deformation induced crystallographic preferred orientation of hcp iron an experimental study using a deformation dia apparatus abstract shear and uniaxial deformation experiments on hexagonal close packed iron hcp fe was conducted using a deformation dia apparatus at a pressure of 13 17 gpa and a temperature of 723 k to determine its deformation induced crystallographic preferred orientation cpo development of the cpo in the deforming sample is determined in situ based on two dimensional x ray diffraction using monochromatic synchrotron x rays in the shear deformation geometry the 0001 and 11 2 0 axes gradually align to be sub parallel to the shear plane normal and shear direction respectively from the initial random texture in the uniaxial compression and tensile geometry the 0001 and 11 2 0 axes respectively gradually align along the direction of the uniaxial deformation axis these results suggest that basal slip 0001 11 2 0 is the dominant slip system in hcp fe under the studied deformation conditions the p wave anisotropy for a shear deformed sample was calculated using elastic constants at the inner core condition by recent ab initio calculations strength of the calculated anisotropy was comparable to or higher than axisymmetric anisotropy in earth s inner core

strength and water weakening of mantle minerals olivine wadsleyite and ringwoodite rheological properties of olivine and its high pressure polymorphs both dry and hydrous have been studied by monitoring x ray diffraction peak broadening as a function of pressure temperature and time the measurements were carried out up to a pressure of 10 gpa and a temperature of 600 c for olivine and wadsleyite and a pressure of 20 gpa and a temperature of 1000 c for ringwoodite the observations indicate that olivine is much weaker than the other two minerals for this range of conditions and furthermore water weakens olivine dramatically but only slightly weakens wadsleyite and ringwoodite when the temperature increases from 25 c to 400 c the yield strength in the olivine drops by 39 without water and 62 with water however there is hardly any change in strength for wet or dry wadsleyite and ringwoodite

upper and mid mantle anisotropy beneath the tonga slab measurements of source side splitting in s waves fromevents within the tonga slab reveal anisotropy in the upperand mid mantle beneath the slab we observed splitting forevents originating at both upper mantle and transition zonedepths anisotropic fast directions are trench parallel orsub parallel for both upper mantle and transition zoneevents delaytimes dt decreasewithdepthforuppermantleevents the source of anisotropy for the upper mantle eventsis likely in the sub slab mantle and is likely indicative oftrench parallel flow due to slab rollback the source ofanisotropy for the deeper earthquakes is more difficult toconstrain but the pattern of splitting measurements arguesfor an uppermost lower mantle anisotropic source and slabinduced deformation may be responsible for this anisotropyas well additional constraints from mineral physics studiesare necessary to interpret the mid mantle anisotropy signalin terms of geodynamical processes

slab stagnation in the shallow lower mantle linked to an increase in mantle viscosity subducting slabs can stagnate in the lower mantle high pressure laboratory experiments show that the viscosity of a dominant mantle phase increases dramatically at shallow lower mantle depths which could cause the slabs to halt their descent

sub slab anisotropy beneath the sumatra and circum pacific subduction zones from source side shear wave splitting observations understanding the dynamics of subduction is critical to our overall understanding of plate tectonics and the solid earth system observations of seismic anisotropy can yield constraints on deformation patterns in the mantle surrounding subducting slabs providing a tool for studying subduction dynamics while many observations of seismic anisotropy have been made in subduction systems our understanding of the mantle beneath subducting slabs remains tenuous due to the difficulty of constraining anisotropy in the sub slab region recently the source side shear wave splitting technique has been refined and applied to several subduction systems worldwide making accurate and direct measurements of sub slab anisotropy feasible and offering unprecedented spatial and depth coverage in the sub slab mantle here we present source side shear wave splitting measurements for the central america alaska aleutians sumatra ryukyu and izu bonin japan kurile subduction systems we find that measured fast splitting directions in these regions generally fall into two broad categories aligning either with the strike of the trench or with the motion of the subducting slab relative to the overriding plate trench parallel fast splitting directions dominate beneath the izu bonin japan and southern kurile slabs and part of the sumatra system while fast directions that parallel the motion of the downgoing plate dominate in the ryukyu central america northern kurile western sumatra and alaska aleutian regions we find that plate motion parallel fast splitting directions in the sub slab mantle are more common than previously thought we observe a correlation between fast direction and age of the subducting lithosphere older lithosphere 95 ma is associated with trench parallel splitting while younger lithosphere 95 ma is associated with plate motion parallel fast splitting directions finally we observe source side splitting for deep earthquakes transition zone depths beneath japan and sumatra suggesting the presence of anisotropy at midmantle depths beneath these regions

