Seismic Tomography Studies Research Articles

Seismic evidence of an extremely thin crust along a 70 Ma slow-spreading crustal segment in the equatorial Atlantic Ocean

The oceanic crust at slow-spreading ridges is formed by a combination of magmatic and tectonic processes. Seismic tomographic studies, commonly used to determine crustal structures, indicate that mature slow-spreading crust has an average thickness of 6.1 ± 0.9 km, with a lower to upper crustal thickness ratio of 2.3. However, tomographic methods are based on high-frequency approximations and can only provide information about large-scale crustal structures. Using seismic full-waveform inversion applied to ocean bottom seismometer data, we report the presence of nearly uniform and extremely thin crust (4 ± 0.2 km) with a lower to upper crustal thickness ratio of 0.74 along an ∼100 km, 70 Ma crustal segment formed at the slow-spreading Mid-Atlantic Ridge in the equatorial Atlantic Ocean. This thin crust is underlain by a Moho transition zone that is 1 ± 0.5 km thick, resulting in a combined crustal and Moho transition zone thickness of 5 ± 0.6 km. The crustal thinning is primarily due to the thinning of the lower crust, suggesting that a significant part of the crust is formed by lava flows and dike intrusions. The presence of an extremely thin crust indicates a decrease in mantle temperature by at least 50 °C, suggesting a vast extent of the upper mantle thermal minimum in the equatorial Atlantic Ocean, and the absence of influence of the mantle from the Siera Leone mantle plume 70 m.y. ago.

A 3‐D Seismic Tomographic Study of Spreading Structures and Smooth Seafloor Generated by Detachment Faulting—The Ultra‐Slow Spreading Southwest Indian Ridge at 64°30′E

AbstractAt ultra‐slow spreading ridges, with full spreading rates less than ∼20 mm/yr, spreading is accommodated both by highly spatially and temporally segmented magmatism, and tectonic extension along large‐scale detachment faults that exhume ultramafic material to the seafloor. In the most magma‐poor regions, detachment faulting alternates in polarity over time, producing a “flip‐flopping” effect of subsequent detachment dips. The resulting seafloor in these regions displays a morphology termed “smooth seafloor” comprising elongate, broad ridges with peridotite/serpentinite lithologies. We conducted tomographic travel‐time inversion of a 3‐D wide‐angle seismic data set acquired over a region of smooth seafloor around 64°30′E along the Southwest Indian Ridge (SISMOSMOOTH; Cruise MD199), to produce a seismic velocity volume through the crustal section and into the uppermost mantle. We observe patterns of velocity anomalies that correspond with variations in the bathymetry arising from the mode of spreading and are interpreted as changes in the degree of alteration with depth resulting from spatial and temporal variations in fluid‐rock interaction, controlled by faulting and tectonic damage processes. The detachment faults do not show simple planar structures at depth but instead mirror the shapes of the bathymetric ridges that they exhume. Magmatic input is overall highly limited, but there is one region on the lower part of an exhumed detachment footwall where a thickness of volcanic material is observed that suggests a component of syn‐tectonic volcanism, which could contribute to detachment abandonment.

