imaging alpine crust using ambient noise wave equation tomography we present an improved crustal vs model and moho depth map using ambient noise wave equation tomography the so called ambient noise wave equation tomography is a method to invert seismic ambient noise phase dispersion data based on elastic waveform simulation which accounts for 3 d and finite frequency effects we use cross correlations of up to four years of continuous vertical component ambient seismic noise recordings from 304 high quality broadband stations in the alpine region we use model lsp eucrust1 0 obtained from traditional ambient noise tomography as initial model and we iteratively improve the initial model by minimizing frequency dependent phase traveltime differences between the observed and synthetic waveforms of rayleigh waves in the period range 10 50 s we obtain the final model after 15 iterations with 65 total misfit reduction compared to the initial model at crustal depth the final model significantly enhances the amplitudes and adjusts the shapes of velocity anomalies at moho and upper mantle depth the final model corrects an obvious systematic velocity shift of the initial model the resulting iso velocity moho map confirms a moho step along the external side of the external crystalline massifs of the northwestern alps and reveals underplated gabbroic plutons in the lower most crust of the central and eastern alps ambient noise wave equation tomography turns out to be a useful tool to refine shear wave velocity models obtained by traditional ambient noise tomography based on ray theory

mapping upper mantle anisotropy beneath se france by sks splitting indicates neogene asthenospheric flow induced by apenninic slab roll back and deflected by the deep alpine roots the presence of two regional seismic networks in southeastern france provides us high quality data to investigate upper mantle flow by measuring the splitting of teleseismic shear waves induced by seismic anisotropy the 10 three component and broadband stations installed in corsica provence and western alps efficiently complete the geographic coverage of anisotropy measurements performed in southern france using temporary experiments deployed on geodynamic targets such as the pyrenees and the massif central teleseismic shear waves mainly sks and skks are used to determine the splitting parameters the fast polarization direction and the delay time delay times ranging between 1 0 and 1 5 s have been observed at most sites but some larger delay times above 2 0 s have been observed at some stations such as in northern alps or corsica suggesting the presence of high strain zones in the upper mantle the azimuths of the fast split shear waves define a simple and smooth pattern trending homogeneously wnw ese in the nice area and progressively rotating to nw se and to ns for stations located further north in the alps this pattern is in continuity with the measurements performed in the southern massif central and could be related to a large asthenospheric flow induced by the rotation of the corsica sardinia lithospheric block and the retreat of the apenninic slab we show that seismic anisotropy nicely maps the route of the slab from the initial rifting phase along the gulf of lion 30 22 ma to the drifting of the corsica sardinia lithospheric block accompanied by the creation of new oceanic lithosphere in the liguro provencal basin 22 17 ma in the external and internal alps the pattern of the azimuth of the fast split waves follows the bend of the alpine arc we propose that the mantle flow beneath this area could be influenced or perhaps controlled by the alpine deep penetrative structures and that the alpine lithospheric roots may have deflected part of the horizontal asthenospheric flow around its southernmost tip

rollback orogeny model for the evolution of the swiss alps the construction of the european alps and the himalayas has been related to the convergence and subsequent collision of two continental plates nearly all models of orogeny build on this concept and all of them relate the stacking of nappes and the build up of topography to compressional forces at work in response to the collision between two continental plates for the central european alps however these models fail to explain the first order observations of a mountain belt which particularly includes the striking isostatic imbalance between the low surface topography and the thick crust beneath the alps here we review and synthesize data on the geologic architecture of the central alps the chronology and pattern of crustal deformation and information about the deep crustal structure derived from seismic tomography furthermore we discuss the intrinsic and explicit assumptions in the kinematic models of alpine evolution in the context of plate tectonic considerations we combine these views with progress in understanding that has been gained through subduction and collision isostatic mass and force balancing and with information that has been collected on the modern seismic regime we conclude that a rollback orogeny model for the european plate offers the most suitable concept to explain the ensemble of surface and deep lithosphere observations in this model gravity forces drive the evolution of the orogen and the construction of surface topography is accomplished without the requirement of a hard collision between two continents

earthquakes in the western alpine mantle wedge abstract the assessment of seismic activity in the shallow continental mantle has long been hindered by the low resolution of both seismic imaging and earthquake locations in young collision zones here we combine the most recent and high resolution image of the lithospheric structure of the western alps with a high quality dataset of anomalously deep earthquakes recorded in the same area in the past 25 yrs we show that these earthquakes are aligned on an active lithospheric strike slip fault and we provide evidence that this fault is located in the mantle wedge beneath the adriatic moho our results i provide direct evidence that deep material can be seismogenic or not depending on the lithology ii confirm the role of serpentinization in favoring the aseismic creep of mantle rocks and iii demonstrate that the upper mantle can be stiff and seismogenic not only in cold cratons but also in young orogenic belts

how deep can we find the traces of alpine subduction 1 slab like seismic velocity heterogeneities below the alpine chain interpreted as subducted lithosphere are imaged by tomographic studies down to only about 300 km depth a non negligible discrepancy therefore exists between tomographic and geological data the latter indicating at least 500 km of tertiary convergence at trench yet a recently published tomographic study detects a pronounced high velocity anomaly at the bottom of the upper mantle right below the alpine area combining tomographic images of the mantle geological findings and plate system kinematics we investigate how the presence of this feature in the transition zone below the alps can be traced back to the tertiary alpine subduction and possibly explain the observed discrepancy we propose that a part of the fast velocity body now residing just above the 660 km discontinuity once belonged to the alpine slab torn off by an event occurred at about 30 35 ma

motion of the pacific plate relative to eurasia and its potential relation to cenozoic extension along the eastern margin of eurasia extension along the eastern margin of eurasia has been regarded commonly as a far field effect of the india eurasia collision however some aspects of the timing and location of this extension make a link between the collision and extension difficult we suggest that some extensional features commonly interpreted as effects of the collision may be related more simply to changes in plate convergence rates along the eastern plate boundary of eurasia an analysis of the motion of the pacific plate relative to eurasia suggests that the rate of pacific eurasia convergence varied significantly during the tertiary period from a late cretaceous convergence rate of 120 140 mm yr the rate declined substantially during early tertiary time and reached a minimum in eocene time of 30 40 mm yr in oligocene to earliest miocene time the average convergence rate increased moderately to 70 95 mm yr then decreased again to 65 70 mm yr during early to middle miocene time from late miocene to the present the rate of convergence increased to an average of 100 110 mm yr the paleocene through middle miocene episode of relatively slow convergence correlates with a period of widespread extension along the eastern margin of eurasia decreased convergence may have been related to a net reduction in horizontal compressional stress transmitted between the pacific and eurasian plates which resulted in widespread extension adjacent to the margin of eurasia

3d modeling of crustal shortening influenced by along strike lithological changes implications for continental collision in the western and central alps abstract the european alps show major along strike variations in the partitioning of crustal shortening between the upper and the lower plates either explained by lateral variation in temperature and rheology or by along strike changes in crustal scale lithology here we present the first 3d thermomechanical geodynamic model to test the potential impact of along strike lithological and associated rheological changes and lateral temperature variations on crustal shortening during continental collision in the western and central alps numerical modeling results suggest a minor impact of lateral thermal variations and lithological changes in the lower plate for the alpine orogeny in contrast lateral lithological changes in the upper plate structure defined by the presence of the buried ivrea body in the western alps should play a major role in determining the present day architecture of the alpine belt our models suggest that the ivrea body acts as a strong rigid buttress constraining crustal shortening to the lower plate promoting the formation of the jura fold and thrust belt and supporting the higher surface elevation observed in the western alps on the other hand in the central alps where the ivrea body is absent much broader distribution of crustal shortening results in lower surface elevation numerical modeling results also suggest that crustal shortening in the central alps is focused within the upper plate during the final stages of continental collision favored by the indentation of thickened upper crust of the lower plate and by detachment of the adriatic upper crust from the underlying lithosphere

strain factorization by identical finite increments and its use abstract progressive deformation of layered rocks up to their present finite strain and bedding attitudes is simulated by an identical increment path model a deformation matrix connecting the initial and deformed states is formulated with the conceptual help of a hypothetical three step deformation path two sequential non coaxial rigid body rotations bring an original horizontal bed to a pre strain attitude a pure shear follows the rotations thus the deformation matrix is calculated by premultiplying the matrices representing the three stages of deformation sixteen identical tectonic deformation increments are then obtained by four times sequentially extracting the square root of that deformation matrix the method has been applied to the campbell block a late jurassic fault bounded turbidite sliver in the northern sierra nevada california the block was deformed into a train of folds during the nevadan orogeny fourteen oriented samples were collected for march strain measurements based on the strains and bedding attitudes the block was divided into 11 deformation bands of three types by applying the proposed model to the campbell block it was found that the deformation style of the block consists of a non coaxial deformation path the regional strain is accommodated among the several deformation bands by strain partitioning the collective deformation indicated by this model is transpressional