anisotropy in the deep earth abstract seismic anisotropy has been found in many regions of the earth s interior its presence in the earth s crust has been known since the 19th century and is due in part to the alignment of anisotropic crystals in rocks and in part to patterns in the distribution of fractures and pores in the upper mantle seismic anisotropy was discovered 50 years ago and can be attributed for the most part to the alignment of intrinsically anisotropic olivine crystals during large scale deformation associated with convection there is some indication for anisotropy in the transition zone particularly in the vicinity of subducted slabs here we focus on the deep earth the lower mantle and core where anisotropy is not yet mapped in detail nor is there consensus on its origin most of the lower mantle appears largely isotropic except in the last 200 300 km in the d region where evidence for seismic anisotropy has been accumulating since the late 1980s mostly from shear wave splitting measurements recently a picture has been emerging where strong anisotropy is associated with high shear velocities at the edges of the large low shear velocity provinces llsvps in the central pacific and under africa these observations are consistent with being due to the presence of highly anisotropic mgsio 3 post perovskite crystals aligned during the deformation of slabs impinging on the core mantle boundary and upwelling flow within the llsvps we also discuss mineral physics aspects such as ultrahigh pressure deformation experiments first principles calculations to obtain information about elastic properties and derivation of dislocation activity based on bonding characteristics polycrystal plasticity simulations can predict anisotropy but models are still highly idealized and neglect the complex microstructure of polyphase aggregates with strong and weak components a promising direction for future progress in understanding the origin of seismic anisotropy in the deep mantle and its relation to global mantle circulation is to link macroscopic information from seismology and microscopic information mineral physics through geodynamics modeling anisotropy in the inner core was proposed 30 years ago to explain faster p wave propagation along the direction of the earth s axis of rotation as well as anomalous splitting of core sensitive free oscillations there is still uncertainty about the origin of this anisotropy in particular it is difficult to explain its strength based on known elastic properties of iron as it would require almost perfect alignment of iron crystals indeed the strongly anomalous p travel times observed on paths from the south sandwich islands to alaska may or may not be due to inner core anisotropy and will need to be explained before consensus can be reached on the strength of anisotropy in the inner core and its origin

fabric development in mg fe o during large strain shear deformation implications for seismic anisotropy in earth s lower mantle abstract large strain shear deformation experiments were performed on mg 1 x fe x o x 0 25 1 0 one of the important minerals in earth s lower mantle deformation experiments were made on coarse grained 15 20 m grain size hot pressed aggregates at conditions of t t m 0 46 0 65 t temperature t m melting temperature and 0 4 10 3 to 0 9 10 3 differential stress shear modulus up to the shear strain of 7 8 under these conditions deformation occurs by dislocation creep the microstructural development in mg fe o is found to be sluggish and the complete dynamic recrystallization and nearly steady state fabric lattice preferred orientation are achieved only after shear strains of 4 at nearly steady state mg fe o shows strong fabrics characterized by the 1 1 0 axes being parallel to the shear direction and the poles of the 1 0 0 planes and to a lesser extent the poles of the 1 1 1 planes normal to the shear plane the seismic anisotropy corresponding to the deformation fabrics in mg fe o was calculated the nature of anisotropy corresponding to a given flow geometry changes significantly with strain as a result of fabric evolution anisotropy changes also with depth pressure due to the large variation of elastic anisotropy of mg fe o with depth seismic anisotropy caused by the deformation fabric of mg fe o is very weak in the shallow lower mantle v sh v sv anisotropy assuming that mg fe o occupies 20 volume fraction of the lower mantle and little shear wave splitting of vertically travelling waves a result that is consistent with the seismological observations in the d layer of the circum pacific regions thus the deformation fabric of mg fe o is a vital candidate of the cause of seismic anisotropy in these regions anisotropy caused by the lattice preferred orientation of mg fe o has a distinct azimuthal anisotropy with a strong 4 term azimuth the direction of propagation of the fastest slowest sh or p wave is parallel perpendicular to the flow direction and the slowest fastest sh or p wave is at 45 from these two directions

the deformational journey of the nazca slab from seismic anisotropy s a and c m e are currently supported by australian research council grant de190100062

deformation of mg0 9 fe0 1 sio3 perovskite aggregates up to 32 gpa room temperature investigations on the shear stress and deformation mechanisms of mg0 9fe0 1 sio3 perovskite are performed in situ up to 32 gpa using radial x ray diffraction and the diamond anvil cell as a deformation apparatus the uniaxial stress supported by the perovskite aggregate is found to increase continuously with pressure up to 10 9 1 9 gpa at 32 1 gpa our measurements show no development of significant lattice preferred orientations in the sample which indicates that deformation by dislocation glide is not the dominant deformation mechanism under these conditions assuming that the underlying cause for seismic anisotropy in the deep earth is elastic anisotropy combined with lattice preferred orientation our results indicate that silicate perovskite deformed under the conditions of this experiment would not be the source of seismic anisotropy c 2003 elsevier science b v all rights reserved

implications of paleozoic and mesozoic rocks in the soda mountains northeastern mojave desert california for late paleozoic and mesozoic cordilleran orogenesis study of the soda mountains in the northeastern mojave desert revealed new stratigraphic and structural features of this area a section of cambrian to jurassic rocks in the area displays various features of regional importance lower permian rocks have depositional characteristics indicative of mass flow deposition contrasting with underlying shelf strata and platformal rocks exposed in areas to the east these rocks are similar to deep water rocks exposed to the north in eastern california lower triassic rocks are named the silver lake formation and consist of shallow water siltstone and carbonate that contain conglomerate with volcanic and gneissic ciasts these latter rocks record the possible presence of permian triassic igneous and deformational activity in the area a complicated deformational history is also evident from this study we interpret several west dipping low angle faults active during intrusion of middle cretaceous granitoids to be normal faults rather than thrust faults as they were originally considered structures in the soda mountains probably correlate with similar ones in surrounding areas collectively the local structures imply a complicated stress regime during emplacement of the middle cretaceous plutonic rocks these new data imply that large thrusts faults placing paleozoic rocks over mesozoic rocks are not present in this area upper paleozoic and lower mesozoic rocks also tie this area to the western mojave desert indicating that strike slip faults with several hundred kilometers of displacement do not separate the two areas

