shear wave splitting and subcontinental mantle deformation splitting observations are interpreted in terms of the strain induced lattice preferred orientation of mantle minerals especially olivine we consider three hypotheses concerning the origin of the continental anlsotropy 1 strain associated with absolute plate motion as in the oceanic upper mantle 2 crustal stress and 3 the past and present internal deformation of the subcontinental upper mantle by tectonic episodes it is found that the last hypothesis is the most successful namely that the most recent significant episode of internal deformation appears to be the best predictor of q for stable continental regions this is interpreted as fossil anisotropy whereas for presently active regions such as alaska the anisotropy reflects present day tectonic activity in the stable portion of north america there is a good correlation between delay time and lithospheric thickness this is consistent with the anisotropy being localized in the subcontinental lithosphere and suggests that intrinsic anisotropy is approximately constant the acceptance of this hypothesis has several implications for subcontinental mantle deformation first it argues for coherent deformation of the continental lithosphere crust and mantle during orogenies this implies that the anisotropic portion of the lithosphere was present since the deformational episode and rules out the addition of undeformed material to this layer by subsequent underplating or conductive growth of the thermal boundary layer one of the most important issues in the study of orogenies is the need to reconcile the formation of thickened lithosphere with the paradoxically high mantle temperatures often associated with orogenic episodes most efforts to date have focussed on modes of deformation whereby the cold lithospheric mantle is removed by convective instability or delamination and replaced by warm asthenosphere these models however are incompatible with the evidence for preserved coherent lithospheric deformation rather the deformed mantle appears to have been heated in place we suggest that the elevated mantle temperatures may be due to the strain heating accompanying the deformation

mantle flow in regions of complex tectonics insights from indonesia 1 indonesia is arguably one of the tectonically most complex regions on earth today due to its location at the junction of several major tectonic plates and its long history of collision and accretion it is thus an ideal location to study the interaction between subducting plates and mantle convection seismic anisotropy can serve as a diagnostic tool for identifying various subsurface deformational processes such as mantle flow for example here we present novel shear wave splitting results across the indonesian region using three different shear phases local s sks and downgoing s to improve spatial resolution of anisotropic fabrics allows us to distinguish several deformational features for example the block rotation history of borneo is reflected in coast parallel fast directions which we attribute to fossil anisotropy furthermore we are able to unravel the mantle flow pattern in the sulawesi and banda region we detect toroidal flow around the celebes sea slab oblique corner flow in the banda wedge and sub slab mantle flow around the arcuate banda slab we present evidence for deep sub 520 km anisotropy at the java subduction zone in the sumatran backarc we measure trench perpendicular fast orientations which we assume to be due to mantle flow beneath the overriding eurasian plate these observations will allow to test ideas of for example slab mantle coupling in subduction regions

mantle flow in subduction systems the subslab flow field and implications for mantle dynamics 1 the character of the mantle flow field in subduction zones remains poorly understood despite its importance for our understanding of subduction dynamics in particular little attention has been paid to mantle flow beneath subducting slabs in order to identify processes that make first order contributions to the global pattern of subslab mantle flow we have compiled shear wave splitting measurements from subduction zones worldwide from previously published studies and estimated average splitting parameters for the subslab region globally the subslab region is overwhelmingly dominated by trench parallel fast splitting directions we tested for relationships between splitting delay time a measure of the strength of anisotropy and parameters that are indicators of tectonic processes such as trench migration velocity convergence velocity and obliquity age of subducting lithosphere and slab dip curvature seismicity and thickness we used several different plate motion models to describe plate and trench motions and evaluated the differences among the models we find that only one parameter namely the trench migration velocity in a pacific hot spot reference frame appears to correlate well with the strength of the subslab splitting signal this finding supports a model in which the mantle beneath subducting slabs is dominated by three dimensional flow induced by trench migration we explore several implications of our model for various aspects of mantle dynamics including the choice of a suitable reference frame s for mantle flow the existence of a thin decoupling zone between slabs and the subslab mantle and consequences for mass transfer between the upper and lower mantle

on the origin of el chichon volcano and subduction of tehuantepec ridge a geodynamical perspective the origin of el chichon volcano is poorly understood and we attempt in this study to demonstrate that the tehuantepec ridge tr a major tectonic discontinuity on the cocos plate plays a key role in determining the location of the volcano by enhancing the slab dehydration budget beneath it using marine magnetic anomalies we show that the upper mantle beneath tr undergoes strong serpentinization carrying significant amounts of water into subduction another key aspect of the magnetic anomaly over southern mexico is a long wavelength 150 km high amplitude 500 nt magnetic anomaly located between the trench and the coast using a 2d joint magnetic gravity forward model constrained by the subduction p t structure slab geometry and seismicity we find a highly magnetic and low density source located at 40 80 km depth that we interpret as a partially serpentinized mantle wedge formed by fluids expelled from the subducting cocos plate using phase diagrams for sediments basalt and peridotite and the thermal structure of the subduction zone beneath el chichon we find that 40 of sediments and basalt dehydrate at depths corresponding with the location of the serpentinized mantle wedge whereas the serpentinized root beneath tr strongly dehydrates 90 at depths of 180 200 km comparable with the slab depths beneath el chichon 200 220 km we conclude that this strong deserpentinization pulse of mantle lithosphere beneath tr at great depths is responsible for the unusual location singularity and probably the geochemically distinct signature adakitic like of el chichon volcano

seismic evidence for sharp lithosphere asthenosphere boundaries of oceanic plates the mobility of the lithosphere over a weaker asthenosphere constitutes the essential element of plate tectonics and thus the understanding of the processes at the lithosphere asthenosphere boundary lab is fundamental to understand how our planet works it is especially so for oceanic plates because their relatively simple creation and evolution should enable easy elucidation of the lab data from borehole broadband ocean bottom seismometers show that the lab beneath the pacific and philippine sea plates is sharp and age dependent the observed large shear wave velocity reduction at the lab requires a partially molten asthenosphere consisting of horizontal melt rich layers embedded in meltless mantle which accounts for the large viscosity contrast at the lab that facilitates horizontal plate motions

complex anisotropy in d beneath the eastern pacific from sks skks splitting discrepancies article i nfo although observations of seismic anisotropy in the lowermost mantle are abundant the mechanism which generates anisotropy in the d layer is not well understood most observational constraints on d anisotropy come from the splitting of shear waves that propagate nearly horizontally through the lowermost mantle however anisotropy in the d layer can also give rise to discrepancies in shear wave splitting for sks and skks phases for the same event station pair as these phases sample different regions of the lowermost mantle here i report observations of strongly discrepant sks skks splitting observed at broadband stations in western mexico and california in particular strong skks splitting with fast polarization directions near 60 and delay times up to 3 s is observed for a group of raypaths that sample a region of the d layer beneath the eastern pacific ocean a comparison of skks splitting with sks splitting observed from the same events as well as with sks splitting observed at a variety of backazimuths at each station suggests that the anomalous anisotropic structure is in the lowermost mantle my preferred model for the unusual anisotropic geometry in this region is large shear deformation in d at the edge of a region where slab material impinges upon the core mantle boundary resulting in lattice preferred orientation of lower mantle minerals

constraints on mantle anisotropy beneath precambrian north america from a transportable teleseismic experiment data from the apt89 transportable teleseismic experiment was examined to characterize the variations in shear wave splitting along a 1500 km traverse in north america see silver et al 1993 this issue three geologic terranes were sampled the western superior province of the canadian shield the trans hudson orogen and the wyoming craton the primary goal was to determine the extent to which the anisotropy was controlled by archean and proterozoic tectonic episodes of the large suite of teleseismic events recorded 5 were suitable for determining the fast polarization direction and delay time t from the phases sks and skks the resulting observations reveal two important features first the values of display good geologic coherence stations along the northern part of the array in the exposed shield reveal a consistent ene direction for that is generally parallel to exposed geologic structures within the part of the superior province buried beneath phanerozoic cover rotates to a ne sw azimuth as do geophysical indicators of geologic fabric moving onto the trans hudson changes abruptly to a more ew azimuth the stations within the wyoming craton suggest values with a more northerly direction although the waveforms from these stations are more complex second t which ranges between 0 40s and 1 75s shows systematic variations across the traverse the largest delay times approaching 2 s are found in a 250 km wide band from red lake ontario to the u s canada border t is reduced to about 1 s or less both north and south of this region for the stations with detectable splitting we find an inverse relationship between s isotropic station delays derived from the portable data set and t which is most easily explained by the anisotropy being primarily localized in the fast presumably lithospheric mantle the splitting observations overall suggest that mantle anisotropy along this traverse is dominated by precambrian fossil anisotropy that is preserved in the subcontinental lithosphere

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

microbial alteration of subsurface natural gas accumulations bacterial alteration of subsurface reservoired hydrocarbon accumulations extends into the range of the c1 c4 gaseous hydrocarbons preferentially attacking the wet gas components in most instances during the initial stages of bacterial gas alteration propane seems to be selectively attacked resulting in anomalous propane carbon isotopic compositions as well as unusual carbon isotopic distributions among the wet gas components extensive bacterial alteration appears to be capable of removing all but traces of the wet gas components resulting in a dry gas that could be confused with overmature dry gas whose wet components have undergone extensive thermal cracking recognition of microbial alteration in subsurface reservoired hydrocarbon gas accumul tions is important for both correlation purposes and maturity estimates

b type olivine fabric in the mantle wedge insights from high resolution non newtonian subduction zone models several hypotheses have been proposed to explain trench parallel shear wave splitting in the mantle wedge of subduction zones these include 3 d flow effects parallel melt filled cracks and b type olivine fabric we predict the distribution of btype and other fabrics with high resolution thermal and stress models of subduction zones a composite viscous rheology is used that incorporates wet diffusion creep wet dislocation creep and a stress limiting approximated peierls creep law rheological parameters for dislocation and diffusion creep are based on deformation experiments the effects of variable viscous coupling between the slab and mantle are explored using kinematic boundary conditions that change along the top of the slab two end member models are observed which depend on the depth of partial viscous coupling between the slab and mantle 1 deep partial coupling which gives rise to cold high stress conditions in the forearc mantle and high temperature low stress conditions in the arc and back arc mantle 2 full viscous coupling at the base of a 40 km conducting lid which is characterized by high temperature and low stress the case with deep partial coupling which produces a better match with attenuation tomography and heat flow shows a large region with suitable conditions for b type fabric in the forearc mantle and a rapid transition toward the back arc to conditions that are more suitable for a c or e type fabrics full viscous coupling gives rise to high temperature low stress conditions unsuitable for b type fabric our modeling predicts that the best candidates for regions with b type fabric are the forearc mantle and a cold 10 15 km layer above the slab d 2005 elsevier b v all rights reserved

free hydrocarbon gas gas hydrate and authigenic minerals in chemosynthetic communities of the northern gulf of mexico continental slope relation to microbial processes research submersibles and piston cores were used to sample two chemosynthetic communities in the gulf of mexico continental slope at 540 m water depth vent gas from the deep subsurface is the starting material from which other carbon pools are derived including gas hydrate free hydrocarbon gas in sediment and authigenic carbonate rock gas crystallizes as exposed mounds of structure ii gas hydrate and as massive vein fillings in hemipelagic mud venting rates from gas hydrate mounds periodically increase after the temperature of the bottom water increases gas hydrate decomposition is largely restricted to exposed hydrate and at shallow depth in sediment overall gas hydrate is accumulating not decomposing at study sites most free gas in sediment appears to be destroyed in situ by anaerobic microbial oxidation in chemosynthetic communities leading to sequestration of carbon as abundant authigenic carbonate rock depleted in 13c free methane is rapidly oxidized leaving residual methane enriched in 13c some in situ microbial co2 reduction occurs and methane depleted in 13c mixes with vent methane enriched in 13c the c2 c5 hydrocarbons of vent gas initially most depleted in 13c ethane isobutane isopentane are least affected by microbial oxidation whereas hydrocarbons initially enriched in 13c propane normal butane normal pentane are most affected anaerobic microbial oxidation of all c1 c5 hydrocarbon gases not only methane may be significant in chemosynthetic communities microbial processes contribute to the development and stability of chemosynthetic communities by providing required h2s anaerobic microbial processes lead to deposition of diagnostic authigenic minerals related to the carbon carbonate minerals and sulfur cycles pyrite elemental sulfur altering the seafloor seafloor cementation favors fauna such as chemosynthetic tubeworms and seep mussels by forming carbonate hardgrounds on an otherwise unfavorable mud dominated seafloor

