Mantle Anomalies Research Articles

Upper Mantle Structure Beneath the Contiguous US Resolved With Array Observations of SKS Multipathing and Slowness Vector Perturbations

AbstractContinent‐scale observations of seismic phenomena have provided multi‐scale constraints of the Earth's interior. Of those analyzed, array‐based observations of slowness vector properties (backazimuth and horizontal slowness) and multipathing have yet to be made on a continental scale. Slowness vector measurements give inferences on mantle heterogeneity properties such as velocity perturbation and velocity gradient strength and quantify their effect on the wavefield. Multipathing is a consequence of waves interacting with strong velocity gradients resulting in two arrivals with different slowness vector properties and times. The mantle structure beneath the contiguous Unites States has been thoroughly analyzed by previous seismic studies and is data‐rich, making it an excellent testing ground to both analyze mantle structure with our approach and compare with other imaging techniques. We apply an automated array‐analysis technique to an SKS data set to create the first continent‐scale data set of multipathing and slowness vector measurements. We analyze the divergence of the slowness vector deviation field to highlight seismically slow and fast regions. Our results resolve several slow mantle anomalies beneath Yellowstone, the Appalachian mountains and fast anomalies throughout the mantle. Many of the anomalies cause multipathing in frequency bands 0.15–0.30 and 0.20–0.40 Hz which suggests velocity transitions over at most 500 km exist. Comparing our observations to synthetics created from tomography models, we find model NA13 (Bedle et al., 2021, https://doi.org/10.1029/2021GC009674) fits our data best but differences still remain. We therefore suggest slowness vector measurements should be used as an additional constraint in tomographic inversions and will lead to better resolved models of the mantle.

Seamount chains and hotspot tracks: Superficially similar, deeply different

Nearly 80% of the seafloor extension has not been covered by high-resolution bathymetry, impeding direct observation of seamounts. Nevertheless, lists of seamount location and height at a global scale have been produced using different techniques. In this work four of such databases (publicly available) are compared with each other to assess their differences. Results identify large differences among databases that could have exerted strong influences on models of seamount production and associated geodynamic processes. Despite those differences, it is shown that all databases allow the identification of seamount lines both along the present-day Mid Ocean Ridge (MOR) system and on intraplate settings. Notably, those seamount lines do not coincide with the so-called hotspot tracks that commonly were defined by selectively focusing attention on the larger seamounts. Examination of all the databases also shows that distinction based only on seamount size between seamounts produced at Mid-Ocean Ridge (MOR) environments from those associated with mantle-plum fed-hotspot activity has been overestimated. This, combined with the fact that most seamount lines defined by the available databases can be traced back to past locations of MOR indicates that most of the present-day intraplate linear arrays of seamounts, which include some large seamounts, were not produced by the action of underlying mantle anomalies envisaged in the form of mantle plumes. The evidence presented here calls for a reassessment of the form in which volcanic and tectonic activities are conceptually related to each other.

STATISTICAL ANALYSIS OF GEOLOGICAL AND GEOPHYSICAL PARAMETERS OF THE MONGOLIAN-BAIKAL REGION

The origins and formation mechanisms of neotectonic structures in a part of the Mongolian-Siberian region were identified by geodynamic zoning based on multidimensional statistical analysis of numerical arrays describing geological-geophysical and geological-geomorphological processes. These processes in the regional lithosphere were described by a set of 11 geological and geophysical parameters, divided into three main groups using the hierarchical method of cluster analysis. The first group includes the seismic moment, the density of active faults, the rates of current horizontal deformations, and the magnitude of the deep heat flow. The second group involves the thicknesses of the earth’s crust and exogenous active layer, the velocities of current horizontal movements, and the amplitudes of vertical neotectonic movements. The third group includes gravity anomalies and the lithosphere thickness. The spatial grouping of parameters by cluster analysis (K-means method) yields seven clusters, whose spatial position and composition are determined by the geological history, geological structure, geodynamic evolution of the region, and the current deformation rates. Some clusters characterize large rigid lithospheric blocks, while other clusters describe large active fault systems in the given area. The search for latent factors that make the greatest contribution to the dispersion of the geological and geophysical parameter values was carried out using the principal component method, which makes it possible to minimize the number of factors. Four main factors were identified for areas that differ in the morphology and origin of neotectonic structures: (i) higher horizontal compressive and tensile strains, (ii) dynamic effect of mantle anomalies, manifested in uplifts and arching, (iii) activation of thinned lithosphere within the boundaries of lithospheric plates and large blocks, and (iv) active shear deformation of the earth’s crust. The results of clustering and factor analysis of numerical arrays describing the geological-geophysical and geological-geomorphological processes within the Mongolian-Siberian region can be interpreted in the framework of physical mesomechanics.

