high velocity anomaly adjacent to the western edge of the pacific low velocity province koki idehara 1 satoru tanaka2 and nozomu takeuchi3 1department of earth and planetary science university of tokyo 7 3 1 hongo bunkyo ku tokyo 113 0033 japan e mail idehara eps s u tokyo ac jp 2institute for research on earth evolution japan agency for marine earth science and technology 2 15 natsushima cho yokosuka 237 0061 japan 3earthquake research institute university of tokyo 1 1 1 yayoi bunkyo ku tokyo 113 0032 japan

hematite u th he thermochronometry constrains intraplate strike slip faulting on the kuh e faghan fault central iran abstract the kuh e faghan strike slip fault system kff located to the northern edge of the lut block in central iran developed through a neogene quaternary pulsed history of eastward fault propagation and fault related exhumation this system is a consequence of the residual stresses transmitted from the arabia eurasia convergent plate boundary here we integrate structural and textural analysis with new and previously published apatite fission track aft and apatite u th he apatite he results chlorite thermomentry and hematite u th he data from hematite coated brittle fault surfaces to constrain the timing of tectonic activity and refine patterns of late miocene pliocene burial and exhumation associated with the propagation of the kff twenty nine hematite u th he hematite he dates from three striated hematite coated slip surfaces from the kff fault core and damage zone yield individual dates from 12 2 ma petrographic analysis and chlorite thermometry of a polyphase fossil fluid system in the kff fault core document that fluid circulation and mineralization transitioned from a closed system characterized by pressure solution and calcite growth to an open system characterized by hot hydrothermal t 239 10 c fluids and hematite formation hematite microtextures and grain size analysis reveal primary and secondary syntectonic hematite fabrics no evidence of hematite comminution and similar hematite he closure temperatures 60 85 c in each sample integration of these results with thermal history modeling of aft and apatite he data shows that kff activity in the late miocene is characterized by an early stage of fault nucleation fluid circulation hematite mineralization and eastward propagation not associated with vertical movement that lasted from 12 to 7 ma hematite he aft and apatite he data track a second phase of fault system activity involving fault related exhumation initiating at 7 ma and continuing until present time our new data constrain the onset of the recognized late miocene pliocene tectonic reorganization in north central iran

timing of thrust activity in the high zagros fold thrust belt iran from u th he thermochronometry 1 apatite and zircon u th he cooling ages are used to quantify the timing of exhumation associated with thrust faulting in the high zagros fold thrust belt single grain cooling age data were collected from 1 cambrian sandstone in various thrust sheets 2 sandstone and basement clasts derived from structurally controlled salt plugs or fault bounded slices and 3 syntectonic neogene siliciclastics strata in the northwestern kuhrang and central kuh e lajin high zagros apatite u th he ahe ages range from 26 7 to 0 38 ma most cooling and exhumation occurred in the early to middle miocene constrained by ahe ages 19 15 ma from the high zagros thrust sheet localized faults and reset cooling ages from bakhtiyari deposits in the southeastern high zagros kuh e dinar cooling occurred later with ahe ages ranging from 16 5 to 0 79 ma here most cooling and exhumation occurred in the late miocene constrained by ahe ages 12 8 ma from the high zagros fault and exhumed paleozoic clasts in synorogenic strata zircon u th he zhe ages from bedrock samples across the high zagros are reflective of the precollisional thermal history the preservation of precollisional zhe ages limits the pre miocene maximum burial temperature of the exhumed strata to 180 c and indicate 7 9 km of maximum exhumation in the central zagros this study shows that thrust activity in the high zagros and continental suturing along the zagros suture was underway by at least 19 ma and initiated no later than latest oligocene to early miocene time 23 ma

