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

mantle deformation and tectonics constraints from seismic anisotropy in the western united states abstract we have examined shear wave splitting in teleseismic shear waves sks skks s from 15 stations in the western united states based on analysis of 123 records from 67 events the varied past and present tectonic styles in this region subduction transform faulting extension and stable domains are expected to cause a wide variety of anisotropic behavior and therefore make it an excellent natural laboratory fast polarization azimuths vary from e w to nearly n s and time delays t range from being barely detectable less than 0 6 s to 1 6 s most stations yielded consistent measurements independent of station event geometry the exceptions were stations situated very close to the san andreas fault which yielded well constrained but inconsistent splitting parameters these have been successfully modeled by two anisotropic layers with different horizontal symmetry axes the upper layer has a fast direction parallel to the fault and t of about 1 s the lower layer with t from 0 6 to 0 9 s is oriented e w near the san francisco bay area and ne sw in the mojave desert other measurements of e w fast are observed as far east as western nevada with large delay times of 1 3 1 5 s but disappear to the north stations in the northwestern basin and range have values of oriented at about 70 with delay times ranging from 0 7 to 1 2 s varies in other regions of the basin and range from 70 in eastern nevada to 20 in the transition zone between the colorado plateau and the basin and range with t of 1 0 s measurements are geographically coherent and in many cases changes correspond to geologic or tectonic boundaries the e w fast feature extends across several tectonic provinces but disappears near the southern boundary of the gorda plate and may therefore be related to the dynamics of plate interactions regional variations in the fast directions of anisotropy within the basin and range suggest that the present extension has not been strong enough to align the fast directions over a great depth extent fast polarization directions within the transition zone between the basin and range and colorado plateau are nearly parallel to the geologic and geophysical boundaries between the two regions and to other geophysical boundaries the size of the delay times suggests that in many of the areas studied either the mantle is almost twice as anisotropic as we estimate or the anisotropy and therefore mantle deformation extends into the sublithospheric mantle we used the two layer method to examine the effect of a postulated lower layer with fast direction parallel to the absolute motion of the north american plate and found the best estimate of contribution from a uniform layer is 0 1 s of t although a contribution of up to 0 7 s cannot be ruled out a value as low as 0 1 s suggests either that olivine is not strongly aligned in a horizontal direction in this layer or that the layer thickness is of the order of 10 km

laboratory and seismological observations of lower mantle isotropy we have carried out seismological and mineral physics investigations to identify the extent and origin of elastic anisotropy in the lower mantle based on observations of shear wave splitting we conclude that the lower mantle is effectively isotropic this result is surprising since the lower mantle is composed of elastically anisotropic minerals silicate perovskite mgo sio 2 that have been extensively strained by geologic processes to reconcile the seismic observations we have measured texture development during deformation and high temperature recrystallization of mantle silicates we show that these experiments provide a direct explanation for the shear wave splitting measurements the lower mantle appears isotropic because silicate perovskite maintains an isotropic texture during deformation and recrystallization

oxygen isotope compositions of phosphate from arvicoline teeth and quaternary climatic changes gigny french jura oxygen isotope compositions of biogenic phosphates from mammals are widely used as proxies of the isotopic compositions of meteoric waters that are roughly linearly related to the air temperature at high and mid latitudes an oxygen isotope fractionation equation was determined by using present day european arvicoline rodents tooth phosphate d 18 op 20 98 f0 59 0 572 f0 065 d 18 ow this fractionation equation was applied to the late pleistocene karstic sequence of gigny french jura comparison between the oxygen isotope compositions of arvicoline tooth phosphate and greenland ice core records suggests to reconsider the previously established hypothetical chronology of the sequence according to the d 18 o value of meteoric water mean air temperature relationships the d 18 o value of arvicoline teeth records variations in mean air temperatures that range from 0j to 15jc