Dynamic subsidence in the Colorado basin, offshore Argentina, South Atlantic

The Colorado basin in the SW Atlantic is one of the key exploratory frontier basins along the Argentine shelf. The stratigraphy shows a typical rift basin, divided into two major seismic successions, a Jurassic-Lower Cretaceous synrift followed by an Upper Cretaceous to Cenozoic postrift. They represent, respectively, the breakup of Gondwana and passive margin evolution. While the synrift basin formation was associated with the development of structural grabens, the preservation of thick (thousands of meters) postrift sequences are still debated because they cannot be accounted for only by thermal subsidence. In this work, we conducted a multimethodological subsidence analysis across the Colorado basin, including part of the continental slope. We performed a 1D backstripping analysis on 10 hydrocarbon exploration wells and 32 pseudo-wells, covering the whole basin area and a uniform stretching model, which were supported using dynamic topography data from the study region. From the backstripping analyses, considering the six major postrift sedimentary successions described in the literature, we could recognize three subsidence stages. An initial rapid subsidence stage during the post-rift (125–83 Ma), which is restricted to the main depocenters, followed by a stage of generalized and low subsidence focused on the main axis of the basin (83–33 Ma), with an increase in the last 33 my (33–0 Ma). The stretching models exhibit a typical asymptotic curve, depicting significant subsidence at the onset of the postrift stage, gradually diminishing to minimal values within a few tens of millions of years. Upon comparing the backstripping curves with the stretching subsidence models, particularly during the final flat segments, a residual negative value emerges over time (locally exceeding −500 m), indicating considerable residual subsidence during the Cenozoic postrift evolution along the Colorado Basin. This could be interpreted as dynamic subsidence. This residual sinking in the Argentina shelf matches with dynamic topography numerical models in the study region as well as the occurrence of accumulated slabs in the mantle detected by seismic tomography studies. We propose that the accumulation of slabs along this segment of the South American subduction zone might be the drivers of dynamic subsidence, which assisted to the preservation of a large thickness of post-rift strata along the SW Atlantic margin.

Spatiotemporal deformation and activity distribution of Irazú and Turrialba volcanoes, Costa Rica: Are these volcanoes interconnected?

The spatiotemporal distribution of volcanic activity poses a significant challenge to risk mitigation measures, as it is still largely unexplored in most of the volcanic systems. In this study, we re-assess the deformation observed by leveling surveys covering the nationwide tragedy 1963–1965 Irazú eruption in Costa Rica with a state-of-the-art analytical source inversion model. We combine the analytical model results with recent geophysical, geochemical, and petrology data to build a geological model of Irazú and its 10 km-distant Turrialba volcano. Based on the leveling survey, the source inversion model finds a reservoir between 5 and 7 km below the Irazú crater which is deeper than previously published. We also confirm that the source location is on top of the mid-crustal reservoir that was feeding Turrialba between the 2010–2022 eruptions. Using previous seismic tomography, gravity, petrology, and geochemistry study of Turrialba and Irazú, as well as other studies conducted on nearby volcanoes worldwide, we find that Irazú and Turrialba volcanoes likely share a mid-crustal plumbing system which could suggesting that their plumbing systems are interconnected with each other. These findings have important implications on the spatiotemporal distribution of the volcanic activity and for the 2.8 million inhabitants settled within a 50 km radius. Observations during recent episodes indicates that inflation beneath Irazú has the potential to trigger eruptive activity at either Irazú or Turrialba. While further analysis is required to assess the tectonic control on volcanic activity, tectonic processes may shape both short- and long-term volcanic activity. These results have global implications for risk mitigation measures for nearby volcanoes.

Mantle Structure Beneath the Damara Belt in South‐Central Africa Imaged Using Adaptively Parameterized P‐Wave Tomography

AbstractMany seismic tomography studies have indicated that the African Large Low Velocity Province (LLVP) extends from the lower mantle beneath southern Africa into the upper mantle beneath eastern Africa; however, it has been questioned whether the LLVP structure may also extend to the north or northwest beneath south‐central Africa. Debates regarding the upper mantle structure beneath the Damara Belt contribute to this uncertainty. Some studies suggest the Damara Belt is underlain by thermally perturbed upper mantle; however, other studies indicate the region is not associated with anomalous structure. Here, we use a comprehensive P‐wave travel‐time data set and an adaptive model parameterization to develop a new tomographic model for the Damara Belt and surrounding regions. Our results show that seismically slow structure beneath the Damara Belt is relegated to depths greater than ∼1,200 km, indicating that the LLVP is not significantly affecting this region. However, further to the northeast, the LLVP structure obliquely rises and crosses the mantle transition zone near the Irumide Belt, where it then extends into the upper mantle. The seismic structure beneath the Damara Belt and neighboring areas in our model correlates well with tectonic observations at the surface, including variations in heat flow, the distribution of geothermal features, the locations of rifts, and estimates of dynamic topography.

