provenance of plumes in global convection models in global convection models constrained by plume motions and subduction history over the last 230 myr plumes emerge preferentially from the edges of thermochemical structures that resemble present day large low shear velocity provinces llsvps beneath africa and the pacific ocean it has been argued that large igneous provinces lips erupting since 200 ma may originate from plumes that emerged from the edges of the llsvps and numerical models have been devised to validate this hypothesis although qualitative assessments that are broadly in agreement with this hypothesis have been derived from numerical models a quantitative assessment has been lacking we present a novel plume detection scheme and derive monte carlo based statistical correlations of model plume eruption sites and reconstructed lip eruption sites we show that models with a chemically anomalous lower mantle are highly correlated to reconstructed lip eruption sites whereas the confidence level obtained for a model with purely thermal plumes falls just short of 95 a network of embayments separated by steep ridges form in the deep lower mantle in models with a chemically anomalous lower mantle plumes become anchored to the peaks of the chemical ridges and the network of ridges acts as a floating anchor adjusting to slab push forces through time the network of ridges imposes a characteristic separation between conduits that can extend into the interior of the thermochemical structures this may explain the observed clustering of reconstructed lip eruption sites that mostly but not exclusively occur around the present day llsvps

sequestration of atmospheric co2 in a weathering derived serpentinite hosted magnesite deposit 14c tracing of carbon sources and age constraints for a refined genetic model abstract the attunga magnesite deposit is texturally and geochemically distinct from other spatially associated serpentinite hosted magnesite deposits in the great serpentinite belt new south wales australia such as the hydrothermal piedmont magnesite deposit or widespread silica carbonate alteration zones cryptocrystalline magnesite at attunga predominantly occurs in nodular masses and irregular desiccated veins that occupy pre existing cracks and pore spaces resulting from fracturing and weathering of the host rock incipient weathering of the serpentinite host rock is accompanied by a decrease in volume and the mobilisation of mgo and cao from the serpentinite pore spaces and permeability created during weathering and fracturing of the host rock provide access for co2 mgo and cao bearing meteoric waters which led to an increase of volume during carbonation sio2 is only mobilised during more advanced stages of weathering and late stage infiltration of sio2 bearing waters and precipitation of opal a lead to local silicification of the serpentinite stable carbon and oxygen isotope signatures show that nodular magnesite at attunga has formed under near surface conditions incorporating carbon from c3 photosynthetic plants and oxygen from meteoric waters radiocarbon concentrations in the magnesite preclude subducted carbonaceous sediments as the source of carbon and together with distinct stable carbon and oxygen isotope signatures indicate that magnesite at attunga precipitated from low temperature supergene fluids even though there is no direct geochemical and isotopic evidence some textural observations and field relationships for weathering derived magnesite deposits suggest the prior existence of a possibly early triassic hydrothermal magnesite deposit at attunga the presence of a pre existing magnesite deposit may entail the localised formation of the weathering derived magnesite at attunga but the predominance of weathering related textures and geochemical signatures indicate that weathering is the integral magnesite mineralisation process at attunga conventional radiocarbon ages of about 50 ka represent a maximum age constraint for the formation of the magnesite deposit during quaternary weathering a significant amount of atmospheric co2 has been sequestered via the biosphere and carbonation of serpentinite at attunga

oblique convergence during the cimmerian collision evidence from the triassic aghdarband basin ne iran abstract the lower middle triassic aghdarband basin ne iran consists of a strongly deformed arc related marine succession deposited along the southern margin of eurasia in a highly mobile tectonic context this basin is a key area for the study of the cimmerian events as the triassic units show severe deformations which occurred short time after the collision of iran with eurasia and were sealed by the middle jurassic succession in this work we document the structural setting and evolution of this area based on detailed mesoscopic structural analyses of faults and folds paleostress reconstruction and revision of the triassic stratigraphy the triassic sequences are deeply involved in a n verging thrust stack interacting with an important left lateral transpressional fault zone characterized by strike slip faults vertical folds and high angle reverse faults generating intricate positive flowers systematic folds asymmetry indicates that they developed in a left lateral transpressional zone coeval to thrust imbrication to the south due to a marked strain partitioning the extent of the transpressional zone shows that important left lateral movements developed parallel to the belt during the cimmerian collision in response to oblique convergence between iran and eurasia inversion of triassic syn sedimentary faults possibly inherited from palaeozoic structures of the kopeh dagh basement and favouring strain partitioning is suggested by unconformities significant differences in the sedimentary successions repeated olistoliths scarp related coarse breccias and rapid tectonic drowning occurring especially along the northern tectonic boundary of the basin paleostress analyses point to a complex stress pattern showing a 45 rotation of the stress field along the left lateral fault system related to a complete deformation partitioning in two domains respectively characterized by pure reverse dip slip and strike slip motions the main direction of compression possibly oriented ne sw in present days coordinates favoured the development of large shear zones disrupting the eastern portion of the cimmerian orogen

seasat altimetry and the south atlantic geoid 1 spectral analysis this paper presents a discussion of the spectral characteristics of geoid profiles derived from seasat alitmetry in the south atlantic individual profile spectra are computed and averaged in seven separate geodynamic provinces mid atlantic ridge flanks walvis and rio grande aseismic ridges and argentine and cape basins three distinct wavebands can be identified in most spectra for wavelengths shorter than 35 km the spectra are typical of white noise with a 7 cm rms amplitude typical of seasat altimeter measurement noise for wavelengths between 300 and 800 km our uppermost limit since original profiles are 1700 km long the spectra are in reasonable agreement with kaula s empirical rule and are compatible with a pratt thermal boundary layer compensation mechanism with an ill defined compensation depth of 30 60 km the paper centers on the description of the 35 300 km part of the spectra computation of an impedance for each province requires good knowledge of its bathymetry which is far from satisfactory as yet in the south atlantic we show that for wavelengths longer than 80 km the squared amplitude spectrum of bathymetry varies as the inverse of wave number we propose that this relationship might also hold at shorter wavelengths although it is not observed due to presently insufficient data using these spectral estimates and correcting for the mean depth of the topography we derive impedance actually response function estimates separately for each province the data are well accounted for by a regional compensation scheme based on elastic flexure models elastic thickness the best resolved parameter ranges from 2 to 5 km for the ridge and flanks and the crustal thickness from 4 to 13 km crustal thickness appears to be larger on the east flank demonstrating a general lack of symmetry between east and west in the south atlantic which appears to hold at all wavelengths and is also expressed in the bathymetry the rio grande rise involves low but nonzero elastic thickness and large crustal thickness 25 35 km in agreement with previously proposed models of local airy compensation the walvis ridge appears to imply a larger elastic thickness in excess of 8 km if the crustal thickness is less than 20 km which may be due to emplacement on older lithosphere therefore despite a lack of sufficient bathymetric control geoid data can be used at the scale of the entire south atlantic to provide reasonable information on thermomechanical compensation in the various provinces the spectral structure emphasizes the 35 300 km spectral window and should be used in constructing band pass filters that can separately enhance features related to plate kinematics or sublithospheric convection

the changing nature of mantle hotspots through time implications for the chemical evolution of the mantle the melting products of mantle plumes can place constraints on the geochemical structure of the mantle and show how it has varied through time the highest temperature melts komatiites and picrites derived from each plume are assumed to represent the best available sample of the thermal boundary layer from which the plume originates a survey of the geochemistry of picrites and komatiites shows that they underwent a fundamental change at about the archean proterozoic boundary archean komatiites have depleted or neutral geochemistry whereas most protero zoic and younger picrites have originated from enriched oib type mantle although rare depleted plumes persist to the tertiary e g gorgona if plumes originate from a thermal boundary layer near the core mantle interface the composition of this layer must have changed from a mixture of depleted and neutral mantle during the archean to mainly enriched mantle in the proterozoic this change coincides with a drop in the maximum mgo content of the ma

theory of slickenline patterns based on the velocity gradient tensor and microrotation abstract brittle fault zones commonly are characterized by a multitude of lineated shear planes having a wide distribution of orientations in our model we assume the faulted rock comprises an aggregate of rigid blocks whose surfaces are the shear planes we define two independent scales of motion to distinguish the local rigid rotation of the blocks with their shear planes the micromotion from the average deformation of the material on a macroscopic scale the macromotion the macromotion is defined by the macrovelocity gradient tensor which is separated into symmetric and antisymmetric parts the deformation rate and the macrospin tensors respectively the micromotion is defined by a microspin tensor on any shear plane slickenlines form parallel to the direction of the maximum rate of shear which we assume is determined by the direction of the maximum component of the macrovelocity gradient tensor tangent to the shear plane and by a component of the microspin in the shear plane for a restricted kinematic model patterns of slickenline orientations are defined on a uniform distribution of shear plane orientations by the values of d and w d is a ratio of differences in the principal values of the deformation rate tensor and w is a ratio of the net spin to the maximum rate of macroshear we distinguish between instantaneous slickenline patterns and finite deformation patterns the two are different if the shear planes rotate relative to the principal deformation rate axes in which case finite deformation slickenlines form curved or crossing lineations if w 0 the instantaneous slickenline patterns have orthorhombic symmetry 0 d d 1 or 0 if w 0 the instantaneous slickenline patterns have monoclinic symmetry for pure shear in an isotropic body instantaneous slickenline patterns have orthorhombic symmetry w 0 d 0 5 whereas for pure shear accommodated by shear on a planar anisotropy w 1 d 0 5 the pattern is monoclinic for simple shear with the rigid rotation of local shear planes defined by the shear induced spin w 0 d 0 5 the instantaneous slickenline pattern is orthorhombic but the finite deformation pattern is monoclinic for simple shear with no shear plane rotation w 1 d 0 5 the instantaneous pattern is monoclinic special cases of our hypothesis are equivalent to the hypothesis that slickenlines are parallel to the direction of maximum resolved shear stress e g angelier 1979 1984 the slickenline patterns predicted for this case are always of orthorhombic or higher symmetry and are equivalent to our instantaneous patterns for which w 0 and to our finite deformation patterns for which in addition there is no microspin

