Using Heat Flow Density Values Obtained in the Gulf of Cadiz and Gorringe Bank, Atlantic Ocean

The geothermal heat flow measured at the surface of the Earth is originated by different heat sources located at different depths of the planet. The main sources of heat flow in the crust are associated with radioactive decay of Uranium, Thorium and Potassium, in rocks. In some regions, additional heat sources must be considered such as exothermic chemical reactions. The value of the heat flow coming from deep regions, designated by “heat from the mantle”, must be obtained using indirect methods. In this work, the geoid height was used as indicator of alterations “in heat from the mantle” values, considering that the density decrease in regions with geoid height increase is related to high temperature values in the upper part of the mantle. The region on study is located in the Atlantic Ocean, SW of Cape St. Vincent and Cadiz Gulf. Temperature-depth values were obtained in twelve points of the region considering heat flow by conduction in the vertical direction, using published heat flow and thermal conductivity data. Layered models were made using data obtained in published seismic profiles. Moho depth values were used as lower boundary of the crust and mantle heat flow variations were made according geoid height increases. Ocean depth values between 2.5 and 4.3 km were used. A value of 5°C was used for temperature at the upper boundary (ocean bottom) of the models. Temperature calculus stops when a value of 1350 °C was attained. Lithosphere thickness is obtained considering this temperature value as temperature at the bottom of the lithosphere. Heat flow density values from 36 to 65.8 mW m−2 were used in the work with “heat from the mantle” values from 33 to 35 mw m−2. Curie Point Temperature (600°C) depths from 33 to 36 km were obtained. Lithosphere thickness values about 97 km were obtained in all the models.

Icelandic-type crust enigma solved

Although on the mid-Atlantic extensional plate boundary, the crust beneath Iceland is unlike any produced by sea-floor spreading 1,2. It is up to 40 km thick – 6-7 times thicker than typical oceanic crust 3,4, its seismic velocity gradients are inconsistent with a solely mafic petrology, its lower part has a density approaching that of the mantle, and lithosphere thickness is up to 100 km. Furthermore, downward continuation of surface high thermal gradients predict partial melt in the lower crust that is at odds with the subsolidus state detected by seismic measurements. For the last ~ 50 years it has been assumed that its unusually large thickness results from excess magma production in a hot source 5 but it cannot explain these observations. We model jointly the seismic and thermal data and show that they are consistent with stretched continental crust 6 overlain by a few kilometres of young flood basalts. This resembles a volcanic passive margin under construction. It is mechanically feasible 7 and compatible with the composition of Icelandic lavas. This solution is the first proposed that can explain the geophysical properties of Icelandic-type crust. It has major implications for Atlantic Ocean opening, the structure of passive margins, extent of continental crust in the oceans, natural resources and limits of national seaboards.

Tectonic escape of Sicily Microplate in the context of Africa-Europe collision

<p>Sicily is in the centre of an area where complex geodynamic processes work together, these are: the Tyrrhenian-Apennine System evolution, the African-Ionian slab subduction and Africa-Europe collision.</p><p>During the last 5 Ma it was involved in a process of escape towards east-southeast: while on one side Africa acted as an intender pushing toward north, on the other side the fragmentation and retreat of the African-Ionian slab created space to the east.</p><p>The aim of this study is to reconstruct the kinematic evolution of Sicily, here considered as an independent plate starting from 5 Ma ago, and its role in the context of the Tyrrhenian-Apennine system.</p><p>The plates and microplate involved in the evolution are Europe, Africa and Calabria. The boundaries between these and Sicily are the margin of the Sicily microplate and are lithospheric structures known from the literature and identifiable from high resolution bathymetric maps, seismic sections, geodetic data, focal mechanism of recent earthquakes, gravimetric maps, lithosphere thickness maps and so on.</p><p>Briefly the margin between Sicily and Europe is along the Elimi chain, a E-W trending morpho-structure with transpressive kinematics, the margin with Calabria microplate is along the right-lateral Taormina line and the margin with Africa is expressed along the Malta Escarpment, south of Etna Mount, with transpressive kinematics and along the Sicily Channel, where a series of troughs (Pantelleria, Linosa and Malta) were interpreted in literature as pull-apart basins related to a dextral trascurrent zone.</p><p>The Euler pole of rotation between Sicily and Africa was found starting from the structures in the Sicily Channel and using the GPlates software, then we were able to find also Sicily-Europe and Sicily-Calabria poles and the respective velocity vectors and to compare these with the geological data and better refine the model.</p>

