compositional variation and the origin of deep crustal reflections abstract identifying the sources of crustal reflections is essential for deriving more geological information from deep crustal reflection profiles theoretical and model studies help place constraints on the role of compositional variation in producing deep crustal reflections analysis of laboratory derived velocities and densities of rock types typical for the continental crust indicates that reflection coefficients are generally small but significantly 17 of the possible reflection coefficients have magnitudes between 0 1 and 0 2 comparison between reflection coefficients derived from well logs and reflections observed in associated seismic profiles indicates that constructive interference associated with geological layering is at least as important as the magnitude of the reflection coefficients for producing detectable reflections constructive interference can increase reflection amplitude by two to three times but is limited to a relatively narrow range of layer thickness for a typical 10 40 hz seismic wavelet and typical crustal velocities of about 6 km s constructive interference occurs for layer thickness ranging between about 35 and 80 m layers thinner than 35 m interfere destructively if reflections result from compositional variation seismic models of hypothetical and observed geologic relations provide analogs for interpreting complex reflection patterns observed in deep crustal reflection profiles such models show reflection patterns similar to those observed in the reflection profiles the models indicate that the reflections could originate in the complexly deformed and intruded terranes that are common in the crystalline crust and it may not be necessary to appeal to unobserved phenomena such as special lamellae or fluid filled fractures to explain the reflections

synthetic seismograms for a complex crustal model the algorithm of the original reflectivity method has been vectorized and implemented on a cdc cyber 205 computer calculation times are shortened by a factor of 20 to 30 compared with a general purpose computer with a capacity of several million floating point operations per second mflop the rapid calculation of synthetic seismograms for complex models high frequency sources and all offset ranges is a provision for modeling not only particular phases but the whole observed wavefield as an example we model refraction data of the black forest southwest germany and are able to derive rather tight constraints on the physical properties of the lower crust

properties of the lower crust from modeling refraction and reflection data data of a refraction and a reflection seismic survey in the black forest of southwest germany are used for extensive one and two dimensional modeling the data are available along approximately the same line and therefore the same piece of crust is probed by two seismic methods we utilize this favorable circumstance for detailed model calculations concerning both data sets in this paper we focus on the lower crust its properties vary on the scale of a wavelength and thus full solutions of the elastic equations are required we use the reflectivity method for the evaluation of refraction seismograms and numerical solutions of the acoustic wave equation for the reflection response the lamellae of the lower crust appear as reflecting elements of several hundred meters length randomly distributed in space rocks with extremely low velocities 5 6 km s must be present indicating that anisotropy and or fluids play an important role in the lower crust ideas concerning lower crustal formation evolution and extinction of lamellaetion are developed

intermediate and deep seismicity and lateral structure of subducted lithosphere in the circum pacific region in this paper we present a region by region review of the wadati benioff zone structure of most of the world s seismically active subduction zones focusing primarily on the intermediate and deep seismicity lateral changes in wadati benioff zone structure are common in every major subduction zone in this study we use these changes to define possible boundaries between portions or segments of lithosphere with differing subduction geometries although earthquake data seldom have the resolution to show conclusively whether these boundaries separate independent blocks of lithosphere the available data indicate that the active process at most of these segment boundaries is ductile deformation of the subducting plate rather than tearing we found the strongest evidence for the existence of tears where tears are geometrically necessary such as where a transform boundary terminates a trench as at the new hebrides tonga and south sandwich trenches weak evidence suggesting other tears does exist in some regions such as taiwan the japan izu bonin corner and the philippines the causes of these changes in structure are in most cases unclear only about 34 of the possible segment boundaries coincide with subducting bathymetric features some boundaries occur where there is apparent lateral strain caused by anomalous trench geometry we have designed a simple modeling procedure which incorporates published plate motion and the observed geometry of trenches and wadati benioff zones to estimate the lateral strain in subduction zones throughout the circum pacific region although no observed subduction zone has a perfectly strain free geometry there is a broad range of geometries for which the lateral strain is small indeed the observed geometry of most subduction zones involve relatively little lateral strain comparison with centroid moment tensor focal mechanisms indicates that in zones where the modeling predicted little lateral strain the mechanisms of intermediate and deep earthquakes show no effects of lateral stress and downdip stresses are clearly dominant in regions such as the mariana arc where the model predicts very large lateral extension lateral tension is very evident in the focal mechanisms in regions such as the hokkaido corner where the modeling predicts large compressional strains the plate appears to buckle and bending stresses parallel to the trench are evident in general this study finds that subducted lithosphere is remarkably cohesive and rigid and only rarely deforms by breaking or stretching

