dynamical lagrangian remeshing dlr a new algorithm for solving large strain deformation problems and its application to fault propagation folding abstract we present a new algorithm to solve the problem of rock deformation the method is based on a classical lagrangian finite element solver in which the connectivity between the nodes of the numerical mesh is updated at the end of each time or deformation step the method which we term dynamic lagrangian remeshing dlr relies on the theory of delaunay triangulation and produces optimal finite element meshes in the sense that nodes are always connected to their set of nearest neighbour nodes because it is based on a lagrangian method in which the numerical grid is attached to material particles and advected with the deformation the dlr algorithm is ideally suited to track material boundaries and properly represent free surfaces in addition in comparison to classical lagrangian solvers the dlr method is not limited to small strain problems and may be used to model discontinuities such as faults and narrow shear zones we have used the dlr method to study the evolution of a crustal layer undergoing finite shortening driven by a velocity discontinuity along its base the numerical model predicts the formation of a large fold that is progressively cut by a fault originating at the velocity discontinuity and propagating through the crustal layer the model predictions provide support for the fault propagation folding model of suppe 1

lithospheric scale folding numerical modelling and application to the himalayan syntaxes we describe the eastern and western himalayan syntaxes which are large scale antiforms situated at geodynamically similar locations and the metamorphic evolution of which is coeval in the india asia collisional history to understand the mechanical plausibility of the structural interpretation we present two dimensional finite element modelling of lithospheric folding the models reveal the coeval development of adjacent synformal basins analogous to the peshawar and kashmir basins on both sides of the western syntaxis similarities between geological data and calculated models indicate that lithospheric buckling is a basic response to large scale continental shortening and an efficient mountain building process

three dimensional lithospheric structure below the new zealand southern alps uppermost mantle seismic structure below the southern alps in south island new zealand is investigated by teleseismic p wave travel time residual inversion the three dimensional tomographic images show a near vertical high velocity 2 4 structure in the uppermost mantle that directly underlies thickened crust along the nnessw axis of the southern alps the center of the high velocity anomaly lies to the east of the alpine fault which bounds pacific and australian plate rocks the oblique collision of these plates resulted in the uplift of the southern alps during the past 5 7 m y also a high velocity anomaly 3 5 corresponding to the hikurangi subduction zone lies to the northeast of the southern alps anomaly and low velocity anomalies 3 underlying parts of northwestern and southern south island may be signatures of late tertiary extension and volcanism the data consist of teleseismic arrival times from the new zealand national seismograph network and arrival times recorded during the 1995 1996 southern alps passive seismic experiment crustal heterogeneity was accounted for by back projecting the rays through an independently obtained three dimensional crustal velocity and moho depth model the southern alps uppermost mantle velocity anomalies are most simply explained by lithospheric thickening below the center of convergence accompanied by thinning and asthenospheric upwelling adjacent to the region of convergence

the plume head continental lithosphere interaction using a tectonically realistic formulation for the lithosphere summary current debates on the existence of mantle plumes largely originate from interpretations of supposed signatures of plume induced surface topography that are compared with predictions of geodynamic models of plume lithosphere interactions these models often inaccurately predict surface evolution in general they assume a fixed upper surface and consider the lithosphere as a single viscous layer in nature the surface evolution is affected by the elastic brittle ductile deformation by a free upper surface and by the layered structure of the lithosphere we make a step towards reconciling mantle and tectonic scale studies by introducing a tectonically realistic continental plate model in large scale plume lithosphere interaction this model includes i a natural free surface boundary condition ii an explicit elastic viscous ductile plastic brittle rheology and iii a stratified structure of continental lithosphere the numerical experiments demonstrate a number of important differences from predictions of conventional models in particular this relates to plate bending mechanical decoupling of crustal and mantle layers and tension compression instabilities which produce transient topographic signatures such as uplift and subsidence at large 500 km and small scale 300 400 200 300 and 50 100 km the mantle plumes do not necessarily produce detectable large scale topographic highs but often generate only alternating small scale surface features that could otherwise be attributed to regional tectonics a single large wavelength deformation predicted by conventional models develops only for a very cold and thick lithosphere distinct topographic wavelengths or temporarily spaced events observed in the east african rift system as well as over french massif central can be explained by a single plume impinging at the base of the continental lithosphere without evoking complex asthenospheric upwelling