strong heterogeneity caused by deep mantle layering 1 numerical simulations are used to characterize the volumetric heterogeneity associated with proposed chemical layering in the deep mantle a globally continuous undulating layer generates enormous amounts of volumetric heterogeneity which is caused by lateral variations in the layer boundary depth and by strong hot upwellings arising from the top of the layer in the middle lower mantle region in contrast the heterogeneity in the deepest mantle is very weak partial cancellation of the temperature and compositional contributions to seismic velocity could mask the layer itself but the strong signatures of the thermochemical boundary layer and induced upwellings remain even when fields are filtered to approximate the resolution of global seismic tomographic models this distinctive heterogeneity is not observed in global seismic tomographic models dense material that is restricted to discontinuous piles generates heterogeneity that is more consistent although not entirely with seismic observations with strong heterogeneity at the core mantle boundary radial correlation functions display a characteristic narrowing in the vicinity of any chemical layer but are very sensitive to the applied seismic tomographic filtering

transpression and strain partitioning in the caribbean island arc fabric development kinematics and ar ar ages of syntectonic emplacement of the loma de cabrera batholith dominican republic abstract an integrative structural and geochronologic study of the loma de cabrera batholith lcb cordillera central dominican republic and its country rocks reveals the interplay of deformation metamorphism and plutonism produced in the caribbean island arc during late cretaceous oblique convergence the results emphasize the interference between three contemporaneous strain fields 1 a northern and southern domains produced by 40 ar 39 ar in hornblende during sinistral transpressional shearing comparison of the structural data with strain models of oblique plate convergence suggest that the lmsz is a preserved ductile signature of strike slip partitioning within a sinistral transpressional intra oceanic subduction zone in the lcb microstructural data indicate that the magmatic to high temperature solid state deformation initially occurred over a wider band of heterogeneously distributed shear deformation and was partitioned in narrow bands of mid to low temperature deformation connected with the lmsz during the cooling of the batholith field and geochronologic studies also suggest that shortening across the southern domain took place concurrently with sinistral strike slip movement along the crustal scale la guacara and macutico fault zones also consistent with a transpressional setting for the late cretaceous caribbean magmatic arc shear and fault zones were variably reactivated during upper eocene oligocene thrusting and miocene to recent uplift of the cordillera central

paleo depth of fossil faults estimated from paleostress state applications from the alps and the apennines italy abstract estimating the paleo depth of a fossil fault usually requires the application of complex and highly specialized techniques we propose a new method that is based mainly on field data and simplifies the process calculation of the reduced stress tensor with four variables through inversion of fault slip data calculation of the complete stress tensor i e the two remaining unknowns using the friction and the rupture laws estimation of the paleo depth the friction law is defined by the lower boundary of the cloud of points in the mohr space represented by inherited faults this operation fixes the abscissa origin of the mohr diagram to calculate the rupture law we estimate both intact rock and outcrop quality parameters and need pairs of conjugate faults their dihedral angle fixes a point on the mohr circle where the rupture law has to be tangent thus defining the scale of the axes and the values of the principal stresses to define the paleo depth we estimate the average density of overlying rocks and the hydraulic conditions i e the pore pressure testing of this workflow in three well constrained geological case studies proved its promising reliability showing a good agreement with independent stratigraphic and structural evidence

paleomagnetic evidence for tertiary anticlockwise rotation in southwest puerto rico late cretaceous and eocene paleomagnetic results from calcareous mudstone limestone and volcaniclastic rocks in southwest puerto rico yield a significant anticlockwise discordance in magnetization direction relative to north american reference data progressive thermal demagnetization has revealed stable characteristic high blocking temperature magnetization components that display an intrasite bipolarity the direction of this stable magnetization in tilt corrected coordinates is consistently either northwest and down or southeast and up the pole position 39 4 n lat 209 6 e long dp 9 3 dm 16 7 n 10 suggests post eocene anticlockwise rotation of the sampling region relative to north america by 45 about a vertical axis a rotating block domain model in which north south compression is superimposed on wnw ese strike slip faults in sw puerto rico can explain the inferred rotation

thermal isotasy in the south atlantic a new approach we present a new way to estimate the thermal diffusivity and the thickness of the oceanic lithosphere based on admittances computed from geoid and bathymetry profiles which roughly parallel the ridge this method appears to operate in die medium wave band 500 1500 km and may be of help in remote oceanic areas where only poor bathymetry is available we apply the method to 32 profiles on each flank of the south atlantic spreading ridge between the equator and 35 s and obtain the solution domain for both the plate thickness and the thermal diffusivity anomalously high admittance values obtained for the west flank at wavelengths around 700 km may be due to deeper asthenospheric density heterogeneities

analysis of fault slip inversions do they constrain stress or strain rate fault slip data commonly are used to infer the orientations and relative magnitudes of either the principal stresses or the principal strain rates which are not necessarily parallel or equal at the local scale of an individual fault the shear plane and slip direction define the orientations of the local principal strain rate axes but not in general the local principal stress axes at a large scale the orientations of p and t axes maxima for sets of fault slip data do not provide accurate inversion solutions for either strain rate or stress the quantitative inversion of such fault slip data however provides direct constraints on the orientations and relative magnitudes of the global principal strain rates to interpret the inversion solution as constraining the global principal stresses requires that 1 the fault slip pattern must have a characteristic symmetry no lower than orthorhombic 2 the material must be mechanically isotropic and 3 there must be a linear constitutive relationship between the global stress and the global strain rate isotropic linear elastic constitutive equations are appropriate to describe the local deformation surrounding an individual slip discontinuity fault slip inversions however constrain the characteristics of a large scale cataclastic flow which is described by constitutive equations that are probably but to an unknown degree anisotropic and nonlinear such material behavior would not strictly satisfy the requirements for the stress interpretation thus at the present state of knowledge fault slip inversion solutions are most reliably interpreted as constraining the principal strain rates

mid eocene early oligocene sinistral transcurrent faulting in puerto rico associated with formation of the northern caribbean plate boundary zone left lateral transcurrent wrench faulting through the southern puerto rico fault zone sprfz in central southern puerto rico is indicated by fault orientations and motion direction indicators the sprfz is a west northwest trending 3 8 km wide diffuse system of faults and folds that within the study area bounds paleocene and eocene volcaniclastic rocks the eocene belt and is flanked by cretaceous volcanic and volcaniclastic rocks structural data show that much of central southern puerto rico is cut by strike and oblique slip faults whereas locally in the southeastern part of the eocene belt rocks are pervasively thrust northeastward over cretaceous rocks of central puerto rico structural data and the distribution of fault types support two local evolutionary scenarios 1 different structural horizons of the transpressional sprfz i e upper thrust dominated horizon and lower strike slip dominated horizon were juxtaposed along post transpressional normal faults that border the ponce basi

instabilities of fluid conduits in a flowing earth are plates lubricated by the asthenosphere summary a laboratory and theoretical study of the stability of conduits of buoyant fluid in a viscous shear flow has been conducted the object of the study is to explain the formation of discrete islands in island chains such as the hawaiian emperor seamount chain and to investigate a new method by which the variation of shear with depth in the mantle may be determined the conduits were made by injecting oil into a more viscous oil of greater density initially a growing chamber of lower viscosity oil formed near the injector but when the chamber got sufficiently large it rose as a buoyant spheroid behind this trailed a vertical cylindrical conduit through which fluid could continue to rise to the surface as long as the source continued if the more viscous fluid was sheared laterally the conduit was gradually rotated to a more horizontal position the diameter of the conduit increased with time due to a decreasing component of gravitational force along the axis of the conduit when the conduit was tilted to more than 60 with the vertical it began to go unstable by developing bumps which ultimately initiated a new chamber which rose to a new spot in addition if the reynolds number of the conduit was greater than approximately ten an axisymmetric wavy instability appeared in the walls of the conduit and the conduit had to be tilted less before a new chamber was initiated if shear under the pacific plate has to tilt buoyant mantle plumes to as much as 60 to form the relatively regular island chains associated with hot spots most of the shear would be found in a zone with a vertical extent of less than 200 km

