in the back-arc extensional environment at ~490 to 450 Ma. [ABSTRACT FROM AUTHOR]

The bulk rock composition of granitoids reflects the composition of their source and the conditions of partial melting, which are functions of the geodynamic setting in which they formed. Granitoids in active continental margins (continental arcs) are dominated by calc-alkaline rocks with subordinate alkaline compositions, although how these different magma compositions formed is not well understood. Neoproterozoic magmatic rocks are widely distributed along the western margin of the Yangtze Block in South China to form the >1000-km long Panxi continental arc system, which is dominated by granitoids with minor mafic–ultramafic and intermediate plutons. The granitoids are subdivided into sodic and potassic variants that occur as belts along the western and eastern sides of the continental arc, respectively. Sodic granitoids from the western part consist of tonalite, granodiorite, and monzogranite with crystallisation ages ranging from 870 Ma to 740 Ma. They have low K2O/Na2O ratios (0.1–1.0) and high Na2O contents (3.5–6.7 wt%), high but variable SiO2 (61–75 wt%) concentrations, and negative to positive whole-rock εNd(t) values (−1.7 to +2.9). Zircon grains from the sodic granitoids have εHf(t) values ranging from +0.3 to +9.6 and δ18O from 3.90‰ to 7.71‰. The potassic granitoids from the eastern side consist of monzogranite and syenogranite with crystallisation ages from 820 Ma to 790 Ma. They have high K2O/Na2O ratios (0.6–2.2), K2O (2.6–5.9 wt%) and SiO2 contents (69–78 wt%), but whole-rock εNd(t) (−0.9 to +2.9) and zircon εHf(t) (+1.8 to +12.9), and δ18O values (2.98‰ to 6.41‰) similar to those of the sodic granitoids. The isotopic compositions of both the sodic and potassic granitoids are similar to those of spatially- and temporally-related mantle-derived (mafic to ultramafic) rocks, and are considered to have been derived from juvenile mafic continental crust. Phase equilibrium modelling shows that the H2O content of the granitoid source rocks played a key role in their petrogenesis, both in lowering solidus temperatures and in controlling the compositions of the derived partial melts. Our results indicate that calc-alkaline sodic granitoids can be formed by water-fluxed melting of juvenile mafic crust at 750–900°C and 9–12 kbar in which the required H2O was derived from the dewatering of underplating mafic arc magmas. By contrast, the potassic granitoids were generated by fluid-absent (H2O-undersaturated) partial melting of a similar juvenile mafic source at 725–900°C and 6–9 kbar. We conclude that the sodic granitoids were derived from partial melting of the newly-formed mafic lower crust in the continental arc, whereas the potassic granitoids were likely generated in the back-arc setting induced by upwelling of asthenospheric mantle. [ABSTRACT FROM AUTHOR]

A number of metamorphic core complexes (MCCs) developed in the North China Craton and adjacent regions in the Early Cretaceous and were characterized by consistent extensional orientations. These MCCs formed in the continental interior and were conceptually attributed to the retreat of the Palaeo-Pacific (Izanagi) Plate, but the exact physical mechanism remains enigmatic. Here we utilize 2-D thermomechanical simulations to study how mechanical conditions of the continental crust respond to stresses derived from oceanic subduction and their roles in the formation of MCCs. Our results demonstrate that pre-existing weaknesses are key for localized formation within the continental interior. These weaknesses first undergo compression to form thrust faults in response to shallow subduction of the oceanic slab. These thrust faults gradually transform into extensional ones as the oceanic slab starts to retreat, eventually causing the synchronous exhumation of middle-to-lower crustal rocks that form the MCCs. The P-T paths of metamorphic rocks in the core of MCCs reveal a two-stage exhumation, with isothermal decompression followed by rapid isobaric cooling. Sensitivity tests show that (1) stronger upper crust and weaker lower crust favour MCC formation, while lithospheric strength could exert an influence on the formation time of MCCs and (2) when the continental crust is hot (T Moho = 800 °C), a new magmatic dome could form along the continental margin. We suggest that pre-existing weaknesses in the North China Craton played a key role in generating the quasi-simultaneous MCC series in response to the retreating Palaeo-Pacific Plate. [ABSTRACT FROM AUTHOR]

We investigate the nature of seismic anisotropy in the Kumaun–Garhwal Himalaya by analysing the core-refracted shear wave splitting phases (SK(K)S, PKS) recorded at 42 broad-band seismic stations. The mean fast anisotropy directions are mostly in ∼E–W and a few in the ∼NE–SW directions. The mean delay time decreases progressively from ∼1.0 s beneath the Sub Himalaya to ∼0.5 s at the Higher Himalaya. The observed seismic anisotropy suggests that the ongoing deformation beneath the Kumaun–Garhwal Himalaya has a large contribution by the relative rotation of the asthenospheric flow by the compression along the Indian Plate motion direction in this collision zone. Also, the southward transportation of the mid-crustal flow is probably responsible for the reduction in the delay times in the Main Central Thrust zone and further north than in the Sub-to-Lower Himalaya. The splitting parameters indicate a complex pattern of deformation beneath the Kumaun–Garhwal Himalaya with contribution from crust, lithospheric mantle and asthenospheric mantle. [ABSTRACT FROM AUTHOR]

The Southern Ocean (SO) is the least understood environment on earth, but anthropogenic impacts related to climate change, fishing activities and tourism are already well-established in the region. Herein, we investigate biodiversity patterns in the Atlantic Sector of the SO by investigating a considerable number of samples collected from a wide depth range (89–6224 m). Three topics are addressed: (1) which taxa occur in the SO; (2) if there are distinct shelf, slope and abyssal faunas; and (3) which abiotic parameters explain the ostracod assemblages. We also tested putative ecological preferences of ostracod genera, widely used in palaeoceanographical reconstruction. For this, almost 5000 living ostracods from 51 epibenthic sledge samples were identified to 140 species. Statistically significant differences were observed among the faunas from the shelf, slope and abyssal faunas, indicating that the shelf fauna survived in refugia during the glacial periods of the Quaternary. Our analyses showed that the main variables influencing ostracod distribution is depth, followed by nitrate and phosphate. Temperature, salinity and oxygen content do not seem to be as important as previously proposed. Putative ecological preferences of ostracod genera, for example, low oxygen for Cytherella , could not be confirmed, suggesting that this topic requires further investigation. [ABSTRACT FROM AUTHOR]

The segmented trapdoor spiders (Liphistiidae) are the sole surviving family of the suborder Mesothelae, which forms the sister lineage to all other living spiders. Liphistiids have retained a number of plesiomorphic traits and their present-day distribution is limited to East and Southeast Asia. Studying this group has the potential to shed light on the deep evolutionary history of spiders, but the phylogeny and divergence times of the family have not been resolved with confidence. We performed phylogenomic and molecular dating analyses of 2765 ultraconserved element loci from 185 liphistiid taxa. Our analyses show that the crown group of Liphistiidae appeared in the mid-Cretaceous at 102 Ma (95% credibility interval 92–113 Ma), but it was not until the Neogene that much of the diversification within the family occurred in mainland Southeast and East Asia. This diversification was coincident with tectonic events such as the extension of the East Asian continental margin, as well as geological upheavals in Indochina induced by the collision between India and Asia. Our study highlights the important role of major tectonic events in shaping the evolutionary history, present-day diversity, and geographical distribution of mesothele and liphistiid spiders. [biogeography; concatenation; Liphistiidae; molecular dating; summary coalescent; UCEs.] [ABSTRACT FROM AUTHOR]

