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Subducting the buoyant crustal material of an aseismic oceanic ridge has been regarded as a dominant contributor to flat slab subduction. However, normal‐dip subduction is also observed in some cases where ridges are subducting. In this study, we compare the subduction of two ridges on the Nazca Plate: Nazca Ridge (flat slab) and Iquique Ridge (normal‐dip slab). Anisotropy determined by shear wave splitting observation suggests that the low‐velocity anomalies found beneath the ridges are mapping anisotropic structure into isotropic velocities. After a tomographic inversion incorporating anisotropy models for both ridges, we find that the low‐velocity anomalies found beneath the Nazca Ridge are not anisotropic and therefore likely represent warm mantle, and those beneath the Iquique Ridge are caused by anisotropy. We conclude that subslab mantle buoyancy has a larger impact on the subduction angle than the crustal material of the ridge. Plain Language Summary: Understanding the subduction process and how the mantle flows is pivotal in understanding the planetary evolution of Earth. Yet, several subduction characteristics remain unsolved, and the angle of plate subduction, which is often categorized into <30° (shallow), ∼30–35° (normal), >35° (steep), is one of those. Subduction of an oceanic ridge on a plate has been proposed as a cause of a shallow subduction angle since the thick crust of the ridge is less dense than the surrounding mantle. However, normal angles have been observed in some cases where oceanic ridges are subducting. In this study, we compare the Nazca Ridge and the Iquique Ridge, on the Nazca Plate subducting beneath South America. The subduction angles of the Nazca and Iquique Ridges are shallow and normal, respectively. When we incorporate directional variations in seismic wave velocities, which are produced by mantle flow, in seismic tomographic imaging, we find that the subducting oceanic ridge may not be a primary factor producing shallow angle subduction. Instead, the warm mantle beneath the Nazca Ridge may provide the buoyancy to support the Nazca Plate. Comparably, since the mantle beneath the Iquique Ridge is not warm, the subduction angle would stay normal. Key Points: Despite having different subduction angles, subslab low‐velocity anomalies are found beneath both Nazca Ridge and Iquique RidgeWe introduced hypothetical seismic anisotropy in tomographic inversion to explore the origin of the subslab low‐velocity anomaliesThe low‐velocity anomalies beneath the Nazca Ridge and Iquique Ridge may come from buoyant warm mantle and anisotropy, respectively [ABSTRACT FROM AUTHOR]

As the mantle plume rises from the deep mantle and reaches the base of a tectonic plate, it changes its traveling direction from vertical to horizontal. The horizontal spread of plume material is often radially asymmetric. An example is a plume found below the Canary Hotspot. Previous studies have suggested that the channeling of the Canary Plume toward the westernmost Mediterranean (Alboran Sea) and consequent lithospheric delamination may have contributed to the low‐velocity anomalies found beneath the Moroccan Atlas Mountains. Regional upwelling and edge‐driven convection have been proposed as other candidates to explain the origin of the low‐velocity anomalies. In this study, we incorporated anisotropy as an a priori constraint in teleseismic P‐wave travel‐time tomography as mantle flow can develop seismic anisotropy. We inverted a new set of travel‐time delays by removing the hypothetical anisotropy‐imposed travel‐time delays from the observations. Our improved results are more consistent with the hypothesis that the low‐velocity anomalies come from the mantle material of the Canary Plume. Plain Language Summary: The propagation velocity of seismic waves is sensitive to the temperature. Low seismic velocities in the mantle found from seismic tomography, a technique used to image the velocity structure of the inner Earth with seismic waves, are commonly interpreted as high‐temperature, hydrous regions, the presence of partial melting, changes in the compositional chemistry, and grain sizes of mantle minerals. Thus, seismic tomography has been the primary tool for revealing mantle structures, such as hot plumes rooted in the deep mantle that play significant roles in mantle dynamics. Previous studies have shown that rising plume material can drag the surrounding mantle and consequently cause directional dependence of the seismic velocities, which can also affect the results of seismic tomography. This study improved the quality of seismic tomography results by considering the directional dependence of wave speeds in the mantle to elucidate the evolution of the mantle plume and its interaction with the upper plate in Morocco. The imaged low‐velocity conduit below the high‐altitude Moroccan Atlas indicates the lateral travel of the mantle plume originating from the Canary Hotspot in northwest Africa, which may have dragged the surrounding mantle beneath the Moroccan Atlas. Key Points: Teleseismic P‐wave tomography is used in combination with anisotropy models to study the origin of the low‐velocity anomalies below the Moroccan AtlasWe incorporate anisotropy as an a priori constraint in tomography and show the reduction in low‐velocity anomalies below the Moroccan AtlasThe lateral travel of plume material from the Canary Hotspot may be the origin of low‐velocity anomalies [ABSTRACT FROM AUTHOR]