induced compositional layering in a convecting fluid layer by an endothermic phase transition abstract an endothermic phase change in the presence of chemical heterogeneities can act as an efficient filter such that heavy material is sequestered below the phase change and light material is trapped above the phase change due to the fact that the endothermic phase change damps thermal convection it can act as a trap door for chemical heterogeneities ascending flows which are chemically dense will need more thermal buoyancy to rise through the transition than ascending flows whose composition is closer to the background composition likewise descending flows which are chemically light need more thermal buoyancy to sink through the transition the results of a suite of numerical calculations of two dimensional double diffusive convection in a fluid layer with an endothermic phase change suggests that if the 670 km seismic discontinuity has a steep enough clapeyron slope compositional layering may be induced and dynamically maintained by the transition furthermore these calculations also show that the phase change and compositional boundaries will approximately coincide

neotectonics of puerto rico and the virgin islands northeastern caribbean from gps geodesy the boundary between the north american and caribbean plates is characterized primarily by left lateral motion along predominantly east west striking faults seismicity and marine geophysical survey data are consistent with at least two and possibly three microplates in the diffuse boundary zone in the northeastern caribbean 1 the gonave 2 the hispaniola and 3 the puerto rico northern virgin islands prvi we discuss results from gps geodetic measurements acquired since 1994 to test the microplate hypothesis define prvi translation and rotation within the boundary zone and constrain prvi neotectonics gps derived velocities are analyzed with respect to both north american and caribbean plate reference frames integrated displacements across prvi are limited to a few millimeters per year consistent with a rigid prvi and permitting calculation of an average velocity for prvi the motions of prvi relative to north america and the caribbean are 16 9 1 1 mm yr toward n68 e 3 1 and 2 4 1 4 mm yr toward s79 w 26 1 respectively in contrast with some recent models ongoing rotation of prvi about a nearby 25 distant vertical axis is not supported by the geodetic data in addition we argue against eastward tectonic escape of prvi and favor a simple progressive increase in velocity across the plate boundary zone requiring that the summed magnitude of strike slip fault slip rates will equal the total plate motion rate between the caribbean and north america gps data are consistent with components of left lateral strike slip faulting along the muertos trough south of puerto rico and shortening across the puerto rico trench comparison of gps velocities for prvi with respect to north america with total north america caribbean relative motion suggests up to 85 of north american caribbean plate motion is accommodated by the puerto rico trench and offshore faults north of puerto rico differences in gps derived velocities from hispaniola and prvi yield east west extension across the n s trending mona rift of a few millimeters per year when estimated elastic strain accumulation effects along the north hispaniola deformed belt and the septentrional fault zone are considered the opening rate implies an age of the mona rift of 2 3 million years agreeing with marine geophysical data that support a young age for the structure

unravelling polyphase brittle tectonics through multi software fault slip analysis the case of the voltri unit western alps italy abstract we have analyzed the brittle structures in a key area of the alps apennines transition zone nw italy here two orogens have interfered with each other since oligocene times producing a complex structural evolution and a heterogeneous fault population only relative chronologies can be reconstructed as stratigraphic constraints are lacking we have performed the inversion of fault slip data with two softwares for paleostress calculations combined with field observations at selected structural stations and photo interpretation the resulting incompatible stress tensors can be grouped in this sequence i a strike slip tensor with nnw sse trending 1 event a ii a strike slip tensor with ne sw trending 1 event b and iii an extensional transtensional tensor with nw se or ne sw trending 3 event c we correlated our results with structures of known age from adjacent areas event a is possibly rupelian early chattian linked to far field incipient rifting in the future ligurian provencal basin event b fits oligo miocene shortening the faults may belong to a sinistral strike slip zone that accommodated the oblique component of deformation during the rotation of the corsica sardinia block event c is attributed to a pliocene quaternary neotectonic event therefore combining different inversion procedures with a detailed structural analysis has successfully unravelled the polyphase brittle tectonics

solution to the volume problem in serpentinization kernel pattern consists of a rim of completely serpentinized peridotite surrounding a core of partly serpentinized or unserpentinized peridotite it is found adjacent to shear zones in some serpentinites kernels are commonly rectangular with length width ratios of 1 to 3 the boundary between the completely serpentinized rim and the unserpentinized core is commonly cut at a high angle by cross fractures the cross fractures which contain either picrolite or chrysotile asbestos taper out in the core and widen in the rim they are almost always perpendicular to the core rim boundary and point into the core of the kernel in this sense they are radially distributed in the rim the kernel pattern can be observed in thin section in outcrop and on the megascopic scale of tens of metres cross fractures represent expansion fractures in serpentinite in the rim of the kernel caused by an increase in volume acompanying serpentinization of the peridotite in the core of the kernel as the serpentinization front penetrates into the kernel the serpentinized peridotite expands however the serpentinite in the rim cannot expand and must fracture to accommodate the increase in volume as serpentinization proceeds more expansion occurs extending the fractures farther into the core and requiring the fractures in the existing serpentinite to widen these fractures are subsequently filled by serpentine minerals the resulting pattern consists of a rim of serpentinite cut by serpentine veins that surrounds a core of peridotite measured densities of cross fractures in the rims of kernels are consistent with those expected from the magnitude of the volume change accompanying serpentinization of peridotite

geoid anomalies across pacific fracture zones summary cooling of the oceanic lithosphere and related subsidence of the ocean floor are usually understood in terms of simple conductive thermal models half space and plate cooling models which predict the same behaviour for heat flow sea floor depth and geoid on very young sea floor but large deviations at greater ages since geoid data are much more sensitive to the thermal structure of the lithosphere at depth than other kinds of data satellite derived geoid anomalies in particular geoid steps observed across fracture zones are currently analysed to discriminate between the two thermal models in this study we present results for the geoid step h estimated along altimeter profiles of the seasat satellite across five fracture zones fzs of the ne pacific clipperton clarion molokai murray and mendocino and across the udintsev fz in the s pacific for each fz the quantity geoid offset h divided by the age offset t is plotted as a function of plate age except for mendocino and clipperton fzs along which the variation of h t with age is consistent with the prediction of the thermal plate model with a plate thickness of 90 100 km the dependence on age of h t along the other fzs is different from that predicted by conductive cooling models large fluctuations of the h t age relationship are observed along murray clarion and udintsev fzs suggesting that the thermal structure of the oceanic lithosphere under a fz is more complex than is to be expected from conductive cooling alone according to recent convection models small scale convective instabilities developed underneath the fz in a shallow low viscosity layer could explain the observed h t age behaviour

experiments on the interaction of thermal convection and compositional layering at the base of the mantle accumulation and survival of compositional heterogeneity at the base of the convecting mantle is investigated using laboratory experiments with temperature dependent viscous fluids two models of heterogeneity in the d layer are considered 1 a reservoir of dense material above the core mantle boundary and 2 continuous recharge of the d layer by subduction of cold negatively buoyant high viscosity slabs of oceanic lithosphere survival of a dense reservoir depends on rp the ratio of stabilizing compositional buoyancy to destabilizing thermal buoyancy rapid mixing occurs for rp 1 for rp 1 mixing occurs by slow entrainment only the reservoir model predicts d layer heterogeneity is concentrated beneath lower mantle upwellings differentiation of composite slabs consisting of dense crustal and light depleted components occurs in a hot thermal boundary layer composite slabs pile up in the basal thermal boundary layer and upon heating the light component is ejected by rayleigh taylor instabilities the dense component accumulates in the boundary layer arrayed in a pattern of interconnected spokes and is slowly entrained into thermal plumes according to these experiments the time required for conversion of slabs to plumes in d is 1 2 gyr the correlation between lateral variations in d layer thickness and lateral variations in lower mantle seismic velocity suggests that d may consist of dense material perhaps former oceanic lithosphere which has sunk through the lower mantle