Controversy between advocates of 'active' (plume-activated) versus 'passive' (driven by external tectonic stresses) modes of continental rifting and break-up has persisted for decades. To a large extent, inconsistencies between observations and models are rooted in the conceptual model of plumes as voluminous upwellings of hot material sourced from the deep mantle. Such large-scale plumes are expected to induce intensive magmatism and topographic uplift, thereby triggering rifting. In this case of an 'active' rifting-to-break-up system, emplacement of plume-related magmatism should precede the onset of rifting that is not observed in many rifted continental margins, thus providing a primary argument in favour of an antiplume origin for continental break-up and supercontinent fragmentation. However, mantle plumes are not restricted to whole-mantle ('primary') plumes emanating from the mantle-core boundary but also include 'secondary' plumes originating from the upper mantle transition zone or shallower. Over the last decades a number of such 'secondary' plumes with horizontal diameters of only ∼100–200 km (therefore, sometimes also called 'baby' plumes) have been imaged in the upper mantle below Europe and China. The longevity of such small-scale plumes and their impact on geodynamics of continental break-up have so far not been explored. We present results of a systematic parametrical analysis of relatively small thermal anomalies seeded at the base of the lithosphere. In particular, we explore the effects of variations in initial plume temperature (T  = 1500–1700 °C) and size (diameter of 80–116 km), characteristics of the overlying lithosphere (e.g. 'Cratonic', 'Variscan', 'Mesozoic' and oceanic) and intraplate tectonic regimes (neutral or far-field extension of 2–10 mm yr–1). In tectonically neutral regimes, the expected decay time of a seismically detectable 'baby'-plume varies from ∼20 to >200 Myr and is mainly controlled by its initial size and temperature, whereas the effect of variations in the thermotectonic age of the overlying lithosphere is modest. These small but enduring plumes are able to trigger localized rifting and subsequent continental break-up occurring from ∼10 to >300 Myr after the onset of far-field extension. Regardless of the thermomechanical structure of the lithosphere, relatively rapid (tens of Myr) break-up (observed in models with a hot plume and fast extension) favours partial melting of plume material. In contrast, in the case of a long-lasting (a few hundreds of Myr) pre-break-up phase (relatively cold plume, low extension rate), rifting is accompanied by modest decompressional melting of only 'normal' sublithospheric mantle. On the basis of the models presented, we distinguish two additional modes of continental rifting and break-up: (1) 'semi-active' when syn-break-up magmatism is carrying geochemical signatures of the deep mantle with deformation localized above the plume head not anymore connected by its tail to the original source of hot material and (2) 'semi-passive' when the site of final lithospheric rupture is controlled by a thermal anomaly of plume origin but without invoking its syn-break-up melting. These intermediate mechanisms are applicable to several segments of the passive continental margins formed during Pangea fragmentation. [ABSTRACT FROM AUTHOR]

The Davie Fracture Zone (DFZ) evolved during the Jurassic and Cretaceous breakup and subsequent drift of Gondwana off East Africa. This old weak zone has been reactivated during the evolution of the East African Rift System. Recent faulting of Cenozoic sediments in the Kerimbas Basin off northern Mozambique shows that they are affected by the neotectonics. The question is if and how the crustal fabric in our research area has been modified by the rifting process. We present two seismic refraction profiles acquired offshore northern Mozambique to investigate its regional crustal structure and tectonic history. The profiles show a continent–ocean transition zone that widens from around 40 km at 13°S to more than 100 km at 11°S. In the west the transitional crust is up to 12 km thick. To the east, around 150 km off the Mozambique coast lies oceanic crust whose thickness varies from 4.9 to 6.5 km along the northern line and from 6.5 to 7.5 km along the southern one. The latter presents an unusual high-velocity lower crustal body (7.0–7.2 km s−1), about 40 km wide and 3.8 km thick, underlying the oceanic crust. The body may consist of underplated melt with the same source as the nearby Paisley Seamount, which has not yet reached its isostatic equilibrium. Despite well documented recent seismicity along the margin, neither of the profiles reveal significant crustal modifications or reduced crustal seismic velocities that might be related to ongoing extensional tectonics as part of of the East African Rift System. Neither profile reveals seismic evidence for the presence of a major fracture zone or sheared continental margin parallel to the margin. Instead, the profiles' broad continent–ocean transitions are consistent with their formation during an early Jurassic stage of plate divergence oblique to the margin. Later, after 157 Ma, the azimuth of relative plate motion between East and West Gondwana changed to be parallel to the margin, and parts of the continent–ocean transitions may have been locally reactivated in a strike-slip sense. However, details on the plate movements during the directional change of the seafloor spreading between 157 and 144 Ma are not available. The oceanic crust formed by the initial divergent oblique extension became faulted/modified by the strike-slip movements between both plates. Instead of a narrow deformation zone, the DFZ is charcaterized by a broad, diffuse zone of transtensional deformation. [ABSTRACT FROM AUTHOR]

Models of gravity corrections and anomalies are created and distributed over the territory of the Republic of Croatia, including the Adriatic and Dinarides regions. Published models cover the study area between 42.0°N < φ < 46.6°N and 13.0°E < λ < 19.5°E with a 1′ × 1′ resolution. Distributed models include gravity corrections and anomalies of normal gravity, atmosphere, free air, Bouguer, terrain, complete Bouguer and topographic–isostatic according to the Airy–Heiskanen model. A high-resolution digital elevation/bathymetry model was prepared from SRTMv3.0 and GEBCO2019 models. It was then used for the computation of topographic and topographic–isostatic gravity effects using rectangular prisms with constant crustal density. Gridding of the scattered gravity data was performed with the Kriging interpolation method using complete Bouguer anomalies by implementing the remove–grid–restore methodology. Developed models were interpreted providing insights into the topography-reduced (anomalous) Earth's gravity field reflecting the variations in mass/density distribution within the crust and upper mantle over the study area. [ABSTRACT FROM AUTHOR]

We propose a tectonic interpretation for the outer-SDRs (SDRs: Seaward-Dipping Reflectors) and Pannikar central ridge in the aborted Laxmi Basin west of India from wide-angle seismic reflection data. The outer-SDRs comprise syn-tectonic extrusives (lavas and/or volcaniclastics) emplaced above passively exhumed mid-to-lower mafic crust of continental origin. They erupted following sudden lithosphere weakening associated with isolation of a continental block (a 'C-Block'). Continuous magmatic addition during crustal extension allowed stretching of the lower crust whilst maintaining constant or even increasing thickness. A similar process occurred at both conjugate margins allowing bulk, pure-shear plate separation and formation of linear magnetic anomalies. The Laxmi example can explain enigmatic features observed in mature oceans such as presence of distal buoyant plateaus of thick continental crust away from the margins. [ABSTRACT FROM AUTHOR]

The present-day tectonics of the southern Red Sea region is complicated by the presence of the overlapping Afar and southern Red Sea rifts as well as the uncertain kinematics and extent of the Danakil block in between. Here we combine up to 16 yr of GPS observations and show that the coherent rotation of the Danakil block is well described by a Danakil-Nubia Euler pole at 16.36°N, 39.96°E with a rotation rate of 2.83 deg Myr–1. The kinematic block modeling also indicates that the Danakil block is significantly smaller than previously suggested, extending only to Hanish-Zukur Islands (∼13.8°N) with the area to the south of the islands being a part of the Arabian Plate. In addition, the GPS velocity field reveals a wide inter-rifting deformation zone across the northern Danakil-Afar rift with ∼5.6 mm yr–1 of east–west opening across Gulf of Zula in Eritrea. Together the results redefine some of the plate boundaries in the region and show how the extension in the southern Red Sea gradually moves over to the Danakil-Afar rift. [ABSTRACT FROM AUTHOR]

