Magdala Tesauro

EuCRUST-07: A new reference model for the European crust

[1] We present a new digital model (EuCRUST-07) for the crust of Western and Central Europe and surroundings (35N–71N, 25W–35E). Available results of seismic reflection, refraction and receiver functions studies are assembled in an integrated model at a uniform grid (15 0 � 15 0 ). The model consists of three layers: sediments and two layers of the crystalline crust. Besides depth to the boundaries, we provide average P-wave velocities in the upper and lower parts of the crystalline crust. The new model demonstrates large differences in the Moho depth compared to previous compilations, over ±10 km in some specific areas (e.g. the Baltic Shield). Furthermore, the velocity structure of the crust is much more heterogeneous than in previous maps. EuCRUST-07 offers a starting point for numerical modeling of deeper structures by allowing correction for crustal effects beforehand and to resolve trade-off with mantle heterogeneities. Citation: Tesauro, M., M. K. Kaban, and S. A. P. L. Cloetingh (2008), EuCRUST-07: A new reference model for the European crust, Geophys. Res. Lett., 35, L05313, doi:10.1029/2007GL032244.

Density, temperature and composition of the North American lithosphere: new insights from a joint analysis of seismic, gravity and mineral physics data: 1. Density structure of the crust and upper mantle.

We introduce a new method to construct integrated 3-D models of density, temperature, and compositional variations of the crust and upper mantle based on a combined analysis of gravity, seismic, and tomography data with mineral physics constraints. The new technique is applied to North America. In the first stage, we remove the effect of the crust from the observed gravity field and topography, using a new crustal model (NACr2014). In the second step, the residual mantle gravity field and residual topography are inverted to obtain a 3-D density model of the upper mantle. The inversion technique accounts for the notion that these fields are controlled by the same factors but in a different way, e.g., depending on depth and horizontal dimension. This enables us to locate the position of principal density anomalies in the upper mantle. Afterward, we estimate the thermal contribution to the density structure by inverting two tomography models for temperature (NA07 and SL2013sv), assuming a laterally and vertically uniform “fertile” mantle composition. Both models show the cold internal part and the hot western margin of the continent, while in some Proterozoic regions (e.g., Grenville province) NA07 at a depth of 100 km is >200°C colder than SL2013sv. After removing this effect from the total mantle anomalies, the residual “compositional” fields are obtained. Some features of the composition density distribution, which are invisible in the seismic tomography data, are detected for the first time in the upper mantle. These results serve as a basis for the second part of the study, in which we improve the thermal and compositional models by applying an iterative approach to account for the effect of composition on the thermal model.

Density, temperature, and composition of the North American lithosphere—New insights from a joint analysis of seismic, gravity, and mineral physics data: 2. Thermal and compositional model of the upper mantle

Temperature and compositional variations of the North American (NA) lithospheric mantle are estimated using a new inversion technique introduced in Part 1, which allows us to jointly interpret seismic tomography and gravity data, taking into account depletion of the lithospheric mantle beneath the cratonic regions. The technique is tested using two tomography models (NA07 and SL2013sv) and different lithospheric density models. The first density model (Model I) reproduces the typical compositionally stratified lithospheric mantle, which is consistent with xenolith samples from the central Slave craton, while the second one (Model II) is based on the direct inversion of the residual gravity and residual topography. The results obtained, both in terms of temperature and composition, are more strongly influenced by the input models derived from seismic tomography, rather than by the choice of lithospheric density Model I versus Model II. The final temperatures estimated in the Archean lithospheric root are up to 150°C higher than in the initial thermal models obtained using a laterally and vertically uniform “fertile” compositional model and are in agreement with temperatures derived from xenolith data. Therefore, the effect of the compositional variations cannot be neglected when temperatures of the cratonic lithospheric mantle are estimated. Strong negative compositional density anomalies ( 92, characterize the lithospheric mantle of the northwestern part of the Superior craton and the central part of the Slave and Churchill craton, according to both tomographic models. The largest discrepancies between the results based on different tomography models are observed in the Proterozoic regions, such as the Trans Hudson Orogen (THO), Rocky Mountains, and Colorado Plateau, which appear weakly depleted (>−0.025 g/cm3 corresponding to Mg # ∼91) when model NA07 is used, or locally characterized by high-density bodies when model SL2013sv is used. The former results are in agreement with those based on the interpretation of xenolith data. The high-density bodies might be interpreted as fragments of subducted slabs or of the advection of the lithospheric mantle induced from the eastward-directed flat slab subduction. The selection of a seismic tomography model plays a significant role when estimating lithospheric density, temperature, and compositional heterogeneity. The consideration of the results of more than one model gives a more complete picture of the possible compositional variations within the NA lithospheric mantle.

