Journal articles on the topic 'GPS; Continental lithosphere'

The northern Canadian Cordillera is remarkably tectonically and seismically active, extending from a terrane collision zone on the continental margin to an active fold and thrust belt at the eastern mountain front. The source and distribution of the deformation are constrained by (i) precision global positioning system (GPS) measurements; (ii) the seismicity distribution, mechanisms, and rates; (iii) the thermal regime; (iv) estimates of lithosphere thickness and strength; and (v) topography and gravity. The ongoing oblique collision of the Yakutat block in the northeast corner of the Gulf of Alaska has produced large deformation and uplift in the adjacent Saint Elias and Chugach mountains and appears to be responsible for the current deformation 800 km to the northeast. Northern Cordillera GPS velocities are ∼5 mm/year northeast relative to the North American Craton. Deformation rates across the eastern mountain front from earthquake statistics are similar, i.e., ∼4 mm/year of thrust shortening across the Mackenzie Mountains and right-lateral strike-slip in the Richardson Mountains. This large-scale motion is explained by a quasi-rigid displacement of the upper crust over a lower crust detachment. The detachment zone is a consequence of the high temperature of the northern Cordillera lithosphere and a weak eastern Cordillera deformation front. Regional Moho temperatures of 800–950 °C are indicated by very high heat flow and other indicators of deep temperature and by the thin lithosphere effective thickness (Te). The northern Cordillera model may have application in other areas, such as the earlier thrusting in the southern Canadian Rocky Mountains driven by terrane collision along the Pacific margin.

Abstract This article describes the regional effects of Cenozoic subduction along the outboard margin of the Northern Cordillera (Alaska, USA, and Western Canada), and thereby acquaints the reader with several chapters of the e-book Dynamic Geology of the Northern Cordillera (Alaska, Western Canada, and Adjacent Marine Areas). This article and the e-book are written for earth-science students and teachers. The level of writing for the article and the source e-book is that of popular science magazines, and readers are encouraged to share this article with students and laypersons. The main thrust of the article is to present and describe a suite of ten regional topographic, bathymetric, and geologic maps, and two figures portraying deep-crustal sections that illustrate the regional effects of Cenozoic subduction along the outboard margin of the North American Cordillera. The regional maps and cross sections are described in a way that a teacher might describe a map to students. Cenozoic subduction along the margin of the Northern Cordillera resulted in the formation of the following: (1) underthrusting of terranes and oceanic lithosphere beneath Southern Alaska; (2) landscapes, including narrow continental shelves along Southern and Southeastern Alaska and Western Canada (the Canadian Cordillera) and continental-margin mountain ranges, including the Alaska Peninsula, Chugach Range, Saint Elias Mountains, and Cascade Mountains; (3) sedimentary basins; (4) an array of active continental strike-slip and thrust faults (inboard of subduction zones); (5) earthquake belts related to subduction of terranes and oceanic plates; (6) active volcanoes, including continental-margin arcs (the Aleutian, Wrangell, and Cascade Arcs) linked to subduction zones, and interior volcanic belts related to strike-slip faulting or to hot spots; (7) lode and placer mineral deposits related to continental margin arcs or subduction of oceanic ridges; (8) hot springs related to continental-margin arcs; (9) plate movements as recorded from GPS measurements; and (10) underthrusting of terranes and oceanic lithosphere beneath the Northern Cordillera.

Abstract Crustal thickening and uplift of southern Tibet have been widely associated with India-Asia continental collision during the Cenozoic. However, recent studies indicated that the crust of the northwestern (NW) Lhasa Terrane was thickened during the late Mesozoic. Here we report geochronological and geochemical data for the Gaerqiong diorite porphyries (GPs) and Xiongma plutons (XPs) in the NW Lhasa terrane, southern Tibet. Zircon U-Pb dating suggests that these intrusive rocks were generated at ca. 85 and ca. 88 Ma, respectively. The GPs are characterized by high MgO, Cr, and Ni contents, and they have adakitic affinities. These geochemical features, combined with their depleted εNd(t) (+1.7 to +2.0), 87Sr/86Sr(i) (0.705103–0.705259), and zircon εHf(t) (+5.2 to +10.2) isotopic compositions, indicate that the GPs were produced by partial melting of the delaminated juvenile continental crust. In contrast, the XPs are composed of host granites and mafic microgranular enclaves (MMEs). The MMEs have low SiO2 and high MgO contents, and low εHf(t) (–14.0 to –5.8) values, indicating that their parental magmas were derived from an enriched mantle. The host granites have high SiO2 and low MgO contents, and variable εNd(t) (–7.4 to –6.3) and zircon εHf(t) (–11 to –4.1) values. These observations, combined with the presence of MMEs in the Xiongma granites, suggest that the host granites were the result of mixing of crust- and mantle-derived magmas. Detailed study of these two plutons, combined with the previous researches, suggests that Late Cretaceous (ca. 90 Ma) magmatism in the NW Lhasa Terrane occurred in a post-collisional extensional setting related to delamination of the regionally thickened lithosphere after collision of the Lhasa-Qiangtang Terranes. We propose that the crust of the NW Lhasa Terrane reached a maximum thickness (average of >50 km) before the Late Cretaceous (ca. 90 Ma). This crustal thickening was caused by underplating of mafic magmas during slab roll-back and break-off of the southward-subducting Bangong-Nujiang oceanic lithosphere and subsequent tectonic thrusting during Qiangtang-Lhasa Terrane collision, respectively. Given that crustal thickening generally results in elevated terrain, the regional uplift (driven by isostasy due to crustal thickening) probably commenced before the Late Cretaceous (ca. 90 Ma).

Regional maps of lithospheric deformation and thermal history have been derived for the eastern continental margin of Canada. Subsidence associated with the rifting and cooling stages of rifted margin formation was calculated from gridded maps of sediment thickness and bathymetry along the Labrador, Grand Banks, and Nova Scotian margins. A two-layer lithospheric extension model was used to compute the deformation and thermal evolution of each region. Deformation results show that the crust and lower lithosphere have generally stretched by different amounts, and that either crustal or subcrustal lithospheric stretching dominates beneath the various basins. Thermal modelling results for the older Nova Scotian and Grand Banks margins show a strong correlation between thermal gradient, crustal stretching, and sediment thickness, and the predicted thermal gradient pattern for the younger Labrador margin correlates extremely well with predicted stretching of the still-cooling subcrustal lithosphere. Predictions of sediment maturity (vitrinite reflectance) of basin deposits were obtained from the derived time – temperature histories. Model results have been constrained with observations from individual boreholes and extrapolated away from these well-constrained areas into regions beyond the frontiers of present exploration. Results are presented as maps showing depths to present-day peak thermal maturity zones and the ages at which earliest post-rift sediments reached peak maturity levels. This reconnaissance approach has led to predictions of thermal maturity zones suitable for oil or gas generation in western Orphan Basin and beneath the continental slopes.

