lateral velocity resolution from three dimensional seismic refraction data summary a method for estimating the lateral velocity resolution from 3 d seismic refraction traveltimes is presented the method is non linear in that synthetic data calculated from checkerboard models are inverted using an iterative tomographic approach incorporating smoothness constraints and updated ray paths at each iteration two applications are presented 1 real data from the faeroe basin experiment using the ray coverage and noise level of approximately 50 000 picks and 2 ideal data from the same experiment corresponding to 100 per cent data recovery equivalent to about three times as many ray paths the refraction data constrain the sedimentary and basement structure to approximately 12 km depth a comparison of the results from the two data sets provides a qualitative check on the resolution method illustrates the pitfalls of using ray coverage as an indicator of resolution and demonstrates the potential resolution of currently feasible 3 d refraction experiments 32 checkerboard models consisting of alternating positive and negative velocity anomalies superimposed on the preferred final model of the basin were tested each with a diverent cell size position or orientation the problem of ray bending associated with the 5 per cent checkerboard anomalies is shown to be small using models with four cell sizes allowed for resolution estimates from 3 to 15 km this range being imposed by the frequency content of the data and the areal dimensions of the study area 44 26 km an operator centred on each model node measures the local semblance between the known and recovered checkerboard models a determination of the smallest well resolved cell size at each model node is then made using a 0 7 semblance threshold to provide a spatially dependent lateral resolution estimate resolution for the real data set varies from an average of just over 3 km at 2 km depth to 10 km at 10 km depth where there is a local maximum in ray coverage due to a concentration of ray bottoming points for the ideal data set the average lateral resolution drops ov roughly linearly from better than 3 km at 2 km depth to 6 3 km at 10 km depth the largest diverence between the resolution associated with the ideal and real data occurs at 7 2 km depth where it averages 4 6 km for the former and 10 8 km for the latter due to the near absence of ray bottoming points between 7 and 9 km the results show that the final model for the faeroe basin obtained by regularized inversion with smoothness constraints is consistent with the resolution estimates in the sense that the model does not contain structure with a wavelength smaller than the resolution estimate at any point the results for the ideal data show that good quality 3 d refraction experiments can provide a lateral resolution of no worse than the receiver spacing shot spacing for land data throughout most of the sampled volume

geochronology of early to middle paleozoic tectonic development in the southwest newfoundland gander zone southwest newfoundland s gander zone was once considered part of the collided eastern margin of an early paleozoic iapetus ocean distinguished from the rest of the gander zone by the extensive exposure of a polydeformed amphibolite to greenschist facies sequence considered devonian in age overlying presumed precambrian basement it was later found that truly devonian strata were confined to the cape ray fault system the local northwestern boundary of the zone u pb zircon isotopic ages of 452 13 51 ma and 458 22 29 ma on metavolcanic rocks in the upper part of the sequence southeast of the fault system indicate that ordovician and possibly older rocks underlie the gander zone itself a temporal framework for the three main stages of regional deformation corresponding to pre metamorphic recumbent folding d 1 and transpression d 2 and d 3 is presented using u pb and 40 ar 39 ar dates of granitoid and metamorphic rocks d 1 is bracketed as late o

the complex 3 d transition from continental crust to backarc magmatism and exhumed mantle in the central tyrrhenian basin geophysical data from the medoc experiment across the northern tyrrhenian backarc basin has mapped a failed rift during backarc extension of cratonic variscan lithosphere in contrast data across the central tyrrhenian have revealed the presence of magmatic accretion followed by mantle exhumation after continental breakup here we analyse the medoc transect e f which extends from sardinia to the campania margin at 40 5 n to define the distribution of geological domains in the transition from the complex central tyrrhenian to the extended continental crust of the northern tyrrhenian the crust and uppermost mantle structure along this 400 km long transect have been investigated based on wide angle seismic data gravity modelling and multichannel seismic reflection imaging the p wave tomographic model together with a p wave velocity derived density model and the multichannel seismic images reveal seven different domains along this transect in contrast to the simpler structure to the south and north the stretched continental crust under sardinia margin abuts the magmatic crust of cornaglia terrace where accretion likely occurred during backarc extension eastwards around secchi seamount a second segment of thinned continental crust 7 8 km is observed two short segments of magmatically modified continental crust are separated by the 5 km wide segment of the vavilov basin possibly made of exhumed mantle rocks the eastern segment of the 40 5 n transect e f is characterized by continental crust extending from mainland italy towards the campania margin ground truthing and prior geophysical information obtained north and south of transect e f was integrated in this study to map the spatial distribution of basement domains in the central tyrrhenian basin the northward transition of crustal domains depicts a complex 3 d structure represented by abrupt spatial changes of magmatic and non magmatic crustal domains these observations imply rapid variations of magmatic activity difficult to reconcile with current models of extension of continental lithosphere essentially 2 d over long distances

crustal characteristic variation in the central yamato basin japan sea back arc basin deduced from seismic survey results abstract the crustal structure of the yamato bank the central yamato basin and the continental shelf in the southern japan sea back arc basin is obtained based on a seismic survey using ocean bottom seismographs and seismic shot to elucidate the back arc basin formation processes the central yamato basin can be divided into three domains based on the crustal structure the deep basin the seamount and the transition domains in the deep basin domain the crust without the sedimentary layer is about 12 13 km thick very few units have p wave velocity of 5 4 6 0 km s which corresponds to the continental upper crust in the seamount and transition domains the crust without the sedimentary layer is about 12 16 km thick the p wave velocities of the upper and lower crusts differs among the deep basin the seamount and the transition domains these results indicate that the central yamato basin displays crustal variability in different domains the crust of the deep basin domain is oceanic in nature and suggests advanced back arc basin development the seamount domain might have been affected by volcanic activity after basin opening in the transition domain the crust comprises mixed characters of continental and oceanic crust this crustal variation might represent the influence of different processes in the central yamato basin suggesting that crustal development was influenced not only by back arc opening processes but also by later volcanic activity in the yamato bank and continental shelf the upper crust has thickness of about 17 18 km and p wave velocities of 3 3 4 1 to 6 6 km s the yamato bank and the continental shelf suggest a continental crustal character

seismological features of island arc crust as inferred from recent seismic expeditions in japan abstract crustal studies within the japanese islands have provided important constraints on the physical properties and deformation styles of the island arc crust the upper crust in the japanese islands has a significant heterogeneity characterized by large velocity variation 5 5 6 1 km s and high seismic attenuation qp 100 400 for 5 15 hz the lateral velocity change sometimes occurs at major tectonic lines in many cases of recent refraction wide angle reflection profiles a middle crust with a velocity of 6 2 6 5 km s is found in a depth range of 5 15 km most shallow microearthquakes are concentrated in the upper middle crust the velocity in the lower crust is estimated to be 6 6 7 0 km s the lower crust often involves a highly reflective zone with less seismicity indicating its ductile rheology the uppermost mantle is characterized by a low pn velocity of 7 5 7 9 km s several observations on pmp phase indicate that the moho is not a sharp boundary with a distinct velocity contrast but forms a transition zone from the upper mantle to the lower crust recent seismic reflection experiments revealed ongoing crustal deformations within the japanese islands a clear image of crustal delamination obtained for an arc arc collision zone in central hokkaido provides an important key for the evolution process from island arc to more felsic continental crust in northern honshu a major fault system with listric geometry which was formed by miocene back arc spreading was successfully mapped down to 12 15 km

formation of the japan sea basin reassessment from ar ar ages and nd sr isotopic data of basement basalts of the japan sea and adjacent regions abstract many studies have examined the japan sea basalts recovered during ocean drilling program odp leg127 128 of these the 40ar 39ar dating undertaken is important in constraining the timing of the formation of the japan sea however the implications of their results do not appear to be fully appreciated by the geological community in this paper i reassess the 40ar 39ar age data of the basalts with reference to nd sr isotopic data the 40ar 39ar dating was performed on basalts somewhat enriched in large ion lithophile elements and recovered from odp sites 794 795 and the lower part of 797 yielding the plateau ages of 21 2 17 7 ma these basalts show the nd sr isotopic signature of a moderately depleted mantle source end 0 6 6 9 in contrast the basalts from the upper part of site 797 have yet to be dated due to their low k content although their nd isotopic compositions are similar to that of morb end 8 4 10 4 by analogy to the secular nd sr isotopic trends reported for sikhote alin and northeast japan the age of the upper basalts at site 797 may be inferred to be younger than the lower basalts probably around 16 ma the nd sr isotopic compositions of the japan sea basalts have been interpreted in terms of eastward asthenospheric flow as have the lavas of the sikhote alin and northeastern japan the timing of volcanic activity in the japan sea region i e from 21 2 to 14 86 ma is consistent with the timing of rotational crustal movements inferred from paleomagnetic studies of the japanese islands i e 14 8 4 2 ma for southwest japan and 16 5 14 4 ma for northeast japan