Petrogenetic insights on tephritic magmatism from Davis Bank (South Atlantic Ocean - Vitória-Trindade Ridge (VTR), Brazil): The role of a CO2-enriched mantle source

Davis Bank is one of the largest physiographic features in the Vitória-Trindade Ridge (VTR), which is located offshore of the Eastern Brazilian margin, midway between the coastline and the South Atlantic mid-ocean ridge. The VTR is related to a linear volcanic track of ca. 1200 km in length, composed of several volcanic seamounts that are lithologically characterized by nephelinitic/tephritic alkaline rocks. The Davis Bank lies between the parallels 20°29′ and 20°32′ S and the meridians 29°17′ and 29°21′ W, and is located southeast of the Abrolhos Volcanic Complex (AVC), at the middle of the volcanic ridge, far from the Brazilian coast. This work focuses on the 40Ar/39Ar age determination of the rocks recovered from the seamount and characterization of Sr–Nd isotopes, mineral chemistry and whole-rock composition, aiming to access information about physical properties of magmas, source composition, magmatic evolution of the volcanic ridge, and the regional tectonic interpretation in the context of other seamounts along the Vitória-Trindade Ridge (VTR). Davis Bank presents 40Ar/39Ar age of ca. 21 Ma and is in an oceanic lithosphere of ca. 100 km thickness, which represents a recharging area in the oceanic lithospheric mantle for CO2-metasomatism enrichment. It is associated with mantle anomalies caused by a hotspot or mantle plume that was influenced by the E-W direction of leaking transform fracture zones in the South Atlantic Ocean. Age constraints indicate decreasing ages eastward of the AVC, favoring the hotspot hypothesis for the generation of magmatic rocks in the Vitória-Trindade Ridge.

Geochemical Signatures of Felsic Volcanic Rocks in Modern Oceanic Settings and Implications for Archean Greenstone Belts

Abstract Felsic volcanic rocks are abundant in ancient greenstone belts and important host rocks for volcanogenic massive sulfide (VMS) deposits. About half of all VMS deposits are hosted by dacite or rhyolite, an association that reflects anomalous heat flow during rifting, partial melting of basaltic crust, and fractional crystallization in high-level magma chambers. For over 30 years, geochemical signatures of these rocks (e.g., F classification of Archean rhyolites) have been widely used to identify possible hosts for VMS deposits in ancient greenstone belts. However, comparisons with modern oceanic settings have been limited, owing to a lack of samples of felsic volcanic rocks from the sea floor. This is changing with increasing exploration of the oceans. In this study, we have compiled high-quality geochemical analyses of more than 2,200 unique samples of submarine felsic volcanic rocks (>60 wt % SiO2) from a wide range of settings, including mid-ocean ridges, ridge-hot-spot intersections, intraoceanic arc and back-arc spreading centers, and ocean islands. The compiled data show significant geochemical diversity spanning the full range of compositions of rhyolites found in ancient greenstone belts. This diversity is interpreted to reflect variations in crustal thickness, the presence or absence of slab-derived fluids (dry melting versus wet melting), and mantle anomalies. Highly variable melting conditions are thought to be related to short-lived microplate domains, such as those caused by diffuse spreading and multiple overlapping spreading centers. Systematic differences in the compositions of felsic volcanic rocks in the modern oceanic settings are revealed by a combination of principal components analysis, unsupervised hierarchical clustering, and supervised random forest classification of the compiled data. Dacites and rhyolites from midocean ridge settings have moderately depleted mantle signatures, whereas rocks from ridge-hot-spot intersections and ocean islands reflect enriched mantle sources. Felsic volcanic rocks from arc-back-arc systems have strongly depleted mantle signatures and well-known subduction-related chemistry (strong large ion lithophile element enrichment in combination with strong negative Nb-Ta anomalies and low heavy rare earth elements [HREEs]). This contrasts with felsic volcanic rocks in Archean greenstone belts, which show high field strength element and HREE enrichment (so-called FIIIb-type) due to a less depleted mantle, a lack of wet melting, and variable crustal contamination. The differences between modern and ancient volcanic rocks are interpreted to reflect the lower mantle temperatures, thinner crust, and subduction-related processes in present-day settings. We suggest that the abundance of FIIIb-type felsic volcanic rocks in Archean greenstone belts is related to buoyant microplate domains with thickened oceanic crust that were better preserved on emerging Archean cratons, whereas in post-Archean tectonic settings most of these rocks are subducted.

