seismic evidence for overpressured subducted oceanic crust and megathrust fault sealing water and hydrous minerals play a key role in geodynamic processes at subduction zones however there are few seismological constraints on the presence of water within the subducting plate audet et al model converted teleseismic waves to constrain the seismological properties of subducted oceanic crust from the cascadia continental margin to its intersection with the forearc mantle their observations suggest that water is pervasively present in fluid form at high pore pressures indicating that the megathrust is a low permeability boundary these results may hold important implications for our understanding of seismogenesis subduction zone structure and the mechanism of episodic tremor and slip this study models converted teleseismic waves to constrain the seismological properties of subducted oceanic crust from the cascadia continental margin to its intersection with the forearc mantle the observations suggest that water is pervasively present in fluid form at high pore pressures indicating that the megathrust is a low permeability boundary these results may hold important implications for our understanding of seismogenesis subduction zone structure and the mechanism of episodic tremor and slip water and hydrous minerals play a key part in geodynamic processes at subduction zones1 2 3 by weakening the plate boundary aiding slip and permitting subduction and indeed plate tectonics to occur4 the seismological signature of water within the forearc mantle wedge is evident in anomalies with low seismic shear velocity marking serpentinization5 6 7 however seismological observations bearing on the presence of water within the subducting plate itself are less well documented here we use converted teleseismic waves to obtain observations of anomalously high poisson s ratios within the subducted oceanic crust from the cascadia continental margin to its intersection with forearc mantle on the basis of pressure temperature and compositional considerations the elevated poisson s ratios indicate that water is pervasively present in fluid form at pore pressures near lithostatic values combined with observations of a strong negative velocity contrast at the top of the oceanic crust our results imply that the megathrust is a low permeability boundary the transition from a low to high permeability plate interface downdip into the mantle wedge is explained by hydrofracturing of the seal by volume changes across the interface caused by the onset of crustal eclogitization and mantle serpentinization these results may have important implications for our understanding of seismogenesis subduction zone structure and the mechanism of episodic tremor and slip

delay times and shear wave splitting in the mediterranean region summary we measure sks splitting parameters and p and s delay times in the mediterranean region using broad band recordings from the temporary midsea array and permanent networks the midsea seismic array has substantially improved data coverage along the european mediterranean margins and in northern africa shear wave splitting is observed for 95 per cent of the stations with splitting times up to 2 s we propose five anisotropic domains based on the characteristics of the observations in the western part of the region we observe an alignment with plate motion suggesting an asthenospheric origin for anisotropy in the central and near eastern part the fast directions are less coherent laterally and influenced by extension and subduction processes along the dead sea fault the fast directions are parallel to the relative motion between arabia and africa which we attribute in part to coherent vertical deformation our measurements of teleseismic relative p and s wave delay times within the mediterranean region reach peak to peak values of 2 5 and 8 s respectively consistent with international seismological centre isc delay times we observe the earliest arrivals along the hellenic subduction zone and some of the latest along the dead sea fault we find that s wave delays are roughly three times as large as those of p waves from a slight frequency dependence of our cross correlation based delay time measurements we estimate mediterranean lateral heterogeneity to have a dominant size of one or more hundred kilometres we further determine that the anisotropy implied by our sks splitting measurements does not significantly bias the pattern of teleseismic delay times

seismic investigation of the transition from continental to oceanic subduction along the western hellenic subduction zone 1 the western hellenic subduction zone whsz exhibits well documented along strike variations in lithosphere density i e oceanic versus continental subduction rates and overriding plate extension differences in slab density are believed to drive deformation rates along the whsz however this hypothesis has been difficult to test given the limited seismic constraints on the structure of the whsz particularly beneath northern greece here we present high resolution seismic images across northern and southern greece to constrain the slab composition and mantle wedge geometry along the whsz data from two temporary arrays deployed across greece in a northern line nl and southern line sl are processed using a 2d teleseismic migration algorithm based on the generalized radon transform images of p and s wave velocity perturbations reveal n60e dipping low velocity layers beneath both nl and sl the 8 km thick layer beneath sl is interpreted as subducted oceanic crust while the 20 km thick layer beneath nl is interpreted as subducted continental crust the thickness of subducted continental crust inferred within the upper mantle suggests that 10 km of continental crust has accreted to the overriding plate the relative position of the two subducted crusts implies 70 85 km of additional slab retreat in the south relative to the north overall our seismic images are consistent with the hypothesis that faster sinking of the denser oceanic portion of the slab relative to the continental portion can explain the different rates of slab retreat and deformation in the overriding plate along the whsz

geodynamic models of crustal scale episodic tectonic accretion and underplating in subduction zones we investigate the dynamics of crustal scale episodic accretion at convergent margins using a finite element model of subduction the numerical model incorporates effects from flexure subduction loading and large scale variations in material properties the model is used to assess the influence of these parameters on the discrete transfer of units from the lower to the upper plate during subduction the primary controls investigated are spatial variations in density material strength and thickness of material layers these controls correspond to natural examples where episodic accretion results from the thermal structure of subducting lithosphere giving rise to areas of thermally activated ductile creep attempted subduction of small crustal blocks density frictional and or ductile strength variations interaction between well attached basement blocks and overlying detachable sediments and spatial changes in thickness of sediment and crust in the experiments accretion of small continental terranes within a model subduction zone can cause crustal scale fold nappes and shear zones to develop with accompanying tectonic underplating and or frontal accretion the model results may provide insight into real cases of episodic accretion for example the emplacement of the upper penninic nappe stack of the central alps

multiparameter two dimensional inversion of scattered teleseismic body waves 2 numerical examples in this paper we investigate the formal inversion of synthetic teleseismic p coda waves for subsurface elastic properties using a ray theoretic approach which assumes single scattering bostock et al this issue we consider a model comprising an idealized lithospheric suture zone whose geometrical configuration is drawn from previous deep crustal seismic studies two dimensional pseudospectral synthetic seismograms representing plane waves propagating through this model are preprocessed to extract an estimate of the scattered wave field generated by short wavelength structure these data are employed in a series of numerical simulations which examine the dependence of multiparameter inversion results on a range of input parameters in particular we demonstrate 1 the contrasting sensitivity which forward and backscattered waves display to structural recovery 2 the diminution of the problem null space accompanied by increased source coverage 3 improvements in model reconstruction achieved through simultaneous treatment of multiple scattering modes and 4 the robustness of the method for data sets with noise levels and receiver geometries that approach those of field experiments

objective regionalization of rayleigh wave dispersion data by clustering algorithms an application to the mediterranean basin abstract the main target of the present study is an objective and automated regionalization of rayleigh wave dispersion data for the mediterranean basin without a priori seismotectonic constraints and to determine the corresponding regional shear velocity structures the database used is formed by almost 200 rayleigh wavetrains corresponding to 42 regional events with surface wave magnitude greater than 4 5 recorded at the mednet very broad band stations in the mediterranean area path averaged group velocities for the rayleigh wave fundamental mode are derived for each available epicentre station trajectory crossing the mediterranean basin after this a principal component analysis and a clustering process are applied to local group velocities obtained for 13 different periods from 10 to 70 s in order to classify the mediterranean basin into several homogeneous regions the stochastic inversion of the averaged group velocity dispersion curve obtained for each region provides the respective shear velocity structures down to a depth of 150 160 km the characteristics of these areas and their possible correlation with the main seismotectonic features of the mediterranean region are discussed the regional models reveal significant lateral changes in the elastic structure with the main differences concerning particularly the upper 35 40 km within this depth range low shear velocities varying from 2 8 to 3 9 km s 1 characterize the eastern mediterranean whereas higher velocities ranging from 3 0 to 4 2 km s 1 are deduced for the western mediterranean these results suggest a thicker crust in the eastern part but with a greater thickness of sedimentary layers however for depths of between 80 and 110 km lower shear velocities are obtained in the western part while higher shear velocities are derived for the eastern mediterranean sea in the aegean sea greece the south of italy sicily and tunisia this velocity pattern suggests an averaged thicker lithosphere under the latter areas as the top of the asthenosphere is detected at a mean depth of 75 km for the remaining regions this thicker lithosphere can be related to processes associated with the convergence of the eurasian and african plates and subduction under the calabrian and hellenic arcs

crustal investigation of the hellenic subduction zone using wide aperture seismic data abstract we present the results of a wide aperture seismic onshore offshore study crete seismic experiment in the cretan region as part of the hellenic arc compressional system three seismic lines were carried out on and around the island of crete in order to investigate the crustal structure of the region up to 119 three component recording stations were deployed on each profile that observed seismic energy generated by a 48 l airgun array and eight 20 kg landshots a total of 6208 shots were fired upon completing the fieldwork the vertical components of all stations were evaluated 300 common receiver gather crg sections of the ocean bottom seismographs obs and land stations as well as 100 common source gather csg sections of the land shots and selected airgun shots were compiled and modeled in order to generate a 2d p wave velocity depth model for each profile the accuracy of the model depends on the depth and position along the profiles and does not exceed 5 for both depth and p wave velocity we identified strong lateral variations in crustal and sedimentary thickness mainly in a north south direction but also along strike east west the crust is continental and has a maximum thickness of 32 5 km below northern central crete its subdivision in an upper v p 5 8 6 3 km s locally up to 6 5 km s and a lower v p 6 4 6 9 km s part is justified by a first order discontinuity with v p velocity a contrast of up to 0 6 km s the eastern part of crete shows a significantly thinner crust of 24 to 26 km to the north the crustal thickness decreases to 15 km below the central cretan sea the prominent decrease of the moho depth north of central crete is interpreted to represent the northern end of a microcontinent that was subducted in oligocene times and later surfaced by buoyant escape stockhert et al 1999 thomson et al 1999 the p t t d history of the high pressure rocks of crete greece denudation by buoyant escape in exhumation processes normal faulting ductile flow and erosion ring u lister g willet s brandon m eds spec publ of the geol soc of london p 154 to the south and southwest of the island the continental crust gradually thins to a minimum of 17 km and at approximately 100 km off the southern coast of crete it is in contact with oceanic crust below the mediterranean ridge upper mantle velocities were determined to be 7 7 km s below the cretan sea and 8 0 km s south of crete below the continental cretan crust a 6 to 7 km thick layer with v p velocities between 6 6 and 7 1 km s was identified on each line and could be followed by reflections to a depth of 42 km it is decoupled from the overlying continental crust at central crete and is interpreted as oceanic crust presently under subduction towards the nne below the aegean sea

seismic structure and the active hellenic subduction in the ionian islands in the region of the ionian islands of western greece the active margin of the hellenic domain passes from oceanic subduction in the south to continental collision in the north linked by the right lateral cephalonia transform fault a slightly landward dipping interface revealed at 13 km depth by a single previous line in the channel between cephalonia and zante has been suggested as the interplate subduction boundary new marine multichannel reflection profiles and obs refraction and wide angle reflection data confirm the reflector as a regional feature these data evidence its extension to the south where large low angle thrust earthquakes occur offshore to zante the new profiles establish a coincidence between the focal depths of these large subduction events and the imaged bright reflective level confirming its tentative interpretation as the interplate boundary which generally appears with a positive reflection polarity in this context the ionian islands outcrop corresponds to a shallowing of the interplate boudary from south to north in the south offshore zante the interplate boundary comprises a stratified zone that may be considered as the sedimentary cover of the ionian basin oceanic like crust which forms the lower plate here the shallower position and single cycle reflection character of the interplate further north suggest that the lower plate could there be the apulian paleomargin to that basin

