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We test here an earlier hypothesis of ours that the Massif Central hotspot, combined with the Iceland mantle plume, exercised first-order control of the Paleogene sedimentary record in the Anglo-Paris Basin. The Anglo-Paris Basin formed a southwestern arm of the Paleogene North Sea. Correlation of Paleogene depositional sequences across the North Sea Basin suggests that control of the sedimentary record in the Anglo-Paris Basin was exercised by both hotspots. The Cretaceous chalk in the basin was uplifted in two main episodes during the early and middle Paleocene. Also during the Paleocene, there was broadly synchronous vertical uplift of the unconformably underlying Mesozoic rocks on both the northwest and southeast flanks of the Anglo-Paris Basin. The resulting opposed regional dips are still obvious at the present day. This regional tilting of the Mesozoic into the Paleogene basin was caused by uplift by the early Iceland plume to the northwest, and by uplift associated with the inception of the early Massif Central hotspot to the southeast. Crustal shortening played a minor role in these hotspot-created vertical movements. In the Paleocene-early Eocene, high-frequency changes in relative regional sea-level in the Anglo-Paris Basin were controlled by apparently synchronous time-dependent behaviour of the Iceland and Massif Central hotspots. In contrast, from the early Eocene onwards, control of sea-level appears to have been exercised separately, with the two hotspots pulsing at different times.

The Oman-United Arab Emirates ophiolite has been used extensively to document the geological processes that form oceanic crust. The geometry of the ophiolite, its extension into the Gulf of Oman, and the nature of the crust that underlies it are, however, unknown. Here, we show the ophiolite forms a high velocity, high density, >15 km thick east-dipping body that during emplacement flexed down a previously rifted continental margin thereby contributing to subsidence of flanking sedimentary basins. The western limit of the ophiolite is defined onshore by the Semail thrust while the eastern limit extends several km offshore, where it is defined seismically by a ~40–45°, east-dipping, normal fault. The fault is interpreted as the southwestern margin of an incipient suture zone that separates the Arabian plate from in situ Gulf of Oman oceanic crust and mantle presently subducting northwards beneath the Eurasian plate along the Makran trench., The Semail ophiolite provides evidence for geological processes that form oceanic crust, however, its deep structure remains debated. Here, the authors use geophysical imaging to determine that the ophiolite is bound by a thrust fault in the west, and a normal fault in the east, bounding a rapidly subsiding basin, implying the ophiolite may not be rooted in the Gulf of Oman crust.

Title:\ud Authors: Conor Andrew Bacon, Jessica Helen Johnson, Robert Stephen White, Nicholas Rawlinson\ud Journal: Journal of Geophysical Research: Solid Earth\ud \ud Plain Language Summary\ud Iceland is well known for its earthquakes and volcanoes, which have helped to produce an awe-inspiring primordial landscape over the last 20 million years or so. The emergence of Iceland in the North Atlantic Ocean can be attributed to the interaction of the Mid-Atlantic Ridge, where new oceanic crust forms by rifting between the North American and Eurasian plates, and a rising conduit of hot mantle from deep in the Earth, known as a mantle plume. The confluence of these two phenomena has produced excessive melting of mantle rocks, with the resultant melt accreted and cooled to form the Icelandic crust. We investigate how extensional stresses related to the divergence of the two tectonic plates have influenced the upper 3–4 km of the crust around Askja volcano, in the deep interior of Iceland. To do so, we exploit information contained in recordings of earthquakes from the neighborhood of Askja, which suggests that cracks formed parallel to the Mid-Atlantic Ridge, which permeate the upper crust, gradually close with depth. This relationship between the regional stress field associated with rifting and brittle deformation in the uppermost crust breaks down around Askja itself, where magmatic processes likely cause local changes in the stress field.\ud \ud Abstract\ud The Icelandic crust is a product of its unique tectonic setting, where the interaction of an ascending mantle plume and the Mid-Atlantic Ridge has caused elevated mantle melting, with the melt accreted and cooled in the crust to form an oceanic plateau. We investigate the strength and orientation of seismic anisotropy in the upper crust of the Northern Volcanic Zone using local earthquake shear-wave splitting, with a view to understanding how the contemporary stress field may influence sub-wavelength structure and processes. This is achieved using a data set comprising urn:x-wiley:21699313:media:jgrb55395:jgrb55395-math-000150,000 earthquakes located in the top 10 km of the crust, recorded by up to 70 stations over a 9 year period. We find that anisotropy is largely confined to the top 3–4 km of the crust, with an average delay time of 0.10 ± 0.05 s, and an average orientation of the fast axis of anisotropy of N014°E ± 27°, which is perpendicular to the spreading direction of the Eurasian and North American plates (N106°E). These results are consistent with the presence of rift-parallel cracks that gradually close with depth, the preferential opening of which is controlled by the regional stress field. Lateral variations in the strength of shear wave anisotropy (SWA) reveal that regions with the highest concentrations of earthquakes have the highest SWA values (∼10%), which reflects the presence of significant brittle deformation. Disruption of the orientation of the fast axis of anisotropy around Askja volcano can be related to local stress changes caused by underlying magmatic processes.

\ud Successful inverse modeling of observed longitudinal river profiles suggests that fluvial landscapes are responsive to continent-wide tectonic forcing. However, inversion algorithms make simplifying assumptions about landscape erodibility and drainage planform stability that require careful justification. For example, precipitation rate and drainage catchment area are usually assumed to be invariant. Here, we exploit a closed-loop modeling strategy by inverting drainage networks generated by dynamic landscape simulations in order to investigate the validity of these assumptions. First, we invert 4,018 African river profiles to determine an uplift history that is independently calibrated, and subsequently validated, using separate suites of geologic observations. Second, we use this tectonic forcing to drive landscape simulations that permit divide migration, interfluvial erosion and changes in catchment size. These simulations reproduce large-scale features of the African landscape, including growth of deltaic deposits. Third, the influence of variable precipitation is investigated by carrying out a series of increasingly severe tests. Inverse modeling of drainage inventories extracted from simulated landscapes can largely recover tectonic forcing. Our closed-loop modeling strategy suggests that large-scale tectonic forcing plays the primary role in landscape evolution. One corollary of the integrative solution of the stream-power equation is that precipitation rate becomes influential only if it varies on time scales longer than ∼1 Ma. We conclude that calibrated inverse modeling of river profiles is a fruitful method for investigating landscape evolution and for testing source-to-sink models.\ud \ud Plain Language Summary\ud There is excellent geologic evidence that large portions of the African landscape were lifted up above sea level over the last 30 million years by upward flow of hot mantle rocks beneath the tectonic plate. The strongest evidence comes from marine deposits which contain fossil fish and sea snakes that are now perched at elevations of hundreds of meters in the middle of the North African desert. Mantle processes gave rise to an egg-carton pattern of gigantic swells and depressions that characterizes much of the continent. As the landscape evolved, it was sculpted and eroded by the action of massive rivers such as the Niger, the Nile and the Congo. Height along the length of each of these rivers varies and appears to preserve a memory of landscape growth. In that sense, rivers appear to act as tape recorders of tectonic processes such as mantle flow. Here, we use computer simulations of an evolving landscape to test the idea that rivers contain mantle memories. These simulations, which include complexities such as variable rainfall, allow rivers to develop naturally as landscapes grow. Our results suggest that the African landscape and its drainage patterns contain valuable information about deep Earth processes.