Plume‐Induced Rifting of Thickened Crust: 2D Numerical Model and Implications for N–S Rifts in Southern Tibet

AbstractN–S rifting is one of the most typical tectonics in southern Tibet, but its formation mechanism remains controversial. Geophysical observations indicated spatial correlations between rifts and lithospheric mantle anomalies, presumably caused by asthenospheric upwelling. Here, we investigate possible plume‐induced rifting via a series of 2‐D thermomechanical models of plume interactions with a heterogeneous lithosphere. The numerical results indicate that a small‐scale mantle plume could promote the formation of a single giant rift throughout the whole thickened crust. In addition, presence of a weak mid‐crustal zone facilitates the rifting development in the upper crust while inhibiting the formation of a giant crustal rift. Instead, multiple rifts develop in the upper crust and thickened crust, jointly controlled by a heterogeneous weak crustal zone and mantle plume. Our numerical results thus emphasize the distinct roles of the weak mid‐crustal zone and small‐scale mantle plume in promoting N‒S rifting in southern Tibet.

Timing of rapid cooling and erosional decay of two volcanic islands of the Canary Archipelago: implications from low-temperature thermochronology

The Canary Archipelago comprises seven volcanic islands formed by the activity of the Canary mantle anomaly that might have been caused by an ascending plume at the NW-African passive margin. The “Basal Complex (BC)”, which contains the islands pre-shield rock formations, is exposed in the northwest and central Fuerteventura and NW-La Gomera and preserves the archive of giant landslides that caused the removal of most of the shield-stage volcanic rocks. Tools, like low-temperature thermochronology (LTT) are sensitive to rapid cooling activities that accompany landslides. In addition, integrating LTT data with time–temperature (t–T) numerical modelling are a powerful tool for reconstructing the thermo-tectonic evolution as well as defining and quantifying long-term landscape evolution in a variety of geological settings. To unravel part of the long-term landscape evolution of Fuerteventura and La Gomera, zircon and apatite fission-track, and (U–Th)/He data combined with t–T numerical modelling were applied to 39 samples representing the main rock units of the BCs and younger magmatic rocks on both islands. In Fuerteventura, the Northwest and Central Basal Complexes reveal rapid cooling/exhumation of more than 200 °C at ~ 20 Ma. The quantification of the thickness of the rock column using the t–T cooling path would need the knowledge of the palaeo-heat flow. The published thickness of the moved rock column in Fuerteventura and La Gomera does not point to an extreme high heat flow. Therefore, the formation of a giant landslide leads to the removal of ~ 2.0 (± 0.5) km of the volcano rock column. Offshore, such a landslide has led to part of the Puerto Rosario large debris avalanche. The “Central Basal Complex” revealed two more rapid cooling/exhumation events at ~ 16 Ma and ~ 14 Ma that might also be related to landslides. The three landslides might be responsible for the formation of the nowadays Puerto Rosario Debris Avalanche Unit offshore. What might have caused the landslides in Fuerteventura. Age data published provide evidence for magmatic and tectonic activity that occur at the time of the formation of the giant landslides. In addition, the Miocene climate significant changes lead to changes in precipitation, and such changes might also provide a destabilisation of pyroclastic units. Therefore, the causes of the giant landslides might be related to more than only one process. The La Gomera BC has experienced two rapid cooling/exhumation events: the first at ~ 9 Ma, which might have caused ~ 2.0 (± 0.2) km of erosion forming the offshore Tazo avalanche, also known as the Tazo landslide. The second rapid cooling at ~ 8.0 Ma is located at the northwest of the Island and might have been caused by the Garajonay caldera collapse and followed by landslides. The landslides are assumed to have formed the Segments I, II, III, and VIII of the submarine debris avalanches offshore. Like Fuerteventura, both landslides might have been triggered by tectonic and magmatic activities as well as due to variation in precipitation caused by climate variation.

