itrf2008 plate motion model 1 the itrf2008 velocity field is demonstrated to be of higher quality and more precise than past itrf solutions we estimated an absolute tectonic plate motion model made up of 14 major plates using velocities of 206 sites of high geodetic quality far from plate boundaries deformation zones and glacial isostatic adjustment gia regions derived from and consistent with itrf2008 the precision of the estimated model is evaluated to be at the level of 0 3 mm a wrms no gia corrections were applied to site velocities prior to estimating plate rotation poles as our selected sites are outside the fennoscandia regions where the gia models we tested are performing reasonably well and far from gia areas where the models would degrade the fit antarctica and north america our selected velocity field has small origin rate bias components following the three axis x y z respectively 0 41 0 54 0 22 0 64 and 0 41 0 60 95 per cent confidence limits comparing our model to nnr nuvel 1a and the newly available nnr morvel56 we found better agreement with nnr morvel56 than with nnr nuvel 1a for all plates except for australia where we observe an average residual rotation rate of 4 mm a using our selection of sites we found large global x rotation rates between the two models 0 016 ma and between our model and nnr morvel56 of 0 023 ma equivalent to 2 5 mm a at the earth surface

geology geochronology and rift basin development in the central sector of the main ethiopia rift based on stratigraphic relationships and k ar dating of volcanic rocks from both of the escarpments flanking plateaus and from the rift floor of the central sector of the main ethiopian rift six major volcanic episodes are recognized in the rift9s development over a time span from the late oligocene to the quaternary using the k ar data correlation of volcanic units from the six periods of activity throughout the study area forms the basis for establishing six time stratigraphic chronozones for the central sector that are related to volcanism in the ethiopian cenozoic volcanic province the oldest basalt and rhyolite flows exposed along the rift margins of the central sector are time correlative to or older than those in river canyons for example blue nile on the adjacent northwest plateau a thinned mesozoic stratigraphic sequence along the guraghe western rift margin suggests that doming may have preceded volcanism and rifting of the cenozoic by late miocene time at least by 8 3 ma and 9 7 ma the eastern and western faulted margins respectively of the rift had formed at guraghe and at agere selam as indicated by containment of flows of that age within the rift wall during eruption a paroxysm of calc alkaline ignimbrite activity produced voluminous flows nearly fully contained within the rift during the pliocene epoch the munesa crystal tuff 3 5 ma a prominent marker tuff exposed on both rift margins is present at depth in a geothermal well beneath the rift floor and indicates a minimum of 2 km of downthrow in the central sector since its eruption structural and stratigraphic relationships in the central sector indicate a two stage rift development this is characterized by an early phase late oligocene or early miocene of a series of alternating opposed half grabens along the rift with alternating polarity such as that in the present gregory and western rifts of east africa and symmetrical rifts that evolved from these grabens in late miocene or early pliocene time thus evolution from alternating half graben to a full symmetrical graben with a medially located neovolcanic zone that is bifurcated to marginal grabens in the northern part of the study area may be a fundamental part of the rifting process the study indicates that there are major petrologic and tectonic differences between the main ethiopian rift and the gregory kenyan rift

micas in experimentally shocked gneiss biotites and muscovites from a gneiss have been experimentally shocked between 18 and 70 gpa using powder propellant guns at nasa johnson space center and at the california institute of technology this study shows that shock in biotite and muscovite can produce homogeneous and devolatilized glasses within microseconds shock deformed micas display fracturing kinking and complex extinction patterns over the entire pressure range investigated however these deformation features are not a sensitive pressure indicator localized melting of micas begins at 33 gpa and goes to completion at 70 gpa melted biotite and muscovite are optically opaque but show extensive microvesiculation and flow when observed with the sem electron diffraction confirms that biotite and muscovite have transformed to a glass the distribution of vesicles in shock vitrified mica shows escape of volatiles within the short duration of the shock experiment experimentally shocked biotite and muscovite undergo congruent melting compositions of the glasses are similar to the unshocked micas except for volatiles h2o loss and k loss these unusual glasses derived from mica may be quenched by rapid cooling conditions during the shock experiment based on these results the extremely low h2o content of tektites may be reconciled with a terrestrial origin by impact release of volatiles in shock melted micas affects the melting behavior of coexisting dry silicates during the short duration of the shock experiment transportation and escape of volatiles released from shock melted micas may provide plausible mechanisms for the origin of protoatmospheres on terrestrial planets hydrothermal activity on phyllosilicate rich meteorite parent bodies and fluid entrapment in meteorites

temporal and spatial correlations between changes in plate motions and the evolution of rifted basins in africa we present the results of a backstripping study incorporating over 200 wells in 14 different basins or groups of basins located on the african interior or continental margins these results presented as mean tectonic subsidence rates averaged over appropriate periods are compared with the opening histories and changes in plate motions of the atlantic and indian oceans and the tethys mediterranean area for seven time steps from the late jurassic until present the results indicate a temporal and spatial correlation between phases of rapid subsidence in african basins and changes in plate motions and therefore are promising in terms of applicability of large scale plate motions in the studies of paleo stresses in sedimentary basins especially at times of initial rifting and formation of subduction zones reactivation of the basins appears to be primarily controlled by the orientation of the basin and underlying basement structures with respect to the stress direction the basins are preferably formed within weaker zones mostly pan african shear zones and the distance from the active plate boundary to the weak zone plays a minor role in whether or not it will be reactivated the presence of three important hot spots which were active during the early stages of break up of gondwana may have been important in preweakening the lithosphere at their sites this may have played an additional role in the localization of the basin deformation

shock temperatures of sio2 and their geophysical implications the temperature of sio 2 in high pressure shock states has been measured for samples of single crystal quartz and fused quartz pressures between 60 and 140 gpa have been studied using projectile impact and optical pyrometry techniques at lawrence livermore national laboratory both data sets indicate the occurrence of a shock induced phase transformation at 70 and 50 gpa along the and fused quartz hugoniots respectively the suggested identification of this transformation is the melting of shock synthesized stishovite with the onset of melting delayed by metastable superheating of the crystalline phase some evidence for this transition in conventional shock wave equation of state data is given and when these data are combined with the shock temperature data it is possible to construct the stishovite liquid phase boundaries the melting temperature of stishovite near 70 gpa pressure is found to be 4500 k and melting in this vicinity is accompanied by a relative volume change and latent heat of fusion of 2 7 and 2 4 mj kg respectively the solid stishovite hugoniot centered on quartz is well described by the linear shock velocity particle velocity relation u s 1 822 up 1 370 km s while at pressures above the melting transition the hugoniot centered on quartz has been fit with u s 1 619 u p 2 049 km s up to a pressure of 200 gpa the melting temperature of stishovite near 100 gpa suggests an approximate limit of 3500 k for the melting temperature of sio 2 bearing solid mantle mineral assemblages all of which are believed to contain si 4 in octahedral coordination with o 2 thus 3500 k is proposed as an approximate upper limit to the melting point and the actual temperature in the earth s mantle moreover the increase of the melting point of stishovite with pressure at 70 gpa is inferred to be 11 k gpa using various adiabatic temperature gradients in the earth s mantle and assuming creep is diffusion controlled in the lower mantle the current results could preclude an increase of viscosity by more than a factor of 10 3 with depth across the mantle

shock compression and isentropic release of granite summary new equation of state data for a weathered granite shocked to about 125 gpa are reported and combined with the westerly granite data of mcqueen marsh fritz 1967 the shock velocity us particle velocity up relations can be fitted with two linear regressions us 4 40 0 6up for a range of up up to about 2 km s 1 and us 2 66 1 49up for a range of about 2 to 5 km s 1 the third order birch murnaghan equation of state parameters are kos 51 57 gpa and k os 1 4 1 8 for the low pressure regime and kos 251 30 gpa and an assumed k os 4 for the high pressure regime compressive waveforms in dry and water saturated granite were measured at 10 15 gpa using the visar technique the measured wave profiles were successfully modelled using a maxwellian stress relaxation material model water saturated granite is characterized by a 25 per cent lower yield strength and a 75 per cent longer material relaxation time than dry granite