Funder: Geoscience Australia, Funder: Shell Global, Shell Exploration and Production Company; Id: http://dx.doi.org/10.13039/100011092, Spatio‐temporal changes of upper mantle structure play a significant role in generating and maintaining surface topography. Although geophysical models of upper mantle structure have become increasingly refined, there is a paucity of geologic constraints with respect to its present‐day state and temporal evolution. Cenozoic intraplate volcanic rocks that crop out across eastern Australia provide a significant opportunity to quantify mantle conditions at the time of emplacement and to independently validate geophysical estimates. This volcanic activity is divided into two categories: age‐progressive provinces that are generated by the sub‐plate passage of mantle plumes and age‐independent provinces that could be generated by convective upwelling at lithospheric steps. In this study, we acquired and analyzed 78 samples from both types of provinces across Queensland. These samples were incorporated into a comprehensive database of Australian Cenozoic volcanism assembled from legacy analyses. We use geochemical modeling techniques to estimate mantle temperature and lithospheric thickness beneath each province. Our results suggest that melting occurred at depths ≤80 km across eastern Australia. Prior to, or coincident with, onset of volcanism, lithospheric thinning as well as dynamic support from shallow convective processes could have triggered uplift of the Eastern Highlands. Mantle temperatures are inferred to be ∼50–100°C hotter beneath age‐progressive provinces that demarcate passage of the Cosgrove mantle plume than beneath age‐independent provinces. Even though this plume initiated as one of the hottest recorded during Cenozoic times, it appears to have thermally waned with time. These results are consistent with xenolith thermobarometric and geophysical studies.

The generation and evolution of continental topography are fundamental geologic and geomorphic concerns. In particular, the history of landscape development might contain useful information about the spatiotemporal evolution of deep Earth processes, such as mantle convection. A significant challenge is to generate observations and theoretical predictions of sufficient fidelity to enable landscape evolution to be constrained at scales of interest. Here, we combine substantial inventories of stratigraphic and geomorphic observations with inverse and forward modeling approaches to determine how the North American landscape evolved. First, stratigraphic markers are used to estimate postdepositional regional uplift. Present‐day elevations of these deposits demonstrate that >2 km of long‐wavelength surface uplift centered on the Colorado‐Rocky‐Mountain plateaus occurred in Cenozoic times. Second, to bridge the gaps between these measurements, an inverse modeling scheme is used to calculate the smoothest spatiotemporal pattern of rock uplift rate that yields the smallest misfit between 4,161 observed and calculated longitudinal river profiles. Our results suggest that Cenozoic regional uplift occurred in a series of stages, in agreement with independent stratigraphic observations. Finally, a landscape evolution model driven by this calculated rock uplift history is used to determine drainage patterns, denudation, and sedimentary flux from Late Cretaceous times until the present day. These patterns are broadly consistent with stratigraphic and thermochronologic observations. We conclude that a calibrated inverse modeling strategy can be used to reliably extract the temporal and spatial evolution of the North American landscape at geodynamically useful scales.

Global inventories of stable sea-level markers for the peak of the last interglacial period, Marine Isotopic Stage (MIS) 5e, play a pivotal role in determining sea-level changes and in testing models of glacial isostatic adjustment. Here, we present surveying and radiometric dating results for emergent terraces from northern Madagascar, which is generally regarded as a stable equatorial site. Fossil coral specimens were dated using conventional and open-system corrected uranium series methods. Elevation of the upper (undated) terrace decreases from 33.8 m to 29.5 m over a distance of 35 km. An intermediate terrace has an average radiometric age of 130.7 ± 13.2 ka (i.e. MIS 5e). Its elevation decreases from 9.3 m to 2.8 m over a distance of 80 km. The record of the lowest terrace is fragmentary and consists of beach rock containing rare corals with ages of 1.6–3.8 ka. The spatial gradient of the MIS 5e marker is inconsistent with glacio-isostatic adjustment calculations. Instead, we propose that variable elevations of this marker around Madagascar, and possibly throughout the Indian Ocean, at least partly reflect spatial patterns of dynamic topography generated by sub-plate mantle convection.

We present a novel numerical method to simulate global seismic wave propagation in realis- tic aspherical 3-D Earth models across the observable frequency band of global seismic data. Our method, named AxiSEM3D, is a hybrid of spectral element method and pseudo-spectral method. It describes the azimuthal dimension of global wavefields with a substantially reduced number of degrees of freedom via a global Fourier series parametrisation, of which the num- ber of terms can be locally adapted to the inherent azimuthal complexity of the wavefields. AxiSEM3D allows for material heterogeneities, such as velocity, density, anisotropy and at- tenuation, as well as for finite undulations on radial discontinuities, both solid-solid and solid- fluid, and thereby a variety of aspherical Earth features such as ellipticity, surface topography, variable crustal thickness, undulating transition zone and core-mantle boundary topography. Undulating discontinuities are honoured by means of the “particle relabelling transformation”, so that the spectral element mesh can be kept spherical. The implementation of the particle relabelling transformation is verified by benchmark solutions against a discretised 3-D spectral element method, considering ellipticity, topography and bathymetry (with the ocean approxi- mated as a hydrodynamic load) and a tomographic mantle model with an undulating transition zone. For the state-of-the-art global tomographic models with aspherical geometry but without a 3-D crust, efficiency comparisons suggest that AxiSEM3D can be 2 to 3 orders of magnitude faster than a discretised 3-D method for a seismic period at 5 s or below, with the speed-up in- creasing with frequency and decreasing with model complexity. We also verify AxiSEM3D for localised small-scale heterogeneities with strong perturbation strength. With reasonable com- puting resources, we have achieved a corner frequency up to 1 Hz for 3-D mantle models.

\ud Crystallization temperatures of primitive olivine crystals have been widely used as both a proxy for, or an intermediate step in calculating, mantle temperatures. The olivine-spinel aluminum-exchange thermometer has been applied to samples from mid-ocean ridges and large igneous provinces, yielding considerable variability in olivine crystallization temperatures. We supplement the existing data with new crystallization temperature estimates for Hawaii, between 1282 ± 21 and 1375 ± 19°C. Magmatic temperatures may be linked to mantle temperatures if the thermal changes during melting can be quantified. The magnitude of this temperature change depends on melt fraction, itself controlled by mantle temperature, mantle composition and lithosphere thickness. Both mantle composition and lithosphere thickness vary spatially and temporally, with systematic differences between mid-ocean ridges, ocean islands and large igneous provinces. For crystallization temperatures to provide robust evidence of mantle temperature variability, the controls of lithosphere thickness and mantle lithology on crystallization temperature must be isolated. We develop a multi-lithology melting model for predicting crystallization temperatures of magmas in both intra-plate volcanic provinces and mid-ocean ridges. We find that the high crystallization temperatures seen at mantle plume localities do require high mantle temperatures. In the absence of further constraints on mantle lithology or melt productivity, we cannot robustly infer variable plume temperatures between ocean-islands and large igneous provinces from crystallization temperatures alone; for example, the extremely high crystallization temperatures obtained for the Tortugal Phanerozoic komatiite could derive from mantle of comparable temperature to modern-day Hawaii. This work demonstrates the limit of petrological thermometers when other geodynamic parameters are poorly known.\ud \ud Plain Language Summary\ud The temperature inside the Earth varies a lot. There are many ways of measuring the mantle's temperature in the present-day, but to understand how our planet has changed through time, we need to know how hot its interior was in the past. One of the ways we can estimate mantle temperatures from ancient and modern rocks is from their crystal chemistry. By measuring the aluminum content in crystals of olivine, we can estimate their crystallization temperature. We do this for crystals from Hawaii. To turn the crystallization temperatures into the mantle temperature we need to know the proportions of minerals the mantle is made of. However, we often don't know all of this information. Using a new model of mantle melting we can calculate how uncertain the mantle temperature is when we only have a crystallization temperature. We find that the mantle under Hawaii is 1582 ± 65°C, much hotter than normal mantle, which has a temperature 1364 ± 23°C. We also apply our method to crystallization temperatures from other locations, including ancient volcanic rocks. We find that crystallization temperatures from large igneous provinces, formed by unusually hot mantle, are consistent with their mantle having a similar temperature to Hawaii

Widespread marine red beds in the European Telychian (lower-Silurian) are one expression of an interval of unusually widespread oxic conditions in low palaeolatitude Silurian seas. This work examines in detail the geochemical and magnetic susceptibility record of cores from southern Poland, which also express the Telychian oxygenation event in grey-mudstones. The geochemical data provide an evaluation of redox conditions, palaeoweathering, sediment provenance, primary palaeoproductivity and upwelling. Sediment provenance is evaluated against possible sources on the East European Craton. The data suggest that the magnetic susceptibility is carried by both paramagnetic Fe-silicates and Fe-oxides. Magnetic data are supplemented by magnetic hysteresis and isothermal remanent magnetisations, and mineralogical data on selected samples. In Poland the oxygenation event is clearly expressed in larger Fe2O3 and magnetic susceptibility, caused by enhanced palaeoweathering, changes in sediment provenance and a redox influence on the preservation of Fe-oxides. A much briefer oxygenation event is detected in the upper Rhuddanian. Palaeoproductivity fluxes indicate that the Telychian oxygenation event was caused by a reduction in primary oceanic palaeoproductivity, possibly linked to a reduction in nutrient delivery to the margin of the East European Craton, inferred to be caused by increased aridity. The increased aridity stimulated enhanced delivery of Fe-enriched aeolian dust from soils, generating a magnetic susceptibility and Fe2O3 expression of the Telychian oxygenation event.

