influence of a pre existing basement weakness on normal fault growth during oblique extension insights from discrete element modeling abstract we use discrete element modeling to investigate three dimensional fault geometry and the three dimensional evolution of a fault network that develops above a 60 dipping planar pre existing weakness striking 60 relative to the extension direction the evolution of the fault network comprises three stages i reactivation of pre existing structure and nucleation of new faults 0 10 extension ii radial propagation and interaction between reactivated structure and new faults 15 20 extension and iii linkage between reactivated structure and adjacent faults 20 25 extension during the first stage the pre existing structure mostly reactivates forming a long and under displaced fault new faults are mainly extension perpendicular and dip at 60 during the second stage saw tooth fringes grow upwards from the upper tip of the reactivated structure which becomes the major fault and influence the density and orientation of surrounding faults during the third stage the reactivated structure links laterally and vertically with adjacent faults creating non planar fault geometries following linkage the reactivated structure enhances the displacement of linked faults along branch lines our study demonstrates that pre existing weak faults can be reactivated propagating upwards in an irregular saw tooth pattern and affecting fault density orientation dip and displacement and providing the nucleation site of new faults

surface stresses associated with arrested dykes in rift zones many theoretical models predict that arrested dykes may generate major grabens at rift zone surfaces arrested dyke tips in eroded rift zones however are normally not associated with major grabens or normal faults that could be generated by dyke induced stresses ahead of the tips and normal faults and grabens tend to be less common in those parts of eroded rift zones where dykes are comparatively abundant similarly there are feeder dykes as well as dykes arrested a few metres below the surface that do not generate faults or grabens at the surface here i propose that this discrepancy between theoretical models and field observations may be explained by the mechanical layering of the crust numerical models presented here show that abrupt changes in young s moduli layers with high dyke normal compressive stresses stress barriers and weak horizontal contacts have large effects on the dyke induced stress fields for the models considered the surface tensile stresses induced by arrested dykes are normally too small to lead to significant fault or graben formation at the rift zone surface the only significant dyke induced surface tensile stresses 2 mpa in these models are for a dyke tip arrested at 1 km depth below the surface of a rift zone with a weak contact at 400 m depth and subject to extension that tensile stress however peaks above the ends of the weak horizontal contact which in the model considered occur at distances of 4 km to either side of the dyke and shows no simple relation to the depth to the dyke tip thus for a layered crust with weak contacts straightforward inversion of surface geodetic data to infer dyke geometries may result in unreliable results

three dimensional variable viscosity convection of an infinite prandtl number boussinesq fluid in a spherical shell we investigate a three dimensional spherical shell model of mantle convection with strongly temperature dependent viscosity numerical calculations of convection in an infinite prandtl number boussinesq fluid heated from below at a rayleigh number of ra 10 5 are carried out for the isoviscous case and for a viscosity contrast across the shell of 1 000 in the isoviscous case convection is time dependent with quasi cylindrical upflow plumes and sheet like downflows when viscosity varies strongly across the shell convection is also time dependent but major quasi cylindrical downflows with spider like extensions occur at both poles and interconnected upflow plumes occur all around the equator the surface expression of mantle convection in the earth downwelling sheets at trenches upwelling plumes at hot spots and upwelling sheets at midocean ridges resembles structures seen in both the isoviscous and variable viscosity models the dominance of spherical harmonic degree l 2 in the variable viscosity model agrees with the l 2 dominance in the earth s geoid topography and seismic tomography the overall pattern of convection in the variable viscosity case is similar to the distribution of major highlands and volcanic rises on venus

localisation of normal faults in multilayer sequences abstract existing conceptual growth models for faults in layered sequences suggest that faults first localise in strong and brittle layers and are later linked in weak and ductile layers we use the discrete element method for modelling the growth of a normal fault in a brittle ductile multilayer sequence the modelling reveals that faults in brittle ductile sequences at low confining pressure and high strength contrast localise first as mode i fractures in the brittle layers low amplitude monoclinal folding prior to failure is accommodated by ductile flow in the weak layers the initially vertically segmented fault arrays are later linked via shallow dipping faults in the weak layers faults localise therefore as geometrically and kinematically coherent arrays of fault segments in which abandoned fault tips or splays are a product of the strain localisation process and do not necessarily indicate linkage of initially isolated faults the modelling suggests that fault tip lines in layered sequences are more advanced in the strong layers compared with weak layers where the difference in propagation distance is most likely related to strength and or ductility contrast layer dependent variations in fault localisation rates generate fringed rather than smooth fault tip lines in multilayers

interpreting gravity geoid and topography for convection with temperature dependent viscosity application to surface features on venus while venus and earth are similar in size venus does not appear to have developed the same system of mobile surface plates which dominates convective heat transfer from the interior of earth numerical models with temperature dependent viscosity do not produce plate boundaries and are more closely reminiscent of venus than earth we demonstrate that many puzzling aspects of surface features on venus are also found in the analysis of the surface observables in these models despite having no strong lithosphere our calculations exhibit very high geoid topography ratios and uncompensated short wavelength topography surface heat flux values are not required to be very different from earth scaled values and the mantle remains below the likely solidus

some effects of a dry crustal flow law on numerical simulations of coupled crustal deformation and mantle convection on venus models of coupled crustal deformation and mantle convection on venus are reevaluated in light of recent experimental evidence suggesting that the strength of dry basalt is comparable to that of olivine a previous model which assumed a relatively weak basalt flow law more appropriate for hydrated crust is compared to a similar model that assumes a strong basalt flow law more appropriate for dry crust a principal difference is the timescale for significant crustal thickening of the order of 108 years for the weak basalt model and 109 years for the strong basalt model these results can be understood in the context of theoretical scaling relations for convecting temperature dependent media which imply that in the absence of concentrated zones of near surface weakness of the type associated with plate margins on earth the lithospheric overturn time which sets the timescale required for convective thickening of crust scales dominantiy with the rayleigh number defined by the average viscosity of the lithosphere theoretical scalings as well as numerical models suggest crustal thickening timescales of the order of 108 years remain possible for high bulk mantle rayleigh numbers and effective viscosity contrasts from lithosphere to interior mantle of the order of 103 such timescales imply a relatively thin lithosphere and heat loss comparable to that of the present day earth the large values inferred for the thickness of the mechanical lithosphere on venus if correct would thus tend to favor timescales of the order of 109 years if one assumes relatively low convective mantle heat loss over an equivalent time this in turn would argue against the likelihood of highland plateaus being manifestations of crustal thickening above mantle downflows if instead one assumes that convective heat removal and implied lithospheric overturn rates were significantly higher over the last several hundred million years then highland formation due to crustal thickening cannot be ruled out based solely on the strength of dry crust

experiments of dike induced deformation insights on the long term evolution of divergent plate boundaries the shallow transport of magma occurs through dikes causing surface deformation our understanding of the effects of diking at the surface is limited especially on the long term for repeated intrusive episodes we use analogue models to study the upper crustal deformation induced by dikes we insert metal plates within cohesive sand with three setups in setup a the intrusion rises upward with constant thickness and in setups b and c the intrusion thickens at a fixed depth with final rectangular setup b or triangular setup c shape in section setup a creates a doming delimited by reverse faults with secondary apical graben without close correspondence in nature in setups b and c a depression flanked by two uplifted areas is bordered by inward dipping normal faults propagating downward and for deeper intrusions in setup b also by inner faults reverse at the surface this deformation is similar to what is observed in nature suggesting a consistent physical behavior dikes in nature initially propagate developing a mode i fracture at the tip subsequently thickened by magma intrusion without any host rock translation in the propagation direction as in setup a the deformation pattern in setups b and c depends on the intrusion depth and thickness consistently to what is observed along divergent plate boundaries the early deformation in setups b and c is similar to that from a single rifting episode i e lakagigar iceland and dabbahu afar whereas the late stages resemble the structure of mature rifts i e krafla iceland confirming diking as a major process in shaping divergent plate boundaries

flexural ridges trenches and outer rises around coronae on venus high resolution altimetry collected by the magellan spacecraft reveals trench and outer rise topographic signatures around major coronae e g eithinoha heng o artemis and latona in addition magellan synthetic aperature radar images show circumferential fractures in areas where the plates are curved downward both observations suggest that the lithosphere around coronae is flexed downward by the weight of the overriding coronal rim or by the negative buoyancy of subducted lithosphere we have modelled the trench and outer rise topography as a thin elastic plate subjected to a line load and bending moment beneath the corona rim the approach was tested at northern freyja montes where the best fit elastic thickness is 18 km in agreement with previously published results the elastic thicknesses determined by modelling numerous profiles at eithinoha heng o artemis and latona are 15 40 37 and 35 km respectively at eithinoha artemis and latona where the plates appear to be yielding the maximum bending moments and elastic thicknesses are similar to those found at the middle america mariana and aleutian trenches on earth respectively estimates of effective elastic thickness and plate curvature are used with a yield strength envelope model of the lithosphere to estimate lithospheric temperature gradients at heng o artemis and latona temperature gradients are less than 10 k km which correspond to conductive heat losses of less than one half the expected average planetary value we propose two scenarios for the creation of the ridge trench and outer rise topography differential thermal subsidence and lithospheric subduction the topography of heng o is well matched by the differential thermal subsidence model however at artemis and latona the amplitudes of the trench and outer rise signatures are a factor of 5 too large to be explained by thermal subsidence alone in these cases we favor the lithospheric subduction model wherein the lithosphere outboard of the corona perimeter subducts rolls back and the corona diameter increases

effects of overpressure variations on fracture apertures and fluid transport abstract for an isolated rock fracture in a homogeneous isotropic rock subject to constant internal fluid overpressure or constant external driving stress the ideal opening displacement aperture profile or variation is that of a flat ellipse for many mineral veins dykes tension fractures normal faults and other rock fractures however the opening displacements show irregular aperture thickness variations very different from that of a flat ellipse here we present field data on typical fracture aperture variations as well as new numerical and analytical models to explain these data we present the overpressure variation by fourier cosine series a very flexible method that can be used to model abrupt overpressure and driving stress variations in vertical and lateral sections for fractures of various sizes and types we calculate the opening displacements of typical hydrofractures and discuss the results with reference to mineral veins and dykes we also present numerical models showing that when a fracture dissects layer or parts of a single layer such as a lava flow with different stiffnesses young s moduli the opening displacement may show irregular variation even when the overpressure is constant from the cubic law the volumetric flow rates in the large aperture segments of the mineral veins and dykes discussed in the paper may have been as much as 3 5 times the flow rates in the small aperture segments we propose that differences in volumetric flow rates are related to irregular opening displacement variations in feeder dykes is one principal reason for the development of crater cones a universal feature of volcanic fissures