the many surface expressions of mantle dynamics plate tectonic theory suggests that present day topography can be explained by the repeated interactions between the tectonic plates moving along earth s surface however mounting evidence indicates that a significant proportion of earth s topography results from the viscous stresses created by flow within the underlying mantle rather than by the moving plates this dynamic topography is transient varying as mantle flow changes and is characterized by small amplitudes and long wavelengths it is therefore often hidden by or confused with the more obvious topographic anomalies resulting from horizontal tectonic movements however dynamic topography can influence surface processes and thus enter the geological record it has for example played a role in the establishment of amazon drainage patterns in turn surface processes such as the erosion of topographical anomalies could affect mantle flow this emerging view of dynamic topography suggests that the concept of plate tectonics as the driver of surface deformation needs to be extended to include the vertical coupling between the mantle and the surface unravelling this coupling back in time with the help of models and the geological record can potentially provide unprecedented insights into past mantle dynamics

tectonic setting of kimberlites kimberlites can be viewed as time capsules in a global plate tectonic framework their distribution illustrates clustering in time and space kimberlite ages span the assembly and break up of a number of supercontinents such as rodinia and gondwana these supercontinents show time lines with i broad periods devoid of kimberlite magmatism corresponding to times of continent stability and ii narrow kimberlite emplacement windows corresponding to times of fundamental plate reorganizations this episodicity implies that kimberlite emplacement events are intrinsically related to particular stages in the life cycle of supercontinents the onset of kimberlite magmatism is closely associated with thermal perturbations thermal insulation mantle upwelling beneath a stagnant or sluggish supercontinent these perturbations may have caused uplift and the onset of continental break up through fracture zones propagating into the supercontinent subsequent spreading and ocean floor development is marked by apparent cusps and jogs in plate motion paths resultant strain is accommodated along trans lithospheric corridors with episodic uplift and erosion and focused kimberlite melt migration the corridors are manifest as discontinuities in the lithosphere mantle measured as geophysical gradients and as changes in mantle lithosphere composition within the crust these corridors are expressed as a terrane boundaries b incipient continental rifts c fracture zones or d major dyke swarms some kimberlite populations are clustered along parallel sets of corridors widely distributed across a large part of a subcontinent and repeated magmatism is seen within many of the clusters the association of kimberlite occurrences with discontinuities may be ascribed to favorable conditions for melt production and to resultant melt focusing along high strain zones that contain fractures and faults such conditions may be attained during different stages in the evolution of continents a supercontinent formation b incipient rifting driven by far field stresses and onset of continental break up and c strain accommodation along the continental continuation of oceanic fracture zones during spreading type c may show concomitant kimberlite magmatism in separate continents after break up

a new velocity field for africa from combined gps and doris space geodetic solutions contribution to the definition of the african reference frame afref 1 we analyzed 16 years of gps and 17 years of doppler orbitography and radiopositioning integrated by satellite doris data at continuously operating geodetic sites in africa and surroundings to describe the present day kinematics of the nubian and somalian plates and constrain relative motions across the east african rift the resulting velocity field describes horizontal and vertical motion at 133 gps sites and 9 doris sites horizontal velocities at sites located on stable nubia fit a single plate model with a weighted root mean square residual of 0 6 mm yr maximum residual 1 mm yr an upper bound for plate wide motions and for regional scale deformation in the seismically active southern africa and cameroon volcanic line we confirm significant southward motion 1 5 mm yr in morocco with respect to nubia consistent with earlier findings we propose an updated angular velocity for the divergence between nubia and somalia which provides the kinematic boundary conditions to rifting in east africa we update a plate motion model for the east african rift and revise the counterclockwise rotation of the victoria plate and clockwise rotation of the rovuma plate with respect to nubia vertical velocities range from 2 to 2 mm yr close to their uncertainties with no clear geographic pattern this study provides the first continent wide position velocity solution for africa expressed in international terrestrial reference frame itrf2008 a contribution to the upcoming african reference frame afref except for a few regions the african continent remains largely under sampled by continuous space geodetic data efforts are needed to augment the geodetic infrastructure and openly share existing data sets so that the objectives of afref can be fully reached

obliquity along plate boundaries most of the plate boundaries are activated obliquely with respect to the direction of far field stresses as roughly only 8 of the plate boundaries total length shows a very low obliquity ranging from 0 to 10 sub orthogonal to the plate displacement the obliquity along plate boundaries is controlled by i lateral rheological variations within the lithosphere and ii consistency with the global plate circuit indeed plate tectonics and magmatism drive rheological changes within the lithosphere and consequently influence strain localization geodynamical evolution controls large scale mantle convection and plate formation consumption and re organization thus triggering plate kinematics variations and the adjustment and re orientation of far field stresses these geological processes may thus result in plate boundaries that are not perpendicular but oblique to the direction of far field stresses this paper reviews the global patterns of obliquity along plate boundaries using gplate we provide a statistical analysis of present day obliquity along plate boundaries within this framework by comparing natural examples and geological models we discuss deformation patterns and kinematics recorded along oblique plate boundaries

location of the nubia somalia boundary along the southwest indian ridge the northern boundary between the nubian west african and somalian east african plates is marked by the existence of the east african rift which is the locus of roughly east west stretching due to the separation of these plates south of 20 s any expression of deformation or seismicity due to the relative motion of these two presumably distinct plates vanishes or is extremely subtle although the boundary must continue until it intersects another plate boundary until recently there were no observational limits on the location of that intersection and the recent limits were very broad being thousands of kilometers wide here from an analysis of the locations of the old edge of magnetic anomaly 5 which is over 11 ma seafloor we show that the main locus of deformation between nubia and somalia near the southwest indian ridge over the past 11 m y can be no wider than a few hundred kilometers from 100 km west of the du toit fracture zone to 50 km east of the andrew bain fracture zone complex deformation is most likely concentrated along a closely spaced set of several fracture zones known collectively as the andrew bain fracture zone complex which is only 100 km wide the displacement along the nubia somalia boundary over the past 11 m y is 23 km 6 km 95 confidence limits indicating a displacement rate of 2 mm yr much slower than typical rates of seafloor spreading and subduction but comparable to that along the owen fracture zone and dalrymple trough which separate the arabian plate from the indian plate and along the azores gibraltar line which separates the eurasian plate from the nubian plate

endorheic exorheic transitions of the rio grande and east african rifts this research was supported by nsf ear 1516680 and spanish government project mite cgl2014 59516

experimental constraints on shock induced microstructures in naturally deformed silicates abstract planar deformation features pdfs in various minerals have long been accepted as evidence of impact induced deformation the uniqueness of this association was challenged in the context of the k t boundary extinction debate after mosaicism and microstructures similar to pdfs were reported from the products of explosive volcanism as a result of this debate a significant volume of new experimental and observational data on the development of shock induced microstructures has become available over the last ten years the results reveal that factors such as pre shock temperature pulse duration and crystallographic orientation of target minerals to the shock wave have a primary influence on how these microstructures develop data from diamond anvil cell and high pressure friction experiments reveal that the same solid state amorphization process that produces shock induced pdfs at low temperatures also occurs at much lower strain rates in static experiments the experimental data indicate that the amorphization process is thermally activated and that the character of the resulting pdfs is a function of the applied strain rate shock induced amorphization occurs along those crystallographic planes that are most readily transformed to the high pressure phase during very short pulse durations and produces pdfs that are visible at the optical scale lower strain rate deformation produces tem scale amorphization with orientations that are more homogeneously distributed throughout the target mineral and produces no optically visible pdfs the data confirm the uniqueness of multiple intersecting sets of optically visible pdfs as a diagnostic indicator of hypervelocity impact the data also support the hypothesis that the amorphization process can occur at a wide range of strain rates and that the limiting pressure for the process is controlled by the phase stability of the target mineral under the applied loading conditions not by the hel the data also suggest that the onset pressure the maximum pressure and the pressure range for producing optically visible pdfs decrease with increasing temperature and decreasing strain rate as the pressure range for optically visible pdf formation decreases it is replaced by homogeneous amorphization as observed in the anvil cell experiments thus the uniqueness of multiple intersecting sets of optically visible pdfs to hypervelocity impact is not due to a unique process but rather to a specific set of loading conditions that produce an optically visible microstructure likewise single sets of microdeformations observed in volcanic rocks are produced by the same process but at different loading conditions that preclude the development of multiple intersecting sets of pdfs the data also indicate that shock mosaicism which occurs above the hel represents a plastic response of the target mineral to loading rates that are too large to be accommodated by crystal plastic mechanisms observational data for some naturally deformed samples from the k t boundary the vredefort dome and volcanic rocks along with the experimental observations are used to constrain the range of conditions under which the natural microstructures form and to understand the differences between microstructures produced by impact volcanic and other natural processes finally some possible mechanisms for producing the microstructures observed in volcanic rocks are proposed