3 d pn tomography reveals continental subduction at the boundaries of the adriatic microplate in the absence of a precursor oceanic slab abstract slab pull generated by subducting oceanic lithosphere is generally considered as a major trigger for the onset of continental subduction however this may be in conflict with the occurrence of uhp terranes bearing no evidence of oceanic lithospheric rocks involved in the exhumation cycle here we image the uppermost mantle p velocity structure beneath the central mediterranean suggesting the possibility that the initiation of continental subduction may not require a precursor oceanic slab we combine i a three step inverted 3 d pn tomography model of the adriatic microplate with ii available geologic constraints and palinspastic reconstructions of the africa eurasia plate boundary zone our pn tomography model reveals elongated regions with vp

hp uhp exhumation during slow continental subduction self consistent thermodynamically and thermomechanically coupled model with application to the western alps abstract the exhumation of ultra high pressure uhp crustal material often occurs at the rear of sedimentary accretion wedges e g alps himalaya norway however the mechanisms of deep 100 km burial in slow 10 mm yr 1 and slow at the later stage

near surface diagnostics of dripping or delaminating lithosphere 1 in various geological regions it has been postulated that the mantle lithosphere has been thinned or completely removed two of the primary removal mechanisms that have been put forward include 1 delamination a wholesale peeling away of a coherent block of the mantle lithosphere and 2 lithospheric dripping viscous rayleigh taylor instability of the mantle lithosphere using computational models we investigate several near surface observables to determine if these may be diagnostic of either often ambiguous removal mechanism surface topography associated with delamination has a broad region of uplift above the lithospheric gap and a localized and mobile zone of subsidence at the delaminating hinge with dripping lithosphere the topographic expression is symmetric and fixed above the downwelling delamination of mantle lithosphere is more efficient than dripping for thermal heating of the crust the onset is more rapid and the elevated temperatures persist longer the resultant crustal p t t paths show modest pressure variations and high temperature increases with large scale delamination or dripping delamination also causes contraction directly above the migrating hinge and distal extension dripping lithosphere induces superimposed contraction and extension above and symmetric about the viscous instability for all the observables if only a portion of the mantle lithosphere is removed by viscous instability delamination inherently removes all of the mantle lithosphere the differences between the two removal mechanisms are even more pronounced with only partial removal of the mantle lithosphere uppermost mantle lithosphere remains well coupled to the crust leading to lower surface temperature variations and broad zones of crustal deformation thickening

seismic evidence for the absence of deeply subducted continental slabs in the lower lithosphere beneath the central orogenic belt of china abstract a study of teleseismic p wave tomography was carried out along the central orogenic belt of china cobc consisting of the qinling tongbai hong an dabie and sulu orogens that were created by northward subduction of the south china block beneath the north china block to subarc depths in the paleozoic to mesozoic the results show the presence of high velocity anomaly underneath the cobc and its adjacent cratonic regions but the absence of high velocity anomaly in the lower lithosphere beneath the cobc the high velocity anomaly is deep underneath the cobc possibly representing the relic of the subducted paleo tethyan oceanic slab predating the continental deep subduction of the south china block the high velocity anomaly is also deep underneath the southeastern part of the north china block possibly being a relic of the subducted paleo pacific slab however no relic of the deeply subducted continental slab is seismically detected in the lower lithosphere beneath the dabie and sulu orogens though the occurrence of ultrahigh pressure metamorphic rocks in the two orogens has clearly demonstrated continental deep subduction to subarc depths in the triassic instead there is only the relatively flat and uniform moho boundary beneath the cobc region indicating that the subduction thickened continental lithosphere along the cobc has lost its root it is inferred that the orogenic root beneath the cobc region has been foundered into the asthenospheric mantle at the postcollisional stage the foundering would have taken place in the early cretaceous as indicated by the extensive occurrence of magmatic and migmatitic rocks along the cobc as a consequence the seismic tomography cannot detect the deeply subducted continental slab remnant from the triassic period it is possible that the tomographic visibility of continental subduction in the lower lithosphere is limited to the past 100 myr

orogenic structure of the eastern alps europe from transalp deep seismic reflection profiling abstract the transalp group comprising of partner institutions from italy austria and germany acquired data on a 340 km long deep seismic reflection line crossing the eastern alps between munich and venice although the field work was split into four campaigns between fall 1998 and summer 2001 the project gathered for the first time a continuous profile across the alps using consistent field acquisition and data processing parameters these sections span the orogen itself at its broadest width as well as the editor fred davey and the two adjacent basins vibroseis and explosion data complementary in their depth penetration and resolution characteristics were obtained along with wide angle and teleseismic data the profile shows a bi vergent asymmetric structure of the crust beneath the alpine axis which reaches a maximum thickness of 55 km and 80 100 km long transcrustal ramps the southward dipping sub tauern ramp and the northward dipping sub dolomites ramp strongly reflective patterns of these ramps can be traced towards the north to the inn valley and towards the south to the valsugana thrust belt both of which show enhanced seismicity in the brittle upper crust the seismic sections do not reveal any direct evidence for the presence of the periadriatic fault system the presumed equivalent to the insubric line in the western alps according to our new evolutionary model the sub tauern ramp is linked at depth with remnants of the subducted penninic ocean the crocodile type model describes an upper lower crustal decoupling and wedging of both the european and the adriatic african continents

hints on the deformation penetration induced by subductions and collision processes seismic anisotropy beneath the adria region central mediterranean adria is a small region surrounded by three mountain belts the alps the apennines and the dinarides built up by long evolution of subduction and collisional systems we present 253 shear wave splitting measurements obtained by studying more than 100 teleseismic events for 12 stations sks splitting measurements show 3 d complexity and quite strong upper mantle deformation we carefully analyzed results in terms of back azimuthal coverage and interpret measurements as related to adria rotation and to subductions evolution in the northern part of adria the anisotropy pattern follows the arcuate shape of the alps the same pattern parallel to the mountains occurs along the apennines but fast directions show a sudden change in the adria foreland this lateral variation has been analyzed to isolate a distinct adria mantle anisotropic pattern which is identified as ne sw fast direction along the western microplate boundary and as n s fast direction at trieste this pattern might be induced by drag effect of the counterclockwise rotation of adria lithosphere that behaves as an independent microplate as identified by gps data our measurements suggest that the geodynamic process that generated the alps is more efficient deforming a larger volume of mantle than its apennine counterpart moreover the mantle circulation we hypothesize looking at the regional scale patterns of anisotropy requires the existence of an escape route beneath the alps apennines transition through which the mantle flows and feed circulation in the tyrrhenian mantle system as suggested by previous geodynamic models and as seen by some tomographic studies

exhumation of high pressure rocks driven by slab rollback rocks metamorphosed under high pressure hp and ultra high pressure uhp conditions in subduction zones come back to the surface relatively soon after their burial and at rates comparable to plate boundary velocities in the mediterranean realm their occurrence in several belts related to a single subduction event shows that the burial exhumation cycle is a recurrent transient process using the calabria apennine and aegean belts as examples we show that the exhumation of hp rocks is associated in time and space with the subduction of small continental lithosphere blocks that triggers slab rollback creating the necessary space for the exhumation of the buoyant continental crust that was deeply buried just before the buoyancy force of the subducted crust increases until this crust detaches from the downgoing slab it then exhumes at a rate that depends directly on the velocity of trench retreat to become part of the overriding plate heated from below by the asthenosphere that flows into the opening mantle wedge the exhumed crust weakens and undergoes core complex type extension responsible for a second stage of exhumation at a lower rate the full sequence of events that characterizes this model crust mantle delamination slab rollback and trench retreat hp rock exhumation asthenosphere heating and core complex formation arises entirely from the initial condition imposed by the subduction of a small continental block no specific condition is required regarding the rheology and erosion rate of hp rocks the burial exhumation cycle is transient and can recur every time a small continental block is subducted

application of a three dimensional finite element method to strain field analyses abstract i have programmed a three dimensional finite element model to spatially integrate distributed strains the mathematics is based on cobbold and percevault 1983 the method finds the pre strain configuration of a region by unstraining rectangular prisms into parallelepipeds then rotating and translating them iteratively to minimize the interelement deviations a numerical singular value decomposition calculates the necessary rotations and the final cycle of further strains to make a best fit of the elements the deviation of elements from holes formed by their nearest neighbors indicates the degree of compatibility of strain measurements the program reproduces the rotations required in examples with analytic solutions even though only a single horizontal layer of elements is used the fitting procedure correctly calculates rotations in all three dimensions from the constraints of strain compatibility three dimensional strains measured in an archean greenstone belt of northeastern minnesota were integrated using a layer of nearly equant finite elements rotations calculated from the pre tectonic configuration accord with field observations of fabrics indicating variable amounts of shear rotations about horizontal axes are minimal precluding large vertical shears an estimate of 40 true horizontal north south shortening across the belt can be made from the undeformed configuration

slab breakoff a model for syncollisional magmatism and tectonics in the alps slab breakoff is the buoyancy driven detachment of subducted oceanic lithosphere from the light continental lithosphere that follows it during continental collision in a recent paper davies and von blanckenburg 1994 have assessed the physical conditions leading to breakoff by quantitative thermomechanical modeling and have predicted various consequences in the evolution of mountain belts breakoff will lead to heating of the overriding lithospheric mantle by upwelling asthenosphere melting of its enriched layers and thus to bimodal magmatism breakoff will also lead to thermal weakening of the subducted crustal lithosphere thereby allowing buoyant rise of released crustal slices from mantle depths in this paper we present a test of this model in the tertiary evolution of the european alps in the alps both basaltic and granitoid magmatism occur between 42 and 25 ma following the closure of oceanic basins by subduction and continental collision the granitoids are now well established to result from mixing of basalt with assimilated continental crust to identify the tectonically crucial origin of the partial mantle melts we have compiled all published geochemical and isotopic data of numerous mafic dykes occurring throughout the whole alpine arc their trace element and isotopic composition suggests that they have been formed by low degree melting of the mechanically stable lithospheric mantle we see no evidence for melting of asthenospheric mantle it was thus not decompressed to depths shallower than 50 km once initiated rapid lateral migration of slab breakoff will result in a linear trace of magmatism in locally thermal weakened crust this explains why all alpine magmatic rocks intruded almost synchronously along a strike slip fault the periadriatic lineament a compilation of ages from penninic high pressure rocks subducted to depths of up to 100 km shows that subduction took place at circa 55 40 ma followed by uplift at 40 35 ma from the short time interval between their uplift and the onset of magmatism we infer that both processes have been induced by the breakoff the slab breakoff model fulfills its predictions in the case of the alps and therefore supports the assumptions made in the theoretical model on a geological basis we believe that the characteristic association of magmatic activity with the return of high pressure rocks to the surface allows the identification of this process in the earth s mountain belts

mantle dynamics in the mediterranean the mediterranean offers a unique opportunity to study the driving forces of tectonic deformation within a complex mobile belt lithospheric dynamics are affected by slab rollback and collision of two large slowly moving plates forcing fragments of continental and oceanic lithosphere to interact this paper reviews the rich and growing set of constraints from geological reconstructions geodetic data and crustal and upper mantle heterogeneity imaged by structural seismology we proceed to discuss a conceptual and quantitative framework for the causes of surface deformation exploring existing and newly developed tectonic and numerical geodynamic models we illustrate the role of mantle convection on surface geology a coherent picture emerges which can be outlined by two almost symmetric upper mantle convection cells the downwellings are found in the center of the mediterranean and are associated with the descent of the tyrrhenian and the hellenic slabs during plate convergence these slabs migrated backward with respect to the eurasian upper plate inducing a return flow of the asthenosphere from the backarc regions towards the subduction zones this flow can be found at large distance from the subduction zones and is at present expressed in two upwellings beneath anatolia and eastern iberia this convection system provides an explanation for the general pattern of seismic anisotropy in the mediterranean first order anatolia and adria microplate kinematics and may contribute to the high elevation of scarcely deformed areas such as anatolia and eastern iberia more generally the mediterranean is an illustration of how upper mantle small scale convection leads to intraplate deformation and complex plate boundary reconfiguration at the westernmost terminus of the tethyan collision