Interaction of the Indian craton with the Reunion plume

<p>One of the fundamental characteristics of cratons is the presence of thick lithosphere of more than 200 km, whereas any standard non-cratonic lithosphere thickness is about 100 km thick. The thickness of Indian craton has remained quite controversial. Under the Indian plate, most seismic studies fail to recognise a thick lithosphere; however, a few studies using other geophysical methods (e.g., magnetotellurics) argue for a thick Indian craton. In the last 30 years, more than ten research articles estimated the thickness of the Indian craton that varied from less than 100 km to 260 km. Such controversy arose primarily because of the Reunion plume and Indian craton interaction at ~65 Ma. Some studies suggested that due to the Reunion plume's eruption underneath the Indian craton, the thick lithosphere of the Indian craton was thinned down. This thin lithosphere is attributed as one of the primary reasons for the acceleration of the Indian plate since 65 Ma. On the other hand, several studies advocated that the Reunion plume did not affect the thickness of the Indian craton. Still now, no study has actually investigated the nature of plume-craton interaction under the Indian plate and how the craton was deformed in the presence of a plume. In this study, we develop time-dependent global mantle convection models using CitcomS to understand the evolution of Indian craton for the last 100 Ma. The models are initiated at 100 Ma and are driven forward  up to the present day using reconstructed plate velocities at every 1 Myr interval. Our results show that it is possible to thin down the thicker cratonic lithosphere due to the eruption of the Reunion plume. We also observe that the plume could get bifurcated due to the craton, and eruptions could occur on both the eastern and western parts of the Indian continental lithosphere.</p>

Relations between earthquake distributions, geological history, tectonics and rheology on the continents

This paper is concerned with the distribution of earthquakes, particularly their depths, with the temperature of the material in which they occur, and with the significance of both for the rheology and deformation of the continental lithosphere. Earthquakes on faults are generated by the sudden release of elastic energy that accumulates during slow plate motions. The nonlinear high-temperature creep that localizes such energy accumulation is, in principle, well understood and can be described by rheological models. But the same is not true of seismogenic brittle failure, the main focus of this paper, and severely limits the insights that can be obtained by simulations derived from geodynamical modelling of lithosphere deformation. Through advances in seismic tomography, we can now make increasingly detailed maps of lithosphere thickness on the continents. The lateral variations are dramatic, with some places up to 300 km thick, and clearly relate to the geological history of the continents as well as their present-day deformation. Where the lithosphere thickness is about 120 km or less, continental earthquakes are generally confined to upper crustal material that is colder than about 350°C. Within thick lithosphere, and especially on its edges, the entire crust may be seismogenic, with earthquakes sometimes extending into the uppermost mantle if the Moho is colder than 600°C, but the continental mantle is generally aseismic. Earthquakes in the continental lower crust at 400–600°C require the crust to be anhydrous and so are a useful guide or proxy to both composition and strength. These patterns and correlations have important implications for the geological evolution of the continents. They can be seen to have influenced features as diverse as the location of post-collisional rifting; cratonic basin formation; the location, origin and timing of granulite-facies metamorphism; and the formation, longevity and strength of cratons. In addition, they have important consequences for earthquake hazard assessment in the slowly deforming edges and interiors of continental shields or platforms, where the large seismogenic thickness can host very large earthquakes. This article is part of a discussion meeting issue ‘Understanding earthquakes using the geological record'.

The role of lithospheric thickness in the formation of ocean islands and seamounts: contrasts between the Louisville and Emperor-Hawaiian hotspot trails

The Hawaii-Emperor and Louisville seamounts form the two most prominent time-progressive hotspot trails on Earth. Both formed over a similar time interval on lithosphere with a similar range of ages and thickness. The Hawaii-Emperor seamounts are large and magma productivity appears to be increasing at present. The Louisville seamounts, by contrast, are smaller and the trail appears to be waning. We present new major- and trace element data from five of the older (74–50 Ma) Louisville seamounts drilled during International Ocean Drilling Program (IODP) Expedition 330 and compare these to published data from the Emperor seamounts of the same age. Despite drilling deep into the shield-forming volcanic rocks at three of the Louisville seamounts, our data confirm the results of earlier studies based on dredge samples that the Louisville seamounts are composed of remarkably uniform alkali basalt. The basalt composition can be modelled by ∼1.5–3% partial melting of a dominantly garnet lherzolite mantle with a composition similar to that of the Ontong Java Plateau mantle source. Rock samples recovered by dredging and drilling on the Emperor Seamounts range in composition from tholeiitic to alkali basalt and require larger degrees of melting (2–10%) and spinel- to garnet lherzolite mantle sources. We use a simple decompression melting model to show that melting of mantle with a potential temperature of 1500ºC under lithosphere of varying thickness can account for the composition of the shield-forming tholeiitic basalts from the Emperor seamounts, while post-shield alkali basalt requires a lower temperature (1300–1400ºC). This is consistent with the derivation of Hawaii-Emperor shield-forming magmas from the hotter axis of a mantle plume and the post-shield magmas from the cooler plume sheath as the seamount drifts away from the plume axis. The composition of basalt from the Louisville seamounts shows no significant variation with lithosphere thickness at the time of seamount formation, contrary to the predictions of our decompression melting model. This lack of influence of lithospheric thickness is characteristic of basalt from most ocean islands. The problem can be resolved if the Louisville seamounts were formed by dehydration melting of mantle containing a small amount of water in a cooler plume. Hydrous melting in a relatively cool mantle plume (Tp = 1350–1400 °C) could produce a small amount of melt and then be inhibited by increasing viscosity from reaching the dry mantle solidus and melting further. The failure of the plume to reach the dry mantle solidus or the base of the lithosphere means that the resulting magmas would have the same composition irrespective of lithosphere thickness. A hotter mantle plume (Tp ≈ 1500 °C) beneath the Emperor seamounts and the Hawaiian Islands would have lower viscosity before the onset of melting, melt to a larger extent, and decompress to the base of the lithosphere. Thus our decompression melting model could potentially explain the composition of both the Emperor and Louisville seamounts. The absence of a significant lithospheric control on the composition of basalt from nearly all ocean islands suggests that dehydration melting is the rule and the Hawaiian islands the exception. Alternatively, many ocean islands may not be the product of mantle plumes but instead be formed by decompression melting of heterogeneous mantle sources composed of peridotite containing discrete bodies of carbonated and silica-oversaturated eclogite within the general upper mantle convective flow.

