abrupt change in mantle fabric across northern apennines detected using seismic anisotropy 1 we estimated sks splitting parameters for 18 earthquakes observed over 2 years on a dense linear seismometer array crossing the apennines orogen northern italy an abrupt within 30 km change in fast polarization occurs near the crest of the apennines southwest of the crest fast polarization parallels the apennines strike wnw ese rotating to ew orientation near the tyrrhenian sea northeast of the crest toward the po plain fast polarization is nne ssw roughly normal to the orogen s strike delay times 0 6 2 9s do not show a remarkable pattern splitting parameters beneath the apennines argue against anisotropy restricted to the crust and suggest that the mantle region deformed by apennines slab rollback is spatially limited we hypothesize that the rollback process has been uneven evolving to a contortion of the northern edge of the apennines slab and perhaps incipient detachment lithospheric anisotropy beneath the po plain may also be significant

probing seismic anisotropy in north iberia from shear wave splitting the mantle anisotropic features at the northwestern part of the iberian peninsula have been investigated by the shear wave splitting technique from different temporary array deployments during the last 6 years successive seismic transects were instrumented from east to west to sample areas affected by the alpine compressional tectonics western pyrenees and cantabrian mountains as well as hinterland parts of the iberian variscan belt a remarkable consistency is found in the retrieved anisotropic parameters throughout the study area with an average fast velocity direction close to e w delay times up to 1 5 s are observed in most transects but lower values not exceeding 1 s are measured at the westernmost part although the averaged values are compatible among the different stations a significant variation of the splitting parameters is observed in each station with respect to the backazimuth direction this azimuthal dependence denotes a complex distribution at depth of the anisotropic features and cannot be explained by considering single layer anisotropic models either with hexagonal or orthorhombic symmetry systems and or dipping axes of symmetry synthetic models including two distinct anisotropic layers with an orthorhombic symmetry provide a satisfactory fit in all the retained models the two layers exhibit different thicknesses the most prominent contribution to the observed anisotropy comes from a lower layer consistently oriented close to e w whereas the thinner second layer located above modulates the result and its anisotropic parameters may change along the transect to account for the observed differences in splitting delay times the dominant e w layer throughout could not be associated to major compressional events as in the westernmost part the variscan terrains exhibit n s main lineaments it may rather correspond to an anisotropic imprint around the lithosphere asthenosphere transition related to the eastward displacement of the iberian plate due to the mesozoic extensional processes during the opening of the north atlantic and bay of biscay domains the weaker anisotropic layer could be associated either with crustal anisotropic materials or more likely to an additional anisotropic signature within the lithosphere led by major variscan and alpine orogenic processes

a reconnaissance teleseismic study of the upper mantle and transition zone beneath the archean slave craton in nw canada abstract the objective of this study is to investigate upper mantle structure below the archean slave craton site of the oldest known rocks on earth and numerous diamondiferous kimberlites and thence to gain an understanding of early craton formation and kimberlite genesis to this aim a temporary array consisting of 13 sites equipped with broadband seismometers recorded teleseisms between november 1996 and may 1998 this data set has been augmented with recordings from the yellowknife seismic array our three most robust observations and their interpretation are 1 p wave travel time tomography reveals the oldest part of the craton the central slave basement complex to be underlain by the fastest seismic velocities indicating that this block remains distinct well into mantle depths 2 receiver function analysis requires only the moho as major s wave velocity discontinuity and points to a fairly constant crustal thickness throughout the slave province and 3 sks splitting analysis shows little variation in delay times and fast polarization directions across the array leading us to conclude that the present day plate motion of north america is the primary cause for mantle fabric beneath the entire array furthermore the data let us rule out the possibility that the mackenzie plume had any seismologically detectable effect on the slave lithosphere more speculative results of our investigation namely a possible genetic link between a low seismic velocity anomaly at depth with the overlying lac de gras kimberlite field and a possible archean origin of two shallow low velocity anomalies will require further investigation

