nonlinear waveform and delay time analysis of triplicated core phases 1 we introduce a new method to measure differential travel times and attenuation of seismic body waves the problem is formulated as a nonlinear inverse problem which is solved by simulated annealing using this technique we have analyzed triplicated pkp waves recorded by the temporary eifel array in central europe these examples demonstrate the potential of the technique which is able to determine differential travel times and waveforms of the core phases even when they interfere on the seismograms or when additional depth phases are present the pkp differential travel times reveal the presence of large amplitude and small scale heterogeneities along the pkp ab ray paths and favor a local radial inner core model with 0 9 velocity perturbation in its top 150 km and small velocity perturbations below the quality factor in the top 300 km of this inner core region is estimated from pkp differential attenuation its preferred value is 330 with a lower bound of 75

stress state in the cuu long and nam con son basins offshore vietnam abstract the results presented in this paper are the first published estimates of the complete stress tensor in the cuu long and nam con son basins offshore vietnam we analysed in situ stress and pore pressure fields in the sedimentary formations using data from petroleum exploration and production wells to evaluate the stress state in these basins the data were obtained from the seafloor to 4300 m burial depth and include both hydrostatic and overpressured sections minimum horizontal stresses were obtained from extended leak off tests and mini fracture tests maximum horizontal stresses were estimated from drilling induced fracture parameters and borehole breakout widths in twelve wells using rock properties measured in the laboratory or estimated empirically from wireline logs seven data points are located in sediments and seventeen data points in igneous basement rocks at depths greater than 3000 m the estimated magnitudes of h are 70 110 of the v magnitudes considering the errors in the stress magnitude estimates their relative magnitudes suggest that a borderline normal strike slip stress regime presently exists in normally pressured sequences of the nam con son and cuu long basins of the twenty four data points twenty have effective stress ratios at a critical stress state for faulting on the assumption that there are faults present that are optimally oriented for failure with friction coefficients of 0 5 the results suggest that the stress state in these basins is generally critical

evidence for a sharp lateral variation of velocity at the core mantle boundary from multipathed pkpab rapid changes in differential travel times between the various pkp branches have been attributed to strong lateral velocity variations at the base of the mantle differential time pkpab df residuals showing jumps of up to 2 s for paths only 100 km apart have been reported such extreme changes in travel times for signals with comparable wavelengths suggest ray bifurcation with multipaths containing slow and fast contributions to pkpab here we report on some well documented normal and multipathed pkpab phases along with rapidly varying differential time residuals observed for fiji events with paths sampling the northern edge of the mid pacific large slow structure beneath hawaii systematic analyses of these data with two dimensional models suggest the presence of a ridge shaped structure at the core mantle boundary containing abrupt reductions of p wave velocity of up to 10 this narrow ultralow velocity zone ulvz is located at the boundary between a large scale low velocity and high velocity structure corroborating recent dynamic models which propose that ulvzs preferentially occur at the boundary between hot and cold mantle domains and could be related to plume activity at hawaii

three dimensional structure of the african superplume from waveform modelling summary previous 2 d modelling of seismic waveforms and traveltimes has revealed a large scale ridge like velocity anomaly beneath africa which is usually referred to as the african superplume the structure starts from the southern indian ocean and extends northwestwards into the atlantic ocean with its base on the core mantle boundary the structure has relatively sharp lateral boundaries with the shear velocity inside 3 per cent lower than the ambient mantle while the compressional velocity is almost normal the 3 d structure is best illustrated by seismic waveforms recorded by the south africa array generated by earthquakes in the western pacific ocean the diffracted s phase travels along the axis of the structure for over 60 with the northern and southernmost paths sampling its edges the s waveforms are simple but delayed by up to 20 s for paths sampling the middle of the structure whereas they display two arrivals for paths along the boundaries these complex waveforms can be explained by 3 d multipathing due to rapid lateral variations in shear wave velocity along the edges of the structure these sharp features are confirmed by modelling broadband records associated with the proposed ridge structure with two independent methods the spectral element method and a ray based 3 d code dwkm

the red river fault system in the tonkin gulf vietnam abstract the red river fault system in the tonkin gulf offshore haiphong was studied using seismic profiles calibrated by deep wells well characterised left lateral strike slip occurred continuously within a narrow 30 km wide zone southwest of the vinh minh fault between 30 ma and 5 5 ma however the corresponding amount of offset probably does not exceed a few tens of kilometres no sign of post 5 5 ma right lateral motion can be detected prior to 30 ma there is a widespread extension in a wider 100 km wide zone which could be related to a significant amount of left lateral motion the motion of the fault splays to the northeast of the vinh minh fault prior to 30 ma was absorbed in the rifting of the gulf of beibu a 15 5 ma unconformity separates the transtensional regime from a later transpresional regime this 15 5 ma date coincides with the cessation of sea floor spreading in the south china sea

evidence for a chemical thermal structure at base of mantle from sharp lateral p wave variations beneath central america compressional waves that sample the lowermost mantle west of central america show a rapid change in travel times of up to 4 s over a sampling distance of 300 km and a change in waveforms the differential travel times of the pkp waves which traverse earth s core correlate remarkably well with predictions for s wave tomography our modeling suggests a sharp transition in the lowermost mantle from a broad slow region to a broad fast region with a narrow zone of slowest anomaly next to the boundary beneath the cocos plate and the caribbean plate the structure may be the result of ponding of ancient subducted farallon slabs situated near the edge of a thermal and chemical upwelling

cenozoic paleostress evolution in south central vietnam implication for changing dynamics of faulting along the eastern indochina continental margin abstract the tectonics of the indochina continental margin before and after the neogene quaternary basalt eruption is poorly understood the analysis of 1500 striated faults observed in different rocks mostly from volcano plutonic rocks with ages ranging from cretaceous to quaternary at more than 140 locations in south central vietnam by direct inversion method allowed us to determine four paleostress regimes that affected eastern indochina from oligocene up to present day the first tectonic paleostress regime with oligocene age was responsible for the sinistral and dextral motions along the nw se and ne sw striking faults respectively this stress state is typical for a strike slip tectonic regime with maximum principal stress 1 and minimum principal stress 3 axes oriented nearly in e w and n s direction and both plunging at average angle of 80 and 100 respectively the paleostress ellipsoid shape ratio 2 3 1 3 ranges from 0 23 to 0 77 with an average of 0 50 0 15 the subsequent strike slip tectonic stress state prevailing from the late oligocene to very early miocene was characterized by 1 and 3 axes trending in nnw sse and ene wsw and plunging at 120 90 and 120 130 respectively the paleostress ellipsoid shape ratio ranges from 0 12 to 0 88 with a 0 52 0 23 average under this paleostress regime the n1100 to n1400 and n1600 to n200 striking faults moved with dextral and sinistral movements respectively the third paleostress state with assigned early miocene to late miocene age was responsible for dextral and sinistral motions along n1400 to n1600 and n200 to n400 striking faults coeval with emplacement of n s striking basaltic dykes this pattern shows consistency and homogeneity of the 1 and 3 axes in n s trend plunging at 120 90 and 3 in e w trend plunging at 130 110 respectively the paleostress ellipsoid shape ratio varies from 0 41 to 0 89 with an average of 0 53 0 16 the fourth tectonic stress regime occurring since the late miocene to the present day is characterized by a consistency of sub horizontal nne striking 1 plunging 110 100 and ese striking 3 plunging at 130 11 and an average of ellipsoid shape ratio 0 45 0 24 under the latest tectonic stress state the n s and n450 to n600 striking faults continue to move as dextral and sinistral faults respectively the first three paleostress regimes have evolved clockwise and were induced by the force exerted from the eastern himalayan syntaxis during the northward indian plate motion while the fourth tectonic stress regime is most likely linked to the force originated from the sunda subduction

a comparison of synthetic seismograms for 2d structures semianalytical versus numerical teleseismic wave fields occasionally exhibit rapid changes in travel times and waveforms over distances less than several great circle degrees when observed at broadband arrays these rapid changes in wave field suggest the existence of significant structural transitions occurring over scales of several hundred kilometers or less in the mid and deep mantle although approximate analytical methods based on raytracing can be readily adapted to structures having arbitrarily small scale lengths it is important to validate their accuracy against the predictions of numerical methods here we compare synthetics from an approximate ray based method wkbj modified wkm against the pseudospectral method for a 2d model of the s velocity anomaly associated with the south african plume this model consists of a uniform 3 decrease in s velocity over a broad 10 region of the mid and deep mantle beneath south africa contiguous at its bottom with a thin 100 to 200 km thick zone of low velocity extending 30 westward toward south america along the core mantle boundary transitions between anomalous and radially symmetric structures of the test model are sharp occurring over l0 km or less sv and sh wave fields synthesized by the wkm and pseudospectral methods in this model generally agree with each other well slight mismatches in the two methods can be understood as the result of either differences in model parameterization or the effects of asymptotic approximations in the ray based wkm method

present day stress field of southeast asia article i nfo it is now well established that ridge push forces provide a major control on the plate scale stress field in most of the earth s tectonic plates however the sunda plate that comprises much of southeast asia is one of only two plates not bounded by a major spreading centre and thus provides an opportunity to evaluate other forces that control the intraplate stress field the cenozoic tectonic evolution of the sunda plate is usually considered to be controlled by escape tectonics associated with india eurasia collision however the sunda plate is bounded by a poorly understood and complex range of convergent and strike slip zones and little is known about the effect of these other plate boundaries on the intraplate stress field in the region we compile the first extensive stress dataset for southeast asia containing 275 a d quality 177 a c horizontal stress orientations consisting of 72 stress indicators from earthquakes located mostly on the periphery of the plate 202 stress indicators from breakouts and drilling induced fractures and one hydraulic fracture test within 14 provinces in the plate interior this data reveals that a variable stress pattern exists throughout southeast asia that is largely inconsistent with the sunda plate s approximately ese absolute motion direction the present day maximum horizontal stress in thailand vietnam and the malay basin is predominately north south consistent with the radiating stress patterns arising from the eastern himalayan syntaxis however the present day maximum horizontal stress is primarily oriented nw se in borneo a direction that may reflect plate boundary forces or topographic stresses exerted by the central borneo highlands furthermore the south and central sumatra basins exhibit a ne sw maximum horizontal stress direction that is perpendicular to the indo australian subduction front hence the plate scale stress field in southeast asia appears to be controlled by a combination of himalayan orogeny related deformation forces related to subduction primarily trench suction and collision and intraplate sources of stress such as topography and basin geometry

