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

middle archean continent formation by crustal delamination the processes that created the first large cratonic areas such as the pilbara and the kaapvaal remain poorly understood models based on the uniformitarian extrapolation of present day arc volcanic processes to a hotter early earth have not adequately explained the observations in these terranes here we propose an alternative mechanism for the formation of the earliest continental crust the formation of continental crust may be achieved by delamination of the lower eclogitic part of an oceanic plateau like protocrust such delamination results in uplift extension and the production of tonalite trondhjemite and granodiorite ttg suites as recorded in middle archean cratons the available geologic and geophysical observations in combination with model calculations permit this scenario as an alternative to subduction based hypotheses

island arc and mid ocean ridge volcanism modelled by diapirism from linear source regions abstract the gravitational instability of buoyant regions of partial melt in the upper mantle has been proposed as the underlying cause of regularly spaced volcanism at both island arcs and mid ocean ridges motivated by this suggestion we report a laboratory study of diapirism initiated by the release of buoyant fluid from a linear source into a homogeneous viscous environment fixed volume and fixed flux releases of buoyant fluid are studied a fixed volume release was obtained by rapidly towing a point source of fluid through a body of denser fluid the horizontal cylinder of fluid which was left behind by the source subsequently underwent a gravitational instability yielding plumes with a characteristic spacing expressions for the mean spacing of the plumes and for the growth rate of instability are derived by dimensional arguments and are compared with the experimental results both quantities are found to be independent of the viscosity of the buoyant fluid a surprising result which we have confirmed theoretically in contrast the plume spacing and growth rate produced by the gravitational instability of a layer of buoyant fluid each depend on the cube root of its viscosity in the light of these differing results we conclude that the geometry of the buoyant region of partial melt must be considered when developing gravitational instability models of island arc and mid ocean ridge volcanism we also conclude that if the expressions for a cylindrical region are appropriate then the observed spacing of volcanoes cannot be used to infer the viscosity and hence the melt fraction of the source region a fixed flux release of fluid was achieved by injecting buoyant fluid through a narrow slot in the base of a large tank of denser fluid the interface between the two fluids was found to be unstable and diapirs were formed however the rising diapirs were followed by a continuous uniform sheet of buoyant fluid we suggest therefore that thermal or rheological effects need to be included in fluid mechanical models if volcanism which is periodic in position and time is to be explained

rheological stratification of the lithosphere abstract the rheology of the lithosphere is estimated on the basis of pore pressure dependent frictional failure in the brittle regime and power law steady state creep in the ductile regime different petrological models of the continental and oceanic lithosphere combined with different geotherms and thicknesses are used to generate models covering a variety of geodynamic situations results show that the depth dependence of lithospheric rheology varies with the tectonic province and that in many instances the continental lithosphere has one at the bottom of the crust or two the previous one plus another at mid crustal levels soft ductile layers sandwiched between brittle layers the depth distribution of geophysical parameters seismicity seismic wave velocity and electrical conductivity matches the theological predictions well the lithospheric rheological profiles are used to analyze observed variations in the structural style of deformed continental margin and oceanic terranes in the southeastern canadian cordillera the large scale characteristics of the system show a satisfactory agreement with the inferred rheological structure sub horizontal decollements should be the rule rather than the exception where the lithospheric rheology is strongly stratified

the role of subduction induced subsidence in the evolution of the karoo basin abstract the stratigraphic record indicates that during the late carboniferous to early triassic south africa experienced an episode of large scale subsidence that resulted in the development of the karoo basin the southern margin of the region was contemporaneously dominated by an episode of compressional tectonics and there is evidence for subduction of the paleo pacific plate beneath the karoo basin we propose that the long wavelength component of the basin subsidence resulted from the deflection of the lithosphere due to mantle flow coupled to the adjacent subduction this was followed by a recovery phase during which the entire region was uplifted on cessation of subduction and subsequent loss of dynamic support for subsidence contributing to the present day high elevation topography of south africa numerical models of mantle convection are presented and show that a shallow dipping slab is able to reconcile the observed long wavelength subsidence of the basin a significant component of the n