shaping mobile belts by small scale convection claudio faccenna and thorsten becker compute global mantle flow based on a recent high resolution seismic tomography model to investigate the role of buoyancy driven and plate motion induced mantle circulation for the mediterranean they show that mantle flow provides an explanation for much of the observed dynamic topography and microplate motion in the region and conclude that vigorous small scale convection in the uppermost mantle may also hold the key for understanding other complex mobile belts such as those in the north american cordillera and the himalayan tibetan collision zone here global mantle flow is computed on the basis of recent high resolution seismic tomography to investigate the role of buoyancy driven and plate motion induced mantle circulation in the mediterranean region the findings show that mantle flow explains much of the observed dynamic topography and microplate motion in the region small scale convection in the uppermost mantle may also hold the key to understanding complex mobile belts elsewhere mobile belts are long lived deformation zones composed of an ensemble of crustal fragments distributed over hundreds of kilometres inside continental convergent margins1 2 the mediterranean represents a remarkable example of this tectonic setting3 the region hosts a diffuse boundary between the nubia and eurasia plates comprised of a mosaic of microplates that move and deform independently from the overall plate convergence4 surface expressions of mediterranean tectonics include deep subsiding backarc basins intraplate plateaux and uplifting orogenic belts although the kinematics of the area are now fairly well defined the dynamical origins of many of these active features are controversial and usually attributed to crustal and lithospheric interactions however the effects of mantle convection well established for continental interiors5 6 7 should be particularly relevant in a mobile belt and modelling may constrain important parameters such as slab coherence and lithospheric strength here we compute global mantle flow on the basis of recent high resolution seismic tomography to investigate the role of buoyancy driven and plate motion induced mantle circulation for the mediterranean we show that mantle flow provides an explanation for much of the observed dynamic topography and microplate motion in the region more generally vigorous small scale convection in the uppermost mantle may also underpin other complex mobile belts such as the north american cordillera or the himalayan tibetan collision zone

crustal and upper mantle structure in the eastern mediterranean from the analysis of surface wave dispersion curves summary the dispersive properties of surface waves are used to infer earth structure in the eastern mediterranean region using group velocity maps for rayleigh and love waves from 7 to 100 s we invert for the best 1 d crust and upper mantle structure at a regular series of points assembling the results produces a 3 d lithospheric model along with corresponding maps of sediment and crustal thickness a comparison of our results to other studies finds the uncertainties of the moho estimates to be about 5 km we find thick sediments beneath most of the eastern mediterranean basin in the hellenic subduction zone and the cyprus arc the ionian sea is more characteristic of oceanic crust than the rest of the eastern mediterranean region as demonstrated in particular by the crustal thickness we also find significant crustal thinning in the aegean sea portion of the backarc particularly towards the south notably slower s wave velocities are found in the upper mantle especially in the northern red sea and dead sea rift central turkey and along the subduction zone the low velocities in the upper mantle that span from north africa to crete in the libyan sea might be an indication of serpentinized mantle from the subducting african lithosphere we also find evidence of a strong reverse correlation between sediment and crustal thickness which while previously demonstrated for extensional regions also seems applicable for this convergence zone

disruption of the hellenic arc late miocene extensional detachment faults and steep pliocene quaternary normal faults or what happened at corinth 1 extensional faults exposed in the peloponnesus and mainland greece most of which are described here for the first time record a transition from regional extension of the aegean domain to the modern tectonic system the east peloponnesus detachment system trends north northwest from the southern peloponnesus to 30 km north of the gulf of corinth dips gently northeast and is late miocene early pliocene in age it has a minimum displacement of 25 30 km and appears to be the youngest of the regional scale extensional systems with significant displacement that formed parallel to the hellenic arc the partially coeval east sterea extensional system which extends from the gulf of corinth to the aegean sea contains low angle normal faults that both crosscut and trend parallel to older structures of the hellenic arc late miocene to early pliocene displacement within this zone disrupted the arc parallel structures of the hellenides upper pliocene quaternary normal faults which trend approximately east west and generally dip steeply at the surface continue the disruption of the hellenic arc much of the subsidence within the gulf of corinth appears to be unrelated to the younger faults and is instead related to the motion on the east peloponnesus detachment which crosscuts the modern graben

seismicity of the hellenic subduction zone in the area of western and central crete observed by temporary local seismic networks temporary local seismic networks were installed in western crete in central crete and on the island gavdos south of western crete respectively in order to image shallow seismically active zones of the hellenic subduction zone more than 4000 events in the magnitude range between 0 5 and 4 8 were detected and localized the resulting three dimensional hypocenter distribution allows the localization of seismically active zones in the area of western and central crete from the mediterranean ridge to the cretan sea furthermore a three dimensional structural model of the studied region was compiled based on results of wide angle seismics surface wave analysis and receiver function studies the comparison of the hypocenter distribution and the structure has allowed intraplate and interplate seismicity to be distinguished high interplate seismicity along the interface between the subducting african lithosphere and the aegean lithosphere was found south of western crete where the interface is located at about 20 to 40 km depth an offset between the southern border of the aegean lithosphere and the southern border of active interplate seismicity is observed in the area of crete the offset varies laterally along the hellenic arc between about 50 and 70 km a southwards dipping zone of high seismicity within the aegean lithosphere is found south of central crete in the region of the ptolemy trench it reaches from the interface between the plates at about 30 km depth towards the surface in comparison the aegean lithosphere south of western crete is seismically much less active including the region of the ionian trench intraplate seismicity within the aegean plate beneath crete and north of crete is confined to the upper about 20 km between 20 and 40 km depth beneath crete the aegean lithosphere appears to be seismically inactive in western crete the southern and western borders of this aseismic zone correlate strongly with the coastline of crete

geoelectric investigation of the hellenic subduction zone using long period magnetotelluric data abstract the strongest evidence up to date for a subduction zone in the hellenic region is a clearly identified wadati benioff zone below the central aegean sea to a maximum depth of 180 km alternative seismic tomography models suggest that subduction process continues deeper than the wadati benioff zone to a maximum depth of at least 600 km so far the lack of deep electrical studies in the region impeded scientists from imposing other control factors than seismic to the proposed models for the hellenic subduction zone hsz a long period magnetotelluric lmt study was carried out in the southern part of the greek mainland to study the deep electrical characteristics of the hsz and examine whether prominent modelled features correlate with structures identified by the seismic methods the study comprised collection processing and modelling of magnetotelluric mt data in the period range 100 10000 s from ten sites located along a 250 km ne sw trending profile the dimensionality of the data was examined at a pre modelling stage and it was found that they do not exhibit three dimensional 3 d features the latter enabled to construct both one dimensional 1 d and two dimensional 2 d models the proposed geoelectric model for hsz was based on 2 d modelling since it had better maximum depth resolution of about 400 km and revealed structures not detected by 1 d modelling attempts the model structure which was related to the african and euro asian lithosphere is relatively resistive 800 m and has an average thickness of 150 170 km although the bottom of the lithosphere is adequately resolved the wadati benioff zone that delineates the top of the subducting lithospheric slab is not identified by any electrical feature the modelled structure associated with the subducting part of the african lithosphere penetrates a relatively conductive

study of the crustal thickness and the subducting lithosphere in greece from gravity data the area of greece has experienced a complex tectonic history dominated by the subduction of the african plate beneath eurasia in this study the variations of the crustal thickness in the area of greece were obtained by means of the multiple source werner deconvolution mswd method applied to gravity data thicknesses of 40 49 km are estimated beneath the hellenides mountain belt to the west eastward thinning of the crust to thicknesses ranging from 25 km in the north to 30 km in the south is seen in the aegean region these results are in good agreement with recent seismological results demonstrating that the mswd method successfully treated the problem using the crustal model we derived we computed the gravity effect of the crust and extracted it from the bouguer anomaly we also extracted the gravity effect of the subducting lithosphere from the bouguer anomaly producing a residual map where most of the original gravity variation has been successfully removed the remaining anomalies appear related to near surface features and an area of low velocity mantle in the central aegean sea

s velocity structure and radial anisotropy in the aegean region from surface wave dispersion summary the lithospheric structure of the aegean region is investigated by analysis of rayleigh wave fundamental mode dispersion measurements isotropic 1 d models for almost 100 two station ray paths across the region display distinct variations in the moho depth and crustal s wave velocities the descending slab of the subducting african plate can be resolved down to 120 km depth beneath the volcanic arc three different regions are distinguished in terms of moho depth 1 the forearc with large crustal thicknesses between 38 and 48 km and an average of 43 km 2 the northern aegean with an average moho depth of 28 km and 3 the southern aegean central volcanic arc i e cyclades and sea of crete with an even thinner crust of around 25 km lateral variations in structure between 25 and 55 km depth indicate a marked difference between the western and eastern forearc collocated with pronounced changes in trench and slab geometry as well as published deformation rates s velocities between 25 and 55 km depth are low everywhere beneath the forearc but increase from the peleponnesus to crete an abrupt change occurs between western and central crete in terms of the visibility of the aegean moho and the seismic structure of the lithospheric mantle wedge an aegean mantle wedge with s velocities above 4 4 km s 1 is only observed to the east of central crete whereas to the west velocities of less than 4 0 km s 1 occur down to the plate contact these low velocities above the slab may indicate the presence of a melange of metamorphic rocks at the depths a low velocity asthenospheric layer is observed beneath the sea of crete and the cyclades below 40 km depth between the thinned lithosphere above and the slab below the observed radial anisotropy in the northern part of the aegean is likely to be due to preferred orientation of anisotropic minerals within the lower crust possibly caused by lateral ductile flow associated with recent lithospheric extension

dynamics of continental collision influence of the plate contact summary observations shows that continental collision may evolve in different ways resulting in a wide range of tectonic responses in search of the controlling conditions and parameters we start from the results of our previous work which demonstrated that the properties of the plate contact are important for the overall dynamics of convergent plate margins two fundamental types of subduction plate contact can be distinguished one based on a fault and the other based on a weak subduction channel in this study we investigate how the plate contact affects the initial stage of continental collision we use a finite element method to solve the heat and the time dependent momentum equations for elastic power law viscous and plastic rheologies for the same rheological properties and driving forces varying the nature of the plate contact leads to three types of responses the presence of a subduction channel promotes coherent and when the boundary conditions allow it plate like subduction of the continental margin in models with a subduction fault coherent subduction of the incoming continental lithosphere occurs when the colliding passive margin has a gentle slope the approaching continental sliver starts to subduct and the subduction is characterized by a non plate like behaviour slower subduction velocity than in channel models and strong slab deformation if the continental margin is steep and the strength of the incoming continental crust is high fault models result in locking of the trench eventually leading to slab break off if the crustal strength is relatively low shear delamination of part of the crust is expected in the channel model this type of delamination never occurs the tectonic settings used in our experiments prescribed plate velocity of the subducting plate versus fixed subducting plate corresponding to a landlocked basin setting do not significantly influence the nature of the model response we conclude that initial stages of continental collision are strongly affected by whether the subduction contact is a fault or a channel neither the slab pull magnitude nor the tectonic setting is very important to the overall geodynamics at this stage the plate contact type along with the slope of the incoming passive margin and the rheology of the continent controls whether the incoming crust 1 subducts entirely 2 separates partially or entirely from the lithospheric mantle or 3 blocks the trench likely leading to slab break off

two ship deep seismic soundings in the basins of the eastern mediterranean sea pasiphae cruise summary new results from a two ship refraction and oblique reflection deep seismic cruise are presented to discuss the nature of the crust in the ionian sirte and herodotus abyssal plains these expanding spread profiles were processed and analysed in both the x t and p domains arrival times of reflected and refracted branches are matched by ray tracing in both domains in spite of a shallow evaporitic sequence messinian evaporites deposited on top of a thick sedimentary pile responsible for velocity inversions on many profiles we obtain excellent velocity control down to moho depth pmp and pn 8 4 8 6 km s 1 arrivals are observed the three basins have a relatively thin crust 8 to 11 km overlain by a thick sedimentary cover up to 10km in the herodotus abyssal plain the moho boundary and main crustal units identified in the basins can be followed beneath the calabrian prism to the west and beneath the mediterranean ridge to the east the crustal structure is of oceanic type for both the ionian and sirte basins where typical oceanic layer 2 and 3 are recognized the thin crust of the herodotus basin may be interpreted either as oceanic or thinned continental crust about 10 km thick the top of the crust of the herodotus basin is much deeper therefore the herodotus basin is probably significantly older than the ionian basin triassic versus early cretaceous in age

crustal structure beneath broad band seismic stations in the mediterranean region summary we have analysed receiver functions to derive simple models for crustal structure below 12 broad band seismological stations from the midsea project and 5 permanent broad band stations in the mediterranean region including northern africa to determine an accurate moho depth we have reduced the trade off between crustal velocities and discontinuity depth using a new grid search method which is an extension of recently published methods to determine crustal thickness in this method the best fitting synthetic receiver function containing both the direct conversion and the reverberated phases is identified on a model grid of varying moho depth and varying poisson s ratio the values we found for moho depth range from around 20 km for intra oceanic islands and extended continental margins to near 45 km in regions where the eurasian and african continents have collided more detailed waveform modelling shows that all receiver functions can be well fit using a 2 or 3 layer model containing a sedimentary layer and or a mid crustal discontinuity on comparing our results with moho maps inferred from interpolated reflection and refraction data we find that for some regions the agreement between our receiver function analysis and existing moho maps is very good while for other regions our observations deviate from the interpolated map values and extend beyond the geographic bounds of these maps

