general features of the uppermost mantle stratification from long range seismic profiles abstract long range seismic studies made by russian institutions during the last two decades reached depths of 700 800 km on the continent peace nuclear explosions were used as a source and 100 km in the ocean ocean bottom stations recording 500 kg explosions in water comparison of the wave fields of these and other seismic profiles in russia western europe and the south atlantic shows that the classical lithosphere asthenosphere model is too simple to explain the observed data a stratified medium with alternating weak and rigid layers is a more realistic model of the upper mantle one of the layers is globally observed at a depth of 80 to 100 km it is located in the thermal lithosphere beneath old platforms at the bottom of the lithosphere under active tectonic areas and inside the asthenosphere below mid oceanic ridges relatively high velocities 8 4 to 8 5 km s are typical for this boundary both in the old platforms and in the high heat flow oceanic areas a change of structural pattern where the block structure of the uppermost mantle is transformed into a horizontally homogeneous one is typical at that boundary level such features suggest that the boundary separates the brittle and more ductile upper mantle layers and may be considered as the bottom of the global mechanical lithosphere

modern concepts of earth dynamics anticipated by alfred wegener in 1912 in his first publication on continental drift alfred wegener anticipated sea floor spreading the functional relationship between bathymetry and age or temperature below the sea floor perhaps mantle convection and some aspects of plate tectonics some of these insights such as sea floor spreading and bathymetry with age did not appear in his later work others such as convection and plate tectonics were taken up when new evidence became available his intuition led him to these insights and he had a very clear perception of the distinction between facts and speculation

lateral variations of thermo mechanical properties in the tyrrhenian northern apennine region abstract we present lithospheric strength profiles calculated for a transect running from the island of corsica in the west to the apennine foredeep in the east strength profiles are constructed at locations selected on the basis of the availability of seismic heat flow gravity data and earthquake foci distribution rheological modelling points to the presence of strong lateral variations in the thermo mechanical state of the lithosphere the tyrrhenian tuscany area has a low rigidity in relation with the intense stretching active from the early miocene and with very high heat flow associated with asthenospheric rise on the contrary hinterland corsica and apenninic foreland areas adriatic area show a higher level of lithospheric strength effective elastic thickness estimates confirm this scenario exhibiting a strong reduction from the northern tyrrhenian to the inner apennines and then an abrupt increase in correspondence of the adriatic plate the distribution of earthquake focal depths confirms the presence of a brittle rheology in the first kilometres of the crust in the tuscany area whereas close to the plate boundary much lower thermal gradients partly induced by the westward plunging adriatic plate enable seismic activity to occur at larger depths

oligo miocene rift of sardinia and the early history of the western mediterranean basin the geodynamic evolution of the western mediterranean basin in spite of many studies is still uncertain there is some consensus for interpreting this basin as a kind of small oceanic marginal basin its opening has generally been related to a subduction process which was active during the oligocene miocene somewhere east of sardinia corsica1 7 as the margins of the basin are deeply buried below miocene to present sediments direct lithological and stratigraphical data which could explain the events responsible for its formation are rare8 10 or missing altogether to obtain such data detailed field studies have been undertaken in sardinia fig 1 and the first results are presented here this approach is justified by the fact that in that island oligocene and miocene sediments were deposited in a rift fossa tettonica sarda of verdabasso11 which is the easternmost arm of the complex rift system that affected the european plate during oligocene and miocene times one of these arms evolved towards a small oceanic basin the western mediterranean or algero provencal basin while others such as the gulf of valencia and the sardinia rift aborted and remained at the rift stage its exceptional exposures make it possible to examine the sardinia rift to clarify the sequence of events which created it and to establish a sedimentological model which we believe is directly applicable to the western mediterranean basin