A Seismic Tomography, Gravity, and Flexure Study of the Crust and Upper Mantle Structure Across the Hawaiian Ridge: 2. Ka'ena

AbstractThe Hawaiian Ridge, a classic intraplate volcanic chain in the Central Pacific Ocean, has long attracted researchers due to its origin, eruption patterns, and impact on lithospheric deformation. Thought to arise from pressure‐release melting within a mantle plume, its mass‐induced deformation of Earth's surface depends on load distribution and lithospheric properties, including elastic thickness (Te). To investigate these features, a marine geophysical campaign was carried out across the Hawaiian Ridge in 2018. Westward of the island of O'ahu, a seismic tomographic image, validated by gravity data, reveals a large mass of volcanic material emplaced on the oceanic crust, flanked by an apron of volcaniclastic material filling the moat created by plate flexure. The ridge adds ∼7 km of material to pre‐existing ∼6‐km‐thick oceanic crust. A high‐velocity and high‐density core resides within the volcanic edifice, draped by alternating lava flows and mass wasting material. Beneath the edifice, upper mantle velocities are slightly higher than that of the surrounding mantle, and there is no evidence of extensive magmatic underplating of the crust. There is ∼3.5 km of downward deflection of the sediment‐crust and crust‐mantle boundaries due to flexure in response to the volcanic load. At Ka'ena Ridge, the volcanic edifice's height and cross‐sectional area are no more than half as large as those determined at Hawai'i Island. Together, these studies confirm that volcanic loads to the west of Hawai'i are largely compensated by flexure. Comparisons to the Emperor Seamount Chain confirm the Hawaiian Ridge's relatively stronger lithospheric rigidity.

A «necklace» of large clusters of strategic raw materials over a stagnant oceanic slab in East Asia

<p>An analysis of geological-geophysical, metallogenic, geochronological, and seismic tomographic studies in territories joining Southeast Russia, East Mongolia, and Northeast China led to the conclusion that deep geodynamics significantly influenced the formation of highly productive ore-magmatic systems in the Late Jurassic-Early Cretaceous. This influence was likely manifested through the initiation of decompression processes around stagnant slab boundaries in the Late Mesozoic. Decompression and advection, which are particularly active near the natural boundaries of the slab, act as triggers for the intense interaction of under and over subduction asthenospheric fluids with adjacent sections of the mantle and for the directed upwelling of powerful flows of matter and energy into the lithosphere. These flows determine the locations of intermediate and peripheral magma chambers: Primary chambers in the lower lithosphere among the metasomatized mantle and lower crust and associated chambers in the middle and upper cratonized parts of the lithosphere. Large ore clusters containing noble metals (Au, PGE), uranium, fluorite, and Cu-Mo-porphyry deposits are associated with late- and postmagmatic derivatives of the emerging magma chambers over the frontal and peripheral (paleotransform) boundaries of a stagnant Pacific slab. These large Late Mesozoic ore clusters and districts form a distinctive "necklace" of strategic materials in East Asia.</p>

CAUSES OF CONTINENTAL LITHOSPHERE DELAMINATION BENEATH THE ARABIAN-EURASIAN AND TIEN-SHAN (KYRGYZSTAN) COLLISION ZONES

The mantle processes occurring in collisional zones give rise to the occurrence of many tectonic and geodynamic processes on the surface which is associated with a high seismicity level. Seismic tomography studies showed that beneath some collision zones, such as for example, the Arabian-Eurasian and Tien-Shan, the mantle part of the continental lithosphere delaminates from the crust, with a further separation and plunge into the mantle which is also called delamination. This paper deals with a comparative analysis of the earlier obtained different-scale 3D models for seismic tomography of the crust and mantle of the Arabian-Eurasian and Tien-Shan collision zones to identify similarities and differences between the inhomogeneities observed. The paper also provides a review of the numerical modeling studies. A comparative analysis of seismotomographic models in combination with the results of mathematical modeling and the data on tectonic evolution allows making speculations about the causes of delamination in the studied regions.