crustal velocity and moho structure beneath the gulf of corinth greece summary the gulf of corinth goc greece is a continental rift with high rates of seismicity and extensional strain how this strain is accommodated in the crust and whether there are variations in the mechanism along strike remain open questions in part because of a lack of wide angle reflection refraction studies that constrain crustal velocity structure in 2001 an extensive multichannel seismic survey was conducted within the goc one component of which included the wide angle recording of sources from within the gulf at stations on land surrounding the gulf in this paper we use wide angle data in two separate but allied studies to constrain crustal velocities and depth to the moho a 2 d inversion of refraction and reflection traveltimes along an axial profile through the goc constrains the shallow crustal velocity structure images the moho at 29 km depth in the east dipping to 39 km in the west and images the eastward subducting african slab beneath the western goc at a depth of 74 km the 1 d average of the 2 d velocity model was used in a tomographic inversion of pmp reflection times to solve for depth to the moho throughout the corinth region this model shows generally thick isostatically compensated crust 37 km beneath the hellenide mountains except immediately south of the goc and a singular region of thin crust 30 km beneath the perahora peninsula at the eastern end of the gulf a comparison with moho depths derived from gravity inversion shows a general agreement with crust thickening from east to west but a number of differences in detail the 3 d crustal thickness variations are more complex than those predicted by either pure shear or simple shear models of continental extension and suggest significant pre rift structural variability

reflection refraction seismics in the gulf of corinth hints at deep structure and control of the deep marine basin abstract the gulf of corinth is a natural laboratory for the study of seismicity and crustal deformation during continental extension seismic profiling along its axis provides a 24 fold normal incidence seismic reflection profile and wide angle reflection refraction profiles recorded by sea bottom seismometers obs and land seismometers at wide angle incidence the land receivers document the moho at 40 km depth under the western end of the gulf north of aigion rising to 32 km depth under the northern coast in the east of the gulf both refraction and normal incidence reflection sections image the basement under the deep marine basin that has formed by recent extension the depth to the base of the sedimentary basin beneath the gulf constrained by both methods is no more than 2 7 km with 1 km of water underlain by no more than 1 7 km of sediment less than what was expected from past modeling of uplift of the south coast in the east of the gulf unlike the flat sea bottom the basement and sedimentary interfaces show topography along this axial line several deeps are identified as depocenters which suggest that this axial line is not a strike line to the basin it appears instead to be controlled by several faults oblique to the s60 e overall trend of the south coast of the gulf their more easterly strikes being consistent with the instantaneous direction of extension measured by earthquake slip vectors and by gps

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

effect of a retreating subduction zone on deformation in simple regions of plate convergence plane strain two dimensional finite element models are used to calculate deformation patterns within model mountain belt systems where subduction zone retreat the process by which space is created between the subducting pro and overriding retro lithospheres either by motion of one or both is active the model uses coulomb plastic and thermally activated power law viscous rheologies overall deformation is determined by balancing the internal strength of the model layer against the sum of applied boundary stresses plus the gravitational stress induced by mass redistribution within the deforming model the effect of incorporating retreat in the subduction setting diffuses deformation as the moving retreating subduction zone deforms a larger cross sectional area of material for a shorter period of time than equivalent models without subduction zone retreat material is deformed while within the subduction zone but once the material has passed completely through the moving subduction zone it is passively appended onto the retrolithosphere and is no longer deformed thus relative to a convergent setting without subduction zone retreat the retreating convergent zone has an areally extensive zone of simple deformation a deformation factor is derived which characterizes the deformation shortening the material experienced within the subduction zone variable is the ratio of convergence rate rate at which material enters the subduction zone to subduction rate rate at which material is subducted variables and f the fraction of the layer that is subducted together determine the dominant style of deformation in the orogenic zone subduction zone retreat is shown to have significant and predictable effects on the model morphology e g basin or mountain formation and on the model geology e g its presence suggests lower topography lower metamorphic grade rocks at the surface and simpler deformational structures which agree with observations

the mediterranean ridge a mass balance across the fastest growing accretionary complex on earth depth migration of seismic reflection profiles across the mediterranean ridge accretionary complex between the african and eurasian blocks illustrates profound variations in the geometry and internal structure along strike structural interpretations of four cross sections together with bathymetric and acoustic surface information and drilling data are used to volumetrically balance the amount of subduction versus accretion with time results suggest the existence of three distinct scenarios with a jump in decollement in the west intense backthrusting in the central part between libya and crete and transcurrent tectonism in the east the onset of accretion coincides with exhumation of thrust sheets 19 ma followed by rapid sediment accretion with thick evaporite bearing incoming successions facilitating outward growth of the wedge the minimum rate of accretion 20 25 of the total sediment supply is observed in the central portion where the ridge suffers maximum deformation here the indenting leading edge of the african plate apparently forces the sediment into subduction or local underplating in contrast an estimated 40 60 of the available sedimentary input was accreted in the western domain where collision is less accentuated the results support the hypothesis that highly destructive forearc collisional events like slab break off and exhumation of thrust sheets can be followed by periods of accretion and continuous growth of accretionary wedges

seismic imaging of subduction zone metamorphism combined analysis of high resolution seismic images of the alaska and cascadia subduction zones reveals where metamorphic fl uids are released both images show the subducted oceanic crust as a dipping low velocity layer with a clear termination depth however in alaska the crust is thicker 15 20 km compared to 8 km and terminates at greater depth 120 km com pared to 40 km than in cascadia based on metamorphic reaction estimates and geodynamic models we demonstrate that the termination depth corresponds to eclogitization of the crust triggered by dehydration of water bearing minerals and that the location of this reaction is dependent on the thermal structure of the subducted slab

timing kinematics and cause of aegean extension a scenario based on a comparison with simple analogue experiments abstract since the early 1970s most geodynamic models proposed for the eastern mediterranean consider that the initiation of extension in the aegean during the late middle miocene or later was a direct consequence of the westward extrusion of anatolia away from the arabia eurasia collision front in the late 1990s a few models have taken into account the increasing evidence for an earlier initiation of aegean extension arising mostly from the analysis of deformation within the various alpine metamorphic complexes of the area based largely on our own investigations we first present a review of the data that reliably constrain the minimum age for the initiation of extension in these complexes it shows that regional scale extension with a pattern of stretching orientations similar to that of the pliocene pleistocene was already strongly active in the aegean before the onset of arabian indentation into eurasia lower miocene vs middle or upper miocene this implies that the initiation of aegean extension did not result from the lateral extrusion of anatolia instead extension may have started owing to a process of gravity spreading of the continental lithosphere that had previously been thickened during alpine collision the main arguments favouring this interpretation are presented and a preliminary scenario is proposed for the neogene evolution of the eastern mediterranean in which lateral extrusion of anatolia occurred lately during southward spreading of the aegean lithosphere in order to test this hypothesis further physical experiments simulating the horizontal spreading of a continental lithosphere toward a free boundary have been carried out the analysis of various elements of the deformation field displacements strain rotations fault pattern and kinematics both in the experiments and in the aegean domain reveals an excellent agreement between the two not only do early stages of aegean extension appear to fit the gravity spreading hypothesis but so does the recent to present internal deformation of the aegean transverse e w active shortening observed in the northern aegean is commonly taken as one of the most striking pieces of evidence that aegean extension results from the lateral extrusion of anatolia the experiments indicate that such transverse shortening may equally develop as a result of pure gravity spreading associated with a pattern of faults and rotations that compares well with that of the northern aegean pure dextral shearing along some strands of the north aegean trough fault system and the related development of an asymmetric pattern of faults in the northern aegean are eventually the only features that evidently arise from the extrusion of anatolia since less than 3 ma finally we argue that during the early stages of aegean extension since at least 21 ma and until ca 11 ma the distribution of extensional strain on a regional scale has been controlled by the distribution of earlier thickening post thickening thermal weakening through partial melting of deep levels of the crust probably accounts for the localization of extensional strain within the areas of thick crust restoring the aegean in its most likely configuration at the onset of bulk crustal extension suggests that 1 the extensional strain rate remained roughly constant or has eventually decreased since the onset of aegean extension and 2 the amounts of extension before and after 16 ma have been roughly equal

imaging crustal discontinuities and the downgoing slab beneath western crete the migration of teleseismic receiver functions yields high resolution images of the crustal structure of western crete data were collected during two field campaigns in 1996 and 1997 by networks of six and 47 short period three component seismic stations respectively a total of 1288 seismograms from 97 teleseismic events were restituted to true ground displacement within a period range from 0 5 to 7 s the application of a noise adaptive deconvolution filter and a new polarization analysis technique helped to overcome problems with local coda and noise conditions the computation and migration of receiver functions results in images of local crustal structures with unprecedented spatial resolution for this region the crust under crete consists of a continental top layer of 15 20 km thickness above a 20 30 km thick subducted fossil accretionary wedge with a characteristic en echelon fault sequence the downgoing oceanic moho lies at a depth of 40 60 km and shows a topography or undulation with an amplitude of several kilometres as a consequence of slab depth and distribution of local seismicity the mediterranean ridge is interpreted as the recent accretionary wedge

multiparameter two dimensional inversion of scattered teleseismic body waves 3 application to the cascadia 1993 data set this is the third paper in a three part series that examines formal inversion of the teleseismic p wave coda for discontinuous two dimensional 2 d variations in elastic properties beneath dense arrays of three component broadband seismometers in this paper the method is applied to data from the incorporated research institutions for seismology program for array seismic studies of the continental lithosphere iris passcal cascadia 1993 experiment undertaken across central oregon two major features are imaged in the resulting model the continental moho becomes evident 150 km from the coast beneath the western cascades and extends through the eastern end of the profile at 35 40 km depth in the western portion of the model oceanic crust of the subducting juan de fuca plate dips shallowly 12 at the coast and more steeply 27 below the willamette valley and is evident to depths of 100 km beneath the high cascades the abrupt increase in plate dip at 40 km depth coincides with an apparent thickening of the oceanic crust followed by a diminution in its signature building on previous work we argue that these results are consistent with the consequences of prograde metamorphic reactions occurring within the oceanic crust progressive dehydration at lower grade facies conditions culminates in the transformation to eclogite producing a pronounced increase in the seismic velocity density and dip of the subducting plate and structural complexity in the overlying wedge

the oceanic nature of the african slab subducted under peloponnesus thin layer resolution from multiscale analysis of teleseismic p to s converted waves summary in the hellenic subduction zone the lithospheric slab may comprise continental and oceanic units juxtaposed downdip and along strike for stations along eastern peloponnesus teleseismic p wave receiver function rf processing in the standard frequency band produces an image of a low velocity layer lvl at the top of the slab apparently twice thicker than for an oceanic crust to assess if this could come from a lack of resolution of the standard processing we develop a multiscale approach with the rfs based on the wavelet response of the medium akin to the wavelet transform of the velocity depth function the synthetic response in conversion is obtained for a multiscale singularity formed by two opposite velocity steps at the boundaries of a crust embedded in mantle material this indicates that only wavelet periods shorter than about 0 8 s will allow to identify clearly a 7 km thin oceanic crust going to longer periods leads to underestimate or overestimate the time thickness of the lvl due to interference phenomena the analysis of the response in conversion from full waveform synthetic seismograms in a dipping slab model validates a multiresolution approach to real observations with earthquakes of broad enough spectrum towards high frequencies yielding energy to provide wavelet periods significantly shorter than 1 s the p to s conversions obtained allow us to resolve for the first time a standard oceanic crust at the slab top beneath the eastern coast of peloponnesus this documents the subduction of a purely oceanic slab of most reduced buoyancy since 4 5 myr under the rapidly southwestward extending upper plate continental material