convectively induced transition in mantle rheological behavior mantle flow occurs through both linear and nonlinear creep mechanisms and the style of convection is strongly influenced by the dominant creep mechanism in a composite rheology we show here by numerical modelling of convection with a composite flow law that the nonlinear strain rate dependence of mantle rheology would dominate over the linear newtonian portion for sufficiently vigorous convection such as might have prevailed in the early earth this rheological transition occurs at a rayleigh number value which increases dramatically with higher values of the activation energy for activation energy values for dislocation and diffusion creep of about 100 and 70 kj mole respectively we find a transition between rayleigh numbers 106 and 107 corresponding values of the transition stress agree with estimates for the lower mantle from empirical data for perovskite analogue these results suggest that early in the earth s thermal history a transition would have taken place from a primarily non newtonian flow to a newtonian temperature dependent style of mantle convection with a high activation energy greater than 125 kj mole the mantle would have been in a newtonian regime throughout most of its thermal history

hotspot evolution and venusian tectonic style because hotspots represent an important manifestation of heat loss on venus their geological evolution is of fundamental importance for any attempt to understand venusian tectonics eistla regio is a 7500 km long moderately elevated region inferred to overlie one or more large mantle upwellings or hotspots it also contains many shield volcanoes and coronae believed due to the rise of thermal plumes in the mantle central eistla regio includes two large volcanoes sappho and anala and several coronae in close proximity detailed mapping in this region results in two conclusions of tectonic significance 1 sappho and anala occur near the intersection of two major extensional deformation zones and 2 the coronae are older than the large volcanoes several of the coronae occur as a chain along guor linea one of the major extensional deformation zones stratigraphic relationships indicate that the coronae began forming very soon after the emplacement of the widespread regional plains materials thus central eistla regio was the site of a swarm of plumes that first formed coronae and then later formed shield volcanoes the expected result of such a swarm would be thermal thinning of the elastic lithosphere with time however model results geological observations and gravity data suggest that the change from coronae to shield volcanoes was accompanied by a thickening of the lithosphere with time this thickening is interpreted to be the result of global cooling of the lithosphere following the most recent episode of near global resurfacing the global cooling must have occurred faster than local heating of the lithosphere due to the impingement of thermal plumes

temperature dependent newtonian and non newtonian convection implications for lithospheric processes convection studies of temperature dependent newtonian and non newtonian rheology in aspect ratio seven boxes have been carried out for volume averaged rayleigh numbers between o 105 and o 107 large lateral viscosity variations o 104 are found for non newtonian cases with total temperature dependent viscosity variations up to 250 flow structures for both rheologies are distinguished by large scale circulations with relatively stable descending limbs much larger hot thermal anomalies are found near the surface for non newtonian rheology lithospheric thinning is facilitated by non newtonian rheology because of stress softening and the lubrication of the descending limbs by hot diapirs

migration of synthetic seismograms for crustal structures with random heterogeneities summary fast frequency wavenumber migration is used to migrate synthetic seismograms calculated by a hybrid method for crustal models with complex 2 d scattering structures in the lower crust scatterers with predominantly horizontal or vertical orientation and scatterers without preferred orientation isotropic scatterers are investigated horizontal scatterers or lamellae have frequently been suggested for the lower crust on the basis of modern reflection seismological experiments in none of the cases studied here are the scattering structures imaged correctly the reason is mainly multiple scattering which can produce coherent arrivals in the seismogram sections and hence coherent signatures in the migrated sections which have no relation to structure imaging is generally acceptable for horizontal scatterers but for isotropic or vertical scatterers the migrated signatures are also horizontal and thus do not represent reality it is concluded that the highly detailed line drawings which are popular in crustal reflection seismology are less reliable than believed as far as the internal structure of scattering zones and the scatterer orientations are concerned horizontal or subhorizontal structures in the lower crust may be less common than assumed the paper also briefly addresses a few methodical aspects of fast frequency wavenumber migration for depth dependent background structure in particular the concept of sounding beams their width and the construction of a migrated section from several sounding beams

