toward a boot strap hypothesis of plate tectonics feedbacks between plates the asthenosphere and the wavelength of mantle convection abstract the solid earth system is characterized by plate tectonics a low viscosity zone beneath plates the asthenosphere and long wavelength flow in the convecting mantle we use suites of numerical experiments to show 1 how long wavelength flow and the operation of plate tectonics can generate and maintain an asthenosphere and 2 how an asthenosphere can maintain long wavelength flow and plate tectonics plate subduction generates a sub adiabatic temperature gradient in the mantle which together with temperature dependent viscosity leads to a viscosity increase from the upper to the lower mantle this allows mantle flow to channelize in a low viscosity region beneath plates an asthenosphere forms dynamically flow channelization in turn stabilizes long wavelength convection the degree of dynamic viscosity variations from the upper to the lower mantle increases with the wavelength of convection and drops toward zero if the system transitions from plate tectonics to a single plate planet the plate margin strength needed to initiate that transition increases for long wavelength cells long wavelength flow allows plate tectonics to exist over a wider range of plate margin strength the coupled feedbacks allow for a linked causality between plates the asthenosphere and the wavelength of mantle flow with none being more fundamental than the others and the existence of each depending on the others under this hypothesis the asthenosphere is defined by an active process plate tectonics which maintains it and is maintained by it and plate tectonics is part of an emergent self sustaining flow system that bootstraps itself into existence

the diversity of tectonic modes and thoughts about transitions between them plate tectonics is a particular mode of tectonic activity that characterizes the present day earth it is directly linked to not only tectonic deformation but also magmatic volcanic activity and all aspects of the rock cycle other terrestrial planets in our solar system do not operate in a plate tectonic mode but do have volcanic constructs and signs of tectonic deformation this indicates the existence of tectonic modes different from plate tectonics this article discusses the defining features of plate tectonics and reviews the range of tectonic modes that have been proposed for terrestrial planets to date a categorization of tectonic modes relates to the issue of when plate tectonics initiated on earth as it provides insights into possible pre plate tectonic behaviour the final focus of this contribution relates to transitions between tectonic modes different transition scenarios are discussed one follows classic ideas of regime transitions in which boundaries between tectonic modes are determined by the physical and chemical properties of a planet the other considers the potential that variations in temporal evolution can introduce contingencies that have a significant effect on tectonic transitions the latter scenario allows for the existence of multiple stable tectonic modes under the same physical chemical conditions the different transition potentials imply different interpretations regarding the type of variable that the tectonic mode of a planet represents under the classic regime transition view the tectonic mode of a planet is a state variable akin to temperature under the multiple stable modes view the tectonic mode of a planet is a process variable that is something that flows through the system akin to heat the different implications that follow are discussed as they relate to the questions of when did plate tectonics initiate on earth and why does earth have plate tectonics this article is part of a discussion meeting issue earth dynamics and the development of plate tectonics

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

magma transport of heat on io a mechanism allowing a thick lithosphere models indicate that the high heat flow from io would result in a very thin approximately 5 km silicate lithosphere overlying a molten interior if all heat was transported through the lithosphere via conduction however the presence of mountains with relief in excess of 10 km would seem to demand a thick lithosphere at least locally a significant fraction of io s heat flow may be transported via advection through volcanoes advective heat transfer permits a thicker lithosphere than does pure conduction possibly reconciling io s high heat flow with the rugged topography

appraisal of highly fluoride zones in groundwater of kurmapalli watershed nalgonda district andhra pradesh india fluoride f contamination study had been carried out to see its allocation in kurmapalli watershed nalgonda district andhra pradesh india the study area is located about 60 km se of hyderabad city the groundwater is the main source of water for their living the groundwater in villages and its surrounding are affected by fluoride contamination and consequently the majority of the people living in these villages has health hazards and is facing fluorosis the purpose of this study is to identify the wells with high f raise awareness in people study the water chemistry and also find out the source of f in groundwater a total of 32 groundwater samples were collected from different wells in both shallow aquifers and deeper fractures zones during october 2004 the chemical analysis of groundwater has been done fluoride values vary from 0 7 to 19 0 mg l it is noted that the maximum value 19 0 mg l is one of the highest values found in groundwater in india and 78 of the total samples show f concentrations that exceeds the permissible limit value 1 5 mg l the highest value of f is found at madanapur bore well which is located at central part of the watershed the f value of this bore well was monitored from october 2004 to october 2006 during this period the f concentration varies from 17 8 to 21 0 mg l with mean 19 3 mg l there is no correlation of f with chemical parameters except calcium the ca has shown inverse proportional with f water rock interaction studies were also carried out to understand the behavior of f in groundwater at prominent f affected areas rock samples were collected and analyzed and found their enrichment of f the anthropogenic possibility of f is almost negligible the rocks of this area are enriched in f from 460 to 1 706 mg kg it is indicated that the rock water interaction is the main source of f in groundwater the highest values of f are found in middle part of the region and are related to the occurrence of fluoride rich rocks and their chemical kinetic behavior with groundwater

mantle convection with a brittle lithosphere thoughts on the global tectonic styles of the earth and venus summary plates are an integral part of the convection system in the fluid mantle but plate boundaries are the product of brittle faulting and plate motions are strongly influenced by the existence of such faults the conditions for plate tectonics are studied by considering brittle behaviour using byerlee s law to limit the maximum stress in the lithosphere in a mantle convection model with temperature dependent viscosity when the yield stress is high convection is confined below a thick stagnant lithosphere at low yield stress brittle deformation mobilizes the lithosphere which becomes a part of the overall circulation surface deformation occurs in localized regions close to upwellings and downwellings in the system at intermediate levels of the yield stress there is a cycling between these two states thick lithosphere episodically mobilizes and collapses into the interior before reforming the mobile lid regime resembles convection of a fluid with temperature dependent viscosity and the boundary layer scalings are found to be analogous this regime has a well defined nusselt number rayleigh number relationship which is in good agreement with scaling theory the surface velocity is nearly independent of the yield stress indicating that the plate motion is resisted by viscous stresses in the mantle analysis suggests that mobilization of the earth s lithosphere can occur if the friction coeycient in the lithosphere is less than 0 03 0 13 lower than laboratory values but consistent with seismic field studies on venus the friction coeycient may be high as a result of the dry conditions and brittle mobilization of the lithosphere would then be episodic and catastrophic

geological consequences of super sized earths 1 the discovery of terrestrial scale extrasolar planets and their calculated abundance in the galaxy has prompted speculation on their surface conditions and thermal structure both are dependent on the tectonic regime of a planet which is itself a function of the balance between driving forces and the resistive strength of the lithosphere here we use mantle convection simulations to show that simply increasing planetary radius acts to decrease the ratio of driving to resisting stresses and thus super sized earths are likely to be in an episodic or stagnant lid regime this effect is robust when associated increases in gravity are included as the more dominant effect is increased fault strength rather than greater buoyancy forces the thermo tectonic evolution of large terrestrial planets is more complex than often assumed and this has implications for the surface and conditions habitability of such worlds

tectonics of artemis chasma a venusian plate boundary abstract artemis chasma an 2600 km diameter arcuate trough located in southwestern aphrodite terra venus has been the site of

formation of ridges in a stable lithosphere in mantle convection models with a viscoplastic rheology numerical simulations of mantle convection with a viscoplastic rheology usually display mobile episodic or stagnant lid regimes in this study we report a new convective regime in which a ridge can form without destabilizing the surrounding lithosphere or forming subduction zones using simulations in 2 d spherical annulus geometry we show that a depth dependent yield stress is sufficient to reach this ridge only regime this regime occurs when the friction coefficient is close to the critical value between mobile lid and stagnant lid regimes maps of convective regime as a function of the parameters friction coefficients and depth dependence of viscosity are provided for both basal heating and mixed heating situations the ridge only regime appears for both pure basal heating and mixed heating mode for basal heating this regime can occur for all vertical viscosity contrasts while for mixed heating a highly viscous deep mantle is required