at what stress level is the central indian ocean lithosphere buckling internal contrasts in strength are responsible for lithospheric buckling these are quantified by comparing the indian ocean data to two dimensional visco elasto plastic numerical models where the material properties depend on temperature and pressure the central indian basin is known for its intraplate seismicity and long wavelength undulations of the sea floor and associated gravity signatures to simulate the amplitudes of undulations that reach 1 km within about 11 ma of compression and 60 km of shortening the required mean yield strength of the lithosphere is 400 mpa if either a hydrostatic fluid pressure is considered in the crust or a mechanical decoupling at the depth of the moho small crustal wavelengths are superimposed on the long wavelength deformation in agreement with observations it is then possible to match the alternative indication that buckling commenced only 4 ma ago with a total amount of shortening of 30 km and with a required yield strength of 200 mpa about 10 of homogeneous thickening accompanies buckling taking into account variable thermal conductivity demonstrates that a change in the geotherm is sufficient to increase buckling amplitudes by 10 within 5 ma fl 2000 elsevier science b v all rights reserved

teleseismic p wave delays and modes of shortening the mantle lithosphere beneath south island new zealand a high speed zone in the mantle directly beneath the southern alps of new zealand is required by the recorded pattern of teleseismic p waves two parallel lines of 80 seismographs spaced at 2 km intervals recorded three earthquakes from the western pacific with epicentral distances of 52 53 and 78 azimuthal bearings were all within 15 degrees of the mean trends of the seismograph lines differences between measured delays and those predicted from the crustal structure reach 0 8 s along one line and 1 0 s along the other with the rays for the earliest arriving signals passing the depth of 120 km beneath the center of the island assuming these early arrivals are due to structure within the mantle shallower than 200 km they imply that the core of the high speed zone lies beneath the thickest crust which has been shortened by 100 km of convergence during the past 6 7 myr although the shape and position of the high speed body cannot be fixed uniquely a roughly symmetric body centered about a depth of 120 km 80 100 km wide with a depth extent of 100 km and with a maximum speed advance of 7 satisfies the observations the pattern of residuals does not fit with those predicted by simple models of intracontinental subduction in which crust and mantle lithosphere are detached and one slab of mantle lithosphere underthrusts the other rather the residuals favor thickening of mantle lithosphere by a more homogenous straining of it as if mantle lithosphere beneath continental crust behaved as a continuum an excess mass in the mantle is also required by the observed gravity anomalies once allowance is made for the seismically determined crustal thickness this high density mantle anomaly provides sufficient force per unit length to maintain the crustal root which is approximately twice as thick as that necessary to support the topography

the quaternary is here to stay the proposals to drop the name quaternary to extend the neogene period to the present day and to confirm the base of the pleistocene at 1 8 ma are reviewed it is concluded that there is no scientific case for dropping the name quaternary and that an overwhelming volume of evidence points to 2 6 ma as the most appropriate base for the quaternary pleistocene when extensive northern hemisphere mid latitude glaciation occurred under what were essentially similar physical boundary conditions of continents oceans seaways and mountain ranges that characterise and are unique to the quaternary it is desirable that all stratigraphical and geochronological evidence is tied rigorously into the global stratigraphic timescale copyright 2007 john wiley sons ltd

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

the deforming bed characteristics of a stratified till assemblage in north east anglia uk investigating controls on sediment rheology and strain signatures the glacial coastal exposures of north norfolk are a type site for subglacial glaciotectonic deforming bed sediments this investigation of the lower stratified diamict within the north sea drift at west runton reveals two distinct lamina types type 1 laminae are the product of primary extensional glaciotectonism with ductile intergranular pervasive shear predominating over brittle shear type 2 laminae also exhibit structures that can be attributed to ductile intergranular pervasive shear and brittle shear but the lateral continuity of type 2 laminae and the presence of dropstone like structures supports a primary subaqueous origin with secondary subglacial deformation when coupled with micromorphological analysis these findings show that ductile viscous creep mechanisms control sedimentary architecture and that shear stratification in particular has the potential to affect the rheological properties of the sediment pile and the hydraulic routing of basal water ultimately influencing critical effective pressure fluctuations and the thresholds controlling the subglacial drainage system

continental plate collision unstable vs stable slab dynamics we study possible scenarios for the evolution of continental collision zones by using a dynamic thermomechanical model that includes brittle elastic ductile rheology surface erosion and explicit metamorphic changes this paper focuses primarily on the influence of four key parameters 1 geotherm or thermotectonic age which controls the rheological profile 2 lower crustal composition weak or strong rheology 3 convergence rate and 4 metamorphic changes in the downgoing crust the experiments suggest that de pending on these parameters plate convergence is accommodated by four distinct mech anisms stable subduction shortening by pure shear thickening or folding and rayleigh taylor instabilities it appears that stable oceanic type subduction can only occur in the case of cold lithospheres moho temperature tmoho 550 8c and basically needs high convergence rates 4 5 cm yr depending on the lower crustal rheology strong or weak either the whole upper and lower crust or only the lower crust can be involved in subduction it appears that in the case of weak metamorphic rheologies phase changes only slightly improve chances for stable subduction lithospheric shortening becomes a dominant mechanism when tmoho 550 8c or convergence rates are 4 5 cm yr pure shear thickening becomes important in all cases of hot lithospheres tmoho 650 8c large scale folding is favored in the case of tmoho 5 500 650 8c and is more effective in the case of mechanical coupling between crust and mantle e g strong lower crust grav itational rayleigh taylor instabilities overcome other mechanisms for very high values of tmoho 800 8c and may lead to development of subvertical cold spots

