associations between burial diagenesis of smectite chemical remagnetization and magnetite authigenesis in the vocontian trough se france results of a paleomagnetic rock magnetic geochemical and petrographic study on jurassic and cretaceous carbonates in the vocontian trough support a hypothesized connection between burial diagenetic alteration of smectite and the widespread occurrence of a chemical remanent magnetization crm carried by magnetite where smectite has altered to other clay minerals limestones are characterized by a prefolding secondary normal polarity magnetization throughout the basin the magnetization is interpreted to be a crm based on low burial depths which cannot cause thermoviscous resetting where significant smectite is still present the crm is absent weakly developed and where the clays show no evidence for burial alteration the units are characterized by a primary magnetization crm intensity also varies with the amount of smectite and burial isothermal anhysteretic and natural remanent magnetization intensities increase where smectite has altered both stratigraphically and geographically this is interpreted to indicate magnetite authigenesis associated with clay diagenesis superparamagnetic magnetite is more dominant in highly altered units based on the results of low temperature experiments all sections away from the alps have 87sr 86sr values that are similar to coeval seawater and stable isotopes of carbon and oxygen show no sign of alteration orogenic type fluids therefore are not a likely agent of remagnetization near the alps the rocks are characterized by an additional reversed polarity component which is interpreted to reflect acquisition of the crm through a reversal a postfolding magnetization is also present there and strontium isotopic ratios are higher than elsewhere in the basin and might indicate some alteration by orogenic type fluids we conclude that burial diagenesis of smectite is the likely cause for the development of the widespread crm in the vocontian trough and that this mechanism might explain widespread chemical remagnetization elsewhere

anisotropy of magnetic susceptibility and p wave velocity in core samples from the taiwan chelungpu fault drilling project tcdp the anisotropy of magnetic susceptibility ams and anisotropy of p wave velocity apv were characterized in core samples retrieved from the tcdp hole a at depths ranging from 600m to 1400m thus cutting across the thrust fault that ruptured during the 1999 chi chi earthquake our data show that the ams was similar for all the samples regardless of the lithologic contrasts with a magnetic fabric characteristic of weakly deformed sedimentary rocks that had undergone layer parallel shortening this robust observation applies to both the siltstones and sandstones at all depths investigated in contrast the apv data reflect fundamental differences between the elastic fabrics in the two facies while apv of the siltstone samples is controlled by a preferred orientation of the matrix minerals the apv of the sandstones is greatly influenced by microcrack porosity with a preferred orientation parallel to the maximum principal direction of paleostress and tectonic deformation these ams and apv data obtained in tcdp hole a at locations not directly impacted by the coseismic rupture provide a first order petrofabric framework for the characterization of hydromechanical anisotropy in the chelungpu fault system

a geodynamic model of mantle density heterogeneity using cenozoic and mesozoic plate motion reconstructions we derive a model of present day mantle density heterogeneity under the assumption that subducted slabs sink vertically into the mantle the thermal buoyancy of these slabs is estimated from the observed thermal subsidence cooling of oceanic lithosphere slab velocities in the upper mantle are computed from the local convergence rate we assume that slabs cross the upper lower mantle interface and continue sinking into the lower mantle with a reduced velocity for a velocity reduction factor between 2 and 5 our slab heterogeneity model is as correlated with current tomographic models as these models are correlated with each other we have also computed a synthetic geoid from our density model for a viscosity increase of about a factor of 40 from the upper to lower mantle our model predicts the first 8 spherical harmonic degrees of the geoid with statistical confidence larger than 95 and explains 84 of the observed geoid assuming that the model c21 and s21 terms are absent due to a long relaxation time for earth s rotational bulge otherwise 73 of the geoid variance is explained the viscosity increase is consistent with our velocity reduction factor for slabs entering the lower mantle since downwelling velocities are expected to scale roughly as the logarithm of viscosity loge 40 3 7 these results show that the history of plate tectonics can explain the main features of the present day structure of the mantle the dynamic topography induced by this heterogeneity structure consists mainly of about 1 km amplitude lows concentrated along the active continental margins of the pacific basin our model can also be used to predict the time variation of mantle heterogeneity and the gravity field we find that the age of the geoid defined as the time in the past before which the geoid becomes uncorrelated with the present geoid is about 50 m y our model for the history of the degree 2 geoid which is equivalent to the history of the inertia tensor should give us a tool to study the variations in earth s rotation pole indicated in paleomagiietic studies

magnetic susceptibility anisotropy of rocks saturated with ferrofluid a new method to study pore fabric abstract the shape effect of ferrofluid filled cavities on the anisotropy of magnetic susceptibility ams is examined for different geometries principal directions of magnetic susceptibility correspond to principal axes of the shape of the cavity elongation directions and axial ratios are reflected in the ams response which for simple geometries is explained by the anisotropy of the demagnetizing factor the experiments can be carried further for the analysis of pore structure in natural rock samples techniques developed for the injection of fractionated ferrofluid into a porous sandstone successfully lead to a homogeneous saturation of the sample the method potentially provides a new means to determine three dimensional pore fabric anisotropy and to relate it to the anisotropy of various petrophysical parameters

effect of post perovskite rheology on the thermal evolution of the earth abstract secular cooling of the earth mantle is a complex process affected by many factors here we present the results of a modelling study focused on efficiency of cooling in the presence of rheologically distinct post perovskite we evaluate combined effects of variable thermal expansivity and diffusivity initial thermal condition and heat source model and concentrate on the effects of rheologically weak post perovskite cooling of the core is included in the model core is assumed to be an isothermal heat reservoir with temperature controlled by heat flux through core mantle boundary our 2d axisymmetric convection model has pressure temperature and phase dependent viscosity and includes the effects of an endothermic phase transition at 660 km depth and an exothermic perovskite post perovskite phase transition in the lowermost mantle in agreement with previous studies we conclude that depth dependent material parameters tend to delay secular cooling presence of the weak post perovskite on the other hand significantly enhances core cooling and its effect on core temperature is opposite and comparable in magnitude

polar motions excited by a convecting viscous mantle the role played by time dependent mantle convection on exciting long term polar motions is examined by means of a viscous model for sufficiently low effective viscosity polar wander speeds of 0 1 deg myr would require contributions from the off diagonal elements of the inertia tensor with magnitudes around 105 times smaller than those found in viscoelastic models used for postglacial rebound contributions from the large scale mantle flow to the angular momentum vector can be comparable to those due to changes in moment of inertia tensor for non linear rheology or for young planets the relative roles of the two contributions depend on the non linear rheology of the mantle and its convective vigor

intermediate magnetite formation during dehydration of goethite abstract the dehydration of goethite has been studied by low temperature induced magnetization ltim and x ray diffraction on well characterized acicular crystals fresh samples were heated in air to temperatures between 155 c and 610 c goethite and hematite were the magnetically dominant phases after all runs except 500 c and 610 c for which only hematite was found however partially dehydrated goethites after the 238 402 c runs had broad peaks or inflections in the ltim curves around 120 k suggesting the formation of an intermediate spinel phase these samples were next given a saturation remanence in a field of 2 t at 10 k and the remanence was measured continuously during zero field warming to 300 k there was a decrease in remanence at the verwey transition 120 k diagnostic of magnetite the possible formation of a small amount of magnetite is of serious concern in studies of goethite bearing sediments and rocks chemical remanent magnetization crm of this strongly magnetic spinel phase could significantly modify the direction as well as the intensity of the original goethite crm as well it would be a new source of paleomagnetic noise as far as primary remanence carried by other mineral phases is concerned

the potential influence of radiative heat transfer on the formation of megaplumes in the lower mantle abstract recent seismic tomographic models have revealed broad low velocity anomalies in the lower mantle beneath africa and the central pacific which suggest a break in the symmetry between hot and cold regions in lower mantle dynamics we have considered the possible impact from radiative heat transfer which can be described by a nonlinear temperature dependent coefficient in the thermal conductivity in 2d numerical simulations results for rayleigh numbers up to 106 show a strong stabilizing influence from radiative heat transfer on mantle upwellings and the production of extremely hot thermal anomalies in the interior this nonlinearity is responsible for producing a strong attractor in the mantle convective system which greatly simplifies its time dependent dynamics the possible link of the simplified lower mantle time dependent dynamics with polar wander is discussed the main point here is that slow time dependence of the huge anomalies in the lower mantle can be the main mechanism controlling long term rotational dynamics