dynamics of retreating slabs 2 insights from three dimensional laboratory experiments 1 a laboratory analogue of a three layer linear viscous slab upper mantle lower mantle system is established in a silicone putty honey and crystallized honey tank experiment the same setup as in the numerical investigation part 1 is used we focus on the interaction of the slab with the induced passive mantle flow by widely varying the mantle volume flux boundary conditions in our numerical experiments the lateral volume flux was set to zero in interpreting the results relative to the real earth the base of the box is taken as the bottom of the mantle convection system while the lateral boundaries may be associated with the presence of other nearby slabs dynamic force equilibrium assessed on the basis of an analytical review of forces is described for four different phases 1 the subduction initiation instability 2 the accelerating dynamic free fall phase of the slab 3 the dynamic interaction with the 660 km discontinuity and 4 a final phase of steady state trench retreat phase 3 is an important feature not observed in the numerical experiments this highly dynamic phase of interrupted trench retreat can therefore be attributed to boundary conditions on mantle volume flux on the basis of integration constants of force equilibrium in phases 2 and 4 we identify two different classes of volume flux one in which the lateral boundary can be considered open and the other class where it is closed closed boundary condition cases are obtained if any of the lateral box boundaries are 600 km away from the slab assuming a one to one relation between trench retreat and back arc spreading enigmatic observations of episodic opening of back arc basins can be explained by our experimental observations

diapiric spoke patterns abstract not all finger like diapirs evolve from or into linear walls as previously supposed that would imply that all contemporaneous finger diapirs lie in rows and many do not physical models demonstrate that diapiric linear walls or rows of fingers have to be forced by lateral forces lateral boundaries or internal edge effects by contrast the kinematics of the simplest isothermal gravity overturns responsible for finger like diapirs are geometrically kinematically and dynamically similar to the spoke patterns of vigorous thermal convection this is because rayleigh taylor rt diapirism and rayleigh benard rb thermal convection are end members of a continuous dynamic spectrum due to any combination of compositional or thermally induced density inversions all simple rt overturns are dynamically similar but the vigour of thermal convection varies with the heat flow the kinematics of spoke patterns are difficult to study in rapid multiple overturns of thermal convection in low viscosity fluids however spoke patterns in slow isothermal diapiric analogues can be analysed at any stage during their single overturn such studies indicate that two sets of spokes conduct fluids of different densities horizontally along the top and bottom boundaries to finger like diapiric stems or plumes where they rise or sink to spread in polygonal bulbs at the opposite boundary spokes converge to hubs at the bottom boundary beneath stems feeding diapiric bulbs at the top boundary and vice versa if the movement cells are considered centred on rising plumes the cell corners are mapped by sinking plumes the differences between spoke patterns and previously understood diapiric patterns are illustrated by geological examples with emphasis on relevance to the search for oil and gas physical arguments suggest that spoke patterns are probably the most common geometries of gravity driven overturns in layered rocks of both the crust and the mantle

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

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

the interaction of mantle plumes with surface thermal and chemical boundary layers applications to hotspots on venus large volcanic swells on venus are believed to be a manifestation of mantle upwelling or hotspots the study of these regions provides important information on the interior of the planet numerical experiments are carried out to examine the interaction of mantle plumes with the thermal lithosphere and a layer of depleted mantle a product of pressure release melting using an axisymmetric finite element and finite difference code that incorporates temperature dependent viscosity and pressure release melting the lithosphere is defined as a high viscosity lid plumes are initiated and maintained by a prescribed temperature at the base of the computational domain the topographic uplift the geoid to topography ratio and the volume of pressure release melt are compared to estimated values for possible hotspots on venus to constrain the properties of plumes and the lithosphere the effects of lithospheric thickness depleted layer viscosity and thickness mantle temperature and plume temperature and duration on the surface observables are predicted models with a thermal lithospheric thickness of approximately 100 150 km are consistent with observations assuming a mantle temperature of 1300 c a maximum plume temperature of approximately 1500 c and mantle plume durations of 150 250 m y to be consistent with estimated volumes of volcanics at venusian hotspots a significantly thinner thermal lithosphere requires a much cooler mantle a thicker lithosphere requires a hotter mantle models with a depleted layer thickness of 100 250 km in addition to a 100 km thick thermal lithosphere predict the range of parameters found on venus interpretation of large volcanic swells on venus based on the evolutionary sequence predicted here implies that beta atla western eistla and imdr regiones overlie either active or recently active mantle plumes the geoid to topography ratio at beta regio is much larger than values found at other hotspots and may indicate that it is in an early stage of evolution or that the chemical or thermal boundary layers are significantly thicker bell dione and themis regiones appear to represent very late stage possibly now extinct hotspots an estimate of the total plume buoyancy flux for venus is much lower than the value obtained for earth the small areal distribution of volcanism is consistent with a low level of ongoing resurfacing at a few active hotspots

