Asthenospheric Diapir Research Articles

Are the Norwegian mountains compensated by a mantle thermal anomaly at depth?

The Norwegian mountains or Scandes represent a long mountain range stretching from southern Norway to the Arctic and is characterised by a rugged topography and peaks up to 2.5 km high. The origin of this mountain chain far away from any plate boundary remains a matter of passionate debates inside the geoscientific community. Hot mantle “fingers” originating from the Iceland Plume, impacting the base of the Scandinavian lithosphere and creating asthenospheric diapirs is one of the most accepted hypotheses for explaining Cenozoic uplift in Norway. In order to test this hypothesis we conducted integrated gravity and thermal modelling. We used the dense NGU gravity grid (i.e. one measurement every ~ 3 km) and modelled the depth extent and the mass deficit associated to the compensating loads located below the southern Scandes. Assuming that the density deficit below the Scandes is purely thermal in origin, thermal modelling allowed for testing the magnitude of the potentially associated temperature anomaly and its impact on surface heat flow. Recently acquired heat flow data were used in order to constrain the results from the thermal modelling. The results of our integrated geophysical modelling rule out the possibility that the present-day topography of the southern Scandes is compensated by a deep-seated asthenospheric diapir. However, our modelling does not exclude that thermal processes in the deep mantle could have initiated or assisted recent uplift of the southern Scandes.

Specifics of magmatism in marginal continental and marginal-sea pull-apart basins: Western periphery of the Pacific Ocean

The paper presents data on pull-apart (synchronous with strike-slip faulting) extensional structures formed in relation to Indo-Eurasian collision and including continental marginal rifts in East Asia and adjacent marginal sea basins. The evolution of Cenozoic pull-apart basins (developing synchronously with strike-slip faulting) in the western surroundings of the Pacific ocean corresponds to a basaltoid sequence in which the onset of rifting and the stage of maximum extension are marked by the first and last members of this sequence that have, respectively, calc-alkaline and tholeiitic depleted composition. The predominance of intermediate members with mixed isotopic-geochemical signatures testifies to the interaction of diverse magmatic melts. The opening of pull-apart basins (including those of marginal sea) was associated with magmatism whose sources were localized, judging from geochemical indicators, in the modified continental lithospheric mantle and depleted asthenosphere. The sources in the lithospheric mantle that was affected by long-lasting metasomatic recycling in the geological past dominated during the initial stages of continental extension and gave way to depleted asthenospheric sources. This model is consistent with the deep structure of the territories: extensional basins correspond to asthenospheric upwelling, with the ascent of asthenospheric diapirs positively correlated with the intensity of extension of the continental lithosphere and the degree of depletion of the accumulated basaltoids. The discovery of widespread calc-alkaline rocks (which are genetically related to the ancient metasomatized lithospheric mantle) in zones of continental rifting and marginal basins of the strike-slip fault nature significantly broadens the compositional range of volcanics typical of extensional geodynamic environments. At the same time, this testifies to the polygeodynamic nature of calc-alkaline volcanics, which can accumulate without any relations with coeval subduction zones.

Oman diopsidites: a new lithology diagnostic of very high temperature hydrothermal circulation in mantle peridotite below oceanic spreading centres

Mafic–ultramafic dykes scattered in the mantle section of ophiolites are generally crystallisation products from common silicate melts. In the frame of a global survey of these melt migration relics in the Oman mantle harzburgites, we discovered a peculiar lithology made essentially of pure diopside ( 0.95 < M g M g + F e total < 1 ) whose characteristics do not match a magmatic or mantle origin. Arguments against a magmatic or mantle origin for these diopsidites combine compositional and textural evidence. In spite of their refractory composition, they are strongly depleted in Cr (Cr 2O 3 0.2 wt.%). By the same way other minor elements (Al, Ti, Na…) and rare earth elements have peculiarly low abundances and plot away from magmatic differentiation trends. In a few samples, the paragenetic association includes pure anorthite (An% up to 0.99), minor amounts of forsterite (Fo > 0.95) or traces of andradite. When not deformed, the diopsides are automorphic and their texture points to metamorphic growth in a matrix composed of antigorite and/or carbonate. Diopsidites have textures reminiscent to that of skarns developing in contact metamorphic halos or that of rodingite frequently present in the serpentine bodies of the ophiolitic crust. They frequently appear as dykes (former cracks), with a few mm to few tens of cm wide transitional zones, which contain high amounts of hydrous minerals, between the diopsidite facies and its host rock. The diopsidites are not randomly distributed in the Oman ophiolite, being more abundant near former asthenospheric diapirs emplaced at shallow depth in the lithosphere. We interpret the diopsidites as the footprint of very high temperature circulation of seawater and carbonated fluids (> 800 °C), which may have leached plagioclase rich lithologies before penetrating the mantle (as shown by a well developed positive Eu anomaly). Our data confirm the prediction of McCollom and Shock [T.M. McCollom, E.L. Shock, Fluid-rock interactions in the lower oceanic crust: Thermodynamic models of hydrothermal alteration, J. Geophys. Res. 103 (B1) (1998) 547–575.] who proposed that common anhydrous minerals like pyroxene, plagioclase and olivine may crystallise from high temperature fluids intermediate between silicate melts and supercritical water. This confirms that there is no clear-cut thermal and chemical boundary between the fields of magmatic and hydrothermal crystallisations.

