the defect microstructure of oxidized mantle olivine from dish hill california the defect microstructure and oxidation state of fe in metasomatized olivine crystals from peridotite xenoliths from dish hill california have been studied by transmission electron microscopy tem and mdssbauer spectroscopy previous petrographic studies suggested that these composite xenoliths were metasomatized under upper mantle conditions mijssbauer data indicate that between i and 60 o of the fe in the olivine crystals is fe3 tem revealed the presence of layers 0 6 nm wide and parallel to 001 of olivine the individual layers do not offset the surrounding olivine periodicities and are interpreted to be laihunite an fe3 derivative of olivine rather than humite group minerals the layers are found in l strong association with dislocations with abundances decreasing rapidly with distance from the dislocation cores 2 at grain boundaries where layers of considerable length decrease in abundance with distance from the grain boundary and 3 in local regions where discontinuous layers possibly formed by homogeneous nucleation are evenly distributed throughout the olivine these patterns suggest hat the layers formed by alteration of olivine tem images from many areas revealed a very fine domain structure l0 to 20 nm in diameter possibly indicating short range order offe3 and vacancies some dislocations are associated with trails of submicrometer sized fluid inclusions that contain iron oxide crystals a few tens of nanometers wide these features may be healed fractures that provided pathways for oxidized fe bearing fluids the association between laihunite and dislocations probably reflects the role ofdislocations as rapid diffusion pathways and as sites for heterogeneous nucleation in olivine that is supersaturated with respect to fe3 the subsequent reactivity rheological and transport properties of olivine may be modified substantially by laihunite layers on dislocations and at grain boundaries

faster seafloor spreading and lithosphere production during the mid cenozoic concurrent changes in seawater chemistry sea level and climate since the mid cretaceous are thought to result from an ongoing decrease in the global rate of lithosphere production at ridges the present day area distribution of seafloor ages however is most easily explained if lithosphere production rates were nearly constant during the past 180 m y we examined spatial gradients of present day seafloor ages and inferred ages for the subducted farallon plate to construct a history of spreading rates in each major ocean basin since ca 140 ma revealing dramatic cenozoic events globally seafloor spreading rates increased by similar to 20 during the early cenozoic due to an increase in plate speeds in the pacific basin since then subduction of the fast spreading pacific farallon ridge system has led to a 12 decrease in average global spreading rate and an 18 or more decrease in the total rate of lithosphere production by the most conservative estimates these rapid changes during the cenozoic defy models of steady state seafloor formation and demonstrate the time dependent and evolving nature of plate tectonics on earth

dynamic causes of the relation between area and age of the ocean floor estimates for the area and age of the ocean floor are at odds with assumptions for mantle convection which imply that an older sea floor rather than a new one would be preferentially subducted over time previous efforts to explain these relationships have been based on geologic evidence and simple models coltice et al p 335 1 created numerical three dimensional convection models representing more realistic physical boundaries including a spherical earth the existence of continents and supercontinents over time and realistic rheologies a combination of continents and plate like behavior of the ocean floor sufficed to produce the observed relationship between plate area and plate age which explains why some old oceanic crust still remains 1 lookup doi 10 1126 science 1219120

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

sea level fluctuations driven by changes in global ocean basin volume following supercontinent break up abstract long term variations in eustatic sea level in an ice free world which existed through most of the mesozoic and early cenozoic eras are partly driven by changes in the volume of ocean basins previous studies have determined ocean basin volume changes from plate tectonic reconstructions since the mesozoic however these studies have not considered a number of important elements that contribute to ocean basin volume such as regional differences in sedimentation or uncertainties within the plate tectonic model itself such as spreading asymmetries and the incomplete representation of back arc basins in the mesozoic additionally studies on long term changes in sea level related to the extension and rifting of passive margins have not been performed on a global scale and likely significantly underestimated the influence of this process in order to improve reconstructions of sea level on geologic time scales and assess the uncertainty in deriving the volume of ocean basins based on a global plate kinematic model we investigate the influence of back arc basins spreading asymmetry large igneous provinces lips sediment thickness and passive margins on ocean basin volume since 200 ma we find that less constrained plate tectonic elements such as the presence of back arc basins or spreading asymmetry may contribute up to 120 m or 150 m to sea level respectively changes in the sea level related to sedimentation and lips are respectively 75 165 m and 45 m changes in sea level associated with passive margin formation are almost negligible at present day though were much larger in the cretaceous and the assumed sedimentation style strongly influences the rate and magnitude of sea level change we incorporate predictions for these components during times where ocean basins are predominantly synthetic reconstructions and find that sea level driven by fluctuating ocean basin volume has changed by 200 m since the jurassic which is comparable to previous estimates our revised estimates will need to be combined with other processes driving long term sea level change including mantle convection driven dynamic topography and glacio eustasy for constructing a complete eustatic sea level curve understanding and quantifying the uncertainties in the volume of ocean basins has implications for modelling subduction flux the oceanic carbon cycle and heatflow and is important for exploring earth s evolutionary cycles especially during times in the geologic past where much of the ocean basin history has been lost

gplates building a virtual earth through deep time gplates is an open source cross platform plate tectonic geographic information system enabling the interactive manipulation of plate tectonic reconstructions and the visualization of geodata through geological time gplates allows the building of topological plate models representing the mosaic of evolving plate boundary networks through time useful for computing plate velocity fields as surface boundary conditions for mantle convection models and for investigating physical and chemical exchanges of material between the surface and the deep earth along tectonic plate boundaries the ability of gplates to visualize subsurface 3 d scalar fields together with traditional geological surface data enables researchers to analyze their relationships through geological time in a common plate tectonic reference frame to achieve this a hierarchical cube map framework is used for rendering reconstructed surface raster data to support the rendering of subsurface 3 d scalar fields using graphics hardware accelerated ray tracing techniques gplates enables the construction of plate deformation zones regions combining extension compression and shearing that accommodate the relative motion between rigid blocks users can explore how strain rates stretching shortening factors and crustal thickness evolve through space and time and interactively update the kinematics associated with deformation where data sets described by geometries points lines or polygons fall within deformation regions the deformation can be applied to these geometries together these tools allow users to build virtual earth models that quantitatively describe continental assembly fragmentation and dispersal and are interoperable with many other mapping and modeling tools enabling applications in tectonics geodynamics basin evolution orogenesis deep earth resource exploration paleobiology paleoceanography and paleoclimate