eoalpine and mesoalpine tectonics in the southern alps according to structural and stratigraphic data eoalpine and mesoalpine tectonics can be clearly recognized in the southern alps and distinguished from the better known neoalpine deformation

first seismic evidence for continental subduction beneath the western alps the first discovery of ultrahigh pressure coesite in the european alps 30 years ago led to the inference that a positively buoyant continental crust can be subducted to mantle depth this had been considered impossible since the advent of the plate tectonics concepts although continental subduction is now widely accepted there remains debate because there is little direct geophysical evidence of a link between exhumed coesite at the surface and subducted continental crust at depth here we provide the first seismic evidence for continental crust at 75 km depth that is clearly connected with the european crust exactly along the transect where coesite was found at the surface our data also provide evidence for a thick suture zone with downward decreasing seismic velocities demonstrating that the european lower crust underthrusts the adriatic mantle these findings from one of the best preserved and long studied ultrahigh pressure orogens worldwide shed decisive new light on geodynamic processes along convergent continental margins

india asia convergence driven by the subduction of the greater indian continent the most spectacular example of plate convergence on earth was the motion of the indian plate towards eurasia and the subsequent collision density estimates of the greater indian continent after its upper crust is scraped off at the himalayan front suggest that this continental plate is readily subductable potentially explaining why the convergence did not halt on collision the most spectacular example of a plate convergence event on earth is the motion of the indian plate towards eurasia at speeds in excess of 18 cm yr 1 ref 1 and the subsequent collision continental buoyancy usually stalls subduction shortly after collision as is seen in most sections of the alpine himalayan chain however in the indian section of this chain plate velocities were merely reduced by a factor of about three when the indian continental margin impinged on the eurasian trench about 50 million years ago plate convergence accompanied by eurasian indentation persisted throughout the cenozoic era1 2 3 suggesting that the driving forces of convergence did not vanish on continental collision here we estimate the density of the greater indian continent after its upper crust is scraped off at the himalayan front and find that the continental plate is readily subductable using numerical models we show that subduction of such a dense continent reduces convergence by a factor similar to that observed in addition an imbalance between ridge push and slab pull can develop and cause trench advance and indentation we conclude that the subduction of the dense indian continental slab provides a significant driving force for the current india asia convergence and explains the documented evolution of plate velocities following continental collision

high resolution 3 d p wave model of the alpine crust summary the 3 d p wave velocity structure of the alpine crust has been determined from local earthquake tomography using a set of high quality traveltime data the application of an algorithm combining accurate phase picking with an automated quality assessment allowed the repicking of first arriving p phases from the original seismograms the quality and quantity of the repicked phase data used in this study allows the 3 d imaging of large parts of the alpine lithosphere between 0 and 60 km depth our model represents a major improvement in terms of reliability and resolution compared to any previous regional tomographic studies of the alpine crust first order anomalies like crust mantle boundary moho and the ivrea body in the western alps are well resolved and in good agreement with previous studies in addition several consistent small scale anomalies are visible in the tomographic image a clear continuation of the lower european crust beneath the adriatic moho in the central alps is not observed in our results the absence of such a signature may indicate the eclogitization of the subducted european lower crust in the central alps in agreement with previous results the additional analysis of focal depths in our new 3 d p wave model shows that all studied earthquakes in the northern foreland have occurred within the european crust waveforms and focal depths suggest that at least one of the analysed events south of the alps is located in the adriatic mantle

trace element and nd sr isotope constraints on origin of the chifeng flood basalts north china abstract the chifeng flood basalt field is a component part of the neogene basalt belt in north china the basalts erupted on the northern edge of the north china craton during the miocene and are subalkaline tholeiites in composition with normative olivine and quartz they show a limited variation in sio2 contents ranging from 48 47 to 51 61 wt and evolved nature as indicated by both mg 47 4 65 2 and large variations in ni and cr contents they share the geochemical characteristics of the high ti group of the gondwana continental flood basalts with tio2 2 10 2 48 wt ti y 399 584 zr y 4 35 7 21 and sr 299 ppm being very similar to the paranapanema high ti basalts brazil in contrast to the paranapanema lavas which generally have large negative nb sr and p anomalies and negative end values the chifeng basalts have no striking sr and p anomalies with variable nb anomalies and positive end values the chifeng basalts are characterised by low rb ba and rb sr high ti y ti yb and ba y and variable but high nb la ratios and show progressive enrichment of k2o p2o5 nb ba sr and zr with decreasing end values which preclude the possibility that the magmas were contaminated by crustal materials their 87 sr 86 sr ratios and end values are in the ranges of 0 7039 to 0 7052 and of 1 6 to 4 4 respectively showing more restricted variations compared with those of the mantle xenoliths from the hannuoba basalt field nd sr isotopic compositions and the trace element ratios such as la nb k nb rb nb ba nb and zr nb show the oib like geochemical signatures the chifeng basalts were derived from a heterogeneous source region in the asthenosphere with little interaction with lithospheric mantle it is likely that a combination of varied degrees of partial melting with the chemical heterogeneity of a source region could result in negative correlation arrays of end vs k2o p2o5 rb sr ba nb zr ti yb ba y and la sm in which relatively low lil and hfs element contents and ratios and high end values would be produced by the relatively high degrees of partial melting of the heterogeneous source region

4 d evolution of se asia s mantle from geological reconstructions and seismic tomography abstract how the collision between india and asia is related to processes deeper in the mantle is unclear here we compare geological reconstructions of block motions within asia since 50 ma with the tomographically imaged three dimensional 3 d morphology of subducted lithosphere to obtain insight into the spatiotemporal evolution of mantle structure past positions of the convergent margin show remarkable similarities with slab geometry at specific depths the striking change in slab geometry from a linear structure beneath 1100 km to an increasingly distorted shape at depths of less than 700 km results from collision the slab contours match the progressive deformation of asia s margin including india s indentation and sundaland s extrusion ever since the onset of collision the indian plate appears to have overridden its own sinking mantle and it does not seem at present to underthrust tibet significantly north of the zangbo suture if correct this observation would provide further evidence against models of plateau build up involving indian lithosphere the tomographic images beneath india confirm that asian deformation has absorbed at least 1500 km of convergence since collision began from the match between the southeastward motion of sundaland between 40 and 20 ma and the principal change in slab structure between 700 and 1100 km depths we infer that lateral advection in the mantle is small and that the sinking rate beneath sunda was 2 cm yr in the lower mantle and 5 cm yr above the transition zone

pb isotopic compositions of archean komatiites and sulfides abstract pb isotopic compositions were measured in komatiites and other volcanic rocks from several archean greenstone belts and in magmatic and exhalative sulfides associated with these volcanics thin komatiite flows generally yielded pb pb isochron ages consistent with other geochronological or geological information but thicker basaltic flows gave ages that are too young these results suggest that the thin komatiite flows were hydrothermally altered during eruption and that this event is recorded by the pb pb age thicker basaltic flows resisted alteration during eruption but were affected by later events their pb pb ages probably correspond to the time of metamorphism and not to the time of emplacement magmatic and exhalative sulfides from the abitibi belt in canada fall on the same trend in the 207 pb 204 pb 206 pb 204 pb diagram as komatiites basalts sedimentary rocks and k feldspars from granites the calculated 1 value is 7 8 0 1 a similar suite of rocks and minerals from the kambalda area in australia has distinctly higher 207 pb 204 pb and a higher 1 of 8 3 nd isotopic data demonstrate that some kambalda rocks are highly contaminated with older continental crust the differences in initial pb isotopic compositions are attributed to the presence or absence and the age of underlying granitoid basement kambalda greenstones are underlain and contaminated by old 3 0 ga granitoid crust but significantly older granitoid basement appears absent from the abitibi belt the pb isotopic compositions of kambalda greenstones are dominated by the crustal component and do not represent the composition of the mantle source of these rocks the abitibi greenstones are not highly influenced by crustal contamination and their pb isotopic compositions may reflect those of archean mantle

vertical gps ground motion rates in the euro mediterranean region new evidence of velocity gradients at different spatial scales along the nubia eurasia plate boundary we use 2 5 to 14 years long position time series from 800 continuous global positioning system gps stations to study vertical deformation rates in the euro mediterranean region we estimate and remove common mode errors in position time series using a principal component analysis obtaining a significant gain in the signal to noise ratio of the displacements data following the results of a maximum likelihood estimation analysis which gives a mean spectral index 0 7 we adopt a power law white noise stochastic model in estimating the final vertical rates and find 95 of the velocities within 2 mm yr with uncertainties from filtered time series 40 smaller than from the unfiltered ones we highlight the presence of statistically significant velocity gradients where the stations density is higher we find undulations of the vertical velocity field at different spatial scales both in tectonically active regions like eastern alps apennines and eastern mediterranean and in regions characterized by a low or negligible tectonic activity like central iberia and western alps a correlation between smooth vertical velocities and topographic features is apparent in many sectors of the study area glacial isostatic adjustment and weathering processes do not completely explain the measured rates and a combination of active tectonics and deep seated geodynamic processes must be invoked excluding areas where localized processes are likely or where subduction processes may be active mantle dynamics is the most likely process but regional mantle modeling is required for a better understanding