geographic correlation between hot spots and deep mantle lateral shear wave velocity gradients abstract hot spot volcanism may originate from the deep mantle in regions exhibiting the earth s most pronounced lateral s wave velocity gradients these strong gradient regions display an improved geographic correlation over s wave velocities to surface hot spot locations for the lowest velocities or strongest gradients occupying 10 of the surface area of the core mantle boundary cmb hot spots are nearly twice as likely to overlie the anomalous gradients if plumes arise in an isochemical lower mantle plume initiation should occur in the hottest thus lowest velocity regions or in the regions of strongest temperature gradients however if plume initiation occurs in the lowest velocity regions of the cmb lateral deflection of plumes or plume roots are required the average lateral deflections of hot spot root locations from the vertical of the presumed current hot spot location ranges from 300 to 900 km at the cmb for the 10 30 of the cmb covered by the most anomalous low s wave velocities the deep mantle may however contain strong temperature gradients or be compositionally heterogeneous with plume initiation in regions of strong lateral s wave velocity gradients as well as low s wave velocity regions if mantle plumes arise from strong gradient regions only half of the lateral deflection from plume root to hot spot surface location is required for the 10 30 of the cmb covered by the most anomalous strong lateral s wave velocities we find that strong gradient regions typically surround the large lower velocity regions in the base of the mantle which may indicate a possible chemical in addition to thermal component to these regions

effects of low viscosity post perovskite on thermo chemical mantle convection in a 3 d spherical shell 1 numerical simulations of thermo chemical multi phase mantle convection in a 3 d spherical shell are performed to determine how a low viscosity of post perovskite affects dynamics and structures in the deep mantle low viscosity post perovskite weakens the deepest part of slabs allowing them to more effectively spread over the core mantle boundary cmb and it also results in a greater volume of basalt segregating both of which increase the size of dense chemical piles the horizontal lengthscale of regions of pooled slab material and the steepness of piles edges in composition and phase consistent with the existence of steep sharp sided edges found in seismic analyses cmb heat flux is strongly enhanced in regions of low viscosity post perovskite consistent with a theoretical prediction and both cmb and surface heat flux are increased on average by a low viscosity of post perovskite which could have important implications for the evolution of earth s core and mantle

near field response to transpression along the southern san andreas fault based on exhumation of the northern san gabriel mountains southern california 1 near field mountain building is common along transpressive faults but how vertical deformation relates to oblique plate motion versus local structural complexity has not been universally established we have investigated the mechanisms of exhumation along the southern san andreas fault in the northern san gabriel mountains to examine the effects of pure shear dominated transpression obliquity 20 although rugged topography suggests near field uplift low temperature thermochronometry indicates minimal exhumation apatite u th he ages are 10 ma along narrow ridges within the fault zone implying near field rock uplift is not pervasive along this oblique plate boundary ages farther from the fault are younger 5 ma and suggest a deformation event occurred during the transition of activity from the san gabriel fault to the modern san andreas strand these results reveal a heterogeneous crustal response to transpression and imply that secondary structures or geometrical complexities play a primary role in near field bedrock uplift along transpressive faults

seismic evidence links the subducted mongol okhotsk slab to deformation in d near the northeastern margin of the perm anomaly abstract numerical simulations have indicated that the large low shear velocity provinces llsvps dynamically interact with subducted slabs in d and slabs in turn play an important role in influencing the shape and location of the llsvps in this study we find evidence for the existence of the d discontinuity near the northeastern margin of the perm anomaly a smaller llsvp beneath eurasia by analyzing the scd the lower mantle triplication phases generated from two deep events occurring in the northwestern pacific subduction zone and recorded at stations in europe and turkey the d discontinuity in the north is characterized with a 2 velocity increase occurring at a height of 300 km above the core mantle boundary cmb whereas the discontinuity in the south is characterized with a 3 velocity increase occurring at a height of 260 km above the cmb the d discontinuity is most likely attributed to a phase transition from perovskite to post perovskite in conjunction with the presence of relic slab materials likely related to the subducted mongol okhotsk slab our observations along with previous seismic anisotropic detections provide a clue to dynamically link the subducted slab to deformation taking place near the edges of the llsvp