frequency dependent shear wave splitting beneath the japan and izu bonin subduction zones abstract despite its importance for our understanding of physical processes associated with subduction the geometry of mantle flow in subduction zones remains poorly understood particularly in the mantle wedge above subducting slabs constraints on mantle flow and deformation can be obtained by measurements of shear wave splitting a valuable tool used to characterize the geometry and strength of seismic anisotropy a complete characterization of shear wave splitting is particularly important for understanding the mantle wedge beneath japan which overlies multiple subduction zones with complex slab morphologies previous studies indicate that the upper mantle beneath japan exhibits complicated anisotropy that manifests itself in complex splitting patterns to characterize better the geometry of mantle anisotropy beneath japan we analyzed direct s waves from local earthquakes originating in the subducting slabs for evidence of shear wave splitting using data from 54 broadband seismic stations in japan s f net array in addition both local s and teleseismic sks phases were examined using data from four f net stations in the izu bonin arc in order to characterize any frequency dependence of splitting parameters that may indicate the presence of complex anisotropy we carried out our splitting analysis in two different frequency bands 0 02 0 125 hz and 0 125 0 5 hz our measurements indicate that shear wave splitting due to upper mantle anisotropy beneath japan is highly complex and exhibits both dramatic spatial variations and a strong dependence on frequency

nonvolcanic tremor observed in the mexican subduction zone nonvolcanic tremor nvt activity is revealed as episodes of higher spectral amplitude at 1 8 hz in daily spectrograms from the continuous seismological records in guerrero mexico the analyzed data cover a period of 2001 2007 when in 2001 2002 a large slow slip event sse had occurred in the guerrero oaxaca region and then a new large sse occurred in 2006 the tremor burst is dominated by s waves more than 100 strong nvt bursts were recorded in the narrow band of 40 150 km 2 to the south of iguala city and parallel to the coastline depths of nvt hypocenters are mostly scattered in the continental crust between 5 and 40 km depth tremor activity is higher during the 2001 2002 and 2006 sse compared with that for the quiet period of 2003 2005 while resistivity pattern in guerrero does not correlate directly with the nvt distribution gravity and magnetic anomaly modeling favors a hypothesis that the nvt is apparently related to the dehydration and serpentinization processes

climate variability over the last 9900 cal yr bp from a swamp forest pollen record along the semiarid coast of chile abstract we present a fossil pollen analysis from a swamp forest in the semiarid coast of chile 32 05 s 71 30 w at the northern influence zone of southern westerly wind belt a 10 000 cal yr bp calendar years before 1950 palynological sequence indicates a humid phase characterized by dense swamp forest taxa dated between 9900 and 8700 cal yr bp the presence of pollen starved sediments with only scant evidence for semiarid vegetation indicates that extreme aridity ensued until 5700 cal yr bp the swamp forest recovered slowly afterwards helped by a significant increase in moisture at 4200 cal yr bp a new swamp forest contraction suggests that another slightly less intense drought occurred between 3000 and 2200 cal yr bp the swamp forest expansion begins again at 2200 cal yr bp punctuated by a highly variable climate comparisons between the record presented here with other records across the region imply major variations in the extent of the southern westerlies during the holocene this variability could have been caused either by latitudinal displacements from the present mean position of southern westerlies wind belt or by changes in the intensity of the south pacific subtropical anticyclone both of which affect winter precipitation in the region

constraints on subduction geodynamics from seismic anisotropy 1 much progress has been made over the past several decades in delineating the structure of subducting slabs but several key aspects of their dynamics remain poorly constrained major unsolved problems in subduction geodynamics include those related to mantle wedge viscosity and rheology slab hydration and dehydration mechanical coupling between slabs and the ambient mantle the geometry of mantle flow above and beneath slabs and the interactions between slabs and deep discontinuities such as the core mantle boundary observations of seismic anisotropy can provide relatively direct constraints on mantle dynamics because of the link between deformation and the resulting anisotropy when mantle rocks are deformed a preferred orientation of individual mineral crystals or materials such as partial melt often develops resulting in the directional dependence of seismic wave speeds measurements of seismic anisotropy thus represent a powerful tool for probing mantle dynamics in subduction systems here i review the observational constraints on seismic anisotropy in subduction zones and discuss how seismic data can place constraints on wedge slab and sub slab anisotropy i also discuss constraints from mineral physics investigations and geodynamical modeling studies and how they inform our interpretation of observations i evaluate different models in light of constraints from seismology geodynamics and mineral physics finally i discuss some of the major unsolved problems related to the dynamics of subduction systems and how ongoing and future work on the characterization and interpretation of seismic anisotropy can lead to progress particularly in frontier areas such as understanding slab dynamics in the deep mantle

flat slab thermal structure and evolution beneath central mexico recent seismic and magnetotelluric experiments aimed at better characterizing the shape and state of the subducting slab and continental crust beneath central mexico exposed significant differences with conclusions of previous studies a new slab geometry is revealed in which the subducting cocos slab is perfectly flat between 120 to 290 km from the trench after which it plunges into the asthenosphere at a dip angle of 65 in sharp contrast with the previously proposed 20 dip angle seismic tomography studies show negative p wave velocity anomalies 2 to 4 in the mantle wedge beneath the mexican volcanic belt and positive anomalies 2 to 3 for the subducted cocos slab magnetotelluric experiments exposed a very low resistivity area 1 10 m located within the continental crust just below the mexican volcanic arc finally several spots of non volcanic tremors nvts have been recorded inside the continental crust above the flat slab segment while all these experiments provide a better picture of the subduction system beneath central mexico several key processes need further investigation in this study we take advantage of these new observations to better constrain the thermal structure beneath central mexico two different thermal models are computed for a mantle potential temperature t p of 1 350 and 1 450 c respectively the new thermal structures are then converted into p wave velocity anomalies and compared with the observed v p anomalies we found that a t p of 1 450 c produced larger v p anomalies that do not fit the observations however using a t p of only 1 350 c our predicted v p anomalies are positive 2 to 3 for the cold slab and negative 2 to 4 in the mantle wedge these v p estimates are consistent with the observed seismic tomography from p wave arrivals and therefore we conclude that a t p of 1 350 c is a better estimate for the mantle potential temperature beneath central mexico the new thermal model in conjunction with phase diagrams for sediments hydrated basalt and lithospheric mantle have been used to estimate the amount and location of fluids released from the subducting cocos slab several dehydration pulses have been identified along the slab interface where most of the fluids stored in sediments and oceanic crust are released into the overlying continental crust above the flat slab we found a good correlation between the pattern of these dehydration pulses and the location of nvts suggesting that slab dehydration is responsible for triggering the tremors we suggest that nvt bursts localized above the flat slab segment represent the manifestation of ongoing continental crust hydration and weakening a process that has been going on since 15 ma ago when the cocos slab entered into a flat slab regime such continuous weakening would have reduced the suction forces that kept the slab in a flat regime in the last 15 ma allowing the slab to easily roll back the continuous low resistivity region recorded beneath the volcanic front in central mexico might represent the evidence of slab dehydration and crust weakening over time

mid holocene radiocarbon ages in the subtropical andes 29 35 s climatic change and implications for human space organization this article discusses the distribution of radiocarbon age signatures obtained from archeological sites between 29 and 35 s in central chile and midwest argentina the goal of this analysis is to establish bases from which to interpret regional trends in the distribution of the archaeological record that connect these areas which have been traditionally considered to be geographically decoupled we propose a standardized methodology for selecting ages that provide a reliable human signature variations in date frequencies in a regional scale are discussed with the use of summed probability distributions radiocarbon voids at the regional level previously identified in midwest argentina are explored regional chronological information is compared to the available paleoenvironmental records thereby emphasizing the possible role of climate pulses in the spatial organization of human populations significant arid conditions between 7800 and 5700 cal bp are coincidental with a focused occupation of the andes mountains an area which may have offered stable resources and thus was more effectively occupied than other environmental bands

structure and anisotropy of the mexico subduction zone based on rayleigh wave analysis and implications for the geometry of the trans mexican volcanic belt 1 we develop a three dimensional model of shear wave velocity and anisotropy for the mexico subduction zone using rayleigh wave phase velocity dispersion measurements this region is characterized by both steep and flat subduction and a volcanic arc that appears to be oblique to the trench we give a new interpretation of the volcanic arc obliqueness and the location of the tzitzio gap in volcanism based on the subduction morphology we employ the two station method to measure rayleigh phase velocity dispersion curves between periods of 16 s to 171 s the results are then inverted to obtain azimuthally anisotropic phase velocity maps and to model 3 d variations in upper mantle velocity and anisotropy our maps reveal lateral variations in phase velocity at all periods consistent with the presence of flat and steep subduction we also find that the data are consistent with two layers of anisotropy beneath mexico a crustal layer with the fast directions parallel to the north american absolute plate motion and a deeper layer that includes the mantle lithosphere and the asthenosphere with the fast direction interpreted in terms of toroidal mantle flow around the slab edges our combined azimuthal anisotropy and velocity model enables us to analyze the transition from flat to steep subduction and to determine whether the transition involves a tear resulting in a gap between segments or is a continuous deformation caused by a lithospheric flexure our anisotropy results favor a tear which is also consistent with the geometry of the volcanic belt

mantle flow in the rivera cocos subduction zone summary western mexico where the young and small rivera plate and the adjacent large cocos plate are subducting beneath the north american plate is a unique region on earth where tearing of subducting oceanic plates as well as fragmentation of the overriding continental plate is occurring today characterizing the mantle flow field that accompanies the subduction of the rivera and adjacent cocos plates can help to clarify the tectonics and magma genesis of this young plate boundary here we report observations of seismic anisotropy as manifested by shear wave splitting derived from local s and teleseismic sks data collected by the mapping rivera subduction zone array that was deployed from 2006 january to 2007 june in southwestern mexico and from data collected by two of mexico s servicio sismologico nacional stations sks and local s wave splitting parameters indicate that the fast directions of the split sks waves for stations that lie on the central and southern jalisco block are approximately trench normal following the convergence direction between the rivera plate and jalisco block s wave splitting from slab events show a small averaged delay time of 0 2 s for the upper 60 km of the crust and mantle therefore the main source of anisotropy must reside in the entrained mantle below the young and thin rivera plate trench oblique fast sks split directions are observed in the western edge of the rivera plate and the western parts of the cocos slab the curved pattern of fast sks split directions in the western jalisco block and beneath the rivera cocos slab gap indicates 3 d toroidal mantle flow around the northwestern edge of the rivera slab and the rivera cocos gap which profoundly affect the finite strain field in the northwestern edge of the rivera slab and the mantle wedge both the tomographic images and shear wave splitting results support the idea that the rivera and western cocos plates not only moved in a downdip direction but also have recently rolled back towards the trench and that the colima rift is intimately related to the tearing between the rivera and cocos plates

