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Comprehensive set of seismic and potential field data from the whole European Variscan belt is used to interpret the structure and evolution of the European Variscides as defined by Martínez Catalán et al. (2021). The gravity data show the presence of high amplitude, short-wavelength gravity anomalies correlated with the outcrops of eclogites, ultramafic rocks and ophiolites delineating the main body of the Mid-Variscan Allochthon (MVA) and the Devonian Mid-Variscan suture (MVS). The medium amplitude and elongated long-wavelength gravity highs, aligned parallel to the Variscan structural grain, correspond to the low-grade Proterozoic rocks of the MVA and Devonian arc – back-arc system. On the other hand, the short wavelength negative gravity anomalies developed in the central part of the belt coincide with Carboniferous (330–310 Ma) per- to meta-aluminous magmatic bodies. The magnetic data show two belts correlated with Carboniferous Rhenohercynian and Devonian Mid-Variscan magmatic arc granitoids. The Rhenohercynian and Mid-Variscan subduction systems are also well-imaged by moderately dipping primary reflectors in reflection seismic lines. Younger moderately dipping reflectors in the upper-middle crust coincide with outcrops of Carboniferous detachments, limiting granite plutons and core complexes along-strike the core of the Variscan orogeny. Deep crustal reflectors are considered as an expression of lower crustal flow resulting from extensional re-equilibration of the previously thickened Variscan crust. A P-wave velocity logs synthesis shows a high-velocity cratonic crust surrounding a thin Variscan orogenic crust defined by low-velocity lower and middle crusts. The latter crustal type coincides with regional outcrops of 330–310 Ma per- to meta- aluminous granitoids and associated gravity lows along-strike the belt. All these data are used to define the primary polarity of Devonian subduction systems defining the European Variscan belt (Schulmann et al., 2022) and discuss the Carboniferous extension forming specific structure of the Variscan crust. This geodynamic evolution is integrated into a paleomagnetically constrained model of the movements of continental plates and intervening oceans (Edel et al., 2018; Martínez Catalán et al., 2021).REFERENCES:Catalan, J.R.M., Schulmann, K. and Ghienne, J.F., 2021. The Mid-Variscan Allochthon: Keys from correlation, partial retrodeformation and plate-tectonic reconstruction to unlock the geometry of a non-cylindrical belt. Earth-Science Reviews, 220, 1–65.Edel, J.B., Schulmann, K., Lexa, O. and Lardeaux, J.M., 2018. Late Palaeozoic palaeomagnetic and tectonic constraints for amalgamation of Pangea supercontinent in the European Variscan belt. Earth-science reviews, 177, 589-612.Schulmann, K., Edel, J.B., Catalán, J.R.M., Mazur, S., Guy, A., Lardeaux, J.M., Ayarza, P. and Palomeras, I., 2022. Tectonic evolution and global crustal architecture of the European Variscan belt constrained by geophysical data. Earth-Science Reviews, 234, p.104195.

The Central Asian Orogenic Belt (CAOB) is a Paleozoic accretionary-collisional orogen located at the eastern Pangea in between the Siberian Craton to the north and the North China and Tarim cratons to the south. Several contradictory geodynamic models were proposed to explain the tectonic assemblage: oroclinal bending and strike-slip duplication of a giant intraoceanic arc or a progressive lateral accretion of linear continental and oceanic terranes towards the Siberian Craton. However, none is generally accepted. A multidisciplinary and multiscale approach integrating potential field analysis and modelling provides new insights into understanding the crustal structures beneath the CAOB.First, we present a synthesis of the previous geophysical studies, which constitute the constraints for the modelling. Second, based on global gravity and magnetic anomaly grids, the large-scale statistical analysis of their lineaments reveals the distribution of the contrasting tectonic zones. Then, the topography of the Moho is determined by 3D forward modelling of the GOCE gravity gradients, which is then integrated into 2D and 3D crustal scale models of southern and central Mongolia. A geodynamic model is derived from the resulting crustal architectures. Thus, the combination of these methods allows us to: (1) unravel the existence and distribution of suspect terranes in accretionary systems; (2) correlate the contrasting tectonic zones with the gravity and magnetic signals and the thickness of the crust, thereby revealing the inheritance of Paleozoic and Mesozoic orogenic history; and (3) determine the significance and possible origin of the major anomalies, which are related to tectonic processes such as lower crustal relamination, presence of deep-seated fault zones and sutures, or delimitation of main tectonomagmatic domains. Finally, with the case study of Central Mongolia, we demonstrate the real benefit and the significant progress, which can be achieved by using potential field analysis combined with seismic receiver function and geological analyses.