first palaeomagnetic results from the cycladic massif greece and their implications for miocene extension directions and tectonic models in the aegean abstract the aegean region is characterised by major fault bound blocks with markedly contrasting structural trends recent models account for these variations either through differential block rotation or through a process of radial extension the cycladic massif in the back arc region to the hellenic trench represents a critical area where apparently divergent extension directions are observed this contribution provides the first palaeomagnetic results from this area magnetic remanence and ams data are presented for middle miocene granodiorites on the islands of mykonos and naxos these rocks are exposed in the footwalls of major extensional detachments within the metamorphic core complexes that characterise the cycladic massif a clear correlation between the orientation of an l s fabric developed within the intrusions and the magnetic fabric data is observed demonstrating the presence of a normal magnetic fabric of extensional origin the mean extension directions inferred from the orientations of the maximum axes of the magnetic susceptibility ellipsoids are markedly divergent at 064 mykonos and 351 naxos northeasterly in situ magnetic remanences are observed on mykonos whereas those on naxos are directed to the nnw neither direction corresponds to the appropriate palaeomagnetic reference direction for the miocene standard palaeomagnetic tilt corrections cannot be applied at these sites because of the lack of palaeohorizontal markers deviations of in situ remanences away from the reference direction are potentially due to rotation around any one of an infinite number of net tectonic rotation axes each of these may be arbitrarily decomposed into components of tilting and vertical axis rotation geologically realistic solutions which restore the in situ magnetisations back to the reference direction may be found by consideration of regional structural trends assuming that tilting is controlled by the major block bounding faults these solutions indicate significant components of differential vertical axis rotations across the cyclades mykonos is bound by nw se trending regional faults and is shown to have rotated clockwise by ca 22 in contrast naxos is bound by ene wsw trending structures and has rotated anticlockwise by ca 33 the apparent divergence of extension directions inferred from both the magnetic anisotropy and structural data is resolved by correcting the mean ams data for subsequent rotation using the rotation solutions derived from the magnetic remanence data the data imply that the primary middle miocene extension direction in the cycladic massif was nearly uniform along a nne ssw azimuth we consider that the uniform extension direction and opposing senses of block rotation determined for this portion of the cycladic massif are most consistent with a recent model describing deformation within the aegean in terms of a system of broken fault bounded slats within this scenario mykonos occupies a position on a clockwise rotating slat which continues northwest at least as far as evvia a major ene wsw trending fault between the islands previously identified on shallow seismic sections separates this domain from an anticlockwise rotating slat which includes naxos

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

alpine orogenic p t t deformation history of the catena costiera area and surrounding regions calabrian arc southern italy the nappe edifice of north calabria revised with insights on the tyrrhenian apennine system formation 1 the nappe structured belt of calabria constitutes the eastward termination of the southern branch of the alpine mediterranean belt that delimits the northern edge of the africa plate contrasting hypotheses for the origin and tectonic significance of the north calabrian nappe edifice have been proposed and kinematic data from north calabria have been used to support different interpretations of the alps apennines linkage and the polarity of the tethyan subduction in the apennine region we reconstruct the architecture of the north calabria nappe edifice through a multidisciplinary approach which integrates structural investigations with metamorphic thermobarometry and 40ar 39ar geochronology results from this study indicate that north calabria consists of a tertiary nappe stack resulting from superimposed top to the west extensional shearing late oligocene to middle miocene in age onto a previously structured top to the east compressional belt eocene to oligocene in age this study also documents that the top to the west extensional tectonics was achieved by means of regionally sized extensional detachment fault systems stretching apart and translating as allochthonous fragments the previously accreted units thinning operated by top to the west extensional detachment tectonics also resulted in the direct juxtaposition of non alpine or slightly alpine metamorphosed units upper plate complex onto the previously exhumed deep seated portions of the orogenic wedge metamorphosed under blueschist facies metamorphic conditions lower plate complex these findings support a new tectonic scenario for the orogenic history of north calabria which may be adequately framed within the tertiary apennine tyrrhenian system evolution

