seismic tomography a window into deep earth abstract the goal of this paper is to provide an overview of the current state of the art in seismic tomography and trace its origins from pioneering work in the early 1970s to its present status as the pre eminent tool for imaging the earth s interior at a variety of scales due to length limitations we cannot hope to cover every aspect of this diverse topic or include mathematical derivations of the underlying principles rather we will provide a largely descriptive coverage of the methodology that is targeted at readers not intimately familiar with the topic the relative merits of local versus global parameterization ray tracing versus wavefront tracking backprojection versus gradient based inversion and synthetic testing versus model covariance are explored a variety of key application areas are also discussed including body wave traveltime tomography surface wave tomography attenuation tomography and ambient noise tomography established and emerging trends many of which are driven by the ongoing rapid increases in available computing power will also be examined including finite frequency tomography full waveform tomography and joint tomography using multiple datasets several practical applications of seismic tomography including body wave traveltime attenuation and surface waveform are presented in order to reinforce prior discussion of theory

gulf of mexico tectonic history hotspot tracks crustal boundaries and early salt distribution a late jurassic mantle plume may have generated hotspot tracks on the north american plate and the yucatan penninsula tectonic block as the gulf of mexico opened ca 150 ma the tracks are identified from deep basement structural highs that have been mapped by integrating seismic refraction and gravity data they are associated with high amplitude distinctive gravity anomalies that provide the basis for a kinematic reconstruction that restores the western ends of the hotspot tracks with a 20 clockwise rotation of the yucatan block or almost one half the total rotation required to open the gulf of mexico basin the duration of track generation is estimated to have been about 8 10 m y or almost one half the total time required to open the gulf of mexico basin prior to this rotation extension of continental crust over a 10 12 m y interval was the result of approximately 22 of counterclockwise rotation and crustal thinning autochthonous salt appears to be confined to the continental flanks of the hotspot tracks confirming that salt was deposited during continental extension and not after ocean floor had begun to form a prominent gravity anomaly along the western boundary of the basin is interpreted to be produced by a marginal ridge which was created along the ocean continent transform boundary as the basin opened the eastern flank of this marginal ridge and the northernmost easternmost and southernmost terminations of the hotspot tracks are interpreted to coincide with the oceanic continental crustal boundary in the basin dale e bird consults in the oil and gas exploration industry as a potential fields specialist since 1981 his experience in the industry includes acquisition processing interpreting and marketing geophysical data including positions with aerodat world geoscience marathon oil company digicon and aero service he earned a ph d from the university of houston in 2004 and his research interests are regional geology and tectonics and the integration and interpretation of gravity and magnetic data he is a member of several local and international geological and geophysical societies kevin burke has been a professor of geology in the university of houston since 1983 having previously lived and worked in several parts of the world his main research is in tectonics the large scale evolution of planetary lithospheres his current interests include african and asian geology and the derivation of large igneous provinces from the core mantle boundary stuart a hall is a professor of geosciences specializing in the study of gravity and magnetic fields he received a b sc honors degree in physics from the university of birmingham united kingdom 1968 and a ph d in geophysics from the university of newcastle upon tyne united kingdom 1976 his current research interests are mainly focused on the use of geophysical data to investigate the development of small ocean basins and orogenic belts he is a member of the society of exploration geophysicists american geophysical union royal astronomical society and sigma xi research society john f casey is a professor of geology and chair of the department of geosciences at the university of houston specializing in structure tectonics and geochemisty he received a b a degree in geology from the university of pennsylvania in 1975 and a ph d in geology with honors from the state university of new york albany in 1980 his research has focused on the tectonics and geochemistry of midocean ridges transform faults and ophiolite bearing orogenic belts

