crustal thickness and velocity structure beneath singapore s seismic network abstract we estimated the crustal thickness and velocity structure beneath the five stations comprising the republic of singapore s seismic network our data set was composed of 697 teleseismic receiver functions and 7 months of broad band data that was cross correlated to produce inter station green s functions surface wave group velocities were extracted from the green s functions to obtain dispersion data for a path from central sumatra to singapore in order to provide a complimentary data set to the receiver functions crustal thickness was estimated via an h k stacking technique and high resolution 1d p wave velocity profiles were generated beneath each station by jointly inverting receiver function stacks and the group velocity data using a linearised time domain inversion scheme crustal thickness beneath four stations was found to be between 28 0 km and 32 0 km while one station in the northeast of singapore indicates 24 0 km thick crust this implies a significant crustal thinning beneath singapore over the lateral extent of 50 0 km inversion results exhibit several crustal features that are observable in the derived models at all five stations indicating that they exist across singapore as a whole there appears to be an upper crustal high velocity zone beneath singapore underlain by a velocity inversion station ntu shows slower near surface velocities than the other stations consistent with its situation above the sedimentary jurong formation these results expand the available global velocity data set as well as being useful for assessing the seismic hazard in singapore

models of crustal thickness for south america from seismic refraction receiver functions and surface wave tomography abstract an extensive compilation of crustal thicknesses is used to develop crustal models in continental south america we consider point crustal thicknesses from seismic refraction experiments receiver function analyses and surface wave dispersion estimates of crustal thickness derived from gravity anomalies were only included along the continental shelf and in some areas of the andes to fill large gaps in seismic coverage two crustal models were developed a by simple interpolation of the point estimates and b our preferred model based on the same point estimates interpolated with surface wave tomography despite gaps in continental coverage both models reveal interesting crustal thickness variations in the andean range the crust reaches 75 km in southern peru and the bolivian altiplano while crustal thicknesses seem to be close to the global continental average 40 km in ecuador and southern colombia despite high elevations and along the southern andes of chile argentina elevation lower than 2000 m in the stable continental platform the average thickness is 38 5 km 1 st deviation and no systematic differences are observed among archean paleoproterozoic cratons neoproterozoic fold belts and low altitude intracratonic sedimentary basins an exception is the borborema province ne brazil with crust 30 35 km thick narrow belts surrounding the cratons are suggested in central brazil parallel to the eastern and southern border of the amazon craton and possibly along the transbrasiliano lineament continuing into the chaco basin where crust thinner than 35 km is observed in the sub andean region between the mid plate cratons and the andean cordillera the crust tends to be thinner 35 km than the average crust in the stable platform a feature possibly inherited from the old pre cambrian history of the continent we expect that these crustal models will be useful for studies of isostasy dynamic topography and crustal evolution of the continent

crustal thicknesses in se brazilian shield by receiver function analysis implications for isostatic compensation hypothesis are proposed to explain the data 1 a lower density by 30 40 kg m 3 i n the lithospheric mantle under the archean block of the sao francisco craton relative to the proterozoic lithosphere is responsible for maintaining the high elevations in the plateau area relatively low density and high p wave velocity are compatible with a depleted low feo composition for the archean lithosphere 2 alternatively if the density contrasts between archean and proterozoic lithospheres are smaller than the values above then the crust beneath the paranabasin must be more dense than that of the craton higher crustal density and high poisson s ratio would be consistent with magmatic underplating in the lower crust beneath the paranabasin as inferred from other studies index terms 7205 seismology continental crust 1242 9360 information related to geographic region south america 7218 seismology lithosphere and upper mantle keywords crust upper mantle brazil parana basin san francisco craton

continental crust composition constrained by measurements of crustal poisson s ratio deciphering the geological evolution of the earth s continental crust requires knowledge of its bulk composition and global variability the main uncertainties are associated with the composition of the lower crust seismic measurements probe the elastic properties of the crust at depth from which composition can be inferred of particular note is poisson s ratio this elastic parameter can be determined uniquely from the ratio of p to s wave seismic velocity and provides a better diagnostic of crustal composition than either p or s wave velocity alone1 previous attempts to measure have been limited by difficulties in obtaining coincident p and s wave data sampling the entire crust2 here we report 76 new estimates of crustal spanning all of the continents except antarctica we find that on average increases with the age of the crust our results strongly support the presence of a mafic lower crust beneath cratons and suggest either a uniformitarian craton formation process involving delamination of the lower crust during continental collisions followed by magmatic underplating or a model in which crust formation processes have changed since the precambrian era

gravity derived moho for south america crustal structure in south america is one of the least understood among the earth s continental areas variations in crustal thickness are still poorly constrained over large portions of the continent because of scarce or unevenly distributed crustal thickness estimates throughout south america to address this scarce and inhomogeneous data cover we explore the possibility to derive crustal thickness from satellite gravity data in this study we utilize the combined gravity model eigen 6c which is composed of goce and other gravity data the gravity field and steady state ocean circulation explorer goce satellite has a much more uniform spatial resolution than any land based gravity or seismic survey in south america the gravity data inversion is for a simple two layer model with fixed density contrast over the interface the moho the method is not relying on point constraint data and assumes that all of the signal is related to topography of the moho model quality can therefore be assessed by a comparison with point observations on crustal thickness we show that for the stable part of the continent 90 of our estimates are similar within error bounds to seismic observations variations occur in active orogenic zones or regions with suspected non standard moho density contrasts a comparison with seismological models shows a high correlation with the most recent model especially in areas where continental and global models of crustal structure have limitations in terms of wave paths or point constraints the gravity based model provides a unique continuity of crustal structure providing new insights on structure and tectonics and increase our understanding of the earth s structure underneath south america