fluid geochemistry of the sardinian rift campidano graben sardinia italy fault segmentation seismic quiescence of geochemically active faults and new constraints for selection of co2 storage sites abstract sardinia is typically seismically quiescent displaying an almost complete lack of historical earthquakes and instrumentally recorded seismicity this evidence may be in agreement with the presence of a ductile layer in the northern sector of the island as suggested by the he isotopic signature in fluids rising to the surface through quiescent fault systems the fault systems have been found to be segmented and therefore isolated in fluid circulation the study of fluid behaviour along fault systems becomes strategically important when applied to solve some geological risk assessments such as rn indoor or to define geological structures like potential co 2 storage sites both of these have been recently requested by the exploitation in italy of the euratom directive and the evolution of the kyotoprotocol policy four water dominated hydrothermal areas of sardinia located along regional fault systems were considered campidano graben tirso valley logudoro and casteldoria a fluid geochemical survey was carried out taking into account physical chemical and environmental parameters major elements within gaseous and liquid phases a few minor and trace elements selected isotope ratios 2 h 18 o 13 c 3 he 4 he 222 rn concentration and some dissolved gases two different fluids have been recognised as regards both water chemistry and dissolved gases i co 2 rich gases poor in he and rn with a relatively high 3 he 4 he ratio up to r r a 2 32 associated with na hco 3 cl thermal and cold groundwater ii gases rich in he and n 2 poor in co 2 and rn with a low 3 he 4 he ratio associated with alkaline thermal and cold waters the distribution of these two groups of fluids characterises the sardinian tectonic systems in fact gas fluxes are not homogeneous being mainly related to the different fault segments and to the areas where quaternary basalts crop out the underground geochemical evolution of the sardinian fluids as a function of the geological and tectonic systems provides some suggestions for solving one of the most important problems co 2 geological sequestration in order to reduce the co 2 excess produced by human activity the best geological disposal sites are reservoirs with low hydraulic conductivity sealed to fluid movement or aquifers characterised by maximum ph buffering capacity of their mineralogical matrix the knowledge of the role of faults as permeability barriers or as deep fluid uprising pathways is prerequisite

r a daly s early model of seafloor generation 40 years before the vine matthews hypothesis an outstanding theoretical achievement inspired by field work on st helena in 1921 1922 large scale lateral mobility of the earth s lithosphere mobilism was a hotly debated issue in earth sciences during some two decades following publication of wegener s 1912 theory of continental displacement the final acceptance of lithospheric mobility was brought about with the plate tectonics revolution during the late 1960s support for mobilism was rather popular in certain european countries during the 1920s whereas the reactions in north america were mostly hostile one of the very few influential mobilists in the new world was reginald aldworth daly of harvard university the present paper discusses his model of continental displacement which is very remarkable in many aspects we focus on the hitherto neglected fact that daly proposed in the mid 1920s a mechanism to create oceanic crust which would have been totally consistent with the vine matthews hypothesis of seafloor generation published in 1963 it is furthermore suggested that daly s geotectonic proposals were inspired by small scale

heat flow heat production and fission track data from the hercynian basement around the provencal basin western mediterranean abstract in the complex structural framework of the western mediterranean hercynian areas are expected to be thermally preserved from the recent tectonic evolution the thermal regime of these areas is studied using heat flow heat production and fission track data the surface heat flow is significantly higher in corsica 76 10 mw m 2 than in the maures and esterel 58 2 mw m 2 neither heat production nor erosion subsequent to the alpine orogeny in corsica can explain such a difference it is suggested that a deep thermal source related to the asymmetric evolution of the provencal basin could explain the higher heat flow in corsica a model of thermal structure based on the present day thermal regime of the maures and esterei is proposed for the stable hercynian crust in this area the mantle heat flow is 20 25 mw m 2 and the temperature at moho level is 375 500 c depending on the thermal parameter distribution with depth

