a kinematic model for recrystallization and texture development in olivine polycrystals the interpretation of seismic anisotropy in the mantle requires a knowledge of the relationship between the lattice preferred orientation lpo of crystals and the convective flow field in order better to understand this link we present a model for the evolution of lpo in olivine aggregates that deform by both intracrystalline slip and dynamic recrystallization dynamic recrystallization depends on the dislocation density of the grains which is a function of the applied local stress grains with a large density of dislocations lower their bulk strain energy by nucleating strain free sub grains at a rate proportional to a dimensionless nucleation parameter grains with high energy are then invaded by grains with low energy by grain boundary migration at a rate proportional to a dimensionless grain boundary mobility m the value of is constrained by observed lpo patterns in experimentally deformed olivine aggregates and m is constrained by the temporal evolution of the strength of the lpo for m 125 75 and 3 the model predictions agree well with the experimental results numerical calculations of lpo using our model are significantly faster than those based on viscoplastic self consistent or equilibrium based theories making the model especially suitable for applications for complex convective flows

extension during continental convergence with application to the tibetan plateau the tibetan plateau is the product of crustal thickening caused by the collision between india and asia and is the largest active example of extensional tectonics in a zone of continental collision throughout most of the tertiary the tectonics of the plateau were dominated by north south shortening a significant proportion of which took place on east west striking thrust faults for the last 5 m y or so the plateau has been thinning by the mechanism of extension on north south trending normal faults previous investigations of the collision have been able to account for the large scale features of the tertiary deformation but have failed to explain the transition in the late tertiary to quaternary strain rate field of the plateau from north south compression to east west extension the transition could in principle be effected by a reduction in the rate of convergence between india and asia or by a uniform reduction in strength of the whole continental lithosphere of asia it could not however be effected by a reduction in strength of the elevated region alone this produces increased compressional strain rates in the weakened zone an alternative explanation for the transition to extension comes from considering the thermal evolution of thickened continental lithosphere the lower part of the lithosphere consists of a thermal boundary layer which when thickened by horizontal shortening is colder and denser than its surroundings convective instability of the thickened thermal boundary layer and its replacement by hot asthenosphere would rapidly raise the surface elevation and gravitational potential energy of the overlying part of the lithosphere the convective instability would happen in a time brief compared with the collision time scale 50 m y in the case of india and asia but would only occur after there had already been substantial thickening of the lithosphere numerical experiments show that for a range of lithospheric parameters the increase in surface height as much as 2 km and of potential energy 5 to 10 1012 n m 1 resulting from convective instability of the lower lithosphere are sufficient for east west extension to replace north south compression as the dominant feature of the stress field of the tibetan plateau

structural reactivation in plate tectonics controlled by olivine crystal anisotropy structures formed during ancient tectonic events are commonly reactivated during subsequent tectonism numerical models point to mechanical anisotropy arising from the inherited orientation of crystals of the mineral olivine in the lithospheric mantle as the cause of this behaviour

mantle flow in subduction systems the subslab flow field and implications for mantle dynamics 1 the character of the mantle flow field in subduction zones remains poorly understood despite its importance for our understanding of subduction dynamics in particular little attention has been paid to mantle flow beneath subducting slabs in order to identify processes that make first order contributions to the global pattern of subslab mantle flow we have compiled shear wave splitting measurements from subduction zones worldwide from previously published studies and estimated average splitting parameters for the subslab region globally the subslab region is overwhelmingly dominated by trench parallel fast splitting directions we tested for relationships between splitting delay time a measure of the strength of anisotropy and parameters that are indicators of tectonic processes such as trench migration velocity convergence velocity and obliquity age of subducting lithosphere and slab dip curvature seismicity and thickness we used several different plate motion models to describe plate and trench motions and evaluated the differences among the models we find that only one parameter namely the trench migration velocity in a pacific hot spot reference frame appears to correlate well with the strength of the subslab splitting signal this finding supports a model in which the mantle beneath subducting slabs is dominated by three dimensional flow induced by trench migration we explore several implications of our model for various aspects of mantle dynamics including the choice of a suitable reference frame s for mantle flow the existence of a thin decoupling zone between slabs and the subslab mantle and consequences for mass transfer between the upper and lower mantle

rheology of the earth s mantle a historical review abstract historical development of our understanding of rheological properties of the earth s mantle is reviewed rheological properties of the earth s mantle control most of the important geological processes such as the style of mantle convection e g stagnant lid versus plate tectonics and the nature of thermal evolution however inferring the rheological properties of the earth s mantle is challenging because of the presence of multiple mechanisms of deformation that have different dependence on time scale strain rate stress levels and other parameters through the integration of a broad range of observations including the elastic stiffness from tidal deformation and the viscosity from the concept of isostasy a gross picture of rheological stratification of the earth s mantle a strong lithosphere a weak asthenosphere and a strong layer below was proposed in the mid 19th century however not only the physical basis for such a model was weak due to the lack of proper understanding of some materials science issues such as the interplay between elastic and viscous deformation but also the lack of understanding of temperature depth relation associated with convection prevented our understanding of the rheological structure of the earth s interior major progress occurred in the first half of the 20th century in our understanding of the atomistic mechanisms of plastic deformation in solids and much of the theoretical framework on the plastic deformation of solids was established by late 1960s those developments provided a basis for scaling analyses that are critical to the applications of laboratory results to the earth s interior major progress in laboratory studies on rheological properties occurred in the mid 1960s to the early 1970s in griggs lab using a new type of solid medium high pressure deformation apparatus to pressure 2 gpa and temperature 1600 k the basic concepts such as the water weakening non linear rheology and deformation induced lattice preferred orientation were identified by their studies however large uncertainties in the stress measurements with this type of apparatus were recognized in the late 1970s and high resolution experimental studies using synthetic samples were initiated in paterson s lab in the mid 1980s using a high resolution gas medium deformation apparatus however experimental studies with such a gas medium deformation apparatus can be conducted only at low pressures

on the robustness of estimates of mechanical anisotropy in the continental lithosphere a north american case study and global reanalysis lithospheric strength variations both influence and are influenced by many tectonic processes including orogenesis and rifting cycles the long complex and highly anisotropic histories of the continental lithosphere might lead to a natural expectation of widespread mechanical anisotropy anisotropy in the coherence between topography and gravity anomalies is indeed often observed but whether it corresponds to an elastic thickness that is anisotropic remains in question if coherence is used to estimate flexural strength of the lithosphere the null hypothesis of elastic isotropy can only be rejected when there is significant anisotropy in both the coherence and the elastic strengths derived from it and if interference from anisotropy in the data themselves can be plausibly excluded we consider coherence estimates made using multitaper and wavelet methods from which estimates of effective elastic thickness are derived we develop a series of statistical and geophysical tests for anisotropy and specifically evaluate the potential for spurious results with synthetically generated data our primary case study the north american continent does not exhibit meaningful anisotropy in its mechanical strength similarly a global reanalysis of continental gravity and topography using multitaper methods produces only scant evidence for lithospheric flexural anisotropy