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

geoid data and thermal structure of the oceanic lithosphere a long standing question is whether old oceanic lithosphere continues cooling as the boundary layer of a halfspace or approaches thermal equilibrium as modeled by a finite thickness plate although the latter is the most direct inference from seafloor depths and heat flow other explanations have been proposed we investigate this issue using published results for the derivative of the oceanic geoid with age estimated from geoid offsets across fracture zones such data have not been used extensively in analyses of the thermal evolution of the lithosphere primarily because they are inconsistent with two commonly used thermal models a halfspace or a 125 km thick plate recent studies however find that depth and heat flow data are better fit by a thinner 95 km plate model we thus compile published geoid slope results and find that these data though scattered can discriminate between the models geoid slope changes with age rather than being constant as predicted for a cooling halfspace this variation is greater than predicted for a thick plate and is better fit by a thin plate geoid data should thus be useful for improving thermal models of the lithosphere

plate kinematics of the south atlantic chron c34 to present a high resolution seafloor spreading history of the south atlantic since chron c34 is constrained by a combination of seasat altimeter data and underway marine geophysical data a set of 45 finite rotation poles defines the relative position of africa and south america at roughly 2 m y intervals a set of 12 stage poles constrain the relative motion of these two plates at 5 to 10 m y intervals the position of the stage poles continuously migrates reflecting the continuously changing azimuths of the fracture zones major changes in spreading direction are observed in the late cretaceous and early cenozoic as the fracture zones sweep out broad s shaped curves similar to the pattern seen on the kane fracture zone in the central atlantic small offset fracture zones were found to be the most accurate recorders of changes in plate motion large offset fracture zones such as the agulhas falkland fracture zone were the least reliable recorders at 30 s spreading rates decrease throughout the late cretaceous from a high of 75 mm yr at the end of chron c34 to a low of 30 mm yr around chron c27 a period of slow spreading between chron c30 and chron c20 corresponds to a zone of fracture zone proliferation an increase in the amplitude of geoid anomalies over fracture zones greater relief on topographic profiles and locally evidence of intraplate crustal deformation spreading rates increase at chron c20 to about 50 mm yr and then gradually decrease during the late paleogene and neogene a comparison of synthetic fracture zones based on the south atlantic stage poles to the observed trends of fracture zones in the equatorial atlantic indicates that the vema and marathon fracture zones were generated by south atlantic spreading as opposed to central atlantic spreading at least as far back as 35 m y ago and possibly 50 m y ago

muertos trough subduction microplate tectonics in the northern caribbean the relative plate motion between the north american and caribbean plates has been commonly assumed to take place in the form of oblique subduction along the puerto rico trench1 west of hispaniola this plate boundary becomes pure strike slip nevertheless it has been suggested that this boundary is more diffuse with part of the plate motion taken up by subduction along the muertos trough mt south of hispaniola2 5 fig 1 but there has been no direct seismological evidence of subduction to confirm this here we present the results of a study of the 24 june 1984 earthquake beneath mt the focal mechanism shows thrusting at a depth of 32 km which together with reports of a great earthquake in 1751 confirms that mt is an active plate boundary this implies that plate motion is accommodated by several tectonic boundaries delineating microplates caught between the caribbean and north american plates fig 4

grim gravity model improvement using lageos grim3 l1 improved modeling of the earth s geopotential is one of the major tasks of satellite geodesy the availability of a new set of precise lageos satellite laser ranging slr data as well as seasat acquired observations enabled us to compute an improved solution in the grim model series the new geopotential grim3 l1 recombined the satellite perturbation normals of the former grim3 solution with new normal equation subsystems derived from an additional 16 months of lageos slr data oceanic 1 1 mean free air anomalies recovered from seasat altimetry an updated set of terrestrial 1 1 mean free air anomalies and several condition equations for zonal and resonant harmonics the combined normal equations were solved for a complete set of spherical harmonics up to degree and order 36 and the coordinates of 109 tracking stations comparisons with altimetry derived quantities terrestrial gravity anomalies from satellite to satellite tracking longitude accelerations of 24 hour satellites and satellite orbit fits show a substantial improvement over previous grim solutions and indicate that the lageos data have contributed considerably to the reliable estimation of the long wavelength portion of the gravity field

hotspots mantle plumes and core heat loss abstract the heat flux at the core mantle boundary cmb is a key parameter for core dynamics since it controls its cooling however it is poorly known and estimates range from 2 tw to 10 tw the lowest bound comes from estimates of buoyancy fluxes of hotspots under two assumptions that they are surface expression of mantle plumes originating from the base of the mantle and that they are responsible for the totality of the heat flux at the cmb using a new procedure to detect plumes in a numerical model of rayleigh benard convection convection between isothermal horizontal planes with internal heating it is shown that many hot plumes that start from the bottom boundary do not reach the top surface and that the bottom heat flux is primarily controlled by the arrival of cold plumes hot plumes easily form at the bottom boundary but they are mostly due to the spreading of cold plume heads that allow the concentration of hot matter these plumes are generally not buoyant enough to cross the whole system and the hot plumes that reach the top surface result from an interaction between several hot plumes according to this simple dynamical behavior the heat flux at the bottom boundary is shown to be strongly correlated with the advection due to cold plumes and not with advection by hot plumes that arrive at the surface it is then inferred that the heat flux out of hotspots can only give a lower bound to the heat flow at the cmb and that knowing the advection by subducted plates would give a better estimate

cretaceous paleogene thrust emplacement of serpentinite in southwestern puerto rico serpentinite emplacement mechanisms along convergent plate boundaries are complex and not well understood this investigation combined structural and stratigraphic data from serpentinite in puerto rico to suggest an emplacement mechanism along the north american caribbean plate boundary northward dipping thrust faults and foliations along the southern margins of the monte del estado and rio guanajibo serpentinite bodies in southwestern puerto rico indicate southwestward thrust movement of serpentinite structural analysis of faults folds and foliation recorded within the serpentinite bodies and analysis of contact relationships with volcano sedimentary rocks lead to the interpretation that thrust movements record contractional deformations occurring during the maastrichtian paleocene and late eocene early oligocene intervals the conclusion that serpentinite has been most recently emplaced above thrust faults during transpression between the caribbean north america plates is in contrast to the earlier hypothesis that serpentinite was emplaced in response to diapirism late eocene transpression reactivated older faults and also displaced the serpentinite although the timing of the serpentinite emplacement in southwestern puerto rico is similar to emplacement events in cuba and hispaniola there are differences in the origin mechanism and direction of emplacement of the serpentinite masses the heterogeneity in the serpentinite emplacement mechanism and transport direction evokes reconsideration of the deformation history along the caribbean north american plate boundary

normal mode and free air gravity constraints on lateral variations in velocity and density of earth s mantle with the use of a large collection of free oscillation data and additional constraints imposed by the free air gravity anomaly lateral variations in shear velocity compressional velocity and density within the mantle dynamic topography on the free surface and topography on the 660 km discontinuity and the core mantle boundary were determined the velocity models are consistent with existing models based on travel time and waveform inversions in the lowermost mantle near the core mantle boundary denser than average material is found beneath regions of upwellings centered on the pacific ocean and africa that are characterized by slow shear velocities these anomalies suggest the existence of compositional heterogeneity near the core mantle boundary

curved slickenfibers a new brittle shear sense indicator with application to a sheared serpentinite abstract brittle fault zones are commonly characterized by penetrative fracturing of the rock to form an aggregate of blocks so that the rock becomes in essence like a granular material composed of rigid grains the surfaces of the blocks or grains have a wide distribution of orientations and shearing of the blocks past one another on these planes accommodates the large scale motion i e the macromotion in the fault zone during a macromotion that is a non coaxial deformation the rigid blocks and their surfaces may undergo a progressive rigid rotation that is distinct from the macromotion and is described by the microspin if a macroscopic non coaxial deformation has monoclinic symmetry this symmetry should be reflected in the sense of rigid rotation of the blocks and their surfaces on the surfaces of the blocks which are local shear planes the history of displacement may be recorded by mineral fibers that grow progessively with displacement because of the rigid rotation of the local shear planes slickenfiber lineations commonly are curved the sense of curvature from the youngest to the oldest part of the fiber looking down the normal to the local shear plane is different for those planes whose normals are on opposite sides of the unique monoclinic symmetry plane for the macroscopic shearing the intersection of the symmetry plane with the plane of the fault zone defines the macroscopic slip direction and the sense of rigid rotation of the local shear planes determined from the lineations defines the shear sense application of this technique to the disrupted and sheared margins of the feather river peridotite in the northern sierra nevada of california indicates that late deformation involved dextral normal oblique slip along the melones fault zone

asymmetry of the mantle structure beneath the mid atlantic ridge altimetry data from repetitive orbits of the geosat satellite are interpreted together with sea floor depths over the mid atlantic ridge south of iceland and south of the azores geoid topography ratios indicate a strong asymmetry of the upper mantle structure in oceanic areas created since 35 my extending several hundreds kilometers from iceland to the azores a model including interaction between the mar and two layers of mantle convection seems adequate to produce the observed asymmetry

two high pressure low temperature serpentinite matrix melange belts motagua fault zone guatemala a record of aptian and maastrichtian collisions left lateral motion along the north american caribbean plate boundary has juxta posed two high pressure low temperature hp lt belts from separate cretaceous colli sions these two belts have quite different ages and different suites of high pressure as semblages yet they both contain jadeitite a relatively rare rock type this part of the plate boundary zone follows the motagua river valley in guatemala where it separates the maya block north american plate from the chortis block caribbean plate on both sides of the bounding motagua fault tectonic slices of serpentinite matrix melange host the hp lt rocks south of the fault the melange slices contain eclogite lawsonite eclogite glaucophane eclogite and blueschist blocks north of the fault the melange slices contain omphacite metabasite albitite and garnet amphibolite blocks but lack intact eclogite in addition to the dissimilar rock assemblages 40 ar 39 ar geochronology of phen gitic micas yields 77 65 ma for northern and 125 113 ma for southern blocks these data suggest that the southern belt formed during early cretaceous aptian northeastward dipping subduction of the farallon plate and collision of the chortis block with western mexico the block was then displaced southeastward along this suture in contrast the northern belt records subduction related to the maastrichtian collision of an extension of the chortis block perhaps the nicaraguan rise with the maya block