late paleozoic rotations of corsica and sardinia new evidence from paleomagnetic and k ar studies abstract a detailed paleomagnetic investigation of so called permian volcanics was undertaken in southeastern and northwestern sardinia k ar dating on separate minerals representing 30 sites on corsica and sardinia showed that the first calc alkaline cycle had taken place in westphalo stephanian times and the second alkaline one at the end of the stephanian 246 paleomagnetic specimens from 28 sites of southeastern sardinia and 2 sites from the northwest were demagnetized by alternating field and thermal processes the directions of characteristic magnetizations are d 85 i 6 95 5 for the volcanics of southeastern sardinia and d 126 i 1 for the nurra samples these mean directions differ from those of northwestern corsica and northern sardinia gallura the paleomagnetic results display good grouping for each volcanic complex but exhibit a geographical dispersion of the mean directions this scattering suggests significant motions in late hercynian times

a magnetovariational study in sardinia abstract a study of geomagnetic field time variations with a magnetometric array was performed in sardinia during the summer of 1988 nine fluxgate magnetometers with digital acquisition systems were employed to cover the main structural features of the island the recordings do not show significant variations in the amplitudes of the horizontal components of the bay like events whereas the vertical component amplitudes show a strong southward increase fourier maps were drawn at significant periods and induction arrows and z h pseudosections were derived

a two stage rifting in the basins of the corsica sardinian straits abstract a set of seismic and field data from the corsica sardinian straits allowed us to define three well separated oligocene deep basins the castelsardo and porto torres basins forming the northern end of the sardinian graben and the bonifacio basin on the continental shelf of the bonifacio straits a seismostratigraphic investigation of these three basins revealed two major tectonic events a first oligocene to middle aquitanian rifting phase followed by oceanic spreading in the western mediterranean and then a second rifting event between corsica and sardinia in lower burdigalian time the first rifting is noticed in the three basins but the second one only in the porto torres and castelsardo basins

the geometry of continental displacement and its application to arctic geology eugen wegmann s early approaches published in the geologische rundschau in 1943 plate tectonics developed around 1965 as a powerful tool to describe the tectonic movements of the earth s crust the article demonstrates that basically four already existing theoretical concepts subduction seafloor spreading the application of euler s theorem and transform faults had to be combined to arrive at the modern theory alfred wegener father of the theory of continental displacement is often credited as the most direct forerunner of plate tectonics however none of the aforementioned concepts had been developed by him the present article deals with the hitherto not duly credited contributions of the swiss geologist eugen wegmann 1896 1982 he developed in a series of highly original papers published between 1943 and 1948 one of them in the geologische rundschau a critical test of the theory of continental displacement based on the regional geology of the arctic furthermore he gave a very concise account on the geometrical principles of drift movements as a result he developed for the first time 25 years before mckenzie and parker s nature 216 1276 1280 landmark paper on the pacific 1967 the geometrical basis to graphically test plate motion directions however his work has not yet received the credit it deserves neither by scientist nor by historians of science

gas geochemistry as a tool to investigate the earth s degassing through volcanic and seismic areas the soul of the 8th international conference on gas geochemistry

geochemical applications of noble gases abstract noble gases are unique tracers due to their rarity on earth and their inertness several of their isotopes are produced by nuclear reactions whereas others were trapped when the earth formed they have been widely used for understanding the evolution and structure of the earth dating geological events processes characterizing magma genesis and volcanism in different tectonic environments tracing mantle crust interaction even in non volcanic areas and for monitoring earthquake and volcanic activities some of these topics were presented at the 9th international conferences on gas geochemistry that was held in 2007 in taipei taiwan this special volume includes twelve papers which cover a wide range of applications of noble gases geochemistry such as tracing the thermal evolution of the early earth understanding the mantle source of continental basalts establishing the spatial variations of gas compositions in hot springs and fumaroles from volcanic and forearc areas as well as in key non volcanic areas such as faults and seismic zones and tracing the origin and evolution of ore forming fluids of uranium deposits reproducing the noble gas behavior during faulting and finally using argon as a tracer of large scale geologic processes