Plain Language Summary\ud The maximum depth of earthquakes provides a crucial constraint on how deep the lithosphere can sustain sufficient stresses to produce brittle failure, rather than deforming through slow ductile processes. In East Africa, the maximum depth of earthquakes varies. In the northern sections of the East Africa Rift System, earthquakes are confined to the upper crust shallower than ∼15 km, but in the southern and western sections of the rift, we observe earthquakes occurring much deeper, into the lower crust and potentially the uppermost mantle. This variation, and the short distance over which it takes place in Northern Tanzania, indicates that it is controlled by the composition of the lower crust, which is likely to be anhydrous where the deeper earthquakes occur.\ud \ud Abstract\ud The well‐established variation in the depth of earthquakes along both branches of the East African Rift System offers an opportunity to probe the controls on the depth‐extent of seismogenesis, and the length‐scales over which this may vary. We present an updated compilation of well‐determined earthquake depths from teleseismic and regional seismic data for the East African Rift System, combined with a summary of the depth distribution of smaller‐magnitude microseismicity from 13 local network deployments. Moderate‐to‐large magnitude (Mw > 5) earthquakes, unrelated to the movement of magmatic fluids, beneath Afar, the Main Ethiopian Rift, and the northernmost sections of the Eastern Branch, are confined to the upper crust. Seismicity along the Western Branch, and the southern‐most sections of the Eastern Branch extends deeper, into the lower crust, in places to depths close to the local Moho. Along the Eastern Branch, in northern Tanzania, the transition between these two regimes occurs over a distance of ≤40 km, requiring a change to a higher temperature cutoff for the deeper earthquakes; an effect that must be compositional in origin. This compositional variation in the lower crust is most likely related to the degree of hydration. Earthquakes deep within the lower crust are therefore likely to be a proxy for an anhydrous crustal composition.

Africa’s Cenozoic tectonism is often attributed to mantle plumes, particularly below East Africa, but their morphology, number, location, and impact on the African lithosphere are debated. The broad slow wavespeed African Superplume, ubiquitous in large-scale tomographic models, originates below South Africa, reaching the surface somewhere below East Africa. However, whether the diverse East African mantle geochemistry is best reconciled with one heterogeneous upwelling, or current tomographic models lack the resolution to image multiple distinct plumes, remains enigmatic. S-wavespeed tomographic images of Africa are legion, but higher-frequency P-wavespeed whole-mantle models possessing complementary diagnostic capabilities are comparatively lacking. This hinders attempts to disentangle the effects of Cenozoic hotspot tectonism and Pan African (and older) tectonic events on the East African lithosphere. Here we develop a continental-scale P-wave tomographic model capable of resolving structure from upper-to-lower mantle depths using a recently-developed technique to extract absolute arrival-times from noisy, temporary African seismograph deployments. Shallow-mantle wavespeeds are δVP ≈–4% below Ethiopia, but less anomalous (δVP ≥–2%) below other volcanic provinces. The heterogeneous African Superplume reaches the upper mantle below the Kenyan plateau. Below Ethiopia/Afar we image a second sub-vertical slow wavespeed anomaly rooted near the core-mantle boundary outside the African LLVP, meaning multiple disparately sourced whole-mantle plumes may influence East African magmatism. In contrast to other African cratons, wavespeeds below Tanzania are only fast to 90–135km depth. When interpreted alongside Lower Eocene on-craton kimberlites, our results support pervasive metasomatic lithospheric modification caused by subduction during the Neoproterozoic Pan-African orogeny.

© 2021 Elsevier B.V. Magnetostratigraphic studies in the Silurian are absent, and what is understood about the geomagnetic polarity during this time is based on polarity bias-type data from palaeopole-type studies. We provide the first composite magnetic polarity record through the Lower Silurian (Llandovery) from the magnetostratigraphy of six sections. These are integrated with graptolite biostratigraphy and some carbon isotope chemostratigraphy. The palaeomagnetic signal is carried by both haematite and magnetite, with haematite dominating in red-coloured mudstones and mostly magnetite in non-red lithologies. The influence of possible tectonic disruption of the fabric is assessed using anisotropy of magnetic susceptibility. Only the most thermally mature section at Backside Beck shows the imprint of initial tectonic fabric formation. The Llandovery is divided into 6 major normal-reverse-polarity chron couplets (referred to as LL1 to LL6). An additional longer, exclusively normal polarity interval (referred to as WE1n), beginning in Telychian Stage slice Te3, runs into the lower Sheinwoodian. Within these five polarity couplets there are 10 further submagnetozones, and 10 tentative submagnetozones. Average reversal frequency (including the tentative submagnetozones) was ca. 3.0 Myr−1 in the Early Silurian, which is probably an underestimate, due to insufficient sampling density in some parts of the Rhuddanian and Aeronian. This reversal frequency is similar to that in the late Cenozoic, indicating the future potential utility of magnetostratigraphy for high-resolution correlation and dating in the Early Silurian.

Funder: German Research Foundation (DFG), Funder: Projekt DEAL, Previous investigation of seismic anisotropy indicates the presence of a simple mantle flow regime beneath the Turkish-Anatolian Plateau and Arabian Plate. Numerical modeling suggests that this simple flow is a component of a large-scale global mantle flow associated with the African superplume, which plays a key role in the geodynamic framework of the Arabia-Eurasia continental collision zone. However, the extent and impact of the flow pattern farther east beneath the Iranian Plateau and Zagros remains unclear. While the relatively smoothly varying lithospheric thickness beneath the Anatolian Plateau and Arabian Plate allows progress of the simple mantle flow, the variable lithospheric thickness across the Iranian Plateau is expected to impose additional boundary conditions on the mantle flow field. In this study, for the first time, we use an unprecedented data set of seismic waveforms from a network of 245 seismic stations to examine the mantle flow pattern and lithospheric deformation over the entire region of the Iranian Plateau and Zagros by investigation of seismic anisotropy. We also examine the correlation between the pattern of seismic anisotropy, plate motion using GPS velocities and surface strain fields. Our study reveals a complex pattern of seismic anisotropy that implies a similarly complex mantle flow field. The pattern of seismic anisotropy suggests that the regional simple mantle flow beneath the Arabian Platform and eastern Turkey deflects as a circular flow around the thick Zagros lithosphere. This circular flow merges into a toroidal component beneath the NW Zagros that is likely an indicator of a lateral discontinuity in the lithosphere. Our examination also suggests that the main lithospheric deformation in the Zagros occurs as an axial shortening across the belt, whereas in the eastern Alborz and Kopeh-Dagh a belt-parallel horizontal lithospheric deformation plays a major role.

The southwest Atlantic gyre connects several distinct water masses, which means that this oceanic region is characterized by a complex frontal system and enhanced water mass modification. Despite its significance, the distribution and variability of vertical mixing rates have yet to be determined for this system. Specifically, potential conditioning of mixing rates by frontal structures, in this location and elsewhere, is poorly understood. Here, we analyze vertical seismic (i.e., acoustic) sections from a three-dimensional survey that straddles a major front along the northern portion of the Brazil-Falkland Confluence. Hydrographic analyses constrain the structure and properties of water masses. By spectrally analyzing seismic reflectivity, we calculate spatial and temporal distributions of the dissipation rate of turbulent kinetic energy, ε, of diapycnal mixing rate, K, and of vertical diffusive heat flux, FH. We show that estimates of ε, K, and FH are elevated compared to regional and global mean values. Notably, cross-sectional mean estimates vary little over a 6 week period whilst smaller scale thermohaline structures appear to have a spatially localized effect upon ε, K, and FH. In contrast, a mesoscale front modifies ε and K to a depth of 1 km, across a region of O(100) km. This front clearly enhances mixing rates, both adjacent to its surface outcrop and beneath the mixed layer, whilst also locally suppressing ε and K to a depth of 1 km. As a result, estimates of FH increase by a factor of two in the vicinity of the surface outcrop of the front. Our results yield estimates of ε, K and FH that can be attributed to identifiable thermohaline structures and they show that fronts can play a significant role in water mass modification to depths of 1 km.