The structure of the lithosphere and upper mantle beneath the Eastern Mediterranean and Middle East

Surface velocity measurements show that the Middle East is one of the most actively deforming regions of the continents. The structure of the underlying lithosphere and convecting upper mantle can be explored by combining three types of measurement. The gravity field from satellite and surface measurements is supported by the elastic properties of the lithosphere and by the underlying mantle convection. Three dimensional shear wave velocities can be determined by tomographic inversion of surface wave velocities. The shear wave velocities of the mantle are principally controlled by temperature, rather than by composition. The mantle composition can be obtained from that of young magmas. Application of these three types of observation to the Eastern Mediterranean and Middle East shows that the lithosphere thickness in most parts is no more than 50-70 km, and that the elastic thickness is less than 5 km. Because the lithosphere is so thin and weak the pattern of the underlying convection is clearly visible in the topography and gravity, as well as controlling the volcanism. The convection pattern takes the form of spokes: lines of hot upwelling mantle, joining hubs where the upwelling is three dimensional. It is the same as that seen in high Rayleigh number laboratory and numerical experiments. The lithospheric thicknesses beneath the seafloor to the SW of the Hellenic Arc and beneath the NE part of the Arabian Shield are more than 150 km and the elastic thicknesses are 30–40 km.

Thermal State, Thickness, and Composition of the Lithospheric Mantle beneath the Upper Muna Kimberlite Field (Siberian Craton) Constrained by Clinopyroxene Xenocrysts and Comparison with Daldyn and Mirny Fields

To gain better insight into the thermal state and composition of the lithospheric mantle beneath the Upper Muna kimberlite field (Siberian craton), a suite of 323 clinopyroxene xenocrysts and 10 mantle xenoliths from the Komsomolskaya-Magnitnaya (KM) pipe have been studied. We selected 188 clinopyroxene grains suitable for precise pressure (P)-temperature (T) estimation using single-clinopyroxene thermobarometry. The majority of P-T points lie along a narrow, elongated field in P-T space with a cluster of high-T and high-P points above 1300 °C, which deviates from the main P-T trend. The latter points may record a thermal event associated with kimberlite magmatism (a “stepped” or “kinked” geotherm). In order to eliminate these factors, the steady-state mantle paleogeotherm for the KM pipe at the time of initiation of kimberlite magmatism (Late Devonian–Early Carboniferous) was constrained by numerical fitting of P-T points below T = 1200 °C. The obtained mantle paleogeotherm is similar to the one from the nearby Novinka pipe, corresponding to a ~34–35 mW/m2 surface heat flux, 225–230 km lithospheric thickness, and 110–120 thick “diamond window” for the Upper Muna field. Coarse peridotite xenoliths are consistent in their P-T estimates with the steady-state mantle paleogeotherm derived from clinopyroxene xenocrysts, whereas porphyroclastic ones plot within the cluster of high-T and high-P clinopyroxene xenocrysts. Discrimination using Cr2O3 demonstrates that peridotitic clinopyroxene xenocrysts are prevalent (89%) among all studied 323 xenocrysts, suggesting that the Upper Muna mantle is predominantly composed of peridotites. Clinopyroxene-poor or -free peridotitic rocks such as harzburgites and dunites may be evident at depths of 140–180 km in the Upper Muna mantle. Judging solely from the thermal considerations and the thickness of the lithosphere, the KM and Novinka pipes should have excellent diamond potential. However, all pipes in the Upper Muna field have low diamond grades (<0.9, in carats/ton), although the lithosphere thickness is almost similar to the values obtained for the high-grade Udachnaya and Mir pipes from the Daldyn and Mirny fields, respectively. Therefore, other factors have affected the diamond grade of the Upper Muna kimberlite field.