platform subsidence behind an active subduction zone numerical models of viscous mantle flow indicate that a broad subsidence should occur above subduction zones1 4 this platform subsidence is predicted to cause an asymmetrical depression 500 1 000 km wide with a maximum depth of 2 km if filled with water direct evidence linking platform subsidence to mantle flow has remained elusive however here we offer evidence from western new zealand for a complete link in both space and time between subduction initiation and platform subsidence because of the broad wavelength 200 km of the observed 1 500 m subsidence it is unlikely that the subsidence is due to a flexural foredeep the observed subsidence does however match in both wavelength and amplitude that predicted by a model of subduction induced mantle flow the occurrence of platform subsidence has implications not only for understanding mantle convection but also for explaining geoid anomalies over active margins5 and patterns of continental submergence6 and sedimentation7 that could otherwise be interpreted as a eustatic sea level change8

mantle flow tectonics the influence of a ductile lower crust and implications for the formation of topographic uplands on venus convective flow within the interiors of terrestrial planets is a primary source of large scale topographic and tectonic features we treat the crust and mantle as a layered viscous half space and find solutions for flow driven by a buoyancy force distribution within the mantle and by relief at the surface and crust mantle boundary the crust initially responds to mantle upwelling with uplift but continued flow causes significant crustal thinning and subsidence downwelling leads to initial subsidence and subsequent crustal thickening and surface uplift changes in crustal thickness are driven by 1 vertical normal stresses due to mantle flow and 2 shear coupling of horizontal mantle flow into the crust the degree and time scale of crustal deformation are sensitive to crustal thickness and viscosity and the presence of elastic layers in the upper crust and mantle deformation can occur on time scales less than several hundred million years for reasonable values of these parameters strong or elastic upper crustal layers enhance crustal thinning or thickening while large viscosity contrasts between the lower crust and upper mantle or elastic mantle layers tend to diminish deformation for time dependent mantle flow due to a rising or sinking diapiric body crustal deformation depends upon the ratio of diapir radius to crustal thickness and the ratio of crustal to mantle viscosity a sinking diapir can lead to crustal thickening elevated topography and a specific sequence of time varying deformation while regions such as beta regio have long been suggested to result from convective upwelling in the venus mantle we suggest that a mantle downwelling origin is consistent with many of the characteristics of the compressional mountain ranges of western ishtar terra

experiments on shortening of a 4 layer model of the continental lithosphere davy ph and cobbold p r 1991 experiments on shortening of a 4layer model of the continental lithosphere in p r cobbold editor experimental and numerical modelling of continental deformation tecronophysics 188 l 25 we present an experimental method for modelhng lithospheric deformation the method is based on a simplified profile of lithosphere rheology where only four layers are considered brittle crust ductile crust brittle mantle and ductile mantle the strengths of the four layers calculated from laboratory determined flow laws depend strongly on geothenns and slightly on strain rates and deformation regimes the rheological profiles of the lithosphere have been recreated in experimental models built with analog materials sand silicone putty and golden syrup which flow under their own weight during lithospheric shortening the style of deformation depends on the relative strengths of the four layers especially those in the mantle of special importance is the existence or non existence of a brittle mantle the initial stages of compression often produce flexures or buckles without local isostatic compensation buckling exerts a strong control on the localisation of thrust faults in later stages the thrusts tend to form at the inflexion points of buckles the style of crustal thickening depends on the relative thicknesses and resistances of the lithospheric layers in 2 layer lithospheric models without resistant mantle shortening was concentrated upon a small number of thrusts widely spaced in 3 layer models shortening was concentrated into a large number of reverse faults more closely spaced and more steeply dipping than before in clayer models a single thrust formed in the brittle mantle motion on this thrust dragged the crustal layers down forming an asymmetric crustal root with steep imbricate slices

controls on the locus of intraplate deformation in central australia abstract the locus of intraplate deformation in central australia changed from the presently preserved southern margin of the amadeus basin during the late neoproterozoic to early phanerozoic petermann orogeny to the northern margin in the late phanerozoic alice springs orogeny immediately prior to each event the thickness of the amadeus basin sediments varied from as little as 1 5 km in the central parts of the basin to as much as 7 km in the vicinity of the presently preserved margins of the basin with the locus of deformation mimicking the locus of maximum sedimentary thicknesses at the onset of each orogenic event as such these orogenies represent extreme examples of basin inversion a process observed in many intracratonic basins worldwide we show that differential burial of the central australian proterozoic basement complexes beneath the amadeus basin is capable of producing variations in moho temperature of up to 30 c prior to the petermann orogeny and up to 110 c prior to the alice springs orogeny for a brace goetze model of lithospheric rheology the variations in moho temperature equate to variations in effective lithospheric strain rate of 1 4 orders of magnitude implying that variations in thickness of the sedimentary blanket may have played a primary role in localising phanerozoic intraplate deformation in central australia an appealing aspect of this model lies in its corollary that the removal of the sedimentary blanket during denudation of the orogen will be accompanied by dramatic cooling and strengthening of the lithosphere this may provide a plausible explanation for the long term persistence of the extraordinary gravity anomalies 150 mgal developed during these central australian intraplate orogenies

