Thermal Lithosphere Research Articles

Constraining crustal thickness and lithospheric thermal state beneath the northeastern Tibetan Plateau and adjacent regions from gravity, aeromagnetic, and heat flow data

• Changed to: Friction due to subduction caused heat upwelling to the Qiangtang Block. • A NE trend zone with high heat flow extends from the Bayan Har-Qilian Orogen to Alxa. • The Qinling Orogen is a heat channel from Songpan-Ganzi Plateau to North China Plain. The northeastern Tibetan Plateau (NETP) is located at the intersection between the Northern China Block, Southern China Block, and Tibetan Plateau, representing the frontier of the northeastward extrusion of the plateau. Data of gravimetry, aeromagnetic survey, and terrestrial heat flow are integrated to investigate the Moho depth, the Curie surface depth, and the thermal lithospheric thickness beneath the NETP and adjacent regions, which appear to be 30–70 km, 18–42 km, and 80–150 km, respectively. The gravity and the magnetic anomalies well reflect the spatial variation of tectonic structures. The NETP has a deep Moho but a shallow Curie surface. In contrast, the North China Plain, Ordos Basin and Yangtze Block have a reverse situation. The distributions of the Curie surface and heat flow both reveal that heat flow has north-eastwardly crossed the Qilian Mountain and has conducted into the Alxa Block. Moreover, the distribution of heat flow confirms that the Qinling Orogen behaviors as a heat channel (between Ordos Basin and Sichuan Basin) from Songpan-Ganzi Plateau to North China Plain. The eastward hot asthenosphere from the NETP may thin out the LAB beneath the North China Plain, as verified by the thinning thermal lithosphere there.

Lithosphere Mantle Density of the North China Craton

AbstractWe constrain the lithospheric mantle density of the North China Craton (NCC) at both in situ and standard temperature‐pressure (STP) conditions from gravity data. The lithosphere‐asthenosphere boundary (LAB) depth is constrained by our new thermal model, which is based on a new regional heat flow data set and a recent regional crustal model NCcrust. The new thermal model shows that the thermal lithosphere thickness is <120 km in most of the NCC, except for the northern and southern parts with the maximum depth of 170 km. The gravity calculations reveal a highly heterogeneous density structure of the lithospheric mantle with in situ and STP values of 3.22–3.29 and 3.32–3.40 g/cm3, respectively. Thick and reduced‐density cratonic‐type lithosphere is preserved mostly in the southern NCC. Most of the Eastern Block has a thin (90–140 km) and high‐density lithospheric mantle. Most of the Western Block has a high‐density lithospheric mantle and a thin (80–110 km) lithosphere typical of Phanerozoic regions, which suggests that the Archean lithosphere is no longer present there. We conclude that in almost the entire NCC the lithosphere has lost its cratonic characteristics by geodynamic processes that include, but are not limited to, the Paleozoic closure of the Paleo‐Asian Ocean in the north, the Mesozoic Yangtze Craton flat subduction in the south, the Mesozoic Pacific subduction in the east, the Cenozoic remote response to the Indian‐Eurasian collision in the west, and the Cenozoic extensional tectonics (possibly associated with the slab roll‐back) in the center.

Lithospheric thermal regime under the Qinling Orogenic Belt and the Weihe Basin: A transect across the Yangtze and the North China cratons in central China