numerical models of martian mantle evolution induced by magmatism and solid state convection beneath stagnant lithosphere 1 numerical models of magmatism in a convecting mantle are presented to understand martian mantle evolution the viscosity is strongly temperature dependent and the lithosphere is stagnant magmatism is modeled as a permeable flow of basaltic magma generated by decompression melting when the initial mantle temperature is sufficiently high a magma ocean develops to generate a thick basaltic crust and create a compositionally layered mantle the upper layer of the mantle consists of compositionally buoyant residue of the basaltic crust while the lower layer consists of compositionally denser materials that are not as depleted in the basaltic component hot plumes grow from the lower layer making it thinner with time by erosion the hot plumes also induce magmatism which extracts heat producing elements hpes from the mantle the hpe extraction regulates plume magmatism activity and controls the duration of plume magmatism plume magmatism also keeps the mantle temperature below the solidus by efficiently extracting heat as soon as the mantle temperature exceeds the solidus the martian mantle is likely to have evolved as a relaxation from a compositionally layered state formed by the magma ocean and plume magmatism has probably played a crucial role in the relaxation process

phase transitions in pyrolite and morb at lowermost mantle conditions implications for a morb rich pile above the core mantle boundary abstract subduction of mid oceanic ridge basalt morb gives rise to strong chemical heterogeneities in the earth s mantle possibly extending down to the core mantle boundary phase relations in both pyrolite and morb compositions are precisely determined at high pressures and temperatures corresponding to lowermost mantle conditions the results demonstrate that the post perovskite phase transition occurs in pyrolite between 116 and 121 gpa at 2500 k while post perovskite and sio2 phase transitions occur in morb at 4 gpa lower pressure at the same temperature theory predicts that these phase changes in pyrolite and morb cause shear wave velocity increase and decrease respectively near the northern margin of the large low shear velocity province in the lowermost mantle beneath the pacific reflections from a negative shear velocity jump near 2520 km depth are followed by reflections from a positive velocity jump 135 to 155 km deeper these negative and positive velocity changes are consistent with the expected phase transitions in a dense pile containing a mixture of morb and pyrolitic material this may be a direct demonstration of the presence of accumulations of subducted morb crust in the deep mantle

a global survey of possible subduction sites on venus abstract about 10 000 km of trenches in chasmata and coronae have been identified as possible sites of retrograde subduction on venus all the sites have narrow deep trenches elongate along strike with arcuate planforms ridge trench outer rise topographic profiles typical of terrestrial subduction zones large outer rise curvatures 10 7 m 1 fractures parallel to the strike of the trench on the outer trench wall and outer rise and no cross strike fractures across the trench both the northern and southern margins of latona corona are possible subduction sites identification of a major graben between the two principal outer ridges in southern latona corona is evidence of back arc extension in the corona the amount of extension is estimated to be more than 2 11 km the moment exerted by the ridges of southern latona corona is insufficient to bend the lithosphere into the observed outer rise shape a negatively buoyant subducted or underthrust slab is needed depending on the unknown trench migration rate lithospheric subduction can make a significant contribution to mantle cooling on venus venusian chasmata could have a dual character they may be propagating rifts near major volcanic rises and subduction trenches far from the rises in the lowlands subduction and rifting may occur in close proximity on venus unlike on earth rifting induced by hotspots on venus may be necessary to break the lithosphere and allow subduction to occur such a process could result in gradual lithospheric subduction or global episodic overturn of the lithosphere

deepest mantle viscosity constraints from earth rotation anomalies 1 the radial variation of viscosity from earth s surface to the core mantle boundary is most accurately determined on the basis of observations related to the glacial isostatic adjustment gia process beneath a depth of approximately 1250 km the primary constraints available pertain to the anomalies in earth s rotational state that have previously been shown to be intimately linked to the same gia process responsible for postglacial sea level variability it is demonstrated that these anomalies are capable of resolving a difference between d viscosity and that of the overlying region which extends upwards to the 1250 km depth horizon a trade off is shown to exist between the viscosities in these deepest mantle layers that may be resolved by the observed direction of true polar wander

influence of initial cmb temperature and other parameters on the thermal evolution of earth s core resulting from thermochemical spherical mantle convection 1 here we present a wide ranging parameter study of the effects of initial core mantle boundary cmb temperature concentration of radioactive potassium in the core and density difference between harzburgite and mid ocean ridge basalt morb in a coupled spherical model of thermochemical mantle convection and parameterized core heat balance the initial cmb temperature is expected to be much higher than the solidus temperature of silicates at the base of the mantle the results indicate that as with previous purely thermal convection models the final state of the system is only weakly dependent on initial cmb temperature unless the cmb becomes blanketed by a global layer of dense material fully 3 d spherical cases have a very similar core evolution to cases in a 2 d spherical annulus giving confidence in the applicability of 2 d spherical annulus geometry for modeling earth s evolution obtaining a successful thermal evolution in the sense of obtaining the correct present day inner core size and maintaining a geodynamo over geological time is helped by the accumulation of piles of dense material at the cmb subducted morb in the present calculations and a concentration of radiogenic k in the core in the range 400 800 ppm the present day cmb heat flow is predicted to be around 9 tw while this is lower than estimates based on calculating temperature gradients in regions where the postperovskite transition is seismically imaged these tend to be areas of higher than average heat flux and thus likely overestimate the global heat flow

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

surface deformation in volcanic rift zones abstract the principal conduits for magma transport within rift zones of basaltic volcanoes are steeply dipping dikes some of which feed fissure eruptions elastic displacements accompanying a single dike emplacement elevate the flanks of the rift relative to a central depression concomitant normal faulting may transform the depression into a graben thus accentuating the topographic features of the rift if eruption occurs the characteristic ridge trough ridge displacement profile changes to a single ridge centered at the fissure and the erupted lava alters the local topography a well developed rift zone owes its structure and topography to the integrated effects of many magmatic rifting events to investigate this process we compute the elastic displacements and stresses in a homogeneous two dimensional half space driven by a pressurized crack that may breach the surface a derivative graphical method permits one to estimate the three geometric parameters of the dike height inclination and depth to center and the mechanical parameter driving pressure rock stiffness from a smoothly varying displacement profile direct comparison of measured and theoretical profiles may be used to estimate these parameters even if inelastic deformation notably normal faulting creates discontinuities in the profile geological structures open cracks normal faults buckles and thrust faults form because of stresses induced by dike emplacement and fissure eruption theoretical stress states associated with dilation of a pressurized crack are used to interpret the distribution and orientation of these structures and their role in rift formation

the use of infiltration field tests for groundwater artificial recharge when using surface infiltration as a method of recharge infiltration testing is considered an important additional data input along with other hydrogeologic data into the recharge decision as part of an investigation into the potential for groundwater recharge two desert basins in jordan wadi madoneh and wadi butum were investigated to determine the possibility of recharge using floodwater retention structures for each area short duration up to 7 h infiltration tests were conducted to estimate surface infiltration capacity of the upper soil layers in order to present to the authorities preliminary information which could be used along with other factors to aid in the selection of the best site for a pilot recharge project given the highly fractured rock formations that constitute the underlying aquifers in the two areas it was assumed that the upper alluvium layers are the limiting factors in transmitting water to target aquifers the infiltration tests conducted to estimate the recharge characteristics of the recharge sites yielded test results that indicated a representative infiltration rate of 0 44 m day for the wadi madoneh site and 0 197 m day for the wadi butum site the data input was used subsequently with other decision factors to select the most promising site for the pilot project

testing the tracer ratio method for modeling active compositional fields in mantle convection simulations 1 tracer methods are attractive for modeling compositional fields because they offer the potential of zero numerical diffusion composition is typically taken to be proportional to the absolute local concentration of tracers but an increasingly popular method is to have dense and regular tracers with composition being equal to the local fraction of dense tracers this paper tests this ratio method using established benchmarks and by comparing the performance of the two tracer methods and grid based methods for simulating the long term evolution of a convecting mantle with a thick dense stable layer for this scenario the ratio method is found to have several advantages giving sharp stable long term layering with no tracer settling minimal statistical noise and low entrainment even with only 5 tracers per cell the method is equally applicable to finite volume and finite element treatments of the underlying flow entrainment in grid based advection methods is heavily dependent on resolution and numerical details and is reduced 1 order of magnitude by the filter proposed by a lenardic numerical determination of physically correct entrainment rates remains a challenging problem comparing tracer and grid based methods the spatial pattern of the thermal and chemical fields appear to be converging on the finest grids however the estimated entrainment differs significantly

form and dimensions of dykes in eastern iceland abstract a study was made of about 700 dykes in eastern iceland the majority of these belong to three swarms about 73 dip within 10 of the vertical most strike between 10 and 40 ne the strike of the dykes within the southernmost swarm alftafjordur changes along its trace from almost n at the north end to nne ssw southward along the swarm the average thickness of the dykes is about 4 1 m and the thickness does not change notably along the alftafjordur swarm the thinner dykes tend to have smaller grains than the thicker dykes of five dykes followed laterally the longest is over 22 km the thickness of individual dykes changes irregularly along their length and the dyke is often offset where its thickness changes many dykes appear to be completely discontinuous but some parts are connected by veins where the dykes end in a vertical section most of them simply taper away only about 10 5 of the dykes occupy faults the mechanical and thermal effects of the dykes on the country rock are small many of the dykes appear to be non feeders i e dykes that never reached the surface to feed lava flows using the length width ratio the depth of origin of three dykes has been estimated the maximum depth of origin of these three dykes is 7 5 km 9 km and 10 km below the original surface