the gross structure of the crust under corsica and sardinia as part of the european geotraverse project egt a number of international cooperative seismic refraction surveys were carried out along the n s segment from the ligurian sea across the islands of corsica and sardinia to the sardinian channel between sardinia and north africa from 1982 to 1985 for all the experiments offshore explosions served as energy sources ranging from 200 to 1125 kg the main result is that the crust in the ligurian sea is of transitional type with a thickness of only 18 km the two continental blocks of corsica and sardinia have a maximum crustal thickness of 33 and 34 km with average crustal velocities of 6 4 and 6 3 km s respectively other results are that the internal crustal structure as derived from the available data in both islands is rather undifferentiated with only local evidence for a more steplike interface in the lower crust corsica 6 3 to 6 6 km s sardinia 6 5 to 6 8 km s there are also indications of a slight velocity inversion under the southwestern part of sardinia farther south in the sardinian channel the crustal thickness decreases again to 20 km a significant feature is the observed pn velocity this parameter increases from the relatively low value of 7 5 km s in the north ligurian sea to 7 7 km s and 7 9 km s for corsica and sardinia and finally to 8 0 km s under the sardinian channel these values reflect the tectonic evolution of that part of the mediterranean sea

a cross section of the lithosphere along the european geotraverse southern segment from the alps to tunisia abstract the european geotraverse southern segment egt s is a 200 300 km wide strip from the alps to tunisia which crosses the youngest tectonic provinces of europe where continued alpine compression is accompanied by recent extension e g in the ligurian sea and sicily channel the geophysical research along the egt s has been nearly continuous both on land and sea from 1982 to 1987 with deep seismic sounding dss experiments and collection of heat flow gravity magnetic seismological and seismotectonic data the new dss and seismic reflection data testify to the asymmetry of the alpine edifice the deepest part of the crust mantle boundary is found beneath the internal and external crystalline massifs where the european moho disappears after a steep flexure in the seismic reflection data the continuation of the moho discontinuity into the southern alpine domain is however evident in the seismic refraction data seismic evidence and heat flow data indicate incipient rifting in the ligurian sea which is underlain by a mantle with an anomalously low pn velocity of only 7 5 km s the palaeozoic crust of the sardinia corsica block exhibits a bowl shape with a rather undifferentiated internal structure average p wave velocity of 6 3 6 4 km s crustal thickness increases from 20 km at its margins to more than 30 km beneath the centre of the islands a thinned crust with thrusted blocks in its upper part characterizes the sardinia channel finally the preliminary dss results from tunisia show an abrupt deepening of the moho from 20 to approximately 35 km in the tell area very low average crustal velocity which requires a velocity inversion within the lower crust and subcrustal reflectors revealing a structured upper mantle

electrical conductivity investigation of the corso sardinian microplate area abstract the analysis of magnetovariational data from an array quasi simultaneously covering sardinia italy and corsica france from summer 1988 to spring 1991 indicates the existence of two major conducting bodies one north of corsica the other south of sardinia fourier maps also show local conductivity anomalies along the campidano graben sardinia across the bonifacio straits and in the northernmost part of corsica two dimensional 2d modelling and inversion procedures have been carried out in order to find the optimum conductivity and geometrical characteristics of the conductive bodies deduced from fourier maps induction arrows and pseudosections studies as a result of the 2d inversion two zones with enhanced electrical conductivity are found respectively in the sardinia channel and in the bonifacio straits the former can be related to the thermal characteristics of the area the latter shallower can be ascribed to an accumulation of sediments some short period magnetovariational anomalies in northern corsica can be related to recent reheating processes that occurred there as well as to the crustal thinning in the ligurian sea