impact of uncertain reference frame motions in plate kinematic reconstructions a theoretical appraisal abstract geoscientists infer past plate motions which serve as fundamental constraints for a range of studies from observations of magnetic isochrons as well as hotspots tracks on the ocean floor and for stages older than the cretaceous from paleomagnetic data these observations effectively represent time integrals of past plate motions but because they are made at present yield plate kinematics naturally tied to a present day reference frame which may be another plate or a hotspots system these kinematics are therefore different than those occurred at the time when the rocks acquired their magnetisation or when hotspot related marine volcanism took place and are normally corrected for the reference frame absolute motion rfam that occurred since then the impact of true polar wander events on paleomagnetic data and the challenge of inferring hotspot drifts result in rfams being less resolved in a temporal sense and prone to noise this limitation is commonly perceived to hamper the correction of plate kinematic reconstructions for rfams but the extent to which this may be the case has not been explored here we assess the impact of uncertain rfams on kinematic reconstructions using synthetic models of plate motions over 100 million years we use randomly drawn models for the kinematics of two plates separated by a spreading ridge to generate a synthetic magnetisation pattern of the ocean floor the kinematics we infer from such a pattern are outputs that we correct for synthetic rfams using two equivalent methods a classical one as well as another that we propose and test here and then compare to the true motions input we assess the misfits between true and inferred kinematics by exploring a statistically significant number of models where we systematically downgrade the temporal resolution of rfam synthetic data and add noise to them we show that even poorly resolved noisy rfams are sufficient to retrieve reliable plate kinematic reconstructions for relative i e one plate with respect to another and absolute i e relative to the deep mantle plate motions estimates upon rfam correction differ from the true kinematics by less that 10 and 3 respectively

melting in the feosio2 system to deep lower mantle pressures implications for subducted banded iron formations abstract banded iron formations bifs consisting of layers of iron oxide and silica are far denser than normal mantle material and should have been subducted and sunk into the deep lower mantle we performed melting experiments on fe2sio4 from 26 to 131 gpa in a laser heated diamond anvil cell dac the textural and chemical characterization of a sample recovered from the dac revealed that sio2 is the liquidus phase for the whole pressure range examined in this study the chemical compositions of partial melts are very rich in feo indicating that the eutectic melt compositions in the feo sio2 binary system are very close to the feo end member the eutectic temperature is estimated to be 3540 150 k at the core mantle boundary cmb which is likely to be lower than the temperature at the top of the core at least in the archean and paleoproterozoic eons suggesting that subducted bifs underwent partial melting in a thermal boundary layer above the cmb the feo rich melts formed by partial melting of the bifs were exceedingly dense and therefore migrated downward we infer that such partial melts have caused iron enrichment in the bottom part of the mantle which may have contributed to the formation of ultralow velocity zones ulvzs observed today on the other hand solid residues left after the segregation of the feo rich partial melts have been almost pure sio2 and therefore buoyant in the deep lower mantle to be entrained in mantle upwellings they have likely been stretched and folded repeatedly by mantle flow forming sio2 streaks within the mantle marble cake mantle packages enhanced by sio2 streaks may be the origin of seismic scatterers in the mid lower mantle

a refinement of the chronology of rift related faulting in the broadly rifted zone southern ethiopia through apatite fission track analysis to reconstruct the timing of rift inception in the broadly rifted zone in southern ethiopia we applied the fission track method to basement rocks collected along the scarp of the main normal faults bounding i the amaro horst in the southern main ethiopian rift and ii the beto basin in the gofa province at the amaro horst a vertical traverse along the major eastern scarp yielded pre rift ages ranging between 121 4 15 3 ma and 69 5 7 2 ma similarly to two other samples one from the western scarp and one at the southern termination of the horst 103 4 24 5 ma and 65 5 4 2 ma respectively more interestingly a second traverse at the amaro northeastern terminus released rift related ages spanning between 12 3 2 7 and 6 8 0 7 ma in the beto basin the ages determined along the base of the main northwestern fault scarp vary between 22 8 3 3 ma and 7 0 0 7 ma we ascertain through thermal modeling that rift related exhumation along the northwestern fault scarp of the beto basin started at 12 2 ma while in the eastern margin of the amaro horst faulting took place later than 10 ma possibly at about 8 ma these results suggest a reconsideration of previous models on timing of rift activation in the different sectors of the ethiopian rift extensional basin formation initiated more or less contemporaneously in the gofa province 12 ma and northern main ethiopian rift 10 12 ma at the time of a major reorganization of the nubia somalia plate boundary i e 11 2 ma afterwards rift related faulting involved the southern mer amaro horst at 8 ma and only later rifting seemingly affected the central mer after 7 ma

the role of horizontal resolution for polar low simulations polar lows are intense mesoscale cyclones that mainly occur during the winter over the sea in polar regions owing to their small spatial scale with a diameter between 200 and 1000 km simulating polar lows is a challenging task in this study we investigated how increased resolution of a numerical weather prediction model impacts its ability to simulate polar lows we focused on a polar low that was successfully captured by three different flights during the ipy thorpex field campaign in 2008 verifying model results against campaign data showed that decreasing the horizontal grid spacing from 12 to 4 km significantly improved the simulation of the developing polar low and a further decrease to 1 km gave further improvement a model run with latent heating reduced to 10 indicated an extensive influence of diabatic heating in this polar low case and we suggest that the improved model performance at higher resolution could be connected to the model s handling of convection additional simulations starting 24 h later showed that the initial conditions were important for the model performance and when simulating another polar low case we found that the model failed to produce the polar low independent of the resolution this shows that while higher resolution indeed may give improved predictions of polar lows other factors like synoptic situation lateral boundaries and the initial condition may also be important copyright 2011 royal meteorological society

shock induced melting of mgsio3 perovskite and implications for melts in earth s lowermost mantle 1 new shock wave equation of state eos data for enstatite and mgsio3 glass constrain the density change upon melting of mg silicate perovskite up to 200 gpa the melt becomes denser than perovskite near the base of earth s lower mantle this inference is confirmed by shock temperature data suggesting a negative pressure temperature slope along the melting curve at high pressure although melting of earth s mantle involves multiple phases and chemical components this implies that the partial melts invoked to explain anomalous seismic velocities in the lowermost mantle may be dynamically stable index terms 3919 mineral physics equations of state 3944 mineral physics shock wave experiments 8124 tectonophysics earth s interior composition and state 1212 citation akins j a s n luo p d asimow and t j ahrens 2004 shock induced melting of mgsio3 perovskite and implications for melts in earth s lowermost mantle geophys res lett 31 l14612 doi 10 1029 2004gl020237

slip re orientation in oblique rifts oblique extension is expected to result in a combination of dip slip and strike slip displacement along faults with strike orthogonal and oblique to the extension direction respectively this general concept is in disagreement with observations from natural oblique rifts where faults show dip slip kinematics indicating pure extension irrespective of the fault strike with respect to the regional extension direction consequently along oblique structures slip is re oriented and oblique to the applied extension direction besides at fault scale slip is re oriented along strike such that it is dip slip at the fault center and becomes highly oblique slip toward the fault tips here we use analogue experiments to show that this discrepancy can be resolved when a preexisting weak zone wz is present in the crust at the onset of oblique extension the wz is implemented within the lower crust and strikes oblique to the extension direction our experimental results show that an inherited wz within the ductile crust favors the re orientation of slip such that oblique extension results in pure dip slip displacement on faults that strike oblique with respect to the extension direction furthermore we show that slip is re oriented along strike of major faults such that the fault center shows dip slip kinematics whereas its tips display strike slip kinematics these findings call into question the use of paleostress reconstructions to constrain plate kinematics in oblique extensional tectonic settings

applicability of large scale convection and condensation parameterization to meso γ scale hirlam a case study of a convective event abstract this paper presents a case study of a single cold air outbreak event with widespread convective precipitation over southern finland on 25 may 2001 the purpose of the study is to investigate the applicability of the convection and condensation scheme of the high resolution limited area model hirlam on meso scales the study concentrates on the issue of grid size dependent convection parameterization an explicit approach without the convection scheme is also examined at the same time the performance of an experimental nonhydrostatic version of hirlam is evaluated model simulations are conducted with three different horizontal grid spacings 11 5 6 and 2 8 km model results are compared to observed radar reflectivity data utilizing a radar simulation model which calculates radar reflectivities from three dimensional model output the best results are obtained using nonhydrostatic dynamics and a grid size dependent convection scheme with a 5 6 km grid interval however even the best conf