SUMMARYRapid urban growth has led to large population densities in foreland basin regions, and therefore a rapid increase in the number of people exposed to hazard from earthquakes in the adjacent mountain ranges. It is well known that earthquake-induced ground shaking is amplified in sedimentary basins. However, questions remain regarding the main controls on this effect. It is, therefore, crucial to identify the main controls on earthquake shaking in foreland basins as a step towards mitigating the earthquake risk posed to these regions. We model seismic-wave propagation from range-front thrust-faulting earthquakes in a foreland-basin setting. The basin geometry (depth and width) and source characteristics (fault dip and source-to-basin distance) were varied, and the resultant ground motion was calculated. We find that the source depth determines the amount of near-source ground shaking and the basin structure controls the propagation of this energy into the foreland basin. Of particular importance is the relative length scales of the basin depth and dominant seismic wavelength (controlled by the source characteristics), as this controls the amount of dispersion of surface-wave energy, and so the amplitude and duration of ground motion. The maximum ground motions occur when the basin depth matches the dominant wavelength set by the source. Basins that are shallow compared with the dominant wavelength result in low-amplitude and long-duration dispersed waveforms. However, the basin structure has a smaller effect on the ground shaking than the source depth and geometry, highlighting the need for understanding the depth distribution and dip angles of earthquakes when assessing earthquake hazard in foreland-basin settings.

© 2020 Elsevier B.V. A thick Infra-Cambrian to Phanerozoic sedimentary sequence in eastern Arabia conceals a poorly known Neoproterozoic basement that once separated the currently exposed Arabian Shield (western Arabia) from NW India in Gondwana. In this study, the Arabian Platform (eastern Arabia) is targeted by relatively high-resolution passive-source seismic tomography for the first time, which aims to illuminate crust and upper mantle structure in order to make inferences on their nature and origin. This is facilitated by a new broadband seismic network of 55 stations in the United Arab Emirates (UAE), which has recorded sufficient teleseismic body wave data to allow for the determination of three-dimensional seismic structure. Using a grid-based eikonal solver and a subspace inversion technique implemented in FMTOMO, relative arrival-time residuals from teleseismic earthquakes are mapped as 3-D P-wave velocity perturbations in the upper mantle beneath the seismic network. The resultant tomographic images reveal a marked transition from higher velocities in the Gulf of Oman to lower velocities in the west that we interpret to indicate the lithospheric boundary between Cretaceous Tethyan oceanic lithosphere and the Arabian passive continental margin. The northern extent of Tethyan oceanic lithosphere appears to terminate against a possible offshore continuation of the Dibba fault, until now only identified onshore. Further inland, the tomographic model exhibits a number of low- and high- velocity anomalies with a predominant NW orientation. Significantly, one well-defined high velocity anomaly is situated along strike of the Neoproterozoic exposure of volcanic-arc granodiorites found in northern Oman. We relate this anomaly to an intra-oceanic arc created during the Tonian subduction of the Mozambique Ocean, which strongly support the idea of continental growth in the Arabian Platform via magmatic arc accretion.

Improving our understanding of glacial sliding is crucial for constraining basal drag in ice dynamics models. We use icequakes, sudden releases of seismic energy as the ice slides over the bed, to provide geophysical observations that can be used to aid understanding of the physics of glacial sliding and constrain ice dynamics models. These icequakes are located at the bed of an alpine glacier in Switzerland and the Rutford Ice Stream, West Antarctica, two extremes of glacial settings and spatial scales. We investigate a number of possible icequake source mechanisms by performing full waveform inversions to constrain the fundamental physics and stress release during an icequake stick-slip event. Results show that double-couple mechanisms best describe the source for the events from both glacial settings and the icequakes originate at or very near the ice-bed interface. We also present an exploratory method for attempting to measure the till shear modulus, if indirect reflected icequake radiation is observed. The results of this study increase our understanding of how icequakes are associated with basal drag while also providing the foundation for a method of remotely measuring bed shear strength. ©2020. The Authors. ISSN:0148-0227 ISSN:2169-9003 ISSN:2169-9011

Hydraulically forced crevassing is thought to reduce the stability of ice shelves and ice sheets, affecting structural integrity and providing pathways for surface meltwater to the bed. It can cause ice shelves to collapse and ice sheets to accelerate into the ocean. However, direct observations of the hydraulically forced crevassing process remain elusive. Here we report a novel method and observations that use icequakes to directly observe crevassing and determine the role of hydrofracture. Crevasse icequake depths from seismic observations are compared to a theoretically derived maximum dry crevasse depth. We observe icequakes below this depth, suggesting hydrofracture. Furthermore, icequake source mechanisms provide insight into the fracture process, with predominantly opening cracks observed, which have opening volumes of hundredths of a cubic meter. Our method and findings provide a framework for studying a critical process that is key for the stability of ice shelves and ice sheets and, therefore, future sea level rise projections.

Brittle deformation in rocks depends upon loading rate; with increasing rates, typically greater than ~102 s−1, rocks become significantly stronger and undergo increasingly severe fragmentation. Dynamic conditions required for rate‐dependent brittle failure may be reached during impact events, seismogenic rupture, and landslides. Material characteristics and fragment characterization of specific geomaterials from dynamic loading are only approximately known. Here we determine the characteristic strain rate for dynamic behavior in felsic crystalline rocks, including anisotropy, and describe the resulting fragments. Regardless of the type of felsic crystalline rock or anisotropy, the characteristic strain rate is the same within uncertainties for all tested materials, with an average value of 229 ± 81 s−1. Despite the lack of variation of the critical strain rate with lithology, we find that the degree of fragmentation as a function of strain rate varies depending on material. Scaled or not, the fragmentation results are inconsistent with current theoretical models of fragmentation. Additionally, we demonstrate that conditions during impact cratering, where the impactor diameter is less than ~100 m, are analogous to the experiments carried out here and therefore that dynamic strengthening and compressive fragmentation should be considered as important processes during impact cratering.

Relocation of earthquakes recorded by the agency for meteorology, climatology and geophysics (BMKG) in Indonesia and inversions of global positioning system (GPS) data reveal clear seismic gaps to the south of the island of Java. These gaps may be related to potential sources of future megathrust earthquakes in the region. To assess the expected inundation hazard, tsunami modeling was conducted based on several scenarios involving large tsunamigenic earthquakes generated by ruptures along segments of the megathrust south of Java. The worst-case scenario, in which the two megathrust segments spanning Java rupture simultaneously, shows that tsunami heights can reach ~ 20 m and ~ 12 m on the south coast of West and East Java, respectively, with an average maximum height of 4.5 m along the entire south coast of Java. These results support recent calls for a strengthening of the existing Indonesian Tsunami Early Warning System (InaTEWS), especially in Java, the most densely populated island in Indonesia.