lithospheric structure of the texas gulf of mexico passive margin from surface wave dispersion and migrated ps receiver functions the seismic velocity structure beneath texas gulf coastal plain gcp is imaged by migrating ps receiver functions with a seismic velocity model found by fitting surface wave dispersion we use seismic data from a linear array of 22 broadband stations spaced 16 20 km apart a common conversion point ccp stacking technique is applied to earthquake data to improve the s n ratios of receiver functions using an incorrect velocity model for depth migration of a stacked ccp image may produce an inaccurate image of the subsurface to find sufficiently accurate p and s velocity models we first apply a nonlinear modeling technique to fit rayleigh wave group velocity dispersion via very fast simulated annealing vs ranges from 1 5 km s in shallow layers of the gcp to 4 5 km s beneath the llano uplift and just outboard of the balcones fault zone bfz the bfz is characterized by slow velocities that persist to nearly 100 km depth in the stacked image the largest amplitude positive polarity event ranges from the surface at the llano uplift to a maximum depth of 16 km beneath matagorda island we attribute this event to the sediment basement contact which is expected to produce a large impedance contrast another large amplitude and positive polarity event at 35 km depth which likely marks the moho disappears outboard of the luling fault zone the disappearance of the moho beneath the gcp may be due to serpentinization of the upper mantle which would reduce the impedance contrast between the lower crust and upper mantle dramatically

building the pamir tibetan plateau crustal stacking extensional collapse and lateral extrusion in the central pamir 2 timing and rates geothermochronologic data outline the temperature deformation time evolution of the muskol and shatput gneiss domes and their hanging walls in the central pamir prograde metamorphism started before 35 ma and peaked at 23 20 ma reflecting top to n thrust sheet and fold nappe emplacement that tripled the thickness of the upper 7 10 km of the asian crust multimethod thermochronology traces cooling through 700 100 c between 22 and 12 ma due to exhumation along dome bounding normal sense shear zones synkinematic minerals date normal sense shear zone deformation at 22 17 ma age versus elevation relationships and paleoisotherm spacing imply exhumation at 3 km myr south of the domes mesozoic granitoids record slow cooling and or constant temperature throughout the paleogene and enhanced cooling 7 31 c myr starting between 23 and 12 ma and continuing today integrating the central pamir data with those of the east chinese pamir kongur shan and muztaghata domes and with the south pamir shakhdara dome implies i regionally distributed paleogene crustal thickening ii pamir wide gravitational collapse of thickened crust starting at 23 21 ma during ongoing india asia convergence and iii termination of doming and resumption of shortening following northward propagating underthrusting of the indian cratonic lithosphere at 12 ma westward lateral extrusion of pamir plateau crust into the hindu kush and the tajik depression accompanied all stages deep seated processes e g slab breakoff crustal foundering and underthrusting of buoyant lithosphere governed transitional phases in the pamir and likely the tibet crust

potential field evidence for a volcanic rifted margin along the texas gulf coast potential fi eld data along the texas portion of the gulf of mexico indicate a large amplitude coast parallel magnetic maximum and a smaller bouguer gravity high models constrained by seismic refraction data indicate that these maxima manifest a deeply buried volcanic rifted passive margin or other magnetic high in the outer transitional crust buried 12 15 km the source is 220 km wide similar to the voring plateau in norway and the u s east coast this margin which formed during the opening of the gulf of mexico differs in origin from the transform boundary of the northeast mexico margin tehuantepec transform and we infer a jurassic triple junction related to the borderland rift system which is traceable as far as southeast california

seismic vp vs tomography of texas oklahoma with a focus on the gulf coast margin abstract the northwestern gulf of mexico passive margin contains an extensive record of continental collision and rifting as well as deformation associated with orogenic events and heavy sedimentation seismic traveltime tomography that incorporates new data from 328 broadband seismic stations deployed throughout the region reveals features that correlate well with expected mantle structures as well as features that have no obvious expression at the surface among the former are a large fast anomaly that corresponds to the southern extent of the laurentia craton and a large slow anomaly associated with the southern oklahoma aulacogen among the latter are a slow layer that we interpret to be a shear zone at the base of the cratonic and transitional continental lithosphere a zone that is bounded at its top and bottom by discontinuities and high levels of seismic anisotropy identified in companion receiver function and shear wave splitting studies respectively a high velocity body underlying the gulf coastal plain may mark delaminating lower crust if this is true it could provide indirect evidence for an elevated geotherm during the rifting process that created the gulf of mexico