single crystal elasticity of al fe bearing bridgmanite and seismic shear wave radial anisotropy at the topmost lower mantle abstract in this study we investigated the single crystal elasticity of al fe bearing bridgmanite bgm with chemical compositions of mg 0 95 fe 0 033 2 fe 0 027 3 al0 04si0 96o3 fe6 al4 bgm and mg 0 89 fe 0 024 2 fe 0 096 3 al0 11si0 89o3 fe12 al11 bgm using combined experimental results from brillouin light scattering bls impulsive stimulated light scattering isls and x ray diffraction xrd measurements in diamond anvil cells at 25 and 35 gpa based on experimentally measured compressional and shear wave velocities v p v s as a function of azimuthal angles within selected crystal platelets that are sensitive to derivation of nine elastic constants for each composition we reliably derived the full elastic constants of fe6 al4 bgm and fe12 al11 bgm at the two experimental pressures our results show that the combined fe and al substitution results in a reduction of both v s and v p in fe12 al11 bgm up to 2 6 0 5 and 1 5 0 3 respectively compared with those in fe6 al4 bgm at the experimental pressures in particular we observed strong combined fe and al effects on v s splitting anisotropy of al fe bearing bgm at the two experimental pressures fe6 al4 bgm exhibits the highest v s splitting anisotropy of 8 23 9 0 along the 001 direction while the direction shifts to the midway between 100 and 001 directions for fe12 al11 bgm with v s splitting anisotropy of 7 68 11 06 combining the single crystal elasticity data of fe6 al4 bgm and fe12 al11 bgm with the crystallographic preferred orientation cpo results of deformed bgm at relevant lower mantle pressure temperature p t conditions from literature we modeled the seismic v s radial anisotropy of deformed al fe bearing bgm near a subducting slab at conditions relevant to the topmost lower mantle taking into account the fe and al contents in al fe bearing bgm with depth in the earth s topmost lower mantle the results of our model show that the deformation of fe6 al4 bgm and fe12 al11 bgm crystals would produce 0 9 and 0 8 v s radial anisotropy at depths of 670 and 920 km respectively these findings provide mineral physics explanations to the distinct seismically detected v s radial anisotropies at the topmost lower mantle near subducted slabs especially in the tonga kermadec subduction region

evidence for 100 slip in ferropericlase in earth s lower mantle from high pressure high temperature experiments abstract seismic anisotropy in earth s lowermost mantle resulting from crystallographic preferred orientation cpo of elastically anisotropic minerals is among the most promising observables to map mantle flow patterns a quantitative interpretation however is hampered by the limited understanding of cpo development in lower mantle minerals at simultaneously high pressures and temperatures here we experimentally determine cpo formation in ferropericlase one of the elastically most anisotropic deep mantle phases at pressures of the lower mantle and temperatures of up to 1400 k using a novel experimental setup our data reveal a significant contribution of slip on 100 to ferropericlase cpo in the deep lower mantle contradicting previous inferences based on experimental work at lower mantle pressures but room temperature we use our results along with a geodynamic model to show that deformed ferropericlase produces strong shear wave anisotropy in the lowermost mantle where horizontally polarized shear waves are faster than vertically polarized shear waves consistent with seismic observations we find that ferropericlase alone can produce the observed seismic shear wave splitting in d in regions of downwelling which may be further enhanced by post perovskite our model further shows that the interplay between ferropericlase causing vsh vsv and bridgmanite causing vsv vsh cpo can produce a more complex anisotropy patterns as observed in regions of upwelling at the margin of the african large low shear velocity province

mid mantle anisotropy in subduction zones and deep water transport the earth s transition zone has until recently been assumed to be seismically isotropic increasingly however evidence suggests that ordering of material over seismic wavelengths occurs there but it is unclear what causes this we use the method of source side shear wave splitting to examine the anisotropy surrounding earthquakes deeper than 200 km in slabs around the globe we find significant amounts of splitting 2 4 s confirming that the transition zone is anisotropic here however there is no decrease in the amount of splitting with depth as would be the case for a metastable tongue of olivine which thins with depth suggesting this is not the cause the amount of splitting does not appear to be consistent with processes in the ambient mantle such as lattice preferred orientation development in wadsleyite ringwoodite or mgsio3 perovskite we invert for the orientation of several mechanisms subject to uncertainties in mineralogy and deformation and the best fit is given by updip flattening in a style of anisotropy common to hydrous phases and layered inclusions we suggest that highly anisotropic hydrous phases or hydrated layering is a likely cause of anisotropy within the slab implying significant water transport from the surface down to at least 660 km depth