radial profile of mantle viscosity results from the joint inversion of convection and postglacial rebound observables we present new inferences of the radial profile of mantle viscosity that simultaneously fit long wavelength free air gravity harmonics associated with mantle convection and a large set of decay times estimated from the postglacial uplift of sites within previously glaciated regions hudson bay arctic canada and fennoscandia the relative sea level variation at these latter sites is constrained by age height pairs obtained by geological survey rather than the subjective trends which are commonly used in glacial isostatic adjustment gia studies our viscosity inferences are generated using two approaches first we adopt a relative viscosity profile which is known to provide a good fit to the free air gravity harmonics and determine an absolute scaling which yields a best fit to the gia decay time constraints second we perform an iterative nonlinear joint inversion of the two data sets in both cases our inferred profiles are characterized by a significant increase of viscosity 2 orders of magnitude with depth to values of 1022 pa s in the bottom half of the lower mantle the new viscosity profiles are shown to satisfy constraints based on the postglacial uplift of both fennoscandia the classic haskell 1935 number and hudson bay which have commonly been invoked to argue for an isoviscous mantle furthermore the models are used to predict a set of long wavelength signatures of the gia process these include predictions of gia induced variations in 1 the length of day over the late holocene period 2 the earth s precession constant and obliquity over the last 2 6 myr and 3 the present day zonal harmonics of the geopotential jl l 7 the predictions 1 and 3 bound the late holocene and ongoing mass flux between the large polar ice sheets greenland and antarctic and the global oceans to small values 0 4 mm yr equivalent eustatic sea level rise

model inspired interpretation of seismic structures in the central alps crustal wedging and buckling at mature stage of collision analogue models of continental convergence inspire a new interpretation of the deep seismic profile across the central alps the classical collision belt of western europe we conclude that active mountain building processes involve wedging and crustal buckling and that seismic information does not reflect subduction related orogeny but documents recent crustal shortening we also infer that present day subduction of the delaminated european mantle lithosphere takes place below the northern side of the mountain belt

lithospheric instability beneath the transverse ranges of california recent high resolution seismic experiments reveal that the crust beneath the san gabriel mountains portion of the transverse ranges thickens by 10 15 km contrary to earlier studies associated with the transverse ranges there is an anomalous ridge of seismically fast upper mantle material extending at least 200 km into the mantle this high velocity anomaly has previously been interpreted as a lithospheric downwelling both lithospheric downwelling and crustal thickening are associated with the oblique convergence of pacific and north america plates across the san andreas fault though it seems likely that the lithospheric downwelling is driven at least partly by gravitational instability of the cold lithospheric mantle we show by means of numerical experiment that the balance between buoyancy forces that drive deformation and viscous stresses that resist deformation determines the geometry of crustal thickening and mantle downwelling we use a simple two layered lithospheric model in which dense lithospheric mantle overlies relatively inviscid and less dense asthenosphere and is overlain by buoyant crust external plate motion drives convergence which is constrained by boundary conditions to occur within a central convergent zone of specified width a fundamental transition in the geometry of downwelling is revealed by our experiments for slow convergence or low crustal viscosity downwelling occurs as multiple sheets on the margins of the convergent zone for fast convergence or crust that is stronger than mantle lithosphere a single downwelling occurs beneath the center of the convergent zone this complexity in the evolution of the system is attributed to the interaction of crustal buoyancy with the evolving gravitational instability in order for a narrow downwelling slab to have formed beneath the transverse ranges within the last 5 myr the effective lithospheric viscosity of the convergent region is at most about 10 20 pa s