The thermal regime of the lithosphere could be a clue for the processes of geodynamics. Based on the Yuquan-Yulin seismic reflection transect along the Sichuan Basin (Yangtze Craton)–Qinling Orogenic Belt (QOB)–Weihe Basin–Ordos Basin (North China Craton), we modified crustal and mantle lithospheric structures to fit the observed data such as elevation, gravity, geoid height, and heat flow. Then we studied thermal structure, lithospheric thermal thickness and rheological structure of this area. The results suggest that the Moho temperature along the transect ranges from 580 to 740 °C; the thickness of the thermal lithosphere varies from 70 to 190 km. The rheological strength is characterized by the rheological characteristics of “weak crust-strong mantle” in the Sichuan Basin, the south QOB and the Ordos Basin, and “strong crust–weak mantle” in the north QOB and Weihe Basin. The Sichuan Basin has the lowest heat flow, the thickest lithosphere, and the strongest lithospheric strength, while the north QOB and Weihe Basin have the highest heat flow, the thinnest lithosphere, and the weakest lithosphere. The thermal regime of the Sichuan and Ordos basins is affected by the stability of the craton. The thermal regime of the QOB and Weihe Basin respond to the thermal effect of extensional/rifting tectonics. The thermal regime along the transect indicates that this area is mainly affected by the convergence caused by the continental collision of the Indian Plate.

Heat Flow and Seismicity in Bulgaria

The study of intra-earth processes requires a comprehensive approach, which should be based on the understanding different nature and origin depth of terrestrial processes. In this regard, the problem of the relationship between seismicity and heat flow in the lithosphere is of interest. We analyzed seismic observation data in Bulgaria from 1981 to 2018 using statistical methods. A significant correlation was found between the heat flow and the earthquakes magnitude, as well as between the thickness of thermal lithosphere and the magnitude. To assess the released friction heat and hydrodynamic processes during the earth’s crust movements, it is necessary to develop theoretical models based on a comprehensive analysis of the geological structure and geophysical fields in seismically active regions.

3-D shear wave velocity model of the lithosphere below the Sardinia–Corsica continental block based on Rayleigh-wave phase velocities

SUMMARY Rayleigh-wave dispersion curves from both ambient noise and teleseismic events allow us to provide the first high-resolution 3-D shear wave velocity (VS) model of the crust and upper mantle below the Sardinia–Corsica microplate, an important continental block for understanding the evolution of the central-western Mediterranean. For a wide range of periods (from 3 to ∼30 s), the phase velocities of the study area are systematically higher than those measured within the Italian peninsula, in agreement with a colder geotherm. Relative and absolute variations in the VS allow us to detect a very heterogeneous upper crust down to 8 km, as opposed to a relatively homogeneous middle and lower crust. The isosurface at 4.1 km s−1 is consistent with a rather flat Moho at a depth of 28.0 ± 1.8 km (2σ). The lithospheric mantle is relatively cold, and we constrain the thermal lithosphere–asthenosphere boundary at ∼100 km. We find our estimate consistent with a continental geotherm based on a surface heat flow of 60 mW m−2. Our results suggest that most of the lithosphere endured the complex history of deformation experienced by the study area and imply, in general, that deep tectonic processes do not easily destabilize the deeper portion of the continental lithosphere, despite leaving a clear surface signature.

Evolution of the Cenozoic thermal lithosphere of the Jiyang Sub-basin, Bohai Bay Basin: Implications for the lithospheric thinning mechanism

The Jiyang Sub-basin is an important sub-tectonic unit of the Bohai Bay Basin (BBB), which thinned by over 100 km during the Meso-Cenozoic. Changes in the thermal lithospheric thickness, of the Jiyang Sub-basin in the Cenozoic, were assessed using data from 131 single boreholes, together with the thermal history and layered crustal structure. From these, basement subsidence, stretching factor and strain rate were calculated for each borehole. The results show that the Jiyang Sub-basin experienced a peak of lithospheric thinning during the Cenozoic. In the early Cenozoic, the thickness of the lithosphere was about 80–100 km, and the strain rate was mostly low, 10−16–10−22s−1, indicating a weak extensional environment. At the peak of the lithospheric thinning (50 Ma to 35 Ma), the thermal lithosphere thickness was 70–55 km, the tectonic subsidence was about 1000 m, the lithospheric strain rate was higher about 10−15–10−16 s−1, the stretching factor had also increased rapidly to 1.1–1.2. The thinning of the lithosphere in the Eocene is interpreted as a product of collective movements of the Indian, Pacific, and the Philippine Sea plates, although the effect of the Pacific plate was small as - the Izanagi-Pacific mid-ocean ridge is still young age (0–10 Ma). Knowledge of the history of the Cenozoic lithosphere in the Jiyang Sub-basin is of great significance for understanding the tectonic evolution of the BBB and of the North China Craton.