turbidity development and dissipation in paleoplacer gold deposits southern new zealand placer gold mining inevitably produces highly turbid processing waters five historic paleoplacer mining sites in central otago new zealand provided 24 samples which were used in laboratory based settling experiments turbidity was examined in the context of stratigraphy of paleoplacer deposits their environments of formation and the groundwater processes that affected the deposits as well as the underlying bedrock settling rates were characterised by measuring turbidity levels over time up to 40 days using turbidimeter and coulter counter methods high clay mineral mostly kaolinite contents of the materials were confirmed by x ray diffraction grain size distributions of suspended materials were very comparable across all samples with majority of particles falling between 1 2 and 2 5 m the levels of turbidity produced by the auriferous sediments were partly controlled by the level of sorting and winnowing that the sediments were subjected to during transport and deposition debris flow material generated high turbidity initial levels of 120 420 nephelometric turbidity units ntu which settled slowly as did eolian siltstone 1 600 ntu fluvial sediments generally generated lower turbidity which settled more rapidly on the scale of hours to days it was found that cementation of the sediment can reduce turbidity generation by limiting disaggregation of the clay minerals on the other hand the presence of altered lithic clasts within the sediments contributes to higher turbidity production there was poor correlation between the level of bedrock alteration as indicated by chemical index of alteration and the resulting turbidity settling rates were more rapid in experiments conducted in saline solution as opposed to stream water due to floc formation

current understanding of pleistocene temperate stages in ireland abstract research on irish pleistocene temperate stages has been dominated by investigation of deposits with very similar floristic records collectively termed the gortian and traditionally correlated with the continental holsteinian recently detailed investigation of interglacial material from cork harbour suggested the gortian group may contain floristically similar deposits of different ages the pollen record had gortian holsteinian characteristics and dating by measurement of uranium thorium disequilibrium and amino acid racemization suggested that the deposits were younger than stage 11 correlated with the holsteinian by reille and de beaulieu reille de beaulieu quat res 44 1995 205 the suggestion that there may have been more than one holsteinian style temperate stage in europe might explain the floristic similarity of gortian and holsteinian deposits and the apparent younger age of the cork deposits the lack of a last interglacial stratotype is the most outstanding problem in irish quaternary studies the pollen content of sediments at fenit co kerry dated to ca 118 ka bp suggested they may represent a cool temperate phase at the end of the last interglacial recently investigated sub drumlin organic rich deposits at knocknacran co monaghan are also possibly last interglacial in age and these are discussed in detail here

dynamics of retreating slabs 1 insights from two dimensional numerical experiments from the effects of mantle flux part 2 therefore in this paper we apply forces to the slab using simple analytical functions related to buoyancy and viscous forces in order to isolate the role of rheology on slab dynamics we analyze parameters for simplified elastic viscous and nonlinear viscoelastoplastic single layer models of slabs and compare them with a stratified thermomechanical viscoelastoplastic slab embedded in a thermal solution the near surface behavior of slabs is summarized by assessing the amplitude and wavelength of forebulge uplift for each rheology in the complete thermomechanical solutions vastly contrasting styles of slab dynamics and force balance are observed at top and bottom bends however we find that slab subduction can be modeled using simplified rheologies characterized by a narrow range of selected benchmark parameters the best fit linear viscosity ranges between 5 10 22 pa s and 5 10 23 pa s the closeness of the numerical solution to nature can be characterized by a deborah number 0 5 indicating that elasticity is an important ingredient in subduction index terms 8120 tectonophysics dynamics of lithosphere and mantle general 8159 tectonophysics rheology crust and lithosphere 8160 tectonophysics rheology general keywords subduction numerical models lithospheric rheology