horizontal subduction and truncation of the cocos plate beneath central mexico 1 based on analysis of data from a trans mexico temporary broadband seismic network centered on mexico city we report that the subducting cocos plate beneath central mexico is horizontal and tectonically underplates the base of the crust for a distance of 250 km from the trench it is decoupled from the crust by a very thin low viscosity zone the plate plunges into the mantle near mexico city but is truncated at a depth of 500 km probably due to an e w propagating tear in the cocos slab unlike the shallow slab subduction in peru and chile there is active volcanism along the trans mexican volcanic belt tmvb that lies much further inland than regions to either side where subduction dip is not horizontal geodynamical modeling indicates that a thin weak layer such as imaged by the seismic experiment can explain the flat subduction

trench parallel flow beneath the nazca plate from seismic anisotropy shear wave splitting of s and sks phases reveals the anisotropy and strain field of the mantle beneath the subducting nazca plate cocos plate and the caribbean region these observations can be used to test models of mantle flow two dimensional entrained mantle flow beneath the subducting nazca slab is not consistent with the data rather there is evidence for horizontal trench parallel flow in the mantle beneath the nazca plate along much of the andean subduction zone trench parallel flow is attributale utable to retrograde motion of the slab the decoupling of the slab and underlying mantle and a partial barrier to flow at depth resulting in lateral mantle flow beneath the slab such flow facilitates the transfer of material from the shrinking mantle reservoir beneath the pacific basin to the growing mantle reservoir beneath the atlantic basin trenchparallel flow may explain the eastward motions of the caribbean and scotia sea plates the anomalously shallow bathymetry of the eastern nazca plate the long wavelength geoid high over western south america and it may contribute to the high elevation and intense deformation of the central andes

mantle flow at a slab edge seismic anisotropy in the kamchatka region the junction of the aleutian island and the kamchatka peninsula defines a sharp turn in the boundary of the pacific and north american plates terminating the subduction zones of the northwest pacific the regional pattern of shear wave birefringence near the junction indicates that trench parallel strain follows the seismogenic benioff zone but rotates to trench normal beyond the slab edge asthenospheric mantle is inferred to flow around and beneath the disrupted slab edge and may influence the shallowing dip of the benioff zone at the aleutian junction

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

early holocene climate change and human occupation along the semiarid coast of north central chile the brief terminal pleistocene archaeological site at santa julia sj 318 50 0 s 718 45 0 w is the only one with fluted projectile preforms and megafauna consumption known from the chilean semiarid coastline here we present the climatic history at sj during the early holocene reconstructed from pollen and charcoal analyses spanning 13 2 8 6 ka 10 3 calibrated 14 c yr bp elevated charcoal concentrations confirm human activity by 13 2 ka human occupation decreased in intensity and charcoal practically disappears from the record after 10 6 ka followed by wetland expansion at sj between 10 5 and 9 5 ka local dominance of coastal shrubland reveals that dry phases occurred between 11 2 10 5 and 9 5 9 0 ka overall these findings imply that by modulating available resources at both local and landscape levels climate change may have played an important role in explaining the peopling of semiarid coastal chile copyright 2010 john wiley sons ltd

shear wave anisotropy beneath nicaragua and costa rica implications for flow in the mantle wedge 1 we present new shear wave splitting data from local events in costa rica and nicaragua recorded by the temporary july 2004 to march 2006 48 station tucan broadband seismic array observed fast polarization directions in the fore arc arc and back arc range from arc parallel to arc normal over very short distances 5 km when plotted at raypath midpoints making the direct interpretation of individual splitting measurements in terms of flow tenuous even when considering variations in the relationship between lattice preferred orientation and deformation e g b type dislocation creep in olivine therefore we tomographically invert the splitting measurements to find a three dimensional model of crystallographic orientation in the wedge we assume the elastic constants of olivine and orthopyroxene with hexagonal symmetry and use a damped iterative least squares approach to account for the nonlinear behavior of splitting when considering three dimensional ray propagation and distributions of anisotropy the best fitting model contains roughly horizontal arc parallel olivine 100 axes in the mantle wedge down to at least 125 km beneath the back arc and arc which we interpret to indicate along arc flow in the mantle wedge pb and nd isotopic ratios in arc lavas provide additional evidence for arc parallel flow and also constrain the direction northwest from costa rica to nicaragua and minimum flow rate 63 190 mm a with only slightly oblique subduction at 85 mm a of the relatively planar cocos plate the most likely mechanism for driving along arc transport is toroidal flow around the edge of the slab in southern costa rica generated by greater slab rollback in nicaragua two important implications of this arc parallel flow are the progressive depletion of the mantle source for arc lavas from costa rica to nicaragua and the possible need for significant decoupling between the wedge and downgoing plate

localized seismic anisotropy associated with long term slow slip events beneath southern mexico 1 slow slip events sses are recently discovered in the transition zone downdip of the locked zone where megathrust earthquakes repeatedly occur at subduction zone interface structural changes from the locked zone to the transition zone and the nature of in situ deformation in the transition zone however remain unclear we model teleseismic converted p to s waves to investigate seismic structural transition and anisotropy associated with shear deformation near the deep locked zone and the transition zone beneath southern mexico the deep locked zone appears to be consistent with normal oceanic crust velocity in the transition zone we find that seismic anisotropy larger than 5 is associated with the ultra slow velocity layer usl near the top of the subducted oceanic crust the foliation plane is plunging 20 10 degrees steeper than the plate interface and it is consistent with crystallographic preferred orientation developed in s c mylonites we suggest that long term sses likely coincide with a near lithostatic semiductile talc rich shear zone

the dynamic history of the trans mexican volcanic belt and the mexico subduction zone abstract the trans mexican volcanic belt tmvb is a 1000 km long neogene continental arc showing a large variation in composition and volcanic style and an intra arc extensional tectonics it overlies the rivera and cocos slabs which display marked changes in geometry geophysical studies indicate that lithospheric mantle is very thin or absent beneath the forearc and arc the fluids from the slab are released in a 40 to 100 km wide belt beneath the frontal part of the arc and the lower crust beneath the arc is partially molten east of 101 w the tmvb is built on a precambrian to paleozoic crust with thickness of 50 55 km west of 101 w the tmvb is underlain by jurassic to cenozoic marine and continental arcs with a 35 40 km thick crust the evolution of the tmvb occurred in four stages 1 from 20 to 10 ma the initial andesitic arc moved inland showing progressively drier melting and eventually slab melting suggesting flattening of the subducted slab 2 since 11 ma a pulse of mafic volcanism migrated from west to east reaching the gulf of mexico by 7 ma this mafic lavas marks the lateral propagation of a slab tear triggered by cessation of subduction beneath baja california 3 thereafter the volcanic front started moving trenchward with a marked phase of silicic volcanism between 7 5 and 3 ma local emplacement of small volume intraplate like basalts since 5 ma and development of extensional faulting these features are related to slab rollback enhancing asthenophere flux into the mantle wedge and promoting partial melting of the crust 4 the modern arc consists of a frontal belt dominated by flux and slab melting and a rear belt characterized by more differentiated rocks or by mafic lavas with little or no evidence of subduction fluids but higher asthenosphere fingerprint

tectonic interpretation of the trans mexican volcanic belt discussion discussion sur l article de shurbet d h et cebull s e paru dans tectonophysics nld 1984 101 159 165

subduction of oceanic asthenosphere evidence from sub slab seismic anisotropy 1 the oceanic asthenosphere is characterized as a low viscosity channel down to 200 300 km depth separating the cold lithosphere from above and it is intimately linked to a layer of low seismic velocity and prominent seismic anisotropy observed globally beneath ocean basins while subduction of tectonic plates in convergent margins is well recognized the fate of oceanic asthenosphere remains enigmatic we demonstrate that subduction of the oceanic asthenosphere characterized by weak azimuthal anisotropy and strong radial anisotropy explains the essence of sub slab shear wave splitting patterns where the fast splitting direction changes from predominantly trench parallel or sub parallel under relatively steep subduction zones to frequently trench normal under shallow subduction zones to explain the observed splitting time the thickness of the subducted asthenosphere is estimated to be 100 50 km

an olivine fabric database an overview of upper mantle fabrics and seismic anisotropy we present a unique database of 110 olivine petrofabrics and their calculated seismic properties the samples come from a variety of the upper mantle geodynamic environments ophiolites subduction zones and kimberlites with a wide range of micro structures a phenomenological relationship is established between p and s wave seismic anisotropy and the degree of crystal alignment fabric strength seismic anisotropy increases rapidly at low fabric strength before reaching a near saturation level of 15 to 20 for p waves and 10 to 15 for s waves despite a large variation in the symmetry of fabric patterns the average seismic properties of the different fabric micro structural and geodynamic settings have similar anisotropies in both magnitude and symmetry hence it would seem possible to determine some measure of fabric strength from seismic anisotropy if the dimensions of the anisotropic region are known but not geodynamic environment or details of the petrofabric pattern a simple pattern of seismic anisotropy characterises the average sample of the database which has the following features the polarisation plane of the fastest s wave is parallel or sub parallel to the foliation plane the maximum shear wave splitting is parallel to the y structural direction in foliation plane and normal to the lineation the maximum of the p wave velocity is parallel to the high concentration of 100 axes which is sub parallel to the lineation the 100 orientation distribution has the greatest influence on the p wave seismic anisotropy the 100 and 001 orientation distributions have the greatest influence on the symmetry of s wave anisotropy although the magnitude of anisotropy is influenced by the distribution of all three principal axes although the database only contains olivine petrofabrics this statistical study clearly shows that seismic anisotropy can be used to deduce the orientation of the structural frame in the upper mantle 1998 elsevier science b v all rights reserved

geometry and seismic properties of the subducting cocos plate in central mexico the geometry and properties of the interface of the cocos plate beneath central mexico are determined from the receiver functions rfs utilizing data from the meso america subduction experiment mase the rf image shows that the subducting oceanic crust is shallowly dipping to the north at 15 for 80 km from acapulco and then horizontally underplates the continental crust for approximately 200 km to the trans mexican volcanic belt tmvb the crustal image also shows that there is no continental root associated with the tmvb the migrated image of the rfs shows that the slab is steeply dipping into the mantle at about 75 beneath the tmvb both the continental and oceanic moho are clearly seen in both images and modeling of the rf conversion amplitudes and timings of the underplated features reveals a thin low velocity zone between the plate and the continental crust that appears to absorb nearly all of the strain between the upper plate and the slab by inverting rf amplitudes of the converted phases and their time separations we produce detailed maps of the seismic properties of the upper and lower oceanic crust of the subducting cocos plate and its thickness high poisson s and vp vs ratios due to anomalously low s wave velocity at the upper oceanic crust in the flat slab region may indicate the presence of water and hydrous minerals or high pore pressure the evidence of high water content within the oceanic crust explains the flat subduction geometry without strong coupling of two plates this may also explain the nonvolcanic tremor activity and slow slip events occurring in the subducting plate and the overlying crust