phase transformations in subducted oceanic crust and buoyancy relationships at depths of 600 800 km in the mantle abstract phase transformations in a representative oceanic crust composition morb have been studied at pressures up to 28 gpa at 1200 c and 1500 c using a multi anvil apparatus the assemblage of majorite casio 3 rich perovskite stishovite which is displayed by the morb composition at pressures above 20 gpa was found to transform further into an assemblage containing a new aluminous phase at pressures above 25 gpa this al phase has a complex composition and is also relatively sodium rich its x ray diffraction pattern is similar to that observed for mgal 2 o 4 calcium ferrite type and is tentatively indexed on the basis of an orthorhombic cell yielding a zero pressure density of 3 87 2 g cm 3 majorite garnet was found to remain as the most abundant phase at the highest pressure studied 28 gpa and does not appear likely to transform to perovskite until pressures exceed 29 30 gpa these results imply that the basaltic crust component of subducted oceanic lithosphere would be 0 1 0 2 g cm 3 less dense than surrounding mantle pyrolite between the depths of 660 800 km thereby confirming an earlier hypothesis of anderson moreover the underlying layer of harzburgite would be 0 05 g cm 3 less dense than pyrolite throughout most of this depth interval these buoyancy relationships contribute to a barrier which impedes and may sometimes prevent the entry of slabs into the lower mantle

magma influence on propagation of normal faults evidence from cumulative slip profiles along dabbahu manda hararo rift segment afar ethiopia abstract measuring displacement length profiles along normal faults provides crucial information on fault growth processes here based on satellite imagery and topography we analyze 357 normal faults distributed along the active rift of dabbahu manda hararo dmh afar which offers a unique opportunity to investigate the influence of magmatism on fault growth processes our measurements reveal a large variety of slip profiles that are not consistent with elastic deformation their analysis contributes towards a better understanding of the lateral propagation of faults especially when nucleation points and existence of barriers are included using the fault growth model of manighetti et al 2001 we determine the preferred direction of lateral propagation for each fault our results suggest that lateral propagation of faults is easier away from areas where magma has been stored for long time at crustal depth and has thus modified the thermo mechanical properties of the host rock however these areas correspond also to areas where the initiation of fault growth appears as easiest along the rift in combining these results with the analysis of rift width and the position of magma reservoirs along dmh rift we show that fault growth keeps track of the magma presence and or movement in the crust

does shallow dike intrusion and widening remain a possible mechanism for graben formation on mars shallow dike intrusion and widening was proposed several decades ago as a cause of surface faulting and graben formation on earth this hypothesis was subsequently applied to the spectacular linear and or radial graben systems visible on mars however a recent study has suggested that shallow dike intrusion on mars results in contractional folding and uplift adjacent to dike walls rather than extensional faulting and subsidence above the dike tip even in the presence of concurrent regional extension here discrete element numerical modeling is used to re examine the typical style of deformation above the shallow tips of widening dikes on mars the dikes are embedded in a frictional cohesive material representing the martian crust in the experiments presented here subsidence and extensional faulting graben formation are produced above the dike tips even with modest amounts of widening for suggested depths to dike tips on mars an overlying graben is produced in all cases no upright detachment style folds are produced results indicate that dike widening does indeed remain a possible mechanism for graben formation on mars implications for the interpretation of deformation associated with shallow dike intrusion on earth and distant planetary surfaces are discussed

some mineral physics constraints on the rheology and geothermal structure of earth s lower mantle we explore the implications of recent mineral physics measurements of diffusion coefficients and melting temperatures of lower mantle materials on the rheological and geothermal structure of earth s lower mantle we show that mgsio3 perovskite is significantly stronger than mgo periclase and therefore the rheology of the lower mantle depends strongly on the geometry of a weaker phase periclase we calculate viscosities of the lower mantle for two cases 1 where periclase occurs as isolated grains and 2 where periclase occurs as continuous films using mineral physics data and models of two phase rheology we find that the effective viscosity for the former is about 10 1000 times higher than the latter we therefore suggest that the rheology of the lower mantle is structure and hence strain dependent leading to weakening at large strains due to the formation of continuous films of periclase overall depth variation of viscosity depends not only on the pressure dependence of creep but also on the geothermal gradient both mgsio3 perovskite and periclase have relatively small activation energies e g r t m with g 10 14 where r is the gas constant and t m is melting temperature and therefore the depth variation of viscosity is rather small even for a nearly adiabatic temperature gradient however the geothermal gradients consistent with the geodynamical inference of nearly depth independent viscosity are sensitive to the pressure dependence of viscosity which is only poorly understood a superadiabatic gradient of up to 0 6 k km is also consistent with mineral physics and geodynamical observations

simulating the thermochemical magmatic and tectonic evolution of venus s mantle and lithosphere two dimensional models 1 numerical convection models of the thermochemical evolution of venus are compared to present day topography and geoid and recent resurfacing history the models include melting magmatism decaying heat producing elements core cooling realistic temperature dependent viscosity and either stagnant lid or episodic lithospheric overturn in stagnant lid convection the dominant mode of heat loss is magmatic heat pipe which requires massive magmatism and produces very thick crust inconsistent with observations partitioning of heat producing elements into the crust helps but does not help enough episodic lid overturn interspersed by periods of quiescence effectively loses venus s heat while giving lower rates of volcanism and a thinner crust calculations predict 5 8 overturn events over venus s history each lasting 150 myr initiating in one place and then spreading globally during quiescent periods convection keeps the lithosphere thin magmatism keeps the mantle temperature constant over venus s history crustal recycling occurs by entrainment in stagnant lid convection and by lid overturn in episodic mode venus like amplitudes of topography and geoid can be produced in either stagnant or episodic modes with a viscosity profile that is earth like but shifted to higher values the basalt density inversion below the olivine perovskite transition causes compositional stratification around 730 km breakdown of this layering increases episodicity but far less than episodic lid overturn the classical stagnant lid mode with interior temperature rheological temperature scale lower than tcmb is not reached because mantle temperature is controlled by magmatism while the core cools slowly from a superheated start core heat flow decreases with time possibly shutting off the dynamo particularly in episodic cases

mechanisms for the generation of plate tectonics by two phase grain damage and pinning abstract weakening and shear localization in the lithosphere are essential ingredients for understanding how and whether plate tectonics is generated from mantle convection on terrestrial planets we present a new theoretical model for the mechanism of lithospheric weakening and shear localization and hence plate generation through damage grain evolution and zener pinning in two phase polycrystalline lithospheric rocks grain size evolves through the competition of coarsening which drives grain growth with damage which drives grain reduction however in a two phase medium the interface between phases induces zener pinning which impedes grain growth and facilitates damage the size of the pinning surfaces is given by the roughness of the interface and damage to the interface causes smaller pinning surfaces which in turn drive down the grain size forcing the rheology into the grain size dependent diffusion creep regime this process allows damage and rheological weakening to co exist which is normally considered impossible in single phase assemblages moreover pinning greatly inhibits grain growth and shear zone healing which is much faster in single phase materials hence the resulting shear localization is rapid less than 1 myr but the healing time for a dormant weak zone is very slow greater than 100 myrs these effects therefore permit rapidly forming and long lived plate boundaries the model therefore provides a key ingredient and predictive theory for the generation of plate tectonics on earth and other planets

in situ infiltration test using a reclaimed abandoned river bed managed aquifer recharge in shijiazhuang city china managed aquifer recharge mar is necessary for water resources management in arid and semiarid regions infiltration rate is often a decisive limiting factor in site selection for mar in order to avoid scale effects in the application of infiltration rate parameters the largest in situ infiltration test in china was undertaken between august 19 and august 30 2009 to measure the infiltration rate of the field selected for mar in shijiazhuang city china the in situ test lasted for 10 days and about 1 82 107 m3 of water was introduced into the infiltration field groundwater level variations were monitored during the test monitoring showed that the infiltration rate of surface water was 1 5 m day which means that about 10 15 108 m3 a of water could be injected into the target aquifer also groundwater level variations showed that the northern part of the infiltration field had a higher infiltration rate as predicted and the test result supplied a sound foundation for validation of the groundwater numerical simulation which will be of benefit for future predictions of the response of the groundwater level to artificial recharge engineering finally an artificial recharge plan was proposed based on the infiltration test results and the water source conditions which would be useful for the development of mar programs and management of local water resources

fracture movements and graben subsidence during the 2014 barðarbunga dike intrusion in iceland abstract a dike propagated 48 km from the bar arbunga central volcano into one of its fissure swarms in august and early september 2014 the dike intrusion was accompanied by caldera subsidence in the ice covered bar arbunga central volcano as well as a propagating earthquake swarm graben subsidence and fissure eruptions in the fissure swarm most of the dike was emplaced in the glacier covered part of the fissure swarm but a 10 km long section at the distal northern end of the dike is ice free new ground fractures were observed in the ice free part during the dike propagation and prior to the outbreak of a major lava producing eruption the fractures delineate two graben structures in direct continuation of each other here we show the extent and depth of the grabens and maps of fractures and eruptive fissures that were formed or reactivated north of the vatnajokull glacier during the dike intrusion the active graben subsidence was identified 3 days after seismicity reached the ice free area then fractures delineated a 700 1000 m wide and 5 km long graben visible from the northern edge of the vatnajokull glacier farther north a narrower 250 450 m wide graben was seen a short lived minor eruption occurred between these grabens on the 29th of august 2014 2 days after the grabens were identified the northern graben was extended southwards during or before the eruption and included the eruptive fissure on the 31st of august 2014 a much larger eruption began on the same eruptive fissure the fissure quickly obtained its maximum length of 2 km this eruption lasted 6 months a minor eruption lasting 2 days or less occurred on the southern graben on the 5th of september 2014 the first two eruptions took place on the same eruptive fissure that formed the pre existing holuhraun lava presumably in the 18th century measurements in january 2015 show that the southern graben was 3 5 5 5 m deep but the northern graben had been covered by lava from the depth and the opening of the graben we estimate that the dip of the boundary faults of the graben is 75 the narrow grabens observed before the eruptions indicated that magma was at shallow depth likely much less than 300 m 2 days before it opened to the surface such observations can serve as a tool to estimate likelihood of eruptions during dike intrusions

insights of dyke emplacement mechanics from detailed 3d dyke thickness datasets abstract dyke thickness datasets offer new insights into the detailed 3d geometry of dyke swarms and an exceptional opportunity to evaluate theoretical emplacement models the swartruggens kimberlite dyke swarm extends over 7 km along strike and intrudes a dolerite quartzite shale and andesitic lava succession the star kimberlite dykes cut shales and sandstones intersect a large dolerite sill and extend 15 km along strike both dyke swarms comprise anastomosing en echelon segments each several hundred metres long in total 1532 swartruggens dyke thickness measurements were taken to 750 m below the surface over a 250 m depth range and 3354 star dyke thickness measurements were taken over a 520 m depth range the swartruggens dyke thicknesses are 0 05 1 95 m mean 0 64 m whereas the star kimberlites range from