A comprehensive GPS velocity field along the Dead Sea Fault System (DSFS) provides new constraints on along-strike variations of near-transform crustal deformation along this plate boundary, and internal deformation of the Sinai and Arabian plates. In general, geodetically derived slip rates decrease northwards along the transform (5.0 ± 0.2 to 2.2 ± 0.5 mm yr−1) and are consistent with geological slip rates averaged over longer time periods. Localized reductions in slip rate occur where the Sinai Plate is in ∼N–S extension. Extension is confined to the Sinai side of the fault and is associated with prominent changes in transform geometry, and with NW–SE striking, left-lateral splay faults, including the Carmel Fault in Israel and the Roum Fault in Lebanon. The asymmetry of the extensional velocity gradients about the transform reflects active fragmentation of the Sinai Plate along the continental margin. Additionally, elastic block modelling of GPS velocities requires an additional structure off-shore the northern DSF segment, which may correspond with a fault located along the continental margin, suggested by prior geophysical studies. [ABSTRACT FROM AUTHOR]

The southcentral Hikurangi subduction margin (North Island, New Zealand) has a wide, low-taper accretionary wedge that is frontally accreting a >3-km-thick layer of sediments, with deformation currently focused near the toe of the wedge. We use a geological model based on a depth-converted seismic section, together with physically realistic parameters for fluid pressure, and sediment and décollement friction based on laboratory experiments, to investigate the present-day force balance in the wedge. Numerical models are used to establish the range of physical parameters compatible with the present-day wedge geometry and mechanics. Our analysis shows that the accretionary wedge stability and taper angle require either high to moderate fluid pressure on the plate interface, and/or weak frictional strength along the décollement. The décollement beneath the outer wedge requires a relatively weaker effective strength than beneath the inner (consolidated) wedge. Increasing density and cohesion with depth make it easier to attain a stable taper within the inner wedge, while anything that weakens the wedge—such as high fluid pressures and weak faults—make it harder. Our results allow a near-hydrostatic wedge fluid pressure, sublithostatic fluid overpressure at the subduction interface, and friction coefficients compatible with measurements from laboratory experiments on weak clay minerals. [ABSTRACT FROM AUTHOR]

In this paper we test whether or not structural and morphological features inherited from the Eurasian continental margin are affecting the contemporary stress and strain fields in south-central Taiwan. Principal stress directions (σ1, σ2 and σ3) are estimated from the inversion of clustered earthquake focal mechanisms and the direction of the maximum compressive horizontal stress (S H) is calculated throughout the study area. From these data the most likely fault plane orientations and their kinematics are inferred. The results of the stress inversion are then discussed together with the directions of displacement, compressional strain rate and maximum shear strain rate derived from GPS data. These data show that there is a marked contrast in the direction of S H from north to south across the study area, with the direction of S H remaining roughly subparallel to the relative plate motion vector in the north, whereas in the south it rotates nearly 45° counter-clockwise. The direction of the horizontal maximum compression strain rate (εH) and associated maximum shear planes, together with the displacement field display an overall similar pattern between them, although undergoing a less marked rotation. We interpret the southward change in the S H, εH and the dextral maximum shear plane directions, together with that of the horizontal displacement field to be related to the reactivation of east–northeast striking faults inherited from the rifted Eurasian margin and to the shelf/slope break. Inherited faults in the basement are typically reactivated as strike-slip faults, whereas newly formed faults in the fold-and-thrust belt are commonly thrusts or oblique thrusts. Eastwards, the stress inversions and strain data show that the western flank of the Central Range is undergoing extension in the upper crust. S H in the Central Range is roughly parallel to the relative plate convergence vector, but in southwestern Taiwan it undergoes a marked counter-clockwise rotation westwards across the Chaochou fault. Farther north, however, there is no significant change across the Lishan fault. This north to south difference is likely due to different margin structures, although local topographic effects may also play a role. [ABSTRACT FROM AUTHOR]

Evolution of the continental margins of the South China Sea (SCS) is one of the open questions when discussing continental breakup and seafloor spreading. We processed data from a wide-angle seismic profile (OBS2011–1), which passes through the northwestern margin of the SCS, and performed travel time modelling to obtain the seismic velocity structures. The modelling results show a stepwise variation of the crustal thicknesses from continental margin to oceanic basin. Stretching factor of the upper crust is nearly double the estimate of the lower crust along the Zhongsha Trough. The lower crust shows asymmetrical upwelling towards the trough center, accompanied by ∼0.3 km s–1 of the velocity increase due to magmatic addition. The upper and lower crusts have almost the same stretching factor beneath continental blocks, indicating a uniform extension. Crustal structures of the conjugate margins of the Southwest Subbasin show similar velocity range and different thickness distribution, supporting the common origin and asymmetric extension of these two margins. The Ocean–Continent Transition zones (OCT) are much wider in the southern part (∼50 km) than the northern part (∼25 km) crossing the margins. We propose a tectonic model for the asymmetry of both the conjugate margins and the OCTs, favoring the highly stretched upper crust and accompanied by rising of the ductile middle-lower crust controlled by major low-angle faults. The rigid blocks may also act as a kind of hindrance for further evolution of the failed rifts and affect the shape of the OCT. [ABSTRACT FROM AUTHOR]

We present a rheological model of the continental lithosphere during the rift–drift transition, founded on theoretical considerations, numerical modelling, and supported by geophysical and geological evidence from the Red Sea region. The model is based on the fundamental assumption that during the rifting phase the necking lithospheric mantle of the conjugate continental margins retains and accumulates elastic strain. This hypothesis is tested numerically, and thereby it is shown that during the phase of extension the uppermost mantle has the capability to store and maintain recoverable elastic strain over geological times, differently from the upper crust, which exhibits a continuous short-term irreversible deformation by seismic release. After the onset of seafloor spreading, strain recovery and release of the strain energy accumulated in the lithospheric mantle occurs through a phase of non-linear anelastic relaxation. During this phase, the upper crust of the conjugate continental margins experiences post-rift deformation with tectonic inversion of former extensional structures, while the extra space created along the axial zone as a consequence of the rapid contraction of the margins triggers a rapid upwelling of the asthenosphere that induces an initial pulse of fast spreading followed by a steady phase of oceanic crust accretion. We present geophysical evidence supporting this model, including: (1) the observed pattern of oceanic magnetic anomalies in the Red Sea, (2) the distribution of finite crustal strains across the continental margins of Nubia and Arabia, and (3) the distribution of earthquake epicentres along the western margin of the Arabian Plate. We also present new structural data acquired during three geological campaigns performed in 2015 and 2016 along the western Arabian margin, which are consistent with a post-rift phase of compression and inversion of the rift structures. We will also show that a selection of realistic rheological parameters supports non-linear viscoelastic behaviour of the continental lithosphere during the rift–drift transition. Finally, we will show that in the case of the Red Sea ∼40 per cent of the total extensional strain accumulated during the rifting stage has been recovered in the southernmost part of the Arabian margin conjugate to the Nubian Plate (∼19°N), while this percentage decreases to ∼14 per cent around 23.8°N, where the continental margin faces the youngest spreading segment, and it is zero north of this area, where the Red Sea is still in the rifting stage. Taking into account of the age of oceanization along the central and northern Red Sea, this implies an average recovery of ∼10 per cent Myr–1. [ABSTRACT FROM AUTHOR]