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

crustal structure of the eastern borborema province ne brazil from the joint inversion of receiver functions and surface wave dispersion implications for plateau uplift we investigate the crustal structure of the borborema province of ne brazil by developing 44 s wave velocity depth profiles from the joint inversion of receiver functions and fundamental mode rayleigh wave group velocities the borborema province is located in the northeasternmost corner of the south american continent and represents a portion of a larger neoproterozoic mobile belt that formed during the brasiliano pan african orogeny extensional processes in the mesozoic eventually leading to the separation of africa and south america left a number of aborted rift basins in the continental interiors and episodes of diffuse intraplate volcanism and uplift marked the evolution of the province after continental breakup our velocity depth profiles reveal the existence of two crustal types in the province i the thin crustal type which consists of 30 32 5 km thick crust with an upper layer of 3 4 3 6 km s overlying a lower layer of 3 7 3 8 km s and ii the thick crustal type which consists of a 35 37 5 km thick crust with velocities between 3 5 and 3 9 km s down to 30 km depth and a gradational increase in velocity vs 4 0 km s down to upper mantle depths the crustal types correlate well with topography with the thick crustal type being mainly found in the high standing southern borborema plateau and the thin crustal type being mostly found in the low lying sertaneja depression and coastal cuestas interestingly the thin crustal type is also observed under the elevated topography of the northern plateau we argue that the thick crustal type is rheologically strong and not necessarily related to postbreakup mantle processes as it is commonly believed we propose that extensional processes in the mesozoic stretched portions of the brasiliano crust and formed the thin crustal type that is now observed in the regions of low lying topography leaving the rheologically strong thick crust of the southern plateau at higher elevations the crust making the northern plateau would have thinned and subsided during mesozoic extension as part of a greater sertaneja depression to then experience uplift in the cenozoic and achieve its present elevation

a new seismic data set on the depth of the moho in the alps we present the results from receiver function analysis applied to a comprehensive data set in the eastern alps teleseismic events were recorded at 70 stations with an average deployment of 1 year the investigated area includes the eastern part of the eastern alps and their transition to the bohemian massif the pannonian domain and the southern alps the crustal structure at each station is examined with the zhu kanamori zk method which yields well resolved interface depths in laterally homogeneous media with limited layering the application of the zk technique is challenged because of the complex tectonic setting therefore we include additional constraints from recent active source seismic studies in particular the well known crustal p wave velocity and where available the vp vs ratio are kept fixed thus reducing the ambiguity in determining moho depths individual depth values vary strongly between adjacent stations showing that the employment of the zk technique in tectonically complex settings is limited we therefore avoid interpreting the results in detail but rather compare them to existing crustal models of the eastern alps we regard this receiver function study in the easternmost part of the alps as a documentation of a data set that has potential to be exploited in the future

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

seismic studies of the brasilia fold belt at the western border of the sao francisco craton central brazil using receiver function surface wave dispersion and teleseismic tomography abstract the tocantins province in central brazil is composed of a series of ssw nne trending terranes of mainly proterozoic ages which stabilized in the neoproterozoic in the final collision between the amazon and sao francisco cratons no previous information on crustal seismic properties was available for this region several broadband stations were used to study the regional patterns of crustal and upper mantle structure extending the results of a recent e w seismic refraction profile receiver functions and surface wave dispersion showed a thin crust 33 37 km in the neoproterozoic magmatic arc terrane high average crustal vp vs ratios 1 74 1 76 were consistently observed in this unit the foreland domain of the brasilia foldbelt on the other hand is characterized by thicker crust 42 43 km low vp vs ratios 1 70 1 72 were observed in the low grade foreland fold and thrust zone of the brasilia belt adjacent to the sao francisco craton teleseismic p wave tomography shows that the lithospheric upper mantle has lower velocities beneath the magmatic arc and goias massif compared with the foreland zone of the belt and sao francisco craton the variations in crustal thickness and upper mantle velocities observed with the broadband stations correlate well with the measurements along the seismic refraction profile the integration of all seismic observations and gravity data indicates a strong lithospheric contrast between the goias massif and the foreland domain of the brasilia belt whereas little variation was found across the foldbelt craton surface boundary these results support the hypothesis that the brasilia foreland domain and the sao francisco craton were part of a larger sao francisco congo continental plate in the final collision with the amazon plate

response of the mantle to flat slab evolution insights from local s splitting beneath peru the dynamics of flat subduction particularly the interaction between a flat slab and the overriding plate are poorly understood here we study the seismically anisotropic properties and deformational regime of the mantle directly above the peruvian flat slab we analyze shear wave splitting from 370 local s events at 49 stations across southern peru we find that the mantle above the flat slab appears to be anisotropic with modest average delay times 0 28 s that are consistent with 4 anisotropy in a 30 km thick mantle layer the most likely mechanism is the lattice preferred orientation of olivine which suggests that the observed splitting pattern preserves information about the mantle deformation we observe a pronounced change in anisotropy along strike with predominately trench parallel fast directions in the north and more variable orientations in the south which we attribute to the ongoing migration of the nazca ridge through the flat slab system