rapid rotation of southwest japan palaeomagnetism and k ar ages of miocene volcanic rocks of southwest japan summary miocene volcanic rocks have been sampled from the san in district in the central part of southwest japan reliable palaeomagnetic direction and k ar dating data have been obtained from 28 localities a clockwise deflection in declination d 38 5 is observed in the kawai formation having an age of 16 1 1 4 ma whereas a northerly declination is observed in formations younger than 14 2 0 6 ma these data indicate that southwest japan had ceased rotational motion by about 14 ma when compared with the amount of rotation estimated from cretaceous palaeomagnetic data it is apparent that more than 80 per cent of the overall clockwise rotation occurred between about 16 and 14 ma we conclude that the angular velocity of southwest japan about a rotation pole at 129 e 34 n reached 20 myr 1 at about 15 ma the corresponding rate of linear motion of the eastern part of southwest japan was 21 cm yr 1

strontium isotopes as tracers of ecosystem processes theory and methods abstract the strontium sr isotope method can be a powerful tool in studies of chemical weathering and soil genesis cation provenance and mobility and the chronostratigraphic correlation of marine sediments it is a sensitive geochemical tracer applicable to large scale ecosystem studies as well as to centimeter scaled examination of cation mobility within a soil profile the 87sr 86sr ratios of natural materials reflect the sources of strontium available during their formation isotopically distinct inputs from precipitation dryfall soil parent material and surface or groundwater allow determination of the relative proportions of those materials entering or leaving an ecosystem the isotopic compositions of labile soil exchange complex and soil solution strontium and sr in vegetation reflect the sources of cations available to plants strontium isotopes can be used to track the biogeochemical cycling of nutrient cations such as calcium the extent of cation contributions from in situ weathering and external additions to soil from dust and rain can also be resolved with this method in this paper we review the geochemistry and isotopic systematics of strontium and discuss the use of this method as a tracer of earth surface processes

last stage of the japan sea back arc opening deduced from the seismic velocity structure using wide angle data the japan sea is one of the most well studied back arc basins in the northwestern pacific the seismic crustal model however has been inadequate to elucidate the detailed opening model of the japan sea in 2002 to clarify the late stage of the formation style of the japan sea opening a seismic experiment using 35 ocean bottom seismographs obss an air gun array and a multichannel hydrophone streamer was undertaken in the areas from the southwestern yamato basin the oki ridge and the southwestern oki trough to the coast of the southwestern japan island arc the crusts beneath the southwestern yamato basin and the oki ridge are estimated as having approximately 13 km and 19 5 km respectively the upper and lower crusts of the southwestern yamato basin are approximately 3 2 km and 8 km thick respectively those of the oki ridge are approximately 8 2 km and 10 5 km thick respectively the upper crust of the oki ridge thickens more steeply than that of the southwestern yamato basin however the lower crust thickens more gently the crustal structure of the southwestern yamato basin shows the extended continental crust accompanied with the opening of the japan sea a remarkable structural characteristic the upper crust being thinner than the lower crust caused by listric or complicated normal faults developed in the upper crust of the southwestern yamato basin this deformed upper crust is a common structural characteristic in the southern japan sea which includes the yamato basin the southern yamato basin including the southwestern yamato basin has the thinnest upper and lower crusts in the japan sea for that reason it is suggested that the southern yamato basin had the strongest deformation by a back arc opening and that the period of the opening in the southern yamato basin had been longest in the southern japan sea the formation process of the southern yamato basin is inferred to have two stages rifting and extension of continental crust separating the northeastern and southwestern japan island arcs from the asian continent and further the extension affected by the rotation of the southwestern japan island arc

kerguelen plateau a volcanic passive margin fragment the 21 25 km thick crust of the southern kerguelen plateau consists of three units 1 a 2 3 km thick sedimentary cover 2 a 3 6 km thick basaltic layer with velocities ranging from 4 5 to 6 2 km s and 3 a 15 17 km thick lower crust with velocities from 6 6 to 6 9 km s including a 3 6 km thick transition zone located at the base of the crust the low velocity transition zone has an average velocity of 6 7 km s and exhibits several internal wide angle reflections the velocity depth structure of the crust differs significantly from that of other hotspot related oceanic plateaus and suggests that the southern kerguelen plateau may be a fragment of a volcanic passive margin composed of a thinned continental crust overlain by basalt flows

deep structure of the southern kerguelen plateau southern indian ocean from ocean bottom seismometer wide angle seismic data wide angle seismic data collected during the kerguelen ocean bottom seismometer experiment provide the first images of the deep structure of the southern kerguelen plateau and support a new interpretation of the origin of the plateau velocity models based on travel time inversions and reflectivity synthetic seismograms show a 22 km thick crust composed of 1 6 km of sedimentary cover 5 3 km of upper crust 11 0 km of lower crust and a 4 to 6 km thick reflective zone immediately above moho velocities in the upper crust from 3 8 4 5 km s at top to 6 0 6 5 km s at bottom are consistent with the basaltic nature of this layer the top of which was sampled during the ocean drilling program velocities in the lower crust increase continuously from 6 60 km s at the top to 6 90 km s at 19 5 km depth the reflective zone at the base of the crust identified by wide angle reflections is observed only along the nnw sse direction it consists of alternating high and low velocity layers with an average velocity of 6 70 km s in the nnw sse direction and 6 90 km s in the perpendicular direction strong azimuthal anisotropy is also observed in the upper mantle with velocities of 8 60 and 8 00 km s in the nnw sse and e w directions respectively the absence of high velocities at the base of the crust that characterizes many large volume mafic provinces the reflective lower crust and anisotropy in upper mantle suggest that the southern kerguelen plateau represents a stretched continental fragment overlain by basaltic flows isolated from the antarctic margin during the early opening of the indian ocean

crustal structure of the carpathian pannonian region from ambient noise tomography we use ambient noise tomography to investigate the crust and uppermost mantle structure beneath the carpathian pannonian region of central europe over 7500 rayleigh wave empirical green s functions are derived from interstation cross correlations of vertical component ambient seismic noise recordings 2005 2011 using a temporary network of 54 stations deployed during the south carpathian project 2009 2011 56 temporary stations deployed in the carpathian basins project 2005 2007 and 100 permanent and regional broad band stations rayleigh wave group velocity dispersion curves 4 40 s are determined using the multiple filter analysis technique group velocity maps are computed on a grid of 0 2 0 2 from a non linear 2 d tomographic inversion using the subspace method we then inverted the group velocity maps for the 3 d shear wave velocity structure of the crust and uppermost mantle beneath the region our shear wave velocity model provides a uniquely complete and relatively high resolution view of the crustal structure in the carpathian pannonian region which in general is validated by comparison with previous studies using other methods to probe the crustal structure at shallow depths 30 km are relatively fast presumably related to shallowing of the moho consequent on the extensional history of the pannonian region

age and tectonic evolution of neoproterozoic ductile shear zones in the southern granulite terrain of india with implications for gondwana studies 1 the high grade rocks of the southern granulite terrain sgt of peninsular india are bounded to the north by the archean dharwar craton another high grade terrane the mesoproterozoic eastern ghats occurs to the northeast of the sgt the tectonic relationship between these crustal domains is complex we present new geochronological and structural data that indicate a continuation of the dharwar craton into the southern granulite terrain as far south as a newly identified neoproterozoic shear zone here named the karur kamban painavu trichur shear zone kkptsz south of the kkptsz mesoproterozoic dates of the sgt are similar to those recorded in the eastern ghats and the two domains may have been conterminous thirty three new u pb th single zircon and monazite dates of samples from six structural transects across the regional shear zones indicate that the sgt has experienced at least seven thermo tectonic events at 2 5 ga 2 0 ga 1 6 ga 1 0 ga 800 ma 600 ma and 550 ma and two distinct episodes of metasomatism charnockitization between 2 50 2 53 and between 0 55 0 53 ga deformation along a number of major shear zones in the sgt is neoproterozoic to earliest paleozoic in age with an early phase d2 concentrated between 700 800 ma and a later phase d3 between 550 and 600 ma major charnockitization 530 550 ma post dates d3 and is in turn overprinted by granitization retrogression and uplift between 525 and 480 ma the kkptsz active between 560 and 570 ma is either a terrane boundary or a tectonized decollement between cover and archean basement rocks represented by predominantly paragneisses to the south and orthogneisses to the north respectively other regional neoproterozoic shear zones do not appear to separate allochthonous terranes as previously suggested on the basis of nd model ages and rb sr biotite whole rock dates the neoproterozoic cambrian tectonothermal history of the sgt and eastern ghats is similar to that recorded in parts of madagascar east africa and antarctica and is used to reconstruct parts of central gondwana here named the deccan continent with more robust confidence