The sea of Okhotsk Geotraverse: Tectonomagmatic evolution of Cenozoic extension structures and implication for their deep structure

Issues of the origin of marginal seas and genesis of their igneous rocks remain controversial because of the insufficient amount of documentary information (small number of drilled holes, rare network of seismic profiles, and so on). Therefore, data on seismic profiles carried out within the framework of the Geotraverse International Project [1, 2] are of great significance. The geotraverse extends from the margin of Asia (Sikhote Alin) to the northwestern periphery of the Pacific plate (Fig. 1) and includes large Cenozoic extension structures, such as the Tatar continental rift and the South Kuril marginal basin. Comprehensive analysis of seismic data on deep zones of this profile coupled with the available information about the regional tectonic setting and magmatism have made it possible to detect a correlation between the Cenozoic geodynamic mode (correspondingly, tectonics and magmatism) at the crustal and deeper (lithospheric mantle and asthenosphere) levels. Ultimately, these data made it possible to refine the evolution model of marginal basins in the study region and provided new insights into the issue of passive and active rifting. The Tatar continental rift located in the western part of the geotraverse (Fig. 2) represents the northern termination of the Japan basin. Both structures are bounded by a convergent system of submeridional dextral strike-slip faults partly transformed into normal faults. The Tatar rift basement incorporates a thin continental crust with the velocity of seismic waves (hereafter, V ) ranging from 5.8 to 7.2 km/s [4, 7]. The continental crust began to break down after its disintegration in the terminal Paleocene‐Eocene into a system of narrow (5‐10 km) horsts and grabens [2, 4] and the accumulation of terrigenous sediments (up to 1.5 km thick) at the initial rifting stage. The next (Oligocene‐middle Miocene) stage of maximum extension of the Tatar rift is marked by the unconformably overlying transgressive sequence of deep-water clayey and siliceous‐clayey rocks (up to 4 km). The next (middle Miocene) regressive sequence is composed of coastal-marine sediments. In contrast to the almost undeformed cover of middle Miocene‐Holocene terrigenous rocks (1.5‐5 km thick) deposited after the rifting (hereafter, postrift complex), rocks deposited during rifting (rift complex) are characterized by significant dislocations. All of the rift and postrift complexes include basaltoid fields in some places. As will be shown below, these rock complexes have specific isotope‐geochemical parameters. The structure beneath the Tatar rift is characterized by drastic reduction of the continental crust (up to 25 km) and ascent of the asthenospheric layer up to the level of 50 km (Fig. 2). Therefore, this region is marked by high thermal flux (123‐132 mW/m 2 ). The temperature in the upper zone of the asthenospheric diapir is estimated at 1100 ° C [2‐4]. The diapir is bounded by narrow gradient stages [4] that extend to higher lithospheric levels and generally correspond to strike-slip faults, which serve as boundaries of the Tatar and Japan basins. The faults presumably extend to the mantle and asthenosphere.

Petrogenetic variability along the North?South Propagating Spreading Center of the North Fiji Basin

Previous studies on propagating rifts suggested that segmentation of a spreading axis could represent the superficial mark of mantle behavior (Sinton et al., 1983; Nicolas, 1989; Gente et al., 1995). The study of North–South Propagating Spreading Center (NSPSC) from the North Fiji Basin (NFB) brings new insights to this debate. Basalts from the central part of the propagator have more variable incompatible and isotopic ratios then those from its northern tip. A model of dynamic partial melting of a thermally and slightly geochemically and isotopically heterogeneous mantle is proposed. Beneath the central segment, the partial fusion starts deeper (ca. 30 km) and reaches a higher rate (ca. 22%). Further open system differentiation occurs within shallow permanent magma reservoirs along most of the central segment. Below the segment closest to the tip of the propagator the partial fusion starts shallower (ca. 25 km) and stays at a lower rate (ca. 16%). The maximum of differentiation occurs close to the propagator, in small, periodically disconnected, magma bodies resulting in the production of ferrobasalts close to the tip. In order to explain these variations, the presence of an asthenospheric diapir focused beneath the central part of the NS-PSC is proposed. The petrogenetic processes of propagating spreading centers of mature oceanic basins or back-arc basins are identical.