the internal deformation of the peridotite nappe of new caledonia a structural study of serpentine bearing faults and shear zones in the koniambo massif we present a structural analysis of serpentine bearing faults and shear zones in the koniambo massif one of the klippes of the peridotite nappe of new caledonia three structural levels are recognized the upper level is characterized by a dense network of fractures antigorite and polygonal serpentine form slickenfibers along fault planes with distinct kinematics as a result the upper level keeps the record of at least two deformation events the first associated with the growth of antigorite wnw ese extension the second with the growth of polygonal serpentine nw se compression the lower level coincides with the serpentine sole of the nappe which consists of massive tectonic breccias overlying a layer of mylonitic serpentinites the sole records pervasive tangential shear with top to sw kinematics and represents a decollement at the base of the nappe the intermediate level is characterized by the presence of several meters thick conjugate shear zones accommodating ne sw shortening like the sole these shear zones involve polygonal serpentine and magnesite as the main syn kinematic mineral phases the shear zones likely root into the basal decollement either along its roof or occasionally around its base compared to top to sw shearing along the sole the two deformation events recorded in the upper level are older

development of schistosity by dissolution crystallization in a californian serpentinite gouge to understand the behaviour and deformation mechanisms of serpentinites in the seismogenic zone we study the deformation macro and microstructures of serpentinites along the santa ynez fault in the san andreas system at the outcrop scale deformation is localized in a gouge zone that shows three different structures 1 micrometric undeformed fragments clasts of the previously serpentinized peridotite 2 localized shear planes y and r and 3 a penetrative schistosity s observations under sem and tem reveal that the schistosity corresponds to serpentine fibres parallel to each other and whose orientation varies as they wrap around clasts tem micro textures indicate that these long fibres result from continuous syntectonic growth rather than from reorientation of pre existing fibres implying a slow transfer process that occurs at short distances we propose a dissolution diffusion crystallization process for the formation of the schistosity that corresponds to a low strain rate creeping process of deformation that can be effective in aseismic fault segments

dynamic topography and gravity anomalies for fluid layers whose viscosity varies exponentially with depth summary we have derived analytic integral relations between boundary topography and temperature as a function of wavenumber for a fluid layer whose viscosity varies exponentially with depth similar relations between gravity and temperature are also derived it is found that when the viscosity changes little over the depth of the layer less than an order of magnitude the topography kernels for both the surface and bottom boundaries are similar to those for the isoviscous case the differences being more pronounced at intermediate wavelengths twice the layer depth at these wavelengths the extrema of the gravity kernel can be altered by as much as 30 per cent for a factor of two increase or decrease in viscosity over the depth of the layer at wavelengths less than the layer depth there are dependences on both the rate of growth of viscosity and the surface boundary condition the latter dependency vanishing in the isoviscous case for greater viscosity variation several orders of magnitude topography and gravity kernels at all wavelengths are strongly affected viscosity increasing rapidly with depth can approximate a rigid lower boundary condition causing gravity kernels calculated with a free lower boundary condition to be negative for a significant range of source depths for very rapidly varying viscosity the sign of the perturbation pressure near the layer boundaries can be reversed from the isoviscous case resulting in topography kernels that are negative for some source depths

structural analysis of hanging wall and footwall blocks within the rio guanajibo fold and thrust belt in southwest puerto rico the rio guanajibo fold and thrust belt rgft composed of cretaceous serpentinite and volcano sedimentary rocks represents the deformation front of a contractional event in sw puerto rico during the paleogene previous studies inferred structural and stratigraphic relationships from poorly exposed outcrops new road cuts exposed the yauco yf and el rayo formations erf providing insights on the deformation of the hanging wall and footwall we described the nature and orientation of faults and folds and analyzed the kinematic indicators to characterize the deformation the yf occurs in the hanging wall and shows a sequence of folded medium bedded mudstone and thinly bedded shale and sandstone major folds strike nw se and are gentle with steeply inclined axial planes and sub horizontal fold axes minor folds are open with moderately inclined axial planes and gently to moderately inclined se plunging fold axes nw se striking reverse and thrust faults cut layers and show movement to the sw steep left lateral faults strike nw se and ne sw and smaller right lateral strike slip faults strike nne ssw at the footwall the erf consists of bioclastic limestone and polymictic orthoconglomerates and paraconglomerates reverse and strike slip faults cut along lithological contacts results suggest that the hanging wall and footwall accommodated strain along preexisting weaknesses which are dependent on lithology and sedimentary structures the kinematic analysis suggests that shortening in the ne sw direction was partitioned between folding and interlayer shortening accommodated by flexural slip and reverse and left lateral faults that resulted from contraction the rgft represents the paleogene back arc deformation of a bivergent thrust system

linking lowermost mantle structure core mantle boundary heat flux and mantle plume formation abstract the dynamics of earth s lowermost mantle exert significant control on the formation of mantle plumes and the core mantle boundary cmb heat flux however it is not clear if and how the variation of cmb heat flux and mantle plume activity are related here we perform geodynamic model experiments that show how temporal variations in cmb heat flux and pulses of mantle plumes are related to morphologic changes of the thermochemical piles of large scale compositional heterogeneities in earth s lowermost mantle represented by the large low shear velocity provinces llsvps we find good correlation between the morphologic changes of the thermochemical piles and the time variation of cmb heat flux the morphology of the thermochemical piles is significantly altered during the initiation and ascent of strong mantle plumes and the changes in pile morphology cause variations in the local and the total cmb heat flux our modeling results indicate that plume induced episodic variations of cmb heat flux link geomagnetic superchrons to pulses of surface volcanism although the relative timing of these two phenomena remains problematic we also find that the density distribution in thermochemical piles is heterogeneous and that the piles are denser on average than the surrounding mantle when both thermal and chemical effects are included

effect of mantle plumes on the growth of d by reaction between the core and mantle the core mantle boundary is a fundamental compositional discontinuity in the earth where molten iron alloy from the core meets solid silicate minerals from the mantle heat flow from the core to the mantle creates a thermal boundary layer at the base of the mantle at the same time chemical reactions may create a layer of different composition and density than the overlying mantle using a finite element model of convection in a spherical shell that includes formation of dense material at the base of the shell we investigate how a layer forms at the base of the mantle development of a layer at the base of the mantle by this method depends both on the composition of the material forming at the core mantle boundary and on the rate at which the material diffuses into the mantle if the material is less than 3 6 denser than the overlying mantle assuming reasonable choices of lower mantle thermodynamic parameters the reactant material will be swept away by upwelling plumes more dense material on the other hand forms an internally convecting layer that entrains material from above the varying thickness and composition of the layer is consistent with geophysical observations of the characteristics of the deep mantle

high resolution 3 d s velocity structure in the d region at the western margin of the pacific llsvp evidence for small scale plumes and paleoslabs abstract although previous tomographic studies found a large low s velocity province llsvp in the lowermost mantle beneath the pacific due to a lack of resolution it remains unclear whether the llsvp consists of clusters of small scale low velocity anomalies or large scale anomalies we recently deployed a seismic array in thailand which provides a dataset with wide azimuthal coverage at the western pacific llsvp we analyze the new dataset using waveform inversion and find high velocity anomalies extending vertically to a height of 400 km above the core mantle boundary cmb beneath the philippine sea and small scale low velocity patches with a diameter of 300 km at the cmb beneath new guinea the locations of the high velocity anomalies are consistent with the past izanagi plate subduction boundary and the low velocity anomalies can be interpreted as a small scale plume cluster hence we conclude that vertical flow upwelling plumes and downwelling of slabs is dominant in the lowermost mantle beneath the western pacific region

turbulent 3 d thermal convection in an infinite prandtl number volumetrically heated fluid implications for mantle dynamics summary the structure and time dependence of 3 d thermal convection in a volumetrically heated infinite prandtl number fluid is examined for high values of the rayleigh number the methods employed allow the numerical experiments to proceed for long enough times to derive good estimates of mean and fluctuating parts of the structure an iterative multirigid method to solve for the buoyant incompressible viscous flow at each time step of the energy equation is a novel aspect of the methodology a simple explicit time step of the energy equation is utilized that vectorizes well on serial computers and which is ideally suited to massively parallel computers numerical experiments were carried out for rayleigh numbers from 3 106 to 3 107 in a cartesian region with a prescribed temperature at the top boundary and an adiabatic bottom boundary over this complete range of rayleigh number the flow structure consists dominantly of cold nearly axisymmetric plumes that migrate horizontally sweeping off the cold thermal boundary layer that forms at the top of the convecting fluid plumes disappear by coalescing with other plumes new plumes are created by thermal boundary layer instability sheet plumes form only occasionally and do not penetrate to significant depths in the fluid plumes have sizes comparable to the thickness of the thermal boundary layer and an average spacing comparable to the fluid depth no persistent large scale motion in the fluid can be identified its absence may reflect the large subadiabatic stratification that develops beneath the thermal boundary layer as cold plumes penetrate to the bottom boundary without thermally equilibrating with surrounding fluid we consider the possible implications for convection in planetary mantles and for the existence of plate tectonics

