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We present the first evidence for a lower S-wave velocity (Vs ~3.3 to 3.5 km/s) at 8-17 km depth underlying a 4 km thick high-velocity zone with Vs >3.8 km/s beneath the west coast and the neighbouring parts of the Deccan Volcanic Province, India, coinciding with the last phase of volcanism. The velocity structure is derived from joint inversion of receiver function from 9 seismographs operated along ~106 km long W-E profile with the surface wave dispersion data. The low-velocity layer possibly represents the horizontally elongated frozen magma reservoir, the source for the magma eruption at ~65 million years produced due to the interaction of the Reunion hotspot with India. The shallow, high-velocity layer could be basaltic mafic intrusions responsible for the production of massive CO2 degassing. The Moho deepens beneath the west coast to ~45 km due to 10-15 km of underplating as a consequence of magma upwelling.

The analysis of seismic ambient noise has been recently proven a viable alternative to the analysis of seismic waves generated by earthquakes. Although seismic ambient noise had traditionally been discarded from earthquake records, it has now been shown that the cross-correlation of the seismic ambient noise allows for the recovery of the Green’s function between the receivers. Furthermore, seismic noise has the ability to propagate continuously and independently of the occurrence of earthquakes, allowing for high-resolution tomographic studies in regions of low seismicity. Over the last two decades, the cross-correlation of continuous seismic noise recordings between pairs of stations has been widely utilized in surface-wave tomography studies. For the Northeast Brazil region, tomographic studies have been exclusively performed with seismic data from broadband stations; however, in addition to those stations, there exists a large volume of short-period data that might potentially improve the resolution of existing ambient noise tomographies. Thus, the goal of this work is to utilize, in addition to broadband data, short-period data recorded by 22 short-period stations in the Borborema Province. Through cross-correlation and stacking of ambient seismic noise, the emergence of the fundamental mode of the Rayleigh waves and their dispersive character was observed. Once the empirical Green's functions were retrieved, the accuracy of (phase and group) dispersion curves for periods up to 10 s and its tomographic inversion were assessed. Our results demonstrate that short-period dispersion measurements can be successfully integrated in regional tomographic studies for improved resolution.

Como poeta contemporáneo, José Agustín Goytisolo hizo uso de la tradición poética para escribir sus poemas, pero también estuvo in/uido porla métrica más convencional. No obstante, el ritmo de sus versos juega con los metros de la tradición española para hacer una propuesta formal más personal.Este artículo describe el estilo poético de Goytisolo, y cómo creó algunos de sus poemas sobre el patrón formal de otros poetas.

NE Brazil is scarred by a number of aborted rift basins that developed from the same extensional stresses that lead to the opening of the South Atlantic. Extension started in Late Jurassic times, with the formation of an AfroBrazilian Depression south of the Patos Lineament, and continued through the Early Berriasian along two NS trending axes of deformation: Recôncavo-Tucano-Jatobá (RTJ) and Gabon-Sergipe-Alagoas (GSA). In the Late Berriasian - Early Barremian, rifting jumped North of the Pernambuco Lineament to progress along the NE-SW trending Cariri-Potiguar (CP) axis. In the Late Barremian, approximately coinciding with the opening of the Equatorial Atlantic, rifting aborted along the RTJ and CP axes and continued along the GBA trend eventually resulting in continental break-up. Extension-related magmatic activity seems to have been restricted to break-up along the marginal basins, although dyke swarms bordering the Potiguar basin (Rio Ceará-Mirim) seem to be associated to early extension stages in NE Brazil and three subparallel dolerite dykes, with K-Ar dates of 105±9 Ma, were inferred indirectly from aeromagnetic and outcrop data East of the RTJ axis. Aiming at better understanding the structure and evolution of the syn-rift basins of NE Brazil, a total of 20 seismic stations were deployed between October 2018 and January 2021 along the CP and RTJ trends. The deployment, funded by the national oil company Petrobras, included both broadband and short-period stations borrowed from the Pool de Equipamentos Geofísicos do Brasil. These stations complemented a number of permanent broadband stations belonging to the Rede Sismográfica do Brasil. Receiver functions were obtained for each of the seismic stations from teleseismic P-wave recordings and S-wave velocity models were developed from their joint inversion with dispersion velocities from an independent tomographic study. In the RTJ basins, our results show that the crust is about 41 km thick and displays a thick (5-8 km) layer of fast-velocity material (> 4.0 km/s) at its bottom; in the Potiguar basin, our results show a thinner crust of about 30-35 km underlain by an anomalously slow (4.3-4.4 km/s) uppermost mantle. We argue that those anomalous layers are the result of syn-rift and/or post-rift magmatic intrusions, which would have had the effect of increasing velocity at lower crustal levels under the RTJ basins and decreasing velocity at uppermost mantle depths under the Potiguar basin. If correct, ou interpretation would imply that, in spite of an overall lack of evidence at shallow levels, deep magmatic processes have played a role in the formation and evolution of the syn-rift basins of NE Brazil.