A Seismic Tomography, Gravity, and Flexure Study of the Crust and Upper Mantle Structure of the Hawaiian Ridge: 1

AbstractThe Hawaiian Ridge has long been a focus site for studying lithospheric flexure due to intraplate volcano loading, but crucial load and flexure details remain unclear. We address this problem using wide‐angle seismic refraction and reflection data acquired along a ∼535‐km‐long profile that intersects the ridge between the islands of Maui and Hawai'i and crosses 80–95 Myr‐old lithosphere. A tomographic image constructed using travel time data of several seismic phases reveals broad flexure of Pacific oceanic crust extending up to ∼200–250 km either side of the Hawaiian Ridge, and vertically up to ∼6–7 km. The P‐wave velocity structure, verified by gravity modeling, reveals that the west flank of Hawaii is comprised of extrusive lavas overlain by volcanoclastic sediments and a carbonate platform. In contrast, the Hāna Ridge, southeast of Maui, contains a high‐velocity core consistent with mafic or ultramafic intrusive rocks. Magmatic underplating along the seismic line is not evident. Reflectors at the top and bottom of the pre‐existing oceanic crust suggest a ∼4.5–6 km crustal thickness. Simple three‐dimensional flexure modeling with an elastic plate thickness, Te, of 26.7 km shows that the depths to the reflectors beneath the western flank of Hawai'i can be explained by volcano loading in which Maui and the older islands in the ridge contribute ∼43% to the flexure and the island of Hawai'i ∼51%. Previous studies, however, revealed a higher Te beneath the eastern flank of Hawai'i suggesting that isostatic compensation may not yet be complete at the youngest end of the ridge.

On constraining 3D seismic anisotropy in subduction, mid-ocean-ridge, and plume environments with teleseismic body wave data

Conventional seismic tomography studies consider the Earth’s interior as mechanically isotropic, despite seismic anisotropy being widely observed. This current standard approach to seismic imaging is likely to lead to significant artefacts in tomographic images with first-order effects on interpretations and hinders the quantitative integration of seismology with geodynamic flow models. Although a few methodologies have been proposed for carrying out anisotropic tomography, their ability in simultaneously recovering isotropic and anisotropic structures has not been rigorously tested. In this contribution we use geodynamic and seismological modeling to predict the elastic properties and synthetic teleseismic P- and S-wave travel-time datasets for three different tectonic settings: a plume rising in an intraplate setting, a divergent margin, and a subduction zone. Subsequently, we perform seismic anisotropy tomography testing a recently developed methodology that allows for the inversion of an arbitrarily oriented weakly anisotropic hexagonally symmetric medium using multiple body-wave datasets. The tomography experiments indicate that anisotropic inversions of separate and joint P- and S-wave travel-times are capable of recovering the first order isotropic velocity anomalies and anisotropic patterns. In particular, joint P- and S-wave anisotropic inversions show that by leveraging both phases it is possible to greatly mitigate issues related to imperfect data coverage common in seismology and reduce parameter trade-offs. In contrast, by neglecting seismic anisotropy, isotropic tomographic models provide no information on the mantle fabrics and in all cases are contaminated by strong velocity artifacts. In the inversions the magnitude of anisotropy (as well as that of seismic anomalies) is always underestimated owing to regularization procedures and smearing effects. It follows that the true seismic anisotropy of mantle rocks is likely higher than estimated from anisotropic tomographies, and more consistent with predictions from laboratory and numerical micro-mechanical experiments. Altogether, these results suggest that anisotropic body-wave tomography could provide unprecedented information about the Earth’s deep geological structure, and that the latter could be better recovered by complementing teleseismic body-wave travel-times with other geophysical datasets.