vertical gps ground motion rates in the euro mediterranean region new evidence of velocity gradients at different spatial scales along the nubia eurasia plate boundary we use 2 5 to 14 years long position time series from 800 continuous global positioning system gps stations to study vertical deformation rates in the euro mediterranean region we estimate and remove common mode errors in position time series using a principal component analysis obtaining a significant gain in the signal to noise ratio of the displacements data following the results of a maximum likelihood estimation analysis which gives a mean spectral index 0 7 we adopt a power law white noise stochastic model in estimating the final vertical rates and find 95 of the velocities within 2 mm yr with uncertainties from filtered time series 40 smaller than from the unfiltered ones we highlight the presence of statistically significant velocity gradients where the stations density is higher we find undulations of the vertical velocity field at different spatial scales both in tectonically active regions like eastern alps apennines and eastern mediterranean and in regions characterized by a low or negligible tectonic activity like central iberia and western alps a correlation between smooth vertical velocities and topographic features is apparent in many sectors of the study area glacial isostatic adjustment and weathering processes do not completely explain the measured rates and a combination of active tectonics and deep seated geodynamic processes must be invoked excluding areas where localized processes are likely or where subduction processes may be active mantle dynamics is the most likely process but regional mantle modeling is required for a better understanding

ground surface temperature and humidity ground temperature cycles and the ice table depths in university valley mcmurdo dry valleys of antarctica in the upper mcmurdo dry valleys 90 of the measured ice table depths range from 0 to 80 cm however numerical models predict that the ice table is not in equilibrium with current climate conditions and should be deeper than measured this study explored the effects of boundary conditions air vs ground surface temperature and humidity ground temperature cycles and their diminishing amplitude with depth and advective flows darcy flow and wind pumping on water vapor fluxes in soils and ice table depths using the rego vapor diffusion model we conducted a series of numerical experiments that illustrated different hypothetical scenarios and estimated the water vapor flux and ice table depth using the conditions in university valley a small high elevation valley in situ measurements showed that while the mean annual ground surface temperature approximates that in the air the mean annual ground surface relative humidity 85 ice was significantly higher than in the atmosphere 50 ice when ground surface temperature and humidity were used as boundary conditions along with damping diurnal and annual temperature cycles within the sandy soil rego predicted that measured ice table depths in the valley were in equilibrium with contemporary conditions based on model results a dry soil column can become saturated with ice within centuries overall the results from the new soil data and modeling have implications regarding the factors and boundary conditions that affect the stability of ground ice in cold and hyper arid regions where liquid water is rare

extent of the low velocity region in the lowermost mantle beneath the western pacific detected by the vietnamese broadband seismograph array 1 we present evidence showing the extent of the low velocity region in the lowermost mantle beneath the western pacific we analyzed s ss scs s and sscs ss travel times observed by the vietnamese broadband seismograph array deployed as part of the ocean hemisphere project the abrupt changes in scs s and sscs ss travel times suggest that the western geographical boundary of the low velocity region is located around 140 e and is sharp more than 4 velocity contrast within 200 km the dependency of s and ss travel time anomalies of epicentral distances suggests that the strong low velocity region is confined to within 400 km from the cmb core mantle boundary the existence of lateral heterogeneities with a 100 km scale inside the low velocity region is also suggested by the abrupt changes in s and scs waveforms