instabilities from phase transitions and the timescales of mantle thermal evolution we have investigated the effects of mantle phase transitions on the earth s thermal evolution by numerical modelling our results show that the tendency of layering grows with increasing vigor of convection a self consistent model which couples the convective dynamics of the mantle to the secular cooling of the core is constructed for a temperature dependent effective viscosity we found that the thermal history of the earth is characterized by two timescales one is associated with layered convection in the early period which is followed by a transition period with dramatic overturns as the rayleigh number decreases sufficiently low whole mantle convection prevails and a faster cooling timescale ensues

tectonic plates d thermal structure and the nature of mantle plumes it is proposed that subducting tectonic plates can affect the nature of thermal mantle plumes by determining the temperature drop across a plume source layer the temperature drop affects source layer stability and the morphology of plumes emitted from it numerical models are presented to demonstrate how introduction of platelike behavior in a convecting temperature dependent medium driven by a combination of internal and basal heating can increase the temperature drop across the lower boundary layer the temperature drop increases dramatically following introduction of platelike behavior due to formation of a cold temperature inversion above the lower boundary layer this thermal inversion induced by deposition of upper boundary layer material to the system base decays in time but the temperature drop across the lower boundary layer always remains considerably higher than in models lacking platelike behavior on the basis of model inferred boundary layer temperature drops and previous studies of plume dynamics we argue that generally accepted notions as to the nature of mantle plumes on earth may hinge on the presence of plates the implication for mars and venus planets apparently lacking plate tectonics is that mantle plumes of these planets may differ morphologically from those of earth a corollary model based argument is that as a result of slab induced thermal inversions above the core mantle boundary the lower most mantle may be subadiabatic on average in space and time if major plate reorganization timescales are less than those required to diffuse newly deposited slab material

the seismic reflection character of the continental mohorovicic discontinuity the continental mohorovicic discontinuity is most often interpreted as a step function velocity boundary however on deep crustal seismic reflection profiles reflections at depths where refraction data places the moho have laminated character and are laterally discontinuous these observations on continental reflection data point to a model of the moho that has thin layered rather than block structure previous workers who have synthetically generated seismic responses from various crust mantle boundary models have shown that thin layered models of alternating high and low velocity generate responses that best emulate observed moho arrivals geologic interpretations of the thin layering include relatively undeformed metasediments cumulate layering tectonic banding and lenses of partial melt to obtain more direct geologic evidence a synthetic seismogram is generated from geologic cross sections of the ivrea verbano zone an exposed section through the lower crust and upper mantle the synthetic shows laminations similar to those observed on reflection data without more direct evidence no single geologic interpretation of the moho is reasonable indeed the moho may be a laterally variable boundary its composition and structure dependent upon the geologic history of the overlying crust

dynamical consequences on fast subducting slabs from a self regulating mechanism due to viscous heating in variable viscosity convection the authors have studied 2 d time dependent convection for a rheology which is both non newtonian and temperature dependent strong effects associated with viscous heating are found in the downwelling sheets which are heated on both sides with an intensity around o 10 sup 2 times the chondritic value the magnitude of viscous heating increases strongly with the subduction speed the slab interior is weakened by viscous heating and slab breakoff then takes place this process provides a self regulating mechanism for governing the speed of intact slabs able to reach the deep mantle timescales associated with viscous heating are quite short a few million years internal heating by radioactivity decreases the amount of shear heating 13 refs 5 figs

subduction zones observations and geodynamic models abstract this review of subduction and geodynamic models is organized around three central questions 1 why is subduction asymmetric 2 are subducted slabs strong or weak 3 how do subducted slabs interact with phase transformations changes in mantle rheology and possibly chemical boundaries in the mantle based on laboratory measurements of the temperature dependence of olivine one would conclude that the core of a subducting slab is at least 10 000 times more viscous than ambient mantle however there are a number of complementary but independent observations that suggest that slabs are much weaker than this slabs undergo significant deformation in the upper mantle and may thicken to twice their original width by the time they reach the base of the transition zone the lack of a clear correlation between the observed dip angle of deep slabs and plate velocity rate of trench migration and slab age in modern subduction zones is consistent with hypothesis that subduction is a time dependent phenomenon both tank and numerical convection experiments with plates conclude that subduction is not a steady phenomenon but that slabs bend thicken stretch and change dip through time this is at odds with the assumptions used in steady state slab thermal models where slab deformation is not considered