dynamics of subduction initiation at preexisting fault zones we show that a new subduction zone can initiate at a preexisting dipping fault zone with reasonable plate forces consistent with what is observed in the western pacific the dynamics of subduction initiation within a viscoelastic medium has been systematically explored with the finite element method we investigate the compression of oceanic lithosphere with both force and velocity boundary conditions and track the thermal structure in which heat is transported by both advection and diffusion the viscosity of the medium is non newtonian and temperature dependent we also examine the influence of a perfectly plastic yield stress a new method to model the preexisting weak zone as a graded fault with remeshing is introduced the first 400 km of plate convergence is examined during which time there are profound changes in plate boundary dynamics initially topography across the plate boundary is characterized by over 1000 m of uplift close to the trench subsequently a bathymetric depression develops on the overriding plate with an initial subsidence rate of 125 m myr for a slab subducting at 2 cm yr tectonic subsidence of the overriding plate is primarily dependent upon the depth of slab penetration for a plate force of 4 10 12 n m subduction initiates even with a fault shear stress of 3 mpa the resistance most consistent with seismological observations however subduction will not initiate when this fault shear stress is 5 mpa our results are consistent with plate reconstructions which predict the initiation of subduction across preexisting weak zones within oceanic basins

control of folding by gravity and matrix thickness implications for large scale folding 1 we show that folding of a non newtonian layer resting on a homogeneous newtonian matrix with finite thickness under influence of gravity can occur by three modes 1 matrix controlled folding dependent on the effective viscosity contrast between layer and matrix 2 gravity controlled folding dependent on the argand number the ratio of the stress caused by gravity to the stress caused by shortening and 3 detachment folding dependent on the ratio of matrix thickness to layer thickness we construct a phase diagram that defines the transitions between each of the three folding modes our priority is transparency of the analytical derivations e g thin plate versus thick plate approximations which permits complete classification of the folding modes involving a minimum number of dimensionless parameters accuracy and sensitivity of the analytical results to model assumptions are investigated in particular depth dependence of matrix rheology is only important for folding over a narrow range of material parameters in contrast strong depth dependence of the viscosity of the folding layer limits applicability of ductile rheology and leads to a viscoelastic transition our theory is applied to estimate the effective thickness of the folded central asian upper crust using the ratio of topographic wavelength to moho depth phase diagrams based on geometrical parameters show that gravity does not significantly control folding in the jura and the zagros mountains but does control folding in central asia applicability conditions of viscous and thin sheet models for large scale lithospheric deformation derived in terms of the argand number have implications for the plate like style of planetary tectonics

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

on the thermomechanical evolution of compressional orogens summary we present results from a newly developed fully coupled thermomechanical model of the continental crust in which crustal shortening at a convergent plate boundary is driven by a basal velocity discontinuity which represents delamination and subduction in the underlying mantle this new dynamical model incorporates complex rheologies elasticity thermally activated creep and brittle frictional behaviour allows for extremely large deformation and is coupled along its top boundary to a complex erosion sedimentation model the model is based on the dynamic lagrangian remeshing dlr method which uses lagrangian spatial discretization of the crust and therefore allows for an accurate tracking of rock particles as they travel through the deforming orogen this information is in turn used to produce synthetic ptt paths we have used this model to predict the distribution of apparent ages for a wide range of isotopic systems at the surface of an actively deforming orogen we have also predicted other geophysical and geological observables such as the metamorphic grade of exposed rocks regarded here as a first order approximation for total denudation topography surface heat flux and the thickness of sediment deposited in the adjacent foreland basins results clearly demonstrate that the highest exhumation rates and thus the youngest isotopic ages are found in regions of maximum topography near the centre of the orogen but that the most deeply exhumed rocks are found on the side of the orogen in the vicinity of the retro shear zone a crustal scale fault which accommodates most of the crustal shortening within the orogen it is also in this region that the isotopic systems characterized by the greatest closure temperatures display the youngest ages and the highest grade metamorphic rocks are found these conclusions are derived from the assumption of uniform erosion across the orogen in cases where rainfall and thus erosion is orographically controlled the tectonic style of the orogen is different from the uniform erosion case as is the distribution of isotopic ages and metamorphic grades across the orogen

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

rheological modelling and deformation instability of lithosphere under extension summary continental break up which precedes oceanic accretion probably results from an unstable extension of the lithosphere analogous to necking of metals when they are submitted to tension by reason of complexity of the rheology no conclusion about lithospheric extension stability may be reached by an a priori analysis we thus examine directly the evolution when the lithosphere is stretched of lateral inhomogeneities represented in our example by small scale variations of thickness the rheological model is derived from the hypotheses of brace kohlstedt 1980 and is consistent with the results of rock mechanics the lithosphere consists of three or four layers of varying thicknesses and mechanical properties the brittle upper crust and eventually the brittle part of the mantle are assimilated to perfectly plastic media and are described in a state of uniform extension by a constant viscosity in the lower crust and ductile mantle lithosphere the effective viscosity is supposed to be exponential the mechanical model relies on a perturbation method developed by fletcher hallet 1983 among others contrary to previous published results no unstable behaviour of the lithosphere is observed unless the latter is more dense than the asthenosphere in which case a gravitational instability may develop this discrepancy can be explained by differences in assumptions concerning the variation of strength in the lithosphere as yet poorly constrained by the data we observe a great sensitivity of the results to the strength stratification and to the artificial discontinuities of density or viscosity implied by the models