shear wave splitting beneath southern korea and its tectonic implication abstract seismic anisotropy beneath the southern korean peninsula skp was investigated based on the scs waves of deep focused earthquakes recorded at 35 broadband stations the most striking feature of the observed splitting patterns is the preference of the fast directions nw se that are nearly parallel to the direction of the absolute plate motion in the region however the splitting patterns over the region show significant variation in splitting parameters indicating a complex anisotropic structure variations of the splitting directions and dissimilarity in the source domains of basaltic volcanisms suggest that the asthenospheric mantle flow since at least the late cenozoic cannot explain the seismic anisotropy beneath the region comparison to shear wave splitting measurements from eastern china revealed that the nw se fast direction of splitting measurements in the skp is close to that in the north china block ncb while the ne sw fast direction might be related to that in the south china block scb the shallow mantle lithosphere beneath the skp retains the fossil anisotropy amalgamated prior to the late paleozoic before the collision between the ncb and scb and the anisotropic structure was not completely realigned by the major orogenic events during the late paleozoic to mesozoic eras

current plate velocities relative to the hotspots incorporating the nuvel 1 global plate motion model nuvel 1 is a new global model of current relative plate velocities demets et al 1990 which differ significantly from those of prior models here we incorporate nuvel 1 into hs2 nuvel1 a new global model of plate velocities relative to the hotspots hs2 nuvel1 was determined from the hotspot data and errors used by minster and jordan 1978 to determine am1 2 which is their model of plate velocities relative to the hotspots am1 2 is consistent with minster and jordan s relative plate velocity model rm2 here we compare hs2 nuvel1 with am1 2 and examine how their differences relate to differences between nuvel 1 and rm2 hs2 nuvel1 plate velocities relative to the hotspots are mainly similar to those of am1 2 minor differences between the two models include the following 1 in hs2 nuvel1 the speed of the partly continental apparently non subducting indian plate is greater than that of the purely oceanic subducting nazca plate 2 in places the direction of motion of the african antarctic arabian australian caribbean cocos eurasian north american and south american plates differs between models by more than 10 3 in places the speed of the australian caribbean cocos indian and nazca plates differs between models by more than 8 mm yr although 27 of the 30 rm2 euler vectors differ with 95 confidence from those of nuvel 1 only the am1 2 arabia hotspot and india hotspot euler vectors differ with 95 confidence from those of hs2 nuvel1 thus substituting nuvel 1 for rm2 in the inversion for plate velocities relative to the hotspots changes few euler vectors significantly presumably because the uncertainty in the velocity of a plate relative to the hotspots is much greater than the uncertainty in its velocity relative to other plates

shear wave polarization anisotropy in the upper mantle from a deep earthquake abstract shear wave polarization anisotropy in the upper mantle is investigated at the spatially dense seismic stations of the national research institute for earth science and disaster prevention located in central japan by the use of the scs wave of a deep earthquake which occurred beneath sakhalin island on 12 may 1990 a characteristic pattern is found small and large shear wave anisotropy values are observed at seismic stations located on the east and west sides of the volcanic front respectively

shear wave splitting and shear wave splitting tomography of the southern puna plateau fil calixto frank j university of missouri columbia department of geological sciences estados unidos

interpretation of sks waves using samples from the subcontinental lithosphere abstract the seismic properties of five south african kimberlite nodules have been calculated from petrofabric measurements these garnet lherzolite and harzburgite composition nodules 56 80 olivine are known from previous studies to have originated at depths of 120 170 km in the subcontinental lithosphere their in situ seismic properties have been calculated by extrapolating the elastic constants to the appropriate temperature 900 1050 c and pressure 3 0 3 5 gpa conditions the average of the five samples has a maximum s wave anisotropy v s of 3 7 which is sufficiently high to explain previously reported teleseismic sks delay times t in continental shield areas of up to 1 7 s the biggest delay time is observed when the foliation is vertical the lineation horizontal and the fastest s wave polarized parallel to the lineation i e an orientation for transcurrent motion delay times of over 1 s can only be generated by this orientation a strong mantle crust mechanical coupling is suggested for such situations the potential use of seismic anisotropy as an indicator of strain in the lithosphere has been investigated using experimental and simulated fabric data for olivine the most abundant phase 70 in the upper mantle the v p and v s seismic anisotropy increases with fabric strength and finite strain recrystallization tends to reduce fabric strength and seismic anisotropy resulting in saturation values for experimental and simulated fabrics which correspond to approximately 50 60 strain or 8 9 v s for pure olivine aggregates a survey of 25 naturally deformed peridotites of oceanic origin suggests an average maximum s wave anisotropy for the olivine component of 9 or about 8 for a lherzolite or harzburgite rock composition when the orthopyroxene component is taken into account the ophiolite samples are twice as anisotropic as the kimberlite nodules if an average anisotropy value is representative of the lithosphere then the sks delay times represent variations in anisotropic layer thickness for delay times over 1 s as the orientation is constrained to be foliation vertical and lineation horizontal the magnitude of s wave anisotropy is less sensitive than v p to variations in olivine volume fraction in range 50 100 and to deformation or fabric intensity further suggesting that s wave anisotropy is particularly apt for the determination of the anisotropic lithosphere thickness for smaller delay times there is some trade off between structural orientation and thickness