Oceanic residual depth varies on urn:x-wiley:jgrb:media:jgrb54314:jgrb54314-math-0001 5,000 km wavelengths with amplitudes of ±1 km. A component of this short‐wavelength signal is dynamic topography caused by convective flow in the upper ∼300 km of the mantle. It exerts a significant influence on landscape evolution and sea level change, but its contribution is often excluded in geodynamic models of whole‐mantle flow. Using seismic tomography to resolve buoyancy anomalies in the oceanic upper mantle is complicated by the dominant influence of lithospheric cooling on velocity structure. Here, we remove this cooling signal from global surface wave tomographic models, revealing a correlation between positive residual depth and slow residual velocity anomalies at depths

We describe and analyze the results of a three‐dimensional seismic (i.e. acoustic) reflection survey from the Faroe‐Shetland Channel that is calibrated with near‐coincident hydrographic and satellite observations. 54 vertical seismic transects were acquired over a period of 25 days. On each transect, a 250‐‐400 m band of reflections is observed within the water column. Hydrographic measurements demonstrate that this reflective band is caused by temperature variations within the pycnocline that separates warm, near‐surface waters of Atlantic origin from cold, deep waters which flow southward from the Nordic Seas. Tilting of reflective surfaces records geostrophic shear between these near‐surface and deep waters. Measurements of temporal changes of pycnoclinic depth and of reflection tilt are used to infer the existence of an anticyclonic vortex that advects northeastward. Comparison with satellite measurements of sea‐surface temperature and height suggests that this vortex is caused by meandering of the Continental Slope Current. A model of a Gaussian vortex is used to match seismic and satellite observations. This putative vortex has a core radius of 20—30 km and a maximum azimuthal velocity of 0.3‐‐0.4 m s‐1. It translates at 0.01‐‐0.1 m s‐1. Within the pycnocline, diapycnal diffusivity, K , is estimaed by analyzing the turbulent spectral subrange of tracked reflections. K varies between 10‐5.7 and 10‐5.0 m 2 s‐1 in a pattern that is broadly consistent with translation of the vortex. Our integrated study demonstrates the ability of time‐lapse seismic reflection surveying to dynamically resolve the effects that mesoscale activity has upon deep thermohaline structure on scales from meters to hundreds of kilometers.

The dispersion of tracer through a liquid injected into a confined aquifer with vertically\ud varying permeability is studied theoretically. The injected fluid is buoyant and of high\ud viscosity relative to the original fluid in the aquifer, which causes the nose region of the\ud flow, where the thickness of the injected fluid is less than the thickness of the aquifer, to\ud advance with constant velocity and fixed shape. Behind the nose, tracer is sheared at early\ud times owing to the vertically varying permeability. At later times, cross-aquifer diffusion\ud homogenises the tracer distribution, which becomes independent of depth but spreads\ud longitudinally in this shear dispersion regime, which leads to much faster spreading\ud than by diffusion alone. As tracer diffuses symmetrically in the longitudinal direction, it\ud eventually reaches the nose. Subsequently, the nose acts as a no-flux boundary and the\ud concentration profile transitions to a half-Gaussian, with the maximum concentration at\ud the front. The centre of mass of the tracer spreads backwards relative to the fixed nose at\ud a rate proportional to [D(T −T 0 )] 1/2 where D is the dispersion coefficient and T 0 is a time\ud offset owing to the adjustment to shear dispersion and the interaction with the nose. The\ud initial release of tracer may not be vertically uniform owing to the heterogeneity and we\ud show that this can lead to the centre of mass of tracer initially advancing faster than the\ud mean flow. We consider the implications of our results on tracer migration in sedimentary\ud layers.

The Marmara region in Turkey is an important geological setting, both from a tectonic and a seismic hazard/risk perspective. Here we present a recently published map of crustal thickness variation across this complex region (Jenkins et al., 2020), to aid in furthering our understanding of the past and present tectonic processes that formed present‐day structure. The crustal thickness map was created using Ps converted phases and receiver function (RF) analysis of earthquakes recorded at all publicly available seismic stations and stations in the national monitoring network (run by AFAD Disaster and Emergency Management Authority Turkey). RFs were converted from time to depth using a local 3‐D full‐waveform tomographic model and combined in multiphase common conversion point stacks, such that direct P to S converted arrivals and associated multiples are used together to produce continuous maps of the Moho discontinuity. Results reveal the Moho beneath Marmara ranges in depth from 26–41 km, and shows a regional trend of westward thinning, reflecting the effects of the extensional regime in western Anatolia and the neighboring Aegean Sea. The thinnest crust is observed beneath the western end of the Sea of Marmara, and can be attributed to transtensional basin opening. A distinct region of increased crustal thickness bounded by the West Black Sea Fault in the west, and the northern strand of the North Anatolian Fault in the south, defines the ancient crustal terrane of the Istanbul Zone. Isostatic arguments indicate that the thickened crust and lower elevation in the Istanbul Zone require it to be underlain by thicker lithosphere, a conclusion that is consistent with its hypothesized origin near the Odessa shelf.

The rate of carbon dioxide (CO2) dissolution in saline aquifers is the least well‐constrained of the secondary trapping mechanisms enhancing the long‐term security of geological carbon storage. CO2 injected into a heterogeneous saline reservoir will preferentially travel along high permeability layers increasing the CO2‐water interfacial area which increases dissolution rates. We provide a conservative, first‐principles analysis of the quantity of CO2 dissolved and the rate at which free‐phase CO2 propagates in layered reservoirs. At early times, advection dominates the propagation of CO2. This transitions to diffusion dominated propagation as the interfacial area increases and diffusive loss slows propagation. As surrounding water‐filled layers become CO2 saturated, propagation becomes advection dominated. For reservoirs with finely bedded strata, ~10% of the injected CO2 can dissolve in a year. The maximum fraction of CO2 that dissolves is determined by the volumetric ratio of water in low permeability layers and CO2 in high permeability layers.

SUMMARY Orogenic plateaus can exist in a delicate balance in which the buoyancy forces due to gravity acting on the high topography and thick crust of the plateau interior are balanced by the compressional forces acting across their forelands. Any shortening or extension within a plateau can indicate a perturbation to this force balance. In this study, we present new observations of the kinematics, morphology and slip rates of active normal faults in the South Peruvian Altiplano obtained from field studies, high-resolution DEMs, Quaternary dating and remote sensing. We then investigate the implications of this faulting for the forces acting on the Andes. We find that the mountains are extending ∼NNE–SSW to ∼NE–SW along a normal fault system that cuts obliquely across the Altiplano plateau, which in many places reactivates Miocene-age reverse faults. Radiocarbon dating of offset late Quaternary moraines and alluvial fan surfaces indicates horizontal extension rates across the fault system of between 1 and 4 mm yr–1—equivalent to an extensional strain rate in the range of 0.5–2 × 10−8 1 yr–1 averaged across the plateau. We suggest the rate and pattern of extension implies there has been a change in the forces exerted between the foreland and the Andes mountains. A reduction in the average shear stresses on the sub-Andean foreland detachment of ≲4 MPa (20–25 per cent of the total force) can account for the rate of extension. These results show that, within a mountain belt, the pattern of faulting is sensitive to small spatial and temporal variations in the strength of faults along their margins.

We assess the performance of the joint inversion of receiver functions (RF) and surface‐wave dispersion in the characterization of the sedimentary package comprising the Parnaíba basin. This procedure is routinely utilized in passive‐source crustal studies to retrieve S‐wave velocity variations with depth, and has seldom been used with higher‐frequency datasets to investigate fine sedimentary structure. The Parnaíba basin is a Paleozoic cratonic basin composed of five supersequences, accumulating ∼3.5 km of sedimentary rocks interbedded by Late Cretaceous diabase sills. The dataset used for this research was acquired between 2015 and 2017 through deployment of 10 short‐period and one broadband seismic stations distributed along an approximately 100‐kilometer‐long linear array in the center of the basin. The deployment was carried out under the Parnaíba Basin Analysis Project, a multi‐institutional and multidisciplinary effort funded by BP Energy do Brasil. High‐frequency RFs (⁠f

The elemental composition of a sediment is set by the composition of its protolith and modified by weathering, sorting, and diagenesis. An important problem is deconvolving these contributions to a sediment's composition to arrive at information about processes that operate on the Earth's surface. We approach this problem by developing a predictive and invertible model of sedimentary major element composition. We compile a data set of sedimentary rock, river sediment, soil, and igneous rock compositions. Principal component analysis of the data set shows that most variation can be simplified to a small number of variables. We thus show that any sediment's composition can be described with just two vectors of igneous evolution and weathering. We hence define a model for sedimentary composition as a combination of these processes. A 1:1 correspondence is observed between predictions and independent data. The log ratios urn:x-wiley:ggge:media:ggge22195:ggge22195-math-0001 and urn:x-wiley:ggge:media:ggge22195:ggge22195-math-0002 are found to be simple proxies for, respectively, the model's protolith and weathering indices. Significant deviations from the model can be explained by sodium‐calcium exchange. Using this approach, we show that the major element composition of the upper continental crust has been modified by weathering, and we calculate the amount of each element that it must have lost to modify it to its present composition. By extrapolating modern weathering rates over the age of the crust, we conclude that it has not retained a significant amount of the necessarily produced weathering restite. This restite has likely been subducted into the mantle, indicating a crust‐to‐mantle recycling rate of 1.33 ± 0.89 ×1013 kg·year−1.