experimental studies of shear deformation of mantle materials towards structural geology of the mantle a brief outline is given on experimental studies carried out in the minnesota mineral and rock physics laboratory of microstructural evolution and rheology of mantle mineral aggregates or their analogues using a simple shear deformation geometry a simple shear deformation geometry allows us to unambiguously identify controlling factors of microstructural evolution and to obtain large strains at high pressures and temperatures and thus provides a unique opportunity to investigate the structural geology of the mantle we have developed a simple shear deformation technique for use at high pressures and temperatures pressure up to 16 gpa and temperature up to 2000 k in both gas medium and solid medium apparati this technique has been applied to the following mineral systems i olivine aggregates ii olivine basaltic melt iii catio3 perovskite aggregates the results have provided important data with which to understand the dynamics of the earth s mantle including the geometry of mantle convection mechanisms of melt distribution and migration beneath mid ocean ridges and the mechanisms of shear localization limitations of laboratory studies and future directions are also discussed

age geodynamic setting and mantle enrichment processes of a k rich intrusion from the meissen massif northern bohemian massif and implications for related occurrences from the mid european hercynian the plutonic complex of the meissen massif northern margin of the bohemian massif comprises dioritic to mainly monzonitic and granitic rocks the diorite to monzonite intrusions show major and trace element patterns typical for shoshonitic series the chemical signatures of less crustally contaminated diorites are similar to arc related shoshonitic rocks derived from continental lithospheric mantle clm sources previously enriched by subduction of altered oceanic crust laser step heating 40ar 39ar analyses on actinolitic to edenitic amphiboles from geographically different occurrences of the monzonitic intrusion yielded concordant plateau ages as well as total gas ages ranging from 329 1 1 4 to 330 4 1 4 ma and from 330 4 2 1 to 330 6 1 8 ma respectively these cooling ages are indistinguishable from sensitive highresolution ion microprobe shrimp 238u 206pb intrusion ages measured on magmatic zircon rims from the monzonite nasdala et al submitted this shows that the monzonite intrusion is probably not related temporally to active subduction because it postdates eclogites of the adjacent saxonian erzgebirge by approximately 20 ma the shoshonitic magmas intruded during strike slip tectonism along the elbe valley zone the enrichment of their mantle sources may be of upper devonian lower carboniferous age or older intrusions of shoshonitic to ultra potassic k rich rocks during the upper visean namurian are widespread in the moldanubian zone based on similar ages and structural relationships a similar post collisional setting to the meissen shoshonitic rocks can be demonstrated most of these occurrences cut high grade nappe units which were subducted during the upper devonian lower carboniferous in contrast to the meissen massif at least the ultra potassic members of the central and the south bohemian batholiths were derived from clm sources enriched by fluids or melts released from subducted oceanic crust and by greater portions of crustal material despite the similar post collisional geodynamic setting of the k rich intrusions different enrichment processes generated mid european hercynian clm sources with heterogeneous major and trace element and isotopic signatures

a multiscale approach to model the anisotropic deformation of lithospheric plates the association of experimental data showing that the plastic deformation of olivine the main constituent of the upper mantle is highly anisotropic and the ubiquitous seismic anisotropy in the upper mantle which indicates that olivine crystals show coherent orientations over scales of tens to hundreds of kilometers implies that the long term deformation in the upper mantle is anisotropic we propose a multiscale approach based on a combination of finite element and homogenization techniques to model the deformation of a lithospheric plate while fully considering the mechanical anisotropy stemming from a strain induced orientation of olivine crystals in the mantle this multiscale model explicitly takes into account the evolution of crystal preferred orientations cpo of olivine and of the mechanical anisotropy during the deformation we performed a series of numerical experiments simulating the uniaxial extension of a homogeneous 100 olivine but anisotropic plate to test the role of the olivine cpo on the plate mechanical behavior and the link between cpo and mechanical anisotropy evolution even for this simple solicitation different orientations and intensity of the initial olivine cpo result in variable plate strengths and deformation regimes a plate with an initial cpo where the olivine 100 and 010 axes are concentrated at 45 degrees to the extension direction has high resolved shear stresses on the easy 010 100 and 001 100 slip systems of olivine this results in low strength and in deformation by transtension plates with an initial cpo where the maximum of 100 axes is parallel or normal to the extension direction show a high initial strength isotropic plates have an intermediate behavior the progressive rotation of olivine 100 axes toward the imposed stretching direction results in hardening in all models except in those characterized by an initial concentration of olivine 100 axes normal to the imposed extension in which softening is followed by hardening

some geodynamical effects of anisotropic viscosity summary rheological anisotropy in the mantle may arise from various causes the most important are the preferred orientation of single crystals and the orientation of streaks of eclogitic material in a marble cake mantle in both cases the orientation can be achieved by the flow kinematics and in steady state convection the planes of easy slip will eventually be orientated parallel to the stream lines the dynamic equations for a 2 d incompressible anisotropic viscous fluid are derived and some geodynamic consequences are studied a difference of the order 10 100 between shear and normal viscosity is considered post glacial rebound would be affected by the mechanical properties in deeper parts of the mantle the observed uplift history near the ice margin has been taken as argument against strong viscosity stratification in the mantle it is shown here that with strong viscous anisotropy at least in the upper mantle the uplift history is compatible with a significant increase in viscosity between upper and lower mantle the topographic and geoid signal can show a spatial offset from a generating mass anomaly in the mantle this may provide the opportunity for in situ observation of anisotropic flow properties in the mantle in numerical case studies of steady state convection streamline orientated anisotropy is considered at high rayleigh number for bottom heated convection the formation of velocity boundary layers is observed with no flow in the stagnant core of the convection cell with internal heating or strong pressure and temperature dependence of viscosity this is no longer found but in the latter case anisotropy enhances the channelling of flow into lowviscosity regions e g hot rising plumes a few experiments have been performed for time dependent convection the results suggest that the onset of time dependent boundary layer instabilities may be somewhat retarded but would not be strongly suppressed by anisotropy strong time dependence may destroy any large scale coherence in the orientation of anisotropy and thus render the mantle to appear viscously isotropic

viscous anisotropy of textured olivine aggregates part 1 measurement of the magnitude and evolution of anisotropy the development of crystallographic textures in olivine rich rocks leads to a marked anisotropy in viscosity of the upper mantle strongly influencing a variety of large scale geodynamic processes most estimates of the magnitude of viscous anisotropy in the upper mantle are derived from micromechanical models that predict textural and mechanical evolution numerically unfortunately relatively few data exist with which to benchmark these models and therefore their applicability to geodynamic processes remains in question here we present the results from a series of laboratory deformation experiments that yield insight into the magnitude and evolution of the anisotropy of olivine aggregates during deformation along complex loading paths aggregates of fo50 olivine were first deformed in extension in a gas medium apparatus at a temperature of 1473 k confining pressure of 300 mpa and a variety of stresses and strain rates early in the extension experiments samples exhibited viscosities similar to those previously determined for isotropic aggregates extensional deformation was accompanied by formation of crystallographic textures with 100 axes dominantly aligned with the extension axis samples were subsequently deformed in torsion under similar conditions to shear strains of up to 15 5 early in the torsion experiments samples supported stresses a factor of 2 larger than measured at the end of extension experiments demonstrating a marked anisotropy in viscosity textures at the end of torsion experiments exhibited 100 axes dominantly aligned with the shear direction comparable to previous experimental observations evolution of the textures resulting from extension to those resulting from torsion was analyzed through examination of radial sections of torsion samples our results confirm that texture produces viscous anisotropy in olivine aggregates and we provide a simple calibrated parameterization of viscous anisotropy for use in geodynamic models our results also provide an extensive dataset for future calibration of micromechanical models that track the evolution of anisotropy in upper mantle rocks