geographic boundary and shear wave velocity structure of the pacific anomaly near the core mantle boundary beneath western pacific we determine the geographical boundary and shear velocity structure of a very low velocity province at the base of the earth s mantle beneath western pacific we term it the pacific anomaly based on the waveform modeling and travel time analysis of scsh sh phases our seismic data are from the china national digital seismographic network the f net in japan the global seismographic network and several passcal arrays the observed scs sh differential travel time residuals allow the northwestern geographic boundary of the anomaly to be clearly defined the seismic data also suggest that the average shearvelocity reduction inside the anomaly reaches 5 in the lowermost 300km of the mantle waveform modeling of the seismic data sampling the edge of the anomaly suggests that the northwestern boundary is best characterized by a shear velocity model with a velocity jump of about 2 at about 100 145km above the core mantle boundary and a thin 30 km thick basal layer with a shear wave velocity reduction of 13 stacked seismic data sampling the middle of the anomaly however show no evidence for any internal discontinuity with a velocity decrease greater than 2 in the middle of the anomaly overall the seismic data sampling the base of the pacific anomaly can be explained by a negative shear velocity gradient from 0 to 1 top to 13 bottom in the lowermost 220 km of the mantle similar to those of a very low velocity province beneath the south atlantic ocean and the indian ocean such a strong negative shear velocity gradient can be explained by partial melting of a compositional anomaly produced early in the earth s history located within a bottom thermal boundary layer our travel time data also exhibit small scale variations inside the anomaly indicating existence of internal small scale seismic heterogeneities inside the pacific anomaly

very low shear wave velocity at the base of the mantle under the south pacific superswell abstract s wave velocity heterogeneity at the base of the mantle beneath the south pacific superswell has been examined using skks sks and s sdiff sks differential travel times the residuals of skks sks and s sdiff sks with respect to the preliminary reference earth model prem are compared to the residuals predicted using recently published three dimensional s wave velocity models we have obtained a three dimensional model that explains the s sdiff sks residuals well and applied this model to the correction of the skks sks residuals the corrected skks sks residuals beneath the south pacific superswell remain large and positive forward modeling reveals that these residuals can be accounted for by a 5 7 low velocity region with a lateral extent of about 500 km in the lowermost 200 km of the mantle although the magnitude of the velocity reduction and the thickness of the anomalous region trade off against one another and cannot be uniquely determined the existence of a very low s wave velocity region at the base of the mantle is a robust feature this feature lies close to the ultra low velocity zone ulvz identified by p velocity data beneath the southwestern pacific and is located just below a 2 3 low s velocity body in the lower mantle at depths of 2300 2700 km beneath the south pacific superswell

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

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

detection of ridge like structures in the pacific large low shear velocity province abstract waveform tomography is conducted for sh velocity structures of the entire mantle using approximately 3 5 times the data used for obtaining the previous model sh18ce the resultant new model sh18cex exhibits a cluster of ridge like low velocity anomalies in the western part of the pacific large low shear velocity province llsvp the location of the ridge like anomalies is in good agreement with the location of the abrupt change in the topography of the d discontinuity these results suggest that the llsvp is associated with a cluster of ridge like piles rather than a single large pile spread over the entire region the piles probably consist of intrinsically dense material however either their volume or density contrast may not be sufficiently large to develop large scale domes

the role of the zagros orogeny in slowing down arabia eurasia convergence since 5 ma 1 large topographic belts along convergent margins have been recognized with the ability to slowdown the kinematics of subduction over geologically short time periods i e few myr because their associated gravitational spreading provides significant resistive force within the framework of plate tectonics the record of past and present day plate kinematics provides important constraints on the dynamics of the lithosphere because plate motion changes must reflect temporal changes in the balance of driving and resisting forces here we focus on the convergence between the arabian and eurasian plates across the zagros mountain belt relative motion across this plate boundary is reconstructed since 13 ma from published paleomagnetic and geodetic data and features a slowdown of 30 from 5 ma to the present day we employ global dynamic models of the mantle lithosphere system to test whether the most recent uplift across the arabia eurasia collision zone including the zagros orogeny may induce the observed slowdown since 5 ma specifically we use constraints from the geologic record to infer past topography and quantify its influence on the convergence rate between arabia and eurasia we test the sensitivity of our models to assumptions made in estimating the paleoelevation by perturbing the orogeny parameterization within reasonable ranges finally we speculate on the potential effects of tethys slab break off changes of the deformation style within the collision zone and the afar plume on the dynamics of convergence our results indicate that orogenic uplift across the arabia eurasia collision zone played a key role in slowing down convergence since 5 ma