The occurrence of deep-focus earthquakes (h > 300 km) is restricted to a handful of regions worldwide, generally associated with subduction zones. In particular, the South American subduction zone hosts two narrow belts of deep-focus seismicity with depths greater than ~500 km along the Peru-Brazil border and Bolivia/northern Argentina. This subduction zone has a thermal parameter of Φ < 2500 km and is regarded as a warm end-member. Only in 2015, the USGS catalog listed up to 25 deep-focus events in the Peru-Brazil belt, with magnitudes and depths ranging from 4.0 to 7.6 Mw and 515 to 655 km, respectively. Notably, this sequence included a well-investigated doublet of two 7.6 Mw events occurring 5 min apart trailed by a number of aftershocks of magnitude 4.0 Mw or larger. Published focal mechanisms for the main doublet display predominantly double-couple components that closely agree with the GCMT solution (E1: 350°, 39°, -80° and E2: 350°, 30°, -81°), suggesting shear failure at those depths. Mechanisms capable of shear instability at those large depths traditionally include dehydration embrittlement, transformational faulting, thermal runaway or a combination of those. Aiming at investigating the physical mechanism responsible for these deep-focus events, we are using a combination of regional and teleseismic recordings from the Brazilian Seismographic Network (RSBR) and other regional and national networks in the continent to determine focal mechanisms for deep-focus earthquakes (M > 4) that occurred between 2014 and 2022. The mechanisms are being determined through a Cut and Paste approach, which compensates for inaccuracies in the velocity model through independent relative time shifts between observations and predictions for P, SV and SH wave trains sampling both the upper and lower hemispheres of the focal sphere. The results on the 2015 doublet, using the full dataset (regional and teleseismic stations), indicated two very similar normal faults fully consistent with the GCMT solutions, at the preferred depths of 616 (E1) and 621 (E2) km. Preliminary inversions using only regional networks (RSBR) for 15 smaller earthquakes (4.3 < M < 7.1) also yield normal mechanisms with T axes oriented roughly E-W. This apparent uniformity of the focal mechanisms for the South-American deep-focus earthquakes, with near-vertical P axes and near-horizontal (east-west-oriented) T axes, strongly suggests vertical compression along the subducting plate is the main source of stress driving deep-focus seismicity. Down-dip compression is expected from either buoyancy forces, equilibrium phase transformations or a metastable olivine wedge (MOW); however, how earthquakes are nucleated at those depths is harder to explain. Transformational faulting within the MOW has been the preferred mechanism in cold slabs, while in warm slabs its presence has been more debated due to wedge size being expected to decrease with temperature. Transformational faulting in other metastable minerals such as enstatite is our preferred alternative, as dehydration embrittlement and thermal runaway seem to lack the capacity of triggering earthquakes at those large depths.

The Transantarctic Mountains (TAMs), Antarctica, exhibit anomalous uplift and volcanism and have been associated with regions of thermally perturbed upper mantle that may or may not be connected to lower mantle processes. To determine if the anomalous upper mantle beneath the TAMs connects to the lower mantle, we interrogate the mantle transition zone (MTZ) structure under the TAMs and adjacent parts of East Antarctica using 12,500+ detections of P-to-S conversions from the 410 and 660 km discontinuities. Our results show distinct zones of thinner-than-global-average MTZ (∼205–225 km, ∼10%–18% thinner) beneath the central TAMs and southern Victoria Land, revealing throughgoing convective thermal anomalies (i.e., mantle plumes) that connect prominent upper and lower mantle low-velocity regions. This suggests that the thermally perturbed upper mantle beneath the TAMs and Ross Island may have a lower mantle origin, which could influence patterns of volcanism and TAMs uplift.

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

A high-resolution 3-D crustal and upper-mantle shear-wave velocity model of Northeast China is established by joint inversion of receiver functions and Rayleigh wave group velocities. The teleseism...

SummaryThis study investigates the seismic structure and anisotropy in the crust beneath Madagascar and southeastern Africa, using receiver functions. The understanding of seismic anisotropy is essential for imaging past and present deformation in the lithosphere–asthenosphere system. In the upper mantle, seismic anisotropy mainly results from the orientation of olivine, which deforms under tectonic (fossil anisotropy) or flow processes (in the asthenosphere). In the crust, the crystallographic alignment of amphiboles, feldspars (plagioclase) or micas or the alignment of heterogeneities such as fractures, add to a complex geometry, which results in challenges to understanding the Earth's shallow structure. The decomposition of receiver functions into backazimuth harmonics allows to characterize orientations of lithospheric structure responsible for azimuthally varying seismic signals, such as a dipping isotropic velocity contrasts or layers of azimuthal seismic anisotropy. By analysing receiver function harmonics from records of 48 permanent or temporary stations this study reveals significant azimuthally varying signals within the upper crust of Madagascar and southeastern Africa. At 30 stations crustal anisotropy dominates the harmonics while the signature of a dipping isotropic contrast is dominant at the remaining 18 stations. However, all stations’ backazimuth harmonics show complex signals involving both dipping isotropic and shallow anisotropic contrasts or more than one source of anisotropy at shallow depth. Our calculated orientations for the crust are therefore interpreted as reflecting either the average or the interplay of several sources of azimuthally varying signals depending of their strength. However, comparing information between stations allows us to draw the same conclusions regionally: in both southern Africa and Madagascar our measurements reflect the interplay between local, inherited structural heterogeneities and crustal seismic anisotropy generated by the current extensional stress field imposed by the southward propagation of the East-African Rift System. A final comparison of our crustal orientations with SKS orientations attributed to mantle deformation further probes the interplay of crustal and mantle anisotropy on SKS measurements.