The Wyoming Craton underwent tectonic modifications during the Laramide Orogeny, which resulted in a series of basement-cored uplifts that built the modern-day Rockies. The easternmost surface expression of this orogeny - the Black Hills in South Dakota - is separated from the main trend of the Rocky Mountains by the southern half of the Powder River Basin, which we refer to as the Thunder Basin. Seismic tomography studies reveal a high-velocity anomaly which extends to a depth of ~300 km below the basin and may represent a lithospheric keel. We constrain seismic attenuation to investigate the hypothesis that variations in lithospheric thickness resulted in the localization of stress and therefore deformation. We utilize data from the CIELO seismic array, a linear array that extends from east of the Black Hills across the Thunder Basin and westward into the Owl Creek Mountains, the BASE FlexArray deployment centered on the Bighorn Mountains, and the EarthScope Transportable Array. We analyze seismograms from deep teleseismic events and compare waveforms in the time-domain to characterize lateral variations in attenuation. Bayesian inversion results reveal high attenuation in the Black Hills and Bighorn Mountains and low attenuation in the Thunder and Bighorn Basins. Scattering is rejected as a confounding factor because of a strong anticorrelation between attenuation and the amplitude of P wave codas. The results support the hypothesis that lateral variations in lithospheric strength, as evidenced by our seismic attenuation measurements, played an important role in the localization of deformation and orogenesis during the Laramide Orogeny. [ABSTRACT FROM AUTHOR]

The eastern margin of North America has been shaped by a series of tectonic events including the Paleozoic Appalachian Orogeny and the breakup of Pangea during the Mesozoic. For the past ∼200 Ma, eastern North America has been a passive continental margin; however, there is evidence in the Central Appalachian Mountains for post‐rifting modification of lithospheric structure. This evidence includes two co‐located pulses of magmatism that post‐date the rifting event (at 152 and 47 Ma) along with low seismic velocities, high seismic attenuation, and high electrical conductivity in the upper mantle. Here, we synthesize and evaluate constraints on the lithospheric evolution of the Central Appalachian Mountains. These include tomographic imaging of seismic velocities, seismic and electrical conductivity imaging along the Mid‐Atlantic Geophysical Integrative Collaboration array, gravity and heat flow measurements, geochemical and petrological examination of Jurassic and Eocene magmatic rocks, and estimates of erosion rates from geomorphological data. We discuss and evaluate a set of possible mechanisms for lithospheric loss and intraplate volcanism beneath the region. Taken together, recent observations provide compelling evidence for lithospheric loss beneath the Central Appalachians; while they cannot uniquely identify the processes associated with this loss, they narrow the range of plausible models, with important implications for our understanding of intraplate volcanism and the evolution of continental lithosphere. Our preferred models invoke a combination of (perhaps episodic) lithospheric loss via Rayleigh‐Taylor instabilities and subsequent small‐scale mantle flow in combination with shear‐driven upwelling that maintains the region of thin lithosphere and causes partial melting in the asthenosphere. Plain Language Summary: For the past 200 million years, the east coast of North America has been situated in the middle of a tectonic plate. Contrary to the expectations for this setting, a region of the Central Appalachian Mountains centered near the boundary between the U.S. states of Virginia and West Virginia exhibits atypical properties. The unusual observations include volcanic activity in the geologic past far away from a plate boundary, elevated rates of erosion associated with high topography in the Central Appalachians, and anomalous structure in the upper mantle that has been detected using geophysical methods. This article describes, synthesizes, and compares a suite of observations that show that this part of the Central Appalachians is unusual compared to other so‐called passive continental margins. We discuss a range of different models that might describe how the lithosphere, or the rigid part of the crust and upper mantle that defines the tectonic plate, has evolved through time beneath our study region. We show that the lithosphere today is thin, and that past episodes of lithospheric loss involving a portion of dense lithosphere "dripping" into the mantle under the force of gravity may provide a good explanation for the observations. Key Points: There is a present‐day geophysical anomaly in the upper mantle co‐located with unusually young volcanism in the Central AppalachiansWe synthesize constraints from geophysics, petrology/geochemistry, and geomorphology to constrain possible models for lithospheric lossWe favor one or more Rayleigh‐Taylor lithospheric instabilities, perhaps in combination with shear‐driven upwelling [ABSTRACT FROM AUTHOR]