role of a low viscosity zone in stabilizing plate tectonics implications for comparative terrestrial planetology 1 earth s near surface layer its lithosphere is broken into quasi rigid plates that form the upper thermal boundary layer for mantle convection since the discovery of plate tectonics it has been widely conjectured but only recently demonstrated that this peculiar style of convection may be facilitated by an upper mantle low viscosity zone lvz over which the plates glide easily the lvz or asthenosphere concept dates from 19th century investigations of isostatic support of mountain belts and is supported by modern evidence for a seismic low velocity zone and by studies of postglacial rebound and dynamic compensation of the earth s gravity field here we show in both two dimensional 2 d cartesian and 3 d spherical earth models that combining a pronounced lvz and a plastic yield stress to allow localized weakening of the cold thermal boundary layer results in a distinctly plate tectonic style of convection with 30 toroidal surface motion for the 3 d case recycling of water into the upper mantle at subduction zones is a plausible cause of earth s lvz whereas venus is dry and lacks both an lvz and plate tectonics

southern ocean thermocline ventilation abstract an approximate mass volume budget in the surface layer of the southern ocean is used to investigate the intensity and regional variability of the ventilation process discussed here in terms of subduction and upwelling ventilation resulting from ekman pumping is estimated from satellite winds the geostrophic mean component is assessed from a climatology strengthened with argo data and the eddy induced advection is included via the parameterization of gent and mcwilliams together with eddy mixing estimates all three components contribute significantly to ventilation finally the seasonal cycle of the upper ocean is resolved using argo data the circumpolar averaged circulation shows an upwelling in the antarctic intermediate water aaiw density classes which is carried north into a zone of dense subantarctic mode water samw subduction although no consistent net production is found in the light samw density classes a large subduction of subtropical mode water stmw is observed the s

generation of plate like behavior and mantle heterogeneity from a spherical viscoplastic convection model 1 how plate tectonics arises from mantle convection is a question that has only very recently become feasible to address with spherical viscoplastic computations we present mainly internally heated convection results with temperature dependent viscosity and explore parts of the rayleigh number ra yield stress y phase space as well as the effects of depth dependent y bottom heating and a low viscosity asthenosphere convective planform and toroidal poloidal velocity field ratio tpr are affected by near surface viscosity variations and tpr values are close to observed values for our most plate like models at the relatively low convective vigor that is accessible at present most models favor spherical harmonic degree one convection though models with a weaker surface viscosity form degree two patterns and reproduce tomographically observed power spectra an asthenospheric viscosity reduction improves plate like nature as expected for our incompressible computations pure bottom heating produces strong plumes that tend to destroy plates at the surface this implies that significant internal heating may be required both to reduce the role of active upwellings and to form a low viscosity zone beneath the upper boundary layer

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

impact driven subduction on the hadean earth impacts could have driven transient subduction events on the hadean earth according to numerical simulations the scenario reconciles evidence for tectonic activity with that for an otherwise tectonically stagnant early earth

the influence of interior mantle temperature on the structure of plumes heads for venus tails for the earth 1 to form mantle plumes with large heads and narrow trailing conduits requires large 102 viscosity variations within the hot thermal boundary layer at the plume source however new and published data from laboratory experiments show that if a layer of fluid with a temperature dependent viscosity is heated from below and cooled from above convection occurs beneath a stagnant lid and the interior fluid temperature remains close to that of the hot boundary consequently the viscosity reduction across the hot thermal boundary layer is always 10 and upwellings take the form of discrete plume heads thermals subduction of the stagnant layer equivalent to recycling lithospheric plates on the earth will cool the interior of the mantle thus increasing the viscosity ratio hence the existence of earth like mantle plumes may be a consequence of plate tectonics an absence of plate tectonics on venus may lead to upwellings composed of thermals the surface expression of which are coronae as well as a smaller core heat flux than on earth

hysteresis in mantle convection plate tectonics systems 1 we use simulations of mantle convection with surface yielding to show that multiple tectonic regimes are possible for equivalent system parameter values models with the same lithospheric strength parameters and the same vigor of convection can display different modes of tectonics within the region of multiple solutions the evolutionary pathway of the system is the dominant factor that determines the tectonic mode e g whether mantle convection operates in a plate tectonic like mode the extent of the multiple regimes window is found to increase with the temperature dependent viscosity contrast across the system the implication for models that seek to predict the tectonic regimes of planets is that the temporal evolution of the planet needs to be taken into account a further implication is that modeling studies can lead to different conclusions regarding the tectonic state of a planet extra solar planets in particular despite the final model parameter values remaining equivalent

experimental and observational evidence for plume induced subduction on venus venus lacks plate tectonics but some trenches on venus resemble subduction zones laboratory experiments suggest that upwelling plumes can initiate localized subduction of a thin lithosphere such as the one on venus

on eddy scales in the eastern and northern north atlantic ocean as a function of latitude infrared satellite images and two ensembles of satellite tracked buoys have been used to analyze eddy scales in the central and eastern north atlantic between 35 n and 60 n both data sets yield eddy scales which decrease northward from the subtropical atlantic and can be closely related to the rossby radius of the first baroclinic mode

the plate tectonic approximation plate nonrigidity diffuse plate boundaries and global plate reconstructions the central assumption of plate tectonics that plate interiors are rigid remains a useful but uncertain approximation strain rates of stable plate interiors are bounded between 10 i12 10 i11 year i1 and 4 10 i10 year i1 the narrowness of all plate boundaries the other main assumption of plate tectonics as originally conceived is contradicted by many observations both in the continents and in the oceans some diffuse plate boundaries in both continents and oceans exceed dimensions of 1000 km on a side diffuse plate boundaries cover 15 of earth s surface the maximum speed of relative plate motion across any one diffuse plate boundary ranges from 2t o 15 mm year which is faster than some upper bounds on intraplate motion across stable plate interiors 2m m year i1 strain rates in diffuse plate boundaries can be as high as 10 i8 year i1 25 times higher than the upper bound on strain rates of stable plate interiors but 600 times lower than the lowest strain rates across typical narrow plate boundaries the poles of rotation of the plates flanking a diffuse oceanic plate boundary tend to be located in the diffuse boundary which is a consequence of the strong coupling across the boundary

thermal water prospecting results at jelenia gora cieplice sudetes poland versus geothermometric forecasts to provide additional volume of thermal water for balneotherapy and space heating the well cieplice 1 has been deepened from 661 m to 2002 5 m a spontaneous outflow of water with temperatures reaching 87 8 c was obtained thermal logging revealed 97 7 c at 1870 m these results are consistent with earlier estimates based on various chemical geothermometers that indicated water temperatures in the lower parts of the geothermal system possibly exceeding 100 c this was also confirmed by the cationic composition geothermometer the estimates based on the dependence of saturation indices on temperature did not prove correct suggesting 65 c as the most probable maximum temperature of the water