seismic anisotropy in the fore arc region of the hikurangi subduction zone new zealand abstract shear waves from local earthquakes recorded on an array of nine broad band seismograph stations have been used to investigate the anisotropic structure within the crust of the overlying australian plate and within and below the subducting slab underlying the southern north island new zealand earthquakes near the plate interface were used to image the crustal anisotropy whereas deeper down dip subduction zone events were used to sample the subducting slab and the mantle below apart from a small amount of variable near surface anisotropy top few kilometres of the crust the measured fast shear wave directions of both the crust of the overlying plate 51 18 and the mantle 41 15 are similar and are nearly parallel to the strike of the subduction zone and the dominant geological features of the region sks measurements which mainly sample the mantle beneath the subducting plate give a value of 28 11 and measurements made elsewhere in the new zealand region using sks and scs2 produce similar results the measurements of crustal anisotropy are similar to those found further south on the wellington peninsula and confirm the presence of near surface anisotropy the maximum delay times for crustal earthquakes indicate a pervasive shear wave velocity anisotropy of 4 in the crust of the overlying plate and the increase in the delay times with depth for the subduction zone earthquakes translates to a shear wave velocity anisotropy of 1 4 in the mantle below the subducting slab these results when combined with the sks measurements suggest trench parallel flow in the mantle below the subducted slab and a coherence of the deformation processes in the crust of the overlying plate and the mantle beneath the subducting slab

a search for source side mantle anisotropy shear wave splitting due to source side mantle anisotropy is estimated from broadband teleseismic s waves using a suite of 19 subduction zone events more than 50km depth and recorded at several rstn stations we assume that there are at most two anisotropic regions along the ray path one on the source side and one on the receiver side and that receiver side splitting is known from analysis of sks it is found that splitting is not detectable below the slab in most cases there are however two specific regions one beneath peru and one beneath kamchatka that indicate significant source side anisotropy based on two nearby intermediate focus events in each region splitting delay times are found to be 1 0 1 5s with the fast polarization direction perpendicular to the trench assuming 4 intrinsic anisotropy this would correspond to path lengths of 100 170 km below the slab both regions are characterized by abrupt changes in along strike slab properties a change in slab dip in the case of peru and a cessation of deep focus seismicity in the case of kamchatka we speculate that the anisotropy is caused by an associated local disturbance in asthenospheric flow that the shear wave splitting parameters of s and sks are consistent with each other in most cases suggests that large scale anisotropy in the lower mantle is very small probably no more than a fraction of a percent

polarities of p and s waves and shear wave splitting observed from the bucaramanga nest colombia the bucaramanga nest colombia is an intense seismic source centered at 6 48 n 73 10 w and 161 km depth beneath the eastern andes during a 3 week period 161 microearthquakes mb 4 3 were recorded by a 260 by 140 km array of 12 three component university of wisconsin uw seismographs centered above the nest eighty eight percent of the events recorded were from the nest providing a unique opportunity to study natural events that have different source parameters but nearly the same ray paths shear wave splitting analysis shows that the average polarization direction of the first arriving shear waves from the nest events at nine of the seismographs is n12 e 18 after corrections based on the amount of discrepancy between the observed average p wave azimuth and the expected event to station azimuth average s wave polarization is n6 e 15 variation in time delays with azimuth and epicentral distance appears to be due to the varying angles between the ray path and the principal axes of the anisotropic system shear wave splitting observed from 17 events along the wadati benioff zone outside the nest shows that the time delays increase with increasing depth and defines a wedge shaped anisotropic volume above the subducting plate the anisotropic layer has a maximum thickness of approximately 60 km at the depth of the nest and is thinning updip the anisotropy most likely results from aligned anisotropic minerals such as olivine and orthopyroxene in a localized mantle flow in the lithosphere above the wadati benioff zone a region weakened by active fluid migration and mobilized by the heating and shearing along the suducting slab

