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

controls on back arc basin formation the relationship between subduction and back arc spreading has been well known since the early days of plate tectonics however the reasons why back arc basins are associated with some subduction systems but not all has remained elusive we examine the kinematic controls on subduction and back arc basins for both the present day and cenozoic to differentiate between the major competing hypotheses for back arc basin formation and to explain their temporal and spatial distribution our new data set of subduction and back arc basin parameters uses a new set of paleo oceanic age grids muller et al 2005 associated with a moving atlantic indian ocean hot spot reference frame o neill et al 2005 the plate model includes detailed reconstructed spreading histories of back arc basins based on marine geophysical and satellite gravity data our combined rotation and oceanic paleo age model provides the age distribution of subducting lithosphere through space and time convergence rates and the absolute motion of the downgoing and overriding plates we find that back arc basins develop when the age of subducting normal oceanic lithosphere is greater than 55 million years additionally we establish an age dip relationship showing that the intermediate dip angle of the subducting slab is always greater than 30 with back arc spreading our results suggest that back arc basin formation is always preceded by an absolute motion of the overriding plate away from the subduction hinge thereby creating accommodation space between the overriding and subducting plates once back arc extension is established it continues regardless of overriding plate motion indicating back arc spreading is not a simple consequence of overriding plate behaviour the landward migration of the overriding plate as a precursor to back arc extension may indicate that extension on the overriding plate is influenced by the oceanward lateral flow of the mantle however once back arc extension is established rollback of the subduction hinge appears to be the primary force responsible for the continued creation of accommodation space our analysis indicates the driving mechanism for back arc extension is a combination of surface kinematics properties of the downgoing slab the effect of lateral mantle flow on the slab and mantle wedge dynamics

fluid and melt pathways in the central chilean subduction zone near the 2010 maule earthquake 35 36 s as inferred from magnetotelluric data

lithospheric and upper mantle structure of southern tibet from a seismological passive source experiment fifteen seismic stations were operated with about 20 km spacing in southern tibet across the zangbo suture the collision zone between india and asia between may and october 1994 as part of the international deep profiling of tibet and the himalaya project indepth ii for wide angle recording of the controlled source experiment and for passive earthquake recording in addition a dense deployment 4 km spacing of stations within the german depth profiling of tibet and the himalayas gedepth project also recorded a number of teleseismic earthquakes the third data source used in this study is the records of the permanent broadband station at lhasa teleseismic records have been obtained in sufficient quantity and quality to derive an image of the structure of the lithosphere and upper mantle from p to s converted phases important results are as follows the moho at 70 80 km and a second discontinuity at 50 60 km depth are observed over the entire profile south and north of the zangbo suture the data from the gedepth dense array enable the detection of inclined structures penetrating the crust at the zangbo suture a pronounced low velocity zone exists north of the zangbo suture at about 10 20 km depth the locations of the upper mantle discontinuities at 410 and 660 km depth are in agreement with the global reference model iasp91 kennett 1991 over a large region of the himalaya and southern tibet

slag remains from the na slupi site prague czech republic evidence for early medieval non ferrous metal smelting archaeological investigation of the na slupi site in the central part of prague the capital of the czech republic and located in the vicinity of vysehrad castle has demonstrated the presence of early medieval 12th century large scale smelting of non ferrous metals this study is focused on the investigation of smelting remains pb rich red and black slags using a combination of geochemical and mineralogical methods in order to understand the technologies and processes used at this site glassy red slags are composed of high temperature ca k and pb rich silicates wollastonite larnite kalsilite melilite and pb bearing glass and are associated with less abundant litharge fragments red slag impregnated furnace walls composed of litharge massicot pbo and cu2pbo2 they indicate that the cupellation process in small furnaces rather than in crucibles open vessels was used for ag refining at the studied site the black slags found in much lower quantity at this site are composed of maghemite fayalite clinopyroxene melilite and glass and are s poor we believe that they correspond to waste products from the first stage of ag purification prior to final cupellation where slagging additives such as fe rich material silica sand or ashes could be added to remove impurities the pb isotopic composition of both types of slags is similar to the isotopic signature of pb zn ores from the vicinity of cracow poland and supports the hypothesis that pb was probably traded between prague and poland in the 12th century and used for refining precious metals in metallurgical workshops

ore genesis of pb zn deposits in the nappe zone of northern tunisia constraints from pb s c o isotopic systems abstract the jalta and jebel ghozlane ore deposits are located in the extreme north of tunisia within the nappe zone the mineralization of jalta hosted in triassic dolostones and the overlying mio pliocene conglomerates consists of abundant galena barite and cerussite with accessory sphalerite pyrite and jordanite at jebel ghozlane large pb zn concentrations occur in the triassic dolostones and eocene limestones the mineral association consists of galena sphalerite barite and celestite and their oxidation products cerussite smithsonite and anglesite lead isotope ratios in galena from both districts are relatively homogeneous 206 pb 204 pb 18 702 18 823 207 pb 204 pb 15 665 15 677 208 pb 204 pb 38 725 38 875 the 34 s values for sulfates from both areas 12 2 to 16 2 at jalta and 14 3 to 19 4 at jebel ghozlane are compatible with a derivation of sulfur from marine sulfates possibly sourced from the triassic evaporites the 34 s values of the sulfides have a range between 10 and 12 5 at jalta and between 9 1 and 22 1 at jebel ghozlane the large range of values suggests reduction of the sulfate by bacterial and or thermochemical reduction of sulfate to sulfur the high 34 s values of sulfides require closed system reduction processes the isotopically light carbon in late calcites 6 3 to 2 5 and authigenic dolomite 17 6 suggests an organic source of at least some of the carbon in these samples whereas the similarity of the 18 o values between calcite 24 8 and the authigenic dolomite 24 7 of jalta and their respective host rocks reflects oxygen isotope buffering of the mineralizing fluids by the host rock carbonates the secondary calcite isotope compositions of jalta are compatible with a hydrothermal fluid circulation at approximately 100 to 200 c but temperatures as low as 50 c may be indicated by the late calcite of jebel ghozlane 18 o of 35 9 given the geological events related to the alpine orogeny in the nappe zone nappe emplacement bimodal volcanism and reactivation of major faults such as ghardimaou cap serrat and the neogene age of the host rocks in several localities a late miocene age is proposed for the pb zn ore deposits considered in this study remobilization of deep seated primary deposits in the paleozoic sequence is the most probable source for metals in both localities considered in this study and probably in the nappe zone as a whole

seismic evidence for sharp lithosphere asthenosphere boundaries of oceanic plates the mobility of the lithosphere over a weaker asthenosphere constitutes the essential element of plate tectonics and thus the understanding of the processes at the lithosphere asthenosphere boundary lab is fundamental to understand how our planet works it is especially so for oceanic plates because their relatively simple creation and evolution should enable easy elucidation of the lab data from borehole broadband ocean bottom seismometers show that the lab beneath the pacific and philippine sea plates is sharp and age dependent the observed large shear wave velocity reduction at the lab requires a partially molten asthenosphere consisting of horizontal melt rich layers embedded in meltless mantle which accounts for the large viscosity contrast at the lab that facilitates horizontal plate motions

an s receiver function analysis of the lithospheric structure in south america 1 we make use of s receiver function to investigate the structure and thickness of the crust and mantle lithosphere in the south american continent and adjacent areas the moho discontinuity has been detected at all stations and goes from 18 km beneath the coast of the continent to 80 km in the andean region the depth of the lithosphere asthenosphere boundary can be clearly detected below those stations that are located on stable areas from 50 km to 160 km the identification of this phase becomes more difficult when the stations are located near subduction zones we also observed the base of the subducted oceanic lithosphere down to depths of 220 km

provenance of early bronze age metal artefacts in western switzerland using elemental and lead isotopic compositions and their possible relation with copper minerals of the nearby valais abstract ten early bronze age bza1 2200 2000 bc copper artefacts from the central valais region from switzerland were studied for their elemental composition and lead isotope ratios in order to answer the archaeological question of a local copper supply a database for copper minerals across the valais switzerland has been established this database contains 69 data on lead isotope ratios as well as additional information on the minerals and geochemical associations for copper minerals from 38 locations in the valais comparisons of the artefacts were also made with data pertaining to minerals from various deposits from europe and anatolia taken from the literature the provenance of the materials is very diverse some of the data are compatible with the data from the copper mineral deposits of the valais region moreover three copper lunulae were identified as possibly tuscan which suggests contacts between italy and the valais region this pattern also establishes a multiplicity of provenances for the metal and cultural influences in the alpine environment of the rhone valley of switzerland at the beginning of the early bronze age

moho topography in the central andes and its geodynamic implications p to s converted waves at the continental moho together with waves multiply reflected between the earth s surface and the moho have been used to estimate the moho depth and average crustal vp vs variations in the central andes our analysis confirms and significantly complements the moho depth estimates previously obtained from wide angle seismic studies and receiver functions the resulting crustal thickness varies from about 35 km in the forearc region to more than 70 km beneath the plateau and thins 30 km further to the east in the chaco plains beneath the andean plateau the moho is deeper in the north altiplano and shallower in the south puna where the plateau attains its maximum elevation a non linear relation exists between crustal thickness and elevation and bouguer gravity suggesting that the crust shallower than 50 55 km is predominately felsic in contrast to a predominately mafic crust below such a relation also implies a 100 km thick thermal lithosphere beneath the altiplano and with a lithospheric thinning of a few tens of kilometers beneath the puna absence of expected increase in lithospheric thickness in regions of almost doubled crust strongly suggests partial removal of the mantle lithosphere beneath the entire plateau in the subandean ranges at 19 20 s the relation between altitude and crustal thickness indicates a thick lithosphere up to 130 150 km and lithospheric flexure beneath a relative topographic low at the salar de atacama a thick crust 67 km suggests that the lithosphere in this region is abnormally cold and dynamically subsided possibly due to coupling with the subducting plate this may be related to the strongest ms 8 0 known intra slab earthquake in the central andes that happened very close to this region in 1950 the average crustal vp vs ratio is about 1 77 for the altiplano puna and it reaches the highest values 1 80 1 85 beneath the volcanic arc indicating high ambient crustal temperatures and wide spread intra crustal melting 2002 elsevier science b v all rights reserved