shock temperatures in anorthite glass temperatures of caal 2si 2o 8 anorthite glass shocked to pressures between 48 and 117 gpa have been measured in the range from 2500 to 5600 k using optical pyrometry techniques the pressure dependence of the shock temperatures deviates significantly from predictions based on a single high pressure phase either a variable specific heat or the existence of three phase transitions at pressures of about 55 85 and 100 gpa and with transition energies of about 0 5 mj kg 1 each 1 5 mj kg 1 total can explain the shock temperature data the proposed phase transition at 100 gpa can possibly be identified with the stishovite melting transition theoretical models of the time dependence of the thermal radiation from the shocked anorthite based on the geometry of the experiment and the absorptive properties of the shocked material yield good agreement with observations indicating that it is not necessary to invoke intrinsic time dependences to explain the data in many cases observed time dependences were used to calculate absorption coefficients of the shocked material of from about 2 mm 1 to greater than 24 mm 1 an increasing function of shock pressure the assumption that the shocked material radiates as a black body is supported by the theoretical model and by the close agreement between measured and calculated black body spectral radiance as a function of wavelength

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

the effects of changing resolution on mesocale modelling of line convection and slantwise circulations in fastex iop16 data are presented from modelling a rapidly developing cyclone in intensive observation period 16 of the fronts and atlantic storm tracks experiment using the new non hydrostatic version of the met office unified model at a number of different horizontal and vertical resolutions the model produces a good representation of the system including multiple fronts with line convection cloud heads and slantwise cross frontal circulations similar to those seen in the observations results from the model with various grid lengths from 60 km nested down to 2 km with 45 vertical levels show that more realistic detail is produced as the grid length is reduced but that the large scale structures remain consistent by spectral analysis it is found that structures on scales shorter than approximately five grid lengths are attenuated the line convection at the fronts is aliased onto the grid length scale except in the 2 km model where evidence of a three dimensional structure starts to appear the 2 and 4 km models have also been run with enhanced vertical resolutions it is concluded that whilst a 24 km 45 level model is sufficient to represent the overall frontal structure a model with resolution at least 2 km in the horizontal and 90 vertical levels is required to represent small scale multiple slantwise circulations similar to those observed in the dropsonde data crown copyright 2003

plate kinematic evidence for the existence of a distinct plate between the nubian and somalian plates along the southwest indian ridge 1 previously we estimated the angular velocity of the nubian plate relative to the somalian plate from an updated set of spreading rates and transform fault azimuths we found that the nubia somalia plate boundary intersects the southwest indian ridge swir between 26 e and 32 e if both the nubian and somalian plates are rigid and if the boundary between them is narrow these prior results are not completely satisfactory mainly for two reasons 1 the four plate circuit somalia antarctica nubia arabia does not close 2 the largest mw 6 8 recorded african earthquakes that are near but not along the swir occur near 48 e well east of the intersection with the nubia somalia boundary here we investigate these problems through detailed analysis of plate motion data especially those along the swir east of the andrew bain transform fault complex we find an improved fit to the data and improved plate circuit closure if a region of the african lithosphere is interpreted as a new component plate this new plate lies between the nubian and somalian plates along the swir and is separated from the latter by a diffuse boundary that includes the locations of the largest off ridge earthquakes following c j h hartnady we call this new plate lwandle use of this new plate geometry shifts the nubia somalia pole of rotation northeastward to just south of south africa and thus alters estimates of current india eurasia plate motion

dynamic compression of sio2 a new interpretation 1 in light of recent discoveries of post stishovite phases of sio2 with the cacl2 and pbo2 structures we have reassigned the regimes along the hugoniots for initial quartz coesite cristobalite porous coesite and fused silica calculated hugoniots for fused silica cristobalite and porous coesite indicate transition to stishovite and then melt hugoniots for crystal quartz and coesite indicate that transition occurs to stishovite then the cacl2 structure and finally to melt

multi technique equation of state for fe2sio4 melt and the density of fe bearing silicate melts from 0 to 161 gpa we have conducted new equation of state measurements on liquid fe 2 sio 4 in a collaborative multi technique study the liquid density the bulk modulus k and its pressure derivative k were measured from 1 atm to 161 gpa using 1 atm double bob archimedean multi anvil sink float and shock wave techniques shock compression results on initially molten fe 2 sio 4 1573 k fitted with previous work and the ultrasonically measured bulk sound speed c o in shock velocity u s particle velocity u p space yields the hugoniot u s 1 58 0 03 u p 2 438 0 005 km s sink float results are in agreement with shock wave and ultrasonic data consistent with an isothermal k t 19 4 gpa and k 5 33 at 1500 c shock melting of initially solid fe 2 sio 4 300 k confirms that the gruneisen parameter of this liquid increases upon compression where o o q yields a q value of 1 45 constraints on the liquid fayalite eos permit the calculation of isentropes for silicate liquids of general composition in the multicomponent system cao mgo al 2 o 3 sio 2 feo at elevated temperatures and pressures in our model a whole mantle magma ocean would first crystallize in the mid lower mantle or at the base of the mantle were it composed of either peridotite or simplified chondrite liquid respectively in regards to the partial melt hypothesis to explain the occurrence and characteristics of ultra low velocity zones neither of these candidate liquids would be dense enough to remain at the core mantle boundary on geologic timescales but our model defines a compositional range of liquids that would be gravitationally stable

shock temperatures in cao blackbody temperatures of cao shocked to pressures from 140 to 182 gpa have been measured in the 3750 to 6000 k range using the lawrence livermore national laboratory light gas gun these shock temperatures along with hugoniot data for single crystal and porous cao and isothermal data are used to construct equations of state for the high pressure b2 phase of cao the zero pressure density of the b2 phase is between 3 8 and 4 0 mg m 3 and the b1 b2 transition energy is 2 1 to 2 3 mj kg the density and bulk modulus at pressures from 70 to 135 gpa are similar to seismically determined values for the lower mantle of the earth thus the lower mantle could have a substantial inventory of ca bearing minerals and mixed oxide models for the composition of the lower mantle will be insensitive to the quantity of cao assumed

controls of inherited lithospheric heterogeneity on rift linkage numerical and analog models of interaction between the kenyan and ethiopian rifts across the turkana depression inherited rheological structures in the lithosphere are expected to have large impact on the architecture of continental rifts the turkana depression in the east african rift connects the main ethiopian rift to the north with the kenya rift in the south this region is characterized by a nw se trending band of thinned crust inherited from a mesozoic rifting event which is cutting the present day n s rift trend at high angle in striking contrast to the narrow rifts in ethiopia and kenya extension in the turkana region is accommodated in sub parallel deformation domains that are laterally distributed over several hundred kilometers we present both analog experiments and numerical models that reproduce the along axis transition from narrow rifting in ethiopia and kenya to a distributed deformation within the turkana depression similarly to natural observations our models show that the ethiopian and kenyan rifts bend away from each other within the turkana region thus forming a right lateral stepover and avoiding a direct link to form a continuous n s depression the models reveal five potential types of rift linkage across the pre existing basin three types where rifts bend away from the inherited structure connecting via a 1 wide or 2 narrow rift or by 3 forming a rotating microplate 4 a type where rifts bend towards it and 5 straight rift linkage the fact that linkage type 1 is realized in the turkana region provides new insights on the rheological configuration of the mesozoic rift system at the onset of the recent rift episode