subduction of continental lithosphere in the banda sea region combining evidence from full waveform tomography and isotope ratios article i nfo we provide new insight into the subduction of old continental lithosphere to depths of more than 100 km beneath the banda arc based on a spatial correlation of full waveform tomographic images of its lithosphere with he pb nd and sr isotope signatures in its arc volcanics the thickness of the subducted lithosphere of around 200 km coincides with the thickness of precambrian lithosphere as inferred from surface wave tomography while the deep subduction of continental material in continent continent collisions is widely recognised the analogue process in the arc continent collision of the banda region is currently unique the integrated data suggest that the late jurassic ocean lithosphere north of the north australian craton was capable of entraining large volumes of continental lithosphere the banda arc example demonstrates that continental lithosphere in arc continent collisions is not generally preserved thus increasing the complexity of tectonic reconstructions in the particular case of timor the tomographic images indicate that this island is not located directly above the northern margin of the north australian craton and that decoupled oceanic lithosphere must be located at a considerable distance north of timor possibly as far north as the northern margin of the volcanically extinct arc sector the tomographic images combined with isotope data suggest that subduction of the continental lithosphere did not lead to the delamination of its complete crust a plausible explanation involves delamination within the continental crust separating upper from lower crustal units this interpretation is consistent with the existence of a massive accretionary complex on timor island with evidence from pb isotope analysis for lower crust involvement in arc volcanism and with the approximate gravitational stability of the subducted lithosphere as inferred from the tomographic images the subduction of continental lithosphere including crustal material beneath the banda arc indicates that global injection rates of continental material into the mantle may have been underestimated and that continental subduction in arc continent collisions may have contributed more to isotopic signatures of earth s mantle than previously recognised

upper mantle convection beneath the central rio grande rift imaged by p and s wave tomography 1 we present models for upper mantle p and s velocity structure beneath a southwestern united states transect extending from near the center of the colorado plateau across the rio grande rift to the great plains the models were derived from travel times of compressional and shear seismic phases recorded by the la ristra passive seismic array deployed from july 1999 to may 2001 large variations in seismic velocity up to 8 in s and 5 in p are seen across the transect in the upper 200 km of the mantle seismically slow mantle underlies the rio grande rift and jemez lineament and relatively slow mantle is seen beneath the navajo volcanic field within the colorado plateau the relative variations of p and s velocity imply that high temperatures are the primary cause of the slow mantle although a small amount of partial melting or hydration cannot be ruled out sharp boundaries in mantle seismic velocity are coincident with boundaries of proterozoic structural trends implying that ancient lithospheric structure exerts a control on the tectonic and magmatic activity in the region weaker seismic variations are imaged below 200 km depth with three southeastward dipping structures in our images two of the structures have fast seismic anomalies beneath the central colorado plateau and the great plains respectively and the third has anomalously slow seismic wave speed we interpret the western deep seismic anomaly to be foundering farallon slab and the slow anomaly just to the east as upwelling mantle possibly associated with volatile release from the sinking farallon slab beneath the great plains there is also downwelling in the upper mantle the combination of upwelling in the west and downwelling in the east are likely part of an upper mantle convection cell that may include entrained lithosphere from beneath the rift index terms 7218 seismology lithosphere and upper mantle 8121 tectonophysics dynamics convection currents and mantle plumes 8180 tectonophysics evolution of the earth tomography keywords convection rio grande rift colorado plateau

movement of amazon surface water from time variable satellite gravity measurements and implications for water cycle parameters in land surface models 1 the large scale observations of terrestrial water storage from grace satellites over the amazon are analyzed with land surface model lsm outputs of runoff and soil moisture a simple yet effective runoff routing method based on a continuity equation is implemented to model horizontal transport of surface water within the amazon basin the grace observations are analyzed separately for soil moisture and surface water storages generated from runoff relying on their distinct spatial patterns being disperse for soil moisture and localized for surface water various effective velocities for storage transport are tested against the grace observations when the model runoff is routed with an uniform velocity of 30 cm s the annual variation of the resulting surface water storage is generally found to be larger than the satellite measurements and ground gauge data by a factor of 1 5 or higher the peak annual anomaly of surface water storage is observed around the midstream of the amazon main stem however the runoff routing simulations present the peak amplitude consistently around the delta downstream unless the increasing velocity in a downstream region is used as complements to the ground gauge data the satellite observations provide unique spatial information of water cycle parameters our analysis indicates possible shortcomings in the certain lsm mass transport scheme between atmosphere and land surface particularly the production of too large seasonal variations in runoff and maybe too little variations in evapotranspiration and the dynamic characteristics of surface water transport within the amazon basins