11 pages, 7 figures, 1 table, supplementary data https://doi.org/10.1016/j.gr.2018.11.013.-- Data availability: Digital.CSIC https://doi.org/10.20350/digitalCSIC/14694, The lithospheric architecture of the western Borborema Province and northern São Francisco craton of NE Brazil has been investigated through analysis of long-period magnetotelluric data acquired along a 700 km long survey, using 12 instruments. The survey samples several tectonic terrains in the Province and penetrates into the adjacent São Francisco craton after crossing the Araripe Basin, an aborted rift basin filled with Mesozoic sediments that peak at ∼1000 m above mean sea level. High conductivities are observed at shallow depths under the main Precambrian shear zones that pervade the Province – consistent with tectonic reactivation – and as a small patch embedded within the high resistivities that characterize the São Francisco craton. High conductivities (∼25 Ωm) are also observed below 120 km depth between the Patos and Pernambuco lineaments – right under the Araripe Basin – flanked by resistive (>120 Ωm) material immediately to the north and south. This deep, highly conductive body is found consistent with the presence of melt and aqueous fluids, and is interpreted as shallow asthenospheric mantle bounded by thicker lithosphere. We propose that extensional stresses in the Mesozoic stretched and thinned the lithosphere under the Araripe Basin, causing passive upwelling of asthenospheric material and lateral flow of the overlying lithosphere, and resulting in thickening of the lithosphere under the flanks and uplift of the Araripe Basin. We also hypothesize that thermal weakening of the lithospheric mantle – perhaps sustained by channeling of asthenospheric flows under the basin – would have caused regional stresses to concentrate in the brittle upper crust and contribute to basin inversion. We thus propose that a combination of localized horizontal stresses and vertical buoyancy from underlying asthenospheric material are ultimately responsible for the actual topography of the Araripe Basin. © 2019 International Association for Gondwana Research., This work was funded by the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) through grant no400743/2014-0. XG thanks CNPq for the visiting research fellowship held under the Ciência Sem Fronteiras program. JJ also thanks CNPq for his regular research fellowship (process no 304421/2015-4).

The crustal and lithospheric velocity structure of the Fiji area may place important constraints on the evolution of island arcs in the region. Here, we obtain receiver functions from one permanent seismic station and three temporary networks on the Fiji Platform, the Lau Ridge, and other small islands to develop point estimates of crustal thickness under the networks. We obtain preliminary estimates of crustal thickness and Poisson's ratio from H-κ stacking of the receiver functions. We then perform an inversion for shear-wave velocity structure using a joint inversion of receiver functions and Rayleigh wave dispersion curves from ambient seismic noise and teleseismic events. The joint inversion results show an average crustal thickness of ~30 km beneath Viti Levu, the largest Fiji island, suggesting an extensive period of arc crustal formation and thickening. In contrast, we find shallower Moho depths averaging ~22 km beneath the smaller Fiji islands and the Lau Ridge. The limited thickness of the high velocity lid obtained by inversion indicates that the lithosphere is thinned beneath the marginal Fiji islands, which may be due to the early rifting event forming the Lau Basin together with the crustal thinning of this area as well, and to thermal lithospheric erosion processes associated with adjacent back-arc spreading. A relatively slow velocity middle crustal layer, roughly of 11–16 km thickness, exists throughout Fiji, suggesting that remnant arcs contain significant volumes of felsic crustal rocks necessary for continental crust formation.

New maps of S velocity variation for the upper and middle crust making up the northwestern most corner of South America have been developed from cross correlation of ambient seismic noise at 52 broadband stations in the region. Over 1,300 empirical Green's functions, reconstructing the Rayleigh wave portion of the seismic wavefield, were obtained after time and frequency-domain normalization of the ambient noise recordings and stacking of 48 months of normalized data. Interstation phase and group velocity curves were then measured in the 6-38 s period range and tomographically inverted to produce maps of phase and group velocity variation in a 0.5° × 0.5° grid. Velocity-depth profiles were developed for each node after simultaneously inverting phase and group velocity curves and combined to produce 3-D maps of S velocity variation for the region. The S velocity models reveal a ~7 km thick sedimentary cover in the Caribbean region, the Magdalena Valley, and the Cordillera Oriental, as well as crustal thicknesses in the Pacific and Caribbean region under ~35 km, consistent with previous studies. They also display zones of slow velocity at 25-35 km depth under regions of both active and inactive volcanism, suggesting the presence of melts that carry the signature of segmented subduction into the overriding plate. A low-velocity zone in the same depth range is imaged under the Lower Magdalena Basin in the Caribbean region, which may represent either sublithospheric melts ponding at midcrustal levels after breaching through a fractured Caribbean flat slab or fluid migration through major faults within the Caribbean crust. ©2018. American Geophysical Union., E. P. acknowledges support from the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) through a 4 year scholarship to complete his Ph.D. J. J.thanks CNPq for his research fellowship (CNPq, grant 308171/2012-8). M. S.acknowledges support by Brazilian Science Without Border Program, grant 40.2174/2012-7.