relating rapid plate motion variations to plate boundary forces in global coupled models of the mantle lithosphere system effects of topography and friction abstract recent high resolution models of past plate motions and their comparison with plate motion models inferred from space geodetic techniques reveal a number of short term variations in global plate velocities over the past 10 myrs such variations serve as powerful probe into the nature and magnitude of plate boundary forces because they are unlikely to originate from changes in mantle buoyancy forces which evolve on longer time scales here we explore the constraints of the velocity record using a novel coupled modeling approach of global neo tectonic simulations combined with realistic plate driving forces obtained from mantle circulation models mcms to arrive at simple global budgets of mantle lithosphere and plate boundary forces we focus on three plate boundary systems along the nazca south america plate margin the aleutian trench and the india australia plate boundary to show that gravitational spreading from high topography in the andes and tibet contributes substantially to the global plate tectonic force balance and that this contribution is sufficient to explain some 35 of recent velocity changes over the earth s surface including among others the observed 30 convergence reduction between the nazca south america plates our models make a number of specific predictions such as significant lateral variations in plate coupling forces along a given margin revealed by trench parallel gravity and bathymetry anomalies and the occurrence of large earthquakes as well as differences by as much as a factor of five from margin to margin they also support the notion of a relatively young plate boundary separating the india and australia plates which has been previously suggested based on independent observations importantly we find that the modeled nazca south america convergence reduction explains recent spreading rate variations in the south atlantic and south pacific which points to the importance of far field effects on the adjacent continents in explaining the spreading record of oceanic basins our numerical results demonstrate a that detailed budgets of forces acting upon plates can be obtained and b support the notion of strong forcing along weak plate boundaries

bayesian noise reduction in arabia somalia and nubia arabia finite rotations since 20 ma implications for nubia somalia relative motion knowledge of nubia somalia relative motion since the early neogene is of particular importance in the earth sciences because it i impacts on inferences on african dynamic topography and ii allows us to link plate kinematics within the indian realm with those within the atlantic basin the contemporary nubia somalia motion is well known from geodetic observations precise estimates of the past 3 2 myr average motion are also available from paleo magnetic observations however little is known of the nubia somalia motion prior to 3 2 ma chiefly because the southwest indian ridge spread slowly posing a challenge to precisely identify magnetic lineations this also makes the few observations available particularly prone to noise here we reconstruct nubia somalia relative motions since 20 ma from the alternative plate circuit nubia arabia somalia we resort to trans dimensional hierarchical bayesian inference which has proved effective in reducing finite rotation noise to unravel the arabia somalia and arabia nubia motions we combine the resulting kinematics to reconstruct the nubia somalia relative motion since 20 ma we verify the validity of the approach by comparing our reconstruction with the available record for the past 3 2 myr obtained through antarctica results indicate that prior to 11 ma the total motion between nubia and somalia was faster than today furthermore it featured a significant strike slip component along the nubia somalia boundary it is only since 11 ma that nubia diverges away from somalia at slower rates comparable to the present day one kinematic changes of some 20 might have occurred in the period leading to the present day but plate motion steadiness is also warranted within the uncertainties

recent volcano tectonic activity of the ririba rift and the evolution of rifting in south ethiopia abstract the relationships between volcanic activity and tectonics at the southernmost termination of the main ethiopian rift mer east africa still represent a debated problem in the mer evolution new constraints on the timing evolution and characteristics of the poorly documented volcanic activity of the dilo and mega volcanic fields vf near the kenya ethiopia border are here presented and discussed the new data delineate the occurrence of two distinct groups of volcanic rocks 1 pliocene subalkaline basalts observed only in the dilo vf forming a lava basement faulted during a significant rifting phase 2 quaternary alkaline basalts occurring in the two volcanic fields as pyroclastic products and lava flows issued from monogenetic edifices and covering the rift related faults 40ar 39ar dating constrains the emplacement time of the large basal lava plateau to 3 7 ma whereas the youngest volcanic activity characterising the two areas dates back to 134 ka dilo vf to as recent as the holocene mega vf volcanic activity developed along tectonic lineaments independent from those of the rift no direct relations are observed between the pliocene roughly n s trending major boundary faults of the ririba rift and the ne sw oriented structural trend characteristic of the quaternary volcanic activity we speculate that this change in structural trend may be the expression of 1 inherited crustal structures affecting the distribution of the recent volcanic vents and 2 a local stress field controlled by differences in crustal thickness following a major episode of reorganization of extensional structures in the region due to rift propagation and abandonment

postshock temperatures in silica experimental postshock temperatures for crystalline quartz released from shock states between 86 and 127 gpa and for fuset silica released from 59 to 73 gpa have been determined using previously measured spectral radiance data the temperatures range from 3660 to 4150 k and are consistent with the interpretation based on shock temperature measurements that the hugoniot of sio sub 2 crosses the phase boundary between stishovite and liquid the new postshock temperatures were used to determine the grueneisen parameter of liquid silica at high pressures and temperatures and the specific volume of the release state gamma v 3 5 mg m sup 3 and f sub r 0 266 m sup 3 mg respectively although there is some ambiguity as to whether this volume corresponds to a partial or total release state to show the validity of these postshock temperatures a radiative transport model was extended to times after free surface arrival of the shock wave copyright american geophysical union 1988

experimentation in planetary geology the reasons for conducting experimentation are as varied as the topics that are studied whether a specific parameter must be quantified or an attempt is being made to duplicate a peculiar process all experiments are unified by a common ultimate objective to provide a tangible link between theory and observation although a suitable theory is often nonexistent at the time of the experiment experimentation in planetary geology assumes particular importance because of the factors inherent in the field of study itself often enormous spatial scales timescales rivaled only by those of cosmology and the remoteness of the objects of study in addition any geoscientist who has entertained thoughts of developing a realistic mathematical model of even the simplest of processes rapidly becomes acquainted with the intimidating complexity of geological phenomena experimentation is still in many cases the only effective means of learning more about some natural mechanisms

thermochemical structure and dynamics of the african superplume 1 we present a new three dimensional 3 d model of the thermochemical structure of the african superplume region obtained from simultaneous inversion of global seismic and convection related data convection related observations include the global free air gravity field tectonic plate motions dynamic surface topography and the excess ellipticity of the core mantle boundary a 3 d image of the chemically induced density perturbations provides direct evidence that intrinsically dense material is entrained within the superplume and concentrated into a rounded structure 1000 km above the core mantle boundary the thermally induced density perturbations are greater in magnitude than the chemically induced implying overall positive buoyancy throughout the superplume the observed morphology and density signatures are consistent with a thermochemical plume that has risen from a compositionally distinct pile at the base of the mantle and may be currently deforming under the influence of its intrinsic negative chemical buoyancy citation simmons n a a m forte and s p grand 2007 thermochemical structure and dynamics of the african superplume geophys res lett 34 l02301 doi 10 1029 2006gl028009

parameterization of convective precipitation in mesoscale numerical models a critical review abstract current approaches for incorporating cumulus convection into mesoscale numerical models are divided into three groups the traditional approach utilizes cumulus parameterization at convectively unstable points and explicit nonparameterized condensation at convectively stable points the fully explicit approach uses explicit methods regardless of stability the hybrid approach parameterizes convective scale updrafts and downdrafts but detrains a fraction of parameterized cloud and precipitation particles to the grid scale this allows the path and phase changes of such particles to be explicitly predicted over subsequent time steps the traditional approach provides the only alternative for numerical models with grid spacing too large to resolve mesoscale structure as grid spacing falls below 50 km the traditional approach becomes increasingly likely to violate fundamental scale separation requirements of parameterization particularly if mesoscale organization of convection is parameterize

high resolution estimates of nubia somalia plate motion since 20 ma from reconstructions of the southwest indian ridge red sea and gulf of aden

melting temperature of the earth s mantle and core earth s thermal structure abstract although the temperature at the top of the lower mantle is well constrained by phase equilibrium data for the transformation of transition zone minerals to the denser perovskite polymorphs the temperature distribution in the lower mantle is poorly known models depend strongly on the assumptions of the amount of internal heating and the viscosity profile new melting data on iron to pressures of the outer core 2 mbar and the observed strong decrease of eutectic melting depression in the fe feo fes system with increasing pressure however tightly constrain the temperature at the inner core boundary to slightly less than 5000 k this estimate can be reconciled with all recent static melting measurements on iron and lays within the uncertainty of shock temperature measurements the resulting temperature at the core mantle boundary of about 4000 k then requires a large temperature gradient at the bottom of the lower mantle of about 1500 k recent findings of the very high melting temperatures o

from slow to ultraslow a previously undetected event at the southwest indian ridge at ca 24 ma changes in plate motion are thought to be recorded in the trend of fracture zones even though fracture zones provide no information about the spreading rate using newly compiled published and unpublished magnetic data from the southwest indian ridge we calculated fi nite rotation poles for a13 a8 and a6 from which we determined a 50 decrease in spreading rate from slow to ultraslow at ca 24 ma not accompanied by a signifi cant change in spreading direction this spreading rate decrease is concurrent with changes in plate motions at only two of the four adjoining plate boundaries finally we discuss the possible relationships of this event with other absolute or relative plate motion events that occurred at ca 24 ma at the global scale