SUMMARY The volcanism of the Eifel volcanic field (EVF), in west-central Germany, is often considered an example of hotspot volcanism given its geochemical signature and the putative mantle plume imaged underneath. EVF's setting in a stable continental area provides a rare natural laboratory to image surface deformation and test the hypothesis of there being a thermally buoyant plume. Here we use Global Positioning System (GPS) data to robustly image vertical land motion (VLM) and horizontal strain rates over most of intraplate Europe. We find a spatially coherent positive VLM anomaly over an area much larger than the EVF and with a maximum uplift of ∼1 mm yr−1 at the EVF (when corrected for glacial isostatic adjustment). This rate is considerably higher than averaged over the Late-Quaternary. Over the same area that uplifts, we find significant horizontal extension surrounded by a radial pattern of shortening, a superposition that strongly suggests a common dynamic cause. Besides the Eifel, no other area in NW Europe shows significant positive VLM coupled with extensional strain rates, except for the much broader region of glacial isostatic adjustment. We refer to this 3-D deformation anomaly as the Eifel Anomaly. We also find an extensional strain rate anomaly near the Massif Central volcanic field surrounded by radial shortening, but we do not detect a significant positive VLM signal there. The fact that the Eifel Anomaly is located above the Eifel plume suggests that the plume causes the anomaly. Indeed, we show that buoyancy forces induced by the plume at the bottom of the lithosphere can explain this remarkable surface deformation. Plume-induced deformation can also explain the relatively high rate of regional seismicity, particularly along the Lower Rhine Embayment.

The investigation of intracontinental collision structures is conducted based on the complex model of the thermal and mechanical evolution of overthrusting process for the rheologically layered lithosphere, which includes brittle upper crust, the lower crust and lithospheric upper mantle with different effective viscosity values. Finite element models with Lagrangian approach were used for the problem simulation. It was shown that thermal evolution of continental orogens essentially results from the geometry and topography due to thrusting and postcollision stage. This work concentrates on the thermal parameters influence on the evolution of collision zones aimed to the study of possibility of granite melt formation. Calculations for mean continental initial temperature distribution lead to the conclusion of possibility of granite melt formation for the case of “wet” granite solidus. The horizon of temperatures higher than “wet” granite solidus appears at the level of 30-40 km, moving upward to the depth 15-20 km at postcollision stage. The early postcollision evolution shows some heat flow increase due to the thickening of the upper crust with maximum heat generation rate. Further history leads to the stable heat flow values because additional loading redistribution resulting from the denudation of surface uplift and corresponding sedimentation is small due to the local erosion in our model. It was shown that surface heat losses after the termination of horizontal shortening depend to a greater extent on radiogenic heat generation rather than thermal conductivity value in the upper crust.

Abstract. The lateral variation of the stress field in the southern Aegean plate and the subducting Hellenic slab is determined from recordings of seismicity obtained with the CYCNET and EGELADOS networks in the years from 2002 to 2007. First motions from 7000 well-located microearthquakes were analysed to produce 540 well-constrained focal mechanisms. They were complemented by another 140 derived by waveform matching of records from larger events. Most of these earthquakes fall into 16 distinct spatial clusters distributed over the southern Aegean region. For each cluster, a stress inversion could be carried out yielding consistent estimates of the stress field and its spatial variation. At crustal levels, the stress field is generally dominated by a steeply dipping compressional principal stress direction except in places where coupling of the subducting slab and overlying plate come into play. Tensional principal stresses are generally subhorizontal. Just behind the forearc, the crust is under arc-parallel tension whereas in the volcanic areas around Kos, Columbo and Astypalea tensional and intermediate stresses are nearly degenerate. Further west and north, in the Santorini–Amorgos graben and in the area of the islands of Mykonos, Andros and Tinos, tensional stresses are significant and point around the NW–SE direction. Very similar stress fields are observed in western Turkey with the tensional axis rotated to NNE–SSW. Intermediate-depth earthquakes below 100 km in the Nisyros region indicate that the Hellenic slab experiences slab-parallel tension at these depths. The direction of tension is close to east–west and thus deviates from the local NW-oriented slab dip presumably owing to the segmentation of the slab. Beneath the Cretan sea, at shallower levels, the slab is under NW–SE compression. Tensional principal stresses in the crust exhibit very good alignment with extensional strain rate principal axes derived from GPS velocities except in volcanic areas, where both appear to be unrelated, and in the forearc where compressional principal stresses are very well aligned with compressional principal strain rates. This finding indicates that, except for volcanic areas, microseismic activity in the southern Aegean is not controlled by small-scale local stresses but rather reflects the regional stress field. The lateral and depth variations of the stress field reflect the various agents that influence tectonics in the Aegean: subduction of the Hellenic slab, incipient collision with continental African lithosphere, roll back of the slab in the southeast, segmentation of the slab, arc volcanism and extension of the Aegean crust.

In the South Okhotsk Sea province – on the islands of Sakhalin, Kunashir, Iturup, Urup and surrounding sea areas – many occurrences of rare, noble metal and other mineralizations as well as of oil-and-gas fields, gas hydrate accumulations, and isolated areas of active emission of water-hydrocarbon gases are known. Occurrences and deposits of solid, liquid and gaseous mineral resources are controlled by hidden deep fault transform zones: Nosappu (Tuscarora), Iturup, and Urup. These long-lived extended (more than 1000 km) zones are distinguished at the N-W Pacific megaplate margin near the S-E flank of the Kuril-Kamchatka trogue. Using the seismotomographic methods we have established their extension to the west from the seismic focal zone in the oceanic slab that subducted into the transition zone of the mantle. In the areas of strike-slip extension the faults accounted for the active formation of the drainage channels for the penetration of the sea water in the lithosphere with the following serpentinization of its ultramafites, and for decompressional generation of ascending mantle-derived abiogenic fluid flows. The latter penetrated from the underslab asthenosphere in the oversubduction mantle wedge and beneath the lithospheric mantle, where they accounted for the development of the processes of metasomatism. The subsequent migration of flows initiated the creation of primary magma reservoirs in the lower parts of the continental lithosphere, and intermediate and peripheral chambers in the Earth’s crust. The injection of melts from the chambers in the consolidated Earth's crust led to the formation of abyssal, hypabyssal intrusive massifs, arch-dome uplifts and magmatogenic-ore (ore-magmatic) systems predominantly among the rocks of the pre-Pliocene basement. The concentration of oil and gas accumulations mainly from the mantle-derived abiogenic hydrocarbons containing mercury, gold, rhenium, and PGE in the Cenozoic sedimentary basins amidst the reservoirs under the impermeable beds also resulted from deep under- and overslab fluid flows.

