quasi lagrangian large eddy simulations of cross equatorial flow in the east pacific atmospheric boundary layer using a large eddy simulation les the atmospheric boundary layer abl is numerically modeled along 958w from 88 st o 48n during boreal autumn and compared to observations from the east pacific investigation of climate processes in the coupled ocean atmosphere system epic 2001 since the local abl winds are predominantly southerly in this season a quasi lagrangian forcing is used in which the abl air column is forced as if it were advecting northward with the mean september october 2001 meridional wind across the equatorial cold tongue and the rapidly warming ssts to the north pressure gradients and large scale zonal advective tendencies are prescribed as a function of latitude where possible observations from the epic 2001 experiment are used for forcing and for comparison with model results the abl s modeled vertical structure accords with the conceptual model of wallace et al and agrees well with observations surface stability accounts for the minimum in surface wind over the equatorial cold tongue and the maximum over the warm water to the north stability of the lower abl over the cold tongue allows a jet to accelerate at about 500 m height relatively uncoupled to the frictional surface layer vertical mixing over the warm water to the north distributes this momentum to the surface additional simulations were performed to explore the modeled abl s sensitivity to pressure gradients zonal advection free tropospheric humidity and initial conditions the model abl was robust changing the forcings resulted in little change in the modeled structure the strongest sensitivity was of stratocumulus clouds over the cold tongue to cloud top radiative cooling once formed at the southern edge of the cold tongue modeled stratocumulus clouds demonstrate a remarkable ability to maintain themselves over the cold tongue in the absence of surface fluxes by radiative cooling at their tops the persistence of thin stratocumulus clouds in this lagrangian model suggests that horizontal advection of condensate might be an important process in determining cloudiness over the cold tongue

lithospheric deformation during the early stages of continental collision numerical experiments and comparison with south island new zealand 1 the nature of lithospheric deformation during continental plate collision still remains unresolved while it has often been proposed that the mantle lithosphere is accommodated by distributed thickening of a viscous root an alternate hypothesis suggests that significant portions of convergent mantle lithosphere essentially undergo underthrusting or subduction to further consider this issue we model the thermochemical evolution of the lithosphere mantle system using arbitrary lagrangian eulerian finite element techniques we incorporate a mix of viscous thermally activated power law creep and plastic frictional coulomb with strain softening rheologies in the numerical experiments to treat disparate composition in the crust and mantle a range of rheological and mechanical parameters is explored to determine controls on the style of lithospheric deformation the models suggest that during the initial stages of plate collision the mantle lithosphere is characterized by plate like behavior and underthrusting subduction of the upper region in conjunction with distributed thickening and rayleigh taylor type viscous instability of the lower portion depending on the material rheology temperature regime and imposed convergence velocity the deforming mantle lithosphere demonstrates various combinations of these end member behavioral modes the modeling results are interpreted in the context of observed lithospheric deformation across south island new zealand a combined style of underthrusting and distributed thickening is consistent with the observed crustal structure in the young collisional orogen as well as seismic imaging of the geometry of the underlying lithosphere

core complex geometries and regional scale flow in the lower crust a simple flexural model that treats the lithosphere as a thin elastic plate overlying a fluid asthenosphere produces an excellent fit to the domal detachment fault geometries observed in typical core complexes of the basin and range province however this mode of isostatic compensation requires an unacceptably large reduction in local topographic elevation 3 5 km above core complex structures because topographic lows of this magnitude are not observed above core complexes today it requires mountain ranges 3 5 km higher than the surrounding areas to have existed at the site of each future core complex and requires these mountain ranges to have had large crustal roots that were subsequently uplifted to form a flat moho as the core complexes developed an alternate mode of isostatic compensation whereby compensation occurs primarily by regional scale flow of material within the lower crust beneath an elastic midcrust provides an equally good fit to the observed detachment fault geometries while requiring little reduction in local topographic elevation above developing core complex structures and providing an obvious means of maintaining a flat moho beneath these zones of upper crustal doming if this interpretation is correct then the effective elastic plate thickness below the basin and range province probably cannot exceed a maximum of about 4 km with a best guess estimate of about 0 5 to 1 km computation of the temperature structure associated with crustal flow during doming shows that an increase in temperature at the moho of about 100 c follows doming by several million years perhaps explaining the observed coincidence of core complexes with crustally derived igneous rocks subsequent cooling of the mid to lower crust beneath the core complex could result in freezing of the initially ductile lower crust within the core of the dome effectively locking the domal shape into the crust even after the detachment surface is cut by younger faults

some thermal and mechanical consequences of rapid uplift an example from the southern alps new zealand abstract the thermal evolution of continental crust during active collision is modeled through numerical solutions of the two dimensional heat conduction equation for a rapidly moving medium the boundary conditions used in the modeling are derived from geological and geophysical observations from the active collision zone in the south island of new zealand the problem domain over which the solutions are obtained consists of a 40 km horizontal by 25 km vertical spatial plane with a vertical discontinuity at 10 km from the western boundary to the east of this discontinuity vertical uplift rates of up to 10 mm a occur over a timespan of up to 4 ma temperature distributions are calculated at 10 ka intervals over the 4 ma duration a two dimensional high temperature region is established upon initiation of uplift of the eastern block due to the advective component carrying heat upwards more rapidly than it can be dissipated laterally from the problem domain temperatures within the upper 5 km are greater than 400 c after 2 25 ma with geothermal gradients of up to 200 c km attained within the upper 3 km at times greater than 2 5 ma the vertical temperature distribution changes little while the anomalously high temperatures spread laterally into the stationary crust using rheological equations to describe the brittle behaviour of a water saturated upper crust and the ductile behaviour of a quartz dominated lower crust together with the thermal distribution of the conduction models the mechanical evolution of a collision zone is investigated in addition to high crustal temperatures and associated high heat flow rapid uplift produces a weakening of the crust by raising of the depth of transition from brittle to ductile behaviour within the zone of most rapid uplift the brittle ductile transition rises from 13 km to less than 5 km after 1 5 ma of uplift further uplift reduces the brittle layer to 3 km thickness and causes lateral spreading of the low strength zone the reductions in crustal strength caused by the thermal weakening produce a high strain zone within the region of maximum uplift which is incapable of sustaining large differential stresses this causes horizontal and vertical stress transfer and results in shallow seismicity increases in the adjacent crust as well as in intermediate depth seismicity within the high strength upper mantle because the thermal and mechanical anomalies discussed are a function of rapid uplift all regions of active continental collision may be expected to exhibit similar behaviour some mechanical and thermal characteristics of the himalayan collision zone are briefly examined in light of the numerical modeling

numerical modelling of melting processes and induced diapirism in the lower crust summary the processes of partial melting and magmatic diapirism within the lower crust are evaluated using a numerical underplating model fully molten basalt t 1200 c is emplaced at the moho beneath a solid granite t 750oc in order that a melt front grows into the granite if diapirism does not occur this melt front in the granite reaches a minimal depth in the crust before like in the molten basalt crystallization takes place the density contrast between the partially molten granite layer and the overlying solid granite can lead to a rayleigh taylor instability rti which results in diapiric rise of the partially molten granite assuming a binary eutectic system for both the granite and the underplating basalt and a temperature and stress dependent rheology for the granite we numerically solve the governing equations and find a that diapirism occurs only within a certain but possibly realistic range of parameters and b that if diapirs occur they do not rise to levels shallower than 15 or perhaps 12km the growth rate depends on the degree of melting and the thickness of the partially molten layer as well as the viscosity of the solid and the partially molten granite from a comparison of the growth rate with the velocity of a stefan front it is possible to predict whether a melt front will become unstable and result in diapiric ascent or whether a partially molten layer is created which remains at depth we carry out such a comparison using our thermodynamically and thermomechanically consistent model of melting and diapirism

tectonics and topography for a lithosphere containing density heterogeneities the purpose of this paper is to clarify the dynamic role of lithospheric density heterogeneities in particular with respect to mountain building and other processes of intraplate deformation density anomalies within or just beneath the lithosphere constitute major sources for tectonic stress fields the product of their magnitude by their depth is shown to characterize their ability to induce deformation this rule of the density moment directly yields the lithospheric thickening or thinning rate when applied to structures of large lateral extent for anomalies of lateral extent that is small in comparison with their depth the deformation is vertically inhomogeneous and has been computed with the help of simple physical models of a stratified viscous newtonian lithosphere the analytical treatment is based on fourier transform continent continent collision thickens not only the crust but the entire lithosphere the cold root underlying a mountain chain induces strong regional compressive stresses able to sustain the mountain bulding process without further help from forces transmitted from far away thus the continental lithosphere is in a somewhat metastable mechanical state adiabatic i e rapid thickening or thinning tends to grow further once initiated tectonic phases of strong compression correspond to the climax of such instabilities the response of models with cold lithospheric roots of various intensities has been computed both in two and three dimensions they yield velocity distributions and stress fields instructive comparisons are made with earthquake focal mechanisms and in situ stress measurements in the alpine and appalachian regions in the presence of lateral variations of the mechanical properties of the lithosphere the tectonic style is not only function of the local topography and of the nature of its compensation deformations in neighbouring provinces are coupled as shown by 3 dimensional models for example thickening sustained by a cold lithospheric root may generate extension in peripheral zones of weakness these last results illustrate the point that the computation of regional tectonic stresses requires the knowledge of the density anomalies within the lithosphere on the one hand and of geometrical constraints related to lateral mechanical heterogeneities on the other