coincident seismic reflection refraction studies of the continental lithosphere a global review nearly 50 coincident seismic reflection refraction studies to depths of at least the moho provide an improved understanding of the continental lithosphere some conclusions include the following 1 a transparent upper crust a common observation on vertical reflection profiles cannot generally be correlated with velocity gradients or low velocity zones rather a commonly transparent upper crust may be explained by short wavelength steeply dipping features in the brittle upper crust and to a lesser degree by signal contamination from source generated noise 2 the reflective lower crust in extensional terranes appears to be characterized by a high average seismic velocity 6 6 7 3 km s and to consist of laminated high and low velocity layers with typical thicknesses of 100 200 m 3 landward dipping reflectors observed in the middle to lower crusts of convergent zones have been identified as paired high and low velocity slabs which represent oceanic crust and mantle accreted via underplating to the continental margin 4 the crust mantle boundary may differ sufficiently when imaged with vertical incidence and wide angle data to justify the retention for the present of the concept of separate reflection and refraction mohos while there is good evidence that these features are coincident within measurement uncertainties in most regions recently recorded data from the basin and range admit the possibility for noncoincidence in that area 5 upper mantle reflections which cannot be migrated into the lower crust remain rare despite isolated unequivocal examples thus the upper mantle appears to be relatively homogeneous at seismic reflection wavelengths and to lack the laminations inferred for the lower crust the wide angle method will likely provide the most reliable information on the velocity structure and physical state of this portion of the lithosphere for some years to come 6 there appear to be clear and consistent basic differences between convergent and extensional terranes which have been identified from coincident experiments these differences may be sufficiently universal to infer the tectonic history of poorly exposed terranes 7 no truly three dimensional coincident experiment i e including three dimensional migration has been conducted but some three dimensional data have been collected using both methods measurements of attenuation poisson s ratio and anisotropy within the crust using coincident data sets remain frontiers

modelling of viscoelastic plume lithosphere interaction using the adaptive multilevel wavelet collocation method summary modelling of mantle flows with sharp viscosity contrasts in a viscoelastic medium is a challenging computational problem in geodynamics because of its multiple scale nature in space and time we have employed a recently developed adaptive multilevel wavelet collocation algorithm to study the dynamics of a small rising diapir interacting with a stiff lithosphere in a maxwell viscoelastic mantle in this kinematic model we have prescribed the upward velocity of the diapir and then we need to integrate in time only the momentum equation governing the temporal evolution of the pressure stress and velocity components which together constitute a sixth order system in time the total number of collocation points did not exceed 10 4 compared to more than 10 6 gridpoints using conventional evenly spaced grid methods the viscosity of the diapir is 10 x4 times lower than that of the surrounding mantle while the viscosity of the thin lithosphere about 5 10 per cent of the entire layer depth is 10 4 10 8 times stiffer than the ambient mantle our results demonstrate the efficacy of wavelets to capture the sharp gradients of the stress and pressure fields developed in the diapiric impingement process the interaction of the viscoelastic lithosphere with the rising viscoelastic diapir results in the localization of stress within the lithosphere the magnitude of the stress fields can reach around 100 300 mpa our simple kinematic model shows clearly that viscoelasticity can potentially play an important role in the dynamics of the lithosphere especially concerning the potential severage of the lithosphere by mantle upwellings