Stress and strain distributions in the Yakutsk-Vilyui large igneous province (LIP) are numerically simulated under geotectonic extension. A two-dimensional model of the geological structure of a part of the Yakutsk-Vilyui LIP is developed using the geophysical data from the profile “Craton-1980”. However, these geophysical data can only be a source of the geometrical model and elastic properties of Earth’s layers. To describe non-elastic strains during the geological process, the Drucker-Prager-Nikolaevsky model of plasticity is adopted. For elastoplastic analysis of the geotectonic process, the “Jelly Sandwich” shear strength model for the continental lithosphere is used, which is based on the variation of the strength properties with depth. Zones of shear stress concentration and plastic strain localization are observed as a result of the extension in the Lindenskaya basin and Khapchagaiskaya reclamation complying with oil and gas deposit locations in the Yakutia region. Stress components have non-linear distributions determined by the dependence of strength properties on the depth and structural inhomogeneity of continental lithosphere. The pressure distribution obtained in the simulation can partially complement the geological information employed when analyzing the possibility of phase transitions in the rocks in different locations of the studied region.

The techniques are referred to as analyzing the relationship between the events of the lithosphere getting thermal and the changes of weakened release energies of strong earthquakes that might be related with the ones of increased exploitation quantities of the global three-large fossil fuels of coal,oil and gas ,and the relationship between the events of earth crust expansion getting thermal with the accumulated increasing of the land crust expansion thickness from calculation and the accumulated increasing of fossil fuels being exploited,et al. The three mechanism-modes of earth's epidermis warming over the past 100 years since 1890 from earth interior changes with relation to fossil fuels being exploited were suggested that the weakened release energies of global strong earthquakes caused by the increasing of earth thermal stress energies and overall warming of lithosphere, the “weightlessness” and expansion and getting thermal of continental crust,the increasing of earth currents and heat quantities generated by added origid geological and tectonic activities in mining areas and earth's surface getting thermal.

AbstractGalena from four REE-rich (Khibina, Sallanlatvi, Seblyavr, Vuoriyarvi) and REE-poor (Kovdor) carbonatites, as well as hydrothermal veins (Khibina) all from the Devonian Kola Alkaline Province of northwestern Russia was analysed for trace elements and Pb and S isotope compositions. Microprobe analyses show that the only detectable elements in galena are Bi and Ag and these vary from not detectable to 2.23 and not detectable to 0.43 wt.% respectively. Three distinct galena groups can be recognized using Bi and Ag contents, which differ from groupings based on Pb isotope data. The Pb isotope ratios show significant spread with 206Pb/204Pb ratios (16.79 to 18.99), 207Pb/204Pb (15.22 to 15.58) and 208Pb/204Pb ratios (36.75 to 38.62). A near-linear array in a 207Pb/204Pb vs.206Pb/204Pb ratio diagram is consistent with mixing between distinct mantle sources, one of which formed during a major differentiation event in the late Archaean or earlier. The S isotopic composition (δ34S) of galena from carbonatites is significantly lighter (–6.7 to –10.3% Canyon Diablo Troilite (CDT) from REE-rich Khibina, Seblyavr and Vuoriyarvi carbonatites, and – 3.2% CDT from REE-poor Kovdor carbonatites) than the mantle value of 0%. Although there is no correlation between S and any of the Pb isotope ratios, Bi and Ag abundances correlate negatively with δ34S values. The variations in the isotopic composition of Pb are attributed to partial melting of an isotopically heterogeneous mantle source, while those of δ34S (together with Bi and Ag abundances) are considered to be process driven. Although variation in Pb isotope values between complexes might reflect different degrees of interaction between carbonatitic melts and continental crust or metasomatized lithosphere, the published noble gas and C, O, Sr, Nd and Hf isotopic data suggest that the variable Pb isotope ratios are best attributed to isotopic differences preserved within a sub-lithospheric mantle source. Different Pb isotopic compositions of galena from the same complex are consistent with a model of magma replenishment by carbonatitic melts/fluids each marked by quite different Pb isotopic compositions.

Abstract. The goal of the paper was to verify triggering of earthquakes by the length of day variations, i.e. the sidereal 13.66 days Earth's rotation variations, in contrast with tidal biweekly 14.76 days variations (full and new Moon), which for hundred years of investigation give negative results. Earthquake triggering governed by sidereal variations caused by variable Moon's declination accelerates and decelerates the Earth's rotation. Profound Schuster's test proved that earthquakes are triggered both in Earth's deceleration and acceleration. For this investigation the most prominent earthquakes from 2010–2011 were used from Mid-Atlantic Ridge, Southeast Indian Ridge, Sumatra and Andaman Sea, Chile trench, Haiti and Honshu region including important older earthquakes of Sumatra 26 December 2004 and Denali Fault 3 November 2002. Dominant number of earthquake occurring in extremes of length of day variations initiated the calculation of forces acting in these time intervals. Calculated forces of tidal force acting on Earth's flattening and the westward drift are strong enough to trigger earthquakes and the movement of plates follows from GPS performed immediately after earthquakes on continents and from increased number of earthquakes of the side of the mid-ocean ridge belonging to the moving plate. Generally the Northern Hemisphere moves quicker westward than the Southern one. Earthquakes are repeated in 19 yr Metonic cycle. Repetitions caused by tidal force acting on Earth's fattening are exact in date. Repetitions caused by westward drift are delayed for several months.

In the South of Kamchatka, modern geodynamic processes are actively taking place. A deep geological and geophysical model of the structure of the Earth’s crust and upper mantle along the regional profile of the Apacha Village-Mutnaya Bay in the zone of Tolmachevsky active magmatic center is presented. The profile passes near the South-Western border of the Karymshinskaya volcano-tectonic structure (VTS) and crosses the Ahomtenskaya VTS. The model created on the basis of integrated interpretation of materials of the earthquake converted-wave method (ECWM), gravity and magnetotelluric sounding (MTS). The thickness of the Earth’s crust along the profile varies from 30-33 km at the edges reaching 44-46 km, in its central part. The dominant feature of the model is a high-density formation – a block of the Earth’s crust, saturated with intrusions of the main and ultrabasic composition. The formation of the block is associated with a permeable zone between the crust and the upper mantle. In the block correlation of seismic boundaries is disturbed and in a density model the area with massive heterogeneity is allocated. A significant increase in depth to the M-Boundary in the center of the model is explained by the presence of a “bloated” transition layer between bark and mantle in this place. The thickness of the layer is about 10 km, and the density of the mantle reaches 3.4 g/cm3. It is assumed that this is a site of eklogization of breeds in a zone of paleosubduction of oceanic lithosphere under a continental. The area is favorable for the accumulation of meteor waters, which are in contact with high-temperature environment and postmagmatic solutions of intrusions, which leads to the formation of hydrothermal systems. The genetic connection of Karymshinsky gold-ore cluster with the intrusive array of medium-sour composition, allocated in the zone of the Tolmachevsky active Magmatic Center is shown.