Tethyan vs. Cordilleran ophiolites: a reappraisal of distinctive tectono-magmatic features of supra-subduction complexes in relation to the subduction mode

Supra-subduction zone (SSZ) ophiolites deserve special attention because they represent fundamental markers of intraoceanic convergence and generation of new lithosphere above subduction zones. Moreover, owing to their structural characteristics and location in the overriding plate, these complexes are far better represented and preserved than Mid-Ocean-Ridge-Basalt (MORB) ophiolites in orogenic belts. In terms of their structure, tectonics, and magmatic features, SSZ ophiolites may be classified in two main types: (1) “Tethyan complexes” (such as those of the Albanide-Hellenide belt), which mostly consist of complete and extensive volcanic, dyke, plutonic, and mantle sections with prevalent Island Arc Tholeiitic (IAT) affinity and the significant presence of Boninites, obducted as relatively intact lithospheric slabs onto collisional continental margins; (2) “Cordilleran complexes” (including the circum-Caribbean ophiolites of Guatemala, Cuba, and Venezuela), mostly represented by dismembered sections of arc volcanic, plutonic, and subordinate mantle sequences with tholeiitic to Calc-Alkaline (IAC) magmatic affinity and acidic differentiates, commonly associated with metamorphic “subduction complexes” and tectonically emplaced onto or juxtaposed against the continental margin within polygenetic terranes. These two types appear to be related to significantly different subduction modes and intraoceanic plate dynamics whereby SSZ ophiolites were generated. The Tethyan complexes can best be compared with west Pacific-type subduction with accentuated steepening and retreat of the subducted slab, accompanied by progressive decoupling of the converging plates, intense mantle diapirism, and tensional events in the upper plate with generation of large IAT-sheeted dike complexes in “open” oceanic spreading systems. Continuous slab sinking and roll back allow increasing asthenospheric diapirism and interarc spreading from the arc axis to the forearc region with generation of boninites and Back Arc Basin Basalts (BABB) with intermediate MORB/IAT composition. By contrast, the genesis of the Cordilleran complexes requires a subduction mode characterized by a steady state regime, with moderate and constant dip of the subducted slab and limited extension in the backarc region. The magmatic evolution of these ophiolites from IAT to IAC and the significant presence of rhyodacite (and tonalite) differentiates coherently indicate a more mature stage of arc magmatism, as well as the occurrence of efficient differentiation processes developing under nearly “closed-system” conditions in independent magma chambers. Tethyan complexes are likely to be obducted as large and relatively intact slabs onto the continental margins through the interposition of metamorphic soles, which represent relics of the MORB lithosphere underplating the SSZ ophiolites. On the other hand, the common emplacement of Cordilleran complexes within polygenetic terranes appears to be controlled by prolonged accretionary mechanisms that trap, against the continental margin, parts of the arc structure, subduction complexes, melanges, and volcanoclastic products.

The Sea of Okhotsk crust from deep seismic sounding data

The crustal structure of the Sea of Okhotsk was investigated by way of reinterpreting the data obtained along 22 deep seismic sounding (DSS) profiles. Eight profiles traversed the Sea of Okhotsk. These seismic surveys were performed during the International Geophysical Year in 1958-1959 by the researchers from the Institute of the Physics of the Earth, USSR Academy of Sciences. Seven profiles were surveyed in 1963-1964 near the shores of Sakhalin Island by geophysicists from the Institute of the Physics of the Earth (USSR Academy of Sciences) and from the Sakhalin Multidisciplinary Research Institute. The new interpretation of the seismic data confirmed the earlier ideas of the reduced crustal thickness in the deep-sea basins of the Sea of Okhotsk, such as, the Kuril Basin, the Deryugin and Tinro basins, the sedimentary trough of the Tatar Strait, where the Moho surface showed low boundary velocities of seismic waves, not higher than 7.6-7.8 km s 1 . It appears that the sedimentary basins of the Sea of Okhotsk reside above asthenospheric diapirs including magma chambers. A system of rifts and spreading centers was mapped in the northern and central parts of the Sea of Okhotsk, in the Tatar Strait, and in the Kuril Basin. Paleosubduction zones that had been active during the late Cretaceous and Early Paleogene time and are marked by ophiolite belts at the present time have been traced in the earth crust near the eastern shores of Sakhalin. Remnants of paleosubduction zones were mapped using seismic data in the Sea of Okhotsk along the Okhotsk-Chukotka volcanic belt, which seem to be the fragments of a lithospheric plate that had been going down under the active continental margin in Mesozoic time.

Tectonic evidence in the Ronda peridotites, Spain, for mantle diapirism related to delamination

The contribution of asthenospheric diapirs to the exhumation of orogenic lherzolites from the mantle to Earth's surface is a major issue in the evolution of orogenic belts. Detailed maps of the trajectories of the foliation in the Carratraca massifs of the Ronda peridotites (Betic Cordillera, southern Spain) provide evidence for a narrow mantle diapir that was formed in early Miocene time. When set in its geologic and petrologic context, this diapir documents the injection of hot asthenosphere into older and cooler lithospheric mantle, possibly in response to the delamination of the lithosphere that had been thickened during the Mesozoic and Cenozoic convergence of the African and Iberian plates from Cretaceous time onward.