International audience

A multidisciplinary approach integrating potential field analysis with geological and geochemical data provides new insights into the understanding of the crustal structure and evolution of the Mongolian collage. Magnetic and gravity data demonstrate the inconsistency between the geologically defined terranes and the geophysical domains in the southwestern part of the Mongolian collage. The combination of potential field analysis and modelling with whole rock geochemistry and isotopic mapping of Carboniferous–Permian granitoids indicates the presence of a homogeneous lower crust composed of a felsic to intermediate juvenile material beneath geophysically heterogeneous upper crust. This feature is interpreted as a result of a trench-directed lower crustal emplacement of an arc type crust underplating deformed Paleozoic oceanic crust. The potential field data also confirmed the occurrence of two orthogonal late Devonian and Permian–Triassic deformation upper crustal fabrics at the scale of the southwestern Mongolian collage. The prominent magnetic highs correspond to the tectono-metamorphic domains and magmatic provinces. The gravity anomalies highlight a periodicity of the signal correlating with alternating Permian–Triassic high and low strain zones, forming a zone of major deformation wrapping around the hinge of Mongolian orocline. The geometry and kinematics of dextral and sinistral transpressive faults are explained to result from the reactivation of Permian–Triassic deformation zones in the Cenozoic stress field.

3D forward gravity modelling combined with receiver function analysis characterize the structures of the southern part of the Mongolian collage. Recently, a multidisciplinary approach integrating potential field analysis with geology and magmatic geochemistry demonstrate that relamination of an allochtonous felsic to intermediate lower crust played a major role in southern Mongolia structure. Relamination of material induces a homogeneous layer in the lower crust, which contrasts with the highly heterogeneous upper crustal part composed of different lithotectonic domains. The seismic signals of the 48 stations of the MOBAL2003 and the IRIS-PASSCAL experiments were analyzed to get the receiver functions. The resulting crustal thickness variation is first compared with the topography of the Moho determined by the 3D forward modeling of the GOCE gravity gradients. In addition, seismic stations south of the Hangay dome display significant signal related to the occurrence of a low velocity zone (LVZ) at lower crustal level. The receiver function analysis also revealed a significant difference between the crustal structures of the Hangay dome and the tectonic zones in the south. Finally, these seismic analysis inputs such as crustal thickness, strike and dips of the seismic interfaces as well as the boundaries and the lithologies of the different tectonic zones constitute the starting points from the 3D forward gravity modelling. The combination of these two independent methods enhances the occurrence and the extent of a low velocity and a low density zone (LVLDZ) at lower crustal level beneath central Mongolia. These LVLDZ may demonstrate the existence of the relamination of a hydrous material in southern Mongolia.