exposed cross sections through the continental crust implications for crustal structure petrology and evolution abstract the continental crust is exposed in cross section at numerous sites on the earth s surface these exposures which appear to have formed by obduction along great faults during continental collision may be recognized by exposures of deep crustal rocks exhibiting asymmetric patterns of metamorphic grade and age across the faults and by distinctive bouguer anomaly patterns reflecting dipping basement structure and an anomalously deep mantle from an examination of five complexes which meet these criteria it is concluded that the most prominent layering in the crust is not compositional but metamorphic the lower crust consists of granulite facies rocks of mafic to intermediate composition while the intermediate and shallow levels consist predominantly of amphibolite facies gneisses and greenschist facies supracrustal rocks respectively post metamorphic granitic intrusions are common at intermediate to shallow levels position of discontinuities in refraction velocity where present commonly correspond to changes in composition or metamorphic grade with depth the continental crust is characterized by lateral and vertical heterogeneities of varying scale which are the apparent cause of the complex seismic reflections recorded by cocorp field observations coupled with geochemical data indicate a complex evolution of the lower crust which can include anatexis multiple deformation polymetamorphism and reworking of older crustal material the complexity of the crust is thus the result of continuous evolution by recycling and metamorphism through time in a variety of tectonic environments

ultrafast upwelling bursting through the upper mantle a high resolution calculation of strongly time dependent thermal convection in the upper mantle with non newtonian temperature dependent rheology shows that for an effective rayleigh number of around 106 extremely fast upwellings at times exceeding 10 m yr can be generated a few hundred kilometers below the lithosphere there is a clear separation of timescales between this fast jet and the more slowly convecting mantle within this fast vertical shear layer is embedded a thermal boundary layer with a width of the order of 50 km the development of the fast non newtonian upwelling is characterized by the growth of the plume head to a large enough size before the plume takes off rapidly at a depth of around 350 km upon impinging the base of the lithosphere this fast plume thins the lithosphere and the flow then becomes a horizontally moving hot sheet extending out for around 1000 km this scenario is found to repeat itself at the same location about 10 myr after the first plume impingement

the potential role of non newtonian rheology in the resurfacing of venus several analyses of the impact cratering record on venus have concluded that 80 of venus was resurfaced sometime in the last 300 m y 700 m y the cratering record further indicates that the resurfacing took place over a 30 m y 100 m y interval since this period there has been little large scale tectonic activity on venus this study proposes that such behavior may result from the interaction of phase change induced avalanches with a non newtonian lithosphere the mechanism proposed here suggests that because of non newtonian rheology the venusian lithosphere becomes highly mobile during brief periods when avalanches occur afterwards due to the reduction in stress levels the lithosphere becomes rigid and large scale tectonics ceases

rheological transition in mantle convection with a composite temperature dependent non newtonian and newtonian rheology numerical simulations of mantle convection with a composite temperature dependent newtonian and non newtonian creep law have revealed a transition in the dominant creep mechanism with the increasing vigour of convection newtonian creep is found to dominate in the low rayleigh number regime with sufficiently high effective rayleigh number the overall creep mechanism in the convective flow becomes non newtonian the transitional rayleigh number increases strongly with the activation energy these results would suggest a scenario that in the early epochs of earth the flow in the mantle would have been governed by non newtonian rheology and would have exhibited both strong spatial and temporal fluctuations with time the flow mechanism would behave like a newtonian fluid and would have a different time dependent character in time dependent newtonian dominated flows there are still localized features with distinctly non newtonian character our analysis of the relative contributions to the lateral viscosity field supports the idea that the inference of the nature of lateral viscosity heterogeneities by seismic tomography may be strongly contaminated by the dominant non newtonian contributions to the total lateral viscosity field

the effects of adiabatic and viscous heatings on plumes we have assessed the influence of viscous and adiabatic heatings on the transient development of plumes by employing the time dependent convection equations in the extended boussinesq limit both temperature dependent and temperature and pressure dependent viscosities are employed for constant thermal expansion of coefficient we find the vigor of plume flows is diminished with larger magnitudes of the thermal expansivity this effects is caused by the local domination of adiabatic decompression over viscous heating inside the developing plume for both the cartesian and axisymmetric domains for deep mantle flows a thermal expansivity decreasing with depth is required for thin plume structures to exist values of the thermal expansivity contrast which can promote plume growth are in accord with estimates from recent studies in seismic equation of state copyright american geophysical union 1987