nondestructive continuous physical property measurements of core samples recovered from hole b taiwan chelungpu fault drilling project 1 the taiwan chelungpu fault drilling project was undertaken in 2002 to investigate the faulting mechanism of the 1999 mw 7 6 taiwan chi chi earthquake hole b penetrated the chelungpu fault and core samples were recovered from between 948 42 and 1352 60 m depth three major zones designated fzb1136 fault zone at 1136 m depth in hole b fzb1194 and fzb1243 were recognized in the core samples as active fault zones within the chelungpu fault nondestructive continuous physical property measurements conducted on all core samples revealed that the three major fault zones were characterized by low gamma ray attenuation gra densities and high magnetic susceptibilities extensive fracturing and cracks within the fault zones and or loss of atoms with high atomic number but not a measurement artifact might have caused the low gra densities whereas the high magnetic susceptibility values might have resulted from the formation of magnetic minerals from paramagnetic minerals by frictional heating minor fault zones were characterized by low gra densities and no change in magnetic susceptibility and the latter may indicate that these minor zones experienced relatively low frictional heating magnetic susceptibility in a fault zone may be key to the determination that frictional heating occurred during an earthquake on the fault

a global geotectonic reference system inferred from genozoic tectonics a system of spherical coordinates with special reference to the major active tectonic features of the earth is presented this geotectonic reference network or geo net is defined by the pacific pole p at 170 w 0 n and the african pole a at 10 e 0 n fig 1 with respect to the geo net geo latitudinal and geo longitudinal oceanic ridges ridge segments and transform faults may be distinguished the former trending parallel to the small circles the latter parallel to the great circles of the geo net fig 3 generally the great circles run perpendicularly or at high angle to the cenozoic mountain belts and oceanic trenches

long wavelength s wave velocity structure throughout the mantle summary long wavelength s wave velocity anomalies are obtained for the entire mantle by inverting 6ooo long period 40 100s sh body waves and lo00 long period 100 500 s love waves in the shallow mantle the patterns of fast and slow velocity anomalies correlate well with age and tectonics in the depth range of 400 1000 km we find two antipodal fast velocity anomalies one in the western pacific and the other in south america they form a fairly strong i 2 anomaly moreover the 1 2 anomaly is the largest spherical harmonic component throughout the entire mantle except for in the layer of 1000 1300 km depth a fast velocity anomaly near the caribbean previously reported by several other investigators is found between the depths of 1000 and 1300km with diminishing amplitudes it can be traced to canada and the aleutians and may be related to old subducted slabs in the same depth interval similar size anomalies can be found in the middle of the pacific and in the indian ocean these could be checked by more detailed studies to judge the reliability of this study heterogeneity power has a peak at the surface and a secondary peak at the bottom of the mantle d consistent with the existence of chemical and or thermal boundary layers at the base of the mantle we also find a fast velocity anomaly which surrounds the pacific ocean this feature has been found by other researchers previously but its interpretation is difficult due to the trade off between velocity anomaly and core mantle boundary deformation

the dynamical impact of electronic thermal conductivity on deep mantle convection of exosolar planets abstract we have modelled the time dependent dynamics of exosolar planets within the framework of a two dimensional cartesian model and the extended boussinesq approximation the mass of the super earth models considered is 8 times the earth s mass and the thickness of the mantle is 4700 km based on a constant density approximation and a similar core mass fraction as in the earth the effects of depth dependent properties have been considered for the thermal expansion coefficient the viscosity and thermal conductivity the viscosity and thermal conductivity are also temperature dependent the thermal conductivity has contributions from phonons photons and electrons the last dependence comes from the band gap nature of the material under high pressure and increases exponentially with temperature and kicks in at temperatures above 5000 k the thermal expansivity decreases by a factor of 20 across the mantle because of the high pressures greater than 1 tpa in the deep mantle we have varied the temperatures at the core mantle boundary between 6000 and 10 000 k accordingly the rayleigh number based on the surface values varies between 3 5 1 0 7 and 7 1 0 7 in the different models investigated three phase transitions have been considered the spinel to perovskite the post perovskite transition and the post perovskite decomposition in the deep lower mantle we have considered an arrhenius type of temperature dependence in the viscosity and have extended the viscosity contrast due to temperature to over one million the parameter values put us well over into the stagnant lid regime our numerical results show that because of the multiple phase transitions and strongly depth dependent properties particularly the thermal expansitivity initially most of the planetary interior is strongly super adiabatic in spite of a high surface rayleigh number because of the presence of partially layered and penetrative convective flows throughout the mantle very much unlike convection in the earth s mantle but with the passage of time after several billion years the temperature profiles become adiabatic the notable influence of electronic thermal conductivity is to heat up the bottom boundary layer quasi periodically giving rise to strong coherent upwellingss which can punch their way to the upper mantle and break up the layered convective pattern

anisotropy of magnetic susceptibility of heated rocks abstract heating produces changes which does not always correspond to simple enhancement of the magnetic fabric two methods are proposed to determine the anisotropy of magnetic susceptibility of the ferrimagnetic minerals formed or that have disappeared by chemical change during successive heating the first diagonalizes the tensor from the difference between each tensor term before and after heating the second employs linear regression for each tensor term made with the values obtained throughout a thermal treatment when the same magnetic fabric is obtained from several thermal steps it cannot be related to randomly oriented ferrimagnetic minerals instead the newly formed fabric must be related to characteristics of the pre existing rock by comparing this ferrimagnetic minerals fabric with the initial whole rock fabric we can distinguish cases where heating simply enhances pre existing fabric from those where thermal treatment induces a different fabric relative to the pre heating fabric this different fabric may simply be an inverse fabric or one whose principal susceptibility axes are oriented in a different direction related to petrostructural elements other than those defining the initial fabric

geological and industrial implications of extensive dilatancy anisotropy extensive dilatancy anisotropy is the hypothesized distribution of stress aligned fluid filled microcracks pervading most rocks in the earth s crust the geometry of the cracks and the aligning stress field can be monitored by analysing the waveforms of shear waves propagating through the rockmass the ability to estimate some of the parameters of the crack and stress geometry by analysing shear wave particle displacements has widespread implications

widespread chemical remagnetization orogenic fluids or burial diagenesis of clays paleomagnetic rock magnetic and geochemical results from mesozoic carbonates in the vocontian trough in southeast france support a hypothesized acquisition of a chemical remanent magnetization crm during burial diagenesis of smectite and are inconsistent with an orogenic fluid remagnetization mechanism the geographic and vertical distribution of a widespread crm which resides in magnetite is parallel to the degree of burial diagenesis of clays the crm is absent only where there is no evidence for clay diagenesis the crm is poorly developed where significant smectite is still present and is well developed with higher natural and anhysteretic remanent intensities where smectite has completely altered to illite strontium isotope results do not support alteration by orogenic type fluids a commonly invoked mechanism for similar crms the results suggest that the burial diagenesis of clays is a viable remagnetization mechanism for limestones in the vocontian trough

hotspots mantle plumes flood basalts and true polar wander persistent long lived stationary sites of excessive mantle melting are called hotspots hotspots leave volcanic trails on lithospheric plates passing across them the global constellation of fixed hotspots thus forms a convenient frame of reference for plate motions through the orientations and age distributions of volcanic trails left by these melting anomalies hotspots appear to be maintained by whole mantle convection in the form of upward flow through narrow plumes evidence suggests that plumes are deflected little by horizontal flow of the upper mantle mantle plumes are largely thermal features and arise from a thermal boundary layer most likely the mantle layer just above the core mantle boundary experiments and theory show that gravitational instability drives flow beginning with the formation of diapirs such a diapir will grow as it rises fed by flow through the trailing conduit and entrainment of surrounding mantle the structure thus develops a large spherical plume head and a long narrow tail on arrival at the base of the lithosphere the plume head flattens and melts by decompression producing enormous quantities of magma which erupt in a short period these are flood basalt events that have occurred on continents and in ocean basins and that signal the beginning of major hotspot tracks the plume supported hotspot reference frame is fixed in the steady state convective flow of the mantle and is independent of the core generated axial dipole paleomagnetic reference frame comparison of plate motions measured in the two frames reveals small but systematic differences that indicate whole mantle motion relative to the earth s spin axis this is termed true polar wander and has amounted to some 12 since early tertiary time the direction and magnitude of true polar wander have varied sporadically through the mesozoic probably in response to major changes in plate motions particularly subduction zone location that change the planet s moments of inertia

the effect of post pangea subduction on global mantle tomography and convection there is an excellent correlation between tomographic patterns and global tectonics in the upper 200 km of the mantle below this depth there is little relationship with present tectonic provinces most of the power in the tomography is in the longest wavelengths l 1 3 with l 2 generally dominant this is quite different from expectations based on mantle convection models the question then arises what causes the long wavelength variations mapped by seismic techniques we investigate the hypotheses that past locations of supercontinents and subduction may control temperature variations in the mantle we look for correlations between pangea and post pangeatic subduction and seismic velocity variations at various depths the best correlations with time integrated slab locations occur near the base of the uppermantle suggesting that slabs bottom out in the mesosphere the pangea hemisphere has a colder than average uppermantle probably due to circum and intra pangea subduction hotspots and ridges avoid regions cooled by subduction over the past 180 ma the presence of large continents at the surface and large areas of cold slab at depth explain the dominance of long wavelengths in the mantle s thermal structure despite the high rayleigh number