International audience; A multidisciplinary approach integrating potential field analysis with geological and geochemical data provides new insights into the understanding of the crustal structure and evolution of the Mongolian collage. Magnetic and gravity data demonstrate the inconsistency between the geologically defined terranes and the geophysical domains in the southwestern part of the Mongolian collage. The combination of potential field analysis and modelling with whole rock geochemistry and isotopic mapping of Carboniferous–Permian granitoids indicates the presence of a homogeneous lower crust composed of a felsic to intermediate juvenile material beneath geophysically heterogeneous upper crust. This feature is interpreted as a result of a trench-directed lower crustal emplacement of an arc type crust underplating deformed Paleozoic oceanic crust. The potential field data also confirmed the occurrence of two orthogonal late Devonian and Permian–Triassic deformation upper crustal fabrics at the scale of the southwestern Mongolian collage. The prominent magnetic highs correspond to the tectono-metamorphic domains and magmatic provinces. The gravity anomalies highlight a periodicity of the signal correlating with alternating Permian–Triassic high and low strain zones, forming a zone of major deformation wrapping around the hinge of Mongolian orocline. The geometry and kinematics of dextral and sinistral transpressive faults are explained to result from the reactivation of Permian–Triassic deformation zones in the Cenozoic stress field.

Machine learning applications in geophysical studies are often used to predict geophysical observations in areas with sparse or not data or recognize patterns and similarities in data. In our study, we test different techniques to improve the information of constraining data by machine learning and to improve strengthen the modelling of lithospheric structures with potential field data. Constraining data like seismic information, surface geology, rock classifications etc. is often used during the interpretation step of lithospheric modelling to aid the qualitative interpretation. Consensus between additional data and the own model is assessed by comparison and used to describe the model goodness consistency. First Wwe test, how this additional data can be used before the modelling by using machine learning techniques to quantify the data. We focus on supervised learning to predict crustal structure in areas with little constraints, on trained learning in data-rich areas. Second, we test the spatial analysis of surface data to determine lithospheric boundaries in depth. These tests are performed in North America and the Central Asian Orogenic belt (CAOB) to compare the results in areas with respectively good and spare data coverage. That approach can be used to link the large variety of surface and deep information in the CAOB region.The combination of the different geophysical data available with the geological data should improve our tectonic modelling.

This study integrates gravity modelling and analysis with seismic constraints through the prism of seismic anisotropy to characterize the structures of southern Mongolia, in particular at the lower crustal but also the upper mantle levels. Recently, gravity signal analysis and forward modelling combined with magmatic geochemistry and thermodynamic modelling demonstrate that relamination of allochtonous felsic to intermediate lower crust played a major role in southern Mongolia structure. Relamination of material induces a homogeneous layer in the lower crust, which contrasts with the highly heterogeneous upper crustal part composed of different lithotectonic domains. The seismic signals of the seven southernmost stations of the MOBAL2003 experiment were analyzed to get the receiver functions. The data treatment was performed following a new protocol, which reduces the noise on the different components. This treatment reveals the variation of the crustal thickness of cca. 10 km along the first 450 km of the profile. In addition, some seismic stations display significant signals related to the occurrence of a low velocity zone (LVZ) at lower crustal and upper mantle levels. The depth of the Moho discontinuity and the dips of the seismic interfaces obtained from the seismic inversions as well as the boundaries of the different tectonic zones constitute the starting points from the 2D forward gravity modelling along the southern part of the MOBAL 2003 profile. Moreover, the density values applied to the different blocks were determined according to the global lithological composition of the different units and the vergences of the tectonic contacts were constrained by the geodynamic studies. The gravity modelling reveals the occurrence of a low density zone in the lower crust beneath the four southernmost seismic stations, which corresponds to the LVZ observed with the receiver function analysis. The combination of the independent methods enhances the occurrence of a low velocity and a low density zone (LVLDZ) at lower crustal level beneath the southernmost part of the MOBAL 2003 seismic profile. These LVLDZ may demonstrate the existence of the relamination of a hydrous material in southern Mongolia.