large scale structure at the core mantle boundary from diffracted waves at the base of the earth s mantle is a region called d which serves as a thermal and chemical boundary layer between the silicate mantle and the liquid iron outer core1 2 tomographic models of mantle compressional wave velocity vp have their worst resolution in d owing to limited ray sampling3 5 but have hinted at large scale lateral variations there here i use a new technique which has its greatest resolution within d to produce a map of the large scale vp variations within d the technique compares the arrival times of waves that have been refracted across and diffracted around the core mantle boundary cmb the diffracted waves travel a long way in d making them excellent probes of this layer and the differential technique removes many effects arising from ray paths outside of d ref 6 the new map provides an image of the continent sized variations at the cmb that is complementary to existing shear wave data7 8 and similarities between the map and projections of ancient subducted lithosphere9 verify previous suggestions10 of a strong coupling between surface plate tectonics and the base of the mantle

structural petrological and thermal evolution of a tertiary ductile strike slip shear zone diancang shan yunnan the diancang shan a horst massif within the red river fault zone in yunnan people s republic of china preserves a structural petrological and thermal record of two distinct phases of tectonic activity a left lateral ductile shear that terminated between 20 and 17 ma and a ductile to brittle phase of normal faulting which began at 4 7 ma and remains active mylonitic rocks in the core of the range display an early steep high temperature ht schistosity and a horizontal stretching lineation that are both parallel to the trend of the belt kinematic indicators indicate that shear was left lateral the complex shape of the ht schistosity at the southern termination of the massif likely results from a large scale oblique left lateral c shear plane that dismembered the shear zone and separated the ailao shan and the diancang shan as left lateral deformation terminated thermochronological and thermobarometric results suggest that the gneisses were partially unroofed during this event along the eastern edge of the diancang shan the ht fabrics were overprinted by low temperature structures during activation of east dipping normal faults cooling associated with this normal right lateral faulting along the diancang shan and perhaps activation of the right lateral normal movement on the range front fault farther south along the ailao shan began at 4 7 0 1 ma these results tend to support the view that extrusion of indochina occurred along the left lateral red river shear zone between 35 and 19 17 ma initiation of right lateral normal slip during the late miocene may relate to eastward extensional collapse of the thickened tibetan crust or more probably to initiation of the second phase of extrusion

pkp differential travel times implications for 3 d lower mantle structure differential pkp ab df travel time measurements are compared directly with predicted residuals for mantle tomographic models to investigate the heterogeneity of the lowermost mantle much of the ab df time is accumulated in the lowermost portions of the mantle resulting in heavy weighting of 3 d lowermost mantle structure on the differential travel time perturbations comparisons of the observations with tomographic models show strong model dependence the best variance reductions are achieved using the recent p wave model of van der hilst et al 1997 up to 20 but the heterogeneity in the lowermost mantle may be 8 times larger than what shown by the tomographic model the predictions for present s wave tomographic models correlate poorly with the observations if we assume p velocity anomalies are proportional to s velocity anomalies we argue that s velocity anomalies in the lowermost mantle do not always track p velocity anomalies our study shows no obvious evidence of slab effect on the observed ab df anomalies and thus the use of pkp phases is likely to provide a powerful tool in mapping the p velocity structure in the lowermost mantle

shear velocity variation within the d region beneath the central pacific 1 small scale variability of shear velocity vs structure in the d region beneath the central pacific is imaged using 442 broadband tangential component s waveforms recorded in western north america for 37 intermediate and deep focus tonga fiji earthquakes double array stacking of spatially binned subsets of data reveals lateral variations in the relative timing and amplitude of deep mantle discontinuity reflections on scale lengths of about 130 km across the 6 8 region of d sampled waveform modeling using localized one dimensional structures indicates variations of the vs increase at the d discontinuity ranging from 0 5 to 2 3 and discontinuity depths ranging from 2490 to 2735 km deepening and weakening from southwest to northeast across the study area two abrupt vs reductions are also detected within the d layer we thus present a laterally variable three layer model of the d region beneath the central pacific the complex structure may be associated with lateral thermal and chemical gradients that produce a lens of postperovskite material above an ultralow velocity zone on the margin of a large low shear velocity province

sediments at the top of earth s core unusual physical properties at the core mantle boundary have been inferred from seismic and geodetic observations in recent years we show how both types of observations can be explained by a layer of silicate sediments which accumulate at the top of the core as earth cools compaction of the sediments expels most of the liquid iron but leaves behind a small amount of core material which is entrained in mantle convection and may account for the isotopic signatures of core material in some hot spot plumes extraction of light elements from the liquid core also enhances the vigor of convection in the core and may increase the power available to the geodynamo

scatter and bias in differential pkp travel times and implications for mantle and core phenomena differential travel times commonly used to eliminate unwanted near source and near receiver heterogeneity from travel time studies can be affected by slabs and hypocentral errors when phase pair slownesses differ greatly in particular d and core structure studies can be biased if differential pkp times are used here we present differential pkp travel time measurements having significant azimuthal travel time anomalies consistent both in size and pattern with a near source slab effect and explore other sources of error in differential times

a two dimensional p sv hybrid method and its application to modeling localized structures near the core mantle boundary a p sv hybrid method is developed for calculating synthetic seismograms involving two dimensional localized heterogeneous structures the finite difference technique is applied in the heterogeneous region and generalized ray theory solutions from a seismic source are used in the finite difference initiation process the seismic motions after interacting with the heterogeneous structures are propagated back to the earth s surface analytically with the aid of the kirchhoff method anomalous long period sks spdks observations sampling a region near the core mantle boundary beneath the southwest pacific are modeled with the hybrid method localized structures just above the core mantle boundary with lateral dimensions of 250 to 400 km can explain even the most anomalous data observed to date if s velocity drops up to 30 are allowed for a p velocity drop of 10 structural shapes and seismic properties of those anomalies are constrained from the data since synthetic waveforms are sensitive to the location and lateral dimension of seismic anomalies near the core mantle boundary some important issues such as the density change and roughness of the structures and the sharpness of the transition from the structures to the surrounding mantle however remain unresolved due to the nature of the data

large scale mantle convection and the history of subduction a large scale pattern of mantle convention is evident in the shape of the earth s gravity field1 lateral variations in seismic velocity2 3 the distribution of hotspots4 and long wavelength bathv metric anomalies5 paradoxically these signals of mantle convection bear little resemblance to present day plate motions1 4 although they seem to be related to past subduction zone configurations6 here we quantify this relationship by correlating a time integrated history of subduction calculated in the hotspot reference frame for the past 180 myr with regions of the deep mantle inferred from other geophysical observations to be colder than average our results support suggestions6 9 that a direct relationship exists between large scale thermal heterogeneity in the lower mantle and subduction during the cenozoic and mesozoic

paleostress reconstructions and geodynamics of the baikal region central asia part 2 cenozoic rifting abstract investigations on the kinematics of rift opening and the associated stress field present a renewed interest since it has recently been shown that the control of the origin and evolution of sedimentary basins depends to a large extent on the interplay between lithospheric strength and applied stresses it appears that changes of stress field with time are an important factor that either controls or results from the rifting process the object of this paper is to study the changes of fault kinematics and paleostress field with time in the baikal rift system during the cenozoic reduced paleostress tensors were determined by inversion from fault slip data measured in the central part of the rift and its southwestern termination between 1991 and 1995 results show that the stress field varies as well in time as in space two major paleostress stages are determined corresponding broadly to the classical stages of rift evolution late oligocene early pliocene and late pliocene quaternary the first paleostress stage is related to the rift initiation and the second to the major stage of rift development similarities between the recent paleostress field and the present day stress field inverted from focal mechanisms indicate that the second paleostress stage is still active therefore we propose to use proto rift for the late oligocene early pliocene stage and active rift for the late pliocene quaternary stage of rift development during the proto rift stage the stress field was characterized by a compressional to strike slip regime a progressive change from transpression to transtension is suspected for the central part of the rift baikal and barguzin basins during this period in the western termination of the rift sayan massif tunka depression a strongly compressional stress field with oblique thrusting kinematics is well constrained in the late miocene early pliocene interval the active rift stage was initiated by a marked change in fault kinematics and stress regime in the late pliocene in the central part of the rift the stress regime changed into pure extension while in the southwestern extremity it changed into pure strike slip fault kinematics suggests that rifting was initiated by an extrusion mechanism due to the interaction of far field compressional stress on a mechanically heterogeneous crust with the southwards pointing wedge of the siberian craton acting as a passive indentor the cenozoic time space evolution of the stress field is believed to reflect the increasing influence of locally generated buoyancy extensional stresses associated with density anomalies of the lithosphere on intraplate stresses generated by the india eurasia convergence and the west pacific subduction

