dynamic modelling of stress accumulation in central italy we develop a viscoelastic dynamic model to calculate the stress distribution in central italy induced by different geodynamic processes likely active in the region africa eurasia convergence adriatic underthrusting subduction and asthenospheric upwelling underneath tuscany model results are compared with the hypocentral distribution of earthquakes and with the orientation of p and t axes available for the region the first order characteristics of the seismotectonic observations in central italy are subcrustal seismicity the pair extension compression in tuscany and in the outer apennines and the flexural behaviour evident in long wavelength orientation of t axes these features are reproduced by the combination of two processes underthrusting of the adriatic plate under the apennines and asthenospheric upwelling underneath tuscany the comparison with available data does not give insights about slab pull as an ongoing process

before the big chill a preliminary overview of arthropods from the middle miocene of iceland insecta crustacea abstract well preserved arthropods are reported from miocene sedimentary rocks of the skar sstrond mokollsdalur 9 8 ma and hre avatn stafholt 7 6 ma formations in iceland fossil remains of terrestrial and or freshwater animals have rarely been reported from the island before here we provide the first overview of the surprisingly rich tortonian fauna from the hrutagil locality and additional messinian aged trichopteran larval cases from the stafholt locality the hrutagil fauna includes representatives of cladocera crustacea branchiopoda and seven insect orders including several morphotypes of the orders plecoptera dermaptera hemiptera cercopoidea aphididae coleoptera hymenoptera trichoptera drusinae and diptera bibionidae previous studies on the miocene of iceland have been based principally on pollen analysis and the macrobotanical record with little attention paid to other aspects of the island s palaeontology this study provides the first comprehensive systematic description of miocene arthropods from the northern north atlantic region and offers the opportunity for a rare glimpse into the late cenozoic arthropod fauna of iceland in the context of transatlantic migration and palaeobiogeography and the onset of major global cooling events

evolution of retreating subduction boundaries formed during continental collision retreating subduction boundaries formed where the rate of subduction exceeds the rate of overall plate convergence appear to be commonly developed features within regions of early or incomplete continent continent collision they are characterized by regional extension within the overriding plate and at their leading edge by thin skinned arcuate thrust belts that are concave towards the overriding plate as is illustrated by examples from the mediterranean region the formation of retreating subduction boundaries is intimately related to the process of continental collision during the early stages of collision retreating subduction boundaries are commonly formed by lateral ejection from zones of crustal shortening along the main collision boundary retreating plate boundaries can also form before the main collision and the associated thrust belts emplaced as precollisional accretionary assemblages because the driving mechanism for retreating subduction boundaries appears to be gravity acting on a dense subducted slab slab pull subduction usually ceases when and only when thick buoyant continental crust enters the subduction zone thus differences in the evolution and duration of retreating subduction systems can be largely attributed to the size and configuration of the deep water regions available to be subducted in some cases retreating subduction boundaries may escape into the open ocean where they form nearly isolated local tectonic systems in these systems the rate of subduction is approximately compensated by the rate of upper plate extension and migration of the system across the oceanic region may be very rapid for example the horseshoe seamounts located about 800 km offshore in the eastern north atlantic may be the active expression of an east dipping westwardly migrating retreating subduction boundary that has evolved from the betic cordillera rif system active in miocene time and may now be progressing across the atlantic at approximately 50 mm yr an analogous situation may be represented by the scotia arc system a westward dipping retreating subduction system located between the south american and antarctic plates which may have escaped into the south atlantic ocean from a zone of crustal shortening in the andes and is now progressing across the atlantic at a rate of about 80 mm yr

ecology of the eocene lake messel revealed by analysis of small fish coprolites and sediments from a drilling core abstract a research drilling project in the eocene messel pit germany provided the opportunity to conduct an ecological analysis of the developmental history of the former lake messel the analysis of samples from different horizons of the drilling core showed that small fish coprolites were quite abundant in the biogenic sediments down to a drilling depth of 132 m only in the most basal layers of the biogenic oil shale sediments were no coprolites found this can be explained by the hypothesis that when these layers were deposited no fish were living in the young lake messel another peculiar feature of these lower horizons is the presence of a new type of ephippia from water fleas which strongly resembles the ephippia of extant ctenodaphnia magna the absence of these cladocera in the upper horizons might be an example of top down control in a fossil ecosystem detailed analysis of the contents of the fish coprolites shows that the larvae of phantom midges chaoboridae were by far the most important element of the former ecosystem over the whole life span of the lake the ecosystem of ancient lake messel appears to have been quite stable chaoborid pupae also turned out to be an important part of the food web chironomid midge larvae are documented for the first time in various horizons throughout the drilling core dinoflagellates are present in high abundance mammalian coprolites are rare in the core