thermally softened continental extensional zones arcs and rifts as precursors to thickened orogenic belts abstract intra continental deformation of soft zones during continental collision requires weak continental lithosphere which is able to be shortened across considerable width during later convergence this enables significant thickening with formation of an orogenic root we have examined models with a history of lithospheric thinning by pure shear during an earlier phase of intra continental extension with associated heating geologically this situation is appropriate to intra continental rifts and back arc basins if thinning of elevated thermal structure is decoupled from the thinning of lithology then a weak soft lower crust and sub arc rift mantle result this weak structure has a favoured rheology for subsequent convergent thickening while the lithosphere is still hot these regions are associated with formation of granulites and metamorphic assemblages typical of high temperature low pressure ht lp if convergence starts while the heat input is still active then the failed rifts and arcs are shut off by lateral wedging of the hard lower crust and upper mantle of shoulder regions into the softened arc rift domain such sites are ideal for the formation and for the exhumation of metamorphic core complexes subsequent thickening during convergence leads to ht eclogites when the previous arc rift was hot and to medium t eclogites for a thickened standard geotherm these p t paths are counterclockwise and their shapes are strongly dependent on the amount of previous thinning and type of initial geotherm if the compression starts long after cessation of the extensional event and associated thermal anomaly then the geotherm of the extended area relaxes and the whole region hardens in this case no homogeneous thickening occurs and deep continental roots cannot form

delamination and detachment of a lithospheric root abstract orogens can be formed by shortening of the lithosphere on time scales much shorter than that of thermal diffusion for time scales of a few ma the major part of the uplift of an orogen which may occur during the transition from crustal shortening to extension may be due to delamination of the mantle lithosphere rather than to convective thinning the negative buoyancy forces of a lithospheric root may cause lateral shortening while the lithospheric root delaminates peels off from the crust detaches and is replaced by hot upwelling asthenosphere this provides a large input of heat into the crust geological observations show that the late orogenic phase may be characterized by volcanic activity high temperature low pressure metamorphism extension high heat flow and a rapid within a few ma increase of the topography we present dynamically and thermally consistent numerical models of the evolution of an orogenic root mantle rheology is based on laboratory data of olivine while models with the rheology of olivine do not allow for delamination due to its very high viscosity the incorporation of a quasi brittle rheology is necessary for modelling delamination of the mantle lithosphere successful delamination models are always associated with a rapid change in topography we examined the parameter range of our models and found different regimes of behaviour of the orogenic root of the mantle lithosphere no delamination delamination delamination and full detachment for a fixed viscosity of the upper crust of 1023 pa s delamination occurs for a range of viscosities of 1023 pa s to 1020 pa s for the lower crust depending on the thickness of the lithospheric root but full detachment of the delaminated lithospheric slab only occurs if the viscosity of the lower crust is greater than approximately 1021 pa s hot asthenosphere is welling up into the gap opening during delamination further implications of our models are lithospheric roots or unsupported slabs of at least 100 170 km depth extent are needed to provide sufficient negative buoyancy to allow delamination and detachment in all delamination models significant amounts of lower crustal material are subducted into the mantle in applying our models to selected orogens we argue that mantle delamination may have played an important role in the variscan and the himalayan orogenies

near surface diagnostics of dripping or delaminating lithosphere 1 in various geological regions it has been postulated that the mantle lithosphere has been thinned or completely removed two of the primary removal mechanisms that have been put forward include 1 delamination a wholesale peeling away of a coherent block of the mantle lithosphere and 2 lithospheric dripping viscous rayleigh taylor instability of the mantle lithosphere using computational models we investigate several near surface observables to determine if these may be diagnostic of either often ambiguous removal mechanism surface topography associated with delamination has a broad region of uplift above the lithospheric gap and a localized and mobile zone of subsidence at the delaminating hinge with dripping lithosphere the topographic expression is symmetric and fixed above the downwelling delamination of mantle lithosphere is more efficient than dripping for thermal heating of the crust the onset is more rapid and the elevated temperatures persist longer the resultant crustal p t t paths show modest pressure variations and high temperature increases with large scale delamination or dripping delamination also causes contraction directly above the migrating hinge and distal extension dripping lithosphere induces superimposed contraction and extension above and symmetric about the viscous instability for all the observables if only a portion of the mantle lithosphere is removed by viscous instability delamination inherently removes all of the mantle lithosphere the differences between the two removal mechanisms are even more pronounced with only partial removal of the mantle lithosphere uppermost mantle lithosphere remains well coupled to the crust leading to lower surface temperature variations and broad zones of crustal deformation thickening

mantle dynamics uplift of the tibetan plateau and the indian monsoon convective removal of lower lithosphere beneath the tibetan plateau can account for a rapid increase in the mean elevation of the tibetan plateau of 1000 m or more in a few million years such uplift seems to be required by abrupt tectonic and environmental changes in asia and the indian ocean in late cenozoic time the composition of basaltic volcanism in northern tibet which apparently began at about 13 ma implies melting of lithosphere not asthenosphere the most plausible mechanism for rapid heat transfer to the midlithosphere is by convective removal of deeper lithosphere and its replacement by hotter asthenosphere the initiation of normal faulting in tibet at about 8 3 ma suggests that the plateau underwent an appreciable increase in elevation at that time an increase due solely to the isostatic response to crustal thickening caused by india s penetration into eurasia should have been slow and could not have triggered normal faulting another process such as removal of relatively cold dense lower lithosphere must have caused a supplemental uplift of the surface folding and faulting of the indo australian plate south of india the most prominent oceanic intraplate deformation on earth began between about 7 5 and 8 ma and indicates an increased north south compressional stress within the indo australian plate a tibetan uplift of only 1000 m if the result of removal of lower lithosphere should have increased the compressional stress that the plateau applies to india and that resists india s northward movement from an amount too small to fold oceanic lithosphere to one sufficient to do so the climate of the equatorial indian ocean and southern asia changed at about 6 9 ma monsoonal winds apparently strengthened northern pakistan became more arid but weathering of rock in the eastern himalaya apparently increased because of its high altitude and lateral extent the tibetan plateau provides a heat source at midlatitudes that should oppose classical symmetric hadley circulation between the equator and temperate latitudes and that should help to drive an essentially opposite circulation characteristic of summer monsoons for the simple case of axisymmetric heating no dependence on longitude of an atmosphere without dissipation theoretical analyses by hou lindzen and plumb show that an axisymmetric heat source displaced from the equator can drive a much stronger meridianal monsoonlike circulation than such a source centered on the equator but only if heating exceeds a threshold whose level increases with the latitude of the heat source because heating of the atmosphere over tibet should increase monotonically with elevation of the plateau a modest uplift 1000 2500 m of tibet already of substantial extent and height might have been sufficient to exceed a threshold necessary for a strong monsoon the virtual simultaneity of these phenomena suggests that uplift was rapid approximately 1000 m to 2500 m in a few million years moreover nearly simultaneously with the late miocene strengthening of the monsoon the calcite compensation depth in the oceans dropped plants using the relatively efficient c4 pathway for photosynthesis evolved rapidly and atmospheric co2 seems to have decreased suggesting causal relationships and positive feedbacks among these phenomena both a supplemental uplift of the himalaya the southern edge of tibet and a strengthened monsoon may have accelerated erosion and weathering of silicate rock in the himalaya that in turn enhanced extraction of co2 from the atmosphere thus these correlations offer some support for links between plateau uplift a downdrawing of co2 from the atmosphere and global climate change as proposed by raymo ruddiman and froehlich mantle dynamics beneath mountain belts not only may profoundly affect tectonic processes near and far from the belts but might also play an important role in altering regional and global climates

coupled crust mantle dynamics and intraplate tectonics two dimensional numerical and three dimensional analogue modeling 1 tectonic deformation of some intraplate regions may be caused by the rayleigh taylor rt instability of dense subcrustal lithosphere mantle lithosphere as it descends into the mantle we report on a series of scaled three dimensional 3 d analogue and 2 d numerical experiments of coupled crust mantle dynamics that study such a process the models investigate the effects of two geometries of rt instability linear versus axisymmetric and use different rheological stratifications of the crust the growth of the rt instability is strongly influenced by crustal rheology while rt growth rates are not greatly influenced by the geometry of the mantle lithosphere instability itself the surface is experiments having a viscous only crust are characterized by significant crustal contraction and thickening above the mantle downwelling and extension and thinning in adjacent regions in experiments with a brittle upper crust the surface deformation is much more subdued however strong mantle flow induced deformation still occurs in the ductile lower crust beneath the quiescent brittle upper crust when the lower crust is relatively strong localized styles of extensional and contractional structure develop in the brittle upper crust owing to the greater degree of coupling between upper crust and the mantle instability in all the experiments a portion of the ductile lower crustal material is entrained within the downgoing rt instability deep into the mantle the interplay between dynamic topography and crustal thickening thinning induced by the underlying mantle flow governs various time dependent phases of subsidence and uplift of the model surface the experiments help to account for a number of first order tectonic behaviors of the lithospheres of earth and other terrestrial planets for example intraplate orogens and basins complex localized tectonic structure in intraplate settings and deep crustal seismic fabrics may arise as the variable crustal response to underlying mantle lithosphere rt instabilities

thermo mechanical modeling of subduction of continental lithosphere abstract we consider two dimensional thermo chemical mantle convection models to investigate the deformation of the continental lithosphere that follows the oceanic lithosphere into the subduction zone the models account for the compositional buoyancy forces by considering lithospheric plates with distinct crustal layers continental convergence results in crust mantle detachment in the subducting plate and crustal thickening in both subducting and overriding plates the depth of detachment ranges from 90 to 200 km depending on the strength of the lithosphere and the density of the continental crust as the crust thickens the convergence velocity in the collision zone decreases and the locus of subduction gradually shifts toward the interior of the subducting plate in models with greater viscosity the subducting mantle lithosphere maintains its integrity and does not break up in models with weaker rheology the positive buoyancy of the thickened crust can overcome the strength of the subducting lithosphere and causes the oceanic slab to break off and sink into the mantle the breakoff occurs over a time interval of 10 20 million years which is roughly the time needed for the crust to reach its maximum thickness crustal detachment takes place over a wide range of lithospheric strength suggesting that crustal buoyancy has an important role in the dynamics of continental subduction