A complex Meso-Cenozoic history characterizes the offshore Indus Basin Pakistan, including rifting, drifting, collision, transpression and recent inverted extension that is poorly understood. We use reflection seismic data to document the geometry of the structures that accommodated the deformation and offshore well data to correlate the sedimentary sequences which were affected by the different tectonic episodes. Synthetic seismograms have been prepared for calibrating seismic data with well information for identification of key reflections representative of major lithological horizons. Seismic stratigraphy helped in dividing the sedimentary successions into rifted margin and transpressive margin sequences. Additionally, the basement represented by chaotic and semitransparent reflections, and Moho discontinuity by moderate amplitude bright reflections are also identified on seismic records. Syn-depositional normal faulting in extensional regime (Cretaceous–Oligocene) are identified on seismic profiles that converted into strike slip faults in Miocene and listric faults in Recent sediments forming inverted extensional regime. We infer that the basin inversion from extension to inverted extension and transpression occurred in recent times. Number of depositional features including shelves, deltaic deposits, canyons and channel levee complexes have been identified that developed in different tectonic episodes. Five tectonic subsidence curves using backstripping procedure have been calculated in order to constrain the geodynamic evolution of the Meso-Cenozoic sedimentary succession of the offshore Indus Pakistan. The analysis of tectonic subsidence curves, covering a time span of 140 Ma, allowed us to quantify the total subsidence and to document the activities of syn-rift and post-rift faulting systems. Different stages, in terms of duration and magnitude of subsidence-uplift trends, have been identified in the evolution of the basin. Two uplift events are prominent in Eocene (∼40 Ma) and Miocene time (∼12 Ma). A rapid resumption of subsidence with non-conformity in Early Palaeocene is seen along the margin. [ABSTRACT FROM AUTHOR]

We present new SKS splitting measurements obtained from a temporary seismic broad-band network in western Iran across the Arabia–Eurasia collision zone. The average delay time over the entire network was found to be 1.27 ± 0.27 s. In the Zagros where the lithosphere attains its greatest thickness, the fast-axes are predominantly subparallel to the trend of the mountain ranges, suggesting a lithospheric origin of the anisotropy caused by transpressional deformation. In contrast in the Alborz, the fast-axes become subparallel to the absolute plate motion vectors and we propose that anisotropy is mainly controlled by the direction of the asthenospheric flow field. The central Iran region shows a more complex pattern of anisotropy that could be the result of variable and small-scale deformation fields in the crust and the shallow sublithospheric mantle. [ABSTRACT FROM AUTHOR]

The presence of high mountains along passive margins is not unusual, as shown by their presence in several regions (Scandinavia, Greenland, East US, SW Africa, Brazil, West India and SE Australia). However, the origin of this topography is notwell understood. Themountain range between the Scandinavian passive margin and the Fennoscandian shield is a good example. A simple Airy isostatic model would predict a compensating root beneath the mountains but existing seismic measurements of variations in crustal thickness do not provide evidence of a root of sufficient size to produce the necessary compensation. In order to better constrain the physical properties of the crust in northern Scandinavia, two broad-band seismic networks were deployed between 2007 and 2009 and between 2013 and 2014. A new map of crustal thickness has been produced from P-receiver function analysis of teleseismic data recorded at 31 seismic stations. The map shows an increase in crustal thickness from the Atlantic coast (38.7 ± 1.8 km) to the Gulf of Bothnia (43.5 ± 2.4 km). This gradient in thickness demonstrates that the Moho topography does not mirror the variation in surface topography in this region. Thus, classical Airy isostatic models cannot explain how the surface topography is supported. New maps showing variation in Poisson's ratio and Moho sharpness together with forward and inverse modelling provide new information about the contrasting properties of the Fennoscandian shield and crust reworked by the Caledonian orogeny. A sharp Moho transition (R > 1) and low value of Vs (3.5 ± 0.2 km s-1) are observed beneath the orogen. The shield is characterized by a gradual transition across the Moho (R < 1) and Vs of 3.8 ± 0.1 km s-1 which is more typical of average continental crust. These observations are explained by a Fennoscandian shield underplated with a thick layer of high velocity, high density material. It is proposed that this layer has been removed or reworked beneath the orogen. [ABSTRACT FROM AUTHOR]

We present a new model of the lithosphere and asthenosphere structure down to 300 km depth beneath the Pyrenees from the joint inversion of recent gravity and teleseismic data. Unlike previous studies, crustal correction was not applied on teleseismic data in order (i) to preserve the consistency between gravity data, which are mainly sensitive to the density structure of the crust lithosphere, and traveltime data, and (ii) to avoid the introduction of biases resulting from crustal reductions. The density model down to 100 km depth is preferentially used here to discuss the lithospheric structure of the Pyrenees, whereas the asthenospheric structure from 100 to 300 km depth is discussed from our velocity model. The absence of a high density anomaly in our model between 30 and 100 km depth (except the Labourd density anomaly) in the northern part of the Pyrenees seems to preclude eclogitization of the subducted Iberian crust at the scale of the entire Pyrenean range. Local eclogitization of the deep Pyrenean crust beneath the western part of the Axial Zone (west of Andorra) associated with the positive central density anomaly is proposed. The Pyrenean lithosphere in density and velocity models appears segmented from east to west. No clear relation between the along-strike segmentation and mapped major faults is visible in our models. The Pyrenees' lithosphere segments are associated with different seismicity pattern in the Pyrenees suggesting a possible relation between the deep structure of the Pyrenees and its seismicity in the upper crust. The concentration of earthquakes localized just straight up the central density anomaly can reflect the subsidence and/or delamination of an eclogitized Pyrenean deep root. The velocity model in the asthenosphere is similar to previous studies. The absence of a high-velocity anomaly in the upper mantle and transition zone (i.e. 125 to 225 km depth) seems to preclude the presence of a detached oceanic lithosphere beneath the European lithosphere. [ABSTRACT FROM AUTHOR]

A 3-D subsurface temperature distribution within the Lofoten--Vesterålen segment of theMid- Norwegian continental margin and adjacent areas has been studied to understand the thermal effect of late Cenozoic erosion of old sedimentary and crystalline rocks and subsequent deposition of glacial sediments during the Pleistocene. A lithosphere-scale 3-D structural model of the Lofoten--Vesterålen area has been used as a realistic approximation of the geometries of the sedimentary infill, underlying crystalline crust and lithospheric mantle during the 3-D thermal modelling. The influence of late Cenozoic erosion and sedimentation has been included during the 3-D thermal calculations. In addition, the 3-D thermal modelling has been carried out by taking also into account the influence of early Cenozoic continental breakup. The results of the 3-D thermal modelling demonstrate that the mainland is generally colder than the basin areas within the upper part of the 3-D model. The thermal influence of the early Cenozoic breakup is still clearly recognizable within the western and deep parts of the Lofoten-Vesterålen margin segment in terms of the increased temperatures. The thermal effects of the erosion and deposition within the study area also indicate that a positive thermal anomaly exists within the specific subareas where sedimentary and crystalline rocks were eroded. A negative thermal effect occurs in the subareas affected by subsidence and sedimentation. The erosion-related positive thermal anomaly reaches its maximum of more than +27 °C at depths of 17-22 km beneath the eastern part of the Vestfjorden Basin. The most pronounced deposition-related negative anomaly shows a minimum of around -70 °C at 17-20 km depth beneath the Lofoten Basin. The second negative anomaly is located within the northeastern part of the Vøring Basin and has minimal values of around -48 °C at 12-14 km depth. These prominent thermal anomalies are associated with the subareas where relatively high erosional and depositional rates were observed for late Cenozoic time. [ABSTRACT FROM AUTHOR]