Variations of the lithospheric strength and elastic thickness in North America

We evaluate the effect of temperature variations on strength and effective elastic thickness (Te) of the lithosphere of the North American (NA) continent. To this purpose, we use two thermal models that are corrected for compositional variations and anelasticity effects in the upper mantle. These thermal models are obtained from a joint inversion of gravity data and two recent seismic tomography models (NA07 and SL2013sv). The crustal rheology was defined using NACr14, the most recent NA crustal model. This model specifies seismic velocities and thickness for a three-layer model of the crystalline crust. Strength in the lithosphere and in the crust has similar distributions, indicating that local geotherms play a dominant role in determining strength rather than crustal composition. A pronounced contrast is present in strength between cratonic and off-cratonic regions. Lithospheric strength in the off-cratonic regions is prevalently localized within the crust and Te shows low values ( 150 km). In contrast to previous results, our models indicate that Phanerozoic regions located close to the edge of the cratons, as the Appalachians, are characterized by low strength. We also find that locally weak zones exist within the cratons (e.g., beneath the intracratonic Illinois Basin and Midcontinent rift). Seismic tomography models NA07 and SL2013sv differ mainly in some peripheral parts of the cratons, as the Proterozoic Canadian Platform, the Grenville, and the western part of the Yavapai-Mazatzal province, where the integrated strength for the model NA07 is 10 times larger than in model SL2013sv due to a temperature difference (>200°C) in the uppermost mantle. The differences in Te between the two models are less pronounced. In both models, Proterozoic regions reactivated by Meso-Cenozoic tectonics (e.g., Rocky Mountains and the Mississippi Embayment) are characterized by a weak lithosphere due to the absence of the mechanically strong part of the mantle lithospheric layer. Intraplate earthquakes are distributed along the edges of the cratons, demonstrating that tectonic stress accumulates there, while the cores of the cratons remain undeformed. In both models, intraplate earthquakes occur in weak lithosphere (∼0.5 × 1013 Pa s, Te ∼ 15 km) or near the edges of strong cratonic blocks, characterized by pronounced contrasts of strength and Te.

Three-dimensional density model of the upper mantle in the Middle East: Interaction of diverse tectonic processes

We present a three-dimensional density model of the lithosphere and upper mantle for the Middle East and surroundings based on seismic, gravity, and seismic tomography data and analyze the main factors responsible for the density variations. The gravity effect of the crust is calculated and removed from the observed field using the most recent crustal model. The residual gravity anomalies are jointly inverted with the residual topography to image the density distribution within the upper mantle. The inversion is constrained by an initial density model based on seismic tomography. The obtained density variations span in a large range (±60 kg/m3), revealing strong asymmetry in the density structure of the Arabian plate. The uppermost mantle layer in the Arabian Shield is relatively dense. However, below a depth of ~100 km we observe a strong low-density anomaly. In contrast, the mantle density in the Arabian platform increases at the same depths. The most pronounced decrease of the mantle density occurs in the Gulf of Aden, Red Sea, and East African Rift. Underneath the northern Red Sea the low-density anomaly is limited to the depth ~150 km, while in the southern part it is likely linked to a mantle plume. The densest mantle material is found under the South Caspian basin, which is likely associated with an eclogite body in the uppermost mantle. In the collision zones (the Zagros Belt and the Hellenic Arc), the high-density lithosphere shows the location of the subducting plates.

Glacial isostatic uplift of the European Alps

Following the last glacial maximum (LGM), the demise of continental ice sheets induced crustal rebound in tectonically stable regions of North America and Scandinavia that is still ongoing. Unlike the ice sheets, the Alpine ice cap developed in an orogen where the measured uplift is potentially attributed to tectonic shortening, lithospheric delamination and unloading due to deglaciation and erosion. Here we show that ∼90% of the geodetically measured rock uplift in the Alps can be explained by the Earth’s viscoelastic response to LGM deglaciation. We modelled rock uplift by reconstructing the Alpine ice cap, while accounting for postglacial erosion, sediment deposition and spatial variations in lithospheric rigidity. Clusters of excessive uplift in the Rhône Valley and in the Eastern Alps delineate regions potentially affected by mantle processes, crustal heterogeneity and active tectonics. Our study shows that even small LGM ice caps can dominate present-day rock uplift in tectonically active regions.