evidence for lithospheric detachment in the central andes from local earthquake tomography abstract data from three temporary seismic networks were merged for tomographic inversion although the deployments did not coincide in time spatial overlap was achieved by re occupying existing sites travel times and t operators of about 1600 earthquakes were inverted for 3d models of p p s and p wave attenuation q p 1 all three attributes provide a consistent image of the entire subduction zone on a lithospheric scale the tomographic images reveal low velocities and high attenuation in the crust and mantle underlying the western cordillera and most of the puna plateau indicative of weak rheology and mostly asthenospheric mantle in contrast forearc and eastern foreland are characterized by high q p values corresponding to cold temperatures in accordance with thermal models in the backarc between 23 s and 24 s a high velocity high q p structure beneath the eastern cordillera and eastern puna is interpreted as detaching continental lithosphere that has been thickened in the orogenic process south of this structure the mantle is characterized by low velocities high p s ratios and low q p values here it is believed that lithosphere originally underlying andean crust has already been removed this is supported by new estimates of crustal thickness and volcanic activity

the role of frictional strength on plate coupling at the subduction interface 1 at a subduction zone the amount of friction between the incoming plate and the forearc is an important factor in controlling the dip angle of subduction and the structure of the forearc in this paper we investigate the role of the frictional strength of sediments and of the serpentinized peridotite on the evolution of convergent margins in numerical models we vary thickness of a serpentinized layer in the mantle wedge 15 to 25 km and the frictional strength of both the sediments and serpentinized mantle friction angle 1 to 15 or static friction coefficient 0 017 to 0 27 to control the amount of frictional coupling between the plates with plastic strain weakening in the lithosphere our numerical models can attain stable subduction geometry over millions of years we find that the frictional strength of the sediments and serpentinized peridotite exerts the largest control on the dip angle of the subduction interface at seismogenic depths in the case of low sediment and serpentinite friction the subduction interface has a shallow dip while the subduction zone develops an accretionary prism a broad forearc high a deep forearc basin and a shallow trench in the high friction case the subduction interface is steep the trench is deeper and the accretionary prism forearc high and basin are all absent the resultant free air gravity and topographic signature of these subduction zone models are consistent with observations we believe that the low friction model produces a geometry and forearc structure similar to that of accretionary margins conversely models with high friction angles in sediments and serpentinite develop characteristics of an erosional convergent margin we find that the strength of the subduction interface is critical in controlling the amount of coupling at the seismogenic zone and perhaps ultimately the size of the largest earthquakes at subduction zones

hypocentral trend surface analysis probing the geometry of benioff zones a hypocentral trend surface is a continuous function of latitude and longtitude fitted by least squares to a set of hypocenters so that it predicts depth to the middle of a benioff zone in this paper we take a relatively simple approach to hypocentral trend surface analysis the hypocentral trend surface is constructed from a spherical surface harmonic expansion whose coefficients are selected so as to minimize the standard vertical deviation between hypocenters and trend surface the vertical deviation of a hypocenter from the hypocentral trend surface is called its residual consideration of a hypocentral trend surface cannot be divorced from consideration of the associated residuals and their spatial distribution any analysis of benioff zone geometry trades off presumed thickness of the zone with its presumed shape this trade off can be investigated by examining suites of candidate hypocentral trend surfaces together with their associated residual distributions generated by varying the number of degrees of freedom available to the trend surface hypocentral trend surfaces are generated for three high quality sets of hypocenters obtained by local seismic networks in honshu japan cook inlet alaska and south peru hypocenters beneath honshu generate a bimodal distribution of residuals about their trend surface the vertical separation of the upper and lower sheets of this double benioff zone averages about 33 km no double benioff zone configuration is discovered below cook inlet or south peru regional trend surfaces are established for intermediate depth teleseismic data from south and middle america in south america 0 40 s teleseismic data suggest that along strike transitions between flat and moderately steeply dipping sections of the benioff zone are achieved by flexure of a coherent slab rather than by fragmentation of the slab into tear bound flaps or separate tongues of lithosphere hypocentral trend surface analysis of teleseismic data from south peru suggests a geometry similar to that indicated by hypocentral trend surface analysis of the local data of hasegawa and sacks an eastward shallowing benioff zone beneath southwest mexico is closely associated with the geometry of the trans mexican volcanic arc the benioff zone beneath central america is more steeply dipping the flat slab steep slab transition near 96 w may relate to the difference in age of oceanic lithosphere on either side of the tehuantepec ridge while the strong flexure of the subducted nazca plate beneath ecuador is closely associated with the carnegie ridge the nazca ridge extropolates slightly north of the flexure in subducted lithosphere beneath south peru the flexure appears instead more closely associated with the pronounced inflection in the map view shape of the leading edge of the overriding south american plate

insights on the upper mantle beneath the eastern alps analyses of ps and sp receiver functions from datasets collected by permanent and temporary seismic stations image a seismic discontinuity due to a negative velocity contrast across the entire eastern alps the receiver functions show the presence of the discontinuity within the upper mantle with a resolution of tens of kilometers laterally it is deeper 100 130 km below the central portion of the eastern alps and shallower 70 80 km towards the pannonian basin and in the central alps comparison with previous studies renders it likely that the observed discontinuity coincides with the lithosphere asthenosphere boundary lab east of 15 e longitude while it could be associated with a low velocity zone west of 15 e

short and long term effects of gps measured crustal deformation rates along the south central andes in this study we present the contemporary crustal deformation field along the central and southern andes 17 42 s estimated from four global positioning sys tem gps campaigns conducted in 1994 97 we find that the majority of the observed crustal deformation field is relatively homogenous roughly parallel to the plate conver gence direction with decreasing velocities away from the trench we attribute this type of deformation pattern to the inter seismic phase of an earthquake deformation cycle caused by 100 locking of the thrust interface between the subducting nazca and the overrid ing south american plates we have also detected a strong post seismic deformation sig nal in the vicinity of the 1995 mw8 0 antofagasta 22 26 s and 1960 mw9 5 chile 38 43 s earthquakes this type of deformation can be described as short term in nature compared to geological timescales the above conclusions are based on the results of the 3 d andean elastic dislocation model aedm by subtracting the aedm predicted deformation rates from the observations we obtained a residual velocity field that high lights the post seismic as well as more long term deformation eects for example we find a strong evidence for the continuing crustal shortening across the back arc reach ing its maximum 4 mm yr in the very north of our study area in addition between latitudes 29 34 s there is an indication of e w oriented extension within the fore arc in accordance with the recent geologic findings for the n s oriented normal faulting

upper mantle s velocity structure of central and western south america we have used broadband seismograms recorded at portable and permanent seismic stations in south america to image the three dimensional s velocity structure beneath central and western south america we have interactively fitted the waveforms of s and surface wave trains in 500 seismograms from 90 earthquakes and 60 different stations the constraints on upper mantle s velocity structure provided by the waveform fits have been combined with independent estimates of depth to the mohorovicic discontinuity yielding model sa99 the s velocity structure of sa99 reveals a highly heterogeneous upper mantle with a maximum peak to peak difference of 1 km s above 100 km the heterogeneity seems to be correlated with tectonic and geologic domains sa99 shows high lithospheric s velocities for the precambrian cratons the amazon basin is also underlain by high s velocities while three other intraplatform basins are underlain by low s velocities at various depths sa99 shows extremely low velocities in the mantle wedge beneath the volcanically active andes we explain these low s velocities as originating from realistic amounts of volatiles released by the subducting nazca crust and resulting amounts of partial melt within the peridotitic mantle wedge without requiring elevated temperatures while sa99 could be consistent with diffuse influx for corner flow in the mantle wedge it does not support models that require influx of subcontinental upper mantle into the mantle wedge through a laterally uniform low viscosity channel

seismicity and shape of the subducted nazca plate an updated compilation of earthquake locations and focal mechanism solutions from the international seismological centre and preliminary determination of earthquakes is the basis of a comprehensive study of the geometry of the wadati benioff zone beneath western south america the new data support previous mapping of a sharp flexure rather than a tear in the subducted nazca plate beneath southern peru and provide evidence for a similar flexure in the southward transition from nearly horizontal subduction to a slab with 30 dip at latitude 33 s in contrast the transition from 30 slab dip beneath bolivia to a nearly horizontal dip in the region between 28 s 32 s is more gradual occurring over several hundred kilometers of along strike distance between 20 s and 32 s this southward flattening corresponds to a broadening of a horizontal benchlike part of the subducted plate formed between 100 and 125 km depth the transition in continental tectonic style near 27 s 28 s from a wide volcanically active plateau to a narrow nonvolcanic cordillera appears not to be associated with the main slab flattening which begins to the north of these latitudes but with a more abrupt change in curvature of the subducted slab from convex upward to concave upward immediately below the plate boundary interface the concept of gaussian curvature is applied to slab bending to explain how subduction geometry is affected by the shape of the south american plate we hypothesize that the polarity of vertical curvature in the subducting slab is governed by the orientation of lateral curvature of the plate margin focal mechanism solutions for intermediate and deep earthquakes are grouped by geographic region and inverted for the orientation and relative magnitudes of the principal stresses results of the inversion indicate that downdip extension dominates in the slab above 350 km while downdip compression dominates at greater depths

imaging the transition from flat to normal subduction variations in the structure of the nazca slab and upper mantle under southern peru and northwestern bolivia

water transportation through the philippine sea slab subducting beneath the central kyushu region japan as derived from receiver function analyses 1 receiver function analyses are performed to detect seismic velocity discontinuities in the uppermost mantle beneath the kyushu subduction zone japan the philippine sea slab subducting beneath kyushu is young 26 50 ma and steeply dipping at greater than 30 we detect a seismic velocity contrast larger than 10 corresponding to the oceanic moho down to 90 km in depth implying that the subducting oceanic crust contains more than 3 0 wt water down to this depth we also detect a discontinuity with downward decreasing seismic velocity at depths of 50 80 km which is parallel to the oceanic moho and 10 km shallower than it this fact indicates that there is a sharp discontinuity between the mantle wedge and the hydrous oceanic crust the existence of such a sharp discontinuity would require a large temperature gradient around the boundary or a permeability barrier at the upper boundary of the slab we delineate the continental moho with downward decreasing seismic velocity and the upper boundary of the slab with upward decreasing seismic velocity beneath the forearc region which implies the existence of serpentinite and or free fluid which causes high pore pressure in the forearc mantle