thermochemical structures beneath africa and the pacific ocean large low velocity seismic anomalies have been detected in the earth s lower mantle beneath africa and the pacific ocean that are not easily explained by temperature variations alone the african anomaly has been interpreted to be a northwest southeast trending structure with a sharp edged linear ridge like morphology the pacific anomaly on the other hand appears to be more rounded in shape mantle models with heterogeneous composition have related these structures to dense thermochemical piles or superplumes it has not been shown however that such models can lead to thermochemical structures that satisfy the geometrical constraints as inferred from seismological observations here we present numerical models of thermochemical convection in a three dimensional spherical geometry using plate velocities inferred for the past 119 million years we show that earth s subduction history can lead to thermochemical structures similar in shape to the observed large lower mantle velocity anomalies we find that subduction history tends to focus dense material into a ridge like pile beneath africa and a relatively more rounded pile under the pacific ocean consistent with seismic observations

thermal diffusivity of the lithosphere derived from altimetry and bathymetry profiles across the southwest indian ridge in standard models for computing oceanic lithosphere thickness the lithosphere s thermal diffusivity is one of the basic parameters required and present estimates have a substantial uncertainty extending the work of crough 1979 lister 1982 and gibert and courtillot 1990 we propose a new method for the determination of thermal diffusivity of oceanic lithosphere in the near ridge crest domain the method requires geoid height and depth data both of which reflect wavelengths associated with the lithospheric cooling process and expressed as a function of seafloor age over the southwest indian ridge the thermal diffusivity so found 0 55 0 17 10 2cm2s 1 is consistent with data from the south atlantic differences in diffusivity calculated on either side of the ridge and caused by asymmetrical subsidence are possibly linked to a differential thermal regime beneath the ridge and or to the general motion of the ridge our findings are interpreted in terms of a lithospheric boundary layer less than 14age kmmyr thick and initial temperature between 1050 and 1390 c

metamorphic history of serpentinite mylonites from the happo ultramafic complex central japan serpentinite mylonites from the happo ultramafic complex show evidence of two stages of mylonitization at different temperature conditions peridotite mylonites exhibit two types of olivine porphyroclasts and neoblasts produced at the earlier stage the olivine neoblasts have a stretching lineation with a fabric suggesting plastic deformation along 0 1 0 0 0 1 in addition to the olivine fabric the stable association of olivine orthopyroxene and tremolite in the peridotites that survived later serpentinization and the si and na contents of tremolite suggest that the earlier mylonitization took place at temperatures between 700 and 800 c later mylonitization was associated with high temperature serpentinization to form serpentinite mylonites in contrast to a common type of serpentinite in orogenic belts the serpentinite mylonites are cohesively foliated rich in olivine and diopside and poor in antigorite the diopside has low al cr and na contents typical of a retrograde origin and the olivine has a homogeneous composition except in areas subjected to contact metamorphism at a later stage modal composition and mineral chemistry suggest that the serpentinite mylonites were formed by a hydration reaction of tremolite and olivine to produce diopside and antigorite under stable conditions of olivine at temperatures between 400 and 600 c later stage mylonitization has preferentially been superimposed on the earlier stage mylonite zone with a common direction of foliation the difference in temperature between the two mylonitization stages suggests that the shear zone was episodically active during the emplacement of the happo complex conditions of relatively high temperature for serpentinization at a convergent plate boundary and high permeability caused by the early mylonitization favoured the formation of the serpentinite mylonites

paleomagnetic evidence for late miocene counterclockwise rotation of north coast carbonate sequence puerto rico a paleomagnetic study of the essentially undeformed middle tertiary carbonate sequence along the north coast of puerto rico reveals statistically significant pre pliocene discordance of characteristic component directions against those expected from cratonic north america for much of the section despite generally weak to moderately weak magnetic intensities confirmation of the magnetization as primary in origin comes from the presence of two distinct components of magnetization intrasite bipolarity and or the reproducibility of measurements the mean geographic direction for the upper oligocene to middle miocene strata is 335 2 degree 32 9 degree and the corrected mean paleomagnetic pole is 207 6 degree 66 5 degree n 3 alpha 95 4 3 degree this suggests a counter clockwise ccw block rotation of puerto rico and its microplate of 24 5 degrees plus minus 5 8 degrees during the late miocene using a width of 250 km for the northern caribbean plate boundary zone ncpbz between the north american plate and caribbean plate the mean left lateral displacement implied is 1 8 to 2 4 cm yr which agrees fairly well with published relative motion rates for the two plates average rotation rate for 50 ma to 20 ma was 0 7 degree my but perhaps as great as 4 degree my in the miocene resolution of mean paleolatitude indicates northward motion of a degree or lessmore during the period of rotation causes of this short lived rotation may include 1 tectonic escape from the inhibiting presence of the bahama banks and beata ridge during eastward motion of puerto rico along the sinistral transpressive puerto rico trench and muertos trough fault systems or 2 changes in relative plate motions of the caribbean and north american plate during the late miocene less

mantle updrafts and mechanisms of oceanic volcanism convection in an isolated planet is characterized by narrow downwellings and broad updrafts consequences of archimedes principle the cooling required by the second law of thermodynamics and the effect of compression on material properties a mature cooling planet with a conductive low viscosity core develops a thick insulating surface boundary layer with a thermal maximum a subadiabatic interior and a cooling highly conductive but thin boundary layer above the core parts of the surface layer sink into the interior displacing older colder material which is entrained by spreading ridges magma characteristics of intraplate volcanoes are derived from within the upper boundary layer upper mantle features revealed by seismic tomography and that are apparently related to surface volcanoes are intrinsically broad and are not due to unresolved narrow jets their morphology aspect ratio inferred ascent rate and temperature show that they are passively responding to downward fluxes as appropriate for a cooling planet that is losing more heat through its surface than is being provided from its core or from radioactive heating response to doward flux is the inverse of the heat pipe mantle plume mode of planetary cooling shear driven melt extraction from the surface boundary layer explains volcanic provinces such as yellowstone hawaii and samoa passive upwellings from deeper in the upper mantle feed ridges and near ridge hotspots and others interact with the sheared and metasomatized surface layer normal plate tectonic processes are responsible both for plate boundary and intraplate swells and volcanism

thermal stresses in the oceanic lithosphere evidence from geoid anomalies at fracture zones models for the thermal and mechanical evolution of the oceanic lithosphere predict the progressive development of large thermal stresses in the thickening plate however there has so far been little direct evidence for the magnitude and distribution of thermal stresses we present theoretical models which examine the effect of thermal stresses at fracture zones and show that an anomaly of the predicted form can be observed in geoid profiles which cross fracture zones specifically our models predict the development of thermal bending moments which depend on lithosphere thickness or age and therefore change across fracture zones including the effect of varying thermal bending moments thin plate theory predicts vertical nonisostatic displacements of the lithosphere by plate flexure the predicted amplitude of the resulting geoid anomaly is large enough to be observed in seasat altimeter profiles furthermore the general form of this anomaly differs sufficiently from other predicted components of the geoid anomaly at fracture zones to be discernable the anomaly due to thermal stresses has been clearly identified in geoid profiles across the clarion and the udintsev fracture zones the amplitude of this observed anomaly is well predicted if cooling lithosphere begins to accumulate elastic stresses at a temperature of 700 c consistent with the maximum depth of seismicity in the oceanic lithosphere the distribution of thermal stresses with depth is also consistent with focal mechanisms of intraplate earthquakes

seasat altimetry and the south atlantic geoid 2 short wavelength undulations this paper is a discussion of the short wavelength 25 100 km undulations of the geoid recovered from seasat altimeter data in the south atlantic these undulations are known to be highly correlated to seafloor topography and they considerably improve our knowledge about global bathymetry of the south atlantic which remains poorly surveyed the orientation of fracture zones fz with respect to the seasat profiles ensures an efficient recovery of the fz pattern in the younger 65 ma parts of the south atlantic between 20 n and 50 s we used 22 well marked fzs regularly distributed between 12 n and 47 s to compute eulerian poles describing the finite relative movement between africa and south america from the present back to 35 ma fz trends south of the equator are well accounted for with a single rotation and standard statistical tests do not indicate a need for distinct and successive rotations however these tests indicate an improvement of the fit when distinct poles are adjusted to separate subsets of our 22 fzs either fzs north and south of the equator or fzs north and south of tristan da cunha this points out the possibility of the existence of either corridors in which slight relative movements occur or diffuse elastic deformation over the entire south atlantic although this is really a second order effect we also present and discuss a map of the roughness of the geoid for the 25 to 110 km wave band strong disagreements between the roughness map and the gebco maps exist in regions of sparse ship track coverage for example the rio grande rise has an associated roughness feature which appears elongated and more symmetric with respect to the walvis ridge than what is displayed on the gebco maps the roughness map also clearly documents a long wavelength 400 km segmentation of the mid atlantic ridge between the equator and 35 s