Orogenic tectonism on Io

We catalog 143 Ionian mountains (montes) and mountain‐like features (mensae, tholi, plana, and small peaks) in order to investigate orogenic tectonism on Io. From this comprehensive list, we select 96 mountains for which there are sufficient coverage and resolution to discern spatial relationships with surrounding geologic features. Three of the 96 mountains are probably volcanoes, 92 appear to be tectonic massifs, and 1 is ambiguous. Of the 92 tectonic mountains, 38 abut paterae (volcanic or volcano‐tectonic craters with irregular or scalloped margins). This juxtaposition is unlikely to be a coincidence as the probability of it occurring by chance is ∼0.1%. We propose instead that orogenic faults may act as conduits for magma ascent, thus fueling patera formation near mountains. As resurfacing buries a shell of material from Io's surface to the base of the lithosphere, its effective radius is reduced and it heats up. We calculate the lithospheric volume change due to subsidence and thermal expansion as a function of lithospheric thickness. Conservation of volume dictates that this material must be uplifted at Io's surface. By estimating the total volume of the mountains, we are able to place a lower limit of 12 km on Io's lithospheric thickness. We hypothesize that, in some cases, mountain formation may be facilitated by asthenospheric diapirs impinging on the base of the lithosphere. The resulting lithospheric swell could focus the compressive stresses that drive orogenic tectonism. This model is one of several possible mechanisms for uplifting isolated mountains such as are observed on Io.

MAR volcanism in the Sierra Leone Fracture Zone region, Central Atlantic

We have studied major and trace element (including REE) geochemistries of basalts and chilled basaltic glasses from the MAR axial zone in the vicinity of the Sierra Leone FZ (5 -7 10 0 N). The links of basalts of various compositions with particular ocean- floor geological structural features have been analyzed as well. Three basaltic varieties have been discriminated. Almost ubiquitous are high-Mg basalts that are derivatives of N-MORB tholeiitic melts and that are produced in the axial zone of spreading. Variety 2 is alkaline basalts widespread on the southwest flank of the MAR crestal zone in the Sierra Leone region, likely generated through deep mantle melting under plume impact. Variety 3 is basalts derivative from T- and P-MORB-like tholeiitic melts and originating through addition of a deeper mantle material to depleted upper mantle melts. Magma generation parameters, as calculated from chilled glass compositions, are dierent for depleted tholeiites (44-55 km, 1320-1370 C) and enriched tholeiites (45-78 km, 1330-1450 C). Mantle plume impact is shown to aect not only tholeiitic basalt compositions but also magma generation conditions in the axial spreading zone, resulting in higher Ti and Na concentrations in the melts parental to the rift-related basalts occurring near the plume. T- and P-MORBs are also developed near the areas where mantle plumes are localized. The high-Mg basalts are shown to come in several types with distinctive Ti and Na contents. Nearly every single MAR segment (bounded by sinistral strike slips and the Bogdanov FZ) is featured by its own basalt type suggesting that it has formed above an asthenospheric diapir with its unique magma generation conditions. These conditions are time variable. The likely causes of the temporal and spatial instability of mantle upwelling beneath this portion of the MAR are singular tectonic processes and plume activity. In the sulfide-bearing rift morphostructures (the so-called Ore area and the Markov Basin), basalts make up highly evolved suites generated through olivine and plagioclase fractionation, which is suggestive of relatively long-lived magma chambers beneath the sulfide-bearing rift morphostructures. The functioning of these chambers is a combined eect of a singular geodynamic regime and plume activity. In these chambers, melts undergo deep dierentiation leading to progressively increasing concentration of the sulfide phase, eventually to be supplied to the hydrothermal plumbing system.

Interaction between small‐scale mantle diapirs and a continental root

A possible mechanism for adding material to a continental root is by means of upwellings from the convecting mantle subject to pressure release partial melting. We present results of numerical modeling of the interaction of melting diapirs with continental roots in an Archean setting characterized by a mantle potential temperature of 1750°C in a two‐dimensional (2‐D) Cartesian geometry. In an extension of earlier work [de Smet et al., 2000b] we have investigated the influence of mantle rheology on the behavior of diapirs. We have in particular looked at the difference in behavior of diapirs using a composite rheology combining both grain size sensitive diffusion creep and dislocation creep mechanisms. We have used the grain size, here taken to be uniform, as a control parameter to obtain model cases with varying contribution from the two creep mechanisms. The diapirs in the composite rheology model rise much faster than they do in a purely Newtonian model. Observed diapiric ascent times from 230 km depth to the top of the ascent path at about 80 km depth are approximately 1 Myr for a Newtonian model (averaged 14 cm/yr) compared to about 50 thousand years for a composite rheology model (averaged 3 m/yr) with the same parameters for the Newtonian component. This clearly indicates the large impact of the dislocation creep component of the viscous deformation process. We have also investigated the effect of an increase in the viscosity due to dehydration during partial melting. This increase has a strong influence on the development of rising diapirs. The ascent velocity and lateral spreading of the diapirs at the end of their ascent are effectively reduced when a viscosity increase by a factor of 10 is applied, and the effect becomes stronger for larger factors. Average vertical velocities range from 1.4 cm/yr for a factor 10 to 2 mm/yr for a factor 200. The most striking result of the viscosity increase due to dehydration is the reduction of the ascent velocity, thereby stretching the characteristic timescale of the diapiric intrusion process to a value between 5 and 50 Myr for dehydration viscosity prefactor values of 10 and 200, respectively. In contrast with the strong difference between the Newtonian and the composite rheology models, small differences are found in the overall dynamics between the composite rheological models, characterized by different values of the uniform grain size. The composite rheological models exhibit self‐regulating behavior where substantial differences between the relative contributions of the two creep components result in very similar effective viscosities, due to a local dominance of dislocation creep at high stresses and corresponding similar flow dynamics. Stress levels and P,T paths in the modeling results are consistent with estimates obtained from Precambrian peridotite bodies which are interpreted to have originated from asthenospheric diapirism.