constraints on absolute s velocities beneath the aegean sea from surface wave analysis summary the 3 d structure of the lithosphere beneath the aegean sea is investigated through surface wave dispersion analysis rayleigh and love waves recorded by 12 broad band stations installed for a duration of 6 months in the aegean region are processed through array analysis and wiener filtering data from three geofon stations in the area of crete were also used the resulting two station phase velocities are used to determine lateral variations of rayleigh wave phase velocities between periods of 20 and 100 s by a 2 d ray tomography method the obtained phase velocities are inverted to calculate variation of s wave velocity with depth using a combination of linearized inversion and a monte carlo based non linear inversion the absolute s wave velocity is resolved to a depth of approximately 200 km a highvelocity anomaly of 3 per cent is observed in the southern aegean attributed to the hellenic subduction in the northern part of the aegean in the prolongation of the north anatolian fault which is influenced by strong extensional movements we found low absolute s wave velocities at 50 100 km depth this supports a model of a distributed deformation of the upper mantle in the area separate rayleigh and love wave phase velocity inversions along common profiles reveal a strong love rayleigh discrepancy in the northern aegean down to at least 150 km depth i e most probably including the top of the asthenosphere

subduction and exhumation of continental crust insights from laboratory models summary when slivers of continental crust and sediment overlying oceanic lithosphere enter a subduction zone they may be scraped off at shallow levels subducted to depths of up to 100 200 km and then exhumed as high pressure hp and ultra high pressure uhp rocks or subducted and recycled in the mantle to investigate the factors that influence the behaviour of subducting slivers of continental material we use 3 d dynamically consistent laboratory models a laboratory analogue of a slab upper mantle system is set up with two linearly viscous layers of silicone putty and glucose syrup in a tank a sliver of continental material also composed of silicone putty overlies the subducting lithosphere separated by a syrup detachment the density of the sliver viscosity of the detachment geometry of the subducting system attached plate versus free ridge and dimensions of the sliver are varied in 34 experiments by varying the density of the sliver and viscosity of the detachment we can reproduce a range of sliver behaviour including subduction subduction and exhumation from various depths and offscraping sliver subduction and exhumation requires sufficient sliver buoyancy and a detachment that is strong enough to hold the sliver during initial subduction but weak enough to allow adequate sliver displacement or detachment for exhumation changes to the system geometry alter the slab dip subduction velocity pattern of mantle flow and amount of rollback shallower slab dips with more trench rollback produce a mantle flow pattern that aids exhumation steeper slab dips allow more buoyancy force to be directed in the up dip direction of the plane of the plate and aide exhumation of subducted slivers slower subduction can also aide exhumation but if slab dip is too steep or subduction too slow the sliver will subduct to only shallow levels and not exhume smaller slivers are most easily subducted and exhumed and influenced by the mantle flow

nappe stacking resulting from subduction of oceanic and continental lithosphere below greece we quantitatively investigate the relation between nappe stacking and subduction in the aegean region if nappe stacking is the result of the decoupling of upper crustal parts 5 10 km thick from subducting lithosphere then the amount of convergence estimated from balancing the nappe stack provides a lower limit to the amount of convergence accommodated by subduction the balanced nappe stack combined with the estimated amount of completely subducted lithosphere indicates 700 km of jurassic and 2400 km of post jurassic convergence from seismic tomographic images of the underlying mantle we estimate 2100 2500 km of post jurassic convergence we conclude that 1 the imaged slab represents the subducted lithosphere that originally underlay the nappes 2 since the early cretaceous subduction in the aegean has occurred in one single subduction zone and 3 the composition of the original basement of the nappes indicates that at least 900 km of sub upper crust continental lithosphere subducted in the aegean

moho topography under central greece and its compensation by pn time terms for the accurate location of hypocenters the example of the gulf of corinth 1995 aigion earthquake abstract in this paper we expand over the whole of central greece the moho map centered on the gulf of corinth from tomographic inversion of pmp traveltime profile data recorded by several tens of temporary stations our approach is based on pn moho refracted waves from a large regional earthquake recorded by both temporary stations and the permanent hellenic network the moho map shows the large moho depth under the hellenides belt it also highlights the shallower moho domain towards the aegean sea south and east of the corinth gulf the domain of shallow moho is limited along a ne sw prolongation ahead of the north anatolian fault from the north aegean trough to the western tip of the gulf of corinth towards the gulf of patras the pn time terms provide corrections for the permanent stations that can be used together with the 1d velocity depth model for a first order compensation of lateral heterogeneity and contribute to the accurate and fast location of earthquake hypocenters as a test we relocated the 1995 aigion earthquake in this way using only the sparse data of the permanent stations hypocentral coordinates then shift close to those derived by a dedicated dense array deployed after the earthquake implying improvement of the routine location

deflection gravity anomalies and tectonics of doubly subducted continental lithosphere adriatic and ionian seas three profiles across the southern adriatic and northern ionian seas span an intermediate foreland area that is or has recently been subducted both westward under the apennine calabrian thrust belt and eastward under the hellenide belt gravity and deflection data from the base of the pliocene along the three profiles are qualitatively similar to those predicted for a thin elastic sheet subject to end loads at both plate ends more detailed analysis of the profiles shows very good agreement between observations and predictions if the intermediate foreland lithosphere is treated as a thin elastic sheet with an effective elastic thickness between about 15 and 20 km equivalent to a flexural rigidity between 2 1022 and 6 1022 nm these results coupled with seismicity data suggest that active convergence within the hellenides extends as far north as southwestern yugoslavia analysis of the northernmost profile across the southern adriatic sea shows a well constrained elastic plate thickness of 14 km and an initial water depth prior to loading of about 1 km the distance between the two subduction systems on this profile is estimated at about 275 km analysis of the two more southerly profiles yields a minimum effective elastic plate thickness of about 15 km but no maximum value could be determined the initial water depths for these two profiles are well constrained at 1 3 and 1 6 km prior to loading and the distance between the two subduction systems is estimated at about 175 km on both profiles along the northwest trend of the two opposing subduction systems the change in distance between the two systems occurs abruptly and the hellenic subduction system appears to be segmented into a series of northwest trending roughly linear subduction segments the calculated deflection profiles also show pronounced asymmetry on the northernmost profile and an opposite sense of asymmety on the southernmost profile

p and s deep velocity structure of the hellenic area obtained by robust nonlinear inversion of travel times we derived a new model for the p and s velocity structure of the lithosphere in the subduction area of the hellenic arc and the aegean sea from the inversion of travel times of local events the inversion technique applied is nonlinear since three dimensional ray tracing is incorporated at the same time an appropriate preconditioning of the final linearized system is used in order to reduce ray density effects on the results the p s coherency is controlled by an additional damping of the vp vs ratio the study focuses mainly on the structure of the hellenic subduction in the southern aegean region interesting features and details of the subducted slab can be recognized in the final tomographic images on the western part the shallower section of the slab is dipping at a very small angle 10 this changes to approximately 25 for the deeper part of the subduction resulting in a prominent kink at a depth of about 70 km which is in accordance with the general characteristics of the associated benioff zone the smaller eastern part of the slab is not resolved with the same detail but a clearly steeper subduction zone can be recognized moreover detailed information about the crustal thickness variations are inferred from the velocity structure and correlate very well with the existing results from refraction experiments finally the results confirm the recent suggestion concerning the existence of a low velocity crustal layer at shallow depths 10 15 km in the accretionary prism of the alpine belt

the role of slab detachment processes in the opening of the western central mediterranean basins some geological and geophysical evidence abstract a review of the geological and geophysical data from the central and western mediterranean region and the present day upper mantle structure derived from tomographic studies are utilized in order to define the oligocene recent geodynamic evolution for the area in line with previous work we suggest that the miocene quaternary opening of the western and central mediterranean basins is the result of back arc extension due to the roll back toward the southeast of a northwestward subducting african slab in a geodynamic setting pinned between the alpine and betic collisional zones we find however that this general pattern is complicated by four different detachment events which occurred beneath the alps early oligocene the betic chain aquitanian northern africa langhian and the apennines late miocene pliocene we show that each of these events determines a major tectonic reorganization within the european plate

the continuity of active fault systems in greece summary evidence from geomorphology the distribution of large earthquakes and geodetic measurements suggests that the active faulting in mainland greece and the north aegean sea is concentrated into a small number of discrete linear zones that bound relatively rigid blocks on land the zones are most clearly identified where the faulting is associated with large escarpments particularly in hard footwall rocks the zones can become indistinct and diffuse near their ends sometimes because they enter high unstable topography where landsliding obscures the fault morphology but mostly because the extension rates decrease along strike as a consequence of the relative rotation of the blocks they bound the main graben systems of chalkidiki southern thessaly and the north gulf of evia all seem to connect with the strike slip faulting in the offshore aegean and to die out in the west a configuration related to clockwise rotations in east central greece by contrast the gulf of corinth which is the fastest opening graben system in greece opens more rapidly in the west than the east both these features of the gulf of corinth are consequences of the motion of the south aegean and peloponnese as a single block late pliocene and quaternary geology and geomorphology indicate that the boundaries of the rigid blocks in central greece have changed over that time with faulting migrating into the hanging walls sometimes changing in orientation these changes are probably related to the way faulting in the seismogenic layer adapts to block rotation so as to maintain the general features of a velocity field that is controlled by larger scale effects such as buoyancy forces and the seaward migration of the subducting slab beneath crete the image of greece as a mosaic of rigid blocks whose boundaries change with time is a useful framework for seismic hazard evaluation but we emphasize that some moderate sized earthquakes do occur away from the main fault zones within the relatively rigid blocks and especially near their diffuse ends

uppermost mantle velocities and anisotropy beneath europe pn data collected within southern europe and the mediterranean are used to tomographically image variations in both seismic velocity and seismic anisotropy seismic anisotropy is an essential part of the inversion and without it several low velocity features within the uppermost mantle could not be properly imaged the technically active mantle of southern europe has much lower seismic velocities 7 6 8 1 km s than the more stable mantle of the sub african plate of the adriatic sea 8 3 km s however the most dramatic features within europe s uppermost mantle relate to the apennine dinaride and hellenide arcs these arcs all have extremely low 5 amounts of arc parallel anisotropy the tyrrhenian and aegean back arc regions also show low velocities 7 7 7 9 km s but less anisotropy the same may be true for the pannonian basin but the tomography has poorer resolution there a model explaining these observations focuses on subducted water metasomatizing the mantle wedge the addition of water causes melting creates arc volcanism lowers the seismic velocity and enhances the formation of anisotropy due to preferential olivine orientation for collisional arcs of the northern mediterranean arc parallel anisotropy has formed in response to compression across them and extension along them within back arc regions water is no longer a major factor instead convection associated with subduction and back arc extension controls the anisotropy

lithosphere tearing at step faults response to edges of subduction zones abstract slab edges are a relatively common feature in plate tectonics two prominent examples are the northern end of the tonga subduction zone and the southern end of the new hebrides subduction zone near such horizontal terminations of subduction trenches ongoing tearing of oceanic lithosphere is a geometric consequence we refer to such kinks in the plate boundary as a subduction transform edge propagator or step other steps are the north and south ends of the lesser antilles trench the north end of the south sandwich trench the south end of the vrancea trench and both ends of the calabria trench volcanism near steps is distinct from typical arc volcanism in some cases slab edges appear to coincide with mantle plumes using 3d mechanical models we establish that step faults are stable plate tectonic features in most circumstances in the probably rare cases that the resistance to fault propagation is high slab break off will occur relative motion along the transform segment of the plate boundary often is non uniform and the step is not a transform plate boundary in the rigid plate tectonics sense of the phrase step propagation may result in substantial deformation rotation topography and sedimentary basins with a very specific time space evolution surface velocities are substantially affected by nearby steps

greater india basin hypothesis and a two stage cenozoic collision between india and asia cenozoic convergence between the indian and asian plates pro duced the archetypical continental collision zone comprising the himalaya mountain belt and the tibetan plateau how and where india asia convergence was accommodated after collision at or be fore 52 ma remains a long standing controversy since 52 ma the two plates have converged up to 3 600 35 km yet the upper crustal shortening documented from the geological record of asia and the himalaya is up to approximately 2 350 km less here we show that the discrepancy between the convergence and the shortening can be explained by subduction of highly extended continental and oceanic indian lithosphere within the himalaya be tween approximately 50 and 25 ma paleomagnetic data show that this extended continental and oceanic greater india promontory resulted from 2 675 700 km of north south extension between 120 and 70 ma accommodated between the tibetan himalaya and cratonic india we suggest that the approximately 50 ma india asia collision was a collision of a tibetan himalayan microconti nent with asia followed by subduction of the largely oceanic greater india basin along a subduction zone at the location of the greater himalaya the hard india asia collision with thicker and contiguous indian continental lithosphere occurred around 25 20 ma this hard collision is coincident with far field deforma tion in central asia and rapid exhumation of greater himalaya crys talline rocks and may be linked to intensification of the asian monsoon system this two stage collision between india and asia is also reflected in the deep mantle remnants of subduction imaged with seismic tomography