effect of water and stress on the lattice preferred orientation of olivine the influence of water and stress on the lattice preferred orientation lpo of olivine aggregates was investigated through large strain shear deformation experiments at high pressures and temperatures p 0 5 2 1 gpa t 1470 1570 k under both waterpoor and water rich conditions the specimens are hot pressed synthetic olivine aggregates or single crystals of olivine water was supplied to the sample by decomposition of a mixture of talc and brucite deformation experiments were conducted up to shear strain 6 using the griggs apparatus where water fugacity was up to 13 gpa at the pressure of 2 gpa the water content in olivine saturated with water increases with increasing pressure and the solubility of water in olivine at p 0 5 2 gpa was 400 1200 ppm h si several new types of lpo in olivine are found depending on water content and stress samples deformed in waterpoor conditions show a conventional lpo of olivine where the olivine 100 axis is subparallel to the shear direction the 010 plane subparallel to the shear plane type a however we identified three new types type b c and e of lpo of olivine depending on the water content and stress the type b lpo of olivine which was found at relatively high stress and or under moderate to high water content conditions is characterized by the olivine 001 axis subparallel to the shear direction the 010 plane subparallel to the shear plane the type c lpo which was found at low stress and under water rich conditions is characterized by the olivine 001 axis subparallel to the shear direction the 100 plane subparallel to the shear plane the type e lpo which was found under low stress and moderate water content is characterized by the olivine 100 axis subparallel to the shear direction the 001 plane subparallel to the shear plane observations by transmission electron microscopy tem and scanning electron microscopy sem show that the dislocations in water poor samples type a are curved and both b 100 and b 001 dislocations have a similar population numerous subgrains are seen in water poor samples in backscattered electron images in contrast water rich samples both type b and type c contain mostly b 001 dislocations and dislocations are straight and sub grain boundaries are rare compared to those in water poor samples these observations suggest that 1 dominant slip systems in olivine change with water fugacity and stress and 2 grain boundary migration is enhanced in the presence of water seismic anisotropy corresponding to the fabrics under water rich condition is significantly different from that under water poor condition consequently the relationship between seismic anisotropy and flow geometry in water rich regions is expected to be

lowermost mantle anisotropy beneath the north pacific from differential s scs splitting summary seismic anisotropy is an important tool for studying the nature origin and dynamics of the lowermost mantle d we introduce differential s scs splitting as a tool for removing the effect of near source and near receiver anisotropy to estimate splitting accrued in the d region this is applicable to events recorded at epicentral distances between 60 and 85 near source anisotropy has often been ignored in previous studies of lowermost mantle anisotropy we apply differential s scs splitting to records from canadian national seismic network stations of western pacific earthquakes these sample the lowermost mantle beneath the north pacific the residual splitting in scs which we attribute to d shows lag times between 1 0 and 3 9 s given the near horizontal ray path of scs in d we interpret the recovered fast directions as the orientation of the fast shear wave in the plane defined by the vertical and transverse directions and observe a clearly non vti transverse isotropy with a vertical axis of symmetry style of anisotropy the largest population of results shows an approximately southeasterly dipping symmetry axis which we speculate might be explained by descending palaeoslab material being swept horizontally across the core mantle boundary towards an upwelling region beneath the central pacific non vti symmetry and the many possible contributions to d anisotropy from lower mantle minerals melt and subducted materials suggest that our understanding of the lowermost mantle could be greatly improved by trying to resolve a more general style of anisotropy

seismicity and state of stress in guerrero segment of the mexican subduction zone 1 we take advantage of a relatively dense network of seismic stations in the guerrero segment of the mexican subduction zone to study seismicity and state of stress in the region we combine our results with recent observations on the geometry of the subducted cocos plate imaged from receiver function rf analysis an ultraslow velocity layer mapped in the upper crust of the subducted slab and episodic slow slip events sses and nonvolcanic tremors nvts reported in the region to obtain a comprehensive view of the subduction process seismicity and focal mechanisms confirm subduction of the cocos plate below mexico at a shallow angle reaching a depth of 25 km at a distance of 65 km from the trench the plate begins to unbend at this distance and becomes horizontal at a distance of 120 km at a depth of 40 km some of the highlights of the inslab seismicity are as follows 1 a cluster of earthquakes in the depth range of 25 45 km immediately downdip from the strongly coupled part of the plate interface revealing both downdip compressional and extensional events this seismicity extends from 80 to 105 km from the trench and may be attributed to the unbending of the slab 2 the slab devoid of seismicity in the distance range of 105 160 km 3 sparse inslab seismicity revealing downdip extension in the distance range of 160 240 km the nvts are also confined to this distance range the episodic sses occur on the horizontal segment in the distance range of 105 240 km where an ultraslow velocity layer in the upper crust of the slab has been mapped from waveform modeling of converted sp phases thus in the distance range of 105 160 km sses occur but nvts and inslab earthquakes are absent this suggests that metamorphic dehydration reactions in the subducting oceanic crust and upper mantle begin at a distance of 160 km giving rise to both the inslab earthquakes and nvts no inslab earthquake occurs beyond 240 km the receiver function images and p wave tomography suggest that the slab begins a steep plunge at a distance of 310 km reaching a depth of 500 km around 340 km from the trench the negative buoyancy of such a slab should give rise to large extensional stress in the slab yet inslab seismicity is remarkably low which may be explained by a slab that is not continuous up to a depth of 500 km but is broken at a shallower depth the resulting slab window may permit subslab material to flow through the gap this may provide an explanation for the recent rift related basalts found near mexico city the fore arc upper plate seismicity which during the period of study 1995 2007 consisted of a moderate earthquake mw5 8 near the coast h 12 km its numerous aftershocks and two shallow events farther inland demonstrates a trench normal extension in the upper plate near this convergent margin a state of stress that may be explained by tectonic erosion and or seaward retreat of the trench seismicity location of the mantle wedge and rupture areas of mexican earthquakes suggest that the downdip limit of rupture during large great earthquakes in guerrero may be 105 15 km shallow dipping interplate thrust earthquakes are not the only type of events that affect the seismic hazard in the region the magnitudes of inslab downdip compressional and extensional earthquakes that occur within 20 km inland from the coast in the depth range of 25 45 km may reach 6 5 and 7 5 respectively in addition we now identify normal faulting earthquakes in the upper plate these sources need to be taken into account in the hazard estimation

on subducting slab entrainment of buoyant asthenosphere laboratory and numerical experiments and boundary layer analysis of the entrainment of buoyant asthenosphere by subducting oceanic lithosphere implies that slab entrainment is likely to be relatively inefficient at removing a buoyant and lower viscosity asthenosphere layer asthenosphere would instead be mostly removed by accretion into and eventual subduction of the overlying oceanic lithosphere the lower hot side of a subducting slab entrains by the formation of a 10 30 km thick downdragged layer whose thickness depends upon the subduction rate and the density contrast and viscosity of the asthenosphere while the upper cold side of the slab may entrain as much by thermal freezing onto the slab as by mechanical downdragging this analysis also implies that proper treatment of slab entrainment in future numerical mantle flow experiments will require the resolution of 10 30 km thick entrainment boundary layers

upper mantle anisotropy beneath japan from shear wave splitting in this study we utilize data from 64 broadband seismic stations of the japanese f net network to investigate the threedimensional pattern of anisotropy in the subduction system beneath japan we have compiled a database of approximately 1900 high quality splitting measurements selected by visual inspection of over 25 000 records of s sks and skks phases at f net stations covering a wide range of incidence angles incoming polarization angles and backazimuths analysis of the variations of measured splitting parameters with these parameters allows us to consider complexities in structure such as multiple anisotropic layers dipping symmetry axes and small scale lateral variations in anisotropic properties here we focus on the presentation of the splitting measurements themselves a detailed interpretation in terms of tectonics and mantle flow is beyond the scope of this paper in the southern part of the f net array along the ryukyu arc we find that fast directions are consistently trench parallel with splitting times of 1 s or more moving northward along the array the measured splitting patterns become more complicated with significant variations in apparent splitting parameters that indicate complex anisotropic structure additionally measured fast directions vary significantly over short length scales and stations separated by less than 100 km often exhibit very different splitting behavior this increase in complexity of anisotropic structure coincides geographically with the complicated slab morphology of the subducting pacific plate at stations on hokkaido to the north and kyushu to the south we see some evidence that the fast direction of anisotropy may rotate from trench parallel close to the trench to subduction parallel further away from the trench which may correspond either to a change in stress conditions and or volatile content or to a change in flow regime 2005 elsevier b v all rights reserved

intramolecular 13c isotope distributions of butane from natural gases abstract a single step method allowing the measurement of 13c intramolecular distribution of butane isomers n and i butane from natural gas samples was developed the method builds up on the one that has been developed for propane using on line pyrolysis followed by isotope analysis of the pyrolytic fragments the isotopic filiation between n butane and its pyrolytic fragments was elucidated using samples spiked with 13c on terminal positions the isotopic filiation for i butane could not be shown experimentally due to the lack of commercially available enriched i butane we thus used a recent model named reaction mechanism generator rmg to assess the origin of the fragments arising from i butane pyrolysis butanes from natural gas samples from the carnarvon basin australia and the arkoma basin usa were analyzed the results are consistent with a model for thermogenic natural gas generation and suggest that combined intramolecular isotope composition of propane and n butane can be related to the gas maturity in addition the method allows the detection and shed light on the mechanism of n butane and i butane biodegradation in natural gas reservoirs

anisotropic uppermost mantle in young subducted slab underplating central mexico knowledge of the rate of plate spreading at mid ocean ridges is critical for estimating plate motions and the outward flux of heat from earth s interior magnetic lineations and anisotropy crystals that have aligned themselves with mantle flow preserved in oceanic lithosphere provide a means for estimating plate spreading rates up to 180 million years ago however reconstructions beyond this time are difficult because most older oceanic lithosphere has been subducted into the mantle here we use converted seismic waveforms to show that anisotropy is preserved in the subducted part of the cocos plate beneath central mexico we observe strong p and s wave anisotropy in the topmost 2 6 km of the subducted oceanic mantle the strength of the anisotropy is comparable to that measured in the surface portion of the cocos plate we also show that p wave azimuthal anisotropy and plate spreading rate at present day mid ocean ridges exhibit a linear relationship on the basis of this relationship we suggest that the subducted portion of the cocos plate formed at a half spreading rate of about 8 cm yr 1 at the east pacific rise about 15 million years ago our results imply that subducted oceanic lithosphere could preserve an archive of ancient plate spreading rates on earth

gas geochemical evidences for biodegradation of shale gases in the upper triassic yanchang formation ordos basin china abstract fractionation of chemical components and hydrogen carbon isotopic compositions associated with biodegradation of shale gases were determined from typical shale profiles of the upper triassic yanchang formation ordos basin china the results show that the light hydrocarbons c1 c5 of the shale gases were partially degraded by microorganisms and the degradation degree decreased with depth in the profile of well yy22 however the degradation degree of wells fy2 is basically unchanging at different depths and there is no obvious evidence for biodegradation in well yy18 biodegradation of c1 and c2 can occur and the degradation of hydrocarbon gas does not occur in any obvious sequence but instead there are only small differences between the relative degradation rates the relative degradation rates in descending sequence were c3 n c4 n c5 i c5 i c4 c2 c1 with intensification of degradation 13c1 of shale gas showed no obvious change whereas a marked alteration took place in 2hc1 the 13c values of c3 and n c4 did not alter significantly during early biodegradation thus use of exceptional heavy isotopic compositions of those two components to recognize moderate levels of biodegradation is limited in contrast the 13c and 2h values of c2 became more positive even at a low degree of biodegradation hence these are effective parameters for identifying biodegraded gases with a low degree of degradation no obvious change of isotopic composition was observed for the n c5 and i c5 during degradation due to the low relative degradation rates but high content of c1 and c2 biodegradation of c1 and c2 will severely destroy and decrease the gas content of shale gas there is a positive relationship between gas content and total organic carbon toc of shale samples without biodegradation in contrast samples that have experienced biodegradation show an obvious decrease in gas content with toc