in situ observation of crystallographic preferred orientation of deforming olivine at high pressure and high temperature abstract simple shear deformation experiments on polycrystalline olivine and olivine single crystal were conducted at pressures of 1 3 3 8 gpa and temperatures of 1223 1573 k to understand the achievement of steady state fabric strength and the process of dynamic recrystallization development of crystallographic preferred orientation cpo of olivine was evaluated from two dimensional x ray diffraction patterns and shear strain was measured from x ray radiographs the steady state fabric strength of the a type fabric was achieved within total shear strain of 2 at strains higher than 1 an increase in concentration of the 0 1 0 axes mainly contributes to an increase in fabric strength at strains higher than 2 the magnitude of v sh v sv i e ratio of horizontally and vertically polarized shear wave velocities scarcely increased in most of the runs the v sh v sv of peridotite 70 vol olivine 30 vol minor phases having the steady state a type olivine fabric coincides with that of recent global one dimensional models under the assumption of horizontal flow suggesting that the seismic anisotropy observed in the shallow upper mantle is mostly explained by the development of a type olivine fabric experimental results on the deformation of single crystal olivine showed that the cpo of olivine is influenced by the initial orientation of the starting single crystal because strain is concentrated in the recrystallized areas and the relic of the starting single crystal remains in the upper mantle the old cpo of olivine developed in the past may affect the olivine cpo developed in the present

water distribution in the lower mantle implications for hydrolytic weakening abstract the presence of water in lower mantle minerals is thought to have substantial effects on the rheological properties of the earth s lower mantle in what is generally known as hydrolytic weakening this weakening will have profound effects on global convection but hydrolytic weakening in lower mantle minerals has not been observed experimentally and thus the effect of water on global dynamics remains speculative in order to constrain the likelihood of hydrolytic weakening being important in the lower mantle we use first principles methods to calculate the partitioning of water strictly protons between mineral phases of the lower mantle under lower mantle conditions we show that throughout the lower mantle water is primarily found either in the minor ca perovskite phase or in bridgmanite as an al3 h pair ferropericlase remains dry however neither of these methods of water absorption creates additional vacancies in bridgmanite and thus the effect of hydrolytic weakening is likely to be small we find that water creates significant number of vacancies in bridgmanite only at the deepest part of the lower mantle and only for very high water contents 1000 ppm we conclude that water is thus likely to have only a limited effect on the rheological properties of the lower mantle

stress relaxation experiments of olivine under conditions of subducted slab in earth s deep upper mantle abstract stress relaxation experiments of olivine were conducted under high pressure and high temperature conditions up to 10 gpa and 1273 k using a kawai type multi anvil apparatus a pre sintered san carlos olivine sample rod was inserted between two dense al 2 o 3 pistons to yield high stress at high pressure within an octahedral pressure medium stress was determined from the two dimensional diffraction pattern taken using monochromatic x rays and an imaging plate and sample length was determined from an x ray radiograph in these experiments pressure was first increased at room temperature and then the temperature was increased and kept at 673 873 1073 and 1273 k four relaxation cycles in total were carried out in two experimental runs the magnitude of deviatoric stress was calculated from five diffraction peaks with the following hkl s 0 2 1 1 0 1 1 3 0 1 3 1 and 1 1 2 the calculated deviatoric stress was significantly different depending on which diffraction peak was used up to a factor of 2 due to plastic deformation within the polycrystalline sample the deviatoric stress decreased with increasing temperature in all of relaxation cycles at given temperatures the final state value of deviatoric stress increased with increasing pressure the upper bound for the plastic strain rate in the final state was determined to be 10 7 s 1 based on a comparison between the total sample length determined from the radiograph and the d spacings along the piston direction determined from x ray diffraction present results suggest a positive activation volume for the low temperature rheology of olivine

mantle dynamics inferred from the crystallographic preferred orientation of bridgmanite seismic shear wave anisotropy is observed in earth s uppermost lower mantle around several subducted slabs the anisotropy caused by the deformation induced crystallographic preferred orientation cpo of bridgmanite perovskite structured mg fe sio3 is the most plausible explanation for these seismic observations however the rheological properties of bridgmanite are largely unknown uniaxial deformation experiments have been carried out to determine the deformation texture of bridgmanite but the dominant slip system the slip direction and plane has not been determined here we report the cpo pattern and dominant slip system of bridgmanite under conditions that correspond to the uppermost lower mantle 25 gigapascals and 1 873 kelvin obtained through simple shear deformation experiments using the kawai type deformation dia apparatus the fabrics obtained are characterized by 100 perpendicular to the shear plane and 001 parallel to the shear direction implying that the dominant slip system of bridgmanite is 001 100 the observed seismic shear wave anisotropies near several subducted slabs tonga kermadec kurile peru and java can be explained in terms of the cpo of bridgmanite as induced by mantle flow parallel to the direction of subduction