present day vertical isostatic readjustment of the western alps revealed by numerical modelling and geodetic and seismotectonic data the active tectonics of the western alps reveals contrasting regimes ongoing extension at the heart of the chain and transpression compression at its external sectors the active processes currently affecting this region are still a matter of debate the classical models proposed in the literature invoke eurasia adria plate collision counterclockwise motion of the adria micro plate slab retreat of the subducted continental lithosphere and slab detachment more recently several authors prefer the hypothesis of tectonics driven by isostasy buoyancy forces to better understand the influence of these processes on the velocity strain and stress fields at the surface and in the crust we developed 2d viscoelastic numerical models along a vertical cross section perpendicular to the western alps we run our models with different driving forces in order to investigate one by one the geodynamic processes proposed in the literature results are compared with available geodetic geological and seismotectonic data in order to bring into coincidence model predictions and observations an important vertical isostatic readjustment must be included in the modelling together with a slight horizontal compression 0 5 mm year 1 probably due to africa eurasia convergence we show that the subduction process in this alpine region is likely to be dead and that buoyancy forces may be dominating the present day tectonics the geological society of london 2010

continental plate collision p t t z conditions and unstable vs stable plate dynamics insights from thermo mechanical modelling there was much debate recently on the mechanisms of continental convergence and related pressure temperature p t conditions both in modeling and petrologic community depending on the mechanisms of convergence subduction collision folding or rt instability one can argue about the possibility of large scale deviations of pressure and temperature in the accretion prism and below it from reference i e lithostatic conditions commonly used for petrologic reconstructions of evolution of exhumed metamorphic rocks these deviations can be caused for example by tectonic overpressure that as suggested in some studies may be responsible for formation of uhp ultra high pressure rocks however overpressure in exhumation zones can be built only in specific collision scenarios associated with plate coupling pure shear folding in this study we analyze conditions that define various mechanisms of convergence and consequently of exhumation these mechanisms can be represented as a superposition of 1 simple shear subduction 2 pure shear collision 3 folding and 4 rayleigh taylor instability we study these scenarios using a thermo mechanical model that accounts for brittle elastic ductile rheology surface processes and metamorphic changes it appears that stable oceanic type subduction may occur in the case of cold lithospheres tmohob550 c and relatively high convergence rates n3 5 cm yr depending on the lower crustal rheology strong or weak either the whole upper and lower crust or only the lower crust can be involved in subduction in case of weak metamorphic rheologies phase changes improve chances for stable subduction pure shear becomes a dominant mechanism when tmohon550 c or convergence rates are lower than 3 cm yr large scale folding is favored in case of tmoho 500 650 c and is more effective in the case of mechanical coupling between crust and mantle e g strong lower crust gravitational rayleigh taylor instabilities overcome other mechanisms for very high values of tmoho n800 c and may lead to development of subvertical cold spots however it is reasonable to assume that in most cases continental collision is initiated at oceanic subduction rate which is rarely slower than 5 cm yr this rate is sufficient to drive continental subduction during the first several myr of collision in this case the subduction channel is characterized by nearly lithostatic pressure conditions large scale zones of tectonic overpressure may be built outside the channel but do not affect the exhumed rocks overpressure may be built inside the channel in the short moment of its closure we suggest that most continental orogenic belts could have started their formation from continental subduction this evokes small tectonic overpressures and thus deep origin of the uhp rocks that may be brought to the surface via the suggested multi level mechanism of exhumation

subduction of continental crust in the western alps as a result of recent deep reflection and refraction seismology the crustal structure of the western alps is now quite well defined however this raises the question of what is present below the moho such as a crustal eclogitic root this study attempts to estimate the volume of this eclogitic root on the basis of palinspastic reconstructions even with a minimum estimate of the crustal material involved in the subduction processes which took place during the alpine orogeny a significant eclogitized crustal root must be present down to depths of around 100 km below the po plain a maximum estimate suggests that a large part of this root could now be recycled in the asthenosphere