Contribution of numeric dynamic modelling to the understanding of the seismotectonic regime of the northern Apennines

In this paper we investigate the forces possibly active in the area of the northern Apennines by means of two-dimensional finite element modelling assuming a viscoelastic rheology. The forces included in the models are related to continental convergence between Africa and Eurasia, to negative buoyancy of the subducted Adriatic lithosphere and to positive buoyancy of anomalously hot mantle material underneath Tuscany. Model-predicted stress distribution is compared with seismotectonic data available for the area of the northern Apennines and with earthquakes distribution. Our results indicate that continental convergence cannot be the only mechanism acting in the study area since it causes insignificant subcrustal stresses, in disagreement with intermediate seismicity observed under the Apennines. Asthenospheric upwelling under Tuscany (back-arc region) is shown to play a crucial role in the present-day dynamics of the Tyrrhenian–Apennines. Positive buoyancy causes an upwards and eastwards flow that generates extensional bending stresses in Tuscany and at the top of the slab under the Apennines. Horizontal pushing of the slab is proposed as an alternative mechanism to slab pull to generate subcrustal stresses. Activation of slab pull does not result in a significant change in orientation of the principal stress axes at shallow depths with respect to the model in which only asthenospheric diapirism is active. Therefore, the existence of a continuous gravitationally sinking slab cannot be ruled out on the basis of comparison with seismotectonic observations. Both models successfully reproduce extension in Tuscany and compression in the outer margin of the Apenninic belt, but fail to reproduce extension along the inner portion of the chain. Our modelling highlights the importance of better constraining the lateral extent of the asthenospheric diapir in order to find out whether the extension and tectonic uplift in the Apennines is caused by asthenospheric upwelling as in the area of Tuscany.

Geodynamical interpretation of crustal and upper mantle electrical conductivity anomalies in Sayan-Baikal province

There exists a number of models that account for the deep structure of the Sayan-Baikal province on the basis of different geophysical data. The models based on magnetotelluric soundings differ from those obtained from seismology with no regard to magnetotelluric soundings (MTS) results. To decide between the active and passive mechanisms of Baikal rifting, we propose to correlate the modeling results to the available seismological and petrological data, as well as to the model of asthenospheric diapirism put forward by Zorin et al. (1989, 1990, 1995). Inasmuch as the reality of MTS-based models depends on removal of static-shift effects, the reported three-dimensional modeling of the Baikal rift deep structure was preceded by normalization of the original MT curves by the global magnetovariation curve. The enhanced electrical conductivities of the crust and upper mantle of the region is interpreted in terms of geodynamics where the Cenozoic activity is suggested to be driven by convective mantle flow and triggered by the associated plate collisions. The origin and evolution of the Baikal rift can be explained in terms of reactivation of a pre-existing lithospheric inhomogeneity between the Siberian craton and the Sayan-Baikal fold area.

THE MANTLE RIDGE OF THE GALICIA PASSIVE MARGIN: GEOCHEMICAL EVIDENCES FOR THE DENUDATION OF A PIECE OF THE LITHOSPHERIC MANTLE DURING OCEANISATION