on the relationships between slab dip back arc stress upper plate absolute motion and crustal nature in subduction zones 1 statistical analysis of modern oceanic subduction zone parameters such as the age of a downgoing plate or the absolute plate motions is performed in order to investigate which parameter controls the dip of a slab and conversely what the influence of slab geometry is on upper plate behavior for that purpose parameters have been determined from global databases along 159 transects from all subduction zones that are not perturbed by nearby collision or ridge plateau seamount subduction on the basis of tomographic images slabs that penetrate through or lie on the 670 km discontinuity are also identified the results of the statistical analysis are as follows 1 back arc stress correlates with slab dip i e back arc spreading is observed for deep dips deeper than 125 km larger than 50 whereas back arc shortening occurs only for deep dips less than 30 2 slab dip correlates with absolute motion of the overriding plate the correlation is even better when the slab lies on or even more penetrates through the 670 km discontinuity 3 slabs dip more steeply by about 20 on average beneath oceanic overriding plates than beneath continental ones 4 slabs dip more steeply on average by about 10 near edges 5 slab dip does not correlate with the magnitude of slab pull the age of subducting lithosphere at the trench the thermal regime of the subducting lithosphere the convergence rate or the subduction polarity east versus west the present study provides evidence that the upper plate absolute motion plays an important role on slab dip as well as on upper plate strain retreating overriding plates are often oceanic ones and thus may partially explain the steeper slab dips beneath oceanic upper plates one can infer that low slab dips correlate well with compression in continental advancing upper plates whereas steep dips are often associated with extension in oceanic retreating upper plates excess weight of old slabs is often counterbalanced by other forces probably asthenospheric in origin such as lateral mantle flow near slab edges or anchor forces to determine slab dip

modelling tectonic styles and ultra high pressure uhp rock exhumation during the transition from oceanic subduction to continental collision the tectonic styles of the transition from oceanic subduction to continental collision and their implications for exhumation of uhp material are tested in a series of numerical thermal mechanical models that address subduction collision dynamics at constant convergence rate reference models examine the sensitivity to continental mantle lithosphere density pro continental margin temperature and rheology and retro continental margin rheology three tectonic styles are observed 1 continental subduction without significant deformation of the over riding continent favoured by relatively dense mantle lithosphere and a strong retro continental margin 2 deformation and subduction of the leading edge of the retro continent favoured by low density mantle lithosphere and strong retro continental crust and 3 backthrusting of the retro continental margin and subsequent double subduction favoured by low density mantle lithosphere and weak retro continental crust continental u hp material is formed in all three tectonic styles although it is only efficiently exhumed in the first case three mechanisms contribute to u hp exhumation their importance depending on the evolving geometry of the subduction zone the strength of the continental margin crust and the density of the subducting mantle lithosphere weaker hotter continental margins decouple in the subduction channel and are subsequently exhumed by buoyancy and or driven circulation of material and or forced expulsion in response to insertion of strong continent into the subduction channel plunger effect stronger colder continental material is subducted without detachment and is then transported laterally underplating the retro continental mantle lithosphere neither surface erosion nor slab break off are required for uhp exhumation

dynamics of sediment subduction accretion at convergent margins short term modes long term deformation and tectonic implications a conceptual model is proposed for the short term operation of subduction accretion systems at convergent margins the operation is described in terms of the mass fluxes among the four system components namely the accretionary or prowedge wedge p an uplifted plug u a retrowedge r and the subduction zone the latter consists of a conduit c of slowly moving and deforming material and an underlying subduction channel where material moves and deforms more rapidly a broad range of convergent margin types can be accommodated within the conceptual model framework with differing short term modes that depend on which of the p u r c components that are active finite element geodynamical model experiments are used to investigate whether these modes are dynamically feasible these experiments determine excitation of modes under controls that include model entry and exit mass fluxes flexural loading by accreted material and slab pull forces boundary velocities corresponding to subduction and subduction zone advance retreat distribution thickness and density of accreted sediment and internal frictional properties of the coulomb sediments that all modes with the exception of p in which only a prowedge is created are seen in the models provides support for the concepts and allows conditions that favor particular modes to be recognized in addition the finite deformation predicted by the models is used to infer the long term tectonic styles associated with both single mode evolutions and those that exhibit mode switching a steady state mass balance can occur for modes 0 pure subduction p c i e where components p and c are active or other modes when the surface mass denudation plus the subduction flux balances accretion plus tectonic underplating the p c mode is favored by subduction zone retreat and or an increasing subduction load and creates an apparent landward dipping backstop p u c is predicted to be the most common mode and it creates an apparent seaward dipping backstop p u r c is an inefficient mode but is favored by easily detached retrocrust or sediment and or a decreasing subduction load

kinematic links between subduction along the hellenic trench and extension in the gulf of corinth greece a multidisciplinary analysis abstract rapid northeast vergent subduction along the hellenic trench at 35 mm yr exists in concert with widespread extensional and strike slip faulting within the upper plate lithosphere of western greece integration of regional geomorphic geologic seismic gps remote sensing and field data demonstrates that young and active deformation in the area extending from the hellenic subduction boundary near kephalonia to the gulf of corinth consists of an interconnected network containing highly localized zones of deformation these bound a series of crustal fragments with relatively little internal deformation these deformation zones merge to form triple junction like features at the western end of the gulf of corinth and in the amvrakikos gulf at the western end of the corinth gulf most of its 14 2 mm yr of extension is relayed to the northwest along a prominent zone of left slip and extension through lake trichonis and the amphilochia fault zone 11 2 mm yr the remaining displacement across the western gulf of corinth is relayed into 7 2 mm yr of right slip on the southwest striking achaia fault zone which traverses the northwestern margin of the peloponnesus a second triple junction like feature occurs in the amvrakikos basin where the left slip amphilochia fault zone the right slip kephalonia transform fault 15 2 mm yr and the convergent thrust front of northern hellenides 4 2 mm yr are joined thus the extensional deformation in the gulf of corinth can be shown to be connected to convergence and subduction along the hellenic trench through a series of discrete deformation zones that are not dissimilar from those observed in the global plate tectonic system

aegean crustal thickness inferred from gravity inversion geodynamical implications since oligo miocene times the aegean domain has undergone regional extension due to the southward retreat of the hellenic subduction zone boundary conditions have been more recently modified by the westward extrusion of anatolia a new map of the aegean crustal thickness inferred from gravity inversion is proposed here to better constrain the variations in space and time of crustal thinning that has accumulated since oligo miocene times moho topography is obtained by inversion of satellite marine gravity data data are first corrected for terrain anomalies and deep mantle effects african subducting slab they are then filtered between 50 and 300 km to avoid short wavelength intracrustal effects results are consistent with previous 2d geophysical studies seismic refraction receiver functions and show that an overall regional isostatic compensation of the crust holds for the aegean area with a mean crustal thickness of 25 km three different provinces north aegean cyclades and cretan sea can be identified thinner crust is observed both in the north aegean region ne sw trending of thinning with crustal thickness lower than 24 km and in the cretan sea crustal thickness of 22 23 km between these two regions the cyclades are marked by a rather flat moho at 25 km a two stage model of the aegean extension could well explain the observed crustal thickness variation from oligocene to middle miocene gravitational collapse of the hellenides due to the southward retreat of the african slab reduced the aegean continental crust from 50 km by reference to continental greece and anatolia to a mean value of 25 km at the scale of the whole aegean from upper miocene to present the westward extrusion of anatolia modified the extension and the associated crustal thinning in the north aegean domain during this second episode crustal thinning related to the southward retreat of the african slab tends to localize in the cretan sea the cyclades likely behave as a rigid block translated toward the south west without significant deformation in agreement with the gps velocity field and the lack of major earthquakes d 2004 elsevier b v all rights reserved

slab dip vs lithosphere age no direct function one paradigm of subduction relates the dip of the slab to the buoyancy of the downgoing lithosphere along subduction zones with the negative buoyancy proportional to the age of the oceanic lithosphere we measured the dip of the slab down to depths of 250 km along 164 sections crossing 13 subduction zones and compared it with the age of the subducting oceanic lithosphere both at the trench and at depth we show here that this relationship is far more irregular than previously suggested and that it is not possible to simply correlate the increase of the slab dip to the increasing age of the downgoing cooler lithosphere younger oceanic lithosphere may show steeper dip than older segments of slabs e g central america vs south america in contrast with predictions of models considering only slab pull the combination of slab age and subduction rate better accounts for slab dip however the correlation is not satisfactory correlation coefficient equal to 0 450 these results suggest that supplemental forces or constraints have to be accounted for such as thickness and shape of the hangingwall plate absolute plate velocity presence of lateral density variations in the hosting upper mantle effects of accretion erosion subduction of oceanic plateaus and slab deformation due to the motion of the mantle relative to the subducting plate d 2005 elsevier b v all rights reserved

temporal variation in the stress field of the aegean region we use a forward numerical model to analyse the intra plate stress field in an area of active extension of continental lithosphere the aegean region recent tomographic images of the hellenic subduction zone offer the unique opportunity to address the relation between the evolving aegean stress field and temporal variations in the subduction process to this extent we apply our model to both the present day and the late pliocene situation regarding the current situation it is found that subduction related forces largely explain the observed pattern of tension results obtained for the pliocene indicate that detachment of the slab subducted below western greece may have controlled the observed temporal variation in the pattern of tension

imaging the source region of cascadia tremor and intermediate depth earthquakes the subduction of hydrated oceanic crust releases volatiles that weaken the plate boundary interface trigger earthquakes and regulate transient phenomena such as episodic tremor and slip ets it is not clear how dehydration can separately induce earthquakes within the subducting plate and ets partly because few data exist on their relationship to subduction zone structures we present results of a seismic experiment in the washington cascades united states that images a region producing both earthquake types migration of scattered teleseis mic waves provides images of low velocity subducting crust at depths 40 45 km with sharp boundaries above and below it the sharp upper boundary indicates a layer of weak sediment or an overpressured fault zone that terminates abruptly downdip at 40 45 km depth regular earthquakes are at the top of the mantle within the downgoing plate everywhere the plate is 95 km deep but ets only exists where the sharp upper boundary occurs the ets location supports models of slow slip that require near lithostatic fluid pressure whereas regular earthquakes nucleate closer to the origin of metamorphic dehydration very low shear stresses on the plate boundary may limit seismicity to ets and similar phenomena

gps derived strain rate field within the boundary zones of the eurasian african and arabian plates we use the gps velocity field 1988 1998 for eastern mediterranean and asia minor to determine the crustal deformation strain rate field in an area bounded by 35 n and 43 n and 20 e and 48 e we calculate the normal and shear strain rate components associated with the major faults and compare these qualitatively with seismological data uncertainties in the calculation of the strain rates reach 50 nstrain yr 1 in sparsely observed parts of anatolia whereas we estimate errors 20 nstrain yr 1 in the aegean and marmara regions the largest compressional strain rate components in the eastern part of the study area occur along the greater caucasus mountain front reaching 70 nstrain yr 1 1 nstrain yr 1 0 0317 10 15 s 1 the north anatolian fault zone is the clearest feature in the shear strain rate field it is expressed as a pronounced dextral strike slip fault zone reaching rates of up to 170 nstrain yr 1 this holds true also for the izmit area where the august 17 1999 earthquake occurred central anatolia is almost strain free whereas extension prevails in western anatolia the principal axes of extension vary around the n s direction with strain rates of up to 85 nstrain yr 1 these extensional areas coincide with graben features and normal faulting earthquakes the central and southwestern aegean sea is strain free with values far below 40 nstrain yr 1 the seismic cluster around the dodekanissa islands southeastern aegean sea coincides with nw se oriented extension attaining strain rates of up to 90 nstrain yr 1 this area of extension also exhibits recent active volcanism the entire hellenic arc shows compressional strain rates perpendicular to the arc the pliny strabo troughs along the eastern segment of the arc show left lateral shear strain rates reaching 80 nstrain yr 1 significant extension is found in central greece with a nne ssw oriented maximum of 120 nstrain yr 1 centered around the gulf of corinthos the kephalonia fault zone in nw greece is a distinct dextral fault zone separating apulia from the rapidly moving aegean microplate right lateral shear strain rates reach 150 nstrain yr 1