Upper mantle temperature structure of the North China Craton

We investigate three-dimensional (3-D) temperature distribution of the upper mantle at depths of 50–200 km beneath the North China Craton (NCC) and surrounding areas using high-resolution P- and S-wave velocity models as well as mineral physics and geothermal data and methods. The thickness of thermal lithosphere is also estimated assuming its bottom has a mantle adiabatic temperature of 1300 °C. Our results show that the study area exhibits an alternate distribution pattern with three weak zones and two stable blocks. The Qilian orogen, the Shanxi rift system and the Tanlu fault system are characterized by lower seismic velocity, higher temperature, higher heat flow, and thinner thermal lithosphere. The three mechanically weak zones may be related to subductions of the Indian plate and the Pacific plate beneath the Eurasian continent, resulting in thermochemical erosion of the upwelling asthenospheric material, which have played an important role in the Cenozoic partial destruction of the lithosphere beneath the NCC. The Ordos block and part of the North China Basin have higher seismic velocity, lower temperature and thicker thermal lithosphere, suggesting that the two relatively stable blocks have suffered from less damage and still kept a remanent Archean lithospheric keel.

Synthesizing Seemingly Contradictory Seismic and Magnetotelluric Observations in the Southeastern United States to Image Physical Properties of the Lithosphere

AbstractAlthough seismic velocity and electrical conductivity are both sensitive to temperature, thermal lithosphere properties are derived almost exclusively from seismic data because conductivity is often too strongly affected by minor highly conductive phases to be a reliable indicator of temperature. However, in certain circumstances, electrical observations can provide strong constraints on mantle temperatures. In the southeastern United States (SEUS), magnetotelluric (MT) data require high resistivity values (>300 Ωm) to at least 200‐km depth. As dry mantle mineral conduction laws provide an upper bound on temperature for an observed resistivity value, the only interpretation is that lithospheric temperatures (<1330 °C) persist to 200 km. However, seismic tomography shows that velocities in this region are generally slightly slow with respect to references models; this observation has led to a view of relatively thin (<150 km), eroded thermal lithosphere beneath the SEUS. We show that MT and seismic (tomography, attenuation, receiver function) results are consistent with thick (~200 km), coherent thermal lithosphere in this region. Reduced seismic velocities (relative to reference models) can be explained by considering the effect of finite grain size (anelasticity). Calculated velocity as a function of temperature is overall slower when including anelastic effects, even at reasonable grain sizes of 1 mm to 1 cm; this permits mantle temperatures that are colder than would typically be inferred. We argue for a geodynamic scenario in which the present thermal lithosphere in the SEUS formed in association with the Central Atlantic Magmatic Province and has subsequently survived intact for ~200 Ma.

The first-approximation model showing the occurrence of epicontinental sedimentary basins due to convective instability of the thermal lithosphere