slow slip events and strain accumulation in the guerrero gap mexico 1 global positioning system gps time series in guerrero mexico reveal the existence of large slow slip events sses at the boundary between the cocos and north american plates in this study we examined the last three sses that occurred in 2001 2002 2006 and 2009 2010 and their impact on the strain accumulation along the guerrero subduction margin gps displacements were inverted to retrieve the slip distribution during each sse and the inter sse coupling of the subduction interface the three analyzed sses have equivalent moment magnitudes of between 7 50 and 7 65 their lateral extents are variable and they all show significant slip in the guerrero seismic gap during the inter sse epochs the interplate coupling is high in the area where slow slip subsequently occurs in the guerrero gap the shallow portion of the plate interface from the trench to the coast is weakly coupled the average slip deficit accumulated in the guerrero gap over a period of 12 years which corresponds to three cycles of sse is only 1 4 of the slip deficit accumulated on both sides of the gap moreover the regions of large slip deficit coincide with the rupture areas of recent large earthquakes we conclude that the sses in the guerrero gap release a significant part of the strain accumulated during the inter sse period if large subduction thrust earthquakes occur in the guerrero gap their recurrence time is probably increased compared to adjacent regions

tomography and thermal state of the cocos plate subduction beneath mexico city 1 the geometry and thermal state of the subducting cocos plate beneath mexico city has been enigmatic because of the absence of a deep wadati benioff zone we present a tomographic image of the slab based on inversion of 8869 teleseismic p wave travel times measured on a portable broadband seismic network the images combined with receiver function analysis show that the slab runs flat from the coast to near mexico city where it dives into the mantle just before the trans mexican volcanic belt with a dip of 75 it continues down to a depth of 500 km at a distance of 400 km from the trench where the tomography reveals that the dipping portion ends as well as standard block tomography we invert the travel time residuals for the parameters of a thermal slab model and find a slab thickness of 40 km that is consistent with the 15 ma age of the cocos plate the combination of a young hot plate and truncation at depth can explain the lack of deep seismicity due to high temperatures and lower negative buoyancy compared with an older thicker nontruncated plate

comment on major hydrological regime change along the semiarid western coast of south america during the early holocene by ortega et al quaternary research 78 3 pp 513 527

enso tropical extratropical climate teleconnections and mechanisms for holocene debris flows along the hyperarid coast of western south america 17 24 s abstract el nino the warm phase of the enso cycle involves ocean climate anomalies in the tropical pacific ocean and in the extratropics which frequently result in heavy rainfall episodes along the equatorial and subtropical regions of western south america here we investigate meteorological mechanisms producing heavy rains floods and debris flows along the less known hyperarid coasts of southern peru and northern chile to evaluate the paleoclimate significance of holocene debris flow deposits our results reveal that heavy rainfall over the coast of southernmost peru occurs either during austral summer at the mature stage of el nino in connection with warmer sea surface temperatures and anomalous jet streams off northern chile or during the previous austral winter spring associated with equatorward shifted pacific south america psa atmospheric teleconnection patterns at antofagasta in northern chile such events occur almost exclusively in the latter season when deeper psa related anomaly poles extend their influence equatorwards beyond central chile during non el nino conditions short lived heavy rainfall episodes in southernmost peru can be associated with similar albeit weaker or less persistent circulation anomalies in addition to that a seasonally enhanced low level southerly flow provides orographic uplift rainfall favourable conditions by the andes at the 18 s coastal bend ultimately the trigger for rainfall events in all seasons and phases of the enso cycle was invariably connected with mid troposphere wave disturbances from mid latitudes the chronostratigraphy of debris flow deposits from both areas and its comparison with other paleoclimate records at the western side of the andes suggests that the enso related teleconnection patterns operated only during the second half of the holocene supporting an onset of modern el nino manifestations at 5300 5500 cal bp and increased frequency of major events during recent times we suggest that several debris flows dated between 12 900 and 8400 cal bp in southernmost peru previously interpreted as an indication of strong el nino events were associated with short and intense heavy rainfall episodes similar to those described here during the late winter spring season in non el nino conditions concomitant with stronger low level southerlies strengthened south eastern pacific subtropical anticyclone and intensified coastal upwelling

seismic anisotropy of oceanic upper mantle shear wave splitting methodologies and observations we develop methodologies to obtain accurate measurements of shear wave splitting and apply these techniques to examine the pattern of oceanic upper mantle anisotropy to obtain high quality estimates of receiver splitting at island stations we devise a stacking method that finds the optimum splitting parameters and 95 error region for teleseismic shear wave phases from a suite of earthquake events to obtain additional measurements in oceanic regions we develop techniques to measure splitting parameters and errors for ss phases that sample upper mantle anisotropy at their bounce points however we find the data are often of low resolution and anomalous characteristics are sometimes found that make splitting difficult to interpret ten second splitting of ss is observed across the banjo seismic array but we cannot unambiguously attribute this signal to mantle anisotropy at the bounce the receiver splitting methods are used to assess the adequacy of a two anisotropic layer model for the pacific region with fast polarization azimuth in the lithosphere oriented in the fossil spreading direction and in the asthenosphere oriented in the absolute plate motion direction this model has been proposed to explain surface wave data in the pacific ocean but our splitting results demonstrate that oceanic anisotropy patterns are more heterogeneous than would be predicted while the island splitting measurements could reflect the influence of individual hotspot upwellings hotspot effects do not appear to be universally dominant we propose that splitting observations alternatively indicate broad scale differences in the underlying character of oceanic upper mantle anisotropy associated with coherant patters of lithospheric structure and asthenospheric flow in particular splitting surface wave models and regional studies all support a model where lithospheric anisotropy throughout the south pacific has been erased or reoriented toward the absolute plate motion direction whereas more limited observations in the north pacific indicate that the fossil lithospheric signature appears to be preserved

shear wave splitting from local events beneath the ryukyu arc trench parallel anisotropy in the mantle wedge we present shear wave splitting measurements from local slab earthquakes at eight seismic stations of the japanese f net array located in the ryukyu arc we obtained high quality splitting measurements for 70 event station pairs and found that the majority of the measured fast directions were parallel to the strike of the trench and perpendicular to the convergence direction splitting times for individual measurements ranged from 0 25 to 2 s most values were between 0 75 and 1 25 s both the fast directions and the split times were similar to results for teleseismic s k ks and s wave splitting at the same stations which suggests that the anisotropy is located in the mantle wedge above the slab we considered several mantle deformation scenarios that would result in predominantly trench parallel fast directions and concluded that for the ryukyu subduction system the most likely explanation for the observations is corner flow in the mantle wedge combined with b type olivine fabric in this model the flow direction in the wedge is perpendicular to the trench but the fast axes of olivine crystals tend to align perpendicular to the flow direction resulting in trench parallel shear wave splitting

major hydrological regime change along the semiarid western coast of south america during the early holocene water availability in the semiarid western coast of chile 30 32 s is conditioned by high interannual precipitation variability reflecting the transition between arid subtropical and moist mid latitude climates in the southeastern pacific ocean a paleoclimate reconstruction based on the latest pleistocene holocene geological record from the quebrada santa julia archeological site in chile 31 50 s and on modern meteorological mechanisms producing alluvial episodes in this region indicates a major change in the rainfall regime shortly after 8600 cal yr bp this together with other paleoclimate proxies along the west coast of south america 34 14 s suggests la nina like conditions 13 000 8600 cal yr bp based on sedimentological and geomorphologic evidence we hypothesized that the absence of heavy rainfall events in northern chile and the new hydrological regime that prevailed ca 8600 5700 cal yr bp in north central chile resulted from an increase in the large scale westerly flow over central chile as expected in near neutral enso conditions this atmospheric circulation anomaly is compatible with an equatorward shift of the influence of the southeast pacific subtropical anticyclone relative to the early holocene prior to the onset of modern enso variability

timescales for the evolution of seismic anisotropy in mantle flow 1 we study systematically the relationship between olivine lattice preferred orientation and the mantle flow field that produces it using the plastic flow recrystallization model of kaminski and ribe 2001 in this model a polycrystal responds to an imposed deformation rate tensor by simultaneous intracrystalline slip and dynamic recrystallization by nucleation and grain boundary migration numerical solutions for the mean orientation of the a axes of an initially isotropic aggregate deformed uniformly with a characteristic strain rate show that the lattice preferred orientation evolves in three stages 1 for small times recrystallization is not yet active and the average a axis follows the long axis of the finite strain ellipsoid 2 for intermediate times the fabric is controlled by grain boundary migration and the average a axis rotates toward the orientation corresponding to the maximum resolved shear stress on the softest slip system 3 for the fabric is controlled by plastic deformation and average a axis rotates toward the orientation of the long axis of the finite strain ellipsoid corresponding to an infinite deformation the infinite strain axis in more realistic nonuniform flows lattice preferred orientation evolution depends on a dimensionless grain orientation lag parameter x defined locally as the ratio of the intrinsic lattice preferred orientation adjustment timescale to the timescale for changes of the infinite strain axis along path lines in the flow explicit numerical calculation of the lattice preferred orientation evolution in simple fluid dynamical models for ridges and for plume ridge interaction shows that the average a axis aligns with the flow direction only in those parts of the flow field where 1 calculation of provides a simple way to evaluate the likely distribution of lattice preferred orientation in a candidate flow field at low numerical cost

seismic anisotropy and mantle flow driven by the cocos slab under southern mexico shear wave splitting measurements were made using sks and skks waves recorded by the meso american subduction experiment which was deployed in southern mexico starting at the coast of the pacific ocean and running north toward the gulf of mexico in this segment of the middle america trench the oceanic cocos plate subducts under the continental north american plate the active volcanic arc is located at the southern end of the trans mexican volcanic belt unlike most subduction zones however the volcanic arc is not subparallel to the trench in the fore arc between the trench and the trans mexican volcanic belt the cocos slab subducts subhorizontally beneath the volcanic belt however the slab dives steeply into the mantle a marked difference in the orientation of the fast polarization directions is observed between the fore arc and the back arc in the fore arc the fast axes determined using sks phases are oriented ne sw in the same direction as the relative motion between the cocos and north american plates and are approximately perpendicular to the trench physical conditions in the subslab mantle are consistent with the existence of a type olivine and consequently entrained mantle flow is inferred strong coupling between the slab and the surrounding mantle is observed in the back arc sks fast polarization directions are oriented n s and are perpendicular to the strike of the slab given the high temperatures in the mantle wedge tip the development of a type or similar olivine fabric throughout the mantle wedge is expected the orientation of the fast axes is consistent with corner flow in the mantle wedge

seismic anisotropy of subducting oceanic uppermost mantle from fossil spreading 1 identifying the sources of seismic anisotropy in subduction zone forearcs is key to understanding mantle deformation processes current models based on the interpretation of shear wave splitting measurements favor the flow induced alignment of olivine crystals around down going slabs or the presence of foliated serpentine minerals due to subduction related processes thereby neglecting fossil slab fabric as a significant source of anisotropy we use seismic receiver functions to show evidence for strong anisotropy within anhydrous uppermost mantle of subducting oceanic plates in the forearcs of nankai cascadia mexico and costa rica subduction zones seismic anisotropy models from the inversion of receiver function waveforms are consistent with fossil fabric generated at spreading ridges

mantle flow in subduction systems the mantle wedge flow field and implications for wedge processes 1 the mantle wedge above subducting slabs is associated with many important processes including the transport of melt and volatiles our understanding of mantle wedge dynamics is incomplete as the mantle flow field above subducting slabs remains poorly understood because seismic anisotropy is a consequence of deformation measurements of shear wave splitting can constrain the geometry of mantle flow in order to identify processes that make first order contributions to the pattern of wedge flow we have compiled a data set of local s splitting measurements from mantle wedges worldwide there is a large amount of variability in splitting parameters with average delay times ranging from 0 1 to 0 3 s up to 1 0 1 5 s and large variations in fast directions we tested for relationships between splitting parameters and a variety of parameters related to subduction processes we also explicitly tested the predictions made by 10 different models that have been proposed to explain splitting patterns in the mantle wedge we find that no simple model can explain all of the trends observed in the global data set mantle wedge flow is likely controlled by a combination of downdip motion of the slab trench migration ambient mantle flow small scale convection proximity to slab edges and slab morphology with the relative contributions of these in any given subduction system controlled by the subduction kinematics and mantle rheology there is also a likely contribution from b type olivine and or serpentinite fabric in many subduction zones governed by the local thermal structure and volatile distribution