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

tectonics of pliocene removal of lithosphere of the sierra nevada california pliocene ca 3 5 ma removal of dense eclogitic material under the sierra nevada has been proposed from variations in the petrology and geochemistry of neogene volcanic rocks and their entrained xenoliths from the southern sierra the replacement of eclogite by buoyant warm asthenosphere is consistent with present day seismologic and magnetotelluric observations made in the southern sierra a necessary consequence of replacing eclogite with peridotite is that mean surface elevations and gravitational potential energy both increase an increase in potential energy should increase extensional strain rates in the area if these forces are insuffi cient to signifi cantly alter pacifi c north american plate motion then increased extensional strain rates in the vicinity of the sierra must be accompanied by changes in the rate and style of deformation elsewhere changes in deformation in california and westernmost nevada agree well with these predictions existing geologic evidence indicates that a period of rapid uplift along the sierran crest of more than 1 km occurred between 8 and 3 ma most likely as a consequence of removal of lower lithosphere about this same time extensional deformation was initiated within 50 km of the eastern side of the sierra 5 3 ma and regional shortening began to produce the california coast ranges 5 3 ma we suggest that these events were induced by the 1 2 10 12 n m increase of gravitational potential energy generated by the sierran uplift evidence for pliocene uplift adjoining crustal extension and shortening in directly opposing parts of the coast ranges is found along nearly the entire length of the sierra nevada and implies that lithosphere was removed beneath all of the presentday mountain range the uplifted area lies between two large upper mantle high pwave velocity bodies under the south end of the san joaquin valley and the north end of the sacramento valley these high velocity bodies plausibly represent the present position of material removed from the base of the crust lithospheric removal may also be responsible for shifting of the distribution of transform slip from the san andreas fault system to the eastern california shear zone a prediction that awaits better defi ned slip histories on both faults overall the late cenozoic deformational history of the sierra nevada and vicinity illustrates that locally derived forces can infl uence deformation kinematics within plate boundary zones

structural softening of the lithosphere structural softening is a decrease in the amount of stress needed to deform the lithosphere at a particular rate because of its structural reorganization while all true rheological properties remain constant structural softening is fundamentally different than material softening where the decrease in stress is generated by a change in rheological properties with progressive deformation such as grain size reduction resulting from large shearing strain we study structural softening generated by folding of the crust mantle boundary which is a structural instability that inevitably develops during compression of the mechanically layered lithosphere for ductile rheologies the stress decrease represents a decrease of the effective lithospheric viscosity which is proportional to the ratio of stress to lithospheric shortening strain rate we present analytical and numerical results quantifying the decrease in stress and effective viscosity that occur during shortening at a constant rate the decrease in effective viscosity can be up to 10 fold

effects of elasticity on the rayleigh taylor instability implications for large scale geodynamics summary although parts of the lithosphere may be expected to behave elastically over certain timescales this effect is commonly ignored in models of large scale mantle dynamics recently it has been demonstrated that elasticity and in particular viscoelasticity may have a significant effect on the buckling instability and on the creation of lithospheric scale shearzones it is however less clear whether elasticity also has an effect on mantle convection and density driven lithospheric instabilities the focus of this work is therefore to study the effects of elasticity on the twolayer rayleigh taylor rt instability consisting of a maxwell viscoelastic layer overlying a viscous layer of lower density we analyse this problem by performing systematic numerical simulations that are compared with newly derived analytical solutions it is demonstrated that elasticity can be important for certain parameter combinations it leads to a speedup of the rt instability the cause for this speedup is that the rt instability is only sensitive to the viscous fraction of deformation in the viscoelastic layer elasticity reduces the viscous fraction of deformation at timescales shorter than the maxwell relaxation time t m t m g where is the viscosity and g the elastic shear module for plate tectonics on earth the parameters are such that the effect of elasticity on instability growth is negligible for most boundary conditions whereas elasticity does not or only slightly change the timescales for lithospheric detachment of the upper mantle it does significantly alter the response and stress build up in the overlying crust numerical simulations illustrate this effect for lithospheric detachment and show that peak stresses in a viscoelastic crust are smaller than stresses that develop in a viscous crust moreover if the timescale for delamination of the mantle lithosphere is equal or smaller than the maxwell relaxation time of the crust the topography of the crust is increased compared to viscous models