3 d shear wave velocity model of the lithosphere below the sardinia corsica continental block based on rayleigh wave phase velocities rayleigh wave dispersion curves from both ambient noise and teleseismic events allow us to provide the first high resolution 3 d shear wave velocity vs model of the crust and upper mantle below the sardinia corsica microplate an important continental block for understanding the evolution of the central western mediterranean for a wide range of periods from 3 to 30 s the phase velocities of the study area are systematically higher than those measured within the italian peninsula in agreement with a colder geotherm relative and absolute variations in the vs allow us to detect a very heterogeneous upper crust down to 8 km as opposed to a relatively homogeneous middle and lower crust the isosurface at 4 1 km s 1 is consistent with a rather flat moho at a depth of 28 0 1 8 km 2 the lithospheric mantle is relatively cold and we constrain the thermal lithosphere asthenosphere boundary at 100 km we find our estimate consistent with a continental geotherm based on a surface heat flow of 60 mw m 2 our results suggest that most of the lithosphere endured the complex history of deformation experienced by the study area and imply in general that deep tectonic processes do not easily destabilize the deeper portion of the continental lithosphere despite leaving a clear surface signature

an introduction to the european geotraverse project first results and present plans abstract the european geotraverse egt is an international multidisciplinary project focused on a north south orientated lithospheric profile 4000 km long and of varying width extending from northernmost scandinavia to north africa this profile consists of three interlinking segments northern central and southern comprising a continuous succession of tectonic provinces ranging from the oldest precambrian areas of the baltic shield to the currently active area of the western mediterranean the broad aim of the egt project is to obtain a better three dimensional picture of the structure state and composition of the continental lithosphere to use as a basis for an understanding of its evolution and dynamics all of the 12 major projects that constitute the egt joint programme have now been initiated and several of these projects are nearing completion along the northern segment data from the fennoscandian long range project fennolora a 2000 km long seismic profile across the precambrian baltic shield show that except beneath southern sweden the shield is characterized by a high velocity 40 50 km thick crust including a 5 10 km thick crust mantle transition zone an alternating series of 4 6 high and low velocity zones is present in the subcrustal lithosphere the base of which increases in depth from ca 110 km to ca 230 km from south to north beneath the shield the top of the mantle transition zone lies at a depth of about 450 km the second major project along this segment the eugeno s european geotraverse northern segment southern part project is a multidisciplinary study of the fennoscandian border zone and was largely completed in 1984 with the realization of a large scale seismic experiment preliminary interpretation of the excellent data obtained indicate the presence of strong lateral variations in internal crustal structure beneath the danish basin field work for a third major project a multidisciplinary transect of the archaean and early proterozoic terrains in the northernmost part of the shield the polar profile was carried out in 1985 a series of deep seismic reflection lines has so far been realized in the area of the central segment in the context of german national programmes first interpretations of the seismic data from a 260 km long profile across the two main intra variscan hercynian lineaments have shown the presence of numerous horizons making up a highly reflective zone in the lowermost 10 km of the crustal section studied and distinct changes in reflectivity between the main variscan tectonic zones in 1986 the entire segment will be investigated in detail in an ambitious international programme of integrated geological and geophysical studies a series of seismic experiments termed egt s have been carried out across the southern segment in 1982 1983 and 1985 interpretation of data from the 1982 and 1983 experiments have led to several interesting results including 1 1 the suggestion that two crust mantle like interfaces exist beneath the po basin at depths of about 35 and 50 km and adjacent tectonic units these interfaces marking a deep contact zone between the adriatic and european plates 2 2 in the area between genoa and corsica the ligurian sea is underlain by a greatly thinned distinctly layered section of continental crust and 3 3 corsica and sardinia are underlain by bowl shaped typically variscan continental crusts the 1985 phase of seismic surveys focused on crustal structure beneath tunisia and the adjacent seas in addition two off traverse projects are being realized first a wide aperture network of autonomously recording seismic stations nars installed along the line gothenburg malaga between 1982 and 1984 is already yielding high quality data on the upper 600 700 km of the mantle second an investigation of lithospheric seismic anisotropy in the area of the iberian peninsula is being organized for 1987 1988 finally of great importance are the systematic compilation of existing data and where needed in critical regions collection of new geophysical and geological data presently being carried out for the entire area encompassed by the egt it is expected that these compilations will be completed by 1987 at about the same time that full results from the main large scale seismic experiments become available enabling the construction of an integrated lithospheric cross section along the egt requiring a final phase of intensive multidisciplinary collaboration