anisotropic loci in the mantle beneath central peru abstract seismic anisotropy of the upper mantle beneath the central part of peru is examined by analyzing shear waves observed at broad band stations and a temporary array of short period stations shear wave splitting is seen on various shear phases such as direct s waves from local intermediate to deep earthquakes scs waves from regional deep earthquakes and sks waves from teleseismic earthquakes it is inferred that the shear wave anisotropy in the uppermost 100 km of the subcontinental mantle overlying the subducted nazca plate is 0 5 at most while the anisotropy in the subslab asthenosphere depth range of about 150 350 km is 2 or greater the fast shear wave polarization direction in this depth range as observed at two broad band stations is 30 40 different from the absolute motion of the nazca plate this does not fit simple two dimensional 2 d models of olivine alignment caused by slab induced mantle flow and implies either the existence of a complex flow pattern in the asthenosphere underneath the nazca plate or the presence of an unknown mechanism for the anisotropy formation the lower mantle beneath central peru is found to be effectively isotropic for nearly vertically propagated shear waves

subduction of the nazca plate beneath peru as determined from seismic observations a detailed reinvestigation of seismicity in the southern half of peru using data from a local seismic network and a focal mechanism study using p wave first motions reveal that for the first 100 km of descent the slab as defined by this seismicity enters at a normal dip angle near 30 and that below this depth it is bent to a nearly horizontal angle this horizontal slab extends eastward for about 300 km and then dips steeply below the thick continental lithosphere scsp observations are found to be consistent with the profile of the subducting plate as determined from the local seismicity beneath the region from southern peru to northern chile the nazca plate descends with an almost constant dip angle 30 down to at least 300 km depth between the 30 dipping plate in southern peru and the horizontal profile beneath central peru there is a contortion over a 100 km wide continuous lateral section of the subducting plate we infer that at least in the upper 150 km depth range the descending nazca plate is contorted rather than torn

shear wave splitting near guam abstract polarities of shear waves from intermediate focus events underneath guam are studied for records from a group of ten events shear wave splitting with faster arriving e w components are observed this event group occurred within or above one geographic portion of the wadati benioff zone with depths ranging between 57 and 148 km ray tracing calculations were performed for 3 d and 1 d velocity models constructed for the region to determine expected s wave polarities and ray patterns as well as their sensitivities to variations in velocity structure these were used to infer the probable existence of intrinsic anisotropy at depth and to determine the location and magnitude of anisotropy which can explain the observed shear wave splitting the most probable location of the anisotropy is beneath the crust and above or partially within the subducting slab assuming a maximum depth range of 10 120 km for the location of the anisotropy its amount is about 1 which may be viewed as a lower bound plausible causes of the anisotropy include mantle flow and thin sheet like channels filled with lava or water vapor migrating upward from the subducting slab