deep crustal structure of the indian shield from joint inversion of p wave receiver functions and rayleigh wave group velocities implications for precambrian crustal evolution 1 the s wave velocity structure of the crust and uppermost mantle of the indian shield has been investigated by jointly inverting p wave receiver functions and rayleigh wave group velocities at 38 broadband stations in the subcontinent the indian shield is an amalgamation of several terranes of archean and proterozoic age that were partly flooded by deccan trap volcanism during cenozoic times and that make up a natural laboratory for assessing models of precambrian crustal evolution our results reveal significant variations in crustal thickness and deep crustal velocities 45 50 km thick under the archean west dharwar craton and southern granulite terrane with lower crustal velocities around 4 1 km s 32 35 km thick beneath the archean east dharwar and bundelkhand cratons with lower crustal velocities around 3 8 3 9 km s 50 65 km thick under the proterozoic bhandara craton with lower crustal velocities around 4 2 4 3 km s and 55 km thick under the proterozoic aravalli delhi belt with lower crustal velocities around 4 2 km s s velocities in the 4 1 4 3 km s range in the deep crust can be attributed to mafic lithologies suggesting there has been no secular change in the precambrian evolution of the south indian shield moreover pervasive mafic dike swarming throughout the indian shield suggests that the layer of mafic cumulates is 2 5 1 6 ga old and that it delaminated from some archean terranes our interpretation is that mafic underplating of the terranes making up the indian shield occurred in proterozoic times and that a refractory root developed under the archean terranes after the proterozoic event

crust and shallow mantle structure of south india by inverting interpolated receiver function with surface wave dispersion abstract the 3 d shear wave velocity structure beneath south india crust and shallow mantle up to depth 130 km have been determined from the joint inversion of surface wave dispersion and interpolated receiver function rf the p wave rf interpolation helps to equalize the lateral sampling of the rf and the surface wave dispersion which mainly simplifies the rfs one of the major difficulties due to back azimuthal variations of receiver functions is suppressed by this scheme of spatial interpolation we combine the interpolated receiver functions with rayleigh wave dispersions to estimate the shear wave velocity structure beneath the region this study reveals the significant variations in crustal thickness and nature of intra crustal layering of southern india shield the crustal thickness of south india varies from 40 km in deccan volcanic province dvp 38 km in east dharwar craton edc 44 km in west dharwar craton northern block wdc n 50 km in west dharwar craton southern block wdc s 48 km in southern granulite terrain sgt and 38 km in kerala khondalite belt kkb the average upper crustal thickness is 5 km and that of middle crust is 15 km the lower crust shows significant variations in terms of thickness varies from 8 to 32 km as well as the high velocity mafic layer the shear wave velocity variations in the model correlate strongly with the different geological variations along different tectonic blocks of southern india the dvp surprisingly does not have any distinct signature of volcanism in the underlying crust and shallow mantle the shallow mantle part of entire south india has similar characteristics except for edc where the velocity is higher beyond 85 km depth

large igneous province on the us atlantic margin and implications for magmatism during continental breakup rifted continental margins commonly include sections of igneous rock more than twice as thick as normal oceanic crust explanations for this voluminous magmatic accretion during rifting include plume models1 3 which require a deep seated thermal or chemical anomaly in upwelling mantle and non plume models4 7 which call on broad shallow thermal anomalies and or rapid upwelling of mantle through the melting zone new seismic models from two transects across the continent ocean transition on the us atlantic margin8 10 confirm the presence of a 20 25 km thick igneous section here we argue that the similarity of the crustal structure on these and two previous transects spanning 1 000 km of the margin and the association of thick igneous crust with the east coast magnetic anomaly11 imply that the thick igneous section extends along the entire margin and may have a volume of as much as 3 2 106 km3 the distribution of volcanic and plutonic rocks details of the seismic structure and lack of independent evidence for a hotspot are difficult to reconcile with plume models and suggest that non plume processes created the thick igneous crust

evolution of the sea of japan back arc and some unsolved issues abstract the sea of japan back arc system provides an exceptional opportunity to study a virtually intact continent ocean back arc system in which recent tectonic inversion has exposed entire sequences of back arc structure on land moreover japan s dense seismic geodetic monitoring networks deployed country wide as well as moveable pools of ocean bottom seismometers provide a rich data set through which to investigate deep back arc structure earlier investigations have produced an initial understanding of back arc opening timing structural evolution temporal spatial patterns of magmatic activity many questions remain among them the mechanism of back arc opening pull apart or trench rollback the dynamics of interacting plates location of the philippine sea plate with time the origin of the anomalously thick japan sea ocean crust and possible influences of far field forces india asia collision given existing high resolution geophysical data sets and extensive on land exposures of back arc sequences and structures the sea of japan back arc is a promising context in which to address both local and more universal questions of how back arc systems evolve here we review the tectonic setting and geological evolution of the sea of japan based our own and others work and briefly discuss outstanding questions that invite further investigation

stratigraphic review of eocene to oligocene successions along the eastern japan sea implication for early opening of the japan sea abstract stratigraphic review of the on land successions exposed on the back arc side of japan and sakhalin combined with supplemental radiometric dating reveals the existence of late eocene or oligocene marine sediments along the eastern coast of the japan sea this finding implies that the origin of the japan sea dates back to the oligocene or earlier time much older than ever supposed the host eocene to oligocene subaerial to shallow water successions distribute immediate west of the volcanic front of that time and contain volcanic rocks the chemistry of which is characteristic of continental rift or volcanic arc they are associated with parallel dike swarms or normal faults and could be infillings of an incipient back arc rift collective evidence suggests that the rift was rapidly subsided and spread during the following early to middle miocene time however a back arc wide unconformity was produced during a short period from the latest oligocene to the earliest miocene and was succeeded by extensive bimodal volcanism with rapid migration of the volcanic front to the fore arc side these short term events could have been induced by upwelling or inflow of hot asthenosphere beneath the arc rift system followed by rapid subsidence and spreading of the japan sea

confirming sharp bending of the pacific plate in the northern japan trench subduction zone by applying a traveltime mapping method abstract many destructive earthquakes have occurred in the northern japan trench region owing to the subduction of the pacific plate two plate bending points where the dip of the subducting plate changes markedly have been found and correlations between the bending points and the updip limits of large earthquake rupture zones have been previously found in the regions off aomori 40 n and off miyagi 38 n in this study we investigate whether sharp plate bending is also present in the region off iwate 39 n located approximately halfway between the off aomori and off miyagi regions we analysed the existing off iwate wide angle seismic experimental data by applying a new method in which observed reflection phases are directly projected onto the depth distance domain from the time distance domain as a result we found a plate bending point in the region off iwate this is the first time such a point has been found there distances between the three bending points and the trench axis are almost uniform and sharp plate bending is a common feature in the northern japan trench subduction zone the bending axis which is defined by connecting bending points approximately coincides with the updip limit of the rupture zones of large interplate earthquakes seismic swarm activities have been observed inside the subducting oceanic crust along the bending axis and we infer that some of these activities are caused by bending of the subducting plate in addition p wave velocities of the upper plate vary across the bending axis implying that the position of the bending axis is closely related to physical properties of the upper plate and to seismic activities

structure and growth of the izu bonin mariana arc crust 1 seismic constraint on crust and mantle structure of the mariana arc back arc system 1 a high resolution seismic velocity model is presented for the crust and upper mantle of the mariana arc back arc system mabs based on active source seismic profiling the major characteristics are 1 slow mantle velocity of 8 km s 1 in the uppermost mantle especially and deep reflectors under the mariana arc ma and the west mariana ridge wmr 2 a deep reflector in the upper mantle beneath the relative thick crust of the mariana trough mt axis 3 distribution of lower velocity lower crusts 6 7 6 9 km s 1 beneath the volcanic front and adjacent to the mt and 4 high velocity lower crust 7 2 7 4 km s 1 beneath the boundary regions between the ma and mt and between the wmr and the parece vela basin pvb adding to structural characteristics of crust and upper mantle beneath the mabs of the characteristics described above characteristic 1 suggests that the origins of the slow mantle velocity and the deep reflectors be explained by transfer of the lower crustal residues to the upper mantle across the moho considering that the wmr is extinct arc currently on the other hand characteristic 2 suggests that the origin of deep reflectors beneath the mt axis might be lower velocity materials due to the diffractive signals with strong amplitudes characteristic 3 suggests that the lower velocity lower crust advanced crustal growth and characteristic 4 suggests that the high velocity lower crust beneath arc back arc transition zone is composed of mafic ultramafic materials created by extensive partial melting of mantle peridotites or last stage of the arc magmatism rather than serpentinized peridotite