the moho in subduction zones abstract the moho in subduction zones exists in two distinct forms one associated with the subducting oceanic plate and second with the overriding plate the seismic expression of both forms is linked to the nature of a landward dipping low velocity zone lvz that has been detected in a majority of subduction zones about the globe and that approximately coincides with wadati benioff seismicity we review seismic studies that constrain the properties of the lvz in cascadia where it has been extensively studied for over a quarter century a model in which the lvz is identified with hydrated pillow basalts and sheeted dikes of oceanic crustal layer 2 is consistent with available geological and geophysical data and reconciles previously conflicting interpretations in this model the upper oceanic crust is hydrated through intense circulation at the ridge and becomes overpressured upon subduction as a result of metamorphic dehydration reactions combined with an impermeable plate boundary above and a low porosity gabbroic layer 3 below the resulting seismic velocity contrast approaching 50 for s waves significantly overwhelms that of a weaker underlying oceanic moho at greater depths oceanic crust undergoes eclogitization in a top down sense leading to gradual disappearance of the lvz the large volume change accompanying eclogitization is postulated to rupture the plate boundary allowing fluids to penetrate the cooled forearc mantle wedge pervasive serpentinization and free fluids reduce velocities within the wedge thereby diminishing erasing or even inverting the seismic contrast associated with the moho of the overriding plate this model is tested against observations of lvzs and forearc mantle structure worldwide

thickness of the central and eastern european lithosphere as seen by s receiver functions summary s receiver functions obtained from seismograms of teleseismic events recorded at 78 european permanent broad band stations are used to estimate the thickness of the european lithosphere our results provide new independent information about the lithospheric thickness beneath the precambrian platform of eastern europe and the phanerozoic platform of central europe detailed high resolution images of the lithosphere asthenosphere boundary lab reveal indications for a typical continental lithosphere of about 100 km thickness beneath a majority of stations within central europe whereas in the vicinity of the trans european suture zone tesz the lithosphere thickens to about 130 km a relatively thin lithosphere of 80 km was found beneath the upper rhine graben region suggesting that the cenozoic extension affects the whole lithosphere no clear signal from the lab was detected beneath the alps and carpathians the lab sp phase might be disturbed by complicated structure due to ongoing collision subduction in these regions or the data are not yet sufficiently dense a relatively thicker lithosphere of about 120 km was found beneath the sw part of the bohemian massif that was formed during the variscan orogeny we found an lab depth of about 190 km near a single station located in the vrancea area eastern carpathians which is characterized by the occurrence of intermediate deep earthquakes beneath the stations located in the precambrian platform of eastern europe the lab deepens to approximately 200 km even though the converted phase from the lab is not as sharp as found beneath other stations located in central europe or even is missing

high resolution relocation and mechanism of aftershocks of the 2007 tocopilla chile earthquake summary we study the distribution of the aftershocks of tocopillamw 7 7 earthquake of 2007 november 14 in northern chile in detail this earthquake broke the lower part of the seismogenic zone at the southern end of the northern chile gap a region that had its last megathrust earthquake in 1877 the aftershocks of tocopilla occurred in several steps the first day they were located along the coast inside the co seismic rupture zone after the second day they extended ocean wards near the mejillones peninsula finally in december they concentrated in the south near the future rupture zone of the michilla intermediate depth earthquake of 2007 december 16 the aftershock sequence was recorded by the permanent ipoc integrated plate boundary observatory in chile network and the temporary task force network installed 2 weeks after the main event a total of 1238 events were identified and the seismic arrival times were directly read from seismograms initially we located these events using a single event procedure and then we relocated them using the double difference method and a crosscorrelation technique to measure time differences for clusters of aftershocks we tested a 1 d velocity model and a 2 d one that takes into account the presence of the subducted nazca plate relocation significantly reduced the width of the aftershock distribution in the inland area the plate interface imaged by the aftershocks is thinner than 2km the two velocity models give similar results for earthquakes under the coast and a larger difference for events closer to the trench the surface imaged by the aftershocks had a length of 160km it extends from 30 to 50km depth in the northern part of the rupture zone and between 5 and 55km depth near the mejillones peninsula we observed a change in the dip angle of the subduction interface from 18 to 24 at a depth of 30km we propose that this change in dip is closely associated with the upper limit of the rupture zone of the main event we also studied the focal mechanisms of the aftershocks most of them were thrust events like the mainshock as the aftershock activity was significantly reduced on 2007 december 13 an ml 6 1 event occurred offshore of the mejillones peninsula reactivating the seismicity three days later the michilla intraslab earthquake of mw 6 8 ruptured an almost vertical fault with slab push mechanism the aftershocks locations of this event define a planar zone about 11km in depth situated right bellow the subduction interface

high resolution image of the geometry and thickness of the subducting nazca lithosphere beneath northern chile 1 results obtained from s and p receiver functions produced a clear image of the top and bottom of the subducting nazca lithosphere beneath northern chile using data from the teleseismic events recorded at 15 permanent integrated plate boundary observatory chile ipoc stations we obtained new constraints on the geometry and thickness of the descending nazca lithosphere we observed the subducted crust of the nazca plate at depths ranging from 50 km beneath the coastal cordillera down to 110 km beneath the western cordillera we found significant along strike variations in the geometry of the nazca plate beneath northern chile on closer inspection it appears that the oceanic nazca plate is divided into two distinct segments as it descends beneath the continental south american plate the transition from the relatively steeper 23 and deeper slab to the north of 21 s to the flatter southern segment 19 is shown reasonably clearly by our data this feature could well be associated with variations in the curvature of the plate margin and the geometry of the chile trench which is mainly curved to the north of 21 s we have also mapped the continental moho of the south american plate at depths ranging between 60 and 70 km to the east of the longitudinal valley beneath the coastal cordillera this boundary becomes invisible probably due to the serpentinization of the forearc mantle wedge that reduces the velocity in the uppermost mantle the base of the subducted nazca plate was clearly identified as a sharp boundary in the results obtained from the p and s receiver functions the thickness of the subducted oceanic nazca plate which has an age of 50 my is estimated to be 50 km although this thickness is consistent with that predicted by thermal gradients the explanation of the sharpness of the lithosphere asthenosphere boundary may require another mechanism such as hydration or melting

absolute paleointensity at 1 27 ga from the mackenzie dyke swarm canada 1 paleointensity studies have been conducted on 6 mafic dykes from the 1270 ma mackenzie swarm in the slave province canada the mean direction of the characteristic magnetization coincides with results of an earlier study in which the primary origin of the magnetization was established on the basis of a contact test high unblocking temperatures magnetic mineralogy and grain size experiments suggest that the magnetization is dominated by pseudo single domain or single domain grains of magnetite paleointensity experiments were conducted with a specially designed oven using a revised version of the thellier coe method thirteen successful determinations of paleointensity were obtained for 4 dykes the paleofield estimates vary between 4 3 and 22 1 t yielding virtual dipole moments vdms between 1 3 0 2 and 4 5 0 9 1022 am2 these new results increase the number of low field determinations during the precambrian which largely dominate the database with an averaged field of 3 1 2 5 1022 am2 they also emphasize the importance of additional studies to understand the differences with the strong paleointensities obtained using new techniques

imaging lithospheric structure of the eastern himalayan syntaxis new insights from receiver function analysis 1 we employ the p and s receiver function technique to data from the 44 seismic stations deployed in the eastern himalayan syntaxis to investigate the crustal thickness the average poisson s ratio and the depth of the lithosphere asthenosphere boundary lab the observed crustal thickness exhibits an overall ne deepening trend varying from 55 to 75 km two anomalous areas lie in the west and east of the namche barwa syntaxis characterized by thinner and thicker crust respectively the average poisson s ratios within the study area are low in the north and moderate elsewhere with some high values in the south consistent with felsic and intermediate rocks forming the crust our migrated images reveal that 1 the lab of the tibetan plate exists at relatively shallow depths 110 km and exhibits a gap beneath the namche barwa syntaxis which may have formed by the delamination of mantle lithosphere due to local mantle upwelling and 2 the lab of the asian plate is observed at a depth of 180 km which implies that the asian plate has advanced southward to about 30 n under the lhasa terrane our results provide new insights into the understanding of continental subduction and lithospheric deformation of the eastern himalayan syntaxis

interdisciplinary characterisation and environmental imprints of mining and forestry in the upper durance valley france during the holocene the upper durance valley contains the largest silverelead mines of the french southern alps we investigated the characterisation and impact of these mining activities and associated forestry in the argentiere l argentiere la bessee and faravel freissinieres districts using a multidisciplinary approach that includes archaeological palynological geochemical anthracological and dendrochrono logical studies applied to mining remnants and a peat bog fangeas in the faravel district more specifically we studied the occurrence of lead contamination episodes the chronology of mining activities and their link to the evolution in woodland cover our chronology is based on mining archaeology radiocarbon dating of about thirty charcoal samples and the dendrochronological analysis of more than 170 specimens of exhumed waterlogged wood the anthracological analysis was established with more than 10 000 charcoals from firesetting the main geochemical approach was based on the analysis of lead and its stable isotopes in sediment fractions from a peat core the combination of palynological records and lead isotope imprints were used to characterise i a roman contamination episode unknown to archaeology ii the development of medieval mining activities and iii the reactivation of mining during the modern period and the industrial revolution medieval mining coincides with an extension of the high mountain agropastoral areas the in situ continuous human activities in the same industrial territory led to rational communal management of subalpine forests but also to their parcelling that reached its peak in the high middle ages mining during the modern period coincides with significant stress on woodland areas that also appears to be related to lumber and fuel timber production for shipyards french royal navy as well as the functioning of several military fortresses the mining reactivation during the 19th c matched public measures of forest protection that took shape in the afforestation programme of the uplands

low latitude glaciation in the palaeoproterozoic era one of the most fundamental enigmas of the earth s palaeoclimate concerns the temporal and spatial distributions of precambrian glaciations through four billion years of precambrian history unequivocally glacial deposits have been found only in the palaeoproterozoic and neoproterozoic record nonetheless some of these deposits are closely associated with tropical rather than just polar palaeolatitudinal indicators such as carbonate rocks red beds and evaporites these observations are quantitatively supported by palaeomagnetic results indicating a 5 latitude for neoproterozoic glaciogenic rocks in australia similarly reliable palaeolatitudes for the older palaeoproterozoic glaciogenic rocks have not yet been obtained as such deposits commonly suffer from poor preservation and secondary magnetic overprinting the archaean palaeoproterozoic transvaal supergroup on the kaapvaal craton in south africa is however exceptionally well preserved and is thus amenable to the palaeomagnetic determination of depositional palaeolatitudes within this supergroup the 2 2 billion year old ongeluk lavas are a regionally extensive largely undeformed and unmetamorphosed extrusive volcanic succession which conformably overlies glaciogenic deposits the makganyene diamictite here we report a palaeomagnetic estimate of 11 5 depositional latitude for the lavas and hence for the underlying contemporaneous glacial rocks the palaeoclimate enigma is thus deepened a largely ice free precambrian world was apparently punctuated by two long ice ages both yielding glacial deposits well within tropical latitudes