the origin of hotspot traces evidence from eastern australia interpretation of available isotopic ages and of published geologic maps of igneous rocks in eastern australia indicates a north south time transgressive pattern of cessation of igneous activity along a curvilinear trace beginning about 35 ma at 20 sup 0 s and extending to the present near 37 sup 0 s however volcanic activity began 70 60 ma over the length of the highlands and persisted through the early cenozoic until progressive cessation began the latitudinal rate of termination is compatible with that predicted by plate reconstructions relative to the hawaiian hotspot but the trend of the trace is incompatible with that predicted unless the hawaiian emperor bend is somewhat greater than 50 m y in age relative to the labrecque et al 1977 time scale the volcanic pattern appears to be incompatible with a hotspot or plume hypothesis paleostress and contemporary stress indicators as well as the volcanic pattern support a model in which the trace forms due to intraplate extension normal to the trend of the trace resulting in pressure release and melting at the base of the lithosphere beginning in the late cretaceous this stress field persisted until 35 ma when progressive reorientation of the stress field occurred cessation of igneous activity reflectedmore onset of compression normal to the trend of the trace as a consequence a migrating stress node is recorded in the progressive pattern of extinction of volcanic activity along the trace evidence from other hotspot traces suggests that they too represent intraplate stress controls it is not necessary to postulate deep sources mantle plumes for the origin of hotspot traces intraplate stress due to drag plate margin forces thermal contraction and nonsphericity of the earth can also reflect absolute motion less

laihunite and jarosite in the yamato 00 nakhlites alteration products on mars 1 nakhlites martian clinopyroxenites are believed to have experienced aqueous alteration on the martian surface on the basis of the presence of reddish brown iddingsite on the rim and along cracks of large olivine crystals amorphous to poorly crystalline phyllosilicates smectites and carbonates are major products of aqueous alteration of the yamato 00 nakhlites the yamato 000593 nakhlite and its pair yamato 000749 contain large olivine crystals having opaque black material intimately associated with iddingsite on their rims and along cracks synchrotron radiation x ray diffraction micro raman spectroscopy and transmission electron microscopy revealed that the opaque material is the nonstoichiometric olivine type mineral laihunite 0 42 mg2 fe2 0 79fe3 0 79si1 00o4 typically formed as a high temperature oxidation product under nonequilibrium conditions because laihunite has been decomposed into a mixture of magnetite and amorphous silicate near the fusion crust it is preterrestrial in origin this is the first observation of large olivine crystals in martian meteorites being replaced by laihunite although its presence in olivine in the mesostasis of the miller range 03346 nakhlite has been suggested by micro raman spectroscopy because laihunite in the nakhlites does not form nanometer sized intergrowth with iron oxides fluid assisted high temperature oxidation may have played an important role in transporting away free fe2 cations liberated from olivine during laihunite formation although the yamato 00 nakhlites also contain jarosite we could not confirm the presence of preterrestrial jarosite based only on its occurrences

evolution of oil generative window and oil and gas occurrence in tertiary niger delta basin assuming a simple model of delta development involving progradation and uniform burial at 500 m m y 1 640 ft m y to present depths oil genesis nomographs derived from the time temperature index tti method were constructed for geothermal gradients ranging from 2 2 to 5 1 c 100 m 1 2 2 8 f 100 ft of the niger delta and used in mapping the positions depth temperature of the top of the oil generative window ogw at various times between 40 m y b p and the present during the active subsidence phase oil generation within any megasedimentary unit was initiated at a temperature of 140 146 c 284 294 8 f and depth of 3 000 5 200 m 9 843 17 060 ft within 7 11 m y after deposition of the potential source rocks after cessation of subsidence vertical upward movement of the ogw by 800 1 600 m 2 625 5 249 ft was accompanied by a temperature lowering of 23 54 c 41 97 f producing correspondingly heavier crudes in a central belt of the delta hydrocarbon generation and expulsion from the lower part of the agbada formation predate the cessation of subsidence and structural deformation whereas in other areas it postdates the cessation of subsidence and structural deformation in this central belt the agbada is the major oil source with the akata serving as a gas source in the other areas both the agbada and akata constitute oil sources which implies that the thermal conditions rather than the kerogen type influence the oil gas mix in the niger delta basin

late triassic and early jurassic palaeogeography of the world abstract a new version of global and regional palaeogeographic maps is presented for two time intervals these maps depict the plate tectonic configuration palaeoenvironment and lithofacies during the late triassic carnian norian and early jurassic hettangian toarcian time the individual maps illustrate the general conditions present during the maximum marine transgressions of higher frequency cyclicity within the absaroka sequence of sloss during triassic time pangaea began to stretch initiating the rifting and future break up of the supercontinent the continued northward drift of the cimmerian continent corresponded with the progressive closure and consumption of palaeotethys oceanic crust and the opening of the neotethys ocean the most significant late triassic convergent event was the indosinian orogeny occurring as a result of the consolidation of south china and north china blocks also indochina and indonesia were sutured to south china at the same time the qiangtang block approached the eurasian margin the consolidation of the north chinese and amurian blocks left open a large embayment of panthalassa between amuria and laurasia the so called mongol okhotsk ocean active subduction existed along the margin of this ocean dipping cratonwards towards east siberia the last collisional events of the uralian orogeny took place during the triassic and early jurassic time the conclusion of the uralian orogeny was accompanied by uplift of the adjacent areas of eastern europe and western siberia during the early jurassic the palaeotethys ocean was finally closed and the cimmerian continent collided with asia causing the cimmerian orogeny the time around the triassic jurassic boundary marked an important biotic extinction event plate tectonic activity caused palaeogeographic and palaeoclimatic change which may have contributed to the mass extinction from the plate tectonic and palaeogeographic point of view the following events could have influenced the extinction 1 the closure of palaeotethys and assembly of the asian part of pangaea 2 the break up of pangaea in the future central atlantic area and transition from rifting to drifting and 3 the very extensive basaltic volcanism of the central atlantic magmatic province