tectonic drivers of the wrangell block insights on fore arc sliver processes from 3 d geodynamic models of alaska intracontinental shear zones can play a key role in understanding how plate convergence is manifested in the upper plate in regions of oblique subduction however the relative role of the driving forces from the subducting plate and the resisting force from within intracontinental shear zones is not well understood results from high resolution geographically referenced instantaneous 3d geodynamic models of flat slab subduction at the oblique convergent margin of alaska are presented these models investigate how viscosity and length of the denali fault intracontinental shear zone as well as coupling along the plate boundary interface modulate motion of the wrangell block forearc sliver and slip across the denali fault models with a weak denali fault 1017 pa s and strong plate coupling 1021 pa s were found to produce the fastest motions of the wrangell block 10 mm yr the 3d models predict along strike variation in offs et along the denali fault changing from dextral strike slip motion in the eastern segment to oblique convergence towards the fault apex models further show that the flat slab drives oblique motion of the wrangell block and contributes to 20 models with a short fault and 28 models with a long fault of the observed quaternary slip rates along the denali fault the 3d models provide insight into the general processes of forearc sliver mechanics and also offer a 3d framework for interpreting hazards in regions of flat slab subduction

buoyancy driven instabilities of low viscosity zones as models of magma rich zones the low reynolds number dynamics of a thin layer of fluid bounded below by a flat horizontal boundary and above by a fluid of another viscosity and greater density is reported three distinct stages of growth were observed the first stage is a rayleigh taylor instability in which disturbances of one specific wave number grow most rapidly if e is the ratio of the viscosity of the thick layer to the viscosity of the thin layer fastest growth is for wave number e in the second stage distortion of the interface is large and it is found experimentally that the fluid moves out of the thick layer as circular columns surrounded by relatively broad regions of descending material in the third stage fully matured structures are formed if the upwelling material has less viscosity than the surrounding material the structure develops a rim syncline and a pronounced overhang and eventually ascends as a spherical pocket of fluid fed by a conduit two applications to geophysics are given the first application follows from the fact that a melt source must exist in the mantle below mid ocean ridges this source can be approximated as a cylindrical body with lower viscosity and density compared to the overlying mantle if the cylinder develops a gravitational instability it will develop regularly spaced vertical protrusions estimates of the spacing are compared to high resolution segmentation data and some constraints on viscosity of the mantle below spreading centers are made these are that viscosity of the mantle is 1018 1 p and the ratio of this viscosity to the viscosity of the cylinder is less than 100 in the second application the upwelling conduits are measured experimentally and solitary waves are observed a recently found analog with magma rising up through the pores of a viscous crystalline matrix is discussed

subduction initiation at a strike slip plate boundary the cenozoic pacific australian plate boundary south of new zealand 1 we first present a synthesis of the macquarie ridge complex mrc tectonic structures as well as paleo reconstruction models of the kinematic evolution of the pacific australia plate boundary south of new zealand since the eocene we then ascertain the geodynamical conditions that preceded subduction initiation and identify the nature and structures of the crust that first subducted at the puysegur subduction zone this synthesis is used to produce a subduction initiation model for the puysegur region concomitant to inception of the alpine fault ca 23 ma a 150 km wide transpressive relay zone developed along puysegur bank inherited structures enabling localization of compressive deformation right lateral motion at the relay zone has juxtaposed oceanic and continental crusts facilitating inception of subduction and controlling the subduction vergence subsequently the puysegur subduction zone initiated at the transpressive relay zone ca 20 ma upper and lower plate inherited structures guided and facilitated the lengthening of the subduction zone during the neogene the four individual segments of the mrc represent different stages of incipient subduction whose development depends on local geodynamical conditions and lithospheric heterogeneities the example of the mrc demonstrates that subduction can initiate from an oceanic spreading center through progressive changes in plate kinematics within a 10 15 myr time frame index terms 3040 marine geology and geophysics plate tectonics 8150 8155 8157 8158 8155 tectonophysics plate motions general 9604 information related to geologic time cenozoic keywords subduction initiation pacific australia plate motion cenezoic macquarie ridge

progressive deformation in anisotropic rocks abstract field observations show that the majority of crustal rocks possess a penetrative foliation defined by either compositional layering preferred orientations of crystals or both ductile deformation involving planar anisotropy of viscosity can be characterized by an anisotropy factor q n r s the ratio of the bulk viscosities in pure and simple shear respectively this ratio of the normal and shear viscosities may be determined analytically if the anisotropy is resolvable in a multilayer sequence with individual laminae of isotropic viscosity in that case the resistance to normal compression will be largely controlled by the competent layers whereas the resistance to shear is controlled by the soft layers more specifically the viscosities of the individual laminae add up like resistances in series and in parallel in the expressions for the normal and shear viscosities respectively the reorientation of the bulk stress within an anisotropic multilayer is systematically investigated for a range of anisotropy factors the mode of progressive deformation is controlled only by the anisotropy factor and the orientation of the principal deviatoric stress the rate of deformation can be scaled if the bulk normal viscosity and the magnitude of the principal deviatoric stress are also known the results are illustrated in a series of nomograms showing the spectrum of strain and rotation histories possible in rock volumes deforming with planar anisotropy it appears that with increasing anisotropy factor the deformation spectrum will narrow on simple shear irrespective of the orientation of the background or bulk deviatoric stress axes plane strain and isochoric conditions are assumed throughout the analysis

topographic response to mantle lithosphere removal in the southern sierra nevada region california geological studies of mantle xenoliths entrained in late neogene quaternary lavas from the southern sierra nevada region and regional geophysical studies suggest that the highdensity mantle lithosphere that formed beneath the sierra nevada batholith in conjunction with arc magmatism is being convectively removed as a drip structure this structure as imaged seismically is roughly cylindrical in shape with a diameter of 100 km and extends to 225 km depth centered above this structure is a region 120 km in diameter that is undergoing active subsidence relative to adjacent regions such subsidence is seen in the active fluvial alluvial sediment flooding of mountainous topography of the southwestern sierra and in the development of the adjacent tulare lake basin of the san joaquin valley dynamic modeling of such upper mantle drip structures predicts a phase of overlying surface subsidence during the most vigorous phase of drip formation the southern sierra upper mantle drip and the overlying crust appear to be in this phase of their dynamically coupled evolution

lithospheric deformation beneath the san gabriel mountains in the southern california transverse ranges high resolution tomographic images from los angeles region seismic experiment larse array and southern california seismic network scsn teleseismic data suggest that the entire lithosphere below the san gabriel mountains and san andreas fault in the transverse ranges has thickened in a narrow vertical sheet p wave travel time inversions of the combined data support the presence of the well documented upper mantle high velocity anomaly that extends 200 km into the mantle under the northernmost los angeles basin and transverse ranges and is associated with mantle downwelling due to oblique convergence we find that the high velocity high density upper mantle anomaly comprises a 60 80 km wide sheet of mantle material that lies directly below a substantial crustal root in the san gabriel mountains the velocity perturbations are as large as 3 in the anomaly corresponding to a 2 density increase the tomographic images suggest that deformation in the ductile lower crust and mantle lithosphere may be partially coupled mechanically and thermally if the thickening is occurring together in response to convergence and that it may be a local compressional feature