geodynamical implications from the correlation of surface geology and seismic tomographic structure abstract we have investigated the relationship between the lateral seismic structure in the mantle and the geological features associated with major mantle upwellings the oceanic ridge system and hotspots using the technique of correlation analysis we have identified that these features extend quite deeply into the mantle to a depth of around 1300 km this finding is surprising for the ridges because it indicates that the style of upwelling is rather active and is somewhat two dimensional in character the confidence level of the correlation between ridges and slow seismic regions is maintained at a level of greater than 90 to a depth of 1300 km this is followed by a sharp decrease to 50 at 1700 km a similar correlation pattern is found for the hotspots in both cases the correlation maxima lie close to a depth of 1000 km where remnants of old slabs have recently been revealed by correlation analysis these findings suggest that dynamical processes at a depth of around 1000 km can be linked with surface tectonic activity this correlation disappears at a depth of around 1500 1700 km certain features of the surface tectonic pattern can again be recognized around a depth of 2000 km and in the deepest mantle where major hotspot plumes and the remnant old lithosphere are the dominant features there may be some dynamical relationship between the ridge system and the two megaplumes we have proposed a physical mechanism based on viscous heating within megaplumes to explain the two peaks in the correlation between the hotspots and the seismic heterogeneities

composition of the upper mantle geophysical tests of two petrological models the elastic properties of candidate mantle phases are used to test the viability of olivine rich pyrolitic and cao al 2o 3 rich eclogitic assemblages for the mantle high temperature adiabats for each phase of interest are constructed and compared to mantle seismic properties both pyrolitic and eclogitic assemblages satisfy the seismic properties between 200 and 400 km between 400 and 670 km depth an eclogitic assemblage yields a superior match to velocities and velocity gradients the 400 km seismic discontinuity may represent a chemical boundary between pyrolite and picritic eclogite piclogite or phase transformations in the olivine orthopyroxene components of a piclogitic assemblage containing about 16 olivine high velocity gradients in the transition zone may be explained by the transformation of ca rich cpx to majorite garnet seismic properties at the top of the lower mantle are consistent with pyrolite piclogite or perovskite implying that the 670 km discontinuity may be a chemical boundary

hugoniot equation of state of anorthite glass and lunar anorthosite twenty one hugoniot experiments were conducted on an amorphous material of anorthite composition in the pressure range 8 120 gpa using both routine and new methods two hugoniot measurements at about 120 gpa were made on lunar gabbroic anorthosite apollo 15 418 theoretical hugoniots are constructed for both materials assuming they are disproportionate to their component oxides these accurately predict the p behaviour of the lunar anorthosite hugoniot at 120 gpa and the anorthite glass hugoniot above 50 gpa but overestimate the shock temperatures of anorthite glass the mixed oxide model fails to predict the release paths of either material we conclude that the mixed oxide model is a good description of the bulk properties of the high pressure phases of anorthite but does not represent the actual phases a significant enrichment of calcic refractory material in the earth s lower mantle is not precluded by the bulk properties of the anorthite high pressure phases

shock experiments on pre heated α and β quartz ii x ray and tem investigations quartz single crystals pre heated to different temperatures 20 275 540 and 630 c were experimentally shocked to pressures ranging from 20 to 40 gpa with the shock front propagating parallel to 1010 or 0001 shocked quartz pre heated to 630 c is nearly x ray amorphous at 26 gpa while quartz shocked at room temperature still displays several x ray diffraction lines up to 30 gpa all samples that were still crystalline when shocked above 25 gpa exhibit lattice expansion internal strain and reduced crystallite sizes lattice expansion was up to 1 for the constants a o and co and up to 3 for the cell volume v o quartz shocked parallel to 1010 always shows larger lattice constants and is more strained than quartz shocked parallel to 0001 indicating that shock effects are controlled by the structural anisotropy internal strain is anisotropic with minimum strain in the planes belonging to 1012 the dominant shock defect revealed by tem are sets of parallel lamellae consisting of pure sio 2 glass so called planar deformation features pdfs below 25 gpa pdfs are extremely rare and thin 30 nm at pressures above 25 gpa however pdfs occur frequently and are about ten times thicker this qualitative observation of increasing pdf frequency and thickness with increasing pressure is quantified by calculations of the amount of glass using x ray and density data the results of this study substantiate the crucial role of pdfs for the lattice expansion of quartz and the formation of diaplectic glass and lechatelierite lattice expansion is ascribed to the enormous expansion of pdfs during decompression leading to elastic stretching in adjacent crystalline areas the observation that pdfs occur predominantly parallel to 1012 where minimum strain is present supports this hypothesis the increasing amount of pdfs above 25 gpa is interpreted as the cause for the formation of diaplectic glass shock temperature calculations support the assumption that these pdfs consist under compression of superheated melt the heat is possibly sufficient to melt crystalline areas adjacent to the superheated pdfs at enhanced pressures such molten bands coalesce to a total melt which is quenched upon decompression thus diaplectic glass represents a quenched high pressure melt while lechatelierite sio 2 glass with a fluidal texture is interpreted as quenched low pressure melt

the microstructural response of quartz and feldspar under shock loading at variable temperatures shock recovery experiments were carried out on westerly granite and hospital hill quartzite targets in the peak pressure range 8 to 25 gpa preshock temperatures of 25 450 and 750 c and pulse durations of 2 to 7 s using internally heated momentum traps and explosive plane wave generators optical and transmission electron microscopy analyses of quartz and feldspar shocked at 25 c revealed the previously documented progression with increasing pressure 1 fracturing 2 planar fractures and shock mosaicism 3 shock mosaicism and planar deformation features pdfs and 4 isotropization this same sequence is observed for experiments at elevated preshock temperature but with specific microstructures occurring at lower pressures than those in previous experiments at room temperature at 750 c strong shock mosaicism partially thermally recovered is characteristic of feldspar shocked to 8 gpa whereas 15 gpa is required for its development in quartz and for the generation of pdfs in both minerals the results suggest that threshold pressures for formation of the various microstructures and phases are expected to vary systematically as a function of the preshock temperature of the target material we suggest that pdfs are generated in the shock transition by progressive heterogeneous phase transformation of the crystal structure to form dense glass or high pressure polymorphs the onset pressures for pdfs in specific crystallographic orientations is not influenced strongly by temperature but the character of the pdfs does change as preshock temperature is increased at the same peak shock stress the change from multiple sets of thin pdfs at low temperature to thicker single sets of pdfs at moderate temperature and finally to complete isotropization at high temperatures reflects a change in the phase transformation mechanism as a function of temperature in contrast development of shock mosaicism in quartz and feldspar occurs throughout the duration of shock loading and is better developed at elevated temperatures where the kinetics are enhanced by the additional thermal energy in the target

rapid plate motion variations through geological time observations serving geodynamic interpretation past and current plate motions are increasingly well mapped from high temporal resolution paleomagnetic and geodetic studies revealing rapid variations that occur on short timescales relative to the time it takes for the large scale structure associated with mantle buoyancy to evolve the rates of change of plate velocities hold key information on the geodynamic tectonic and earth s surface processes that may have caused them rapid plate motion changes thus provide us with a unique opportunity to quantify the forcing associated with these processes important mechanisms capable of inducing such rapid changes include evolving plate boundary forces for example those associated with slab sinking or orogeny along convergent margins as well as temporal variations in pressure driven flow within the asthenosphere that link plate velocity variations explicitly to changes in dynamic topography here we focus on a findings from recent kinematic observations and b the quantitative framework that allows t

the rotation of arabia and the levant fault system abstract we establish the kinematics of the rotation of arabia with respect to africa and show that the rotation rate of africa increased by a factor of about four in late serravallian time about 12 13 ma ago we discuss evidence that oceanic accretion started simultaneously in the gulf of aden and in the red sea at this time it is also at this time that the levant and anatolian fault systems were initiated our initial reconstruction of the red sea rift in late oligocene early miocene time approximately superposes the present shore lines which coincide with the mid point of the zones of rapid crustal thinning the future continental slopes the amount of extension during the early phase of continental rifting is obtained by assuming that the total volume of continental crust is conserved during extension and by checking these estimates through closure around the sinai triple junction in the late miocene the southern evant fault zone was a zone of distributed shear deformation through counterclockwise block rotation extending soul h ward within the northernmost red sea since the early pliocene the shear motion has been localized within the gulf of aqaba changing the tectonic pattern in the northern red sea