convergence of tectonic reconstructions and mantle convection models for significant fluctuations in seafloor spreading for 50 years of data collection and kinematic reconstruction efforts plate models have provided alternative scenarios for plate motions and seafloor spreading for the past 200 my however these efforts are naturally limited by the incomplete preservation of very old seafloor and therefore the time dependence of the production of new seafloor is controversial there is no consensus on how much it has varied in the past 200 my and how it could have fluctuated over longer timescales we explore how seafloor spreading and continental drift evolve over long geological periods using independently derived models a recently developed geodynamic modelling approach and state of the art plate reconstructions both kinematic reconstructions and geodynamic models converge on variations by a factor of 2 in the rate of production of new seafloor over a wilson cycle with concomitant changes of the shape of the area age distribution of the seafloor between end members of rectangular triangular and skewed distributions convection models show that significant fluctuations over longer periods 1 gy should exist involving changes in ridge length and global tectonic reorganisations although independent both convection models and kinematic reconstructions suggest that changes in ridge length are at least as significant as spreading rate fluctuations in driving changes in the seafloor area age distribution through time

digital isochrons of the world s ocean floor we have created a digital age grid of the ocean floor with a grid node interval of 6 arc min using a self consistent set of global isochrons and associated plate reconstruction poles the age at each grid node was determined by linear interpolation between adjacent isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust were interpolated by estimating the ages of passive continental margin segments from geological data and published plate models we have constructed an age grid with error estimates for each grid cell as a function of 1 the error of ocean floor ages identified from magnetic anomalies along ship tracks and the age of the corresponding grid cells in our age grid 2 the distance of a given grid cell to the nearest magnetic anomaly identification and 3 the gradient of the age grid i e larger errors are associated with high age gradients at fracture zones or other age discontinuities future applications of this digital grid include studies of the thermal and elastic structure of the lithosphere the heat loss of the earth ridge push forces through time asymmetry of spreading and providing constraints for seismic tomography and mantle convection models

subduction controls the distribution and fragmentation of earth s tectonic plates the theory of plate tectonics describes how the surface of the earth is split into an organized jigsaw of seven large plates 1 of similar sizes and a population of smaller plates whose areas follow a fractal distribution 2 3 the reconstruction of global tectonics during the past 200 my 4 suggests that this layout is probably a long term feature of our planet but the forces governing it are unknown previous studies 3 5 6 primarily based on statistical properties of plate distributions were unable to resolve how the size of plates is determined by lithosphere properties and or underlying mantle convection here we demonstrate that the plate layout of the earth is produced by a dynamic feedback between mantle convection and the strength of the lithosphere using 3d spherical models of mantle convection with plate like behaviour that match the plate size frequency distribution observed for earth we show that subduction geometry drives the tectonic fragmentation that generates plates the spacing between slabs controls the layout of large plates and the stresses caused by the bending of trenches break plates into smaller fragments our results explain why the fast evolution in small back arc plates 7 8 reflects the dramatic changes in plate motions during times of major reorganizations our study opens the way to use convection simulations with plate like behaviour to unravel how global tectonics and mantle convection are dynamically connected

multi scale dynamics and rheology of mantle flow with plates fundamental issues in our understanding of plate and mantle dynamics remain unresolved including the rheology and state of stress of plates and slabs the coupling between plates slabs and mantle and the flow around slabs to address these questions models of global mantle flow with plates are computed using adaptive finite elements and compared to a variety of observational constraints the dynamically consistent instantaneous models include a composite rheology with yielding and incorporate details of the thermal buoyancy field around plate boundaries the local resolution is 1 km which allows us to study highly detailed features in a globally consistent framework models that best fit plateness criteria and plate motion data have strong slabs with high stresses we find a strong dependence of global plate motions trench rollback net rotation plateness and strain rate on the stress exponent in the nonlinear viscosity the yield stress is found to be important only if it is smaller than the ambient convective stress due to strong coupling between plates slabs and the surrounding mantle the presence of lower mantle anomalies affect plate motions the flow in and around slabs microplate motion and trench rollback are intimately linked to the amount of yielding in the subducting slab hinge slab morphology and the presence of high viscosity structures in the lower mantle beneath the slab

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

subduction history reveals cretaceous slab superflux as a possible cause for the mid cretaceous plume pulse and superswell events abstract subduction is a fundamental mechanism of material exchange between the planetary interior and the surface despite its significance our current understanding of fluctuating subducting plate area and slab volume flux has been limited to a range of proxy estimates here we present a new detailed quantification of subduction zone parameters from the late triassic to present day 230 0 ma we use a community plate motion model with evolving plate topologies to extract trench normal convergence rates through time to compute subducting plate areas and we use seafloor paleo age grids to estimate the thickness of subducting lithosphere to derive the slab flux through time our results imply that slab flux doubled to values greater than 500 km3 yr from 180 ma in the jurassic to 130 ma in the mid cretaceous subsequently halving again towards the cretaceous paleogene boundary largely driven by subduction zones rimming the pacific ocean basin the 130 ma spike can be attributed to a two fold increase in mid ocean ridge lengths following the break up of pangea and a coincident increase in convergence rates with average speeds exceeding 10 cm yr with one third of the total 230 0 ma subducted volume entering the mantle during this short 50 myr period we suggest this slab superflux drove a surge in slab penetration into the lower mantle and an associated increase in the vigour of mantle return flow this mid cretaceous event may have triggered or at least contributed to the formation of the darwin rise mantle superswell dynamic uplift of the south african plateau and the plume pulse that produced the ontong java hikurangi manihiki and kerguelen plateaus among others the models presented here contribute to an improved understanding of the time evolving flux of material consumed by subduction and suggest that slab superflux may be a general feature of continental dispersal following supercontinent breakup these insights may be useful for better understanding how supercontinent cycles are related to transient episodes of large igneous province and superswell formation and the associated deep cycling of minerals and volatiles as well as leading to a better understanding of tectonic drivers of long term climate and icehouse to greenhouse transitions