sub slab mantle flow parallel to the caribbean plate boundaries inferences from sks splitting abstract upper mantle deformation near the margins of the caribbean plate is investigated using observations of shear wave splitting in teleseismic and local shear phases the caribbean plate is almost stationary in the hot spot reference frame and is wedged between the north america south america nazca and cocos plates collisional belts and major shear zones encircle the plate data from seismic stations operated by iris geoscope the venezuelan seismological network and the british geological survey have provided nearly 2000 seismic records for analysis analysis of shear wave splitting in teleseismic core phases e g sks at stations reveals fast shear wave polarisations that are conformal to the plate boundary paralleling the major structural features the magnitude of the splitting is in general quite large 1 2 2 1 s in northern venezuela the magnitude of splitting increases towards the caribbean south american collisional front local shear phases from earthquakes up to 200 km deep beneath northeastern venezuela show very small amounts of splitting 0 1 0 3 s analysis of the depth dependence in the magnitude of the splitting suggests that most of the upper mantle wedge is isotropic and the splitting in the local phases is mostly accrued in the crust and uppermost mantle in ne venezuela fast shear wave polarisations in both the local and teleseismic phases closely parallel the fault systems in the region suggesting that the crust and mantle are coupled in this area stations on the island of montserrat and near bucaramanga columbia also show very small magnitudes in local s wave splitting but large amounts of sks splitting in general the small magnitude of the local s phase splitting suggests that the teleseismic phases accrue considerable splitting beneath the top of the slab we interpret the bulk of the splitting in terms of sub slab flow that is forced around the caribbean plate due to the convergence of the surrounding plates

seismic anisotropy reveals focused mantle flow around the calabrian slab southern italy 1 sks splitting at the calabrian subduction zone with delay times t up to 3s reveals the presence of a strong anisotropic fabric fast directions are oriented nne ssw in the calabrian arc c a and rotate nnw sse to the north following the arcuate shape of the subducting plate we interpret the trench parallel as local scale mantle flow driven by the retrograde motion of the slab the absence of trench perpendicular below the southern apennines s a excludes the presence of a seismically detectable return flow at its ne edge this may be due to the relative youth and limited width of the s a slab tear a possible return flow is identified farther north at the boundary of the s a and central apennines different and weaker anisotropy is present below the apulian platform a p this implies that the influence of the slab rollback in the sub slab mantle is limited to less then 100 km from the slab

intraslab seismicity and thermal stress in the subducted cocos plate beneath central mexico we present a model of the subducting cocos slab beneath central mexico that provides an explanation for stresses causing the occurrence of the majority of the intraslab earthquakes which are concentrated in a long flat segment based on the recently developed thermal models for the central mexico subduction zone the thermal stresses due to non uniform temperature contrast in the subducting slab are calculated using a finite element approach the slab is considered purely elastic but due to high temperature at its bottom the behavior is considered as ductile creep the calculation results show a 20 km slab core characterized by a tensional state of stress with stresses up to 70 mpa on the other hand the top of the slab experiences high compressive thermal stresses up to 110 mpa depending on the elastic constants used and location along the flat part of the subducting plate these compressive stresses at the top of the slab are not consistent with the exclusive normal fault intraslab earthquakes and two different sources of stress are proposed the trenchward migration of the mexican volcanic arc for the last 7 ma indicates an increase of the slab dip through time this observation suggests that the gravity torque might exceed the suction torque considering the flat slab as an embedded plate subject to an applied clockwise net torque of 0 5 10 16 1 5 10 16 n m the upper half would exhibit tensional stresses of 40 110 mpa that can actually balance the compressive thermally induced stresses an alternative stress source might come from the slab pull force caused by the slab positive density anomaly based on our density anomaly estimations 75 20 kg m 3 a 350 km slab length dipping at 20 into the asthenosphere induces a slab pull force of 1 7 10 12 4 6 10 12 n m this force produces a tensional stress of 41 114 mpa sufficient to balance the compressive thermal stresses at the top of the flat slab the linear superposition of the thermally and torque or slab pull induced stresses shows tensile stresses up to 60 180 mpa inside the flat slab core also our results suggest that the majority of the intraslab earthquakes inside the flat slab are situated where the resultant stresses are larger than 40 80 mpa this study provides a reasonable explanation for the existence of exclusively normal fault intraslab earthquakes in the flat slab beneath central mexico and also it shows that thermal stresses due to non uniform reheating of subducting slabs play a considerable role in the total stress field

crust and upper mantle heterogeneities in the southwest pacific from surface wave phase velocity analysis abstract direct earthquake to station rayleigh and love wave data observed on high gain broadband records are analyzed in order to improve the lateral resolution of the uppermost mantle in the southwest pacific region we used data of nine permanent geoscope and iris stations located in the southern hemisphere and nine other stations as part of two temporary networks the first one installed in new caledonia and vanuatu hereafter named cavascope network by orstom and the eost from louis pasteur university in strasbourg france and the second one installed in the fiji tonga and niue islands hereafter named spase network by washington university in st louis usa in order to collect more significant details on the surficial structures we included the analysis of short period waves down to 8 s a multiple frequency filtering technique has been used to recover phase velocities of rayleigh and love waves for selected earthquakes with magnitude greater than 5 5 and with known centroid moment tensor cmt about 1100 well distributed seismograms have been processed in the period range 8 100 s and corrections for topography and water depth have been applied to the observed phase velocities the geographical distribution of phase velocity anomalies have then been computed using the tomographic method developed by montagner montagner j p 1986a regional three dimensional structures using long period surface waves ann geophys 4 b3 283 294 due to a poor knowledge of dense well distributed crustal thickness values and corresponding velocity models we did not perform or speculate on the construction of an s wave 3d velocity model therefore we limited this study to the interpretation of the phase velocity distribution the location of phase velocity anomalies are well determined and the deviations are discussed within the framework of the geological context and compared with other tomographic models at long periods from 40 s to 100 s our results agree well with most of previous studies the tomographic imaging shows a large contrast between low and high phase velocities along the solomon new hebrides and fiji tonga trenches the lowest phase velocity anomalies are distributed beneath northern and southern fiji basins and the lau basin corresponding to the volume situated just above the dipping slabs whereas the highest values are displayed beneath the pacific plate and the eastern part of indian plate downgoing under the north fiji basin at shorter periods our results show that the phase velocity distributions are well correlated with the large structural crustal domains the use of local temporary broadband stations in the central part of the studied area gives us the opportunity to observe surface waves showing well dispersed trains allowing extended velocity measurements down to 8 s although aliasing due to multipaths become important the continental regions eastern australia new guinea fiji islands and new zealand show low velocities which are likely due to thick continental crust whereas the tasmanian d entrecasteaux and the northern and southern fiji basins are characterized by higher velocities suggesting thinner oceanic crust additional analysis including the anisotropic case and s wave velocity inversion with depth is in progress

abundance and carbon isotopic composition of neo pentane in australian natural gases abstract here we report the abundance and carbon isotopic composition of neo pentane or 2 2 dimethylpropane in australian natural gases derived from source rocks of late proterozoic to early tertiary age in australian natural gases neo pentane abundance is low ranging from 6 to 295 ppm the 13cneo pentane shows a strong source control and together with the other gaseous hydrocarbons a maturity effect albeit less pronounced than the source effect in severely biodegraded natural gases neo pentane abundance is similar to that in non biodegraded gases as a result of a counter balance between preservation of neo pentane relative to other wet gases and the addition of biogenic methane nevertheless neo pentane can become the most abundant pentane isomer with neo pentane iso n pentane ratios up to 9 9 here the 13cneo pentane is identical between biodegraded and non biodegraded natural gas from the same source providing strong evidence that neo pentane is resistant to biodegradation even within severely biodegraded gases where the carbon isotopic composition of all the other gaseous hydrocarbons has been altered the carbon isotopic composition of neo pentane ranges from 13c 50 8 to 24 4 vpdb and is typically isotopically more depleted in 13c up to 9 7 than iso pentane and n pentane this wide carbon isotopic variability between the pentane isomers most likely results from the interplay between decomposition isomerisation and methylation mechanisms for neo pentane formation in biodegraded gases the iso pentane and n pentane become progressively enriched in 13c resulting in much larger isotopic differences 13cneo pentane iso pentane values of up to 16 6 in this paper we show how the carbon isotopic composition of neo pentane is related to source rock type and age and demonstrate the usefulness of neo pentane in gas gas correlation particularly in biodegraded accumulations

anisotropic structure under a back arc spreading region the taupo volcanic zone new zealand 1 we investigate anisotropy surrounding a continental back arc spreading region the central volcanic region cvr in the north island of new zealand and present a simple method for spatial averaging to examine heterogeneous anisotropy s phases from local earthquakes yield consistent trench parallel fast directions in the southern compressional region but varied directions in the northern extensional region in the forearc east of the cvr is nearly trench parallel suggesting shear in the mantle or trench parallel flow in the western cvr local phases from 50 to 80 km depth give parallel to extension 120 and phases from 150 to 220 km depth yield similar 150 these results suggest asthenospheric flow with olivine a axes oriented in the extension direction down to 100 150 km depth in the eastern cvr local phases show mixed suggesting that it is a transition between the western cvr and the forearc trench parallel for shallow local events within the cvr may be caused by fluid filled cracks trench parallel for deeper local events and for sks phases can be explained by standard olivine fabrics developed by lithospheric shearing by trench parallel flow and by fossil anisotropy in the subducting oceanic lithosphere alternatively in the cvr they could be caused in part by hydrous minerals at the slab mantle upper interface which could cause olivine a axes to orient parallel to the trench due to a maximum shear stress oriented perpendicular to the trench along the slab we suggest that trench perpendicular flow in the extensional region helps to drive fluids away from the slab while in the compressional region trench parallel flow fails to distribute the fluids explaining the location of changes in geophysical properties

shape of the subducted rivera and cocos plates in southern mexico seismic and tectonic implications the geometry of the subducted rivera and cocos plates beneath the north american plate in southern mexico was determined based on the accurately located hypocenters of local and teleseismic earthquakes the hypocenters of the teleseisms were relocated and the focal depths of 21 events were constrained using a body wave inversion scheme the suduction in southern mexico may be approximated as a subhorizontal slab bounded at the edges by the steep subduction geometry of the cocos plate beneath the caribbean plate to the east and of the rivera plate beneath north america to the west the dip of the interplate contact geometry is constant to a depth of 30 km and lateral changes in the dip of the subducted plate are only observed once it is decoupled from the overriding plate on the basis of the seismicity the focal mechanisms and the geometry of the downgoing slab southern mexico may be segmented into four regions 1 the jalisco region to the west where the rivera plate subducts at a steep angle that resembles the geometry of the cocos plate beneath the caribbean plate in central america 2 the michoacan region where the dip angle of the cocos plate decreases gradually toward the southeast 3 the guerrero oaxaca region bounded approximately by the onshore projection of the orozco and o gorman fracture zones where the subducted slab is almost subhorizontal and underplates the upper continental plate for about 250 km and 4 the southern oaxaca and chiapas region in southeastern mexico where the dip of the subduction gradually increases to a steeper subduction in central america these drastic changes in dip do not appear to take place on tear faults suggesting that smooth contortions accommodate these changes in geometry the inferred 80 and 100 km depth contours of the subducted slab lie beneath the southern front of the trans mexican volcanic belt suggesting it is directly related to the subduction thus the observed nonparallelism with the middle american trench is apparently due to the changing geometry of the rivera and cocos plates beneath the north american plate in southern mexico and not to zones of weakness in the crust of the north american plate as some authors have suggested