Seismic tomography of the mantle and primary hydrogen deposits in the Dnieper-Donetsk basin

According to the three-dimensional P-velocity model of the mantle under Eurasia obtained by the Taylor approximation method, the analysis of the velocity structure of the mantle (to depths of 2500 km south of 50° N and 1700 km north) in the territory of the Dnieper-Donetsk depression was carried out in order to determine the possible areas of primary hydrogen release. Primary hydrogen is formed in the core and lower mantle, can be transferred to the surface (according to I.L. Gufeld). According to seismotomography, nine superdeep mantle fluids are isolated on the territory of Ukraine, the routes of which are defined as subvertical columns of alternation of high-speed and low-speed anomalies. In addition to the presence of superdeep mantle fluids in the study area, the following characteristics were analyzed: the depth of the main geodynamic boundary, the influence of the high-velocity transition zone of the upper mantle (propagating northward into the low-velocity transition zone of the upper mantle of the East European platform), the depth of the Moho boundary, gravity mantle anomalies and heat flow. The totality of the studies performed allowed us to conclude that, according to seismic tomography data, the Izyumsky and the eastern part of the Lokhvitsky segment of the Dnieper-Donetsk depression are promising areas for detecting primary hydrogen in the territory of the Dnieper-Donetsk depression.

Discovery of distinct lithosphere-asthenosphere boundary and the Gutenberg discontinuity in the Atlantic Ocean.

The plate tectonic theory requires a rigid lithosphere floating over a weak asthenosphere, separated by the lithosphere-asthenosphere boundary, which has been sometimes interpreted as the Gutenberg discontinuity. Using a deep seismic reflection technique, we report the presence of two continuous reflections covering 27 Ma to 58 Ma oceanic lithosphere in the Atlantic Ocean. We find that the upper reflection deepens with age and follows the ~1250°C isotherm, whereas the deeper reflection lies at a constant depth of ~75 km. We suggest that the upper reflection represents the thermally controlled lithosphere-asthenosphere boundary, whereas the lower reflection is the Gutenberg discontinuity, a frozen-in dehydration boundary separating the dry mantle melting region above from the hydrated mantle below formed at the ridge axis. We also find that thermal mantle anomalies rejuvenate the lithosphere, uplift the lithosphere-asthenosphere boundary, and destroy the Gutenberg discontinuity.

Strong influence of tomographic models on geoid prediction: Case studies from Indian Ocean and Ross Sea geoids

The geoid over the north Indian Ocean has a significantly large negative amplitude, even if the excess flattening of the Earth beyond its equilibrium shape is considered. The various mechanisms proposed for this geoid anomaly vary and acceptable geoid predictions are obtained for specific tomographic models only. In this study, we identified model-independent features in the mantle beneath the Indian Ocean and Ross Sea region by analyzing eight recent global tomographic models. We standardized each of the models and applied cluster analysis to regionalize geophysically significant features in various depth ranges. These regionalizations are compared grid-by-grid to construct vote-maps. They highlight the anomalous features consistent across models and their approximate dimensions. Low velocity anomalies of dVS ∼-1.1% in the ∼400–680 km depth range are consistent in almost all the models beneath the Indian Ocean and Ross Sea. High velocity anomalies of dVS≥1% at depths below 1600 km, incoherent in dimension and orientation, are also observed. High velocity anomalies are most likely subducted slabs while low velocity anomalies could be partial melts generated by hydration of mantle. Additionally, a consistent low velocity structure is seen throughout the mantle beneath the southwestern Indian Ocean and east Africa. It is mostly likely a plume rising from the African LLSVP. It connects to the probable partial melts beneath the Indian Ocean via a remnant trail. Forward modelling of the geoid using vote-maps reveals that the E-W extent of the Indian Ocean Geoid Low is precisely reproduced by the consistent low velocity anomalies in the upper mantle. However, the N-S extent, most likely dependent on lower mantle anomalies, is not expressed since the dimension, orientation and dVS characteristics of high velocity structures are inconsistent in different models. The inter-model agreement is insufficient to identify structure(s) seen across models that can explain the geoid low over the Ross Sea.