three dimensional structure at the base of the mantle beneath the central pacific forward modeling of differential travel times of phases sensitive to lowermost mantle beneath the central pacific reveals lateral heterogeneity that is higher in amplitude than predicted by tomographic models a broad zone of low s velocity 4 percent with respect to standard models which may correspond to the base of a thermal plume narrows and is deflected as it extends to about 1000 kilometers above the core mantle boundary to the east of this zone a localized region of fast s velocity 5 percent suggests strong heterogeneity or anisotropy related to the presence of high pressure and temperature assemblages which may or may not involve core material its presence could also explain the observation of precursors to core reflected phases in this region

an inverse problem in microtectonics for the determination of stress tensors from fault striation analysis abstract a method is proposed to compute some characteristics of a possible stress tensor related to striations measured in a given faulted area if several tectonic phases are responsible for the striations the method separates and determines the successive stress tensors the iterative algorithm consists of alternatively a sorting the data attributed to a tectonic phase and b computing the characteristics of the stress tensor of this phase the method is tested using an application to synthetic and actual data

seismic evidence for ultralow velocity zones beneath africa and eastern atlantic sks waveforms recorded at distances of about 110 are extremely useful to constrain seismic velocity structure at the base of the mantle sks waves near this distance develop a complicated interference pattern with the phases sp dks and skp ds we report anomalous behavior of this interference in a number of recordings of deep earthquakes beneath south america from stations in europe and africa we model these data with two dimensional dome like structures at the base of the mantle which extend laterally by a few hundred kilometers and in which the shear velocity is up to 30 lower than in the preliminary reference earth model prem the spatial extent of these structures their position with respect to the sks core exit points and their seismic characteristics can not be uniquely determined however the presence of a dipping or a concaved upper interface is a key attribute of successful models models that invoke flat layers are insufficiently complex to explain the most erratic waveform behavior the most anomalous data correspond to sampling regions at the base of the mantle beneath the east african rift and beneath the iceland where possibly whole mantle upwellings form

40ar 39ar geochronology of the charnockites and granulites of the kan nack complex kon tum massif vietnam abstract the truong son belt forms the eastern rim of the indochina block in southeast asia the age of the metamorphism mainly along nw se mylonitic shear zones that affects this belt has been formerly determined at about 240 250 ma this age corresponds to the indosinian tectonometamorphic episode the kon tum massif situated to the south of this belt comprises high temperature rocks the kan nack complex including charnockites and granulites the main charnockitic outcrops restricted to the song ba valley establish the intrusive nature of these magmatic rocks within granulite facies material basic charnockitic rocks are mainly quartz enderbites to norites and hornblende pyroxene granulite facies rocks the 40 ar 39 ar age of intrusion cooling of charnockitic magmas is determined from primary magmatic biotites at about 245 ma in the east of the kan nack complex some granulite facies rocks exhibit relicts of primary granulite facies parageneses whereas others show evidence of overprinting by a retrogressive low grade metamorphism ar ar dating confirm this evolution giving ages of 400 ma for primary relict granulite facies phases and 260 270 ma from the most retrogressed samples establishing the youngest limit for the granulite facies metamorphism granulites intruded by charnockites in the song ba valley yield ages of about 250 ma equivalent to the ages of the charnockites and have evidently been completely reset by these high temperature intrusions therefore the kan nack complex of the kon tum massif is not an independent unit with respect to the indosinian orogen but represents the deep crustal part of this belt

global mantle shear velocity model developed using nonlinear asymptotic coupling theory we present a three dimensional shear velocity model of the whole mantle developed using s h waveform data the model is expressed horizontally in terms of spherical harmonics up to degree 12 and vertically in terms of legendre polynomials up to degrees 5 and 7 in the upper and lower mantle respectively what distinguishes this model from other tomographic models published to date is 1 the theoretical normal mode based wave propagation approach where we include across branch mode coupling terms in order to model the body wave sensitivity to structure along the path more accurately 2 the wave packet weighting scheme which allows to balance contributions from high amplitude and low amplitude phases increasing the resolution in some parts of the mantle we also relax the constraints on the moho depth which is allowed to vary in the inversion thus absorbing some uncertainties in crustal structure the resulting model is generally in good agreement with other recent global mantle s velocity models and with some regional models the rms profile with depth has more power than other models in the upper mantle lower mantle transition region and the zone of increased power and low degree structure near the base of the mantle is confined to the last 500 km in depth this model provides a particularly good fit to the non hydrostatic geoid through harmonic degree 12 79 variance reduction as well as good fits to observed splitting functions of s velocity sensitive mantle modes indicating that both large scale and small scale features are really well constrained

deep mantle seismic modeling and imaging detailed seismic modeling and imaging of earth s deep interior is providing key information about lower mantle structures and processes including heat flow across the core mantle boundary the configuration of mantle upwellings and downwellings phase equilibria and transport properties of deep mantle materials and mechanisms of core mantle coupling multichannel seismic wave analysis methods that provide the highest resolution deep mantle structural information include network waveform modeling and stacking array processing and 3d migrations of p and s wave seismograms these methods detect and identify weak signals from structures that cannot be resolved by global seismic tomography some methods are adapted from oil exploration seismology but all are constrained by the source and receiver distributions long travel paths and strong attenuation experienced by seismic waves that penetrate to the deep mantle large and small scale structures with velocity variations ranging from a fraction of a percent to tens of percent have been detected and are guiding geophysicists to new perspectives of thermochemical mantle convection and evolution

depth dependence of anisotropy of earth s inner core both body wave pkp travel times creager 1992 song and helmberger 1993a mcsweeney and creager 1993 shearer 1994 and fits to the splitting of core modes tromp 1993 show general agreement that the top 300 km of inner core is very anisotropic the anisotropy displays axial symmetry around the earth s spin axis with the polar direction 3 faster than the equatorial direction one key problem now is the depth dependence of the inner core anisotropy here we extend our polar path studies to include both long period and short period modeling for the pkp phases at ranges 120 to 173 arrivals from the top of the inner core pkikp and reflections from the inner core boundary pkikp can be observed distinctly in short period records at ranges 130 to 140 and as waveform distortions in the long period records at ranges 130 to 146 these waveforms provide a new set of data for examining the topmost 150 km of the inner core which is not well sampled by the previous body wave travel times record sections of waves traversing the inner core nearly parallel to the earth s spin axis polar paths from three events two beneath the south sandwich islands and one along the macquarie ridge recorded at world wide standardized seismograph network canadian network and long range seismic measurements stations are analyzed our results suggest that the top 150 km of the inner core is only weakly anisotropic less than 1 with strong evidence indicating that the top 60 km is not anisotropic at all

normal mode and free air gravity constraints on lateral variations in velocity and density of earth s mantle with the use of a large collection of free oscillation data and additional constraints imposed by the free air gravity anomaly lateral variations in shear velocity compressional velocity and density within the mantle dynamic topography on the free surface and topography on the 660 km discontinuity and the core mantle boundary were determined the velocity models are consistent with existing models based on travel time and waveform inversions in the lowermost mantle near the core mantle boundary denser than average material is found beneath regions of upwellings centered on the pacific ocean and africa that are characterized by slow shear velocities these anomalies suggest the existence of compositional heterogeneity near the core mantle boundary

intraplate tectonics in asia a precise age for large scale miocene movement along the ailao shan red river shear zone china tertiary left lateral movement along the 1000 km long ailao shan red river shear zone appears to have played an important role in absorbing post collisional northward penetration of india into asia crustal strike slip shear along this zone caused the formation of a gneiss belt metamorphosed to amphibolite grade including anatectic melting both metamorphism and melting were induced by ductile deformation yielding the possibility to date the major tectonometamorphic event that shaped the ailao shan red river belt 17 u pb isotope analyses were performed on small size fractions of zircon monazite and xenotime extracted from two different leucogranitic layers the two samples are located about 50 km apart in the central segment of ailao shan in structurally well controlled settings where the melts crystallized within the strongly foliated gneisses during late stages of deformation all mineral u pb analyses had to be corrected for excess or deficit amounts of radiogenic 206 pb originating from initial 230 th disequilibrium in the 238 u decay series in monazite such disequilibrium 206 pb reaches 20 of total radiogenic 206 pb the corrected u pb ages of monazite and xenotime lie between 22 1 and 23 9 ma whereas zircon yields significantly discordant u pb ages between 30 5 and 33 9 ma pointing to precambrian material in the magma source region inherited components could also be detected in monazite the set of u pb data shows that monazite and xenotime formed simultaneously in both localities substantiating an early miocene age of 23 0 0 2ma for late kinematic crystallization of anatectic melts in the metamorphic belt of the ailao shan red river shear zone

evidence for a thick and localized ultra low shear velocity zone at the base of the mantle beneath the central pacific abstract prominent postcursors to s sdiff waves with delays as large as 26 s are observed in northern america for papua new guinea events these waves sample the northern side of the pacific large low shear velocity province revealed by global shear velocity vs tomographic models the emergence of the postcursors strongly depends on the epicenter to station azimuth indicating that the waveforms are in general strongly affected by 3 dimensional 3d heterogeneities we limit our focus to an azimuthal range around 60 measured clockwise from north at the epicenter where the records show a relatively small azimuthal variation suggesting a relatively small 3d effect there in this azimuthal range we attempt 2d structural modelling along the great circle plane towards stations in southern us first we use a 2d ray theory to search for a range of models which generate a postcursor to the main sdiff phase with a delay time consistent with the observations then for some typical models we calculated waveforms at periods down to 5 s using the spectral element method we obtained several models that provide synthetic waveforms in a fair agreement with the observations the result shows that two types of low vs regions are required to explain the data one is a broad and weak anomaly region with a vs reduction of 5 or so constituting a part of the pacific large low shear velocity province at the base of the mantle the other is a laterally localized strong anomaly region with a more than 25 reduction of vs within a thickness of at least 80 km and a width on the order of 500 km

