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.
Key Points:
3D density distribution in the upper mantle in Asia and surrounding regions
Joint inversion of different data reveals new features of the lithosphere
Nature of thermocompositional anomalies in the 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.
Key Points:
Gravity and seismic tomography data are jointly inverted using a new technique
A 3‐D density model of the North American upper mantle is obtained
The results constrain the origin of the upper mantle heterogeneity
Continental collision forms mountain ranges that have shaped much of Earth's topography. Yet, the process by which material is transported and redistributed in collision zones remains debatable. Here ...we present a series of two-dimensional thermo-mechanical experiments on continent–continent collision zones to investigate the role of crustal strength in terms of geometry, deformation and exhumation. Depending on the crustal rheology, rate of collision and initial temperature distribution, continental collision may form double vergent orogens or result in continental subduction. Double vergent orogens are characterized by subduction of the lithospheric mantle, diffuse fore- and highly localized retro-shears, elevated topographies, and exhumation of high grade metamorphic rocks. In contrast, continental subduction results in subduction of lower continental crust, the formation of a wedge shaped Moho, a foreland propagating deformation zone, “lower” topographic build-up and exhumation of low grade metamorphic rocks. It is the combination of strength variations and ambient conditions that determines the geometry of mountain belts. Strong rheological coupling of upper and lower crust forms double vergent orogens; low rheological coupling of upper and lower crust results in continental subduction.
•We use thermo-mechanical models to investigate continent–continent collision zones.•Collision may result in double vergent orogens or continental subduction.•Crustal mechanics determine this transition.•Strong rheological coupling of upper and lower crust forms double vergent orogens.•Low rheological coupling of upper and lower crust results in continental subduction.
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 (<−0.03 g/cm3), corresponding to Mg # (100 × Mg/(Mg + Fe)) >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.
Key Points:
A new inversion technique using gravity and seismic tomography data is applied
Improved thermal and density models of the upper mantle are developed
In the cratons, density varies more significantly than temperature
We present a new digital model (EuCRUST‐07) for the crust of Western and Central Europe and surroundings (35°N–71°N, 25°W–35°E). Available results of seismic reflection, refraction and receiver ...functions studies are assembled in an integrated model at a uniform grid (15′ × 15′). 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.
The strength and effective elastic thickness (Te) of the lithosphere control its response to tectonic and surface processes. Here, we present the first global strength and effective elastic thickness ...maps, which are determined using physical properties from recent crustal and lithospheric models. Pronounced strength contrasts exist between old cratons and areas affected by Tertiary volcanism, which mostly coincide with the boundaries of seimogenic zones. Lithospheric strength is primarily controlled by the crust in young (Phanerozoic) geological provinces characterized by low Te (~25km), high topography (>1000m) and active seismicity. In contrast, the old (Achaean and Proterozoic) cratons of the continental plates show strength primarily in the lithospheric mantle, high Te (over 100km), low topography (<1000m) and very low seismicity.
Abstract
Traditionally, the emplacement of the Large Igneous Provinces (LIPs) is considered to have caused continental break-up. However, this does not always seem to be the case, as illustrated by, ...for example, the Siberian Traps, one of the most voluminous flood basalt events in Earth history, which was not followed by lithospheric rupture. Moreover, the classical model of purely active (plume-induced) rifting and continental break-up often fails to do justice to widely varying tectonic impacts of Phanerozoic LIPs. Here, we show that the role of the LIPs in rupture of the lithosphere ranges from initial dominance (e.g., Deccan LIP) to activation (e.g., Central Atlantic Magmatic Province, CAMP) or alignment (e.g., Afar LIP). A special case is the North Atlantic Igneous Province (NAIP), formed due to the “re-awakening” of the Iceland plume by the lateral propagation of the spreading ridge and the simultaneous approach of the plume conduit to adjacent segments of the thinner overlying lithosphere. The proposed new classification of LIPs may provide useful guidance for future research, particularly with respect to some inherent limitations of the common paradigm of purely passive continental break-up and the assumption of a direct link between internal mantle dynamics and the timing of near-surface magmatism.
Global distribution of the strength and effective elastic thickness (Te) of the lithosphere are estimated using physical parameters from recent crustal and lithospheric models. For the Te estimation ...we apply a new approach, which provides a possibility to take into account variations of Young modulus (E) within the lithosphere. In view of the large uncertainties affecting strength estimates, we evaluate global strength and Te distributions for possible end-member ‘hard’ (HRM) and a ‘soft’ (SRM) rheology models of the continental crust. Temperature within the lithosphere has been estimated using a recent tomography model of Ritsema et al. (2011), which has much higher horizontal resolution than previous global models. Most of the strength is localized in the crust for the HRM and in the mantle for the SRM. These results contribute to the long debates on applicability of the “crème brulée” or “jelly-sandwich” model for the lithosphere structure. Changing from the SRM to HRM turns most of the continental areas from the totally decoupled mode to the fully coupled mode of the lithospheric layers. However, in the areas characterized by a high thermal regime and thick crust, the layers remain decoupled even for the HRM. At the same time, for the inner part of the cratons the lithospheric layers are coupled in both models. Therefore, rheological variations lead to large changes in the integrated strength and Te distribution in the regions characterized by intermediate thermal conditions. In these areas temperature uncertainties have a greater effect, since this parameter principally determines rheological behavior. Comparison of the Te estimates for both models with those determined from the flexural loading and spectral analysis shows that the ‘hard’ rheology is likely applicable for cratonic areas, whereas the ‘soft’ rheology is more representative for young orogens.