Mapping the mantle transition zone beneath the Indian Ocean geoid low from Ps receiver functions

The most pronounced high-amplitude (−40 to −106 m) geoid deficit with respect to the Earth's reference shape straddles the Indian Ocean. The existing hypotheses suggest a possible linkage of low density and low-velocity anomalies in the mid-and upper mantle and high-velocity anomalies near the base of the mantle to the source behind the lowest geoid. We obtained P to S wave radial receiver functions to investigate these mantle anomalies from an extensive array of Ocean Bottom Seismometers (OBSs) positioned in the centre of the Indian Ocean Geoid Low (IOGL). Our modelling suggests that the mean depths for d410 and d660 range from 386.0 ± 13.6 to 459.7 ± 2.9 km and 643.1 ± 7.4 to 710.2 ± 5.4 km with an average of 432.6 km and 680.2 km, respectively. The average thickness of the transition zone varies from 199.6 to 289.8 km. New results confirm an extensive ~800 km wide depression at d410 and d660 towards the centre of the geoid. The depression potentially implies a rather hot mantle material, in which majorite garnet to perovskite transition may become dominant at 660 km depth. The excess temperature calculated for d410 and d660 topography ranges from 139.5 to 557.5 K and 206 to >1000 K, respectively. The shear velocity anomalies derived from the excess temperature range from −0.89 to −3.52 (%) at d410, and at dd60 range from −1.13 to −7.87 (%). The velocity anomalies are highly underestimated at d660 and could be explained by hydrous mantle upwelling at the lower mantle transition zone. The present study outcomes offer new avenues towards exploring a mantle upwelling in the Indian Ocean and its linkage with geoid undulations.

Mantle gravity anomaly and recent activation in the territory of Ukraine

The study of the zones of modern activation on the territory of Ukraine for a long time was carried out using a complex of geological and geophysical methods. Each of them is not without its drawbacks. The results obtained allowed us to assume that the determination of the mantle anomaly of the gravitational field can be eliminated their significant part and serve as a unified criterion for identifying the zones of modern activation. For this, it is necessary to improve the methodology for calculating the effect of the earth’s crust, supplementing it with taking into account the degree of catagenesis of part of the rocks of the sedimentary layer, the effect of increased concentrations of ore minerals in the depths of regional magnetic anomalies, etc. Thus, a more accurate calculation of the effect along the DSS profiles can be achieved. In addition, it is necessary to ensure the determination of the gravitational effect of the crust in the area between the profiles. Kriging technology was used to solve this problem. It allows you to calculate the predicted values of the parameter using those known in other points and to estimate the error of the result. In our case, the usual kriging option was sufficient. To determine the errors, instead of the commonly used variograms, a comparison of data on one of the DSS profiles with those calculated by kriging from information on adjacent profiles was used. The resulting error turned out to be quite understandable, taking into account the errors of the initial data, and acceptable for using the results for mapping the mantle gravity anomaly, and, consequently, the zones of modern activation of Ukraine. The constructed map demonstrates some difference in the location of the anomaly (and, accordingly, the recent activization — RA zones) from the previously established one. In particular, the anomaly is absent in the Desnyanskiy basin of the Dnieper-Donetsk depression. Actually, the gravitational mantle anomaly was absent there before, the spread of the RA zone into the basin was indicated by other criteria of low reliability. It is possible to record the presence of an anomaly in the territory between the Azov and Donbas regions, where it was not previously diagnosed. This result still needs to be supported by additional data. First of all, due to the observed significant discrepancies between different variants of velocity sections along the DSS profiles.

СВЯЗЬ ПРОСТРАНСТВЕННОГО РАЗМЕЩЕНИЯ РУДНЫХ МЕСТОРОЖДЕНИЙ С ГЛУБИННЫМ СТРОЕНИЕМ ЗЕМНОЙ КОРЫ И ВЕРХНЕЙ МАНТИИ В ВЕРХОЯНО-КОЛЫМСКОМ РЕГИОНЕ

In the report, the authors analyze connections of spatial placement of ore fields with the crust and upper mantle anomalies of the Verkhoyano-Kolymsky region in 3D-statement. Gravity rheological models discover new features of the tectonosfere deep structure; they explain already known structural, petrologic and geochemical characteristics of ore fields with different specialization from a position of their connection with a deep structure of the region.