indosinian nw trending shear zones within the truong son belt vietnam 40ar39ar triassic ages and cretaceous to cenozoic overprints abstract several nw trending ductile and partly mylonitic shear zones cross the truong son belt of central vietnam along the song ma and song ca valleys and north of the kontum block from da nang to aluoi and khe sanh high grade metamorphic rocks of amphibolite facies showing a retrograde evolution and consisting of ortho and paragneisses metavolcanics amphibolites marbles and quartzites are exposed along these structures they display a homogeneous deformation pattern characterized by a generally steeply dipping foliation and a near horizontal to gently plunging mineral stretching lineation indicating a strike slip tectonic regime of deformation along the southernmost fault zone at least various and consistent kinematic indicators including s c structures asymmetric tails of porphyroclasts prove that the strain and metamorphism have been generated by a phase of dominantly non coaxial deformation with a dextral sense of shear 40ar 39ar dating applied on the high grade metamorphic rocks minerals occurring along these zones provide plateau cooling ages closely around 245 ma establishing that this event took place in the lowermost triassic as an early phase of the indosinian orogeny the existence of indosinian movements in vietnam as they have been defined by previous authors during the early century is now accurately confirmed and this is the first insight in the occurrence of ductile strike slip tectonics of indosinian age along nw se fault zones well expressed in central vietnam is a cretaceous thermal and deformational overprint marked by epimetamorphism which took place between 90 and 120 ma as attested by low temperature degassing ages this cretaceous event is not found further north in the song ma zone where younger ages appear as a result of the influence of cenozoic shear movements along the red river fault which displays 20 30 ma ages on the northern flank of song ca in the bu khang phu hoat core complex comparable oligocene lower miocene ages 20 35 ma yielded by biotite and phlogopite reflect a strong overprinting and attest to a rapid uplift of the basement

seismic evidence of hyper stretched crust and mantle exhumation offshore vietnam abstract we study the evolution of the eocene recent phu khanh basin opened during the rifting of the south china sea scs this sub basin formed when continental crust ruptured along the east vietnam boundary fault evbf at the western edge of the scs using high quality long streamer seismic lines we interpret structures that highlight the different phases of the scs rifting and processes related to crustal boudinage extreme crustal thinning and mantle uplift that sometimes places sediments in contact with the moho discontinuity mark the central part of the basin the mantle is shallowest there and marks the final rupture of the continental crust during an intense phase of mantle upwelling there a low angle detachment fault separates several crustal blocks from the moho the cylindrical axis of the moho rise is roughly parallel to the trend of the south china sea propagator above the mantle the upper and lower crusts form large crustal boudins the network of normal faults is dense in the upper crust and occasionally propagates into the lower crust however the lower crust is missing at some places the seismic facies above the moho rise is poorly stratified and might have been affected by a certain degree of metamorphism at the apex of mantle uplift there are frequent indications of fluid circulations including volcanic edifices and gas escapes features three stages of extension are clearly identifiable with ages of the two youngest constrained by well calibration the first and oldest rift sequence is situated between the tilted pre rift basement and the oligocene horizons 32 ma the second is delimited by the oligocene to the mid miocene 15 5 ma horizons and the third is bound by the mid miocene and the upper miocene before 10 5 ma horizons these three rift episodes formed in at least two extension directions the first n s and the second nw se the distinct mid miocene 15 5 ma horizon is tilted and the above layers show a diverging reflection these are in turn sealed by an erosional unconformity before 10 5 ma although tectonic activity appears diachronous from north to south we suggest that cessation of rifting did not occur before 12 10 5 ma this differs from models derived from magnetic anomalies observed in the south china sea 15 5 to 20 ma

paleomagnetism of the earliest cretaceous to early late cretaceous sandstones khorat group northeast thailand implications for tectonic plate movement of the indochina block abstract a total of 400 samples 33 sites were collected from the earliest cretaceous to early late cretaceous sandstones of the khorat group in the indochina block for paleomagnetic study to unravel the tectonic evolution of the region the sites were adopted from 3 traverses located in the northern edge of the khorat plateau northeastern thailand results indicate that almost all the sandstones exhibit similar magnetic values with an average declination d 31 7 inclination i 30 3 59 7 190 9 k 54 4 and a95 3 7 at reference point 17 30 n and 103 30 e the calculated paleolatitude points are inferred to deviate from the present latitude point by 1 2 2 3 only the lowermost part of the cretaceous sandstones can pass a positive fold test at 95 confidence level the relationship between the virtual geomagnetic poles vgps of cretaceous rocks of the indochina plate in thailand and those of the south china plate advocate that there is a major displacement of indochina along the northwest trending red river and associated faults by about 950 150 km with a 16 0 17 0 clockwise rotation relative to the south china plate during earliest cretaceous times paleomagnetic results of the early late cretaceous indochina plate point to a 20 25 clockwise rotation relative to the present occurring since very late cretaceous 65 myrs early neogene times which may be due to the collision between india and asia

tertiary diachronic extrusion and deformation of western indochina structural and 40ar 39ar evidence from nw thailand the wang chao and three pagodas fault zones cut the western part of the indochina block and run parallel to the red river fault evidence of intense ductile left lateral shear is found in the lansang gneisses which form a 5 km wide elongated core along the wang chao fault zone dating by 40 ar 39 ar shows that such deformation probably terminated around 30 5 ma the wang chao and three pagodas faults offset the north striking lower mesozoic metamorphic and magmatic belt of northern thailand 40 ar 39 ar results suggest that this belt suffered rapid cooling in the tertiary probably around 23 ma these results imply that the extrusion of the southwestern part of indochina occurred in the upper eocene lower oligocene it probably induced rifting in some basins of the gulf of thailand and in the malay and mekong basins in the oligo miocene the continuing penetration of india into asia culminated with the extrusion of all of indochina along the ailao shan red river fault this occurred concurrently with the onset of e w extension more to the south plotting in a geographical reference frame the diachronic time spans of movement on left lateral faults east and southeast of tibet implies that the northward movement of the indian indenter successively initiated new strike slip faults located farther and farther north along its path

u pb dating of high temperature metamorphic episodes in the kon tum massif vietnam abstract in vietnam recent studies demonstrated that the kon tum massif was part of the indosinian mountain system together with the truong son belt one sector of the kon tum massif song ba valley is a granulite facies terrain for which ar ar and u pb ages cluster around 245 250 ma in the area of song bien river also metamorphosed under granulite facies conditions ar ar ages of 410 ma are displayed by biotite in this study u pb dating of monazite and zircon has been used to understand the reason for these two groups of ages taking into account the metamorphic mineral assemblages related to granulite facies metamorphism in the two districts for granulites of song ba valley ages of 245 250 ma are confirmed from monazites in the song bien district ages of 465 470 ma displayed by monazites are interpreted to correspond to the metamorphic climax zircons from one of these samples record the age of anatexis that has affected the granulitic rocks the younger age recorded in the song ba valley is considered to reflect high temperature related to charnockitic intrusions emplaced near the end of the indosinian orogeny overprinting the earlier formed granulites and resetting the u pb and ar ar ages for the first time two ht events are dated in the kon tum massif one occurring during the ordovician and the other during the permo triassic

tomographic inversion for three dimensional anisotropy of earth s inner core abstract seismological studies generally suggest that the earth s inner core is anisotropic and the anisotropic structure changes significantly both laterally and with depth previous body wave studies of the inner core have relied on ray tracing or waveform modeling using one dimensional 1d models here we present non linear tomographic inversions of the inner core anisotropy using three dimensional 3d ray tracing spline parameterization and a large collection of pkp differential travel times we adapt a pseudo bending ray tracing pbr method in spherical coordinates for seismic rays that traverse the inner core pkp df phase the method iteratively perturbs each discontinuity point and continuous segment of the ray through 3d earth structure so that its travel time is minimum the 3d anisotropic structure of the inner core is approximated to the first order as 3d heterogeneous but isotropic structure for a given ray the data are corrected using a scaled mantle tomographic model the inner core anisotropy model obtained has the following major features 1 the model has strong hemispherical and depth variation the isotropic velocity in the topmost inner core is greater in quasi eastern hemisphere qeh 40 160 e than in quasi western hemisphere qwh other longitudes the anisotropy is weak in qeh to the depth of 600 700 km below the inner core boundary icb while in qwh the anisotropy increases at much shallower depth about 100 200 km below the icb to about 3 4 then remains at about 2 4 throughout the rest of the inner core 2 the anisotropy form changes abruptly over a depth range of about 150 km at the radius of about 600 km slightly less than half of the inner core radius forming a distinct inner inner core iic the velocity in the iic has maximums at equatorial and polar directions and minimum at an angle of about 40 from the equatorial plane the velocity in the outer inner core oic however changes little for ray directions 0 40 from the equatorial plane 3 despite large variation of the anisotropy the isotropic velocity voigt average throughout the inner core is nearly uniform the results suggest that the oic is likely composed of the same type of iron crystals with uniform chemistry but the iic may be composed of a different type of crystal alignment a different iron phase or a different chemical composition our tests on model parameterization mantle correction and linear and non linear inversion suggest the main features of our model are very robust however fine scale structures are likely to differ particularly in the major transition zones e g in the topmost qwh isotropy to anisotropy between oic and iic change in the form of anisotropy and between qeh and qwh in oic difference in anisotropy strength searches for possible waveform complications from these boundaries need to be aware of the directional dependence and geographical variation to be successful