Modern computational technologies make it possible to simulate practically any concept developed by geologists to investigate the processes of formation of the structures under study, including diametrically opposed ones. Today’s trend is to create complex ‘realistic’ models. Such models are based on a large number of parameters with properly set values and simulate the settings that can be viewed similar to the real situations. However, the adequacy of both the models themselves and the concepts used as the basis for simulation remains the issue of debate. Apparently, it is required to specify a general approach to theoretical constructions in geodynamics, which should ensure that the scope of applicability of the models can be correctly evaluated. Such an approach can be implemented by successive approximations based on the fundamental results of the theory of simple liquids with damping memory, the most general description of irreversible deformation of materials under non-isotropic stress. It is critical to correctly formulate a model in the first approximation. It should be fairly simple and based on reliably established experimental facts, give adequate and clearly interpretable non-trivial results and allow further logical refinement of the details, i.e. the next approximations. This article presents an attempt to strictly follow the requirements and consistently construct a model that can show the occurrence of large epicontinental sedimentary basins, the origin of which has been in the focus of geological studies for many years. Our model is based on the following reliably established facts: (1) at the surface of the planet, in continental areas there is an approximately 300-km-thick thermal boundary layer (TBL), wherein the temperature drop amounts to ~1300–1500 °C; (2) the material of the lithosphere, including the crust, is irreversibly deformed during slow geological processes; (3) the continental crust is the thick layer that is less dense than the material of the mantle. The numerical experiments demonstrate free convection in the upper mantle, which induces countercurrents in the light crust and leads to the occurrence of sedimentary basins above the ascending flows and uplifts above the descending flows, which form platform shields during the transition to the quasi-stationary mode. The parameters of the typical structures formed in the lithosphere and the crust and the sedimentary basins proper are estimated. Revealed are the stages of their evolution, which correlate with the available geological and geophysical data, except for the effects caused, in our opinion, by the higher temperature of the mantle and the dynamics of the resultant melt. (Our next publications will describe modeling with account of decompression melting of the mantle material and separation, migration and freezing of the resultant melt.) The proposed first-approximation model can be used to describe a wide variety of geodynamic processes of similar scales.

Lithospheric structure of the Southwest South China Sea: implications for rifting and extension

ABSTRACT The South China Sea (SCS) is an excellent site for studying the process of conjugate margin rifting, and the origin and evolution of oceanic basins. Compared with the well-defined northern margin of the SCS, the western and southern segments of the SCS margin have not been researched in significant detail. To investigate the regional structure of the southwestern SCS, a gravity model is constructed, along with the lithospheric thermal structure along a wide-angle seismic profile. The profile extends across the conjugate margins of the Southwest Sub-Basin (SWSB) of the SCS and is based on the latest multiple geophysical measurements (including heat flow and thermo-physical parameters). The results show that the average thicknesses of the crust and thermal lithosphere along the profile are about 15 km and 57 km, respectively. The overall amount of extension of continental crust and lithosphere is more than 200 km. Thermal structure of the lithosphere shows that the continental margins are in a warm thermal state. The southwest SCS is characterized by ultra-wide, thinned continental crust and lithosphere, high Moho heat flow, early syn-rift faulted basins, undeformed late syn-rifting, and high seismic velocities in the lower crust. These various pieces of evidence suggest that the break-up of the mantle lithosphere occurred before that of the continental crust favouring a depth-dependent extension of the southwestern SCS margin.

Upper Mantle Seismic Structure of Alaska From Rayleigh and S Wave Tomography

AbstractMantle shear velocity (Vs) structure beneath the Transportable Array (TA) in Alaska and northwestern Canada is imaged by joint inversion of Rayleigh wave dispersion and teleseismic S wave travel times. The study connects previously unsampled parts of northern and western Alaska with portions of southern Alaska imaged with earlier seismic arrays. The new Vs tomography shows contrasting lithospheric structure in the plate interior with lower Vs shallow upper mantle indicative of thinner thermal lithosphere south of the Brooks Range and along the transform margin. Higher Vs down to ~200 km beneath the Brooks Range and northern coast is consistent with the presence of a cold stable lithospheric root that may help guide intraplate deformation to the south. In the subduction‐to‐transform transition, a potential slab fragment is imaged beneath the Wrangell volcanic field where modern subduction has slowed due to the thick buoyant crust of the Yakutat terrane.