► We estimate global strength and Te using end-member rheologies. ► The Te estimates are compared with those obtained by other methods. ► Rheological variations produce large spatial changes in integrated strength and Te.
The extension of the lithosphere, controlling the development of rifted basins, is driven by a combination of plate-boundary forces, frictional forces exerted on the base of the lithosphere by the ...convecting asthenosphere and deviatoric tensional stresses developing over upwelling branches of the asthenospheric convection system. Although mantle plumes are not a primary driving force of rifting, they play an important secondary role by weakening the lithosphere and by controlling the level of rift-related volcanic activity. A distinction between “active” and “passive” rifting is only conditionally justified.
The extension of the lithosphere, depending on its rate and magnitude, and the potential temperature of the asthenosphere, can cause by adiabatic decompression partial melting of the lower lithosphere and upper asthenosphere. In rift systems, the level and timing of volcanic activity is highly variable. The lack of volcanic activity implies “passive” rifting. An initial “passive” rifting stage can be followed by a more “active” one during which magmatism plays an increasingly important role. Magmatic destabilization of the Moho may account for the frequently observed discrepancy between upper and lower crustal extension factors. Combined with evidence for thermal thinning of the mantle–lithosphere, this suggest that the volume of the lithosphere is not necessarily preserved during rifting as advocated by conventional stretching models.
The structural style of rifts is controlled by the rheological structure of the lithosphere, the availability of crustal discontinuities that can be tensionally reactivated, the mode (orthogonal or oblique) and amount of extension, and the lithological composition of pre- and syn-rift sediments. Simple-shear extension prevails in rifts that subparallel the structural grain of the basement. Pure-shear extension is typical for rifts cross-cutting the basement grain. Pre-existing crustal and mantle–lithospheric discontinuities contribute to the localization of rift systems.
The duration of the rifting stage of extensional basins is highly variable. Stress field changes can cause abrupt termination of rifting. In major rift systems, progressive strain concentration on the zone of future crustal separation entails abandonment of lateral rifts. Depending on constraints on lateral block movements, crustal separation can be achieved after as little as 9 My and as much as 280 My of rifting activity.
Syn-rift basin subsidence is controlled by isostatic adjustment of the crust to mechanical stretching of the lithosphere, its magmatic inflation and thermal attenuation of the mantle–lithosphere.
Post-rift basin subsidence is governed by thermal reequilibration of the lithosphere–asthenosphere system. Deep-seated thermal anomalies related to syn-rift pull-up of the asthenosphere–lithosphere boundary have decayed after 60 My by about 65% and after 180 My by about 95%. The magnitude of post-rift subsidence is a function of the rift-induced thermal anomaly and crustal density changes, the potential temperature of the asthenosphere and initial water depths. Intraplate stresses can have an overprinting effect on post-rift subsidence. Stretching factors derived from post-rift subsidence analyses must be corrected for such effects.
The western branch of the East African Rift is composed of an arcuate succession of elongate asymmetric basins, which differ in terms of interaction geometry, fault architecture and kinematics, and ...patterns of uplift/subsidence and erosion/sedimentation. The basins are located within Proterozoic mobile belts at the edge of the strong Tanzanian craton; surface geology suggests that the geometry of these weak zones is an important parameter in controlling rift development and architecture, although other processes have been proposed. In this study, we use lithosphere‐scale numerical models and crustal‐scale analogue experiments to shed light on the relations between preexisting structures and rift architecture. Results illustrate that on a regional scale, rift localization within the mobile belts at the curved craton's western border results in an arcuate rift system, which implies that under a constant extensional stress field, part of the western branch experienced orthogonal extension and part oblique extension. Largest depocenters are predicted to form mostly orthogonal to the extension direction, and smaller depocenters will form along the oblique parts of the rift. The varying extension direction along the rift zone furthermore results in lengthwise varying rift asymmetry, segmentation characteristics, and border fault architecture (trend, length, and kinematics). Analogue models predict that discrete upper crustal fabrics may influence the location of accommodation zones and control the architecture of extension‐related faults at a local scale. Models support that fabric reactivation is responsible for the oblique‐slip kinematics on faults and for the development of Z‐shaped or arcuate normal faults typically documented in nature.
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