shear wave polarization anisotropy in the upper mantle beneath honshu japan shear wave polarization anisotropy in the wedge portion of the upper mantle between a subducting plate and the earth s surface is investigated using three component seismograms of intermediate depth and deep earthquakes recorded at 14 local stations in honshu japan eighty nine high quality seismograms were selected from a period of 3 years the data used in this study are restricted such that incidence angles are smaller than the critical angle of 30 to the earth s surface in order to avoid phase shifts in the shear wave train to find directions of the maximum and minimum velocities in split shear waves where shear waves are resolved into two phases with the maximum time separation each set of the two horizontal component seismograms is rotated in the horizontal plane the split shear waves thus obtained are again recombined after the correction of anisotropy and the anisotropy corrected particle motion is compared with the focal mechanism for a cross check of the observed anisotropy directions of the maximum axes are plotted on azimuthincidence angle stereograms at each station the stereograms and the cross sections of seismic ray paths show that 1 the anisotropic material is distributed at intermediate location between earthquake sources and receiving stations and 2 the anisotropic region is separated into two parts one in the north of the present study area with the polarization of the maximum velocity shear wave trending 0 to 30 from the north north anisotropy and the other in the south with it trending 90 to 120 south anisotropy the maximum time delays between the two shear waves along a vertical seismic ray is about l s for both the anisotropic regions the horizontal extent of the anisotropic area in the north is 50 km at depths of 50 to 150 km perhaps prevalent in west honshu however the depth of anisotropic material is not well constrained because of insufficient coverage of seismic ray paths and angles if we assume the vertical extent of the anisotropic material to be 100 km the maximum velocity contrast would be 4 if we adopt a crack alignment model for the observed anisotropy cracks are inferred to be distributed vertically trending 0 to 30 in the north and 90 to 120 in the south if we assume an olivine alignment model olivine crystals are inferred to be aligned in the north with the a axis of the slip direction trending 0 to 30 and the slip plane normal to the b axis being vertical and in the south anisotropy with the axis trending 90 to 120 and the slip plane being horizontal

spatial distribution and b value of intermediate depth earthquakes beneath central peru we have investigated the spatial distribution of microseismicity at intermediate depth in the wadati benioff w b zone beneath central peru within a 100 to 150 km depth range in the subducting nazca plate 67 event hypocenters define a horizontally subducting zone horizontal zone 450 to 600 km from the trench and a zone of increasing dip resubduction zone 600 to 700 km from the trench although the shape of the w b zone is generally consistent with previous observations from teleseismic data the distribution of seismicity is not in a 550 km wide band extending from 400 to 750 km from the trench the density of teleseismicity mb 4 1971 1981 increases as a function of distance from the trench while the density of microseismicity 1 6 mb 4 2 decreases combining local and teleseismic data the b value changes from 84 in the horizontal zone to 57 in the resubduction zone this result suggests that the minimum earthquake size and or stress drop increases in the resubduction zone low b values may also be intrinsic to proximity to the seismic cut off at 150 km depth

seismic evidence for continuity of the deep slab beneath central and eastern peru seismograms of peru brazil deep focus earthquakes recorded on the nearby carnegie broadband station at cuzco peru cus are characterized by an anomalous large amplitude p wave arrival that follows about 1 5 s after direct p the anomalous arrival is interpreted to be an underside wide angle reflection from the upper surface of the descending nazca plate somewhere in the depth range 150 400 km and in a region where the slab is wholly aseismic particle motion analysis of the cus seismograms shows that the direct p wave has arrival angles as predicted from simple ray tracing the large impulsive anomalous phase termed pr also arrives as a p wave but is of opposite polarity and about 1 5 times the amplitude of the direct p arrival the back azimuth of pr is about 20 clockwise from the path of direct p and its angle of emergence is significantly larger forward modeling calculations indicate that the seismic observations are consistent with a model in which pr originates as a p wave that travels up inside the slab from the source and is then reflected at wide angle from the steeply dipping upper plate boundary results from modeling lead to the following conclusions 1 the descending nazca plate exists in and is probably continuous through the aseismic region between 150 and 500 km depth 2 the plate is very steeply dipping 70 below depths of about 150 or 200 km and 3 the velocity discontinuity at the reflection point must be relatively sharp probably less than about 10 km in thickness

a seismological constraint on the depth of basalt eclogite transition in a subducting oceanic crust the oceanic crust primarily has a basaltic composition1 2 which must transform into eclogite on subduction of the oceanic crust beneath continents3 4 there has been a considerable debate about the role of the basalt eclogite transformation in driving lithospheric plate motion5 and in the thermal regieme of subducting plates6 how important this transformation is in tectonics critically depends on the depth of transformation which is affected by factors such as temperature7 8 bulk rock chemical composition2 water content9 and perhaps stress environment becuase it would be practically impossible to evaluate all these factors correctly we attempt here to constrain the transformation depth by a more direct seismological method