along strike variation in deformation style inferred from kinematic reconstruction and strain rate analysis a case study of the ethiopian rift abstract in this paper we combine kinematic reconstruction and seismic strain rate analysis to understand the along strike variation in strain accommodation in the ethiopian rift er evolution the reconstruction poles close the southern and central er at 19 and 15 myr respectively whereas there is 34 14 km overlap in the northern er at 11 myr using kostrov summation seismic strain rates of 6 81 10 9 yr 1 and 0 06 10 9 yr 1 are obtained for the south central and northern er respectively comparison of the seismic and geodetic strain rates shows that seismic deformation dominates the south and central er contrary to the northern er that deforms aseismically the results obtained indicate that nubia somalia plate reconstructions together with information on the onset of rifting overestimate the kinematics of the northern er we argue that magmatic processes play significant role in accommodating the 2 myr opening of the rift our findings agree well with previous geophysical and geological studies in the ethiopian rift

shock compressed mgsio3 glass enstatite olivine and quartz optical emission temperatures and melting optical emission of mgsio 3 glass enstatite olivine and quartz under shock wave compression was investigated with optical pyrometry at discrete wavelengths ranging from visible to near infrared we develop a new analysis of optical emission that does not require a gray body assumption instead at each wavelength the optical linear absorption coefficients and blackbody spectral radiances l b of shocked and unshocked materials were obtained by nonlinear fitting to the time resolved radiance from the target assembly the absorption spectra of unshocked samples corresponding to the measured values of reproduce those from independent static optical spectroscopic measurements the measured values of ranging from 7 to 56 mm 1 for shocked samples indicate that shock induced high pressure phases including melt can be regarded essentially as black bodies in the optical range investigated although starting phases such as enstatite and olivine have band like spectra at ambient conditions the effect of emission from the air gap at the driver sample interface on the recorded radiance can be resolved but and l b cannot be separated for this component of the signal the shock velocity particle velocity relationships of these silicates derived from radiance history are in accord with previous investigations using independent techniques given the limited amount of shock wave data possible high pressure melting curves of mg perovskite and its assemblage with periclase are deduced their melting temperatures near the core mantle boundary cmb being 6000 500 k and 4000 300 k respectively it is proposed that mg perovskite melts with density increase at the cmb pressure

molecular dynamics modeling of stishovite a morse stretch potential charge equilibrium force field for silica system has been employed to simulate the thermodynamics of stishovite with the molecular dynamics md method the equation of state thermal expansivity and melting curve of stishovite have been obtained this simple force field yielded results in accordance with the static and dynamic experiments the stishovite melting simulation appears to validate the interpretation of superheating of the solid along the hugoniot in the shock melting experiments md simulations show that the thermal expansivity of stishovite at lowermost mantle conditions is a weak function of temperature the phase diagram of silica up to the mega bar regime is proposed based on the experimental and theoretical studies the related physical and geophysical implications are addressed 5 2002 elsevier science b v all rights reserved

the angular velocities of the plates and the velocity of earth s centre from space geodesy summary using space geodetic observations from four techniques gps vlbi slr and doris we simultaneously estimate the angular velocities of 11 major plates and the velocity of earth s centre we call this set of relative plate angular velocities geodvel for geodesy velocity plate angular velocities depend on the estimate of the velocity of earth s centre and on the assignment of sites to plates most geodetic estimates of the angular velocities of the plates are determined assuming that earth s centre is fixed in an international terrestrial reference frame itrf and are therefore subject to errors in the estimate of the velocity of earth s centre in itrf2005 and itrf2000 earth s centre is the centre of mass of earth oceans and atmosphere cm the velocity of cm is estimated by slr observation of lageos s orbit herein we define earth s centre to be the centre of mass of solid earth ce we determine the velocity of ce by assuming that the portions of plate interiors not near the late pleistocene ice sheets move laterally as if they were part of a rigid spherical cap the geodvel estimate of the velocity of ce is likely nearer the true velocity of cm than are the itrf2005 and itrf2000 estimates because 1 no phenomena can sustain a significant velocity between cm and ce 2 the plates are indeed nearly rigid aside from vertical motion and 3 the velocity of cm differs between itrf2005 and itrf2000 by an unacceptably large speed of 1 8 mm yr 1 the velocity of earth s centre in geodvel lies between that of itrf2000 and that of itrf2005 with the distance from itrf2005 being about twice that from itrf2000 because the geodvel estimates of uncertainties in plate angular velocities account for uncertainty in the velocity of earth s centre they are more realistic than prior estimates of uncertainties geodvel differs significantly from all prior global sets of relative plate angular velocities determined from space geodesy for example the 95 per cent confidence limits for the angular velocities of geodvel exclude those of revel sella et al for 34 of the 36 plate pairs that can be formed between any two of the nine plates with the best constrained motion the median angular velocity vector difference between geodvel and revel is 0 028 myr 1 which is up to 3 1 mm yr 1 on earth s surface geodvel differs the least from the geodetic angular velocities that altamimi et al determine from itrf2005 geodvel s 95 per cent confidence limits exclude 11 of 36 angular velocities of altamimi et al and the median difference is 0 015 myr 1 geodvel differs significantly from nearly all relative plate angular velocities averaged over the past few million years including those of nuvel 1a the difference of geodvel from updated 3 2 myr angular velocities is statistically significant for all but two of 36 angular velocities with a median difference of 0 063 myr 1 across spreading centres eight have slowed down while only two have sped up we conclude that plate angular velocities over the past few decades differ significantly from the corresponding angular velocity averaged over the past 3 2 myr

current plate motions across the red sea summary a new objective method is used to estimate precisely 64 rates of seafloor spreading since chron 2a 3 2 ma from all the red sea magnetic profiles available from 15 9 to 26 n the fastest spreading rate 16 mm yr 1 occurs near 18 n whereas the slowest rate 10 mm yr 1 occurs at 25 5 n and is consistent with the rate predicted from arabia nubia data to the south the standard deviation of the spreading rates is 0 8 mm yr 1 much smaller than the median standard deviation of 4 mm yr 1 previously assigned to spreading rates in the global plate motion model nuvel 1 demets et al 1990 the fit to the spreading rates as well as the locations of earthquakes in and near the red sea indicate that spreading south of approximately 17 7 n is less than the full rate of spreading between the arabian and nubian plates the red sea spreading centre is instead usefully interpreted as the boundary between the arabian plate and a danakil microplate that includes the subaerial danakil block and a larger oceanic portion of lithosphere despite the absence of reliable azimuths of transform faults in the red sea all components of the angular velocity of arabia relative to nubia are usefully constrained from just the 45 relevant spreading rates the new compact 95 per cent confidence region of the angular velocity excludes prior estimates based on only four and two spreading rates by chase 1978 and jestin et al 1994 respectively 19 spreading rates in the southern red sea are used to estimate the angular velocity between the danakil microplate and arabia an approach based on singular value decomposition shows that without slip vectors only two of the three components of the angular velocity of the danakil microplate relative to arabia or nubia are usefully constrained but that all three components are usefully constrained if one earthquake slip vector is included

slow downs and speed ups of india eurasia convergence since 20 ma data noise uncertainties and dynamic implications india somalia and north america eurasia relative motions since early miocene 20 ma have been recently reconstructed at unprecedented temporal resolution o 1 myr from magnetic surveys of the carlsberg and northern mid atlantic ridges these new datasets revamped interest in the convergence of india relative to eurasia which is obtained from the india somalia nubia north america eurasia plate circuit unless finite rotations are arbitrarily smoothed through time however the reconstructed kinematics i e stage euler vectors appear to be surprisingly unusual over the past 20 myr in fact the euler pole for the india eurasia rigid motion scattered erratically over a broad region while the associated angular velocity underwent sudden increases and decreases consequently convergence across the himalayan front featured significant speed ups as well as slow downs with almost no consistent trend arguably this pattern arises from the presence of data noise which biases kinematic reconstructions particularly at high temporal resolution the rapid and important india eurasia platemotion changes reconstructed since early miocene are likely to be of apparent nature because they cannot result even from the most optimistic estimates of torques associated for instance with the descent of the indian slab into earth s mantle our previous work aimed at reducing noise in finiterotation datasets via an expanded bayesian formulation which offers several advantages over arbitrary smoothing methods here we build on this advance and revise the india eurasia kinematics since 20 ma accounting also for three alternative histories of rifting in africa we find that india eurasia kinematics are simpler and most importantly geodynamically plausible upon noise reduction convergence across the himalayan front overall decreased until 10 ma but then systematically increased albeit moderately towards the present day we test with global dynamic models of the coupled mantle lithosphere system how a previously proposed uplift of the tibetan plateau by 1 2 km as well as the break up of the indo australian plate into the india and australia units impact the kinematic record from 20 to 10 ma our modelling results indicate that the convergence slow down until 10 ma resulted chiefly from the indo australian break up however the tectonic scenario yielding the best fit to kinematic reconstructions is indeed one where the tibetan mean elevation increased by 1 2 km