rayleigh wave phase velocities small scale convection and azimuthal anisotropy beneath southern california 1 we use rayleigh waves to invert for shear velocities in the upper mantle beneath southern california a one dimensional shear velocity model reveals a pronounced low velocity zone lvz from 90 to 210 km the pattern of velocity anomalies indicates that there is active small scale convection in the asthenosphere and that the dominant form of convection is three dimensional 3 d lithospheric drips and asthenospheric upwellings rather than 2 d sheets or slabs several of the features that we observe have been previously detected by body wave tomography these anomalies have been interpreted as delaminated lithosphere and consequent upwelling of the asthenosphere beneath the eastern edge of the southern sierra nevada and walker lane region sinking lithosphere beneath the southern central valley upwelling beneath the salton trough and downwelling beneath the transverse ranges our new observations provide better constraints on the lateral and vertical extent of these anomalies in addition we detect two previously undetected features a high velocity anomaly beneath the northern peninsular range and a low velocity anomaly beneath the northeastern mojave block we also estimate the azimuthal anisotropy from rayleigh wave data the strength is 1 7 at periods shorter than 100 s and decreases to below 1 at longer periods the fast direction is nearly e w the anisotropic layer is more than 300 km thick the e w fast directions in the lithosphere and sublithosphere mantle may be caused by distinct deformation mechanisms pure shear in the lithosphere due to n s tectonic shortening and simple shear in sublithosphere mantle due to mantle flow

tomographic image of the southern california mantle p wave teleseismic travel time delays recorded by the southern california array are inverted by the method of backprojection tomography to obtain images of variations in the p wave velocity structure to a depth of 750 km two major upper mantle features arc resolved one beneath the transverse ranges region and another beneath the salton trough region the transverse ranges feature appears as a curtainlike east trending high velocity anomaly this feature is 60 km thick extends most deeply on its eastern end to 250 km and attains a maximum velocity 3 greater than average southern california mantle of the same depth the salton trough feature which is not as well resolved as the transverse ranges feature is composed of low velocities in the upper 70 100 km these p wave velocities arc depressed 3 4 compared to average southern california mantle tests of the inversion indicate the major aspects of the imaged structure are authentic

anisotropic rheology of a cubic medium and implications for geological materials summary dislocation creep which is the dominant deformation mechanism in the upper mantle results in a non newtonian anisotropic rheology the implication of non newtonian rheology has been quite extensively studied in geodynamic models but the anisotropic aspect remains poorly in vestigated in this paper we propose to fill this gap by 1 introducing a simple mathematical description of anisotropic viscosity and 2 illustrating the link between plastic crystal defor mation and bulk material rheology the study relies on the highest symmetry of the anisotropic tensor a cubic symmetry for which anisotropy is characterized by one parameter only first order implications of anisotropy are quantitatively explored as a function of the effective rheology of the material is described as a function of the orientation of the crystals and of the imposed stress and the validity of the isotropic approximation is discussed the model applied to ringwoodite a cubic crystal with spinel type structure predicts that the dynamics of the transition zone in the earth s mantle is going to be strongly affected by mechanical anisotropy

numerical simulations of texture development and associated rheological anisotropy in regions of complex mantle flow 1 the development of lattice preferred orientations lpo within olivine aggregates under flow in the upper mantle leads to seismic and rheological or viscoplastic anisotropies we compare predictions from different micromechanical models applying several commonly used theoretical descriptions to an upwelling flow scenario representing a typical oceanic spreading center significant differences are obtained between models in terms of lpo and associated rheology in particular in regions where the flow direction changes rapidly with superior predictions for the recently proposed second order approach this highlights the limitations of ad hoc formulations lpo induced rheological anisotropy may have a large effect on actual flow patterns with 1 2 orders of magnitude variation in directional viscosities compared to the isotropic case citation castelnau o d k blackman and t w becker 2009 numerical simulations of texture development and associated rheological anisotropy in regions of complex mantle flow geophys res lett 36 l12304

lithospheric instability in obliquely convergent margins san gabriel mountains southern california 1 oblique convergence across the san andreas fault in southern california has lead to rapid uplift of the san gabriel mountains since 5 ma tomographic models of this region include a sheet like high velocity anomaly extending to depths 200 km one proposed mechanism for the formation of this feature is gravitational instability of the lithosphere resulting in a cold thermal downwelling we present a systematic study of the sensitivity of lithospheric deformation lithospheric downwelling topography deflection of the moho convergent velocity profile to the viscosity structure in 2 d numerical models of lithospheric instability including a pre existing zone of weakness strike parallel strain within a convergent region should create a local zone of weakness due to the non newtonian response of the lithosphere such a weak zone can focus convergent motion increasing and localizing the growth rate of a lithospheric instability we find that the observed characteristics of deformation depend on the width x w and relative viscous weakening factor f w the ratio of crust to mantle lithospheric viscosity eta c eta m and the absolute viscosity of the lithosphere h m for the san gabriel mountains we find that a range of models is capable of reproducing the observed moho deflection and depth extent of the downwelling within the short time since the onset of convergence however only a small subset of models x w 20 km f w 50 100 eta m 7 5 x 10 20 1 5 x 10 21 pa s eta c eta m 100 reproduce both the narrow high topography and horizontal shortening profile

mantle convection modeling with viscoelastic brittle lithosphere numerical methodology and plate tectonic modeling the earth s tectonic plates are strong viscoelastic shells which make up the outermost part of a thermally convecting predominantly viscous layer brittle failure of the lithosphere occurs when stresses are high in order to build a realistic simulation of the planet s evolution the complete viscoelastic brittle convection system needs to be considered a particle in cell finite element method is demonstrated which can simulate very large deformation viscoelasticity with a strain dependent yield stress this is applied to a plate deformation problem numerical accuracy is demonstrated relative to analytic benchmarks and the characteristics of the method are discussed

holocene glacier behavior around the northern antarctic peninsula and possible causes abstract we obtained 49 new 10be ages that document the activity of the former northern antarctic peninsula ice sheet and subsequently the james ross island ice cap and nearby glaciers from the end of the last glacial period until the last 100 years the data indicate that from 11 to 8 ka marked recession of glacier systems occurred around james ross island including tidewater and local land terminating glaciers glaciers reached heads of bays and fjords by 8 7 ka subsequently local glaciers were larger than present around at least 7 5 7 ka and 5 4 ka at times between 3 9 and 3 6 ka and just after 3 ka between 2 4 and 1 ka and from 300 to 100 years ago after deglaciation the largest local glacier extents occurred between 7 ka and 4 ka comparison with other paleoclimate records including of sea ice extent reveals coherent climate changes over 15 of latitude in the early holocene most of the time a swath of warmth spanned from southern south america to the antarctic peninsula sector we infer such intervals are times of weakening and or poleward expansion of the band of stronger westerlies associated with contraction of the polar vortex conversely increased sea ice and equatorward expansion of the westerlies and the polar vortex favor larger glaciers from patagonia to the antarctic peninsula which typically occurred after 8 ka although warm stretches did take place for example on the antarctic peninsula and in patagonia the interval from 4 to 3 ka was typically warm but conditions were not uniform in either region we also infer that reduced and expanded glacier extents in patagonia and the eastern antarctic peninsula tend to occur when conditions resemble a persistent positive and negative southern annular mode respectively

observation of abnormal thermal and infrasound signals prior to the earthquakes a study on bonin island earthquake m7 8 may 30 2015 scientists from all over the world try to incorporate multi disciplinary precursors to forecast the earthquake on a short term basis the authors here have analyzed outgoing longwave radiations acquired from polar orbiting national oceanic and atmospheric adminstration noaa satellites and long period infrasound waves recorded by the ground observatories in china prior to the recent bonin islands japan region earthquake which occurred on may 30 2015 with the magnitude of 7 8 the anomalous outgoing longwave radiation olr was observed on may 15 2015 and was recorded by noaa 18 satellite during its night pass similarly an abnormal infrasound spike was recorded at the beijing station on may 17 2015 the delay in observing anomalous infrasound waves compared to the olr anomaly is due to these low frequency waves traveling at low speed with the velocity range of 10 15 m s from the analysis of the results it can be inferred that there is a substantial relation between parameters like olr and infrasound waves hence the authors conclude that it is possible to forecast the earthquake on a short term basis with reasonable accuracy