crustal and upper mantle structure beneath the corinth rift greece from a teleseismic tomography study we report here the results of a tomographic lithospheric study in the area of the corinth and evvia rifts greece designed to constrain the mechanism of continental extension sixty seismological stations were deployed in the area for a period of 6 months and 177 teleseismic events were recorded by more than five stations and gave more than 2000 travel time residuals p and pkp phases which were inverted to image the velocity structure down to 200 km depth we use both a linear and a nonlinear method to invert the data set the main result is a long wavelength positive velocity anomaly located in the upper mantle which is interpreted as the subducted african lithosphere the subducted lithosphere is well defined from 7o km depth down to 200 km from synthetic tests as well as from the amplitude of the anomaly more than 7 we conclude that the subduction continues below 200 km in addition a second positive velocity anomaly of about 4 from the surface down to 40 km depth located north of the gulf of corinth has been found this is interpreted as the result of a crustal thinning of several kilometers 5 km shifted to the north from the gulf of corinth and trending obliquily nw se we suggest that this crustal thinning is mainly related to the miocene widespread extension in the aegean and that the quaternary corinth rift initiated where the crust was already thinned the different styles of deformation of the eastern and western part of the rift are consistent with this interpretation no clear velocity anomaly can be related to the evvia rift

the tectonic expression slab pull at continental convergent boundaries examination of five thrust belt systems developed at continental subduction boundaries suggests that they comprise two distinct groups that display pronounced and systematic differences in structural style topographic elevation denudation metamorphism postcollisional convergence and foredeep basin geometry and facies the distinctive geological features developed within each thrust belt group appear to be causally linked to the relative rates of subduction and convergence via the magnitude of horizontal compressional stress transmitted across the subduction boundary at subduction boundaries where the rate of overall plate convergence is less than the rate of subduction termed here retreating subduction boundaries the transmission of horizontal compressive stress across the plate boundary is small and regional deformation of the overriding plate is by horizontal extension the tectonic expression of these retreating subduction boundaries includes topographically low mountains little erosion or denudation low grade to no metamorphism little to no involvement of crystalline basement in shortening little to no postcollisional convergence anomalously deep foredeep basins and a protracted history of flysch deposition within the adjacent foredeep basin analysis of deflection and gravity data across three retreating subduction boundaries apennine carpathian and hellenic systems shows that subduction is driven by gravitational forces acting on dense subducted slabs at depths between about 40 and 80 km carpathians 50 and 150 km apennines and 50 and 250 km hellenides the total mass anomalies represented by the slabs are approximately 3 1012 6 1012 and 12 1012 n m respectively the slabs are partially supported by flexural stresses transmitted through the subducted lithosphere to the foreland and partially supported by dynamic viscous stresses in the asthenosphere at subduction boundaries where the rate of overall plate convergence is greater than the rate of subduction termed here advancing subduction boundaries the transmission of horizontal compressive stress across the plate boundary is large and regional deformation of the overriding plate is by horizontal shortening the tectonic expression of these advancing subduction boundaries includes topographically high mountains antithetic thrust belts large amounts of erosion and denudation exposure of high grade metamorphic rocks at the surface extensive deformation of crystalline basement to midcrustal depths protracted postcollisional convergence tens of millions of years and a protracted history of molasse deposition within the adjacent foredeep basins analysis of gravity and deflection data across two advancing subduction boundaries developed within the continental lithosphere western to eastern and southern alps and himalayas shows that the thrust sheets have been translated for great distances over the foreland lithosphere relative to the point at which the subduction forces are applied thus obscuring any flexural and gravity signals from the subducted slab however it appears that far field stresses presumably related to global plate motions drive most of the convergent motion across these subduction boundaries the concept that orogenic belts formed above retreating subduction boundaries have recognizable tectonic signatures that differ from those of orogenic belts formed above advancing subduction boundaries suggests that it may be possible to interpret the plate boundary settings in which ancient orogenic belts evolved appendix b is available with entire article on microfiche order from the american geophysical union 2000 floridaavenue n w washington d c 20009 document t92 004 2 50 payment must accompany order

the hellenic subduction zone a tomographic image and its geodynamic implications new tomographic images of the hellenic subduction zone demonstrate slab penetration in the aegean upper mantle to depths of at least 600 km beneath greece the lower part of the slab appears to be detached at a depth of about 200 km whereas it still seems to be unruptured beneath the southern aegean schematically we derive minimum time estimates for the duration of the hellenic subduction zone that range from 26 to 40 ma this is considerably longer than earlier estimates which vary between 5 and about 13 ma

moho topography beneath the corinth rift area greece from inversion of gravity data summary our aim is to understand better the rifting process by imaging the moho depth variation beneath corinth and evvia we present here the results of a gravity inversion analysis in the region of the corinth and evvia rift system and compare them to those obtained independently from teleseismic tomography and receiver function analyses the results of these different studies appear to be consistent and show 1 a 10 km crustal thickening in the western part of the area beneath the hellenides mountains 2 nw se trending periodic crustal thinning and 3 a maximum crustal thinning north of the gulf of corinth this 4 km thinning is unlikely to be the result of the rifting alone which seems to have been reactivated since only 1 ma we propose here a geodynamical scenario in two major steps to explain the evolution of corinth area aegean miocene extension involving boudinage resulted in periodic crustal thinning consistent with observations these lithospheric instabilities could have favoured rupture initiation in particular areas especially near the city of corinth then the reactivation of the corinth rift extension 1 myr ago led to westward rift propagation the offset observed between the maximum crustal thinning and the gulf of corinth could be accommodated by a lowangle normal fault at about 10 15 km depth the corinth rift is thus asymmetrical and was initiated in places of crustal weakness due to miocene lithospheric instabilities

quaternary evolution of the corinth rift and its implications for the late cenozoic evolution of the aegean summary we present geological and morphological observations at different scales to constrain rates of faulting and the distribution of deformation in the seismically active aegean region we focus first on the 130 km long corinth rift an asymmetric graben where a flight of terraces of marine origin are uplifted we show that the edges of the terraces lie in the footwall of the normal fault bounding the corinth rift and correspond to sea level highstands of laic pleistocene age using a detailed analysis of aerial and spot imagery supported by field observations we have mapped 10 terrace platforms and strandlines ranging in elevation from 10 to 400 m over distances of 2 to 20 km from the fault the elevation of the terraces inner edges was estimated at 172 sites with an error of 5m this data set contains a precise description of the uplift and flexure of 10 different palaeohorizontal lines with respect to the present sea level to date the deformation we correlate the corinth terraces with late pleistocene oxygen isotope stages of high sea level stands and with global sea level fluctuations using a thick elastic plate model consistent with our current understanding of the earthquake cycle and a boundary element technique we reproduce the geometry of the shorelines to constrain both mechanical parameters and the slip on the fault we show that the seismogenic layer behaves over the long term as if its elastic modulus were reduced by a factor of about 1000 all the terraces are fitted for fault slip increasing in proportion to terrace age and the component of regional uplift is found to be less than 0 3 mm yr 1 the best fits give a slip rate of 11 3 mm yr 1 on the main rift bounding fault over the last 350 kyr other geological and morphologic information allows us to estimate the total age of the main fault 1 ma and to examine the mechanical evolution of the corinth rift the minimum observed sediment thickness in the gulf places an extreme check on the results of the modelling and a lower bound on slip rate of 6 7 mm yr 1 40 per cent less than estimated with modelling even this slip rate is nearly 10 times higher than for comparable features in most of the aegean and elsewhere in the world at a larger scale the spacing and asymmetry of the rift systems in the aegean suggest strain localization in the upper mantle with slow extension starting 15 myr ago or earlier the more recent 1 myr rapid phase of rifting in corinth partly reactivated this earlier phase of extension the younger faulting in corinth appears to result from its present location in the inhomogeneous stress field process zone of the south westward propagating tip of the southern branch of the north anatolian fault we extend these relations to propose a mechanical model for the late cenozoic evolution of the aegean as the arabia europe collision progressed in eastern turkey it caused anatolia to move to the west and the north anatolian fault to propagate into the aegean where the early slow extension started to be modified about 5 ma ago the process of propagation dramatically increased the activity of some but not all of the earlier rifts the model we present is compatible with tectonic observations as well as with the seismicity the palaeomagnetic rotations and the displacement field now observed with gps and slr

crustal motion and deformation in greece from a decade of gps measurements 1993 2003 abstract the hellenic plate boundary region located in the collision zone between the nubian arabian and eurasian lithospheric plates is one of the seismo tectonically most active areas of europe during the last 15 years gps measurements have been used to determine the crustal motion in the area of greece with the aim to better understand the geodynamical processes of this region an extended reoccupation network covering whole greece has been measured periodically in numerous gps campaigns since the late eighties and a continuous gps network has been operated in the region of the ionian sea since 1995 in this paper we present a new detailed high quality solution of continuous and campaign type measurements acquired between 1993 and 2003 during the gps processing a special effort was made to obtain consistent results with highest possible accuracies and reliabilities data of 54 mainly european igs and euref sites were included in the gps processing in order to obtain results which are internally consistent with the european kinematic field and order to allow for a regional interpretation after an overview of the results of the igs euref sites the results from more than 80 stations in greece are presented in terms of velocities time series trajectories and strain rates previous geodetic geological and seismological findings are generally confirmed and substantially refined new important results include the observation of deformation zones to the north and to the south of the north aegean trough and in the west hellenic arc region arc parallel extension of about 19 mm yr along the hellenic arc and compression between the ionian islands and the greek mainland due to continuous long term observations of 4 8 years it was possible to extract height changes from the gps time series in greece we observe a differential subsidence of the order of 2 mm yr between the northern and central ionian islands across the kefalonia fault zone the differential subsidence of the central ionian islands with respect to the northwestern greek mainland amounts to 4 mm yr

new geophysical insight into the origin of the denali volcanic gap summary volcanic gaps are segments of subduction zones that lack the volcanic activity usually found at these convergent margins they are regions where the necessary conditions to produce melt may appear favourable but where volcanoes are surprisingly absent from the surface in this study we present a new model that can explain the occurrence of such volcanic gaps it is based on seismic imaging and geodynamic modelling of the denali volcanic gap a 400 km wide region at the eastern end of the alaska aleutian subduction zone here the thick crust of the pacific plate and yakutat terrane subduct at shallow angle beneath north america a high resolution seismic profile clearly images the subducting crust undergoing progressive dehydration between 50 and 120 km depth and a negative subhorizontal velocity contrast at 60 km depth in the overlying mantle wedge we interpret this 60 km discontinuity as marking the top of a layer of partial melt that pools at the base of the overriding plate in steadystate subduction models melt accumulates at the apex of a vaulted mantle wedge the pinch zone from where it may break through the overlying lithosphere to the surface beneath the denali volcanic gap the pinch zone is absent or greatly reduced because shallow subduction of the yakutat terrane progressively cools the system and causes the slab to advance and replace the hot core of the mantle wedge this regime can be seen as the opposite of subduction rollback it prevents the formation of a pinch zone reduces the length of the melting column and causes melt to pool at the base of the overriding plate thus inhibiting magma generation and extraction