Heat flow of northern Norway, new data and geodynamic implications

The present contribution introduces new heat flow data from northern Norway, a region of the Baltic Shied and Scandinavian Caledonides that has been poorly covered until now. We computed heat flow values based on data gathered in ten boreholes reaching total depths ranging between ∼390 m and 960 m below ground level. Abundant core material for five of the studied boreholes allowed for precise determination of thermal conductivity profiles. The new determinations represent significant improvement with respect to the few pre-existing heat flow values that were based on shallow drillholes and lake measurements. The obtained heat flow values range between ∼40 and 70 mW/m2, after corrections, and suggest significant heat flow decrease (i.e. ∼10–20 mW/m2) from chiefly Proterozoic terrains to the Archean nucleus in NE Norway. The results suggest also sharp decrease in heat flow from the NE Atlantic to the Lofoten-Vesterålen margin but rather smooth gradients from the Barents Shelf to the continent. The former may be associated with abrupt deepening of the base of the lithosphere below the Lofoten-Vesterålen margin, as already suggested by previous seismic tomography studies and consistent with drastic strain focusing and the formation of a narrow margin. In contrast, gradual deepening of the base lithosphere towards the continent appears to be in agreement with the observed diffuse deformation that took place in the comparatively wide Western Barents Shelf.

The effect of the mantle and core matter phase state on the course of geodynamic processes

The study of the course of geodynamic processes in the lower crust and upper mantle proves that an additional energy contribution is made by a change in the phase state of matter with increasing pressure and temperature. The gas phase, composed of hydrogen, oxygen and carbon, turns into a fluid that combines the properties of a liquid and a gas. The result is a change in the behavior of fluid-crystal and fluid-melt systems which significantly accelerates melting and physicochemical interactions in the thermal asthenosphere. These conclusions are confirmed by numerous experimental studies and the results of the study of xenoliths representing the crust and mantle of cratons and active regions.
 Seismic tomography studies show distinct patterns of inhomogeneities in physical pro­per­ties, reflecting inhomogeneities in the mantle structure. Many works hypothesize, with substantiation, that plumes or fluid flows arise at the boundary of the core and mantle and are factors of all geodynamic processes. Modern ideas about the composition of the Earth's core are based on the statement that it is composed of molten iron with minor impurities of other elements. However, calculations of the energy balance and physical modeling of the redistribution of matter in the core itself show that the removal of volatile components or convective currents do not provide enough energy for the formation of plumes.
 The assumption that the substance of the core is an electrically conductive ionic liquid in which chemical compounds have completely dissociatedand the electronic structure has no gapradically changes the idea of the energetics of the core and the possibility of initiating plume processes. The properties of a substance in a similar phase state are fundamentally different from the properties of a liquid.

Identifikasi Struktur Kerak Di Bawah Permukaan Jawa Timur Berdasarkan Hasil Tomografi Seismik Menggunakan Model Kecepatan Gelombang P Dan Gelombang S

Tectonic conditions that are vulnerable to earthquakes have become a trigger for researchers to conduct studies on subsurface conditions in East Java. A seismic tomographic study based on the P-wave and S-wave velocity models was applied to identify crustal structures in East Java. The research data totaled 3,893 earthquake events and 28 stations from the Meteorology, Climatology and Geophysics Agency (BMKG) in the 2009-2017 period covering coordinates 5oSL- 11oSL and 110o - 115o EL. The modeling results in this study are expressed in velocity perturbations against the ak135 one-dimensional reference velocity between -5% to 5%. The results of this study indicate that in the central of East Java, there is a negative velocity anomaly which is identified as a sedimentary layer that fills the basin in the Kendeng Zone and volcaniclastic deposits in the Southern Mountains Zone to a depth of 5 km. The dominant negative velocity anomaly in southern East Java is associated with the presence of intrusive and bedrock in the Southern Mountain Zone. The transition zone between the crust and upper mantle in East Java is observed at a depth of 35 km below the surface.