the role of pre existing mechanical anisotropy on shear zone development within oceanic mantle lithosphere an example from the oman ophiolite structural and fabric analysis of the well exposed hilti mantle section oman ophiolite suggests that shear zone development which may have resulted from oceanic plate fragmentation was influenced by pre existing mantle fabric present at the paleoridge detailed structural mapping in the mantle section revealed a gently undulating structure with an east west flow direction a nw se strike slip shear zone cuts across this horizontal structure the crystal preferred orientation cpo of olivine within the foliation is dominated by 010 axial patterns rather than more commonly observed 010 100 patterns suggesting that the horizontal flow close to the moho involved non coaxial flow olivine cpo within the shear zone formed at low temperature is characterized by 001 100 patterns and a sinistral sense of shear the olivine cpo becomes weaker with progressive mylonitization and accompanying grain size reduction and ultimately develops into an ultra mylonite with a random cpo pattern the olivine 010 axis is consistently sub vertical even where the horizontal foliation has been rotated to a sub vertical orientation within the shear zone these observations suggest that the primary mechanical anisotropy mantle fabric has been readily transformed into a secondary structure shear zone with minimum modification this occurred as a result of a change of the olivine slip systems during oceanic detachment and related tectonics during cooling we propose that primary olivine cpo fabrics may play a significant role in the subsequent structural development of the mantle thus the structural behavior of oceanic mantle lithosphere during subduction and obduction may be strongly influenced by initial mechanical anisotropy developed at an oceanic spreading center

postcollisional lithospheric evolution of the southeast carpathians comparison of geodynamical models and observations seismic evidence and thermal and topographic transients have led to the interpretation of lithospheric removal beneath the southeast carpathians region a series of numerical geodynamic experiments in the context of the tectonic evolution of the region are conducted to test the surface crustal response to lithosphere delamination and slab break off the results show that a delamination type removal plate like migrating instability causes a characteristic pattern of surface uplift subsidence and crustal extension shortening to occur due to the lithospheric deformation and dynamic thermal forcing of the sublithospheric mantle these features migrate with the progressive removal of the underlying lithosphere model results for delamination are comparable with observables related to the geodynamic evolution of the southeast carpathians since 10 ma the mantle structure inferred by seismic tomography migrating patterns of uplift 1 5 km and subsidence 2 km in the region crustal thinning in the carpathian hinterland and thickening at the focsani depression and regional extension in the carpathian corner e g opening of brasov basin correlating with volcanism e g harghita and persani volcanics in the last 3 myr

azimuthal anisotropy in the d layer beneath the caribbean 1 the lowermost mantle beneath central america has anisotropic seismic velocity structure manifested in shear wave splitting of signals from south american earthquakes recorded at north american broadband recording stations prior studies of deep mantle anisotropy in this region have characterized the structure as having vertical transverse isotropy vti which is sufficient to explain a general trend of early tangential sh component arrivals however vti models cannot quantitatively match systematic waveform complexities in the onset of many of the shear waves that graze this region after accounting for splitting effects of upper mantle anisotropy beneath the recording stations we model the corrected waveform data using full wave theory for mantle velocity models with an anisotropic d layer this is the first attempt to quantitatively model a large data set including azimuthal anisotropy in d the models include transverse isotropy with either a vertical or tilted symmetry axis the latter resulting in azimuthal anisotropy for some initial shear wave polarizations tilted transverse isotropy tti produces small reversed polarity arrivals on the sv components at the arrival time of sh consistent with the data geographical variations in the azimuth of the tti symmetry axis are indicated by the data the lack of azimuthal coverage prevents unique resolution of the tti orientation and also precludes distinguishing between tti and other azimuthal anisotropy structures such as that predicted for lattice preferred orientation of minerals nonetheless our modeling demonstrates the need for laterally varying anisotropic structure of more complex form than vti for this region

viscous anisotropy of two phase composites and applications to rocks and structures abstract anisotropy of viscosity in rocks has a significant effect on the development of geological structures as demonstrated in several theoretical and analogue model studies this paper develops the results of a self consistent mechanics approach to modelling the bulk viscous properties of two phase composites to reveal the strength of the viscous anisotropy anisotropy factor is defined as the ratio of normal to shear viscosity n s the value of is determined for different types of composites of two viscous phases with varying viscosity ratio m 5 10 100 ranging from bilaminates to those with elliptical shape fabrics given by axial ratio r bilaminates with equal layer thickness are the most highly anisotropic systems where simply depends on m these results are applied to rocks modelled as two phase composites of competent and incompetent lithologies with the aim of quantifying the viscous anisotropy that is associated with natural rock deformation and the formation of geological structures it is deduced that m 5 or 10 is an appropriate maximum viscosity contrast for many metasedimentary rocks e g psammite pelite composites but that m 100 might be effectively reached in limestone marl pairs or in rocks containing surfaces of slip or zones of recrystallisation or grain boundary sliding these m values give rise to maximum values of 3 5 5 and 25 5 respectively an anisotropy factor of 25 is effectively a composite of a rigid and a viscous phase and may only be achieved in solid melt mixtures anisotropic rocks whether layered or containing a strong shape fabric will develop folds more easily than isotropic rocks characteristic structures are similar folds chevron folds kink bands and shear bands geometric features of folds especially fold angularity may provide methods of quantifying the viscous anisotropy of rocks and composites

convective instability of a boundary layer with temperature and strain rate dependent viscosity in terms of available buoyancy summary cold mantle lithosphere is gravitationally unstable with respect to the hotter buoyant asthenosphere beneath it leading to the possibility that the lower part of the mantle lithosphere could sink into the mantle in convective downwelling such instabilities are driven by the negative thermal buoyancy of the cold lithosphere and retarded largely by viscous stress in the lithosphere because of the temperature dependence of viscosity the coldest and therefore densest parts of the lithosphere are unavailable for driving the instability because of their strength by comparing theory and the results of a finite element representation of a cooling lithosphere we show that for a newtonian fluid the rate of exponential growth of an instability should be approximately proportional to the integral over the depth of the lithosphere of the ratio of thermal buoyancy to viscosity both of which are functions of temperature and thus depth we term this quantity available buoyancy because it quantifies the buoyancy of material sufficiently weak to flow and therefore available for driving convective downwelling for non newtonian viscosity with power law exponent n and temperature dependent pre exponential factor b the instabilities grow superexponentially as described by houseman molnar 1997 and the appropriate timescale is given by the integral of the nth power of the ratio of the thermal buoyancy to b the scaling by the available buoyancy thus offers a method of determining the timescale for the growth of perturbations to an arbitrary temperature profile and a given dependence of viscosity on both temperature and strain rate this timescale can be compared to the one relevant for the smoothing of temperature perturbations by the diffusion of heat allowing us to define a parameter similar to a rayleigh number that describes a given temperature profile s tendency toward convective instability like the rayleigh number this parameter depends on the cube of the thickness of a potentially unstable layer therefore mechanical thickening of a layer should substantially increase its degree of convective instability and could cause stable lithosphere to become convectively unstable on short cales we estimate that convective erosion will in 10 myr reduce a layer thickened by a factor of two to a thickness only 20 to 50 per cent greater than its pre thickened value thickening followed by convective instability may lead to a net thinningof a layer if thickening also enhances the amplitude of perturbations to the layer s lateral temperature structure for the mantle lithosphere the resulting influx of hot asthenosphere could result in rapid surface uplift and volcanism