THE CENOZOIC CRUSTAL STRESS STATE OF MONGOLIA ACCORDING TO GEOLOGICAL AND STRUCTURAL DATA (REVIEW)

This article gives a chronological review of the main published research results concerning the Cenozoic crustal stress-strain state in Mongolia and adjacent territories. The studies commenced in the southern Baikal rift zone in the 1970s and were extended further southwards to cover mobile regions neighbouring the Siberian platform. Geological, structural and morphostructural data were collected and analysed to define the crustal stress types and spatial characteristics. The authors have consolidated their reconstructions of the crustal stress-strain state of Mongolia, which were based on tectonic fracturing data and displacements along fractures in fault zones active in the Cenozoic. We consolidated a database of reconstructed stress tensors, which now contains more than 750+ solutions. The Late Cenozoic stress field was mapped. The map shows domains differing in types of the paleostress state of the crust. The reconstructions were compared to our calculations of the present-day crustal stress state, which were based on earthquake focal mechanisms, and to calculations by other authors. At the Late Cenozoic and current stages, the maximum horizontal compression axis (S Hmax ) has varying orientations, from submeridional (Western Mongolia) to NE and ENE (Eastern Mongolia). The role of compression increases from the northern domains, where the reconstructions show shear and transtension, to the southern domains with prevailing transpression and compression. Regular changes occur in the stress state and rupture parageneses along the largest latitudinal faults, North Khangai and Dolinoozersky; such changes are related to left-lateral strike-slip faulting. We analysed the sequence of the occurrence of stress fields differing in types and spatial characteristics, and revealed the main regularities in the evolution of the crustal stress-strain state in time. In the Cenozoic history of crust deformation in Mongolia, we can distinguish several episodes that differ in the dominant impacts of various tectonic force sources or combinations of such impacts. At the beginning of the Cenozoic, tectonic structures developed mainly under the influence of the interaction of East Asia and the Pacific Plate, which was manifested in the southeastern domains of the study area. The long-term SE-trending asthenospheric flow caused crustal stretching, which initiated the formation of tectonic structures comprising the Baikal rift system. Starting from the Pliocene, crustal stretching took place in combination with NNE compression caused by the India–Eurasia convergence. As a result, shearing occurred along the large faults. At this background, the Khangai and Khentei uplifts (including crust extension zones at their crests) are large structures that developed due to the dynamic effect of local mantle anomalies.

Hybrid local and teleseismic P-wave tomography in North Tanzania: role of inherited structures and magmatism on continental rifting

SUMMARY While local earthquake tomography is typically used to image the crust, this technique has restricted depth penetration due to short receiver-source distances. Regional tomography however aims to image the upper mantle from teleseismic events but suffers from poor resolution from 0 down to 40 km depth. We present here a hybrid method that combines the two approaches taking advantage of the short-wavelength resolution within the crust to better constrain the ray path at depth, and thus to improve the lithospheric imaging. Using this new method enhances the continuity or disruption of mantle anomalies towards the surface. Such hybrid tomographic images of crust-to-upper mantle structures are then critical to understand the relation and interplay between the thermal and mechanical lithospheric processes and the role in the localization of the deformation at the surface. We apply our approach to the North Tanzanian Divergence (NTD), where those processes interact with a cold cratonic lithosphere. Our new tomographic images clearly demonstrate the impact of deep-seated processes on surface features. First, strong lateral velocity anomalies and clustered seismicity in the crust are consistent with the surface geology of the NTD (rifted basins, volcanoes and border faults). Then, at a lithospheric scale, the velocity distribution highlights the major role of inherited structures in guiding the rift opening. In particular, our study suggests a strong influence of the Masai cratonic block, south of the NTD, in the rift evolution. The transition from the north–south axial valley into three diverging rift arms (Eyasi, Natron-Manyara and Pangani) is likely due to the change in rheology and to the presence of magma along inherited sutures between the craton and the mobile belts.