On 2016 December 25, an Mw 7.6 earthquake broke a portion of the Southern Chilean subduction zone south of Chiloé Island, located in the central part of the Mw 9.5 1960 Valdivia earthquake. This region is characterized by repeated earthquakes in 1960 and historical times with very sparse interseismic activity due to the subduction of a young (~15 Ma), and therefore hot, oceanic plate.We estimate the coseismic slip distribution based on a kinematic finite-fault source model, and through joint inversion of teleseismic body waves and strong motion data. The coseismic slip model yields a total seismic moment of 3.94 x 1020 N·m that occurred over ~30 s, with the rupture propagating mainly downdip, reaching a peak slip of ~4.2 m. Regional moment tensor inversion of stronger aftershocks reveals thrust type faulting at depths of the plate interface. The fore- and aftershock seismicity is mostly related to the subduction interface with sparse seismicity in the overriding crust. The 2016 Chilo´e event broke a region with increased locking and most likely broke an asperity of the 1960 earthquake. The updip limit of the main event, aftershocks, foreshocks and interseismic activity are spatially similar, located ~15 km offshore and parallel to Chiloé Islands west coast. The coseismic slip model of the 2016 Chiloé earthquake suggests a peak slip of 4.2 m that locally exceeds the 3.38 m slip deficit that has accumulated since 1960. Therefore, the 2016 Chiloé earthquake possibly released strain that has built up prior to the 1960 Valdivia earthquake. [ABSTRACT FROM AUTHOR]

Alithosphere-scale 3-D density/magnetic structural model of the Møre and Vøring segments of the Mid-Norwegian continental margin and the adjacent areas of the Norwegian mainland has been constructed by using both published, publically available data sets and confidential data, validated by the 3-D density and magnetic modelling. The obtained Moho topography clearly correlates with the major tectonic units of the study area where a deep Moho corresponds to the base of the Precambrian continental crust and the shallower one is located in close proximity to the younger oceanic lithospheric domain. The 3-D density modelling agrees with previous studieswhich indicate the presence of a high-density/high-velocity lower-crustal layer beneath the Mid-Norwegian continental margin. The broad Jan Mayen Corridor gravity low is partially related to the decreasing density of the sedimentary layers within the Jan Mayen Corridor and also has to be considered in relation to a possible low-density compositionand/or temperature-related zone in the lithospheric mantle. According to the results of the 3-D magnetic modelling, the absence of a strong magnetic anomaly over the Utgard High indicates that the uplifted crystalline rocks are not so magnetic there, supporting a suggestion that the entire crystalline crust has a low magnetization beneath the greater part of the Vøring Basin and the northern part of the Møre Basin. On the contrary, the crystalline crust is much more magnetic beneath the Trøndelag Platform, the southern part of the Møre Basin and within the mainland, reaching a culmination at the Frøya High where the most intensive magnetic anomaly is observed within the study area. [ABSTRACT FROM AUTHOR]

Differences between seismic and geodetic strain rates can highlight regions of potential seismic hazard. In China, many of the most devastating historical and recent earthquakes have occurred around the margins of the Ordos Plateau. We construct an earthquake catalogue for the region that covers the 700 yr period from 1315 to 2015, and is thought to be complete for events with Ms > 6. We use Kostrov summations to determine a seismic strain rate tensor for each of the margins of the Ordos Plateau. We also use the latest campaign GPS data to calculate both a continuously varying geodetic strain rate field for the region and individual geodetic strain rate tensors for each of the plateau margins. We then compare our seismic and geodetic strain rate results to identify regions with seismic moment deficits and also calculate expected recurrence intervals from our geodetic moment rates. We find notable seismic strain rate deficits in northern Ningxia (to the west of the Ordos Plateau), in the Hetao and Linhe Grabens (to the north of the plateau), and in the Shanxi Grabens (to the northeast of the plateau). In theWeihe Graben (to the south of the plateau), although there is an overall seismic strain rate excess for the last 700 yr, there is some indication of relative quiescence for the last few centuries. Assuming no aseismic accommodation of deformation, these deficits indicate enhanced potential for large earthquakes in these regions in the future. Either temporal clustering of earthquakes or individual, large-magnitude events with long recurrence intervals could explain these patterns of moment release. Meanwhile, in southern Ningxia (to the southwest of the plateau) we find a seismic strain rate excess, indicating a likely lower level of seismic hazard in this region. In southern Ningxia and the Shanxi Grabens the pattern of strain suggested by the Kostrov summations is consistent with the geodetic strains. In northern Ningxia, the Hetao and Linhe Grabens, and the Weihe Graben--where there are fewer historical earthquakes and sparser GPS station coverage--the seismic and geodetic patterns of strain release are inconsistent. [ABSTRACT FROM AUTHOR]

The variation of oceanic basement roughness at mid-oceanic ridges is a complex trade-off between spreading rate that largely controls the thermal state of the lithosphere and its composition controlling the rheology and thus also the strength of the lithosphere. Here we estimate top basement roughness (i.e. the root-mean-square deviation of residual basement relief) over initial oceanic crust bordering the Iberia, Newfoundland, Bay of Biscay, Goban Spur, Flemish Cap, Australian and Antarctic rifted margins to provide new insights into the spreading processes at the nascent plate boundary. Although ultraslow seafloor spreading is suggested in those areas, the lack of undisputable oceanicmagnetic anomalies prevents anywell-constrained determination of the initial spreading rates. We compare these estimated roughness values with those determined over ultraslow-spreading crust formed at the Mid Atlantic Ridge, Southwest Indian Ridge, Arctic ridges, Mid-Cayman Spreading Center, Sheba ridge and South Pandora Ridge. The roughness values obtained at these ultraslow-spreading ridges range from 100 to >500 m and include 200-240 m roughness values which are typical of slow-spreading ridges. Roughness values larger than ~300 m are characteristic of magma-poor sections of ultraslow-spreading ridges. The top basement roughness values determined within the inferred initial oceanic domain bordering the investigated magma-poor rifted margins are all higher than 200 m. Mean roughness values of the inferred initial oceanic domains alongside the conjugate Iberia and Newfoundland margins are greater than 300 m similarly to magma-poor sections of ultraslow-spreading ridges. The top basement in the initial oceanic crust alongside the conjugate Flemish Cap and Goban Spur margins shows roughness values and a tilted block morphology typical of slow-spreading ridges. We suggest that the roughness and the morphology of the top basement bordering the conjugate Australian and Antarctic margins indicate large tectonic extension and intermediate magma supply at either slow- or ultraslow-spreading rates. We show that estimating roughness values within transitional domains of magma-poor margins like exhumed mantle domains is less pertinent as polyphased tectonism and magmatism may have affected these domains leading to highly variable top basement roughness values that cannot be linked to a single process. [ABSTRACT FROM AUTHOR]

We constrain and model the geometry of the Zagros foreland to assess the equivalent elastic thickness of the northern edge of the Arabian plate and the loads that have originated due to the Arabia-Eurasia collision. The Oligo-Miocene Asmari formation, and its equivalents in Iraq and Syria, is used to estimate the post-collisional subsidence as they separate passive margin sediments from the younger foreland deposits. The depth to these formations is obtained by synthesizing a large database of well logs, seismic profiles and structural sections from the Mesopotamian basin and the Persian Gulf. The foreland depth varies along strike of the Zagros wedge between 1 and 6 km. The foreland is deepest beneath the Dezful embayment, in southwest Iran, and becomes shallower towards both ends. We investigate how the geometry of the foreland relates to the range topography loading based on simple flexural models. Deflection of the Arabian plate is modelled using point load distribution and convolution technique. The results show that the foreland depth is well predicted with a flexural model which assumes loading by the basin sedimentary fill, and thickened crust of the Zagros. The model also predicts a Moho depth consistent with Free-Air anomalies over the foreland and Zagros wedge. The equivalent elastic thickness of the flexed Arabian lithosphere is estimated to be ca. 50 km. We conclude that other sources of loading of the lithosphere, either related to the density variations (e.g. due to a possible lithospheric root) or dynamic origin (e.g. due to sublithospheric mantle flow or lithospheric buckling) have a negligible influence on the foreland geometry, Moho depth and topography of the Zagros. We calculate the shortening across the Zagros assuming conservation of crustal mass during deformation, trapping of all the sediments eroded from the range in the foreland, and an initial crustal thickness of 38 km. This calculation implies a minimum of 126 ± 18 km of crustal shortening due to ophiolite obduction and post-ollisional shortening. [ABSTRACT FROM AUTHOR]