the 2007 m7 7 tocopilla northern chile earthquake sequence implications for along strike and downdip rupture segmentation and megathrust frictional behavior 1 in 2007 a m7 7 earthquake occurred near the town of tocopilla within the northern chile seismic gap main shock slip derived from coseismic surface deformation was confined to the depth range between 30 and 55 km we relocated 1100 events during six months before and one week after the main shock aftershock seismicity is first congruent to the main shock slip and then it spreads offshore west and northwest of mejillones peninsula mp waveform modeling for 38 aftershocks reveals source mechanisms that are in the majority similar to the main shock however a few events appear to occur in the upper plate some with extensional mechanisms juxtaposing the tocopilla aftershocks with those following the neighboring 1995 antofagasta earthquake produces a striking symmetry across an ew axis in the center of mp events seem to skirt around mp probably due to a shallower moho there we suggest that the seismogenic coupling zone in northern chile changes its frictional behavior in the downdip direction from unstable to mostly conditionally stable for both earthquake sequences aftershocks agglomerate in the conditionally stable region whereas maximum inter seismic slip deficit and co seismic slip occurs in the unstable region the boundary between the unstable and conditionally stable zones parallels the coastline we identify a similar segmentation for other earthquakes in chile and peru where the offshore segments break in great m 8 earthquakes and the onshore segments in smaller m 8 earthquakes using critical taper analysis we demonstrate a causal relationship between varying slip behavior on the interface and forearc wedge anatomy that can be attributed to spatial variations in the rate dependency of friction

ree and isotope sr s and pb geochemistry to constrain the genesis and timing of the f ba pb zn ores of the zaghouan district ne tunisia abstract the f ba pb zn ore deposits of the zaghouan district located in ne tunisia occur as open space fillings or stratabound orebodies hosted in jurassic cretaceous and tertiary layers the chondrite normalized rare earth element ree patterns may be split into three groups i normal marine patterns characterizing the wallrock carbonates ii light ree lree enriched slide shaped patterns with respect to heavy ree hree with small negative ce and eu anomalies characteristic of the early ore stages iii bell shaped ree patterns displaying lree depletion as well as weak negative ce and eu anomalies characterizing residual fluids of subsequent stages the 87 sr 86 sr ratios 0 707654 0 708127 8 show that the sr of the epigenetic carbonates dolomite calcite and ore minerals fluorite celestite are more radiogenic than those of the country triassic jurassic cretaceous lower miocene sedimentary rocks the uniformity of this ratio throughout the district provides evidence for the isotopic homogeneity and consequently the identity of the source of the mineralizing fluids this signature strongly suggests that the radiogenic sr is carried by upper paleozoic basinal fluids the 34 s values of barite associated to mineralizations are close to those of the triassic sea water 17 the 34 s values of sulfide minerals range from 13 6 to 11 4 suggesting two sulfur reduced end members bsr tsr with a dominant bsr process taking account of the homogeneity in the pb isotope composition of galenas 18 833 18 954 0 001 15 679 15 700 0 001 and 38 690 38 880 0 004 for the 206 pb 204 pb 207 pb 204 pb and 208 pb 204 pb ratios respectively a single upper crustal source for base metals is accepted the late paleozoic basement seems to be the more plausible source for f pb zn concentrated in the deposits the genesis of the zaghouan district ore deposits is considered as the result of the zaghouan fault reactivation during the late miocene period

two dimensional velocity models of the nazca plate subduction zone between 19 5 s and 25 s from wide angle seismic measurements during the cinca95 project the interpretation of seismic refraction wide angle reflection data from the 1995 crustal investigations off and on shore nazca central andes cinca95 project has resulted in the derivation of nine e w two dimensional 2 d velocity cross sections for the region between the peru chile trench and the coast between 19 5 s and 25 s with three of the cross sections extending a farther 100 200 km inland these sections define the major lithospheric structures of the upper south american plate and the lower nazca plate down to uppermost mantle depths of 30 60 km beneath this part of the present day forearc region in addition to showing the nazca plate subducting at an increasing angle of 9 25 down to 30 50 km depth near the coast these cross sections show a portion of the moho dipping eastward from 43 50 km near the coast to 55 64 km up to about 240 km inland owing to a gap of almost 50 km in the data coverage of the moho near the coast it is uncertain from this data set alone whether the moho defined east of the coast should be correlated with the lower oceanic nazca plate or the upper continental south american plate however a comparison with other seismological data suggests that the moho identified here east of the coast defines the base of the continental crust of the upper south american plate immediately behind the downgoing nazca plate thus the hypothesis of delouis et al 1996 that the zone of seismic coupling between the plates correlates with the region where continental crust is in contact with the plate boundary is supported the cross sections also show between 20 s and 22 5 s a boundary extending from upper crustal levels down to 25 30 km depth subparallel to and 5 10 km above the top surface of the subducting nazca plate the discussion focuses on either that the boundary is a fossil extensional structure or preferably that the wedge below the boundary represents gabbroic material from layer 3 of the oceanic crust tectonically underplated onto the lower part of the upper plate

seismic evidence for stratification in composition and anisotropic fabric within the thick lithosphere of kalahari craton 1 based on joint consideration of s receiver functions and surface wave anisotropy we present evidence for the existence of a thick and layered lithosphere beneath the kalahari craton our results show that frozen in anisotropy and compositional changes can generate sharp mid lithospheric discontinuities mld at depths of 85 and 150 200 km respectively we found that a 50 km thick anisotropic layer containing 3 s wave anisotropy and with a fast velocity axis different from that in the layer beneath can account for the first mld at about 85 km depth significant correlation between the depths of an apparent boundary separating the depleted and metasomatised lithosphere as inferred from chemical tomography and those of our second mld led us to characterize it as a compositional boundary most likely due to the modification of the cratonic mantle lithosphere by magma infiltration the deepening of this boundary from 150 to 200 km is spatially correlated with the surficial expression of the thabazimbi murchison lineament tml implying that the tml isolates the lithosphere of the limpopo terrane from that of the ancient kaapvaal terrane the largest velocity contrast 3 6 4 7 is observed at a boundary located at depths of 260 280 km beneath the archean domains and the older proterozoic belt this boundary most likely represents the lithosphere asthenosphere boundary which shallows to about 200 km beneath the younger proterozoic belt thus the kalahari lithosphere may have survived multiple episodes of intense magmatism and collisional rifting during the billions of years of its history which left their imprint in its internal layering

space geodetic estimation of the nazca south america euler vector abstract gps data from four sites in the nazca plate easter island galapagos robinson crusoe and san felix islands and from five sites in the stable core of the south american plate enabled us to estimate the euler vector of the nazca plate with respect to south america the observed velocities of easter island 6 6 cm yr at 102 3 galapagos 5 1 cm yr at 90 0 robinson crusoe 6 6 cm yr at 80 1 and san felix 6 0 cm yr at 82 1 are significantly slower than the global plate model nuvel 1a velocites for those four sites our estimated euler pole is located at 48 8 n 91 7 w with a rate of 0 59 m y the nuvel 1a and earlier space geodetic studies give rotation rates that are 20 faster

high resolution image of the oceanic moho in the subducting nazca plate from p s converted waves short period p s converted waves outline the oceanic moho of the descending nazca plate in northern chile at 24 s comparison with hypocentral locations obtained with temporary local seismic networks show that earthquakes inside the subducting oceanic lithosphere of the nazca plate are located predominantly in the oceanic crust at depths between 70 and 120 km and probably in the upper mantle at depths below 120 km the results suggest that dehydration embrittlement as mechanism for intermediate depth earthquakes is active both in the former oceanic crust and upper mantle of subducting lithosphere

multiresolution imaging of mantle reflectivity structure using ss and p p precursors summary knowledge of the mantle reflectivity structure is highly dependent on our ability to efficiently extract and properly interpret small seismic arrivals among the various data types and techniques long period ss pp precursors and high frequency receiver functions are routinely utilized to increase the confidence of the recovered mantle stratifications at distinct spatial scales however low resolution and a complex fresnel zone are glaring weaknesses of ss precursors while over reliance on receiver distribution is a formidable challenge for the analysis of converted waves from oceanic regions a promising high frequency alternative to receiver functions is ppprecursors which are capable of resolving mantle structures at verticalandlateralresolutionof 5and 200km respectively owingtotheirspectralcontent shallow angle of incidence and near symmetric fresnel zones this study presents a novel processing method for both ss or pp and ppprecursors based on deconvolution stacking radon transform and depth migration a suite of synthetic tests is performed to quantify the fidelity and stability of this method under different data conditions our multiresolution survey of the mantle at targeted areas near nazca south america subduction zone reveal both olivine and garnet related transitions at depths below 400km we attribute a depressed 660 to thermal variations whereas compositional variations atop the upper mantle transition zone are needed to explain the diminished or highly complex reflected scattered signals from the 410km discontinuity we also observe prominent ppreflections within the transition zone and the anomalous amplitudes near the plate boundary zone indicate a sharp 10km thick transition that likely resonates with the frequency content of ppprecursors the migration of ss precursors in this study shows no evidence of split 660 reflections but potential majorite ilmenite 590 640km and ilmenite perovskite transitions 740 750km areidentifiedbasedonsimilarlyprocessedhigh frequencyppprecursors additionalfindings of severely scattered energy in the lithosphere and distinct lower mantle reflections at 800km could be potentially important but require further verifications overall our improved imaging methods and the strong sensitivity of ppprecursors to the existence depth sharpness and strengthofreflective structuresoffersignificant futurepromisefortheunderstanding ofmantle mineralogy and dynamics