long term patterns and short term dynamics of stream solutes and suspended sediment in a rapidly weathering tropical watershed 1 the 326 ha rio icacos watershed in the tropical wet forest of the luquillo mountains northeastern puerto rico is underlain by granodiorite bedrock with weathering rates among the highest in the world we pooled stream chemistry and total suspended sediment tss data sets from three discrete periods 1983 1987 1991 1997 and 2000 2008 during this period three major hurricanes crossed the site hugo in 1989 hortense in 1996 and georges in 1998 stream chemistry reflects sea salt inputs na cl and so4 and high weathering rates of the granodiorite ca mg si and alkalinity during rainfall stream composition shifts toward that of precipitation diluting 90 or more in the largest storms but maintains a biogeochemical watershed signal marked by elevated k and dissolved organic carbon doc concentration doc exhibits an unusual boomerang pattern initially increasing with flow but then decreasing at the highest flows as it becomes depleted and or vigorous overland flow minimizes contact with watershed surfaces tss increased markedly with discharge power function slope 1 54 reflecting the erosive power of large storms in a landslide prone landscape the relations of tss and most solute concentrations with stream discharge were stable through time suggesting minimal long term effects from repeated hurricane disturbance nitrate concentration however increased about threefold in response to hurricanes then returned to baseline over several years following a pseudo first order decay pattern the combined data sets provide insight about important hydrologic pathways a long term perspective to assess response to hurricanes and a framework to evaluate future climate change in tropical ecosystems

the importance of radiative heat transfer on superplumes in the lower mantle with the new post perovskite phase change abstract a new post perovskite phase has been identified recently from both x ray observations and first principles calculations this novel phase change occurs at a depth about 200 km above the core mantle boundary for temperatures of around 2800 k using a two dimensional extended boussinesq model we have studied the dynamical consequences of this deep mantle phase transition on mantle convection with particular emphasis on the effects on lower mantle plume structures we have employed a depth dependent viscosity with a viscosity maximum in the mid lower mantle and two phase transitions one at 670 km depth and the other at a depth of 2650 km the phase transition at 670 km is endothermic while we have examined both exothermic and endothermic possibilities for the new phase transition our results show the following situations favorable for the development of superplumes 1 endothermic phase transition for both constant and radiative thermal conductivities and 2 exothermic phase transition and radiative thermal conductivity smaller unstable plumes are found for exothermic phase transition and constant thermal conductivity extremely high lateral temperature increases exceeding 1500 degrees are found inside the lower mantle plume for constant thermal conductivity this lateral temperature contrast is mitigated by the presence of radiative thermal conductivity in order for superplumes to prevail in the lower mantle with a deep exothermic phase change we must invoke some form of radiative thermal conductivity for stabilizing and promoting large upwellings with a width exceeding at least 500 km strong lateral heterogeneities in both the seismic velocity and density fields can be produced in the plumes and the surrounding mantle for this new phase transition with a large clapeyron slope close to 10 mpa k

structural geology of a classic thrust belt earthquake the 1999 chi chi earthquake taiwan mw 7 6 abstract we document the structural context of the 1999 chi chi earthquake m w 7 6 in western taiwan which is one of the best instrumented thrust belt earthquakes the main surface break and large slip 3 10 m is on two segments of the shallow otherwise aseismic bedding parallel chelungpu sanyi thrust system which shows nearly classic ramp flat geometry with shallow detachments 1 6 km in the pliocene chinshui shale and mio pliocene kueichulin tungkeng formations however rupture is complex involving at least six faults including a previously unknown deeper thrust 8 10 km on which the rupture began we compare the coseismic displacements with a new 3d map of the chelungpu sanyi system the displacements are spatially and temporally heterogeneous and well correlated with discrete geometric segments of the 3d shape of the fault system geodetic displacement vectors are statistically parallel to the nearest adjacent fault segment and are parallel to large scale oblique fault corrugations the displacement magnitudes are heterogeneous at several scales which requires in the long term other non chi chi events or significant aseismic deformation the chelungpu thrust has a total displacement of 14 km but the area of largest chi chi slip 10 m is on a newly propagated north chelungpu chinshui detachment 0 3 km total slip which shows abnormally smooth rupture dynamics

the impact of variability in the rheological activation parameters on lower mantle viscosity stratification and its dynamics the recently discovered spin state crossover in iron in major mantle minerals at high pressures should exert dramatic influences on transport properties such as the activation creep parameters in the deep mantle wentzcovitch et al 2009 have computed the elastic parameter of ferropericlase that is affected by this crossover the bulk modulus at high realistic conditions using first principles density functional theory plus hubbard u as a consequence of the spin crossover there is a significant softening of the bulk modulus and an attendant reduction of the activation energy in the creep law using a parameterized model capturing the basic physics we have studied the dynamical consequences within the framework of a 2 d cartesian convection model our models reveal a series of asthenospheres and are characterized first by a low viscosity channel below the 670 km boundary second asthenosphere caused by an increase of temperature and decrease in grain size reduction superplascity with the post spinel transition which results in partially layered convection further the variability of the rheological parameters leads to the prevalent formation of viscosity minimum at a depth of about 1600 km third asthenosphere caused by spin crossovers followed by a viscosity hill in the bottom half of the lower mantle the latter due to the increase of the activation energy in some low spin irons at the bottom of the mantle our numerical simulations reveal a tendency to the formation of small scale convection below the 670 km boundary and bigger stabilized plumes and or plume clustering in the deep lower mantle other material properties at the bottom of the mantle decrease of thermal expansivity radiative thermal conductivity also exert significant influence on the multi scale lower mantle plume dynamics

how wide is a road the association of roads and mass wasting in a forested montane environment a spatial data base of 1609 landslides was analysed using a geographic information system to determine landslide frequency in relation to highways a 126 km long transportation network in a 201km 2 area of humid tropical mountainous forested terrain in puerto rico was used in conjunction with a series of 20 buffer disturbance zones varying from 5 to 400 m in length measured perpendicular to the highways average landslide frequency in the study area at distances greater than 85 m from roads was about six landslides per square kilometre at distances of 85 m or less on either side of a highway landslide frequency was about 30 landslides per square kilometre on average this elevated disturbance rate affected 330 m 2 km 2 a 1 within the 170 m swath the mass wasting rate outside of the disturbance zone affected 40 m 2 km 2 a 1 these results indicate that the rate of mass wasting disturbance is increased from five to eight times in a 170 m wide swath along road corridors the lateral extent of the environmental impact of roads in the study area is greater than is commonly perceived the approach described herein demonstrates a simple method to assess the spatial association of mass wasting with highways 1997 by john wiley sons ltd

sediment production from forest road surfaces erosion on roads is an important source of fine grained sediment in streams draining logged basins of the pacific northwest runoff rates and sediment concentrations from 10 road segments subject to a variety of traffic levels were monitored to produce sediment rating curves and unit hydrographs for different use levels and types of surfaces these relationships are combined with a continuous rainfall record to calculate mean annual sediment yields from road segments of each use level a heavily used road segment in the field area contributes 130 times as much sediment as an abandoned road a paved road segment along which cut slopes and ditches are the only sources of sediment yields less than 1 as much sediment as a heavily used road with a gravel surface

on the scale of mantle heterogeneity abstract a data set comprising 3313 residuals measured from digital records of 429 earthquakes is analyzed for the heterogeneity of the earth s mantle the smoothed ss residuals reveal a large pattern of heterogeneity and agree well with the major tectonic features on the earth s surface and the residual patterns predicted by upper mantle and whole mantle models the spectrum of ss residuals indicates that the heterogeneity in the earth s mantle which probably reflects the patterns of mantle convection is dominated by long wavelength features our analysis of the power of ss residuals as a function of harmonic degree shows a distinct difference in the rate of decrease of the power below and above degree 8 the rate is first slower than l 1 and then it exceeds l 2 this is also true for ss residuals predicted for model sh 425 2 and love wave phase velocities at 100 s measured by other researchers this indicates that there is a preference for features with dimensions larger than 2500 3500 km we find that the power spectrum of the velocity anomalies associated with the subducted slabs is two orders of magnitude lower for 1 l 8 than that of velocity anomalies obtained by inversion of waveform data for the ocean basins correlation of the ss residuals with the square root of age of the oceanic lithosphere is highly significant however the age correlation predicts only 10 of the variance this means that the thermal signature of the spreading center represents only a small fraction of the thermal anomalies introduced into the upper mantle by convection