16 pages, 14 figures, 1 table, supplementary material https://doi.org/10.1016/j.tecto.2021.229024, The Proterozoic Borborema province of NE Brazil was under extensional stresses during the Mesozoic, causing stretching and thinning of the lithosphere. During Cenozoic times, compression associated with the development of the South Atlantic and collision along the Andean front dominated throughout the Province. One of the main consequences of such compression might have been the formation of the Chapada de Araripe, an inverted rift basin located in the Western Province. We undertook a passive seismic experiment along a NS profile crossing the Borborema province and a short EW profile across the Chapada de Araripe, with the goal of providing constraints on the origin of the Chapada de Araripe. A variety of mechanisms can contribute to basin uplift, so understinding basin inversion is in turn important for understanding the tectonic evolution and geodynamics of NE Brazil. Through joint inversion of receiver functions and surface-wave dispersion, we show that the crust is typically 34–38 km thick under the western Province; in contrast, the crust is 43–46 km thick under the northern craton. No apparent crustal thinning is observed under the Araripe Basin, due to the presence of a relatively thick (10–14 km) high-velocity (4.1–4.3 km/s) lowermost crust. Our velocity models also image a drop in S-velocity (from 4.6 km/s to 4.3 km/s) at 135–145 km depth under the southern half of the profile, which we interpret as a mid-lithospheric discontinuity, and at 120–140 km depth under the Araripe Basin, likely representing the lithosphere-asthenosphere boundary. Our findings confirm the presence of thin lithosphere under the Araripe Basin - originally postulated by a collocated MT survey - that might have facilitated tectonic inversion of this basin through regional stress concentration and thermal buoyancy. These findings also suggest additional buoyancy contributed by a mafic lowermost crust right under the basin, Financial support was provided to AMN by PETROBRAS (grant PRH-PB 229, project PIB10314-2013) to complete her PhD at the Universidade Federal do Rio Grande do Norte. The acquisition of the dataset was possible through a grant from the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) (CNPq-400743/2014-0) and the national oil company Petróleo Brasileiro S.A. (Petrobras). JJ additionally thanks CNPq for his research fellowship (CNPq-304421/2015-04), With the institutional support of the ‘Severo OchoaCentre of Excellence’ accreditation (CEX2019-000928-S)

We investigate the portability of a spectral amplitude method in determining moment tensor solutions for mine‐related seismicity recorded by deep mining networks to tectonic earthquakes recorded by surface aftershock networks. The original methodology inverts spectral amplitudes—with polarity attached—for direct P , SV , and SH waves recorded at deep‐seated geophones, so surface recordings must first be cleaned from free‐surface effects to recover the incident P and S waves. For precritical incidence, the correction is easily achieved by dividing the vertical component of the P and SV waveforms by the corresponding free‐surface reflection coefficients; for postcritical incidence, a more sophisticated correction that accounts for waveform distortion introduced by the coefficient’s phase shift is needed. Correction of SH components is achieved through division by a factor of 2. The proposed corrections are applied to 16 earthquakes recorded at local distances ( S ‐to‐ P critical reflection prevented the use of many SV amplitude measurements, inversion of the remaining spectral amplitudes allowed the recovery of deviatoric moment tensors for most of the events. Comparison with an independent first‐motion fault‐plane mechanism developed for the area shows consistency with our moment tensor solutions. Additionally, the ported methodology allows estimation of moment magnitudes for the selected events.

Using receiver-functions and surface-wave dispersion curves, we study the crustal and upper-mantle structure of Saudi Arabia. Our results reveal first-order differences in crustal thickness between the Arabian Shield in the west and the Arabian Platform in the east. Moho depths generally increase eastward, while crustal thickness varies strongly in the west over the volcanic regions and near the Red Sea. Localized zones of increased P-wave speed in the west may indicate solidified magmatic intrusions within the area of recent volcanism. Our receiver-function analysis for deep converted phases reveals that the transition zone thickness between the 410 km and the 660 km discontinuities is not anomalously thinned, refuting the hypothesis of a small localized mantle plume as the origin for the volcanic activity in western Saudi Arabia. Our results suggest that the volcanism in western Arabia may be due to the lithospheric mantle being heated from below by lateral flow from the Afar and (possibly) Jordan plumes. This triggers localized melts that ascend adiabatically through the lithosphere as magma diapirs. Recent xenolith measurements that provide information on temperatures and depths of melting are overall consistent with this hypothesis. However, further dedicated localized tomographic studies are needed to decipher the details of the origin of the volcanism and its relation to the overall geodynamics of the region.

We investigate the lithospheric shear-wave velocity structure of Saudi Arabia by conducting H-κ stacking analysis and jointly inverting teleseismic P-receiver functions and fundamental-mode Rayleigh wave group velocities at 56 broadband stations deployed by the Saudi Geological Survey (SGS). The study region, the Arabian plate, is traditionally divided into the western Arabian shield and the eastern Arabian platform: The Arabian shield itself is a complicated melange of crustal material, composed of several Proterozoic terrains separated by ophiolite-bearing suture zones and dotted by outcropping Cenozoic volcanic rocks (locally known as harrats). The Arabian platform is primarily covered by 8 to 10 km of Paleozoic, Mesozoic and Cenozoic sedimentary rocks. Our results reveal high Vp/Vs ratios in the region of Harrat Lunayyir, which are interpreted as solidified magma intrusions from old magmatic episodes in the shield. Our results also indicate slow velocities and large upper mantle lid temperatures below the southern and northern tips of the Arabian shield, when compared with the values obtained for the central shield. We argue that our inferred patterns of lid velocity and temperature are due to heating by thermal conduction from the Afar plume (and, possibly, the Jordan plume), and that volcanism in western Arabia may result from small-scale adiabatic ascent of magma diapirs.