Lateral Variations of Attenuation in the Crust of Alaska Using Lg Q Tomography

ABSTRACT We have conducted a crustal seismic (QLg) attenuation tomography study across Alaska using recordings from the EarthScope USArray from 2014 to 2019. The resolving power of the inversion is 150 × 150 km for most of Alaska, and it is 75 × 75 km in central and southern Alaska. Numerous fault systems and high mountain ranges are present across Alaska and accommodate compression in the north–south direction and shearing of southern Alaska toward the west. These mountain ranges include the Brooks range in the north, the Alaska range in central Alaska, and the Aleutian range in the southwest. The average LgQ for all of Alaska is significantly higher than in the western United States and Canada. This lower average attenuation impacts seismic hazard estimates for the region. According to the tomographic results, we see a significant variation of the QLg values from low to high across the southern part of the Brooks range. Also, we found higher attenuation in the southeast region of Alaska, where the Wrangell volcanoes are located. Moreover, we see an area of lower attenuation associated with weak frequency dependence in the south-central region of Alaska next to Anchorage. Another anomaly with lower attenuation can be seen extending from central Alaska to southeast Alaska, possibly associated with the Yukon–Tanana terrane. There are a few areas like southwest Alaska associated with the Togiak terrane and an area next to Fairbanks in Alaska’s interior that shows lower attenuation with lower frequency dependence and higher attenuation with higher frequency dependence, respectively, for low frequencies up to 3 Hz. Our model’s highest η zones (η≳95) are mostly confined to major tectonic terranes and other major tectonic elements such as faults and fractures. Regional variations in crustal attenuation can impact local seismic hazard estimates if incorporated into the hazard analysis.

The deep structure of the Trans-European Suture Zone (based on seismic survey and GSR data) and some insights in to its development

Deep crust and mantle structure of the Trans-European Suture Zone (TESZ) is considered on the basis of geological and geophysical investigations in the Baltic Sea-Black Sea section. The crustal structure of TESZ was studied on the basis of wide-angle depth seismic sounding (WDS), which was performed by international scientific teams with the participation of the Institute of Geophysics of NAS of Ukraine (IGF NASU). TESZ mantle structure was studied down to a depth of 800 km by the 3D P-velocity model of the Eurasian mantle according to the Taylor approximation method developed in the Institute of Geophysics of NASU. It is concluded that the deep crustal and mantle structure of the zone is a result of the simultaneous action of plate- and plum tectonic processes. TESZ was formed on two major collision alstages: in the late Ordovician — early Silurianas a result of the accession of the Avalonia microcontinent to the East European Platform (EEP), and in the late Carboniferous – early Permian with the accession of the European Hercynian (Varisian) terranes to EEP. The TESZ crustal structure is a trough of 150 (sometimes up to 200) km wide and several to 21 km deep, built by the allochthonous complex of paleozoids that underwent Caledonian and Hercynian orogens beyond the trough. Mantle structure of the TESZ, according to seismic tomographic studies, is of dual nature: on the one hand, the zone is traced subvertically to a depth of 700 km, on the other, within the zone there are everywhere inclined layers — slips to the depth of 350—600 km, that is the traces of subduction processes, which precededorac companied TESZ formation. Both structural features overlapeachother, which complicates paleotectonic and geohistorical analysis of TESZ formation. TESZ sinking to greater depths in the mantle can be explained by its increased permeability for advection of ultra-deep mantle fluids, established hereborogensic tomographic and paleomagnetic methods. Several variants of TESZ formation are assumed — A- or B-subduction during north eastern plate thrusting under the south western one in all variants.