Ultra-high-pressure (UHP) eclogites and ultramafites and associated fluid inclusions from the Western Gneiss Region, Norwegian Caledonides, have been analysed for F, Cl, Br and I using electron-probe micro-analysis, time-of-flight secondary ion mass spectrometry and neutron-irradiated noble gas mass spectrometry. Textures of multi-phase and fluid inclusions in the cores of silicate grains indicate formation during growth of the host crystal at UHP. Halogens are predominantly hosted by fluid inclusions with a minor component from mineral inclusions such as biotite, phengite, amphibole and apatite. The reconstructed fluid composition contains between 11.3 and 12.1 wt% Cl, 870 and 8900 ppm Br and 6 and 169 ppm I. F/Cl ratios indicate efficient fractionation of F from Cl by hydrous mineral crystallisation. Heavy halogen ratios are higher than modern seawater by up to two orders of magnitude for Br/Cl and up to three orders of magnitude for I/Cl. No correlation exists between Cl and Br or I, while Br and I show good correlation, suggesting that Cl behaved differently to Br and I during subduction. Evolution to higher Br/Cl ratios is similar to trends defined by eclogitic hydration reactions and seawater evaporation, indicating preferential removal of Cl from the fluid during UHP metamorphism. This study, by analogy, offers a field model for an alternative source (continental crust) and mechanism (metasomatism by partial melts or supercritical fluids) by which halogens may be transferred to and stored in the sub-continental lithospheric mantle during transient subduction of a continental margin.

Within the framework of the basic model of migrating seismicity, the main geoinformation criteria for identifying quasi-linear chains of earthquakes in epicentral fields of complex geometry and variable density of shock distribution have been determined by numerical methods. The developed model is used to study the migrations of earthquake foci and identify zones of hidden seismically active faults: it provides statistical criteria for the presence of quasi-linear chains of seismicity migration and zones of hidden seismic faults in arrays of various volumes of seismological data as an excess at a given level of significance of the average numbers of chains of earthquakes, spatially temporal distribution of shocks. The established dependences of the average number of selected chains of random events on the sample size and site shape make it possible to reveal the presence of seismicity migration and zones of hidden faults under the given criteria of “significance”, “representativeness” and ISMA. The developed methods, implemented programs and patents lay the theoretical and practical basis for GIS technology for identifying hidden faults and studying the migrations of earthquake foci in the lithosphere of the Baikal rift system. Since the main laws governing the formation of the internal structure of continental fault zones and seismicity are determined by the fundamental properties of the progressive deformation of transgressive shearing realized within them and are uniform, the developed GIS technologies can be applied in other seismically active regions.

Abstract. The Himalayas are the archetype of continental collision, where a number of long-standing fundamental problems persist in the Greater Himalayan Sequence (GHS): (1) contemporaneous reverse and normal faulting, (2) inversion of metamorphic grade, (3) origin of high- (HP) and ultrahigh-pressure (UHP) rocks, (4) mode of ductile extrusion and exhumation of HP and UHP rocks close to the GHS hanging wall, (5) flow kinematics in the subduction channel, and (6) tectonic overpressure, here defined as TOP = P∕PL where P is total (dynamic) pressure and PL is lithostatic pressure. In this study we couple Himalayan geodynamics to numerical simulations to show how one single model, upward-tapering channel (UTC) flow, can be used to find a unified explanation for the evidence. The UTC simulates a flat-ramp geometry of the main underthrust faults, as proposed for many sections across the Himalayan continental subduction. Based on the current knowledge of the Himalayan subduction channel geometry and geological/geophysical data, the simulations predict that a UTC can be responsible for high TOP ( > 2). TOP increases exponentially with a decrease in UTC mouth width, and with an increase in underthrusting velocity and channel viscosity. The highest overpressure occurs at depths < −60 km, which, combined with the flow configuration in the UTC, forces HP and UHP rocks to exhume along the channel's hanging wall, as in the Himalayas. By matching the computed velocities and pressures with geological data, we constrain the GHS viscosity to be ≤ 1021 Pa s, and the effective convergence (transpression) to a value ≤ 10 %. Variations in channel dip over time may promote or inhibit exhumation (> or < 15°, respectively). Viscous deformable walls do not affect overpressure significantly enough for a viscosity contrast (viscosity walls to viscosity channel) of the order of 1000 or 100. TOP in a UTC, however, is only possible if the condition at the bottom boundary is no-outlet pressure; otherwise it behaves as a leaking boundary that cannot retain dynamic pressure. However, the cold, thick, and strong lithospheres forming the Indian and Eurasian plates are a good argument against a leaking bottom boundary in a flat-ramp geometry, and therefore it is possible for overpressure to reach high values in the GHS.

Geophysical model inputs were the results of a survey on an anomalous magnetic field and a gravitational field of the Black Sea’s north-western shelf. Thegeophysical profiles of the complex effective parameter (CEP) are calculated and graphed. Complex effective parameter characterizes the relationship between the effective densities and the magnetization by their spatial distribution. Effective parameters (magnetization, density, CEP) were calculated within the studyarea with their distribution on the optimum depth. The profiles are meridional and parallel to each other, direction of the profiles from south to north. The distance between the profiles is 50 kilometers. The generalized deep structure of the study area was elucidated using the graphed profiles. The distribution of CEP on vertical sections within the shelf zone of the western Black Sea basin emphasizes the position in the space of tectonic elements. That is gives an idea about the nature and structure of the region’s lithosphere and their relationship with the spatial distribution of deposits and manifestations of hydrocarbons. Structural and geological interpretation of the CEP profile data was performed. According to the spatial consistency of the correlation by structures, the profiles are conditionally divided into two groups, the western and the eastern. Structural differences in profiles are explained by the presence of the Odesa-Sinop fault zone between the groups. According to the results of profiles interpretation and works of previous researchers, paleogeodynamic processes were established. That significantly complicated the geological structure of the Black Sea’s north-western shelf. The interpretation of the CEP field distribution gives additional arguments in favour of the Earth crust evolution on the north-western shelf of the Black Sea in the conditions of a passive continental margin with short periods of reverse motions with obligatory subduction due to the activation of rifting, the nature of which is yet to be studied. According to the results of interpretation, the presence of the Earth’s crust destruction zone was established. With the help of spatial analysis, the spatial regularities of the gas seeping manifestations with the zone of destruction of the Earth’s crust of continental type and sites of rising of the mantle surface are established.

Hydrogen gas is seeping from the sedimentary basin of São Franciso, Brazil. The seepages of H2 are accompanied by helium, whose isotopes reveal a strong crustal signature. Geophysical data indicates that this intra-cratonic basin is characterized by (i) a relatively high geothermal gradient, (ii) deep faults delineating a horst and graben structure and affecting the entire sedimentary sequence, (iii) archean to paleoproterozoïc basements enriched in radiogenic elements and displaying mafic and ultramafic units, and (iv) a possible karstic reservoir located 400 m below the surface. The high geothermal gradient could be due to a thin lithosphere enriched in radiogenic elements, which can also contribute to a massive radiolysis process of water at depth, releasing a significant amount of H2. Alternatively, ultramafic rocks that may have generated H2 during their serpentinization are also documented in the basement. The seismic profiles show that the faults seen at the surface are deeply rooted in the basement, and can drain deep fluids to shallow depths in a short time scale. The carbonate reservoirs within the Bambuí group which forms the main part of the sedimentary layers, are crossed by the fault system and represent good candidates for temporary H2 accumulation zones. The formation by chemical dissolution of sinkholes located at 400 m depth might explain the presence of sub-circular depressions seen at the surface. These sinkholes might control the migration of gas from temporary storage reservoirs in the upper layer of the Bambuí formation to the surface. The fluxes of H2 escaping out of these structures, which have been recently documented, are discussed in light of the newly developed H2 production model in the Precambrian continental crust.