The Caribbean plate subducts beneath northwest South America at a shallow angle due to a large igneous province that added up to 12 km of buoyant crust. The overriding plate lacks volcanism and exhibits Laramide‐style uplifts over 500 km from the trench. Here, we illuminate the subduction structures through finite frequency teleseismic P‐wave tomography and connect those structures to the Laramide‐style deformation on the overriding plate. We use a new data set collected from the Caribbean‐Mérida Andes seismic experiment comprised of 65 temporary broadband stations integrated with permanent stations from the Colombian and Venezuelan national networks. We identify three segments of subducting Caribbean plate with one segment completely detached from the surface. The timing of the detachment aligns with other regional events, including the uplift of the Mérida Andes, about 10 Ma. Slab buoyancy post‐detachment likely resulted in recoupling with the overriding plate, reactivation of Jurassic‐aged rift structures and subsequent uplift of the Mérida Andes. Mantle counterflow over the broken segment induced by rollback of the attached slab likely contributed to the uplift of the Mérida Andes. We conclude that the northern limit of subduction lies south of the Oca‐Ancón fault, though the fault itself may be the surface expression of the boundary. The southern limit of subduction lies south of our study area. Plain Language Summary: At the convergent boundary between the southern Caribbean and northwest South America the Caribbean plate dips, or subducts, beneath South America at angles less than 30° for up to ∼300 km inland before steepening. Such low‐angle subduction systems are unique because they produce mountains hundreds of kilometers inland. Using new data collected by the Caribbean‐Mérida Andes seismic array, which continuously recorded earthquakes for two years, we imaged the Caribbean plate to depths of 660 km and identified at least three segments, including a segment detached from the surface. The timing of the detachment ∼5–15 million years ago aligns with the uplift of the Mérida Andes of Venezuela ∼500 km inland. Both the attached and detached segments interacted with the underside of South America, compressing parts of it, and causing uplift of the Mérida Andes. The northern limit of subduction lies south of the Oca‐Ancón fault of northern Colombia and Venezuela. The southern expanse of subduction includes the Bucaramanga earthquake cluster, but the boundary lies south of our study area. By understanding how the actively subducting plate, the Caribbean, deforms the upper plate, South America, we can better assess seismic risk and understand similar systems that are no longer active. Key Points: At least three Caribbean segments subduct under South America, one detached under the Merida Andes. Detachment due to lithospheric weaknessPost‐break recoupling and rollback of slab contribute to uplift of Mérida AndesNorthern subduction boundary identified [ABSTRACT FROM AUTHOR]

Seismic body‐wave tomography studies typically assume an isotropic upper mantle, possibly mapping anisotropy into artificial isotropic velocity anomalies in the resulting images. The Eastern Mediterranean with its oceanic, continental, and extinct subduction systems, as well as dense station coverage, provides an ideal setting to explore this issue. To examine the influence of seismic anisotropy, our study deals with both synthetic and real data inversions in which realistic seismic anisotropy models derived from 3D mantle convection simulations and shear wave splitting measurements are taken as a priori constraints. Spatial large‐scale velocity perturbations are mostly consistent between models derived with and without considering anisotropy. Small differences in the magnitude (up to 2%) and shape of velocity perturbations occur, and some structures are less diffuse when including anisotropy. Additionally, good backazimuthal coverage of teleseismic events and a larger data set improve the resolution of our model with respect to previous tomography studies and allow us to better interpret first‐order isotropic velocity anomalies. Key features, such as the half‐arc subducting oceanic plate in the southern Aegean and a wide and deep tear in the slab beneath southwestern Turkey, are clearly visible in all models. Our final tomography images also provide evidence for a shallow horizontal tear in the northern Hellenides and a vertical tear between two parts of the Cyprian slab. In eastern Anatolia, slab‐related high‐velocity anomalies are absent due to the continental collision and break‐off. Plain Language Summary: Understanding current and prior mantle flow in the upper mantle is fundamental for the reconstruction of plate tectonics. P‐wave tomography is used as a method to monitor seismic velocity changes at great depths. These anomalies give clues about various geodynamic events, for example, downgoing plates and upwelling mantle material. However, whenever mantle flow is involved, intrinsically anisotropic minerals tend to align in certain directions, which makes the velocities directionally dependent and hence could lead to distortion of the results. Therefore, it may be important to include directional parameters in the calculations, to achieve a better resolution and a more accurate knowledge of the subsurface. Considering the effect of seismic anisotropy is particularly important in the very tectonically active region of the Eastern Mediterranean. We found that by including seismic anisotropy, the magnitude and geometry of some anomalies change but spatially large anomalies do not change significantly. We were able to recover the shape of the slab in detail and found evidence for break‐offs and tears in western Greece and Turkey, nearby Cyprus and eastern Turkey in all models. Key Points: Velocity perturbations show several tears and break‐off areas in the Hellenic and Cyprian slabsMain P‐wave velocity perturbation features are similar in isotropic and anisotropy corrected modelsDiscrepancies of up to ±2% between models highlight the importance of anisotropy to resolve small structures [ABSTRACT FROM AUTHOR]