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

the effect of rheological parameters on plate behaviour in a self consistent model of mantle convection abstract in a three dimensional numerical model of mantle convection we have explored the self consistent formation and evolution of plates this was done by investigating the influence of temperature stress and pressure dependence of the viscosity on plate like behaviour further the role of a depth dependent thermal expansivity and of internal heating was examined first order self consistent plate tectonics can be obtained for temperature and stress dependent viscosity we observe virtually undeformed moving plates and the phenomenon of trench migration however plate motion only appears during short intervals which are interrupted by long phases characterised by an immobile lid an improvement is achieved by adding pressure dependence of the viscosity and of the thermal expansivity to the model this leads to extended plates with continuous plate motion and the appearance of a low viscosity zone internal heating accelerates subduction and gives an increase in plate velocity the plates are otherwise left intact we find that the surface velocity is largely determined by subduction rather than by upwellings plate like behaviour is restricted to a narrow parameter window the window is bordered by an episodic or a stagnant lid type of flow on the one side and by a domain in which convection is akin to constant viscosity flow on the other side with these different convective regimes the behaviour of various terrestrial planets can be described self consistently occasionally even a change from the plate regime to the stagnant lid regime occurs such a type of transition has possibly changed the style of tectonics on some terrestrial planets such as mars

currents fronts and eddy fluxes in the canary basin hydrographic data from two cruises in the canary basin meteor 57 july 1981 poseidon 86 april 1982 are analysed with respect to current distribution and lateral heat flux in the azores madeira region the first part of the data base consists of long transects of xbt and g e k measurements between cape finisterre north west spain and the northern canary basin where several year long current meter records exist further information is obtained by thermosalinograph surface data and by expendable current profilers xcp geostrophic currents are derived from xbt profiles using the tight temperature salinity relationship in the depth range of the warmwassersphare the results compare well with the g e k and xcp current observations the second part consists of ctd data from an eddy resolving box shaped ctd survey 500 500 km2 centered at the mooring location 33 n 22 w the observations are supplemented by satellite buoy trajectories horizontal parameter distribution is shown in terms of objectively contoured maps bands of spatially enhanced energetic structures seen in the long transects are further resolved by the box survey as a deep jet like current system cross the canary basin in a west east direction associated with this azores current is a frontal zone with near surface temperature and salinity steps of order 2 k and 0 3 practical salinity units the dynamic topography field can be decomposed into a linear background field a rossby wave and a mesoscale eddy field we find that major contributions to the meridional eddy heat flux are confined to the vicinity of the azores current frontal zone it is shown that the principal balance in the temperature equation is between heating by the mean horizontal advection terms and cooling by the eddy flux divergence

evidence for retrograde lithospheric subduction on venus annular moats and outer rises around large venus coronae such as artemis latona and eithinoha are similar in arcuate planform and topography to the trenches and outer rises of terrestrial subduction zones on earth trenches and outer rises are modeled as the flexural response of a thin elastic lithosphere to the bending moment of the subducted slab this lithospheric flexure model also accounts for the trenches and outer rises outboard of the major coronae on venus accordingly it is proposed that retrograde lithospheric subduction may be occurring on the margins of the large venus coronae while compensating back arc extension is occurring in the expanding coronae interiors similar processes may be taking place at other deep arcuate trenches or chasmata on venus such as those in the dali diana chasmata area of eastern aphrodite terra

a window for plate tectonics in terrestrial planet evolution abstract the tectonic regime of a planet depends critically on the contributions of basal and internal heating to the planetary mantle and how these evolve through time we use viscoplastic mantle convection simulations with evolving core mantle boundary temperatures and radiogenic heat decay to explore how these factors affect tectonic regime over the lifetime of a planet the simulations demonstrate i hot mantle conditions coming out of a magma ocean phase of evolution can produce a hot stagnant lid regime whilst a cooler post magma ocean mantle may begin in a plate tectonic regime ii planets may evolve from an initial hot stagnant lid condition through an episodic regime lasting 1 3 gyr into a plate tectonic regime and finally into a cold senescent stagnant lid regime after 10 gyr of evolution as heat production and basal temperatures wane and iii the thermal state of the post magma ocean mantle which effectively sets the initial conditions for the sub solidus mantle convection phase of planetary evolution is one of the most sensitive parameters affecting planetary evolution systems with exactly the same physical parameters may exhibit completely different tectonics depending on the initial state employed estimates of the early earth s temperatures suggest earth may have begun in a hot stagnant lid mode evolving into an episodic regime throughout most of the archaean before finally passing into a plate tectonic regime the implication of these results is that for many cases plate tectonics may be a phase in planetary evolution between hot and cold stagnant states rather than an end member

spatial scales of mesoscale variability in the north atlantic as deduced from geosat data two years of geosat data were used to systematically investigate the mesoscale variability and spatial scales of the north atlantic we first calculated sea surface height variability and eddy kinetic energy these compare well with the eddy kinetic energy obtained with surface drifters then the spatial scales of mesoscale variability were characterized in 10 latitude 10 longitude areas by their wave number spectra and autocorrelation functions comparison with existing in situ data shows good agreement and it is believed that the total error budget is globally below that of the oceanic signal the wave number spectra show significant differences as a function of latitude and longitude and these can be related to different types of forcing instability of a mean current or fluctuating wind scales typically decrease from west to east and south to north simple proportionality with respect to the first internal rossby radii does not apply everywhere

four views of a portion of the north atlantic subtropical gyre abstract data from four cruises to the triangle centered at 27 n 32 30 w were smoothed by fitting second degree polynomials at each of about 30 different density surfaces the density intervals were p 0 3 corresponding to about 50 db intervals in pressure from the polynomials determination was made of central values horizontal derivatives and laplacians of the fields of pressure salt oxygen and dynamic height in addition maps of the fits and deviations of each nation from the smoothed fits were produced from the steady advective diffusive equation and the smoothed fits to the age oxygen and dynamic height fields the lateral isopycnal diffusivity as estimated to be kh 0 5 103 m2 s 1 although the salt field was reasonably stable from cruise to cruise the variability of the baroclinic velocity shear was found to be as large as the baroclinic shear itself the maps suggest a wobble of the gyre the standard deviation of the fluctuations at each nation from the smoothed fits when norma

quantitative tests for plate tectonics on venus if plate tectonics exists on venus the rate of topographic dropoff from spreading centers should be about half that on earth due to greater rock fluid density contrast and lower temperature differential between the surface and interior using 1 1 mean elevations 21 000 km of ridge were identified on venus and 33 000 km on earth heights h at distances up to 1000 km to either side of the ridges were analyzed to make least squares determinations of the parameters ho h and n in where s is off crest distance the venus ridges have a much less well defined mode in crest heights ho than the earth ridges and a somewhat smaller exponent n on the average indicating greater concavity fixing n to the theoretical value of 0 5 plate velocities were inferred from the h s using h t from north atlantic and north pacific data on earth these velocity solutions were compared to those from plate rotations which indicate the h t is probably about 40 too high for a global mean the resulting plate creation rates on venus assuming comparable total heat generation and taking into account the density and temperature differences are 0 7 km yr or less and indicate that not more than 15 of venus s energy is delivered to the surface by plate tectonics in contrast to 2 9 km yr and 70 energy delivery on earth