application of simple shear model to the evolution of passive continental margins of the gulf of mexico basin the asymmetrical distribution of different crustal types under the gulf of mexico basin and sharp differences in the sedimentary record between the northern and southern parts of the gulf are the main arguments used to invoke the lithospheric simple shear model in interpreting the evolution of the basin9s conjugate passive margins in the framework of this model we interpret the northern and southern gulf margins as proximal and distal margins respectively

mantle flow and lithosphere asthenosphere coupling beneath the southwestern edge of the north american craton constraints from shear wave splitting measurements abstract high quality broadband seismic data recorded by the usarray and other stations in the southwestern united states provide a unique opportunity to test different models of anisotropy forming mechanisms in the vicinity of a cratonic margin systematic spatial variations of anisotropic characteristics are revealed by 3027 pairs of splitting parameters measured at 547 broadband seismic stations the western and southern edges of the north american craton show edge parallel fast directions with larger than normal splitting times and the continental interior is characterized by smaller splitting times and spatially consistent fast directions that are mostly parallel to the absolute plate motion direction of north america at the majority of the stations no significant systematic azimuthal variations of the splitting parameters are observed suggesting that a single layer of anisotropy with a horizontal axis of symmetry can adequately explain most of the observations the spatial coherency of the splitting parameters indicates that the observed anisotropy is likely caused by shearing between the partially coupled lithosphere and asthenosphere based on previous results of seismic tomography and geodynamic modeling we propose a model involving deflecting of asthenospheric flow by the cratonic root as the cause of the observed edge parallel fast directions and large splitting times along the western and southern edges of the north american craton

finite frequency tomography in southeastern tibet evidence for the causal relationship between mantle lithosphere delamination and the north south trending rifts 1 while several mechanisms have been suggested to explain the evolution of the tibetan plateau observational constraints on the deep lithospheric processes have been sparse and previous seismic studies were mostly along profiles perpendicular to the collision front of the indian and eurasian plates in this study we show tomographic evidence for the delamination of the mantle lithosphere beneath southeastern tibet a process in which the entire mantle lithosphere peels away from the crust along the moho and thus is a mechanism for rapid thinning of the lithosphere our p and s wave velocity models show the presence of a low velocity anomaly in the crust and upper mantle down to 300 km depth beneath a north south trending rift zone in southeastern tibet this low velocity anomaly is situated above a tabular high dipping angle high velocity anomaly that extends into the upper mantle transition zone the vp vs ratio of this high velocity anomaly suggests that temperature variations are not the only cause of the anomaly and a highly melt depleted mantle is required these observations suggest a causal relationship between the delamination of mantle lithosphere and the formation of the north south trending rift in southeastern tibet

study of plate deformation and stress in subduction processes using two dimensional numerical models two dimensional finite element modeling is used to model subduction of an oceanic lithospheric plate beneath continental lithosphere the subduction process is initiated along a preexisting inclined fault and continues until reaching 400 km of total convergence the lithosphere is assumed to be underlain by an in viscid asthenosphere different rheological laws have been considered for the lithosphere including elasticity and elastoplasticity the modeling shows that both the stress system in the plates and the surface topography are strongly dependent on two main parameters the density contrast between lithosphere and asthenosphere l a and the coefficient of friction along the subduction plane varying these two parameters allows explanation of the main characteristics of real subduction zones and results in two major regimes manifested by extension or compression in the arc back arc system extension and back arc rifting corresponds to a positive density contrast and a low coefficient of friction while negative values and or high friction leads to a compressional regime the coexistence of trench arc compression and back arc tension is only possible for a coefficient of friction lower than 0 1 the results of the numerical experiments agree with those of experimental modeling conducted under similar physical assumptions