geoid anomalies across ascension fracture zone and the cooling of the lithosphere twenty three seasat altimeter profiles crossing the ascension fracture zone are used together with seafloor ages inferred from a kinematic model to estimate a geoid slope age relation between ages of 5 and 50 ma the data do not definitely contradict a half space cooling model slope 9 5 3 cm ma if biases are accepted resulting in large scatter however the data would seem to support a puzzling explanation in terms of a plate cooling model involving two distinct thermal thicknesses one of 70 km for ages 35 ma although preliminary this result is similar to that found in the pacific with a much larger data set

global mean sea surface based upon the seasat altimeter data a mean sea surface map has been computed for the global ocean areas between 70 n latitude and 62 s latitude based upon the 70 day set of seasat altimeter data recorded in 1978 a combination of regional crossover adjustment techniques and a global reference surface using the best available geodetic parameters for seasat has formed the basis for the computation the overall rms value of the crossover residuals in the open ocean areas was 13 cm after the adjustment a contour map of the crossover residuals is very similar in nature to earlier contour maps of the collinear pass results for mesoscale variability the mean sea surface is presented in the form of a global contour map a 0 5 0 5 grid and a computerilluminated image

kinematic analysis of serpentinite structures and the manifestation of transpression in southwestern puerto rico abstract faults and shear zones recorded in the monte del estado and rio guanajibo serpentinite masses in southwestern puerto rico show previously unrecognized southwestward tectonic transport the orientations of planar and linear structures and the sense of slip along faults and shear zones determined by offset rock layers drag folds in foliations and steps in slickensided surfaces and or s c fabrics from 1846 shear planes studied at more than 300 stations reveal two predominant groups of faults 1 northwesterly striking thrust faults and easterly striking left lateral faults and 2 northwesterly striking right lateral faults and easterly striking thrust faults shortening and extension p and t axes calculated for geographic domains within the serpentinite reveal early north trending shortening followed by southwestward directed movement during which older structures were re activated the sw directed shortening is attributed to transpression that accompanied late eocene left lateral shearing of the serpentinite a third younger group comprising fewer faults consists of northwesterly striking left lateral faults and north directed thrusts that also may be related to the latest transpressional deformation within puerto rico deformational events in puerto rico correlate to tectonic events along the caribbean north american plate boundary

the effect of a shallow low viscosity zone on the apparent compensation of mid plate swells abstract a simple model for mid plate swells is that of convection in a fluid which has a low viscosity layer lying between a rigid bed and a constant viscosity region finite element calculations have been used to determine the effects of the viscosity contrast the layer thickness and the rayleigh number on the flow and on the perceived compensation mechanism for the resulting topographic swell as the viscosity decreases in the low viscosity zone the effective local rayleigh number for the top boundary layer of the convecting cell increases also because the lower viscosity facilitates greater velocities in the low viscosity zone the low viscosity layer produces proportionally greater horizontal flow near the conducting lid causing the base of the conducting lid to appear like a free boundary the change in the local rayleigh number and in the effective boundary condition both cause the top boundary layer to thin through a green s function analysis we have found that the low viscosity zone damps the response of the surface topography to the temperature anomalies at depth whereas it causes the gravity and geoid response functions to change sign at depth counteracting the positive contributions from the shallower temperature variations by increasing the viscosity contrast the conbined effects of the thinning of the boundary layer and the behaviour of the response functions allow the apparent depth of compensation to become arbitrarily small therefore shallow depths of compensation cannot be used to argue against dynamic support of mid plate swells furthermore we compared the distribution of the effective compensating densities which is used to obtain the geoid to that of pratt compensation which is often used to calculate the depth of compensation from geoid and topography data for mid plate swells for all of our calculations including those with no low viscosity layer the effective gravitational mass distribution is more complex than assumed in simple pratt models so that the pratt models are not an appropriate gauge of the compensation mechanism

seismic geodynamic determination of the origin of excess ellipticity of the core mantle boundary we demonstrate that the excess dynamic ellipticity of the core mantle boundary cmb inferred from the period of the earth s retrograde free core nutation rfcn may be explained in terms of viscous stresses generated by mantle convective flow the 3d elastic structure and radial viscosity profile which define the model used in the viscous flow calculations are constrained to simultaneously reconcile a large set of seismic and geodynamic data

thermal cooling of the oceanic lithosphere new constraints from geoid height data abstract up to now tests of thermal models of the oceanic lithosphere as it cools and moves away from the ridge crest have been based mainly on topography and heat flow data however large areas of the ocean floor deviate from the normal subsidence due to thermal contraction and heat flow data are not very sensitive to the form of the model cooling of the lithosphere causes a short wavelength step in the geoid across fracture zones that can also be used to constrain thermal models we have analyzed geoid data at fracture zones from the seasat altimeter measurements in the entire pacific ocean and redetermined parameters of the cooling models we find that the data reveal two distinct regimes of cooling one for seafloor ages in the range 0 30 ma the other beyond 30 ma this does not appear to be correlated with particular fracture zones but rather it is representative of the whole area studied i e the entire south pacific and northeast pacific ocean these two trends may be interpreted in terms of two different asymptotic thermal thicknesses of the plate model the smaller thermal thickness 65 km found for ages from the results obtained in this study we conclude that the half space cooling model is unable to explain the data that beyond 30 ma a simple plate model gives a satisfactory fit to the data but in the younger plate portion ages

three dimensional convection planforms with internal heat generation thermal convection planforms in an infinite prandtl number isoviscous fluid layer with basal and internal heating are determined using three dimensional finite difference calculations with basal heating only at rayleigh number ra sub t 1 5 times 10 sup 5 the planform consists of a weakly time dependent spoke pattern with connected networks of equally buoyant up and downwellings with volumetric heat sources the planform is strongly time variable it consists of negatively buoyant flow concentrated in thermal trenches discrete arcuate sheets and spots cylindrical columns separated by diffuse nonbuoyant upwellings rapid planform changes occur primarily by trench propagation as observed experimentally by weinstein and olson this issue the analogy between thermal trenches in internally heated convection and convergent plate boundaries on earth suggests that the global pattern of subduction may evolve by propagation of wadati benioff zones along the strike of the slab

on cooling of the earth s core abstract we have constructed a self consistent model for cooling of the earth s core in which the thermal history of the core is computed as a function of the time evolution of the heat flux delivered to the mantle across the core mantle boundary the temperature profile in the convecting core is first assumed to be adiabatic and its evolution in time is calculated with the only constraint that energy be globally conserved when the temperature at the centre drops below the freezing point of the core alloy the inner core starts growing and cools by conduction it is found that it cannot have reached its present size in more than 1 7 billion years if the heat flux delivered to the mantle becomes less than that conducted down the adiabat the temperature profile becomes subadiabatic in a shell at the top of the core through which heat is evacuated by conduction although it is stable against thermal convection this shell is not necessarily stagnant and may be the seat of motions owing to compositional convection

extended boussinesq thermal chemical convection with moving heat sources and variable viscosity we have studied with an aspect ratio four box the thermal chemical convective evolution with strongly temperature and depth dependent viscosity and moving heat sources within the extended boussinesq framework in which both adiabatic and viscous heating are included and a depth dependent thermal expansivity is assumed in the equation of state our focus is to show how this type of mantle evolution with an averaged ra of 0 106 may develop with a linear chemical stratification and a uniformly hot mantle as an initial condition the effects of extended boussinesq and depth dependent thermal expansivity are to prevent the effective destruction of the chemical heterogeneities our results show that this initial condition would after the age of the earth lead to a lava lamp mode consisting of a thick chemically stratified and intensely internally heated layer with a thickness of around a quarter of the whole mantle thickness however in this isolated internally convecting layer exceedingly high temperatures greater than 4500 k would be reached in the deep mantle plumes can be launched from the top of this thick denser layer this lava lamp stage would give way to the formation of denser hill like structures at the core mantle boundary then upwellings with deep lower mantle origins can be induced by the interaction of the downwellings with the d layer our simulations show the possibility for some long range mass transfer interaction between these widely separated chemical hills promoted by the fast horizontal flow induced by the sinking currents along the low viscosity zone due to temperature dependent rheology at the core mantle boundary