localized boundary layer below the mid pacific velocity anomaly identified from a pcp precursor dense record sections from deep earthquakes in fiji and argentina recorded on hundreds of short period stations in california at distances of 81 to 85 are used to investigate the detailed p wave velocity structure above the core mantle boundary cmb in the fiji data a secondary phase arriving 2 to 4 s after the direct p is identified as a precursor to pcp this phase provides good evidence for a reflection off the top of a thin low velocity layer above the cmb comparisons to synthetic seismograms indicate a layer thickness of 10 km and a velocity reduction of 5 10 compared to the overlying mantle a record section from an argentina event does not show the pcp precursor indicating that the low velocity layer is not a global feature this thin low velocity layer is in the same place as a much larger s wave velocity anomaly in the lower mantle and is probably indicative of a boundary layer just above the cmb under the mid pacific

various influences on plumes and dynamics in time dependent compressible mantle convection in 3 d spherical shell the effects on the dynamics of compressible mantle convection from increasing rayleigh number viscosity stratification and internal heating are investigated in a time dependent 3 d spherical shell model resolution of up to spherical harmonics of degree 192 and 150 higher order finite difference grid points unevenly distributed in the radial direction have been employed surface rayleigh number varies from 5 104 to o 107 the viscosity profiles used range from constant to a depth dependent viscosity profile the adams williamson adiabatic assumption is adopted for describing the equation of state both depth dependences of thermal expansivity and thermal conductivity are included we have focused on the viscosity stratification in the lower mantle the mean temperature profiles are very sensitive to the input parameters such as the amount of internal heating and the dissipation number in the isoviscous model the number of hot plumes decreases for low rayleigh number 5 104 and increases dramatically with higher rayleigh number 106 in the model with a high viscosity zone in the lower mantle the number of plumes is small and does not change too much when the rayleigh number varies from 3 106 to 8 106 in the purely base heated viscously stratified models well organized triple junctions of the downwelling sheets are developed near the surface and interconnected cold walls of downwellings are produced around upwelling plumes the presence of internal heating increases the number of upwelling plumes and breaks up the interconnection of downwelling sheets strong viscous heating in the compressible solutions is found near the top and bottom at the higher values of rayleigh number the laterally averaged value of viscous dissipation is comparable with the chondritic strength of internal heating whether the maximum viscous heating is found in upwelling plumes or downwelling flows depends on the presence of internal heating and the mechanical boundary conditions the effects of compressibility on local structures and plume morphologies can be significant however these compressible effects are moderate on the global patterns

late cenozoic and modern stress fields in the western fars iran implications for the tectonic and kinematic evolution of central zagros 1 the zagros iran developed during mio pliocene times in response to arabia eurasia convergence the western fars highlights a major bend of the deformation front and displays a remarkable set of nearly n s right lateral strike slip faults the kazerun borazjan karebass sabz pushan sarvestan faults oblique at high angle to the belt the region likely plays a major kinematic role by accommodating the change in shortening modes from partitioned in the western central zagros to nonpartitioned in the eastern zagros the inversion of focal mechanisms from small and moderate earthquakes shows a consistent n020 030 compression with a low ratio between differential stresses this regime accounts for the combination of strike slip and thrust type mechanisms through likely 2 3 permutations fault slip analysis reveals two successive late cenozoic regional compressional trends ne sw then n020 the latter is in good agreement with the present day stress the significance of the ne sw compression is discussed alternatively in terms of stress deviations or block rotations in relation to the strike slip fault system fieldwork and satellite imagery suggest that these faults behave first as transfer faults during folding of the cover and later as strike slip faults in agreement with the succession of stress regimes and the evolution of the dominant deformation style from thin skinned to thick skinned the first order stability of the collision related state of stress since 5 ma supports that the arabia eurasia convergence did not give rise to partitioning in the western fars but rather was and is still accommodated by distributed deformation involving both shortening and strike slip motion throughout the cover and the basement

reactivation of a collisional suture by miocene transpressional domes associated with the red river and song chay detachment faults northern vietnam abstract elongate miocene gneissose and granitic domes in northern vietnam formed in a dextral transpressional ductile shear regime possibly associated with large scale restraining step overs along dextral faults initial anticlinal d1 doming involved folding of both basement and hanging wall rocks with d1 secondary folds that verge toward the anticlinal axes such folds reflect dome scale flexural slip folding with continued shortening d2 detachment faults developed on the flanks of the anticlines along the hanging wall basement interface so that the basement was extruded vertically into the overlying hanging wall rocks the detachment faults were associated with d2 drag folds that verge away from the anticlinal axes the hanging wall assemblage lacks a well ordered stratigraphy displaying primarily block in matrix fabric we identified bedded cherts associated with umbers and alkalic basaltic intrusions within these hanging wall rocks a first report of such rocks from vietnam the association of cherts umbers and basaltic intrusions and extrusions with block in matrix units with clastic rocks strongly suggest that the hanging wall rocks comprise part of a subduction complex because the base of a subduction complex is a former subduction megathrust horizon the hanging wall basement interface represents a reactivated collisional suture such a suture was probably associated with the indosinian orogeny and the basement should be the indochina continental block this structure may have influenced the position of miocene dextral faulting in addition to controlling the position of the dome detachments the well known red river fault marks the boundary of one of the domes but in this region it appears to be a detachment normal fault rather than a dextral strike slip fault however the association with the dome evolution with large scale restraining step overs suggests that dextral faulting associated with dome development may lie further away from the dome axes than the detachment

horizontal transition from fast to slow structures at the core mantle boundary south atlantic recent tomographic studies of the deep mantle have revealed large scale low velocity zones beneath africa and relatively fast velocity zones beneath south america here we conduct a concentrated core mantle boundary cmb study across this transition zone from fast to slow beneath the south atlantic deep south american earthquakes recorded in africa provide s and scs waveform data at core grazing distances of 85 105 the waveform interference pattern produced at these ranges provides an excellent sample of the cmb structure in the vicinity of the scs bounce point which can be used to map out regions of low velocity zones 10 reductions the strongest anomalies occur at the western edge roughly beneath the tristat plateau with weaker structures to the east beneath the so called great africa plume these results agree with dynamic models where strong local upwellings develop at the edges of d fast structures

small scale lateral shear velocity and anisotropy heterogeneity near the core mantle boundary beneath the central pacific imaged using broadband scs waves core reflected scs waves from 49 large m 5 1 deep tonga fiji events recorded in western north america are used to study a localized region of the core mantle boundary cmb under the central pacific ocean a total of 248 observations from the berkeley digital seismic network bdsn caltech united states geological survey usgs terrascope and the incorporated research institutions for seismology iris broadband arrays span epicentral distances of 73 85 scs reflection points sample a cmb patch southeast of the hawaiian islands at latitude 4 to 16 n and longitude 156 to 144 w near the proposed source location of the hawaiian plume highly variable scs travel times amplitudes waveforms and shearwave splitting indicate that the lowermost mantle in this region is heterogeneous both laterally and radially scsh sh differential travel times are on average 4 s larger than predicted by the preliminary reference earth model prem this is due to delayed scs arrivals and is largely accounted for by a model with a strong velocity decrease in d the scsh sh residuals also show a spatial trend not accounted for by a radial model indicating a lateral decrease in lower mantle shear velocity to the northeast this lateral velocity gradient appears to cause focusing of scs energy scsh sh amplitude ratios are larger than predicted by prem or by a model with a strong negative gradient with depth in d which enhances scs scs splitting corrected for lithospheric anisotropy beneath the receivers indicates spatial variations in d anisotropy with a systematic change in the orientation of the fast polarization direction from transverse to the ray path fast scsh to parallel to the ray path fast scsv along a northeast traverse these spatial trends suggest lateral gradients in the boundary layer shear flow on scale lengths of a few hundred kilometers which may be related to dynamical flow near the near the root of the large hawaiian plume

the red river fault zone in the yinggehai basin south china sea abstract the red river fault a major strike slip fault of onshore southeast asia is suggested to be closely related to the formation of the south china sea and uplift of the tibetan plateau yet published documentation of the offshore segment of the red river fault is sparse high resolution seismic reflection and borehole data obtained by the petroleum industry along the northwestern margin of the south china sea permit mapping of the offshore segment of the red river fault in the yinggehai basin as well as demonstration of the timing of its shear movement and slip reversal two boundary faults and two basin center faults are mapped three successive deformation phases for the offshore red river fault zone include sinistral movement from 30 to16 ma slip reversal between 16 and 5 5 ma and dextral movement with low rates after 5 5 ma although no robust piecing points were observed for the sinistral movement period the horizontal displacement of dextral movement is estimated to be tens of kilometers this study supports a two stage model for the evolution of the india asia collisional orogen before the middle miocene sinistral movement of the red river fault likely was linked to spreading of the south china sea consistent with the concept of continental extrusion after the middle miocene distributed shortening better explains a low rate of dextral accommodation of the red river fault in response to the continuing india asia collision

pkp bc df travel time residuals and short scale heterogeneity in the deep earth differential pkp bc pkp df travel time residuals for quasi polar paths in the earth present large excursions from the trends predicted by simple models of inner core anisotropy we analyze a set of such paths classified according to specific source and station regions and show that observations are compatible with a constant anisotropy model of at most 1 5 in the outer 400 km of the inner core to explain local departures of up to 3 sec from the predictions of such a model strong lateral variations on scale lengths shorter than a few hundred km need to be invoked either in the core or in the mantle independent results of d modelling argue in favor of an important contribution from the latter

characterization of stress perturbations near major fault zones insights from 2 d distinct element numerical modelling and field studies jura mountains abstract we consider and discuss the presence of discontinuities in the crust as a major source of stress perturbations based on 2 d distinct element modelling we reconstruct the local stress field around a vertical discontinuity in various geological contexts the resulting stress distribution reveals that major directional stress changes occur near the tips of the discontinuity so that stress deviations can reach values as large as 50 we establish simple relationships controlling stress changes around a pre existing fault zone as a function of 1 the remote differential stress magnitude 1 3 2 the friction coefficient on the discontinuity and 3 the strike of the discontinuity relative to the far field stress as a geological example we present the morez fault zone in the internal jura paleostress reconstruction in forty two sites indicates that the trends of the mio pliocene compression are n110 on average near the fault whereas they are n130 in the surrounding areas a comparison between the results of the tectonic study and those of theoretical modelling suggests that the 20 counterclockwise deviation is directly related to the reactivation of this large weak zone we thus evaluate the role of mechanical decoupling along pre existing zones of weakness especially with consideration to the accommodation of the alpine deformation in the jura belt