midmantle deformation between the australian continent and the fiji tonga subduction zone 1 shear wave splitting measurements are performed at temporary stations deployed across the australian continent for direct s waves from events occurring in the neighboring subduction zones especially fiji tonga unlike core refracted shear waves direct s waves are not polarized at the core mantle boundary and are therefore subject to influences on the source side on the receiver side and along the ray path in between source side contamination due to the development of lattice preferred orientation of olivine can be minimized by considering deep events with an epicentral location below 410 km depth the olivine spinel phase transition depth anisotropic contributions from the transition zone or lower mantle can only be reliably inferred in the case of high delays that cannot be generated solely by the upper mantle structure underneath a station the analysis of a comprehensive data set for direct s waves from deep events leads to an average delay time of 0 9 s at stations deployed across the continent despite the limitations of such an arithmetic average the result highlights the lack of evidence for midmantle deformation between the fiji tonga subduction zone and the australian continent unlike previously suggested in studies dealing with the permanent seismological stations only a striking consistency between measurements performed across several stations belonging to an array in western australia suggests a very coherent lithospheric structure underneath the yilgarn craton

characterization by scanning precession electron diffraction of an aggregate of bridgmanite and ferropericlase deformed at hp ht scanning precession electron diffraction is an emerging promising technique for mapping phases and crystal orientations with short acquisition times 10 20 ms pixel in a transmission electron microscope similarly to the electron backscattered diffraction ebsd or transmission kikuchi diffraction tkd techniques in a scanning electron microscope in this study we apply this technique to the characterization of deformation microstructures in an aggregate of bridgmanite and ferropericlase deformed at 27 gpa and 2 130 k such a sample is challenging for microstructural characterization for two reasons i the bridgmanite is very unstable under electron irradiation ii under high stress conditions the dislocation density is so large that standard characterization by diffraction contrast are limited or impossible here we show that detailed analysis of intracrystalline misorientations sheds some light on the deformation mechanisms of both phases in bridgmanite deformation is accommodated by localized amorphous shear deformation lamellae whereas ferropericlase undergoes large strains leading to grain elongation in response to intense dislocation activity with no evidence for recrystallization plastic strain in ferropericlase can be semiquantitatively assessed by following kernel average misorientation distributions

evidence of midmantle anisotropy from shear wave splitting and the influence of shear coupled p waves 1 until recently the midmantle region of the earth the transition zone and the uppermost lower mantle has been considered isotropic recent work by wookey et al 2002 presented evidence of anisotropy in the midmantle region near the tonga kermadec subduction zone inferred from shear wave splitting in teleseismic s phases at australian seismic stations this data set is revisited to examine the possibility of contamination by shear coupled p waves using reflectivity modeling we explore the effects of these phases on the measurement of shear wave splitting contamination of such measurements is demonstrated for short epicentral distances 30 wave field decomposition is a simple technique which can be used to separate these phases from the main s wave arrival the tonga australia data set is reprocessed after wave field decomposition shear wave splitting ranging between 0 7 and 6 2 s is observed a range comparable to that observed by wookey et al 2002 as before the polarization of the fast shear wave is observed to be predominantly horizontal however the magnitudes of splitting for several events at short epicentral distances are significantly reduced suggesting some influence of shear coupled p waves in the original analysis the results are compared with possible models of mantle anisotropy and the results can be best explained by significant anisotropy in the midmantle region the best candidate location for this anisotropy is in the uppermost lower mantle and scenarios involving the alignment of lower mantle or subducted materials by regional dynamic processes are suggested to account for this

the anisotropic signal of topotaxy during phase transitions in d abstract while observations and modelling of seismic anisotropy in the lowermost mantle offers the possibility of imaging mantle flow close to the core mantle boundary current models do not explain all observations here we seek to explain a long wavelength pattern of shear wave anisotropy observed in anisotropic tomography where vertically polarised shear waves travel faster than horizontally polarised shear waves in the central pacific and under africa but this pattern is reversed elsewhere in particular we test an explanation derived from experiments on analogues which suggest that texture may be inherited during phase transitions between bridgmanite perovskite structured mgsio3 and post perovskite and that such texture inheritance may yield the long wavelength pattern of anisotropy we find that models that include this effect correlate better with tomographic models than those that assume deformation due to a single phase in the lowermost mantle supporting the idea that texture inheritance is an important factor in understanding lowermost mantle anisotropy it is possible that anisotropy could be used to map the post perovskite stability field in the lowermost mantle and thus place constraints on the temperature structure above the core mantle boundary