Abstract. Fréchet (sensitivity) kernels are an important tool in glacial isostatic adjustment (GIA) investigations to understand lithospheric thickness, mantle viscosity and ice-load model variations. These parameters influence the interpretation of geologic, geophysical and geodetic data, which contribute to our understanding of global change. Recently, sensitivity kernels have been extended to laterally heterogeneous Earth models using the finite-element formulation, which enabled detailed studies on the sensitivity of the different geodetic observations of GIA such as GPS and terrestrial and space gravimetry. In this study, we discuss global sensitivities of relative sea-level (RSL) data of the last 18 000 yr. This also includes indicative RSL-like data (e.g. lake levels) on the continents far off the coasts. We present detailed sensitivity maps for four parameters important in GIA investigations (ice-load history, lithospheric thickness, background viscosity, lateral viscosity variations) for up to 9 dedicated times. Assuming an accuracy of 2 m of RSL data of all ages, we highlight areas around the world where, if the environmental conditions allowed its deposition and survival until today, RSL data of at least this accuracy may help to quantify the GIA modelling parameters above. The sensitivity to ice-load history variations is the dominating pattern covering in times of 14 ka BP and older almost the whole world. Lithospheric thickness variations are mainly only possible to be determined in certain high-latitude areas around the large former and current ice sheets. Background viscosity as well as lateral viscosity variations can be traced at most coast and shelf areas around the world, especially when dated to be older than 10 ka BP. The latter three are almost everywhere overlapped by the ice-load history pattern. In general we find that the more recent the data are, the smaller is the area of possible RSL locations which could provide enough information on the four GIA modelling parameters. But, we also note that when the accuracy of RSL data can be improved, e.g. from 2 m to 1 m, these areas become larger allowing better inference of background viscosity and lateral heterogeneity. Although the patterns depend on the chosen models and error limit, our results are indicative enough to outline areas where one should look for helpful RSL data of a certain time period.

The subduction of an oceanic plate is studied as the motion of a high-viscosity Newtonian fluid. The subducting plate spreads along the 670-km depth boundary under the influence of oppositely directed horizontal forces. These forces are due to oppositely directed horizontal temperature gradients. We consider the flow structure and heat transfer in the layer that includes both the oceanic lithosphere and the crust and moves underneath a continent. The heat flow is estimated at the contact between the subducting plate and the surrounding mantle in the continental limb of the subduction zone. Our study results show that the crustal layer of the subducting plate can melt and a thermochemical plume can form at the 670-km boundary. Our model of a thermochemical plume in the subduction zone shows the following: (1) formation of a plume conduit in the crustal layer of the subducting plate; (2) formation of a primary magmatic chamber in the area wherein the melting rate equals the rate of subduction; (3) origination of a vertical plume conduit from the primary chamber melting through the continent; (4) plume eruption through the crustal layer to the surface, i.e. formation of a volcano. Our experiments are aimed to model the plume conduit melting in an inclined flat layer above a local heat source. The melt flow structure in the plume conduit is described. Laboratory modeling have revealed that the mechanisms of melt eruption from the plume conduit differ depending on whether a gas cushion is present or absent at the plume roof.

Plate reconstructions indicate that if the Yellowstone plume existed prior to 50 Ma, then it would have been overlain by oceanic lithosphere located to the west of the North American plate (NAP). In the context of models supporting long-lived easterly directed subduction of oceanic lithosphere beneath the NAP, the Yellowstone plume would have been progressively overridden by the NAP continental margin since that time, the effects of which should be apparent in the geological record. The role of this ‘ancestral’ Yellowstone plume and its related buoyant swell in influencing the Late Mesozoic–Cenozoic tectonic evolution of the southwestern United States is reviewed in the light of recent field, analytical and geophysical data, constraints provided by more refined paleogeographic constructions, and by insights derived from recent geodynamic modeling of the interaction of a plume and a subduction zone. Geodynamic models suggesting that the ascent of plumes is either stalled or destroyed at subduction zones have focused attention on the role of gaps or tears in the subducted slab that permit the flow of plume material from the lower to the upper plate during subduction. These models imply that the ascent of plumes may be significantly deflected as plume material migrates from the lower to the upper plate, so that the connection between the hot spot track calculated from plate reconstructions and the manifestations of plume activity in the upper plate may be far more diffuse compared to the more precise relationships in the oceanic domain. Other geodynamic models support the hypothesis that subduction of oceanic plateau material beneath the NAP correlates with the generation of a flat slab, which has long been held to have been a defining characteristic of the Laramide orogeny in the western United States, the dominant Late Mesozoic–Early Cenozoic orogenic episode affecting the NAP. Over the last 20 years, a growing body of evidence from a variety of approaches suggests that a plume existed between 70 and 50 Ma within the oceanic realm close to the NAP margin in a similar location and with similar vigour to the modern Yellowstone hot spot. If so, interaction of this plume with the margin would have been preceded by that of its buoyant swell and related oceanic plateau, a scenario which could have generated the flat slab subduction that characterizes the Laramide orogeny. Unless this plume was destroyed by subduction, it would have gone into an incubation period when it was overridden by the North American margin. During this incubation period, plume material could have migrated into the upper plate via slab windows or tears or around the lateral margins of the slab, in a manner consistent with recent laboratory models. The resulting magmatic activity may be located at considerable distance from the calculated hot spot track. The current distribution of plumes and their buoyant swells suggests that their interaction with subduction zones should be common in the geological record. If so, the Late Mesozoic–Cenozoic evolution of western North America may represent a relatively modern analogue for such processes.RÉSUMÉLes reconstitutions de plaques montrent que si le panache de Yellowstone avait existé avant 50 Ma, il aurait été recouvert par la lithosphère océanique située à l'ouest de la plaque nord-américaine (PNA). Dans le contexte de modèles de subduction de longue durée vers l’est de la lithosphère océanique sous la PNA, avec le temps, la marge continentale de la PNA aurait progressivement neutralisé le panache de Yellowstone, et on devrait en voir les effets dans le registre géologique. Le rôle de ce panache de Yellowstone « ancestral » et de son renflement de surface régional associé sur l'évolution tectonique du Sud-ouest des États-Unis au Mésozoïque–Cénozoïque tardif est reconsidéré ici à la lumière de données récentes, de terrain, analytiques et géophysiques, de contraintes découlant de constructions paléogéographiques affinées, et d’idées nouvelles découlant d’une modélisation géodynamique récente de l'interaction d'un panache et d'une zone de subduction. Les modèles géodynamiques suggérant que l'ascension des panaches soient bloquée ou détruite dans les zones de subduction ont attiré l'attention sur le rôle d’hiatus ou de déchirures dans la plaque subduite qui permettent le passage du matériau du panache de la plaque inférieure à la plaque supérieure pendant la subduction. Ces modèles impliquent que le flux ascendant des panaches peut être sensiblement dévié alors que le matériau du panache migre de la plaque inférieure à la plaque supérieure, de sorte que la connexion entre la trace du point chaud calculée à partir des reconstructions de la plaque et les manifestations de l'activité du panache dans la plaque supérieure peut être bien plus diffuse que sa contrepartie du domaine océanique. D'autres modèles géodynamiques appuient l'hypothèse selon laquelle la subduction du matériau de plateau océanique sous la PNA correspond à la génération d'une plaque plate, particularité qui a longtemps été considérée comme caractéristique déterminante de l'orogenèse de Laramide dans l'ouest des États-Unis, épisode orogénique dominante de la fin du Mésozoïque au début du Cénozoïque affectant la PAN. Au cours des 20 dernières années, un nombre croissant d'éléments de preuve provenant d'une variété d'approches suggèrent qu'un panache existait bien entre 70 et 50 Ma dans le domaine océanique près de la marge la PNA, en un endroit et avec une vigueur similaires au point chaud de Yellowstone moderne. Le cas échéant, l'interaction de ce panache avec la marge aurait été précédée de celle de son renflement de surface et du plateau océanique connexe, scénario qui aurait pu générer la subduction de la plaque plate qui caractérise l'orogenèse Laramide. À moins que ce panache n'ait été détruit par subduction, il serait entré dans une période d'incubation lorsqu’il a été recouvert par la marge nord-américaine. Au cours de cette période d'incubation, le matériau du panache aurait pu migrer dans la plaque supérieure par des fenêtres ou déchirures de la plaque ou autour des marges latérales de la plaque, conformément aux modèles récents de laboratoire. La trace de l'activité magmatique résultante pourrait se trouver alors à une distance considérable de la trace du point chaud calculée. La distribution actuelle des panaches et de leurs renflements de surface suggère que leur interaction avec les zones de subduction devrait être un phénomène courant dans le registre géologique. Si tel est le cas, l'évolution du Mésozoïque–Cénozoïque tardif de l'Amérique du Nord occidentale peut représenter un analogue relativement moderne pour de tels processus.