rheology and strength of the lithosphere abstract mechanical properties of lithosphere are of primary importance for interpretation of deformation at all spatial and time scales from local scale to large scale geodynamics and from seismic time scale to billions of years depending on loading conditions and time scale lithosphere exhibits elastic brittle plastic or viscous ductile properties as can be inferred from rock mechanics data a large part of the long term lithospheric strength is supported in the ductile or ductile elastic regime while it also maintains important brittle strength yet at short seismic time scale s the entire lithosphere responds in elastic brittle elastic regime even though rock mechanics experiments provide important insights into the rheological properties of the lithosphere their conditions e g time scales strain rates temperature and loading conditions are too far from those of real earth therefore these data cannot be reliably extended to geological time and spatial scales strain rates 10 17 to 10 13 s 1 without additional parameterization or validation based on geological time scale observations of large scale deformation for the oceanic lithosphere the goetze and evan s brittle elastic ductile yield strength envelopes yses were validated by geodynamic scale observations such as the observations of plate flexure however oceanic lithosphere behavior in subduction zones and passive continental margins is strongly conditioned by the properties of the continental counterpart whose rheology is less well understood for continents and continental margins the uncertainties of available data sources are greater due to the complex structure and history of continental plates for example in a common continental rheology model dubbed jelly sandwich the strength mainly resides in crust and mantle while in some alternative models the mantle is weak and the strength is limited to the upper crust we address the problems related to lithosphere rheology and mechanics by first reviewing the rock mechanics data te flexure and ts earthquake data and long term observations such as folding and subsidence data and then by examining the physical plausibility of various rheological models for the latter we review the results of thermo mechanical numerical experiments aimed at testing the possible tectonic implications of different rheology models in particular it appears that irrespective of the actual crustal strength the models implying weak mantle are unable to explain either the persistence of mountain ranges for long periods of time or the integrity of the subducting slabs although there is certainly no single rheology model for continents the jelly sandwich is a useful first order model with which to parameterize the long term strength of the lithosphere it is concluded that dry olivine rheology laws seem to represent well the long term behavior of mantle lithosphere in oceans margins and continents as to the continent and margin crust rheology analysis of the results of thermo mechanical models and of te data based on the most robust variants of flexural models suggests that continental plates with te 30 50 smaller than their theoretical mechanical thickness hm i e te 20 60 km should be characterized by a weak lower or intermediate crustal rheology enabling mechanical decoupling between crust and mantle older plates such as cratons are strong due to crust mantle coupling and specific properties of the cratonic mantle lithosphere

numerical simulation of the stratus to cumulus transition in the subtropical marine boundary layer part i boundary layer structure abstract a stratus to cumulus transition sct that resembles observations occurred in lagrangian numerical simulations of the subtropical marine boundary layer over the northeastern pacific ocean southwest of california the lagrangian approach involves translating the domain along the climatological boundary layer trajectory at a rate equal to the observed surface wind speed the sst is increased at a corresponding rate the simulations did not include drizzle the diurnal cycle divergence changes or mesoscale circulations and thus demonstrate that these processes are not essential for an sct a 2d numerical cloud model that can explicitly represent large convective eddies is used turbulence at scales smaller than the large eddies is parameterized using a third moment turbulence closure this type of model requires no cloud regime specific input and is computationally economical for multiday simulations the results suggest that there are four stages in the transition from the stratus topped boundary

mantle downwelling and crustal convergence a model for ishtar terra venus the ishtar terra region contains the highest topography known on venus over 10 km above mean planetary radius as well as abundant tectonic features many of apparently compressional origin these characteristics suggest that ishtar is a crustal convergence zone overlying downwelling mantle in order to explore quantitatively the implications of this hypothesis for ishtar s origin we present models of viscous crustal flow driven by gradients in topography assuming a free slip surface boundary condition we find that if the crustal convergence hypothesis is correct then the crustal thickness in the plains surrounding ishtar can be no more than about 25 km thick this upper bound assumes a cold 10 k km 1 geotherm and the stiffest available diabase flow law if the geothermal gradient is larger or the rheology is weaker the crust must be even thinner for net crustal convergence to be possible this upper bound is in good agreement with several independent estimates of crustal thickness of 15 30 km in the plains of venus based on modeling of the spacing of tectonic features and of impact crater relaxation if the surface layer of venus provides a no slip boundary then our models allow the crustal thickness in the plains to be up to 50 km but the likely existence of faults that cut through the crust makes a no slip surface layer unlikely our upper bound on crustal thickness is much less than that derived from an airy isostasy model of ishtar s gravity anomaly much of the observed gravity anomaly must be due to density anomalies in the mantle beneath ishtar although we treat ishtar as a crustal convergence zone our crustal flow model shows that under some circumstances near surface material may actually flow away from ishtar providing a possible explanation for grabenlike structures in fortuna tessera

three dimensional crust and upper mantle structure of the eurasian continent group velocities of fundamental mode rayleigh waves of 109 wave paths within eurasia have been measured by an improved frequency time analysis technique stochastic inversion theory is applied to the mixed path measurements to extract pure path group velocities of 10 10 grid elements that form the subdivisions of eurasia then a three dimensional crust and upper mantle structure of the eurasian continent to a depth of 300 km is constructed the average crustal thickness of eurasia is found to be about 40 km abnormally thick crust from 50 to 65 km is obtained in areas of tien shan the altai mountains afghanistan pamir tibet and burma among them tibet has the least dense crust and highest elevation this implies that at least part of the elevation of tibet is due to isostatic compensation the upper mantle seismic velocity of the tectonically active regions of asia is considerably lower than that of the european and siberian cratons if high velocity implies high rigidity this explains that large scale tectonic deformations of eurasia occur in regions with weak upper mantle that deforms more plastically regions with stronger upper mantle remain relatively undeformed this is especially evident for the tertiary tectonic deformation as a consequence of the collision of india with eurasia

structure and evolution of the himalaya tibet orogenic belt the 1981 french chinese expedition to tibet focused on the lhasa block extending earlier coverage 400 km north of the tsangpo suture the lhasa block stood between 10 and 15 n latitude over most of the upper cretaceous and eocene and if gondwanian in origin had detached from gondwana by early permian seismic profiles reveal a complex moho topography resulting both from multiple continental thrusting and large scale strike slip faulting subduction related granitoids representing mixtures of mantle and crustal components and anatectic granitoids have been analysed and dated this study emphasizes the role of smaller blocks in the accretion of the continental mosaic

convective removal of thermal boundary layer of thickened continental lithosphere a brief summary of causes and consequences with special reference to the cenozoic tectonics of the tibetan plateau and surrounding regions england p 1993 convective removal of thermal boundary layer of thickened continental lithosphere a brief summary of causes and consequences with special reference to the cenozoic tectonics of the tibetan plateau and surrounding regions in m j r wortel u hansen and r sabadini editors relationships between mantle processes and geological processes at or near the earth s surface tectonophysics 223 67 73 the tectonics of tibet and its surroundings underwent marked changes at about 8 ma normal faulting of the plateau appears to have initiated at about this time widespread highly potassic volcanism interpreted as the result of rapid heating of the mechanical boundary layer began about 12 8 ma such rapid heating could be the consequence of convective removal of the lower lithosphere and its replacement by asthenosphere this process would have raised the surface height of the plateau and could have provided the driving force for extension the diffuse deformation in the indian ocean probably began at about 8 ma analysis of the buckling of a plastic lithosphere implies that the horizontal force per unit length required to drive this deformation is consistent with uplift of the tibetan plateau by 1 3 km the indian monsoon intensified at about 8 ma this intensification is consistent with the rapid passage of the surface height of the plateau across a threshold required to disrupt the pre existing atmospheric circulation of the region taken together these observations imply that the surface height of the tibetan plateau rose by between 1 and 3 km in a few million years around 8 ma such a rapid increase in surface height affecting an area the size of the tibetan plateau could not be achieved by crustal thickness changes but could have been caused by the convective removal of a substantial thickness of lower continental lithosphere of the region

on the coupling of wind stress and sea surface temperature a simple quasi equilibrium analytical model is used to explore hypotheses related to observed spatial correlations between sea surface temperatures and wind stress on horizontal scales of 50 500 km it is argued that a plausible contributor to the observed correlations is the approximate linear relationship between the surface wind stress and stress boundary layer depth under conditions in which the stress boundary layer has come into approximate equilibrium with steady free atmospheric forcing warmer sea surface temperature is associated with deeper boundary layers and stronger wind stress while colder temperature is associated with shallower boundary layers and weaker wind stress two interpretations of a previous hypothesis involving the downward mixing of horizontal momentum are discussed and it is argued that neither is appropriate for the warm to cold transition or quasi equilibrium conditions while one may be appropriate for the cold to warm transition solutions of a turbulent large eddy simulation numerical model illustrate some of the processes represented in the analytical model a dimensionless ratio a is introduced to measure the relative influence of lateral momentum advection and local surface stress on the boundary layer wind profile it is argued that when a 1 and under conditions in which the thermodynamically induced lateral pressure gradients are small the boundary layer depth effect will dominate lateral advection and control the surface stress