a three dimensional crustal velocity model of the southwestern alps from local earthquake tomography a temporary network of 65 short period seismological stations was installed in the southwestern alps during the second half of 1996 it complemented the permanent monitoring networks obtaining an average interstation distance of 10 km travel time data from 446 local earthquakes and 104 quarry blasts were inverted simultaneously for hypocenter parameters and three dimensional velocity structure the p wave velocity model displays strong lateral contrasts both at shallow and deeper levels a low velocity anomaly stands out at shallow depths beneath the digne and castellane nappes in the southwestern part of the investigated area farther east the monviso ophiolitic massif appears to have a much larger extension at depth than previously assumed the largest and strongest anomaly is located under the dora maira massif and the westernmost po plain it correlates with the well known ivrea body which is classically interpreted as a wedge of adriatic upper mantle at the best resolved depths 10 and 15 km it appears as a rather thin 10 to 15 km north south elongated high velocity 7 4 to 7 7 km s 1 anomaly with very sharp edges extending to the south as far as 10 km north of the surface trace of the frontal penninic thrust special care was taken with regard to the quantitative estimation of the resolution for the main anomalies using the inversion of synthetic travel time data

widespread extension in the core of the western alps revealed by earthquake analysis the western alps are an active collision belt whose current stress field is inhomogeneous muller et al 1992 we report new seismological data which significantly improve our knowledge of this stress field about 1600 earthquakes which occurred in the western alps during the last 10 years were precisely located and 79 new focal solutions were computed the analysis of this database shows that widespread extension affects all the internal zones of the belt to better constrain the associated stress regime six stress tensors were computed using the gephart and forsyth 1984 method they show that the current tectonics of the western alps are contrasted with close variation in the stress regime transpression to the front of the belt contrasting with extension in the core of the belt the extensional direction is radial to the arcuate geometry of the belt and bounded outboard by the former thrust of the internal zones onto the external zone suggesting extensional reactivation of this inherited crustal discontinuity such widespread extension within the inner part of an actually ongoing collision belt cannot be explained by simple collision related tectonics we propose that intrabelt buoyancy forces such as those produced by a slab retreat or slab break off interfere with the boundary forces driven by the ongoing europe africa convergence

late cenozoic history of the polar north atlantic results from ocean drilling abstract dsdp odp drilling in the northern north atlantic reveals that so far the onset of northern hemisphere cooling deduced from the occurrence of ice rafted debris ird in deep sea sediments can be traced back to the middle miocene the greenland ice sheet has probably the longest history on the northern hemisphere leg 151 site 909 fram strait recovered a unique continuous sediment record dating from early middle miocene the composition of the coarse fraction indicates that the oldest ird pulses are documented around 14 ma in the fram strait site 909 and around 12 6 ma on the voring plateau leg 104 site 642 in the norwegian greenland sea fram strait site 909 indicates a further stepwise increase of northern hemisphere cooling by serveral ird pulses during late miocene between 10 8 and 8 6 ma around 7 2 6 8 and 6 3 ma which ultimately led to a drastic intensification of northern hemisphere glaciation during plio pleistocene the deep sea sediment records show that plio pleistocene strengthening of ird pulses have been observed since approximately 4 0 ma in the westernmost norwegian greenland sea and labrador sea by contrast strengthening of ird pulses are observed at approximately 3 2 2 7 ma on the eastern side of the norwegian greenland sea including the barents sea margin sediment composition and physical properties of the drilled mostly terrigenous sections reveal that orbital parameters have controlled major aspects of shallow and deep water environments with frequency domains related to 41 ka periods of obliquity dominant in the pre glacial miocene pliocene and early quaternary and to 100 ka periods of eccentricity dominant during the past 600 000 700 000 years the stratigraphy of ice rafted material reveals a frequently and rapidly changing history of the dynamics of various segments in ice sheets around the norwegian greenland sea it is particularly evident from the composition of the ice rafted material that the eurasian ice sheets delivered ird to the eastern part of the norwegian greenland sea whereas greenland source areas are represented in the drill sites of the east greenland continental margin and in the labrador sea the stratigraphic sequences on the yermak plateau suggest that the svalbard ice sheet may have extended far to the north prior to oxygen isope stage 16 but not since then invasions of temperate atlantic waters during interglacials can be observed as short lived and rather irregular events it never reached the area off eastern greenland except during the pliocene interval when tundra floras were able to develop on north eastern greenland and planktonic foraminifers suggesting the presence of temperate water masses in the fram strait as been documented in the drill sites from the yermak plateau

graptolites as indicators of regional maturity in lower paleozoic sediments selwyn basin yukon and northwest territories canada variations in reflectance and bireflectance of graptolites and bitumen are documented from lower paleozoic rocks of howard s pass xy zn pb deposit and adjacent portions of the selwyn basin two types of graptolite morphologies are recognized in these rocks a nongranular type comprising blocky and lath shaped varieties and a granular type the nongranular graptolites generally display higher reflectance and bireflectance than granular graptolites bitumen epi impsonite to meso impsonite is common in these rocks and displays a wide range of reflectance and bireflectance values at the howard s pass xy property graptolite reflectance values display a slight increase with depth and anomalously high values 7 46 10 25 are documented in the sulphide rich upper active member in surrounding regions of the selwyn basin remote from howard s pass graptolite reflectance values are lower 3 02 6 26 conodont colour alteration index values reported by others show no detectable variation in the same