sp receiver function imaging of a passive margin transect across texas s gulf coastal plain abstract the gulf coast of texas has been the subject of intensive geological and geophysical investigation in pursuit of hydrocarbons but studies that penetrate beyond the upper crust are limited to a few refraction profiles and regional surface wave investigations the passing of earthscope s transportable array has facilitated regional investigations of the lithosphere but its 70 km station spacing does not allow many important tectonic features to be imaged a broadband seismic transect across the texas gulf coastal plain was therefore performed in order to image deep structure beneath this passive margin and the transition to the neighboring craton a 2d sp receiver function common conversion point ccp stacked image produced for this transect reveals several discontinuities in the sub crustal lithosphere the region nearest the shoreline is underlain by an anomalous 18 km thick low velocity layer that produces a strong negative pulse in the sp receiver functions the drop in velocity is too large to be due to any reasonable change in fe or mg content but could be produced by partial melt or mantle hydration it is unlikely that partial melt would still be found in a 160 180 year old passive margin such as the gulf coast but hydration possibly introduced by a through going balcones fault system and resulting serpentinization could produce the observed anomaly an event with negative polarity appears at a depth of 110 km which we interpret to be the lithosphere asthenosphere boundary lab thermal variations alone would not produce a sufficiently sharp discontinuity to be imaged by sp converted phases recent shear wave splitting studies revealed unusually large delay times in this region along with fast polarization directions that differ from measurements on the laurentian craton large delay times may imply significant flow which could also produce frictional heating due to shearing and partial melt which would steepen the velocity gradients an additional layer is bounded by discontinuities at 145 165 km depth with negative polarity and 185 205 km depth with positive polarity the appearance of these discontinuities suggests that the mantle beneath the lab is more complicated here than elsewhere and that the transition from lithosphere to asthenosphere occurs over a broad 85 100 km thick depth interval

teleseismic tomography of the upper mantle beneath the southern lachlan orogen australia with the goal of better understanding the deep structure and tectonic setting of the lachlan orogen 50 short period seismic stations were deployed across the southern end of the great dividing range in victoria southeast australia between 2005 and 2006 to record distant earthquakes a total of 7452 relative pwave arrival time residuals from 169 teleseismic events have been extracted from the continuous records using an adaptive stacking technique which exploits the coherency of global phases across the array these residuals are mapped as three dimensional perturbations in p wavespeed in the upper mantle beneath the array using a recently developed iterative non linear tomographic procedure which combines a grid based eikonal solver and a subspace inversion technique the capability of the new scheme to include interface geometry is utilised in order to investigate the effects of a priori moho topography on the resolution of upper mantle structure the resultant images show a pattern of p wavespeed anomalies that lacks a predominant orientation and therefore does not favour a purely w e subduction accretion model for the formation of the lachlan orogen one of the main features of the three dimensional model is a zone of elevated wavespeed beneath the northern end of the array which extends to a depth of approximately 150 km and contrasts with significantly lower wavespeeds to the south this anomaly which does not appear to be an artifact of relative arrival time residual contributions from the adjoining mountainous terrane may reflect the presence of a substantial piece of proterozoic lithosphere incorporated within the phanerozoic subduction accretion setting of the lachlan orogen another key feature of the solution model is a zone of relatively low velocity beneath the newer volcanic province northwest of melbourne which extends from the crust to a depth of approximately 200 km this is likely to represent the signature of elevated temperatures associated with a diffuse mantle source for the quaternary volcanism in victoria 2008 elsevier b v all rights reserved

relationship between observed upper mantle structures and recent tectonic activity across the southeastern united states the lithospheric structure of the southeastern united states is a product of earlier episodes of continental collision and breakup the region is located in the interior of the north american plate away from active plate margins however there is ongoing tectonism in the region with multiple zones of seismicity uplifting arches and cenozoic intraplate volcanism the mechanisms controlling this activity and the state of stress remain enigmatic two important factors are plate strength and preexisting inherited structures here we present new tomographic images of the upper mantle beneath the southeastern united states revealing large scale structural variations in the upper mantle examples include the relatively thick lithospheric mantle of stable north america that abruptly thins beneath the paleozoic appalachian orogeny and the slow upper mantle of the proterozoic reelfoot rift our results also indicate fast seismic velocity patterns that can be interpreted as ongoing lithospheric foundering this provides a viable explanation for seismicity uplifting and young intraplate volcanism we postulate that not only tectonic inheritance but also continuing lithospheric foundering may control the ongoing activity of the region long after it became a passive margin based on distinct variations in the geometry and thickness of the lithospheric mantle and foundered lithosphere we propose that piecemeal delamination has occurred beneath the region throughout the cenozoic removing a significant amount of reworked deformed mantle lithosphere ongoing lithospheric foundering beneath the eastern margin of stable north america explains significant variations in thickness of lithospheric mantle across the former grenville deformation front