seismic anisotropy beneath southern iberia from sks splitting abstract seismic anisotropy of the south iberian upper mantle is investigated using shear wave splitting of sks phases we analyzed teleseismic events recorded by sixteen permanent broadband stations installed on the southern iberian peninsula and in northern africa and we determined fast polarization directions and delay times t between fast and slow components the area of investigation extends across two important geological structures in the variscan iberian peninsula the variscan iberian massif in its center and the gibraltar arc in the southeast that represents the most westerly alpine belt in the western mediterranean shear wave splitting measurements from stations in the betic domain show homogeneous ene wsw fast directions nearly parallel to the trend of the mountain belt and smooth spatial variations stations in the north toward the southern part of the variscan iberian massif show homogeneous fast directions however trending ns to ne sw different from those recorded in the betic these observations may reflect a post hercynian variscan deformation of the ossa morena zone related to the main stages in the tectonic evolution of this part namely transpressional stage transtensional stage and shortening episode or a deformation related to the posterior alpine orogeny along the gibraltar arc we observe a smoothly varying trend changing from ene wsw in the eastern betics to ns in the area of gibraltar and ceuta following more or less the general trend of the mountain belt around the alboran sea and the coastline since a similar rotation is also visible in results from pn anisotropy this suggests that the anisotropy is vertically coherent starting from just below the moho comparing the anisotropy pattern expected from various geodynamic models with the observed sks splitting suggests that the anisotropy is best explained by a model of slab rollback rather than by delamination models

late orogenic doming in the eastern betic cordilleras final exhumation of the nevado filabride complex and its relation to basin genesis the geometry timing and kinematics of late orogenic extension in the betic cordilleras pose the problem of a decoupling of upper crustal and lower crustal deformation regimes perpendicular directions of extension in metamorphic domes and nearby sedimentary basins remain unexplained this paper puts kinematic constraints on the final exhumation of the nevado filabride complex focusing on the formation of metamorphic domes and their relations with the adjacent basins structural fabrics and kinematic indicators below the main shear zones as well as their relations with both published changing metamorphic p t conditions and geochronological data were studied our approach describes 1 a consistent top to the west shear parallel to dome axes of during d2 i e during decompression with distributed ductile flow and the onset of strain localization along major shear zones 2 further strain localization along the major shear zones under greenschist facies conditions during d3 leading to s c mylonites formation accompanied with a rock strong thickness reduction 3 the divergence of shear direction on either limbs of domes during d3 showing the appearance of the dome geometry and 4 a local evolution toward n s brittle extension d4 in the upper plate and formation of sedimentary basins continuous ductile to brittle top to the west shear is compatible with the slab retreat hypothesis from the miocene the formation of domes which adds gravitational forces responsible for the final stages of exhumation is thus characterized by important kinematics changes necessary to explain coeval n s opened basins later from the upper tortonian a contractional event d5 amplified the earlier domal structures forming the present north vergent folds

shear wave splitting in a young continent continent collision an example from eastern turkey an edited version of this paper was published by the american geophysical union agu copyright 2003 agu see also http www agu org pubs crossref 2003 2003gl017390 shtml http atlas geo cornell edu turkey publications sandvol et al 2003b htm

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

sks splitting measurements in the apenninic tyrrhenian domain italy and their relation with lithospheric subduction and mantle convection 1 we present a compilation of singular shear wave splitting measurements in the italian region we used the method of silver and chan 1991 to determine the splitting parameters of fast direction and delay time from broadband sks waveforms the stations analyzed were mainly deployed in temporary experiments a few are permanent mednet broadband stations the splitting parameters reveal the presence of strong seismic anisotropy in the mantle beneath italy the fast polarization directions define two anisotropic domains a tyrrhenian domain with e w prevalent fast directions and an apennines adriatic domain with nw se to nnw sse prevalent fast directions the stations along the boundary between these two main domains show fast polarization directions that vary for earthquakes coming from different back azimuths this variability is a sign of a complex upper mantle structure also shown by seismic tomography but allows us to constrain the depth of the anisotropic layer in the 100 250 km range at least in the boundary zone between the two domains data at station vsl sardinia suggest the presence of depth dependent anisotropic structure where the shallower signature is consistent with the tyrrhenian domain results and the deeper one is possibly related to the presence of a subhorizontal slab laying on the 670 km discontinuity we argue that the complex pattern of fast directions is strictly related to the upper mantle structure and is caused by asthenospheric flows induced by slab rollback and contemporary deformation of the overriding tyrrhenian plate

seismic anisotropy beneath the northern apennines italy mantle flow or lithosphere fabric shear wave splitting estimates from recordings of 10 portable seismographic stations during the first year of the retreat seismic deployment in combination with broadband data from the italian national seismic network are associated with seismic anisotropy within the upper mantle beneath the northern apennines anisotropic parameters derived from both shear wave splitting and p travel time residuals vary geographically and depend on event back azimuth reflecting complexity in the underlying mantle strain field variations of the splitting time delays and fast polarization seem to exclude a 2 d sublithosphere corner flow associated with the apennines subduction as the main source of the inferred anisotropy the anisotropic signal may be generated by a frozen in fabric of the adriatic and tyrrhenian lithosphere domains or by flow variations induced by episodic and fragmentary slab rollback