instabilities from phase transitions and the timescales of mantle thermal evolution we have investigated the effects of mantle phase transitions on the earth s thermal evolution by numerical modelling our results show that the tendency of layering grows with increasing vigor of convection a self consistent model which couples the convective dynamics of the mantle to the secular cooling of the core is constructed for a temperature dependent effective viscosity we found that the thermal history of the earth is characterized by two timescales one is associated with layered convection in the early period which is followed by a transition period with dramatic overturns as the rayleigh number decreases sufficiently low whole mantle convection prevails and a faster cooling timescale ensues

dynamics of a terrestrial magma ocean under planetary rotation a study in spherical geometry abstract during a later stage of the earth s accretion impacts of mars sized bodies on the earth caused one or more deep terrestrial magma oceans of global extent during that time the earth rotated very fast with a rotation period between two and five hours accordingly the effect of planetary rotation is not only of key importance for the chemical structure and the development of chemical heterogeneities but also sets the stage for the initiation of plate tectonics on earth this study investigates the effect of planetary rotation on the dynamics of silicate crystals in spherical geometry during an early stage of magma ocean solidification our results in a rotating spherical shell reveal a substantial influence of planetary rotation on the early magma ocean crystallization leading to crucial differences in solidification with respect to crystal accumulation depth and latitude without and with slow planetary rotation crystals are kept in suspension at all latitudes differently at moderate and high rotation rate clear dependencies of the crystal dynamics on latitude do arise at the poles all crystals settle at the bottom while in the equatorial region they stay entrained in the bottom half however at mid latitudes they are kept in suspension and are distributed over the entire depth of the magma ocean at even higher rotation rate all crystals accumulate at the bottom of the spherical shell suggesting that crystallization would start at the bottom at lower latitudes crystals are repeatedly re entrained by convective motions hindering the development of a stable layering at low latitudes whereas at higher latitudes a potentially stable gravitational layering does develop our numerical experiments reveal that planetary rotation is likely to induce an inhomogeneous solidification of a terrestrial magma ocean with respect to depth and latitude depending on the rotational strength this has important consequences on the solidification time and further favors major chemical segregation resulting in large scale chemical heterogeneities developing during magma ocean crystallization the origin and distribution of mantle heterogeneities like the llsvps could be an imprint of planetary rotation on the magma ocean crystallization during earth s early evolution

the chi chi earthquake sequence active out of sequence thrust faulting in taiwan we combined precise focal depths and fault plane solutions of more than 40 events from the 20 september 1999 chi chi earthquake sequence with a synthesis of subsurface geology to show that the dominant structure for generating earthquakes in central taiwan is a moderately dipping 20 to 30 thrust fault away from the deformation front a second subparallel seismic zone lies about 15 kilometers below the main thrust these seismic zones differ from previous models indicating that both the basal decollement and relic normal faults are aseismic

radiative heat transfer in a hydrous mantle transition zone abstract the structure and dynamics of earth s interior depend crucially upon heat flow and thus upon the thermal conductivity of its constituents we measured optical absorbance spectra of hydrous wadsleyite and hydrous ringwoodite at simultaneous high pressure and high temperature conditions up to 26 gpa and 823 k in order to determine their radiative conductivities and to study the potential influence of hydration in the transition zone on thermal conductivity of the mantle we report radiative thermal conductivities of 1 5 0 2 wm 1 k 1 for hydrous wadsleyite and 1 2 0 1 wm 1 k 1 for hydrous ringwoodite at transition zone conditions the analytically derived radiative thermal conductivities of anhydrous wadsleyite and ringwoodite are 40 and 33 higher respectively the total thermal conductivities calculated from temperature and pressure dependent optical absorption measurements maintain an energy transmission window in the infrared and visible spectral range at high pressures and temperatures the results indicate that the mantle transition zone may contribute significantly to heat transfer in the mantle and demonstrate the importance of radiative heat transfer in controlling geodynamic processes in earth s mantle

symmetries of volcanic distributions on mars and earth and their mantle plume dynamics the symmetries in the distribution of major volcanic centers on mars have been analyzed by employing a technique involving the angular self similarity of point fields the distribution of all central vent volcanoes exhibits an axisymmetry along with a reflecting symmetry about a plane whose normal is the axis of axisymmetry the latter axis is skewed by about 30 from the rotational axis and is suggested to represent the early axis of symmetry of the interior dynamics the high level of symmetry of mars volcanic distribution may be the result of a focusing of seismic energy from a major impact in the hellas area that formed the hellas basin and may have triggered the onset of volcanic activity in the greater tharsis area if the highland paterae near the hellas region are not taken into account in the analysis an asymmetry with respect to the reflecting plane appears this is probably representative of the late hesperian and amazonian when the once global volcanic activity had retreated to the tharsis province these angular symmetries suggest a simple pattern of long wavelength axisymmetric martian mantle convection involving a large stationary plume as shown by our three dimensional 3 d spherical shell simulations on the basis of these calculations we venture to propose that the volcanic history of mars reflects the transformation of a bipolar mantle plume structure into a single megaplume by the process of plume plume collision acting in concert with the deep martian phase transitions this transformation might have overcome the original pattern induced by a major impact forming the hellas basin we have repeated the procedure for the earth s hotspot distribution which revealed both axisymmetry and a certain level of reflecting symmetry this similarity between the two planets is striking the symmetries of the earth s hotspot distribution are probably due to the interaction of the lower mantle megaplumes with the endothermic perovskite to spinel phase transition

polar wandering of a dynamic earth summary the rotational behaviour of a stratified visco elastic planet submitted to changes in its inertia tensor is studied in a viscous quasi fluid approximation this approximation allows for large displacements of the earth rotation axis with respect to the entire mantle but is only valid for mass redistribution within the planet occurring on the time scale of a few million years such a motion called true polar wander tpw is detected by palaeomagneticiens assuming that the earth s magnetic field remains on average aligned with the spin axis our model shows that a downgoing cold slab induces a tpw which quickly brings this slab to the pole for a mantle of uniform viscosity the same slab is slowly moved toward the equator when a large viscosity increase with depth takes place in the mantle our model is also suitable to investigate the effects of a non steady state convection on the earth s rotation we discuss these effects using a simple mass redistribution model inspired by the pioneering paper of goldreich toomre 1969 it consists of studying the tpw induced by a random distribution of slabs sinking into the mantle for such a mass redistribution only a strongly stratified mantle can reduce the earth s pole velocity below l ma which is the upper bound value observed by palaeomagnetic investigations for the last 200 ma our model also shows that when corrected for the hydrostatic flattening the earth s polar inertia generally corresponds to the maximum inertia as it is presently observed however this may not be the case during some short time periods we also discuss the amount of excess polar flattening that can be related to tidal deceleration this frozen component is found to be negligible

hard turbulent thermal convection and thermal evolution of the mantle this article summarizes the results of hard turbulent convection obtained in laboratory experiments and numerical simulations its applications to mantle convection are illustrated by two dimensional numerical solutions to 1 newtonian 2 non newtonian convection and 3 newtonian convection with multiple phase transitions in newtonian mantle convection the transition from soft to hard turbulence is marked by the appearance of disconnected plumes spectral analysis of the time series of the nusselt number reveals the presence of a spectral scaling subrange for hard turbulence but not for soft turbulence in hard turbulence there is correspondence between the spectra in frequency and wavenumber domains the slope of the seismic wave spectra measured from seismology suggests that the mantle convection today is strongly time dependent the transition to hard turbulence takes place at much lower nusselt numbers for non newtonian than for newtonian rheology for the mantle this would have important ramifications non newtonian plumes behave quite differently from newtonian ones in that large curvatures are developed in their trajectories in the hard turbulent regime mantle convection with phase transitions tends to become more layered with increasing rayleigh numbers the style of mantle convection might have changed from a layered to a more whole mantle type of flow with time catastrophic overturns associated with strong gravitational instabilities in the transition zone could be responsible for superplume events

antipodal hotspot pairs on the earth the results of statistical analyses performed on three published hotspot distributions suggest that significantly more hotspots occur as nearly antipodal pairs than is anticipated from a random distribution or from their association with geoid highs and divergent plate margins the observed number of antipodal hotspot pairs depends on the maximum allowable deviation from exact antipodality at a maximum deviation of 700 km 26 to 37 of hotspots form antipodal pairs in the published lists examined here significantly more than would be expected from the general hotspot distribution two possible mechanisms that might create such a distribution include 1 symmetry in the generation of mantle plumes and 2 melting related to antipodal focusing of seismic energy from large body impacts