shdiff svdiff splitting in an isotropic earth 1 the vertically polarized svdiff seismic phase sometimes arrives later than the horizontally polarized shdiff both phases propagate in the d layer at the base of the mantle and the difference in their arrival times is usually interpreted in terms of seismic anisotropy in d using numerical simulations we demonstrate that a significant 1 s delay of svdiff relative to shdiff is present even in the iasp91 isotropic reference earth model and in a model with a high s velocity in d relative to iasp91 the delay is accompanied by strong amplitude decay of svdiff with distance this relationship resembles the effect of anelasticity in a model with a low s velocity in d the waveform of svdiff can be very different from shdiff and this difference can be mistaken for a delay of shdiff relative to svdiff in a laterally heterogeneous isotropic d svdiff can also be delayed because unlike shdiff svdiff has strong amplitude decay along fast paths its traveltime is thus dominated by slow propagation paths and tends to be positively biased for d models with an s velocity difference of 3 between two hemispheres there are regions where shdiff arrives up to 15 s earlier than the first clearly visible svdiff this is observed in a corridor several hundred kilometers wide if these effects are misinterpreted in terms of d anisotropy in actual seismic data estimates of anisotropy are biased and may contribute to the idea of seismic anisotropy being different in high and low s velocity regions of d

traveltime sensitivity kernels for pkp phases in the mantle abstract we investigate the finite frequency effects of perturbations of compressional wave velocity on the traveltimes of pkp phases owing to their long paths in the mantle pkp phases have a large fresnel zone at the cmb and thus sample a large volume of the d layer we compute traveltime frechet derivatives of the three pkp phases in the mantle with respect to the ak135 reference model taking into account the coupling between the different branches pkp kernels do not have a simple banana doughnut shape because the different pkp branches interfere with each other and because of the presence of shadow zones and caustics in addition we compute the differential kernels ab bc bc df and ab df the geometry of pkp differential kernels in d show significant qualitative differences from those predicted by simple ray tracing this suggests that the tomographic techniques commonly used to image lower mantle structures could be improved by using 3d frechet kernels

constructing synthetics from deep earth tomographic models recent studies of deep mantle structure indicate strong heterogeneity to conduct high resolution waveform modelling of these structures we have developed a new method to construct 2 d synthetics directly from block style tomographic models unlike the wkbj approximation which utilizes rays overshooting and undershooting receivers our method wkm approximation uses rays that arrive at the receiver first the ray paths from the 1 d layered reference model are used to localize each ray segment where the anomalous velocities are applied by overlay as in tomography next new p i t i p i ray parameter ti traveltime are computed to satisfy snell s law along with their numerical derivative p t which is used to construct a synthetic seismogram similar to the wkbj method as a demonstration of the usefulness of this method we generated wkm synthetics for the d region of high velocities beneath central america based on grand s tomography model reasonable fits to broad band data are obtained by condensing his distributed anomalies into his lowermost mantle layer such a 2 d model predicts synthetics containing a laterally varying s cd triplication similar to observations

tectonic evolution of high grade metamorphic terranes in central vietnam constraints from large scale monazite geochronology abstract several metamorphic complexes in southeast asia have been interpreted as precambrian basement characterized by amphibolite to granulite facies metamorphism in this paper we re evaluate the timing of this thermal event based on the large scale geochronology and compositional variation of monazites from amphibolite to granulite facies metamorphic terranes in central vietnam most of the samples in this study are from metamorphic rocks n 38 and granitoids n 11 in the kontum massif gneisses n 6 and granitoids n 5 from the hai van migmatite complex and the truong son belt located to the north of the massif were also studied two distinct thermal episodes 245 230 ma and 460 430 ma affected kontum massif gneisses while a single dominant event at 240 220 ma is recorded in the gneisses from the hai van complex and the truong son belt monazites from granitoids commonly yield an age of 240 220 ma mesoproterozoic ages 1530 1340 ma were obtained only from monazite cores that are surrounded by c 440 ma overgrowths thermobarometric results combined with concentrations of y 2 o 3 ce 2 o 3 and heavy rare earth elements in monazite and recently reported pressure temperature paths suggest that triassic ages correspond to retrograde metamorphism following decompression from high to medium pressure temperature conditions ordovician silurian ages reflect low pressure temperature metamorphism accompanied by isobaric heating during prograde metamorphism some samples were affected by both metamorphic events we conclude that high grade metamorphism observed in so called precambrian basement terranes in central vietnam occurred during both the permian triassic and the ordovician silurian while peraluminous granitoid magmatism is triassic additionally our preliminary analyses for u pb zircon age and whole rock chemistry of granitic gneisses from the truong song belt suggests the presence of the ordovician silurian volcanic arc magmatism in the region based on the pressure temperature time protolith evolutions metamorphic rocks from central vietnam provide a continuous record of subduction accretion collision tectonics between the south china and indochina blocks in the ordovician silurian the region was characterized by active continental margin tectonics followed by continental collision during the late permian to early triassic and subsequent exhumation during the late triassic the results also suggest that the timing of metamorphism and protolith formation as well as the geochemical features in other southeast asian terranes should be verified to achieve a better understanding of the precambrian to early mesozoic tectonic history in asia

ridge like lower mantle structure beneath south africa recent scs s results from probing the deep structure beneath southern africa display strong delays of up to 10 s at distances beyond 90 such delays could be explained by long period tomographic models containing smooth weak features with the addition of rough strong d structure 3 9 drops in shear velocities however these structures cannot explain the sks s differentials sampling the same region to explain the sks s and scs s data sets simultaneously requires instead a large scale ridge like structure with a relatively uniform 3 reduction in shear velocity the structure is about 1000 km wide and extends at least 1200 km above d it is orientated roughly nw se and leans toward the east at latitudes from 15 to about 30 it proves difficult to explain such sharp features with thermal effects alone and thus the importance of high resolution waveform modeling to establish their existence to derive the above results we developed a special algorithm by matching simulated synthetics to observed broadband waveforms this is achieved by computing the various arrivals separately using generalized ray theory for a reference model and allowing the arrivals to shift in relative times to match the data tomographic models can then be constructed or existing tomographic models can be altered to match these data and new 2 d synthetics can be constructed as well to better fit the waveform data these updated synthetics can again be decomposed and reassembled and the process can be repeated this algorithm is applied to a combination of analog and digital data along a corridor from south america producing the high resolution 2 d model described above

interpreting d seismic structure using synthetic waveforms computed from dynamic models we have formulated dynamic models of processes hypothesized to give rise to observed seismological structure at the base of the mantle one dimensional seismic models are determined from thermo chemical convection calculations from which synthetic waveforms are computed of the three scenarios studied 1 a thermal slab 2 a thermal slab interacting with simple chemical layer and 3 a thermal slab interacting with a phase transition the strength of seismic phases computed from 3 best correlates with the global geography of the observed d triplication seismic structures predicted by 1 or 2 are inconsistent with the global geography and strength of the triplication our successful model motivates a new class of 1 d seismic models with two mild gradients and a small discontinuity to test this hypothesis we searched for seismic models that both contain these features and fit d seismic observations our preferred model contains a gentle positive gradient initiated 350 km above the cmb and a 1 jump in s velocity near 200 km a strong negative gradient begins about 100 km above the cmb similar to previous studies representing the lower thermal boundary layer with the reduced first order discontinuity the p wave triplication becomes too small to observe and provides a simple explanation for the observed weakness of the p in proportion to the s triplication

the basins of sundaland se asia evolution and boundary conditions abstract most of the basins developed in the continental core of se asia sundaland evolved since the late cretaceous in a manner that may be correlated to the conditions of the subduction in the sunda trench by the end of mesozoic times sundaland was an elevated area composed of granite and metamorphic basement on the rims which suffered collapse and incipient extension whereas the central part was stable this promontory was surrounded by a large subduction zone except in the north and was a free boundary in the early cenozoic starting from the palaeogene and following fractures initiated during the india eurasia collision rifting began along large faults mostly n s and nnw sse strike slip which crosscut the whole region the basins remained in a continental fluvio lacustrine or shallow marine environment for a long time and some are marked by extremely stretched crust phu khanh natuna n makassar or even reached the ocean floor spreading stage celebes flores western sundaland was a combination of basin opening and strike slip transpressional deformation the configuration suggests a free boundary particularly to the east trench pull associated with the proto south china sea subduction java sulawesi trench subduction rollback in the early miocene australian blocks reached the sunda subduction zone and imposed local shortening in the south and southeast whereas the western part was free from compression after the indian continent had moved away to the north this suggests an important coupling of the sunda plate with the indo australian plate both to se and nw possibly further west rollback had ceased in the java sumatra subduction zone and compressional stress was being transferred northwards across the plate boundary the internal compression is expressed to the south by shortening which is transmitted as far as the malay basin in the late miocene most of the sunda plate was under compression except the tectonically isolated andaman sea and the damar basins in the pliocene collision north of australia propagated toward the north and west causing subduction reversal and compression in the short lived damar basin docking of the philippine plate confined the eastern side of sundaland and created local compression and uplift such as in nw borneo palawan and taiwan transpressional deformation created extensive folding strike slip faulting and uplift of the central basin and arakan yoma in myanmar minor inversion affected many thailand rift basins all the other basins record subsidence the uplift is responsible for gravity tectonics where thick sediments were accumulated sarawak ne luconia bangladesh wedge