a simple global model of plate dynamics and mantle convection cooling and thickening of lithospheric plates with age and subduction result in large scale horizontal density contrasts tending to drive plate motions and mantle flow we quantify the driving forces associated with these density contrasts to determine if they can drive the observed plate motions first two dimensional models are computed to evaluate the effects of assumed rheologies and boundary conditions we are unable to obtain platelike behavior in viscous models with traction free boundary conditions the piecewise uniform velocities distinctive of plate motion can be imposed as boundary conditions and the dynamic consistency of the models evaluated by determining if the net force on each vanishes if the lithosphere has a newtonian viscous rheology the net force on any plate is a strong function of the effective grid spacing used leading to ambiguities in interpretation incorporating a rigid plastic lithosphere which fails at a critical yield stress into the otherwise viscous model removes these ambiguities the model is extended to the actual three dimensional spherical plate geometry the observed velocities of rigid plastic plates are matched to the solution of the viscous stokes equation at the lithosphere asthenosphere boundary body forces from the seismically observed slabs from the thickening of the lithosphere obtained from the actual lithospheric ages and from the differences in structure between continents and oceans are included interior density contrasts such as those resulting from upwellings from a hot bottom boundary layer are assumed to occur on a scale small compared to plate dimensions and are not included the driving forces from the density contrasts within the plates are calculated and compared to resisting forces resulting from viscous drag computed from the three dimensional global return flow and resistance to deformation at converging boundaries the rms residual torque is 30 of the driving torque the density contrasts within the plates themselves can reasonably account for plate motions body forces from convection in the interior may provide only a small net force on the plates at converging boundaries the lithosphere has a yield stress of 100 bars drag at the base of the plates is 5 bars and resists plate motion the net driving forces from subducting slabs and collisional resistance are localized and approximately balance driving forces from lithospheric thickening are distributed over the areas of the plates as is viscous drag the approximate balance of these two forces predicts plate velocities uncorrelated with plate area as observed the model represents a specific case of boundary layer convection the dynamical results are consistent with either upper mantle or mantle wide convection

evolving viscous anisotropy in the upper mantle and its geodynamic implications asthenospheric shear causes some minerals particularly olivine to develop anisotropic textures that can be detected seismically in laboratory experiments these textures are also associated with

drip instabilities of continental lithosphere acceleration and entrainment by damage summary geological evidence indicates large volume lithospheric drip instabilities form under regions of continental lithosphere the size of these drips suggests that either the base of the lithosphere is volumetrically increased or the highly viscous upper lithosphere participates in the drip previous theoretical models using simple rheologies have been unable to produce unforced large volume drip instabilities thus large drip instabilities are typically induced by thickening or destabilizing the mobile base of the lithosphere through tectonic forcing such as convergent thickening or dense magmatic emplacement following extension here we propose a theory of lithospheric drip instabilities that may arise naturally from the thermal and rheological structure of earth independent of specific tectonic forcings using damage physics relevant for earth we find a large portion of the lithosphere may be mobilized and entrained into growing drip instabilities for a critical amount of damage the growth is accelerated sufficiently that large volume drip instabilities may form within geologically feasible time frames we therefore suggest large volume lithospheric drip instabilities may arise independently of tectonic settings through damage assisted mobilization and entrainment of the highly viscous lithosphere

viscous anisotropy of textured olivine aggregates 2 micromechanical model the significant viscous anisotropy that results from crystallographic alignment texture of olivine grains in deformed upper mantle rocks strongly influences a large variety of geodynamic processes our ability to explore the effects of anisotropic viscosity in simulations of these processes requires a mechanical model that can predict the magnitude of anisotropy and its evolution unfortunately existing models of olivine textural evolution and viscous anisotropy are calibrated for relatively small deformations and simple strain paths making them less general than desired for many large scale geodynamic scenarios here we develop a new set of micromechanical models to describe the mechanical behavior and textural evolution of olivine through a large range of strains and complex strain histories for the mechanical behavior we explore two extreme scenarios one in which each grain experiences the same stress tensor sachs model and one in which each grain undergoes a strain rate as close as possible to the macroscopic strain rate pseudo taylor model for the textural evolution we develop a new model in which the director method is used to control the rate of grain rotation and the available slip systems in olivine are used to control the axis of rotation only recently has enough laboratory data on the deformation of olivine become available to calibrate these models we use these new data to conduct inversions for the best parameters to characterize both the mechanical and textural evolution models these inversions demonstrate that the calibrated pseudo taylor model best reproduces the mechanical observations additionally the pseudo taylor textural evolution model can reasonably reproduce the observed texture strength shape and orientation after large and complex deformations a quantitative comparison between our calibrated models and previously published models reveals that our new models excel in predicting the magnitude of viscous anisotropy and the details of the textural evolution in addition we demonstrate that the mechanical and textural evolution models can be coupled and used to reproduce mechanical evolution during large strain torsion tests this set of models therefore provides a new geodynamic tool for incorporating viscous anisotropy into large scale numerical simulations

prediction of anisotropy from flow models a comparison of three methods observations of anisotropy in earth are used regularly as constraints for models of deformation using various assumptions about the relationship between deformation and the resulting anisotropic fabric we compare three methods for calculating fabric from velocity fields tracking of finite strain ellipses a kinematic crystallographic code and the evolution of directors we find that the use of the finite strain ellipse provides only limited prediction capabilities as it cannot reproduce experimental observations that involve recrystallization the crystallographic code we tested a variant of the popular code d rex provides a more complete fabric prediction but at a much higher computational cost the directors method provides an intermediate solution while it does not include some of the more complex crystallographic processes that d rex does the results of this method closely resemble those of d rex at a lower computational cost the directors are also capable of tracking anisotropy at much larger strains than d rex we conclude that when computation speed is important for example in self consistent geodynamic models with anisotropic rheology the directors method provides an appropriate approximation

rayleigh taylor instability under a shear stress free top boundary condition and its relevance to removal of mantle lithosphere from beneath the sierra nevada 1 the separation of zones of apparent downwelling flow at the ends of the sierra nevada suggests a relatively large wavelength 500 km of unstable growth but rayleigh taylor instabilities for plausible rheological structures with a fixed top boundary condition require much shorter wavelengths 100 km for maximum growth rates to understand this difference we analyze analytical solutions and perform numerical 2 d plane strain experiments for rayleigh taylor instability of a dense layer overlying a less dense substratum representing the instability between the mantle lithosphere and the underlying asthenosphere focusing on the effects of a shear stress free boundary condition at the top the overall effect of this condition is an enhancement of growth rate factors at long wavelengths which depends greatly on the exponential viscosity variation with depth of the layer with large or little variation across the unstable layer the solutions approximate those with a fixed top boundary condition or for constant viscosity respectively an intermediate zone showing the enhanced growth rates includes ratios of layer thickness to viscosity e folding length h l of 1 8 for newtonian viscosity and 1 4 for nonlinear viscosity the free top condition is likely applicable to geologic situations where the lower crust is weak olivine flow laws and low temperature estimates at 35 km depth 255 355 c place the sierra nevada viscosity scaling ratio h l between 5 and 9 thus longer wavelengths than commonly assumed for rayleigh taylor instabilities seem permissible when viscosity decreases with depth and the top surface of the layer is only weakly constrained