paleomagnetic data from the caborca terrane mexico implications for cordilleran tectonics and the mojave sonora megashear hypothesis two ancient magnetizations have been isolated in rocks of the caborca terrane northwest mexico the characteristic magnetizations of neoproterozoic and paleozoic miogeoclinal shelf strata arc derived lower jurassic marine strata and jurassic volcanic and volcaniclastic rocks are of dual polarity and east northeast declination or south south west and shallow inclination magnetizations in neoproterozoic and paleozoic miogeoclinal strata are interpreted as secondary j and to be of similar age to those observed in lower and middle jurassic rocks remanence acquisition is bracketed between about 190 and 160 ma the overall mean d 15 0 i 8 5 n 38 sites six localities k 19 1 95 5 5 suggests a moderate to large clockwise rotation of 12 to 50 depending on reference direction assumed of the caborca terrane and rocks of the sonoran segment of the cordilleran volcanic arc with respect to the north america craton when compared with expected inclinations observed values are not anomalously steep arguing against statistically significant southward latitudinal displacement of the caborca block after remanence acquisition late cretaceous intrusions yield primary dual polarity steep inclination k magnetizations d 341 4 i 52 3 n 10 sites five localities k 38 3 95 7 9 and have locally remagnetized neoproterozoic and jurassic strata when present secondary k magnetizations in neoproterozoic strata are of higher coercivity and higher unblocking temperature than the characteristic j magnetization importantly the regional internal consistency of data for late cretaceous intrusions suggests that effects of tertiary tilt or rotation about a vertical axis over the broad region sampled 5000 km2 are not substantial late cretaceous primary k magnetizations and secondary k magnetizations yield a combined mean of d 348 1 i 50 7 n 10 localities 47 sites k 53 5 95 6 7 indicating at most small 10 clockwise rotation of the caborca region with respect to the craton permissible post late cretaceous latitudinal displacement is near or below the detection limit of paleomagnetism 300 km limited data from lower cretaceous strata of the bisbee group d 339 9 i 47 9 n 4 sites suggest that the modest clockwise rotations inferred on the basis of j magnetizations in jurassic and older strata occurred in jurassic time together the lack of evidence for southward displacement yet evidence for statistically significant clockwise rotation and the overall similarity of jurassic magnetizations in the cordilleran arc with those of the caborca block despite the fact that some of them are clearly secondary are not consistent with the mojave sonora megashear hypothesis of late jurassic left lateral strike slip motion of the crust of northern mexico

predicted glide systems and crystal preferred orientations of polycrystalline silicate mg perovskite at high pressure implications for the seismic anisotropy in the lower mantle abstract we use first first principle methods and the peierls nabarro model to evaluate the resistance to glide characterized by the peierls stress of glide systems for end member mgsio 3 perovskite at mantle pressures 010 100 is the easiest glide system in mg perovskite at all pressures peierls stresses increase systematically with pressure for all systems except 001 010 indicating the importance of lattice friction at lower mantle pressures the ratio of the maximum peierls stress for each system relative to the 010 100 value defines their critical resolved shear stress crss these crss are used in a visco plastic self consistent homogenization model to predict the evolution of crystal preferred orientations cpo during deformation of polycrystalline mg perovskite in axial compression 100 tends to align with the compression direction in agreement with in situ observations in axial compression experiments in simple shear 010 concentrates near the shear direction and 100 although more dispersed tends to align near the shear plane consistent with the dominant activity of the easier 010 100 system the calculated seismic anisotropy for a 100 mg perovskite aggregate using the cpo in simple shear and the elastic constants of mgsio 3 perovskite at lower mantle pressures and temperatures is weak 3 for p waves with and 2 for s waves and decreases with increasing temperature and pressure p waves show the fastest propagation parallel to the lineation and s waves fast polarization is in the foliation at 38 gpa and normal to the lineation at 88 gpa this weak anisotropy is consistent with global seismological observations of a nearly isotropic lower mantle there are however two regions where strain induced mg perovskite cpo could contribute to anisotropy a low temperature regions in the uppermost lower mantle where the predicted s wave polarization anisotropy may attain 1 6 with a fast polarization parallel to the foliation b in high high temperature domains in the d layer where mg perovskite may be the major stable phase leading to polarization of fast s waves normal to the lineation for propagation directions at high angle to the lineation and an apparent isotropy for all other propagation directions

modeling lower mantle anisotropy development in a subducting slab a model is presented that simulates anisotropy development in a subducting slab from the upper lower mantle boundary 661 km to the core mantle boundary 2891 km two phases are considered orthorhombic mgsio3 perovskite and cubic magnesiowuestite mg fe o single crystal elastic properties at mantle conditions are obtained from existing density functional theory calculations and quasiharmonic approximation it is assumed that deformation is accommodated by slip a polycrystal plasticity model predicts strong texture development for perovskite and weaker texture for magnesiowuestite when averaging single crystal elastic properties with the orientation distribution this results for both phases in weak p wave anisotropy and shear wave splitting in the upper part of the lower mantle but pronounced anisotropy near the core mantle boundary up to 4 for vp and 7 for vs the anisotropy pattern is complex and asymmetric and local heterogeneity is expected these predictions are consistent with seismic observations

triassic plutonism in southern california southward younging of arc initiation along a truncated continental margin earliest cordilleran magnatism in the southwestern united states is recorded by a belt of triassic plutons that inme protemzoic basemint of the mojave crustal province and its cratona miogeoclinal cover the belt extends from the western mojave desert through the trausverse ranges to the colorado river trough triassic plutons are predominantly alkali calcic fe and sr mriched quartz mzrdior tes and monzonites the northern part of the belt is comlx sed of two older plutonic mites 241 231 ma which are high k to shoshonitic the southern part of the belt is a younger 218 213 ma sodic alkalic suite the plutonic record in southern california suggests a short lived southward younging continental margin arc sang for eraplacement of triassic plutons sulim on a continental margin modified by s mistral tranqform faulting triassic plutonism in this region was followed by a magmatic lull prior to the onset of voluminous middle to late jurassic cordilleran arc magmatism