p wave tomography of the mantle under the alpine mediterranean area 1 we study the upper mantle p wave velocity structure below the euro mediterranean area down to 1000 km depth by seismic travel time tomography we invert summary residuals constructed with both regional and teleseismic first arrival data reported by the international seismological centre isc 1964 1995 introducing some alternative strategies in the travel time tomographic approach and a new scheme to correct teleseismic data for global mantle structure our high resolution model pm0 5 is parameterized with three dimensional 3 d linear splines on a grid of nodes with 0 5 spacing in both horizontal directions and 50 km vertical spacing we obtain about 26 root mean square rms reduction of residuals by inversion in addition to roughly 31 reduction after summary rays formation and selection sensitivity analyses are performed through several test inversions to explore the resolution characteristics of the model at different spatial scales the distribution of large scale fast anomalies suggests that two different stages of a convection process presently coexist in very close regions the mantle dynamics of western central europe is dominated by blockage of subducted slabs at the 660 km discontinuity and ponding of seismically fast material in the transition zone contrarily in the eastern mediterranean fast velocity material sinks into the lower mantle suggesting that the flow of the cold downwelling here is not blocked by the 660 km discontinuity on a smaller scale the existence of tears in the subducted slab lithospheric detachment all along both margins of the adriatic plate as proposed by some authors is not supported by our tomographic images

tracking adria indentation beneath the alps by detrital zircon u pb geochronology implications for the oligocene miocene dynamics of the adriatic microplate the adriatic microplate is a key player in the western mediterranean tectonic puzzle but its oligocene miocene dynamics are not yet fully understood in fact even though the timing and magnitude of adriatic slab rollback and backarc extension in the apennines have long been established the timing of progressive adria indentation beneath the central alps and major strike slip motion along the insubric fault are still poorly constrained here we tackle these issues by utilizing detrital zircon u pb geochronology on adriatic foredeep turbidites i e by comparing the geochronologic fingerprints of the exhuming tectonic domes of the central alps ticino and toce subdomes with those of the oligocene miocene turbidites chiefly derived from their erosion we analyzed 11 sandstone samples ranging in age from 32 to 18 ma the ratio between variscan and caledonian zircon grains which are dominant in the toce and ticino subdomes respectively sharply increases at ca 24 23 ma this major provenance change marks the westward shift of the adriatic indenter beneath the central alps and the associated right lateral activity of the insubric fault the coexistence of strike slip motion at the northern boundary of the adriatic microplate at ca 24 23 ma and trench retreat during scissor type backarc opening to the west requires a near vertical rotation axis located at the northern tip of the ligurian provencal basin we propose that the rotation axis position was controlled by the interaction between the european and the adriatic slabs which may have collided at depth by the end of the oligocene triggering the westward shift of the adriatic indenter beneath the central alps

shear velocity structure of the tyrrhenian sea tectonics volcanism and mantle de hydration of a back arc basin the tyrrhenian sea in the mediterranean formed as the result of roll back of the adriatic and ionian subducting plates it is mostly underlain by strongly thinned continental lithosphere but contains two small oceanic basins in the southern tyrrhenian the youngest one located just behind the active magmatic arc its regional setting with dense station coverage provides a unique opportunity to derive a high resolution 3 d shear velocity model of this back arc basin and surrounding onshore areas using interstation rayleigh wave dispersion measurements our tomographic model extending to a depth of approximately 160 km displays a pronounced nearly ring shaped low shear velocity zone between 70 and 110 km depth which surrounds the older oceanic vavilov basin the sharp velocity decrease at 70 km depth can be explained by the transition from a relatively dry lithospheric mantle to more hydrous asthenospheric mantle material the tectonic evolution of the region and the correlation of the low velocity anomaly with subduction related volcanism indicate that the low velocity anomaly reflects hydrous mantle material in present or former mantle wedge regions we suggest that the absence of the low velocity zone beneath the vavilov basin is due to mantle dehydration caused by the creation of morb crust whereas temperature effects may dominate the asthenospheric shear velocity differences between various back arc basins we find that the variations in shear velocity structure within the tyrrhenian area are best explained by variations in mantle water content

matrix computation of deformed lineation patterns abstract the superposition of distortional strains on originally small circle lineation loci leads to a range of lineation patterns such patterns can be calculated using matrix techniques matrix computation together with modern computer graphics capabilities greatly facilitate investigation of complex lineation loci the computational scheme can be used to display the results of a range of geometric combinations of original small circles and superposed strains the resultant patterns are similar to those found by recent field studies of lineations associated with individual mesoscopic folds such deformed loci appear to provide a useful data set for quantitative strain analyses

mountain building in taiwan insights from 3 d geodynamic models taiwan is widely considered to be a typical example of an arc continent collision surrounded by two opposite dipping subduction zones the manner by which the interaction of the two neighboring slabs caused plate collision and mountain building is insufficiently understood various hypotheses have been proposed but the geodynamic feasibility of those remains to be tested here we present 3 d thermomechanical models to study the geodynamic evolution process of a taiwan like setting after an initial transform fault was consumed in our model setup the boundary between the eurasian plate and the south china sea is northeast trending the results show that all simulations result in toroidal mantle flow around the slab edges and that slab breakoff as well as a small scale mountain belt with high topography and crustal exhumation occurs in most cases the eurasian continental crust is exhumed in a dome like manner exposing higher grade metamorphic rocks facilitated by high erosion rates and a weak continental lower crust rheology but inhibited by the presence of a weak arc a high topography within the orogen as well as continental slab detachment can develop for the convergence direction of n307 degrees and large convergence rates our modeling results are thus generally consistent with the eurasian slab tearing model proposed for taiwan based on seismic tomographic studies and we suggest that the main characteristic features in taiwan can be explained by the combined effects of fast erosion a weak lower crust fast convergence and a small convergence azimuth

a synchronous alpine and corsica sardinia rotation 1 we report on the paleomagnetism of 34 sites from lower oligocene middle miocene sediments exposed in the tertiary piedmont basin tpb northern italy the tpb is formed by a thick 4000 m and virtually undeformed sedimentary succession unconformably lying upon alpine nappes decapitated by extensional exhumation which in turn are tectonically stacked over the adriatic foreland paleomagnetic directions from 23 mostly oligocene sites were chronologically framed using new biostratigraphic evidence from calcareous nannoplankton our data along with published paleomagnetic results show that the tpb rotated 50 counterclockwise with respect to africa in aquitanian serravallian times the rotation was likely driven by underneath nappe stacking and was synchronous with further bending of the alpine chain both the rotation magnitude and its timing are similar to those documented for the corsica sardinia microplate therefore the formation of the western alpine arc or at least part of its present day curvature occurred during the rollback of the apenninic slab and related back arc spreading of the liguro provencal basin and drift of the corsica sardinia block this suggests a common dynamics driving both the alpine and the apennine slab motions paleomagnetic data also document that the adriatic plate has undergone no paleomagnetic rotation since mid late miocene times anisotropy of magnetic susceptibility data suggests that the tpb an enigmatic basin arising from a controversial tectonic setting formed in an extensional regime characterized by a stretching direction approximately orthogonal to the main trend of the underlying chain

litho asthenospheric structures of northern italy as inferred from teleseismic p wave tomography abstract regional three dimensional inversions of teleseismic p wave travel time residuals recorded by high frequency regional and local seismic networks operating along the western alps and surrounding regions were carried out and lithosphere and upper mantle p wave velocity models down to 300 km were obtained residuals of more than 500 teleseismic events recorded by 98 fixed and temporary seismic stations have been inverted the comparison between real residuals and the ones obtained from tomographic model indicates that the method is able to solve the feature of the regional heterogeneities where the resolution is good coherent lithospheric and upper mantle structures are imaged in the shallower layers high and low velocity anomalies follow the structural behaviour of the alpine apenninic chains showing the existence of very strong velocity contrasts in the deepest layers velocity contrast decreases however two deep seated high velocity structures are observed the most extended in depth and approximately trending ne sw has been interpreted as a wreck of the oldest subduction responsible of the alpine orogenesis the second one connected to the northwestern sector of the apenninic chain appears to vanish at depths greater than 180 km and is probably due to still active apenninic roots cross sections depict the spatial trend of perturbations and in particular outline the sub vertical character of the alpine and apenninic anomalies under the ligurian sea the 3 d inversion confirms the uplift of the asthenosphere in agreement with the tectonic evolution of the basin

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

divergence in subduction zones and exhumation of high pressure rocks eocene western alps abstract exhumation of high pressure rocks has long remained a controversial issue in the earth sciences in this article we analyze the tectono metamorphic stratigraphic and plate motion constraints from the western alps region providing new insights on exhumation mechanisms and tectonic evolution during the earliest orogenic stages eocene eclogites of the western alps form a 20 25 km wide belt on the upper plate side of the orogen eclogite belt exposed beneath extensional shear zones at the rear of a lower pressure accretionary wedge units of the eclogite belt show the youngest peak pressure assemblages within the subduction zone and experienced superfast tectonic exhumation since 45 40 ma the role of erosion was negligible during the whole of this stage eocene foreland basins remained starved and the massive arrival of axial belt detritus began well after exhumation was completed tectonic reconstructions based on fixed boundaries exhumation models e g channel flow and or implying fast erosion at the surface e g slab breakoff are thus not consistent with geological evidence in the lack of erosion exhumation through the overburden requires divergence within the subduction zone we demonstrate that this was not attained by rollback of the lower plate europe but was instead attained by nneward motion of the upper plate adria africa away from the western alps trench such motion induced localized extension within the weak portion of the upper plate at the rear of the accretionary wedge and allowed tectonic emplacement of the eclogite belt in the upper crust at rates much faster than subduction rates tectonic exhumation ceased in the oligocene when oblique divergence along the western alps traverse changed into oblique convergence the onset of slow erosional unroofing was synchronously recorded by pressure temperature paths in all major tectonic units of the western alps and by arrival of orogenic detritus in sedimentary basins this work demonstrates that divergence between upper plate and trench is a viable mechanism to exhume large and coherent eclogite units in continental subduction zones our exhumation model can be applied to other eclogite belts showing a similar exhumational record including the western gneiss region the dabie sulu and eastern papua new guinea

subduction polarity reversal at the junction between the western alps and the northern apennines italy abstract the controversial relationship between the orogenic segments of the western alps and the northern apennines is here explored integrating recently published 3d tomographic models of subduction with new and re interpreted geological observations from the eclogitic domain of the voltri massif ligurian alps italy where the two belts joint each other the voltri massif is here described as an extensional domain accommodating the opposing outward migration of the alpine and apennine thrust fronts since about 30 35 ma using tomographic images of the upper mantle and paleotectonic reconstructions we propose that this extensional setting represents the surface manifestation of an along strike change in polarity of the subducted oceanic slab whose polarity changed laterally in space and in time our tectonic model suggests that the westward shift of the alpine thrust front from the oligocene onward was the consequence of the toroidal asthenospheric flow induced by the retreat of the apenninic slab