a very slow basal layer underlying large scale low velocity anomalies in the lower mantle beneath the pacific evidence from core phases a multi phase analysis using long period world wide standardized seismograph network and canadian network data has been conducted using core phases for deep focus events from the southwest pacific these include sks s2ks svd iff and sp dks the last phase emerges from sks near 106 and is associated with a p wave diffracting along the bottom of the mantle patterns in s2ks sks differential travel times t s2ks sks correlate with those in sp dks sks t spdks sks t s2ks sks values strongly depend on variations in v s structure in the lower third of the mantle whereas t spdks sks values mainly depend on v p structure and variations in a thin zone 100 km or less at the very base of the mantle anomalously large t s2ks sks and t spdks sks values relative to the preliminary reference earth model prem are present for fiji tonga and kermadec events recorded in north and south america along with anomalously large sv diff amplitudes well into the core s shadow more northerly paths beneath the pacific to north america for indonesian and solomon events display both prem like and anomalous times a model compatible with the observations is presented and contains a thin very low velocity layer at the base of the mantle that underlies the large volumetric lower mantle low velocity regions in the southwest pacific a low velocity layer of 20 100 km thickness with reductions of up to 5 10 relative to prem can reproduce t spdks sks as well as sv diff amplitudes large scale more than 1000 km lower mantle v s heterogeneity 2 4 can explain long wavelength trends in t s2ks sks the exact thickness and velocity reduction in the basal layer is uncertain owing to difficulties in resolving whether anomalous structure occurs on the source and or receiver side of wavepaths at the cmb

denudation history of onshore central vietnam constraints on the cenozoic evolution of the western margin of the south china sea abstract apatite fission track analysis is used to monitor the timing and rate of upper crustal cooling and denudation across central vietnam in response to rifting of the south china sea results show regional denudation in the syn and post rift phases of the south china sea occurred at a similar rate 40 10 m myr consistent with the sedimentation record in adjacent offshore basins this suggests that extension was not the dominant factor controlling cenozoic denudation probably because the study area was sufficiently distant from the actively rifted continental margin a significant change in regional cooling rate during the upper miocene inferred denudation rates also shift from 34 m myr to 390 500 m myr is linked to enhanced erosion and deposition of prograding sediments in adjacent offshore basins this shift in denudation is associated and contemporaneous with initiation of regional basaltic magmatism and development of topography

lower mantle tomography and phase change mapping a lower mantle s wave triplication scd has been recognized for many years and appears to be explained by the recently discovered perovskite pv to postperovskite ppv phase change seismic observations of scd display 1 rapid changes in strength and timing relative to s and scs and 2 early arrivals beneath fast lower mantle regions while the latter feature can be explained by a clapeyron slope of 6 mpa k and a velocity jump of 1 5 when corrected by tomographic predictions it does not explain the first feature here we expand on this mapping approach by attempting a new parameterization that requires a sample of d near the scs bounce point vs where the phase height hph and velocity jump are functions of vs these parameters are determined by modeling dense record sections collected from usarray and passcal data where grand s tomographic model is the most detailed in d structure beneath central america we also address the range of to generate new global models of the phase boundary and associated temperature variation we conclude that a near 9 mpa k is most satisfactory but requires to be nonuniform with a range from about 1 0 to 4 0 with some slow region samples requiring the largest values moreover the edges of the supposed buckled slabs delimitated by both p and s waves display very rapid changes in phase boundary heights producing scd multipathing these features can explain the unstable nature of the scd phase with easy detection to no detection commonly observed the fine structure at the base of the mantle beneath these edges contains particularly strong reflections indicative of local ultralow velocity zones which are predicted in some dynamic models

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

exhumation history of the red river shear zone in northern vietnam new insights from zircon and apatite fission track analysis abstract a new set of zircon and apatite fission track ages from the ailao shan and day nui con voi dncv metamorphic massifs of the red river shear zone rrsz and neighboring rocks in northern vietnam is presented a complex along strike diachronous denudation history is revealed the southern sector of the dncv cooled to about 100 c by the late oligocene whereas its central compartment was affected by the later thermotectonic evolution of the song chay dome to the e of the rrsz whose final exhumation occurred during the early miocene the northern sector of the rrsz is characterized by the 35 ma phan si pang pre to synkinematic intrusion fission track ages from a vertical section within the phan si pang granite indicate rapid exhumation and cooling the paleozoic tectonic block to the west of the rrsz fission track ages between 40 and 30 ma was exhumed and cooled earlier than the fault mylonite belt fission track ages of 30 ma and younger and also than the eastern block its structural level is consistent with field observations that suggest the rrsz in northern vietnam to be a transtensional system with a regional ne sw oriented extension component

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

tectonic analysis of fault slip data sets using data that include the direction and the sense of motion on individual fault surfaces determined by slickenside lineations it is possible to reconstruct reduced stress tensors that correspond to the orientation of stress axes and to the ratio 2 3 1 3 between principal stress values 1 2 3 compression being positive no assumption is made concerning the orientation of fault planes relative to stress axes so that reactived faults are taken into account as well as newly created ones qualitative and quantitative methods for analysis of fault slip data were developed during the last 10 years the practical limitation of the methods and the necessity for critical field observations are emphasized these methods can be applied to focal mechanisms of earthquakes a more complex analysis of heterogeneous data sets involving an iterative separation of different stress systems is also discussed this analysis enables one to distinguish successive faulting events careful qualitative study in the field is in all cases essential

some dynamical consequences of partial melting in earth s deep mantle we use regional scale numerical models of mantle convection to investigate the simple hypothesis that seismically anomalous thin patches of earth s lowermost mantle termed ultralow velocity zones or ulvz are derived from partial melting of ordinary mantle the models span the lower 500 km of earth s mantle employ both temperature and melting related contributions to buoyancy and include a cold randomly moving downwelling introduced from above to maintain a thermal boundary layer partial melting of ordinary mantle introduces a ubiquitous partially molten layer above an isothermal core mantle boundary as a consequence of its isothermal and isobaric conditions although it naturally develops variations in thickness greater than two orders of magnitude with the thickest portions occurring at the base of upwelling plumes and a thin layer elsewhere we find that only a dense partially molten mixture produces partial melt distributions that are compatible with seismic observations of ulvz however if such a melt percolates downward a dense basal liquid layer accumulates above the core mantle boundary the apparent requirement of a volumetrically dense and non percolating melt phase in the lowermost mantle presents serious problems for the hypothesis that ulvz arise from melting of ordinary mantle and suggests that such features likely form as a consequence of more complex processes furthermore these considerations suggest that the solidus of ordinary mantle is a reasonable upper bound on the present day temperature of the cmb 2007 published by elsevier b v

the effect of d on pkp ab df travel time residuals and possible implications for inner core structure abstract the hypothesis that the deep inner core is anisotropic is based on pkp travel time observations at large distances and relies on a small number of very anomalous measurements for paths quasi parallel to the earth s rotation axis here we analyze a global dataset of pkp ab df travel times residuals and discuss their significant dispersion 2 s and coherent large scale patterns we show that the trends observed for quasi equatorial paths are consistent with predictions from recent tomographic mantle models when the latter are modified to account for strong heterogeneity at the base of the mantle under the pacific ocean and africa as documented in several recent studies likewise for polar paths we show that a large part of the signal could be explained by deep mantle structure the effects of complex structure in the deep mantle on pkp ab df travel times should be carefully considered in order to reliably estimate the anisotropic structure of the central part of the inner core

oligocene miocene bu khang extensional gneiss dome in vietnam geodynamic implications a large oligocene miocene extensional gneiss dome in vietnam has implications for the kinematics of the ailao shan red river shear zone and rifting of the south china sea a major northeast dipping extensional shear zone separates the bu khang dome from less metamorphosed units above shearing deformation occurred during the exhumation of high pressure rocks 40 ar 39 ar ages on micas reveal oligocene miocene ages from 36 to 21 ma after a high pressure episode of probable oligocene age the area was subjected to northeast southwest extension during the opening of the south china sea a kinematic model involving left lateral shear and block rotation within a north south dextral shear zone integrates our observations in the bu khang dome with south china sea rifting

mapping sea ice types from sentinel 1 considering the surface type dependent effect of incidence angle automated classification of sea ice types in synthetic aperture radar sar imagery is complicated by the class dependent decrease of backscatter intensity with incidence angle ia in the log domain this decrease is approximately linear over the typical range of space borne sar instruments a global correction does not consider that different surface types show different rates of decrease in backscatter intensity here we introduce a supervised classification algorithm that directly incorporates the surface type dependent effect of ia we replace the constant mean vector of a gaussian probability density function in a bayesian classifier with a linearly variable mean during training the classifier first retrieves the slope and intercept of the linear function describing the mean value and then calculates the covariance matrix as the mean squared deviation relative to this function the ia dependence is no longer treated as an image property but as a class property based on training and validation data selected from overlapping sar and optical images we evaluate the proposed method in several case studies and compare to other classification algorithms for which a global ia correction is applied during pre processing our results show that the inclusion of the per class ia sensitivity can significantly improve the performance of the classifier