3D Crustal Model of Western and Central Europe as a Basis for Modelling Mantle Structure

EuCRUST-07 is a new 3D model of the crust for western and central Europe. It offers a starting point in any kind of numerical modelling, which requires an a priori removal of the crustal effect. The digital model (35oN, 71oN; 25oW, 35oE) consists of three layers: sediments and two layers of the crystalline crust. The latter are characterized by average P-wave velocities (V p ), which were defined based on seismic data. The model was obtained by assembling together at uniform 15′×15′ grid available results of deep seismic reflection, refraction and receiver function studies. The Moho depth variations were reconstructed by merging the most robust and recent Moho depth maps existing for the European region and compiled using published interpretations of seismic profiles. EuCRUST-07 demonstrates large differences in Moho depth with previous compilations: over ±10 km in some specific areas (e.g., the Baltic Shield). The basement is outcropping in some part of eastern Europe, while in western Europe it is up to ∼16 km deep, with an average value of 3–4 km, reflecting the presence of relatively shallow basins. The velocity structure of the crystalline crust turns out to be much more heterogeneous than demonstrated in previous compilations, having an average V p varying from 6.0 to 6.9 km/s. In comparison to existing models, the new model shows average crustal velocity values distributed over a larger and continuous range. The sedimentary thickness appears underestimated by CRUST2.0 by ∼10 km in several basins (e.g., the Porcupine basin), while it is overestimated by ∼3–6 km along part of the coastline (e.g., the Norwegian coast). EuCRUST-07 shows a Moho 5–10 km deeper than previous models in the orogens (e.g., the Cantabrian Mountains) and in the areas where the presence of magmatic underplating increases anomalously the crustal thickness. EuCRUST-07 predicts a Moho shallower 10–20 km along parts of the Atlantic margin, and in the basin (e.g., the Tyrrhenian Sea), where previous models overestimate the average crustal velocity. Furthermore, the results of EuCRUST-07 are used to make inferences on the lithology for various parts of Europe. The new lithology map shows the eastern European tectonic provinces represented by a granite-felsic granulite upper crust and a mafic granulite lower crust. By contrast, the younger western European tectonic provinces are mostly characterized by an upper and lower crust of granite-gneiss and dioritic composition, respectively.

3D density model of the upper mantle of Asia based on inversion of gravity and seismic tomography data

We construct a new-generation 3D density model of the upper mantle of Asia and its surrounding areas based on a joint interpretation of several data sets. A recent model of the crust combining nearly all available seismic data is employed to calculate the impact of the crust on the gravity anomalies and observed topography and to estimate the residual mantle anomalies and residual topography. These fields are jointly inverted to calculate the density variations in the lithosphere and upper mantle down to 325 km. As an initial approximation, we estimate density variations using a seismic tomography model. Seismic velocity variations are converted into temperatures and then to density variations based on mineral physics constraints. In the Occam-type inversion, we fit both the residual mantle gravity anomalies and residual topography by finding deviations to the initial model. The obtained corrections improve the resolution of the initial model and reflect important features of the mantle structure that are not well resolved by the seismic tomography. The most significant negative corrections of the upper mantle density, found in the Siberian and East European cratons, can be associated with depleted mantle material. The most pronounced positive density anomalies are found beneath the Tarim and South Caspian basins, Barents Sea, and Bay of Bengal. We attribute these anomalies to eclogites in the uppermost mantle, which have substantially affected the evolution of the basins. Furthermore, the obtained results provide evidence for the presence of eclogites in the oceanic subducting mantle lithosphere.

Lithospheric strength variations in Mainland China: Tectonic implications

We present a new thermal and strength model for the lithosphere of Mainland China. To this purpose, we integrate a thermal model for the crust, using a 3-D steady state heat conduction equation, with estimates for the upper mantle thermal structure, obtained by inverting a S wave tomography model. With this new thermal model and assigning to the lithospheric layers a “soft” and “hard” rheology, respectively, we estimate integrated strength of the lithosphere. In the Ordos and the Sichuan basins, characterized by intermediate temperatures, strength is primarily concentrated in the crust, when the rheology is soft, and in both the crust and upper mantle, when the rheology is hard. In turn, the Tibetan Plateau and the Tarim basin have a weak and strong lithosphere mainly on account of their high and low temperatures, respectively. A comparison of temperatures, strength, and effective viscosity variations with earthquakes distribution and their seismic energy released indicates that both the deep part of the crust and the upper mantle of the Tibetan Plateau are weak and prone to flow toward adjacent areas. The high strength of some of the tectonic domains surrounding Tibet (Tarim, Ordos, and Sichuan basins) favors the flow toward the weak western part of South China block.

Reconsidering Effective Elastic Thickness Estimates by Incorporating the Effect of Sediments: A Case Study for Europe

In the present study we analyzed the influence of density heterogeneity in the sedimentary cover on estimates of the effective elastic thickness (EET) of the lithosphere based on a cross‐spectral analysis of gravity and topography data. The fan wavelet coherence technique was employed to calculate EET for most of Europe and adjoining southern mountain belts. We employed Bouguer gravity anomalies and topography corrected for the effect of density variations within sediments. Correcting for sediments considerably suppresses the effect of unexpressed subsurface loads and substantially reduces EET estimates in areas with negligible topography variations as it was demonstrated for North Europe and East European Platform. The results show a good correspondence between the EET patterns and tectonic fragmentation of Europe and better agree with independent estimates based on the strength model of the lithosphere. Therefore, considering of the effect of sediments is essential for correct determinations of EET in flat areas.