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

shear wave anisotropy in the upper mantle beneath the aegean related to internal deformation seismic anisotropy deduced from sks splitting measured at 25 stations installed in the aegean does not show a homogeneous pattern it is not restricted to the north anatolian fault but is distributed over a region several hundreds kilometers wide little anisotropy is observed in continental greece or along the hellenic arc however significant anisotropy is observed in the north aegean sea large values of delay times suggest that anisotropy is due to a long path within the upper mantle and to strong intrinsic anisotropy our results both in fast polarization directions and in values of delay time do not support the idea that anisotropy is associated with inherited tectonic fabric nor are they consistent with the present day aegean motion relative to an absolute frame in contrast the direction of fast polarization and the magnitude of delay times correlate well with the present day strain rate observed at the surface deduced from both geodetic measurements and seismicity this anisotropy is not horizontally restricted to major surface faults but is spread over a wide region

a block model of distributed deformation by faulting it is clear from palaeomagnetic evidence that large and hitherto unexpected rotations about a vertical axis are common in regions of distributed continental deformation we propose a simple two dimensional model to illustrate how fault movement and block rotation within a zone of distributed deformation may be related to the relative motion of the rigid plates that bound the zone one surprizing feature of this model is that the component of strike slip motion on the faults within the deforming zone is in the opposite sense to the strike slip component across the zone as a whole thus slip vectors within the zone are not the same as that between the bounding rigid plates in contrast to deformation at oceanic plate boundaries examples of active fault geometries similar to those of our model can be seen in greece iran and the western usa

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 scale geometry and kinematics of late orogenic extension in the central aegean cyclades and ewia island abstract the aegean continental domain is known to be the site of widespread back arc extension since at least the middle miocene overprinting structures related to the mesozoic cenozoic hellenic orogeny features attributed to early thrusting include the overall ductile deformation within two broad belts that have suffered hp lt metamorphism across the aegean this study summarizes the results of a structural study in the central aegean area cyclades and evvia island examining in particular the relationship between ductile and brittle deformation on a regional scale extension appears to be responsible for most of the ductile deformation within hp rock units that have experienced penetrative greenschist facies and higher grade metamorphic overprinting on each island studied progressive extensional deformation has occurred through the development of a major normal sense detachment zone down to depths of about 18 25 km the geometry of the extensional system on the scale of the central aegean is described taking into account the characteristic features of the regional scale ductile strain field at least two probably three subparallel major north dipping detachment zones are identified trending nw se in the northwestern cyclades and e w in the southeastern cyclades both with initial dips estimated at around 30 45 the present geometry of the extensional system implies that these detachment zones and related metamorphic core complexes interfere with each other two main evolutionary models are considered which can account for such a kinematic link between parallel detachment zones structural data favour the sequential development of synthetic major detachments zones in a direction opposite to the sense of dip of the detachments early oligocene lower miocene detachment type extension in the area studied is related to a late orogenic post thickening back arc setting

neogene to quaternary rifting and inversion in corsica retreat and collision in the western mediterranean 1 stratigraphic structural and fission track data have been integrated in order to unravel the neogene to quaternary tectonic evolution of northeastern corsica results indicate that during early to middle miocene the study area was affected by a rapid cooling at low temperatures accompanied by exhumation and rifting basin inversion occurred during the tortonian again followed by extension in the messinian to early pliocene compressional deformations are finally recorded by early pliocene to quaternary deposits alternating rifting and inversion events are recorded not only in northeastern corsica but also in many peri tyrrhenian regions this alternation can be explained in terms of intraplate propagation of the compressional stress related to the africa europe convergence and to a discontinuous slab retreat along the apennine subduction system a pause in the slab retreat implies a break of the back arc extension in the upper plate and it allows a temporary propagation of africa europe compression