dike induced faulting in rift zones of iceland and afar geodetic data and field observations demonstrate that the emplacement of dikes in volcanic rift zones frequently generates normal faulting and graben subsidence at the earth9s surface elastic modeling of the vertical ground surface displacements above dikes and faults indicates that the extent of graben subsidence can be achieved only if fault slip extends virtually to or beyond the dike plane at depth a mechanical model that includes dikes and frictional faults shows that dike opening tends to compress and lock faults located to either side of the dike therefore slip extending into or beyond the dike cavity must occur either 1 on faults that intersect the dike near its top above the zone of dike induced compression or 2 on faults that slip ahead of the dike as it propagates laterally data from iceland indicate that slip occurred on deep faults that presumably slipped in advance of the laterally propagating dike

evolution of a lateral dike intrusion revealed by relatively relocated dike induced earthquakes the 2014 15 barðarbunga holuhraun rifting event iceland abstract understanding dikes is vital as they serve both as bodies that build the crust and as conduits that feed eruptions and must be monitored to evaluate volcanic hazard during the 2014 15 bar arbunga rifting event iceland intense seismicity accompanied the intrusion of a 50 km lateral dike which culminated in a 6 month long eruption we here present relocations of earthquakes induced by the lateral dike intrusion using cross correlated sub sample relative travel times the 100 m spatial resolution achieved reveals the complexity of the dike propagation pathway and dynamics jerky segmented and allows us to address the precise relationship between the dike and seismicity with direct implications for hazard monitoring the spatio temporal characteristics of the induced seismicity can be directly linked in the first instance to propagation of the tip and opening of the dike and following this after dike opening indicate a relationship with magma pressure changes i e dike inflation deflation followed by a general post opening decay seismicity occurs only at the base of the dike where dike imposed stresses combined with the background tectonic stress from regional extension over 200 yr since last rifting are sufficient to induce failure of pre existing weaknesses in the crust while the greatest opening is at shallower depths emplacement oblique to the spreading ridge resulted in left lateral shear motion along the distal dike section studied here and a prevalence of left lateral shear failure fault plane strikes are predominately independent of the orientation of lineations delineated by the hypocenters indicating that they are controlled by the underlying host rock fabric this high resolution study provides unprecedented opportunity for comparison with both geodetic and field frozen dike observations and development and consolidation of analytical and analogue models with implications for rifting processes and real time monitoring of magma intrusion

the isostatic stagnant lid approximation and global variations in the venusian lithospheric thickness we use the long wavelength venusian topography and geoid to constrain the global lithospheric thickness by assuming that the stagnant lid overlying the convective mantle is in isostatic equilibrium the isostatic stagnant lid isl approximation two dimensional and three dimensional numerical simulations of convection show that the isl approximation is reasonably accurate in the tested parameter range of temperature dependant viscosity convection with bottom heating and internal heating the errors in predicting the thickness of the stagnant lid i e the lithosphere are somewhat larger for internally heated convection we attribute this to a flatter lid and as a result to a larger relative contribution to topography and geoid anomalies from the deeper parts of the layer application of the isl approximation to venus suggests that the average lithospheric thickness might be as large as 600 km which is on the high end of previously proposed estimates

self consistent generation of tectonic plates in time dependent three dimensional mantle convection simulations 1 presented here are self consistent three dimensional simulations of mantle convection some of which display an approximation of plate tectonic behavior that is continuous in space and time plate behavior arises through a reasonable material description of silicate deformation with a simple yield stress being sufficient to give first order plate like behavior however the required yield strength or fault frictional coefficient is much less than experimentally determined values toroidal poloidal ratios are within geologically observed limits the sensitivity of the system to yield strength and the form of strength envelope is systematically investigated optimum plate character is obtained in a narrow range of yield strength below which diffuse boundaries and above which episodic behavior and eventually a rigid lid are observed models with mobile lids develop very long wavelength horizontal structure the longest wavelength possible in the domain two dimensional models display much greater time dependence than three dimensional models

resurfacing events on venus implications on plume dynamics and surface topography 1 global resurfacing events explain the relatively young and uniformly aged surface of venus in numerical models featuring these events a variety of plume classes occur ranging from strong stable to smaller wind driven upwellings we investigate this plume dynamics in a 3d cartesian geometry paying special attention to the surface topography while the stable upwellings of the stagnant lid phase correlate with an elevation of the surface the wind driven upwellings of the resurfacing phase do not necessarily correlate with a positive topography signal we rather observe that a thick plate can dominate the topography above upwellings

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

modelling of surface stresses and fracturing during dyke emplacement application to the 2009 episode at harrat lunayyir saudi arabia abstract correct interpretation of surface stresses and deformation or displacement during volcanotectonic episodes is of fundamental importance for hazard assessment and dyke path forecasting here we present new general numerical models on the local stresses induced by arrested dykes in the models the crustal segments hosting the dyke vary greatly in mechanical properties from uniform or non layered elastic half spaces to highly anisotropic layers with strong contrast in young s modulus the shallow parts of active volcanoes and volcanic zones are normally highly anisotropic and some with open contacts the numerical results show that for a given surface deformation non layered half space models underestimate the dyke overpressure thickness needed and overestimate the likely depth to the tip of the dyke also as the mechanical contrast between the layers increases so does the stress dissipation and associated reduction in surface stresses and associated fracturing in the absence of open contacts the distance between the two dyke induced tensile and shear stress peaks and fractures if any at the surface is roughly twice the depth to the tip of the dyke the width of a graben if it forms should therefore be roughly twice the depth to the tip of the associated arrested dyke when applied to the 2009 episode at harrat lunayyir the main results are as follows the entire 3 7 km wide fracture zone graben formed during the episode is far too wide to have been generated by induced stresses of a single arrested dyke the eastern part of the zone graben may have been generated by the inferred arrested dyke but the western zone primarily by regional extensional loading the dyke tip was arrested at only a few hundred metres below the surface the estimated thickness of the uppermost part of the dyke being between about 6 and 12 m for the inferred dyke length strike dimension of about 14 km this yields a dyke length thickness ratio between 2400 and 1200 similar to commonly measured ratios of regional dykes in the field

mantle flow tectonics and the origin of ishtar terra venus it is proposed that western ishtar terra formed due to compression and crustal thickening above a cylindrical mantle downwelling a model for crustal deformation due to downwelling successfully reproduces many observed characteristics of western ishtar although axisymmetric downwellings occur in numerical models of constant viscosity mantle convection there is no evidence for their existence in earth s mantle where downwellings are sheet like either modes of downflow in venus and earth are fundamentally different or differences in near surface conditions and material behavior selectively emphasize surface expressions of the different downwelling modes

heat transport efficiency for stagnant lid convection with dislocation viscosity application to mars and venus mantle convection on mars and venus is likely to occur in the regime known as stagnant lid convection we perform thermal boundary layer analyses as well as finite element simulations of stagnant lid convection with non newtonian viscosity which is believed to be more appropriate for the lithosphere and upper mantle and discuss one particular application of the results the efficiency of heat transport on the terrestrial planets as in the case of newtonian viscosity the efficiency of heat transfer in the stagnant lid regime is extremely low compared to plate tectonics for example in the absence of plate tectonics the mantle temperature on earth which is already close to the solidus would be about 700 1500 k higher for the present day value of the surface heat flux for venus the critical heat flux which can be removed without widespread melting is only 10 20 m w m2 for mars it is 15 30 m w m2 therefore there are no doubts that in the absence of mobile plates the mantle temperature would significantly exceed solidus during planetary evolution it is hypothesized that this could cause one or a combination of two possible processes 1 differentiation of radiogenic isotopes into the crust during early planetary magmatism and 2 initiation of some kind of plate tectonics as a result of plate weakening due to melting

fault and graben growth along active magmatic divergent plate boundaries in iceland and ethiopia recent studies highlight the importance of annual scale dike induced rifting episodes in developing normal faults and graben along the active axis of magmatic divergent plate boundaries mdpb however the longer term 102 105 years role of diking on the cumulative surface deformation and evolution of mdpb is not yet well understood to better understand the longer term normal faults and graben along the axis of mdpb we analyze fissure swarms in iceland and ethiopia we first focus on the simplest case of immature fissure swarms with single dike fed eruptive fissures these consist of a 1 km wide graben bordered by normal faults with displacement up to a few meters consistent with theoretical models and geodetic data a similar structural pattern is found with asymmetric and multiple graben within wider mature fissure swarms formed by several dike fed eruptive fissures we then consider the lateral termination of normal faults along these grabens to detect their upward or downward propagation most faults terminate as open fractures on flat surface suggesting downward fault propagation this is consistent with recent experiments showing dike induced normal faults propagating downward from the surface however some normal faults also terminate as open fractures on monoclines which resemble fault propagation folds this suggests upward propagation of reactivated buried faults promoted by diking these results suggest that fault growth and graben development as well as the longer term evolution of the axis of mdpb may be explained only through dike emplacement and that any amagmatic faulting is not necessary

effect of host rock rheology on dyke shape thickness and magma overpressure the size and thickness of dykes is of fundamental importance for volcano dynamics because dykes are the primary path for magma transport and because large numbers of dykes often comprise a major p

mountains and calderas on io possible implications for lithosphere structure and magma generation abstract the combination of voyager images and newly acquired galileo images with low illumination and resolutions ranging from 2 to 6 km pixel now allows determination of the global distribution of mountains and volcanic centers on io the mountains generally do not have characteristics typical of terrestrial volcanic landforms they are evenly distributed across the surface and show no obvious correlation with known hot spots or plumes relative elevations determined by shadow measurements and stereoscopy indicate that mountains in the newly imaged area range in elevation up to at least 7 6 km the origin of the mountains remains uncertain some appear to be multitiered volcanic constructs others enclosing the partial remains of large circular depressions appear to be remnants of old volcanoes yet others show extensive tectonic disruption volcanic centers also appear to be distributed evenly across the surface except for an apparently somewhat lower density at high latitudes the low latitudes have one volcanic center per 7 10 4 km 2 and on average the centers are spaced roughly 250 km apart the global distribution of high mountains suggests that the lithosphere over most of io is thick although the thickness cannot be calculated the previously suggested 30 km appears reasonable as a lower limit the high rates of resurfacing combined with the likely dissipation of most of the tidal energy in the asthenosphere and underlying mantle implies a very low temperature gradient in the upper part of the lithosphere and steep gradients in the lower lithosphere the slow rate of separation of melt from host rock in the magma source regions as a consequence of the low gravity on io coupled with the high rate of magma production will likely result in larger melt fractions than is typical for source regions on earth the variety of volcanic landforms suggests that volcanic products with a range of compositions are deposited on the surface this mixture will be carried downward through the lithosphere as a consequence of the 0 5 1 5 cm yr resurfacing rates during descent the more volatile components will tend to be driven off early but complete or near complete melting at the base of the lithosphere may result in rehomogenization of the silicate mixture that remains