on the existence of a second scale of convection in the upper mantle summary this paper is concerned with an examination of the possibility that there might exist a small scale of convective circulation beneath the oceanic lithosphere recent suggestions that this might be the case have been made in an effort to understand why the bathymetry of the sea floor deviates from the prediction of boundary layer theory for ages in excess of about 100 ma the energy source which sustains the secondary motion is supposed to be found in the steep temperature gradient near the planetary surface which is itself presumably maintained by the large scale convective circulation associated with plate creation and destruction here we investigate the extent to which the temperature dependence of viscosity may act so as to stabilize the upper boundary layer against disruption by such secondary instability if the viscosity profile is monotonie and the asymptotic upper mantle viscosity is about 1022poise as suggested by post glacial rebound data then the existence of the second scale is extremely unlikely on the other hand if a sufficiently pronounced low viscosity zone does exist under old sea floor then the development of such a second scale cannot be ruled out completely some recently obtained geophysical evidence is reviewed which suggests that this is unlikely to be the case

upper mantle viscosity derived from the difference in rebound of the provo and bonneville shorelines lake bonneville basin utah twenty four new field measurements of elevation of provo level and bonneville level shoreline terraces provide data for reanalysis of isostatic rebound in the lake bonneville basin analysis of the differential rebound between the provo shoreline maximum rebound of 43 m and the bonneville shoreline maximum rebound of 69 m requires that the latter be an equilibrium shoreline within the possible measurement errors the provo shoreline formed after the elastic lithosphere had attained at least 91 equilibration to the removal of the water load between the bonneville and provo shorelines within the errors the provo may also represent an equilibrium shoreline from the new data presented in this paper the best estimate of the upper limit of effective viscosity of the uppermost mantle assuming a half space model and a 2000 year time interval between the bonneville and provo shorelines is 2 1019 n s m 2 2 1020 p in addition comparison of shoreline rebound profiles for both shorelines with theoretical plate flexure models indicates that the mean flexural rigidity of the basin and range lithosphere in this region is 1 1023 n m or slightly less

constraints on the seismic wave velocity structure beneath the tibetan plateau and their tectonic implications we combine observations of group and phase velocity dispersion of rayleigh waves of the waveform of a long period pl phase of pn and sn velocities from unreversed refraction profiles using earthquakes and of teleseismic s p travel time residuals to place bounds on the seismic wave velocity structure of the crust and upper mantle under tibet from surface wave measurements alone the tibetan crustal thickness can be from 55 to 85 km with corresponding uppermost mantle shear wave velocities of about 4 4 to 4 9 km s respectively the pn and sn velocities were determined to be 8 12 0 06 and 4 8 0 1 km s respectively using travel time data at lhasa from earthquakes in and on the margins of tibet combining these results the crustal thickness is most likely to be between 65 80 km with an average shear wave velocity in the upper crust less than 3 5 km s a synthesis of one pl waveform does not provide an additional constraint on the velocity structure but is compatible with the range of models given above in contrast to observations obtained for eight earthquakes in the himalaya measurements of both teleseismic s and p wave arrival times for nine earthquakes within tibet show unusually large intervals between p and s compared with the jeffreys bullen tables thus the pn and sn velocities apparently do not reflect high velocities in the mantle to a great depth beneath tibet from the dependence of the seismic velocities of olivine on pressure and temperature and from the similarity of the measured pn and sn velocities beneath tibet and beneath shields and platforms the velocities at the moho beneath tibet are compatible with the temperature being 250 300 higher than beneath shields and platforms i e 750 c if the temperature beneath the platforms is close to 500 c such a temperature could reach or exceed the solidus of the lower crust simple one dimensional heat conduction calculations suggest that the volcanic activity could be explained by the recovery of the geotherm maintained by a mantle heat flux of about 0 9 hfu at the base of the crust if the distribution of radioactive heat production elements were not concentrated at the top of the crust radioactive heating could also contribute significantly to the recovery of the geotherm and thus lower the required mantle heat flow thus the idea of a thickened crust in response to horizontal shortening is compatible both with these data and with these calculations appendices are available with entire article on microfiche order from the american geophysical union 2000 florida avenue n w washington d c 20009 document j81 004 1 00 payment must accompany order

heat focussing granite genesis and inverted metamorphic gradients in continental collision zones abstract the thermal conductivity of sedimentary rocks is usually much lower than that of crystalline rocks gneisses and granites continental collision zones are characterized by large scale thrusts which superpose crystalline basement and sedimentary layers this leads to heat refraction effects of significant magnitude sedimentary layers act as a barrier to the transport of heat we show that the peculiar geometry of a gently sloping sedimentary layer thrust into more conductive rocks is very efficient in diverting the vertical flux of heat temperatures can vary by more than 50 along the horizontal and reach a marked maximum at the top of crystalline layers these results are in general agreement with the main features of orogenic belts as exemplified by the himalayas they explain why the young leucogranites of the main central thrust mct are confined to a narrow belt at the top of the basement slab they also account for the inverted metamorphic gradient observed there in a crustal accretionary wedge folding and thrusting modify the structure of the upper crust by thickening the sedimentary cover and by introducing sedimentary rocks at depth this leads to high temperatures at shallow depths in specific places les zones de collision continentale se caracterisent par de grands chevauchementsafaible pendage qui empilent les unes sur les autres des couches de socle cristallin et des couches sedimentaires cette structure particuliere engendre d importants effets de refraction thermique en effet leur conductibilitethermiqueetant normalement bien plus faible que celle des roches cristallines les formations sedimentaires font office de barrieres pour le transfert thermique le flux de chaleur se trouve fortement devie dans un plan horizontal la temperature peut varier de plus de 50 les maximaetant situesal interface superieur des dalles cristallines en ces endroits precis les isothermes profonds remontent tres significativement ces resultats rendent bien compte de plusieurs faits essentiels observes dans l himalaya zoneographie inverse du metamorphisme localisation des leucogranites le long d une bandeetroite situee au sommet de la dalle du tibet

the onset of extension during lithospheric shortening a two dimensional thermomechanical model for lithospheric unrooting summary we model the evolution of the lithosphere during its shortening and consequent gravitational collapse with special emphasis on the induced variations in the surface stress regime and dynamic topography in particular we analyse the conditions leading immediately after lithospheric failure to local extension eventually coeval with compression diverent crustal rheologies and kinematic conditions as well as thermally imposed mechanical rupture are considered numerical calculations have been performed by using a 2 d finite element code that couples the thermal and mechanical equations for a newtonian rheology with a temperature dependent viscosity the results show that after the failure of a gravitationally unstable lithospheric root the replacement of lithospheric mantle by warmer asthenospheric material induces a considerable variation in the dynamic topography and in the surface stress regime the occurrence of local extension its intensity and its spatial distribution depend mainly on whether convergence continues throughout the process or ceases after or before the lithospheric failure similarly uplift subsidence and topographic inversion are controlled by kinematic conditions and crustal rheology mechanical rupture produces drastic changes in the surface stress regime and dynamic topography but only for a short time period after which the system tends to evolve like a continuous model

a thermal model for the distribution in space and time of the himalayan granites abstract in southern tibet crustal thickening due to the india asia collision has led to the formation of two granite belts one is located at the southern edge of the accretionary wedge of tethyan sedimentary rocks close to the contact with basement gneisses of the tibetan slab the other is found within the wedge itself close to the kangmar thrust trace available ages suggest that the granites appeared first in the southern belt and then in the kangmar belt this sequence seems to violate the chronology of thrusting another feature of the himalayas is that melting started only about 20 ma after the onset of thickening which is much less than the thermal time constant of thick crust we give a thermal model based on the assumption of conductive heat transfer which explains these features the model relies on the geometry of a sedimentary accretionary wedge bounded by low angle thrust faults and on the existence of a thermal conductivity contrast between old basement and young sedimentary rocks the wedge of sedimentary rocks acts as an insulating cap and its southern edge heats up along the contact with basement rocks on a horizontal cross section there is a temperature maximum along this southern edge which explains why melting starts there the early thermal evolution is sensitive to local conditions and granites first appear in the vicinity of the most radiogenic parts of the basement the distribution of granites in space and time is seemingly random reflecting different melting events in different radiogenic environments in the heterogeneous basement this model predicts a relationship between radioactivity and age which is compatible with available data the results emphasize that there are large horizontal temperature variations across a thickened region and that granite ages are not related simply to the timing of tectonic phases

near surface thermal chemical boundary layer convection at infinite prandtl number two dimensional numerical experiments chemical differentiation and convective removal of internal heat make the earth s lithosphere a thermal and a chemical boundary layer thin layers of chemically light material form near the earth s surface and become embedded within the cold thermal boundary layer associated with interior heat removal the likelihood of near surface thermal and chemical boundary layer interactions influencing the earth s thermotectonic evolution prompts the models presented herein a simplified system consisting of a chemically light layer within the upper thermal boundary layer of a denser thermally convecting layer is explored through a suite of numerical experiments to see how its dynamic behaviour differs from similar well studied thermal boundary layer systems a major cause of differences between the two systems resides in the ability of the deformable near surface chemical layer to alter the effective upper thermal boundary condition imposed on the convectively unstable layer below in thermal equilibrium regions of chemical boundary layer accumulation locally enforce an effectively near constant heat flux condition on the thermally convecting layer due to the finite thermal conductivity of chemical boundary layer material for cases in which chemical accumulations translate laterally above the unstable layer the thermal coupling condition between chemical boundary layer material and the unstable layer below is one of non equilibrium type i e the thermal condition at the top of the convectively unstable layer is time as well as space variable a second major cause of differences is that for the thermal chemical system chemically induced rheologic variations can offset or compete with those due to temperature more specifically the presence of chemically weak material can lubricate convective downwellings allowing for enhanced overturn of an on average strong upper thermal boundary layer both of these factors have low order effects on internal flow structure and heat loss and lead to dynamic behaviour in which chemical boundary layer deformation is not only driven by flow in the thermally convecting interior layer but also feeds back and alters this flow some implications of this in regard to elucidating how near surface chemical boundary layer deformation e g continental tectonics might interact with and influence mantle convection are discussed