patterns of total and incremental strain in subsiding troughs experimental centrifuged models of inter diapir synclines the experimental technique of centrifuge modelling has been used to elucidate patterns of total strain and progressive deformation associated with troughs that form by gravitational subsidence of a dense cover into a less dense basement model structures are analogous to synclines found between mantled gneiss domes and the models provide data with which to test the hypothesis that archean greenstone belts are products of vertical gravity tectonics experimental results include the following 1 strain within the main body of subsiding belts is dominated by strong horizontal contraction and vertical extension in natural structures this strain pattern would produce vertical foliation and steeply plunging mineral lineation 2 within cover units near trough margins the flattening plane trends parallel with or at a low angle to the cover basement interface at all levels in the trough 3 patterns of strain within subsiding troughs at least as expressed in surface deformation reflect the geometry of the

cooling of the earth in the archaean consequences of pressure release melting in a hotter mantle a model is presented to describe the cooling of the earth in the archaean at the higher archaean mantle temperatures pressure release melting starts deeper and generates a thicker basaltic or komatiitic crust and depleted harzburgite layer compared with the present day situation intrinsic compositional stability and lack of mechanical coherency renders the mechanism of plate tectonics ineffective it is proposed that the archaean continents stabilised early on top of a compositionally stratified root in the archaean oceanic lithosphere hydrated upper crust can founder and recycle through its high pressure phase eclogite eclogite remelting and new pressure release melting generates new crustal material migration of magma and latent heat release by solidification at the surface provides an efficient mechanism to cool the mantle by several hundreds of degrees during the archaean this can satisfactorily explain the occurrence of high extrusion temperature komatiites and lower extrusion temperature basalts in greenstone belts as being derived from the same source by different mechanisms

archean magmatism and deformation were not products of plate tectonics abstract the granite and greenstone terrains that dominate upper crust formed from about 3 6 to about 2 6 ga and record magmatic and tectonic processes very different from those of a younger time they indicate heat loss by the archean earth primarily by voluminous magmatism from a mantle much hotter than that of the present plate tectonic processes were not then operating the distinctive array of petrologic structural and stratigraphic features that characterize phanerozoic convergent plate systems ophiolites magmatic arcs accretionary wedges fore arc basins etc have no viable analogues in archean terrains purported archean plate tectonic indicators consist merely of rock types that even superficially resemble actual phanerozoic indicators only when considered in isolation from their association and structure archean ultramafic and mafic volcanic rocks neither resemble ophiolitic rocks in petrology nor occur in ophiolite type successions they often depositionally overlie felsic basement rocks and often overlie and are intercalated with sedimentary and felsic volcanic rocks and they require a mantle about 200 c hotter than now archean graywackes are coherent strata derived from nearby volcanic rocks late in the histories of their regions and they lack the setting and disruption that characterize modern accretionary wedges the lithologic structural and stratigraphic assemblages that typify proterozoic and phanerozoic rifted and reassembled margins also lack archean analogues and no evidence has been found for archean rifting rotation and reassembly of continental plates conversely characteristic archean assemblages lack modern equivalents in any tectonic setting ultramafic lavas erupted at liquidus temperatures are voluminous granite and greenstone terrains have no modern analogues greenstone belts are typically anastomosing networks of upright synforms formed by crowding aside by and sinking between large rising diapiric elliptical composite batholiths the batholiths include both the products of new crustal melts and variably remobilized mid crustal gneisses the greenstone belts are defined by late deformation of regionally semiconcordant volcanic and sedimentary successions and are not relics of successively formed volcanic chains little deformation generally preceded the diapirism and metamorphism was primarily of contact type the regionally uniform areal density and accordant crustal level of the diapiric batholiths their contacts primarily against the oldest strata of the synforms their general age 10 20 million years younger than most of the flanking stratiform rocks and considerations of high archean radiogenic heat productivity all fit the explanation that the batholiths were mobilized by partial melting of hydrous lower crust by radiogenic heating diapirism was accompanied by modest regional orthogonal shortening and extension of the hot upper crust producing the orientations of the batholiths rise of the batholiths greatly increased the petrologic fractionation of the crust and the concentration of radionuclides high in it resulting in cooling of the deeper crust and subjacent mantle and thus cratonization the upper crust containing the granite and greenstone aggregates was decoupled from the gneissic middle crust which underwent flattening and extension subparallel to the elongation of the shallow batholiths this deep deformation may have been driven by flow of dense restites toward delamination loci from which they sank into the mantle the early earth was probably wholly molten the surface of the earth like that of the moon must have been wholly recycled by impacts before 3 9 ga and heavily modified by them until 3 8 ga zircons as old as 4 2 ga have been found as clastic grains in much younger archean quartzites and polycyclic migmatites last partly melted and reconstituted under hydrous conditions only after 3 6 ga contain relict zircons as old as 4 0 ga the lithologies of the early earth protoliths in which these zircons formed have not been established but impact melts and breccias must be represented and magma ocean fractionates may be the nature of the transition in tectonic style into the granite and greenstone mode is unknown plate tectonic rifting and convergence were operating by about 2 0 ga and were in an essentially modern mode by about 0 8 ga the nature of the transition from the granite and greenstone mode at about 2 6 ga to plate mode by about 2 0 ga has yet to be defined the change may have been facilitated by the increasing content of water and carbon dioxide in the mantle as dense but hydrated delaminated archean crust sank into it