basaltic accumulation instability and chaotic plate motion in the earliest mantle inferred from numerical experiments 1 a series of self consistent numerical models of mantle convection with magmatism and moving plates are presented to clarify the dynamics of the strongly heated mantle of the earliest earth broad hot accumulations of subducted basaltic crusts develop on the core mantle boundary and remain there for geologic time only when the internal heating rate is below a threshold above the threshold the basaltic accumulations become unstable instead the thermal buoyancy induced by the strong internal heating repeatedly causes a massive upwelling or burst of hot materials from deep mantle the mantle burst induces vigorous magmatic activity on the surface the magmatic activity in turn damages the lithosphere and disintegrates it into smaller fragments in contrast to the well ordered plate motion observed below the threshold the lithospheric fragments chaotically move and new subduction zones sporadically develop the mantle burst and chaotic motion of lithospheric fragments effectively stir the mantle and the mantle remains chemically rather homogeneous in spite of the chemical differentiation due to the vigorous magmatism the earth s mantle convection has probably changed its regime from one dominated by frequent mantle bursts and chaotic motion of lithospheric fragments to one dominated by stable basaltic accumulations and well ordered plate motion as the internal heat source decays the suggested regime transition fits in with the earth s tectonic evolution inferred from geologic observations of the continents of various ages

dike induced stresses and displacements in layered volcanic zones abstract during a volcanic unrest period with dike injection one of the main scientific tasks is to assess the geometry and the propagation path of the dike and in particular the likelihood of the dike reaching the surface to erupt currently the dike path and geometry including depth and opening aperture are both partly determined from geodetic surface data using mostly dislocation models that assume the volcanic zone volcano to be an elastic half space of uniform mechanical properties by contrast field observations of volcanic zones volcanoes active and extinct show that they are composed of numerous layers whose mechanical properties primarily young s modulus vary widely and whose contacts commonly arrest dikes here we provide field observations and numerical models on the effects of a typical variation in young s modulus in an active volcanic zone on the internal and surface stresses and displacements induced by a dike whose tip is arrested at 0 5 km depth below the surface of the volcanic zone above the layer or unit hosting the dike are four layers of equal thickness we vary the young s modulus or stiffness of the fourth layer the one adjacent to the layer or unit hosting the dike from 10 gpa to 0 01 gpa while all the other layers units maintain their young s moduli in the model runs the results show that as the fourth layer becomes more compliant or soft 0 1 0 01 gpa dike induced stresses and displacements lateral and vertical above the layer including those at the surface become suppressed but the stresses and displacements of the layer unit hosting the dike increase and their peaks do not coincide in location or magnitude with those of the other layers thus the dike induced internal deformation of the volcanic zone increases as the fourth layer becomes softer also the tensile and shear stress peaks at the surface occur at locations widely different from those of maximum surface uplift more specifically for a comparatively stiff fourth layer 1 10 gpa the surface tensile and shear stresses peak at lateral distances of 0 5 0 7 km from the projection of the dike to the surface essentially no tensile shear stresses reach the surface when the fourth layer is as soft as 0 1 0 01 gpa so that there are no stress peaks by contrast the maximum surface displacements uplift peak at lateral distances of 2 8 3 3 km from the dike projection to the surface if tension fractures or faults in particular the boundary faults of a graben are induced by the dike they should form at the tensile shear stress peaks and not as is commonly suggested at the location of the surface displacement peaks our results thus suggest that any dike induced graben is likely to be of a width about twice the depth to the tip top of the arrested dike the results demonstrate that elastic half space models overestimate the dike induced surface stresses and thus the depth to the tip top of the associated dike in particular the models presented here indicate that for typical dikes little or no dike induced surface deformation would be expected until the dike tip propagates to depths below the surface of less than a kilometre

mantle phase transitions and layered chaotic convection we report preliminary results from a sequence of very high resolution simulations of the mantle convection process based upon an axisymmetric anelastically compressible spherical model with both olivine spinel and spinel post spinel phase transitions our analyses strongly suggest that at earth like rayleigh numbers near 107 the circulation may be quite strongly layered although a very long timescale transient exists that is characterized by brief excursions into the whole mantle state we suggest the use of a new diagnostic tool that may be employed to determine whether or not the layered state is characteristic of the present day earth

groundwater recharge in semi arid carbonate aquifers under intensive use the estepa range aquifers seville southern spain quantifying groundwater recharge in carbonate aquifers located in semi arid regions and subjected to intensive groundwater use is no easy task one reason is that there are very few available methods suitable for application under such climatic conditions and moreover some of the methods that might be applied were originally designed with reference to non carbonate aquifers in addition it is necessary to take into account the fact that in any given aquifer groundwater recharge is modified by the groundwater exploitation here we focus on four methods selected to assess their suitability for estimating groundwater recharge in carbonate aquifers affected by intensive exploitation the methods were applied to the estepa range aquifers of seville southern spain which are subjected to different degrees of exploitation two conventional methods were used chloride mass balance and daily soil water balance these results were compared with the results obtained by means of two non conventional methods designed for application to the carbonate aquifers of southern spain the aplis and eras methods the results of the different methods are analogous comparable to those obtained in nearby non exploited carbonate aquifers confirming their suitability for use with carbonate aquifers in either natural or exploited regimes in a semi arid climate

graben formation and dike arrest during the 2009 harrat lunayyir dike intrusion in saudi arabia insights from insar stress calculations and analog experiments

alluvial aquifer recharge enhanced by a natural dam feasibility assessment based on multidisciplinary characterization khuzestan southwest iran in the kushkak valley khuzestan southwest iran an anticlinal structure has partially impounded an ephemeral stream this natural impounded area has been chosen for an artificial recharge site due to its current rate of recharge capability to store water and favorable situation for the construction of man made barriers to stream flow the aquifer to be recharged is the kushkak unconfined aquifer which consists of medium to coarse grained alluvial deposits that overlie consolidated conglomerate rock in this semi arid area with infrequent relatively heavy falls of rain alluvial aquifer recharge can be an important process that sustains shallow over exploited groundwater bodies in this investigation a multidisciplinary approach including hydrometerological studies and a detailed hydrogeochemical survey have been carried out other essential prerequisite parameters for the scheme were also taken into account to determine the suitability of this location for groundwater artificial recharge the assessment has brought out that 1 the proposed reservoir will conserve a major part of the water being lost 2 annual runoff of about 0 27 mcm can be injected into the aquifer through recharge from impounded water 3 hydrochemical data from surface water and from the kushkak aquifer water demonstrates that dilution and change in compositional trend in the groundwater proximal to the impounded alluvial bed areas would be expected based on the infiltration capacity of this site and 4 cost benefit ratio of the project is 1 2 and it is assumed to recover the investment within six years

small mantle upwellings are pervasive on venus and earth observations of venus and africa an analogous terrestrial plate indicate that upwellings a few hundred kilometers across are pervasive in the ambient upper mantle of both venus and earth because of their small size when they reach the lithosphere the upwellings affect the surface only under special circumstances on venus they form volcano tectonic structures called coronae where the lithosphere has been thinned by rifting on earth the small upwellings produce topographic swells where they impinge on a plate stationary with respect to the hot spot reference frame as is africa the small upwellings presence throughout the mantle explains both the corona paradox in which coronae are associated with rifts on venus but do not appear to drive rifting and the basin and swell topography of the african plate the morphology of coronae requires that the small upwellings have some diapiric properties their pervasiveness transient nature and generally small size make them fundamentally different from traditional hawaiian style plumes and they may be detached diapirs within a mantle convecting in the hard turbulence regime

recent hotspot volcanism on venus from virtis emissivity data the questions of whether venus is geologically active and how the planet has resurfaced over the past billion years have major implications for interior dynamics and climate change nine hotspots areas analogous to hawaii with volcanism broad topographic rises and large positive gravity anomalies suggesting mantle plumes at depth have been identified as possibly active this study used variations in the thermal emissivity of the surface observed by the visible and infrared thermal imaging spectrometer on the european space agency s venus express spacecraft to identify compositional differences in lava flows at three hotspots the anomalies are interpreted as a lack of surface weathering we estimate the flows to be younger than 2 5 million years and probably much younger about 250 000 years or less indicating that venus is actively resurfacing