presence of a layered lithosphere beneath the zagros collision zone abstract the continental collision between the arabian and eurasian plates following the subduction of the neo tethys ocean beneath eurasia resulted in a complex deformation across the zagros mountains we used more than 700 s receiver functions obtained from 61 permanent stations to provide a detailed image of the lithospheric interaction between the eurasian and arabian plates our results suggest the existence of two different lithospheric blocks a 200 km thick lithosphere was observed beneath the zagros collision zone and most likely represents the arabian lithosphere which has been strongly deformed thickened and depleted based on our results shortening due to the collisional process is differently accommodated by the crust and mantle lithosphere of the zagros collision zone we localized the thick arabian lithosphere beneath the zagros sanandaj sirjan zone ssz and urumieh dokhtar magmatic assemblage udma whereas the crustal thickening 70 km seems to be concentrated just beneath the ssz furthermore we found a thin lithosphere of about 80 90 km that we interpret here as the iranian lithosphere beneath central iran and alborz additionally our results suggest the presence of remnants of the fossil neo tethys subduction at depths ranging between 80 and 150 km within the arabian lithosphere this dipping structure can be seen beneath the zagros ssz and udma and seems to disappear towards the northeast beneath central iran and alborz these findings may support the idea of a breakoff of the oceanic neo tethyan slab beneath central iran which results in an asthenospheric upwelling and thinning of the iranian lithosphere beneath central iran and alborz the boundary between the arabian and iranian plates seems to be located northeast of the udma in the northwest of zagros and southwest of the udma in the central zagros region

copper production at baratti populonia southern tuscany in the early etruscan period 9th 8th centuries bc abstract populonia is commonly regarded as one of the most important iron working centres in europe in the 1st millennium bc however multiple layers of copper slags have been recently discovered at the bottom of the slag deposit occurring along the shoreline of the underlying baratti gulf copper slag layers have been radiocarbon dated to the 9th 8th centuries bc i e to an early metallurgical stage predating the later large scale iron production the overall features of copper slags indicate a single step smelting process of copper rich ores containing variable amounts of zn and pb chemical and textural features of copper slags indicate an optimised high efficiency metallurgical process

impact of ancient metal smelting on arsenic pollution in the pecora river valley southern tuscany italy abstract a significant as anomaly has been reported in the literature for stream sediments and unlithified quaternary deposits of the pecora river valley in southern tuscany extending from the colline metallifere pyrite base metals district to the tyrrhenian sea the as anomaly spreads over several square kilometers around a core that exceeds 500 ppm several source contributions from natural to anthropogenic have been invoked to explain the observed as distribution in the pecora valley including the metal working industry which was active in this area particularly in etrusco roman times and in the middle ages in order to evaluate the contribution of ancient mining and metallurgical activities in the pecora valley to elevated as concentrations in the environment a detailed mineralogical and geochemical survey of metallurgical slags and smelted ore minerals was undertaken from six different sites through the pecora valley poggio butelli etrusco roman iron slags sata creek arialla marsiliana forra and cascata sites all medieval base metals slags the as content of etrusco roman slags is relatively low few tens of ppm whereas medieval slags show variable but higher amounts of base metals ag ranging from tens to tens of thousands ppm and as up to 267 ppm with average contents of about 40 ppm arsenic is mostly partitioned in sulfides disseminated through the glassy groundmass rather than in solid solution with the glassy matrix remnants of the ore used for base metal and ag smelting during the middle ages had the highest as contents up to about 1000 ppm the overall as concentration in the slags independent of age and type is generally lower than that found in stream sediments and quaternary alluvial deposit outcrops of the upper pecora valley textural observations indicate that exogenous alteration of slags is minor and mostly confined to superficial exposures of matte droplets or along fractures metal particles embedded within the glassy groundmass even those located a few micrometres from the surface appear to be unaltered it is suggested therefore that as and contaminant metal contributions to the pecora valley by archaeometallurgical slags was minor at least for slags from the last 2500 to 700 a bp similarly based on gross mass balance calculations airborne release of as due to smelting roasting operations in the past cannot account for the observed extent of the as anomaly in stream sediments and quaternary deposits although residues of unsmelted or partially smelted fe cu pb zn charges or fluxes for base metal metallurgy contain much greater amounts of as than smelting slags their relatively low abundance would limit the overall input of as into the pecora river valley on the other hand concurrent research seems to indicate a major geogenic contribution to the as anomaly in the pecora valley

large scale trench normal mantle flow beneath central south america abstract we investigate the anisotropic properties of the fore arc region of the central andean margin between 17 25 s by analyzing shear wave splitting from teleseismic and local earthquakes from the nazca slab with partly over ten years of recording time the data set is uniquely suited to address the long standing debate about the mantle flow field at the south american margin and in particular whether the flow field beneath the slab is parallel or perpendicular to the trench our measurements suggest two anisotropic layers located within the crust and mantle beneath the stations respectively the teleseismic measurements show a moderate change of fast polarizations from north to south along the trench ranging from parallel to subparallel to the absolute plate motion and are oriented mostly perpendicular to the trench shear wave splitting measurements from local earthquakes show fast polarizations roughly aligned trench parallel but exhibit short scale variations which are indicative of a relatively shallow origin comparisons between fast polarization directions from local earthquakes and the strike of the local fault systems yield a good agreement to infer the parameters of the lower anisotropic layer we employ an inversion of the teleseismic waveforms based on two layer models where the anisotropy of the upper crustal layer is constrained by the results from the local splitting the waveform inversion yields a mantle layer that is best characterized by a fast axis parallel to the absolute plate motion which is more or less perpendicular to the trench this orientation is likely caused by a combination of the fossil crystallographic preferred orientation of olivine within the slab and entrained mantle flow beneath the slab the anisotropy within the crust of the overriding continental plate is explained by the shape preferred orientation of micro cracks in relation to local fault zones which are oriented parallel to the overall strike of the andean range our results do not provide any evidence for a significant contribution of trench parallel mantle flow beneath the subducting slab

accurate hypocentre determination in the seismogenic zone of the subducting nazca plate in northern chile using a combined on offshore network the coupled plate interface of subduction zones commonly called the seismogenic zone has been recognized as the origin of fatal earthquakes a subset of the after shock series of the great antofagasta thrust type event 1995 july 30 mw 8 0 has been used to study the extent of the seismogenic zone in northern chile to achieve reliable and precise hypocentre locations we applied the concept of the minimum 1 d model which incorporates iterative simultaneous inversion of velocity and hypocentre parameters the minimum 1 d model is complemented by station corrections which are influenced by near surface velocity heterogeneity and by the individual station elevations by relocating mine blasts which were not included in the inversion we obtain absolute location errors of 1 km in epicentre and 2 km in focal depth a study of the resolution parameters ale and dspr documents the importance of offshore stations on location accuracy for offshore events based on precisely determined hypo centres we calculate a depth of 46 km for the lower limit of the seismogenic zone which is in good agreement with previous studies for this area for the upper limit we found a depth of 20 km our results of an aseismic zone between the upper limit of the seismogenic zone and the surface correlates with a detachment zone proposed by other studies the results are also in agreement with thermal studies for the antofagasta forearc region

filling the gap in a double seismic zone intraslab seismicity in northern chile abstract double seismic zones dszs of intermediate depth intraslab seismicity are observed in many subduction zones around the globe and have been related to dehydration reactions in the downgoing crust and mantle lithosphere these reactions occur at to first order constant temperatures which explains the observed linear arrangements of seismicity that appear to follow isotherms of thermal models intermediate depth seismicity in northern chile however exhibits a pattern of intraslab seismicity that substantially deviates from a classical dsz whereas two parallel seismicity planes are present in the updip part of the slab these abruptly change into a 25 30 km thick homogeneously seismogenic volume at a depth of 80 100 km seismicity rate and moment release significantly increase in this depth interval in order to understand which processes evoke this configuration and what distinguishes the northern chile subduction zone from more conventional subduction zone settings e g japan we performed a detailed seismological investigation of slab seismicity in northern chile using data from the ipoc permanent network we determined 600 moment tensors of intraslab earthquakes processed and evaluated location uncertainties for 8 years of high resolution earthquake hypocenter data and performed statistical analysis of the different seismicity populations we observe that earthquakes both in the highly active cluster and the dsz above exhibit consistently downdip extensive source mechanisms that align with the dip angle and direction of the slab this implies strong slab pull which is also evident from slab steepening outlined by hypocenters towards the downdip termination of the highly active cluster moreover events in the cluster show a very weak aftershock productivity and a high background event rate which leads to a temporal distribution of seismicity that is close to a purely random process we find that the position of the highly seismogenic volume spatially coincides with 1 the disappearance of the velocity contrast between oceanic crust and the underlying mantle in receiver function images 2 the transition from the cold nose i e the stagnant part to the hot part of the mantle wedge as evidenced by seismic attenuation images and 3 with an increase of the slab dip angle based on these different pieces of evidence we speculate that high tensile stresses and heat input from above could lead to a sudden burst of kinetically delayed metamorphic reactions there that then enables the observed increased seismicity rates since these reactions have overall a negative volume change that leads to slab densification and hence further increases slab pull the spatial pattern of seismicity we observe could result from a runaway type process which would explain its abrupt start and high moment release rates

imaging the colliding indian and asian lithospheric plates beneath tibet 1 a high resolution image of the base of the lithosphere from s to p converted seismic waves revealed the collision architecture of the indian and asian continental plates beneath the tibetan plateau the base of the indian lithosphere dips northward from a depth of 160 km beneath the himalayas to a depth of 220 km just south of the bangong suture the base of the asian lithosphere is nearly horizontal at the depth of 160 180 km from central to northern tibet there is a vertical gap of about 50 km between indian and asian lithospheres our observation of a well defined thick lithosphere throughout the entire plateau is not consistent with models of wholesale convective instability of a thickened mantle lithosphere which would predict a very thin asian lithosphere the hypothesized sequential southward subduction of tibetan and asian continental lithospheres leading to the growth of the tibetan plateau if correct cannot be occurring below 180 km depth our results along with available geological and geophysical data strongly support that the plateau is predominantly formed by a relatively coherent north dipping subducted indian continental lithosphere in the south which presently can be traced to the middle of the plateau and a south dipping subducted asian lithosphere in the north at a shallower depth

lithospheric structure beneath the northern central andean plateau from the joint inversion of ambient noise and earthquake generated surface waves the central andean plateau cap as defined by elevations in excess of 3 km extends over 1 800 km along the active south american cordilleran margin making it the second largest active orogenic plateau on earth the uplift history of this high plateau with an average elevation around 4 km above sea level remains uncertain as paleoelevation studies along the cap suggest a complex non uniform uplift history as part of the central andean uplift and the geodynamics of high topography caught project we image the s wave velocity structure of the crust and upper mantle using surface waves measured from ambient noise and teleseismic earthquakes to investigate the upper mantle component of plateau uplift we observe three main features in our s wave velocity model including 1 a positive velocity perturbation associated with the subducting nazca slab 2 a negative velocity perturbation below the subandean crust that we interpret as anisotropic brazilian cratonic lithosphere and 3 a high velocity feature in the mantle above the slab that extends along the length of the altiplano from the base of the moho to a depth of 120 km a strong spatial correlation exists between the lateral extent of this high velocity feature and the relatively lower elevations of the altiplano basin suggesting a potential relationship determining if this high velocity feature represents a small lithospheric root or foundering of orogenic lithosphere requires more integration of observations but either interpretation implies a strong geodynamic connection with the uppermost mantle and the current topography of the northern cap