deep crustal structure off akita eastern margin of the japan sea deduced from ocean bottom seismographic measurements abstract a seismic exploration was carried out to detect the heterogeneity of the seismic wavespeed structure associated with the plate convergence at the eastern margin of the japan sea two airgun obs ocean bottom seismograph profiles were shot off akita japan where a seismic gap seems to exist but the location of the plate boundary has not been confirmed one of the profiles was 60 km long trending nne ssw named line obs 9 at the northeastern end of the yamato basin and the other was 170 km long trending wnw ese line nt95 1 parallel to the direction of the supposed plate convergence the crustal structure beneath line obs 9 consists of six layers the uppermost layer is sediment three layers are identified beneath the top sedimentary layer and their p wavespeeds are estimated to be 3 3 3 4 km s 5 1 5 4 km s and 5 8 6 3 km s which corresponds to the upper crust underneath these layers a layer with p wavespeed ranges of 6 3 7 2 km s comprises the middle and lower crust the depth of moho is inferred to be 19 km these wavespeed values are comparable with those of the present japanese island arc while the thickness of the crust is one half of that of the japanese arc the crustal model supports the scenario that the yamato basin is formed by extension of the island arc the crustal model for line nt95 1 shows a transition from the extended island arc structure beneath the yamato basin to a thicker crust similar to the japanese arc p wavespeed heterogeneity related to the plate boundary is not detected however a significant change in the structural model along the profile is found around the region where the largest change in the seafloor topography exists in that region the wavespeeds in the middle crust have lower values than those of the neighboring area and the moho begins to deepen towards the japanese island arc from comparison with the relationship between p wavespeed structure and aftershock distribution of the 1993 hokkaido nansei oki earthquake southwest of hokkaido m jma 7 8 the region may correspond to a possible fault position of the pending earthquake

constraints on the evolution of the japan sea based on 40ar 39ar ages and sr isotopic ratios for volcanic rocks of the yamato seamount chain in the japan sea 40 ar 39 ar and sr isotope analyses were performed on basalts and andesites dredged from the yamato seamount chain in the japan sea the 40 ar 39 ar plateau ages range from about 11 to 17 ma though most samples show ages between 10 and 14 ma the seamounts seem to have formed within a period of a few million years although some of them might have formed earlier based on the present results together with previously reported radiometric age data it is thought that the yamato basin formed during some period prior to 17 ma and probably later than around 25 ma taking into account the radiometric age data on rocks from the japan basin it is conjectured that the opening of the japan sea might have started almost at this time or a little earlier the observed 87 sr 86 sr ratios range from 0 70357 to 0 70388 suggesting incorporation of some time integrated components enriched in incompatible elements such as continental crustal materials this may indicate that in the japan sea area at least the yamato basin had not developed enough to show the characteristics of typical n type morb source materials without being affected by pre existing continental crustal materials

seismic constraints of the formation process on the back arc basin in the southeastern japan sea to clarify the formation process of the back arc basin in the japan sea which is located next to the northwestern pacific a seismic survey using ocean bottom seismographs and an air gun array was undertaken in areas from the northern yamato basin to the coast of the northeastern japan island arc off awa shima island the crust beneath the northern yamato basin off awa shima island is approximately 16 km thick the upper and lower crusts are respectively about 5 km thick with a steep velocity gradient and about 10 km thick with a gentle velocity gradient in the basin there are very few units with p wave velocity of 5 4 6 0 km s corresponding to the continental upper crust the crustal structure of the northern yamato basin has characteristics of thicker oceanic crust the high velocity lowermost crust in the northern yamato basin with 7 2 7 4 km s might show melt formed by a slightly high mantle temperature during the opening of the basin however the crust beneath the areas from the sado ridge to the coast which is approximately 25 26 km thick is slightly thinner than that of the continental crust and island arc crust the crustal structure beneath this area is inferred to be a rifted continental and or a rifted island arc crust

oceanic crust in the japan basin of the japan sea by the 1990 japan ussr expedition in september of 1990 a seismic refraction and reflection survey was conducted in the japan basin in the northeastern part of the japan sea as a part of the japan ussr joint expedition twenty six ocean bottom seismometers obss were deployed on two 200 km long lines explosives and an airgun were fired as controlled seismic sources on the two mutually perpendicular lines airguns were also used as the source for the multi channel reflection profiles the crustal structure deduced is that of a typical oceanic basin the crustal thickness is about 8 5 km including 2 km of sediment we obtained a more detailed crustal structure than that obtained previously from the dense airgun shooting data the crustal structure is well resolved to show layer 1a layer 1b layer 2a 2b 2c layer 3 and the mantle the crust basically consists of laterally homogeneous layers but the moho deepens slightly westward

possible magmatic underplating beneath the west coast of india and adjoining dharwar craton imprint from archean crustal evolution to breakup of india and madagascar abstract the shear wave velocity of the crust along a 660 km profile from the west to the east coast of south india is mapped through the joint inversion of receiver functions and rayleigh wave group velocity the profile consisting of 38 broadband seismic stations covers the archean dharwar craton proterozoic cuddapah basin and rifted margin and escarpment the moho is mapped at a depth of 40 km beneath the mid archean western dharwar craton wdc cuddapah basin cb and the west and east coasts formed through the rifting process this is in contrast with a thin 35 km crust beneath the late archean eastern dharwar craton edc along the profile the average thickness of the upper middle and lower crust is 4 km 12 4 km and 24 4 km respectively above the moho we observe a high velocity layer hvl vs 4 km s of variable thickness increasing from 3 1 km beneath the edc to 11 3 km beneath the wdc and the cb and 18 2 km beneath the west coast of india the seismic wave velocity in this layer is greater than typical oceanic lower crust we interpret the high velocity layer as a signature of magmatic underplating due to past tectonic processes its significant thinning beneath the edc may be attributed to crustal delamination or relamination at 2 5 ga these results demonstrate the dual signature of the archean dharwar crust the change in the geochemical character of the crust possibly occurred at the end of archean when komatiite volcanism ceased the unusually thick hvl beneath the west coast of india and the adjoining region may represent underplated material formed due to india madagascar rifting which is supported by the presence of seaward dipping reflectors and a 85 90 ma mafic dyke in the adjoining island

crustal structure of the southern okinawa trough symmetrical rifting submarine volcano and potential mantle accretion in the continental back arc basin back arc basins are a primary target to understand lithospheric evolution in extension associated with plate subduction most of the currently active back arc basins formed in intraoceanic settings and host well developed spreading centers where seafloor spreading has occurred however rift structure at its initial stage a key to understand how the continental lithosphere starts to break in a magma rich back arc setting is poorly documented here we present seismological evidence for structure of the southern okinawa trough an active rift zone behind the ryukyu subduction zone we find that the southern okinawa trough exhibits an almost symmetric rift system across the rift axis yaeyama rift and that the sedimentary layers are highly cut by inward dipping normal faults the rift structure also accompanies a narrow 2 7 km wide on axis intrusion resulted from passive upwelling of magma on the other hand an active submarine volcano is located 10 km away from the rift axis the p wave velocity vp model derived from seismic refraction data suggests that the crust has been significantly thinned from the original 25 km thick arc crust and the thinnest part with 12 km thickness occurs directly beneath the rift axis the velocity model also reveals that there exists a thick layer with vp of 6 5 7 2 km s at lower crustal levels and may indicate that mantle materials accreted at the bottom of the crust during the crustal stretching the abrupt crustal thinning and the velocity depth profile suggest that the southern okinawa trough is at a transitional stage from continental rifting to seafloor spreading

structure and evolution of the cauvery shear zone system southern granulite terrain india evidence from deep seismic and other geophysical studies abstract the southern granulite terrain with exposed archean lower crustal rocks is studied using various geophysical tools the crustal structure derived from seismic reflection and refraction wide angle reflection studies is used to understand the tectonic evolution of the region deep seismic reflection section along the kolattur palani segment shows an oppositely dipping reflection fabric near the moyar bhavani shear zone which is interpreted as a signature of collision between the dharwar craton and another crustal block in the south the thickened crust due to collision was delaminated during the orogenic collapse and modified the central part covering the cauvery shear zone system located between the moyar bhavani and karur oddanchatram shear zones the delaminated lower crust is altered by magmatic underplating as evidenced by the high velocity layer just above the moho the velocity model of the region indicates crustal thickening at the boundary of the dharwar craton and moyar bhavani shear zone and thinning further south back scattered seismic wave field with negative moveout and the moho offset indicate the spatial location and strike slip nature of the shear zones present study suggests that the late archean collision and suturing of the dharwar craton with the southern crustal block at the moyar bhavani shear zone may be responsible for the evolution of late archean granulites late neoproterozoic rifting is observed along the paleo fault zones the seismic studies constrained by gravity magnetic and magnetotelluric data suggest that the moyar bhavani and karur oddanchatram shear zones of the cauvery shear zone system mark terrane boundaries suture zones