modeling of transient water table response to managed aquifer recharge a lagoon in muscat oman natural and anthropogenic conditions in the gulf countries lead to substantial changes in groundwater gw dynamics gw mound under a lagoon supplied by treated wastewater tww from a municipal sewage treatment plant stp in muscat oman is studied using analytical and numerical modeling techniques the water table mound in an unconfined aquifer laterally bounded by the sea receives recharge from a lagoon fed by a stp in a seasonally varying regime the onset and duration of the filling period and correspondingly infiltration from the lagoon bed is dictated by the wastewater processing and tww use following the period of recharge induced growth the gw mound decays when recharge ceases and the void space above the water table becomes available for future injection in the next annual cycle of infiltration and tww subsurface storage the green ampt model application illustrates that the first phase of the process vertical infiltration with front propagating from the lagoon bed to the regional flat water table is typically short several hours the second phase when tww gets in a direct hydraulic contact with gw is the longest and with a mound spreading to hundreds of meters during winter months no analytical solutions to this moving boundary value problem are known and approximate models based on the boussinesq approximation and its linearizations are utilized the hantush type drawup of the water table in the vicinity of the lagoon is studied by the watson analytical solution to the diffusion equation for a vertically averaged hydraulic head in an unbounded domain with an instantaneous jump of the head over a disk shaped zone this drawup is compared with modflow simulations which take into account a finite size of the flow domain catchment and discharge into the sea through a constant head segment the mishra guyonnet s formula assumes a spatially uniform flux from the lagoon bed disk in a model plain into a growing mound and is used for calculation of the total discharge which progressively decreases from the early stages of the green ampt infiltration to a theoretically zero value if the second phase lasts indefinitely long we assess characteristic times and phases of the mound dynamics during the rise and fall stages using regional hydrogeological parameters site geometry and typical patterns of wastewater disposal sensitivity analysis of the locus of the phreatic surface at a given instance and location is carried out with storativity hydraulic conductivity undisturbed aquifer thickness and lagoon radius as input variables for an example of an aquifer of saturated depth of 40 m undisturbed vadose zone thickness of 10 m saturated hydraulic conductivity of 10 m day effective porosity of 0 2 a circular lagoon size of 90 m and water depth there of 1 m we found that the infiltration front propagates to the water table in less than 0 2 days after 40 days of a continuous mound growth in a hypothetical piezometer located 400 m from the lagoon the water table drawup was in the range of 2 0 2 7 m if the aquifer hydraulic parameters and lagoon size are varied 20 from the average value this approach will permit to plan more detailed site investigation facilitate management decisions on the rate of water release to the lagoon and foresee waterlogged areas

isostatic compensation of equatorial highlands on venus abstract spherical harmonic models for venus global topography and gravity incorporating magellan data are used to test isostatic compensation models in five 30 30 regions representative of the main classes of equatorial highlands the power spectral density for the harmonic models obeys a power law scaling with spectral slope 2 brown noise for the topography and 3 kaula s law for the geoid similar to what is observed for earth the venus topography spectrum has lower amplitudes than earth s which reflects the dominant lowland topography on venus observed degree geoid to topography ratios gtrs on venus are significantly smaller than degree gtrs for uncompensated topography indicative of substantial compensation assuming a global airy compensation most of the topography is compensated at depths greater than 100 km suggesting a thick lithosphere on venus for each region considered we obtain a regional degree of compensation c from a linear regression of bouguer anomaly versus bouguer gravity data geoid anomaly n versus topography variation h data for each sample were compared in the least squares sense to theoretical correlations for pratt airy and thermal thinning isostasy models yielding regional gtr zero elevation crustal thickness h and zero elevation thermal lithosphere thickness y l 0 respectively we find the regional compensation to be substantial c 52 90 and the h n data correlations in the chosen areas can be explained by isostasy models applicable on the earth and involving variations in crustal thickness airy and or lithospheric thermal thinning thickness however a thick crust and lithosphere y l 0 300 km must be assumed for venus compensation mechanisms are distributed as follows for the high plateaus of ovda gtr 9 m km and thetis gtr 11 m km mainly airy isostasy with h 50 60 km for the chasmata area sample gtr 15 m km airy with perhaps a thermal component finally a very strong thermal component for the broad swells of beta gtr 32 m km and atla gtr 25 m km with the lithosphere thinned to 100 km aphrodite terra which shows a marked eastward increase in pratt and airy model parameter values could be a candidate for thermal isostasy decay from east to west

climate and interior coupled evolution on venus climate interior coupled evolution is investigated for venus by merging a partial melting parameterized mantle convection model with a gray radiative convective atmospheric model a positive feedback process can operate by the release of water to the atmosphere via mantle melting leading to an increase in atmospheric opacity and the radiative temperature gradient the resulting amplification of the greenhouse surface temperature raises the mantle temperature leading to an increase in the partial melting rate using thin lid convection a coupled model for venus running over an interval of 3 to 1 ga shows a significant increase in surface temperature partial melting extent and extrusive magma flux compared to a model where there is no communication between greenhouse modulated surface temperature and partial melting in the interior coupled and uncoupled models that transition with variable timing to stagnant lid convection either shut down partial melting via lithospheric thickening or evolve to large amounts of melt due to increased interior temperatures

stagnant lid convection on venus the effect of strongly temperature dependent viscosity on convection in the interior of venus is studied systematically with the help of finite element numerical models for viscosity contrasts satisfying experimental constraints on the rheology of rocks venus is likely to be in the regime of stagnant lid convection this regime is characterized by the formation of a slowly creeping very viscous lid on top of the mantle venusian lithosphere and is in agreement with the tectonic style observed on venus stagnant lid convection explains large geoid to topography ratios on venus by the thermal thinning of a thick lithosphere the thickness of the lithosphere can be as large as 400 550 km for beta regio and 200 400 km on average geoid and topography data and experimental data on the rheology of rocks provide constraints on the viscosity of the mantle 10 20 10 21 pa s the convective stresses in the interior 0 2 0 5 mpa the stresses in the lid 100 200 mpa the velocity in the interior 0 5 3 cm yr 1 and the heat flux beneath the lithosphere 8 16 mw m 2 parameterized convection calculations of thermal history of venus are difficult to reconcile with a thick present day lithosphere however a sufficiently thick lithosphere can be formed if a convective regime with mobile plates was replaced by stagnant lid convection around 0 5 b y ago one of the possible explanations for the cessation of venusian plate tectonics is that during the evolution of venus stresses in the lid dropped below the yield strength of the lithosphere this model predicts a drastic drop in the heat flux thickening of the lithosphere and suppression of melting and is consistent with the hypothesis of cessation of resurfacing on venus around 0 5 b y ago

venus crater distribution and plains resurfacing models detailed analysis of the distribution of craters on venus using mth nearest neighbor analysis coupled with models based upon surface morphology constraints indicates that the hypothesis of complete spatial randomness csr cannot be rejected but is not a unique model of the observed crater distribution based on morphologic mapping the extensive volcanic plains can be divided into four units that have a spread in age of the order of 0 5t the mean surface age of the planet this four unit plains model along with its derivatives produce test statistics that indicate such models also cannot be rejected further the probability of obtaining a result at least as extreme as the observed test statistic given that the null hypothesis model corresponds to venus is true is lowest for the csr model there is no particular reason to pick a csr model along with its implications for catastrophic resurfacing as a constraint on the evolution of venus and there are geological reasons to choose the multiage models we find that we cannot distinguish statistically among models that have two three or four distinct production ages within the plains however the hypothesis that the variation in crater density within all of the plains is due to a single random process can be rejected for two reasons first the binomial probability that such a process could exist within each of the plains units is 0 05 except the smallest and youngest unit pl1 second using a chi squared statistic to test the hypothesis that four plains units have the same age gives a p value of 10 4 indicating confident rejection of the hypothesis thus csr cannot be used as a constraint on models of resurfacing or planetary evolution of venus because of the non uniqueness in matching such a model to the observed crater distribution and the strong indication of distinct ages within the plains with a significant spread in age geological and geophysical constraints provide our best clues for understanding venus

rift topography linked to magmatism at the intermediate spreading juan de fuca ridge new seismic observations of crustal structure along the juan de fuca ridge indicate that the axial rift topography reflects magma induced deformation rather than alternating phases of magmatism and tectonic extension as previously proposed contrary to predictions of the episodic models crustal magma bodies are imaged beneath portions of all ridge segments surveyed at average depths of 2 1 2 6 km the shallow rift valley or axial graben associated with each juan de fuca segment is 50 200 m deep and 1 8 km wide and is well correlated with a magma body in the subsurface analysis of graben dimensions height and width shows that the axial graben narrows and graben height diminishes where the magma body disappears rather than deepening and broadening as expected for rift topography due to tectonic extension we propose an evolutionary model of axial topography that emphasizes the contribution of dike intrusion to subsidence and fault slip at the seafloor in this model an evolving axial topography results from feedbacks between the rheology of the crust above a magma sill and dike intrusion rather than episodic magma delivery from the mantle

melt segregation from partially molten source regions the importance of melt density and source region size the compressibility of basic melt at 1 atmosphere is about an order of magnitude higher than that of mantle minerals consequently the density contrast between melt and the principal residual crystals in mantle source regions is expected to decrease with increasing source region depth the increasingly olivine normative character of primary melts produced at greater depths is also expected to result in a decrease in this density contrast with increasing source region depth once vertical permeability is established by melt generated during partial melting buoyancy driven melt percolation can under some circumstances segregate melt from the residual crystals in its source region on a geologically rapid time scale limits to this process are provided by cooling of the source region freezing melt in and rigidity of the crystalline matrix mechanically trapping melt source region size influences these limits strongly consequently small partially molten diapirs km in diameter may be able to trap large melt fractions 30 but larger source regions would be unable to do so the reduction in density contrast with pressure reduces the buoyant force driving melt percolation and provides another limit to melt segregation diapirs at depth may thus stably contain large fractions of melt but may decompress and unload their melt during ascent this effect would be enhanced in small diapirs and may be relevant to the genesis of komatiitic magma melt compression may also be a factor in explaining why the very different maximum depths inferred for typical basic melt segregation from source regions on different planets 500 km on the moon 250 km on mars 100 km on earth correspond to similar pressures 25 35 kbar at greater pressures melt may no longer be capable under ordinary conditions of segregating upwards by buoyancy this may also help to explain why depleted peridotites overlie more fertile peridotites and how deep regions of the mantle are able to remain fertile over geologic time

a conceptual model for the relationship between coronae and large scale mantle dynamics on venus 1 among the most enigmatic features on venus are the more than 500 quasi circular structures collectively referred to as coronae associated volcanism and tectonism suggest that coronae result from small transient mantle plumes although only about 20 have clearly associated positive gravity anomalies indicative of current activity coronae are concentrated within a region defined by the dynamically active highland areas of beta atla and themis regiones bat coronae avoid the lowest or highest topography and geoid subsets of the corona population indicate that the spatial distribution of coronae has evolved from a previously less concentrated global distribution to the current bat concentration recent laboratory experiments on thermal convection at high rayleigh number using fluids with temperature dependent viscosity suggest a straightforward interpretation of these relationships small scale upwellings from a hot lower thermal boundary layer tend to be excluded by longer wavelength downwelling structures from the cold upper boundary layer the relative paucity of coronae in the broad lowland plains regions is consistent with the inference of mantle downwellings beneath the plains coronae are largely excluded from compensated highlands perhaps because the thickened crust is not easily disrupted in these regions laboratory experiments indicate that small transient boundary layer instabilities can coexist with and be focused toward large scale upwellings possibly explaining the spatial concentration of coronae in the bat region thus coronae provide important insights into interacting scales of convective flow in venus s mantle and into the evolution of venus s mantle dynamics direct tests of our model are possible through geological mapping