along arc variation in water distribution in the uppermost mantle beneath kyushu japan as derived from receiver function analyses 1 we investigated the seismic velocity discontinuities in the uppermost mantle of kyushu a subduction zone in japan using receiver function analyses developed especially for discontinuities with high dipping angles we elucidated the regional variation of discontinuities as being the continental moho oceanic moho and upper boundary of the philippine sea slab from the geometry of these discontinuities and contrast in s wave velocities we discovered that the oceanic crust of the philippine sea slab has a low velocity and is possibly hydrated down to 70 km beneath south kyushu 80 90 km beneath central kyushu and less than 50 km beneath north kyushu the fore arc mantle wedge beneath central kyushu has a low velocity zone and possibly contains hydrated materials and free fluid but such a low velocity zone does not exist in the fore arc mantle wedge beneath north kyushu and south kyushu beneath south kyushu water dehydrated from the slab could move to the back arc side and cause arc volcanism beneath central kyushu water dehydrated from the slab could move to the fore arc side and cause a gap in volcanism and hydration of fore arc mantle materials beneath north kyushu the oceanic crust does not appear to convey water abundantly in the mantle wedge the low velocity hydrated fore arc mantle extends landward beneath the northern part of central kyushu and could produce volcanic rocks partially contaminated by slab derived fluid

a lead isotope database of copper ores from the southeastern alps a tool for the investigation of prehistoric copper metallurgy abstract the southeastern alps were an important source of copper metal in prehistory at least from the eneolithic and through the bronze age as documented by the abundant and substantial presence of smelting slags evidence of mining activity is scarce because of limited ad hoc investigation and because of the subsequent systematic erasing by post medieval exploitation moreover until recently the profusion of archaeometallurgical and archaeological investigations focusing on the prehistoric exploitation of northern alpine central european and balkan ore sources has somehow obscured the early role of the italian southern alps as a major copper producing area the recent advances in the systematic characterization of the copper ores in the southeastern alps including alto adige trentino veneto and nearby regions by lead isotope analysis supported by mineralogical and geochemical interpretation offer now the appropriate tools to re evaluate the extent of prehistoric mining and the local patterns of ore exploitation the developed database is a powerful tool to identify the metal derived from local production it is suggested that 1 based on the abundance and chronological distribution of smelting slags evidence two major periods of mining exploitation took place the first in the middle of the 3rd millennium bc and the second during the late bronze age and 2 based on the discrimination of copper sources and the available analyses most of the metal circulating in northern italy and in the greater po valley region was actually produced from southern alpine ores

thermal petrological controls on the location of earthquakes within subducting plates we find that in young and warm subducting plates earthquakes occur just below the moho in older plates earthquakes occur throughout the subducting oceanic crust as well as the subducting mantle we document this behavior in several subduction zones where there are independent constraints on earthquake locations and slab structure specifically for northern and southern japan alaska and cascadia the differences in earthquake depth relative to subducting crust may reflect large differences in temperature and thus locations of major dehydration reactions in colder slabs the crust passes through blueschist facies dehydration reactions while in cascadia and nankai the major dehydration reactions in crust may be due to zoisite and amphibole breakdown or associated melting the cold paths allow more mineral bound h2o to be retained within the crust at shallow depths eventually released upon dehydration over shorter time intervals than warm paths the cold path dehydration reactions also result in net positive volume changes of solid fluid with solid volume decreasing less than the volume of h2o produced on hot paths the net volume changes are negative with solid volumes decreasing more than the volume of h2o produced the difference in behavior could drive a net increase in pore pressure upon dehydration for the cold but not the hot crustal paths the difference in rate of release in h2o and difference in sign of net system volume change may promote seismogenesis in cold subduction zones but inhibit it in the crust of warm slabs within the mantle of the downgoing plate earthquakes mostly occur where serpentine is stable or breaks down in both settings

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

seismic imaging of a convergent continental margin and plateau in the central andes andean continental research project 1996 ancorp 96 1 a 400 km long seismic reflection profile andean continental research project 1996 ancorp 96 and integrated geophysical experiments wide angle seismology passive seismology gravity and magnetotelluric depth sounding across the central andes 21 s observed subduction of the nazca plate under the south american continent an east dipping reflector nazca reflector is linked to the down going oceanic crust and shows increasing downdip intensity before gradual breakdown below 80 km we interpret parts of the nazca reflector as a fluid trap located at the front of recent hydration and shearing of the mantle the fluids being supplied by dehydration of the oceanic plate patches of bright quebrada blanca bright spot to more diffuse reflectivity underlie the plateau domain at 15 30 km depth this reflectivity is associated with a low velocity zone p to s wave conversions the upper limits of high conductivity and high vp vs ratios and to the occurrence of neogene volcanic rocks at surface we interpret this feature as evidence of widespread partial melting of the plateau crust causing decoupling of the upper and lower crust during neogene shortening and plateau growth the imaging properties of the continental moho beneath the andes indicate a broad transitional character of the crust mantle boundary owing to active processes like hydration of mantle rocks in the cooler parts of the plate margin system magmatic underplating and intraplating under and into the lowermost crust mechanical instability at moho etc hence all first order features appear to be related to fluid assisted processes in a subduction setting

lithospheric thickness beneath the dabie shan central eastern china from s receiver functions summary p and s receiver functions obtained from a portable array of 34 broad band stations in east central china provide a detailed image of the crust mantle and lithosphere asthenosphere boundaries lab in the dabie shan and its adjacent areas clear s to p converted waves produced at the lab show a thin lithosphere beneath the whole study area based on our results the thickest lithosphere of 72 km is observed beneath the southern part of the area within the yangtze craton whereas beneath the north china platform the lithosphere is only 60 km thick s receiver functions also reveal in good agreement with p receiver functions a maximum depth of the moho beneath the dabie shan orogen at approximately 40 km furthermore we interpret the structural difference at 32 latitude as the probable location of

evolving force balance during incipient subduction nearly half of all active subduction zones initiated during the cenozoic all subduction zones associated with active back arc extension have initiated since the eocene hinting that back arc extension may be intimately associated with an interval several tens of myr following subduction initiation that such a large proportion of subduction zones are young indicates that subduction initiation is a continuous process in which the net resisting force associated with forming a new subduction zone can be overcome during the normal evolution of plates subduction initiation is known to have occurred in a variety of tectonic settings old fracture zones transform faults and extinct spreading centers and through polarity reversal behind active subduction zones although occurring within different tectonic settings four known subduction initiation events izu bonin mariana ibm along a fracture zone tonga kermadec along an extinct subduction boundary new hebrides within a back arc and puysegur fiordland along a spreading center were typified by rapid uplift within the forearc followed by sudden subsidence other constraints corroborate the compressive nature of ibm and tonga kermadec during initiation using an explicit finite element method within a two dimensional domain we explore the evolving force balance during initiation in which elastic flexure viscous flow plastic failure and heat transport are all considered in order to tie theory with observation known tectonic settings of subduction initiation are used as initial and boundary conditions we systematically explore incipient compression of a homogeneous plate a former spreading center and a fracture zone the force balance is typified by a rapid growth in resisting force as the plate begins bending reaching a maximum value dependent on plate thickness but typically ranging from 2 to 3 1012 n m for cases that become self sustaining this is followed by a drop in stress once a shear zone extends through the plate the formation of a throughgoing fault is associated with rapid uplift on the hanging wall and subsidence on the footwall cumulative convergence not the rate of convergence is the dominant control on the force balance viscous tractions influence the force balance only if the viscosity of the asthenosphere is 1020 pa s and then only after plate failure following plate failure buoyancy of the oceanic crust leads to a linear increase with crustal thickness in the work required to initiate subduction the total work done is also influenced by the rate of lithospheric failure a self sustaining subduction zone does not form from a homogeneous plate a ridge placed under compression localizes subduction initiation but the resisting ridge push force is not nearly as large as the force required to bend the subducting plate the large initial bending resistance can be entirely eliminated in ridge models explaining the propensity for new subduction zones to form through polarity reversals a fracture zone fz placed in compression leads to subduction initiation with rapid extension of the overriding plate a fz must be underthrust by the older plate for 100 150 km before a transition from forced to self sustaining states is reached in fz models the change in force during transition is reflected by a shift from forearc uplift to subsidence subduction initiation is followed by trench retreat and back arc extension moderate resisting forces associated with modeled subduction initiation are consistent with the observed youth of pacific subduction zones the models provide an explanation for the compressive state of western pacific margins before and during subduction initiation including ibm and tonga kermadec in the eocene and the association of active back arcs with young subduction zones on the basis of our dynamic models and the relative poles of rotation between pacific and australia during the eocene we predict that the northern segment of the tonga kermadec convergent margin would have initiated earlier with a progressive southern migration of the transition between forced and self sustaining states

broad depth range seismic imaging of the subducted nazca slab north chile abstract in this paper we present a compilation of modern seismic and seismological methods applied to image the subduction process in north chile south america we use data from active and passive seismic experiments that were acquired within the framework of the german collaborative research center sfb267 deformation processes in the andes the investigation area is located between 20 and 25 s and extends from the trench down to 100 km depth in the depth range between the sea bottom and 15 km we process an offshore seismic reflection profile using a recently developed velocity model independent stacking procedure we find that the upper part of the subducting oceanic lithosphere in this depth range is characterized by a horst and graben structure this structure supports an approximately 3 km thick coupling zone between the plates in the depth range between 15 and 45 km we analyse the spatial distribution of aftershocks of the antofagasta earthquake 1995 the aftershock hypocenters are concentrated in an approximately 3 km thick layer finally in the depth range between 45 and 100 km we apply kirchhoff prestack depth migration to the onshore ancorp profile a double reflection zone is observed between 45 and 60 km depth which may represent the upper and lower boundary of the subducted oceanic crust over the whole range down to more than 80 90 km depth we obtain an image of the subducting slab at that depth the hypocenters of local earthquakes deviate significantly in the direction perpendicular to the slab face from the reflective parts of the slab consequently our results yield a complete seismic image of the downgoing plate and the associated seismic coupling zone