shear wave splitting above the subducting juan de fuca plate shear wave splitting observed in three component broadband recordings of local earthquakes near southern vancouver island british columbia provide the first constraints on shear wave anisotropy in the continental crust above the subducting juan de fuca plate waveforms of 22 shallow earthquakes 15 30 km depth and 16 deep earthquakes 40 60 km depth were examined for shear wave splitting the fast polarization direction is well defined at 312 19 for the deep earthquakes and 293 27 for the shallow earthquakes these fast polarization directions are approximately parallel to the continental margin and coincide with the orientation of the pressure axes of small crustal earthquakes in this region interpreting this polarization pattern in terms of the extensive dilatancy anisotropy model indicates a margin parallel compressive stress regime comparing the average time delays between the fast and slow shear waves for the deep earthquakes 0 32 s and the shallow earthquakes 0 20 s suggests that the source of the seismic anisotropy is stronger in the upper 20 km of the continental crust here the average differential shear wave anisotropy is estimated at 2 3 these observations support the theory of weak coupling between the downgoing juan de fuca plate and the overlying north america plate as the principal stress is perpendicular to the direction of subduction and the s wave splitting above the subducting plate is nearly perpendicular to the sks splitting direction in the upper mantle beneath the subducting plate

azimuthal anisotropy in the lithosphere from observations of long period s waves summary the paper gives a detailed description of the technique for measuring azimuthal anisotropy which was suggested earlier by vinnik kosarev makeyeva 1984 and used by kind et ai 1985 the technique is based on the observations of long period converted phases like sks additionally we describe a generalization of this approach which makes use of long period s waves of arbitrary polarization both modifications were applied to records of the grf array in southern germany the results of this analysis if combined with the data on p velocities bamford 1977 suggest that the direction of the fast velocity in the lithosphere of the region varies with depth

seismic anisotropy and mantle flow seismic anisotropy in the mantle is primarily due to deformation induced lattice preferred orientation lpo of olivine crystals the aim of this paper is to better understand how such lpo is produced by flow and to learn whether observations of seismic anisotropy can be used to infer the local direction of flow within the mantle the basis of the work is a two dimensional theory which describes how lpo evolves during an arbitrary time dependent deformation ribe 1989 i investigate the evolution of lpo beneath ocean ridges above subducted slabs and in thermal convective flows the ocean ridge model predicts that the seismic anisotropy produced at a fast spreading ridge should be greater in magnitude than that produced at a slow spreading ridge and that the fast axis should be more nearly horizontal the subduction zone model predicts that strong seismic anisotropy should be produced above subducted slabs with the fast axis aligned with the slab regardless of the subduction angle the evolution of lpo in typical convective flows is extremely complex and depends strongly on such factors as the mode of heating and the surface boundary conditions in general there is no simple relation between the lpo and the local flow direction in any of the models considered the attempt to use seismic anisotropy to map flow patterns on a global scale is therefore probably unwarranted however the lpo does become aligned with the flow direction when the flow is a progressive simple shear such a flow occurs in limited portions of the mantle beneath the lithosphere at a fast spreading ridge above subducted slabs and in boundary layers beneath rigid surfaces such as continents observations of seismic anisotropy may therefore indicate the sense of shear at shallow depths in the mantle