A new model unifies the descriptions of sand transport by wind and water, a result that could lead to a better understanding of the movements of sediments on the surfaces of planets.

In this study, we analysed 2708 receiver functions (RFs) using data recorded by 53 seismographic stations that surround Mt. Merapi and Mt. Merbabu – two volcanos in Central Java - to map the boundary between Earth's crust and upper mantle. We observe that a number of RFs from this new dataset have complex signals and do not exhibit typical RF characteristics; in particular, where the converted Ps signal from the Moho discontinuity is the clearest and strongest amplitude arrival following the P onset. This effect may be related to complex shallow velocity structure due to the presence of magmatic rocks and sediments. Further analysis of the RF results using the H-κ method suggests that Moho depth varies between 27 and 32 km beneath the array, with no apparent correlation between crustal thickness and surface topography, as one might expect from Airy isostacy. For instance, the Moho is quite shallow beneath Mt. Merapi (up to 27 km depth), despite its elevation of nearly 3 km. This may be a consequence of dynamic support from an active upper mantle coupled with erosion and/or weakening of the lower crust due to the active volcanic plumbing system. To the north of Mt. Merapi, the Moho is deeper (30–31 km depth) below Mt. Merbabu. Vp/Vs ratio estimates from the H-κ method are relatively high (~1.9) beneath the Mt. Merapi and Kendeng Basin area, which may indicate the presence of a zone of hydrous and active partial melting in the underlying crust. Lower Vp/Vs ratios (~1.7) are found beneath Mt. Merbabu, which may be due to its relative lack of volcanic activity compared to Mt. Merapi.

We present a simple model for geophysical systems involving sources of deformation, such as magmatic intrusions, subglacial lakes, and the subsurface storage of CO2. We consider the idealized system of a uniform elastic layer overlying a localized region of constant pressure that is surrounded by a Winkler foundation composed of springs. We investigate the effect of source depth and foundation stiffness on the resulting displacement profiles at both the surface and the level of the source. The system is characterized by three key features: the maximum uplift, the maximum subsidence, and the distance to the point of zero displacement. For each of these, we determine asymptotic scaling behavior in the limits of a thin/thick layer and a soft/stiff foundation and form composite curves that allow specific parameter values to be determined from field data. Both two-dimensional and axisymmetric pressure patches are considered, and in the thin-layer limit we derive analytical solutions.

We present an experimental investigation of steady particle-driven gravity currents\ud with Reynolds numbers in the range 500–1600, and with the ratio of the initial\ud current speed to the fall speed of the particles, S = u 0 /u fall , in the range 5 < S < 160.\ud We identify three regimes: (i) For S < 10, the particles settle close to the source\ud at a velocity corresponding to their fall speed, consistent with the observation\ud of sedimenting fronts in classical settling column experiments. (ii) In the range\ud 10 < S < 40, a steady gravity current develops within the tank. The experiments show\ud that the depth of the gravity current gradually decreases away from the source and dye\ud added to the source liquid appears above the gravity current along its entire length,\ud suggesting that there is a sedimentation front, so that the volume and momentum\ud fluxes of the current gradually decrease with distance from the source. We find that\ud as S increases, the descent speed of the sedimentation front decreases relative to the\ud fall speed of the particles, and the run-out length of the gravity current increases.\ud We note that the density of the interstitial fluid corresponds to the density of the\ud ambient fluid, so that any reduction in buoyancy of the gravity current is attributed\ud to the sedimentation of particles on the floor of the tank and we do not observe\ud lofting of the interstitial fluid. (iii) For 40 < S < 160, the gravity currents reach the\ud end of our experimental tank and we no longer observe a sedimentation front. For\ud these experiments, it appears that the entrainment at the top of the current begins to\ud match the sedimentation and so the current depth does not change significantly over\ud the scale of the tank, but a larger scale experimental system would be needed to\ud explore the full run-out behaviour for these larger values of S. For the intermediate\ud case, 10 < S < 40, we develop a model for the conservation of volume, momentum\ud and buoyancy fluxes in the current, accounting for the sedimentation front and the\ud release of fluid at the top surface of the gravity current, and we compare this with\ud our new experimental data.

Sand dunes rarely occur in isolation, but usually form vast dune fields. The large scale dynamics of these fields is hitherto poorly understood, not least due to the lack of longtime observations. Theoretical models usually abstract dunes in a field as self-propelled autonomous agents, exchanging mass, either remotely or as a consequence of collisions. In contrast to the spirit of these models, here we present experimental evidence that aqueous dunes interact over large distances without the necessity of exchanging mass. Interactions are mediated by turbulent structures forming in the wake of a dune, and lead to dune-dune repulsion, which can prevent collisions. We conjecture that a similar mechanism may be present in wind driven dunes, potentially explaining the observed robust stability of dune fields in different environments.

©2018. American Geophysical Union. All Rights Reserved. Western North America has an average elevation that is ∼2 km higher than cratonic North America. This difference coincides with a westward decrease in average lithospheric thickness from ∼240 to 260 basaltic samples. Forward and inverse modeling of carefully selected major, trace, and rare earth elements were used to determine melt fraction as a function of depth. Basaltic melt appears to have been generated by adiabatic decompression of dry peridotite with asthenospheric potential temperatures of 1340 ± 20 °C. Potential temperatures as high as 1365 °C were obtained for the Snake River Plain. For the youngest (i.e.

We study the Kashmir seismic gap using data from a network of local broadband stations spanning southeastern Kashmir Valley and Jammu. We detected and located several hundred earthquakes using continuous data recorded in our network. The earthquakes (ML 1.0–5.0) were relocated using probabilistic and relative location methods to obtain a subset of events with depth and spatial uncertainty ≤1.5 km. The earthquakes were found to cluster along two adjacent lines parallel to the strike of the Himalaya but at different depths. The SW cluster was found to be shallower (4–15 km) than the NE cluster (13–18 km). The events in the SW cluster shallowed toward NW from SE. We calculated the source mechanism of larger earthquakes (ML≥3.0) using global and local data sets. The source mechanism results show dominant thrust motion of the earthquakes in the NE cluster. Considering nodal planes dipping to the northeast as fault planes, we obtained the model of a steep frontal ramp close to the locked portion of the Main Himalayan Thrust. The model obtained for SW cluster of seismicity showed the presence of a lateral ramp dipping to the SE. Normal faulting was observed in the SW cluster. Our analysis shows two possible causes for the existence of these normal faults—the existing strike‐slip motion loading/unloading stresses on the lateral heterogeneities within the decollement or the transfer of potential energy by sediment yield of the river Chenab. The elevated flat situated northwest of the lateral ramp gives rise to very shallow microseismicity (4–7 km).