3h and 3he in the beta triangle observations of gyre ventilation and oxygen utilization rates abstract isopycnal maps of 3h 3he age with about 100 km resolution have been obtained for a 1000 km scale area in the eastern subtropical north atlantic the midscale texture of the maps is consistent with isopycnal diffusivities of order 500 m 2 s 1 analysis of the distributions within the context of advection diffusion equations reveals that the larger scale 3h 3he age gradients observed within the area are not seriously affected by mixing on those surfaces that outcrop in the region of ekman downwelling thus isopycnal velocities can be estimated from 1 but record only the velocity component normal to the outcrop the velocities thus obtained agree well with geostrophic estimates although the comparison is flawed by the fundamental mismatch in timescales between the two techniques backward extrapolation of the maps to zero age outcrop positions indicate that the contribution of unventilated older recirculated water to the gyre flow above 600 m depth is small in this area no more than 20

spatial variability of subducting water in the north atlantic a pilot study hydrographic and tritium 3he data obtained on a small scale survey conducted in 1986 south of the azores show strong horizontal and isopycnic gradients across the azores front examination of small scale spatial variability reveals a dominant scale for salinity oxygen and tritium 3he age of the order of 160 km comparable to estimates of sea surface height correlation scales from geosat data this spatial variability correlates strongly with large scale horizontal gradients two mesoscale resolving surveys separated by 300 km reveal spatial scales and amplitudes of thickness variability that decrease rapidly with age becoming incoherent on scales greater than 18 km within 1 year and approaching internal wave background values there appears to be a substantial reduction in mode thickness between the two surveys which may be the result of non conservation of potential vorticity or the result of interannual changes in formation subduction rates computed from tritium 3he age gradients are comparable to or larger than ekman pumping estimates based on climatological data

the levels of fluorine in the sediments of the aquifer and their significance for fluorosis in coastal region of laizhou bay china the levels of fluorine in sediment cores obtained from changyi county pz core and laizhou county ts core are used to discuss the fluorine sources in groundwater and its enrichment dynamics the sediments in the aquifer are mainly constituted of granite gravels the levels of fluorine in the pz and ts cores range from 130 to 468 mg kg 139 528 mg kg with average values of 324 348 mg kg respectively which show relatively lower levels than the national average of fluorine in the soil or sediment thus the fluorosis in this area should not be attributed to the levels of fluorine in sediments the average fluorine concentrations in the aquifer from top to bottom are 154 139 200 222 mg kg for the ts core and 154 130 266 272 mg kg for the pz core respectively which are the lowest of the cores and extremely lower levels than the fresh granites such a fact indicates that a vast amount of fluorine has been leached into the groundwater moreover the fluorine leachability is estimated to be approximately 70 although the previous documents showed fluorine contents of the granite surrounding laizhou bay were almost equal to or even lower than the average levels of fluorine in fresh granites meanwhile a simulation experiment also reveals that fluorine release from rocks increases with the addition of seawater and brine water therefore the seawater intrusion may potentially enhance the fluorine leachability and should be an important dynamic of fluorine enrichment

long wavelength convection poiseuille couette flow in the low viscosity asthenosphere and the strength of plate margins summary mixed heated 3 d mantle convection simulations with a low viscosity asthenosphere reveal relatively short and long wavelength regimes with different scalings in terms of surface velocity and surface heat flux and show that mantle flow in the lithosphere asthenosphere region is a poiseuille couette flow the poiseuille couette velocity magnitude ratio d u allows us to characterize solid state flow in the asthenosphere and to predict the regime transition the transition from dominantly pressure driven poiseuille flow at shorter wavelengths to dominantly shear driven couette flow at long wavelengths depends on the relative strength of lithosphere and asthenosphere and is associated with a switch in the dominant resistance to convective motion in the poiseuille regime significant resistance is provided by plate bending whereas in the couette regime most resistance is due to vertical shear in the bulk mantle the couette case corresponds to classical scaling ideas for mantle convection whereas the poiseuille case with asthenospheric velocities exceeding surface velocities is an example of a sluggish lid mode of mantle convection that has more recently been invoked for thermal history models of the earth our simulations show that both modes can exist for the same level of convective vigour i e rayleigh number but at different convective wavelengths additional simulations with temperature and yield stress dependent viscosity show consistent behaviour and suggest an association of the regime crossover with the relative strength of plate margins our simulations establish a connection between the strength of plate margins solid state flow in the asthenosphere and the wavelength of mantle convection this connection suggests that plate tectonics in the sluggish lid mode is wavelength dependent and potentially more robust than previously envisioned

short term episodicity of archaean plate tectonics by combining geochemical data and geodynamical models evidence is provided to address the existence and style of archaean plate tectonics a topic of vigorous debate for decades using careful analyses of lithostratigraphic archaean assemblages and numerical model results we illustrate that a short term episodic style of subduction was a viable style of tectonics in the early earth modeling results show how due to the low strength of slabs in a hotter earth frequent slab break off events prevented a modern style long lived subduction system and resulted in frequent cessation and re initiation of the subduction process on a typical time scale of a few million years results fit with geochemical observations that suggest frequent alternation of arc style and non arc style volcanism on a similarly short time scale such tectonics could provide the link between early pre plate tectonic style of tectonics or stagnant lid convection and modern style plate tectonics in which short term episodes of proto subduction evolved over time into a longer term more successful style of plate tectonics as mantle temperature decayed

thermal stresses in planetary elastic lithospheres the role of thermal stresses in the tectonics of the moon is reexamined in this paper a model is used that considers a spherical elastic shell overlying a fluid core it is shown that the thermal stresses generated by temperature changes within the shell usually dominate over the thermal contraction or expansion of the core during the entire evolution of the moon the cooling of the lithosphere is likely to have dominated the cooling of the interior and the result would be tensional lithospheric thermal stresses however during the recent evolution of the moon the change in the near surface thermal stresses is compressional it is argued that the surface compressional features on mercury are not due to the thermal contraction of the interior

subduction origin on early earth a hypothesis i propose a hypothesis for the origin of subduction on early earth that is directly coupled with mantle upwelling and the formation of mafi c crust the hypothesis invokes spatial and temporal overlap of both endogenic and exogenic processes broad quasi cylindrical mantle upwelling and large bolide impact respectively leading to subduction and spreading two signature processes of modern plate tectonics the spatial and temporal intersection of these processes could have occurred variably across early earth and thus subduction could have begun at different locations and times globally the hypothesis postulates that the ability of a terrestrial planet to evolve plate tectonics results from a balance between the strength of its lithosphere and the size bolide it can attract and survive both factors to a fi rst order are a function of planet size

on the emergence of plate tectonics at present young oceanic lithosphere is positively buoyant and it does not become negatively buoyant until it is older than about 20 m y if in the past the mantle was hotter the oceanic crust would have been thicker and the lithosphere9s age of neutral buoyancy greater on the other hand the hotter mantle would have had a lower viscosity and convected faster so the average age of oceanic plates at subduction would have been less than the present 100 m y at some time in the past average oceanic lithosphere would have only just become negatively buoyant as it reached a subduction zone it is estimated here that this condition occurred when the mantle was only about 50 c hotter than at present and as recently as 0 9 1 4 b y ago transformation of basaltic crust to eclogite might have enhanced the vigor of the plates but possibly only modestly and intermittently in earlier times plate tectonics could have operated but more slowly so that it could not by itself have accomplished the necessary rate of heat removal from the earth a different tectonic mode must also have operated perhaps subcrustal delamination or dripping plate tectonics would have gradually taken over from the earlier mode plumes are a omplementary mode of mantle convection driven by a lower thermal boundary layer and so they could have operated in conjunction with both regimes of the upper boundary layer

continental crust formation on early earth controlled by intrusive magmatism the global geodynamic regime of early earth which operated before the onset of plate tectonics remains contentious as geological and geochemical data suggest hotter archean mantle temperature and more intense juvenile magmatism than in the present day earth two crust mantle interaction modes differing in melt eruption efficiency have been proposed the io like heat pipe tectonics regime dominated by volcanism and the plutonic squishy lid tectonics regime governed by intrusive magmatism which is thought to apply to the dynamics of venus both tectonics regimes are capable of producing primordial tonalite trondhjemite granodiorite ttg continental crust but lithospheric geotherms and crust production rates as well as proportions of various ttg compositions differ greatly which implies that the heat pipe and plutonic squishy lid hypotheses can be tested using natural data here we investigate the creation of primordial ttg like continental crust using self consistent numerical models of global thermochemical convection associated with magmatic processes we show that the volcanism dominated heat pipe tectonics model results in cold crustal geotherms and is not able to produce earth like primordial continental crust in contrast the plutonic squishy lid tectonics regime dominated by intrusive magmatism results in hotter crustal geotherms and is capable of reproducing the observed proportions of various ttg rocks using a systematic parameter study we show that the typical modern eruption efficiency of less than 40 per cent leads to the production of the expected amounts of the three main primordial crustal compositions previously reported from field data low medium and high pressure ttg our study thus suggests that the pre plate tectonics archean earth operated globally in the plutonic squishy lid regime rather than in an io like heat pipe regime