a linkage between the north atlantic oscillation and its downstream development due to the existence of a blocking ridge 1 in this work the spatial growth of the north atlantic oscillation nao and its relationship to downstream development are examined in contrast to positive nao events which are primarily initiated over the western north atlantic negative persistent nao events tend to be preceded by a blocking ridge in the vicinity of northern europe this blocking ridge contributes to the growth of negative nao events through a low frequency eddy interaction it is also shown that a noticeable downstream development is dominant only for negative persistent nao events that are preceded by the blocking ridge while the blocking ridge exists over inland eurasia it induces anticyclonic circulation near the surface over siberia as the nao grows a stationary wave train that originates from the north atlantic transfers wave energy downstream toward siberia the surface anticyclone is then reinforced by the wave train and begins to migrate eastward this process explains the distinct impact of the negative nao on the downstream eurasian region

seismic anisotropy reveals focused mantle flow around the calabrian slab southern italy 1 sks splitting at the calabrian subduction zone with delay times t up to 3s reveals the presence of a strong anisotropic fabric fast directions are oriented nne ssw in the calabrian arc c a and rotate nnw sse to the north following the arcuate shape of the subducting plate we interpret the trench parallel as local scale mantle flow driven by the retrograde motion of the slab the absence of trench perpendicular below the southern apennines s a excludes the presence of a seismically detectable return flow at its ne edge this may be due to the relative youth and limited width of the s a slab tear a possible return flow is identified farther north at the boundary of the s a and central apennines different and weaker anisotropy is present below the apulian platform a p this implies that the influence of the slab rollback in the sub slab mantle is limited to less then 100 km from the slab

thrust or detachment exhumation processes in the aegean insight from a field study on ios cyclades greece 1 whether back arc extension in the aegean is symmetric or asymmetric at the regional scale is a key question to the understanding of the dynamics of back arc extension new field observations structural analysis of finite and instantaneous strain and their relationship with metamorphism on ios island cyclades lead us to investigate the significance of the south cycladic shear zone scsz two successive tectonometamorphic events have been documented in the hanging wall and the footwall of the scsz first penetrative top to the south shearing is observed in the upper cycladic blueschists and the lower cycladic basement close to their contact it is expressed in the hanging wall by shear bands and pressure shadows associated to blueschist facies minerals contemporaneous with overthrusting of the cycladic blueschists over the basement unit second large extensional top to the north shear zones lately crosscut the whole stack these constraints allow us to reinterpret the kinematics of the aegean domain during synorogenic and postorogenic periods when replaced in the reconstructed geometry of the hellenic eocene orogenic wedge the observed top to the south thrusting is coeval with extensional shear zones observed in syros that could represent the roof of the cycladic blueschists an extrusion structure is then proposed for the exhumation of the cycladic blueschists in the subduction zone top to the north asymmetric extension together with the andros tinos mykonos and paros naxos detachments can be correlated to oligo miocene extension in the back arc domain

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

subduction rollback slab breakoff and induced strain in the uppermost mantle beneath italy differences in seismic anisotropy revealed by split sks waves that traverse the upper mantle beneath the italian region reveal four areas of internally coherent anisotropic strength providing evidence for mantle strain partitioning when compared to uppermost mantle structure imaged by tomography the sequence of these areas displays a straightforward parallelism under proper assumptions the correspondence between areas of coherent splitting degree and areas of coherent velocity perturbations offers new insights into the late evolution of subduction in italy and their effect on mantle strain

relation between surface velocity field and shear wave splitting in the south island of new zealand 1 the relative motion between the pacific and australian plates across the central south island of new zealand as measured by repeated gps measurements is distributed over a region more than 80 km wide the surface distribution of velocity is fit almost equally well by a model in which deformation of the lower lithosphere is concentrated into very narrow slip zones below one or two major faults and by a model in which the lower lithosphere deforms in a broad shear zone observations of the splitting of sks waves permit distinction between the two models on the basis of the finite strain they predict deformation in the deep lithosphere that was limited to narrow shear zones would not produce measurable splitting hence any splitting observed should not correlate with surface tectonics however the distribution of fast propagation directions revealed by observations of shear wave splitting do reflect surface tectonics in particular the orientations of fast propagation directions are consistent with the finite strain calculated by assuming that the distribution of velocity at depth within the lithosphere resembles the distribution of velocity measured at the surface by gps the observed distribution of splitting can be reproduced with an rms misfit of 15 provided that the velocity field is integrated over at least the time interval since significant convergence across the southern alps began at about 6 5 ma