anisotropy of magnetic remanence a brief review of mineralogical sources physical origins and geological applications and comparison with susceptibility anisotropy the magnetic fabric of rocks and sediments is most commonly characterized in terms of the anisotropy of low field magnetic susceptibility ams however alternative methods based on remanent magnetization measured in the absence of a magnetic field rather than induced magnetization measured in the applied field have distinct advantages for certain geological applications this is particularly true for 1 adjunct studies in paleomagnetism in order to assess the fidelity with which a natural remanence records the paleofield orientation 2 studies of weakly magnetic or weakly deformed rocks for which susceptibility anisotropy is very difficult to measure precisely and 3 quantitative applications such as strain estimation the fundamental differences between susceptibility and remanence and their respective anisotropies are due to several factors 1 susceptibility arises from all of the minerals present in a sample whereas remanence is carried exclusively by a relatively small number of ferromagnetic minerals 2 ferromagnetic minerals are generally more anisotropic than para and diamagnetic minerals 3 for ferromagnetic minerals remanence is inevitably more anisotropic than susceptibility and 4 a number of common minerals including single domain magnetites possess an inverse anisotropy of susceptibility i e they tend to have minimum susceptibility parallel to the long axis of an individual particle remanence is immune to this phenomenon as a consequence of all these factors remanence anisotropy may generally provide a better quantitative estimate of the actual distribution of particle orientations in a rock sample

lattice thermal conductivity of mgsio3 perovskite and post perovskite at the core mantle boundary abstract thermal conductivity is essential for controlling the rate of core heat loss and long term thermal evolution of the earth but it has been poorly constrained at the high pressures of earth s lowermost mantle we measured the lattice component of thermal diffusivity heat transport by scattering of phonons of both mgsio 3 perovskite pv and post perovskite ppv at high pressures of up to 144 gpa and at room temperature lattice thermal conductivity of pv dominant lowermost mantle assemblage obtained in this study is about 11 w m k while ppv bearing rocks exhibit 60 higher conductivity since such pv value is comparable to the conventionally assumed lowermost mantle conductivity our findings do not significantly alter but support the recent notion of high core mantle boundary heat flow along with a young inner core and high temperatures in the early deep earth

lower mantle dynamics with the post perovskite phase change radiative thermal conductivity temperature and depth dependent viscosity the new post perovskite phase near the core mantle boundary has important ramifications on lower mantle dynamics we have investigated the dynamical impact arising from the interaction of temperature and depth dependent viscosity with radiative thermal conductivity up to a lateral viscosity contrast of 10 4 on both the ascending and descending flows in the presence of both the endothermic phase change at 670 km depth and an exothermic post perovskite transition at 2650 km depth the phase boundaries are approximated as localized zones we have employed a two dimensional cartesian model using a box with an aspect ratio of 10 within the framework of the extended boussinesq approximation our results for temperature and depth dependent viscosity corroborate the previous results for depth dependent viscosity in that a sufficiently strong radiative thermal conductivity plays an important role for sustaining superplumes in the lower mantle once the post perovskite phase change is brought into play this aspect is especially emphasized when the radiative thermal conductivity is restricted only to the post perovskite phase these results revealed a greater degree of asymmetry is produced in the vertical flow structures of the mantle by the phase transitions mass and heat transfer between the upper and lower mantle will deviate substantially from the traditional whole mantle convection model streamlines revealed that an overall complete communication between the top and lower mantle is difficult to be achieved 2005 elsevier b v all rights reserved

production of small volcanoes in the superswell region of the south pacific abstract we use sea beam swath bathymetry and wide beam profiler data to investigate the production of small 50 700 m high volcanoes in the region of the superswell 1 in french polynesia seamount population parameters are estimated for three study regions we find that seamount abundances increase threefold going southward across the marquesas fracture zone the inferred northern boundary of the superswell in addition we document that south of the marquesas fracture zone seamount abundances on average are equally as high on young crust 0 18 my generated at the east pacific rise 9 22 as on the adjacent superswell region 20 60 my old crust the fact that seamount abundance estimates are the same in the two regions south of the marquesas fracture zone leads to an uncertainty in whether the seamounts have been produced by ridge or off axis processes and suggests the following possibilities 1 processes at the east pacific rise have produced more small seamounts south of the fracture zone than to the north for the last 60 my 2 off axis processes possibly related to the superswell have led to excess seamount volcanism on both the superswell region and the region adjacent to the east pacific rise or 3 off axis seamount production is anomalously high on the superswell and the production of seamounts has increased at the east pacific rise since 20 my

predominance of long wavelength heterogeneity in the mantle analysis of travel time anomalies of long period shear waves recorded by the global digital seismographic network indicates that mantle heterogeneities which probably reflect patterns of mantle convection occur most prominently on length scales greater than about 6 000 km when projected onto a two dimensional map at the earth s surface models of mantle convection will need to be able to produce a similar power spectrum

anelastic tomography a new perspective on upper mantle thermal structure abstract anelastic tomography provides new insights into the thermal structure of the earth s upper mantle lateral variations in q are largest in the upper 300 km but remain significant down to at least 500 600 km they correlate with the distribution of plates in the upper 300 km in agreement with cooling plate models and with the distribution of hotspots at greater depths two major upwellings are revealed under hawaii and northeastern africa which may originate at the bottom of the transition zone or deeper low q structure beneath the main ridge systems on the other hand does not appear to extend vertically below about 300 km

mantle convection as a boundary layer phenomenon summary the boundary layer nature of vigorous thermal convection is explored using high resolution numerical solutions to the governing hydrodynamic equations these solutions are obtained for a series of idealized models of the earth s mantle in which the viscosity is assumed to be constant a detailed analysis of the local energy balance within the horizontal and vertical thermal boundary layers is presented in terms of which a test of the fundamental assumptions of boundary layer theory is provided the results of this test have important geophysical consequences since the asymptotic predictions of boundary layer theory have been employed extensively in the context of thermal history modelling although boundary layer theory closely predicts the correct power law behaviour of various quantities it does not determine their absolute values accurately vertical advection is shown to play an important role in the energy balance within horizontal boundary layers at all rayleigh numbers horizontal and vertical advection dominate the energy balance within vertical plumes while horizontal diffusion plays a very minor role when heating is partially from within the fluid vertical advection into the upper thermal boundary layer can produce significant departures in the thermal structure from that found when heating is entirely from below for a free upper boundary this results in a relative flattening of the variations of surface topography and heat flow across the convection cells for a constant velocity upper boundary similar to plate motion the bathymetry flattens but the heat flow does not this result agrees with marine observations rigidity of the thermal boundary layer below the upper surface is not included explicitly in the model and it is not known whether the inclusion of this feature in future models would significantly alter the topographic expression if not the observed departure of the oceanic bathymetry from a 6 dependence at old ocean floor ages could be attributed to a small amount of internal heating in a mantle wide convective circulation

non linear effects from variable thermal conductivity and mantle internal heating implications for massive melting and secular cooling of the mantle abstract the temperature dependence of the phonon portion of the thermal conductivity k t p devised by hofmeister science 283 1699 1706 decreases with temperature the same as in the dependence of mantle viscosity such a functional relationship of k t k t 0 thus there can be a tradeoff between the phonon and photon contributions in the conductivity in the presence of internal heating we have conducted two dimensional calculations of mantle convection up to a surface rayleigh number of around five million and an internal heating of chondritic abundance with the extended boussinesq approximation in which the dissipation number has been set to 0 47 and depth dependent thermal expansivity decreasing by a factor of 5 across the mantle the value of the constant mantle viscosity and the amount of internal heating are varied for an enhanced radiative contribution j geophys res 84 b4 1603 1610 the radiative component of the thermal conductivity can exceed the phonon contribution in the upper mantle our results show that in all cases with basal heating the average mantle temperature of the variable conductivity models are higher than those of the corresponding constant conductivity models but the interior thermal difference between the two conductivity models decreases 1 with greater vigor of convection 2 an increase of internal heating and 3 an increase in the radiative contribution to the conductivity the interior mantle temperature is significantly hotter more than 500 c than the constant conductivity model for the k t p model with the less enhanced radiative component science 283 1699 1706 these results would suggest that some sort of massive melting in the young earth might have occurred with k t p and that there should not be so much radioactivity in the lower mantle today without incurring the wrath of some melting we have also studied the effects of k t p on slowing down the mantle secular cooling process by monitoring the gradual decrease in mantle temperature following an imposition of an adiabatic boundary condition at the core mantle boundary a decay time of 3 6 gy has been taken for the mantle radioactivity and we have varied the initial amount of radioactive heating from chondritic value to four times the chondritic value a significant delay in the cooling process of at least 1 2 gy is found for a surface rayleigh number of between 5 10 6 to 5 10 7 the mantle temperature can be heated up by 300 400 c for initial radiogenic heating value characteristic of the archean we find the strongest deviations from the constant conductivity case for a silicate model by hofmeister science 283 1699 1706 and intermediate values for an enhanced radiative conductivity model comparable to the model of shankland et al j geophys res 84 b4 1603 1610 such high mantle temperatures maintained for a long time by variable thermal conductivity would have important consequences on the thermal and petrological evolution of the mantle