dynamics of retreating slabs 1 insights from two dimensional numerical experiments from the effects of mantle flux part 2 therefore in this paper we apply forces to the slab using simple analytical functions related to buoyancy and viscous forces in order to isolate the role of rheology on slab dynamics we analyze parameters for simplified elastic viscous and nonlinear viscoelastoplastic single layer models of slabs and compare them with a stratified thermomechanical viscoelastoplastic slab embedded in a thermal solution the near surface behavior of slabs is summarized by assessing the amplitude and wavelength of forebulge uplift for each rheology in the complete thermomechanical solutions vastly contrasting styles of slab dynamics and force balance are observed at top and bottom bends however we find that slab subduction can be modeled using simplified rheologies characterized by a narrow range of selected benchmark parameters the best fit linear viscosity ranges between 5 10 22 pa s and 5 10 23 pa s the closeness of the numerical solution to nature can be characterized by a deborah number 0 5 indicating that elasticity is an important ingredient in subduction index terms 8120 tectonophysics dynamics of lithosphere and mantle general 8159 tectonophysics rheology crust and lithosphere 8160 tectonophysics rheology general keywords subduction numerical models lithospheric rheology

an endemic ichnoassemblage from a late miocene paleolake in se iceland abstract in orisdalur valley a small relict of a sedimentary body was identified in southeastern iceland it probably represents a remnant of the deep tectonically arranged paleolake late miocene 8 9 ma and filled by volcaniclastic material from nearby active volcanic centers in the profile of tuffitic sandstone siltstone and claystone the ripple bedding layers molds and flute casts indicate periodic mass flow episodes in the sedimentary profile the characteristic arrangement of sediments is evident showing features of the bouma sequences in the claystone layers deposited during episodes of lowest kinetic energy a specific ichnoassemblage was found represented by thorichnus ramosus igen et isp nov t corniculatus igen et isp nov mammillichnis jakubi isp nov helminthoidichnites multilaqueatus comb nov vatnaspor jachymi igen et isp nov and five preliminarily identified trace fossils the assemblage belongs to mermia ichnofacies the nonmarine representative of an ichnofacies developed in a turbiditic environment most of identified trace fossils are so far endemic

estimating present day displacement fields and tectonic deformation in active mountain belts an example from the chartreuse massif and the southern jura mountains western alps abstract determination of relative movements between the alpine foreland and the external crystalline massif is a key point for the understanding of the present day tectonics of the western alps in this study we try to test the continuity of the present day tectonics with the mio pliocene deformation in particular we will test if the present day displacements are localized along the thrusts of the jura mountains or along a blind thrust in the bas dauphine molasse basin definition of relative movements is achieved by several methods including a comparison of two high precision leveling networks to estimate vertical displacements horizontal deformation measurements performed by triangulation triangulation and triangulation gps comparison in situ stress measurements performed in the different tectonic units and geomorphologic observations that constrain the location and the magnitude of the quaternary deformation comparison of leveling data demonstrates 1 an uplift of the southern bas dauphine molasse basin relative to its northern part 0 8 mm year also revealed by geomorphologic analysis 2 a significant uplift of the most external jurassian anticlines 0 8 to 2 mm year also recorded by the deformation of a paleo river bed and 3 an important uplift up to 2 mm year of the subalpine massifs the horizontal strain estimated from comparison of horizontal geodetic data triangulation gps shows 1 a nw se directed shortening between the eastern chartreuse massif and the bas dauphine molasses basin approximately 3 mm year 2 an e w directed shortening in the jura mountains approximately 4 3 mm year and 3 a dextral strike slip motion consistent with focal mechanisms along a nne ssw direction between the eastern chartreuse massif and the eastern belledonne massif these data reveal a present day strain partitioning between the belledonne external crystalline massif and the bas dauphine molasses basin the westward motion of the subalpine massifs is partitioned along two southern jurassian thrust folds and a dextral nne ssw strike slip shear zone between the chartreuse massif and the belledonne massif this strain partitioning is also accompanied by a stress partitioning between the alpine foreland and the external crystalline massifs