olivine grain growth in partially molten fe ni s a proxy for the genesis of pallasite meteorites abstract the origin of pallasites has been the focus of a number of recent studies yet their formation process remains elusive while the mechanism leading to the genesis of the sub group termed mixed type pallasites containing polygonal rounded and fragmental olivines simultaneously is unclear here we test the hypothesis of mixing of olivine fragments with fe ni s after a non destructive impact followed by annealing employing both experimental analogues and numerical models the experimental series evidenced that the addition of sulfur to olivine fe ni accelerates olivine grain growth though the growth rate is reduced when fe ni s is not fully molten this is shown to be the consequence of competing growth of olivine and fe ni grains numerical models satisfying available formation constraints from natural samples indicate that planetesimals with radii 200 km are favorable for the genesis of rounded olivine bearing pallasites by annealing of fragments in partially molten fe ni s moreover early mixing in the planetesimal can form regions containing olivine grains with different grain sizes that could explain the formation of mixed type pallasites

magmagenesis associated with extension in orogenic belts examples from the himalaya and tibet abstract structures in mountain belts which accommodate layer extension may be synchronous with either ongoing thickening or reduction in crustal thickness either case may be associated with magmagenesis because they contribute to the thermal regime in the lithosphere and the chemistry of magmas produced can reveal source compositions and thereby conditions of melting in the high himalaya exhumation of upper amphibolite facies metamorphic rocks in the footwall of a major normal shear zone occurred during crustal thickening this rapid decompression enhanced fluid absent melting reactions in micaceous pelites and produced the high himalayan leucogranite suites which were subsequently intruded into the zone of normal shear similar leucogranites are seen in the cretaceous north lhasa terrane nlt in association with a calc alkaline series these suites are also crustally derived but their chemistry indicates higher melting temperatures than in the himalaya suggesting that heat flow into the base of the crust was enhanced this thermal anomaly was most probably associated with lithosphere attenuation following an episode of thickening the tibetan plateau is currently a topographically elevated region of thick crust that appears to be undergoing orogenic collapse active structures are stretching and thinning the crust the plateau is underlain by a thin hot mantle lithosphere supporting theoretical models that suggest that a lithospheric root of a thick orogen would become unstable and be removed by convection consequent thermal perturbations in the mantle lithosphere cause melting in suitably fertile horizons leading to widespread volcanism comparisons have been drawn between the tibetan model and the late stages of the variscan belt in europe magmatic rocks of diverse composition occur throughout the late variscan many of them indicating derivation from a previously enriched lithospheric source suggesting thermal anomalies in the subcontinental mantle akin to those inferred for tibet taken with structural and sedimentary evidence these indications strongly support the hypothesis that the variscan orogen underwent orogenic collapse in the tibetan style

slab breakoff a model of lithosphere detachment and its test in the magmatism and deformation of collisional orogens we present a model proposing that oceanic lithosphere detaches from continental lithosphere during continental collision slab breakoff allowing an explanation of syn to post collisional magmatism and metamorphism continental collisions are preceded by subduction of dense oceanic lithosphere and followed by attempted subduction of buoyant continental lithosphere this situation of opposing buoyancy forces leads to extensional deformation in the subducted slab a narrow rifting mode of deformation will result if there is strain localization slab breakoff results we have assessed the plausiblity of this process by quantitatively evaluating an upper bound for the strength of the lithosphere and have compared it with the change in buoyancy force during continental sbduction whether breakoff will occur and the depth at which it will occur is a strong function of temperature and hence the subduction velocity for a subduction velocity of 1 cm yr breakoff could occur at depths of between 50 and 120 km while at higher velocities it is still likely to occur but at deeper depths as a result of the rifting during breakoff the asthenosphere upwells into the narrow rift and following breakoff it impinges on the mechanical lithosphere of the overriding plate the resulting conducted thermal perturbation leads to melting of the metasomatised overriding mantle lithosphere producing basaltic magmatism that leads to granitic magmatism in the crust dry asthenospheric mantle will melt only if breakoff occurs at a depth shallower than ca 50 km breakoff removes the force at the downdip side of the continental crust while the enhanced heating leads to a reduction of the strength of the underlying crust both effects facilitate the freeing of buoyant crustal sheets which can then rise towards the surface leading to the rapid exhumation of eclogite facies continental crust the cessation of subduction and replacement of the cold oceanic lithosphere by asthenosphere leads to rapid uplift of the orogen we have tested the variety of predicted expressions of slab breakoff in the geological record of the alps the aegean islands and the dabie shan a comparison of the various belts highlights 1 that the magmatism and metamorphism are found near the suture and the centre of the orogen demonstrating the general steepening of suture during collision 2 that a vague inverse correlation exists between maximum depth of metamorphism and volume of syn orogenic magmatism and 3 that the melts can be emplaced in both compressive and extensional environments we suggest that slab breakoff is an important step in the evolution of many orogenic belts and it allows an explanation for the combined presence in collisional orogens of magmatism with a mantle parentage and ultra high pressure metamorphics

numerical simulations and a conceptual model of the stratocumulus to trade cumulus transition abstract a two dimensional eddy resolving model is used to study the transition from the stratocumulus topped boundary layer to the trade cumulus boundary layer the 10 day simulations use an idealized lagrangian trajectory representative of summertime climatological conditions in the subtropical northeastern pacific the sea surface temperature is increased steadily at 1 5 k day 1 reflecting the southwestward advection of the subtropical marine boundary layer by the trade winds while the free tropospheric temperature remains unchanged results from simulations with both a fixed diurnally averaged shortwave radiative forcing and a diurnally varying shortwave forcing are presented a two stage model for the boundary layer evolution consistent with these simulations is proposed in the first stage decoupling is induced by increased latent heat fluxes in the deepening boundary layer after decoupling cloud cover remains high but the cloudiness regime changes from a single stratocumulus layer to sporadic

brittle and ductile deformation in a zone of rapid uplift central southern alps new zealand schists exposed in the central southern alps new zealand 1 5 km east of the alpine fault zone have been rapidly uplifted during the late cenozoic a regionally consistent sequence of mesoscopic structures discordant to the schistosity is recognized extending backward in time from recent brittle displacements of deglaciated surfaces through structures exhibiting brittle ductile transitional behaviour to ductile deformational features kinematic analysis of these structures gives consistent principal subhorizontal shortening directions similar to the present day principal horizontal shortening direction indicating their relation to late cenozoic uplift of the southern alps analysis of deformed veins suggests a minimum shortening strain of about 50 perpendicular to the foliation the veins crosscut upright mesoscopic to macroscopic folds which commonly develop high strain zones on their limbs within such zones a strong stretching lineation plunges gently sw approximately perpendicular to the stretching lineation in the mylonites along the alpine fault the folds and high strain zones do not appear to be related to late cenozoic uplift but may have originated during an earlier phase of dominantly strike slip motion fluid inclusion studies give depth temperature estimates of around 3 km and 285 c for the development of the brittle structures and 6 8 km at 310 350 c for the brittle ductile transition the deformed veins show evidence of extensive fluid infiltration during the later phases data from fluid inclusions and metamorphic assemblages give an estimate of 15 20 km and 400 450 c for their deformation these data combined with uplift rates determined by other studies allow the construction of a depth temperature time path which indicates nearly isothermal decompression associated with rapid uplift a shallow brittle ductile transition 6 8 km is consistent with numerical modeling of thermal and mechanical behavior of the crust during rapid uplift associated with continental collision and with the presence of high heat flow in the area because of the small temperature change over a large amount of decompression pressure may be an important factor in controlling the depth of the brittle ductile transition the marked temperature drop above the brittle ductile transition is thought to be enhanced by the influence of convection as an effective cooling mechanism

mid triassic felsic igneous rocks from the southern lancangjiang zone sw china petrogenesis and implications for the evolution of paleo tethys abstract the southern lancangjiang magmatic belt is an important component of the triassic san jiang igneous zone in southwestern china and is mainly composed of the lincang lc batholith and a mesozoic volcanic belt that contains abundant rhyolites our new la icp ms zircon u pb results together with previous shrimp and la icp ms ages define two emplacement ages for the lincang batholith at 230 ma and 220 ma corresponding to the eruption times of volcanic rocks of the manghuai and xiaodingxi formations respectively most of the 230 ma lc granites possess high a cnk values 1 1 and display strongly peraluminous characteristics similar to s type granite whereas the manghuai formation mhf rhyolites show an affinity to a type granite especially their elevated feot feot mgo and ga al ratios both of the granitic and volcanic rocks have strongly fractionated ree patterns la yb n 2 66 33 2 and 13 1 19 7 respectively and have conspicuous negative eu anomalies eu eu 0 06 0 65 and 0 52 0 74 respectively with a similar depletion in hfse nb zr hf p ba and sr the sr nd isotopic data and t dm2 model ages suggest that the lc granitic magma had a dominantly crustal source whereas the mhf rhyolites were derived from mixing between middle upper crustal rocks similar to the source of the lc granitic magma with a small volume of mantle derived melt in combination with regional studies our new geochemical data and geochronological results show that the mid triassic magmatism was generated in a post collisional tectonic setting the spatial distribution pattern of the mesozoic igneous rocks along the lancangjiang zone favors a slab breakoff model which resulted in post collisional extension and asthenospheric upwelling that induced large scale partial melting of the middle lower crust to produce voluminous amounts of felsic magma therefore the occurrence of the mid triassic post collisional magmatism clearly indicates that the final continent continent arc collision between the gondwana derived sibumasu and indochina blocks of paleo tethys was completed by the early mid triassic