deformation and mantle flow beneath the sangihe subduction zone from seismic anisotropy abstract subduction of oceanic lithosphere is the most direct feedback between the earth s surface and deep interior however the detail of its interaction with the broader convecting mantle is still unclear mantle flow around subduction zones can be constrained using seismic anisotropy but despite many such studies a simple global picture is lacking the sangihe subduction zone where the molucca sea microplate is subducting westward beneath the eurasian plate is part of the tectonically complex sulawesi philippine region and an ideal natural laboratory to study complex subduction processes we investigate the anisotropic structure of the sangihe subduction zone with shear wave splitting measurements of local s and sks phases at two stations mni in sulawesi dav in the philippines as well as downgoing s phases at five stations at teleseismic distances combining different phases allows a better vertical resolution of anisotropic fabrics than is possible with a single phase the broad depth distribution of local events 60 630 km allows us to observe a change in splitting behaviour at 380 km depth above fast directions are trench parallel and delay times t are 0 34 0 53 s with no increase with depth we suggest this anisotropy is caused by aligned cracks possibly melt filled beneath the volcanic arc and fossil anisotropy in the overriding plate below 380 km is predominantly trench normal and t are slightly higher 0 53 0 65 s as no correlation is observed with inferred distance travelled inside the slab we attribute this anisotropy to shear layers atop the slab which are coherent from 200 to 400 km depth and perhaps extend into the transition zone sks and source side measurements show larger t 1 53 and 1 33 s respectively and trench parallel since these phases predominantly sample sub slab mantle we consider along strike lateral flow associated with the double sided subduction of the molucca sea microplate to be the most likely explanation we thus infer three dominant regions of anisotropy at the sangihe subduction zone one within the overriding lithosphere one along the slab wedge interface and one below the subducting molucca sea slab the mantle wedge above 200 km depth and the slab itself do not seem to contribute notably to the measured anisotropy this study demonstrates the insight seismic anisotropy can provide into mantle dynamics even in tectonically complex subduction systems

book review orogenic andesites and plate tectonics minerals and rocks 16 james b gill springer berlin 1981 xiv 390 pp 109 figs dm 98 00 approx u s 44 60 cloth

seismic anisotropy in the south western pacific region from shear wave splitting 1 we perform shear wave splitting measurements to determine seismic anisotropy in the upper mantle under the remote and still not well understood south western pacific region more precisely the new hebrides subduction zone we obtained 29 good and 35 fair splitting measurements at 8 stations and 93 good and fair null measurements from the data provided by various networks according to the splitting parameters the seismic stations are grouped into eastern and western stations anisotropy of the subducting plate western stations appears to be the result of absolute plate motion apm of the australian plate from fast orientations alone it would be difficult to judge which process is responsible for the anisotropic structure either the absolute plate motion or trench parallel toroidal flow their delay times reveal that these anisotropic parameters can be explained without the presence of the subduction zone and the apm provides a simple explanation for the observed anisotropy on the other hand delay times at eastern stations are higher than that of western stations these stations are located close to the subduction zone and the nearly trench parallel orientation of fast polarization axes and their higher splitting delay times indicate the presence of subduction zone anisotropy at eastern stations is apparently due to the additional effect from toroidal flow near the subduction zone which increases the total anisotropy

testing models of sub slab anisotropy using a global compilation of source side shear wave splitting data recently a number of source side shear wave splitting measurements that directly constrain anisotropy in the upper mantle beneath subducting slabs have been published such measurements have yielded an observational foundation on which to base our understanding of the dynamics of the sub slab mantle here we compile measurements from recent studies of source side splitting beneath slabs that employ identical measurement and processing techniques we use this compilation to test the predictions of a number of recently proposed conceptual models for the dynamics of the sub slab mantle including those that invoke three dimensional return flow beneath slabs strong radial anisotropy in suboceanic asthenosphere that is entrained via subduction and a model based on the correlation between sub slab splitting behavior and the age of the subducting lithosphere we find that a model in which fast splitting directions are determined by slab age matches the observations better than either the 3 d return flow or radial anisotropic models based on this observation we propose that the sub slab mantle is characterized by two distinct anisotropic and mantle flow regimes beneath younger lithosphere 95 ma the entrained layer is thin and effectively serves as decoupling layer the dynamics of the sub slab region beneath old lithosphere is therefore dominated by three dimensional return flow this variation in the amount of mechanical coupling may be facilitated by the onset of small scale convection beneath older lithosphere

a review of the geodynamic evolution of flat slab subduction in mexico peru and chile abstract subducting plates around the globe display a large variability in terms of slab geometry including regions where smooth and little variation in subduction parameters is observed while the vast majority of subduction slabs plunge into the mantle at different but positive dip angles the end member case of flat slab subduction seems to strongly defy this rule and move horizontally several hundreds of kilometers before diving into the surrounding hotter mantle by employing a comparative assessment for the mexican peruvian and chilean flat slab subduction zones we find a series of parameters that apparently facilitate slab flattening among them trench roll back as well as strong variations and discontinuities in the structure of oceanic and overriding plates seem to be the most important however we were not able to find the necessary and sufficient conditions that provide an explanation for the formation of flat slabs in all three subduction zones in order to unravel the origin of flat slab subduction it is probably necessary a numerical approach that considers also the influence of surrounding plates and their corresponding geometries on 3d subduction dynamics

detailed mapping of seismic anisotropy with local shear waves in southeastern kamchatka summary the kamchatka peninsula lies over a vigorous subduction zone where pacific and north american plates converge at a rate of almost 80 mm yr 1 earthquakes associated with the subduction process provide an excellent source of seismic data for the study of anisotropic properties of the upper mantle and crust overlying the downgoing lithospheric slab we collected a large set of shear waves from events within the slab recorded by a variety of seismic stations in kamchatka data from permanent and temporary networks cover the entire 700 km length of the subduction zone with 50 200 km spacing between observing sites resulting in an unprecedented coverage of the supraslab mantle wedge we estimated shear wave splitting in selected s waves using two techniques and applied quality tests to ensure measurement stability fast directions vary from station to station and they can vary with backazimuth at individual stations and with direction of propagation for individual sources in over 350 measurements we recovered meaningful splitting delays up to 1 s with most delays in the 0 2 0 6 s range additionally in nearly 200 measurements splitting could not be resolved yielding null observations anisotropic properties of the kamchatka supraslab mantle wedge vary greatly along the volcanic arc and forearc of the subduction zone observed anisotropic indicators in the arc and forearc correlate spatially with some tectonic features e g volcanoes inland of the volcanic arc most splitting values indicate trench parallel fast polarization we do not observe depth dependence in local s wave splitting delays consistent with a shallow coherence of anisotropic texture in the vicinity of petropavlovsk kamchatsky observed anisotropic indicators are coherently trench normal and thus consistent with 2 d corner flow however splitting above the fragmented slab edge near the klyuchevskoy volcanic centre is variable and trench oblique birefringence between petropavlovsk and klyuchevskoy is weak overall our observations are incompatible with a regional slab driven corner flow regime

motion of the rivera plate since 10 ma relative to the pacific and north american plates and the mantle abstract to better understand the influence of rivera plate kinematics on the geodynamic evolution of western mexico we use more than 1400 crossings of seafloor spreading magnetic lineations along the pacific rivera rise and northern mathematician ridge to solve for rotations of the rivera plate relative to the underlying mantle and the pacific and north american plates at 14 times since 9 9 ma our comparison of magnetic anomaly crossings from the undeformed pacific plate to their counterparts on the rivera plate indicates that significant areas of the rivera plate have deformed since 9 9 ma dextral shear along the southern edge of the plate from 3 3 2 2 ma during a regional plate boundary reorganization deformed the rivera plate farther into its interior than previously recognized in addition seafloor located north of two rupture zones within the rivera plate sutured to north america after 1 5 ma anomaly crossings from these two deformed regions thus cannot be used to reconstruct motion of the rivera plate finite rotations that best reconstruct pacific plate anomaly crossings onto their undeformed counterparts on the rivera plate yield stage spreading rates that decrease gradually by 10 between 10 and 3 6 ma decrease rapidly by 20 after 3 6 ma and recover after 1 ma the slowdown in pacific rivera seafloor spreading at 3 6 ma coincided with the onset of dextral shear across the then incipient southern boundary of the rivera plate with the pacific plate the available evidence indicates that the rivera plate has been an independent microplate since at least 10 ma contrary to published assertions that it fragmented from the cocos plate at 5 ma motion of the rivera plate relative to north america has changed significantly since 10 ma in concert with significant changes in pacific rivera motion a significant and robust feature of rivera north america motion not previously recognized is the cessation of margin normal convergence and thus subduction from 2 6 to 1 0 ma along the entire plate boundary followed by a resumption of trench normal subduction along the southern half of the rivera north america plate boundary after 1 0 ma motion of the rivera plate relative to the underlying mantle since 10 ma has oscillated between periods of landward motion and seaward motion the evidence suggests that the torque exerted by slab pull on this young and hot oceanic plate is either minimal or is effectively counterbalanced by forces that resist its motion

crust and upper mantle seismic anisotropy variations from the coast to inland in central and southern mexico subduction zones are among the most dynamic tectonic environments on earth deformation mechanisms of various scales produce networks of oriented structures and faulting systems that result in a highly anisotropic medium for seismic wave propagation in this study we combine shear wave splitting inferred from receiver functions and the results from a previous sks wave study to quantify and constrain the vertically averaged shear wave splitting at different depths along the 100 station mesoamerican subduction experiment array this produces a transect that runs perpendicular to the trench across the flat slab portion of the subduction zone below central and southern mexico strong anisotropy in the continental crust is found below the trans mexican volcanic belt tmvb and above the source region of slow slip events we interpret this as the result of fluid melt ascent the upper oceanic crust and the overlying low velocity zone exhibit highly complex anisotropy while the oceanic lower crust is relatively homogeneous regions of strong oceanic crust anisotropy correlate with previously found low v p v s regions indicating that the relatively high v s is an anisotropic effect upper mantle anisotropy in the southern part of the array is in trench perpendicular direction consistent with the alignment of type a olivine and with entrained subslab flow the fast polarization direction of mantle anisotropy changes to n s in the north likely reflecting mantle wedge corner flow perpendicular to the tmvb

three dimensional flow in the subslab mantle three dimensional models of mantle flow at subduction zones make it possible to explain the common occurrence of trench parallel subslab seismic anisotropy subslab flow becomes inherently three dimensional when slab driven flow interacts with a wide variety of ambient background mantle flow conditions this interaction depends on slab geometries mechanical coupling parameters and lower mantle viscosities deflection of subslab mantle flow is a robust feature for all model parameters and geometries as the slab acts as an obstruction to the ambient background mantle flow background mantle flow can become trench perpendicular or trench parallel subslab flow depending on whether the ambient background mantle flow is deflected beneath the bottom of the slab or toward the edge of the slab the first case is especially prominent in models with short slabs that do not penetrate into the lower mantle the second case is especially prominent in models with long steep slabs the results are also highly sensitive to the amount of mechanical coupling between the subducting plate and the mantle beneath it high levels of coupling create a boundary layer of trench perpendicular entrained flow pushing the deflection due to the obstructing slab away from the slab we compare our subslab flow model predictions with a global set of seismic anisotropy fast directions in the subslab mantle and find generally good agreement between the anisotropy observations dominantly trench parallel or trench perpendicular and the mantle flow directions predicted for decoupled systems