life at the top of the greenhouse eocene world a review of the eocene flora and vertebrate fauna from canada s high arctic early middle eocene ca 53 38 ma sediments of the eureka sound group in canada s arctic archipelago preserve evidence of lush mixed conifer broadleaf rain forests inhabited at times by alligators turtles and diverse mammals including primates tapirs brontotheres and hippo like coryphodon this biota reflects a greenhouse world offering a climatic and ecologic deep time analog of a mild ice free arctic that may be our best means to predict what is in store for the future arctic if current climate change goes unchecked in our review of the early middle eocene arctic flora and vertebrate fauna we place the arctic fossil localities in historic geographic and stratigraphic context and we provide an integrated synthesis and discussion of the paleobiology and paleoecology of these eocene arctic forests and their vertebrate inhabitants the abundance and diversity of tapirs and plagiomenids both rare elements in midlatitude faunas and the absence of artiodactyls early horses and the hyopsodontid condylarth hyopsodus well represented at midlatitude localities are peculiar to the eocene arctic the eocene arctic macrofloras reveal a forested landscape analogous to the swamp cypress and broadleaf floodplain forests of the modern southeastern united states multiple climate proxies indicate a mild temperate early middle eocene arctic with winter temperatures at or just above freezing and summer temperatures of 20 c or higher and high precipitation at times this high precipitation resulted in freshwater discharge into a nearly enclosed arctic ocean basin sufficient to cause surface freshening of the arctic ocean supporting mats of the floating fern azolla fluctuating arctic ocean sea level due to freshwater inputs as well as tectonics produced temporary land bridges allowing land plants and animals to disperse between north america and both europe and asia

neogene lateral extension during convergence in the central alps evidence from interrelated faulting and backfolding around the simplonpass switzerland abstract the simplon fault zone occurs at the western end of the lepontine metamorphic dome in the central alps and represents a major low angle 25 dip normal fault associated with wsw ene directed lateral extension movement began in the early miocene and continued until at least 3 ma accumulating a total vertical throw of around 10 km corresponding to 24 km displacement parallel to the fault this displacement is partitioned between a broad zone of ductile mylonites in the footwall and a more discrete cataclastic detachment fault the simplon line in the central region simplon village to zwischbergen some 3 km of the total vertical displacement is accommodated by the discrete cataclastic zone due to the higher exposed structural level the importance of the cataclastic component increases towards the northwest in the northern steep belt around the simplonpass retrograde greenschist facies mylonite formation and younger cataclastic faulting associated with lateral extension occurred contemporaneously with sse vergent large scale backfolding representing nnw sse shortening and crustal thickening the combined effect was an overall shortening across the orogen accommodated by an extension parallel to the orogen allowing lateral escape of continental material normal faulting and the resultant lateral escape was not just superficial it extended to depths of at least 15 km and probably to depths exceeding 25 km the overall geometry and kinematics of the simplon fault zone and in particular the interplay between folding and faulting is very similar to that described from the metamorphic core complexes and detachment faults of the cordillera of the southwestern usa within the central alps extension was associated with rapid exhumation of the lepontine metamorphic dome as evidenced by very rapid cooling rates determined from mineral ages in the footwall similar to the cenozoic tectonics of the himalayas and tibet orogen parallel extension in the mountain belt may have been promoted by an overall reduction in the horizontal compressive stress in this direction more limited extension of the same age and orientation is known from a large area of the european foreland greater extension in the mountain belt compared with the foreland was accommodated by major strike slip transfer fault systems and was linked to coeval overthrusting of the lowlands towards the southwest in the external southwestern alps of haute provence france coeval folding and orogen parallel extension such as at the simplonpass were developed in a dextral transpressive regime due to oblique wnw ese convergence across the alps during the neogene

glacier shrinkage and modeled uplift of the alps 1 present day glacier reduction in the alps estimated from glacier inventories and induced viscoelastic response of a stratified earth s model is responsible for sizable uplift rates patches of 0 4 0 5 mm yr due to ice mass loss of largest ice complexes overprint a characteristic area of slower uplift of 0 1 0 2 mm yr signature of the phenomenon in the whole alpine chain viscous stress relaxation in the lower crust due to glacier mass loss after the end of the little ice age is expected to produce uplift rates of 0 32 mm yr leading to a total viscoelastic response up to 0 8 mm yr our predictions in the western alps show that viscoelastic response to present day glacier shrinkage forms a substantial fraction half of the observed uplift data attempts to constrain the contributions arising from active alpine tectonics and drainage must account for this uplift signal citation barletta v r c ferrari g diolaiuti t carnielli r sabadini and c smiraglia 2006 glacier shrinkage and modeled uplift of the alps geophys res lett 33 l14307 doi 10 1029 2006gl026490