Large-scale asymmetry in thickness of crustal accretion at the Southeast Indian Ridge due to deep mantle anomalies

Abstract Hot mantle plumes and ancient cold slabs have been observed beneath modern mid-ocean ridges, but their specific and detailed effects on mid-ocean ridge crustal accretion are poorly understood. The oceanic lithosphere beneath the Southeast Indian Ocean displays unique morphological, geophysical, and geochemical characteristics, which may reflect the influence of both mantle anomalies and upwelling plumes on seafloor spreading. In this study, we combined gravity-derived oceanic crustal thickness with plate tectonic reconstructions to investigate patterns of asymmetry in thickness of crust accreted at the Southeast Indian Ridge over the last 50 m.y. Our results reveal several distinct features: (1) small-scale, short-lived asymmetries in the thickness of crustal accretion of up to 0.75 km are alternatively distributed on the southern and northern flanks of the 90°–120°E Southeast Indian Ridge segment. These can be explained by variations in mantle depletion or mantle temperature. (2) Two large-scale, long-lived (duration of ∼50 m.y.) asymmetries in crustal accretion of >2.5 km are observed around the Kerguelen Plateau and Balleny Islands, which we attribute to excess crust from the off-axis Kerguelen and Balleny mantle plumes. (3) Two large-scale, long-lived (duration of ∼50 m.y.) asymmetries in crustal accretion of 0.75–2.5 km are observed on the northern flank of the westernmost (70°–80°E) Southeast Indian Ridge and the southern flank of the eastern (120°–140°E) Southeast Indian Ridge segment, respectively. We attribute these to asymmetry in mantle temperature of up to 20–53 °C. We suggest these asymmetric temperatures across the Southeast Indian Ridge are associated with the foundered lithospheric fragments of the Indian Craton triggered by the African Large Low-Shear-Velocity Province during the breakup of Gondwanaland and an intraplate subducted slab of the Paleo-Tethys Ocean, respectively. The remnant craton fragments and subducted oceanic slab may have moved north in concert with the northward-migrating Southeast Indian Ridge beginning at 50 m.y. ago.

AusLAMP 3D MT imaging of an intracontinental deformation zone, Musgrave Province, Central Australia

The central Australian Musgrave Province at the junction of the South, North and West Australian cratons has undergone and continues to retain evidence of significant whole-of-crust, and most likely ‘whole-of-lithosphere’ tectono-magmatic processes. The area is known for some of the largest geophysical anomalies related to significant Moho offsets of up to 15 km, which resulted from repeated intracratonic reworking since the Neoproterozoic. New magnetotelluric (MT) data have been collected across the Musgrave Province in Western Australia and South Australia as part of the Australian Lithospheric Architecture Magnetotelluric Project (AusLAMP). Station spacing was sim 50,hbox {km} between 96 sites over an area of 500 × 700 km. Long-period MT impedance and tipper data over a bandwidth of 8 s to 10,000 s period have been inverted using a smooth 3D inverse algorithm. The 3D model shows two predominant resistivity trends. There are deep (>65,hbox {km}) north–south mantle conductors that we infer to be related to the Palaeo- to Mesoproterozoic north-trending arc-related rocks that experienced ultra-high temperature metamorphism and widespread magmatism during the Mesoproterozoic Musgravian Orogeny. These conductors are preserved in the crust south of the Musgrave Province. The upper mantle also contains a localised resistive zone that possibly represents generation of mafic- to ultramafic magmas during the c. 1090–1040 Ma Giles Event. The crust (<65,hbox {km} depth) contains strong east–west crustal conductors interpreted to reflect the east–west structural grain that initiated during the c. 1090–1040 Ma Giles Event and overprinted the older N–S-oriented mantle anomalies. These E–W crustal conductors coincide with magnetic anomalies that represent crustal-scale structures, and high gravity anomalies associated with significant Moho offsets resulting from further reactivation during the c. 630–520 Ma Petermann and c. 450–300 Ma Alice Springs orogenies.