shear wave anisotropy beneath the andes from the banjo seda and pisco experiments we present the results of a detailed shear wave splitting analysis of data collected by three temporary broadband deployments located in central western south america the broadband andean joint experiment banjo a 1000 km long east west line at 20 s and the projecto de investigacion sismologica de la cordillera occidental pisco and seismic exploration of the deep altiplano seda deployed several hunderd kilometers north and south of this line we determined the splitting parameters fast polarization direction and t splitting delay time for waves that sample the above and below slab regions teleseismic ks and s scs waves from local deep focus events as well as s waves from intermediate focus events that sample only the above slab region all but one of the ks stacks for the banjo stations show e w fast directions with t varying between 0 4 and 1 5 s however for ks recorded at most of the seda and pisco stations and for local deep focus s events north and south of banjo there is a rotation of to a more nearly trench parallel direction the splitting parameters for above slab paths determined from events around 200 km deep to western stations yield small delay times 0 3 s and n s fast polarization directions assuming the anisotropy is limited to the top 400 km of the mantle olivine stability field these data suggest the following spatial distribution of anisotropy for the above slab component as one goes from east where ks reflects the above slab component to west changes from e w to n s and delay times are substantially reduced this change may mark the transition from the brazilian craton to actively deforming e w shortening andean mantle we see no evidence for the strain field expected for either corner flow or shear in the mantle wedge associated with relative plate motion the small delay times for above slab paths in the west require the existence of significant spatially varying below slab anisotropy to explain the ks results the implied anisotropic pattern below the slab is not easily explained by a simple model of slab entrained shear flow beneath the plate instead flow induced by the retrograde motion of the slab in combination with local structural variations may provide a better explanation

fluid immiscibility in metamorphic rocks evidence of fluid immiscibility in metamorphic rocks comes in the best of all worlds from fluid inclusion observations in fact fluid immiscibility should be anticipated over the full range of metamorphic conditions provided that appropriate bulk fluid compositions are present in a review paper entitled metamorphic fluids the evidence from fluid inclusions crawford and hollister 1986 summarized the role of fluid immiscibility in metamorphism was only recognized after serious study of fluid inclusions in metamorphic rocks was underway in the late 1970s and has yet to attract the attention of more than a handful of petrologists studying metamorphic rocks since then tremendous progress has been made a huge amount of fluid inclusion studies have convincingly shown that immiscible fluids can be present in rocks of all bulk compositions and at all p t conditions thermodynamic databases of minerals solid solutions and fluid mixtures along with the development of gibbs free energy minimization programs now allow for detailed prediction of the evolution of phase assemblages and mineral compositions in p t space significant progress has also been made in understanding reaction kinetics diffusional and convective mass transport and the interpretation of fluid rock interaction during metamorphism given the vast fluid inclusion evidence of immiscibility it would appear however that phase petrology and mass transport in any specific rock where fluids are involved are not correctly interpreted if possible or proven immiscibility is not considered relatively few studies directly link mineral reactions with immiscible fluids therefore this review aims at working out the effects of fluid immiscibility on the progress of metamorphic reactions and fluid transport over a wide range of conditions questions arise directly from the fluid inclusion record in metamorphic rocks this chapter however is organized the other way round at first it summarizes phase relations of fluid systems pertinent to

evidence for shallow and pervasive seismic anisotropy in the wellington region new zealand the shear waves of local earthquakes were recorded during a 5 month deployment of seven three component digital seismographs on the wellington peninsula new zealand the seismographs were spaced an average of less than 5 km apart and over 300 local earthquakes were recorded with phase arrivals within the shear wave window a significant number 37 of the earthquakes recorded showed clear evidence of shear wave splitting identifiable fast and slow shear wave arrivals with similar pulse shapes consistent polarization directions at particular stations were also observed even when poor signal to noise ratio or scattering meant that no slow shear wave arrival could be identified however there were large station to station differences in the polarization directions correcting for the observed shear wave splitting improved the fit between the measured shear wave polarizations and those calculated assuming a double couple focal mechanism the cause of the observed shear wave splitting is therefore most likely to be seismic anisotropy large station to station differences in the polarization alignments ranging from 61 24 to 137 18 suggest that most anisotropy is confined to the top 2 3 km of the crust however there is evidence from one station for a small amount of pervasive anisotropy if such a trend existed on the other stations it could not be identified because of the large scatter in the data points the measured delay times between split shear waves vary from 0 02 to 0 22 s with a mean value of 0 1 0 06 s this translates to a near surface shear wave velocity anisotropy of about 10 with up to 2 pervasive anisotropy possible throughout the crust this data set indicates that extensive dilatancy anisotropy cannot be the sole cause of crustal seismic anisotropy and that foliations in the rock fabric and the fracture zones of active faults may also be important there is no evidence for temporal change in the shear wave splitting parameters during the period of the experiment