We report new estimates of crustal thickness (Moho depth), Poisson's ratio and shear-wave velocities for eleven broadband seismological stations in Nigeria and Ghana. Data used for this study came from teleseismic earthquakes recorded at epicentral distances between 30° and 95° and with moment magnitudes greater than or equal to 5.5. P-wave receiver functions were modeled using the Moho Ps arrival times, H–k stacking, and joint inversion of receiver functions and Rayleigh wave group velocities. The average crustal thickness of the stations in the Neoproterozoic basement complex of Nigeria is 36 km, and 23 km for the stations in the Cretaceous Benue Trough. The crustal structure of the Paleoproterozoic Birimian Terrain, and Neoproterozoic Dahomeyan Terrain and Togo Structural Unit in southern Ghana is similar, with an average Moho depth of 44 km. Poisson's ratios for all the stations range from 0.24 to 0.26, indicating a bulk felsic to intermediate crustal composition. The crustal structure of the basement complex in Nigeria is similar to the average crustal structure of Neoproterozoic terrains in other parts of Africa, but the two Neoproterozoic terrains in southern Ghana have a thicker crust with a thick mafic lower crust, ranging in thickness from 12 to 17 km. Both the thicker crust and thick mafic lower crustal section are consistent with many Precambrian suture zones, and thus we suggest that both features are relict from the collisional event during the formation of Gondwana.

SUMMARY New estimates of crustal thickness, Poisson’s ratio and crustal shear wave velocity have been obtained for 39 stations in Angola, Botswana, the Democratic Republic of Congo, Malawi, Mozambique, Namibia, Rwanda, Tanzania and Zambia by modellingP-wave receiver functions using the H–κ stacking method and jointly inverting the receiver functions with Rayleigh-wave phase and group velocities. These estimates, combined with similar results from previous studies, have been examined for secular trends in Precambrian crustal structure within the southern African subcontinent. In both Archean and Proterozoic terranes we find similar Moho depths [38–39 ± 3k mSD (standard deviation)], crustal Poisson’s ratio (0.26 ± 0.01 SD), mean crustal shear wave velocity (3.7 ± 0.1 km s −1 SD), and amounts of heterogeneity in the thickness of the mafic lower crust, as defined by shear wave velocities ≥4.0 km s −1 . In addition, the amount of variability in these crustal parameters is similar within each individual age grouping as between age groupings. Thus, the results provide little evidence for secular variation in Precambrian crustal structure, including between Meso- and Neoarchean crust. This finding suggests that (1) continental crustal has been generated by similar processes since the Mesoarchean or (2) plate tectonic processes have reworked and modified the crust through time, erasing variations in structure resulting from crustal genesis.

We investigate the crustal structure of the Borborema Province of NE Brazil by developing 44 S wave velocity-depth profiles from the joint inversion of receiver functions and fundamental mode, Rayleigh wave group velocities. The Borborema Province is located in the northeasternmost corner of the South American continent and represents a portion of a larger Neoproterozoic mobile belt that formed during the Brasiliano-Pan African orogeny. Extensional processes in the Mesozoic—eventually leading to the separation of Africa and South America—left a number of aborted rift basins in the continental interiors, and episodes of diffuse intraplate volcanism and uplift marked the evolution of the Province after continental breakup. Our velocity-depth profiles reveal the existence of two crustal types in the Province: (i) the thin crustal type, which consists of 30–32.5 km thick crust, with an upper layer of 3.4–3.6 km/s overlying a lower layer of 3.7–3.8 km/s and (ii) the thick crustal type, which consists of a 35–37.5 km thick crust, with velocities between 3.5 and 3.9 km/s down to ∼30 km depth and a gradational increase in velocity (VS≥4.0 km/s) down to upper mantle depths. The crustal types correlate well with topography, with the thick crustal type being mainly found in the high-standing southern Borborema Plateau and the thin crustal type being mostly found in the low-lying Sertaneja depression and coastal cuestas. Interestingly, the thin crustal type is also observed under the elevated topography of the northern Plateau. We argue that the thick crustal type is rheologically strong and not necessarily related to postbreakup mantle processes, as it is commonly believed. We propose that extensional processes in the Mesozoic stretched portions of the Brasiliano crust and formed the thin crustal type that is now observed in the regions of low-lying topography, leaving the rheologically strong thick crust of the southern Plateau at higher elevations. The crust making the northern Plateau would have thinned and subsided during Mesozoic extension as part of a greater Sertaneja depression, to then experience uplift in the Cenozoic and achieve its present elevation.