joint inversion for three dimensional s velocity mantle structure along the tethyan margin abstract for purposes of studying the lateral heterogeneity as well as for ultimately predicting seismograms for this region we construct a new 3 d s velocity model by jointly inverting a variety of different seismic data we jointly invert regional waveforms surface wave group velocity measurements teleseismic s arrival times and crustal thickness estimates from receiver functions refraction lines and gravity surveys these data types have complementary resolving power for crust and mantle structures vertical and lateral variations shallow and deep mantle features local and global structure therefore a joint inversion of these data sets might help unravel the complexity of this tectonically diverse area these measurements are made from a combination of mantle investigation of the deep suture between europe and africa midsea program for array seismic studies of the continental lithosphere passcal geoscope geofon global seismographic network gsn international deployment of accelerometeres ida mednet national networks and local deployments throughout the study region which extends from the western mediterranean region to the hindu kush and encompasses northeastern africa the arabian peninsula the middle east and part of the atlantic ocean for reference we have fitted the waveforms of regional s and rayleigh waves from over 3800 seismograms using partitioned waveform inversion we include over 3000 crustal thickness estimates from receiver functions gravity measurements and refraction profiles we include rayleigh wave group velocities for hundred thousands of paths transecting the region we have over 3000 teleseismic s arrival times measured through cross correlation and over 170000 from picks originally reported to the international seismological centre isc

topographic asymmetry of the south atlantic from global models of mantle flow and lithospheric stretching the relief of the south atlantic is characterized by elevated passive continental margins along southern africa and eastern brazil and by the bathymetric asymmetry of the southern oceanic basin where the western flank is much deeper than the eastern flank we investigate the origin of these topographic features in the present and over time since the jurassic with a model of global mantle flow and lithospheric deformation the model progressively assimilates plate kinematics plate boundaries and lithospheric age derived from global tectonic reconstructions with deforming plates and predicts the evolution of mantle temperature continental crustal thickness long wavelength dynamic topography and isostatic topography mantle viscosity and the kinematics of the opening of the south atlantic are adjustable parameters in thirteen model cases model predictions are compared to observables both for the present day and in the past present day predictions are compared to topography mantle tomography and an estimate of residual topography predictions for the past are compared to tectonic subsidence from backstripped borehole data along the south american passive margin and to dynamic uplift as constrained by thermochronology in southern africa comparison between model predictions and observations suggests that the first order features of the topography of the south atlantic are due to long wavelength dynamic topography rather than to asthenospheric processes the uplift of southern africa is best reproduced with a lower mantle that is at least 40 times more viscous than the upper mantle

crustal structure of the arabian plate new constraints from the analysis of teleseismic receiver functions an edited version of this paper was published by elsevier science copyright 2005 elsevier science see also http dx doi org 10 1016 j epsl 2004 12 020 http atlas geo cornell edu saudiarabia publications al damegh 202005 htm

upper and middle crustal velocity structure of the colombian andes from ambient noise tomography investigating subduction related magmatism in the overriding plate e p acknowledges support from the coordenacao de aperfeicoamento de pessoal de nivel superior capes through a 4 year scholarship to complete his ph d j j thanks cnpq for his research fellowship cnpq grant 308171 2012 8 m s acknowledges support by brazilian science without border program grant 40 2174 2012 7

receiver function analysis of the north american crust and upper mantle summary data from 27 permanent broad band stations spread over north america and sampling diverse tectonic regimes of varying ages from the archaean to the phanerozoic were analysed using the receiver function approach receiver functions a few hundred at most of the locations provide valuable constraints on the composition of the crust shaped by diverse processes characteristic of their time of formation the chemically inert and stable nature of the archaean domains of canada is well manifested in their low poisson ratio 0 26 felsic lower than global averages for precambrian age crust accompanied by a thicker than global average 35 km archaean crust data from the canadian cordillera and innuitian orogens of phanerozoic times indicate the possible operation of two distinctly different crustal processes resulting in thin crusts 35 km with dominantly felsic and more mafic compositions respectively these processes are restricted to the region above 410 km without disturbing the integrity of the mantle transition zone mtz which is evident from the remarkably homogeneous nature of the 410 and 660 km discontinuities beneath the canadian landmass contrastingly the thermally complex phanerozoic southern california region suggests the presence of a heterogeneous mantle transition zone possibly owing to localized 660 topography with a relatively uniform 410 overlain by a crust heterogeneous in composition and perhaps maturing to that of a continent

upper mantle structure of south america from joint inversion of waveforms and fundamental mode group velocities of rayleigh waves 1 a new tomographic s wave velocity model for the upper mantle beneath south america is presented we developed and applied a new method of simultaneously inverting regional s and rayleigh waveforms and fundamental mode rayleigh wave group velocities to better constrain upper mantle s velocity structure and moho depth we used 5700 rayleigh wave group velocity dispersion curves and 1537 regional wave trains with paths principally passing through the south american continent the joint inversion of this data set provided a new three dimensional 3 d upper mantle s velocity model and a moho depth model for south america which fits both the group velocity and regional waveform data sets well new features of the final three dimensional 3 d s velocity and moho depth model correlate well with known geotectonic units on a regional scale the moho depth ranges from 30 km in the central chaco basin to 42 km beneath the amazonian craton and 45 70 km beneath the orogenic andean belt the imaged s velocity indicates an average lithosphere thickness of around 160 km for the amazonian craton high velocities are imaged beneath the amazon and part of the parana and parnao ba basins down to about 150 km low to very low velocities are imaged