Mumbai, located on the western Indian continental margin, exposes Danian-age Deccan magmatic units of diverse compositions, dipping seaward due to the Panvel flexure. The Ghatkopar-Powai tholeiitic sequence contains seaward-dipping (thus pre-flexure) flows and subvertical (thus post-flexure) dykes. We present new 40Ar/39Ar ages of 62.4 ± 0.7 and 62.4 ± 0.3 Ma (2σ) on two flows, and 62.2 ± 0.3, 62.8 ± 0.3 and 61.8 ± 0.2Ma on three dykes, showing that this sequence is much younger than the main 66-65 Ma Deccan sequence in theWestern Ghats escarpment. The mutually indistinguishable ages of the Ghatkopar-Powai tholeiites overlap with available 40Ar/39Ar ages of 62.6 ± 0.6 and 62.9 ± 0.2Ma for the seaward-dipping Dongri rhyolite flow and 62.2 ± 0.6 Ma for the Saki Naka trachyte intrusion, both from the uppermost Mumbai stratigraphy. The weighted mean of these eight 40Ar/39Ar ages is 62.4 ± 0.1 Ma (2 SEM), relative to an MMhb-1 monitor age of 523.1 ± 2.6Ma (2σ), and indicates essentially contemporaneous volcanism, intrusion and tectonic flexure. This age also coincides with the rift-to-drift transition of the Seychelles and Laxmi Ridge-India breakup and the emplacement of the Raman-Panikkar-Wadia seamount chain in the axial part of the Laxmi Basin. Pre-rift magmatism is seen in the 64.55 Ma Jogeshwari basalt in Mumbai and 63.5-63.0 Ma intrusions in the Seychelles. Post-rift magmatism is seen in the 60.8-60.9 Ma Manori trachyte and Gilbert Hill basalt intrusions in Mumbai and 60-61 Ma syenitic intrusions in the Seychelles. The Mumbai area thus preserves the pre-, syn- and post-rift onshore tectonomagmatic record of the breakup between the Seychelles and the Laxmi Ridge-India. Voluminous submarine volcanism forming the Raman, Panikkar and Wadia seamounts in the Laxmi Basin represents the offshore syn-rift magmatism. [ABSTRACT FROM AUTHOR]

It remains challenging to obtain absolute shear wave velocities of heterogeneities of small lateral extension in the uppermost mantle. This study presents a cross-section of Vs across the strongly heterogeneous 3-D structure of the western European Alps, based on array analysis of data from 92 broad-band seismic stations from the CIFALPS experiment and from permanent networks in France and Italy. Half of the stations were located along a dense sublinear array. Using a combination of these stations and off-profile stations, fundamental-mode Rayleigh wave dispersion curves were calculated using a combined frequency-time beamforming approach. We calculated dispersion curves for seven arrays of approximately 100 km aperture and 14 arrays of approximately 50 km aperture, the latter with the aim of obtaining a 2-D vertical cross-section of Vs beneath the western Alps. The dispersion curves were inverted for Vs(z), with crustal interfaces imposed from a previous receiver function study. The array approach proved feasible, as Vs(z) from independent arrays vary smoothly across the profile length. Results from the seven large arrays show that the shear velocity of the upper mantle beneath the European plate is overall low compared to AK135 with the lowest velocities in the internal part of the western Alps, and higher velocities east of the Alps beneath the Po plain. The 2-D Vs model is coherent with (i) a~100 km thick eastward-dipping European lithosphere west of the Alps, (ii) very high velocities beneath the Po plain, coherent with the presence of the Alpine (European) slab and (iii) a narrow low-velocity anomaly beneath the core of the western Alps (from the Briançonnais to the Dora Maira massif), and approximately colocated with a similar anomaly observed in a recent teleseismic P-wave tomography. This intriguing anomaly is also supported by traveltime variations of subvertically propagating body waves from two teleseismic events that are approximately located on the profile great circle. [ABSTRACT FROM AUTHOR]

We present new rotations that describe the relative positions and velocities of the Pacific and North America plates at 22 times during the past 19.7 Myr, offering ?1-Myr temporal resolution for studies of the geotectonic evolution of western North America and other plate boundary locations. Derived from ≈18 000 magnetic reversal, fracture zone and transform fault identifications from the Pacific-Antarctic-Nubia-North America plate circuit and the velocities of 935 GPS sites on the Pacific and North America plates, the new rotations and GPS-derived angular velocity indicate that the rate of motion between the two plates increased by ≈70 per cent from 19.7 to 9±1 Ma, but changed by less than 2 per cent since 8 Ma and even less since 4.2 Ma. The rotations further suggest that the relative plate direction has rotated clockwise for most of the past 20 Myr, with a possible hiatus from 9 to 5 Ma. This conflicts with previously reported evidence for a significant clockwise change in the plate direction at ≈8-6 Ma. Our new rotations indicate that Pacific plate motion became obliquely convergent with respect to the San Andreas Fault of central California at 5.2-4.2 Ma, in agreement with geological evidence for a Pliocene onset of folding and faulting in central California. Our reconstruction of the northern Gulf of California at 6.3 Ma differs by only 15-30 km from structurally derived reconstructions after including 3-4 km Myr-1 of geodetically measured slip between the Baja California Peninsula and Pacific plate. This implies an approximate 15-30 km upper bound for plate non-rigidity integrated around the global circuit at 6.3 Ma. A much larger 200±54 km discrepancy between our reconstruction of the northern Gulf of California at 12 Ma and that estimated from structural and marine geophysical observations suggests that faults in northwestern Mexico or possibly west of the Baja California Peninsula accommodated large amounts of obliquely divergent dextral shear from 12-6.3 Ma. Pacific-North America plate motion since 16 Myr estimated with our new rotations agrees well with structurally summed deformation along two transects of western North America between the Colorado Plateau and western California, with a difference as small as 40 km out of 760 km of margin-parallel motion. A strong resemblance between a 20-Myr-to-present flow line reconstructed with our new rotations and the traces of the 700-km-long Queen Charlotte Fault and continental slope west of Canada suggests that the plate margin geometry was influenced by the passage of the Pacific plate and Yakutat block. The new rotations also suggest that (1) oblique convergence west of Canada initiated at 12-11 Ma, 5-8 Myr earlier than previously estimated, (2) no significant margin-normal shortening has occurred in areas of Canada located east of the Haida Gwaii archipelago since 20 Ma and (3) Pacific plate underthrusting of Haida Gwaii has accommodated the margin-normal component of plate motion since 12-11 Ma. Our rotations suggest an ≈70 per cent increase in the rate that the Pacific plate has been consumed by subduction beneath the Aleutian arc since 19.7 Ma, with still-unknown consequences for the rate of arc magmatism. [ABSTRACT FROM AUTHOR]