present day geodynamics in the bend of the western and central alps as constrained by earthquake analysis summary the contrasted tectonics of the western central alps is examined using a synthesis of 389 reliable focal mechanisms the present day strain regime is mapped and interpolated for the entire alpine belt based on a newly developed method of regionalization the most striking feature is a continuous area of extension which closely follows the large scale topographic crest line of the alpine arc thrusting is observed locally limited to areas near the border of the alpine chain a majority of earthquakes within the alps and its forelands are in strike slip mode stress inversion methods have been applied to homogenous subsets of focal plane mechanisms in order to map regional variations in stress orientation the stress state is confirmed to be orogen perpendicular both for 3 in the inner extensional zones and 1 in the outer transcurrent transpressional zones extensional areas are well correlated with the part of the belt which presents the thickest crust as shown by the comparison with the bouguer anomaly and the average topography of the belt in the northwestern swiss alps extension is also correlated with currently uplifting zones these observations and our strain stress analyses support a geodynamic model for the western alps in which the current activity is mostly a result of gravitational body forces earthquakes do not provide any direct evidence for ongoing convergence in the alpine system but a relationship with ongoing activity of complex block rotations of the apulian microplate cannot be ruled out

closure of the africa eurasia north america plate motion circuit and tectonics of the gloria fault we examine the closure of the current plate motion circuit between the african north american and eurasian plates to test whether these plates are rigid and whether the gloria fault is an active transform fault we also investigate the possible existence of microplates that have been previously proposed to lie along these plate boundaries and compare the predicted direction of motion along the african eurasian plate boundary in the mediterranean with the direction of slip observed in earthquakes from marine geophysical data we obtain 13 transform fault azimuths and 40 3 m y average spreading rates 34 of which are determined from comparison of synthetic magnetic anomaly profiles to 140 observed profiles slip vectors from 32 earthquake focal mechanisms further describe plate motion detailed magnetic surveys north of iceland provide 11 rates in a region where prior plate motion models had few data magnetic profiles north of the azores triple junction record a rate of 24 mm yr 4 mm yr slower than used by prior models gloria and sea beam surveys accurately measure the azimuths of seven transform faults our plate motion model fits six of the seven within 2 two transform faults surveyed by gloria side scan sonar lie near famous area transform faults a and b and give azimuths 13 clockwise of them because recent studies show that short offset transforms such as transforms a and b are in many places oblique to the direction of plate motion we exclude azimuths from transforms with less than 35 km offset the best fitting and closure enforced vectors fit the data well except for a small systematic misfit to the slip vectors on right lateral slipping transforms slip vectors tend to be a few degrees clockwise of plate motion and mapped fault azimuths whereas on left lateral slipping transforms slip vectors tend to be a few degrees counterclockwise of plate motion and mapped fault azimuths we search the long eurasia north america boundary for evidence of an additional plate but find no systematic misfits to the data in particular if a spitsbergen plate exists and moves relative to eurasia its motion is less than 3 mm yr an africa eurasia euler vector determined by adding the eurasia north america and africa north america euler vectors is consistent with the gloria fault trend and with slip vectors from eastern azores gibraltar ridge focal mechanisms a small circle centered at the africa eurasia closure enforced pole fits the trace of the gloria fault the model in which closure was enforced predicts 4 mm yr slip across the azores gibraltar ridge and west northwest convergence near gibraltar 45 more oblique than suggested by a recent model based on compressive axes of focal mechanisms moreover our model predicts directions of plate motion that agree well with northwest trending slip vectors from thrust earthquakes between gibraltar and sicily because closure enforced vectors fit the data nearly as well as the best fitting vectors we conclude that the data are consistent with a rigid plate model and with the gloria fault being a transform fault

quaternary erosion induced isostatic rebound in the western alps the high elevation and deep incision of the alps have traditionally been used as an argument for recent tectonic activity that has elevated the belt and increased erosion rates normal faulting and horizontal extension however dominate current tectonic activity and isostatic compensation of thinning crust should lead not to increased but to decreased mean elevations here we test the idea that enhanced quaternary erosion of the alps and isostatic compensation of the mass removed can account for the distribution of present day geodetically measured rates of vertical movement in the western alps using geophysical relief and kuhlemann s estimated average erosion rate for the alps we quantify the spatial distribution of erosion and the volume of eroded rock respectively from these we obtain a map of rock eroded within a given time span the calculated isostatic response of the alpine lithosphere to erosional unloading for a variety of values of the flexural rigidity of the alpine lithosphere reaches a maximum of 500 m since 1 ma in the inner swiss alps and vertical movement extends across the entire belt including peri alpine basins assuming a steady erosion rate since 1 ma this rebound accounts for half of the measured vertical motion of 1 1 mm yr in the southern valais thus a significant fraction 50 of the present day vertical movement results from the isostatic response to enhanced erosion during plio quaternary time