The long and often complicated tectonic history of continental lithosphere results in lateral strength heterogeneities which in turn affect the style and localization of deformation. In this study, we produce a model for the attenuation structure of Iberia and northern Morocco using a waveform-matching approach on P-wave data from teleseismic deep-focus earthquakes. We find that attenuation is correlated with zones of intraplate deformation and seismicity, but do not find a consistent relationship between attenuation and recent volcanism. The main features of our model are low to moderate Δ t ⁎ in the undeformed Tertiary basins of Spain and high Δ t ⁎ in areas deformed by the Alpine orogeny. Additionally, low Δ t ⁎ is found in areas where the Alboran slab is thought to be attached to the Iberian and African lithosphere, and high Δ t ⁎ where it has detached. These features are robust with respect to inversion parameters, and are consistent with independent data. Very mild backazimuthal dependence of the measurements and comparison with previous results suggest that the source of the attenuation is sub-crustal. In line with other recent studies, the range of Δ t ⁎ we observe is much larger than can be expected from lithospheric thickness or temperature variations. [ABSTRACT FROM AUTHOR]

The occurrence of intermediate depth seismicity in intracontinental settings is rare, but it has been postulated with various degrees of certainty in several regions. One such region is the Middle Atlas of Morocco. Since 1960, 75 intermediate depth earthquakes have been reported in this region by Spain's National Geographic Institute (IGN). The apparent deep nature of these events is hard to reconcile with well-established geophysical evidence of a thin lithosphere under the Middle Atlas, but has been associated with mantle delamination processes. We relocate 4 events with IGN-reported depths > 80 km that were recorded by a relatively dense temporary deployment; using a recent regional 3D velocity model obtained through teleseismic body and surface wave tomography. The relocation procedure uses a grid-search approach to minimize the mean normalized misfit, where each travel-time misfit is normalized by the estimated pick uncertainty. We find that our observed arrivals are much better fit by shallow (< 5 km) depths than the reported depths of > 80 km. We propose that these shallow foci earthquakes are the result of regional crustal deformation of this region caused by the present convergence between Africa and Eurasian Plate. We infer that if there are any ongoing delamination processes in the area, they are aseismic. This study is an example of how local earthquake locations in tectonically complex areas can be significantly improved by using a dense local seismic array and a well-constrained 3-D velocity model. [ABSTRACT FROM AUTHOR]

Crustal density models derived from seismic velocity models by means of velocity-density conversions typically reproduce the main features of the observed gravity anomaly over the area but often show significant misfits. Given the uncertainty in the relationship between velocity and density, seismically derived density models should be regarded as an initial estimate of the true subsurface density structure. In this paper, we present a method for estimating the adjustments necessary to a seismically derived density model to improve the fit to gravity data. The method combines the Genetic Algorithm paradigm with linear inversion as a way to approach the non-linear and linear aspects of the problem. The models are divided into three layers representing the sedimentary column, the crystalline crust and the lithospheric mantle; the depths of these layers are determined from the seismic velocity model. Each of the layers is divided into a number of provinces and a density adjustment (Δρ) value is found for each province so that the residual gravity (difference between the observed gravity anomaly and the anomaly calculated for the seismically derived model) is minimized while keeping Δρ between predefined bounds. The preferred position of the province boundaries is found through the artificial evolution of a population of solutions. Given the stochastic nature of the algorithm and the non-uniqueness of the problem, different realizations can yield different solutions. By performing multiple realizations we can analyse a set of solutions by taking their mean and standard deviation, providing not only an estimate of the Δρ distribution in the subsurface but also an estimate of the associated uncertainty. Synthetic tests prove the ability of the algorithm to accurately recover the location of province boundaries and the Δρ values for a known model when using noise-free synthetic data. When noise is added to the data, the algorithm broadly recovers the features that define the known model despite greater standard deviations of the solutions and the occurrence of artefacts in the mean solutions. The algorithm was applied to four profiles across the Caribbean-South America Plate boundary. Some general patterns in the distribution of Δρ were observed consistently in the profiles and are correlated with the interpretations of the velocity models. Positive Δρ values in the sedimentary layer, negative Δρ values for island arc and extended island arc crust with an abrupt change to positive values in South American crust, and positive values in the mantle under the continent and island arc with a transition to negative values under the Caribbean oceanic crust. [ABSTRACT FROM AUTHOR]