slab low velocity layer in the eastern aleutian subduction zone summary local earthquakes in the vicinity of the alaskan peninsula s shumagin islands often produce arrivals between the main p and s arrivals not predicted by standard traveltime tables based on traveltime and polarization these anomalous arrivals appear to be from p to s conversions at the surface of the subducted pacific plate beneath the recording stations the p to s conversion occurs at the top of a low velocity layer which extends to at least 150 km depth and is 8 f 2 per cent slower than the overlying mantle the slab is 7 per cent faster than the mantle the low velocity layer contains the foci of the earthquakes in the upper plane of the double seismic zone and confines ps ray paths to lie within it these observations indicate that layered structures persist to positions well past the surface location of the volcanic front reactions forming high pressure minerals do not yield slab like velocities until beyond the point that subduction zone magma genesis occurs if the subducted oceanic crust forms the layer it is subducted essentially intact

shear wave reflectivity imaging of the nazca south america subduction zone stagnant slab in the mantle transition zone 1 in this study we utilize over 5000 ss waveforms to investigate the high resolution mantle reflectivity structure down to 1200 km beneath the south american convergent margin our results indicate that the dynamics of the nazca subduction are more complex than previously suggested the 410 and 660 km seismic discontinuities beneath the pacific ocean and amazonian shield exhibit limited lateral depth variations but their depths vary substantially in the vicinity of the subducting nazca plate the reflection amplitude of the 410 km discontinuity is greatly diminished in a 1300 km wide region in the back arc of the subducting plate which is likely associated with a compositional heterogeneity on top of the upper mantle transition zone the underlying 660 km discontinuity is strongly depressed showing localized depth and amplitude variations both within and to the east of the wadati benioff zone the width of this anomalous zone 1000 km far exceeds that of the high velocity slab structure and suggesting significant slab deformation within the transition zone the shape of the 660 km discontinuity and the presence of lower mantle reflectivity imply both stagnation and penetration are possible as the descending nazca slab impinges upon the base of the upper mantle citation contenti s y j gu a okeler and m d sacchi 2012 shear wave reflectivity imaging of the nazca south america subduction zone stagnant slab in the mantle transition zone geophys res lett 39 l02310 doi 10 1029 2011gl050064

episodic growth of the gondwana supercontinent from hafnium and oxygen isotopes in zircon it is thought that continental crust existed as early as 150 million years after planetary accretion but assessing the rates and processes of subsequent crustal growth requires linking the apparently contradictory information from the igneous and sedimentary rock records for example the striking global peaks in juvenile igneous activity 2 7 1 9 and 1 2 gyr ago imply rapid crustal generation in response to the emplacement of mantle super plumes rather than by the continuous process of subduction yet uncertainties persist over whether these age peaks are artefacts of selective preservation and over how to reconcile episodic crust formation with the smooth crustal evolution curves inferred from neodymium isotope variations of sedimentary rocks detrital zircons encapsulate a more representative record of igneous events than the exposed geology and their hafnium isotope ratios reflect the time since the source of the parental magmas separated from the mantle these model ages are only meaningful if the host magma lacked a mixed or sedimentary source component but the latter can be diagnosed by oxygen isotopes which are strongly fractionated by rock hydrosphere interactions here we report the first study that integrates hafnium and oxygen isotopes all measured in situ on the same precisely dated detrital zircon grains the data reveal that crust generation in part of gondwana was limited to major pulses at 1 9 and 3 3 gyr ago and that the zircons crystallized during repeated reworking of crust formed at these times the implication is that the mechanisms of crust formation differed from those of crustal differentiation in ancient orogenic belts

attenuation tomography in the western central andes a detailed insight into the structure of a magmatic arc high quality data from 1498 local earthquakes recorded by the pisco 94 proyecto de investigation sismologica de la cordillera occidental 1994 and ancorp 96 andean continental research project 1996 temporary seismological networks allowed the detailed determination of the three dimensional 3 d attenuation structure qp 1 beneath the recent magmatic arc in the western central andes 20 to 24 s assuming a frequency independent qp 1 in a frequency band between 1 and 30 hz whole path attenuation t was estimated from the amplitude spectra of the p waves using spectral ratios and a spectral inversion technique the damped least squares inversion tomography of the data reveals a complex attenuation structure crust and mantle of the forearc and subducting slab are generally characterized by low attenuation qp 1000 crust and mantle beneath the magmatic arc show elevated attenuation the strongest anomaly of extremely low qp is found in the crust between 22 and 23 s beneath the recent volcanic arc qp 100 n s variations can be observed the western flank of the crustal attenuation anomaly follows the curved course of the volcanic front north of 21 s the attenuation is less developed in the northern part of the study area the low qp zone penetrates in the forearc mantle down to the subducting slab in the south a deeper zone of high attenuation is resolved between 23 and 24 s directly above the subducting slab low qp in the mantle correlates with earthquake clusters the strong crustal attenuation is confined to the distribution of young ignimbrites and silicic volcanism and is interpreted as a thermally weakened zone with partial melts the attenuation pattern in the upper mantle might reflect the variable extent of the asthenosphere and maps variations of subduction related hydration processes in the mantle wedge from slab derived fluids

crustal thickness variations in the central andes we estimated the crustal thickness along an east west transect across the andes at lat 20 s and along a north south transect along the eastern edge of the altiplano from data recorded on two arrays of portable broadband seismic stations banjo and seda waveforms of deep regional events in the downgoing nazca slab and teleseismic earthquakes were processed to isolate the p to s converted phases from the moho in order to compute the crustal thickness we found crustal thickness variations of nearly 40 km across the andes maximum crustal thicknesses of 70 74 km under the western cordillera and the eastern cordillera thin to 32 38 km 200 km east of the andes in the chaco plain the central altiplano at 20 s has crustal thicknesses of 60 to 65 km the crust also appears to thicken from north 16 s 55 60 km to south 20 s 70 74 km along the eastern cordillera the subandean zone crust has intermediate thicknesses of 43 to 47 km crustal thickness predictions for the andes based on airy type isostatic behavior show remarkable overall correlation with observed crustal thickness in the regions of high elevation in contrast at the boundary between the eastern cordillera and the subandean zone and in the chaco plain the crust is thinner than predicted suggesting that the crust in these regions is supported in part by the flexural rigidity of a strong lithosphere with additional constraints we conclude that the observation of airy type isostasy is consistent with thickening associated with compressional shortening of a weak lithosphere squeezed between the stronger lithosphere of the subducting nazca plate and the cratonic lithosphere of the brazilian craton

evidence for a missing crustal root and a thin lithosphere beneath the central alborz by receiver function studies summary the alborz mountains build the northern part of the alpine himalayan orogen in western asia they are located south of the caspian sea and form an east west range across the north of iran this region is one of the most active tectonic areas as it undergoes extensive crustal deformation and shortening in the present work we used data from 11 permanent stations of the tehran telemetry seismic network to estimate the thickness of the crust and mantle lithosphere beneath the central alborz region by p and s receiver function methods results of both p and s receiver functions revealed a relatively large crustal thickness beneath this region 51 54 km which can be associated with the shortening process related to the orogenic belt no remarkable crustal thickening has been detected below the high topography of central alborz a thick crust 67 5 km is observed locally in the region beneath the damavand volcano which is possibly related to the magmatic addition at the base of the crust beneath the volcanic region the s receiver functions exhibit the existence of a seismic discontinuity in the upper mantle at a depth of 90 km which we interpreted as the base of the lithosphere the missing crustal root and the relatively thin lithosphere beneath the alborz may imply that sublithospheric mantle could be responsible for and support the elevation of the alborz

a change in the geodynamics of continental growth 3 billion years ago models for the growth of continental crust rely on knowing the balance between the generation of new crust and the reworking of old crust throughout earth s history the oxygen isotopic composition of zircons for which uranium lead and hafnium isotopic data provide age constraints is a key archive of crustal reworking we identified systematic variations in hafnium and oxygen isotopes in zircons of different ages that reveal the relative proportions of reworked crust and of new crust through time growth of continental crust appears to have been a continuous process albeit at variable rates a marked decrease in the rate of crustal growth at 3 billion years ago may be linked to the onset of subduction driven plate tectonics

anomalous crust of the bolivian altiplano central andes constraints from broadband regional seismic waveforms a one year deployment of broadband seismographs in the bolivian altiplano recorded numerous intermediate depth earthquakes at near regional distances we modeled the associated broadband waveforms of two earthquakes to estimate an average crustal structure for the altiplano the resulting model is characterized by an anomalously low mean p velocity of 6 0 km s a low poisson s ratio of 0 25 and a crustal thickness of 65 km the combination of the low mean velocity and low poisson s ratio can be explained only by a predominantly quartz rich felsic bulk composition this constraint precludes significant volumes of magmatic addition from the mantle contributing to the great thickness of the altiplano crust but is consistent with thickening by compressive shortening concentrated in a weak felsic layer

receiver function study of the cascadia megathrust evidence for localized serpentinization 1 we characterize the crustal and upper mantle structure of the cascadian fore arc using receiver function analysis of data from long running observatories located along the north american margin in oregon and washington we identify coherent sv polarized phases consistent with p s conversion from the top of the subducting juan de fuca plate throughout the area west of the volcanic arc in many instances we also find evidence for deeper features likely associated with the subducted oceanic crust at many locations sh polarized energy in receiver functions is associated with the surface of the subducting plate these sh polarized phases display significant directional variation and show clear polarity reversals consistent with the presence of seismic anisotropy previously a layer of serpentinized material within the megathrust has been proposed to explain an anisotropic low velocity high poisson s ratio layer identified by receiver function analysis at corvallis oregon global seismic network station cor our new results suggest that this feature is widespread in cascadia but the specific receiver function signature varies this anisotropic supraslab layer is supported by receiver function estimates beneath seismic stations which similar to cor are 100 km from the coast and 40 km above the slab surface however the orientation of rock fabric implied by the anisotropy varies greatly from station to station modeling with synthetic seismograms supports the existence of serpentinite beneath station gnw green mountain washington closer to the coast where the slab is shallower an anisotropic layer is likely beneath some stations but the evidence is less conclusive we propose that the anisotropic signature beneath the inland stations is related to serpentinization of the mantle wedge at greater depths best explained by the presence of the polymorph antigorite hypothetically suctioned upward by slab rollback a deformed sliver of serpentinite rich rock above the slab interface would likely display the observed large spatial variation in anisotropic symmetry axis within the layer consistent with our observations