The northern part of the Iberian continental passive margin is characterised by the presence of a ridge made of mantellic and magmatic rocks. This ridge, roughly parallel to the margin and continuous over several hundred kilometres, was emplaced at the end of the major rifting event that preceded North-Atlantic ocean opening. Petrological and geochemical studies on mantle rocks, whose results are presented here, are essential to characterise mantle affinity and processes that have affected this part of the mantle during or before continental break-up. In this work we would like to answer some questions like: Does this mantle correspond to the upwelling of the upper asthenospheric mantle at the end of the rifting event or rather correspond to the denudation of lithospheric mantle that was beneath the American-Iberian continent before oceanisation? Does the mantellic and magmatic ridge represent the first North-Atlantic oceanic lithosphere? The studied peridotites were sampled by submersible on the northwest part of the Galicia bank during the Galinaute II campaign (Boillot et al., 1995). As all samples are weathered, we have only performed analyses on fresh minerals. All peridotites have common mantle assemblage (Ol+Opx+Cpx+spinel) but according to microscopic observations and geochemical results we have defined three different types of peridotites. The first type is characterised by aluminous spinel (Cr* 0.2) and rare Cpx which is poorer in Na and Ti, richer in Cr, and have rather flat REE patterns, but, strikingly, lower middle and heavy REE contents compared to type 1 Cpx. Type 2 Cpx has also the lowest observed eNd values (-2.9 and +4.4. The third type of peridotite is rather different: it is characterised by a strong high-temperature strain, by the occurrence of scattered pargasitic amphiboles and dioritic dykelets made of pargasite and plagioclase, and by the presence of plagioclase rim around spinel. Dioritic dykelets are contemporaneous with the rifting high-temperature strain of the host mantle (Feraud et al., 1988). Amphibole in these dykelets shows rather homogeneous geochemical features: +5.6 and +5.8 eNd values and REE contents about one hundred times chondrite, with middle REE maxima and LaN/SmN ratios < 1. Scattered amphibole in peridotites shows the same kind of REE patterns but variable REE contents ranging from 5 to 80 the chondrite composition. From all these results, the first point to note is that the Galicia mantle ridge has been locally influenced by fluid circulation (type 3 peridotites), which was channelled inside active simple shear zones during rifting; moreover, the fluid associated with strain have probably favoured partial recrystallisation of the mantle from spinel to plagioclase facies. Type 1 and 2 peridotites probably give us more information on mantle affinity before oceanic opening, the most important fact being their contrasted eNd values. In this geodynamical context of passive rifting we can’t explain the lowest eNd values by metasomatism of the asthenospheric mantle reservoir by enriched fluids during the rifting or oceanisation. Moreover, the eNd values calculated at rifting time are always so different that we can’t explain the present isotopic values by different degrees of partial melting and so, different Sm/Nd time integrated evolution of an initially homogeneous asthenospheric mantle. Therefore, geochemical features of the Galicia ridge samples indicate that the mantle was already heterogeneous before rifting and probably represents a part of a sub-continental lithospheric mantle. To conclude, we can say that the Galicia mantle ridge is neither an asthenospheric diapir mantle emplaced during the rifting event, nor the mantellic part of the first North-Atlantic oceanic lithosphere, but likely is a piece of the lithospheric mantle that was beneath the American-Iberian continent before North-Atlantic opening. The Galicia mantle shows geochemical heterogeneities similar to what is known in many orogenic lherzolite massifs (Beni-Bousera, Ronda, Pyrenees, etc.) but these heterogeneities have probably been amplified during the rifting processes.

Middle Proterozoic–early Palaeozoic evolution of central Baltoscandian intracratonic basins: evidence for asthenospheric diapirs

The three intracratonic sedimentary basins located in central Baltoscandinavia, namely the Bothnian Gulf basin, the Bothnian Sea basin and the Baltic basin, developed in response to Middle Proterozoic and Late Proterozoic tectonic events, separated in time by about 800 Ma. Only the Baltic basin was subsequently affected by Caledonian orogenesis and Mesozoic rifting. Crustal extension was minor or did not take place during the Proterozoic basin evolution phases. However, according to the Moho topography, crustal thinning did take place. This was probably a result of subcrustal magmatism. On a craton-wide scale, the ages of granitoids, which intruded during the Middle Proterozoic basin formation, generally decrease from east to west. This fact, combined with the evidence provided by mantle-derived flood basalt magmatism, points to a moving asthenospheric diapir as the cause for basin development. Asthenospheric upwelling was probably also responsible for the second, Late Proterozoic, basin evolution phase, as evidenced by the lack of crustal thinning and extension, and the occurrence of tholeiitic intrusions. In addition, a Late Proterozoic thermally induced palaeo-high, located at about the position of the intracratonic basins, is compatible with indications from glaciations. As the ages of Late Proterozoic intracratonic basins also decrease from east to west across the craton, the location of asthenospheric diapirism during this time interval was also moving. For the Fennoscandian lithosphere, the presence of fundamental lithospheric weakness zones (e.g. terrane boundaries) might be an explanation for the formation of two generations of basins originating from asthenospheric upwelling at about the same location in the Fennoscandian Shield. The spacing and size of the Proterozoic intracratonic basins suggest that the asthenospheric diapirism was not deep seated. Therefore, sublithospheric convective processes might be the cause for the asthenospheric upwellings. Such processes are related to Rayleigh–Taylor instabilities in the sublithospheric mantle. Emplacement of an asthenospheric diapir causes a thermal bulge at the surface of the lithosphere. Modelling results demonstrate that erosion of the surficial high, succeeded by cooling of the lithosphere, can explain the accumulation of early Palaeozoic sediments in the Bothnian Sea basin, taking into account post-Ordovician vertical and lateral erosion of the basin fill.