Both the Caribbean and Nazca plates subduct beneath northwestern South America. The configuration of the two subducted slabs and the nature of any interaction between them has long been a matter of debate. Based on the location of intermediate-depth seismicity and active and extinct volcanism, as well as on seismic imaging, several different tectonic scenarios have been proposed. In this paper, we use teleseismic data recorded by the Colombian National Network and the temporary CARMA array in Venezuela and Colombia to produce a finite-frequency tomography model for the region. Our results show several distinct subduction segments. Through synthetic tests, we show that our results require a zone of overlap between Nazca and Caribbean subduction north of the "Caldas Tear" as has been proposed by previous studies. Additionally, we find that the Bucaramanga Nest occurs within the Caribbean Plate and coincides with bending of the slab in two planes, where both the strike and the dip of the slab change. We infer that elevated stresses are an important factor in producing the very high rates of seismicity in the nest. • Teleseismic tomography illuminates the geometry of subduction in NW South America. • There is a zone of overlap between Caribbean and Nazca subduction. • The offset at the Caldas Tear corresponds to seismicity in two different slabs. • The Bucaramanga Nest occurs within the Caribbean Plate. [ABSTRACT FROM AUTHOR]

Abstract: The Falcón Basin in northwestern Venezuela has a complex geological history driven by the interactions between the South American and Caribbean plates. Igneous intrusive bodies that outcrop along the axis of the basin have been associated with crustal thinning, and gravity modeling has shown evidence for a significantly thinned crust beneath the basin. In this study, crustal scale seismic refraction/wide-angle reflection data derived from onshore/offshore active seismic experiments are interpreted and forward-modeled to generate a P-wave velocity model for a ∼450km long profile. The final model shows thinning of the crust beneath the Falcón Basin where depth to Moho decreases to 27km from a value of 40km about 100km to the south. A deeper reflected phase on the offshore section is interpreted to be derived from the downgoing Caribbean slab. Velocity values were converted to density and the resulting gravimetric response was shown to be consistent with the regional gravity anomaly. The crustal thinning proposed here supports a rift origin for the Falcón Basin. [Copyright &y& Elsevier]

Seismic tomography images high-velocity mantle beneath the Wyoming craton extending to >250 km depth. Although xenoliths and isostatic arguments suggest that this mantle is depleted of basaltic component, it is not typical craton: its NE elongate shape extends SW of the Wyoming craton; xenoliths suggest that the base of Archean mantle was truncated from ∼180–200 to ∼140–150 km depth since the Devonian, and that the deeper mantle is younger than ∼200 Ma. The Sevier–Laramide orogeny is the only significant Phanerozoic tectonic event to have affected the region, and presumably caused the truncation. Apparently, the base of the Wyoming craton was removed and young, depleted mantle was emplaced beneath the Wyoming craton during the Sevier–Laramide orogeny. We suggest that the Wyoming craton experienced a ∼75 Ma phase of growth through a three-stage process. First, flat-slab subduction removed 40–50 km off the base of the Archean Wyoming craton. This was followed by emplacement of basalt-depleted ocean plateau mantle lithosphere of the Shatsky Rise conjugate, which arrived in the early Laramide. The geologic recorded of vertical motion in the Wyoming region suggests that the plateau's crust escaped into the Earth's interior at 70–75 Ma. Initiation of Colorado Mineral Belt magmatism at this time may represent a slab rupture through which the ocean crust escaped. [ABSTRACT FROM AUTHOR]