systematic variation in anisotropy beneath the mantle wedge in the java sumatra subduction system from shear wave splitting the tectonic context of south east asia is dominated by subduction one such major convergent boundary is the java sunda trench where the australian indian plates are being subducted beneath the eurasian plate we measure shear wave splitting in local and teleseismic data from 12 broadband stations across sumatra and java to study the anisotropic characteristics of this subduction system which can provide important constraints on dynamical processes involved splitting in s waves from local earthquakes between 75 and 300 km deep show roughly trench parallel fast directions and with time lags 0 1 1 3 s 92 0 6 s splitting from deeper local events and sks however shows larger time lags 0 8 2 0 s and significant variation in fast direction in order to infer patterns of deformation in the slab we apply a hybrid modelling scheme we raytrace through an isotropic subduction zone velocity model obtaining event to station raypaths in the upper mantle we then apply appropriately rotated olivine elastic constants to various parts of the subduction zone and predict the shear wave splitting accrued along the raypath finally we perform grid searches for orientation of deformation and attempt to minimise the misfit between predicted and observed shear wave splitting splitting from the shallow local events is best explained by anisotropy confined to a 40 km over riding plate with horizontal trench parallel deformation however in order to explain the larger lag times from sks and deeper events we must consider an additional region of seismic anisotropy in or around the slab the slab geometry in the model is constrained by seismicity and regional tomography models and many sks raypaths travel large distances within the slab models placing anisotropy in the slab produce smaller misfits than those with anisotropy outside for most stations there is a strong indication that inferred flow directions are different for sub sumatran stations than for sub javanese with 60 change over 375 km the former appear aligned with the subduction plate motion whereas the latter are closer to perpendicular parallel to the trench direction there are significant differences between the slab being subducted beneath sumatra and that beneath java age of seafloor maximum depth of seismicity relative strength of the bulk sound and shear wave velocity anomaly and location of volcanic front all vary along the trench we speculate therefore that the anisotropy may be a fossilised signature rather than due to contemporary dynamics

shear wave splitting beneath western united states in relation to plate tectonics we have examined shear wave splitting in teleseismic shear waves from 26 broadband stations in the western united states fast polarization directions and delay times t show spatial variations that are coherent within geologic provinces stations located near the san andreas fault show clear evidence for fault parallel anisotropy in the crust and upper mantle 115 125 km thickness this can be explained by the finite strain associated with the relative plate motion between the north american and pacific plates the lateral extent of this strain field is probably narrow to the west because stations 55 km west of the san andreas fault do not show fault parallel anisotropy in southern california station lac located 80 km east of the san andreas fault shows large fault parallel anisotropy this suggests that the pacific north american plate boundary in the mantle might be displaced to the east in southern california a deeper e w oriented fast direction of anisotropy underlies the fault parallel anisotropic layer in the vicinity of the san andreas fault an e w fast feature is also present beneath the western basin and range and the foothills of the sierra nevada although local variations are present the magnitude of delay times suggests that this feature resides in the asthenosphere we interpret this feature as the asthenospheric flow in the slabless window left behind the farallon plate the flow induced anisotropy may partially be frozen in at shallow depths station orv is located near the southern edge of the gorda slab where no anisotropy is detected the absence of anisotropy at this location could therefore mark a boundary between farallon associated flow and regions where e w oriented asthenospheric flow did not occur the lack of evidence for ne sw fast orientation within the walker lane shear belt of western nevada suggests that this crustal feature does not extend into the mantle or that is not as well developed as that beneath the san andreas fault stations located over the young subducting gorda plate mark a change in the fast direction to nearly ne sw this direction aligns well with the maximum compressive stress direction in the overlying north american plate and the ne sw directed internal shearing of the gorda plate the anisotropic thicknesses calculated from delay times suggest roughly double that expected for purely lithospheric contributions this implies that the anisotropic thickness may include some of the asthenosphere alternatively using a higher anisotropy of 8 can bring thicknesses in line with other measures of lithospheric thicknesses the correspondence between the fast directions and the present plate tectonic deformations suggest that mapping upper mantle deformation through seismic anisotropy is a viable method and that asthenospheric flow may be a significant contributor to seismic anisotropy

insights into mantle structure and flow beneath alaska based on a decade of observations of shear wave splitting sks shear wave splitting measurements from three program for array seismic studies of the continental lithosphere experiments broadband experiment across the alaska range alaska receiving cross transect of the inner core and multidisciplinary observations of subduction which form a north south transect across alaska show a remarkably simple pattern of two large anisotropy domains in the northern domain extending from the 70 km contour of the subducting pacific plate north to the arctic ocean fast directions are consistently in the ne sw direction these directions are essentially parallel to the absolute plate motion direction in northern alaska and parallel to the strike of the subducting plate above the mantle wedge suggesting that they represent some combination of plate scale asthenospheric flow in the upper mantle and flow along the subducting plate in the mantle wedge a strong wedge component beneath the alaska range is required to explain systematics of splitting delay times in the southern domain which extends south from the 70 km depth contour to the subducting plate fast directions are in the nw se direction a 90 rotation from the northern domain these fast directions are parallel to the dip of the subducting plate in the direction of convergence and represent entrained flow beneath the subducting slab the pacific plate absolute motion approximately parallels local convergence two major factors seem to control flow in these regions absolute plate motion in the north and the subduction of the pacific plate in the south although both subduction driven wedge flow and absolute plate motion contribute to the southern part of the northern regime

constraints on upper mantle anisotropy surrounding the cocos slab from sk k s splitting 1 sks and skks splitting observations are used to constrain the pattern of mantle flow in the central american subduction zone beneath costa rica and nicaragua after removing the effects of shallow wedge anisotropy on sk k s waveforms a best fitting model of anisotropy beneath the cocos plate and in the deeper mantle wedge is determined fast polarization directions and model symmetry axis orientations in both regions as well as the shallow wedge are dominated by roughly arc parallel azimuths and therefore are not consistent with sublithospheric mantle flow entrained by the subducting cocos plate or simple two dimensional corner flow in the wedge in conjunction with geochemical data and local s splitting tomography the sk k s splitting observations and anisotropy models are consistent with flow to the wnw within the mantle wedge on the caribbean side of the cocos plate possibly drawn through a slab window beneath panama and southern costa rica anisotropy in the pacific mantle beneath the cocos plate is also best explained by flow with a component that is roughly parallel to the strike of the slab although the absolute direction of this flow is not uniquely constrained

paleoenvironmental changes in the semiarid coast of chile 32 s during the last 6200 cal years inferred from a swamp forest pollen record pollen analysis of two sediment records from a coastal swamp forest site in the chilean semiarid region 31 50 s 71 28 w shows an alternation of dry and wet phases during the past 6100 cal yr b p the most prominent vegetation changes occur at 4200 cal yr b p with the expansion of the swamp forest taxa luma chequen and escallonia sp followed by a regression of the forest beginning at 3200 cal yr b p and ending with its replacement by a xerophytic scrub between 1800 and 1300 cal yr b p the swamp forest reexpanded after 1300 cal yr b p and persisted with minor variation until the present we interpret the establishment of the swamp forest at the study site to be the result of a rising watertable in response to increased rainfalls from 4200 cal yr b p onward our results indicate that in north central chile the second half of the holocene was climatically more variable than previously thought suggesting significant changes in the position and or intensity of the westerlies wind belt and possibly in the frequency of el nino southern oscillation events

sub slab seismic anisotropy and mantle flow beneath the caribbean and scotia subduction zones effects of slab morphology and kinematics subduction systems are vitally important to plate tectonics and mantle convection but questions remain about many aspects of subduction dynamics particularly the nature of sub slab mantle flow observations of seismic anisotropy can shed light on the pattern of mantle flow in subduction systems but major uncertainties remain regarding the interpretation of sub slab anisotropy here we present measurements of shear wave splitting due to anisotropy in the sub slab mantle beneath the caribbean and scotia subduction zones these subduction systems are morphologically similar with high arc curvature and short arc length but there are differences between them in terms of their kinematic and tectonic settings we apply the source side shear wave splitting technique to direct teleseismic s phases originating from slab earthquakes we restrict our analysis to seismic stations at which we have examined the receiver side splitting in detail to ensure accurate corrections for anisotropy in the upper mantle beneath the station we observe a well defined pattern of trench parallel fast directions f i n the northern half of the caribbean subduction zone with a transition to dominantly trenchperpendicular f at the southern end there is more scatter in the measurements for scotia but we observe generally trench parallel f in the northern portion of the subduction system with a mix of trench parallel perpendicular and oblique fast directions to the south our preferred interpretation of these splitting patterns is that they reflect 3 d return flow of the sub slab mantle due to trench rollback in both systems we infer that sub slab flow is being driven from south to north beneath scotia this is likely driven by differential migration of the scotia trench along strike beneath the caribbean we hypothesize that mantle flow around the southern edge of the slab is inhibited by the presence of the south american continental keel enabling trench perpendicular stretching in the subslab mantle and forcing mantle flow to escape to the north the similarities and differences we observe between the two systems yield insight into the relative contributions of slab morphology and plate kinematics in controlling mantle flow beneath subducting slabs

seismic anisotropy beneath the continents probing the depths of geology seismic anisotropy beneath continents is analyzed from shear wave splitting recorded at more than 300 continental seismic stations anisotropy is found to be a ubiquitous property that is due to mantle deformation from past and present orogenic activity the observed coherence with crustal deformation implies that the mantle plays a major if not dominant role in orogenies no evidence is found for a continental asthenospheric decoupling zone suggesting that continents are coupled to general mantle circulation

deflection of mantle flow beneath subducting slabs and the origin of subslab anisotropy global compilations of subslab shear wave splitting parameters show a mix of trench parallel and trench perpendicular fast directions that often directly contradict predictions from two dimensional models of slab entrained flow here we show that subslab anisotropy is consistent with three dimensional geodynamic models that feature the interaction between subducting slabs and regional mantle flow each model represents a specific region for which high quality source side shear wave splitting data are available we compare the distribution of finite strain in the models with shear wave splitting observations showing that both trench parallel and trench perpendicular fast directions can be explained by deflection of regional mantle flow around or beneath subducted slabs subslab maximum elongation directions calculated from our models depend on a combination of geometry factors such as slab dip angle and maximum depth mechanical parameters such as decoupling between the slab and the subjacent mantle and the orientation and magnitude of the regional mantle flow

palynological evidence for increased aridity on the central chilean coast during the holocene abstract the late quaternary vegetation of the semiarid coast of central chile is inferred from the palynological analysis of profiles from quereo 31 55 s and quintero 32 47 s prior to 11 400 yr b p wet conditions are suggested by the abundance of pollen indicators of swamp and aquatic taxa such as cyperaceae and myriophyllum and by the presence of traces of arboreal pollen since ca 10 000 yr b p a trend toward increasingly drier conditions is implied by the almost complete absence of arboreal and aquatic taxa and a general decrease in the diversity of the semiarid shrubland indicators from 3000 yr b p onward the pollen records show the reappearance of swamp and aquatic taxa presumably associated with wetter conditions which led to recolonization by forest taxa at 1720 yr b p in quintero the drier climate detected along the semiarid coast of central chile during most of the holocene extended inland to the andean foot hills within the present mediterranean type climate zone of chile and also affected the distribution of the winter deciduous nothofagus forests and the northern boundary of the temperate rain forests