We have investigated the crustal architecture of the Borborema Province of NE Brazil by constructing common conversion point (CCP) receiver function stacks from teleseismic P-waveforms recorded at 64 seismic stations in the region. The Borborema Province represents the western portion of a larger Neoproterozoic mobile belt that experienced extension in the Mesozoic, leading to the formation of a number of intra-continental rift basins and, eventually, continental breakup. After continental breakup, episodes of uplift in the Province — perhaps related to coeval episodes of Cenozoic volcanism — helped shape the high topographies of the Borborema Plateau. Our receiver function CCP stacks image clear P-to-S conversions at the crust–mantle boundary and confirm independent evidence for a 36–38 km thick crust under the southern portion of the Plateau and a thinner 30–32 km thick crust in the surrounding regions, including the northern Plateau. The cross-sections also reveal the presence of an intra-crustal discontinuity at 9–18 km depth under the regions of thin crust that fades away under the thick southern Plateau. We argue that the thin crust in the Borborema Province is the result of Mesozoic crustal stretching and that the intra-crustal discontinuity represents a low-angle detachment zone that helped accommodate extension in the crust. The thick crust under the southern Plateau would then represent a rheologically stronger portion of the Borborema Province that resisted deformation by Mesozoic extension, while the thin crust under the northern Plateau would be a portion of formerly depressed thin crust that was uplifted during the Cenozoic.

We investigate variations in crustal thickness and bulk VP/VS ratio across the Borborema Province of NE Brazil by analyzing teleseismic P-to-S conversions recorded at 52 seismic stations in the Province. The Borborema Province represents the western portion of a larger Neoproterozoic mobile belt that resulted from the assembly of Gondwanaland, and that split from the African continent during Mesozoic times. The evolution of the Province after continental breakup was marked by episodes of diffuse intraplate magmatism, perhaps leading to uplift of the Borborema Plateau in the Cenozoic. A number of geodynamic models have been proposed to explain coeval Cenozoic magmatism and uplift in the Province, which invoke either thermal anomalies under the Plateau and related mantle upwellings, channeling along lithospheric thin spots from a distant mantle plume, and small-scale convection at the continental edge. Alternatively, plateau uplift might have resulted from thickening of the crust after depth-dependent stretching of the continental lithosphere in the Mesozoic. Most of the models imply mafic underplating of the Plateau's crust in order to fully explain its elevated topography, but the volume of such mafic underplate varies among them. Our results show that: (i) the crust is 32–40 km thick under the Borborema Plateau, (ii) the crust is generally thinner – about 30–33 km – under the lower topographies surrounding the Plateau, and (iii) VP/VS ratios are in the 1.68–1.80 range for both regions of higher and lower topography. No apparent correlation is observed between VP/VS ratio and crustal thickness. Our results suggest that compositional differences between thick and thin crust across the Borborema Province are minimal, and that models of plateau uplift involving a small volume of mafic underplate provide a more plausible explanation for the observed topography of the Borborema Plateau.

Although prior work suggests that a mantle plume is associated with Cenozoic rifting and volcanism in West Antarctica, the existence of a plume remains conjectural. Here we use P wave receiver functions (PRFs) from the Antarctic POLENET array to estimate mantle transition zone thickness, which is sensitive to temperature perturbations, throughout previously unstudied parts of West Antarctica. We obtain over 8000 high-quality PRFs using an iterative, time domain deconvolution method filtered with a Gaussian width of 0.5 and 1.0, corresponding to frequencies less than ∼0.24 and ∼0.48 Hz, respectively. Single-station and common conversion point stacks, migrated to depth using the AK135 velocity model, indicate that mantle transition zone thickness throughout most of West Antarctica does not differ significantly from the global average, except in two locations; one small region exhibits a vertically thinned (210 ± 15 km) transition zone beneath the Ruppert Coast of Marie Byrd Land and another laterally broader region shows slight, vertical thinning (225 ± 25 km) beneath the Bentley Subglacial Trench. We also observe the 520 discontinuity and a prominent negative peak above the mantle transition zone throughout much of West Antarctica. These results suggest that the mantle transition zone may be hotter than average in two places, possibly due to upwelling from the lower mantle, but not broadly across West Antarctica. Furthermore, we propose that the transition zone may be hydrated due to >100 million years of subduction beneath the region during the early Mesozoic.

The lithosphere of Madagascar was initially amalgamated during the Pan-African events in the Neoproterozoic. It has subsequently been reshaped by extensional processes associated with the separation from Africa and India in the Jurassic and Cretaceous, respectively, and been subjected to several magmatic events in the late Cretaceous and the Cenozoic. In this study, the crust and uppermost mantle have been investigated to gain insights into the present-day structure and tectonic evolution of Madagascar.We analysed receiver functions, computed from data recorded on 37 broad-band seismic stations, using the H–κ stacking method and a joint inversion with Rayleigh-wave phase-velocity measurements. The thickness of the Malagasy crust ranges between 18 and 46 km. It is generally thick beneath the spine of mountains in the centre part (up to 46 km thick) and decreases in thickness towards the edges of the island. The shallowest Moho is found beneath the western sedimentary basins (18 km thick), which formed during both the Permo-Triassic Karro rifting in Gondwana and the Jurassic rifting of Madagascar from eastern Africa. The crust below the sedimentary basin thickens towards the north and east, reflecting the progressive development of the basins. In contrast, in the east there was no major rifting episode. Instead, the slight thinning of the crust along the east coast (31–36 km thick) may have been caused by crustal uplift and erosion when Madagascar moved over the Marion hotspot and India broke away from it. The parameters describing the crustal structure of Archean and Proterozoic terranes, including average thickness (40 km versus 35 km), Poisson’s ratio (0.25 versus 0.26), average shear-wave velocity (both 3.7 km s–1), and thickness of mafic lower crust (7 km versus 4 km), show weak evidence of secular variation. The uppermost mantle beneath Madagascar is generally characterized by shear-wave velocities typical of stable lithosphere (∼4.5 km s–1). However, markedly slow shear-wave velocities (4.2–4.3 km s–1) are observed beneath the northern tip, central part and southwestern region of the island where the major Cenozoic volcanic provinces are located, implying the lithosphere has been significantly modified in these places.