pkp bc versus pkp df differential travel times and aspherical structure in the earth s inner core pkp bc versus pkp df differential travel times show coherent patterns of residuals indicating aspherical structure within the earth s inner core using a waveform correlation method we measure 318 differential pkp bc versus pkp df travel times from 5 years of global digital seismograph network gdsn short period data at ranges between 145 and 155 in addition we obtain 19 470 probable bc versus df travel times at these ranges by searching through 23 years of the international seismological centre isc catalog plots of residuals versus turning point location and residuals versus ray direction show clear patterns which indicate that the differential travel times are highly correlated along similar ray paths this pattern is robust with respect to range and source depth these anomalies are almost certainly due to aspherical inner core structure since pkp bc and pkp df have nearly coincident ray paths in the mantle and outer core the gdsn and isc residual patterns are in general agreement but the isc residuals underpredict the gdsn residuals by about a factor of 2 the isc data have better spatial coverage particularly for n s ray paths within the inner core these n s ray paths show positive residuals of about 0 5 s indicating faster inner core p velocities in this direction a result consistent with previous studies of absolute pkp df travel times but derived from an independent data set due to the sparse ray coverage of our data either heterogeneity or anisotropy within the inner core with about 1 velocity variations could explain these residual patterns however inversions of the isc data for heterogeneity require many more free parameters to achieve the same variance reduction as a simple two parameter anisotropy model

crustal anatexis and formation of two types of granitic magmas in the kontum massif central vietnam implications for magma processes in collision zones abstract asia grew in the late permian by the collision of a number of micro continents syn to post collisional magmatism occurred along the continental collision zones in this study we report two types of granitic rocks garnet granite grt granite and orthopyroxene granite opx granite from the kontum massif central vietnam which is situated on the continental collision zone between the south china and indochina cratons these granitic rocks were formed at ca 250 ma when high temperature ht and ultrahigh temperature uht metamorphism took place in the same zone based on the petrological and geochemical features compared with previously reported experimental results garnet bearing granite is derived from pelitic gneisses by partial melting whereas orthopyroxene bearing granite is produced by the partial melting of garnet bearing mafic granulites we inferred that a significantly high geothermal gradient is required to produce vietnamese granitic magmatism and related ht to uht metamorphism this geotherm may be attributed to upwelling mantle plume beneath the kontum massif during the late permian

high temperature metamorphism in a major strike slip shear zone the ailao shan red river people s republic of china abstract petrographic and thermobarometric analysis provides constraints on thept path of the mylonitic gneisses of the left lateral ailao shan red river shear zone which has accommodated the lateral extrusion of indochina during the tertiary two different paragenesis p1 and p2 are coeval with this deformation and correspond respectively to the amphibolite and greenschist facies microprobe analysis reveals that p1 garnets bear a chemical zonation from core to rim this zonation indicates a temperature increase during garnet growth conditions of formation of garnet rims p1b which are estimated using biotite garnet and plagioclase garnet thermobarometers are close to the granitic solidus 710 70 c and4 5 1 5 kbar p2 conditions are estimated to be approximately 500 c and

jurassic dextral movement along the dien bien phu fault nw vietnam constraints from 40ar 39ar geochronology abstractthe nne ssw trending dien bien phu fault which dextrally displaces the nw se trending song ma suture in northwestern vietnam is widely considered one of the most seismically active faults in indochina in order to better understand the fault s activity this study reports new 40ar 39ar geochronological data for mica schists and mylonites from several areas along the dien bien phu fault showing 40ar 39ar ages of 194 212 ma for mica schists and 158 198 ma for mylonites these ages suggest that after the collision of the south china and indochina blocks dextral shearing deformation initiated along the dien bien phu fault in the jurassic significantly earlier than previously thought in light of the relevant tectonic events in indochina the successive suturing collision of the indochina simao and sibumasu blocks may have been responsible for the initiation of dextral shearing along the dien bien phu fault which in turn resulted in offset of the song ma suture

new tectono geochronological constraints on timing of shearing along the ailao shan red river shear zone implications for genesis of ailao shan gold mineralization abstract several world class gold deposits are located along the ailao shan als belt in eastern tibet china the genesis of gold mineralization along the belt however has been the subject of debates in the last decades which highlights the importance of dating shearing magmatism and mineralization along the ailao shan red river shear zone asrr through detailed field observations and microscopic analysis a group of leucocratic intrusions from within and outside the shear zone along the als belt are investigated in the present paper pre syn and post shearing intrusions are grouped based on structural and microstructural analysis la icp ms and sims dating of the intrusions revealed the existence of two age populations a group of ages older than 30 ma and the other younger than 28 ma the former are distributed both within and outside the shear zone and the latter in contrast occur only within the shear zone our new results show that the ductile shearing along the asrr shear zone initiated since ca 30 ma ago the dating results place major constraints on timing of shearing along the asrr shear zone and have profound implications on the genesis of gold mineralization along the als belt the present study reveals that ductile shearing along the asrr shear zone was resulted from extrusion of the indochina block late during the indian eurasian plate collision meanwhile we conclude that the gold mineralization took place prior to the shearing but in close relation to an early magmatism 30 ma ascribed to post collisional extension collapse involving mantle processes early during the plate collision

slabs in the lower mantle and their modulation of plume formation numerical mantle convection models indicate that subducting slabs can reach the core mantle boundary cmb for a wide range of assumed material properties and plate tectonic histories an increase in lower mantle viscosity a phase transition at 660 km depth depth dependent thermal expansivity and depth dependent thermal diffusivity do not preclude model slabs from reaching the cmb we find that ancient slabs could be associated with lateral temperature anomalies 500 c cooler than ambient mantle plausible increases of thermal conductivity with depth will not cause slabs to diffuse away regional spherical models with actual plate evolutionary models show that slabs are unlikely to be continuous from the upper mantle to the cmb even for radially simple mantle structures the observation from tomography showing only a few continuous slab like features from the surface to the cmb may be a result of complex plate kinematics not mantle layering there are important consequences of deeply penetrating slabs our models show that plumes preferentially develop on the edge of slabs in areas on the cmb free of slabs plume formation and eruption are expected to be frequent while the basal thermal boundary layer would be thin however in areas beneath slabs the basal thermal boundary layer would be thicker and plume formation infrequent beneath slabs a substantial amount of hot mantle can be trapped over long periods of time leading to mega plume formation we predict that patches of low seismic velocity may be found beneath large scale high seismic velocity structures at the core mantle boundary we find that the location buoyancy and geochemistry of mega plumes will differ from those plumes forming at the edge of slabs various geophysical and geochemical implications of this finding are discussed

influence of thermochemical piles on topography at earth s core mantle boundary abstract numerous seismic studies reveal the presence of two large low velocity anomalies beneath africa and the central pacific efforts to characterize these anomalies have yielded a variety of interpretations over the years both isochemical and thermochemical previous interpretations have included large isochemical superplumes clusters of smaller thermal plumes and doming thermochemical superplumes a conceptual mantle model that is presently growing favor involves long lived thermochemical piles in anticipation that nutation studies will provide better topographic constraints on the core mantle boundary cmb in the future we examine the effects of thermochemical piles at this boundary in this study we perform numerical modeling of thermochemical and isochemical convection as a function of convective vigor and temperature dependent viscosity to predict the topography at earth s cmb our results show that in isochemical convection downwellings always lead to negative depressed cmb topography while upwellings cause positive elevated topography consistent with previous studies however in thermochemical convection cmb topography and its relationship to downwellings and thermochemical piles are significantly affected by temperature dependent viscosity for isoviscous or weakly temperature dependent viscosity the piles cause negative cmb topography while positive topography is often below cold downwellings however for realistic more strongly temperature dependent viscosity the most negative cmb topography is below downwellings while the topography below the piles is often slightly more positive and or flat similar to upwellings in isochemical models we also show that although thermochemical piles are intrinsically more dense the large thermal buoyancy associated with them leads to overall relative buoyancy that is on par with slabs consequently thermochemical models lead to an overall reduction in magnitude of cmb topography with respect to isochemical models

a plea for more caution in fault slip analysis abstract the analysis of fault slip data is a widely used tool in structural geology this method consists of three major steps a data acquisition by measurements in outcrops b data separation into homogeneous subsets and calculation of the principal stress or strain axes with one of several available methods and c data interpretation in a regional tectonic context which includes the assignment of ages and the derivation of regional patterns each one of these three steps may introduce artefacts and misinterpretations the major problem during step a is sampling bias due to unfavourable outcrop geometry or due to neglecting faults with subtle slip sense indicators step b contains numerous possibilities to influence the calculation results e g by arbitrary separation of data into subsets choice of the analysis method and uncritical use of software age assignment in step c can in most cases be made only indirectly e g by the assumption that measured outcrop scale faults have the same age as parallel map scale faults or by the inference that subsets from different sites with parallel oriented principal axes are syngenetic neither assumption is necessarily valid with this paper we want to sharpen researchers awareness of the problems related to fault slip analysis

a strongly negative shear velocity gradient and lateral variability in the lowermost mantle beneath the pacific a new approach for constraining the seismic shear velocity structure above the core mantle boundary is introduced whereby sh sks differential travel times amplitude ratios of sv sks and sdiff waveshapes are simultaneously modeled this procedure is applied to the lower mantle beneath the central pacific using da ta from numerous deep focus southwest pacific earthquakes recorded in north america we analyze 90 broadband and 248 digitized analog recordings for this source receiver geometry sh sks times are highly variable and up to 10 s larger than standard reference model predictions indicating the presence of laterally varying low shear velocities in the study area the travel times however do not constrain the depth extent or velocity gradient of the low velocity region sv sks amplitude ratios and sh waveforms are sensitive to the radial shear velocity profile and when analyzed simultaneously with sh sks times rnveal up to 3 shear velocity reductions restricted to the lowermost 190 50 km of the mantle our preferred model for the central eastern pacific region ml has a strong negative gradient with 0 5 reduction in velocity relative to the preliminary reference earth model prem at 2700 km depth and 3 reduction at 2891 km depth and slight velocity reductions from 2000 to 2700 km depth 0 0 5 lower than prem significant small scale 100 500 km shear velocity heterogeneity 0 5 1 is required to explain scatter in the differential times and amplitude ratios