trans hudson orogen of north america and himalaya karakoram tibetan orogen of asia structural and thermal characteristics of the lower and upper plates 1 the trans hudson orogen tho of north america and the himalaya karakoram tibetan orogen hkto of asia preserve a paleoproterozoic and cenozoic record respectively of continent continent collision that is notably similar in scale duration and character in tho the tectonothermal evolution of the lower plate involves 1 early thin skinned thrusting and barrovian metamorphism 2 out of sequence thrusting and high t metamorphism and 3 fluid localized reequilibration anatexis and leucogranite formation the crustal evolution of the indian lower plate in hkto involves 1 early subduction of continental crust to ultrahigh pressure uhp eclogite depths 2 regional barrovian metamorphism and 3 widespread high t metamorphism anatexis and leucogranite formation the shallow depths of the high t metamorphism in hkto are consistent with early to mid miocene ductile flow of an indian lower plate midcrustal channel from beneath the southern tibetan plateau to the greater himalaya melt weakening of the lower plate in tho is not observed at a similar scale probably due to the paucity of pelitic lithologies tectonothermal events in the upper plate of both orogens include precollisional accretion of crustal blocks emplacement of andean type plutonic suites and high t metamorphism syncollisional to postcollisional events include emplacement of garnet biotite muscovite leucogranites anatectic granites and sporadic metamorphism up to 90 myr following the onset of collision in tho comparing the type and duration of tectonothermal events for tho and hkto supports the notion of tectonic uniformitarianism for at least the later half of dated earth history and highlights the complementary nature of the rock record in an older exhumed orogen compared to one undergoing present day orogenesis

neogene deformation of the back arc shelf of southwest japan and its impact on the palaeoenvironments of the japan sea abstract the evolutionary process of the back arc basins of southwest japan is presented on the basis of geological and geophysical data sets surface geology and borehole information demonstrate rapid subsidence throughout the back arc margin in the early miocene as a result of continental rifting uplift in the middle miocene and extensive truncation in the late miocene on the back arc margin tend to become more intense eastward and westward respectively palaeomagnetic studies show that the middle miocene uplift is linked to collision of the izu bonin arc with the eastern part of southwest japan and confined deformation of the island arc offshore seismic data illustrate that the late miocene unconformity event was caused by intra arc folding under n s compression coeval resumption of subduction of the philippine sea plate is the probable cause of this episode seismic profiles also indicate that the folds converge into the tsushima strait the crustal contraction brought about closure of the tsushima strait at the western end of southwest japan blocking the warm tsushima current and so impacting strongly on the neogene palaeoenvironments of the japan sea

automated detection extraction and measurement of regional surface waves our goal is to develop and test an effective method to detect identify extract and quantify surface wave signals for weak events observed at regional stations we describe an automated surface wave detector and extractor designed to work on weak surface wave signals across eurasia at intermediate periods 8 s 40 s the method is based on phase matched filters defined by the rayleigh wave group travel time predictions from the broadband group velocity maps presented by ritzwoller and levshin 1998 and ritzwoller et al 1998 and proceeds in three steps signal compression signal extraction or cleaning and measurement first the dispersed surface wave signals are compressed in time by applying an anti dispersion or phase matched filter defined from the group velocity maps we refer to this as the compressed signal second the surface wave is then extracted by filtering noise temporally isolated from the time compressed signal this filtered signal is then redispersed by applying the inverse of the phase matched filter finally we adaptively estimate spectral amplitude as well as group and phase velocity on the filtered signal the method is naturally used as a detector by allowing origin time to slide along the time axis we describe preliminary results of the application of this method to a set of nuclear explosions and earthquakes that occurred on or near the chinese lop nor test site from 1992 through 1996 and one explosion on the indian rajasthan test site that occurred in may of 1998

crustal velocity structure across the southern korean peninsula from seismic refraction survey 1 the first crustal scale controlled source seismic refraction experiment in the southern korean peninsula kcrust2002 was carried out along a 300 km long profile across this peninsula in december 2002 iterative processing and modeling produced a laterally varying layered crustal velocity model the crust is thickest 34 km below the okcheon fold belt in the middle of the transect and thinnest 28 km at the eastern end where the cretaceous gyeongsang basin is characterized by 5 km of low velocity material that constitutes the upper crust the p velocities in upper and lower crust range from 5 4 to 6 0 km s and from 6 4 to 6 7 km s respectively the average crustal poisson s ratio is found to be 0 25 0 27 vp vs 1 73 1 78 along the profile a mid crustal velocity discontinuity is recognized in the northwestern part of the transect the underlying mantle has velocities in the range of 7 9 8 1 km s

quantitative strontium isotope models for weathering pedogenesis and biogeochemical cycling abstract isotopes of strontium sr are a useful tracer for weathering atmospheric fluxes cation biocycling and pedogenesis we present basic models for application of strontium isotopes to the soil vegetation atmosphere system the mathematical formulations fall into the general categories of 1 steady state models in which isotopic ratios remain constant over the time scale of interest and 2 time dependent models in which isotope ratios change through time in the steady state models fluxes of sr and other elements to the system are constant steady state models can be used to infer short term weathering rates from river and stream isotope compositions to determine fluxes to a single or multiple layer soil exchange solution system and to quantify nutrient fluxes to vegetation time dependent models involve a change in isotopic ratio from some initial value to a new value over the time period of interest in some cases the change may represent a shift from one steady state situation to a new one after a shift in one or more of the fluxes feeding the system examples of applications of time dependent models include identifying the dominant cation sources to an evolving soil exchange solution system and calculating weathering rates by measuring the isotopic compositions of primary soil minerals we use time dependent models to explain differences in the isotopic ratios of labile and carbonate sr from arid sites in hawaii with a basalt parent material isotopic signature and new mexico with an atmospheric isotopic signature these models suggest that the difference is due to a combination of low atmospheric strontium fluxes and high weathering rates in the hawaiian profile compared to the new mexico calcrete profile

extensional structure in northern honshu arc as inferred from seismic refraction wide angle reflection profiling a recent extensive seismic wide angle experiment revealed a new image of crustal and uppermantle structure across northern honshu arc japan the western part of the arc recorded the crustal deformation by the miocene back arc spreading of the sea of japan the crust is composed of highly deformed tertiary sedimentary layers a relatively low velocity 5 75 5 9 km s crystalline basement and a 15 km thick lower crust with a velocity of 6 6 7 0 km s clear westward crustal thinning from 32 to 27 km represents the extensional deformation by the backarc spreading the crust attains the maximum thickness 32 35km east of the backbone range for which the magmatic intrusion underplating since 10 15 ma is a predominant factor the eastern part of the arc has a less deformed upper crust and a reflective middle lower crust probably remaining a stable block since the time of the backarc spreading

p wave velocity structure of the margin of the southeastern tsushima basin in the japan sea using ocean bottom seismometers and airguns abstract the tsushima basin is located in the southwestern japan sea which is a back arc basin in the northwestern pacific although some geophysical surveys had been conducted to investigate the formation process of the tsushima basin it remains unclear in 2000 to clarify the formation process of the tsushima basin the seismic velocity structure survey with ocean bottom seismometers and airguns was carried out at the southeastern tsushima basin and its margin which are presumed to be the transition zone of the crustal structure of the southwestern japan island arc the crustal thickness under the southeastern tsushima basin is about 17 km including a 5 km thick sedimentary layer and 20 km including a 1 5 km thick sedimentary layer under its margin the whole crustal thickness and thickness of the upper part of the crust increase towards the southwestern japan island arc on the other hand thickness of the lower part of the crust seems more uniform than that of the upper part the crust in the southeastern tsushima basin has about 6 km s layer with the large velocity gradient shallow structures of the continental bank show that the accumulation of the sediments started from lower miocene in the southeastern tsushima basin the crustal structure in southeastern tsushima basin is not the oceanic crust which is formed ocean floor spreading or affected by mantle plume but the rifted extended island arc crust because magnitudes of the whole crustal and the upper part of the crustal thickening are larger than that of the lower part of the crustal thickening towards the southwestern japan island arc in the margin of the southeastern tsushima basin high velocity material does not exist in the lowermost crust for that reason the margin is inferred to be a non volcanic rifted margin the asymmetric structure in the both margins of the southeastern and korean peninsula of the tsushima basin indicates that the formation process of the tsushima basin may be simple shear style rather than pure shear style

broadband ambient noise surface wave tomography across the united states 1 this study presents surface wave dispersion maps across the contiguous united states determined using seismic ambient noise two years of ambient noise data are used from march 2003 through february 2005 observed at 203 broadband seismic stations in the us southern canada and northern mexico cross correlations are computed between all station pairs to produce empirical green functions at most azimuths across the us coherent rayleigh wave signals exist in the empirical green functions implying that ambient noise in the frequency band of this study 5 100 s period is sufficiently isotropically distributed in azimuth to yield largely unbiased dispersion measurements rayleigh and love wave group and phase velocity curves are measured together with associated uncertainties determined from the temporal variability of the measurements a sufficient number of measurements 2000 is obtained between 8 and 25 s period for love waves and 8 and 70 s period for rayleigh waves to produce tomographic dispersion maps both phase and group velocity maps are presented in these period bands resolution is estimated to be better than 100 km across much of the us from 8 40 s period for rayleigh waves and 8 20 s period for love waves which is unprecedented in a study at this spatial scale at longer and shorter periods resolution degrades as the number of coherent signals diminishes the dispersion maps agree well with each other and with known geological and tectonic features and in addition provide new information about structures in the crust and uppermost mantle beneath much of the us