Ambient seismic noise has traditionally been regarded as an unwanted perturbation that >contaminates> earthquake data. Over the last decade, however, it has been shown that consistent information about subsurface structure can be extracted from ambient seismic noise. By cross-correlation of noise simultaneously recorded at two seismic stations, the empirical Green's function for the propagating medium between them can be reconstructed. Moreover, for periods less than 30 s the seismic spectrum of ambient noise is dominated by microseismic energy and, because microseismic energy travels mostly as surface-waves, the reconstruction of the empirical Green's function is usually proportional to the surface-wave portion of the seismic wavefield. In this paper, we present 333 empirical Green's functions obtained from stacked cross-correlations of one month of vertical component ambient seismic noise for different pairs of seismic stations in the Borborema Province of NE Brazil. The empirical Green's functions show that the signal obtained is dominated by Rayleigh waves and that dispersion velocities can be measured reliably for periods between 5 and 20 s. The study includes permanent stations from a monitoring seismic network and temporary stations from past passive experiments in the region, resulting in a combined network of 34 stations separated by distances between approximately 40 and 1,287 km. Fundamental-mode group velocities were obtained for all station pairs and then tomographically inverted to produce maps of group velocity variation. For short periods (5'10 s) the tomographic maps correlate well with surface geology, with slow velocities delineating the main rift basins (Potiguar, Tucano, and Reco'ncavo) and fast velocities delineating the location of the Precambrian Sa'o Francisco craton and the Rio Grande do Norte domain. For longer periods (15'20 s) most of the velocity anomalies fade away, and only those associated with the deep Tucano basin and the Sa'o Francisco craton remain. The fading of the Rio Grande do Norte domain fast-velocity anomaly suggests this is a supracrustal structure rather than a lithospheric terrain, and places new constraints on the Precambrian evolution of the Borborema Province.

Crustal structure in South America is one of the least understood among the Earth's continental areas. Variations in crustal thickness are still poorly constrained over large portions of the continent because of scarce or unevenly distributed crustal thickness estimates throughout South America. To address this scarce and inhomogeneous data cover we explore the possibility to derive crustal thickness from satellite gravity data. In this study, we utilize the combined gravity model EIGEN-6C, which is composed of GOCE and other gravity data. The Gravity field and steady-state Ocean Circulation Explorer (GOCE) satellite has a much more uniform spatial resolution than any land-based gravity or seismic survey in South America. The gravity data inversion is for a simple two-layer model with fixed density contrast over the interface, the Moho. The method is not relying on point constraint data and assumes that all of the signal is related to topography of the Moho. Model quality can therefore be assessed by a comparison with point observations on crustal thickness. We show that for the stable part of the continent 90% of our estimates are similar, within error bounds, to seismic observations. Variations occur in active orogenic zones or regions with suspected non-standard Moho density contrasts. A comparison with seismological models shows a high correlation with the most recent model. Especially in areas where continental and global models of crustal structure have limitations in terms of wave paths or point constraints the gravity based model provides a unique continuity of crustal structure providing new insights on structure and tectonics and increase our understanding of the Earth's structure underneath South America.

The crustal structure of the northern part of the Khartoum Basin has been investigated using data from 3 permanent seismic stations within 40 km of Khartoum and two modeling methods, H–k stacking of receiver functions and a joint inversion of receiver functions and Rayleigh wave group velocities. The Khartoum Basin is one of several Mesozoic rift basins in Sudan associated with the Central African Rift System. Results from the H–k-stacking indicate that crustal thickness beneath the Khartoum Basin ranges between 33 and 37 km, with an average of 35 km, and that the crustal Vp/Vs ratio ranges from 1.74 to 1.81, with an average of 1.78. From the joint inversion of receiver functions and Rayleigh wave group velocities, we obtained similar results for Moho depth, as well as an average shear wave velocity of 3.7 km/s for the crust. These results provide the first seismic estimates of Moho depth for a basin in Sudan. When compared to average crustal thickness for unrifted Proterozoic crust in eastern Africa, our results indicate that at most only a few km of crustal thinning may have occurred beneath the Khartoum Basin. This finding is consistent with estimates of effective elastic plate thickness, which indicate little modification of the Proterozoic lithosphere beneath the basin, and suggests that there may be insufficient topography on the lithosphere–asthenosphere boundary beneath the Sudanese basins to channel plume material westward from Ethiopia.

