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

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

late cenozoic bending of the bolivian andes new paleomagnetic and kinematic constraints 1 new paleomagnetic measurements of essentially undeformed late cenozoic volcanic rocks in the bolivian andes south america constrain rigid body rotations about vertical axes during the last 13 myr on both limbs of the bolivian orocline in the eastern cordillera thermal and alternating field demagnetization was carried out on samples from 52 sites in three major volcanic complexes 1 13 2 ma los frailes ignimbritic volcanics outcropping at 19 5 s 2 9 5 ma morococala ignimbritic volcanics outcropping at 18 s and 3 13 5 ma shoshonitic to acidic lavas and intrusives outcropping at 17 5 s well defined magnetic components were isolated which are interpreted to represent the earth s magnetic field at the time of volcanic activity the mean magnetic vector for site groupings suggests regional tectonic rotations about vertical axes with respect to stable south america of 10 8 clockwise for the 13 2 ma los frailes volcanic complex 8 9 clockwise for only the miocene 13 5 ma los frailes volcanics and 1 18 anticlockwise for the combined 13 5 ma morococala and eucalyptus volcanic complexes these data are consistent with observed shortening gradients on the eastern margin of the bolivian andes in the sub andean zone suggesting rotation and shortening are synchronous a joint inversion for both tectonic rotation and the amount of shortening assuming a linear variation in the amount of rotation along the length of the southern limb of the bolivian orocline shows that rotation of the eastern cordillera in the last 10 15 myr relative to stable south america varies from zero at the oroclinal hinge at 18 s to a maximum of 13 5 clockwise at 22 s decreasing to zero south of 23 s with an average rotation in the range 5 to 10 clockwise concomitant with this shortening in the sub andean zone decreases from a maximum of 86 km at the oroclinal hinge to 47 km at 22 s and then 33 km at 23 s the lack of evidence for large scale internal faulting in the eastern cordillera since 9 ma suggests either bending of the eastern cordillera was accommodated by more distributed small scale faulting or that relative rotation here mainly occurred prior to 9 ma the small average rotation of the eastern cordillera on the southern limb of the orocline is consistent with only very small rotation 10 of the forearc region in northern chile during the last 20 myr rotation of the northern limb is less well constrained but it is also likely to be small 5 and anticlockwise

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

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

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

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

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

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

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

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

a geochemical study of the impact of irrigation and aquifer lithology on groundwater in the upper yakima river basin washington usa the yakima river a major tributary of the columbia river is currently overallocated in its surface water usage in part because of large agricultural water use as a result groundwater availability and surface water groundwater interactions have become an important issue in this area in several sub basins the yakima river water is diverted and applied liberally to fields in the summer creating artificial recharge of shallow groundwater major ion trace element and stable isotope geochemistry of samples from 26 groundwater wells from a transect across the yakima river and 24 surface waters in the kittitas sub basin were used to delineate waters with similar geochemical signatures and to identify surface water influence on groundwater major ion chemistry and stable isotope signatures combined with principal component analysis revealed four major hydrochemical groups one of these groups collected from shallow wells within the sedimentary basin fill displays temporal variations in no3 and so4 along with high 18o and d values indicating significant contribution from yakima river and or irrigation water two other major hydrochemical groups reflect interaction with the main aquifer lithologies in the basin the columbia river basalts high na groundwaters and the volcaniclastic rocks of the ellensburg formation ca mg hco3 type waters the fourth major group has interacted with the volcaniclastic rocks and is influenced to a lesser degree by surface waters the geochemical groupings constrain a conceptual model for groundwater flow that includes movement of water between underlying columbia river basalt and deeper sedimentary basin fill and seasonal input of irrigation water

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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