three dimensional lithospheric electrical structure of southern granulite terrain india and its tectonic implications 1 the crustal as well as the upper mantle lithospheric electrical structure of the southern granulite terrain sgt is evaluated using the magnetotelluric mt data from two parallel traverses one is an 500 km long n s trending traverse across sgt and another a 200 km long traverse data space occam 3 d inversion was used to invert the mt data the electrical characterization of lithospheric structure in sgt shows basically a highly resistive several thousands of ohm meters upper crustal layer overlying a moderately resistive a few hundred ohm meters lower crustal layer which in turn is underlain by the upper mantle lithosphere whose resistivity shows significant changes along the traverse the highly resistive upper crustal layer is interspersed with four major conductive features with three of them cutting across the crustal column bringing out a well defined crustal block structure in sgt with individual highly resistive blocks showing correspondence to the geologically demarcated salem madurai and trivandrum blocks the 3 d model also brought out a well defined major crustal conductor located in the northern half of the madurai block the electrical characteristics of this south dipping conductor and its close spatial correlation with two of the major structural elements viz karur oddanchatram kodaikanal shear zone and karur kamban painavu trichur shear zone suggest that this conductive feature is closely linked to the subduction collision tectonic processes in the sgt and it is inferred that the archean dharwar craton neoproterozoic sgt terrain boundary lies south of the palghat cauvery shear zone the results also showed that the achankovil shear zone is characterized by a well defined north dipping conductive feature the resistive block adjoining this conductor on the southern side representing the trivandrum block is shown to be downthrown along this north dipping crustal conductor relative to the madurai block suggesting a northward movement of trivandrum block colliding against the madurai block the lithospheric upper mantle electrical structure of the sgt up to a depth of 100 km may be broadly divided into two distinctly different segments viz northern and southern segments the northern lithospheric segment over a major part is characterized by a thick resistive upper mantle while the southern one is characterized by a dominantly conductive medium suggesting a relatively thinned lithosphere in the southern segment

seismic evidence for stretched continental crust in the japan sea the japan sea is a major marginal sea in the western pacific comprised of basins and bathymetric highs technically these features were created by multiple rifting and spreading processes as exemplified in the creation of the japan basin and yamato basin however there are smaller scale features for which seismic crustal models have been inadequate to infer or verify their origin and evolution recent advances in marine seismological surveys enable us to obtain heterogeneous seismic crustal structures in sufficient detail to characterize these features in september of 1992 a seismic experiment was conducted to obtain the detailed seismic crustal structure in the southwestern part of the japan sea including the basin and bank areas seventeen ocean bottom seismometers obss were deployed on two lines explosives and airguns were fired as controlled seismic sources the model of the crust obtained in the northern tsushima basin is about 13 km thick including a 1 5 km thick sedimentary layer beneath the bank area the crust is 22 km thick and the upper crust is relatively homogeneous with a p wave velocity of about 6 km s the bank is interpreted to be a stretched continental crust fragment

crustal structure of the grenville province in southeastern labrador from refraction seismic data evidence for a high velocity lower crustal wedge the crustal structure of the eastern grenville and makkovik provinces was determined using two onshore offshore refraction seismic lines of the lithoprobe eastern canadian shield onshore offshore transect ecsoot a gravity high in the hawke river terrane correlates with increased p wave velocities in the upper 30 km of the crust 6 2 6 7 km s in the upper and middle crust and 6 9 7 1 km s below which we interpret as structure inherited from the labradorian orogen velocities in the adjacent groswater bay terrane are 6 0 6 55 km s in the upper and middle crust and 6 6 6 95 km s in the lower crust the entire grenville crust is underlain by a 15 20 km thick high velocity lower crustal hvlc wedge consisting of an upper layer 7 1 7 4 km s and a lower layer 7 6 7 8 km s the hvlc wedge is interpreted as an underplated layer formed during iapetan rifting this interpretation is based on the correlation with the 615 ma long range dykes onshore and the eastward termination of the wedge at the cartwright

active right lateral strike slip fault zone along the southern margin of the japan sea abstract we describe an active right lateral strike slip fault zone along the southern margin of the japan sea named the southern japan sea fault zone sjsfz onshore segments of the fault zone are delineated on the basis of aerial photograph interpretations and field observations of tectonic geomorphic features whereas the offshore parts are interpreted from single multichannel seismic data combined with borehole information in an effort to evaluate late quaternary activity along the fault zone four active segments separated by uplifting structures are identified in this study the east northeast trending sjsfz constitutes paired arc parallel strike slip faults together with the median tectonic line mtl both of which have been activated by oblique subduction of the philippine sea plate during the quaternary they act as the boundaries of three neotectonic stress domains around the eastern margin of the eurasian plate the near trench outer zone and nw se compressive inner zone of southwest japan arc and the southern japan sea deformed under e w compression from south to north

extensive seismic anisotropy in the lower crust of archean metamorphic terrain south india inferred from ambient noise tomography abstract we use rayleigh and love wave empirical green s function egf recovered from the cross correlation of seismic ambient noise to study the spatial distribution of radial anisotropy in the southern india crust the corresponding dispersion curves in the period 2 to 32 s are measured from ambient noise data recorded at 57 sites and the strength of anisotropy computed from the discrepancy between shear velocities obtained from rayleigh v sv and love v sh at various depths down to 40 km in upper crust up to a depth of 20 km the region is characterized by anisotropy coefficients of 2 to 2 that could be explained due to a combination of fluid filled open cracks and foliated metamorphic rocks at deeper levels beyond 20 km except for the archean metamorphic terrain most part of south india has anisotropies of up to 5 this may be due to rocks with varying degree of metamorphism beneath the archean metamorphic terrain the anisotropy is recorded up to 9 in the depth range of 20 40 km this high anisotropy is unlikely to be the manifestation of any recent geodynamic process considering that the region has low surface heat flow 30 mw m 2 we propose that the observed strong anisotropy in the metamorphic belt of southern india crust could best be explained as due to the presence of micaceous minerals or amphiboles in the deep crust that are formed possibly during the evolution of granulite terrain at 2 5 ga

opening mode of the japan sea inferred from the palaeomagnetism of the japan arc the western edge of the pacific plate is marked by a chain of back arc basins stretching from the aleutians through japan and the philippines to new zealand most of the basins lie behind volcanic island arcs far from continents and are the result of extension of the oceanic lithosphere1 but the japan sea is bounded on one side by continental crust the sikhote alin and korean peninsula of the asian continent and on the other by an active arc with continental fragments it can thus be said to be associated more with continental rifting1 4 than with oceanic processes we report here palaeomagnetic data from the japan arc which indicate that since the early miocene north east japan has rotated counter clockwise through 47 around a vertical axis while south west japan has rotated clockwise through 56 refs 5 8 we attribute this differential rotation which took place concurrently between 21 and 11 myr ago to the back arc opening of the japan sea8 with a double door mode that is there are two fan shaped spreading basins facing each other such a fan shaped opening is probably peculiar to the geological phenomenon of rifting of a continental fragment from thick continental lithosphere

precambrian tectonics and crustal evolution in south india about 3 4 ga ago voluminous calc alkaline magmas represented by the granitoid gneisses of southern india were emplaced into a now poorly preserved non continental crust unstable ensialic basins initiated at about 3 0 ga were filled with volcanic and sedimentary rocks up to about 2 6 ga this basement cover sequence was deformed at the close of the archaean first by northward accretion and thickening of several crustal slabs formerly separated by prisms of stable shelf sediments structures produced by this episode were refolded and dislocated by large n s strike slip shear belts to impart intense steep planar fabrics to large volumes of the crust fluids rich in co possibly derived from sedimentary material driven beneath the crustal slabs by the thickening mechanism purged the deep crust of h o to form granulites upward migration of hot h o rich fluids encouraged partial melting at intermediate crustal levels these late archaean tectonic and thermal events began soon after the close of basin f

the relationship between late cenozoic tectonic events and stress field and basin development in northeast japan the regional tectonic stress field basin development and crustal deformation of the ne japan arc in the interval between 32 ma to the quaternary can be synthesized based on dike vein and fault orientation data as well as on the compilation of the regional geology an extensional stress field became prevalent from 32 ma and major normal faulting started at 25 20 ma which resulted from back arc rifting normal faulting trending nearly parallel to the arc propagated from the present japan sea coast to the forearc side following the trenchward migration of the main volcanic field from 20 to 15 ma normal faults with an oblique trend to the arc developed due to the counterclockwise rotation of ne japan the rapid clockwise rotation of sw japan since 16 ma produced a nw se directed transtensional stress regime in the ne japan arc due to crustal stretching associated with this pull apart movement the back arc side of the ne japan arc subsided rapidly to middle bathyal environments after the termination of the opening of the japan sea at about 14 ma a neutral stress regime prevailed which included phases of both weak extension and compression lithospheric cooling eventually led to thermal subsidence of the back arc region and igneous underplating caused uplift of the axial zone of the volcanic arc the increase in velocity of the westward motion of the pacific plate at around 4 ma produced strong compression across the arc reactivating most of the miocene normal faults and uplifting the volcanic arc the greatest crustal shortening occurred in areas that were stretched the most in the miocene within the volcanic arc which implies tectonic inversion