We have investigated the nature of the 520 km discontinuity under Africa by migrating P-wave receiver functions at 30 permanent broadband stations in the continent. Worldwide investigations with SS precursors have revealed that the detection of this discontinuity is intermittent and that it may split into two separate discontinuities, ~ 60 km apart, in some regions. Several explanations have been proposed to account for both the intermittent detection and the splitting of the 520-km discontinuity, ranging from lateral temperature variations in the transition zone to compositional variations due to changes in minor phases containing Fe, Ca, and water. Our results reveal that detection of the 520 km discontinuity is intermittent under Africa and that – when observed – it does not split. Detectability does not correlate with published maps of transition zone temperature, temperature estimates from transition zone thickness, or published potential temperature estimates from primitive magmas throughout East Africa. It does not correlate with published maps of transition zone water content, as well. Lateral variation in iron content due, for instance, to varying proportions of the transition zone׳s constituent minerals offers a plausible explanation for the observed intermittency in detection, but better maps of transition zone iron content need to be developed to assess a correlation. Focusing and defocusing effects due to small-scale variations around the stations are also a possibility. The lack of splitting, on the other hand, is found consistent with low-Ca contents and the general absence of present-day subduction around the continent. However, a regional increase in mantle fertility under West Africa – as previously reported with analysis of SS precursors – cannot be ruled out with our analysis.

We investigate the structure of Precambrian crust in East Africa by using estimates of Moho depth and Poisson's ratio for 37 new seismic stations in Uganda and Tanzania and for 32 previously deployed seismic stations in Kenya and Tanzania. The dataset includes estimates of crustal structure distributed between all of the major Archean and Proterozoic terrains obtained from modeling P-wave receiver functions. The average crustal thickness for the Ubendian belt is 42 km and between 37 and 39 km for all of the terrains. The average Poisson's ratio for all of the terrains is either 0.25 or 0.26, indicating a felsic to intermediate bulk crustal composition. A similar composition and thickness of the crust in East Africa for terrains spanning some 4 byr of Earth history is different than in many other Precambrian regions. Our results suggest that there may have been few changes over Earth's history in the processes that formed the East African crust, or else that processes have been at play to homogenize crustal structure, such as the flow of lower crustal material in orogenic systems or through the foundering of eclogites into the mantle. The finding that crustal structure is similar between the Archean and Proterozoic terrains indicates that crustal structure, through its influence on lithospheric rheology and strength, has not had a first-order effect on the location of rifting. We also find that there is little correlation between Moho depth and elevation across the East African Plateau, indicating that variations in crustal thickness exert few, if any, influences on topography in East Africa. This conclusion lends support to many studies arguing that mantle structure and processes provide the primary buoyant support for the plateau.

The structure of the crust in the environs of the Bushveld Complex has been investigated by jointly inverting high-frequency teleseismic receiver functions and 2–60s period Rayleigh wave group velocities for 16 broadband seismic stations located across the Bushveld Complex. Group velocities for 2–15s periods were obtained from surface wave tomography using local and regional events, while group velocities for 20–60s periods were taken from a published model. 1-D shear wave velocity models obtained for each station show the presence of thickened crust in the center of the Bushveld Complex and a region at the base of the crust where shear wave velocities exceed 4.0km/s. The shear wave velocity models also suggest that velocities in some upper crustal layers may be as high as 3.7–3.8km/s, consistent with the presence of mafic lithologies. These results favor a continuous-sheet model for the Bushveld Complex in which the outcropping mafic layers of the western and eastern limbs are continuous at depth beneath the center of the complex. However, detailed modeling of receiver functions at one station within the center of the complex indicates that the mafic layering may be locally disrupted due to thermal diapirism triggered by the emplacement of the Bushveld Complex or thermal and tectonic reactivation at a later time.

SUMMARY We investigate crustal Vp–Vs ratios and thickness along the Transantarctic Mountain (TAM) front and on Ross Island, Antarctica to determine if the TAM crust has been modified by the Neogene magmatism associated with Ross Island. A seismic low velocity zone (LVZ) in the upper mantle beneath Ross Island extends laterally ∼80 km under the TAM front, and mantle temperatures within the LVZ may be sufficiently elevated for partial melting to have occurred and modified the crust. Data for the study come from 16 temporary seismic stations that were part of the TAM Seismic Experiment and three permanent stations. Estimates of Vp/Vs (κ) and crustal thickness (H) have been obtained from receiver functions analysed using the H–κ stacking method for 10 of the stations, and for the remaining stations, crustal thickness has been calculated by using the Moho Ps arrival time with an assumed Vp/Vs value. A Vp/Vs value of 1.88 is obtained for Ross Island, consistent with the mafic composition of the volcanic rocks from Mt. Erebus. Vp/Vs values for stations in the TAM situated away from the LVZ range from 1.63 to 1.78, with a mean of 1.73, while values for stations in the TAM lying above the LVZ range from 1.67 to 1.78, with a mean of 1.72. This result indicates that there is little difference in bulk crustal composition for areas above and away from the LVZ, and together with a Vp/Vs value (1.73) that is typical for felsic to intermediate composition crust, suggests that the crust along the TAM front has not been altered significantly by mafic magmatism. Crustal thickness estimates along the coast are quite variable, ranging from 18 to 33 km, and increase to 39 km inland beneath the crest of the TAM. On Ross Island, crustal thickness estimates range between 19 and 27 km. In this study, we investigate crustal structure along the Transantarctic Mountain (TAM) front in the vicinity of Ross Island, Antarctica, to determine the extent to which the crust has been modified by the Neogene volcanism associated with Ross Island and the Terror Rift. The TAM represent Earth’s largest non-collisional mountain