Lithospheric controls on late Cenozoic construction of the Mongolian Altai

The Altai is one of the great Cenozoic intracontinental and intraplate mountain ranges of central Asia, but it is one of the less studied mountain belts on Earth from a modern structural geology standpoint, and few western scientists are familiar with its tectonic evolution. The range is located dominantly in Mongolia with important sectors in Russia, China, and Kazakhstan and structurally links with the Chinese Tien Shan and Russian Sayan ranges. The Altai is tectonically active and is best understood as two kinematically distinct mountain belts that intersect at 46°N, 96°E: the right‐lateral transpressive western Altai and the left‐lateral transpressive Gobi Altai. Transpressional deformation dominates the late Cenozoic deformation of the Altai and is manifested by throughgoing strike‐slip faults, restraining bends, thrust fault‐bounded ranges linked by strike‐slip faults, and possibly inverted Mesozoic graben. Regionally, there is strong correlation between Cenozoic fault trends and older basement strike trends. Cenozoic deformation regimes appear to be dictated by the angular relationship between preexisting basement structural trends and the prevailing NE directed maximum compressive stress. The Altai is constructed on Paleozoic terranes dominantly consisting of arc and subduction complex assemblages and passive margin sedimentary rocks juxtaposed to the west, southwest and south of the Precambrian block that underlies the Hangay Dome area of central Mongolia. Because the Hangay Dome is characterized by diffuse extension and not transpression, regionally upwarped topography, late Cenozoic volcanism, and elevated heat flow, it is kinematically separate from the Altai region and has previously been interpreted to overlie a mantle plume or asthenospheric diapir. It is proposed that anomalously hot mantle beneath the Hangay Dome coupled with the regionally domed surface topography creates a radial horizontal stress field that acts against the regional NE directed maximum horizontal stress within the Altai. These conditions may deflect postulated NE flowing lithospheric mantle/lower crust that is believed to be driving the shallow upper crustal deformation in the Altai.

Fluid circulation related to deformation in the Zabargad gneisses (Red Sea rift)

A fluid inclusion study in relation to textures has been performed on Pan-African granulite-facies gneisses from Zabargad Island (Red Sea rift, Egypt). The gneiss samples were collected at a maximum distance of 60 m above the contact with mantle peridotites with which they are co-structured by a high- T plastic deformation during an isothermal decompression starting from conditions estimated at 35 km depth and 850°C (base of the Pan-African crust). A first metasomatism of the crustal section during decompression is represented by CO 2-fluid inclusions trapped at minimum conditions of 15 km depth and 800°C. The CO 2-fluid inclusion planes are controlled in orientation by the high- T plastic flow structures, but have been subsequently deformed during medium- T plastic deformation in quartz. The second metasomatism is represented by solid-rich aqueous fluid inclusions which were trapped at minimum P-T conditions of 3.7 km depth and 450°C corresponding to a very high thermal gradient. They trace the introduction either of seawater or of fluids resulting from leaching of the Middle Miocene evaporites, within the gneisses during their residence at a shallow depth below the seafloor. Two models are proposed in the literature in order to reconstruct the geodynamic history of Zabargad Island: one [Brueckner, H.K., Elhaddad, M.A., Hamelin, B., Hemming, S., Kröner, A., Reisberg, L., Seyler, M., 1995, A Pan African origin and uplift for the gneisses and peridotites of Zabargad Island, Red Sea: a Nd, Sr, Pb and Os isotope study, J. Geophys. Res. 100, 22283–22297] suggests that most of the decompression P-T path is Pan-African in age and only the aqueous metasomatism is related to the rifting history of the Red Sea; the second [Nicolas, A., Boudier, F., Montigny, R., 1987, Structure of Zabargad Island and early rifting of the Red Sea, J. Geophys. Res. 92, 461–474; Boudier, F., Nicolas, A., Ji, S., Kienast, J.R., Mevel, C., 1988, The gneiss of Zabargad Island: deep crust of a rift, Tectonophysics 150, 209–227] supposes the existence of an asthenospheric diapir intrusive through the deep Pan-African crust leading to a continuous deformation event during early rifting of the Red Sea. Although it is impossible to date trapping of fluid inclusions and to know with certainty the origin of the fluids, our preference is for the second model but slightly modified. The high- T decompression path traced by CO 2-fluid inclusions, relayed by solid-rich aqueous inclusions, traces a continuous process of crustal thinning. Such a continuous path would assign to a single event the thinning and uplift of the deep crust and uppermost lithosphere to shallow basin bottom during the Red Sea early rifting. In such an interpretative model, the northern and central peridotite bodies are representative of the uppermost continental lithosphere and only the southern body could belong to an ascending asthenospheric diapir centred on the southeast of Zabargad Island.

Cenozoic postrift domal uplift of North Atlantic margins: An asthenospheric diapirism model