SKS arrivals from ocean bottom seismometer (OBS) data from an offshore southern California deployment are analysed for shear wave splitting. The project involved 34 OBSs deployed for 12 months in a region extending up to 500 km west of the coastline into the oceanic Pacific plate. The measurement process consisted of removing the effects of anisotropy using a range of values for splitting fast directions and delay times to minimize energy along the transverse seismometer axis. Computed splitting parameters are unexpectedly similar to onland parameters, exhibiting WSW-ENE fast polarization directions and delays between 0.8 and 1.8 s, even for oceanic plate sites. This is the first SKS splitting study to extend across the entire boundary between the North America and Pacific plates, into the oceanic part of the Pacific plate. The splitting results show that the fast direction of anisotropy on the Pacific plate does not align with absolute plate motion (APM), and they extend the trend of anisotropy in southern California an additional 500 km west, well onto the oceanic Pacific plate. We model the finite strain and anisotropy within the asthenosphere associated with density-buoyancy driven mantle flow and the effects of APM. In the absence of plate motion effects, such buoyancy driven mantle flow would be NE-directed beneath the Pacific plate observations. The best-fit patterns of mantle flow are inferred from the tomography-based models that show primary influences from foundering higher-density zones associated with the history of subduction beneath North America. The new offshore SKS measurements, when combined with measurements onshore within the plate boundary zone, indicate that dramatic lateral variations in density-driven upper-mantle flow are required from offshore California into the plate boundary zone in California and western Basin and Range. [ABSTRACT FROM AUTHOR]

Powerful subduction zone earthquakes rupture thousands of square kilometres along continental margins but at certain locations earthquake rupture terminates. To date, detailed knowledge of the parameters that govern seismic rupture and aftershocks is still incomplete. On 2015 September 16, the Mw 8.3 Illapel earthquake ruptured a 200 km long stretch of the Central Chilean subduction zone, triggering a tsunami and causing significant damage. Here, we analyse the temporal and spatial pattern of the coseismic rupture and aftershocks in relation to the tectonic setting in the earthquake area. Aftershocks cluster around the area of maximum coseismic slip, in particular in lateral and downdip direction. During the first 24 hr after the main shock, aftershocks migrated in both lateral directions with velocities of approximately 2.5 and 5 kmhr-1. At the southern rupture boundary, aftershocks cluster around individual subducted seamounts that are related to the downthrusting Juan Fernández Ridge. In the northern part of the rupture area, aftershocks separate into an upper cluster (above 25 km depth) and a lower cluster (below 35 km depth). This dual seismic-aseismic transition in downdip direction is also observed in the interseismic period suggesting that it may represent a persistent feature for the Central Chilean subduction zone. [ABSTRACT FROM AUTHOR]

Recent tectonic reconstructions of the South Atlantic have partitioned the ocean basin into several segments based upon one or more proposed intraplate South American deformation zones. In several of these reconstructions, opening of the southern segment(s) by seafloor spreading prior to Aptian-Albian time is accompanied by contemporaneous strike-slip motion along an intraplate boundary extending southeastward from the Andean Cochabamba--Santa Cruz bend to the Rio Grande Fracture Zone (RGFZ). We have examined new magnetic data over the Pelotas, Santos and Campos Basins, offshore Argentina and Brazil, acquired by IONGXT in tandem with long-offset, long record seismic reflection data, and identified seafloor spreading anomalies M4, M3, M2 and M0 (~131, ~129, ~128 and ~125 Ma). Integrating these results with our earlier work, we have been able to correlate magnetochrons M4, M3, M2 and M0 north and south of the RGFZ on the South American margin, and north and south of the Walvis Ridge on the African side. Our results are therefore inconsistent with diachronous opening models that involve substantial continental strike-slip motion north of RGFZ during M4 to M0 time. Although the ocean basin may have opened from south to north, our results indicate that seafloor spreading began north of the RGFZ earlier than previously proposed. [ABSTRACT FROM AUTHOR]

Owing to its vast natural resources and the influence of the Congo River and associated submarine canyon, the Angolan continental margin is of both socioeconomic and ecological interest. The deep-sea ecosystems of the region are nevertheless understudied, and much of the deep-sea fauna remains undescribed. Here, we document the scavenging amphipods of the Angolan deep-sea habitat, which provides valuable new insights into the ecology of Angolan deep-sea scavengers. This can be used as an ecological baseline, against which resourceextraction impacts can be measured. A total of 7996 scavenging amphipods, representing 10 species, were identified. At least four species were new to science. The relatively low scavenger diversity of the region, combined with the large sample sizes, may be indicative of abundant food falls in the region due to the presence of the submarine canyon system. The dominant species across all samples, Abyssorchomene distinctus (Birstein and Vinogradov, 1960), was the focus of a population-level study, which was used to describe the population structure of this species and identify species traits. Of the 826 individuals of A. distinctus dissected and measured, 533 were unsexed juveniles, 149 were male, and 144 were female. Females were significantly larger than males, which is indicative of non-mateguarding pre-copulatory behaviour, but had significantly shorter antennae, which may indicate that males use chemical cues during mate searching. Two, three, and five discrete size-based cohorts were identified for juveniles, males, and females respectively. No ovigerous females were caught but brood size of A. distinctus was estimated to be 10-38 offspring based on ovary contents. [ABSTRACT FROM AUTHOR]

In Europe, common input data types for seismic hazard evaluation include earthquake catalogues, seismic zonation models and ground motion models, all with well-constrained epistemic uncertainties. In contrast, neotectonic deformation models and their related uncertainties are rarely considered in earthquake forecasting and seismic hazard studies. In this study, for the first time in Europe, we developed a seismic hazard model based exclusively on active fault and geodynamic deformation models. We applied it to the External Dinarides, a slow-deforming fold-and-thrust belt in the Central Mediterranean. The two deformation models furnish consistent long-term earthquake rates above the Mw 4.7 threshold on a latitude/longitude grid with 0.2° spacing. Results suggest that the use of deformation models is a valid alternative to empirical-statistical approaches in earthquake forecasting in slow-deforming regions of Europe. Furthermore, we show that the variability of different deformation models has a comparable effect on the peak ground motion acceleration uncertainty as do the ground motion prediction equations. [ABSTRACT FROM AUTHOR]

The Makran subduction zone is one of the last convergent margins to be investigated using space-based geodesy. While there is a lack of historical and modern instrumentation in the region, a sparse sampling of continuous and campaign measurements over the past decade has allowed us to make the first estimates of convergence rates. We combine GPS measurements from 20 stations located in Iran, Pakistan and Oman along with hypocentral locations from the International Seismological Centre to create a preliminary 3-D estimate of the geometry of the megathrust, along with a preliminary fault-coupling model for the Makran subduction zone. Using a convergence rate which is strongly constrained by measurements from the incoming Arabia plate along with the backslip method of Savage, we find the Makran subduction zone appears to be locked to a depth of at least 38 km and accumulating strain. We also find evidence for a segmentation of plate coupling, with a 300 km long section of reduced plate coupling. The range of acceptable locking depths from our modelling and the 900 km along-strike length for the megathrust, makes the Makran subduction zone capable of earthquakes up to Mw = 8.8. In addition, we find evidence for slow-slip-like transient deformation events on two GPS stations. These observations are suggestive of transient deformation events observed in Cascadia, Japan and elsewhere. [ABSTRACT FROM AUTHOR]