International audience; We present a new analogue model of oroclinal buckling, which is based on recent advances in our geological understanding of the Mongolian orocline in the Central Asian Orogenic Belt. The model simulates an amplification of a preexisting, gently arcuated accretionary system. In the analogue models, we established three neighboring subdomains that represent contrasting lithospheric domains separated by a steep “subduction” interface. The shortening of the model is parallel to the pseudo-linear interface between these subdomains and produces two different deformation patterns in the amplified interlimb areas of the orocline. We used analogue modelling techniques to investigate both the flow of a ductile lithosphere and the brittle deformation of an upper crust that is associated with the development of an orocline. The modelling results show that: (1) two types of regional-scale folds develop both of which are upright. The largest are steeply plunging oroclines and the smaller have sub-horizontal axes; (2) the overall deformation distribution displays significant thickening/thinning and exhumation of the lower ductile layers with variations of upward transfer of crustal material; (3) the hinge regions of the two adjacent oroclines studied consist of different materials, one contains oceanic and the other felsic lower crustal rocks. It is found that the geometry of the lithosphere beneath the two regions is very different; (4) the geometry of the orocline is non-cylindrical and exhibits an increasing plunge of fold axes when traced from the upper-crust to deeper lithospheric layers. The resulting model is used to geophysically constrain deep crustal structures and surface deformation patterns in Mongolia.

International audience

The main purpose of this study is to construct a new 3-D model of the Central Asian Orogenic Belt (CAOB) crust, which can be used as a starting point for future lithospheric studies. The CAOB is a Paleozoic accretionary orogen surrounded by the Siberian Craton to the north and the North China and Tarim Cratons to the south. This area is of great interest due to its enigmatic and still not completely understood geodynamic evolution. First, we estimate an initial crustal thickness by inversion of the vertical gravity component of the Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) and DTU10 models. Second, 3-D forward modeling of the GOCE gravity gradients is performed, which determines the topography of the Moho, the geometry, and the density distribution of the deeper parts of the CAOB and its surroundings, taking into account the lateral and vertical density variations of the crust. The model is constrained by seismic refraction, reflection, and receiver function studies and geological studies. In addition, we discuss the isostatic implications of the differences between the seismic Moho and the resulting 3-D gravity Moho, complemented by the analysis of the lithostatic load distribution at the upper mantle level. Finally, the correlation between the contrasting tectonic domains and the thickness of the crust reveals the inheritance of Paleozoic and Mesozoic geodynamics, particularly the magmatic provinces and the orocline which preserve their crustal features.

Granitoid magmatism and its role in differentiation and stabilization of the Paleozoic accretionary wedge in the Chinese Altai are evaluated in this study. Voluminous Silurian-Devonian granitoids intruded a greywacke-dominated Ordovician sedimentary succession (the Habahe Group) of the accretionary wedge. The close temporal and spatial relationship between the regional anatexis and the formation of granitoids, as well as their geochemical similarities including rather unevolved Nd isotopic signatures and the strong enrichment of large-ion lithophile elements relative to many of the high field strength elements, may indicate that the granitoids are product of partial melting of the accretionary wedge rocks. Whole-rock geochemistry and pseudosection modeling show that regional anatexis of fertile sediments could have produced a large amount of melts compositionally similar to the granitoids. Such process could have left a high-density garnet- and/or garnet-pyroxene granulite residue in the deep crust, which can be the major reason for the gravity high over the Chinese Altai. Our results show that melting and crustal differentiation can transform accretionary wedge sediments into vertically stratified and stable continental crust. This may be a key mechanism contributing to the peripheral continental growth worldwide.