volcanism and tectonics on venus we review recent developments in the study of volcanism and tectonics on venus venus s crust is basaltic dry and probably about 30 km thick the mantle convects giving rise to plumes and has a similar composition and mean temperature 1300 c but a higher viscosity 10 20 pa s than that of the earth inferred melt generation rates constrain the lithospheric thickness to between 80 and 200 km the elastic thickness of the lithosphere is about 30 km on average the present day lack of plate tectonics may be due to strong faults and the high viscosity of the mantle most of the differences between earth and venus processes can be explained by the absence of water venus underwent a global resurfacing event 300 600 ma ago the cause and nature of which remains uncertain the present day surface heat flux on venus is about half the likely radiogenic heat generation rate which suggests that venus has been heating up since the resurfacing event

the thermal evolution of an earth with strong subduction zones it is commonly supposed that plate tectonic rates are controlled by the temperature dependent viscosity of earth s deep interior if this were so a small decrease in mantle temperature would lead to a large decrease in global heat transport this negative feedback mechanism would prevent mantle temperatures from changing rapidly with time we propose alternatively that convection is primarily resisted by the bending of oceanic lithosphere at subduction zones because lithospheric strength should not depend strongly on interior mantle temperature this relationship decreases the sensitivity of heat flow to changes in interior mantle viscosity and thus permits more rapid temperature changes there the bending resistance is large enough to limit heat flow rates for effective viscosities of the lithosphere greater than about 1023 pa s and increases with the cube of plate thickness as a result processes that affect plate thickness such as small scale convection or subduction initiation could profoundly influence earth s thermal history

climate change as a regulator of tectonics on venus tectonics volcanism and climate on venus may be strongly coupled large excursions in surface temperature predicted to follow a global or near global volcanic event diffuse into the interior and introduce thermal stresses of a magnitude sufficient to influence widespread tectonic deformation this sequence of events accounts for the timing and many of the characteristics of deformation in the ridged plains of venus the most widely preserved volcanic terrain on the planet

earth dynamics and the development of plate tectonics this volume brings together contributions from the royal society discussion meeting on earth dynamics and the development of plate tectonics held in march 2018 other planets in the solar system do not exhibit plate tectonics so why does it occur on earth how did it develop and when did earth adopt this tectonic regime in evaluating evidence from the geological record it is critical to distinguish between local and global phenomena in a discussion of the why how and when of the transition to plate tectonics on earth thus evidence of local or episodic subduction in the geological record for example does not necessarily provide evidence for the development of a sustainable global network of mobile belts that forms the basis for a mosaic of plates the tectonic regime at any point in the evolution of a planet appears to depend on the initial conditions set by crystallization of the last magma ocean these conditions determine the thermal state hot or cold at the start of sub solidus mantle convection which is subsequently driven by the relative contributions of basal and internal heating to the mantle through time plate tectonics is linked to the ability of mantle convection to form plate boundaries which requires localized weakening of the lithospheric lid how and when did this become possible consideration of the tectonic regime on venus which may be an analogue for the early tectonic development of earth evidence from the rock record rock deformation experiments geodynamic models extrapolated back to the thermal conditions appropriate to the archaean and geochemical models for the development and growth of the continental crust have led to the currently popular view that plate tectonics developed from a stagnant lid regime however if mantle convection is able to form weak plate boundaries at the higher mantle temperatures expected during the

fluoride ion contamination in the groundwater of mithi sub district the thar desert pakistan groundwater samples were collected from various localities of mithi sub district of the thar desert of pakistan and analysed for fluoride ion along with other chemical parameters the area is mainly covered by sand dunes and kaolin granite at variable depths results showed that collected water samples were severely contaminated by the presence of fluoride ion and most of the samples have higher concentration than prescribed who standards 1 5 mg l for drinking water fluoride ion concentrations ranged between 0 09 and 11 63 mg l with mean and median values of 3 64 and 3 44 mg l respectively in this area whereas distribution pattern showed high concentrations in the vicinity of islamkot and mithi towns the content of f has also been correlated with other major ions found in the groundwater of the study area the positive correlation of f with na and hco3 showed that the water with high na and hco3 stabilizes f ions in the groundwater of the thar desert the ph versus f plots signifies high fluoride concentration at higher ph values implying that alkaline environment favours the replacement of exchangeable oh with f in the groundwater of mithi area the saturation indices si of fluorite caf2 and calcite caco3 in the groundwater samples showed that most of the samples are oversaturated with respect to calcite whereas majority of samples have been found under saturated with respect to fluorite the log tds and na na ca ratio reflected supremacy of weathering of rocks which promotes the availability of fluoride ions in the groundwater piper diagram has been used to classify the hydrofacies in the cation triangle all samples are na type while the anion triangle reflects major dominance of cl type with a minor influence of hco3 and so4

the tectonic evolution of western ishtar terra venus the origin and evolution of ishtar terra is modeled using numerical simulations of crust mantle interaction on venus based on our modeling we favor a two phase evolutionary sequence for ishtar the first phase is driven by bulk mantle flow due to thermal convection and results in the formation of a thickened crustal block over a mantle downwelling the second phase a result of the thermal perturbation exerted on the mantle by the region of thickened crust is driven by the sinking of a lithospheric slab this slab driven tectonic pulse leads to low angle subduction and orogenesis at the peripheries of the crustal block throughout its evolution ishtar remains coupled to flow within the deep mantle due to the lack of an asthenosphere and ultimately bulk mantle flow comes to dominate as the effects of the slab driven pulse wane

application of selected geothermometers to exploration of low enthalpy thermal water the sudetic geothermal region in poland the most important intakes of thermal waters within the sudetic geothermal region occur in three separate hydrogeothermal systems 1 l dek 2 duszniki and 3 cieplice all these waters are of meteoric origin and circulate in crystalline rocks to different depths their outflow temperatures are between less than 20 c and to about 87 c to evaluate the geothermal fields in the light of their prospectiveness to further exploration of thermal energy resources we took an effort to apply selected isotopic and chemical geothermometers to assess the maximum possible temperatures which may be found in the reservoirs the only chemical geothermometers which give a reliable range of reservoir temperatures are sio2 chalcedony na ka ca and partly na k ones the oxygen isotopic geothermometer in the so4 h2o system gives a real range of estimated reservoir temperatures only for deeply circulating waters in the cieplice area on the other hand in the case of co2 rich waters in the duszniki area where outflow temperatures do not exceed 30 c application of chemical or isotopic temperature indicators always leads to erroneous results due to the lack of equilibrium in the thermodynamic system of water rock interaction

the influence of air sea interaction on the ventilated thermocline abstract air sea interaction influences the ventilated thermocline by forcing the mixed layer to deepen and cool poleward when there is flow from the mixed layer into the interior the mixed layer depth and density fields help to set the potential vorticity of the subducted fluid the importance of this process is assessed by incorporating a depth varying mixed layer in a ventilation model which is forced by ekman pumping and implied surface heating the formulation of the ventilation problem is simplified by only allowing density surfaces to outcrop along latitude circles and by assuming that there is no zonal inflow along the eastern boundary the surface heating enables a cross isopycnal flow within the mixed layer the volume of ventilated fluid within the subtropical gyre is increased by including the depth varying mixed layer and this fluid partly originates from the western boundary as well as from the ekman layer the depth varying mixed layer increases the depth at which isopycnals are subduc