evidence for low viscosity garnet rich layers in the upper mantle abstract the rheological properties of upper mantle rocks play an important role in controlling the dynamics of the lithosphere and mantle convection experimental studies and microstructures in naturally deformed mantle rocks usually imply that olivine controls the upper mantle rheology here we show for the first time evidence from the geometry of folded compositional layers in mantle rocks from western norway that garnet rich rocks can have lower solid state viscosities than olivine rich rocks modeling of melt free and dry rheology of garnet and olivine confirms that the reversed viscosity contrast between garnet rich and olivine rich layers for this folding event can be achieved over a relatively wide range of temperatures at low stress conditions when the fine grained garnet deforms by diffusion creep while the coarse grained olivine deforms by dislocation creep and or diffusion creep in general modeling of the fold viscosity contrast shows that in the stable subcontinental lithospheric mantle or convecting mantle such a reversed viscosity contrast can be formed due to diffusion creep processes in fine grained garnets in a dry mantle environment or at conditions where the garnet pyroxene layer is partially molten i e close to solidus liquidus conditions in the upper mantle alternatively in cold plate tectonic settings e g in subduction zones some water weakening is a feasible mechanism to create the reversed viscosity contrast between garnet and olivine

high temperature creep of olivine single crystals 2 dislocation structures dislocation structures and densities in experimentally deformed single crystals of san carlos olivine were examined using the oxidation decoration technique the high temperature deformation experiments were conducted at various temperatures t differential stresses and oxygen fugacities o2 the samples were buffered against either orthopyroxene opx or magnesiowustite mw solid state powders and compressed along one of the three 45 orientations in samples that were deformed and subsequently quenched under load seven distinct dislocation structures were observed a for samples compressed parallel to 110 c at 16 50 mpa and o2 10 6atm two different dislocation arrangements were identified for opx buffered samples at 1400 t 1475 c the dislocation structure was composed of curved screw segments that had usually cross slipped and straight edge segments of comparable length at 1300 t 1350 c for opx buffered samples and at 1350 t 1500 c for mw buffered samples the dislocation structure is dominated by arrays of very long edge or mixed dislocations with only a few screw dislocations b for both opx buffered and mw buffered samples compressed parallel to 101 c at 1340 t 1400 c and 30 60 mpa the dislocation configurations are characterized by zigzag near edge dislocations at o2 10 5atm and by straight cross slipped screw dislocations plus pinned edge dislocations at o2 10 9atm c for samples compressed parallel to 011 c at t 1400 c and 70 150mpa the primary dislocations change from long straight edge dislocations for o2 10 4atm and opx buffer to a combination of straight edge dislocations and polygon shaped half loops for o2 10 4atm and mw buffer to gently curved near edge dislocations for o2 10 9atm for both solid state buffers abundant long straight screw dislocations were present in each case for the 011 c samples all of the above variations in dislocation structure are paralleled by changes in the measured power law equation that describes the behavior of creep mechanisms of olivine thus the different dislocation structures are associated with different rate controlling creep mechanisms for all of the dislocation structures the dislocation density increases with differential stress according to the relation 1 4 dislocation structures in samples that were deformed and then statically annealed have also been studied for both 110 c and 011 c samples deformed over a wide range of experimental conditions the dislocation structures on the slip planes generally consist of short curved edge or mixed dislocation segments in 101 1 samples deformed at low o2 50 mpa and 1400 c the dislocation structure was much less altered by annealing than the dislocation structures in 110 c and 011 c samples the density of dislocations decreased much less for these 101 c samples than for the 110 c and 011 c samples due to climb of the edge segments the straight screw segments found in quenched samples became curved and many low angle 100 tilt boundaries connected by straight 100 screw dislocations developed during the annealing process

mantle dynamics and seismic anisotropy abstract observations of seismic anisotropy yield some of the most direct constraints available on both past and present day deformation in the earth s mantle insight into the character of mantle flow can also be gained from the geodynamical modeling of mantle processes on both global and regional scales we highlight recent progress toward understanding mantle flow from both observations and modeling and discuss outstanding problems and avenues for progress particularly in the integration of seismological and geodynamical constraints to understand seismic anisotropy and the deformation that produces it to first order the predictions of upper mantle anisotropy made by global mantle circulation models match seismological observations well beneath the ocean basins but the fit is poorer in regions of greater tectonic complexity such as beneath continental interiors and within subduction systems in many regions of the upper mantle models of anisotropy derived from surface waves are seemingly inconsistent with shear wave splitting observations which suggests that our understanding of complex anisotropic regions remains incomplete observations of anisotropy in the d layer hold promise for improving our understanding of dynamic processes in the deep earth but much progress remains to be made in characterizing anisotropic structure and relating it to the geometry of flow geochemical heterogeneity or phase transitions major outstanding problems related to understanding mantle anisotropy remain particularly regarding the deformation and evolution of continents the nature of the asthenosphere subduction zone geodynamics and the thermo chemical state of the lowermost mantle however we expect that new seismological deployments and closer integration of observations with geodynamical models will yield rapid progress in these areas

anisotropic viscosity changes subduction zone thermal structure abundant observations of seismic anisotropy in subduction zones attest that the material in the mantle wedge has a strong fabric and therefore should be mechanically anisotropic in this paper we examine the effect of anisotropic viscosity on the thermal structure of subduction zone mantle wedges and quantify its importance relative to other thermal and rheological factors using two dimensional finite element kinematic models we find that anisotropic viscosity results in two substantial changes a hotter slab wedge interface and time variability of the melt production rate and excess temperatures although not as significant as the effect of temperature dependent viscosity anisotropy leads to an increase of up to 35 c in the temperature along the slab wedge interface a hotter slab wedge interface can change the depth extent of the seismogenic zone limit the depth to which hydrous minerals can carry water and influence flux melting time variability of the thermal field is a novel result of adding anisotropic viscosity to our models this time variability results from heterogeneity in material alignment and could explain temporal changes in subduction zone magmatism without invoking a change in the wedge geometry slab age or composition

on the role of anisotropic viscosity for plate scale flow 1 we study the role of partial melt or lattice fabric induced viscous anisotropy for lithosphere asthenosphere interactions using mantle flow models mechanical anisotropy has only a moderate influence on plate scale flow and global geoid predictions for the cases considered and anisotropic weakening effects are similar to those due to effective isotropic viscosity reduction while anisotropy modifies details of mantle flow and may be relevant for time dependent scenarios it may be safely ignored for a range of other studies these findings increase our confidence in previous isotropic modeling they also imply that melt rich layers may be a dynamically valid explanation for the lithosphere asthenosphere boundary as far as our anisotropic models are applicable