high radiogenic heat producing granites and metamorphism an example from the western mount isa inlier australia the origins of metamorphism 600 c and 3 4 kbar in the western mount isa inlier australia remain controversial for a number of reasons 1 no synmetamorphic intrusive bodies can be recognized 2 high temperatures appear to be sustained for periods 100 m y and 3 metamorphism follows an extended phase of thermal subsidence we show that the burial of granite batholiths enriched in radiogenic elements beneath the thick insulating sedimentary succession of the mount isa group deposited in response to repeated rift sag cycles was capable of generating steep upper crustal thermal gradients immediately prior to the isan orogeny these gradients are appropriate to peak metamorphic conditions such that the ensuing isan orogeny required no significant additional heat input this result is significant in that it may provide a mechanism for understanding the origins of high temperature metamorphism in other terranes where the involvement of transient heating is not obvious

layered convection induced by phase transitions we report a systematic study on the conditions under which an endothermic phase transition can enforce layered convection two dimensional numerical calculations of convection in a domain containing a divariant phase change were performed in the framework of the extended boussinesq approximation i e considering the effects of adiabatic gradient latent heat and frictional heating in the energy equation we find that the critical value of the negative clapeyron slope which must be surpassed in order to induce layered convection decreases in magnitude with increasing rayleigh number ra in the range 104 ra 2 106 near the critical clapeyron slope vacillations between double and single layer convection or strongly leaking double layer convection are possible the breakdown into layers is influenced very little by the latent heat release but depends solely on the phase boundary deflection caused by lateral temperature differences the value of the critical clapeyron slope also seems little affected by the width of the transition zone or by its depth a possible superplastic rheology within the transition zone would tend to favor layered convection scaling the model results to the 670 km discontinuity in the earth s mantle as a possible endothermic phase boundary we estimate the critical clapeyron slope to be in the range of 4 to 8 mpa k 40 to 80 bar k the possibility that the spinel perovskite periclase transition is within this range appears to be remote but certainly cannot be neglected

development of the late proterozoic to mid paleozoic intracratonic amadeus basin in central australia a key to understanding tectonic forces in plate interiors the intracratonic amadeus basin in central australia is a complex composite basin covering 17 000 km with at least nine distinct episodes of evolution between 900 and 300 ma these have been identified in neighboring central australian basins and are characterized by intervals of renewed subsidence followed by intervals of erosion and changes in basin shape several unconformities separating the tectonostratigraphic sequences represent periods of mild regional uplifts and correlate with major episodes of compressive tectonism at the evolving margin of the early australian plate in the absence of direct geological evidence for thermal events in the amadeus basin after the initial period of subsidence ended possibly at about 800 ma the correlations found between events in the amadeus basin those in the neighboring interconnected basins and tectonic events at the continental or plate margins emphasize that regional horizontal stress fields contribute significantly to primary basin forming mechanisms this suggests that stresses generated at continental or plate margins may propagate to the plate interior with the lithosphere acting as a stress guide throughout the history of the amadeus basin both periods when compressional stresses were dominant and when extensional tectonics controlled subsidence can be identified there were also transitional periods when neither compression nor extension dominated so that other processes masked the effects of weak horizontal stresses the most recent compressional thrust belt concentrated at the northern margin between 300 and 400 ma and has partly obscured the record of earlier basin shape and size