crustal structure of a young margin pair new results across the liguro provencal basin from wide angle seismic tomography tomographic inversion of two wide angle seismic profiles acquired during the sardinia cruise 2006 on the conjugate gulf of lions west sardinia margins pair offers insight into the deep structure of this young basin it is the first combined conjugate margins study in this area based on deep seismic data modelling of the two conjugate profiles reveals that the conjugate margins are symmetric in term of crustal velocity structure with a partitioning in three similar regions 1 a zone of low crustal vertical velocity gradients and a crustal thickness of 18 to 10 km interpreted to be thinned continental crust 2 a transitional zone characterised by high lower crustal velocities non typical of continental or normal oceanic crust and 3 a 4 to 5 km thick crust showing vertical velocity gradients and relative velocity ranges typical of oceanic crust this latter region is interpreted to be a thin oceanic crust resulting from a tectonic heritage and the possible influence of a cool slab back arc basin although the three regions are found on both sides of the margin they are asymmetric in terms of the width of these domains wide and smooth along the gulf of lions margin and narrow and abrupt along its conjugate sardinia side the width of region 2 is larger on the gulf of lions side 90 km than on the sardinia side 40 km crustal thickness in region 3 is about 1 km thinner on the sardinia side than on its conjugate in region 1 the crustal thickness is lower on the sardinia side at a given distance from the shelf break the differences in width of region 2 and in crustal thickness might be due to the origin as a back arc basin of the nw mediterranean basin with the final breakup located closer to the western sardinia margin also probably linked to the complex tectonic history of the region our results do not support a simple shear mechanism of the opening of the basin along a lithospheric detachment fault

an anomalously thick layering of the crust of the yamato basin southeastern sea of japan the final stage of back arc spreading abstract the yamato basin is believed to have been formed by back arc spreading of the japanese islands a seismic velocity structure beneath the basin was modeled on the basis of seismic data acquired during the delp 85 wakashio cruise conducted by the japanese research group of the international lithosphere project ilp the dataset consists of multichannel reflection profiles airgun shot refraction profiles recorded by ocean bottom seismometers obss and explosion refraction profiles also recorded by the obss the model indicates that the crustal layering is twice as thick as in a normal oceanic basin but the velocity in each layer and that of the underlying uppermost mantle is very similar to that of oceanic basin crust the crust is 14 km thick at the middle of the oblong basin along its strike and thinner towards the rim of the basin tending toward 11 km stratigraphy of the sedimentary layer and the acoustic basement suggest that volcanism during the late stage of spreading is responsible for the thick layering and the seamount chain located along the spreading axis of the basin

underplating in the himalaya tibet collision zone revealed by the hi climb experiment we studied the formation of the himalayan mountain range and the tibetan plateau by investigating their lithospheric structure using an 800 kilometer long densely spaced seismic array we have constructed an image of the crust and upper mantle beneath the himalayas and the southern tibetan plateau the image reveals in a continuous fashion the main himalayan thrust fault as it extends from a shallow depth under nepal to the mid crust under southern tibet indian crust can be traced to 31 n the crust mantle interface beneath tibet is anisotropic indicating shearing during its formation the dipping mantle fabric suggests that the indian mantle is subducting in a diffuse fashion along several evolving subparallel structures

high frequency ambient noise tomography of southeast australia new constraints on tasmania s tectonic past 1 the island of tasmania at the southeast tip of australia is an ideal natural laboratory for ambient noise tomography as the surrounding oceans provide an energetic and relatively even distribution of noise sources we extract rayleigh wave dispersion curves from the continuous records of 104 stations with 15 km separation unlike most passive experiments of this type which observe very little coherent noise below a 5 s period we clearly detect energy at periods as short as 1 s thanks largely to the close proximity of oceanic microseisms on all sides the main structural elements of the eastern and northern tasmanian crust are revealed by inverting the dispersion curves between 1 and 12 s period for both group and phase velocity maps of particular significance is a pronounced band of low velocity observed across all periods that underlies the tamar river valley and continues south until dissipating in southeast tasmania together with evidence from combined active source and teleseismic tomography and heat flow data we interpret this region as a diffuse zone of strong deformation associated with the mid paleozoic accretion of oceanic crust along the eastern margin of proterozoic tasmania which has important implications for the evolution of the tasman orogen of eastern australia in the northwest a narrower low velocity anomaly is seen in the vicinity of the arthur lineament which may be attributed to local sediments and strong deformation and folding associated with the final phases of the tyennan orogeny

structural pattern in the dharwar craton high grade and low grade supracrustal belts sargur and dharwar set in a matrix of gneisses dotted with granitic plutons of different ages make up the dharwar craton the overall structural pattern is one of subparallel linear belts and stringers whose regional trend changes from nw se in the northern part to n s in the south both the high grade and low grade belts show three principal episodes of folding however the correlation between the deformation episodes in the sargur and the dharwar rocks is controversial and the structural relations between them are not unequivocally established parallelism of the first phase structures in the sargur and the dharwar rocks can be interpreted either as synchronous development or rotation of the earlier sargur structures into parallelism with the younger dharwar structures during later deformation the extension of the supracrustal belts is parallel to the f 2 axial traces of the second deformation granite diapirism and basement uplift further modified

seismic structure of the extended continental crust in the yamato basin japan sea from ocean bottom seismometer survey abstract we present the result of a seismic experiment conducted using ocean bottom seismometers and an airgun in the yamato basin of the japan sea the japan sea is one of the most well studied back arc basins in the western pacific the japan sea is believed to have been formed by back arc opening however the timing and formation processes of the opening of individual basins in and around the japan sea are not clear to reveal the crustal structure of the yamato basin it is important to consider the formation process of the japan sea therefore we conducted a seismic survey and estimated the p wave seismic velocity structure beneath the 170 km profile using a 2 d ray tracing method a layer with a p wave velocity of 3 4 4 0 km s underlies the sedimentary sections which is thought to consist of a sill and sediment complex the upper crust below the profile varies greatly in thickness the thickness of the upper crust is 3 5 km in the thinnest part and 7 km in the thickest part the thickness of the lower crust is approximately 8 km and is relatively constant over the profile the total thickness of the crust is approximately 15 km including the sedimentary layer the distribution of p wave velocities and the thickness indicate that the crust in the yamato basin is neither a typical continental nor a typical oceanic crust from the point of view of seismic velocity the obtained structure is more similar to a continental crust than to an oceanic crust the large lateral thickness variation in the upper crust and the uniform thickness of the lower crust suggest that the crust in the study area was formed by rifting extension of continental crust during the opening of the japan sea the margins of the continent or of island arcs can be divided into two types volcanic rifted margins and non volcanic rifted margins volcanic rifted margins are normally classified by the presence of a high velocity body in the lower crust at the volcanic rifted margin the high velocity body is formed in the lower crust we did not observe such a high velocity layer at depth on our profile in contrast the acoustic basement consists of a sill and sediment complex therefore we infer that the study area was affected by volcanism during its formation however the magmatism was not significantly massive

redefining dharwar craton southern granulite terrain boundary in south india from new seismological constraints abstract the southern part of the dharwar craton is an archean granulite terrain that continues further south and joins with the proterozoic southern granulite terrain sgt the linkage and the boundary between the dharwar craton and the sgt has been a subject of debate over the last 75 years based on adequate coverage of broadband seismological stations 80 over south india we generated3 d shear velocity image of the crust through joint inversion of the p receiver function and rayleigh wave group velocity dispersions derived from the cross correlation of ambient noise time series analysis the most significant finding is an unusually thick 15 28 km high velocity layer hvl with vs 4 0 km sin the lowermost crust beneath the south dharwar craton that continues southward in a most coherent fashion well beyond the southern limb of palghat cauvery shear zone pcsz widely considered to be the contact zone of the dharwar and sgt elsewhere in south india the high velocity layer is about 4 8 km thick similar to the global average we interpret this hvl as a mafic magmatic underplate of archean ancestry formed during granulite metamorphism the study suggests that the dharwar craton continues further south of the mapped orthopyroxene boundary to the palghat shear zone in a continuous manner