ocean basin evolution and global scale plate reorganization events since pangea breakup we present a revised global plate motion model with continuously closing plate boundaries ranging from the triassic at 230 ma to the present day assess differences among alternative absolute plate motion models and review global tectonic events relatively high mean absolute plate motion rates of approximately 9 10 cm yr 1 between 140 and 120 ma may be related to transient plate motion accelerations driven by the successive emplacement of a sequence of large igneous provinces during that time an event at 100 ma is most clearly expressed in the indian ocean and may reflect the initiation of andean style subduction along southern continental eurasia whereas an acceleration at 80 ma of mean rates from 6 to 8 cm yr 1 reflects the initial northward acceleration of india and simultaneous speedups of plates in the pacific an event at 50 ma expressed in relative and some absolute plate motion changes around the globe and in a reduction of global mean plate speeds from about 6 to 4 5 cm yr 1 indicates that an increase in collisional forces such as the india eurasia collision and ridge subduction events in the pacific such as the izanagi pacific ridge play a significant role in modulating plate velocities

bulk mineralogical changes of hydrous micrometeorites during heating in the upper atmosphere at temperatures below 1000 c small particles 200 m in diameter from the hydrous carbonaceous chondrites orgueil ci murchison cm2 and tagish lake were experimentally heated for short durations at subsolidus temperatures under controlled ambient pressures in order to examine the bulk mineralogical changes of hydrous micrometeorites during atmospheric entry the three primitive meteorites consist mainly of various phyllosilicates and carbonates that are subject to decomposition at low temperatures and thus the brief heating up to 1000 c drastically changed the mineralogy changes included shrinkage of interlayer spacing of saponite due to loss of molecular water at 400 600 c serpentine and saponite decomposition to amorphous phases at 600 and 700 c respectively decomposition of mg fe carbonate at 600 c recrystallization of secondary olivine and fe oxide or metal at 700 800 c and recrystallization of secondary low ca pyroxene at 800 c the ambient atmospheric pressures controlled species of secondary fe phase taenite at pressures lower than 10 2 torr magnesiowstite from 10 3 to 10 1 torr and magnetite from 10 2 to 1 torr the abundance of secondary low ca pyroxene increases in the order of murchison orgueil and tagish lake and the order corresponds to saponite abundance in samples prior to heating mineralogy of the three unmelted micrometeorites f96ci024 kw740052 and kw740054 were investigated in detail in order to estimate heating conditions the results showed that they might have come from different parental objects carbonaterich tagish lake type carbonate poor tagish lake or ci type and cm type respectively and experienced different peak temperatures 600 700 and 800 900 c respectively at 60 80 km altitude upon atmospheric entry

forced regressions in a sequence stratigraphic framework concepts examples and exploration significance sequence stratigraphic concepts suggest that stratal geometries develop and are largely controlled by changes in relative sea level on the shelf lowstand deposits which form during falls and subsequent stillstands of relative sea level can be recognized by the presence of an unconformity at the base the isolated and basinward position relative to the previous shoreline and the abrupt seaward translation of shallow water and shoreline facies into the basin across an unconformity surface this seaward translation of facies and shoreline regression in response to relative sea level lowering is termed a forced regression a forced regression is independent of variations of sediment flux and is in contrast with normal regressions that occur in response to excess sediment flux relative to space available on the shelf i e accommodation forced regressions commonly are associated with a zone of sedimentary bypass subaerial exposure and possible fluvial erosion between the newly formed and preceding shorelines certain shelf sands previously interpreted as offshore or mid shelf sand bodies thus can be reinterpreted as stranded lowstand shorelines associated with forced regressions this alternative interpretation has economic significance insofar as it suggests different subsurface correlations and reservoir geometries with the potential for development of new play types and enhanced recovery in o der fields examples of forced regression can be observed at a variety of scales and ages several such examples include the modern east coulee fan delta and the lower cretaceous viking formation in alberta canada the quaternary rhone delta and the quaternary hudson valley system

a sequential data assimilation approach for the joint reconstruction of mantle convection and surface tectonics with the progress of mantle convection modelling over the last decade it now becomes possible to solve for the dynamics of the interior flow and the surface tectonics to first order we show here that tectonic data like surface kinematics and seafloor age distribution and mantle convection models with plate like behaviour can in principle be combined to reconstruct mantle convection we present a sequential data assimilation method based on suboptimal schemes derived from the kalman filter where surface velocities and seafloor age maps are not used as boundary conditions for the flow but as data to assimilate two stages a forecast followed by an analysis are repeated sequentially to take into account data observed at different times whenever observations are available an analysis infers the most probable state of the mantle at this time considering a prior guess supplied by the forecast and the new observations at hand using the classical best linear unbiased estimate between two observation times the evolution of the mantle is governed by the forward model of mantle convection this method is applied to synthetic 2 d spherical annulus mantle cases to evaluate its efficiency we compare the reference evolutions to the estimations obtained by data assimilation two parameters control the behaviour of the scheme the time between two analyses and the amplitude of noise in the synthetic observations our technique proves to be efficient in retrieving temperature field evolutions provided the time between two analyses is 10 myr if the amplitude of the a priori error on the observations is large 30 per cent our method provides a better estimate of surface tectonics than the observations taking advantage of the information within the physics of convection

interaction between sea level changes and depositional tectonics implications for hydrocarbon prospectivity in the western coastal swamp depobelt niger delta basin nigeria this paper presents the potential depositional framework of sandstones and unconsolidated sand reservoirs seals and traps in the western part of the coastal swamp depobelt of the niger delta basin nigeria through integration of sequence stratigraphy and seismic imaging the integration of well logs and biostratigraphic foraminifera and palynological data with three dimensional seismic volume established the sequence stratigraphic framework which has improved our understanding of the impact of sea level changes in the distribution of sand facies four third order seismic depositional sequences sequences 1 4 of serravallian deposits were identified based on the vail stratigraphic concept depositional processes structures thickness and seismic signatures vary in each sequence sequence 1 dominated by mud and very fine grained sand facies represents the outer shelf shelf margin setting and has experienced shale tectonics ductile deformation sequences 2 and 3 consist of alternating sand shale facies and have experienced gravitational tectonics although sequence 2 represents middle shelf inner shelf setting sequence 3 represents inner shelf transitional setting sequence 4 consists of thick fluvial coastal plain sand facies and represents deltaic plain or continental sediments these deposits reflect postgravitational tectonics within the third order sequences are fourth order sequences that recorded the interaction between small scale transgressive regressive cycles and depositional tectonics the potential reservoirs within these sequences are stacked channel sandstones shoreface sandstones and heteroliths the juxtaposition of the potential reservoirs against transgressive shales provides potential seals and traps structural interpretation shows synthetic and antithetic faults in addition event maps at various ages document structural closures that indicate the existence of possible hydrocarbon leads and prospects