deep temperatures and lithospheric thickness along the european geotraverse abstract one dimensional steady state heat conduction models in the crust and upper mantle for 24 areas along the european geotraverse egt show that temperatures are higher beneath young geological regions at the moho they range from 450 to 825 c heat flowing out from the mantle varies from 18 to 72 mw m2 and the contribution of crustal heat flow density evaluated by the correlation between seismic velocity and the abundance of radioactive elements accounts for about 48 of the surface heat flow density the temperature gradient below moho from about 5 to 23 mk m is low where the surface heat flow density reaches its minimum values the depth at which the geotherms reach 0 85 of the mantle melting temperature is greater in the precambrian and caledonian areas with a maximum value of about 195 km under the early proterozoic scandinavian shield in the hercynian and alpine areas this depth is generally lower higher values maximum 150 km coincide with the orogenic belts and the continental block of sardinia and corsica under the ligurian sea and the sardinian channel the lithospheric thickness decreases remarkably to about 50 km globally the lithosphere thickness is controlled by the mantle heat flow density which decreases according to a power law with an increase in the lithosphere asthenosphere depth

the tethys sea and the alpine himalayan orogenic belt mega elements in a new global tectonic system abstract analysis of meso cainozoic palaeomagnetic data for africa india and eurasia has led to the development of a new mobilistic alpine plate tectonic model characterized by a hierarchical system of plates in relative rotation the new model which discounts seafloor spreading implies that there have been no significant palaeogeographic changes in the overall distribution of continental and oceanic regions the mid oceanic ridges are interpreted as transpressive tectonic features caused by rotation of megaplates containing both continental and oceanic crust the isostatic uplift due to crustal lithospheric thickening giving rise to the general ridge topography as well as to the ridge parallel structural grain the new plate tectonic theory gains strong support from a variety of geophysical geological and palaeoclimatological evidence and several observations that have remained enigmatic or awkward within the context of the orthodox model can be readily accounted for in the new tectonic framework the model maintains the tethys as a relatively narrow epicontinental sea which during its maximum extent stretched latitudinally from the caribbean across the central atlantic to se asia the alpine himalayan orogenic belt developed along the boundary of two megaplates in relative rotation which provided a transpressive tectonic regime the location of the plate boundary to the north of the mediterranean has important implications for discussion of mediterranean microplates for example it now seems that italy has been subjected to 10 15 of clockwise microplate rotation previous conclusions in favour of 30 40 of anticlockwise rotation are regarded as artefacts which arise from incorrectly linking the mediterranean region to the european palaeomagnetic frame instead of to the african one the model suggests further that the indo pakistani plate was closely tied to eurasia this challenges the conventional view that the peninsula was part of an alleged gondwanaland the new pre drift configuration implies that the indo pakistani plate rotated 135 clockwise at around the cretaceous tertiary boundary before redocking with asia in approximately its present relative orientation

differential fluid migration behaviour and tectonic movement in lower silurian and lower cambrian shale gas systems in china using isotope geochemistry abstract isotope geochemistry has been introduced as a means to trace origin of the hydrocarbon and characterize highly productive shale gas systems recently to assess the impact of tectonic movement and the sealing of shale gas systems isotope geochemistry pressure coefficients and the distribution of bitumens are analysed many samples yield isotope geochemical data with typical carbon isotopic reversals 13 c 1 13 c 2 and hydrogen isotopic reversals 13 d c2h6 13 d ch4 in the lower silurian shale gas isotopically reversed gases are considered to originate in sealed self contained petroleum systems besides isotope reversals order degree of shale gas has positive correlation with gas production isotopically normal gases from the lower cambrian indicate that this formation was a continued relatively open petroleum system when oil and gas generated the pressure coefficients of the lower silurian shale gas reservoir range from 1 45 to 2 03 indicating that the reservoir is overpressurized whereas the lower cambrian shale gas reservoir possesses a normal pressure system overpressurization of the lower silurian shale gas reservoir also indicates that it is a well sealed system the distribution and isotope geochemistry of bitumens in the sinian dolomite and cambrian shale suggests that the source rock of the sinian hydrocarbon is the cambrian shale an unconformity induced from tectonic movement during the tongwan period is interpreted to be the fluid migration tunnel and the cause of the differential shale gas content and production finally the isotopic reversals associated with maturity pressure coefficients and tectonic evolution can both assess the preservation conditions of the reservoir and explain the differential fluid migration behaviour