The present volume marks the completion of a large research project by the Geological Survey of Denmark and Greenland (GEUS), focused on the northern part of the Palaeoproterozoic Nagssugtoqidian orogen of central West Greenland, and carried out by a team of Danish and international participants. The project comprised geological mapping as well as structural, geochronological, geochemical and economic geological studies. This volume contains reports on both Archaean and Palaeoproterozoic geology as well as a study of neotectonic brittle structures. The field work was carried out in 2000-2003 in the region between Nordre Strømfjord and Jakobshavn Isfjord (see e.g. van Gool & Piazolo 2006, this volume, fig. 1). The project had two immediate purposes, namely to establish an overview of the mineral resource potential of supracrustal rocks in the region between 66° and 70°15'N, and produce four new geological sheets in the Survey's 1:100 000 map series. The first collection of papers about the Nagssugtoqidian orogen, published by the Geological Survey of Greenland (GGU, now part of GEUS), dates back to 1979 (Korstgård 1979). The investigations in this period were mainly based on field descriptions and structural analysis of coastal areas in the southern and central parts of the orogen, combined with limited petrographical, palaeomagnetic and geochronological studies; the results also comprised the first 1:100 000 geological map from within the Nagssugtoqidian orogen (Olesen 1984). The Proterozoic age of the orogen had been established, but it was believed that most, if not all of the quartzofeldspathic basement gneisses were of Archaean origin. Subsequent work in the Nagssugtoqidian orogen by GGU in the 1980s showed that besides Archaean orthogneisses and supracrustal rocks, the central part of the orogen also comprises the root zone of a Palaeoproterozoic magmatic arc and associated panels of Palaeoproterozoic volcanic and metasedimentary rocks (Kalsbeek et al. 1987). These results were confirmed during further investigations by the Danish Lithosphere Centre (DLC) in 1994-1999, and the plate-tectonic collisional history of the southern and central Nagssugtoqidian orogen was described in detail (van Gool et al. 2002). However, these studies added little to previous knowledge of the northern parts of the orogen in the Kangaatsiaq-Aasiaat-Qasigiannguit region, knowledge that was largely based on coastal reconnaissance by Henderson (1969) at the time when the entire orogen was still believed to consist of Archaean rocks. Another project preceding the present work was carried out by GGU in 1988-1991 immediately north of the Nagssugtoqidian orogen, in the southernmost part of the likewise Palaeoproterozoic Rinkian fold belt (Disko Bugt project, Kalsbeek 1999). It was shown that also the latter region comprises Palaeoproterozoic (meta)sedimentary rocks, and that most of the Archaean basement is strongly overprinted by Palaeoproterozoic structures that were formed during overall W- or NW-directed lateral tectonic transport. Although these structures might be related to similar structures in the Nagssugtoqidian orogen, the relationship between the Nagssugtoqidian orogen and the Rinkian fold belt remained speculative. The only previous economic geological study of regional extent in central West Greenland was an airborne reconnaissance study supplemented by local field work, which was carried out in the early 1960s by Kryolitselskabet Øresund A/S. This work resulted in the discovery of a massive sulphide deposit at Naternaq (Lersletten), which was studied again in some detail in 2001 by the Survey (Østergaard et al. 2002) but not reported on in the present volume. The present volume comprises 12 papers with topics ranging geochronologically from mid-Archaean to Palaeogene, and geographically from the southern Nagssugtoqidian foreland to the central part of the Rinkian fold belt. Many of the papers deal with the northern part of the Nagssugtoqidian orogen and are related to the recent field work in that region, while a few contributions are rooted in DLC- or other projects. The papers have been arranged in approximate chronological order and are grouped in terms of their main subjects. The two first papers, by Hollis et al. and Moyen & Watt, deal with Archaean supra- and infracrustal rocks in the northern Nagssugtoqidian orogen: their origin, ages, and structural and metamorphic evolution. These papers provide insight into the age and origin of the continental crustal orthogneisses and granites that underlie most of the region, and discuss the relationships between the supracrustal and plutonic components, using zircon U-Pb age determinations and major and trace element geochemical characteristics. Also the question of Palaeoproterozoic tectonic overprint is discussed, with the conclusion from both study areas that most of the observed structures are Archaean. The third paper with focus on Archaean geology, by Stendal et al., describes a small gold prospect at Attu likewise in the northern Nagssugtoqidian orogen, and discusses the age of the prospect and its host rocks using Pb-Pb geochronology of magnetite. It is concluded that the host rocks at Attu may be as old as 3162 ± 43 Ma, and that the gold prospect itself is around 2650 Ma in age. The fourth paper, by Mayborn & Lesher, is a thorough review of the Kangâmiut dyke swarm in the southern Nagssugtoqidian orogen and its foreland. It includes new whole-rock and mineral chemical data, and a list of sampling sites and corresponding field data. The emplacement mechanism and depth of the dyke swarm are discussed in detail, and it is concluded that the dykes were emplaced during the initial rifting prior to the Nagssugtoqidian collision and that they are unrelated to subduction processes (contrary to the belief by some previous authors). The next three papers provide geochronological constraints on the ages of supra- and infracrustal rocks and the deformation and metamorphism in the northern Nagssugtoqidian orogen, and on late orogenic uplift in the central Rinkian fold belt. In the first of these papers Thrane & Connelly employ zircon U-Pb age determinations (mainly using the laser ICP-MS method), and for the first time provide unequivocal documentation that the Naternaq supracrustal belt is of Palaeoproterozoic age. Other zircon age data from a synkinematic granite southeast of Kangaatsiaq show that the large fold structures in this region are of Archaean age. The subsequent paper by Stendal et al. presents Pb-Pb ages and isotopic signatures of magnetite in amphibolites; the obtained ages are younger than 1800 Ma and are related to cooling of the orogen. Stepwise leaching Pb-Pb ages of monazite and allanite in pegmatites fall in the range of 1750-1800 Ma, and are interpreted to date the emplacement of these rocks. The third paper in this group, by Sidgren et al., deals with new 40Ar/39Ar ages of around 1790 Ma (hornblende) and 1680 Ma (muscovite) from Archaean and Palaeoproterozoic rocks in the central Rinkian fold belt, which are interpreted as orogenic cooling ages. The hornblende ages are significantly older than such hornblende ages previously obtained from the central and northern Nagssugtoqidian orogen, pointing to different uplift histories in the two regions. This may in turn suggest that the Rinkian continental collision preceded that in the Nagssugtoqidian orogen. Four of the remaining five papers deal with the Nagssugtoqidian structural evolution. In the first of these, van Gool & Piazolo present a new method of structural analysis, where a geographical information system (GIS) is used as a framework for visualisation and analysis of large amounts of structural data. The paper graphically presents an overview of thousands of data points within an area of approximately 160 × 180 km in the central and northern parts of the Nagssugtoqidian orogen. This interesting data set points directly towards the two next papers, where crustal-scale structures in the same region and their origin are discussed: Sørensen et al. address the prominent Nordre Strømfjord shear zone just south of this block, and describes the structural and metasomatic transition into the shear zone by means of aeromagnetic and lithological map patterns and geochemical data. Another paper, by Mazur et al., addresses a prominent break in the structural pattern within the Kangaatsiaq-Aasiaat area, where the southern part acted as a rigid block during the Nagssugtoqidian orogeny and thus preserved its Archaean structure. The fourth paper in this group, by Korstgård et al., combines rock and aeromagnetic data to discuss the relationship between structure, metamorphic facies and total magnetic field intensity anomalies in the southern Nagssugtoqidian orogen. The authors show that static metamorphic boundaries are gradual, whereas boundaries along deformation zones are abrupt. The last paper, by Wilson et al., is a novel remote sensing and field geological analysis of onshore brittle structures related to the complex Ungava fault zone in the Davis Strait, which developed during the Cretaceous-Palaeogene opening of the Labrador Sea - Davis Strait - Baffin Bay seaway. The study area is located in the central Nagssugtoqidian orogen, and the authors carefully establish a distinction between old Nagssugtoqidian and younger structures in the basement rocks and identify five main sets of young lineaments. They conclude that the onshore fault patterns are predominantly of strike-slip nature, and that they reflect the stress fields that governed the opening of the seaway. Acknowledgements The editors are grateful to the 14 external reviewers, each of whom reviewed one or more of the individual papers, for their thorough and constructive work.