With the advent of more precise dating methods, it has become apparent that zircon dates from granite plutons frequently indicate older emplacement ages than other dating methods. Here we attempt to reconcile a number of dating methods from the c. 5 km2 Caledonian Shap granite, Northern England. The results reveal a more complex and protracted evolution than indicated by application of any single dating method. Zircon U-Pb dates give a weighted mean age of 415.6 ± 1.4 (2σ) Ma. A mafic enclave, dated at 412 ± 2 (2σ) Ma (revised Rb-Sr feldspar age from Davidson et al., 2005), contains resorbed K-feldspar and zircon crystals scavenged from the host crystal mush. These ages are at odds with field relations in the thermal aureole that suggest final emplacement at approximately 404 Ma or later during Acadian deformation. Previously reported Re-Os ages on molybdenites associated with magmatic fluids, have given ages of 405.2 ± 1.8 (2σ) Ma (Selby et al., 2008) and confirm the overlap of at least some magmatic activity with Acadian deformation. A similar emplacement age is supported by Rb-Sr whole-rock-mineral and biotite K-Ar dates when adjusted for revised decay constants (402 ± 3 Ma and 401 ± 7 Ma, respectively, Wadge et al., 1978). The lower closure temperatures of these systems relative to the U-Pb system in zircon means that they are more likely to record the timing of final granite emplacement. These data suggest that most zircons grew before final granite emplacement, by about 10 Ma on average. We suggest that the majority of zircon crystals record pre-emplacement magmatic activity within a deeper part of the system. Mafic enclaves and their scavenged cargo of crystals record the assembly of a mid-crustal batholith where crystals remained at least locally mobile at 412 Ma. Gravity data support the existence of an extensive, 1500 km2 intrusive body, originally at about 15 km depth beneath Shap. This batholith is likely to have remained below the granite solidus for much of its existence due to conductive heat loss, but episodic influxes of silicic magma between c. 412 and 405 Ma are thought to have enabled periods of rejuvenation. These influxes are recorded by complex compositional zoning patterns within K-feldspar megacrysts. The Shap granite itself is likely to represent a rejuvenated crystal slurry, emplaced as a cylindrical cupola above the main magma body during Acadian transpression. This study highlights the importance of integrating different dating techniques and that final emplacement of granites can only be indicated by the youngest zircon ages.

We employ an empirical approach to study the phenomenon of surface wave mode conversion due to lateral heterogeneity, and, as an example, assess its impact on a specific waveform inversion methodology used for surface wave tomography. Finite difference modelling in 2-D media, using a method that allows modelling of a single surface wave mode at a time, is combined with frequency domain decomposition of the wavefield onto a basis of local mode eigenfunctions, to illuminate mode conversion as a function of frequency and heterogeneity parameters. Synthetic waveforms generated by the modelling are inverted to study the effects of mode conversion on the inversion process. For heterogeneities in the upper mantle depth range of ∼40–300 km, we find that heterogeneity strengths of about 5 per cent (with sharp lateral boundaries), or lateral boundary length scales of 10–15 times the seismic wavelength (with 10 per cent maximum strength) produce significant mode conversion at periods of 30 s and shorter. These are significant in the sense that, depending on source strength, converted mode amplitudes can be well above typical noise levels in seismology. Correspondingly, waveform inversion with higher modes reveals the inadequacy of the path average approximation at these periods, with the potential for errors as large as 7 per cent in inferred group velocities, which will translate into errors in the inverted shear-velocity structure.

International audience; Imaging and resolving the lateral continuity of 3-D crustal structures enhance our ability to interpret seismicity and to understand how orogens are created. We apply a Bayesian, hierarchical inversion approach based on a transdimensional trees-structured wavelet parameterization to recover phase velocity maps at 2-40 s periods. We then invert phase velocity dispersion to constrain a 3-D shear velocity model of the crust beneath south-central Iran. Together with accurate earthquake centroid depths and focal mechanisms, the pattern of 3-D velocity variations supports recent suggestions that most large earthquakes in the Zagros occur within the lower sedimentary cover or close to the sediment-basement interface. Furthermore, we find evidence for Arabian basement underthrusting beneath central Iran, although only in one location does it appear to generate earthquakes. Our new 3-D tomographic model clarifies and throws new light on the crustal structure of the SE Zagros and its relation to seismicity and active faulting.

The Cambridge Deep Earth Seismology group has an exhibition at the Sedgwick Museum, Cambridge, aimed at increasing understanding of our planet and changing perceptions of geophysics – and geophysicists. Group members Jennifer Jenkins, Jess Bartlet and Sanne Cottaar tell us more.

We investigate the mantle transition zone beneath the Chile‐Argentina flat subduction region by means of P‐to‐S conversions at mantle discontinuities from teleseismic events recorded at 103 seismic stations. From the analysis of receiver functions, we obtain clear converted phases from the 410 and 660 discontinuities, and we identify a robust precursory signal to P660s, of negative amplitude, that we name P590s. We observe little frequency dependence in the amplitude of the P410s converted phase, while the P660s is less visible toward higher frequencies. The 410 is on average deeper than 410 km by 10 ± 1 km in the higher‐frequency bands, and it is relatively sharp, being consistent with a 10% velocity jump over less than 20 km. The observed 660 depth varies with frequency; it is deeper by up to 18 ± 2 km for lower frequencies and close to reference at higher frequencies, being consistent with a 13% broad velocity gradient over 30–40 km, probably caused by a composite of multiple phase transitions. The transition zone thickness is controlled by the frequency‐dependent depth variability of the 660. Our findings of relative depth, width, and velocity jump of the detected discontinuities, combined with tomographic images of the mantle transition zone, cannot be explained by thermal variations alone. Compositional constraints from mineral physics show that a near pyrolitic mantle is consistent with the ratio of the estimated velocity jumps. However, the negative P590s phase in this region could be signal from the velocity reduction due to basalt accumulation at the base of the transition zone.

SUMMARY We have constructed a 3-D shear wave velocity (Vs) model for the crust and uppermost mantle beneath the Middle East using Rayleigh wave records obtained from ambient-noise cross-correlations and regional earthquakes. We combined one decade of data collected from 852 permanent and temporary broad-band stations in the region to calculate group-velocity dispersion curves. A compilation of >54 000 ray paths provides reliable group-velocity measurements for periods between 2 and 150 s. Path-averaged group velocities calculated at different periods were inverted for 2-D group-velocity maps. To overcome the problem of heterogeneous ray coverage, we used an adaptive grid parametrization for the group-velocity tomographic inversion. We then sample the period-dependent group-velocity field at each cell of a predefined grid to generate 1-D group-velocity dispersion curves, which are subsequently inverted for 1-D Vs models beneath each cell and combined to approximate the 3-D Vs structure of the area. The Vs model shows low velocities at shallow depths (5–10 km) beneath the Mesopotamian foredeep, South Caspian Basin, eastern Mediterranean and the Black Sea, in coincidence with deep sedimentary basins. Shallow high-velocity anomalies are observed in regions such as the Arabian Shield, Anatolian Plateau and Central Iran, which are dominated by widespread magmatic exposures. In the 10–20 km depth range, we find evidence for a band of high velocities (>4.0 km s–1) along the southern Red Sea and Arabian Shield, indicating the presence of upper mantle rocks. Our 3-D velocity model exhibits high velocities in the depth range of 30–50 km beneath western Arabia, eastern Mediterranean, Central Iranian Block, South Caspian Basin and the Black Sea, possibly indicating a relatively thin crust. In contrast, the Zagros mountain range, the Sanandaj-Sirjan metamorphic zone in western central Iran, the easternmost Anatolian plateau and Lesser Caucasus are characterized by low velocities at these depths. Some of these anomalies may be related to thick crustal roots that support the high topography of these regions. In the upper mantle depth range, high-velocity anomalies are obtained beneath the Arabian Platform, southern Zagros, Persian Gulf and the eastern Mediterranean, in contrast to low velocities beneath the Red Sea, Arabian Shield, Afar depression, eastern Turkey and Lut Block in eastern Iran. Our Vs model may be used as a new reference crustal model for the Middle East in a broad range of future studies.

Environmental fluid dynamics underlies a wealth of natural, industrial and, by extension, societal challenges. In the coming decades, as we strive towards a more sustainable planet, there are a wide range of grand challenge problems that need to be tackled, ranging from fundamental advances in understanding and modeling of stratified turbulence and consequent mixing, to applied studies of pollution transport in the ocean, atmosphere and urban environments. A workshop was organized in the Les Houches School of Physics in France in January 2019 with the objective of gathering leading figures in the field to produce a road map for the scientific community. Five subject areas were addressed: multiphase flow, stratified flow, ocean transport, atmospheric and urban transport, and weather and climate prediction. This article summarizes the discussions and outcomes of the meeting, with the intent of providing a resource for the community going forward.