The general structure of the Chinese Altai has been traditionally regarded as being formed by five tectono-stratigraphic ‘terranes’ bounded by large-scale faults. However, numerous detrital zircon studies of the Paleozoic volcano-sedimentary sequences shown that the variably metamorphosed Cambro-Ordovician sequence, known as the Habahe Group, is present at least in four ‘terranes’. It structurally represents deepest rocks unconformably covered by Devonian and Carboniferous sedimentary and volcanic rocks. Calc-alkaline, mostly Devonian, granitoids that intruded all the terranes revealed their syn-subduction related setting. Geochemistry and isotope features of the syn-subduction granitoids have shown that they originated mainly from the melting of youthful sediments derived from an eroded Ordovician arc further north. In contrast, Permian alkaline granitoids, mostly located in the southern part of the Chinese Altai, reflect a post-subduction intraplate setting. The metamorphic evolution of the metasedimentary sequences shows an early MP-MT Barrovian event, followed by two Buchan events: LP-HT mid-Devonian (ca. 400–380 Ma) and UHT-HT Permian (ca. 300–270 Ma) cycles. The Barrovian metamorphism is linked to the formation of a regional sub-horizontal possibly Early Devonian fabric and the burial of the Cambro-Ordovician sequence. The Middle Devonian Buchan type event is related to intrusions of the syn-subduction granitoids during an extensional setting and followed by Late Devonian-Early Carboniferous NE-SW trending upright folding and crustal scale doming during a general NW-SE shortening, responsible for the exhumation of the hot lower crust. The last Permian deformation formed NW-SE trending upright folds and vertical zones of deformation related to the extrusion of migmatites, anatectic granitoids and granulite rocks, and to the intrusions of gabbros and granites along the southern border of the Chinese Altai. Finally, the Permo-Triassic cooling and thrust systems affected the whole mountain range from ca. 265 to 230 Ma. In conclusion, the Chinese Altai represents different crustal levels of the lower, middle and upper orogenic crust of a single Cambro-Ordovician accretionary wedge, heterogeneously affected by the Devonian polyphase metamorphism and deformation followed by the Permian tectono-thermal reworking event related to the collision with the Junggar arc. It is the interference of Devonian and Permian upright folding events that formed vertical boundaries surrounding the variously exhumed and eroded crustal segments. Consequently, these crustal segments should not be regarded as individual suspect terranes.

The Central Asian Orogenic Belt (CAOB) in southern Mongolia consists of E-W trending Neoproterozoic cratons and Silurian-Devonian oceanic tectonic zones. Previous study revealed that the Early Paleozoic accretionary wedge and the oceanic tectonic zone are underlain by a layer giving a homogeneous gravity signal. Forward gravity modelling suggests that this layer is not formed of high-density material typical of lower oceanic crust but is composed of low- to intermediate-density rocks resembling continental crust. The nature of this lower crust is constrained by the whole-rock geochemistry and zircon Hf isotopic signature of abundant Late Carboniferous high-K calc-alkaline and Early Permian A-type granitoids intruding the two Early Paleozoic domains. It is possible to explain the genesis of these granitoids by anatexis of juvenile, metaigneous (tonalitic-gabbroic) rocks of Late Cambrian age, the source of which is presumed to lie in the “Khantaishir” arc (520–495 Ma) further north. In order to test this hypothesis, the likely modal composition and density of Khantaishir arc-like protoliths are thermodynamically modelled at granulite- and higher amphibolite-facies conditions. It is shown that the current average density of the lower crust inferred by gravity modelling (2730 ± 20 kg/m3) matches best metamorphosed leucotonalite to diorite. Based on these results, it is now proposed that Mongolian CAOB has an architecture in which the accretionary wedge and oceanic upper crust is underlain by allochthonous lower crust that originated in a Cambrian arc. A tectonic model explaining relamination of allochthonous felsic to intermediate lower crust beneath mafic upper crust is proposed.

The Chinese Altai, a key component of the western Central Asian Orogenic Belt, is considered to be formed through multiple accretions of different terranes. However, the deformational histories of each terrane (tectonic domain), i.e. structural records before and after the accretion, are rarely studied, which has hindered our understanding of the accretionary processes. To fill the gap, a systematic macro- and microscopic structural analysis was carried out on two contrasting litho-tectonic units, i.e. the early Paleozoic low-grade Alegedayi Ophiolitic Complex (AOC) juxtaposed to the high grade Tarlang Granitic Massif (TGM). Selected rock samples were analyzed using zircon U–Pb isotopic dating to constrain the timing of polyphase deformation. Our structural and geochronological data suggest that the two litho-tectonic units were initially detached and located in different crustal levels and experienced distinct phases of deformation under contrasting P-T conditions. They were mutually accreted with each other in the early Devonian and jointly underwent a WNW-ESE-directed shortening deformational event (D1) at ∼390 Ma. The change of tectonic regime was further enhanced by a subsequent NNE-SSW-directed shortening deformation (D2) after ∼ 380 Ma. The shortening process ended before the crustal-scale sinistral strike-slip shearing deformation along the Erqis fault zone at 290 - 240 Ma. Results of this study provide solid field-based evidence for a model that the Chinese Altai initially underwent a nearly E–W-oriented subduction-accretional event in the middle Paleozoic, before it was reoriented to a nearly N–S-oriented convergence.