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

matrix and fine grained rims in the unequilibrated co3 chondrite alha77307 origins and evidence for diverse primitive nebular dust components abstract the mineralogical and chemical characteristics of dark matrix and fine grained rims in the unequilibrated co3 chondrite alha77307 have been investigated in detail by scanning electron microscopy electron microprobe analysis and transmission electron microscopy matrix and fine grained rims on chondrules and other objects are compositionally homogeneous on the scale of 10 m and there are no compositional differences between matrix and rims mineralogically both the matrix and rims are extremely diverse and consist of a highly unequilibrated assemblage of si and fe rich amorphous material olivine pyroxene fe ni metal magnetite pentlandite pyrrhotite anhydrite and mixed layer phyllosilicate phases several distinct components can be recognized within the matrix and rims based on their textural and compositional characteristics which appear to represent basic fine grained units of nebular dust the microstructures of these different components show that they have experienced significantly different thermal histories unlike the ordinary chondrites an origin for any significant component of the matrix or rims in alha77307 from chondrules is improbable based on compositional and microstructural evidence the matrix and rim components in alha77307 formed by disequilibrium condensation processes as fine grained amorphous dust that is represented by the abundant amorphous component in the matrix such condensation could have occurred under a variety of conditions at different times and locations within the solar nebula or possibly earlier in a circumstellar environment subsequent thermal processing of this primitive condensate material in a variety of environments in the nebula resulted in partial or complete recrystallization of the fine grained dust the intimate association of fine grained components with disparate compositions and thermal histories shows that mixing of fine grained dust within the nebula must have been extremely thorough

new artificial granular materials for analogue laboratory experiments aluminium and siliceous microspheres abstract laboratory tests have been carried out to determine the physical properties of high sphericity and low density fine grained dry granular materials in particular we tested hollow aluminium microspheres and hollow siliceous microspheres shear tests have been carried out with a casagrande apparatus for different values of normal stress from 12 27 to 491 8 kpa in order to determine the frictional properties of the microspheres diagrams of the shear stress against displacement have been used to describe the strength of the investigated materials at different confining pressures the shear tests show that hollow aluminium and siliceous microspheres successfully replicate the mechanical behaviour of weaker layers within natural sedimentary successions undergoing deformation the use of these artificial granular materials in specifically designed experiments indicated that they can be successfully used for constructing more natural multilayers in the laboratory

seismic and tectono metamorphic characters of the lower continental crust in phanerozoic areas a consequence of post thickening extension deep seismic profiles of phanerozoic continental crust commonly show a highly reflective lower crust rheological considerations suggest that the seismic fabric of the lower crust can be attributed to the tectonic transposition of various petrological heterogeneities in the main flow plane the variscan provinces of western europe have been affected during phanerozoic times by several extensional and compressional events the geometrical relationships between seismic and geological structures indicate that the layering of the lower crust was acquired during the late carboniferous to permian when the thickened variscan crust was affected by gravitational collapse petrological and geochronological analyses of deep crustal rocks xenoliths and exposed sections indicate that the lower crust has recorded a major high t medium p granulite facies metamorphism during the late variscan extension whereas on the surface upper carboniferous to permian basins were being deposited a similar scenario characterizes other phanerozoic orogenic belts in the caledonian provinces of the british islands the lower crust is seismically reflective it has undergone medium p granulitic metamorphism during the deposition of devonian sedimentary basins at the end of the caledonian orogeny in the same way collapse of the mesozoic belt in the western part of north america is responsible during the cenozoic for pervasive crustal extension whose consequence is a seismic layering of the lower crust accompanied by a low p granulite grade metamorphic event while in the mid and upper crusts low angle ductile normal faults give rise to the basin and range province therefore it is proposed that there is a genetic relationship between 1 post thickening crustal extension 2 low to medium p granulite facies metamorphism of the lower crust and 3 seismic layering of the lower crust

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

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

lithostratigraphic correlation of blaini formation late proterozoic lesser himalaya india with other late proterozoic tillite sequences the blaini formation of the lesser himalaya has long been considered permo carboniferous in age on simple lithological correlation with gondwana tillites in southern india rare and non repeatable finds of fossils from the type area were thought to confirm this recently lower cambrian faunas have been recorded from the overlying tal and krol formations and possible metazoan trails recorded from the blaini itself in addition the blaini formation is even on a lithostratigraphic comparison much more like other late proterozoic glacial sequences the term blaini formation should only be used for rocks of the type area not for widely scattered glacial deposits which may or may not be correlatable with the type section the persistent belief in the paleozoic age of the blaini and adjacent formations has for long retarded unterstanding of lesser himalaya stratigraphy and tectonics