Stochastic Modeling of the Thermal Structure to Decipher the Lithospheric Thickness: Application to Dharwar Craton

A wide range of speculation regarding lithospheric thermal thickness in a region calls for quantifying the uncertainties associated with model parameters. In this paper, we present an attempt to decipher it by solving the differential equations governing the heat transfer through the lithosphere. Adequately quantifying the temperature field is very important, since it is highly dependent on the controlling thermal parameters. Thermal conductivity is modeled as a random parameter with a known mean value and correlation structure. We compute the temperatures in the crust and lithospheric mantle, along with their error bounds, using an analytical solution to the steady-state heat equation and the Adomian method of decomposition. We apply these solutions in the Archean Dharwar craton, which is divided into western Dharwar craton (WDC) and eastern Dharwar craton (EDC). We obtain a Moho temperature estimate of 304 ± 58 °C in WDC, whilst in EDC it is 375 ± 74 °C for a 20% coefficient of variability in thermal conductivity. The range of lithospheric thickness obtained for WDC is 150–280 km, and 110–160 km for the EDC. These values are in agreement with other geophysical and geological findings in the region. The thermal lithosphere obtained from this study is based on the mantle solidus and the surface wave seismological studies considering radial anisotropy reveal that the lithosphere is characterized by higher shear wave velocities, and decrease at the lithosphere–asthenosphere boundary. Average shear velocity model for two different cratonic blocks (EDC and WDC) is used to show the LAB thickness for the eastern and western Dharwar craton. The two independent studies show a correlation in the lithospheric thickness.

Seismic Imaging of Thickened Lithosphere Resulting From Plume Pulsing Beneath Iceland.

Ocean plates conductively cool and subside with seafloor age. Plate thickening with age is also predicted, and hot spots may cause thinning. However, both are debated and depend on the way the plate is defined. Determining the thickness of the plates along with the process that governs it has proven challenging. We use S‐to‐P (Sp) receiver functions to image a strong, persistent LAB beneath Iceland where the mid‐Atlantic Ridge interacts with a plume with hypothesized pulsating thermal anomaly. The plate is thickest, up to 84 ± 6 km, beneath lithosphere formed during times of hypothesized hotter plume temperatures and as thin as 61 ± 6 km beneath regions formed during colder intervals. We performed geodynamic modeling to show that these plate thicknesses are inconsistent with a thermal lithosphere. Instead, periods of increased plume temperatures likely increased the melting depth, causing deeper depletion and dehydration, and creating a thicker plate. This suggests plate thickness is dictated by the conditions of plate formation.

Thermal regime of the lithosphere and geothermal potential in Xiong'an New Area

Investigation into the thermal regime of lithosphere is an essential part of geothermal research. The thermal state of lithosphere can be expressed as the vertical distribution of temperature. It has been found that the thermal regime of lithosphere can control the heat flow distribution and the geothermal mechanism of local geothermal fields. In this work, equilibrium temperature logs were obtained from 27 wells, and thermal conductivity and heat generation data were collected from 148 rock samples from different wells. Besides, 55 high-quality terrestrial heat flow values were extracted. Based on these data, the distribution of heat flow across Xiong'an New Area was mapped. Later, the thickness of the thermal lithosphere in Xiong'an and the deep crustal temperature were analyzed using the one-dimensional steady-state heat conduction equation. The crustal structure beneath this area was derived from the seismic wave velocity profile, and it was then used to illustrate the connection between the geothermal fields’ heat sources and regional tectonic setting. The results indicate that high heat flow is mainly distributed around the basement uplifts, such as Niutuozhen uplift and Rongcheng uplift. The average heat flux in the study area is 70.5 mW/m2, higher than the 61.5 mW/m2 in mainland China. The temperature-depth profiles show great temperature variation across the new area. At the depth of 40 km, the crustal temperature ranges from about 750°C to 1100°C. Despite the large temperature variation, this area shows high thermal state. The temperature variation with depth may be attributed to the heat flow variability and the high thermal state may be caused by the large residual heat flow from the lithosphere boundary. The average reduced heat flow in the new area is high at around 44.35mW/m2, which is associated with the high thermal state of this area.