earthquake relocation using a 3d a priori geological velocity model from the western alps to corsica implication for seismic hazard the region between the inner zones of the alps and corsica juxtaposes an overthickened crust to an oceanic domain which makes difficult to ascertain the focal depth of seismic events using routine location codes and average 1d velocity models the aim of this article is to show that even with a rather lose monitoring network accurate routine locations can be achieved by using realistic 3d modelling and advanced location techniques previous earthquake tomography studies cover the whole region with spatial resolutions of several tens of kilometres on land but they fail to resolve the marine domain due to the absence of station coverage and sparse seismicity to overcome these limitations we first construct a 3d a priori p and s velocity model integrating known geophysical and geological information significant progress has been achieved in the 3d numerical modelling of complex geological structures by the development of dedicated softwares e g 3d geomodeller capable at once of elaborating a 3d structural model from geological and geophysical constraints and possibly of refining it by inversion processes calcagno et al 2008 then we build an arrival time catalogue of 1500 events recorded from 2000 to 2011 hypocentres are then located in this model using a numerical code based on the maximum intersection method font et al 2004 updated by theunissen et al 2012 as well as another 3d location technique the nonlinloc software lomax and curtis 2001 the reduction of arrival time residuals and uncertainties dh dz with respect to classical 1d locations demonstrates the improved accuracy allowed by our approach and confirms the coherence of the 3d geological model built and used in this study our results are also compared with previous works that benefitted from the installation of dense temporary networks surrounding the studied epicentre area the resulting 3d location catalogue allows us to improve the regional seismic hazard assessment more particularly in the south of the argentera massif and in the ligurian basin

european geotraverse a seismic refraction profile across the ligurian sea abstract a seismic refraction profile was shot in the ligurian sea between italy and corsica in september 1983 the profile which is part of the southern portion of the european geotraverse was shot using small dynamite charges and the seismograms were recorded by ocean bottom seismometers and land stations resulting in a number of partially overlapping reversed profiles the refraction data were interpreted by means of the velocity intercept and ray tracing methods using seismic reflection derived velocity depth information for the sedimentary section refraction velocities of 4 8 6 0 and 6 6 km s 1 were calculated for the base of the sedimentary section crystalline basement and lower crust respectively an upper mantle velocity of 7 4 km s 1 was inferred using the refraction profiles recorded on corsica the interpretation of the data indicates a prominent high in the crystalline basement in the northern part of the profile while the sedimentary section increases in thickness from about 1 5 km over the basement high to over 6 km between the basement high and corsica and includes a considerable thickness of pre miocene sediments the crust mantle boundary shows shoaling towards the centre of the profile where a minimum crustal thickness of 16 km is reached the model therefore is that of a stretched and thinned continental crust which was rifted in oligo miocene times the presence of a low velocity upper mantle may be indicative of the present renewal of subcrustal activity under the ligurian sea