Research Article| October 01, 1996 Cenozoic postrift domal uplift of North Atlantic margins: An asthenospheric diapirism model Max Rohrman; Max Rohrman 1Faculty of Earth Sciences, Vrije Universiteit, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands Search for other works by this author on: GSW Google Scholar Peter van der Beek Peter van der Beek 1Faculty of Earth Sciences, Vrije Universiteit, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands Search for other works by this author on: GSW Google Scholar Author and Article Information Max Rohrman 1Faculty of Earth Sciences, Vrije Universiteit, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands Peter van der Beek 1Faculty of Earth Sciences, Vrije Universiteit, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1996) 24 (10): 901–904. https://doi.org/10.1130/0091-7613(1996)024<0901:CPDUON>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Max Rohrman, Peter van der Beek; Cenozoic postrift domal uplift of North Atlantic margins: An asthenospheric diapirism model. Geology 1996;; 24 (10): 901–904. doi: https://doi.org/10.1130/0091-7613(1996)024<0901:CPDUON>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Postrift domal uplifts are a distinct feature of the northern North Atlantic rifted margins. Uplifted regions (southern Norway, Finnmark, Svalbard, northern United Kingdom) occur with a typical spacing of ∼ 900 km. The amount of domal uplift is substantial (1–2 km); thermochronological, geomorphological, and seismic stratigraphic data suggest a coincident Neogene onset of uplift, postdating North Atlantic breakup and predating onset of glaciation. The Norwegian domes are characterized by strongly negative gravity anomalies and reduced mantle P-wave and S-wave velocities. Previous models for uplift around the North Atlantic (e.g., glacial erosion, intraplate stresses, secondary convection) fail to explain the above characteristics. We present an asthenospheric diapir model based on a Rayleigh-Taylor instability, wherein a hot “Icelandic” asthenosphere layer meets cold cratonic lithosphere. The generated asthenospheric diapirs and associated partial melting offer a viable explanation for updoming. Application of this model to the south Norwegian dome succesfully explains most of the currently available data. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Alkali basalts and leucitites in an extensional intracontinental plate setting: The late Cenozoic Calatrava Volcanic Province (central Spain)

The Calatrava Volcanic Province (CVP) of central Spain is characterised by a intracontinental plate magmatic association of leucitites, melilitites, nephelinites and olivine basalts extruded during the late Miocene to Quaternary. Most of the rocks represent relatively primitive magmas and less than 45% have experienced small degrees ( < 25%) of crystal fractionation. Melilitites, nephelinites and olivine basalts form a suite generated by variable degrees of partial melting ( F = 5–17%) of a nearly homogeneous enriched (up to x18 chondritic values for the highly incompatible elements and × 3 for the moderately incompatible) mantle source, composed of Ol+Opx+Cpx+Gt+Phl. The leucitites appear to be derived by low degrees of partial melting ( ~ 4%) from a different mantle source, characterized by higher Rb, Ba, K and 87Sr enrichment and the presence of residual apatite. Phlogopite contributed to the leucititic liquids in a higher proportion than in the basaltic suite whereas clinopyroxene participated in lower amounts. The geochemical characteristics of the primary liquids suggest a contribution from two source components: (1) a subcontinental lithospheric component which appears to be a strongly enriched garnet-lherzolite with phlogopite ± apatite and (2) an asthenospheric diapir component with characteristics similar to the HIMU reservoir. These conclusions combined with geophysical observations indicate that the CVP magmatism could develop in two stages. In a first stage, a HIMU-like mantle diapir would trigger magma generation in the overlying subcontinental lithosphere by melting of pervasive enriched streaks or veins with phlogopite + apatite, giving rise to small amounts of leucititic liquids and to initial extension of the upper crust. In subsequent steps the extension developed, the lithosphere becomes stripped of its enriched components and the asthenospheric diapir starts to melt giving rise to the basaltic liquids which are virtually OIB-like magmas. This model is also supported by the available K-Ar radiometric ages as the leucitites were extruded first (~ 7.6 Ma), followed by the basaltic suite ( 4.6-1.75 Ma) This volcanic region is linked with the rift system which developed from late Miocene to Quaternary times in western/central Europe and shows strong geochemical similarities with this volcanism, in which both lithospheric and asthenospheric components have been detected.

The Ronda peridotites (Los Reales nappe): An example of the relationship between lithospheric thickening by oblique tectonics and late extensional deformation within the Betic Cordillera (Spain)

The Internal (or metamorphic) Zone of the Betic Cordillera (southern Spain) is a 400 km long and ENE-elongated tectonic domain characterized by a widespread record of alternating compressional and extensional processes. In the westernmost part of the Internal Zone several orogenic Iherzolite massifs—the Ronda peridotites—were also involved in such processes. The Ronda peridotites form the lower portion of the Los Reales nappe and are overlain by a metamorphic sequence which varies from granulite- to greenschist-facies conditions. The tectono-metamorphic history of the Los Reales nappe involves two main synmetamorphic steps. A first extensional event at lithospheric scale is preserved within the attenuated continental crust which overlies the Ronda peridotites. It was related to the upwelling of an asthenospheric diapir below the thinned lithosphere, and led to the development of low-pressure-high-temperature metamorphism and of ductile shear zones with N110°E-trending stretching lineations. A subsequent compressional event is represented by the hot thrusting of the Ronda peridotites from the west-southwest to the east-northeast. Finally, late extensional deformations in a dominant N-S direction developed in response to the lithospheric thickening achieved during the former compressional event. The kinematics of the synmetamorphic deformations can be integrated into a model of sinistrally oblique tectonics compatible with the overall strike-slip motion of the African and Iberian plates from Jurassic to Tertiary times, whereas the later deformations should reflect the convergence between Africa and Iberia from Tertiary time onwards.