We present a seismic waveform tomography of the upper mantle beneath the central and eastern Mediterranean down to the mantle transition zone. Our methodology incorporates in a consistent manner the information from body and multimode surface waves, source effects, frequency dependence, wavefront healing, anisotropy and attenuation. This allows us to jointly image multiple parameters of the crust and upper mantle. Based on the data from ∼ 17 000 unique source–receiver pairs, gathered from 80 earthquakes, we image radially anisotropic S velocity, P velocity and density. We use a multi-scale approach in which the longest periods (100–150 s) are inverted first, broadening to a period band of 28–150 s. Thanks to a strategy that combines long-period signals and a separation of body and surface wave signals, we are able to image down to the mantle transition zone in most of the model domain. Our model shows considerable detail in especially the northern part of the domain, where data coverage is very dense, and displays a number of clear and coherent high-velocity structures across the domain that can be linked to episodes of current and past subduction. These include the Hellenic subduction zone, the Cyprus subduction zone and high-velocity anomalies beneath the Italian peninsula and the Dinarides. This model is able to explain data from new events that were not included in the inversion., Solid Earth, 11 (2), ISSN:1869-9510, ISSN:1869-9529

SUMMARYThe mantle transition zone is the region between the globally observed major seismic velocity discontinuities around depths of 410 and 660 km and is important for determining the style of convection and mixing between the upper and the lower mantle. In this study, P-to-S converted waves, or receiver functions, are used to study these discontinuities beneath the Alaskan subduction zone, where the Pacific Plate subducts underneath the North American Plate. Previous tomographic models do not agree on the depth extent of the subducting slab, therefore improved imaging of the Earth structure underneath Alaska is required. We use 27 800 high quality radial receiver functions to make common conversion point stacks. Upper mantle velocity anomalies are accounted for by two recently published regional tomographic S-wave velocity models. Using these two tomographic models, we show that the discontinuity depths within our CCP stacks are highly dependent on the choice of velocity model, between which velocity anomaly magnitudes vary greatly. We design a quantitative test to show whether the anomalies in the velocity models are too strong or too weak, leading to over- or undercorrected discontinuity depths. We also show how this test can be used to rescale the 3-D velocity corrections in order to improve the discontinuity topography maps.After applying the appropriate corrections, we find a localized thicker mantle transition zone and an uplifted 410 discontinuity, which show that the slab has clearly penetrated into the mantle transition zone. Little topography is seen on the 660 discontinuity, indicating that the slab has probably not reached the lower mantle. In the southwest, P410s arrivals have very small amplitudes or no significant arrival at all. This could be caused by water or basalt in the subducting slab, reducing the strength at the 410, or by topography on the 410 discontinuity, preventing coherent stacking. In the southeast of Alaska, a thinner mantle transition zone is observed. This area corresponds to the location of a slab window, and thinning of the mantle transition zone may be caused by hot mantle upwellings.

Large pre-historical earthquakes leave traces in the geological and geomorphological record, such as primary and secondary surface ruptures and mass movements, which are the only means to estimate their magnitudes. These environmental earthquake effects (EEEs) can be calibrated using recent seismic events and the Environmental Seismic Intensity Scale (ESI2007). We apply the ESI2007 scale to the 1992 MS7.3 Suusamyr Earthquake in the Kyrgyz Tien Shan, because similar studies are sparse in that area and geological setting, and because this earthquake was very peculiar in its primary surface rupture pattern. We analyze literature data on primary and secondary earthquake effects and add our own observations from fieldwork. We show that the ESI2007 distribution differs somewhat from traditional intensity assessments (MSK (Medvedev-Sponheuer-Karnik) and MM (Modified Mercalli)), because of the sparse population in the epicentral area and the spatial distribution of primary and secondary EEEs. However, the ESI2007 scale captures a similar overall pattern of the intensity distribution. We then explore how uncertainties in the identification of primary surface ruptures influence the results of the ESI2007 assignment. Our results highlight the applicability of the ESI2007 scale, even in earthquakes with complex and unusual primary surface rupture patterns.

What happens when subduction stops? A team of scientists installed a dense seismic network in Borneo to investigate causes and consequences of subduction termination.

A tract of amphibolite facies granitic gneisses and metagabbros in northern Shetland, U.K., is here named the Uyea Gneiss Complex. Zircon U–Pb dating indicates emplacement of the igneous protoliths of the complex c. 2746–2726 Ma, at a later time than most of the Archaean protoliths of the Lewisian Gneiss Complex of mainland Scotland. Calc-alkaline geochemistry of the Uyea Gneiss Complex indicates arc-affinity and a strong genetic kinship among the mafic and felsic components. Zircon Hf compositions suggest an enriched mantle source and limited interaction with older crust during emplacement. Ductile fabrics developed soon after emplacement, with zircon rims at c. 2710 Ma, but there was little further deformation until Caledonian reworking east of the Uyea Shear Zone. There is no evidence for the Palaeoproterozoic reworking that dominates large tracts of the Lewisian Gneiss Complex and of the Nagssugtoqidian Orogen of East Greenland. The more northerly location of the Uyea Gneiss Complex and extensive offshore basement of similar age implies that, prior to the opening of the North Atlantic Ocean, these rocks were contiguous with the Archaean Rae Craton.

The existence of subcrustal continental earthquakes beneath the Alpine-Himalayan Belt was recog-\ud nised more than 60 years ago. There is general agreement that most of those beneath the western\ud part of the belt in the Mediterranean result from the subduction of oceanic lithosphere. There is less\ud agreement about the origin of those beneath Vrancea in Romania, the Hindu Kush and the Pamir.\ud Because there is little evidence for the former existence of oceanic lithosphere beneath these regions,\ud many authors have argued that these seismic zones result from the separation of the mantle part\ud of the continental lithosphere from the crust before it sinks into the mantle. However, this model\ud has become steadily less satisfactory. Detailed studies of the depth of earthquakes beneath all stable\ud regions of continents have shown that substantial subcrustal earthquakes, with magnitudes greater\ud than 5.5, are rare. We show that this distribution is controlled by temperature, with material hotter\ud than ∼ 600\ud ◦\ud C being aseismic. This simple rule accounts for the distribution of almost all earth-\ud quakes in oceanic and continental lithosphere, including those in subduction zones. We argue that\ud the subcrustal continental earthquakes must also result from the subduction of oceanic lithosphere.\ud This proposal is not new, but has generally been dismissed because of the lack of surface geological\ud evidence that suitable pieces of oceanic lithosphere existed. However, the depth distribution of con-\ud tinental earthquakes makes it steadily harder to avoid.

Numerical modeling plays an essential role in both identifying and assessing sub-surface reservoirs that might be suitable for future carbon capture and storage projects. Accuracy of flow simulations is tested by benchmarking against historic observations from on-going CO2 injection sites. At the Sleipner project located in the North Sea, a suite of time-lapse seismic reflection surveys enables the three-dimensional distribution of CO2 at the top of the reservoir to be determined as a function of time. Previous attempts have used Darcy flow simulators to model CO2 migration throughout this layer, given the volume of injection with time and the location of the injection point. Due primarily to computational limitations preventing adequate exploration of model parameter space, these simulations usually fail to match the observed distribution of CO2 as a function of space and time. To circumvent these limitations, we develop a vertically-integrated fluid flow simulator that is based upon the theory of topographically controlled, porous gravity currents. This computationally efficient scheme can be used to invert for the spatial distribution of reservoir permeability required to minimize differences between the observed and calculated CO2 distributions. When a uniform reservoir permeability is assumed, inverse modeling is unable to adequately match the migration of CO2 at the top of the reservoir. If, however, the width and permeability of a mapped channel deposit are allowed to independently vary, a satisfactory match between the observed and calculated CO2 distributions is obtained. Finally, the ability of this algorithm to forecast the flow of CO2 at the top of the reservoir is assessed. By dividing the complete set of seismic reflection surveys into training and validation subsets, we find that the spatial pattern of permeability required to match the training subset can successfully predict CO2 migration for the validation subset. This ability suggests that it might be feasible to forecast migration patterns into the future with a degree of confidence. Nevertheless, our analysis highlights the difficulty in estimating reservoir parameters away from the region swept by CO2 without additional observational constraints.