seismic structure of the southeast australian lithosphere from surface and body wave tomography abstract teleseismic arrival time residuals from the wombat transportable seismic array experiment are inverted to construct a high resolution 3 d p wave velocity model of the upper mantle beneath southeast australia in order to address one of the principal limitations of teleseismic tomography that long wavelength structure is filtered out when data from multiple arrays operating at different times are used an initial model with lower spatial resolution derived from surface tomography is constructed to preserve the broad scale features that would otherwise be lost although the absolute velocities of the final model are not strongly constrained due to the assumption of radial vp vs ratios and differences in regularisation the relative variations appear robust across all scales these reveal a wealth of features that can be related to the geology and tectonic history of the region the most significant being 1 an easterly dipping velocity transition zone which involves a higher velocity delamerian orogen extending beneath the western subprovince of a lower velocity lachlan orogen 2 a distinct region of low velocity in the upper mantle north of melbourne which can be associated with recent quaternary hot spot volcanism 3 a gradual east southeast decrease in velocity towards the coast which is consistent with lithospheric stretching and thinning near a passive margin and 4 a zone of high velocity north of adelaide that may correspond to the presence of the palaeoproterozoic curnamona province at depth these results have important implications for the palaeozoic evolution of the east margin of gondwana the subsequent break up of australia and antarctica and opening of the tasman sea

p and s wave tomography of the mantle beneath the united states mantle seismic structure beneath the united states spanning from the active western plate margin to the passive eastern margin was imaged with teleseismic p and s wave traveltime tomography including usarray data up to may 2014 to mitigate artifacts from crustal structure 5 40 s rayleigh wave phase velocities were used to create a 3 d starting model major features of the final p and s models include two distinct low velocity anomalies at depths of 60 300 km beneath the central and northern appalachians and passive margin the central appalachian low velocity anomaly coincides with eocene basaltic magmatism and the northern anomaly is located along the cretaceous track of the great meteor hot spot at depths of 300 700 km beneath the central and eastern u s large high velocity anomalies are inferred to be remnants of the farallon slab that subducted prior to 40 ma during the laramide orogeny

structure and evolution of the central gulf of mexico continental margin and coastal plain southeast united states as in many continental margins mesozoic rifting in the gulf of mexico trended subparallel to older tectonic structures created by opening and closure of a prior ocean basin unlike the situation in most other margins however extension in the gulf of mexico did not involve widespread reactivation of these older tectonic features this difference may be related to the unusual facts that 1 the antecedent passive continental margin was a transform rather than a rift margin and 2 the prior orogeny did not substantially thicken the crust in order to characterize the interaction between mesozoic rifting and prior tectonic events two geophysical transects were constructed across the central gulf of mexico coastal plain and continental shelf the transects show that the early paleozoic passive continental margin buried beneath the late paleozoic ouachita orogen was little affected by compressional tectonism or mesozoic extension during opening of the gulf of mexico the thickness of the crust on this ancient margin decreases from 35 km to 10 km over a distance of only 50 km consistent with previous interpretations that this was a transform margin the ouachita fold and thrust belt is shown to be a thin skinned doubly vergent orogen that formed above a well preserved southward dipping subduction system relict cambrian oceanic crust may exist beneath the southern part of the orogen but its presence is not required by the data the thin crust and shallow mantle in the ouachita suture created a zone of relatively high strength that acted as a barrier to mesozoic extension which is restricted to regions south of the orogen extension was initially distributed over a broad area that included the mississippi interior salt basin and regions south of the wiggins arch extension became progressively more focused south of the wiggins arch with time culminating in the onset of seafloor spreading beneath the modern continental rise mesozoic extension factors are 1 4 1 9 beneath the mississippi interior salt basin 2 9 immediately south of the wiggins arch and 4 adjacent to the oldest oceanic crust in the central gulf of mexico

mantle unrooting in collisional settings abstract we present a two dimensional numerical model to study the thermo mechanical evolution of the lithosphere under a convergence regime in order to define the conditions that lead to lithospheric mantle break up and consequent unrooting a newtonian rheology with a temperature dependent viscosity is considered the system is not closed and horizontal flow through lateral boundaries is permitted a horizontal velocity is imposed at the top of the model to simulate compression whereas velocity vanishes at the bottom of the model the initial conditions correspond to a homogeneous lithosphere with a constant heat production in the crust the analysis of variations of maximum shear stress strain rate and total kinetic energy allowed us to define four major stages during the mantle unrooting process orogenic growth initiation of gravitational instability until lithospheric failure sinking of the detached lithosphere and relaxation of the system numerical results also show that the conditions for lithospheric unrooting strongly depend on the convergence velocity the wideness of the deformation zone and the imposed rheology