rayleigh taylor instability as a mechanism for corona formation on venus abstract in this study we explore the idea that coronae have formed on venus as a result of gravitational rayleigh taylor instability of the lithosphere the lithosphere is represented by a system of stratified homogeneous viscous layers low density crust over high density mantle over lower density layer beneath the lithosphere a small harmonic perturbation imposed on the base of the lithosphere is observed to result in gravitational instability under the constraint of assumed axisymmetry topography develops with time under the influence of dynamic stress associated with downwelling or upwelling and spatially variable crustal thickening or thinning topography may therefore be elevated or depressed above a mantle downwelling but the computed gravity anomaly is always negative above a mantle downwelling in a homogeneous asthenosphere the ratio of peak gravity to topography anomaly depends primarily on the ratio of crust to lithospheric viscosity average observed ratios are well resolved for two groups of coronae 40 mgal km 1 consistent with models in which the crust is perhaps 5 times stronger than the lithosphere group 3a rim surrounding elevated central region coronae are inferred to arise from a central upwelling model whereas group 8 depression coronae are inferred to arise from central downwelling observed average coronae radii are consistent with a lithospheric thickness of only 50 km an upper low density crustal layer is 10 20 km thick as inferred from the amplitude of gravity and topography anomalies

the effects of buoyant crust on the gravitational instability of thickened mantle lithosphere at zones of intracontinental convergence summary using both numerical experiments and approximate linear theory we examine the stability of a 2 d three layered viscous system that represents crust over mantle lithosphere overlying asthenosphere if horizontal shortening of lithosphere occurs as in mountain building the development of gravitational instability may follow one of two distinct paths localized downwelling occurs beneath the region of horizontal shortening and crustal thickening if the layer representing crust has a large viscosity coefficient is relatively dense or is thin relative to the underlying layer that represents mantle lithosphere conversely for a low viscosity ratio or buoyant crust downwelling flow develops on the flanks of the zone of convergence and the mantle lithosphere layer thins beneath the central region thinning in such a setting would facilitate volcanism and high temperature metamorphism during the development of a mountain belt a series of numerical experiments with different density and viscosity structures including runs with non newtonian viscosity show that this general description is insensitive to minor differences in the depth distributions of density and viscosity the tendency of structures with relatively thick crustal layers or relatively thin lithospheric mantle to favour paired marginal downwellings may account for sustained volcanism during intracontinental deformation where lithosphere initially is warm thin and weak the western part of the tien shan in asia and the region including and surrounding the alboran sea illustrate some of the features associated with paired marginal downwellings the western transverse ranges of california and the southern alps of new zealand share features associated with single downwelling flow beneath the axes of the belts

new data for the last glacial maximum in great britain and ireland abstract understanding the history of the british and irish ice sheet biis at the last glacial maximum lgm has been advanced by new approaches in particular by cosmogenic nuclide surface exposure dating aminostratigraphy of shelly glacial deposits ams radiocarbon dating and the evidence from continental margin marine cores all of which supersede the previously weak geochronologic control it was formerly believed that great britain and ireland was largely ice free between the last interglacial oxygen isotope sub stage 5e and the late devensian when the lgm occurred as such the biis was effectively out of phase with laurentide and scandinavian ice sheets as well as inferences of ice volume from oxygen isotope stratigraphy the biis during the late devensian maximum was also perceived as having been relatively stationary new evidence shows that the lgm was an important event during the evolution of an earlier biis when the extent of ice was greater repeated iceberg rafting events over the past 50 ka are shown by marine cores while the derivative inference of numerous corresponding glacial advances is supported by several clusters of 36 cl ages on glaciated surfaces and glacial boulders that are indicative of deglacial events between 40 and 12 ka these appear to be associated with heinrich events the earliest being inferred as heinrich 4 at about 40 ka during this advance the biis and scandinavian ice sheet sis were in contact and all of ireland was glaciated the ice sheet appears to have fluctuated several times between 40 and 25 ka although evidence for this is poorly preserved but the 36 cl and 14 c evidence is clear that the biis reached its lgm maximum size about 22 ka soon after heinrich event 2 when the biis and sis were not in contact one cluster of 36 cl and 14 c ages at 21 4 1 3 ka records an initial pulse of deglaciation that was followed by extensive deglaciation about 17 4 0 4 ka just before heinrich event 1 when the ice sheet readvanced contrary to previous views the biis probably existed throughout much of devensian time as a mobile and sensitive ice sheet during which the lgm advance was but one important event in places glacial deposits of the earlier devensian glaciation have previously been incorrectly identified as products of the later lgm glaciation