AbstractPrevious investigation of seismic anisotropy indicates the presence of a simple mantle flow regime beneath the Turkish-Anatolian Plateau and Arabian Plate. Numerical modeling suggests that this simple flow is a component of a large-scale global mantle flow associated with the African superplume, which plays a key role in the geodynamic framework of the Arabia-Eurasia continental collision zone. However, the extent and impact of the flow pattern farther east beneath the Iranian Plateau and Zagros remains unclear. While the relatively smoothly varying lithospheric thickness beneath the Anatolian Plateau and Arabian Plate allows progress of the simple mantle flow, the variable lithospheric thickness across the Iranian Plateau is expected to impose additional boundary conditions on the mantle flow field. In this study, for the first time, we use an unprecedented data set of seismic waveforms from a network of 245 seismic stations to examine the mantle flow pattern and lithospheric deformation over the entire region of the Iranian Plateau and Zagros by investigation of seismic anisotropy. We also examine the correlation between the pattern of seismic anisotropy, plate motion using GPS velocities and surface strain fields. Our study reveals a complex pattern of seismic anisotropy that implies a similarly complex mantle flow field. The pattern of seismic anisotropy suggests that the regional simple mantle flow beneath the Arabian Platform and eastern Turkey deflects as a circular flow around the thick Zagros lithosphere. This circular flow merges into a toroidal component beneath the NW Zagros that is likely an indicator of a lateral discontinuity in the lithosphere. Our examination also suggests that the main lithospheric deformation in the Zagros occurs as an axial shortening across the belt, whereas in the eastern Alborz and Kopeh-Dagh a belt-parallel horizontal lithospheric deformation plays a major role.