group velocity tomography and lithospheric s velocity structure of the south american continent the lithosphere of the south american continent has been studied little especially in northern brazil the amazonian region a 3d lithospheric s velocity model of south america was obtained by first carrying out rayleigh and love wave groupvelocity tomography and then inverting the regionalized dispersion curves fundamental mode group velocities were measured using a multiple filtering technique more than 12 000 paths were examined and about 6000 rayleigh and 3500 love wave dispersion curves with good quality were retrieved checkerboard tests showed that our dataset permits the resolution of features 400 800 km across laterally in the central part of the continent from crustal to upper mantle depths our results confirm previous tomographic results and correlate well with the major geological provinces of south america the 3d s velocity model confirms both regional features of se brazil from p wave travel time tomography and continental scale features of central and western south america from waveform inversion e g lowest velocities in the andean upper mantle three parts of the nazca plate with flat subduction strong low velocity anomalies in the upper mantle depth beneath the chaco basin furthermore our 3d model revealed new features in the south american continent 1 high velocities in the lower crust were consistently found in regions with high bouguer or free air anomalies 2 the ne sw trending transbrasiliano shear zone was delineated by a ne sw low velocity belt at lithospheric depths 3 the eastern amazonian craton appears to have thicker lithosphere than the western

organic accumulation in the lower chihsia formation middle permian of south china constraints from pyrite morphology and multiple geochemical proxies article i nfo the organic rich chihsia formation middle permian on the yangtze platform western hubei south china is considered to be the major hydrocarbon source rock in south china the lower part of the chihsia formation reported here was deposited in an intrashelf basin and is characterised by black laminated marlstones shales intercalated with dark grey limestones dolomites in which metre scale shallowing or cleaning upward cycles are ubiquitous each cycle is composed of basal marlstone shale and upper lime stone the organic matter is concentrated exclusively in the shaley bases of these cycles and shows system atic cyclic variations in abundance up through the succession an integrated approach of pyrite morphology and multiple geochemical proxies was used to determine the mechanism of organic accumulation in the organic rich sediments many redox proxies including pyrite framboid size distribution c s fe relation ships iron speciation and 34 s values of pyrite suggest deposition beneath the dysoxic bottom water which evolved into a more oxic water body when the organic poor limestones were deposited however the coincidence of increased abundance of total organic carbon toc with increased ba and mo contents in dicates that organic matter accumulation was controlled mainly by primary productivity these data indicate that increased organic accumulation resulted mainly from higher organic carbon export with increased nutri ent flux during superimposed sea level rises of different orders this in turn could have increased the oxygen consumption by respiration of microorganisms and organic matter decay leading to the depletion of oxygen in the water columns furthermore the rapid sedimentation rate further limited bioturbation and organic matter decomposition facilitating the preservation of buried organic matter

moho map of south america from receiver functions and surface waves 1 we estimate crustal structure and thickness of south america north of roughly 40 s to this end we analyzed receiver functions from 20 relatively new temporary broadband seismic stations deployed across eastern brazil in the analysis we include teleseismic and some regional events particularly for stations that recorded few suitable earthquakes we first estimate crustal thickness and average poisson s ratio using two different stacking methods we then combine the new crustal constraints with results from previous receiver function studies to interpolate the crustal thickness between the station locations we jointly invert these moho point constraints rayleigh wave group velocities and regional s and rayleigh waveforms for a continuous map of moho depth the new tomographic moho map suggests that moho depth and moho relief vary slightly with age within the precambrian crust whether or not a positive correlation between crustal thickness and geologic age is derived from the pre interpolation point constraints depends strongly on the selected subset of receiver functions this implies that using only pre interpolation point constraints receiver functions inadequately samples the spatial variation in geologic age the new moho map also reveals an anomalously deep moho beneath the oldest core of the amazonian craton

distinct crustal structure of the north american midcontinent rift from p wave receiver functions eighty two broadband seismic stations of the superior province rifting earthscope experiment spree collected 2 5 years of continuous seismic data in the area of the high gravity anomaly associated with the midcontinent rift mcr over 100 high quality teleseismic earthquakes were used for crustal p wave receiver function analysis our analysis reveals that the base of the sedimentary layer is shallow outside the mcr thickens near the flanks where gravity anomalies are low and shallows again in the mcr s center where the gravity anomalies peak this pattern is similar to that found from local geophysical studies and is consistent with reverse faulting having accompanied the cessation of rifting at 1 1 ga intermittent intracrustal boundaries imaged by our analysis might represent the bottom of the mcr s mostly buried dense volcanic layers outside the mcr the moho is strong sharp and relatively flat both beneath the archean superior province and the proterozoic terranes to its south inside the mcr two weaker candidate mohos are found at depths up to 25 km apart in the rift s center the intermediate layer between these discontinuities tapers toward the edges of the mcr the presence of this transitional layer is remarkably consistent along the strike of the mcr including beneath its jog in southern minnesota near the belle plaine fault we interpret these results as evidence for extensive underplating as a defining characteristic of the rift which remains continuous along the minnesota jog where due to its orientation it is minimally affected by the reverse faulting that characterizes the nne striking parts of the rift

paraguay araguaia belt conductivity anomaly a fundamental tectonic boundary in south american platform imaged by electromagnetic induction surveys 1 we discovered and mapped a 1200 km long zone of enhanced electrical conductivity through a large array of magnetometers over a narrow corridor parallel to the paraguay and araguaia belts which surround the amazon craton the anomaly has been further studied using magnetotelluric measurements made along a 300 km profile crossing the paraguay belt modeling shows the anomaly at this location as a very strong 100 km wide upper crust to mid crust conductor located under a region where surficial rocks show an increase in deformation and metamorphism within the belt from the high conductivity of the anomalous structure its observed geometry and surface rock exposures the most likely interpretation of its source is graphitized biogenic material in metasediments now deeply underthrust in a neoproterozoic or early cambrian suture zone this result strongly supports the hypothesis that the collision between the amazon plate and the western parana block has probably closed an ocean in the paraguay belt region contrary to former propositions of an ensialic evolution for this belt although the precise age of this ocean closure have been a matter of debate our finding significantly constrains the tectonic setting of west gondwana amalgamation involving the amazon plate and its surrounding blocks