reflection properties of phase transition and compositional change models of the 670 km discontinuity seismic observations of the amplitude ratio amplitude p 670p amplitude p p suggest a reflection coefficient r 7 13 for reflections from the underside of the 670 km discontinuity this can be compared with calculated reflection coefficients to constrain the nature of the mantle transition region velocity and density profiles through the transition region were calculated for olivine pyroxene garnet and olivine garnet model mineral assemblages for models with identical upper and lower mantle compositions the computed reflection coefficients from the phase transitions pertinent to the 670 km discontinuity are less than 0 5 these phase transitions produce their maximum r if they are effectively discontinuous which gives r 2 3 except for the garnet to perovskite transformation which gives r 8 in general given realistic impedance contrasts across a phase transition the change in material properties must occur over a region less than 3 km thick for r to be of the order of 5 a discontinuous change in chemical composition at 670 km depth coupled with a change in phase can also produce a reflection coefficient of the observed magnitude r 2 3 for a first order change in phase at 670 km with different iron concentrations on each side of the discontinuity and r 4 6 for cases with a discontinuous increase in silica content based on our analysis the thermal and chemical boundary layers which would occur across such a chemical discontinuity would probably have negligible effects on the reflection coefficient thus although limited by considerable uncertainties the mineralogical and seismic data suggest a two layered model of the mantle with a discontinuous change in both phase and composition at 670 km

india asia collision and the cenozoic slowdown of the indian plate implications for the forces driving plate motions the plate motion of india changed dramatically between 50 and 35 ma with the rate of convergence between india and asia dropping from 15 to 4 cm yr this change is coincident with the onset of the india asia collision and with a rearrangement of plate boundaries in the indian ocean on the basis of a simple model for the forces exerted upon the edges of the plate and the tractions on the base of the plate we perform force balance calculations for the precollision and postcollision configurations we show that the observed euler poles for the indian plate are well explained in terms of their locations and magnitudes if 1 the resistive force induced by mountain building in the himalaya tibet area is 5 6 10 12 n m 2 the net force exerted upon the indian plate by subduction zones is similar in magnitude to the ridge push force 2 5 10 12 n m and 3 basal tractions exert a resisting force that is linearly proportional to the plate velocity in the hot spot reference frame the third point implies an asthenospheric viscosity of 2 5 10 19 pa s assuming a thickness of 100 150 km synthetic euler poles show that crustal thickening in the tibetan plateau was the dominant cause of the cenozoic slowdown of the indian plate

high pressure equation of state of molten anorthite and diopside new hugoniot equation of state data for molten diopside at 1773 k and molten anorthite at 1923 k are reported to 38 and 35 gpa respectively the diopside data initial density 2 61 mg m 3 are described by a straight line fit to the shock velocity particle velocity results of u s 3 30 1 44 u p km s 1 and our preferred fit to the anorthite data initial density 2 55 mg m 3 is given by u s 2 68 1 42 u p km s 1 reduction of the data to a third order birch murnaghan isentrope assuming the gruneisen ratio times the density is a constant and the mie gruneisen equation of state gives k 0s 22 4 gpa and k s 6 9 for diopside for anorthite we calculate k 0s 17 9 gpa and k s 5 3 the present data for diopside are used to calculate the diopside solidus at high pressures we expect the solidus to be shallow above 10 gpa but the lack of data on the variation of either the gruneisen parameters of the liquid and crystal or the heat capacity and thermal expansion at elevated pressures makes extrapolation of fusion curves uncertain solidus temperatures of 2400 2500 k and 2560 2705 k for diopside are calculated at 10 and 20 gpa respectively the new data are combined with those of rigden et al 1988 for the di 0 64 a 0 36 eutectic composition to examine the degree to which such liquids mix ideally with respect to volume up to 25 gpa for the eutectic composition at 1400 c we calculate the volumes of the an and di mix nearly ideally to 25 gpa we find that the ratio of the partial molar volumes of the oxides in silicate melts to that of the crystal oxides at 1673 k and 1 atm is 1 0 0 1 for a wide range of oxide components for the low pressure tetrahedrally coordinated oxides e g sio 2 al 2o 3 fe 2o 3 the ratio is 1 3 with respect to oxides such as stishovite corundum and hematite in which the cations are octahedrally coordinated by oxygens if changes in coordination of al and si from tetrahedral at low pressures to octahedral at high pressures occur in melts they do so gradually over an interval of 40 gpa although 1 atm bulk moduli for a wide compositional range of silicate melts are similar the differences in integrated compression to mixed oxide like high pressure configurations are reflected mainly by variations in k t k t is found to vary inversely with fraction of network forming initially tetrahedrally coordinated cations e g al 3 si 4 thus k t which may be uncertain by 1 5 is estimated to vary from 7 for molten anorthite enstatite and diopside to 8 for molten ferrosilite to 10 for molten forsterite and to 11 for molten fayalite

shock compression of liquid silicates to 125 gpa the anorthite diopside join we determined the equation of state eos of three silicate liquid compositions by shock compression of preheated samples up to 127 gpa diopside di ca 2mg 2sio 6 at 1773 k anorthite an caal 2si 2o 8 at 1923 k and the eutectic composition di 64 an 36 at 1673 k were previously studied by shock compression to 38 gpa the new data extend the eos of each composition nearly to the earth s core mantle boundary the previously reported anomaly at 25 gpa for di 64 an 36 eutectic was not reproduced rather all data for this composition fit within error a straight line hugoniot in particle velocity vs shock velocity di also displays a linear hugoniot consistent with ultrasonic data and a third order finite strain eos the full anorthite data set is complex we examine a model with a transition between two structural states and a fourth order finite strain model excluding two points that may not display relaxed behavior we also report an experiment on room temperature solid di 64 an 36 aggregate that clearly demonstrates increase upon compression of the gruneisen parameter of this liquid much as experiment and theory have shown for forsterite and enstatite liquids we construct isentropes and isotherms from our hugoniots using mie gruneisen thermal pressure and evaluate the model of ideal mixing of volumes volume may mix almost linearly at high temperature but deviates strongly when calculated along an isotherm it remains difficult to reach a firm conclusion we compare the densities of liquids to lower mantle solids our results suggest that basaltic liquids rich in cao and al 2o 3 are notably denser than liquids in the mgo sio 2 binary and subject to uncertainties in the behavior of feo and in corrections for thermal pressure such liquids may be the most likely candidates for achieving negative buoyancy in the lowermost mantle

constraining uplift and denudation of west african continental margin by inversion of stacking velocity data an inverse model has been developed to determine the magnitude of denudation at seabed and subsurface unconformities by using root mean square rms stacking velocity data derived from processing a set of seismic reflection profiles this approach provides superior spatial coverage in comparison to other methods such as vitrinite reflectance apatite fission track and sonic velocity modeling which are restricted to borehole locations the model assumes exponential porosity decay with depth and a standard velocity porosity relationship in order to compute a synthetic rms velocity profile denudation values at two levels in the stratigraphy are then adjusted until the fit between the model and the data is optimized successful modeling is dependent upon independent estimates of the initial porosity of sediment since significant trade off occurs between initial porosity and denudation application to the west african shelf shows that 0 5 1 km of denudation occurred along the entire margin probably during late neogene times the amount of denudation decreases oceanward and was probably triggered by regional tilting associated with initiation and or regeneration of continent wide mantle convective upwelling which is thought to have affected much of subequatorial africa a subsurface oligocene unconformity represents as much as 2 5 km of denudation and was probably produced by initiation of an oceanic current