active lithospheric thinning abstract this paper starts with a discussion of the physical mechanisms capable of inducing a rapid upward migration of the lithosphere asthenosphere boundary two models based on a thermomechanical destabilization of the lower lithosphere are proposed first we consider the enhancement of small scale sublithospheric convection modes by a hot thermal plume a perturbation of a few hundred degrees is found to thin the lithosphere and generate uplift within a few tens of million years this efficient erosion from below only stops when it reaches a layer of low density such as the crust or some depleted mantle material second we show that passive lithospheric stretching leads to an unstable lithospheric configuration on the sides of the rift the denser unstretched lithosphere is subject to a lateral convective penetration by the hotter asthenospheric mantle material coming from the rift zone this lateral destabilization generates uplift of the shoulders natural examples can be found along passive continental margins a good example is given by the uplift of the scandinavian caledonides following the break up in the north atlantic

fast plumeheads temperature dependent versus non newtonian rheology in a series of two dimensional cartesian simulations with high resolution on the order of 2 km we have compared for the same effective rayleigh number around 106 the dynamics between newtonian and non newtonian rheologies a viscosity contrast due to temperature of 1200 has been employed throughout with surface dissipation numbers varying between 0 05 and 0 2 peak velocities for non newtonian plumes are at least 50 times faster than the peak newtonian velocities and reach magnitudes in excess of m yr upon impinging the surface plume head structures of the non newtonian rheology contain much greater complexity in the thermal mechanical structure than for newtonian plumes

nature of the lower crust and moho in eastern nevada from wide angle reflection measurements wide angle recordings at offsets between 20 and 40 km in the ruby mountains consistently show 5 strong reflections between 4 and 11 s with enough moveout to estimate velocities to the base of the crust the uppermost layer with a temperature corrected velocity of 6 2 km s and thickness of 9 km corresponds to quartzofeldspathic rocks such as metasedimentary rocks migmatites and deformed granites and is underlain by a 7 km thick layer with velocities of 6 4 km s which corresponds to quartzofeldspathic material interlayered with amphibolites the lower crust consists of a heterogeneous 9 km thick zone with velocities of 6 7 to 6 8 km s corresponding to mafic rocks interlayered with smaller amounts of quartzofeldspathic rock the lowermost crust is marked by the x reflection overlying a 3 km thick layer with velocities between 7 4 7 8 km s corresponding to anomolous material possibly layered cumulates the subhorizontal layered structure of the crust is caused primarily by ductile extension which may well be superposed on material added to the crust by underplating at most approximately one third of the present crust could have been added by underplating in the cenozoic

is the lower mantle rheology newtonian today the issue concerning the dominant creep mechanism in mantle convection has been studied numerically with a rheology incorporating both linear and non linear elements we have employed a rheology suggested by the recently obtained melting temperature of lower mantle constituents for an effective rayleigh number between 105 and 106 this type of strongly temperature and pressure dependent rheology induces a circulation in the earth s mantle which is characterized by a sluggish type of flow in the lower mantle with a prevailing newtonian character while in the upper mantle much more vigorous flows are developed in a predominantly non newtonian rheology

effects of strongly temperature dependent viscosity on time dependent three dimensional models of mantle convection numerical simulations of thermal convection in a wide 8 8 1 cartesian box heated from below with temperature dependent viscosity contrasts of 1000 and rayleigh number 105 show that boundary conditions and aspect ratio have an enormous effect on the preferred flow pattern with rigid upper and lower boundaries spoke pattern flow with small diameter 1 5 cells is obtained consistent with laboratory experiments and previous numerical results however with the arguably more realistic stress free boundaries the flow chooses the largest possible wavelength forming a single square cell of aspect ratio 8 with one huge cylindrical downwelling surrounded by upwelling sheets the addition of stress dependence to the rheology weakens the stiff upper boundary layer resulting in smaller cells though still with upwelling sheets and downwelling plumes

lower crustal reflectivity patterns in wide angle seismic recordings many features of wide angle reflection data have been attributed to finely layered structure in the lower crust reflections with similar character however can be produced by small amplitude 2 d random velocity variations finite difference seismograms for a lower crustal model with isotropic random velocity variations match field data recorded at the black forest germany at source receiver offsets beyond about twice the target depth waves scattered from a random zone produce the appearance of horizontal layering in wide angle data lower crustal reflectivity patterns can thus be explained by models that have lateral and vertical random fluctutations rather than fine scale layering