The Central Asian Orogenic Belt (CAOB) is a typical accretionary orogen divided into numerous lithostratigraphic terranes. In theory, these terranes should be characterized by contrasting magnetic and gravity signatures owing to their dissimilar petrophysical properties. To test this hypothesis, the extent of tectonostratigraphic terranes in southern Mongolia was compared with the potential field data. The analysis reveals that the terrane boundaries are not systematically defined by strong gravity and magnetic gradients. The correlation of the magnetic signal with the geology reveals that the magnetic highs coincide with Late Carboniferous to Early Permian volcanic-plutonic belts. The matched filtering shows a good continuity of signal along the boundaries of these high magnetic anomalies toward the deeper crustal levels which may indicate the presence of deeply rooted tectonomagmatic zones. The axes of high-density bodies in the western and central parts of the study area are characterized by periodic alternations of NW-SE trending gravity anomalies corresponding to up to 20 km wide cleavage fronts of Permo-Triassic age. The matched filtering analysis shows good continuity of signal to the depth of these gravity highs which may indicate presence of deeply rooted high-strain zones. The magnetic signal is interpreted to be as the result of a giant Permo-Triassic magmatic event associated with lithosphere-scale deformation, whereas the gravity pattern is related to the postaccretionary shortening of the CAOB between the North China and Siberia cratons.

The Trans-Altai Zone in southern Mongolia is characterized by thrusting of greenschist-facies Silurian oceanic rocks over Devonian and Lower Carboniferous volcano-sedimentary sequences, by E–W directed folding affecting the early Carboniferous volcanic rocks, and by the development of N–S trending magmatic fabrics in the Devonian–Carboniferous arc plutons. This structural pattern is interpreted as the result of early Carboniferous thick-skinned E–W directed nappe stacking of oceanic crust associated with syn-compressional emplacement of a magmatic arc. The southernmost South Gobi Zone represents a Proterozoic continental domain affected by shallow crustal greenschist-facies detachments of Ordovician and Devonian cover sequences from the Proterozoic substratum, whereas supracrustal Carboniferous volcanic rocks and Permian sediments were folded into N–S upright folds. This structural pattern implies E–W directed thin-skinned tectonics operating from the late Carboniferous to the Permian, as demonstrated by K–Ar ages ranging from ~ 320 Ma to 257 Ma for clay fractions separated from a variety of rock types. Moreover, the geographical distribution of granitoids combined with their geochemistry and SHRIMP U–Pb zircon ages form distinct groups of Carboniferous and Permian age that record typical processes of magma generation and increase in crustal thickness. The field observations combined with clay ages, the geochemical characteristics of the granitoids and their ages imply that the E–W trending zone affected by tectonism migrated southwards, leaving the Trans Altai Zone inactive during the late Carboniferous and Permian, suggesting that the two units were tectonically amalgamated along a major E–W trending strike slip fault zone. This event was related to late Carboniferous subduction that was responsible for the vast volume of granitoid magma emplaced at 300–305 Ma in the South Gobi and at 307–308 Ma in the Trans-Altai Zones. The formation and growth of the crust was initially due only to subduction and accretion processes. During the post-collisional period from 305 to 290 Ma the addition of heat to the crust led to the generation of (per-) alkaline melts. Once amalgamated, these two different crustal domains were affected by N–S compression during the Triassic and early Jurassic (185–173 Ma), resulting in E–W refolding of early thrusts and folds and major shortening of both tectonic zones.