the source location of mantle plumes from 3d spherical models of mantle convection abstract mantle plumes are thought to originate from thermal boundary layers such as earth s core mantle boundary cmb and may cause intraplate volcanism such as large igneous provinces lips on the earth s surface previous studies showed that the original eruption sites of deep sourced lips for the last 200 myrs occur mostly above the margins of the seismically observed large low shear velocity provinces llsvps in the lowermost mantle however the mechanism that leads to the distribution of the lips is not clear the location of the lips is largely determined by the source location of mantle plumes but the question is under what conditions mantle plumes form outside at the edges or above the middle of llsvps here we perform 3d geodynamic calculations and theoretical analyses to study the plume source location in the lowermost mantle we find that a factor of five decrease of thermal expansivity and a factor of two increase of thermal diffusivity from the surface to the cmb which are consistent with mineral physics studies significantly reduce the number of mantle plumes forming far outside of thermochemical piles i e llsvps an increase of mantle viscosity in the lowermost mantle also reduces number of plumes far outside of piles in addition we find that strong plumes preferentially form at near the edges of piles and are generally hotter than that forming on top of piles which may explain the observations that most lips occur above llsvp margins however some plumes originated at pile edges can later appear above the middle of piles due to lateral movement of the plumes and piles and morphologic changes of the piles 65 70 strong plumes are found within 10 degrees from pile edges in our models although plate motion exerts significant controls over the large scale mantle convection in the lower mantle mantle plume formation at the cmb remains largely controlled by thermal boundary layer instability which makes it difficult to predict geographic locations of most mantle plumes however all our models show consistently strong plumes originating from the lowermost mantle beneath iceland supporting a deep mantle plume origin of the iceland volcanism

evidence for strong shear velocity reductions and velocity gradients in the lower mantle beneath africa we present data which indicate that the broad low shear velocity anomaly beneath southern africa is stronger and more extensive than previously thought recordings of earthquakes in the southwestern atlantic ocean at an array of broadband seismic stations in eastern africa show anomalously large propagation time delays of the shear phases s scs and sks which vary rapidly with epicentral distance by forward modeling we estimate that the low velocity anomaly extends from the core mantle boundary about 1500 km up into the mantle and that the average shear velocity within this structure is 3 lower than in standard models such as prem strong velocity contrasts exist at its margins 2 over about 300 km these seismic characteristic are consistent with recent numerical simulations of lower mantle mega plume formation

residual geoid anomalies in atlantic ocean basin relationship to mantle plumes the relations of geoid data from radar altimeter measurements geos 3 to water depth and geochemical variations along the mid atlantic ridge are investigated eight geophysical lines across the ridge in the atlantic ocean were constructed approximately normal to the local ridge trend slopes of geoid per kilometer of topographic relief show an asymmetry between the two sides of the ridge that generally is not matched by topography which has better symmetry between the two flanks subtracting a regional field based on previous work produces residual geoid anomalies whose source must be in the earth s outer 600 km the residual geoid anomalies of the north atlantic show striking correlation with the axis of the mid atlantic ridge but those in the south atlantic do not instead major positive residual geoid anomalies appear over the rio grande and walvis ridges determination of slope s residual geoid per million years crustal age reveals that s increases with greater residual geoid anomaly and with higher shallower crestal ridge topography the slope of 22 cm m y west of the azores high near 38 n suggests that the anomalously high crestal residual geoid and depth anomalies may not be entirely of thermal origin which may normally produce slopes only of about 8 cm m y as found near 28 n away from shallow ridge topography to account for this difference in slopes simply by a variation in mantle temperature would require a difference of about 940 c between the two sites which appears excessive also the decrease in topographic height with crustal age away from the azores high is much greater than predicted by theoretical cooling curves correlation of residual geoid anomalies with depth geochemical variations along the length of the mid atlantic ridge and the chemistry of the rio grande rise and walvis ridge suggest that many regions of anomalous residual geoid and crestal depth values may be in large part a function of crustal thickness and density variations induced by plume intrusion and volcanism in addition to their transient thermal effects

serpentinites of the zermatt saas ophiolite complex and their texture evolution the zermatt saas serpentinite complex is an integral member of the penninic ophiolites of the central alps and represents the mantle part of the oceanic lithosphere of the tethys metamorphic textures of the serpentinite preserve the complex mineralogical evolution from primary abyssal peridotite through ocean floor hydration subduction related high pressure overprint meso alpine greenschist facies metamorphism and late stage hydrothermal alteration the early ocean floor hydration of the spinel harzburgites is still visible in relic pseudomorphic bastite and locally preserved mesh textures the primary serpentine minerals were completely replaced by antigorite the stable assemblage in subduction related mylonitic serpentinites is antigorite olivine magnetite diopside the mid tertiary greenschist facies overprint is characterized by minor antigorite recrystallization textural and mineral composition data of this study prove that the hydrated mineral assemblages remained stable during high pressure metamorphism of up to 2 5 gpa and 650 c the zermatt saas serpentinites thus provide a well documented example for the lack of dehydration of a mantle fragment during subduction to 75 km depth

thermal diffusivity of igneous rocks at elevated pressure and temperature thermal diffusivity measurements of seven igneous rocks were made to temperatures of 400 sup 0 c and pressures of 200 mpa the measuring method was based on the concept of cylindrical symmetry and periodic heat pulses the seven rocks measured were westerly rhode island granite climax stock nevada quartz monzonite pomona washington basalt atikokan ontario canada granite creighton ontario canada gabbro east bull lake ontario canada gabbro and stripa sweden granite the diffusivity of all the rocks showed a positive linear dependence on inverse temperature and excluding the east bull lake gabbro showed a linear dependence on quartz content quartz content varied from 0 to 31 by volume diffusivity in all cases rose or remained steady with increasing confining pressure the pressure effect was strongest at lowest pressures and vanished by levels between 10 and 100 mpa depending on rock type the pressure effect measured as a percentage change in diffusivity is stronger in the four rocks of granite composition than in the three of basaltic composition our results agree well with existing thermal diffusivity measurements at atmospheric pressure

the emplacement and fault history of the coolac serpentinite lachlan fold belt southeastern australia abstract the tumut synclinorial zone located in the lachlan fold belt southeastern australia forms a fault bounded belt of early silurian volcanics and flyschoid metasediments of the tumut trough the mooney mooney fault system containing an extensive ultramafic belt known as the coolac serpentinite forms the eastern margin of the trough the ultramafics together with mafic volcanics and intrusive grabbroic rocks have been previously interpreted as early palaeozoic oceanic crust dismembered and obducted during late silurian deformation of the silurian trough sequence recent work however has unravelled a more complex history involving several periods of movement the ultramafics and early silurian volcanics are intruded by early silurian gabbro and syn kinematic late silurian granodiorite these intrusive relationships indicate that the ultramafic rocks were present in approximately their present structural position prior to deformation of the trough sequence the ultramafics therefore cannot represent early silurian oceanic crust obducted during the late silurian deformation they probably represent either early silurian or cambrian ordovician mantle derived material emplaced within a strike slip fault zone during early silurian oblique extension

thrust emplacement of the hispaniola peridotite belt orogenic expression of the mid cretaceous caribbean arc polarity reversal articulo sobre el emplazamiento fallero en la zona de peridotitas de la hispaniola republica dominicana estudiando si es la expresion orogenica de la polaridad reversa en la zona del caribe en el cretacico medio

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

lateral heterogeneity in the convecting mantle thermally induced lateral heterogeneity in the earth s mantle has been studied in terms of idealized two dimensional constant viscosity numerical models of mantle convection steady model solutions of the temperature and velocity fields were analyzed with respect to both the spatial and spectral signatures of their inherent lateral heterogeneity by examining the changes in spectral signature produced by varying the rayleigh number the degree of internal heating and the depth in the convection cell it has been possible to identify those features of the spectra of lateral heterogeneity which are most indicative of these variations particular attention has been paid to the spectral characteristics of thermal boundary layers within the convecting layer the spectral analysis of known temperature fields represents the forward problem corresponding to the inverse problem of inferring the thermal structure of the mantle from the spectral components of its lateral heterogeneity as determined by seismic tomography the forward problem presented here illustrates a new approach in convection modeling which may ultimately provide important diagnostic criteria for the interpretation of recent tomographic data

kinematic history of a reactivated oceanic suture the mamonia complex suture zone sw cyprus the mamonia complex suture zone in sw cyprus marks an arcuate tectonic suture between an upper cretaceous oceanic microplate of supra subduction zone origin troodos complex and a deformed triassic passive margin sequence mamonia complex mapping of the serpentinite filled fault zones that define the suture zone combined with microstructural studies of its constituent fault rocks has revealed evidence of three main tectonic events of upper cretaceous age 1 high temperature c 600 c transform related dextral strike slip and localized metamorphism of mamonia complex units c 90 83 ma 2 retrograde hydration c 83 73 ma 3 regional contractional reactivation at low temperatures which ceased by the latest maastrichtian c 65 ma the present day structural geometries are interpreted as the result of maastrichtian contractional reactivation of a pre existing steep and irregular structural architecture that formed during transtension the resultant orientation of finite strain axes and thrust displacements display a radial distribution documenting near fault normal contraction along the whole length of the arcuate suture zone this pattern of displacements suggests a model whereby the troodos microplate impinged on the mamonia complex during westerly to southwesterly directed plate motion contractionally reactivating the mechanically weak suture zone