diapirism and topography summary a new numerical technique using markers and a deformable lagrangian mesh is used to study the deformation of the surface above a rising diapir this method allows modelling of a free surface boundary in a self consistent way the method makes it possible to investigate many different problems with non regular geometry the codes were verified through comparison with analytical solutions for the initial stages and with other numerical codes for mature stages our simulations led to the following conclusions 1 the growth rate of the diapir is more strongly influenced by the viscosity contrast than the wavelengths this is due to the strong growth rate difference during the initial stage 2 the maximum elevation above the diapir linearly increases with increasing wavelength and is approximately the same for different viscosity contrasts 3 the elevation will be considerably higher for a low viscosity diapir at a given depth than for an isoviscous diapir comparisons for different thicknesses of the buoyant layer showed that the highest topography is produced when the two layers have equal thickness we showed the dependence of the topographic behaviour on the parameter r the ratio of the density difference between two layers to the density of the upper layer it was found that topography behaves linearly up to r 0 15 it indicates that a posteriori estimation of topography for traditional calculations using the free slip boundary condition works well within this limit

a test of laboratory based rheological parameters of olivine from an analysis of late cenozoic convective removal of mantle lithosphere beneath the sierra nevada california usa summary we use the apparent change in mantle structure beneath the sierra nevada since ca 10 ma which suggests convective removal of eclogite rich mantle lithosphere and scaling laws developed for rayleigh taylor instability to place constraints on the average viscosity coefficient of the mantle lithosphere by treating the lithosphere as a non newtonian fluid obeying power law creep with an exponent of n 3 5 we may compare the inferred values of viscosity coefficient with those obtained from laboratory experiments on olivine and eclogite the values that we obtain overlap those predicted by laboratory based flow laws for the range of geotherms implied by heat flux measurements within the sierra nevada and by metamorphic geothermometry and geobarometry of xenoliths in volcanic rock erupted in the sierra nevada at ca 10 ma thus this comparison offers support for laboratory derived flow laws and specifically for the high stress limit suggested by evans goetze 1979 conversely this agreement shows that the high strength of cold mantle minerals does not prohibit its removal by convective instability

towards a self consistent plate mantle model that includes elasticity simple benchmarks and application to basic modes of convection one of the difficulties with self consistent plate mantle models capturing multiple physical features such as elasticity non newtonian flow properties and temperature dependence is that the individual behaviours cannot be considered in isolation for instance if a viscous mantle convection model is generalized naively to include hypo elasticity then problems based on earth like rayleigh numbers exhibit almost insurmountable numerical stability issues due to spurious softening associated with the co rotational stress terms if a stress limiter is introduced in the form of a power law rheology or yield criterion these difficulties can be avoided in this paper a novel eulerian finite element formulation for visco elastic convection is presented and the implementation of the co rotational stress terms is addressed the salient dimensionless numbers of visco elastic plastic flows such as weissenberg deborah and bingham numbers are discussed in a separate section in the context of geodynamics we present an eulerian formulation for slow temperature dependent visco elastic plastic flows a consistent tangent incremental formulation of the governing equations is derived numerical and analytical solutions demonstrating the effect of visco elasticity co rotational terms are first discussed for simplified benchmark problems for flow around cylinders we identify parameter ranges of predominantly viscous and visco plastic and transient behavior the influence of locally high strain rates on the importance of elasticity and non newtonian effects is also discussed in this context for the case of simple shear we investigate in detail the effect of different co rotational stress rates and the effect of power law creep the results show that the effect of the co rotational terms is insignificant if realistic stress levels are considered e g deviatoric invariant smaller than 1 10 of the shear modulus say we also consider the basic convection modes of stagnant lid episodic resurfacing and mobile lid convection as applicable to a cooling planet the simulations show that elasticity does not have a significant effect on global parameters such as the nusselt number and the qualitative nature of the basic convection pattern our simple benchmarks show however also that elasticity plays a significant role for instabilities on the local scale of an individual subduction zone

numerical modelling of strain localisation during extension of the continental lithosphere dipping reflectors in the upper mantle are observed on reflection seismic data and in some cases have been interpreted to represent mantle shear zones it is not clear however how these shear zones develop and under what circumstances previous attempts to model strain localisation by strain softening in the ductile regime have failed to show the development of mantle shear zones we present here results from a numerical model of lithospheric extension the model uses an elasto visco plastic rheology dependent on temperature pressure deviatoric stress and composition strain softening is incorporated into the mechanical model via a simple ad hoc strain viscosity relationship the results demonstrate that strain localisation can dominate lithospheric deformation in an extensional regime we illustrate the effects of varying the parameters which define the strain viscosity relationship on the geometry of lithospheric scale shear zones and the rift basin we also investigate the effects on shear zone development of introducing a variety of pre extension weak zones in the crust and or mantle our results suggest that if dipping reflectors observed on deep seismic data around the world are evidence of the existence of mantle shear zones there is a need to review our conceptual models of strain strength evolution during the development of continental rifts