large lenses of highly saline mediterranean water isolated compact anticyclonic eddies or salt lenses were found in the canary basin hydrographic surveys of three such lenses show large anomalies of salinity and temperature 0 8 2 5 c they are centered at 1100 m have a vertical extent of up to 900 m and radii of 50 km current meter records indicate anticyclonic velocities up to 29 cm s 1 fine structure with vertical scales of 20 m and less possibly due to intrusive decay appears at the outer edges of the lenses whereas the centers are free of such structure the probability of finding a salt lens at any station in the canary basin is fairly high 0 08

the effects of internal heating and large scale climate variations on tectonic bi stability in terrestrial planets abstract we use 3d mantle convection and planetary tectonics models to explore the links between tectonic regimes and the level of internal heating within the mantle of a planet a proxy for thermal age planetary surface temperature and lithosphere strength at both high and low values of internal heating for moderate to high lithospheric yield strength hot and cold stagnant lid single plate planet states prevail for intermediate values of internal heating multiple stable tectonic states can exist in these regions of parameter space the specific evolutionary path of the system has a dominant role in determining its tectonic state for low to moderate lithospheric yield strength mobile lid behavior a plate tectonic like mode of convection is attainable for high degrees of internal heating i e early in a planet s thermal evolution however this state is sensitive to climate driven changes in surface temperatures relatively small increases in surface temperature can be sufficient to usher in a transition from a mobile to a stagnant lid regime once a stagnant lid mode is initiated a return to mobile lid is not attainable by a reduction of surface temperatures alone for lower levels of internal heating the tectonic regime becomes less sensitive to surface temperature changes collectively our results indicate that terrestrial planets can alternate between multiple tectonic states over giga year timescales within parameter space regions that allow for bi stable behavior any model based prediction as to the current mode of tectonics is inherently non unique in the absence of constraints on the geologic and climatic histories of a planet

the conditions for plate tectonics on super earths inferences from convection models with damage abstract numerical simulations of mantle convection with a damage grainsize feedback are used to develop scaling laws for predicting conditions at which super earths would have plate tectonics in particular the numerical simulations are used to determine how large a viscosity ratio between pristine lithosphere and mantle l m can be offset by damage to allow mobile plate like convection regime diagrams of l m versus the damage number d show that the transition from stagnant lid to mobile convection occurs for higher l m as d increases a similar trend occurs for increasing rayleigh number we hypothesize a new criterion for the onset of plate tectonics on terrestrial planets that damage must reduce the viscosity of shear zones in the lithosphere to a critical value equivalent to the underlying mantle viscosity a scaling law based on this hypothesis reproduces the numerical results for the earth damage is efficient in the lithosphere and provides a viable mechanism for the operation of plate tectonics we scale our theory to super earths and map out the transition between plate like and stagnant lid convection with a planetary plate tectonic phase diagram in planet size surface temperature space both size and surface conditions are found to be important with plate tectonics being favored for larger cooler planets this gives a natural explanation for earth venus and mars and implies that plate tectonics on exoplanets should correlate with size incident solar radiation and atmospheric composition

global decoupling of crust and mantle implications for topography geoid and mantle viscosity on venus the surface of venus is so hot that the lower crust may be weak enough to allow decoupling of mantle and crust an analytic model of such decoupling assumes that the shallow mantle forms the top boundary layers of large scale mantle convection cells crustal flow is driven by the motion of the mantle and by topographically induced pressure gradients the model predicts that the lowest lowlands are sites of mantle upwelling and thinner than average crust highlands are places where mantle downwells and the crust is thick surface heat flow is inversely correlated with elevation consistent with recent estimates of brittle layer thickness variations on venus if the average crustal thickness is about 20 km then the average lower crustal viscosity must be close to 1018 pa s to allow decoupling the observed amplitude of geoid highs over highlands requires an earth like increase in mantle viscosity with depth

a regime diagram of mobile lid convection with plate like behavior abstract terrestrial planetary bodies that undergo solid state convection can exhibit a variety of tectonic styles from stagnant lid one plate planets to those with a mobilized lithosphere for modeled planetary convection the de facto mode of recycling of lithosphere into the planetary interior is typically achieved through 2 sided and symmetric downwelling flows however lithosphere recycling on earth occurs in a distinctly 1 sided mode known as subduction using numerical models of mantle convection in which the viscosity of planetary mantle material is strongly temperature dependent yet maintains a finite material strength as dictated by its yield stress we investigate the continuum of mobile lid convection with plate like behavior the models span a parameter space of rayleigh number and plate strength and explore convective systems with low yield stresses resulting in weak subduction hinges that bend easily and highly deformable subducted slabs three distinct modes are found to occur in convective systems with weak plates the stagnant lid mode 2 sided downwelling mode and a mode that alternates between 1 sided subduction and 2 sided downwellings we classify the style of convective downwelling for a range of models and show that mode selection strongly depends on the combination of surface mobility and strength of the downgoing plate using these measurements we have developed a regime diagram that can predict whether a particular system will be in one of those three modes

water undersaturated mantle plume volcanism on present day mars based on meteorite evidence the present day martian mantle has a combined abundance of up to a few hundred ppm of h2o cl and f which lowers the solidus and enhances the magma production rate adiabatic decompression melting in upwelling mantle plumes is the best explanation for young last 200 myr volcanism on mars we explore water undersaturated mantle plume volcanism using a finite element mantle convection model coupled to a model of hydrous peridotite melting relative to a dry mantle the reduction in solidus temperature due to water increases the magma production rate by a factor of 1 3 1 7 at 50 ppm water and by a factor of 1 9 3 2 at 200 ppm water mantle water also decreases the viscosity and increases the vigor of convection which indirectly increases the magma production rate by thinning the thermal boundary layer and increasing the flow velocity at conditions relevant to mars these indirect effects can cause an order of magnitude increase in the magma production rate using geologic and geophysical observations of the late amazonian magma production rate and geochemical observations of melt fractions in shergottite meteorites present day mars is constrained to have a core mantle boundary temperature of 1750 to 1800 c and a volume averaged thermal rayleigh number of 2 106 to 107 indicating that moderately vigorous mantle convection has persisted to the present day melting occurs at depths of 2 5 6 gpa and is controlled by the rayleigh number at the low pressure end and by the mantle water concentration at high pressure

depth dependent viscosity and mantle stress amplification implications for the role of the asthenosphere in maintaining plate tectonics summary boundary layer theory is used to derive scaling relationships for plate stresses in a mantle convection system with a low viscosity asthenosphere the theory assumes a plate tectonic like mode of mantle convection withflow driven by an active upper boundary layer the theory predicts that the confinement of horizontal mantle flow within a low viscosity sublithospheric channel can lead to an increase in plate stress compared to the case lacking a channel even if the absolute viscosity of the sublithosphere mantle does not change between the two cases thetheoryfurtherpredictsincreasingshearstresswithdecreasinglow viscositychannelthickness if the thickness of tectonic plates is determined dominantly by a dehydrated chemical lithosphere then the plate normal stress is predicted to also increase with decreasing channel thickness we use 3 d spherical shell simulations of mantle convection with temperature depth and stress dependent rheology to test scaling trends the simulations and theoretical scalings demonstrate that a low viscosity layer asthenosphere can amplify convective stresses if the level of convective stress plays a role in maintaining and or reactivating plate boundaries this suggests that a relatively thin low viscosity layer may help to maintain plate tectonics the numerical simulations support this suggestion as they show that an increase in the thickness of a low viscosity channel can cause the system to transition from an active lid modeofconvectiontoastagnantlidstate collectively thesimulationsandtheoreticalscalings lead to the conclusion that the role of the asthenosphere in maintaining plate tectonics does not come principally from a basal lubrication effect associated with a low absolute asthenosphere viscosity but instead from a mantle flow channelization effect associated with a high viscosity contrast from the asthenosphere to the mantle below