Continental crust is formed above subduction zones by well-known process of “juvenile crust growth”. This new crust is in modern Earth assembled into continents by two ways: (i) short-lived collisions of continental blocks with the Laurussian or later Eurasian continent along the “Alpine Himalayan collisional/interior orogens” in the heart of the Pangean continental plates realm; and (ii) long lived lateral accretion of ocean-floor fragments along “circum-Pacific accretionary/peripheral orogens” at the border of the PaleoPacific and modern Pacific oceanic plate.

Tectonic-magmatic reworking of accretionary wedges is a key process responsible for differentiation and stabilization of continental crustal in accretionary orogens. This generic problem can be exemplified by magmatic evolution of the Chinese Altai which represents a high-grade core of the world's largest accretionary system, namely the Central Asian Orogenic Belt (CAOB). In the Chinese Altai, voluminous SilurianDevonian granitoids intruding a greywacke-dominated Ordovician flysch sequence. These intrusions are classically interpreted to originate from predominant (70‒90 %) juvenile (depleted mantle-derived) magma. However, their close temporal and spatial relationship with the regional anatexis of flysch rocks, allows us to examine the possibility that they were mainly derived from flysch rocks.

A new model of the structure and composition of the Variscan crust in the Bohemian Massif is proposed based on 3D gravity modelling, geological data, seismic refraction (CEL09) and reflection (9HR) sections. The Bohemian Massif crust is characterized by a succession of positive and negative anomalies of about 60–80 km wavelength for nearly constant Moho depths. The south-western part of the Bohemian Massif displays a large negative Bouguer anomaly corresponding to high grade rocks (granulites and migmatites) of the Palaeozoic crustal root represented by the Moldanubian domain. The adjacent Neo-Proterozoic Bruno-Vistulian microcontinent displays an important gravity high reflecting mafic and intermediate medium grade metamorphic and magmatic rocks. The deep crustal boundary between the root domain and the Bruno-Vistulian microcontinent is represented by a strong gradient located 50 to 70 km westwards from the surface boundary between these units indicating that the high density basement rocks are covered by a thin sheet of low density granulites and migmatites. North-west from the Moldanubian domain occurs an important gravity high corresponding to the Neo-Proterozoic basement of the Tepla-Barrandian Unit limited in the north by southeast dipping reflectors of the Tepla suture which is characterized by high density eclogites and ultramafics. The footwall of the suture corresponds to low density felsic crust of the Saxothuringian basement. The reflection and refraction seismics and gravity modelling suggest a complex lithological structure of the Moldanubian domain marked by a low density 5–10 km thick lower crustal layer located above the Moho, a 5–10 km thick heavy mafic layer, a 10 km thick mid-crustal layer of intermediate density and a locally developed 2–5 km thick low density layer at the surface. The low density lower crust correlates well with low P-wave velocities in the range 6.0–6.4 km s− 1 in the CEL09 section. This complex geophysical structure and surface geology are interpreted as a result of Carboniferous partial overturn of low density lower crust and high and intermediate density crust in the area of central root and by viscous extrusion of low density orogenic lower crust over the high density Bruno-Vistulian continent. Comparison of these data with geophysical profiling of the Andean and the Tibetan plateaus suggests that modern orogenic systems reveal comparable deep crustal geophysical pattern. Based on these similarities we propose that the Variscan root represents a deep crustal section of above mentioned plateaus, which may have develop by the same orogenic process.