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

geophysical models for the formation and evolution of coronae on venus coronae are large circular features on venus characterized by an annulus of concentric tectonic features interior fracturing volcanism and generally upraised topography they are suggested to form over sites of mantle upwelling and modified by subsequent gravitational relaxation we examine this proposition using two geophysical models to determine whether and under what conditions these mechanisms can produce the topography and tectonics exhibited by coronae in the magellan altimetry data and radar images our results show that mantle diapirism can produce the domical topography of novae which may be coronae in the earliest stage of formation the model stresses induced at the surface by a mantle diapir imply the formation of radially oriented extensional fracturing as observed in novae the dimensions of novae indicate that the diapirs responsible for them are smaller than about 100 km in radius and that the elastic lithosphere is less than 32 km thick diapirs that have reached the top of the mantle are expected to spread and flatten producing plateaulike rather than domical topography we model a flattened diapir at the top of the mantle and show that it will result in plateaulike uplift the volume of the flattened model diapir is similar to that of the spherical diapirs derived for novae we model gravitational relaxation of isostatically uncompensated plateaus and show that they relax to the topographic forms associated with coronae and that the model stresses are consistent with the development of the annulus of tectonic features around coronae

application of gis techniques to determine areas most suitable for artificial groundwater recharge in a coastal aquifer in southern iran abstract special attention has been paid to artificial groundwater recharge in water resource management in arid and semi arid regions parameters considered in the selection of groundwater artificial recharge locations are diverse and complex in this study factors such as slope infiltration rate depth to groundwater quality of alluvial sediments and land use are considered to determine the areas most suitable for groundwater recharge in a coastal aquifer in the gavbandi drainage basin in the southern part of iran thematic layers for the above parameters were prepared classified weighted and integrated in a gis environment by the means of boolean and fuzzy logic to determine the relationships between geomorphological units and the appropriate sites for groundwater artificial recharge land use and geomorphological maps were developed from satellite images the results of the study indicate that about 12 of the study area is considered as appropriate and 8 moderately appropriate sites for artificial groundwater recharge the relationship between geomorphology and appropriate areas for groundwater recharge indicate that the majority of these areas are located on alluvial fans and pediment units at the reconnaissance stage these geomorphological units can be considered as appropriate sites for artificial recharge in regions with similar characteristics

numerical models of magmatism in convecting mantle with temperature dependent viscosity and their implications for venus and earth numerical models are presented for magmatism in a convecting mantle that contains internal heat source mantle convection is modeled by a convection of binary eutectic material with newtonian temperature dependent rheology driven by thermal compositional and melt buoyancy as well as the buoyancy from the 660 km phase transitions mantle magmatism is modeled by a gravitationally induced permeable flow of magma generated by pressure release melting through matrix magma mostly has the eutectic composition that is basaltic composition the numerical models suggest that magmatism and mantle convection strongly influence each other to form a coupled system when the internal heating is sufficiently strong episodic magmatism takes place to chemically differentiate the mantle basaltic materials occupy deeper part of the lower mantle while magma residue occupies the uppermost mantle compositional buoyancy of the differentiated materials makes mantle convection sluggish except when magmatic activities take place the style of magmatism and mantle convection depends on the viscosity contrast across the cold thermal boundary layer tbl along the top surface boundary when the viscosity contrast is large the fluid in the coldest part of top tbl becomes a stagnant lid and a conspicuous lateral heterogeneity arises in the upper mantle magmatism is induced by hot uprising diapirs originating in the top of the lower mantle the diapirs induce convective circulation within the upper mantle when the lower mantle is rather cold but induce flushing event that is a massive flow across the 660 km phase boundary and the resulting vigorous magmatism when the lower mantle becomes sufficiently hot owing to the internal heating when the viscosity contrast is moderate the fluid in the coldest part of top tbl behaves as a moving lid both the moving lid and hot uprising diapirs induce magmatic activity the magmatic activity due to a moving lid resembles ridge volcanism the mantle of venus and that of the early earth are suggested to have been on the regime modeled here and the magmatic activity by flushing event is compared to the magmatism of volcanic plain formation on venus

recent deformation rates on venus constraints on the recent geological evolution of venus may be provided by quantitative estimates of the rates of the principal resurfacing processes volcanism and tectonism this paper focuses on the latter using impact craters as strain indicators the total postimpact tectonic strain lies in the range 0 5 6 5 which defines a recent mean strain rate of 10 18 10 17 s 1 when divided by the mean surface age interpretation of the cratering record as one of pure production requires a decline in resurfacing rates at about 500 ma catastrophic resurfacing model if distributed tectonic resurfacing contributed strongly before that time as suggested by the widespread occurrence of tessera as inliers the mean global strain rate must have been at least 10 155 s 1 which is also typical of terrestrial active margins numerical calculations of the response of the lithosphere to inferred mantle convective forces were performed to test the hypothesis that a decrease in surface strain rate by at least two orders of magnitude could be caused by a steady decline in heat flow over the last billion years parameterized convection models predict that the mean global thermal gradient decreases by only about 5 k km over this time even with the exponential dependence of viscosity upon temperature the surface strain rate drops by little more than one order of magnitude strongly unsteady cooling and very low thermal gradients today are necessary to satisfy the catastrophic model an alternative uniformitarian resurfacing hypothesis holds that venus is resurfaced in quasi random patches several hundred kilometers in size that occur in response to changing mantle convection patterns under such a model the observed crater strain distribution indicates that about 1 of the planet s surface is tectonically active at any time however this model requires a very weak crustal rheology to achieve surface velocities 100 mm yr appropriate to the required patch size without well developed lateral weak zones venus is essentially a one plate planet but one in which the lithosphere is able to respond to topography produced by mantle convection through faulting and limited horizontal movement the net rate of tectonic activity is logarithmically intermediate between earth and mars about 100 times slower than plate tectonics but up to 100 times faster than planets where tectonic stress arises largely from lithospheric cooling and contraction

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

planforms of self consistently generated plates in 3d spherical geometry 1 in the past decade several studies have documented the effectiveness of plastic yielding in causing a basic approximation of plate tectonic behavior in mantle convection models with strongly temperature dependent viscosity strong enough to form a rigid lid in the absence of yielding the vast majority of such research to date has been in either two dimensional or three dimensional cartesian geometry in the present study mantle convection calculations are performed to investigate the planform of self consistent tectonic plates in three dimensional spherical geometry the results are compared to those of similar calculations where a three dimensional cartesian geometry is used we found that when yield stress of the lithosphere is low 20 mpa a great circle subduction zone forms at low intermediate yield stresses 100 mpa plates spreading centers and subduction zones formed and were destroyed over time at high intermediate yield stresses 200 mpa two plates form separated by a great circle boundary that is a spreading centre on one side and a subduction zone on the other side at high yield stresses 400 mpa a rigid lid was observed the great circle subduction zone and the rigid lid are stable over time while at intermediate yield stresses some episodic behavior is observed the spherical cases showed a higher more earth like toroidal poloidal ratio of the surface velocity field than the cartesian cases

propagation of brittle failure triggered by magma in iceland abstract the architectures of normal faults at a divergent plate boundary in iceland are examined by combining surface fault observations with cross sectional studies at different structural levels to constrain a model of failure propagation the structures of holocene faults defining graben are analyzed to characterize the upper most parts of ruptures the shapes of faults resulting from growth and interaction of separate segments are used to better understand failure propagation inferred to occur in the intervening stages of displacement accumulation and lateral propagation pleistocene faults in volcanic sequences exhumed from 800 to 1000 m are analyzed to characterize deeper portions of failure that occurred beneath the central rift zone tertiary dikes exhumed from depths of 1300 1500 m are studied to infer how magma controls the failure initiation field studies in combination with a literature review indicate that the planar ruptures are likely to initiate at depth as magma filled vertical fractures and lengthen upward and laterally as failures propagate to higher crustal levels they are likely to change into inclined normal faults at near surface levels faults link with cooling joints and dilational fractures propagating downward from the surface it is suggested that the inferred stages of fault propagation are characteristic for normal faults developed at spreading ridges

impact of oil excavation activities on soil metallic pollution case study of an iran southern oil field the ahvaz oil field located in southern iran has been identified as a possible point of elemental pollution since elevated levels of metallic pollution as cd cr ni cu zn pb and v have been introduced to the adjacent environment through the oil excavation activities the concentration of metals cr ni cu pb and zn are far below all reference values while that of as cd and v are remarkably above limits enrichment factor ef reports the mean waste status to be enriched by all three elements as cd and v while the igeo values manifest the strongly polluted condition for cd and moderately polluted for as and v in order to quantify the association of metal concentrations with various acid soluble reducible oxidizable and pseudo total phases and consequently to estimate the potential mobility of existent metallic pollution the sequential chemical partitioning technique was used for the samples collected from clean soil drilling mud and wastage pit according to the results achieved by chemical partitioning analysis cd seems to be mobile similar conditions exist for v with relatively lower risk level regarding as the potential mobility is estimated to be low as a remarkable portion of bulk concentration more than 70 is accumulated in the pseudo total phase paying attention to the bulk and partitioning concentrations of three enriched elements in the drilling mud it is concluded that the main cause of wastage pit contamination may be attributed to the nature of drilling mud utilized in excavation activities additionally the geological texture in different depths may also play a role in this scenario

a magmatic loading model for coronae on venus 1 models for the formation of coronae quasi circular volcanotectonic features on venus must explain four critical characteristics coronae display 1 a wide range of diameters 2 complex varied topography 3 fracture annuli and 4 sometimes extensive volcanism previous models have difficulty simultaneously satisfying all four constraints on the basis of observations and interpretations of features on venus and earth and experiments in geophysical fluid dynamics we propose that coronae form in response to magmatic loading of the crust over zones of partial melting at the tops of thermally buoyant heads of transient mantle plumes that impinge on the base of the thermal lithosphere by tying corona formation to a melt zone and not directly to an impinging upwelling our conceptual model can account for the wide range in corona diameters lateral crustal flow facilitated by magmatic heating may lead to central depressions characteristic of many coronae a thermomechanical finite element simulation with a dissipating thermal anomaly leads to crustal thinning and predicts an elevation profile consistent with one of the more populous topographic classes of coronae deformation concentrated at the transition from thicker to thinner lithosphere above the thermal anomaly yields a narrow annulus of enhanced differential stress consistent with formation of fracture annuli estimates of excess heat in an upwelling exceed the thermal energy in the melt that comprises the corona band pass filtered maps of the topography and gravity fields in the beta atla themis region are consistent with upwellings now impinging on the lithosphere beneath seven coronae previously inferred to be active