seismic heterogeneity and small scale convection in the southern california upper mantle we invert traveltime residuals of teleseismic p and s phases for 3 d perturbations in vp vs and vp vs structures in the southern california upper mantle the tomographic inversion uses frequency dependent 3 d sensitivity kernels to interpret traveltime residuals measured in multiple frequency bands and recent advances in regional crustal thickness and velocity models to better isolate the mantle component of traveltime residuals in addition to separate vp and vs tomography we jointly invert the p and s data sets for vp vs perturbations by imposing a smoothness constraint on the lnvs lnvp field the regional upper mantle is very heterogeneous with the greatest amplitude of velocity and vp vs perturbations in the upper 200 km and mantle structures that correlate spatially with the major physiographic provinces of southern california our imaging improves the resolution of the geometry and amplitude of these features in addition to the major structures imaged and discussed in previous papers we find a large high velocity anomaly at depths between 340 and 500 km beneath the borderland transverse ranges and northern peninsular ranges this anomaly is separate from the well known uppermost mantle transverse ranges anomaly which extends no deeper than 175 200 km the strongest low velocity high vp vs anomaly is found beneath and west of the salton trough and is attributed to relatively high partial melt in the asthenosphere as a result of lithospheric thinning the magnitudes of vp vs and vp vs perturbations and knowledge of regional 1 d velocity and attenuation lead us to conclude that the asthenosphere contains up to 1 partial melt extending to depths as great as 150 200 km

mesoproterozoic plate tectonics a collisional model for the grenville aged orogenic belt in the llano uplift central texas the llano uplift of central texas united states exposes the core of a mesoproterozoic orogenic belt that formed along the southern margin of laurentia during grenville time a new collisional model is proposed that reconciles differences in structural stacking apparent tectonic transport and deformation conditions between the eastern and western portions of the uplift and explains uplift and exhumation of high pressure eclogitic rocks emplacement of ophiolitic rocks and subsequent late stage to postcollisional plutonism our model proposes that subduction with southward polarity resulted in collision of an exotic arc with laurentia emplacement of ophiolitic rocks and telescoping of the intervening basinal sediments followed by overriding of the arc and margin of laurentia by a southern continent with transport toward laurentia the model further proposes that convergence led to subduction of the laurentian margin resulting in high pressure metamorphism but buoyancy forces due to subduction of continental crust under the southern continent resulted in uplift and retrotransport away from laurentia in a manner similar to that proposed for the alpine orogeny slab breakoff resulted in upwelling of the asthenosphere leading to intrusion of juvenile granitic plutons subduction along strike caused continued contraction that waned with time the eastern uplift records continent arc continent collision whereas the western uplift records continent continent collision the two regions also expose different crustal levels in the orogen the striking similarity with phanerozoic orogens including emplacement of ophiolites and formation of high pressure rocks implies that plate tectonic processes including subduction were active prior to the neoproterozoic

upper mantle seismic structure beneath se and central brazil from p and s wave regional traveltime tomography summary we present models for the upper mantle velocity structure beneath se and central brazil using independent tomographic inversions of p and s wave relative arrival time residuals including core phases from teleseismic earthquakes the events were recorded by a total of 92 stations deployed through different projects institutions and time periods during the years 1992 2004 our results show correlations with the main tectonic structures and reveal new anomalies not yet observed in previous works all interpretations are based on robust anomalies which appear in the different inversions for p and s waves the resolution is variable through our study volume and has been analyzed through different theoretical test inversions high velocity anomalies are observed in the western portion of the sao francisco craton supporting the hypothesis that this craton was part of a major neoproterozoic plate san franciscan plate low velocity anomalies beneath the tocantins province mainly fold belts between the amazon and sao francisco cratons are interpreted as due to lithospheric thinning which is consistent with the good correlation between intraplate seismicity and low velocity anomalies in this region our results show that the basement of the parana basin is formed by several blocks separated by suture zones according to model of milani ramos the slab of the nazca plate can be observed as a high velocity anomaly beneath the parana basin between the depths of 700 and 1200 km further we confirm the low velocity anomaly in the ne area of the parana basin which has been interpreted by vandecar et al as a fossil conduct of the tristan da cunha plume related to the parana flood basalt eruptions during the opening of the south atlantic