2.5 dimensional model of mantle heterogeneities under the Ukrainian shield according to the gradients of the velocities of seismic waves

We analyze the basic techniques for the investigation of the deep structure of the mantle and the shortcomings of the models of mantle structures derived from them. Thus, we reveal that there is no analysis of the velocity field by means of analytical transformants. Therefore, we developed and tested a new approach to define the mantle boundaries based on the calculations of the sequence of P-waves velocity derivatives. As a result, we obtain some new set of velocity gradient distributions for the principal tectonic structures of the Ukrainian Shield along the composite profile. The boundaries of the mantle discontinuities according to the velocity gradient we define in a special manner to eliminate the false anomalies and the fluctuations of the velocity curves that occur due to the conversion of the hodograph into the mean velocities. The smoothing of the velocity curve we perform with a previously defined wavelength step being equal to 50 km. We treat the calculated velocity gradient anomalies as the useful signal response above the appropriate sections, which have different velocity accelerations levels inside the upper mantle. We assume that the mantle anomalies have the same physical background (density/viscosity distributions, temperature gradients etc.) within each range with the equal acceleration value. However, the singular points determined by the inflections of the gradient curve could be the possible boundaries of additional inhomogeneities within the mantle. We calculate both the 1st and the 2nd derivatives for the velocity curves obtained. The excesses 2.5-D model of the 1-th and 2-th gradient curves (the acceleration of the gradvp itself) determine the position of the max / min anomalies of gradvp at the consolidated seismic profile within the Ukrainian Shield. Finally, we analyze in detail the distribution of velocity gradients of P-waves within the upper mantle in the depth range of 50–750 km. It results in the identification of a series of additional gradient velocity boundaries within three principal structural horizons of the upper mantle (under ~ 200–300 km, ~ 410–500 km, and ~ 600–650 km respectively).

Surface Wave Diffraction Pattern Recorded on AlpArray: Cameroon Volcanic Line Case Study.

Stripe‐like patterns of surface wave arrival angle deviations have been observed by several seismological studies around the world, but this phenomenon has not been explained so far. Here we test the hypothesis that systematic arrival angle deviations observed at the AlpArray broadband seismic network in Europe are interference patterns caused by diffraction of surface waves at single small‐scaled velocity anomalies. We use the observed pattern of Rayleigh waves from two earthquakes under the Southern Atlantic Ocean, and we fit this pattern with theoretical arrival angles derived by a simple modeling approach describing the interaction of a seismic wavefield with small anomalies. A grid search inversion scheme is implemented, which indicates that the anomaly is located in Central Africa, with its head under Cameroon. Moreover, the inversion enables the characterization of the anomaly: The anomaly is inferred to be between 320 and 420 km wide, matching in length the 2,500 km long upper mantle low‐velocity region under the volcano‐capped swells of the Cameroon volcanic line. We show that this approach can be generally used for studying the upper mantle anomalies worldwide.

A New Contribution to the (IPC) Bouguer Map of a Gap Area in Mosul and Duhok Region (Iraq)

The purpose of the current study was to complete an important gap within the Bouguer map of Iraq implemented by the Iraq Petroleum Company (IPC). In addition, the collected data were processed with recent gravity concepts and methods to establish a new regional geological image. A total of 868 gravity points were measured and corrected. The Bouguer anomaly map of the study area was initially drawn up with the parameters and equations used to construct the IPC map. The newly-produced map shows a very strong correlation with the old IPC results. A new Bouguer map of the study area was then constructed using modern parameters and equations. A comparison between the old and new maps illustrates the difference between recent and previous techniques. The isostatic residual map shows the presence of over-compensated areas close to Chia Gara and Mateen and spreads to the Aqra zone with a negative value of up to -20 mGal. In contrast, there is under-compensation in the southern regions with positive isostatic anomalies of about 80 mGal. In the researched area there are zero compensation lines around the larger structures (Chia Gara, Mateen, Mangesh, Aqra, Piris, and Perat). In order to draw up the crustal thickness map, the Moho discontinuity depth was determined using topographic and Bouguer anomaly grids of the study area. In addition, the upper mantle anomalies were calculated by continuing the isostatic residual anomaly to 45 km upward and then removing the past isostatic residual anomaly, that was continued upward to 35 km. Several qualitative approaches were used to isolate the regional and remaining anomalies to analyze the gravity data, with an optimal upward continuation level of 14 kilometers generated for the large and small maps.