scaling laws for convection with temperature dependent viscosity and grain damage numerical experiments of convection with grain damage are used to develop scaling laws for convective heat flow mantle velocity and plate velocity across the stagnant lid and plate tectonic regimes three main cases are presented in order of increasing complexity a simple case wherein viscosity is only dependent on grainsize a case where viscosity depends on temperature and grainsize and finally a case where viscosity is temperature and grainsize sensitive and the grain growth or healing is also temperature sensitive in all cases convection with grain damage scales differently than newtonian convection due to the effects of grain damage for the fully realistic case numerical results show stagnant lid convection fully mobilized convection that resembles the temperature independent viscosity case and partially mobile or transitional convection depending on damage to healing ratio rayleigh number and the activation energies for viscosity and healing applying our scaling laws for the fully realistic case to earth and venus we demonstrate that increasing surface temperature dramatically decreases plate speed and heat flow essentially shutting down plate tectonics due to increased healing in lithospheric shear zones as proposed previously contrary to many previous studies the transitional regime between the stagnant lid and fully mobilized regimes is large and the transition from stagnant lid to mobile convection is gradual and continuous thus planets could exhibit a full range of surface mobility as opposed to the bimodal distribution of fully mobile lid planets and stagnant lid planets that is typically assumed

convection driven tectonics on venus an analysis is presented for convective stress coupling to an elastic lithosphere as it applies to tectonic deformation on venus theoretical solutions are introduced for the response of a mathematically thick elastic plate overlying a newtonian viscous medium with an exponential depth dependence of viscosity a green s function solution is obtained for the viscous flow driven by a harmonic density distribution at a specified depth it is shown that during the transient phase of the solution the field quantities stress velocity displacement in both media depend upon the rheology of both media whereas in steady state the solutions become uncoupled due to this uncoupling stresses obtained from a purely viscous solution can be applied as boundary conditions at the base of an isolated elastic plate i show for such a plate that 1 shear stress imparted at the base of the lower boundary can induce large in plane stretching mode forces and 2 these forces can be estimated directly from free air gravity anomalies without detailed knowledge of the density distribution in the underlying flow free air gravity anomalies for venus imply in plane forces of about 2 3 1012 n m in plane forces of this magnitude are capable of extensive disruption of the venusian lithosphere to demonstrate this an elastic plastic analysis is carried out for the deformation of a model venus lithosphere the calculation is based on a lithospheric strength envelope and uses gravity and topography as a guide to the magnitude of stresses imparted by convective flow the results predict that dynamic uplift of venusian topography must be accompanied by extensive brittle failure and viscous flow in the lithosphere geoid anomalies and geoid to topography ratios for venusian topographic features are large compared to values for thermal swells in earth s ocean basins this suggests that dynamic support of topography must be an important process on venus further a low viscosity zone is probably not present beneath the venusian lithosphere a comparative analysis suggests that tectonic deformation of the lithosphere associated with dynamically supported topography on earth is unimportant

a free plate surface and weak oceanic crust produce single sided subduction on earth 1 earth s lithosphere is characterized by the relative movement of almost rigid plates as part of global mantle convection subduction zones on present day earth are strongly asymmetric features composed of an overriding plate above a subducting plate that sinks into the mantle while global self consistent numerical models of mantle convection have reproduced some aspects of plate tectonics the assumptions behind these models do not allow for realistic single sided subduction here we demonstrate that the asymmetry of subduction results from two major features of terrestrial plates 1 the presence of a free deformable upper surface and 2 the presence of weak hydrated crust atop subducting slabs we show that assuming a free surface rather than the conventional free slip surface allows the dynamical behavior at convergent plate boundaries to change from double sided to single sided a weak crustal layer further improves the behavior towards steady single sided subduction by acting as lubricating layer between the sinking and the overriding plate this is a first order finding of the causes of single sided subduction which by its own produces important features like the arcuate curvature of subduction trenches citation crameri f p j tackley i meilick t v gerya and b j p kaus 2012 a free plate surface and weak oceanic crust produce single sided subduction on earth geophys res lett 39 l03306 doi 10 1029 2011gl050046

the madeira mode water quasi homogeneous layers in vertical profiles of temperature and salinity in the eastern north atlantic near madeira indicate the existence of a subtropical mode water in the eastern basin temperature sections show a maximum horizontal extent of at least 500 km the frequency distribution analysis of homogeneous layers in a historical xbt dataset shows a mode water formation region near and to the north of madeira this mode water is found at increasing depths and displaced to the west and southwest during the course of the year after its formation by wintertime convection it disappears almost completely due to mixing before the next winter volume estimates suggest that this madeira mode water in the eastern atlantic accounts for 15 20 of the total central water formation in the corresponding density range as obtained from tracer studies in the north atlantic gyre

plate tectonics damage and inheritance lithospheric damage combined with transient mantle flow and migrating proto subduction is proposed to explain the apparent emergence of plate tectonics three billion years ago modelling confirms that tectonic plate boundaries and fully formed tectonic plates can arise under conditions characteristic of earth but not of venus plate tectonics the division of the earth s surface into rigid plates separated by linear zones of concentrated deformation is unique among known terrestrial bodies as yet there is no consensus view on how the phenomenon arose here david bercovici and yanick ricard outline an explanation that starts with the microscopic properties of deforming minerals when sufficient lithospheric damage accumulates the theory states shear localization occurs and long lived weak zones develop transient mantle flow and migrating proto subduction then lead to the accumulation of plate boundaries and eventually tectonic plates the authors model this process with a grain evolution and damage mechanism coupled to an idealized model of pressure driven lithospheric flow and show that tectonic plate boundaries and fully formed tectonic plates can form under such conditions the initiation of plate tectonics on earth is a critical event in our planet s history the time lag between the first proto subduction about 4 billion years ago and global tectonics approximately 3 billion years ago suggests that plates and plate boundaries became widespread over a period of 1 billion years the reason for this time lag is unknown but fundamental to understanding the origin of plate tectonics here we suggest that when sufficient lithospheric damage which promotes shear localization and long lived weak zones combines with transient mantle flow and migrating proto subduction it leads to the accumulation of weak plate boundaries and eventually to fully formed tectonic plates driven by subduction alone we simulate this process using a grain evolution and damage mechanism with a composite rheology which is compatible with field and laboratory observations of polycrystalline rocks1 2 coupled to an idealized model of pressure driven lithospheric flow in which a low pressure zone is equivalent to the suction of convective downwellings in the simplest case for earth like conditions a few successive rotations of the driving pressure field yield relic damaged weak zones that are inherited by the lithospheric flow to form a nearly perfect plate with passive spreading and strike slip margins that persist and localize further even though flow is driven only by subduction but for hotter surface conditions such as those on venus accumulation and inheritance of damage is negligible hence only subduction zones survive and plate tectonics does not spread which corresponds to observations after plates have developed continued changes in driving forces combined with inherited damage and weak zones promote increased tectonic complexity such as oblique subduction strike slip boundaries that are subparallel to plate motion and spalling of minor plates

continental tectonics in the aftermath of plate tectonics the success of plate tectonics required an acceptance of continental drift and thus a reinterpretation of the large scale geological history of most of the earth but the basic tenet of plate tectonics rigid body movements of large plates of lithosphere fails to apply to continental interiors where buoyant continental crust can detach from the underlying mantle to form mountain ranges and broad zones of diffuse tectonic activity

self lubricated mantle convection two dimensional models we present results from convection models allowing for self lubrication of downflows models impose a line source of chemically light low viscosity material at the top of a convecting layer of temperature dependent viscosity material low viscosity surface material serves as an analog to hydrated sediment crust and the high viscosity upper portion of the convecting layer as an analog to mantle lithosphere slow near surface motion in the convecting layer entrains low viscosity material into zones of downflow which has a lubricating effect once entrained lubricant is deeper than the cold high viscosity portion of the convecting layer rapid upper boundary layer overturn occurs and system properties change e g heat flux doubles this marks transition to a lubricated state before and after transition transport properties are dominantly determined by respectively the viscosity of mantle lithosphere and that of interior mantle lubricated and nonlubricated states appear as distinct regions in system output space suggesting that exchange between them is akin to a phase transition that such exchange depends on a near surface lubricant implies that the geodynamics of planets lacking such lubricants may fundamentally differ from that of planets possessing them