seafloor spreading evolution in response to continental growth the growth of the continental crust has shaped the evolution of the earth from its interior to its fluid envelopes continents have played a major role in the evolution of global tectonics through their interaction with mantle convection the feedback between continents and mantle convection has been studied for the past 25 years but it is only recently that the dynamic influence of continents on seafloor spreading can be explored thanks to progress in convection modeling in this work we investigate how continental size impacts seafloor spreading activity with state of the art three dimensional spherical convection models we show that increasing the continental area forces higher production rates of new seafloor with stronger fluctuations as a consequence the average age of the seafloor decreases with increasing continental area this study suggests that mantle heat loss experienced significant fluctuations through continental growth and reinforces the estimate of

subduction flux modulates the geomagnetic polarity reversal rate abstract ascertaining the cause of variations in the frequency of geomagnetic polarity reversals through the phanerozoic has remained a primary research question straddling paleomagnetism and geodynamics for decades numerical models suggest the primary control on geomagnetic reversal rates on 10 to 100 ma timescales is the changing heat flux across the core mantle boundary and that this is itself expected to be strongly influenced by variations in the flux of lithosphere subducted into the mantle a positive relationship between the time dependent global subduction flux and magnetic reversal rate is expected with a time delay to transmit the thermal imprint into the lowermost mantle we perform the first test of this hypothesis using subduction flux estimates and geomagnetic reversal rate data back to the early paleozoic subduction area flux estimates are derived from global full plate tectonic models and are evaluated against independent subduction flux proxies based on the global age distribution of detrital zircons and strontium isotopes a continuous phanerozoic reversal rate model is built from pre existing compilations back to 320 ma plus a new reversal rate model in the data sparse mid to early paleozoic cross correlation of the time dependent subduction flux and geomagnetic reversal rate series reveals a significant correlation with a time delay of 120 ma with reversals trailing the subduction flux this time delay represents a value intermediate between the seismologically constrained time expected for a subducted slab to transit from the surface to the core mantle boundary 150 300 ma and the much shorter lag time predicted by some numerical models of mantle flow 30 60 ma while the reason for this large discrepancy remains unclear it is encouraging that our novel estimate of lag time represents a compromise between them although important uncertainties in our proposed relationship remain these results cast new light on the dynamic connections between the surface and deep earth and will help to constrain new models linking mantle convection the thermal evolution of the lowermost mantle and the geodynamo

age spreading rates and spreading asymmetry of the world s ocean crust we present four companion digital models of the age age uncertainty spreading rates and spreading asymmetries of the world s ocean basins as geographic and mercator grids with 2 arc min resolution the grids include data from all the major ocean basins as well as detailed reconstructions of back arc basins the age spreading rate and asymmetry at each grid node are determined by linear interpolation between adjacent seafloor isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust are interpolated by geological estimates of the ages of passive continental margin segments the age uncertainties for grid cells coinciding with marine magnetic anomaly identifications observed or rotated to their conjugate ridge flanks are based on the difference between gridded age and observed age the uncertainties are also a function of the distance of a given grid cell to the nearest age observation and the proximity to fracture zones or other age discontinuities asymmetries in crustal accretion appear to be frequently related to asthenospheric flow from mantle plumes to spreading ridges resulting in ridge jumps toward hot spots we also use the new age grid to compute global residual basement depth grids from the difference between observed oceanic basement depth and predicted depth using three alternative age depth relationships the new set of grids helps to investigate prominent negative depth anomalies which may be alternatively related to subducted slab material descending in the mantle or to asthenospheric flow a combination of our digital grids and the associated relative and absolute plate motion model with seismic tomography and mantle convection model outputs represents a valuable set of tools to investigate geodynamic problems

dissolution of olivine during natural weathering abstract naturally weathered olivine occurring as phenocrysts in hawai ian volcanic rocks from several volcanic centers and regolith outcrop settings and as tectonized olivines from several metadunite bodies in the southern appalachian blue ridge are all similarly corroded by natural weathering conical funnel shaped etch pits occur as individual pits base to base pairs of cone shaped pits or en echelon arrays etch pit shapes and orientations in the smallest etch pit arrays visible in conventional scanning electron microscopy resemble even smaller features previously reported from transmission electron microscope investigations of olivine weathering etch pits occur in samples with chemical and or mineralogical evidence of weathering and or are associated with or proximal or directly connected to fractures or exposed outcrop surface and therefore are formed by weathering and not inherited from pre weathering aqueous alteration e g serpentinization iddingsitization of these parent rocks many etch pits are devoid of weathering products natural weathering of olivine is surface reaction limited similarity of corrosion forms from naturally weathered olivine from multiple igneous and metamorphic parent rock bodies suggests that olivine weathers in the same manner regardless of its specific crystallization recrystallization history eruption weathering exposure ages of the olivine s host rock and the local regolith history

modeling seafloor spreading rates through time a steady state model of crust production and destruction for the past 180 m y was proposed by b parsons and advocated by d rowley such a model has serious implications for models of secular variations in e g global sea level global climate and seawater chemistry this paper presents an analysis of the steady state model and then offers alternative extensions of that model that allow for non steady state production of ocean crust with time results suggest that the observed linear decrease in area versus age of ocean floor does not force a steady state view of seafloor spreading