the mantle flow field beneath western north america although motions at the surface of tectonic plates are well determined the accompanying horizontal mantle flow is not we have combined observations of surface deformation and upper mantle seismic anisotropy to estimate this flow field for western north america we find that the mantle velocity is 5 5 1 5 centimeters per year due east in a hot spot reference frame nearly opposite to the direction of north american plate motion west southwest the flow is only weakly coupled to the motion of the surface plate producing a small drag force this flow field is probably due to heterogeneity in mantle density associated with the former farallon oceanic plate beneath north america

constraining the extent of crust mantle coupling in central asia using gps geologic and shear wave splitting data abstract we have obtained constraints on mechanical crust mantle coupling for tibet and yunnan indo china by comparing the observed surface deformation field inferred from gps and quaternary fault slip rate data with the mantle deformation field inferred from several sks shear wave splitting data sets we first determined whether the anisotropy is dominantly asthenospheric or lithospheric by testing simple models of both types against the observed values of the fast polarization direction which is assumed to be parallel to the horizontal projection of the direction of maximum shear for asthenospheric flow we solved for a best fitting uniform sub asthenospheric velocity model for eurasia the fit however was not satisfactory rms misfit 20 solving for separate uniform flow fields in each region improves the fit 15 for tibet 11 for yunnan although the resulting flow fields are inconsistent with several geophysical and geological constraints and thus considered unlikely we then considered lithospheric models for tibet vertically coherent deformation i e maximum shear direction from surface deformation is parallel to yields an improved match 11 for left lateral shear both the goodness of fit and the dominance of left lateral surface faulting in tibet argue for a lithospheric source of anisotropy the misfit for yunnan is large for either right 53 or left lateral 49 shear which argues for complete crust mantle decoupling in yunnan we show that the fast polarization directions throughout both the tibet and yunnan region can be fit by a single lithospheric dynamic model in which there is strong coupling between crust and mantle beneath tibet but a complete decoupling between crust and mantle beneath yunnan crust this dynamic model predicts left lateral maximum shear directions within the mantle that align with fast polarization directions in both regions 9 these maximum shear directions within the mantle align with the left lateral maximum shear directions in the crustal deformation field in tibet but are not in yunnan our results have the following implications first the coherence between crust and mantle deformation in tibet implies strong crust mantle mechanical coupling since this is the only way that crustal buoyancy forces required to account for the surface deformation field can be transmitted into the mantle this behavior is consistent with a uniform strength lithosphere or strong crust but not with a substantially weaker crust for example it is inconsistent with the popular jelly sandwich rheology and thus precludes behavior such as large scale lower crustal flow in tibet the crust mantle coherence is also incompatible with mantle delamination second crust mantle decoupling within the yunnan lithosphere argues that mantle deformation there is controlled only by boundary conditions crustal buoyancy forces are not transmitted into the mantle beneath yunnan moreover the dynamic model for the mantle shows that the yunnan crust is moving south with respect to the mantle at rates as high as 30 mm yr third there is a fundamental rheological lithospheric transition between tibet and yunnan that may provide a key to understanding this significant orogen

metamorphic core complexes of cordilleran type in the cyclades aegean sea greece the islands of ios and naxos in the cycladic archipelago greece contain elongate domes over which a pervasive north to north northeast stretching lineation is warped these islands may be metamorphic core complexes of cordilleran type involving middle crustal rocks drawn upward and outward from underneath the sedimentary basins now 100 km to the south from ca 25 ma onward during uplift the crystalline rocks now exposed on the islands were deformed noncoaxially in the deeper levels of a major shallow dipping ductile shear zone this shear zone may have connected with low angle normal faults in its upper levels because it is probable that its later history involved the metamorphic complexes being dragged out from under a stretching and fracturing upper plate composed mainly of unmetamorphosed sedimentary and ophiolite suite rocks this model can be extended to explain the exhumation of the high pressure low temperature metamorphic rocks on crete which with the cyclades forms a distended paired metamorphic belt separated by an active sedimentary basin