geological analysis of the silver lake marsokhod field test from ground truth sampling and mapping we compare the results of the observations and conclusions obtained from a remote science team and field geologists during the 1999 marsokhod silver lake field experiment this comparison is used to determine methods that enhance the ability of a remote team using a rover to determine the geology and paleoclimate of a landing site analogous to sites that may be encountered during the mars surveyor program the mobility of the field geologists and their ability to rapidly gain visual and physical data from the field site give an immediate confidence to their interpretation of the geology the finite time of teleoperation and the initial decisions and mechanical problems that caused the rover to linger and collect data in a small area of the test site for the first 5 days of the test limited the ability of the remote science team to gain a similar confidence nevertheless by appropriate use of orbital and descent images as well as the rover s science payload the remote science team interpreted the present and former climates the geology and geological history and present day biology relatively accurately the few erroneous interpretations by the remote team resulted in part from biases brought to the test about the nature of the geology that was likely to be encountered data quality and variety also impacted the quality of the geological interpretations significant misinterpretations in the remotely determined geology resulted from the lack of detail in the optical imagery a less than optimal understanding of the site s topography and misinterpretation of some infrared data all in all this field test showed that the geology paleoclimate and biology of a site can be assessed to a reasonable level of confidence using a remotely controlled vehicle it also showed that it is both feasible and desirable for a distributed science team to participate in long science missions from home institutions as long as data flows efficiently to and from these institutions

seismic anisotropy and mantle flow below subducting slabs abstract subduction is integral to mantle convection and plate tectonics yet the role of the subslab mantle in this process is poorly understood some propose that decoupling from the slab permits widespread trench parallel flow in the subslab mantle although the geodynamical feasibility of this has been questioned here we use the source side shear wave splitting technique to probe anisotropy beneath subducting slabs enabling us to test petrofabric models and constrain the geometry of mantle fow our global dataset contains 6369 high quality measurements spanning 40 000 km of subduction zone trenches over the complete range of available source depths 4 to 687 km and a large range of angles in the slab reference frame we find that anisotropy in the subslab mantle is well characterised by tilted transverse isotropy with a slow symmetry axis pointing normal to the plane of the slab this appears incompatible with purely trench parallel flow models on the other hand it is compatible with the idea that the asthenosphere is tilted and entrained during subduction trench parallel measurements are most commonly associated with shallow events source depth 50 km suggesting a separate region of anisotropy in the lithospheric slab this may correspond to the shape preferred orientation of cracks fractures and faults opened by slab bending meanwhile the deepest events probe the upper lower mantle where splitting is found to be consistent with deformed bridgmanite

bounded by crises an overview of the evolution of marine ostracods during the triassic abstract the triassic was a turning in the history of biodiversity bracketed by two major biotic crises characterised by major biotic climatic and tectonic events it saw the transition from the palaeozoic to the modern evolutionary faunas herein we propose the first synthetic analysis of the diversity of marine and brackish water ostracods over the entire triassic in the light of palaeoecological palaeoenvironmental and palaeogeographical contexts although general diversity trends witnessed poor ostracod communities during most of the early triassic after the end permian crisis the roots of their triassic taxonomic rediversification were visible as early as the dienerian the explosive diversification of the spathian and anisian was followed by a high diversity plateau up to the brink of the end triassic extinction a morphological phylogeny proposes that all permian and triassic ornate bairdiidae derived from petasobairdia in the kungurian with the emergence of the ceratobairdia lineage and abrobairdia lineage while they are generally the poor cousin of trophic chain analyses traces of typical mesozoic drilling predation on late triassic ostracods unexpectedly document the increase in the efficiency of predators drilling abilities through the triassic finally the palaeogeographical distribution of ostracods was very dynamic during this interval with distinct peri palaeo tethyan and peri neotethyan biotas in the early triassic followed by a dispersal and thus a relative homogenisation from the anisian onwards

interconnection of ferro periclase controls subducted slab morphology at the top of the lower mantle abstract the electrical conductivity of mantle rocks during phase transformation from ringwoodite to silicate perovskite and ferro periclase was measured at 25 gpa and various temperatures ranging from 1300 to 1900 k the electrical conductivity was high at the initial stage of annealing suggesting that ferro periclase forms interconnected layers in aggregates of silicate perovskite and ferro periclase that are representative of lower mantle rock at 1900 k the electrical conductivity quickly decreased and reached that of silicate perovskite suggesting the cut off of the interconnected ferro periclase because of rounding of crystals below 1700 k the high conductivity values were maintained for experimental duration the interconnection of ferro periclase which has a lower viscosity than silicate perovskite can be maintained in a cold descending slab over geological time scales 1 my indicating that a colder slab is less viscous than the warmer mantle surrounding it the low viscosity slab can be prevented from penetrating into the deeper part of the lower mantle by the high viscosities encountered at a depth of 1000 km referred to as the viscosity hill that cause stagnation at this depth as observed by seismic tomography