Geothermal field and deep thermal structure of the Tianshan‐Altun region

AbstractThis paper analyses the characteristics of geothermal gradient and terrestrial heat flow in the Tianshan‐Altun region on the basis of available published data, such as geothermal gradient, measured temperature during oil test producing, rock thermophysics, and heat flow. Based on the steady‐state heat conduction equation, this paper has set up a 3D model by adopting basin modelling method to carry out deep temperature field simulation, with the aim to determine deep thermal structural parameters, the lithospheric thermal thickness, and the crustal contribution rate to the terrestrial heat flow. The geological section of Golmud‐Ejin Banner and the geological section of West Kunlun‐Tarim‐Tianshan were selected to represent the regional temperature and its thermal structure. The results show that the Tianshan‐Altun region has a low temperature geothermal field (averaging temperature gradient 2.2 °C/100 m, heat flow 56.3 mW/m2). The Qaidam Basin has the highest value (averaging 3.2 °C/100 m, 57.4 mW/m2). The lithospheric thermal thickness of the Tianshan‐Altun region is between 110 and 210 km. There are great differences of lithospheric thermal thickness on both sides of the Altun‐Tagh Fault. The west has thicker thermal lithospheric thickness (160–210 km) than the east (110–180 km). The contribution rate of the crust to the terrestrial heat flow decreases in the following order: the Qaidam Basin, the Jiudong Basin, the Jungger Basin, the Tarim Basin, and the Tuha Basin. Sedimentary basins have higher temperature and thinner thermal lithosphere than tectonic orogens when at the same depth. It can be observed that great thermal differences exist on both sides of the Altun‐Tagh Fault. There are relative similarities of low temperature, deeper thermal structure, and thicker lithospheric thermal thickness among the Tarim Basin, the Junggar Basin, and the Yining Basin in a wide geological area because of their shared deep geological basement. The Qaidam Basin and the Hexi Corridor Basin, which are located to the east of Altun‐Tagh Fault, have higher thermal background.

Carbonatites in oceanic hotspots

An analysis of the global array of ocean island volcanics shows that carbonatites only form in those hotspots that have the lowest Si- and highest alkali-contents among their primitive melts, such as the Cape Verde and Canary (Islands) hotspots. Fractionated melts from these two hotspots reach, at any given SiO_2, several wt% higher total alkali contents than for ocean islands without carbonatites. This is because their strongly silica-undersaturated primitive melts fractionate at low SiO_2 to high alkali contents, driving the evolving melt into the silicate-carbonatite miscibility gap. Instead, moderately alkaline magmas fractionate toward the alkali-feldspar thermal divide and do not reach liquid immiscibility. Low SiO_2 and high alkalis are the combined result of comparatively deep and low-degree mantle melting, the latter is corroborated by the highest high-field-strength and rare earth element concentrations in the Cape Verde and Canary primitive melts. CO_2 in the source facilitates low melt SiO_2, but enrichment in CO_2 relative to other hotspots is not required. The oceanic hotspots with carbonatites are among those with the thickest thermal lithosphere supporting a deep origin of their asthenospheric parent melts, an argument that could be expanded to continental hotspot settings.

Variability of the geothermal gradient across two differently aged magma-rich continental rifted margins of the Atlantic Ocean: the Southwest African and the Norwegian margins

Abstract. The aim of this study is to investigate the shallow thermal field differences for two differently aged passive continental margins by analyzing regional variations in geothermal gradient and exploring the controlling factors for these variations. Hence, we analyzed two previously published 3-D conductive and lithospheric-scale thermal models of the Southwest African and the Norwegian passive margins. These 3-D models differentiate various sedimentary, crustal, and mantle units and integrate different geophysical data such as seismic observations and the gravity field. We extracted the temperature–depth distributions in 1 km intervals down to 6 km below the upper thermal boundary condition. The geothermal gradient was then calculated for these intervals between the upper thermal boundary condition and the respective depth levels (1, 2, 3, 4, 5, and 6 km below the upper thermal boundary condition). According to our results, the geothermal gradient decreases with increasing depth and shows varying lateral trends and values for these two different margins. We compare the 3-D geological structural models and the geothermal gradient variations for both thermal models and show how radiogenic heat production, sediment insulating effect, and thermal lithosphere–asthenosphere boundary (LAB) depth influence the shallow thermal field pattern. The results indicate an ongoing process of oceanic mantle cooling at the young Norwegian margin compared with the old SW African passive margin that seems to be thermally equilibrated in the present day.