variscan shear zone deformation of late precambrian basement in sw iberia implications for circum atlantic pre mesozoic tectonics abstract polyphase deformation and metamorphism in the badajoz cordoba shear belt sw spain relate to a poly orogenic evolution involving two major tectonothermal episodes the accretion of mid and lower crust tectonic slices in an orogenic wedge complex during pre variscan times and the development of an intracontinental ductile shear zone during the late palaeozoic structural analysis of zones with different intensities of shear dominated variscan deformation allows a recognition of the poly deformed nature of most units of the belt identification of varying deformation mechanisms and a description of the geometry of early structures the implications for the regional tectonics of this area of the southern iberian massif are discussed in the light of the geodynamic evolution of the ibero armorican arc during the variscan orogeny and the reconstruction of the late precambrian cadomian avalonian panafrican orogen of the circum atlantic region

more thoughts on convergent crustal plateau formation and mantle dynamics with regard to tibet evidence suggests that some 10 m y ago a large portion of thickened mantle lithosphere below the tibetan plateau was rapidly removed removal appears to have been abrupt and delayed from the time of substantial thickening fluid dynamical models of convective thermal boundary layer instability have had difficulties explaining abrupt lithospheric removal long after initial thickening such models have implied that either removal should be synchronous with thickening or should not occur at all due to boundary layer stabilization resulting from high lithospheric viscosity we present simple thermal chemical boundary layer convection models that suggest a compromise between previous end member results the models allow for delayed instability of a thickened thermal boundary layer under geologically reasonable parameter conditions instability delay relies on high viscosity of thickened mantle lithosphere stabilizing it against free convective removal and on lateral extent of thickened crust exceeding that of mantle lithosphere the latter condition leads to a peripheral region of thickened crust and relatively unthickened mantle lithosphere and an interior plateau region of both thickened crust and mantle lithosphere if the thermal resistance and or the rate of internal heat production of crust is greater than that of mantle then mantle lithosphere at the peripheral region can be thermally eroded to the point that the base of the crust comes into contact with bulk mantle this creates a peripheral weak zone that allows thickened mantle lithosphere of the interior plateau region to become mechanically detached and sink into bulk mantle the instability is akin to delamination as originally defined and its delay time is roughly the thermal diffusion time across the thinnest portion of mantle lithosphere

evolution of a previously thickened thermochemical lithosphere application to venus magellan images have revealed areas with high topography on venus like maxwell montes interpreted as the result of a lithospheric thickening associated to a global compressional stress regime in addition to compressional structures maxwell montes displays younger extensional structures and magmatism at its summit we present a numerical model to explore the main factors that may explain the chronology of compressional and magmatic events observed on maxwell montes in particular this study aims to quantify the process of destabilization of a thickened lithosphere by taking into account thermal and chemical instabilities indeed the contribution of basaltic eclogitic rocks and depleted mantle rocks is not negligible in mantle dynamical models the initial situation in the upper mantle is represented in a two dimensional cartesian domain by a stationary state of thermal convection the thermal lithosphere is then instantaneously thickened and buoyant tracers of depleted mantle and basalt are placed in the lithospheric mantle and crust respectively this material distribution controls both the thermal evolution of the lithosphere and its recycling after thickening assuming a continuous and slow basalt eclogite transformation the lithosphere acts as an insulating layer during 100 myr before an effective recycling can occur for an instantaneous basalt eclogite transformation combination of basaltic crust and depleted upper mantle prevents recycling of the lithosphere during the first few millions years the recycling is all the more enhanced as the basalt transforms to eclogite because it is an ongoing process it is twice as fast 20 myr as the recycling of a purely thermal lithosphere 40 myr but the recycled rocks of a thermochemical lithosphere represent less than the recycled rocks of a thermal one because of the great stability of the depleted layer this results in a thermochemical lithosphere thicker by several tens of kilometers at an upper mantle scale this chemical diversity favors the mixing of chemical heterogeneities and the perturbation of internal dynamics this involves the impingement of a hot depleted diapir at the base of the lithospheric root for a sufficiently hot mantle a partial melting zone is trapped inside the lithospheric root and induces magmatism about 80 100 myr after the lithospheric recycling this could explain the late volcanic flows observed in maxwell montes detailed modeling of a thermochemical lithosphere also could explain the formation and evolution of maxwell montes as evidenced by volcanic and tectonic features

boundary layer development and gravity waves in conventionally neutral wind farms while neutral atmospheric boundary layers are rare over land they occur frequently over sea in these cases they are almost always of the conventionally neutral type in which the neutral boundary layer is capped by a strong inversion layer and a stably stratified atmosphere aloft in the current study we use large eddy simulations les to investigate the interaction between a large wind farm that has a fetch of 15 km and a conventionally neutral boundary layer cnbl in typical offshore conditions at the domain inlet we consider three different equilibrium cnbls with heights of approximately 300 m 500 m and 1000 m that are generated in a separate precursor les we find that the height of the inflow boundary layer has a significant impact on the wind farm flow development first of all above the farm an internal boundary layer develops that interacts downwind with the capping inversion for the two lowest cnbl cases secondly the upward displacement of the boundary layer by flow deceleration in the wind farm excites gravity waves in the inversion layer and the free atmosphere above for the lower cnbl cases these waves induce significant pressure gradients in the farm both favourable and unfavourable depending on location and case a detailed energy budget analysis in the turbine region shows that energy extracted by the wind turbines comes both from flow deceleration and from vertical turbulent entrainment though turbulent transport dominates near the end of the farm flow deceleration remains significant i e up to 35 of the turbulent flux for the lowest cnbl case in fact while the turbulent fluxes are fully developed after eight turbine rows the mean flow does not reach a stationary regime a further energy budget analysis over the rest of the cnbl reveals that all energy available at turbine level comes from upwind kinetic energy in the boundary layer in the lower cnbl cases the pressure field induced by gravity waves plays an important role in redistributing this energy throughout the farm overall in all cases entrainment at the capping inversion is negligible and also the work done by the mean background pressure gradient arising from the geostrophic balance in the free atmosphere is small

three dimensional numerical simulations of crustal deformation and subcontinental mantle convection abstract 3 d simulations of mantle convection allowing for continental crust are explored to study the effects of crustal thickening on lithosphere stability and of continents on large scale mantle flow simulations begin with a crustal layer within the upper thermal boundary layer of a mantle convection roll in a 1 1 1 cartesian domain convective stresses cause crust to thicken above a sheet like mantle downwelling for mild convective vigor an initial crustal thickness variation is required to induce 3 d lithospheric instability below the zone of crustal convergence the amplitude of the required variation decreases with increasing convective vigor morphologically instability is manifest in formation of drip like thermals that exist within the large scale roll associated with initial crustal thickening a strong surface signature of the drips is their ability to cause deviations from local airy compensation of topography after the initial thickening phase the crustal accumulation that forms serves as a model analog to a continent its presence leads to mantle flow patterns distinctly different from the steady state roll that results in its absence large lateral thermal gradients are generated at its edge allowing this region to be the initiation site for continued small scale thermal instabilities eventually these instabilities induce a restructuring of large scale mantle flow with the roll pattern being replaced by a square cell although preliminary and idealized the simulations do show the fluid dynamical plausibility behind the idea that significant mantle variations can be generated along the strike of a largely 2 d mountain chain by the formation of the chain itself the ability of a model continent to cause a change in fundamental convective planform also suggests that the effects of continental crust on mantle convection may be low order despite the seemingly trivial volume of crust relative to mantle

les of a spatially developing atmospheric boundary layer application of a fringe method for the stratocumulus to shallow cumulus cloud transition abstractan arrangement of a large eddy simulation les is described that facilitates a spatially developing thermally stratified atmospheric boundary layer abl when the inflow and outflow boundary conditions are specified the les of stably stratified abl turns out to be challenging because spurious reflections of waves at the boundary accumulate inside the domain to tackle this problem a fringe method with an auxiliary les running concurrently is applied to enforce upstream downstream boundary conditions an artificial forcing term is applied within a fringe region located at the beginning of the main les domain in order to ensure statistically stationary inflow boundary conditions the auxiliary les which is horizontally homogeneous in a doubly periodic domain is used to determine the inflow condition of the main les domain the present scheme is used to provide an eulerian perspective of the stratocumulus to shallow cumulus cloud sc cu transition one of the key cloud regimes over the subtrop

steady propagation of delamination events delamination of the lithospheric thermal boundary from overlying continental crust propagates laterally from the line of initiation accelerating as the sinking slab of detached lithosphere grows longer this propagation has been numerically modeled with steady state equations in a moving reference frame by matching an interior finite element solution to flexible boundary conditions which represent the mechanical and thermal response of the surroundings the form of the solution depends on the shear coupling of intruding asthenosphere to the top of the sinking slab across a thin layer of crustal material without coupling the tip of the intrusion cools and stiffens to form a wedge dividing the crust cold mode with coupling the intrusion is forced to convect and remains ductile hot mode the cold mode can propagate at all velocities the hot mode has a lower limiting velocity of 1 2 cm year but offers less resistance at higher speeds resistance to delamination includes a constant term from the buoyant crustal downwarp plus a velocity proportional term representing viscous deformation however the proportionality constant of the latter term is only weakly dependent on crust and lithosphere viscosities matching this resistance to loading lines of 100 to 800 km slabs sinking in a mantle of 1022 p velocities of 0 3 8 0 cm year are obtained changes in viscosity affect this rate but cold mode delamination is unstoppable except at continental margins or by failure in the sinking slab the surface expression of delamination is a leading outer rise followed by a submarine trough with a large negative free air anomaly which finally evolves into a 1 km plateau if crustal viscosity and velocity are both low however there is a montonic crustal uplift with no trough thus the present lack of linear supracontinental oceans does not preclude delamination at up to 4 cm year driven by slabs up to 400 km in length