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

an episodic hypothesis for venusian tectonics it is suggested that episodic plate tectonics occurs on venus episodes of rapid plate tectonics are separated by periods of surface quiescence for the last 500 200 m y it is postulated that the surface of venus has been a single rigid plate that has been thickening due to conductive cooling a near uniform surface age is consistent with observed crater densities and the relatively small number of craters modified by surface tectonics or embayed by lava flows a lithosphere that has conductively thickened for some 500 m y has a thickness of about 300 km nearly an order of magnitude greater than the thickness associated with steady state conductive heat loss such a thick lithosphere can support the high topography and associated gravity anomalies on venus as well as the unrelaxed craters studies of lithospheric flexure at coronae are also consistent with a thick elastic lithosphere incipient subduction associated with large coronae may represent the onset of a new episode of rapid plate tectonics on the earth 75 90 of mantle heat transport is attributed to the creation of new oceanic lithosphere at ocean ridges this process is not operative on venus this paper suggests that episodic plate tectonics on venus constitutes the primary mechanism for mantle heat transport on that planet

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

oceanic mega impacts and crustal evolution lunar mare crater counts the terrestrial impact flux and astronomical observations of asteroids and comets define a consistent impact rate of 4 6 10 15 km 2 yr 1 within the inner solar system since the end of the late heavy bombardment 3 8 ga coupled with the observed crater size vs cumulative crater size frequency relationship of n d c 1 8 n cumulative number of craters of diameter d c these rates imply formation on earth of more than 450 d c 100 km diameter craters more than 50 d c 300 km diameter craters and more than 20 d c 500 km diameter craters geochemical and isotopic constraints require that more than 80 of the projectiles impacted on time integrated oceanic crust since the late heavy bombardment the injection of shock energies calculated at 10 8 mt tnt equivalent by a d p 10 km diameter projectile may result in propagating fractures and rift networks thermal perturbations and ensuing magmatic activity examinations of the geologic record for correlated impact and magmatic fingerprints of such events remain inconclusive in view of isotopic age uncertainties potential but unproven connections may be represented by the 1 cretaceous tertiary boundary ca 65 ma impact s onset of the carlsberg ridge spreading deccan volcanism and onset of the mantle plume of the emperor hawaii chain 2 jurassic cretaceous boundary ca 145 ma impacts onset of gondwana breakup including precursors of the east african rift structures 3 permian triassic boundary ca 251 ma impact s siberian norilsk traps and early triassic rifting and 4 the 3 26 ga basal fig tree group east transvaal ir rich and ni rich quench spinel bearing impact spherules and contemporaneous igneous tectonic activity tests of the theory require further identification and isotopic dating of distal ejecta impact spherule condensates and meteoritic geochemical anomalies

occurrence and use of mineral and thermal waters in poland mineral and thermal waters are defined according to polish geological and mining law their occurrence in particular hydrogeological provinces of the country is characterized besides their use in balneotherapeutics their role as a heat carrier and source of chemical raw materials is stressed topics mentioned requiring further research are the origin of saline formation waters and the origin of co2 in carbonated waters

self consistent generation of tectonic plates in time dependent three dimensional mantle convection simulations 2 strain weakening and asthenosphere 1 in a previous paper tackley 2000b it was shown that a combination of temperature dependent viscosity and viscoplastic yielding is sufficient to give rudimentary plate tectonic like behavior in three dimensional models of mantle convection here the calculations are extended to include two complexities that have been suggested as being important in localizing deformation at plate boundaries strain weakening and the presence of a low viscosity asthenosphere introducing an asthenosphere by reducing the viscosity by a factor of 10 where material reaches a solidus 1 dramatically improves plate quality even if the asthenosphere is restricted to regions around spreading centers 2 gives good smoothly evolving platelike behavior over a wide range of yield stress values spanning an order of magnitude and 3 gives bimodal stable solutions over a range of yield stresses either immobile lid or plate behavior may be obtained depending on initial condition by contrast introducing an asthenosphere by using depth dependent viscosity has a much smaller effect on system behavior increasing strain weakening 1 improves localization at spreading centers but leads to an increasingly complex network of spreading centers fragmenting the plates 2 weakens convergent zones and can make downwellings highly episodic and 3 does not lead to pure transform boundaries in these calculations time dependent damage evolution and instantaneous strain rate weakening give very similar results all cases that display platelike behavior have very long wavelength mantle heterogeneity consistent with earth s

sedimentary processes and crustal cycling on venus sediment exists on the venus surface it is observed in venera images between outcrops and boulders of sedimentary rocks sediment is produced by pyroclastic volcanism and chemical weathering chemical weathering is driven by an enhanced activity of water and an elevated surface temperature sediment is transported by wind action and lithified by cementation and diagenesis cementation may be by carbonate or silica cement diagenesis may be by products of chemical weathering acting as cement or by compaction and recrystallization of sediment into a texture with interlocking grains sediment may be transported from the top of sialic continents such as ishtar to the modal plains where it is deposited lithified and integrated into the local crust as new layers are added the bottom of the crust melts and is in part returned to the mantle a steady state chemical exchange might exist by this mechanism of crustal cycling that links atmosphere continents modal plains and mantle

an analytic model of convection in a system with layered viscosity and plates summary we present an analytic boundary layer model for thermal convection with a finite strength plate and depth dependent viscosity the model permits solutions in which convective flow rates in the mantle exceed the plate velocity the energy balance equations for the lithosphere and convective cell are distinct and the model yields the plate velocity plate thickness and heat flow as well as the laterally averaged horizontal flow profile for a convective cell we demonstrate by inspection of the lithospheric energy balance that the dominant plate driving force depends not only on the material properties of the mantle and lithosphere but also on the velocity of the plate multiple solutions are possible with three solution branches for the plate velocity representing three distinct modes of thermal convection the branch of solutions with the largest plate velocity corresponds to the classic boundary layer solution with a plate velocity approximately equal to the maximum mantle velocity this branch reproduces the classic convective scaling laws for an isoviscous fluid and its dynamics are controlled by the mantle material properties the branch of solutions with intermediate plate velocity represents a convective cell with a sluggish lid and a plate velocity that is less than the underlying mantle velocity the dynamics for this solution branch depend on the material properties of both the mantle and the lithosphere finally the lower branch of solutions for the plate velocity yields a convective solution in which the plate velocity is determined entirely by the local dynamics of a thick and strong relative to the mantle lithosphere the dynamics of this solution branch are independent of the material properties of the mantle and depend entirely on the properties of the lithosphere the introduction of an asthenosphere or low viscosity layer lvl beneath the plate can significantly alter the dynamics of the system by affecting plate mantle coupling lowering the lvl viscosity relative to the lower mantle can promote plate motion by providing a lubricating layer however a very low viscosity lvl beneath a strong plate can decouple plate and mantle and inhibit plate motion producing a solution with a slow moving plate and a channelized flow in the lvl thus a lvl can sometimes inhibit rather than promote plate tectonics