cenozoic volcanic rocks of eastern china secular and geographic trends in chemistry and strontium isotopic composition abstract the cenozoic volcanic rocks of eastern china are subalkalic to alkalic basalts erupted in an early tertiary back arc rift environment and from scattered late tertiary and quaternary volcanic centers in a continental area crossed by active faults driven by subduction of the pacific plate and the collision of india and eurasia immobile trace elements and major elements conform very well to each other in classification of the 59 rocks for which complete data are reported and they correctly identify the tectonic setting lil element enrichments of the basalts lie between those of p morb and ocean island alkalic basalts and show a secular increase 87sr 86sr ratios of basalts vary from 0 7029 to 0 7048 alkalic basalts are systematically less radiogenic than geographically coextensive and contemporaneous tholeiitic basalts increase of radiogenic sr with increasing crustal thickness and crustal age and with silica enrichment of the magmas suggests crustal contamination but this is inadequate to explain the lil element enrichment patterns and variable lil element enrichments the preferred hypothesis is that the alkalic magmas come from a deeper source with long term lil element depletion and low rb sr ratio but relatively recent lil element enrichment conversely the tholeiitic magmas are melts of subcontinental mantle lithosphere that is more lil element depleted than the alkalic source at the time of magma genesis but has had an elevated rb sr ratio for much of its post consolidation history

rapid exhumation in the western alps driven by slab detachment and glacial erosion identifying topographic and erosion rate response to tectonic and climatic forcing remains challenging this is in part because of the difficulty in isolating the respective roles of climate and tectonics here we exploit 2500 thermochronometric data points collected over several decades of research using a new inverse technique to image the space time evolution of erosion rate across the european alps over the past 35 m y the most striking feature of our results is a two to three fold increase in erosion rate over the past 2 m y exclusively within the western and central alps this increase appears to be controlled by the inferred high rock uplift rate due to the progressive detachment of the european slab under the western alps the similarity in mean elevation between the western and eastern alps indicates a surprisingly low topographic response to this differential tectonic forcing and points to the role of enhanced glacial erosion in response to surface uplift

a seismotectonic picture of the inner southern western alps based on the analysis of anomalously deep earthquakes abstract the anomalously deep seismicity beneath the western po plain is here analyzed to shed light on the complex and still poorly understood tectonic configuration of the internal side of the western alps area the original dataset including 590 earthquakes deeper than 20 km recorded during the last 25 years has been accurately relocated with hypodd using both catalog and cross correlation differential times we found that the distribution of seismic events faithfully mirrors the presence of two distinct tectonic domains axial belt domains 1 and 2 originally belonging to the paleogene alpine wedge and now anomalously juxtaposed beneath the sedimentary infill of the western po plain shallow low magnitude earthquakes 8 km and accommodated in correspondence of the fault could be linked to its neogene activity coeval with the northward translation of the retreating adriatic slab

subduction history of the tethyan region derived from seismic tomography and tectonic reconstructions 1 in the mantle underneath the tethyan suture zone large volumes of positive velocity anomalies have been imaged by seismic tomography and interpreted as the present day signature of subducted tethyan lithosphere we investigate the mesozoic cenozoic subduction history of the region by integrating independent information from mantle tomography and tectonic reconstructions three different subduction scenarios for the tethyan oceanic lithosphere representative for the available tectonic reconstructions are used to predict the present thermally anomalous volumes associated with the lithospheric surface subducted since the late mesozoic next these predicted thermal volumes and their expected positions are compared to the relevant anomalous volumes derived from seismic tomographic images in this analysis we include among others the possible effects of ridge subduction and slab detachment after the cenozoic continental collisions absolute plate motion and slab thickening in the mantle our preferred subduction model comprises the opening of large back arc oceanic basins within the eurasian margin the model points to slab thickening by a factor of 3 in the mantle in which case the estimated volumes allow for active oceanic spreading 1 2 5 cm yr in the tethyan lithosphere during convergence our results further indicate the occurrence of early oligocene slab detachment underneath the northern zagros suture zone followed by both westward and eastward propagation of the slab tear and diachronous eocene to miocene slab detachment below the eastern to western himalayas free sinking rates of the detached material of 2 cm yr in the lower mantle provide the best fit between the tomographic mantle structure and our tethyan subduction model

finite strains from non coaxial strain paths i computational techniques abstract matrix methods provide a powerful technique for calculating cumulative strain during non coaxial progressive deformation a straightforward derivation of the matrix equations representing the strain ellipsoid suitable for the field geologist is presented multiplication of transformation matrices used to model such strain paths results in non symmetric matrices describing strain ellipsoids with principal axes which cannot be directly solved for by standard eigenvector solution techniques the singular value decomposition is used instead the form of the strain ellipse used is completely general and with appropriate choice of the ellipse description matrix it can effect both pure and simple shear in modeling polyphase folded terrains the rotational component of the deformation is generally known at least in part from the relative orientations of deformed foliations and lineations matrices representing both the rotational and distortional components of strain can be multiplied to compute the resultant finite strain for any specified strain paths examples of some simple deformation paths demonstrate that large variations in the symmetry and magnitude of finite strains with orientation can be expected such variations are consistent with the range of strains observed in multiply folded terrains

volcanic geomorphology an overview abstract the review examines the role of geomorphology in analyzing the volcanoes on earth five objectives are stressed first classifications of volcanic landforms should be improved to take care of the complexity in volcanic landform generation as magmatic systems style of eruption and the erupted material all influence the morphology second geomorphology should contribute to the science of volcanology through its capability in reconstructing growth stages in complex volcanoes and also in analyzing the structural factors which contribute to the catastrophic collapse of volcanoes third geomorphology can contribute to physical volcanology by assessing the effects of topography on transport erosion and deposition of volcanogenic flows and identifying the sources and climatic tectonic conditions which govern the emplacement of volcaniclastic deposits fourth volcanic geomorphology a identifies sedimentary facies associations b constructs facies models for dynamic volcano delivery systems and c analyzes the characteristics of sediment gravity flows in order to determine relevant parameters for modelling their behaviour fifth process oriented geomorphology is critical in developing accurate methods for measuring rates of geomorphic processes that shape ephemeral volcanic constructs and for evaluating and comparing geomorphic impacts on disturbed catchments and the related hydrologic response before during and after eruptions this should help to refine parameters for the exponential decay model finally volcanic geomorphology is essential for risk assessment through geomorphic hazard zonation and composite risk zonation such treatments are necessary in order to face the enhanced challenge posed by the combination of natural hazards and the increasing number of people who are at risk around volcanoes

slab breakoff a model of lithosphere detachment and its test in the magmatism and deformation of collisional orogens we present a model proposing that oceanic lithosphere detaches from continental lithosphere during continental collision slab breakoff allowing an explanation of syn to post collisional magmatism and metamorphism continental collisions are preceded by subduction of dense oceanic lithosphere and followed by attempted subduction of buoyant continental lithosphere this situation of opposing buoyancy forces leads to extensional deformation in the subducted slab a narrow rifting mode of deformation will result if there is strain localization slab breakoff results we have assessed the plausiblity of this process by quantitatively evaluating an upper bound for the strength of the lithosphere and have compared it with the change in buoyancy force during continental sbduction whether breakoff will occur and the depth at which it will occur is a strong function of temperature and hence the subduction velocity for a subduction velocity of 1 cm yr breakoff could occur at depths of between 50 and 120 km while at higher velocities it is still likely to occur but at deeper depths as a result of the rifting during breakoff the asthenosphere upwells into the narrow rift and following breakoff it impinges on the mechanical lithosphere of the overriding plate the resulting conducted thermal perturbation leads to melting of the metasomatised overriding mantle lithosphere producing basaltic magmatism that leads to granitic magmatism in the crust dry asthenospheric mantle will melt only if breakoff occurs at a depth shallower than ca 50 km breakoff removes the force at the downdip side of the continental crust while the enhanced heating leads to a reduction of the strength of the underlying crust both effects facilitate the freeing of buoyant crustal sheets which can then rise towards the surface leading to the rapid exhumation of eclogite facies continental crust the cessation of subduction and replacement of the cold oceanic lithosphere by asthenosphere leads to rapid uplift of the orogen we have tested the variety of predicted expressions of slab breakoff in the geological record of the alps the aegean islands and the dabie shan a comparison of the various belts highlights 1 that the magmatism and metamorphism are found near the suture and the centre of the orogen demonstrating the general steepening of suture during collision 2 that a vague inverse correlation exists between maximum depth of metamorphism and volume of syn orogenic magmatism and 3 that the melts can be emplaced in both compressive and extensional environments we suggest that slab breakoff is an important step in the evolution of many orogenic belts and it allows an explanation for the combined presence in collisional orogens of magmatism with a mantle parentage and ultra high pressure metamorphics

strain partitioning in the axial nw alps since the oligocene 1 this work analyzes the postmetamorphic evolution of the northwestern alps thanks to a multiscale field geology study integrated by fission track analysis it illustrates how the fission track method constraining throws along major faults and age of mesoscale deformation provides interpretation keys to detangle complex structural data sets results are discussed within the framework of the alps apennines knot and give new insights on strain partitioning within transpressional orogenic belts the axial nw alps acted since the late oligocene as an orogen scale shear zone accommodating in the aosta valley dextral strike slip coeval with forward propagation of external thrusts within this transcurrent belt right lateral deformation was partitioned between the fast exhuming western block and the already unroofed eastern block these blocks show contrasting structural characters the former is cut by first order longitudinal structures bounding major tectonostratigraphic units the latter is fragmented by complex patterns of lower order faults interpreted as subsidiary shears the onset of mesoscale extension widespread in the eastern block is diachronous across the belt and migrated through time toward the european foreland transition from syncollisional shortening to orogen scale transcurrence observed in the nw alps corresponds to a regional strain field variation documented in the mid oligocene all around the alps apennines knot which is possibly related to a change of the adria trajectory relative to europe since the late oligocene deformation associated with oblique plate convergence was partitioned between strike slip in the axial belt and thrusting in external areas most of the shortening was accommodated in the helvetic chains 102 km deformation was limited elsewhere to an order of 101 km