Geochemical data help to constrain the sizes of identifiable reservoirs within the framework of models of layered or whole-mantle circulation, and they identify the sources of the circulating heterogeneities as mainly crustal and/or lithospheric, but they do not decisively distinguish between different types of circulation. The mass balance between crust, depleted mantle and undepleted mantle based on 143 Nd/ 144 Nd, Nb/U and Ce/Pb, and the concentrations of very highly incompatible elements Ba, Rb, Th, U, and K, shows that ca. 25- 70% (by mass) of depleted mantle balances the trace element and isotopic abundances of the continental crust. This mass balance reflects the actual proportions of mantle reservoirs only if there are no additional unidentified reservoirs. Evidence on the nature and ages of different source reservoirs comes from the geochemical fingerprints of basalts extruded at mid-ocean ridges and oceanic islands. Consideration of Nd and He isotopes alone indicates that ocean island basalts (oibs) may be derived from a relatively undepleted portion of the mantle. This has in the past provided a geochemical rationale for a two-layer model consisting of an upper depleted and a lower undepleted (‘primitive’) mantle layer. However, Pb-isotopic ratios, and Nb/U and Ce/Pb concentration ratios demonstrate that most or all oib source reservoirs are definitely not primitive. Models consistent with this evidence postulate recycling of oceanic crust and lithosphere or subcontinental lithosphere. Recycling is a natural consequence of mantle convection. This cannot be said for some other models such as those requiring large-scale vertical metasomatism beneath oib source regions. Unlike other trace elements, Nb, Ta, and Pb discriminate sharply between continental and oceanic crust-forming processes. Because of this, the primitive mantle value of Nb/U = 30 (Ce/Pb = 9) has been fractionated into a continental crustal Nb/U = 12 (Ce/Pb = 4) and a residual-mantle (morb (mid-ocean ridge basalt) plus oib source) Nb/U = 47 (Ce/Pb = 25). These residual mantle values are uniform within about 20% and are not fractionated during formation of oceanic crust. By using these concentrations ratios as tracers, it can be shown that the possible contribution of recycled continental crust to oib sources is limited to a few percent. Therefore, recycling must be dominated by oceanic crust and lithosphere, or by subcontinental lithosphere. Oceanic crust normally bears a thin layer of pelagic sediment at the time of subduction, and this is consistent with oib sources that are dominated by subducted oceanic crust with variable but always small additions of continental material. Primordial 3 He, 36 Ar, and excess 129 Xe, in oceanic basalts demonstrate that the mantle has been neither completely outgassed nor homogenized, but they do not constrain the degree of mixing or the size of reservoirs. Also, helium does not correlate well with other isotopic data and may have migrated into the basalt source from other regions. The high 3 He/ 4 He ratios found in some oibs suggest that, even though the basalts are not derived from primordial mantle, their sources may be located close to a reservoir rich in primordial gases. This leads to models in which the oib sources are in a boundary layer within the mantle. The primordial helium migrates into this layer from below. The interpretation of the rare-gas data is still quite controversial. It is often argued that the upper mantle is a well-homogenized reservoir, but the data indicate heterogeneities on scales ranging from 10° to 10 6 m. The 206 Pb/ 204 Pb ratios in the oceanic m antle range from 17 to 21, which is similar to the range in most continental rocks. The degree of mixing cannot be directly inferred from these data unless the size and composition of the heterogeneities and the time of their introduction into the system are known. The relative uniformity of Nb/U and Ce/Pb ratios in the otherwise heterogeneous morb and oib sources indicates that this reservoir was indeed homogenized after the separation of the continental crust, and that the observed isotopic and chem ical heterogeneities were introduced subsequently. Overall, the results are consistent with, but do not prove, a layered mantle where the upper layer contains both morb and oib sources, and the lower, primitive mantle is not sampled by present-day volcanism. Alternative models such as those involving a chemically graded mantle have not been sufficiently explored.

The analysis of geomagnetic field variations is a useful tool to detect electrical conductivity contrasts within the Earth. Lateral resolution of outlined patterns depends on the array dimensions and density of measurement sites over the investigated area. The inspection depth is constrained by the period of geomagnetic variations considered in data processing. Regions with significant geological features such as boundaries of continental plates, marginal areas of contact between tectonic units or other geodynamical processes, are of primary interest for the application of the MagnetoVariational (MV) method. In the last ten years, in the frame of the ElectroMagnetic (EM) sounding techniques in applied geophysics, this method has been applied in Italy by researchers of the Istituto Nazionale di Geofisica, Rome, the Dipartimento di Scienze della Terra, Universitá di Genova and the Czech Science Academy of Prague. The Ivrea body in the Northwestern Alps and their junction with the Apennine chain, the micro-plate of the Sardinian-Corsican system and, recently, the central part of the peninsula along Tyrrhenian-Adriatic lithospheric transects were investigated. Studies in time and frequency-domain used in the first investigations, have been followed by more refined analysis involving tests on the induced EM field dimension, computations of single site Transfer Functions (TFs) through Parkinson arrows' and Fourier maps in the Hypothetical Event technique (HE). It was possible to describe the electrical conductivity distribution in the inner part of the SW Alpine arc and to confirm the presence of lithospheric and asthenospheric anomalies obtained by other geophysical methods. For the Sardinia-Corsica system, 2D and 3D inversion models highlighted the existence of two major conducting bodies, one north of Corsica, and the other south of Sardinia. In Central Italy, the regional electrical conductivity distribution pointed out a deep conductive structure beneath the Apennines and a very resistive root for this part of the mountain chain.

AbstractMantle xenoliths from southern Victoria Land have been collected and extensively studied for over a century. In this chapter, chemical and petrological data are, for the first time, comprehensively collated, and petrogenetic models for the regional mantle are reviewed and assessed. The most common lithologies are spinel lherzolite and harzburgite; plagioclase lherzolite also occurs, and pyroxenite xenoliths found across the province comprise <20% of all mantle xenoliths. The lithospheric mantle in the region has Paleoproterozoic stabilization ages, although pockets of younger mantle may exist. This peridotite mantle comprises a HIMU (high 238U/204Pb = high μ)-component sensu stricto, has been variably carbonated and has undergone multiple melt-depletion events. Regional variations in a sedimentary (EMI: Enriched Mantle I) component to the west, and iron-rich components to the east, reflect a complex history of refertilization and metasomatism. The sources of these fluids are likely to have been oceanic crust subducted during c. 0.5 Ga and older events. Peridotites have been cross-cut by pyroxenite veins, probably in multiple episodes, with the geochemistry of some samples reflecting the involvement of an upper continental crust (EMII: Enriched Mantle II) component. Future research directions should apply advanced isotopic, noble gas and volatile techniques to better understand the upper mantle below this dynamic rifting environment.

Summary Magnetotelluric (MT) data allow for electrical resistivity probing of the Earth's subsurface. Integration of resistivity models in passive margin studies could help disambiguate non-unique interpretations of crustal composition derived from seismic and potential field data, a recurrent issue in the distal domain. In this contribution, we present the first marine MT data in the Barents Sea, derived from industrial controlled-source electromagnetic (CSEM) surveys. We characterize data quality, dimensionality, depth penetration and elaborate an analysis strategy. The extensive MT database consists of 337 receivers located along 7 regional transects, emanating from ∼70,000 km2 of 3D CSEM surveys acquired for hydrocarbon exploration from 2007 to 2019. High-quality MT data are extracted for periods ranging from 0.5 s to 5000 s. The data show no apparent contamination by the active source nor effects related to large time-gaps in data collection and variable solar activity. Along receiver profiles, abrupt lateral variations of apparent resistivity and phase trends coincide with major structural boundaries and underline the geological information contained in the data. Dimensionality analysis reveals a dichotomy between the western domain of the SW Barents Sea, dominated by a single N-S electromagnetic strike, and the eastern domain, with a two-fold, period-dependent strike. 35 receivers show 3D distortion caused by nearby bathymetric slopes, evidenced by elevated skew values. We delineate geographical areas where the 2D assumption is tenable and lay the foundation for future MT modelling strategies in the SW Barents Sea. We performed 2D MT inversion along one of the regional transects, a ∼220 km-long, E-W profile encompassing a major structural high and sedimentary basin approaching the continent-ocean transition. The resistivity model reveals low crustal resistivity values (1–10 Ω.m) beneath the deep sedimentary basins, in marked contrast with high resistivity values (1000–5000 Ω.m) of the thick crystalline crust on the structural high. We interpret this abrupt lateral resistivity variation as a rapid transition from a thick, dry continental crust to a hyperextended and hydrated crustal domain. Integration of resistivity with seismic velocity, density and magnetic susceptibility models may further refine these structural models and the underlying tectonic processes in the SW Barents Sea margin. Our methodology is applicable globally where 3D CSEM surveys are acquired and has a large potential for harvesting new knowledge on the electrical resistivity properties of the lithosphere.