the subduction of young lithosphere studies using a variety of techniques of the subduction beneath western south america and the southwest honshu kyushu region of japan indicate that volcano free segments occur where the subducted and continental lithospheres remain in contact without intervening asthenosphere the subduction is initiated at normal dip angles but the plate deforms at some depth 100 km under central peru to travel horizontally immediately beneath continental lithosphere the most plausible reason for this geometry is that the subducted plate is buoyant a model is developed constrained by age of the plate bathymetry and heat flow estimates of the density of oceanic plates as a function of age show that younger ocean floor may be less dense than the asthenosphere into which it subducts if the high density tranformation of crustal basalt to eclogite is retarded by low temperatures the plate can remain buoyant to considerable depth for long periods of time heat flow data from western south america are consistent with this model

partial melting in a thermo chemical boundary layer at the base of the mantle abstract seismological detections of complex structures in the lowermost mantle boundary layer the d region motivate a conceptual model of a compositionally stratified thermo chemical boundary layer tcbl within which lateral temperature variations sustained by large scale mid mantle flow cause variations of partial melt fraction partial melt fractions of from 0 to 30 in the tcbl occur due to the eutectic of the boundary layer material lying below the high temperature at the core mantle boundary cmb coupled with the presence of steep thermal gradients across the boundary layer regions of the tcbl with the highest temperatures have extensive partial melting producing lateral chemical variations and strong effects on seismic velocities and boundary layer dynamics this tcbl concept provides a relatively simple framework that can plausibly account for diverse seismological observations such as predominance of large scale volumetric elastic wave velocity heterogeneity in the boundary layer laterally extensive but intermittent abrupt shear and compressional velocity increases and decreases at the top of the d region small scale topography of d velocity discontinuities thin ultra low velocity zones at the cmb widespread shear wave anisotropy in d bulk sound velocity anomalies detected in low shear velocity regions of d and large scale upwellings from the most extensively melted regions of the boundary layer neutral or negative buoyancy of the partial melt in the tcbl is required along with an increase in bulk modulus and some density increase of the boundary layer material relative to the overlying mantle more complex models in which the partially melted chemical boundary layer is laterally displaced by mid mantle downwellings are also viable but these appear to require additional special circumstances such as a phase change efficient segregation of slab crustal material or abrupt onset of anisotropy in order to account for rapid velocity increases at the top of d the precise nature of the compositional anomaly or anomalies in d and the eutectic composition of this zone are yet to be determined but it appears likely that partial melting within the lowermost mantle plays a paramount role in the observed seismic velocity heterogeneity increased resolution geodynamic calculations of dense boundary layer structures with partial melting and attendant viscosity reductions and chemical heterogeneity are needed to quantify this tcbl concept

complex anisotropy beneath the peruvian flat slab from frequency dependent multiple phase shear wave splitting analysis flat or shallow subduction is a relatively widespread global occurrence but the dynamics remain poorly understood in particular the interaction between flat slabs and the surrounding mantle flow has yet to be studied in detail here we present measurements of seismic anisotropy to investigate mantle flow beneath the peruvian flat slab segment the largest present day region of flat subduction we conduct a detailed shear wave splitting analysis at a long running seismic station nna located near lima peru we present measurements of apparent splitting parameters fast direction and delay time t for sks scs and local s phases from 80 events we observe well defined frequency dependence and backazimuthal variability indicating the likely presence of complex anisotropy forward modeling the observations with two or three layers of anisotropy reveals a likely layer with a trench normal fast direction underlying a layer with a more trench oblique to trench subparallel fast direction in order to further constrain the anisotropic geometry we analyzed the source side splitting from events originating within the slab measured at distant stations beneath the flat slab segment we found trench normal fast splitting directions in the subslab mantle while within the dipping portion of the slab further to the east likely trench subparallel anisotropy within the slab itself this subslab pattern contradicts observations from elsewhere in south america for normal i e more steeply dipping slab conditions it is similar however to inferences from other shallowly dipping subduction zones around the world while there is an apparent link between slab dip and the surrounding mantle flow at least beneath peru the precise nature of the relationship remains to be clarified