sequence stratigraphic controls on reservoir characterization and architectural analysis a case study of tovo field coastal swamp depobelt niger delta basin nigeria abstract the decline in reservoir pressure and rate of production have prompted a series of studies to better characterize and define reservoir architecture in anticipation of ultimate recovery of the hydrocarbon deposits by a fluid injection program in the tovo field located in the coastal swamp depobelt of the niger delta basin this study utilizes the integration of well logs biostratigraphic and 3d seismic data sets to establish a third order sequence stratigraphic framework consisting of fourth order sequences it facilitates the construction of predictive depositional models that elucidate reservoir facies distribution and architecture three types of reservoir deposits that are generally composed of channel and shoreface sandstone facies have been considered lowstand fluvial sediments with a bulging geometry marine influenced sandstone deposits defined by strike oriented tabular geometry and highstand fluvial channel sandstone deposit characterized by dip oriented lobate geometry intrareservoir shale discontinuous shale stringers and or shale lensoids which are products of channel avulsion overbank sedimentation and marine flooding define the internal reservoir architecture their overall reservoir quality is very good however the fluvial dominated sandstone deposits show a wide range of porosity and permeability across individual wells conversely the marine influenced reservoir deposits show better lateral and vertical sand connectivity as suggestive of near similar in the range of porosity and permeability values across the field implying a better oil sweep efficiency and fluid injection potential the results of this study suggest that the depositional environment of sandstone reservoirs of the basin is complex but not infinite in facies distribution the spatial distribution of these sandstone reservoir facies is related to changes in local accommodation space in the basin this study therefore demonstrates the importance of evaluating the impact of sequence stratigraphic controls on reservoir characterization and architecture of hydrocarbon bearing sandstones for optimum exploration and production in clastic depositional settings

on the predictability limit of convection models of the earth s mantle reconstructing convective flow in the earth s mantle is a crucial issue for a diversity of disciplines from seismology to sedimentology the common and fundamental limitation of these reconstructions based on geodynamic modeling is the unknown initial conditions because of the chaotic nature of convection in the earth s mantle errors in initial conditions grow exponentially with time and limit forecasting and hindcasting abilities in this work we estimate for the first time the limit of predictability of earth s mantle convection following the twin experiment method we compute the lyapunov time i e e folding time for state of the art 3 d spherical convection models varying rheology and rayleigh number our most earth like and optimistic solution gives a lyapunov time of 136 13 myr rough estimates of the uncertainties in best guessed initial conditions are around 5 leading to a limit of predictability for mantle convection of 95 myr our results suggest that error growth could produce unrealistic convective structures over time scales shorter than that of pangea dispersal

constraints on mantle viscosity structure from continental drift histories in spherical mantle convection models abstract earth s continents drift in response to the force balance between mantle flow and plate tectonics and actively change the plate mantle coupling thus the patterns of continental drift provide relevant information on the coupled evolution of surface tectonics mantle structure and dynamics here we investigate rheological controls on such evolutions and use surface tectonic patterns to derive inferences on mantle viscosity structure on earth we employ global spherical models of mantle convection featuring self consistently generated plate tectonics which are used to compute time evolving continental configurations for different mantle and lithosphere structures our results highlight the importance of the wavelength of mantle flow for continental configuration evolution too strong short wavelength components complicate the aggregation of large continental clusters while too stable very long wavelength flow tends to enforce compact supercontinent clustering without reasonable dispersal frequencies earth like continental drift with episodic collisions and dispersals thus requires a viscosity structure that supports long wavelength flow but also allows for shorter wavelength contributions such a criterion alone is a rather permissive constraint on internal structure but it can be improved by considering continental oceanic plate speed ratios and the toroidal poloidal partitioning of plate motions the best approximation of earth s recent tectonic evolution is then achieved with an intermediate lithospheric yield stress and a viscosity structure in which oceanic plates are 103 more viscous than the characteristic upper mantle which itself is 100 200 less viscous than the lowermost mantle such a structure causes continents to move on average 2 2 1 0 slower than oceanic plates consistent with estimates from present day and from plate reconstructions this does not require a low viscosity asthenosphere globally extending below continental roots however this plate speed ratio may undergo strong fluctuations on timescales of several 100 myr that may be linked to periods of enhanced continental collisions and are not yet captured by current tectonic reconstructions

assessing the role of slab rheology in coupled plate mantle convection models reconstructing the 3d structure of the earth s mantle has been a challenge for geodynamicists for about 40 yr although numerical models and computational capabilities have substantially progressed parameterizations used for modeling convection forced by plate motions are far from being earth like among the set of parameters rheology is fundamental because it defines in a non linear way the dynamics of slabs and plumes and the organization of lithosphere deformation in this study we evaluate the role of the temperature dependence of viscosity variations up to 6 orders of magnitude and the importance of pseudo plasticity on reconstructing slab evolution in 3d spherical models of convection driven by plate history models pseudo plasticity which produces plate like behavior in convection models allows a consistent coupling between imposed plate motions and global convection which is not possible with temperature dependent viscosity alone using test case models we show that increasing temperature dependence of viscosity enhances vertical and lateral coherence of slabs but leads to unrealistic slab morphologies for large viscosity contrasts introducing pseudo plasticity partially solves this issue producing thin laterally and vertically more continuous slabs and flat subduction where trench retreat is fast we evaluate the differences between convection reconstructions employing different viscosity laws to be very large and similar to the differences between two models with the same rheology but using two different plate histories or initial conditions

mapping sediment dispersal patterns and associated systems tracts in fourth and fifth order sequences using seismic sedimentology example from corpus christi bay texas a seismic sedimentologic study was performed to map fourth and fifth order systems tracts in oligocene frio strata in corpus christi bay south texas guided by third order sequence stratigraphic correlations from seismic and wire line log data we prepared stratal slices from a three dimensional seismic volume to reveal high resolution 10 m 33 ft levels sediment dispersal patterns and associated systems tracts in a relative geologic time domain on average 1200 m 3940 ft of sediments were deposited in the third order lowstand expansion cycle and at least 16 higher order sequences fourth and fifth order sequences were recognized three types of depositional systems were identified in the frio stratigraphic section 1 offshelf lowstand slope fans that are best characterized by submarine channel and levee systems inside and outside incised submarine channels and by fan shaped sand body geometry 2 lowstand prograding deltaic systems that are composed of strike oriented and lobate deltaic sand bodies and 3 highstand systems that are represented by onshelf barrier lagoon and deltaic systems higher order sequence development was controlled by the interaction of relative sea level change sediment supply and gravity tectonics the top of sediment ridges was eroded or decapitated during many of the higher order sequences sand dispersal patterns are primarily controlled by accommodation resulting from rollover topography associated with growth faulting between the boundary fault and the hinge line atop rollover structures strike oriented sandstone bodies dominate within submarine channels and incised valleys and beyond the hinge line to the distal basin dip oriented sandstone bodies prevail sandstone thickness and dispersal patterns can be predicted by integrating wire line log measurements and seismic amplitude patterns