stretching instabilities and lithospheric boudinage a competent layer with a nonlinear rheology can under extension exhibit pinch and swell instabilities such instabilities can explain small scale regular deformations of rock recently they have also been invoked in relation to the distribution of basins and ranges fletcher and hallet 1983 and the undulation of the bouguer map froidevaux 1986 in the western part of north america for the case of a simple stratified structure different authors have expressed mathematical solutions describing these instabilities in this paper we elucidate the general physical meaning of these nonhomogeneous deformations in the simplest case where the competent layer is fully embedded in a softer material a fundamental mode develops with a preferred wavelength equal to 4 times the thickness of a competent layer the novelty is the occurrence of overtones with alternate symmetrical and antisymmetrical deflections of the interfaces between layers these modes have a large amplitude when the rheology tends to perfect plasticity the presence of a free surface like the earth s surface for the upper crust is shown to double the preferred wavelength of the fundamental mode and to forbid one overtone out of two this new effect is caused by gravity which inhibits the growth of surface topography another shift is of opposite sign to that caused by a free surface numerical solutions for multilayer structures can elucidate the nature of couplings between various layers the case of a three layer lithosphere having a soft lower crust and overlying a mantle asthenosphere has been analyzed in some detail it shows that in the range of wavelengths larger than 30 km up to five preferred modes may be superimposed this should encourage further analysis of the western u s topography and gravity pattern which is known to contain at least two wavelengths about 40 km for the individual basins and ranges and about 200 km for the bouguer troughs in nevada and their associated broad topographies

shear instability in a viscoelastic material as the cause of deep focus earthquakes the mechanism of deep focus earthquakes has been examined by numerical and linear analysis of shear instability in subducting slabs we assume subducting slabs deform such that the spatially averaged strain rate becomes time independent furthermore we assume that rheology depends on temperature in the manner of an activation process using this model we investigate quantitatively the conditions under which shear instability takes place i e the deformation in the material concentrates in a thin layer whose temperature increases explosively until melting occurs first we study instability in a material whose rheological properties are spatially homogeneous shear instability takes place in a homogeneous material when exceeds a certain critical level e c which depends strongly on the stress in the slabs but does not depend on the detail of the creep law the value of the activation energy next we study instability in a material that contains shear zones similar to those often observed by field geologists i e a thin layer made of material with lower vicosity than the surrounding region we find that shear zones trigger shear instability even for the case of provided that g the ratio of characteristic time of thermal diffusion to that of shear heating in shear zones exceeds 1 and that f the ratio of stiffness of elastic deformation in the surrounding region to that of ductile deformation in shear zones becomes lower than 1 applying this condition to subducting slabs with shear zones we find that shear zones trigger shear instability even when the average strain rate is lower than by an order of magnitude from these results and from an estimate of in subducting slabs we conclude that shear instability is a mechanism likely to induce deep focus earthquakes

catastrophic initiation of subduction following forced convergence across fracture zones although the formation of subduction zones plays a central role in plate evolution the processes and geological settings that lead to the initiation of subduction are poorly understood using a visco elastoplastic model we show that a fracture zone could be converted into a self sustaining subduction zone after approximately 100 km of convergence modeled initiation is accompanied by rapid extension of the over riding plate and explains the inferred catastrophic boninitic volcanism associated with eocene initiation of the izu bonin mariana ibm subduction zone using global plate reconstructions we suggest that ibm nucleation was associated with a change in plate motion between 55 and 45 ma we estimate that the forces resisting ibm subduction initiation were substantially smaller than available driving forces

extension of continental lithosphere a model for two scales of basin and range deformation we develop a model for deformation in an extending continental lithosphere that is stratified in density and strength assuming a rheology consistent with seismic focal depths and experimental flow laws the model demonstrates that necking instabilities at two wavelengths will arise due to the presence of a strong upper crust and upper mantle separated by a weak lower crust the magnitudes of the instabilities are directly related to strength contrasts within the lithosphere while the dominant wavelengths of necking are controlled mainly by the thicknesses of the strong layers the results are applied to the basin and range province of the western united states where two scales of deformation can be recognized one corresponding to the spacing of ranges and the other to the width of tilt domains a bouguer gravity anomaly and associated regional topography with a wavelength comparable to the width of tilt domains has also been recognized for plausible density and strength stratifications our results show that the horizontal scale of short wavelength necking is consistent with the spacing of individual basins and ranges while that of the longer wavelength necking is consistent with the width of tilt domains we thus suggest that basin and range deformation may be controlled by two scales of extensional instability extension in the weak lower crust in this model is laterally displaced from regions of upper crustal extension the resultant horizontal shearing in the lower crust may be a mechanism for the initiation of low angle extensional detachments