A review of surface heat-flow data of the northern Middle Atlas (Morocco)

We revised thermal data available from water and oil wells in the northern sector of the Middle Atlas region. To avoid biased estimation of surface heat flow caused by advection likely occurring in shallow aquifers, temperature measurements in water boreholes were carefully inspected and selected. The heat flow in the oil wells was inferred by taking into account the porosity variation with depth, the temperature effect on thermal conductivity of the matrix and the pore fluid, together with the contribution of the radiogenic heat production. Moreover, the possible bias in heat flow caused by convection occurring in confined carbonate aquifers was evaluated. The results of heat flow slightly modify the picture reported in previous investigations. The heat flow value over the investigated region is rather uniform (about 80mWm−2) and is similar in oil wells and in water boreholes. Geothermal calculations indicate that such a surface heat flow is compatible with a ∼70km thick thermal lithosphere and normal thermal conditions in the asthenospheric mantle.

Heterogeneous structure of the lithosphere in the Black Sea from a multidisciplinary analysis of geophysical fields

Magnetic, gravity, geothermal, seismic and tomographic data from the lithosphere were first jointly examined. A multidisciplinary interpretation has resulted in a new and consistent model for lithospheric density, magnetic, thermal and velocity heterogeneities. Faults of different orders for the crystalline crust have been mapped in details. Large deep fault zones were recognized. Among them is the most prominent Odessa-Sinop-Ordu (OSO) fault zone, which played a key role in the opening and development of the Black Sea Depression. A fundamental difference was revealed between the crustal and mantle structure and geophysical parameters of the Western Black Sea Basin (WBSB) and Eastern Black Sea Basin (EBSB). These dissimilarities are in the size of «non-granitic» crust, pattern and intensity of heat flow, topography of the lower boundary of the thermal lithosphere, mantle seismic velocity and structure of magnetic and residual gravity anomalies. Based on new information it was demonstrated that the WBSB and EBSB were diachronously formed on two large distinct continental blocks with independent post-rift development of the sub-basins. The rifting of the western sub-basin commenced earlier than that of the eastern one. The EBSB is characterized by younger thermal activity than the WBSB and consequently it was stabilized later. The Mid Black Sea High (MBSH) is not a single tectonic unit but is formed by two ridges of various crystalline crustal structure and age shifted relative to each other by the faults of the OSO zone.

Interactions of scales of convection in the Earth's mantle

The existence of undulations of the geoid, gravity and bathymetry in ocean basins, as well as anomalies in heat flow, point to the existence of small scale convection beneath tectonic plates. The instabilities that could develop at the base of the lithosphere are sufficiently small scale (<500km) that they remain mostly elusive from seismic detection. We take advantage of 3D spherical numerical geodynamic models displaying plate-like behavior to study the interaction between large-scale flow and small-scale convection. We find that finger-shaped instabilities develop at seafloor ages >60Ma. They form networks that are shaped by the plate evolution, slabs, plumes and the geometry of continental boundaries. Plumes impacting the boundary layer from below have a particular influence through rejuvenating the thermal lithosphere. They create a wake in which new instabilities form downstream. These wakes form channels that are about 1000km wide, and thus are possibly detectable by seismic tomography. Beneath fast plates, cold sinking instabilities are tilted in the direction opposite to plate motion, while they sink vertically for slow plates. These instabilities are too small to be detected by usual seismic methods, since they are about 200km in lateral scale. However, this preferred orientation of instabilities below fast plates could produce a pattern of large-scale azimuthal anisotropy consistent with both plate motions and the large scale organisation of azimuthal anisotropy obtained from recent surface wave models.