crustal structure from the ligurian sea to the northern apennines a wide angle seismic transect abstract a new wide angle seismic reflection profile 250 km long was shot along a line extending from parma in the apennines to the deepest part of the ligurian sea thirty seismic land stations lobs and 25 ocean bottom seismographs obs recorded 54 shots fired at sea the resulting 55 record sections allowed the definition of the transition from continental to oceanic crust in a geologically complex and hitherto poorly understood region the subducted continental crust beneath the northern apennines has a thickness of 40 km consisting of 30 km of igneous crust overlain by 10 km of sediments two sedimentary layers are identified with vp velocities in the range from 2 8 to 5 5 km s the igneous portion of continental crust thins gradually to approx 11 km across a 120 km wide zone extending from the southwestern part of the apennines some 20 km inland into the ligurian sea an easterly thinning low velocity body 7 2 7 5 km s immediately beneath the base of the crust is interpreted as an underplated unit emplaced prior to the onset of seafloor spreading this may have been initiated by magmatic activity along the incipient continental oceanic boundary a basement high coincides with the northeastern edge of the ligurian sea and is interpreted as part of the alpine ophiolite belt indicating strike slip motion the two sedimentary layers above the transition zone are typically 5 km thick and have slightly lower velocities than those further to the northeast a sharp continent ocean break is recognised in the ligurian sea oceanic crust is 6 7 km thick and is overlain by a two layered sedimentary succession approx 5 km thick mantle velocities in the region range from 7 9 km s beneath the thinned crust to 8 1 km s beneath the thick crust the ratio c o of continental to oceanic crustal thickness at the continent ocean transition is approx 1 5 and the overall crust stretching factor at that boundary is approx 3 5 these values are similar to other continent ocean boundaries and indicate that the oceanic crust in the ligurian sea formed largely by stretching with a small pull apart component

seismic receiver functions and the lithosphere asthenosphere boundary abstract the lower boundary of the lithospheric plates has remained as an enigmatic boundary for seismologists since it is relatively poorly observed by seismic means there is traditionally a broad consensus that the asthenosphere is observable as a low velocity zone by seismic surface waves seismic techniques which use shorter period p to s or s to p converted body waves are now far enough developed to be successful in observing such a low velocity zone with a higher resolution the principle of this technique the so called receiver function technique is that a strong teleseismic mother phase e g p s pp or sks incident from below on any seismic discontinuity beneath a station produces a converted phase ps or sp which indicates its depth and properties we discuss details of this technique a sufficient number of such observations exist already to indicate that the top of the low velocity zone is a globally observable discontinuity and it is sharper than previously thought an intriguing observation is that in some cratons the new seismic data indicate that the low velocity zone exists already at shallower depths than obtained from surface waves this confirms earlier results from controlled source observations thybo and perchuc 1997 we discuss possible interpretations of this shallow low velocity zone in cratonic regions

introduction to the special issue on frontiers in gas geochemistry abstract the study of the geochemistry of gases pervades the earth and environmental sciences this is due in no small measure to the well established thermodynamic properties of gases which allow their application to a variety of processes occurring over a wide spectrum of natural conditions in this respect both major and associated minor gases have been proven useful indeed the trace gases have been particularly important given their role as sensitive geochemical tracers examples where gas geochemistry places key constraints on geochemical processes include the degassing history of the solid earth to form the atmosphere and oceans the origin and migration characteristics of hydrocarbon deposits the scale of climate variability the p t characteristics of geothermal reservoirs and the dynamics of the earthquake cycle and volcanic activity to name but a few this volume continues this rich tradition with an eclectic selection of papers aimed at exploring and exploiting gas geochemistry over a myriad set of research themes

on the post 25 ma geodynamic evolution of the western mediterranean during the neogene and quaternary western mediterranean geodynamics were dominated by the eastward migration of the apenninic arc and associated back arc basins the migration was controlled by retreat of the apenninic slab and was associated with boudinage of the lithosphere in the back arc area palaeo reconstruction of the kinematics of the arc suggests about 775 km of migration from the late oligocene to present along a transect from the gulf of lions to calabria a maximum of 135 km of n s converge occurred between africa and europe during the same time span the western mediterranean was thus mainly shaped by the migration of the slab related to west directed subduction it is hypothesized that minor n s convergence deformed the arc but was not the cause of its formation 1998 elsevier science b v all rights reserved