Non-linear resonant coupling of edge waves can occur with tsunamis generated by largemagnitude subduction zone earthquakes. Earthquake rupture zones that straddle beneath the coastline of continental margins are particularly efficient at generating tsunami edge waves. Using a stochastic model for earthquake slip, it is shown that a wide range of edge-wave modes and wavenumbers can be excited, depending on the variability of slip. If two modes are present that satisfy resonance conditions, then a third mode can gradually increase in amplitude over time, even if the earthquake did not originally excite that edge-wave mode. These three edge waves form a resonant triad that can cause unexpected variations in tsunami amplitude long after the first arrival. An M ~ 9, 1100 km-long continental subduction zone earthquake is considered as a test case. For the least-variable slip examined involving a Gaussian random variable, the dominant resonant triad includes a high-amplitude fundamental mode wave with wavenumber associated with the along-strike dimension of rupture. The two other waves that make up this triad include subharmonic waves, one of fundamental mode and the other of mode 2 or 3. For the most variable slip examined involving a Cauchy-distributed random variable, the dominant triads involve higher wavenumbers and modes because subevents, rather than the overall rupture dimension, control the excitation of edge waves. Calculation of the resonant period for energy transfer determines which cases resonant coupling may be instrumentally observed. For low-mode triads, the maximum transfer of energy occurs approximately 20-30 wave periods after the first arrival and thus may be observed prior to the tsunami coda being completely attenuated. Therefore, under certain circumstances the necessary ingredients for resonant coupling of tsunami edge waves exist, indicating that resonant triads may be observable and implicated in late, large-amplitude tsunami arrivals. [ABSTRACT FROM AUTHOR]

This paper presents the gravimetric analysis together with seismic data as an integral application in order to identify the continental-oceanic crust boundary (COB) of the Argentine continental margin from 36°S to 50°S in a continuous way. The gravimetric and seismic data are made up of large grids of data obtained from satellite altimetry and marine research. The methodology consists of three distinct methods: (i) the application of enhancement techniques to gravimetric anomalies, (ii) the calculation of crustal thinning from 3-D gravity inversion modelling of the crust-mantle discontinuity and (iii) 2-D gravimetric modelling supported by multichannel reflection and refraction seismic profiles. In the first method, the analytic signal, Theta map, and tilt angle and its horizontal derivative were applied. In the second method, crustal thickness was obtained as the difference in the depths of the crystalline basement and the crust-mantle discontinuity; the latter was obtained via gravimetric inversion. Finally, 2-D modelling was performed from free-air anomalies in two representative sections by considering as restriction surfaces those coming from the interpretation of seismic data. The results of the joint application of enhancement techniques and 2-D and 3-D modelling have enabled continuous interpretation of the COB. In this study, the COB was determined continuously from the integration of 2-D profiles of the enhancement techniques, taking account of crustal thickness and performing 2-D gravimetric modelling. The modelling technique was complemented by regional studies integrated with multichannel seismic reflection and seismic refraction lines, resulting in consistent enhancement techniques. [ABSTRACT FROM AUTHOR]

We determine the deep structure of the eastern Algerian basin and its southern margin in the Annaba region (easternmost Algeria), to better constrain the plate kinematic reconstruction in this region. This study is based on new geophysical data collected during the SPIRAL cruise in 2009, which included a wide-angle, 240-km-long, onshore-offshore seismic profile, multichannel seismic reflection lines and gravity and magnetic data, complemented by the available geophysical data for the study area. The analysis and modelling of the wide-angle seismic data including refracted and reflected arrival travel times, and integrated with the multichannel seismic reflection lines, reveal the detailed structure of an ocean-to-continent transition. In the deep basin, there is an ~5.5-km-thick oceanic crust that is composed of two layers. The upper layer of the crust is defined by a high velocity gradient and P-wave velocities between 4.8 and 6.0 km s-1, from the top to the bottom. The lower crust is defined by a lower velocity gradient and P-wave velocity between 6.0 and 7.1 km s-1. The Poisson ratio in the lower crust deduced from S-wave modelling is 0.28, which indicates that the lower crust is composed mainly of gabbros. Below the continental edge, a typical continental crust with P-wave velocities between 5.2 and 7.0 km s-1, from the top to the bottom, shows a gradual seaward thinning of ~15 km over an ~35-km distance. This thinning is regularly distributed between the upper and lower crusts, and it characterizes a rifted margin, which has resulted from backarc extension at the rear of the Kabylian block, here represented by the Edough Massif at the shoreline. Above the continental basement, an ~2-km-thick, pre-Messinian sediment layer with a complex internal structure is interpreted as allochthonous nappes of flysch backthrusted on the margin during the collision of Kabylia with the African margin. The crustal structure, moreover, provides evidence for Miocene emplacement of magmatic intrusions in both the deep basin and the continental margin. Based on the crustal structure, we propose that the eastern Algerian basin opened during the southeastward migration of the European forearc before the collision, along a NW-SE elongated spreading centre that ran perpendicular to the subduction trend. Such an atypical geometry is explained by the diverging directions of the subduction rollback during the backarc opening: eastward for the Corsica-Sardinia block, and southward for the Kabylian blocks. This geometry of the forearc can be interpreted as the surface expression of a slab tear at depth, which is responsible for atypical magmatism in the overlying backarc oceanic basin. [ABSTRACT FROM AUTHOR]

The Scythian Platform (ScP) with a heterogeneous basement of Baikalian-Variscan- Cimmerian age is located between the East European Craton (EEC) on the north and the Crimean-Caucasus orogenic belt and the Black Sea (BS) Basin on the south. In order to get new constrains on the basin architecture and crustal structure of the ScP and a better understanding of the tectonic processes and evolution of the southern margin of the EEC during Mesozoic and Cenozoic time, a 630-km-long seismic wide-angle refraction and reflection (WARR) profile DOBRE-5 was acquired in 2011 October. It crosses in a W-E direction the Fore-Dobrudja Trough, the Odessa Shelf of the BS and the Crimean Plain. The field acquisition included eight chemical shot points located every 50 km and recorded by 215 stations placed every ~2.0 km on the land. In addition, the offshore data from existing profile 26, placed in the Odessa Shelf, were used. The obtained seismic model shows clear lateral segmentation of the crust within the study region on four domains: the Fore-Dobrudja Domain (km 20-160), an offshore domain of the Karkinit Trough at the Odessa Shelf of the BS (km 160-360), an onshore domain of the Central Crimean Uplift (Crimean Plain, km 360-520) and the Indolo-Kuban Trough at the Kerch Peninsula (km 520-620) that is the easternmost part of the Crimea. Two contrasting domains of the ScP within the central part of the DOBRE-5 profile, the Karkinit Trough and the Central Crimean Uplift, may represent different stages of the ScP formation. A deep Karkinit Trough with an underlying high-velocity (>7.16 km s-1) lower crust body suggests its rifting-related origin during Early Cretaceous time. The Central Crimean Uplift represents a thick (up to 47 km) crustal domain consisting of three layers with velocities 5.8-6.4, 6.5-6.6 and 6.7-7.0 kms-1, which could be evidence of this part of the ScP originating on the crust of Precambrian craton (EEC). The thick heterogeneous basement of the Central Crimean Uplift shows inclusions of granitic bodies associated with magmatic activity related with Variscan orogeny within the ScP. General bending and crustal scale buckling of the Central Crimean Uplift with a wavelength of 230 km could be an effect of the Alpine compressional tectonics in the adjacent Crimean Mountains. The extended/rifted continental margin of the ScP (EEC) at the Odessa Shelf and buckling/uplifted domain of the Central Crimean Uplift affected by compressional tectonics, are separated by the N-S oriented Western Crimean Fault. The crust of the southern margin of the EEC is separated from the ScP, which originated on the EEC crust tectonised and reworked during the Palaeozoic-Mesozoic, by the crustal fault of ~W-E orientation, which corresponds with the Golitsyn Fault observed at the surface between the EEC and the ScP. The Fore-Dobrudja Domain with a thick (>10 km) heterogeneous basement and two subhorizontal layers in the crystalline crust (with velocities 6.2-6.3 and 6.4-6.65 km s-1) differs from the ScP crust and its origin could be very similar to that of the Trans-European Suture Zone and Palaeozoic West European Platform. [ABSTRACT FROM AUTHOR]