reconstructing first order changes in sea level during the phanerozoic and neoproterozoic using strontium isotopes the eustatic sea level curves published in the seventies and eighties have supported scientific advances in the earth sciences and the emergence of sequence stratigraphy as an important hydrocarbon exploration tool however validity of reconstructions of eustatic sea level based on sequence stratigraphic correlations has remained controversial proposed sea level curves differ because of site to site changes in local tectonics depositional rates and long wavelength dynamic topography resulting from mantle convection in particular the overall amplitude of global phanerozoic long term sea level is poorly constrained and has been estimated to vary between 400 m above present day sea level to 50 m below present day sea level to improve estimates of past sea level we explore an alternative methodology to estimate global sea level change we utilise the phanerozoic neoproterozoic 87sr 86sr record which at first order represents the mix of inputs from continental weathering and from mantle input by volcanism by compensating for weathering with estimates of runoff from a 3d climate model geoclimtec a corrected 87sr 86sr record can be obtained that solely reflects the contribution of strontium from mantle sources at first order the flux of strontium from the mantle through time is due to increases and decreases in the production of oceanic crust through time therefore the changing levels of mantle derived strontium can be used as a proxy for the production of oceanic lithosphere by applying linear oceanic plate age distributions we compute sea level and continental flooded area curves we find that our curves are generally within the range of previous curves built on classical approaches a phanerozoic first order cyclicity of 250 myr is observed that may extend into the neoproterozoic the low frequency i e on the order of 10 to 100 my sea level curve that we propose while open for improvement may be used as baseline for refined sequence stratigraphic studies at a global and basin scale

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

geology and stratigraphy of fluvio deltaic deposits in the ivishak formation applications for development of prudhoe bay field alaska significant remaining reserves in prudhoe bay field are confined within deltaic rocks at the base of the triassic ivishak sandstone the initial stratigraphic characterization of the prudhoe bay reservoir was lithostratigraphically based and it depicted this basal reservoir interval as tabular zones between marine shale and overlying coarse grained fluvial sandstones a reassessment of this interval based on cores and genetic stratigraphic correlations depicts en echelon offlapping fluvially dominated deltaic wedges reservoir quality rocks occur in distributary mouth bar distributary channel and fluvial facies associations a paleogeographic reconstruction of one delta lobe includes an alluvial plain crossed by channels of possibly braided or low sinuosity rivers this alluvial plain graded into a delta plain cut by distributary channels that fed distributary mouth bars on a broad delta front river dominance is inferred from the abundance of unidirectional current structures normally graded beds soft sediment deformation and general absence of wave formed tidal and biogenic structures slumping and growth faulting locally replaced coarsening upward deltaic successions with sharp based overthickened mouth bar and distributary channel deposits mudstones deposited following delta lobe abandonment form laterally extensive flow barriers between lobes compartmentalization is most pronounced distally where deltaic sandstones are overlain by and pass laterally into marine shale proximally fluvial and deltaic sandstones are juxtaposed across erosional contacts improving reservoir continuity this stratigraphic interpretation is corroborated by production and surveillance data plus an interference test locally stratigraphy and poor waterflood performance reflect completions in diachronous sandstones that originated in separate deltaic lobes previously poor well performances were attributed to sandstone pinch outs in some cases production can be enhanced with recompletions rather than infill drilling nonconventional wells planned and completed with the benefit of detailed facies association correlations currently are recovering millions of barrels of previously bypassed oil

martian dunite nwa 2737 integrated spectroscopic analyses of brown olivine received 5 october 2007 revised 13 december 2007 accepted 6 march 2008 published 18 june 2008 1 a second martian meteorite has been identified that is composed primarily of heavily shocked dunite northwest africa nwa 2737 this meteorite has several similarities to the chassigny dunite cumulate but the olivine is more mg rich and most notably is very dark and visually brown carefully coordinated analyses of nwa 2737 whole rock and olivine separates were undertaken using visible and near infrared reflectance

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

temporal evolution of the stress state in a supercontinent during mantle reorganization summary previous numerical simulation models of isoviscous mantle convection in both 2 d and 3 d geometries have revealed that the presence of a supercontinental lid produces large scale horizontal mantle flow thereby reorganizing the thermal structure of the mantle interior large scale upwelling plumes arising from the core mantle boundary cmb beneath the supercontinent are observed in some models these upwelling plumes have been hypothesized to produce tensional stresses in the supercontinent and to be responsible for the subsequent continental lifting and breakup in this study numerical simulations using a 3 d spherical shell model are conducted in order to elucidate temporal changes in stress state in an idealized supercontinent during mantle reorganization the model supercontinent is assumed to be an undeformable highly viscous supercontinental lid hvsl with respect to the mantle the mantle is heated from the base and from within the hvsl is imposed abruptly on well developed convection systems and remains fixed during the simulations i first consider the scenario in which the hvsl is imposed on isoviscous or weakly temperature dependent viscosity convection exhibiting short wavelength thermal structures the simulation results reveal that a tensional stress regime rapidly predominates throughout the hvsl within 50 100 myr in model time in response to the horizontal mantle flow beneath the hvsl the subsequent large scale plumes upwelling from the cmb also induce tensional stresses in the supercontinent but the effects of these stresses on the overall stress state appear to be secondary for most of the models investigated herein the maximum deviatoric tensional stress generated in the hvsl once convection is re established is 30 90 mpa which is probably comparable to the minimum stress magnitude required to break up the supercontinent to investigate the effects of the hvsl on other convection regimes i next consider the scenario in which an hvsl having a viscosity contrast of 10 is imposed on much longer wavelength i e degree one or degree two dominant convection systems with moderately temperaturedependent viscosity the patterns of such low degree dominant convection are affected little by the imposition of the hvsl and notable mantle reorganization does not occur on the other hand when a reasonable yielding stress is imposed at the continental margins newly formed weak upwelling plumes develop due to enhanced downwelling at the margins rather than large scale mantle reorganization however the newly upwelling plumes do not seem to produce a visible tensional stress regime in the hvsl interior and the spatial distributions of the stress regime and stress magnitude in the hvsl depend on the spatial relationship between the hvsl and the convection pattern since the series l models of the present study do not include the yield stress in the oceanic part of the lithosphere plate like behaviour and subducting plates are not actualized the results for series s models in which the oceanic lithosphere freely sinks and the convection pattern is easily organized imply that the broadly tensional regime prevailing in the supercontinent may support the penetration of mantle plumes into the supercontinent the emergence of hotspots and subsequent continental uplift and break up