topography of the ocean floor thermal evolution of the lithosphere and interaction of deep mantle heterogeneities with the lithosphere topography associated with large wave length geoid is derived by two methods first the bathymetry corrected for sediment loading is decomposed using a least squares method as a function of age plus terms proportional to the geoid the topography at old ages is flatter predicted by the law at degrees 2 and 3 very little topography is associated with the geoid the transfer function is not even certain in sign but is never smaller than 0 6 more topography seems associated with higher degree harmonics 6 and 7 second the function of age alone best fitting the topography is determined the correlation coefficients of the residual topography with the geoid are then computed a slight correlation appears for degree 6 but not for degrees 2 and 3 these results suggest a mechanical decoupling between the lower mantle mass heterogeneities and the lithosphere

a spatial domain stokes flow model for the gravity and topography of the middle latitudes of venus a new modeling strategy is developed and applied to the long wavelength gravity topography and internal density structure of venus geodynamic modeling of planetary scale phenomena is usually carried out in the wavenumber transform domain spherical harmonics or fourier transforms however because of limitations associated with the gravity and topography data available for venus a spatial domain approach is preferable smoothed representations of the gravity and topography of venus are developed in terms of quadratic b splines on 10 centers in latitude and longitude over the latitude interval from 45 s to 45 n these models reproduce 83 6 and 98 3 of their respective data variances a similarly constructed model is then presented for the dynamic support of gravity and topography on venus internal mass anomalies are specified at two discrete depths 50 and 500 km with lateral variations determined by two constraints 1 that the slow viscous flow induced by the anomalies will distort the free surface into a form matching the observed topography and 2 that the sum of the gravitational effects of the internal anomalies plus that due to the surface distortion they cause will match the observed gravity there are four major features evident in the gravity and topography data sets atla beta ovda and thetis regiones all four have compensating mass patterns consistent with a mantle plume origin the surficial flow pattern associated with this model does not resemble terrestrial plate tectonic patterns

axisymmetry of mantle aspherical structures recent seismic tomographic models have revealed broad low velocity anomalies in the lower mantle beneath africa and the central pacific that correspond to the poles of the surface geotectonic reference network we test the hypothesis that the tomographic models are axisymmetrical with respect to axes passing near the geotectonic poles the symmetry is apparent at the bottom of the lower mantle as well as in the transition zone in the upper mantle this supports the idea of massive flushing of cold upper mantle material into the bottom part of the lower mantle and points to a stabilizing role of the lower mantle megaplumes on whole mantle processes

pulsating diapiric flows consequences of vertical variations in mantle creep laws abstract recent laboratory work has suggested that the rheology of the lower mantle may be newtonian we have studied the time dependent dynamics of plumes interacting with a rheological interface separating an upper non newtonian mantle and a newtonian lower mantle pulsating diapiric structures with fast time scales are promoted by the interaction of the rising plumes with this rheological boundary surface heat flow signals are discernible as pulses which remain relatively stationary they correlate well with the localized upwellings just below the surface the presence of a mobile lithosphere from increasing the non newtonian power law index helps to produce a large scale circulation in the upper mantle which draws large hot patches away from nearby upwellings in our calculations the resultant averaged effective viscosity of the non newtonian upper mantle is about two orders of magnitude lower than that of the newtonian lower mantle a viscously stratified newtonian model produces more incoherent and broad scale diapiric structures and is less efficient for generating sharply varying time dependent thermal signatures

angular symmetries of hotspot distributions the symmetries in the distribution of hotspots are investigated by an analogy of the angular autocorrelation of scalar fields it has been applied to two lists of hotspot locations revealing the axes as well as the angles of symmetries the existence of angular symmetries suggests that the hotspot distribution observable on the earth s surface is probably influenced by symmetries of deep geodynamic processes

magnetic anisotropy in the trenton limestone results of a new technique anisotropy of anhysteretic susceptibility a new method for determining magnetic anisotropy using anhysteric remanence susceptibility is described the magnetic fabric of a collection of trenton limestone specimens has been determined using this method as well as by conventional anisotropy of magnetic susceptibility the results demonstrate the usefulness of the new method for finding the magnetic fabric of rock units such as the trenton in which the bulk magnetic susceptibilities are low a model is proposed to explain the observed foliated and lineated fabric as a consequence of overburden compaction and regional horizontal stresses the original fabric is inferred to have been isotropic the anisotropy resides in secondary magnetite of late paleozoic age it is argued that the observed magnetic fabric must therefore be alleghenian or younger in age our method has the potential to determine paleostress directions in carbonates elsewhere provided our assumptions are correct

bullen s parameter η a link between seismology and geodynamical modelling valuable information concerning the style of mantle density stratification can be gleaned from the spatial distribution of bullen s parameter by means of numerical modelling of 2 d cartesian convection and by monitoring this with the 2 d field of the local bullen s parameter values which were obtained by post processing the convection results we show that the local adiabaticity is seriously influenced by the presence of an endothermic phase transition at 670 km depth in this situation the upper mantle suffers much more from non adiabatic effects than the lower mantle we have also employed the 3 d distributions of density and seismic velocities from the model of ishii and tromp based on free oscillation observations and constructed 3 d local bullen s parameter for a spherical earth in the lower mantle we find that there is a striking similarity in the range of magnitudes of the local bullen s parameter calculated from convection and those inferred from the splitting of seismic free oscillations the morphologies of subadiabatic regions in the deep mantle under africa and the central pacific would suggest a thermal chemical nature of the superplumes their vertical extent is limited to around 400 km above the core mantle boundary cmb underneath the icelandic area in the north atlantic such an subadiabatic region in the deep mantle above the cmb cannot be discerned we conclude that the mantle is not as adiabatic as has commonly been held fl 2002 elsevier science b v all rights reserved

effects of a realistic mantle thermal conductivity on the patterns of 3 d convection abstract a physical model describing the temperature and pressure dependences of mantle thermal conductivity has recently been provided by a m hofmeister science 283 1999 1699 1706 we have studied numerically the influences of such a temperature and pressure dependent thermal conductivity on 3 d constant viscosity convection a large aspect ratio box of 4 4 1 has been taken for rayleigh numbers greater than 104 to 106 the power law index governing the phonon contribution to the thermal conductivity controls the style of convection for silicates the pattern of convection is quite different from that of constant conductivity whereas the pattern of convection associated with conductivity of oxides is close to the case for constant conductivity both boussinesq and extended boussinesq models with viscous and adiabatic heatings have been examined major effects of variable thermal conductivity on mantle convection are 1 heating up of the lower mantle 2 longer wavelength boussinesq convection 3 shorter wavelength for extended boussinesq convection 4 thick stable plumes with large plumeheads 5 thicker thermal boundary layer of around 500 km at the base of the mantle the contributions from the new terms due to the variable conductivity in the energy equation have magnitudes several times that of chondritic heating at the boundary layers a large region of superadiabatic lower mantle with about a 400 to 600 k excess is caused by variable thermal conductivity this finding has an interesting implication on the recent seismic finding of a lighter density in the deep portion of the lower mantle

dynamical consequences of depth dependent thermal expansivity and viscosity on mantle circulations and thermal structure the effects of both depth dependent thermal expansivity and depth dependent viscosity on mantle convection have been examined with two dimensional finite element simulations in aspect ratio ten boxes surface rayleigh numbers between 107 and 6 107 have been considered the effects of depth dependent properties acting singly or in concert are to produce large scale circulations with a few major upwellings the interior of the mantle is cooled by the many cold instabilities which are slowed down and eventually swept about by the large scale circulation the interior temperature of the mantle can be influenced by the trade off between depth dependent properties and internal heating for chondritic abundance of internal heating and depth dependent thermal expansivity the viscosity increase across the mantle can be no greater than a factor of around ten in order to keep the lower mantle adiabatic the thermal contrasts between the cold blobs and the surrounding mantle are strongly reduced by depth dependent properties whereas the lateral differences between the hot upwelling and the ambient lower mantle can be significant over several hundred degrees depth dependent properties also encourage the formation of a stronger mean flow in the upper mantle which may be important for promoting long term polar motions

catastrophic overturn of the earth s mantle driven by multiple phase changes and internal heat generation the effects of phase changes and strong internal heat generation may combine to bring about brief but extremely intense episodes of rapid thermal convection in the earth s mantle numerical calculations using realistic thermodynamic properties for the exothermic olivine spinel and endothermic spinel perovskite magnesiowustite phase transitions suggest the transition region of the earth s mantle may act as a capacitor for subducting slabs slab material accumulates in the transition region until a threshold level of thermal buoyancy is reached and is then rapidly discharged into the lower mantle in my calculations this occurs as a catastrophic burst of convection lasting 10 myr with elevated heat transfer rates lasting 100 myr such episodes may be analogs to superplume activity which has been hypothesized to give rise to an intense episode of intra plate volcanism and stabilize the geodynamo against reversals the topography on the two phase change boundaries is found to be negatively correlated with the correlation becoming more negative during periods of rapid convection furthermore the results of this study suggest the transition region of the earth s mantle could be 250 k cooler on average and consequently more viscous than the surrounding mantle