p wave anisotropic tomography of the alps the first tomographic images of p wave azimuthal and radial anisotropies in the crust and upper mantle beneath the alps are determined by joint inversions of arrival time data of local earthquakes and teleseismic events our results show the south dipping european plate with a high velocity high v anomaly beneath the western central alps and the north dipping adriatic plate with a high v anomaly beneath the eastern alps indicating that the subduction polarity changes along the strike of the alps the p wave azimuthal anisotropy is characterized by mountain chain parallel fast velocity directions fvds in the western central alps and ne sw fvds in the eastern alps which may be caused by mantle flow induced by the slab subductions our results reveal a negative radial anisotropy i e vph vpv in the low velocity mantle wedge which may reflect the subvertical plate subduction and its induced mantle flow the results of anisotropic tomography provide important new information on the complex mantle structure and dynamics of the alps and adjacent regions

constraints on geological strain rates arguments from finite strain states of naturally deformed rocks a summary of known finite strain states is presented longitudinal strains 1 e as measured in many rocks often range from 1 to 40 and 1 to 0 025 the time span available to produce such measureable strains in young orogenic zones seems to be less than 10 m y possibly less than 1 m y which constrains conventional strain rates into the range of 10 sup 13 s sup 1 to 10 sup 15 s sup 1 for both pure and simple shear the most efficient way and much less efficient way to accumulate incremental strains respectively the ellipticity of the finite strain ellipse increases in a nonlinear manner finite strain variations in adjacent layers which give rise to features such as cleavage refraction arise with only slight differences in the strain rates within these layers

epcrust a reference crustal model for the european plate summary we present a new crustal model for the european plate derived from collection and critical integration of information selected from the literature the model covers the whole european plate from north africa to the north pole 20 n 90 n and from the mid atlantic ridge to the urals 40 w 70 e the chosen parametrization represents the crust in three layers sediments upper crust and lower crust and describes the 3 d geometry of the interfaces and seismologically relevant parameters isotropic p and s wave velocity plus density with a resolution of 0 5 0 5 on a geographical latitude longitude grid we selected global and local models derived from geological assumptions active seismic experiments surface wave studies noise correlation receiver functions model epcrust presents significant advantages with respect to previous models it covers the whole european plate it is a complete and internally consistent model with all the parameters provided also for the sedimentary layer it is reproducible it is easy to update in the future by adding new contributions and it is available in a convenient digital format epcrust could be used to account for crustal structure in seismic wave propagation modelling at continental scale or to compute linearized crustal corrections in continental scale seismic tomography gravity studies dynamic topography and other applications that require a reliable crustal structure because of its resolution our model is not suited for local scale studies such as the computation of earthquake scenarios where more detailed knowledge of the structure is required we plan to update the model as new data will become available and possibly improve its resolution for selected areas in the future

lithospheric architecture of the south western alps revealed by multiparameter teleseismic full waveform inversion the western alps although being intensively investigated remains elusive when it comes to determining its lithospheric structure new inferences on the latter are important for the understanding of processes and mechanisms of orogeny needed to unravel the dynamic evolution of the alps this situation led to the deployment of the cifalps temporary experiment conducted to address the lack of seismological data amenable to high resolution seismic imaging of the crust and the upper mantle we perform a 3 d isotropic full waveform inversion fwi of nine teleseismic events recorded by the cifalps experiment to infer 3 d models of both density and p and s wave velocities of the alpine lithosphere here by fwi is meant the inversion of the full seismograms including phase and amplitude effects within a time window following the first arrival up to a frequency of 0 2 hz we show that the application of the fwi at the lithospheric scale is able to generate images of the lithosphere with unprecedented resolution and can furnish a reliable density model of the upper lithosphere in the shallowest part of the crust we retrieve the shape of the fast dense ivrea body anomaly and detect the low velocities of the po and se france sedimentary basins the geometry of the ivrea body as revealed by our density model is consistent with the bouguer anomaly a sharp moho transition is followed from the external part 30 km depth to the internal part of the alps 70 80 km depth giving clear evidence of a continental subduction event during the formation of the alpine belt a low velocity zone in the lower lithosphere of the s wave velocity model supports the hypothesis of a slab detachment in the western part of the alps that is followed by asthenospheric upwelling the application of fwi to teleseismic data helps to fill the gap of resolution between traditional imaging techniques and enables integrated interpretations of both upper and lower lithospheric structures

divergent plate motion drives rapid exhumation of ultra high pressure rocks abstract exhumation of ultra high pressure u hp rocks by upper plate divergent motion above an unbroken slab first proposed in the western alps has never been tested by numerical methods we present 2d thermo mechanical models incorporating subduction of a thinned continental margin beneath either a continental or oceanic upper plate followed by upper plate divergent motion away from the lower plate results demonstrate how divergent plate motion may trigger rapid exhumation of large volumes of u hp rocks directly to the earth s surface without the need for significant overburden removal by erosion model exhumation paths are fully consistent with natural examples for a wide range of upper plate divergence rates exhumation rates are systematically higher than the divergent rate imposed to the upper plate and the modeled size of exhumed u hp domes is invariant for different rates of upper plate divergence major variations are instead predicted at depth for differing model scenarios as larger amounts of divergent motion may allow mantle wedge exhumation to shallow depth under the exhuming domes the transient temperature increase due to ascent of mantle wedge material in the subduction channel has a limited effect on exhumed continental u hp rocks already at the surface we test two examples the cenozoic u hp terranes of the western alps continental upper plate and eastern papua new guinea oceanic upper plate the good fit between model predictions and the geologic record in these terranes encourages the application of these models globally to pre cenozoic u hp terranes where the geologic record of exhumation is only partly preserved

subsidence and sedimentation on jurassic passive continental margin southern alps italy the southern alps of italy preserve a tectonically intact array of jurassic facies that record the evolution of a part of the margin of the apulian plate from its ancestral beginnings in a complex of permian and triassic rifted continental basins through the initial stages of breakup and stepwise foundering of a carbonate platform breakup was accompanied first by the rapid accumulation of thick prisms of carbonate turbidites in newly formed fault troughs then as the new ligurian oceanic basin began to open farther west and as subsidence gradually slowed accumulation of a succession of slowly deposited biogenous pelagic sediments recorded not only the increasing depths of the seafloor but also fluctuations in oceanographic conditions of fertility carbonate dissolutio levels and the strength of bottom currents estimates of the history of seafloor depths based on a simple subsidence law of the form subsidence k age 12 provide a basis for the construction of a set of curves showing the changing depths of significant carbonate dissolution surfaces during the jurassic in this region the rapid 1 km deepening of the compensation depth for calcite during the late jurassic may be due to a change in regional oceanic vertical circulation patterns from upwelling fertile silica rich carbonate dissolving to downwelling less fertile silica poor carbonate preserving

orogens and slabs vs their direction of subduction abstract subduction zones appear primarily controlled by the polarity of their direction i e w directed or e to nne directed probably due to the westward drift of the lithosphere relative to the asthenosphere the decollement planes behave differently in the two end members in the w directed subduction zone the decollement of the plate to the east is warped and subducted whereas in the e to nne directed it is ramping upward at the surface there are w directed subduction zones that work also in absence of active convergence like the carpathians or the apennines w directed subduction zones have shorter life 30 40 ma than e or ne directed subduction zones even longer than 100 ma the different decollements in the two end members of subduction should control different ptt paths and therefore generate variable metamorphic assemblages in the associated accretionary wedges and orogens these asymmetries also determine different topographic and structural evolutions that are marked by low topography and a fast eastward migrating structural wave along w directed subduction zones whereas the topography and the structure are rapidly growing upward and expanding laterally along the opposite subduction zones the magmatic pair calc alkaline and alkaline tholeiitic volcanic products of the island arc and the back arc basin characterise the w directed subduction zones magmatic rocks associated with e or ne directed subduction zones have higher abundances of incompatible elements and mainly consist of calc alkaline shoshonitic suites with large volumes of batholithic intrusions and porphyry copper ore deposits the subduction zones surrounding the adriatic plate in the central mediterranean confirm the differences among subduction zones as primarily controlled by the geographic polarity of the main direction of the slab the western margin of the adriatic plate contemporaneously overridden and underthrust europe toward the west to generate respectively the alps and the apennines while the eastern margin subducted under the dinarides hellenides these belts confirm the characters of the end members of subduction zones as a function of their geographic polarity similarly to the pacific subduction zones

strain in an archean greenstone belt of minnesota abstract we measured strain at more than 60 locations in metasedimentary and metavolcanic rocks of the vermilion district an e w trending archean greenstone belt in minnesota strain ellipsoid orientations and shapes correlate strongly with n s location in the belt but magnitudes do not flattening strains occur near the present vermilion fault which bounds the greenstone belt to the north with constrictional strains to the south the observed strain patterns can be mathematically modeled by deformation paths which produce the flattening strains with west plunging 1 axes by dextral shear of the constrictional strains with east plunging 1 axes using reasonable geologic constraints the shear plane must dip to the north with a subhorizontal shear direction structures throughout the district also indicate dextral shear a geometrical finite element program uses the measured strains to destrain the rocks and find the configuration which most closely satisfies strain compatibility equations the linear e w strain patterns and minor rotations about horizontal axes during the deformation preclude origin of the greenstone belt by infolding and shear off the flanks of a rising granitic diapir by accounting for rotations which result in the deformed curvature of the original surface a true estimate of 50 n s shortening across the belt can be made the data and deformation models favor the origin of the vermilion district rocks at a convergent margin most likely as a n dipping subduction zone complex with shallow slab dip the origin of the constrictional strains remains enigmatic

high resolution imaging of the pyrenees and massif central from the data of the pyrope and iberarray portable array deployments the lithospheric structures beneath the pyrenees which holds the key to settle long standing controversies regarding the opening of the bay of biscay and the formation of the pyrenees are still poorly known the temporary pyrope and iberarray experiments have recently filled a strong deficit of seismological stations in this part of western europe offering a new and unique opportunity to image crustal and mantle structures with unprecedented resolution here we report the results of the first tomographic study of the pyrenees relying on this rich data set the important aspects of our tomographic study are the precision of both absolute and relative traveltime measurements obtained by a nonlinear simulated annealing waveform fit and the detailed crustal model that has been constructed to compute accurate crustal corrections beneath the massif central the most prominent feature is a widespread slow anomaly that reflects a strong thermal anomaly resulting from the thinning of the lithosphere and upwelling of the asthenosphere our tomographic images clearly exclude scenarios involving subduction of oceanic lithosphere beneath the pyrenees in contrast they reveal the segmentation of lithospheric structures mainly by two major lithospheric faults the toulouse fault in the central pyrenees and the pamplona fault in the western pyrenees these inherited hercynian faults were reactivated during the cretaceous rifting of the aquitaine and iberian margins and during the cenozoic alpine convergence therefore the pyrenees can be seen as resulting from the tectonic inversion of a segmented continental rift that was buried by subduction beneath the european plate