evidence for a post 3 16 ma change in nubia eurasia north america plate motions we combine updated gps velocities from the nubian nu eurasian eu and north american na plates with 500 new 3 16 myr average seafloor spreading rates and nine transform fault azimuths from the northern atlantic and arctic basin seafloor spreading centers to estimate and test for changes in the relative motion between these plates the numerous new seafloor spreading rates and gps velocities improve our ability to detect recent changes in the relative motions of these plates the angular velocity vector that best fits the eu na gps velocities lies significantly north of the 3 ma average pole in accord with previously published geologic evidence that the eu na pole has migrated northward since v3 ma although we also find evidence for a significant post 3 ma change in nu na motion it is less compelling because the nubian plate gps velocity field is sparse and nu na seafloor spreading rates appear to have remained steady within the 1 mm yr 31 uncertainties if we systematically decrease the seafloor spreading rates to correct for outward displacement of seafloor spreading magnetic lineations the nu eu pole derived from gps site velocities lies more than 30 angular degrees south of the tightly constrained 3 ma average estimate and predicts significantly slower and more oblique present day nu eu convergence in the mediterranean both models for nu eu motion pass a key test for their accuracy namely they correctly predict strike slip motion along the well mapped gloria fault east of the azores the change to more oblique nu eu motion may reflect increasing difficulty in maintaining margin normal convergence within this continent continent collision zone 8 2003 published by elsevier b v

the arc of the western alps in the light of geophysical data on deep crustal structure recently two international deep seismic campaigns in the western and central alps etude continentale et oceanique par reflexion et refraction sismique progetto strategico crosta profonda ecors crop roure et al 1996b and national research program 20 nrp 20 pfiffner et al 1997 have been completed here we present a synoptic interpretation of the wealth of geophysical data about deep crustal structure of the alps collected during the past 40 years including the two above mentioned geophysical projects the tectonic reinterpretation of the ecors crop high resolution seismic transect is based on an evaluation of the deep crustal structure by integrating new and literature data on surface geology combined with previously published interpretations regarding the central alps schmid et al 1996 this study reveals substantial differences in geometry and kinematics between transects across the western and central alps respectively at depth the transition between the central and western alps is marked by the western limits of an adriatic lower crustal wedge shaped structure and the northeastern limits of a similar structure made of european lower crustal material at the surface it coincides with a corridor of dextral strike slip along the tonale and simplon lines in addition the thickness of the seismogenic zone shows a remarkable variation from over 40 km beneath the penninic realm of the western alps to less than 20 km in the central alps the formation of the western alpine arc was initiated during convergence and collision before 35 myr ago when the adriatic micro plate moving northward with respect to the european foreland caused sinistral transpression in the western alps while the central and eastern alps underwent head on convergence and collision during the post collisional stage i e after 35 myr ago the arcuate shape of the western alps was accentuated by wnw directed movement and anticlockwise rotation of the adriatic microplate decoupled from the central and eastern alps along the tonale simplon dextral shear zone this led to wedging of lower crustal slices both in the western alps and in the central alps the new tectonic interpretation of the ecors crop transect allows a kinematic model to be established for crustal shortening in the western alps during the past 35 myr the ivrea mantle plays the role of a backstop in our tectonic model we distinguish three episodes of post collisional crustal shortening along the ecors crop transect from 35 to 30 ma the brianconnais basement was back thrusted over the gran paradiso units leading to 30 km of crustal shortening first episode in the early miocene movements concentrated mainly along the penninic frontal thrust and resulted in about 60 km shortening second episode post 12 ma shortening within the external massifs is associated with folding in the jura mountains when the crust was shortened by an additional 30 km third episode

mantle unrooting in collisional settings abstract we present a two dimensional numerical model to study the thermo mechanical evolution of the lithosphere under a convergence regime in order to define the conditions that lead to lithospheric mantle break up and consequent unrooting a newtonian rheology with a temperature dependent viscosity is considered the system is not closed and horizontal flow through lateral boundaries is permitted a horizontal velocity is imposed at the top of the model to simulate compression whereas velocity vanishes at the bottom of the model the initial conditions correspond to a homogeneous lithosphere with a constant heat production in the crust the analysis of variations of maximum shear stress strain rate and total kinetic energy allowed us to define four major stages during the mantle unrooting process orogenic growth initiation of gravitational instability until lithospheric failure sinking of the detached lithosphere and relaxation of the system numerical results also show that the conditions for lithospheric unrooting strongly depend on the convergence velocity the wideness of the deformation zone and the imposed rheology