stagnant slab a review a stagnant slab is a subducted slab of oceanic lithosphere subhorizontally deflected above across or below the 660 km discontinuity this phenomenon has now been widely recognized beneath subduction zones around the circum pacific and in the mediterranean collaboration of seismic and electromagnetic observations mineral physics measurements and geodynamic modeling has begun to provide a consistent picture of stagnant slab

mgsio3 postperovskite at d conditions the postperovskite transition in mgsio3 at conditions similar to those expected at the d discontinuity of earth s lower mantle offers a paradigm for interpreting the properties of this region despite consistent experimental and theoretical predictions of this phase transformation the complexity of the d region raises questions about its geophysical significance here we report the thermoelastic properties of cmcm postperovskite at appropriate conditions and evidences of its presence in the lowermost mantle these are i the jumps in shear and longitudinal velocities similar to those observed in certain places of the d discontinuity and ii the anticorrelation between shear and bulk velocity anomalies as detected within the d region in addition the increase in shear modulus across the phase transition provides a possible explanation for the reported discrepancy between the calculated shear modulus of postperovskite free aggregates and the seismological counterpart in the lowermost mantle

late cenozoic reorganization of the arabia eurasia collision and the comparison of short term and long term deformation rates the arabia eurasia collision deforms an area of 3 000 000 km2 of continental crust making it one of the largest regions of convergent deformation on earth there are now estimates for the active slip rates total convergence and timing of collision related deformation of regions from western turkey to eastern iran this paper shows that extrapolating the present day slip rates of many active fault systems for 3 7 million years accounts for their total displacement this result means that the present kinematics of the arabia eurasia collision are unlikely be the same as at its start which was probably in the early miocene 16 23 ma or earlier in some but not all active fault systems short term 10 year and long term 5 million year average deformation rates are consistent there is little active thickening across the turkish iranian plateau and possibly the interior of the greater caucasus these are two areas where present shortening rates would need more than 7 million years to account for the total crustal thickening and where there are structural and or stratigraphic data for pre late miocene deformation we suggest that once thick crust up to 60 km built up in the turkish iranian plateau and the greater caucasus convergence took place more easily by crustal shortening in less elevated regions such as the zagros simple folded zone the south caspian region and foothills of the greater caucasus or in other ways such as westward transport of turkey between the north and east anatolian faults the time and duration of this changeover are not known for certain and are likely be diachronous although deformation started or intensified in many of the currently active fault systems at 5 2 ma

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

the varzaghan ahar iran earthquake doublet mw 6 4 6 2 implications for the geodynamics of northwest iran on 2012 august 11 a pair of large damaging earthquakes struck the varzaghan ahar region in northwest iran in a region where there was no major mapped fault or any well documented historical seismicity to investigate the active tectonics of the source region we applied a combination of seismological methods local aftershock network calibrated multiple event relocation and focal mechanism studies field observations structural geology and geomorphological and inversions for the regional stress field the epicentral region is north of the north tabriz fault the first main shock is characterized by right lateral strike slip motion on an almost e w fault plane of about 23 km length extending from the surface to a depth of about 14 km the second main shock occurred on an ene striking fault that dips at 60 70a degrees to the nw independent inversions of focal mechanisms and geologically determined fault kinematic data for the active stress state yield a transpressional tectonic regime with sigma 1 oriented n132e for the region northeast of the north tabriz fault the presence of rigid lithosphere of the south caspian basin implies the kinematic adjustment by northward transferring of the contracted masses through both distributed deformation and structural deflections our results suggest that the kinematic adjustment inside a contracting wedge may occur along interacting crosswise or conjugate faults to accommodate low rates of internal deformation at a global scale our results indicate that despite the basic assumption of rigid blocks in geodetic plate modelling internal deformation of block like regions could control the kinematics of deformation and the level of seismic hazard within and around such regions of low deformation rate