the interaction of a subducting lithospheric slab with a chemical or phase boundary we have used a finite element model of time dependent convection to determine the conditions for penetration of the subducted plate into the lower mantle a temperature dependent and non newtonian rheology is applied to achieve platelike behavior of the upper and sinking thermal boundary layer of convection the 650 km discontinuity is taken as either a chemical or phase boundary or as a combination of both it is represented by a marker chain which effects additional buoyancy when distorted out of its equilibrium position when the compositional density contrast is greater than about 5 the descending slab is deflected sidewards at the boundary and two layer convection prevails a resulting depression of the boundary in the range of 50 200 km should be detectable with seismic methods below 5 density difference the slab plunges several hundred kilometers into the lower mantle and below 2 it will probably not stop before reaching the core mantle boundary and extensive mixing would be expected with a pure phase change a negative clapeyron slope of about 6 mpa k 60 bar k is required to establish a type of leaky double layer convection a more moderate slope can aid a small compositional density difference to prevent slab penetration into the lower mantle with the present uncertainties about the physical nature of the 650 km discontinuity a variety of convective styles appears possible on dynamical grounds

from collision to collapse phases of lithospheric evolution as monitored by seismic records abstract deep seismic reflection profiles in europe and elsewhere cover a range of different tectonic units specifically in western and central europe they cross structures relating to the alpine variscan and caledonian orogens with considerable crustal shortening delamination and interfingering the variscan mountain belts in france and germany show collapsed structures from various collision events between 300 and 350 ma ago still further north in middle england and the southwest baltic sea traces of the caledonian collision around 400 ma and associated collapse structures are visible here the terrain east avalonia cadomia docked to the colliding continents of baltica and laurentia in a complex pattern with closing oceans and compressional boundaries which can still be seen in today s seismic sections in britain and the sw baltic sea all the processes of crustal shortening interfingering and delamination were certainly active during the compressional stages of these earlier orogens and have left their marks which are still recognizable in today s seismic image of the crust crustal roots and high elevations have disappeared in the extensional collapse phase thermal events have intruded underplated or otherwise modified the stretched lower crust in the variscan internides massif granite production started and the lower crust assumed an especially strong and thick sheared laminated structure with a plane moho the various tectonic stages are illuminated by a gross analysis of reflectivity patterns we postulate that the fate of these patterns from their origin to their death is imbedded in thermally and rheologically varying creep processes which always accompany the brittle and ductile deformation in the earth s crust

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

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

a mechanism for crustal recycling on venus entrainment of lower crust by convective mantle downflows is proposed as a crustal recycling mechanism on venus the mechanism is characterized by thin sheets of crust being pulled into the mantle by viscous flow stresses finite element models of crust mantle interaction are used to explore tectonic conditions under which crustal entrainment may occur the recycling scenarios suggested by the numerical models are analogous to previously studied problems for which analytic and experimental relationships assessing entrainment rates have been derived we use these relationships to estimate crustal recycling rates on venus estimated rates are largely determined by 1 strain rate at the crust mantle interface higher strain rate leads to greater entrainment and 2 effective viscosity of the lower crust viscosity closer to that of mantle lithosphere leads to greater entrainment reasonable geologic strain rates and available crustal flow laws suggest entrainment can recycle 1 km3 of crust per year under favorable conditions

coldspots and hotspots global tectonics and mantle dynamics of venus magellan radar images and altimetry data and pioneer venus gravity data of major highlands and lowlands on venus are examined with the objective of relating these tectonic and volcanic landforms to convection within venus mantle two roughly circular lowlands atalanta and lavinia planitiae are bowl shaped depressions which contain compressional tectonic features and lack hotspot related features such as coronae and shield volcanoes they are identified as surface expressions of young mantle coldspots or regions of mantle downwelling highlands can be divided into two distinct groups volcanic rises and plateau shaped highlands volcanic rises are domical circular to elongate regions characterized by volcanic construction extensional tectonism large positive gravity and geoid anomalies centered on the highlands and large apparent depths of compensation volcanic rises include beta bell atla and western eistla regiones and are identified as surface expressions of large mantle hotspots or plumes plateau shaped highlands are steep sided elevated regions that range from circular alpha regio to elongate or irregular ovda regio tellus regio in shape their surfaces are dominated by complex ridged terrain rather than the shield volcanoes and flows that dominate volcanic rises most plateau shaped highlands contain compressional structures some of which commonly lie along their margins and have small gravity and geoid anomalies and small apparent depths of compensation compared to volcanic rises plateau shaped highlands which include those mentioned above and western ishtar terra thetis regio and phoebe regio are identified as surface expressions of mantle coldspots or regions of downwelling the highland coldspot features are elevated as a consequence of crustal thickening crustal thickening is absent or minor at lowland coldspots the importance of mantle downwelling in the tectonic deformation of venus suggests that its mantle like the mantle of earth is strongly heated from within the major venusian hotspots attest to the existence of large mantle plumes carrying heat upward from the planet s core