a climate induced transition in the tectonic style of a terrestrial planet abstract we explore the possibility that an increase in the surface temperature of a terrestrial planet due to an enhanced concentration of atmospheric greenhouse gasses and or increased solar luminosity could initiate a transition from an active lid mode of mantle convection e g plate tectonics to an episodic or stagnant lid mode i e single plate planet a scaling theory is developed to estimate the required temperature change as a function of the temperature dependence of mantle viscosity and the yield stress of the lithosphere the theory relies on the assumptions that convective stresses scale with mantle viscosity and that a planet will adjust to surface temperature changes so as to maintain a surface heat flow that balances internal heat production the theory is tested against a suite of numerical simulations of mantle convection the comparisons are favorable the combined theory and numerics suggest that if the yield stress for the earths lithosphere is 30 35 mpa then a surface temperature change of 60 120 could shut down an active lid mode of convection assuming present day conditions lower values are predicted for higher yield stresses and for earlier times in the earth geologic evolution

the effects of multiple phase transitions on venusian mantle convection the effects of multiple phase transitions on the dynamics of the venusian mantle have been investigated with a two dimensional finite element method a depth dependent thermal expansivity has been employed in a purely basal heating configuration with an aspect ratio of four the addition of the olivine spinel transition promotes layered convection more so than models with a single spinel perovskite phase change the shifts in the phase transition depths and the presence of a conductive lid increase the tendency of the venusian mantle circulation to flow through the transition zone the potentially lower rayleigh number in venus from lack of volatiles will also enhance whole mantle convection the style of convection in venus may have changed from layered convection in the past to whole mantle convection today this transition might have been responsible for the major resurfacing event on venus

mechanisms for cessation of magmatic resurfacing on venus 1 proposed mechanisms of resurfacing on venus about 0 3 1 gyr ago usually involve either some form of tectonic resurfacing in which venusian lithosphere is recycled mechanically or magmatic resurfacing where essentially immobile lithosphere is covered by lava the focus of this study is mechanisms of magmatic resurfacing in the stagnant lid regime of mantle convection parameterized convection models suggest that cessation of magmatic resurfacing can occur in several ways 1 the mantle temperature drops sufficiently such that mantle rising adiabatically does not cross the solidus 2 the molten layer migrates below the solid melt density inversion at 250 500 km so that no melt can escape 3 sublithospheric small scale convection stops and conductive thickening of the lid suppresses melting in each case inability of magma to penetrate thickened venusian lithosphere may play a role the timing of melt cessation and duration of late stage resurfacing depend on various factors such as mantle rheology degree of mantle depletion and depth of resurfacing the models indicate that the waning stages of resurfacing which result in surface age variations can last 0 1 1 gyr a general trend of recent lithospheric thickening is also predicted some cases exhibit a rapid transition from relatively thin to thick lid as suggested by previous authors

intermittent layered convection in a model mantle with an endothermic phase change at 670 km it is not yet known whether convection in the earth s mantle occurs in cells that extend throughout the entire depth of the mantle or if the upper and lower mantle convect in independent layers both possibilities seem to be consistent with geochemical and seismologi cal observations1 4 it is generally accepted however that the seismologically observed boundary between the upper and lower mantle at 670 km depth arises from the endothermic phase change spinel perovskite mgo here we investigate the results of including this phase change in a model of mantle convection for realism and to eliminate scaling problems our calculations use commonly accepted values of geophysical parameters for a clapeyron slope y 2 106 a value close to experimental results and theoretical expectations we observe local intermittent mixing between the upper and lower mantle such behaviour may offer a way to reconcile the existence of geophysical evidence for both whole mantle and layered convection

the global resurfacing of venus the impact cratering record on venus is unique among the terrestrial planets fully 84 of the craters are in pristine condition and only 12 are fractured remarkably only 2 5 of the craters and crater related features are embayed by lava although intense volcanism and tectonism have affected the entire planet furthermore the spatial and hypsometric distribution of the craters is consistent with a completely random one including stochastic variations monte carlo simulations of equilibrium resurfacing models result in a minimum of 17 times more embayed craters than observed or unobserved nonrandom crater distributions for resurfacing areas between 0 03 and 100 of the planet s surface these models also are not consistent with the number and nonrandom distribution of volcanoes and the nonrandom distribution of embayed and heavily fractured craters the constraints imposed by the cratering record strongly indicate that venus experienced a global resurfacing event about 300 m y ago followed by a dramatic reduction of volcanism and tectonism this global resurfacing event ended abruptly 10 m y the present crater population has accumulated since then and remains largely intact thermal history models suggest that similar global resurfacing events probably occurred episodically in the past the tesserae statistically have the same average surface age crater retention age as the rest of the planet but they probably represent older rock units deformed by earlier episodes of global resurfacing although they largely survived the latest global resurfacing event their surfaces were severely deformed by it monte carlo simulations indicate that only about 4 6 of the planet has been volcanically resurfaced since the global event and that the lava production rate has been no more than 0 01 0 15 km3 yr during this time this rate is significantly less than the current rate of intraplate volcanism on earth 0 33 0 5 km3 yr most of venus recent volcanism occurs in the beta atla themis region and most of the recent tectonism is associated with the major global scale tectonic disruption zones that lie within and connect the equatorial highlands the approximately 33 of the planet s surface bounded by latitudes 30 n and 30 s longitudes 60 and 300 e contains twice as many heavily fractured craters and 1 4 times more lava embayed craters as the planetary average this region includes most of the major tectonic belts in the equatorial region because the craters are indistinguishable from a statistically random distribution both spatially and hypsometrically this concentration of strongly fractured and embayed craters is considered indicative of a continuing low level of limited extension and volcanic activity in this region over the past 300 m y however these craters are simply fractured and or embayed and very few have been subjected to complete tectonic disruption complete burial and subsequent removal from the surface as was the case during the global resurfacing event we show that neither the present level and style of geologic activity nor anything less than global resurfacing could have produced the observed cratering record the effects of recent geologic activity are much less than those of the earlier global resurfacing event when the record of all the early heavy bombardment and much of the later light bombardment was erased from the surface by massive volcanism and tectonic activity episodic regional resurfacing events that had global effects also occurred on earth e g the mid cretaceous superplume and probably on mars on mars they may have triggered the catastrophic releases of water that formed the outflow channels

a mantle plume model for the equatorial highlands of venus the equatorial highlands of venus consist of four main structures atla beta ovda and thetis regiones each has a circular to oval shaped planform and rises 4 5 km above the mean planetary radius these highlands are associated with long wavelength geoid highs with amplitudes ranging from 35 m at ovda to 120 m at atla they also contain topographic valleys interpreted as extensional rift zones and beta is known to contain shield volcanoes these characteristics are all consistent with the equatorial highlands being formed by mantle plumes an alternative model in which ovda and thetis are interpreted as spreading centers analogous to terrestrial mid ocean ridges fails to explain most of the observed geoid anomalies and topography in these regions some smaller highlands such as bell regio eistla regio and the hathor innini ushas region may also be plume related but most coronae are unlikely to be the direct result of plume activity we have modeled plumes using a cylindrical axisymmetric finite element code and a depth dependent newtonian rheology we compare our model results with profiles of geoid and topography across atla beta ovda and thetis our best model fits are for beta and atla assuming whole mantle convection and that earth and venus have similar mantle heat flows venus must lack an earth like low viscosity zone in its upper mantle in order satisfy the observed geoid and topography for these features this conclusion is consistent with the long wavelength admittance spectrum of venus and with the observed differences in the slopes of the geoid spectra for the two planets one explanation for the different viscosity structures of the two planets could be that the mantle of venus is drier than earth s mantle

surface deformation and shallow dike intrusion processes at inyo craters long valley california the inyo craters are the two largest of four phreatic craters that lie within a 2 5 km long 500 to 700 m wide n s trend of faults and fissures at the south end of the inyo volcanic chain in eastern california the alignment of these features with dike fed silicic volcanic centers of the same age a few kilometers to the north suggests that they were produced during intrusion of a dike at about 650 550 yr b p e w extension south of south inyo crater is ten to several tens of meters suggesting that the dike is at least that thick to understand how the faults and fissures developed we mapped and studied the fault and fissure pattern used a theoretical boundary element model to determine the surface strain profile above a shallow dike in a purely elastic medium and conducted physical model experiments of fault and fissure growth results of the field studies and experiments indicate that deformation develops according to the following sequence 1 extension fractures and perhaps other inelastic deformation develop immediately above the dike top and on the limbs of a shallow syncline which forms above the dike 2 fissures form along two parallel trends at the surface on opposite sides of the dike plane leaving a relatively unfractured region in between 3 dip slip movement on subsurface fractures and linkage of these fractures with surface fissures produces inward facing normal fault scarps that bound a nested graben above the dike experimental graben widths are up to several times narrower than predicted from the locations of theoretical strain maxima above a dilating crack in a linearly elastic medium this fact is apparently due to the growth of a zone of inelastic deformation above the experimental dike top in the subsurface if the ratio between the depth to the dike top and the distance between the outermost surface fissures at inyo craters is within the range of ratios measured experimentally then the depth to the dike top is between 250 m and several times that a slant hole drilled in the summer of 1987 by the department of energy office of basic energy sciences intersected three breccia bodies of possibly intrusive origin directly below the center of south inyo crater between 550 m and 610 m below the crater rim no intact juvenile igneous rock was intersected suggesting that the hole passed above the magma filled dike top perhaps passing through the zone of inelastic deformation which is inferred to exist from our model results

broad accommodation of rift related extension recorded by dyke intrusion in saudi arabia volcanic fields at the eastern margin of the red sea rift were regarded as seismically quiet until a swarm of 30 000 earthquakes struck in 2009 geological analyses reveal the intrusion of a volcanic dyke and indicate that rift related extension is spread over a broad region far from the rift axis