slab segmentation and late cenozoic disruption of the hellenic arc the hellenic subduction zone displays well defined temporal and spatial variations in subduction rate and offers an excellent natural laboratory for studying the interaction among slab buoyancy subduction rate and tectonic deformation in space the active hellenic subduction front is dextrally offset by 100 120 km across the kephalonia transform zone coinciding with the junction of a slowly subducting adriatic continental lithosphere in the north 5 10 mm yr and a rapidly subducting ionian oceanic lithosphere in the south 35 mm yr subduction rates can be shown to have decreased from late eocene time onward reaching 5 12 mm yr by late miocene time before increasing again along the southern portion of the subduction system geodynamic modeling demonstrates that the differing rates of subduction and the resultant trench offset arise naturally from subduction of oceanic pindos lithosphere until late eocene time followed by subduction of a broad tract of continental or transitional lithosphere hellenic external carbonate platform and then by miocene entry of high density oceanic ionian lithosphere into the southern hellenic trench model results yield an initiation age for the kephalonia transform of 6 8 ma in good agreement with observations consistency between geodynamic model results and geologic observations suggest that the middle miocene and younger deformation of the hellenic upper plate including formation of the central hellenic shear zone can be quantitatively understood as the result of spatial variations in the buoyancy of the subducting slab using this assumption we make late eocene middle miocene and pliocene reconstructions of the hellenic system that include quantitative constraints from subduction modeling and geologic constraints on the timing and mode of upper plate deformation

geodetic constraints on the tectonic evolution of the aegean region and strain accumulation along the hellenic subduction zone article i nfo we present evidence that gps velocity estimates of plate motions and fault slip rates agree to within uncertainties with geologic estimates during the most recent phase of the geologic evolution of the e mediterranean region post late miocene on this basis we use the gps differential velocities to estimate the timing of initiation of the principal structures in nw turkey the n aegean sea and central greece including the marmara sea the gulfs of evia goe and corinth goc and the kephalonia transform fault ktf we interpret these ages to indicate that the north anatolian fault propagated across the n aegean opening the goe and goc and initiating the ktf during the past 1 4 ma we further suggest that aegean extension that was earlier more distributed across the aegean basin became focused on this new fault system allowing the southern aegean and peloponnisos to translate sw with little internal deformation as observed today with gps this change in tectonic configuration may account for the clear geologic evidence for crustal thinning throughout the s aegean in apparent contradiction with low present day strain rates we further show that the low present day strain rate along the southern edge of the aegean micro plate requires substantial aseismic slip along the plate interface below crete consistent with the low level of historic subduction type earthquakes along this segment of the subduction zone

three dimensional seismic attenuation structure beneath the aegean region and its tectonic implication abstract the three dimensional attenuation structure beneath the aegean sea and the surrounding regions is determined by inversion of seismic intensity data a large number of seismic intensity data have been accumulated in a uniform scale in the aegean region where the seismic activity is much higher than that of the other mediterranean regions nearly 7000 seismic intensity data from 50 earthquakes that have occurred in these regions are used to determine seismic attenuation structure and source acceleration of the earthquakes the resultant structure reveals a remarkable contrast of attenuation in the top layer depth 0 40 km low q is dominant in the aegean sea while high q is dominant in the surrounding land areas except for the southwestern turkey the low q regions correspond to areas of neogene quaternary grabens where the high seismicity of shallow earthquakes appears in the upper mantle high q zones corresponding to the subducting african plate dominate along the hellenic arc the irregular shape of high q zones might reflect the split or disintegration of the african slab some low q spots corresponding to the distribution of volcanoes exist along the volcanic arc the low q spots might correspond to diapirs causing the subduction volcanism

shear velocity structure in the aegean area obtained by inversion of rayleigh waves summary the purpose of this work is to derive a 3 d tomographic image of the shear wave velocity structure of the crust uppermost mantle in the aegean area using the group velocities of rayleigh wave fundamental mode the database consists of 185 regional earthquakes recorded at broad band stations that were installed for a period of 6 month in the aegean area within the framework of a large scale experiment in a previous work karagianni et al 2002 an averaged group velocity has been determined using the method of frequency time analysis ftan for each epicentre station ray path and the data were used in order to determine the local group velocities for different periods over the area covered by the seismic ray paths taking into account additional resolution results obtained for the local group velocities a grid of 0 5 was adopted for the aegean area and a local dispersion curve was defined for each gridpoint more than 80 local dispersion curves were finally inverted using a non linear inversion approach deriving the corresponding 1 d shear velocity models the interpolation of these models resulted in a 3 d s wave tomographic image of the crust and uppermost mantle in the broader aegean area as expected as a result of the complex tectonic setting of the aegean area strong lateral variations of the s wave velocities of the crust and uppermost mantle of the studied area are found in the southern aegean sea as well as in a large part of the central aegean sea a thin crust of approximately 20 22 km is observed whereas the remaining aegean sea area exhibits a crustal thickness less than 28 30 km on the contrary a crustal thickness of 40 46 km is observed in western greece along the hellenides mountain range whereas in the eastern continental greece the crust has a typical thickness of approximately 30 34 km for shallow depths 10 km low s wave velocities are observed under the sedimentary basins of the north aegean sea the gulf of thermaikos axios basin and western greece at depths ranging from 10 to 20 km low s wave velocities are mainly found in western greece under peloponnesus as well as in rhodes this low velocity zone seems to extend along the hellenic arc and can be correlated to the hellenides mountain range and the alpine orogenesis in agreement with previous p wave tomographic results in the southern aegean sea very low s wave velocities 3 6 4 0 km s 1 are observed at depths of approximately 30 40 km just below the moho discontinuity while in the rest of the inner aegean sea and continental greece the uppermost mantle is characterized by velocities around 4 3 4 4 km s 1 this low velocity zone in the southern aegean sea can be associated with the high temperatures and the presence of significant percentage partial melt in the mantle wedge of the southern aegean subduction zone in agreement with previous 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

correlated shear and bulk moduli to 1400 km beneath the mediterranean region we present a new model for the shear and compressional wave velocity structure of the mediterranean plate boundary region the model is obtained by a combined inversion of s and p data where we solve in one single step for shear wave and bulk sound velocity variations the ratios between the types of seismic velocities inferred from the combined inversion constrain the origin of heterogeneity our model is dominated by fast anomalies from subducted oceanic lithosphere we found that only a slight amount of bulk sound velocity heterogeneity is required to explain the combined set of p and s data thus compressional wave velocity anomalies are dominated by shear modulus heterogeneity and the ratio between bulk sound and shear wave velocity heterogeneity suggests a predominantly thermal origin 2006 elsevier b v all rights reserved

high resolution seismic imaging of the western hellenic subduction zone using teleseismic scattered waves summary the active hellenic subduction system has long been considered an ideal setting for studying subduction dynamics because it is easily accessible and of limited spatial extent it has been the focus of numerous seismological studies over the last few decades but nonetheless the detailed structure of both the slab and the surrounding mantle remain poorly constrained in an intermediate depth range from 30 to 150 km the objective of this paper is to fill this gap the intermediate depth regime is of particular interest because it is pivotal for improving our understanding of the dynamic interaction between subducting lithosphere and the surrounding mantle an interdisciplinary effort aimed at addressing this challenge is currently undertaken by the multidisciplinary experiments for dynamic understanding of subduction under the aegean sea medusa project as part of the medusa initiative a temporary array consisting of 40 densely spaced broad band seismometers from the iris passcal pool has been deployed in southern greece we process the teleseismic data recorded by this array with a migration algorithm based on the generalized radon transform to obtain high resolution images of the subduction zone in 2 d the images reveal a sharp mohoroviy discontinuity moho at depths ranging from 30 km beneath the western margin of the aegean sea to 40 km beneath the central peloponnesus where it outlines the crustal root of the hellenides to the west of the hellenides the continental moho is not identified but we interpret a pronounced discontinuity imaged at 20 km depth as the contact between low velocity sediments and high velocity crystalline basement the images also show the subducted oceanic crust as a low velocity layer that plunges at a constant angle of 21 from west to east the oceanic crust exhibits low velocities to at least 90 km depth indicating that the bulk of fluid transfer from the subducted slab into the mantle wedge occurs below this depth a detailed comparison of images constructed for distinct backazimuthal illuminations reveals deviations in the geometry of the subducted slab these deviations are attributed to structural and or compositional changes taking place directly to the north of the medusa array and are consistent with the existence of a slab tear beneath the central hellenic shear zone

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

slab top dips resolved by teleseismic converted waves in the hellenic subduction zone the variations of the arrival times and polarities with backazimuth and distance of teleseismic p to s converted waves at interfaces bounding the slab crust under the upper plate mantle are used to constrain the depth dip angle and azimuth of the slab of the hellenic subduction zone a grid search is designed to estimate the model parameters dip values of 16 18 with an azimuth of 20 to 40 are thus derived at 3 sites aligned over 50 km along the eastern coast of peloponnesus they are consistent with the variation from 54 to 61 km of the slab top depths constrained below each receiver north of the gulfs of corinth and evvia a similar depth for the top of the slab is found at a distance from the subduction at least 100 km larger this suggests flatter subduction of a different slab segment such a variation in slab attitude at depth across the region from south of the eastern gulf of corinth to north of evvia is a candidate for the control of the recent or active localized crustal thinning of the upper plate we documented in earlier work and of the surface deformation

tomography of the crust and upper mantle in southeast europe compressional velocity structure of the crust and the upper mantle in southeastern europe broader aegean area is studied by inverting residuals of the first p arrivals from earthquakes in this region 16 e 31 e 34 n 43 n the data used are from regional events recorded by the permanent network of stations during the period 1971 1987 enriched with data from experiments with portable seismographs in four regions of this broad area this study confirms the strong variations of crustal thickness in this area as well as the subduction of the eastern mediterranean lithosphere under the southern aegean and gives further detailed information on the crustal and upper mantle structure of the area important new information is the existence of a low velocity crustal layer in western greece and albania and that the velocity anomaly in the mantle under the southern aegean extends much farther and deeper to the northeast than the benioff zone of the intermediate depth earthquakes indicates furthermore evidence is presented about the possible existence of older subduction in the northern aegean and about the influence of the tectonic regime on the velocity field

3 d structure of the lithosphere in the aegean region summary p wave traveltime residuals from about 300 local and 400 teleseismic earthquakes recorded by the athens seismological network were used to study the structure of the lithosphere in the aegean region teleseismic data included readings from local stations which do not routinely report to the isc the large values of the residuals up to f3 s and their variation with azimuth and epicentral distance indicate significant velocity heterogeneity in the upper mantle p wave traveltime residuals were inverted simultaneously for both local and teleseismic earthquakes tomographic results which are best resolved at lithospheric depths reveal a heterogeneous lithosphere down to a depth of 100km or so a high velocity anomaly is found dipping at an average of 45 to the ne away from the hellenic trench and supports the hypothesis of subduction due to a cooler descending slab of oceanic composition an area of low velocities is observed at depths less than 80 km in the aegean back arc corresponding well with observed high heat flow values in the area in contrast an area of relatively higher velocity anomalies is resolved in the lithosphere underlying the sea of crete the tectonic structure of the crust and upper mantle in the eastern mediterranean area results from the complex interaction of several different geodynamic processes not all of which can be described by simple plate tectonic models at present the eurasian and african plates are converging at a rate of about lcmyr in an approximately n s direction minster jordan 1978 with the latter descending

the strain field in northwestern greece and the ionian islands results inferred from gps measurements abstract recent crustal movements detected by the analysis of repeated satellite geodetic measurements reflect the ongoing geodynamic processes in the alpine mediterranean area superimposed on the large scale counterclockwise rotation of the african plate complex dynamic processes are affecting the lithospheric fragments between the african and eurasian plates key features to better understand the driving forces and associated seismic activity in the africa eurasia collision zone are the calabrian and hellenic arcs in this paper geodynamic investigations along the west hellenic arc are discussed they are based on two epochs 1989 and 1993 of satellite geodetic measurements carried out using the us global positioning system gps the results are presented in terms of relative displacements and strain rates within the time span of 4 years southwestern greece has moved to the southwest relative to southeastern italy by an average of 120 mm increasing from 80 mm at lefkada in the center of the ionian islands to 160 mm at the peloponnesus the maximum strain rate is 0 18 strain a located in the vicinity of lefkada where anomalously high earthquake activity is observed the data provide strong evidence for dextral strike slip motion on the order of 25 mm a along the kephalonia fault zone kfz the deformation field of the kfz is interpreted as a transition zone between the kinematics of the apulian platform and the west hellenic fold and thrust belts