water partitioning between nominally anhydrous minerals in the mgo sio2 h2o system up to 24 gpa implications for the distribution of water in the earth s mantle multi anvil experiments have been conducted in the mgo sio2 h2o system at pressures of 15 24 gpa and temperatures of 1200 1600 c to investigate the partitioning of water between mantle phases the water contents were obtained from fourier transform infrared spectroscopy on doubly polished and clear crystals most of the mineral phases coexisted with a hydrous melt the results show that among all magnesium silicate phases 1 mgsio3 perovskite is the only phase which does not dissolve a detectable amount of water and 2 the high pressure polymorphs of mg2sio4 wadsleyite and ringwoodite dissolve about one order of magnitude more water than the polymorphs of mgsio3 clinoenstatite majorite and akimotoite the following partition coefficients of water were measured dwadsleyite clinoenstatite 3 8 at 15 gpa and 1300 c dringwoodite akimotoite 21 at 19 gpa and 1300 c dakimotoite perovskite 1 at 24 gpa and 1600 c and dpericlase perovskite 1 at 24 gpa and 1500 c these results suggest that most of the water in the earth s mantle is partitioned in the transition zone

the 1999 marsokhod rover mission simulation at silver lake california mission overview data sets and summary of results we report on a field experiment held near silver lake playa in the mojave desert in february 1999 with the marsokhod rover the payload descent imager pancam mini tes and robotic arm camera data volumes and data transmission receipt windows simulated those planned for the mars surveyor mission selected for 2001 a central mast with a pan and tilt platform at 150 cm height carried a high resolution color stereo imager to simulate the pancam and a visible near infrared fiberoptic spectrometer operating range 0 35 2 5 m monochrome stereo navigation cameras were mounted on the mast and the front and rear of the rover near the wheels a field portable infrared spectroradiometer operating range 8 14 m simulated the mini tes a robotic arm camera capable of close up color imaging at 23 m pixel resolution was used in conjunction with the excavation of a trench into the subsurface the science team was also provided with simulated images from the mars descent imager and orbital panchromatic and multispectral imaging of the site obtained with the french spot airborne thermal infrared mapping spectrometer and landsat thematic mapper instruments commands sequences were programmed and sent daily to the rover and data returned were limited to 40 mbits per communication cycle during the simulated mission 12 commands were uplinked to the rover it traversed 90 m six sites were analyzed 11 samples were collected for laboratory analysis and over 5 gbits of data were collected twenty two scientists unfamiliar with the location of the field site participated in the science mission from a variety of locations accessing data via the world wide web remote science interpretations were compared with ground truth from the field and laboratory analysis of collected samples using this payload and mission approach the science team synergistically interpreted orbital imaging and infrared spectroscopy descent imaging rover based imaging infrared spectroscopy and microscopic imaging to deduce a consistent and largely correct interpretation of the geology mineralogy stratigraphy and exobiology of the site use of imaging combined with infrared spectroscopy allowed source outcrops to be identified for local rocks on an alluvial fan different lithologies were distinguished both near the rover and at distances of hundreds of meters or more subtle differences such as a contact between dolomite and calcite were identified at a distance of 0 5 km a biomarker for endolithic microbiota a plausible life form to be found on mars was successfully identified microscopic imaging of soils extracted from the surface and subsurface allowed the mineralogy and fluvial history of the trench site to be deduced the scientific productivity of this simulation shows that this payload and mission approach has high science value and would contribute substantially to achieving the goals of mars exploration

ubiquitous lower mantle anisotropy beneath subduction zones seismic anisotropy provides key information to map the trajectories of mantle flow and understand the evolution of our planet while the presence of anisotropy in the uppermost mantle is well established the existence and nature of anisotropy in the transition zone and uppermost lower mantle are still debated here we use three dimensional global seismic tomography images based on a large dataset that is sensitive to this region to show the ubiquitous presence of anisotropy in the lower mantle beneath subduction zones whereas above the 660 km seismic discontinuity slabs are associated with fast sv anomalies up to about 3 in the lower mantle fast sh anomalies of about 2 persist near slabs down to about 1 000 1 200 km these observations are consistent with 3d numerical models of deformation from subducting slabs and the associated lattice preferred orientation of bridgmanite produced in the dislocation creep regime in areas subjected to high stresses this study provides evidence that dislocation creep may be active in the earth s lower mantle providing new constraints on the debated nature of deformation in this key but inaccessible component of the deep earth lower mantle anisotropy is present beneath all subduction zones indicating that dislocation creep is active in the lower mantle according to analysis of 3d global seismic tomography images

ground penetrating radar as a tool for probing the shallow subsurface of mars 1 design and testing of an impulse ground penetrating radar gpr for possible rover deployment on mars in 2009 is well underway the gpr has mass power volume and data rate targets of 0 5 kg 3 w peak 3400 cm3 and 0 3 mb d and will possess easily modified bistatic high frequency e g 500 600 mhz and monostatic rat tail e g 100 mhz antennas the gpr should be capable of measuring in situ radar properties on mars to depths of up to 10 20 m thereby helping to constrain near surface geology and structure initial gpr tests at the site of the 2001 fido rover field trials near cronese lake california confirm the value of the instrument in defining local geologic setting and in providing context for data collected using other rover science payload elements a rover deployed gpr may enable three dimensional mapping of local stratigraphy which in turn can assist in evaluating the history of aqueous activity and the accessibility of near surface water