The structure of the Moldanubian domain is marked by felsic granulites of Ordovician protolith age forming the cores of domes that are separated from mid-crustal Neoproterozoic and Palaeozoic metasedimentary rocks that occur in synclines by a late Ordovician to Silurian metabasic unit. Reflection and refraction seismic sections combined with gravity inversion modelling suggest the presence of a low density layer at the bottom of the crust (interpreted as felsic granulite) overlain by a denser layer (interpreted as amphibolite) with layers of intermediate density at the top (interpreted as metasedimen- tary rocks). It is proposed that the granulite domes surrounded by middle crustal rocks reflect transposed horizontal layering originally similar to that preserved in the deep crust and imaged by the geophysical surveys. This geological and geophysical structure is considered to be a result of Visean gravity redistribution initiated by radioactive heating of felsic crust tectonically emplaced at the bottom of a Palaeozoic orogenic root. The radioactive layer with heat production of 4 l Wm )3 corresponds geochemically and isotopically to Ordovician felsic metaigneous rocks of the Saxothuringian domain that have been emplaced at Moho depth under thickened crust during late Devonian-early Carboniferous continental subduction. Part of the continental crust continued to be subducted and produced fluids ⁄ low-volume melts which directly contaminated and enriched the local lithospheric mantle by lithophile elements, most notably Cs, Rb, Li, Pb, U, Th and K. Thermal incubation of 10-15 Myr was sufficient to heat and convert the underplated felsic layer into granulites via dehydration melting and melt segregation. The process of melt loss was responsible for the removal of radioactive elements and for switching off the heat at the beginning of the exhumation process. At the same time, the metasomatized underlying mantle was heated producing characteristic ultrapotassic magmas. Gravitational instability was then induced by the density contrast between the light granulites and the overlaying denser mafic lower crustal layer and a viscosity drop related to thermal weakening and partial melting of the latter.

Field surveys were performed on the terminal cone of Piton de la Fournaise in 2006 and 2008 to precisely map the self potential (SP) signal and determine the zonation of the hydrothermal activity both on the flanks of the cone and in the summit area, including inside the Bory and Dolomieu craters. SP maps inside the craters have been performed 8. months before the 5-7 April 2007 caldera collapse. Zonations appear both at the scale of the cone and of the summit and allow new interpretation of the electrical signal distribution on the terminal cone of Piton de la Fournaise. Superimposed to the SP maxima linked to the rift-zones, several areas of SP maxima associated with collapse structures have been detected: (1) in the summit area, the Bory and Dolomieu craters show the strongest SP values with amplitudes exceeding 2. V with respect to the base of the cone, and with a sharp lateral variation to the East, corresponding to the inner boundary of the Dolomieu caldera, collapsed on 5-7 April 2007, and (2) in the paleo pit craters surrounding the summit which show amplitudes similar to the Dolomieu-Bory craters. The analysis of the variations of the signal with time evidences a modification of the fluid flow pattern with a higher associated SP signature to the east in 2008. We interpret the amplification of fluid flow to the east in 2008 as a consequence of the eastward motion of the eastern flank of the volcano during the April 2007 eruption. The acquisition of SP data during two periods separated by the April 2007 eruption turns out to be a good opportunity to correlate the SP signal to the Piton de la Fournaise structure and to its evolution in term of hydrothermal and eruptive activity. © 2011 Elsevier B.V., The field work was supported by The Université de la Réunion (BQR 2005–2006).

On June 20th, 1996, a submarine land failure in a fjord near the village of Finneidfjord (Northern Norway) developed into a retrogressive quick clay slide encroaching 100-150 m inland, resulting in the loss of life and significant material damage. Reports in the aftermath of the slide concluded that build up of excess pore pressure may have been the main reason for the slide, though other causes are invoked too. The geological settings at this location involve many of the processes typical for large offshore slides. As the fjord is sheltered and accessible, this location is ideal as a field laboratory for studying submarine slide hazards. Such a project for evaluating instrumentation and site investigation techniques for studying submarine slope stability was initiated in 2004 by the International Centre for Geohazards (ICG, Oslo). Initial field campaigns since 1997 have consisted of sediment coring, VHR seismic surveys, and the deployment of long term in situ instrumentation. A larger campaign is planned in 2008, including shear-wave acquisition, and, to support that campaign with land data, several geophysical investigations were carried out onshore in July 2007 in cooperation with the University of Strasbourg. The field studies have included several students to train them in field practice. Seismic, GPR and resistivity investigations were all carried out, and preliminary results are presented.