archean and proterozoic crustal evolution evidence from crustal seismology seismic velocity models for archean and proterozoic provinces throughout the world are analyzed the thickness of the crust in archean provinces is generally found to be about 35 km except at collisional boundaries whereas proterozoic crust has a significantly greater thickness of about 45 km and has a substantially thicker high velocity 7 0 km s layer at the base we consider two models that may explain these differences the first model attributes the difference to a change in the composition of the upper mantle the higher temperatures in the archean mantle led to the eruption of komatiitic lavas resulting in an ultradepleted lithosphere unable to produce significant volumes of basaltic melt proterozoic crust developed above fertile mantle and subsequent partial melting resulted in basaltic underplating and crustal inflation in the second model convection in the hot archean mantle is considered to have been too turbulent to sustain stable long lived subduction zones by the proterozoic the mantle had cooled sufficiently for substantial island and continental arcs to be constructed and the high velocity basal layer was formed by basaltic underplating

crustal framework of the northwest parana basin brazil insights from joint modeling of magnetic and gravity data abstract the transbrasiliano lineament tbl is a major proterozoic strike slip shear zone crossing the parana basin brazil yet the relationship between the tbl and basin structures is poorly understood magnetic and gravity data are available from the whole northwestern region of the parana basin and provide insights into crustal partitioning and basement geometry joint 2d modeling of these data was performed in three strategic sections of the study area two smaller sections were constrained by available borehole data and receiver function analysis modeling of these sections facilitated modeling of the third larger nw se section which crosses the basin in an area with significant lack of data these three sections together form the basis for a preliminary tectonic model of the northern parana basin the tbl includes strong n30 e trending gravity and magnetic gradients represented by the serra negra fault zone this structure separates two crustal tectonic domains the paranapanema block to the east and the amazon paleocontinent to the west the tectonic domain northeast of the fault is represented by the arenopolis magmatic arc the n60 70 e trending general carneiro fault also reflected in gravity anomalies forms another important magnetic contact our tectonic model shows a region of crustal thinning between the two fault zones where structures associated with possible reactivation form small cretaceous basins the baliza fault may accommodate significant vertical displacements of devonian and carboniferous sections of the parana basin the main structures revealed in the sections are associated with grabens and horsts interpreted as reactivated structures that controlled parana basin sedimentation the northwestern portion of the basin exhibits basement asymmetry caused by dip and strike slip faulting along the tbl

a tectonostratigraphic synthesis of the sub andean basins inferences on the position of south american intraplate accommodation zones and their control on south atlantic opening it has been shown in the accompanying paper that the sub andean foreland can be subdivided longitudinally into a number of tectonostratigraphic domains to test the hypothesis that changes in palaeo depositional setting rely on the presence of a series of transverse zones of structural accommodation data have been digitally compiled from across the south american continent spatial and temporal geological relationships have been analysed and evaluated as a means of identifying the position of tectonostratigraphic domain boundaries structural accommodation zones and patterns of subsidence and intraplate deformation the results suggest that individually these structural accommodation zones represent a composite of deep crustal fractures which on a regional scale interlink to form a transcontinental belt or zone that can accommodate intraplate deformation during episodes of plate reorganization their strong spatial relationship with mesozoic intraplate alkaline igneous activity suggests that they exerted an important control on lithospheric melt siting during gondwana breakup these localized zones of high heat flow have important implications for source rock maturity in the interior phanerozoic intracratonic basins of south america on the south atlantic margin the majority of these crustal lineaments correlate with failed arms of triple junction rifts and define the boundaries to tectonostratigraphic domains recognized along the south atlantic rift system

crustal thickness beneath libya and the origin of partial melt beneath as sawda volcanic province from receiver function constraints this study investigates crustal thickness and properties within the libyan region results obtained from 15 seismic stations belonging to the libyan center for remote sensing and space science are reported in addition to 3 seismic stations publically available using receiver functions the results show crustal thicknesses ranging from 24 km to 36 km with uncertainties ranging between 0 10 km to 0 90 km more specifically crustal thickness ranges from 32 km to 36 km in the southern portion of the libyan territory then becomes thinner between 24 km and 30 km in the coastal areas of libya and thinnest between 24 km and 28 km in the sirt basin the observed high vp vs value of 1 91 at one station located at the as sawda volcanic province in central libya indicates the presence of either partial melt or an abnormally warm area this finding suggests that magma reservoirs beneath the libyan territory may still be partially molten and active thereby posing significant earthquake and volcanic risks the hypothesis of an active magma source is further demonstrated though the presence of asthenospheric upwelling and extension of the sirt basin this study provides a new calculation of unconsolidated sediment layers by using the arrival time of the p to s converted phases the results show sediments thicknesses of 0 4 km to 3 7 km with the vp vs values ranging from 2 2 to 4 8 the variations in crustal thickness throughout the region are correlated with surface elevation and bouguer gravity anomalies which suggest that they are isostatically compensated