petrological constraints on the eclogite and blueschistfacies metamorphism and p t t paths in the western alps phase relationships in the model mafic system and geothermobarometry allow discrimination of four main groups of high p rocks in the nappes of the western alps very high p eclogite facies including kyanite eclogites and coesite pyrope assemblages eclogite facies paragonite zoisite eclogites high t blueschist facies glaucophane garnet lawsonite assemblages and low t blueschist facies glaucophane lawsonite pumpellyite assemblages the blueschist facies eclogite facies transition is promoted chiefly by increasing t low bulk xmg and relatively low h2o the variety of assemblages and the heterogeneous approach to equilibrium observed in the alpine rocks are not only constrained by the intersection of the reaction surfaces in p t x space but also by the effect and timing of the processes which control kinetics i e pervasive deformation and fluid infiltration the faster rate of dehydration reactions relative to hydration reactions along with the fact that different bulk compositions crossed the reaction curves at different temperatures and times all may have induced h2o gradients and contributed to the heterogeneous distribution of deformation through a process of reaction enhanced ductility also mass transfer may have been an effective process in determining the type of high p assemblage in particular rock volumes as regards the p t t paths only the post climax histories are recorded well in the alpine nappes post eclogitic exhumation paths at decreasing temperatures characterize structurally higher nappes which were first subducted during the early alpine cretaceous event in contrast more or less isothermal decompression paths characterize structurally deeper nappes formed by westward propagation of the underthrust surfaces during the early alpine event and the subsequent meso alpine palaeogene collision between the european and african plates in the western alps prevalent eclogite facies conditions were attained during the metamorphic climax of the early alpine subduction while blueschist facies recrystallization characterizes the early alpine exhumation of the eclogitized units and the subsequent intracontinental underthrusts linked to the meso alpine continental collision

travel time tomography of the european mediterranean mantle down to 1400 km abstract the 3 d p wave velocity structure of the mantle below europe the mediterranean region and a part of asia minor is investigated this study is a considerable extension of an earlier tomographic experiment that was limited to imaging upper mantle structure only here the earth s volume under study encompasses the mantle to a depth of 1400 km and we increase the number of international seismological centre isc data for inversion by a factor of four by taking more years of observation and by including data from teleseismic events the most important departure from the earlier study is that we do not use the jeffreys bullen model as a reference model but an improved radially symmetric velocity model the pm2 model which is appropriate for the european mediterranean mantle our inversion procedure consists of two steps first the radial model pm2 is determined from the isc delay times by a nonlinear trial and error inversion of the data as opposed to the jeffreys bullen model the new reference model has a high velocity lithosphere a low velocity zone and seismic discontinuities at depths of 400 and 670 km next the isc data are corrected for effects related to the change in reference model and inverted for 3 d heterogeneity relative to the pm2 model we follow this two step approach to attain a better linearizable tomographic problem in which ray paths computed in the pm2 model provide a better approximation of the actual ray paths than those computed from the jeffreys bullen model hence the two step scheme leads to a more credible application of fermat s principle in linearizing the tomographic equations inversion results for the 3 d heterogeneity are computed for both the uncorrected isc data and for the pm2 data the data fit obtained in the two step approach is slightly better than in the inversion of isc data using the jeffreys bullen reference model a comparison of the tomographic results demonstrates that the pm2 data inversion is to be preferred to assess the spatial resolution an analysis is given of hit count patterns sampling of the mantle by ray paths and results of sensitivity tests with 3 d synthetic velocity models the spatial resolution obtained varies with position in the mantle and is studied both in map view and in cross section in the well sampled regions of the mantle the spatial resolution for larger scale structure can qualitatively be denoted as reasonable to good and at least sufficient to allow interpretation of larger scale anomalies a comparison is made of the results of this study with independent models of s velocity heterogeneity obtained in a number of investigations and with a prediction of the seismic velocity structure of the mantle computed from tectonic reconstructions of the mediterranean region in the context of this comparison interpretations of large scale positive anomalies found in the mediterranean upper mantle in terms of subducted lithosphere are given specifically addressed are subduction below southern spain below the western mediterranean and italy and below the aegean in the last region a slab anomaly is mapped down to depths of 800 km