dynamically supported geoid highs over hotspots observation and theory hotspots are associated with long wavelength geoid highs an association that is even stronger when the geoid highs associated with subduction zones are removed we quantify these associations by expanding the hotspot distribution in spherical harmonics and calculating correlation coefficients as a function of harmonic degree the hotspot distribution spectrum is nearly white except for peaks at degrees 2 and 6 it is correlated positively with the slab residual geoid for degrees 2 6 with low seismic velocity in the lower mantle at degree 2 and with low seismic velocity in the upper mantle at degree 6 we test fluid mechanical models for hotspots including lithospheric delamination and hot plumes by calculating their predicted dynamic geoid responses and comparing them to the observations these models include the effects of temperature dependent rheology our preferred hotspot model based on observations of the geoid and seismic tomography has plumes preferentially occurring in regions of large scale background temperature highs in a mantle which has a substantial viscosity increase with depth although other models are possible alternatively strong plumes in the lower mantle may neck down and attenuate in the upper mantle due to changes in plume viscosity the major low density anomalies causing the geoid highs associated with plumes appear to be in the lower mantle we hypothesize that cold lower mantle underlying the complementary geoid lows is due to deeply subducted oceanic lithosphere and that hotspot plumes tend to be excluded from these cold regions of the mantle

quantification and modeling of foot trail surface erosion in a dry sub tropical setting anthropogenic disturbance often increases surface erosion and this may have potential detrimental effects on downstream aquatic resources foot trails are often overlooked as they represent only a small fragment of the landscape yet they can be important sources of sediment particularly in pristine areas the trail network above east end bay on the island of st croix in the us virgin islands is the sole anthropogenic source of terrestrial sediment in the area concern over the potential for trail erosion to stress nearshore coral reefs of the east end marine park led to trail reconstruction and restoration efforts the objectives of this study were to 1 quantify trail erosion rates 2 identify key factors controlling erosion rates 3 develop an empirical trail erosion model sediment production was measured from 12 trail segments with sediment traps from november 2009 to october 2011 annual trail erosion rates ranged from 0 6 to 81mghayr the lower values were from abandoned trails with a dense vegetation cover while the highest rates were associated with devegetated trails immediately following construction or restoration trail erosion was a function of rainfall slope and vegetation cover density raised to the negative 1 7th power annual trail erosion rates were one to three orders of magnitude higher than measured surface erosion rates on undisturbed hillslopes the absence of rills or gullies suggests that traditional parametric or repeated transect monitoring commonly used to assess trail erosion may greatly underestimate actual sediment production rates the new empirical trail erosion model serves as a tool to assess the effects of trail construction and restoration activities in the generation of sediment from small catchments in east end bay and in other similar settings

orogenic remagnetizations in the front ranges and inner foothills of the southern canadian cordillera chemical harbinger and thermal handmaiden of cordilleran deformation abstract paleomagnetic studies of carbonate rocks in mountain belts have been helpful to the understanding of orogenic mational origin with only one exception the a component has normal polarity in the front ranges and reverse processes the timing of deformation and the nature of fluid flow in actively deforming foreland basins most studies on shallow water carbonates indicate that their remanence is not primary but rather they record remagnetization events contemporaneous with orogeny by considering the remanent directions fold test results and rock magnetic properties and tying these results to complementary structural geochronological and geochemical studies we are able to interpret deformation history hydrological environment and orogenic diagenesis we study the relationship between remagnetization and deformation of paleozoic carbonates of the front ranges and inner foothills of the southern canadian rockies the magnetic properties are remarkably constant along 500 km strike length as sampled at 124 sites through 4 transects primary paleozoic remanent directions which would have shallow inclination are never observed rather the paleomagnetic signal is dominated by geographically persistent remagnetizations characterized by steep inclination as well as a soft present field overprint we observed two distinctive secondary magnetizations named the a and b components carried by fine grained magnetite pervasive diagenesis induced the a component a total chemical remanent remagnetization poles for the a component are better concentrated after bedding correction indicating a pre or early syndefor polarity in the inner foothills pole positions polarity and geological and thermal constraints indicate that the a component was acquired diachronously in advance of the eastward migrating cordilleran tectonic wedge subsequently an intermediate temperature partial thermo remanent remagnetization the b component was superimposed on large regions of the front ranges and inner foothills b component directions are brought into optimal concentration by differential untilting of 0 to 50 indicating that the component was acquired after the rocks were incorporated into the orogenic wedge but before the end of contractional deformation the b component is strongest within a couple of kilometers of the frontal thrust of the front ranges the relative magnitude of the b to a components and the maximum unblocking temperature of the b component decrease away from the frontal thrust over about 30 km both to the west and to the east the b component thermal overprint was attained by 250 c heating in response to tectonic or possibly sedimentary loading it was preserved by a rapid cooling accompanying a differential uplift and erosion of up to 8 km in the vicinity of the frontal thrust late in or post dating its local tilting history the likely cause was uplift of the exposed structural panel by contraction of younger underlying thrust structures

characterization of slip zone associated with the 1999 taiwan chi chi earthquake x ray ct image analyses and microstructural observations of the taiwan chelungpu fault abstract to characterize the fault related rocks within the chelungpu fault we performed x ray computed tomography ct image analyses and microstructural observations of hole b core samples from the taiwan chelungpu fault drilling project we identified the slip zone associated with the 1999 chi chi earthquake within the black gouge zone in the shallowest major fault zone by comparison with previous reports the slip zone was characterized by low ct number cataclastic or ultracataclastic texture and high possibility to have experienced a mechanically fluidized state taking these characteristics and previous reports of frictional heating in the slip zone into consideration we suggested that thermal pressurization was the most likely dynamic weakening mechanism during the earthquake

magnetic inference of in situ open microcracks in sandstone samples from the taiwan chelungpu fault drilling project tcdp abstract this paper presents a thorough magnetic study of centimeter sized rock samples from hole b of the taiwan chelungpu fault drilling project tcdp fabrics were obtained from magnetic susceptibility and anhysteretic remanent magnetization are interpreted using whole rock and magnetic mineralogy characterization we infer the presence in the sandstone lithology of secondary magnetite coating a dense network of microcracks using other evidence from ferrofluid saturation experiments we conclude that this network of microcracks is not sealed but open in situ and should therefore be incorporated in the interpretation of seismic and borehole scale studies in the tcdp area

some dynamical consequences of partial melting in earth s deep mantle we use regional scale numerical models of mantle convection to investigate the simple hypothesis that seismically anomalous thin patches of earth s lowermost mantle termed ultralow velocity zones or ulvz are derived from partial melting of ordinary mantle the models span the lower 500 km of earth s mantle employ both temperature and melting related contributions to buoyancy and include a cold randomly moving downwelling introduced from above to maintain a thermal boundary layer partial melting of ordinary mantle introduces a ubiquitous partially molten layer above an isothermal core mantle boundary as a consequence of its isothermal and isobaric conditions although it naturally develops variations in thickness greater than two orders of magnitude with the thickest portions occurring at the base of upwelling plumes and a thin layer elsewhere we find that only a dense partially molten mixture produces partial melt distributions that are compatible with seismic observations of ulvz however if such a melt percolates downward a dense basal liquid layer accumulates above the core mantle boundary the apparent requirement of a volumetrically dense and non percolating melt phase in the lowermost mantle presents serious problems for the hypothesis that ulvz arise from melting of ordinary mantle and suggests that such features likely form as a consequence of more complex processes furthermore these considerations suggest that the solidus of ordinary mantle is a reasonable upper bound on the present day temperature of the cmb 2007 published by elsevier b v