images of a lower crustal oceanic slab direct evidence for tectonic accretion in the archean western superior province the archean western superior province in canada is the type area for proposed archean plate tectonics seismic images from this region provide direct evidence for assembly of the craton by terrane accretion and for a large slab of remnant oceanic crust preserved at the base of the crust this slab with inferred garnet amphibolite composition adds a critical piece of evidence to previous suggestions that archean subduction was at a shallow angle and that some neoarchean tonalite trondhjemite granodiorite suites distinct from most modern day suprasubduction magmas are melts primarily derived directly from subducted slabs

the source fault of the 1983 nihonkai chubu earthquake revealed by seismic imaging abstract from 2011 to 2012 we conducted marine seismic surveys using a multichannel seismic reflection mcs system and ocean bottom seismometers obss in and around the hypocentral region of the 1983 nihonkai chubu earthquake the survey covered the area from the japan coast of the japan sea to the yamato basin and the japan basin we investigated the relationship between the nihonkai chubu earthquake and the crustal structure obtained from seismic reflection imaging and the p wave velocity structure near the hypocentral region the focus in the present paper is on seismic data that were acquired using both the mcs system and obss based on the results the crustal structure of the japan sea around the hypocentral region of the 1983 nihonkai chubu earthquake is divided into three types island arc crust thick oceanic crust and oceanic crust the nihonkai chubu earthquake occurred near the boundary between the thick oceanic crust and the island arc crust there is a strong possibility that the east dipping reflector detected in the crust was the source fault of the nihonkai chubu earthquake in addition the location of anticlines formed by the east dipping reverse fault occurring in the boundary of the crustal structure associated with the source fault of the nihonkai chubu earthquake was confirmed in the range of latitude from 39 20 to 41 15 n although they were not found south of this segment there is also a concentrated area of reverse faults and folds approximately 100 km west of the source region of the nihonkai chubu earthquake this area is located near the boundary between the thick oceanic crust and the oceanic crust

seismological study of lau back arc crust mantle water magmatic differentiation and a compositionally zoned basin abstract on the basis of seismic tomography analyses a structural model of crust formed along the eastern lau spreading center elsc is presented as evidence for a transition from a hydrous type of oceanic crust to a more typical oceanic crust the seismic data indicate that as the spreading center moved away from the active arc the crust thinned from 8 9 km to 7 km the lower crust changed from high p wave velocity values 7 2 7 4 km s to typical values for oceanic crust 7 0 7 2 km s and the upper crustal volcanic layer changed from a thick low velocity layer to a thinner layer with more typical wave speeds the seismic results in combination with other geophysical and geochemical data suggest that crustal formation along the elsc is strongly controlled by the influence of slab water when a spreading center is near the active arc water from the downgoing slab is entrained in the melting zone beneath the ridges where it enhances melting thereafter the water enhances crustal differentiation within sub ridge magma chambers this creates an anomalous hydrous form of oceanic crust with a thick felsic volcanic layer and a mafic ultramafic lower crust features that are not typically observed in crust formed at mid ocean ridges the lau basin has a zoned structure with an abrupt transition from this type of oceanic crust to more typical oceanic crust which resulted from a rapid change in the influence of slab water as the ridge moved away from the arc the abundance and high rate of production of the hydrous crust suggests that such crust may make up a significant proportion of the arc like crust that forms continents

submarine geology and post opening tectonic movements in the southern region of the sea of japan abstract the submarine geologic structure of the southern region of the sea of japan was analyzed on the basis of seismic reflection profile data and bottom samples the seismic reflection patterns depict sedimentary layers with well developed onlap and toplap terminations and truncations by submarine erosion the planes marked by series of onlap toplap and erosional truncations enabled us to define three stratigraphic groups composed of six formations in this region they are the kasumioki group the hamasakaoki group and the tottorioki group in ascending order detailed analysis of the seismic reflection profiles reveals subtle structures of these sedimentary units the oldest kasumioki group is folded and faulted forming submarine ridges the hamasakaoki group mainly occurs in the depressions between these ridges the youngest tottorioki group covers widely both of the older formations with a pronounced marine onlap fossil evidence indicates that the depositional ages of the kasumioki group the hamasakaoki group and the tottorioki group are early miocene to earliest pliocene late early to late pliocene and late pliocene to holocene respectively these observations provide the basis to understand two main structural features in this region 1 e w trending folds and 2 northwestward tilting blocks the former construct a fold zone composed of two to four ranges of anticlines that have steeper southern flanks truncated by reverse faults the latter are shown by slightly inclined floors formed by submarine erosion step like asymmetric basins bounded by ne sw trending reverse faults and onlap toplap sedimentation patterns the lateral change in bed thickness sedimentation patterns and age of submarine sediments indicate that the e w trending folds were formed in the early middle or late miocene to the early pliocene by n s compression whereas the northwestward tilt blocks were formed in the late early to late pliocene by nw se compression the sedimentary records in this region dictate an abrupt change in tectonic setting in the late early pliocene this change is interpreted to show that the sea of japan has been closing since then due to either the reactivation of the amurian plate or the directional change of the philippine sea plate motion

pds receiver function evidence for crustal scale thrusting relic subduction and mafic underplating in the trans hudson orogen and yavapai province a b s t r a c t article history the trans hudson orogen tho in the north central united states represents a major suturing event between the wyoming and superior archean provinces it is bounded to the south by the ne sw striking yavapai province which was accreted along the southeastern margin of north america between 1 71 and 1 68 ga and was one of a series of major collisional events responsible for the assembly of laurentia in this study pds teleseismic receiver functions were used to investigate the deep crustal structure associated with these collisions using data from over 800 broadband seismic stations distributed throughout the great plains midcontinent region we calculated 0 5 hz receiver functions using 245 m 6 0 teleseismic events the receiver functions were then ccp common conversion point stacked to create a 3d image volume profiles through this image volume show evidence of crustal scale thrusting of the wyoming province in the west over the superior province in the east and a relic subduction zone associated with the yavapai superior boundary we also performed a density analysis of the region using relative amplitude of the 2p1s and 0p1s receiver function phases from 233 stations these data indicate a relatively low moho density contrast throughout the tho and northern yavapai province associated with a region of thickened crust 50 km which we interpret to be evidence of a dense lower crustal layer that is the result of mafic underplating

seismic structure of the central tyrrhenian basin geophysical constraints on the nature of the main crustal domains in this work we investigate the crustal and tectonic structures of the central tyrrhenian back arc basin combining refraction and wide angle reflection seismic was gravity and multichannel seismic mcs reflection data acquired during the medoc mediterraneo occidental 2010 survey along a transect crossing the entire basin from sardinia to campania at 40 n the results presented include a 450 km long 2 d p wave velocity model obtained by the traveltime inversion of the was data a coincident density model and a mcs poststack time migrated profile we interpret three basement domains with different petrological affinity along the transect based on the comparison of velocity and velocity derived density models with existing compilations for continental crust oceanic crust and exhumed mantle the first domain includes the continental crust of sardinia and the conjugate campania margin in the sardinia margin extension has thinned the crust from 20 km under the coastline to 13 km 60 km seaward similarly the campania margin is also affected by strong extensional deformation the second domain under the cornaglia terrace and its conjugate campania terrace appears to be oceanic in nature however it shows differences with respect to the reference atlantic oceanic crust and agrees with that generated in back arc oceanic settings the velocities depth relationships and lack of moho reflections in seismic records of the third domain i e the magnaghi and vavilov basins support a basement fundamentally made of mantle rocks the large seamounts of the third domain e g vavilov are underlain by 10 20 km wide relatively low velocity anomalies interpreted as magmatic bodies locally intruding the mantle

crustal structure and fracture zone in the central basin of the south china sea from wide angle seismic experiments using obs abstract we present two e w trending wide angle seismic profiles obs2013 zn obs2014 zn which cross the boundary zhongnan fault zone between the east sub basin and the southwest sub basin of the south china sea scs we processed the data and used 2d ray tracing to determine the oceanic crust thickness velocity structures and moho depth variations related to the fault zone the simulated velocity models show that the oceanic basin of the scs has a typical oceanic crust covered by a 1 2 km thick sediment layer with a velocity of 2 0 3 5 km s the crust has a thickness of 5 8 km of which the oceanic layer 2 is 1 8 3 km thick with velocity increasing downward from 4 3 km s to 6 4 km s and the oceanic layer 3 is 3 5 km thick with velocity increasing downward from 6 4 km s 7 0 km s the moho depth in the oceanic basin is approximately 6 7 km below seabed the moho discontinuity has a prominent upheaval zone with a low velocity of 7 6 km s whose location corresponds to the low velocity zone in oceanic layer 2 our results suggest the presence of a nw se trending fracture zone 40 60 km wide rather than a major zhongnan fault oriented n s by connecting the upheaval portions of the moho in the two profiles the nw se orientation zhongnan transform fault zone in our study area is consistent with the direction of opening of the south china sea in the last stage of its evolution this large transform fault zone connected and offset the spreading centers of the east and southwest sub basin