the crustal structure of the guayana shield venezuela from seismic refraction and gravity data abstract we present results from a seismic refraction experiment on the northern margin of the guayana shield performed during june 1998 along nine profiles of up to 320 km length using the daily blasts of the cerro bolivar mines as energy source as well as from gravimetric measurements clear moho arrivals can be observed on the main e w profile on the shield whereas the profiles entering the oriental basin to the north are more noisy the crustal thickness of the shield is unusually high with up to 46 km on the archean segment in the west and 43 km on the proterozoic segment in the east a 20 km thick upper crust with p wave velocities between 6 0 and 6 3 km s can be separated from a lower crust with velocities ranging from 6 5 to 7 2 km s a lower crustal low velocity zone with a velocity reduction to 6 3 km s is observed between 25 and 25 km depth the average crustal velocity is 6 5 km s the changes in the bouguer anomaly positive 30 mgal in the west and negative 20 mgal in the east cannot be explained by the observed seismic crustal features alone lateral variations in the crust or in the upper mantle must be responsible for these observations

a deterministic approach toward isostatic gravity residuals a case study from south america a new deterministic method for correcting isostatic effects in gravity data sets overcomes the deficiencies of empirically based methods this technique produces a superior gravity image for south america and has application to other continental scale gravity data sets the basis for the correction is the airy heiskanen isostatic model which assumes that surface topography is supported by crustal thickening the three key parameters 1 the crustal thickness at sea level 2 the surface reduction density and 3 the density contrast between the crust and the mantle are determined directly from the elevation free air gravity and bouguer gravity data sets the surface density parameter of 2 60 g cc is determined using a new fractal technique this technique assumes that the topography is fractal the best value for density is that which minimizes the fractal component caused by topographic effects the new value is substantially different than the 2 67 g cc density assumed by many previous workers for most continental scale data sets the crust mantle density contrast parameter of 0 45 g cc is determined by comparison between the densities determined from crossplots of the bouguer values versus elevation the crustal thickness parameter of 30 km is determined using a spectral method applied to the free air gravity the results of this work are not only an isostatic residual map but a methodology that cross checks the data for quality control the final isostatic residual map can be used with confidence for basin evaluation throughout the continent of south america basins at high elevations like the eastern foreland basins of the andes and the altiplano basin are imaged better by using this method

mapping moho depth variations in central italy from psmoho p delay times evidence of an e w transition in the adriatic moho at 42 n latitude 1 along the italian peninsula adjoin two crustal domains peri tyrrhenian and adriatic whose boundary is not univocal in central italy in this area we attempt to map the extent of the moho in the two terrains from variations of the travel time difference between the direct p wave and the p to s wave converted at the crust mantle boundary called psmoho we use teleseismic receiver functions computed at 38 broad band stations in this and previous studies and assigned each of the recording sites to the adriatic or peri tyrrhenian terrains based on station location geologic and geophysical data and interpretation and consistency of delays with the regional moho trend the results of the present study show that the psmoho arrival time varies from 2 3 to 4 1 s in the peri tyrrhenian domain and from 3 7 to 5 5 s in the adriatic domain as expected the lowest time difference is observed along the tyrrhenian coastline and the largest values are observed in the axial zone of the apennine chain a key new result of this study is a sharp e w boundary in the adriatic domain that separates a deeper moho north of about 42 n latitude from a shallower moho to the south this feature is constrained for a length of about 40 km by the observations available in this study the e w boundary requires a revision of prior mapping of the moho in central italy and supports previous hypotheses of lithosphere segmentation

characteristics and consequences of flow in the lower crust in some places there is strong evidence that the lower continental crust has flowed so as to smooth out variations in crustal thickness caused by differential crustal extension or shortening in order to better understand the processes involved we investigate the behavior of a fluid layer over a fluid half space to see how such a system responds to the deformation of its upper and lower boundaries this simple system can be used to study both the decay of crustal thickness contrasts and the behavior of a thin lithospheric sheet the changing response of the system to variations in density and viscosity contrasts and to different boundary conditions imposed on the fluid interface can easily be studied analytically the most important results are that variations in crustal thickness on a wavelength of a few times the thickness of the flowing channel will decay quickest and that large lateral variations in crustal thickness cause the fluid to develop a steep front which may cause a topographic step above it at the earth s surface deformation within the channel will be principally by simple shear the clear association of lower crustal flow with regions of thickened crust and magmatic activity suggests that both can reduce the viscosity of the lower crust to levels at which flow can occur the smoothing of crustal thickness contrasts leads to differential vertical motions and is thus a method by which substantial tilting can occur without faulting this differential uplift may be responsible for rotating and exhuming some of the detachment faults in metamorphic core complexes in the basin and range province of the western united states it is also a method of causing structural inversion in basins that does not require the reactivation of normal faults as thrusts or reverse faults

magmatism and continental break up in the south atlantic high precision40ar 39ar geochronology abstract a detailed ar ar study of the parana etendeka continental flood basalts cfb has been undertaken using laser spot analysis data provide information not only on the age of the samples but also on the variability of the non radiogenic ar component and state of alteration the results indicate the parana etendeka cfb were erupted over 10 million years between 137 and 127 ma providing a minimum overall eruption rate of 0 1 km3 yr 1 this is an order of magnitude less than that previously proposed for this the deccan and siberian cfb provinces but it is similar to estimates for hawaii iceland and the columbia river cfb in detail the new ar ar analyses indicate that magmatism within the parana etendeka province migrated from nw to se prior to and during the opening of the south atlantic providing an explanation for the asymmetry of the cfb lavas about the south atlantic moreover chemically defined magma types were erupted at different times in different places and so within the paranathey may not be used as reliable chronostratigraphic units rather such magmatic units may reflect the extent of compositionally distinct source regions in the uppermost mantle and indicate that partial melting took place over a wide area under the paranabasin the nw se migration of the onset of magmatism might be interpreted as a plume trace but the inferred rate of movement is 3 times faster than that inferred from subsequent magmatism on the rio grande rise it is argued that the onset of magmatism reflected extensional tectonics normal to the ponta grossa dyke swarm given that magmatism preceded the main phase of rifting and that extension across the ponta grossa was moderate current plume models have difficulty in predicting sufficient melting within the asthenosphere