insights into the evolution of the italian lithospheric structure from s receiver function analysis abstract to advance our understanding of the complex and episodic tectonic history of the central mediterranean we image the characteristics and structure of the convergent african and eurasian lithospheric plates using s receiver functions specifically we investigate the lithospheric structure in terms of seismic velocity discontinuities including the crust mantle boundary moho and the lithosphere asthenosphere boundary lab beneath italy the geometry and continuity of these structures are explored using teleseismic s to p converted phases applied extensively throughout this region for the first time the continental lithospheric thickness varies between 60 and 170 km with an average of 90 km and the pulse characteristics of the converted phases can be characterized by tectonic region and history beneath the northern apennines where subduction is still active we find evidence for a complex lithospheric structure where two different negative s velocity jumps are present at approximately 90 and 180 km depth the shallowest negative phases are interpreted as the base of the lithosphere but the second negative phases are much deeper than expected for the overriding plate lithosphere when compared to hypocenter locations and p wave tomography these deep converted phases correlate with the base of fast velocities perturbations in the upper mantle and regions with subcrustal earthquakes in contrast beneath the southern apennines the lithospheric structure appears less complex with only one significant negative arrival in the receiver functions and where no subcrustal earthquakes are present these pulses are generally deeper 110 km than those beneath the northern apennines and are broader in shape indicating a less sharp velocity contrast farther south along the italian peninsula the structure becomes complex again with two sharp negative amplitude phases at depth these deep converted phases that originate in the uppermost mantle beneath the calabrian arc and sicily are also co located with deep 35 km earthquakes these observations suggest that these deepest negative phases correspond to the base of the actively subducting lithosphere beneath the northern apennines and calabria and where these phases are absent in the receiver functions there is no subducting slab seismic imaging of the lithosphere and uppermost mantle using s to p conversions provides another geophysical dataset to aid in interpretation of the complex segmented slab structure beneath italy

the nature of orogenic crust in the central andes 1 the central andes 16 22 s are part of an active continental margin mountain belt and the result of shortening of the weak western edge of south america between the strong lithospheres of the subducting nazca plate and the underthrusting brazilian shield we have combined receiver function and surface wave dispersion results from the banjo seda project with other geophysical studies to characterize the nature of the continental crust and mantle lithospheric structure the major results are as follows 1 the crust supporting the high elevations is thick and has a felsic to intermediate bulk composition 2 the relatively strong brazilian lithosphere is underthrusting as far west 65 5 w as the high elevations of the western part of the eastern cordillera ec but does not underthrust the entire altiplano 3 the subcrustal lithosphere is delaminating piecemeal under the altiplano ec boundary but is not completely removed beneath the central altiplano the altiplano crust is characterized by a brittle upper crust decoupled from a very weak lower crust that is dominated by ductile deformation leading to lower crustal flow and flat topography in contrast in the high relief inland sloping regions of the ec and sub andean zone the upper crust is still strongly coupled across the basal thrust of the fold thrust belt to the underthrusting brazilian shield lithosphere subcrustal shortening between the altiplano and brazilian lithosphere appears to be accommodated by delamination near the altiplano ec boundary our study suggests that orogenic reworking may be an important part of the felsification of continental crust

stress distribution and geometry of the subducting nazca plate in northern chile using teleseismically recorded earthquakes summary the stress distribution along the subducting nazca plate in northern chile is analysed using focal mechanism solutions obtained from the inversion of long period p sv and sh waveforms of 15 earthquakes mb 5 5 and from 212 events with reported focal mechanisms which occurred between 1962 and 1993 a joint hypocentral determination was carried out to control the depth of 261 events mb 5 0 recorded at teleseismic distances a change from tensional to compressional stress field along the upper part of the subducting slab is associated with the maximum depth extent of the coupled zone this change occurs in northern chile at 200 250 km from the trench at depths of 60 10 km this depth is larger than the maximum depth observed for the thrusting interplate events 40 10 km probably meaning that at depths of between 40 and 60 km large low dip angle thrust events do not nucleate seismic slip however probably extends down to 40 km in depth the shallow dip angle up to 60 km in depth of the wadati benioff zone does not show variations along the strike of the trench however a gradual southward flattening of the slab is observed at distances greater than 200 250 km from the trench this change observed from about 21 s could be associated with a younger and probably more buoyant lithosphere than that observed to the north of this latitude there are two gaps located between the three main clusters of seismicity these gaps are clearly not related to detachments in the descending litosphere the first cluster is located in and beneath the seismogenic interplate contact and is characterized by reverse and thrust faulting events over a scarce tensional activity in the second cluster the compressional seismicity is scarce for teleseismic events and is located beneath the normal faulting events the third cluster corresponds to tensional events therefore these gaps in seismicity could be associated with alternating changes from compressional to tensional stress field in the subducting slab

structural features of the subducting slab beneath the kii peninsula central japan seismic evidence of slab segmentation dehydration and anisotropy 1 seismic evidence of lithospheric slab segmentation dehydration and anisotropic rocks beneath the kii peninsula central japan is detected with receiver function rf analyses by stacking radial and transverse rfs with moveout perturbations we estimate the downdip azimuth dip angle and depth of the slab moho beneath each seismic station comparing the downdip azimuth to the slab convergence azimuth we can classify the slab into three segments the segments correspond to both the fault zones of past megathrust earthquakes and to the spatial distribution of intraslab seismicity the slab moho lies just above the intraslab seismicity in the southwestern kii peninsula slab moho lies deeper at the eastern part of the peninsula at the bottom of seismicity this suggests that earthquakes occur only within the slab mantle under the southern kii peninsula the dip angle of slab moho becomes steeper at 40 to 50 km depth perhaps caused by dehydration transformation of basalt to eclogite at some stations located in the western part of the kii peninsula rf estimated dip angle of the interface is much steeper than that estimated by spatial trends in seismicity since the local seismicity distribution reflects well the larger scale geometry of the slab the rf estimated dip angle may imply anisotropic wave speeds within strongly sheared and or hydrated media around the slab moho these detailed features of the slab may be one key in distinguishing the source regions of megathrust earthquakes in the nankai region

an inverted continental moho and serpentinization of the forearc mantle volatiles that are transported by subducting lithospheric plates to depths greater than 100 km are thought to induce partial melting in the overlying mantle wedge resulting in arc magmatism and the addition of significant quantities of material to the overlying lithosphere asthenospheric flow and upwelling within the wedge produce increased lithospheric temperatures in this back arc region but the forearc mantle in the corner of the wedge is thought to be significantly cooler here we explore the structure of the mantle wedge in the southern cascadia subduction zone using scattered teleseismic waves recorded on a dense portable array of broadband seismometers we find very low shear wave velocities in the cold forearc mantle indicated by the exceptional occurrence of an inverted continental moho which reverts to normal polarity seaward of the cascade arc this observation provides compelling evidence for a highly hydrated and serpentinized forearc region consistent with thermal and petrological models of the forearc mantle wedge this serpentinized material is thought to have low strength and may therefore control the down dip rupture limit of great thrust earthquakes as well as the nature of large scale flow in the mantle wedge

the shear wave velocity structure in the upper mantle beneath eurasia summary we develop an approach that allows us to invert for the mantle velocity structure within a finely parametrized region as a perturbation with respect to a low resolution global tomographic model we implement this technique to investigate the upper mantle structure beneath eurasia and present a new model of shear wave velocity parametrized laterally using spherical splines with 2 9 spacing in eurasia and 11 5 spacing elsewhere the model is obtained from a combined data set of surface wave phase velocities long period waveforms and body wave traveltimes we identify many features as narrow as few hundred kilometres in diameter such as subducting slabs in eastern eurasia and slow velocity anomalies beneath tectonically active regions in contrast to regional studies in which these features have been identified our model encompasses the structure of the entire eurasian continent furthermore including mantleand body wave waveforms helped us constrain structures at depths larger than 250 km which are poorly resolved in earlier models we find that up to 9 per cent faster than average anomalies within the uppermost 200 km of the mantle beneath cratons and some orogenic regions are separated by a sharp gradient zone from deeper 1t o 2 per cent anomalies we speculate that this gradient zone may represent a boundary separating the lithosphere from the continental root which might be compositionally distinct from the overlying lithosphere and remain stable either due to its compositional buoyancy or due to higher viscosity compared with the suboceanic mantle our regional model of anisotropy is not significantly different from the global one

crustal thickness and mantle wedge structure from receiver functions in the chilean maule region at 35 s a temporary passive seismic network of 21 broad band stations was deployed in central chile between 35 s to 36 s the network recorded data prior to the magnitude mw 8 8 2010 maule earthquake at a latitude of the major slip and surface deformation the experiment was conducted to survey crustal and mantle structures and to assess the state of hydration of the mantle wedge we present results of a teleseismic p receiver function study supporting a continental moho at approx 38 km depth phase conversion at this boundary could be observed continuously from the intersection of the subducting slab with the continental moho towards the andes the character of receiver functions indicated little evidence for infiltration of water from the subducting plate into the overlying mantle wedge suggesting that only a small amount of water is released from the subducting plate aftershocks of the maule earthquake and post seismic slip reached depths of 50 km and hence slip spread down dip of the continental moho in the post seismic phase co seismic rupture however occurred updip of the continental moho spare aftershock seismicity is observed at the intersection of the continental moho with the subducting slab

complex patterns of fluid and melt transport in the central andean subduction zone revealed by attenuation tomography abstract we present a high resolution 3 d model of p wave attenuation q p 1 for the central andean subduction zone data from 1500 mostly intermediate depth 60 250 km earthquakes recorded at three temporary seismic networks covering the forearc arc and backarc around 23 s were used for tomographic inversion the forearc is characterised by uniformly high q p values indicating low temperature rocks in accordance with low surface heat flow values prominent low q p anomalies are found beneath the magmatic arc and the backarc in the crust and mantle continuous regions of low q p connect earthquake clusters at 100 km and 200 km depth with zones of active volcanism in the arc and backarc fluids fluxed from the subducted oceanic lithosphere into the overlying mantle wedge where they induce melting explain our observations we propose that low q p regions indicate source and ascent pathways of metamorphic fluids and partial melts ascent of fluids and melts as imaged by seismic q p are not vertical as is often implicitely assumed instead sources of fluids are located at different depth levels and ascent paths are complex and exhibit significant variation within the study area the largest quaternary backarc volcano cerro tuzgle is fed by mantle melts which are imaged as a plume of low q p material that reaches to the strong earthquake cluster at 200 km depth

scattering of trapped p and s waves in the hydrated subducting crust of the philippine sea plate at shallow depths beneath the kanto region japan