slab break offs in the alpine subduction zone after the onset of plate collision in the alps at 32 34 ma the deep structure of the orogen is inferred to have changed dramatically european plate break offs in various places of the alpine arc as well as a possible reversal of subduction polarity in the eastern alps have been proposed we review different high resolution tomographic studies of the upper mantle and combine shear and body wave models to assess the most reliable geometries of the slabs several hypotheses for the tectonic evolution are presented and tested against the tomographic model interpretations and constraints from geologic and geodetic observations we favor the interpretation of a recent european slab break off under the western alps in the eastern alps we review three published scenarios for the subduction structure and propose a fourth one to reconcile the results from tomography and geology we suggest that the fast slab anomalies are mainly due to european subduction adriatic subduction plays no or only a minor role along the tauern window sections possibly increasing towards the dinarides the apparent northward dip of the slab under the eastern alps may be caused by imaging a combination of adriatic slab from the dinaric subduction system and a deeper lying european one as well as by an overturned retreating european slab

stagnant slab a review a stagnant slab is a subducted slab of oceanic lithosphere subhorizontally deflected above across or below the 660 km discontinuity this phenomenon has now been widely recognized beneath subduction zones around the circum pacific and in the mediterranean collaboration of seismic and electromagnetic observations mineral physics measurements and geodynamic modeling has begun to provide a consistent picture of stagnant slab

continuity of the alpine slab unraveled by high resolution p wave tomography the question of lateral and or vertical continuity of subducted slabs in active orogens is a hot topic partly due to poorly resolved tomographic data the complex slab structure beneath the alpine region is only partly resolved by available geophysical data leaving many geological and geodynamical issues widely open based upon a finite frequency kernel method we present a new high resolution tomography model using p wave data from 527 broadband seismic stations both from permanent networks and temporary experiments this model provides an improved image of the slab structure in the alpine region and fundamental pinpoints for the analysis of cenozoic magmatism u hp metamorphism and alpine topography our results document the lateral continuity of the european slab from the western alps to the central alps and the downdip slab continuity beneath the central alps ruling out the hypothesis of slab break off to explain cenozoic alpine magmatism a low velocity anomaly is observed in the upper mantle beneath the core of the western alps pointing to dynamic topography effects a ne dipping adriatic slab consistent with dinaric subduction is possibly observed beneath the eastern alps whereas the laterally continuous adriatic slab of the northern apennines shows major gaps at the boundary with the southern apennines and becomes near vertical in the alps apennines transition zone tear faults accommodating opposite dipping subductions during alpine convergence may represent reactivated lithospheric faults inherited from tethyan extension our results suggest that the interpretations of previous tomography results that include successive slab break offs along the alpine zagros himalaya orogenic belt might be proficiently reconsidered

active and fossil mantle flows in the western alpine region unravelled by seismic anisotropy analysis and high resolution p wave tomography abstract the anisotropy of seismic velocities in the mantle when integrated with high resolution tomographic models and geologic information can be used to detect active mantle flows in complex plate boundary areas providing new insights on the impact of mantle processes on the topography of mountain belts here we use a densely spaced array of temporary broadband seismic stations to analyze the seismic anisotropy pattern of the western alpine region at the boundary between the alpine and apenninic slabs our results are supportive of a polyphase development of anisotropic mantle fabrics possibly starting from the jurassic to present geophysical data presented in this work and geologic evidence taken from the literature indicate that i fossil fabrics formed during tethyan rifting may be still preserved within the alpine and apenninic slabs ii mantle deformation during apenninic slab rollback is not compensated by a complete toroidal flow around the northern tip of the retreating slab iii the previously observed continuous trend of anisotropy fast axes near parallel to the western alpine arc is confirmed we observe that this arc parallel trend of fast axes is located in correspondence to a low velocity anomaly in the european upper mantle beneath regions of the western and ligurian alps showing the highest uplift rates we propose that the progressive rollback of the apenninic slab in the absence of a counterclockwise toroidal flow at its northern tip induced a suction effect at the scale of the supraslab mantle the resulting mantle flow pattern was characterized by an asthenospheric counterflow at the rear of the unbroken western alps slab and around its southern tip and by an asthenospheric upwelling mirrored by low p wave velocities that would have favored the topographic uplift of the alpine belt from the mont blanc to the mediterranean sea

upper mantle structure beneath the alpine orogen from high resolution teleseismic tomography 1 to understand the evolution of the alpine orogen knowledge of the actual structure of the lithosphere asthenosphere system is important we perform high resolution teleseismic tomography with manually picked p wave arrival times from seismograms recorded in the greater alpine region the resulting data set consists of 4199 relative p wave arrivals and 499 absolute p wave arrivals from 76 teleseismic events corrected for the contribution of the alpine crust to the travel times the three dimensional 3 d crustal model established from controlled source seismology data for that purpose represents the large scale alpine crustal structure absolute p wave arrival times are used to compute an initial reference model for the inversion tests with synthetic data document that the combination of nonlinear inversion high quality teleseismic data and usage of an a priori 3 d crustal model allows a reliable resolution of cells at 50 km 50 km 30 km hence structures as small as two cells can be resolved in the upper mantle our tomographic images illuminate the structure of the uppermost mantle to depth of 400 km along strike of the alps the inversion reveals a high velocity structure that dips toward the se beneath the adriatic microplate in the western and central alps in the eastern alps we observe a northeastward dipping feature subducting beneath the european plate we interpret this feature in the western and central alps as subducted mainly continental european lower lithosphere for the east we propose that parts of the vardar oceanic basin were subducted toward the ne forcing continental adriatic lower lithosphere to subduct northeastward beneath the european plate

reconciling plate tectonic reconstructions of alpine tethys with the geological geophysical record of spreading and subduction in the alps article a new reconstruction of alpine tethys combines plate kinematic modelling with a wealth of geological data and seismic tomography to shed light on its evolution from sea floor spreading through subduction to collision in the alps unlike previous models which relate the fate of alpine tethys solely to relative motions of africa iberia and europe during opening of the atlantic our reconstruction additionally invokes independent microplates whose motions are constrained primarily by the geological record the motions of these microplates adria iberia alcapia alkapecia and tiszia relative to both africa and europe during late cretaceous to cenozoic time involved the subduction of remnant tethyan basins during the following three stages that are characterized by contrasting plate motions and driving forces 1 131 84 ma intra oceanic subduction of the ligurian part of alpine tethys attached to iberia coincided with eo alpine orogenesis in the alcapia microplate north of africa these events were triggered primarily by foundering of the older 170 131 ma neotethyan subduction slab along the ne margin of the composite african adriatic plate subduction was linked by a sinistral transform system to e w opening of the valais part of alpine tethys 2 84 35 ma subduction of primarily the piemont and valais parts of alpine tethys which were then attached to the european plate beneath the overriding african and later adriatic plates nw translation of adria with respect to africa was accommodated primarily by slow widening of the ionian sea 3 35 ma recent rollback subduction of the ligurian part of alpine tethys coincided with western alpine orogenesis and involved the formation of the gibraltar and calabrian arcs rapid subduction and arc formation were driven primarily by the pull of the gravitationally unstable retreating adriatic and african slabs during slow convergence of africa and europe the upper european iberian plate stretched to accommodate this slab retreat in a very mobile fashion while the continental core of the adriatic microplate acted as a rigid indenter within the alpine collisional zone the subducted lithosphere in this reconstruction can be correlated with slab material imaged by seismic tomography beneath the alps and apennines as well as beneath parts of the pannonian basin the adriatic sea the ligurian sea and the western mediterranean the predicted amount of subducted lithosphere exceeds the estimated volume of slab material residing at depth by some 10 30 indicating that parts of slabs may be superposed within the mantle transition zone and or that some of this subducted lithosphere became seismically transparent

trace element and nd isotope systematics in detrital apatite of the po river catchment implications for provenance discrimination and the lag time approach to detrital thermochronology abstract detrital thermochronology is often employed to assess the evolutionary stage of an entire orogenic belt using the lag time approach i e the difference between the cooling and depositional ages of detrital mineral grains preserved in a stratigraphic succession the impact of different eroding sources to the final sediment sink is controlled by several factors including the short term erosion rate and the mineral fertility of eroded bedrock here we use apatite fertility data and cosmogenic derived erosion rates in the po river catchment alps apennines to calculate the expected percentage of apatite grains supplied to the modern po delta from the major alpine and apenninic eroding sources we test these predictions by using a cutting edge dataset of trace element and nd isotope signatures on 871 apatite grains from 14 modern sand samples and we use apatite fission track data to validate our geochemical approach to provenance discrimination we found that apatite grains shed from different sources are geochemically distinct apatites from the lepontine dome in the central alps show relative hree enrichment lower concentrations in ce and u and higher 147 sm 144 nd ratios compared to apatites derived from the external massifs derived provenance budgets point to a dominant apatite contribution to the po delta from the high fertility lepontine dome consistent with the range independently predicted from cosmonuclide and mineral fertility data our results demonstrate that the single mineral record in the final sediment sink can be largely determined by high fertility source rocks exposed in rapidly eroding areas within the drainage this implies that the detrital thermochronology record may reflect processes affecting relatively small parts of the orogenic system under consideration a reliable approach to lag time analysis would thus benefit from an independent provenance discrimination of dated mineral grains which may allow to proficiently reconsider many previous interpretations of detrital thermochronology datasets in terms of orogenic wide steady state