crustal and upper mantle structure in the amazon region brazil determined with broadband mobile stations 1 three broadband stations operated from march 1997 to september 1998 in the amazon region north of manaus brazil which including the iris station ptga pitinga covered an area of roughly 60 200 km applying the receiver function technique to determine the crustal structure there is evidence for an increase of the moho depth from about 38 km below the amazon basin to approximately 48 km north of the basin in addition we analyzed the polarization of sks waves to determine anisotropy the observed splitting parameters in a majority of the data set can be accounted for by a layer of axial symmetric anisotropy with the fast direction oriented roughly n110e this direction cannot be associated with the present day plate motion of the south american plate however the anisotropy may be related to a lithospheric fabric that was generated during the early tectonic history of the amazonian craton

rayleigh wave group velocity tomography of the borborema province ne brazil from ambient seismic noise ambient seismic noise has traditionally been regarded as an unwanted perturbation that contaminates earthquake data over the last decade however it has been shown that consistent information about subsurface structure can be extracted from ambient seismic noise by cross correlation of noise simultaneously recorded at two seismic stations the empirical green s function for the propagating medium between them can be reconstructed moreover for periods less than 30 s the seismic spectrum of ambient noise is dominated by microseismic energy and because microseismic energy travels mostly as surface waves the reconstruction of the empirical green s function is usually proportional to the surface wave portion of the seismic wavefield in this paper we present 333 empirical green s functions obtained from stacked cross correlations of one month of vertical component ambient seismic noise for different pairs of seismic stations in the borborema province of ne brazil the empirical green s functions show that the signal obtained is dominated by rayleigh waves and that dispersion velocities can be measured reliably for periods between 5 and 20 s the study includes permanent stations from a monitoring seismic network and temporary stations from past passive experiments in the region resulting in a combined network of 34 stations separated by distances between approximately 40 and 1 287 km fundamental mode group velocities were obtained for all station pairs and then tomographically inverted to produce maps of group velocity variation for short periods 5 10 s the tomographic maps correlate well with surface geology with slow velocities delineating the main rift basins potiguar tucano and reconcavo and fast velocities delineating the location of the precambrian sao francisco craton and the rio grande do norte domain for longer periods 15 20 s most of the velocity anomalies fade away and only those associated with the deep tucano basin and the sao francisco craton remain the fading of the rio grande do norte domain fast velocity anomaly suggests this is a supracrustal structure rather than a lithospheric terrain and places new constraints on the precambrian evolution of the borborema province

central andean crustal structure from receiver function analysis abstract the central andean plateau 15 27 s is a high plateau in excess of 3 km elevation associated with thickened crust along the western edge of the south america plate in the convergent margin between the subducting nazca plate and the brazilian craton we have calculated receiver functions using seismic data from a recent portable deployment of broadband seismometers in the bolivian orocline 12 21 s region and combined them with waveforms from 38 other stations in the region to investigate crustal thickness and crust and mantle structures results from the receiver functions provide a more detailed map of crustal thickness than previously existed and highlight mid crustal features that match well with prior studies the active volcanic arc and altiplano have thick crust with moho depths increasing from the central altiplano 65 km to the northern altiplano 75 km the eastern cordillera shows large along strike variations in crustal thickness along a densely sampled sw ne profile through the bolivian orocline there is a small region of thin crust beneath the high peaks of the cordillera real where the average elevations are near 4 km and the moho depth varies from 55 to 60 km implying the crust is undercompensated by 5 km in comparison a broader region of high elevations in the eastern cordillera to the southeast near 20 s has a deeper moho at 65 70 km and appears close to isostatic equilibrium at the moho assuming the modern day pattern of high precipitation on the flanks of the andean plateau has existed since the late miocene we suggest that climate induced exhumation can explain some of the variations in present day crustal structure across the bolivian orocline we also suggest that south of the orocline at 20 s the thicker and isostatically compensated crust is due to the absence of erosional exhumation and the occurrence of lithospheric delamination

upper mantle s velocity structure of north america we have inverted fundamental and higher mode rayleigh waveforms from 685 vertical component broadband seismograms with wave paths over north america to obtain an image of the upper mantle s velocity structure down to 660 km among the well resolved features of the new model are 1 a high velocity root beneath the north american craton which extends no deeper than 250 km except near the archean core of the craton where depths of 350 km are reached 2 a weak band of low velocity along the eastern margin of the north american craton which reaches into the transition zone 3 a low velocity slab window beneath the western united states down to a depth of 300 km 4 areas of low uppermost mantle velocity beneath the cascade volcanoes the yellowstone hotspot track the colorado plateau the sierra madre occidental and the grabens bordering the jalisco block and 5 a pronounced band of high velocities in the transition zone coinciding with the expected location of the subducted trailing fragments of the farallon plate we introduce several improvements to the method of partitioned waveform inversion which was used to compute the new model rather than to correct for an estimated depth to the mohorovicic discontinuity we leave the crustal thickness as a free though yet poorly resolved parameter in the inversion we also improve the windowing and filtering operator used to select uncontaminated waveforms