timing of tectonic emplacement of the ophiolites and terrane paleogeography in the hellenides the timing of tectonic emplacement of the ophiolites is analyzed in the four oceanic terranes of the hellenides h2 h4 h6 h8 the criteria for this analysis are based on a the post emplacement sedimentary cover or intrusive rocks b the syn emplacement tectonostratigraphic formations and c the youngest rocks involved in the structure of the autochthon and the allochthon unit in each case the timing becomes younger towards the more external tectonic units of the hellenides with i late eocene oligocene age in the external ophiolite belt of the pindos cyclades oceanic terrane h2 ii late jurassic early cretaceous age in the internal ophiolite belt of the vardar axios oceanic terrane h4 iii post liassic pre late jurassic age in the ophiolites of lesvos circum rhodope oceanic terrane h6 and iv pre late jurassic age in the ophiolites of volvi eastern rhodope terrane h8 an ophiolite obduction model can be applied with the ophiolitic nappes always emplaced on top of pre alpine continental terranes with mesozoic shallow water carbonate platforms the geometry of the continental terranes drifting during the mesozoic within the tethys ocean controls the number and dimensions of the tethyan oceanic basins where a continental terrane dies out the two adjacent oceanic basins merge into one larger basin this seems to be the case of the pelagonian terrane h3 which is terminated north of skopje where the pindos oceanic basin h2 merges with the vardar axios oceanic basin h4

lithospheric structure of the aegean obtained from p and s receiver functions combined p and s receiver functions from seismograms of teleseismic events recorded at 65 temporary and permanent stations in the aegean region are used to map the geometry of the subducted african and the overriding aegean plates we image the moho of the subducting african plate at depths ranging from 40 km beneath southern crete and the western peloponnesus to 160 km beneath the volcanic arc and 220 km beneath northern greece however the dip of the moho of the subducting african plate is shallower beneath the peloponnesus than beneath crete and rhodes and flattens out beneath the northern aegean observed p to s conversions at stations located in the forearc indicate a reversed velocity contrast at the moho boundary of the aegean plate whereas this boundary is observed as a normal velocity contrast by the s to p conversions our modeling suggests that the presence of a large amount of serpentinite more than 30 in the forearc mantle wedge which generally occurs in the subduction zones may be the reason for the reverse sign of the p to s conversion coefficient moho depths for the aegean plate show that the southern part of the aegean crustal thickness of 20 22 km has been strongly influenced by extension while the northern aegean sea which at present undergoes the highest crustal deformation shows a relatively thicker crust 25 28 km this may imply a recent initiation of the present kinematics in the aegean western greece crustal thickness of 32 40 km is unaffected by the recent extension but underwent crustal thickening during the hellenides mountains building event the depths of the aegean moho beneath the margin of the peloponnesus and crete 25 28 and 25 33 km respectively show that these areas are also likely to be affected by the aegean extension even though the cyclades crustal thickness of 26 30 km were not significantly involved in this episode the aegean lithosphere asthenosphere boundary lab mapped with s receiver functions is about 150 km deep beneath mainland greece whereas the lab of the subducted african plate dips from 100 km beneath crete and the southern aegean sea to about 225 km under the volcanic arc this implies a thickness of 60 65 km for the subducted african lithosphere suggesting that the aegean lithosphere was not significantly affected by the extensional process associated with the exhumation of metamorphic core complexes in the cyclades

crustal thickness variations in the aegean region and implications for the extension of continental crust 1 we installed 5 broadband and 45 short period temporary seismic stations distributed partly as a dense 100 km long n s linear array and partly as a regional network throughout the menderes massif of western turkey in order to study crust upper mantle structure and seismicity in this study we have combined teleseismic waveform data from these stations with data from several permanent seismic stations to determine crustal thickness variations in the aegean region receiver function studies at seven broadband stations using the h stacking method have yielded crustal thicknesses and vp vs ratios over a broad region of the aegean a more detailed crustal image was obtained in the central menderes massif where we applied common conversion point stacking to receiver functions obtained from the n s linear array the results show a general trend of westward crustal thinning from 36 km in central anatolia to 28 30 km in the central menderes massif to 25 km beneath the aegean sea the results also indicate that crustal thinning in the aegean is not uniform in the n s extensional direction the crust is thinner in the central menderes massif 28 30 km of crustal thicknesses and the cycladic massif 25 26 km than in surrounding regions where crustal thicknesses are 32 34 km the long lived elevated moho under the metamorphic core complexes suggests that the lower crust in the aegean region is at least 3 times more viscous than that in the basin and range province where the moho is much flatter

the removal of free surface interactions from three component seismograms summary the effect of the free surface can be removed from three component seismic recordings to recover the incident upgoing wavefield if the slowness and azimuth of the current wavefront are known as a function of time for a single threecomponent station it is usually possible to estimate an azimuth for an event from the first arriving p waves but slowness estimates are less reliable when more than one wavetype is presented in the seismic wavetrain however the free surface correction operators are generally slowly varying functions of slowness and so some error in slowness can be tolerated effective approximations for the removal of the free surface effects can be made for hard rock sites to cover slowness bands for the main regional phases pn pg sn and lg by applying these operators in turn over group velocity windows appropriate to the particular phases the relative amplitude of the p sv and sh contributions to the wavefield can be estimated because the free surface amplification effects have been removed the amplitudes can be compared directly and provide useful constraints on the radiation characteristics of the source this procedure is therefore helpful for developing discrimination measures for different classes of sources

ionian sea western greece its structural outline deduced from drilling and geophysical data abstract this paper outlines the preliminary structural results obtained by the compilation of geological drilling and geophysical data in western greece most external geotectonic zones of the hellenides are included in the geological setting of the marine and insular domain of western greece ionian pre apulian and apulian zones the gavrovo and pindus zones make a restricted contribution to the marine geological framework continental shelf of southwest peloponesus besides the geographical setting of the geotectonic units their general tectonic characteristics allow the following remarks to be made the main tectonic events younger than eocene are well marked in both ionian and pre apulian zones the detrital sequence shows a fast pinch out towards the apulian platform where the plio quaternary clastic layers overlap what are probably late mesozoic or early tertiary limestones taking into consideration general tectonic features we can distinguish a northern area and a southern one separated by a fault trending 30 n located on the western margin of kephallinia lefkas islands 1 a in the northern area we observe a strong tangential tectonism decreasing towards the apulian platform with transitional characteristics on the pre apulian zone paxos where asymmetrical folding occurs 2 b in the southern area from lefkas kephallinia islands to the s w cape of peloponesus the tangential tectomism is well masked by strong vertical faulting attributed to young plio quaternary deformations the fast subsidence induces huge diapiric structures in the ionian zone salt walls originating from triassic layers intersect a thick pleistocene sequence

the geometry of the wadati benioff zone and lithospheric kinematics in the hellenic arc abstract accurate locations of 961 shallow and intermediate depth earthquakes which occurred between 1956 and 1995 in the hellenic arc 34 39 n 19 28 e are used to define the plate boundaries in this area reliable fault plane solutions for 77 shallow and intermediate depth earthquakes are also used in order to define the interaction between the different plates in the arc an ocean continent type of interaction occurs on a curved surface which is defined by the shallow branch 20 100 km of the wadati benioff zone the intersection of this zone with the earth s surface is a curve which follows the convex outer side of the sedimentary arc western peloponnese west of cythera south coast of crete east coast of rhodes and dips at low angle 30 to the aegean sea coupling between the subducted oceanic crust and the overriding of the aegean lithospheric plate takes place on this surface the deep branch 100 180 km of the wadati benioff zone is dipping freely without coupling at a high angle 45 beneath the south aegean trough and the volcanic arc the high shallow seismicity h 20 km which is observed in the southwestern convex outer side of the arc ionian sea is attributed to the fast southwestward motion of the aegean plate location of strong deep earthquakes h 100 km in the fore arc area of the southwestern part of the hellenic arc west of cythera indicates that oceanic crust is destroyed in this part of the hellenic trench due to roll back of the descending lithospheric slab

high frequency shear wave propagation across the hellenic subduction zone abstract this study investigates shear wave propagation across the hellenic subduction zone using high quality waveforms of 71 intermediate depth events 40 160 km depth the data have been recorded by a permanent seismic network consisting of 22 broadband three component stations that cover most parts of greece shear waves can be efficiently transmitted with low attenuation q s 1100 through the descending slab which extends laterally over much of the forearc area with a northwest southeast strike the slab also appears to be continuous downdip in the depth range 60 160 km even though it was found that a gap smaller than 10 km would not affect shear wave efficiency significantly two highly attenuating areas have been detected one in the central southeast aegean corresponding to the mantle wedge and the other in the northern aegean probably caused by elevated upper mantle temperatures owing to the ongoing extension in that area these results provide additional constraints for the geometry of the hellenic subduction zone and the geodynamic setting of the aegean region

global positioning system constraints on plate kinematics and dynamics in the eastern mediterranean and caucasus we present and interpret global positioning system gps measurements of crustal motions for the period 1988 1997 at 189 sites extending east west from the caucasus mountains to the adriatic sea and north south from the southern edge of the eurasian plate to the northern edge of the african plate sites on the northern arabian platform move 18 2 mm yr at n25 5 w relative to eurasia less than the nuvel 1a circuit closure rate 25 1 mm yr at n21 7 w preliminary motion estimates 1994 1997 for stations located in egypt on the northeastern part of africa show northward motion at 5 6 2 mm yr also slower than nuvel ia estimates 10 1 mm yr at n2 4 e eastern turkey is characterized by distributed deformation while central turkey is characterized by coherent plate motion internal deformation of 2 mm yr involving westward displacement and counterclockwise rotation of the anatolian plate the anatolian plate is de coupled from eurasia along the right lateral strike slip north anatolian fault naf we derive a best fitting euler vector for anatolia eurasia motion of 30 7 0 8 n 32 6 0 4 e 1 2 0 1 myr the euler vector gives an upper bound for naf slip rate of 24 1 mm yr we determine a preliminary gps arabia anatolia euler vector of 32 9 1 2 n 40 3 1 1 e 0 8 0 2 myr and an upper bound on left lateral slip on the east anatolian fault eaf of 9 1 mm yr the central and southern aegean is characterized by coherent motion internal deformation of 2 mm yr toward the sw at 30 1 mm yr relative to eurasia stations in the se aegean deviate significantly from the overall motion of the southern aegean showing increasing velocities toward the trench and reaching 10 1 mm yr relative to the southern aegean as a whole

modeling the influence of moho topography on receiver functions a case study from the central hellenic subduction zone 1 pseudospectral 2d modeling of wave propagation provides a fresh look at receiver function data from the hellenic subduction zone it is shown that distinct moho topography which is suggested for this complex subduction environment from various geophysical data sets can provide a conclusive explanation for previously observed inverted moho phases in the forearc as well as for a complex sequence of apparently dipping conversions in the volcanic arc prominent effects of strongly dipping discontinuities include the change of sign in ps phases refracted past the vertical and multiple p reflections that cannot be separated from ps conversions during receiver function processing unambiguous identification of these phenomena can be hampered by poor azimuthal coverage of the data still in regions where discontinuity topography can reasonably be expected its effects should be considered when interpreting receiver functions as results might be spurious otherwise

volcanism in the baikal rift 40 years of active versus passive model discussion abstract in this review we focus on the volcanism that occurred in transbaikalia siberia after the closure of the mongolia okhotsk ocean the closure happened in the early jurassic after that time lithosphere in transbaikalia went through two phases of rifting in the early cretaceous and again in the late cretaceous until present the latter rifting event is known as the baikal rifting we consider the chronology of the volcanism and basin formation in the baikal rift and show that there has been a complex relationship between the two extension initiated in the central part of the rift system this area is now occupied by lake baikal sedimentary basins initially developed by deepening and widening of the central part of the rift system and then by bilateral propagation of basin formation outwards volcanism was generally offset from the axial rift considering along axis distribution of volcanism it initiated in the central part of the system and propagated bilaterally to the modern rift ends we argue that tectonic stress controlled localization of the eruptive centres extension and shearing probably caused melting at mantle depth suggestive of the passive model of volcanism however when considering the baikal rift and adjacent non rifted regions of mongolia in a wider context of tectonics and volcanism of central and east asia it is not possible to rule out that the volcanism may be associated with mantle transition zone diapirs thus the active model of volcanism may also apply the diapirs are located by regional isostatic gravity anomalies and considered as upwelling parts of the upper mantle convective cell controlled by the pacific subduction and slab stagnation in the mantle transition zone we do not see any geochemical geophysical and geochronological evidence for involvement of deeper mantle to explain volcanism in either baikal rift non rifted regions of mongolia or anywhere else within central and east asia