deep and near surface consequences of root removal by asymmetric continental delamination abstract the occurrence of continental delamination has been proposed for a number of areas characterized by highly variable geodynamic settings in this study we present results of numerical simulations considering different initial setups representative for geodynamic scenarios where delamination could potentially develop to mimic a post collisional orogenic scenario we have designed an initial state characterized by the presence of an area of orogenic lithosphere with both crustal and lithospheric roots in a second setup we have considered a lithospheric root representative of a remnant slab with a flat overlying crust we focus on predicted evolution of surface and near surface observables namely the crustal structure surface heat flow and isostatic and dynamic topography evolution our results show that a high density orogenic lower crust likely related to the presence of eclogite significantly accelerates the sinking of the lithospheric mantle the pattern of local isostatic elevation is characterized by laterally migrating surface uplift subsidence this pattern is shown to be little sensitive to lower crust density variations in contrast predicted dynamic topography is more sensitive to these changes and shows surface subsidence adjacent to the delaminating lithospheric mantle for the model with a high density lower crust and surface uplift above the slab for a model with a less dense lower crust the reason for uplift in this second model is that the effect of the positive buoyancy of the thickened crust overwhelms the effect of negative buoyancy of the slowly sinking lithospheric mantle we infer from our modeling that there is not a specific characteristic pattern of topography changes associated with delamination but it depends on the interplay between highly variable factors as slab sinking velocity asthenospheric upwelling and changes in crustal thickness

activation of the nyainqentanghla shear zone implications for uplift of the southern tibetan plateau neogene extension of the tibetan plateau is manifested as a series of north south trending graben the most prominent of which is the yadong gulu rift the nyainqen tanghla shan a ne sw trending mountain range 100 km nw of lhasa bounds the western margin of the yangbajian graben the central segment of the yadong gulu rift the eastern edge of the nyainqentanghla massif is marked by a low angle 25 o detachment fault shear zone of amphibolite grade mylonites the noar 3 ar thermal history results from samples collected along two deeply incised valleys within the massif reveal that a rapid cooling event propagated from 8 ma in the core of the range to 4 ma within the high strain zone at the eastern boundary assuming that faulting initiated at high angle 60o thermal histories were fit to a numerical simulation of slip on a normal fault to yield estimates of both the age of fault initiation and the slip rate history the form of the isotopically derived thermal histories are similar to general form predicted by the thermal model and suggests that significant movement began at 8 1 ma in the southern valley goring la and proceeded at an average slip rate of 3 mm yr between 8 and 3 ma a more complex history is required to fit the data from the northern valley balum chun but the timing of initiation and average slip rate are similar to the goring la result numerical simulations in which the fault angle is varied indicate that the isotopically derived temperature histories are inconsistent with slip occurring at low angle 40o because the extension direction of the yangbajian graben is representative of most rifts on the southern tibetan plateau our data suggest that crustal thickness and elevation reached close to their present values by 8 1 ma a carbon isotopic shift in pedogenic carbonates from the siwalik formation at about 7 5 ma appears to reflect intensification of the asian monsoon and by inference that the plateau had attained an important threshold elevation by that time formation of a diffuse plate boundary in the indian oceanic lithosphere beginning at 7 5 8 0 ma is also consistent with this history we suggest that the plateau had attained a threshold area and elevation by 8 1 ma sufficient to trigger these three independent manifestations

a review of heterogeneous materials and their implications for relationships between kinematics and dynamics in continents 1 the variation of mechanical properties as a function of depth in the lithosphere controls the relationship between surface deformation and whole lithosphere deformation where mechanical competence elastic strength or viscous stiffness is a continuous function with depth surface deformation can be used to constrain either force balance or rheological parameters for the lithosphere however where competence is vertically discontinuous the surface deformation and deformation at depth may have a range of relationships including complete independence therefore both inversions and forward models designed to test either rheology or stress state are highly sensitive to the a priori choice of a mechanical model

geodynamic models of mature continental collision evolution of an orogen from lithospheric subduction to continental retreat delamination 1 the behavior of deep lithospheric processes in continental collision are frequently speculated on but poorly understood in this study we build on previous work to explore the styles of continental lithosphere deformation during mature collision where 1800 km of convergence has been accommodated by horizontal shortening we conducted a suite of numerical geodynamic experiments that test the sensitivity of mature continent collision to varying mantle lithosphere density mantle lithosphere yield stress lower crustal strength and to the presence of phase change related density changes in the lower crust i e eclogitization of mafic lower crust the models suggest that the early stages of collision are accommodated by subduction of lower crust and mantle lithosphere along a discrete shear zone beneath the overriding plate following this initial stage of subduction the subducting lower crust and mantle lithosphere can retreat from the collision zone permitting the sub lithospheric mantle to upwell and intrude the overriding plate as a result the lower crust and mantle lithosphere of the overriding plate delaminate from the overlying crust with local isostatic adjustment subduction and delamination driven crustal processes plateau like uplift occurs the sub crustal evolution also causes bands of syn convergent crustal extension to develop in models with a rheologically weaker lower crust surface crustal response to the deep lithosphere dynamics becomes more diffuse as an example the numerical experiments satisfy a number of surface observables of the himalayan tibetan orogen namely 1 the current general mantle lithosphere architecture as defined by seismic analyses 2 the long wavelength plateau uplift of the western tibetan plateau 3 the surface heat flow measurements in india the tethyan himalaya qiangtang and in qaidam basin and 4 the anomalous syn convergent extension in the southern portion of the orogen

a possible dependence of tectonic strength on the age of the crust in asia in asia the intensity and style of active tectonics appears to depend on the age of the last orogenic activity the shields have remained essentially undeformed during the india eurasian collision but paleozoic and mesozoic orogenic belts apparently have been reactivated by this collision with a suggestion of greater reactivation and more diffuse deformation of the younger belts if the greater observed heat flow in regions of more recent orogenic activity reflects at least in part a greater heat flow from the mantle beneath the younger belts then the temperatures in the mantle beneath the younger belts should be higher than beneath older belts and shields because of the strong dependence on temperature of the creep strength of minerals particularly of olivine the crust and mantle beneath the hotter younger belts should be much weaker than those beneath older belts this difference in temperature and consequent difference in strength may be the cause of the greater reactivation of younger belts

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

thermal effects of continental collisions thickening a variable viscosity lithosphere abstract based on data that indicate that the lithosphere and the crust are anomalously hot in some convergence zones simple calculations of the thermal effect of crustal thickening are presented the results indicate that post tectonic melting of the lower crust requires high levels of mantle heat flux it has been suggested that this high heat flux can be explained by the convective removal of thermal boundary layers thickened by convergence to test this hypothesis a series of numerical experiments were carried out on the removal of thickened thermal boundary layers whose viscosity depends on temperature and pressure it is found that the boundary layers can be removed quickly 10 m y in some cases however these layers are too thin to greatly alter the thickness of the lithosphere or the heat flux at the top of the mantle a set of calculations was done with internal heat sources which give equilibrium lithospheric thicknesses half that of the thickened lithospheres these indicate that the time scale for convective thinning by one half is extremely long at least 100 m y for initial lithospheric thicknesses of 100 km therefore the lithosphere must be anomalously hot and thin before deformation for extensive crustal melting to occur after thickening in observed times 40 m y for tibet finally it is suggested that subduction induced convection may raise the general mantle temperatures over a broad region behind a convergence zone this effect can inhibit the cooling of the lithosphere for more than 50 m y after the subduction induced convection has ceased

convective thinning of the lithosphere a mechanism for the initiation of continental rifting thinning of the lithosphere by heat carried convectively to its base is a possible mechanism for the initiation of continental rifting the lithosphere asthenosphere boundary marks the transition from a quasi rigid behavior of the mantle on geological time scales to a viscous one the boundary lies at or close to the solidus of the mantle material and its position is a function of temperature and pressure additional heat carried to the boundary will cause it to rise and the lithosphere to thin a one dimensional model is used to study the upward movement of the boundary from an initial equilibrium position as a consequence of an increase in the heat flux from the convecting mantle the ratio of the amount of thinning to the initial thickness is directly proportional to the increase in heat flux from the asthenosphere and inversely proportional to the difference between the final subcrustal geothermal gradient and the slope of the transition temperature profile the rate of thinning is determined by the initial and final equilibrium thicknesses the thermal diffusivity the transition temperature profile and the plume temperature profile it depends only slightly on any latent heat associated with the lithosphere asthenosphere transition during a large fraction of the time between the initial and final equilibrium states the lithosphere thins at a rate which is inversely proportional to the square root of time t even 300 km thick lithospheres may be thinned to 50 km or crustal thicknesses in tens of million years by the heat advected by mantle plumes this heat contributes to an enhanced surface heat flow because of the constant level of radioactive heat production in the crust the ratio of the final to the initial surface heat flow increases much less than the ratio of the final to the initial heat flow from the asthenosphere for large increases in asthenospheric heat flow the lithosphere is almost thinned to the crust before any significant change of surface heat flow occurs uplift due to thermal expansion upon lithospheric thinning is of the order of 102 103 m a rayleigh taylor instability acting on the thinning lithosphere may further increase thinning rates and cause magmatic intrusions of the lithosphere and surface volcanism