causes and consequences of the relation between area and age of the ocean floor the distribution of area of the ocean floor with age t is approximately described by da dt c0 1 t tm where c0 is the rate of crustal generation and tm the maximum age a linear differential area versus age relation can be explained by a balance between crustal generation and consumption where consumption is uniformly distributed with age the present distribution of consumption with age was estimated from the isochron map used to derive the area age relation and a recently published set of angular velocity vectors describing present plate motions the rate of consumption per unit age shows considerable scatter about a mean value equal to c0 tm 1 7 10 8 km2 yr2 but consumption is sufficiently close to being uniformly distributed with age to explain the observed degree of approximation to a linear differential area age relation the trenches appear to be distributed randomly with respect to age provinces in the oceans it was not possible to discern any trend of mean subduction velocities with age apart from the smaller velocities observed for the two youngest age provinces 0 9 ma changes in the rate of plate generation and the distribution of consumption with age result in shifts in the area age distribution in turn these shifts produce changes in the plate driving forces which act to restore the rate of plate generation and distribution of consumption to their initial states the ridge pushing force for example decreases as the rate of crustal generation increases this coupling between driving forces and the area age distribution provides a feedback mechanism limiting the extent of any changes a measure of the magnitude of shifts in the area age distribution is given by global changes in sea level if long term changes in sea level are less than 150 m as suggested by recent estimates then only small changes 12 in the rate of plate generation and distribution of consumption with age are required sea level changes as large as 300 m require about a 50 change in the rate of crustal generation the above area age relation can be combined with simple expressions for depth and heat flow versus age to obtain an empirical hypsometric distribution parameterized in terms of age and exact expressions for the heat loss from the ocean floor the latter is estimated to be 2 9 1013 w with 25 of this heat being lost by seafloor younger than 5 ma of the total heat loss from the mantle 26 is estimated to be transported by small scale convection

past and present seafloor age distributions and the temporal evolution of plate tectonic heat transport abstract variations in earth s rates of seafloor generation and recycling have far reaching consequences for sea level ocean chemistry and climate however there is little agreement on the correct parameterization for the time dependent evolution of plate motions a strong constraint is given by seafloor age distributions which are affected by variations in average spreading rate ridge length and the age distribution of seafloor being removed by subduction using a simplified evolution model we explore which physical parameterizations of these quantities are compatible with broad trends in the area per seafloor age statistics for the present day and back to 140 ma from paleo age reconstructions we show that a probability of subduction based on plate buoyancy slab pull or sqrt age and a time varying spreading rate fits the observed age distributions as well as or better than a subduction probability consistent with an unvarying triangular age distribution and age independent destruction of ocean floor instead we interpret the present near triangular distribution of ages as a snapshot of a transient state of the evolving oceanic plate system current seafloor ages still contain hints of a 60 myr periodicity in seafloor production and using paleoages we find that a 250 myr period variation is consistent with geologically based reconstructions of production rate variations these long period variations also imply a decrease of oceanic heat flow by 0 25 ma during the last 140 ma caused by a 25 50 decrease in the rate of seafloor production our study offers an improved understanding of the non uniformitarian evolution of plate tectonics and the interplay between continental cycles and the self organization of the oceanic plates

tectonic history of the pacific farallon phoenix triple junction from late jurassic to early cretaceous an abandoned mesozoic spreading system in the central pacific basin in the central pacific basin an abandoned ne trending spreading ridge is evident on the satellite gravity map and is confirmed in bathymetric and magnetic data our identification of magnetic anomaly lineations flanking the abandoned ridge suggests that between chrons m21 147 ma and m14 135 ma it was the plate boundary between the pacific plate and a microplate which we here name the trinidad microplate the configuration of the pacific farallon phoenix triple junction until chron m22 149 ma was fault fault ridge ffr after chron m22 the pacific farallon phoenix triple junction changed its configuration the reorganization of the triple junction led to the birth of the trinidad microplate in its vicinity around chron m15 136 ma the spreading ridge between the pacific plate and the trinidad microplate was abandoned and the magellan microplate was born and remained active till chron m9 128 5 ma after welding of the magellan microplate to the pacific plate the triple junction had a ridge ridge ridge rrr configuration until the major reorganization around chron m0 120 ma

fe3 distribution in oxidized olivine a synchrotron micro xanes study synchrotron micro xanes spectroscopy smx is used to examine the amount and distribution of fe super 2 and fe super 3 in five samples of fayalite previously studied by mossbauer spectroscopy rockport fayalite is homogeneous and the fe in it is completely reduced olivine from qianan is almost completely oxidized and probably contains finely intergrown silica laihunite and hematite with hematite predominating pantelleria olivine has an oxidized rim around a reduced core suggesting either a post crystallization reaction with fayalite and oxygen going to laihunite and hematite or a change in prevailing oxidation during growth the texture of olivine from the mourne mountains suggests exsolution from a fayalite laihunite solid solution based on the substitution of three fe super 3 atoms for two fe super 2 and one vacancy that was stable at high temperature laihunite from the type locality lai he village shows 1 3 mu m mottling between light and dark areas in backscatter images but these areas cannot correspond to pure fayalite and laihunite exsolution because the bulk fe super 3 sigma fe of such a combination would not correspond to the smx value of 67 fe super 3 rather the texture observed in backscatter is interpreted to represent alternating areas of magnetite rich and laihunite rich compositions with a similar i e 67 fe super 3 sigma fe extent of oxidation