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

lithospheric thickness jumps at the s atlantic continental margins from satellite gravity data and modelled isostatic anomalies abstract isostatic equilibrium is a good approximation for passive continental margins in these regions geoid anomalies are proportional to the local dipole moment of density depth distributions which can be used to constrain the amount of oceanic to continental lithospheric thickening lithospheric jumps we consider a five or three layer 1d model for the oceanic and continental lithosphere respectively composed of water a sediment layer both for the oceanic case the crust the mantle lithosphere and the asthenosphere the mantle lithosphere is defined by a mantle density which is a function of temperature and composition due to melt depletion in addition a depth dependent sediment density associated with compaction and ocean floor variation is adopted we analyzed satellite derived geoid data and after filtering extracted typical averaged profiles across the western and eastern passive margins of the south atlantic they show geoid jumps of 8 1 m and 7 0 m for the argentinian and african sides respectively together with topography data and an averaged crustal density at the conjugate margins these jumps are interpreted as isostatic geoid anomalies and yield best fitting crustal and lithospheric thicknesses in a grid search approach five parameters are systematically varied namely the thicknesses of the sediment layer the oceanic and continental crusts and the oceanic and the continental mantle lithosphere the set of successful models reveals a clear asymmetry between the south africa and argentine lithospheres by 15 km preferred models predict a sediment layer at the argentine margin of 3 6 km and at the south africa margin of 1 2 5 km moreover we derived a linear relationship between oceanic lithosphere sediment thickness and lithospheric jumps at the south atlantic margins it suggests that the continental lithospheres on the western and eastern south atlantic are thicker by 45 70 and 60 80 km than the oceanic lithospheres respectively

the nature of the crust in southern india implications for precambrian crustal evolution 1 we present crustal thickness and poisson s ratio determinations from receiver function analyzes at 32 sites on the archaean and proterozoic terrains of south india the crustal thickness in the late archaean 2 5 ga eastern dharwar craton varies from 34 39 km similar crustal thickness is observed beneath the deccan volcanic province and the cuddapah basin the most unexpected result is the anomalous present day crustal thickness of 42 51 km beneath the mid archaean 3 4 3 0 ga segment of the western dharwar craton since the amphibolite grade metamorphic mineral assemblages 5 7 kbar paleopressures in this part of western dharwar craton equilibrated at the depths of 15 20 km our observations suggest the existence of an exceptionally thick 57 70 km crust 3 0 ga ago beneath the exhumed granulite terrain in southernmost india the crustal thickness varies between 42 60 km the poisson s ratio ranges between 0 24 0 28 beneath the precambrian terrains indicating the presence of intermediate rock type in the lower crust these observations of thickened crust suggest significant crustal shortening in south india during the archaean

seismic evidence for an ancient rift beneath the cumberland plateau tennessee a detailed analysis of broadband teleseismic p waveforms broadband receiver functions developed from teleseismic p waveforms recorded on the midperiod passband of regional seismic test network station rscp are inverted for vertical velocity structure beneath the cumberland plateau tennessee the detailed broadband receiver functions are obtained by stacking source equalizd horizontal components of teleseismic p waveforms the resulting receiver functions are most sensitive to the shear velocity structure near the station a time domain inversion routine utilizes the radial receiver function to determine this structure assuming a crustal model parameterized by many thin flat lying homogeneous layers lateral changes in structure are identified by examining azimuthal variations in the vertical structure the results reveal significant rapid lateral changes in the midcrustal structure beneath the station that are interpreted in relation to the origin of the east continent gravity high located northeast of rscp the results from events arriving from the northeast show a high velocity midcrustal layer not present in results from the southeast azimuth this velocity structure can be shown to support the idea that this feature is part of a keweenawan rift system another interesting feature of the derived velocity models is the indication that the crust mantle boundary beneath the cumberland plateau is a thick probably laminated transition zonemore between the depths of 40 and 55 km a result consistent with interpretations of early refraction work in the area less

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

structure of the subduction transition region from seismic array data in southern peru data from three seismic arrays installed in southern peru were analysed using receiver functions from p pp and pkp wave phases in order to image the subducted nazca slab the arrays cover the transition region from flat slab subduction in central peru to normal subduction with an angle of about 30 further south a previous study used data from the first array in the normal subduction region to image the moho depth and slab and showed the existence of a mid crustal structure at 40 km depth that is suggested to be a possible underthrusting of the brazilian shield here we discuss new observations from the other two arrays that span the transition between the two subduction regimes and the flat subduction region the results provide an image of the flattened slab from the coast to approximately 300 km inland and also across the transition region from flat to 30 subduction which appears to be a bend rather than a tear in the slab in the flat slab region the slab is well defined near the coast and flattens out at 100 km depth beneath the altiplano the slab appears to start flattening some 400 km in advance of the subduction of the nazca ridge and the flattening is maintained for 1300 km after its passage the moho begins at a depth of around 30 km near the coast and has a maximum depth of 75 km beneath the altiplano consistent with the results of the other arrays both arrays also show a positive impedance mid crustal structure at 40 km depth which if explainable by underthrusting of the brazilian shield would add further support to the observations from the normal subduction region and show the northward and westward extent of the signal the underthrusting hypothesis would explain the missing crust from the shortening budget needed to support the altiplano the vp vs ratios for both arrays exhibit average values between 1 73 and 1 75 indicating a lesser likelihood that there is a high degree of partial melting or magma bodies at depth in this region the receiver function results provide new imaging of the flat slab and transition from normal to flat slab subduction which allows for comparison of different subduction regimes