Seismic Azimuthal Anisotropy Beneath a Fast Moving Ancient Continent: Constraints From Shear Wave Splitting Analysis in Australia

AbstractSeismic azimuthal anisotropy beneath Australia is investigated using splitting of the teleseismic PKS, SKKS, and SKS phases to delineate asthenospheric flow and lithospheric deformation beneath one of the oldest and fast‐moving continents on Earth. In total 511 pairs of high‐quality splitting parameters were observed at 116 seismic stations. Unlike other stable continental areas in Africa, East Asia, and North America, where spatially consistent splitting parameters dominate, the fast orientations and splitting times observed in Australia show a complex pattern, with a slightly smaller than normal average splitting time of 0.85 ± 0.33 s. On the North Australian Craton, the fast orientations are mostly N‐S, which is parallel to the absolute plate motion (APM) direction in the hotspot frame. Those observed in the South Australian Craton are mostly NE‐SW and E‐W, which are perpendicular to the maximum lithospheric horizontal shortening direction. In east Australia, the observed azimuthal anisotropy can be attributed to either APM induced simple shear or lithospheric fabric parallel to the strike of the orogenic belts. The observed spatial variations of the seismic azimuthal anisotropy, when combined with results from depth estimation utilizing the spatial coherency of the splitting parameters and seismic tomography studies, suggest that the azimuthal anisotropy in Australia can mostly be related to simple shear in the rheologically transition layer between the lithosphere and asthenosphere. Non‐APM parallel anisotropy is attributable to modulations of the mantle flow system by undulations of the bottom of the lithosphere, with a spatially variable degree of contribution from lithospheric fabric.

Wavefield distortion imaging of Earth's deep mantle

The seismic wavefield, as recorded at the surface, carries information about the seismic source and Earth's structure along the seismic path, essential for the understanding of the interior of our planet. For 40 years seismic tomography studies have resolved the 3D seismic velocity structure in growing detail using seismic traveltimes and waveforms. These studies have been driving our understanding of the dynamics and evolution of the planet, but are limited in their spatial resolution to imaging scales of a few 100 s to 1000 km due to the constraints of the tomographic inversion. Detailed studies of seismic waveforms can resolve finer scale structure but are often reliant on serendipitous source-receiver combinations and provide very uneven coverage of the planet. Therefore, we often lack an understanding of the fine scale structure of the Earth that is important to understand structures and processes such as mantle plumes or details of slab recycling. Here we show evidence that we can exploit slowness vector deviations of the seismic wavefield to extend our knowledge of Earth structure to smaller scales using large datasets. Analysing seismic array data, we show strong and measurable focussing and defocussing effects of the teleseismic P and Pdiff wavefield sampling the deep Earth. We compare the P-wave results to additional S and Sdiff data and find good agreement between both wavetypes. We can link the wavefield deviations to strong velocity variations assuming sharp boundaries are sampled along the path in the deep mantle. The dataset samples the Pacific and Gulf of Mexico well and shows strong horizontal incidence (backazimuth) deviations in the Pacific (up to 14° westwards) and beneath the Gulf of Mexico (up to 5 to 8° east- and west-ward). The backazimuth deviations are also reflected in slowness deviations in the range of ± 0.8 s/° relating to velocity variations in the range of ± 9 km/s. Using 3D raytracing we are able to forward model the detected backazimuth variations of the P and Pdiff dataset. The high frequencies of the P-waves, density of the ray-paths, and low computational cost of our forward calculation allow us to construct a higher resolution and more detailed model of velocity anomalies under Hawaii than was possible with previous methods. The best-fitting velocity model for the Pacific contains two low-velocity regions located at N25°/W155° and N25°/W165° beneath the tip of the Hawaii Emperor chain. The Pacific anomalies have diameters (D) of 6° and 2° with velocity reductions (dVP) of 8% and 4% with heights (H) above the CMB of 70 km and at least 200 km, respectively. We also detect a fast region of 3% velocity increase in the North Pacific rising at least 300 km above the CMB with a diameter of 12° at N60°/W175°. Beneath the Gulf of Mexico we find ambiguous results with either a slow region (N25°/W85°, H = 200 km, dVP=-3%, D = 2°) or a fast region (N15°/W75°, H = 200 km, dVP = 3%, D = 4°) able to explain the data. We thus show that the directivity information of the seismic wavefield - largely underexploited - can be used to resolve the fine scale velocity structure of the Earth's interior with great accuracy and can deliver additional insight into deep Earth dynamics.