steady state large eddy simulations to study the stratocumulus to shallow cumulus cloud transition abstractthis study presents a series of steady state large eddy simulations less to study the stratocumulus to shallow cumulus cloud transition to represent the different stages of what can be interpreted as an eulerian view of the transition each simulation is assigned a unique sea surface temperature sst and run until statistically steady the les runs are identical in every other aspect these idealized boundary driven steady state less allow for a simple parametric assessment of cloud controlling factors in isolation from initial conditions and time lag effects inherent in the lagrangian view of the transition the analysis of the thermodynamic energy budget reveals that as the cloud regime transitions from stratocumulus to shallow cumulus changes in the cloud radiative cooling term are balanced by changes in the subsidence warming term this leads to a linear regression between the cloud fraction cf and an integral that scales to a first order approximation as the lower tropospheric stabi

intraplate tectonics feedback between radioactive thermal weakening and crustal deformation driven by mantle lithosphere instabilities abstract two dimensional viscous plastic numerical experiments show that weak radioactive crust may be susceptible to significant tectonic activity by the effect of rayleigh taylor type instability of the sub crustal lithosphere below a continental interior in particular crust having a localized or regional enrichment of radioactive elements can respond to an underlying mantle lithosphere downwelling by initial subsidence then thickening and uplift owing to the positive feedback among thickening heating and weakening such models may later undergo orogenic deflation as spontaneous crustal extension thinning and surface subsidence occur while mantle downwelling continues strong homogeneous crust subsides dynamically above mantle downwellings and undergoes essentially no internal deformation a uniform distribution of radioactive elements through the crust or a concentration at depth is much more effective in causing thermal weakening of the crust and subsequent intraplate deformation than with radioactive material concentrated nearer the surface for wide regions of high heat production in the crust the length scale of the intracrustal tectonic zone is controlled by the width of the mantle downwelling in contrast with narrow heat production regions which control their own length scale in general the numerical experiments demonstrate that the presence of radioactive elements may make the crust vulnerable to intraplate tectonic deformation and represent a primary control on the way the strength of the crust evolves during sub crustal forcing the results may explain first order features of intraplate tectonics including the subsidence mechanism for intracontinental basins orogenic crustal contraction thickening and surface uplift and subsequent extension and crustal deflation the processes have particular implications for regions of high radioactive heat production and juvenile crust which would more likely have a higher concentration of radioactive elements in the lower crust

from subduction to collision control of deep processes on the evolution of convergent plate boundary 1 using laboratory experiments we investigate the dynamics of the collisional process that follows the closure of an oceanic basin the evolution of these experiments systematically shows four successive episodes of deformation which correspond to 1 the initiation of oceanic subduction 2 a mature period of oceanic subduction 3 an episode of continental subduction during which the trench absorbs all the convergence and the superficial tectonic regime does not change significantly within the continental plates and 4 continental collision that starts when the trench locks and convergence is absorbed by a system of thrust faults and folds we observe that the amount of continental material that subducts before the onset of collision depends on the slab pull exerted by the subducted oceanic lithosphere the slab pull force in turn depends on the amount of subducted oceanic material on the thickness of the convective mantle and also on the rheology of the slab our experiments indeed suggest that parts of the oceanic slab may separate from the superficial slab to sink rapidly in the mantle decreasing the slab pull level and triggering the rapid onset of collision we observe two possible modes of slab deformation slab break off and development of viscous instabilities we define two dimensionless rheological numbers to characterize the possible occurrence of these modes of deformation in all cases oceanic closure is followed by episode of continental subduction during which the continental lithosphere may reach depths varying between 50 and 450 km prior to the onset of continental collision

subduction initiation and continental crust recycling the roles of rheology and eclogitization abstract a mechanical model with viscous ductile and plastic brittle rheologies is used to investigate the effect of eclogitization on the dynamics of convergence density increases by 300 600 km m3 during eclogitization of crustal rocks and continental lower crust and oceanic crust reach higher density than mantle we explore cases of intracontinental deformation subduction and continental collision we consider a wide range of parameters for friction activation energy and initial thermal state and cases with or without eclogitization the style of deformation appears to be primarily controlled by the presence or absence of weak zones simulations are run with a constant convergence velocity 1 5 or 4 cm year and the evolution of the compressive force through time is thus a critical test of the model viability for intracontinental deformation when the brittle crust is decoupled from the mantle the mantle deforms by symmetrical bending or subduction and a variable amount of lowermost crust is entrained with the sinking mantle in these cases the compressive force remains in the 1012 3 1013 n m acceptable range oceanic subduction only occurs if a low friction shear zone is specified in the brittle realm during the transition from oceanic subduction to collision the oceanic crust in this shear zone remains trapped between colliding crusts eclogitization has no influence on the initial mode of deformation thus it is probably not an important factor in the initiation of subduction in all simulations the influence of eclogitization increases progressively with time in proportion of the amount of lower continental crust and oceanic crust entrained into the mantle the structural evolution of the orogen model depends on eclogitization if initial decoupling occurs at mid crustal level the simulations thus indicate that eclogitization promotes convergence and also in some cases enables the recycling of the whole continental lower crust into the mantle we also suggest that eclogitization could regulate the long term convergence rate in orogens

application of a perturbation recycling method in the large eddy simulation of a mesoscale convective internal boundary layer an arrangement of boundary conditions is described and demonstrated that facilitates the large eddy simulation les of inhomogeneous boundary layers such as internal boundary layers in addition to the domain where the internal boundary layer develops the method requires a section of domain over the upwind surface that is of the order of 10 boundary layer thicknesses and thus similar in size to that needed for the les of the upwind boundary layer in addition to periodic lateral closed upstream and open downstream boundary conditions a simple and efficient perturbation recycling technique which follows from one of lund wu and squires is used to generate a steady supply of fully developed turbulence from the inflow boundary the arrangement is used to simulate a convective internal boundary layer during a cold air outbreak over water results show the method consistently produces a solution that is homogeneous over the upwind surface and inhomogeneous over the downwind surface and that the statistics are stationary after spinup the sensitivity to the placement of the outflow boundary is tested and examples of instantaneous and mean fields generated by the simulations are shown

modeling the behavior of the continental mantle lithosphere during plate convergence in studying orogenic processes the mechanism of viscous rayleigh taylor type removal of gravitationally unstable lithosphere is often invoked to explain the behavior of the mantle lithosphere using numerical models we consider this mechanism and explore alternate styles of deep lithospheric deformation during tectonic convergence the numerical experiments incorporate a mix of viscous and plastic rheologies to model the mechanical evolution of the lithosphere asthenosphere system our results suggest that there are a number of deformational modes of the model mantle lithosphere 1 a dripping or rayleigh taylor type instability 2 an asymmetric underthrusting or subduction 3 symmetric ablative plate consumption 4 slab breakoff the failure and detachment of the strong lithosphere and 5 mixed modes with combinations of these processes the development of the modes is controlled by the rate of convergence associated with the background tectonic regime the density field and the rheology of the mantle lithosphere it is important to determine whether these modes occur in the earth beneath collisional orogens

insights from geodynamical modeling on possible fates of continental mantle lithosphere collision removal and overturn geodynamic modeling demonstrates various modes of behaviour of the tectonically active continental mantle lithosphere at continental collision mantle lithosphere below thickening crust can be accommodated by mixed subduction like consumption and viscous drip like instability depending on the material rheology temperature and convergence velocity late stage slab steepening dual sided and ablative consumption and breakoff can occur as the buoyant crust resists subduction removal of accreted crust by erosion can modify how even the deepest portions of the mantle lithosphere evolves during contraction when gravitational forcing rather than plate shortening dominates mantle lithosphere may be removed through viscous dripping like instability or delamination the removal induces crustal heating modified topography and deformation but distinctive styles of these develop depending on whether mantle lithosphere delaminates or drips with a modified density stratification postulated for the archean r

numerical simulations of the mantle lithosphere delamination 1 sudden uplift extension and increased igneous activity are often explained by rapid mechanical thinning of the lithospheric mantle two main thinning mechanisms have been proposed convective removal of a thickened lithospheric root and delamination of the mantle lithosphere along the moho in the latter case the whole mantle lithosphere peels away from the crust by the propagation of a localized shear zone and sinks into the mantle to study this mechanism we perform two dimensional 2 d numerical simulations of convection using a viscoplastic rheology with an effective viscosity depending strongly on temperature depth composition crust mantle and stress the simulations develop in four steps 1 we first obtain classical sublithospheric convection for a long time period 300 myr yielding a slightly heterogeneous lithospheric temperature structure 2 at some time in some simulations a strong thinning of the mantle occurs progressively in a small area 100 km wide this process puts the asthenosphere in direct contact with the lower crust 3 large pieces of mantle lithosphere then quickly sink into the mantle by the horizontal propagation of a detachment level away from the asthenospheric conduit or by progressive erosion on the flanks of the delaminated area 4 delamination pauses or stops when the lithospheric mantle part detaches or when small scale convection on the flanks of the delaminated area is counterbalanced by heat diffusion we determine the parameters crustal thicknesses activation energies and friction coefficients leading to delamination initiation step 2 we find that delamination initiates where the moho temperature is the highest as soon as the crust and mantle viscosities are sufficiently low delamination should occur on earth when the moho temperature exceeds 800 c this condition can be reached by thermal relaxation in a thickened crust in orogenic setting or by corner flow lithospheric erosion in the overriding lithosphere of subduction zones