paleomagnetic dating of burial diagenesis in mississippian carbonates utah 1 the objective of this study is to test models for the origin of widespread secondary magnetizations in the mississippian deseret limestone the delle phosphatic member of the deseret limestone is a source rock for hydrocarbons and modeling studies indicate that it entered the oil window in the early cretaceous during the sevier orogeny paleomagnetic and rock magnetic results from the deseret limestone and the stratigraphically equivalent chainman shale in central and western utah indicate that the units contain two ancient magnetizations residing in magnetite burial temperatures are too low for the magnetizations to be thermoviscous in origin and they are interpreted to be chemical remanent magnetizations crms fold tests from western utah indicate the presence of a prefolding triassic to jurassic crm geochemical 87sr 86sr 13c and 18o and petrographic analyses suggest that externally derived fluids did not alter these rocks this crm was acquired at the beginning of the oil window and is interpreted to be the result of burial diagenesis of organic matter a second younger crm in western central utah is apparently postfolding and is probably late cretaceous to early tertiary in age on the basis of the thermal modeling the timing overlaps with the oil window these results are consistent with a connection between organic matter maturation and remagnetization modeling of the smectite to illite transformation in the deseret limestone suggests a mean age prior to acquisition of both crms although the range for illitization overlaps with the triassic to jurassic crm the results of this study support the hypothesis that pervasive crms can be related to burial diagenetic processes in addition paleomagnetism can be used to determine the timing of such processes which can benefit hydrocarbon exploration efforts

rock magnetic evidence for the dissolution and authigenic growth of magnetic minerals within anoxic marine sediments of the california continental borderland the rock magnetic properties of marine sediments from the california continental borderland san pedro santa catalina and san nicolas basins have been studied in order to quantitatively assess the effects of sediment diagenesis on magnetic minerals previous studies have noted that the natural remanent magnetization in these sediments primarily carried by detrital magnetite decays to 10 or less of its surface value soon after deposition this decrease is caused by magnetite dissolution related to sediment diagenesis and is unrelated to paleoclimatic variations or changes in the regional influx of detrital magnetic material detailed rock magnetic measurements show that shifts to softer remanent coercivity and differences in the rate and degree of magnetic intensity loss with depth can be related to the dissolution process the shift to softer remanent coercivity is related to a coarsening of the magnetic mineral grain sizes with depth due to preferential dissolution of the finest grained magnetic material the intensity decreases which are linearly proportional to magnetite concentration decreases indicate that dissolution occurs with rate constants ranging from 0 3 to 1 6 kyr 1 the rate constants sulfide concentrations and magnetite grain size estimates from the borderland are consistant with previous studies of magnetite dissolution our results demonstrate the importance of both sulfide and magnetite surface area in the dissolution process anomalous peaks in viscous remanence within the sediments suggest the authigenic growth of greigite and its subsequent transformation to pyrite

dissolution and pyritization of magnetite in anoxie marine sediments concentrations of magnetite were determined with depth in sediments of varying h2s content taken from long island sound foam nwc and sachem sites and from a site in the rapidly depositing subaqueous portion of the mississippi delta for the three long island sound sites dissolution of magnetite during burial is clearly demonstrated and for all four sites the rate of magnetite dissolution is proportional to the concentration of dissolved pore water sulfide by combining magnetite depth distributions with sedimentation rate data and measurements of dissolved sulfide we find that magnetite dissolution is well described by the following rate law dcmagdt 1 1 10 5c0 5bcmagamag where cmag is the concentration of magnetite grams per gram of sediment cs is the concentration of dissolved sulfide mm and amag is the specific surface area of magnetite in the sediment cm2 per gram of magnetite this equation means that for a typical range of magnetite surface areas and dissolved sulfide concentrations the half life of magnetite in anoxic marine sediments ranges from about 50 to 1000 years sem observations of magnetite grains from the different sediment sites reveal that magnetite dissolution may be accompanied by extensive replacement by pyrite this only occurs if magnetite is in contact with high concentrations of h2s 1 mm for relatively long periods of time several hundred years with low concentrations of h2s 1 mm magnetite dissolution occurs without identifiable pyrite replacement we suggest that this feature of magnetite diagenesis may prove to be a useful paleosulfide indicator

spatial variation of suspended sediment concentrations in a tropical andean river system the paute river southern ecuador ecosistemas montanosos tropicales son sensibles a los cambios ambientales provocados por los procesos naturales y antropogenicos la topografia escarpada suelos poco profundos y las practicas no sostenibles de uso del suelo tras la conversion de bosques a menudo conducen a mejora de la tasa de actividad geomorfologica incluyendo la erosion del suelo deslizamientos y actividad fluvial el rapido crecimiento demografico y el desarrollo socio economico se han acelerado los cambios ambientales en los ultimos decenios durante los ultimos 50 anos la erosion del agua en los campos agricolas causados por practicas agricolas inadecuadas se percibe como la principal fuente de sedimentos fluviales en las cuencas tropicales andinos sin embargo debido a datos esporadicos sobre la produccion de sedimentos transporte y export en cuencas tropicales andinos no esta claro en cuanto a si la actividad geomorfologica es controlada principalmente por factores naturales o antropogenicos un programa de investigacion se ha llevado a cabo en el 5176 kilometros 2 rio paute cuenca situada en el sur de ecuador para examinar la variabilidad en la carga de sedimentos en los rios un programa de monitoreo se creo en la parte central de la cuenca para extraer informacion sobre la variabilidad espacial en la carga de sedimentos del rio en relacion con las variables de influencia que describen el clima la litologia la topografia y el uso del suelo veintinueve tramos fluviales se muestrearon durante dos temporadas de lluvias sucesivas en cada visita se tomaron muestras de sedimentos en suspension y se midio la descarga del rio instantanea se recogieron datos geograficos relacionados con la topografia la morfologia del paisaje las precipitaciones la cobertura del suelo y la geologia las cuencas que fueron afectados por una intensa erosion deslizamiento de tierra en la zona aguas arriba tienen una concentracion de sedimentos en suspension ponderada significativamente mayor que las cuencas restantes existe cierta evidencia que sugiere que la aparicion de estas grandes deslizamientos de tierra poco frecuentes se relaciona principalmente con la pendiente topografica en lugar de a las practicas de uso del suelo actuales o actividades humanas

geophysical inferences of thermal chemical structures in the lower mantle lateral variations of the temperature field in the lower mantle have been reconstructed using new results in mineral physics and seismic tomographic data we show that with the application of high pressure experimental values of thermal expansivity and of sound velocities the slow seismic anomalies in the lower mantle under the pacific and africa can be converted into realistically looking plume structures with large dimensions of 0 10 km the outer fringes of the plumes have an excess temperature of around 400 k in the core of the plumes are found tongue like structures with extremely high thermal anomalies these values can exceed 1200 k and are too high to be explained on the basis of thermal anomalies alone we suggest that these major plumes in the deep mantle may be driven by both thermal and chemical buoyancies or that enhanced conductive heat transfer may be important there

effects of strongly temperature dependent viscosity on time dependent three dimensional models of mantle convection numerical simulations of thermal convection in a wide 8 8 1 cartesian box heated from below with temperature dependent viscosity contrasts of 1000 and rayleigh number 105 show that boundary conditions and aspect ratio have an enormous effect on the preferred flow pattern with rigid upper and lower boundaries spoke pattern flow with small diameter 1 5 cells is obtained consistent with laboratory experiments and previous numerical results however with the arguably more realistic stress free boundaries the flow chooses the largest possible wavelength forming a single square cell of aspect ratio 8 with one huge cylindrical downwelling surrounded by upwelling sheets the addition of stress dependence to the rheology weakens the stiff upper boundary layer resulting in smaller cells though still with upwelling sheets and downwelling plumes

runoff and sediment yield from rural roads trails and settlements in the upper konto catchment east java indonesia abstract the potential importance of roads and settlements for the generation of storm runoff and sediment in tropical steeplands is increasingly recognised but rarely quantified this paper presents runoff and sediment yield data for a cobbled and an unpaved road section two large unbounded settlement plots and several trails draining residential areas or terraced fields in a volcanic upland catchment in east java in addition the sediment yield of major landslides associated with roads was quantified the unpaved road section exhibited an average runoff coefficient of about 65 and yielded about 7 kg m 2 yr 1 of sediment both the runoff coefficient and the sediment yield for the cobbled road section plus adjacent yards were lower 38 and 1 9 kg m 2 yr 1 sediment output from a 4160 m2 hillside plot including a network of trails draining terraced fields for which runoff and sediment outputs were shown to be negligible was similar to that for the cobbled road 2 3 kg m 2 yr 1 depending on rainfall however a much higher value was obtained when the overall soil loss from the plot was expressed per square metre of trail surface area ca 42 kg m 2 yr 1 whereas the associated trail runoff coefficient was about 70 the results obtained for several trails and large unbounded plots draining residential areas at two locations were less extreme runoff coefficients of 24 43 soil loss 1 3 3 5 kg m 2 yr 1 landsliding occurred mainly at the end of the rainy season march and was estimated to have contributed ca 2365 m3 of sediment to the main road network during the 1988 89 wet season vs ca 905 m3 in 1989 90 and only ca 150 m3 in 2000 01 it is concluded that despite their relatively small areal extent 5 in the study area rural roads trails and settlements are significant producers of runoff and sediment at the catchment scale and should be included in watershed management programmes designed to reduce catchment sediment yields and reservoir siltation