Insights into the relationship between the Red Sea rift-related structures and the seismo-volcanic activity in Harrat Lunayyir, Saudi Arabia: A seismic tomography study

Harrat Lunayyir is one of the youngest Cenozoic volcanic fields in western Saudi Arabia, which in 2009 experienced a seismic swarm of about 34,000 events with a maximum magnitude of M5.4. Many studies debated about the relationship of a depicted shallow-level magma intrusion with the Red Sea rifting and the channelized northward flow from the Afar mantle plume. In this study, a detailed seismic tomography inversion for the P- and S-wave velocities, and the Poisson’s ratio structures beneath Harrat Lunayyir and the adjacent Red Sea offshore area is presented. The tomographic images showed evidence of magma intrusions beneath the Lunayyir Swarm Area (LSA) characterized by low P- and S-wave velocity and high Poisson’s ratio anomalies down to 21 km depth. These anomalies are in juxtaposition with some high-velocity anomalies attributed to remnants of solidified magma intrusions relevant to previous episodes of volcanicity. Beneath the Red Sea, the lithospheric mantle structure is clearly observed as a high P-wave velocity high-Poisson’s ratio anomaly at a depth of 20–40 km. Traces of high-Vp anomalies with anomalous Poisson’s ratio are observed in the area between the Red Sea and the LSA suggesting that a channel of hot lithospheric mantle material may link the Red Sea rift structure with the LSA. We suggest that the Red Sea drifting and its related processes in the Zabargad Shear Zone might have significantly contributed to the early formation of the Lunayyir volcanic field and could possibly have in an impact on any magmatic activity in the future.

Mantle structure and dynamics at the eastern boundary of the northern Cascadia backarc

The tectonics of southwestern Canada are dominated by the Cascadia subduction zone. The northern Cascadia backarc encompasses a > 400 km wide region of the Southern Canadian Cordillera. Geophysical observations, including seismic tomography and surface heat flow, show that the backarc is characterized by a hot, thin lithosphere (60–70 km). The eastern limit of the backarc approximately underlies the Rocky Mountain Trench, where there is an abrupt eastward increase in lithosphere thickness to the ∼250 km thick North American (Laurentian) Craton. Seismic tomography studies show that the transition in lithosphere thickness occurs over a horizontal distance of 50–100 km, resulting in a subvertical to west-dipping lithosphere step, with a dip angle of 75–90°. Using numerical models, we show that such a structure can be readily destabilised by internal buoyancy forces, edge-driven convection, and shearing by regional mantle flow. To maintain a subvertical step for > 50 Myr, the lowermost craton mantle lithosphere must be both dry and moderately chemically depleted. The observed westward dip may reflect partial lateral extrusion of the lowermost craton lithosphere, as well as shearing from west-directed mantle flow associated with the Cascadia subduction zone. The models also show that the backarc mantle must be relatively weak, such that vigorous convection maintains the hot, thin lithosphere. This also provides a mechanism to explain the observed lateral seismic gradient between the low-velocity backarc mantle and high-velocity craton. Our models demonstrate that the eastern limit of the Cascadia backarc is a region of active mantle flow, including possible slow deformation of the craton edge.

Models for the evolution of seamounts

SUMMARY Seamounts are volcanic constructs that litter the seafloor. They are important for understanding numerous aspects of marine science, such as plate tectonics, the volcanic melt budget, oceanic circulation, tsunami wave diffraction, tidal energy dissipation and mass wasting. Geometrically, seamounts come in many sizes and shapes, and for the purpose of modelling them for morphological, gravimetric or isostatic studies it is convenient to have simple analytical models whose properties are well known. Here, we present a family of seamount models that may be used in such studies, covering both the initial construction phase and later mass-wasting by sectoral collapses. We also derive realistic axisymmetric density variations that are compatible with observed first-order structure from seismic tomography studies.