a joint study of experimental deformation and experimentally induced microstructures of pretextured peridotites a series of deformation tests was performed on samples from natural xenoliths and dunites with well defined textures and microstructures the rock samples were deformed perpendicular to the foliation 45 to the foliation and parallel to the foliation at temperatures from 1000 c to 1200 c at a confining pressure of 300 mpa and a constant strain rate of 10 5 s to obtain insight into the rheological behavior of anisotropic lithospheric mantle material the experiments showed strong work hardening for all samples with a large increase of the flow stress at relatively low strain in all samples both brittle and plastic mechanisms were operative we observed nonlocalized dilatant and localized dilatant deformation features nonlocalized and localized plastic deformation created subgrain boundaries deformation lamellae and an increase in dislocation density in the olivine crystals heterogeneous dislocation activities occurred on the 100 0kl system at 1000 c and on the 100 010 and 101 010 systems at 1200 c strong crystallographic fabrics developed in samples with olivines that were initially oriented with low resolved shear stresses on their slip systems the postdeformation fabric intensity was lower in specimens with strong initial petrofabrics interactions between dislocations of different type led to high work hardening rates during high temperature deformation favoring intragranular microcracking fractures opened parallel to the 100 screw dislocations in the 010 plane and in the hk0 planes oblique to the 100 plane the strength of the deformed samples was temperature and orientation dependent rocks deformed perpendicular to the foliation are strongest and those deformed parallel to the foliation have intermediate rock strength similar to those deformed at 45 to the foliation based on the laboratory results we find important implications for the rheology of the lithosphere 1 peridotites develop semibrittle rheology even at high temperature and show characteristic microstructures and fabric changes 2 the orientation of compressive stress and initial petrofabric are important parameters for defining a deformation regime if pressure temperature and strain rate are fixed 3 interaction of slip systems produces strong work hardening and intragranular microcracking

middle to late paleozoic acadian orogeny in the northern appalachians a laramide style plume modified orogeny the laramide orogeny of the western united states is proposed as a modern analogue for the silurian devonian acadian orogeny and subsequent diachronous voluminous short lived magmatism and basin formation in the northern appalachians shallowing of the benioff zone accounts for several enigmatic features associated with plate convergence in the northern appalachians including 1 wenlockian ludlovian termination of arc related magmatism in the avalon terrane followed by a period of relative magmatic quiescence from 395 to 380 ma and 2 diachronous migration of the acadian deformation front from ca 415 ma in the southeast to ca 370 ma in the northwest extending more than 600 km into the continental interior the flattening of the subduction zone is attributed to overriding of a plume by the convergent margin which may explain 1 the abrupt termination of magmatic quiescence by 380 370 ma and voluminous felsic magmatism and production of plume related lamprophyres in the southeast meguma terrane as the plume thermally eroded the oceanic lithosphere causing melting of the lower crust 2 late devonian regional high temperature low pressure metamorphism in the meguma terrane related to the thermal anomalies above a plume 3 synchronous devonian emplacement of meguma gold deposits and associated siderophile elements possibly derived from fluid circulation above an ascending plume 4 rapid late devonian uplift and erosion of as much as 10 km due to dynamic uplift over a plume 5 migration of magmatism to the north avalon terrane cobequid highlands so that plume related carboniferous magmatism occurred in and around the carboniferous permian maritimes basin 6 the high density lens at the base of the crust beneath the maritimes basin the product of plume derived underplated mafic rocks and 7 a subsidence mechanism for formation of the maritimes basin by cooling of a decapitated plume head

rayleigh taylor instabilities with anisotropic lithospheric viscosity summary rocks often develop fabric when subject to deformation and this fabric causes anisotropy of physical properties such as viscosity and seismic velocities we employ 2 d analytical solutions and numerical flow models to investigate the effect of anisotropic viscosity av on the development of rayleigh taylor instabilities a process strongly connected to lithospheric instabilities our results demonstrate a dramatic effect of av on the development of instabilities their timing location and most notably their wavelength are strongly affected by the initial fabric specifically we find a significant increase in the wavelength of instability in the presence of av which favours horizontal shear we also find that an interplay between regions with different initial fabric gives rise to striking irregularities in the downwellings our study shows that for investigations of lithospheric instabilities and likely of other mantle processes the approximation of isotropic viscosity may not be adequate and that av should be included

an analysis of anisotropy of rocks containing shape fabrics of rigid inclusions abstract this paper presents a theoretical basis for estimation of mechanical anisotropy in homogeneous rocks containing shape fabrics of rigid inclusions the analysis is based on two types of viscous models one containing linear fabrics of prolate a b c inclusions cf l tectonite and the other containing planar fabrics of oblate a b c inclusions cf s tectonite models show contrasting bulk viscosities in stretching normal viscosity and shearing shear viscosity parallel to the fabric the axial ratio r a b and the volume concentration v of rigid inclusions appear to be the principal parameters in determining the viscosity contrast in anisotropic models with linear fabrics normal viscosity p increases monotonically with increase in r whereas shear viscosity s increases to a maximum and then drops down to a near stationary value in anisotropic models with planar fabrics the normal viscosity increases little with increasing flatness of inclusions but the variation assumes a steep gradient when the latter is large shear viscosity on the other hand is relatively less sensitive to the shape of inclusions the ratio of normal and shear viscosities conventionally described as anisotropy factor in both the models is always greater than 1 indicating that normal viscosity will be essentially greater than shear viscosity irrespective of the axial ratio of inclusions forming the fabric models with a linear fabric show contrasting normal viscosities in pure shear flow along and across the linear fabric the anisotropy is expressed by the ratio of longitudinal and transverse normal viscosities anisotropic factor it is revealed that the transverse viscosity is essentially less than the longitudinal viscosity as observed in test models

a kinematic model for the late cenozoic development of southern california crust and upper mantle a model is presented for the young and ongoing kinematic deformation of southern california crust and upper mantle that accounts for both the known late cenozoic geologic record and upper mantle structure in this model the high velocity upper mantle transverse ranges anomaly is created through the convergence and sinking of the entire thickness of subcrustal lithosphere using typical scaling relations we estimate this anomaly to be 500 c colder and 1 more dense than average southern california mantle of the same depth we infer that subcrustal lithosphere from both sides of the convergence zone participates nearly equally in the construction of the anomaly the low velocity upper mantle anomaly beneath the salton trough region is attributed to high temperatures and 1 4 partial melt related to adiabatic decompression during mantle upwelling our model has crustal motions in the central transverse ranges region differing from mantle motions while the subcrustal lithosphere south of the transverse ranges moves toward and sinks beneath the transverse ranges the corresponding crust largely avoids convergence by adopting a more westerly direction of motion the differing velocity fields necessitates of a decoupling zone immediately south of the transverse ranges near the moho we interpret the upper mantle anomalies as results of small scale convection beneath southern california to which we attribute much of the recent tectonic activity in the region

overriding plate controls on subduction evolution geologic and geophysical observations indicate that the thickness density and strength of the lithosphere vary on the earth however the role of the overriding plate lithosphere properties on the evolution and morphology of subduction is not well understood this paper presents time dependent numerical models of subduction that vary the overriding plate thickness strength and density and allow for a plate interface that evolves with time via an anisotropic brittle failure rheology we examine the effect of these parameters on subduction evolution in particular the emergence of a asymmetric versus symmetric subduction b trench retreat versus advance c subduction zone geometry d slab stagnation versus penetration into the lower mantle and e flat slab subduction almost all of the models presented result in sustained asymmetric subduction from initiation trench advance occurs in models with a thick and or strong overriding plate slab dip measured at a depth below the plate boundary interface has a negative correlation with an increase in overriding plate thickness overriding plate thickness exerts a first order control over slab penetration into the lower mantle with penetration most commonly occurring in models with a thick overriding plate periods of flat slab subduction occur with thick strong overriding plates producing strong plate boundary interface coupling the results provide insight into how the overriding plate plays a role in establishing advancing and retreating subduction as well as providing an explanation for the variation of slab geometry across subduction zones on earth where similar patterns of evolution are observed