seismic image and origin of the changbai intraplate volcano in east asia role of big mantle wedge above the stagnant pacific slab abstract we determined p wave tomography of the crust and upper mantle under the changbai volcanic area in northeast asia by using 289 318 arrival time data from 4802 local earthquakes and 9599 teleseismic events a prominent low velocity anomaly is clearly imaged in the crust and upper mantle down to about 410 km depth beneath the changbai volcano a broad high velocity anomaly is revealed in the mantle transition zone and deep earthquakes occurred actively there suggesting that the subducting pacific slab is stagnating in the mantle transition zone under east asia our results suggest that the formation of the changbai and other intraplate volcanoes in northeast asia is related to the upwelling of hot and wet asthenospheric materials in the big mantle wedge above the stagnant pacific slab hence the active intraplate volcanoes in northeast asia are not hotspots related to deep mantle plumes but are caused by plate tectonic processes in the upper mantle

shear wave velocities in the upper mantle of the western alps new constraints using array analysis of seismic surface waves it remains challenging to obtain absolute shear wave velocities of heterogeneities of small lateral extension in the uppermost mantle this study presents a cross section of vs across the strongly heterogeneous 3 d structure of the western european alps based on array analysis of data from 92 broad band seismic stations from the cifalps experiment and from permanent networks in france and italy half of the stations were located along a dense sublinear array using a combination of these stations and off profile stations fundamental mode rayleigh wave dispersion curves were calculated using a combined frequency time beamforming approach we calculated dispersion curves for seven arrays of approximately 100 km aperture and 14 arrays of approximately 50 km aperture the latter with the aim of obtaining a 2 d vertical cross section of vs beneath the western alps the dispersion curves were inverted for vs z with crustal interfaces imposed from a previous receiver function study the array approach proved feasible as vs z from independent arrays vary smoothly across the profile length results from the seven large arrays show that the shear velocity of the upper mantle beneath the european plate is overall low compared to ak135 with the lowest velocities in the internal part of the western alps and higher velocities east of the alps beneath the po plain the 2 d vs model is coherent with i a 100 km thick eastward dipping european lithosphere west of the alps ii very high velocities beneath the po plain coherent with the presence of the alpine european slab and iii a narrow low velocity anomaly beneath the core of the western alps from the brianconnais to the dora maira massif and approximately colocated with a similar anomaly observed in a recent teleseismic p wave tomography this intriguing anomaly is also supported by traveltime variations of subvertically propagating body waves from two teleseismic events that are approximately located on the profile great circle

high resolution teleseismic tomography of upper mantle structure using an a priori three dimensional crustal model summary the effect of an a priori known 3 d crustal model in teleseismic tomography of upper mantle structure is investigated we developed a 3 d crustal p wave velocity model for the greater alpine region encompassing the central and western alps and the northern apennines to estimate the crustal contribution to teleseismic traveltimes the model is constructed by comparative use of published information from active and passive seismic surveys the model components are chosen to represent the present large scale alpine crustal structure and for their significant effect on the propagation of seismic wavefields they are first order structures such as the crust mantle boundary sedimentary basins and the high velocity ivrea body teleseismic traveltime residuals are calculated for a realistic distribution of azimuths and distances by coupling a finite difference technique to the iasp91 traveltime tables residuals are produced for a synthetic upper mantle model featuring two slab structures and the 3 d crustal model on top of it the crustal model produces traveltime residuals in the range between 0 7 and 1 5 s that vary strongly as a function of backazimuth and epicentral distance we find that the non linear inversion of the synthetic residuals without correcting for the 3 d crustal structure erroneously maps the crustal anomalies into the upper mantle correction of the residuals for crustal structure before inversion properly recovers the synthetic slab structures placed in the upper mantle we conclude that with the increasing amount of high quality seismic traveltime data correction for near surface structure is essential for increasing resolution in tomographic images of upper mantle structure

linking the alps and apennines subduction systems new constraints revealed by high resolution teleseismic tomography abstract we report a new model of the upper mantle structure beneath italy obtained by means of p wave teleseismic tomography besides the recent and remarkable development of the italian seismic network a high model resolution has been achieved improving the inversion method upon the ach method used in previous investigations and picking high quality arrival times with the multi channel cross correlation technique the finer details of our vp model yield new insights into the heterogeneous structure of the adria continental lithosphere involved in the collision between the africa and europe plates a wide low vp anomaly located in the northern adria mantle facing the alpine high vp slab supports the idea that the adria lithosphere has been hydrated and thinned during the alpine subduction we argue that this mantle softening may have played a key role in favoring the subsequent delamination of the adria lithosphere in the northern apennines we hypothesize that delamination of continental lithosphere previously thinned in a back arc setting may be considered a key process to favor subduction polarity reversal and recycling of continental material into the mantle circulation conversely in the central southern apennines the velocity structure is consistent with the existence of a deeper oceanic slab that flattens at the base of the upper mantle in agreement with the widely accepted geodynamic evolution of the central mediterranean by slab retreat and back arc spreading the oceanic slab is discontinuously detached from the surface plate suggesting a different structure of the adria lithosphere which resists subduction instead of favoring delamination

from mantle to crust stretching the mediterranean abstract the origin of forces driving the deformation of the continental crust near subduction zones and especially in backarc regions is debated this work is based on a compilation of sks fast splitting directions that give an image of flow lines in the mantle around the mediterranean subduction zones and a comparison with stretching and shear directions in metamorphic core complexes that show the pattern of deformation at the scale of the middle and lower crusts we find that 1 the two sets of directions are parallel in the three main backarc regions namely the alboran sea the tyrrhenian sea and the aegean sea showing that the lithosphere deforms with the same direction of stretching in the crust and the mantle suggesting that 2 crustal deformation is mainly driven from below by slab retreat and 3 the lithospheric fabric is reset within a few millions of years in backarc environments

tomographic constraints on the geodynamic evolution of the italian region in this paper we present p wave tomographic images of the mantle beneath italy obtained by inverting 6000 teleseismic p and pkp wave arrival times accurately repicked recorded in the time period 1988 1994 by the stations of the national seismic network of the istituto nazionale di geofisica we pay great attention in the data selection and picking procedure of seismic phases to obtain a very high quality data set the data were inverted with the well established aki christofferson husebye tomographic technique different reference models and residuals computation have been tried to verify the stability of the results the high quality of the repicked arrival times allows us to enhance the definition of the deep structures beneath both the alps and the apennines looking for their lateral and vertical continuity down to 800 km depth the main finding of this study is a continuous high velocity body located between 250 and 670 km depth beneath the entire apenninic system dipping toward the tyrrhenian area which continues upward segmented in two main anomalies in the northern apenninic and the calabrian arcs we interpret this high velocity feature as the subducted oceanic lithosphere between the eurasian and african plates dipping down to the upper lower mantle boundary beneath the tyrrhenian sea the retrieved images of the lithosphere subducting beneath apennines are reliable in terms of thickness about 80 90 km and p wave velocity contrast 2 4 higher than the normal mantle furthermore our tomographic images which focus on the deep geometry and continuity of the velocity structures provide new keys to understanding the geodynamic evolution of the italian region the segmentation of the high velocity slab upward suggests a complex evolution of the arc trench system and the initially continuous subduction of the ionian adriatic plate progressively developed in subordinate arcs probably due to lateral heterogeneity of the subducting lithosphere

asthenospheric upwelling oceanic slab retreat and exhumation of uhp mantle rocks insights from greater antilles 1 exhumation of garnet bearing peridotites has been associated with low density continental rocks this association does not explain the occurrence of garnetbearing peridotites in the oceanic subduction zone of the greater antilles in hispaniola we use numerical models of intra oceanic subduction to explain exhumation of garnet peridotites without involvement of buoyant continental crust we demonstrate that rheological weakening of the mantle wedge takes place due to its strong hydration during subduction of serpentinized slow spreading ridge this weakening triggers upwelling of the hydrated peridotites and partially molten peridotites followed by upwelling of hot asthenosphere and subsequent retreat of the subducting slab according to numerical modelling of p t paths this process can explain exhumation of uhp 4gpa rocks in an intra oceanic setting citation gorczyk w s guillot t v gerya and k hattori 2007 asthenospheric upwelling oceanic slab retreat and exhumation of uhp mantle rocks insights from greater antilles geophys res lett 34 l21309 doi 10 1029 2007gl031059

characterization of volcanic geomorphology and geology by slope and topographic openness abstract we used a digital elevation model dem of iwate volcano japan to calculate slope and topographic openness for every geological unit in that area further we conducted principal component analysis pca and stepwise discriminant analysis sda to evaluate statistical differences in the morphometric parameters of the geological units we categorized the geological units according to their ages and their formation processes the categorization based on formation processes was as follows lava flow lava central cone stratocone air fall deposit pyroclastic flow deposit volcanic fan deposit and debris avalanche deposit pca allowed us to select geological units suitable for analysis while sda was used to select some morphometric parameters which could explain the differences in the formation processes and ages of the geological units we found that the formation processes correspond well to mean slope and that the geological ages that control the degree of surface dissection correspond well to the standard deviations of slope and negative topographic openness

deep structure of the alps what do we really know abstract in the last five decades the deep structure of the alps has been probed by every geophysical method applicable and the resulting amount of data is unmatched for any other orogen in this study an attempt is made to review the data and the proposed structural models with the aim of separating the certain from the probable and from the speculative information this can be achieved by first reviewing the theoretical resolving power and ambiguity of the applied interpretation methods and then analysing the proposed models the methods reviewed are inversion of surface wave data teleseismic and local earthquake seismic tomography near vertical reflection seismology wide angle reflection and refraction seismology and gravity modelling all information about the moho rated as certain is combined to give a moho map of the alpine area the information rated as certain and probable and additional qualitative arguments are used to discuss a crustal model of the western and central alps represented by two cross sections major structural elements in this crustal model are a thick overthrust penninic nappe system wedging at mid to lower crustal levels a discontinuous moho and strong variations along the strike of the orogen whereas the structures of the european upper crust and of the penninic nappe system are well constrained only few and isolated lower crustal structural elements are rated as certain finally the shape of the lower lithosphere in the alps is discussed by review and comparison of the results from surface wave teleseismic travel time residual and tomographic studies qualitative arguments suggest the existence of a lithospheric root or slab beneath the alps probable tomographic information suggests a south vergent european lithospheric slab beneath the southern alps and the po plain despite the considerable number of studies aimed at resolving the deepest part of alpine orogeny the available quantitative information on the sub moho structure cannot be rated as certain

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