upper mantle anisotropy beneath the geoscope stations seismic anisotropy has been widely studied this last decade particularly by measuring splitting of vertically propagating core shear waves the main interest in this technique is to characterize upper mantle flow beneath seismic stations on the other hand the major restriction in this method is that a single station gives a single anisotropy measurement alternative methods have been developed in order to avoid this restriction an accurate determination of upper mantle seismic anisotropy beneath a seismic station may allow one by doing anisotropy correction to characterize remote or deeper anisotropy the geoscope network is ideal for this purpose because it is composed of a large set about 26 of high quality broadband seismometers globally distributed and because some of these stations have run for more than 10 years and most of them for more than 5 years we selected about 100 events at each site generally of magnitude mb 6 0 and we performed systematic measurements of the splitting parameters fast polarization direction and delay time t on sks skks and pks phases splitting on oceanic islands has been difficult to observe owing to the low quality of the signal but also perhaps owing to complex upper mantle structures beneath the stations station kip kipapa hawaii in the pacific is the only oceanic geoscope station with a clear anisotropy we determined well constrained splitting parameters for 10 of the 17 continental stations that may be explained by a single anisotropic layer the poor correlation between fast polarization directions and the absolute plate motion together with the apparent incoherence between the plate velocities and the observed delay times suggest that a simple drag induced asthenospheric flow alone fails to explain most of the observations for some stations located on or near major lithospheric structures tam tamanrasset algeria for instance we observe a good correlation between fast polarization directions and regional structures at station scz santa cruz california we found clear variations of the splitting parameters as a function of the event backazimuth compatible with two layers of anisotropy three stations can canberra hyb hyderabad india and ssb saint sauveur badole france seem to be devoid of detectable anisotropy

heat producing elements and the thermal and baric patterns of metamorphic belts the character of sedimentary basins before they are deformed and metamorphosed may strongly influence the thermal and baric patterns of metamorphic belts crustal thickening of anoxic sedimentary basins and subsequent thermal reequilibration may produce large areas of high grade metamorphic rocks and granites because the concentrations of the heat producing elements are high in such basins in new england there is a spatial association among granites and high grade metasedimentary rocks rich in u and th that now form the central maine terrane the high content of heat producing elements in these rocks is attributed to fixing of u and th in highly reduced sediments that were deposited in an anoxic basin that formed in the silurian when the basin was thickened during the devonian acadian orogeny the thermal energy generated by the u and th rich sediments produced the observed broad zone of high grade rocks and anatectic granites this hypothesis was tested with thermal calculations that reproduce most of the first order thermal and baric patterns of the acadian appalachians if pretectonic lateral variations in heat production are assumed

seismic anisotropy tracing plate dynamics in the mantle elastic anisotropy is present where the speed of a seismic wave depends on its direction in earth9s mantle elastic anisotropy is induced by minerals that are preferentially oriented in a directional flow or deformation earthquakes generate two seismic wave types compressional p and shear s waves whose coupling in anisotropic rocks leads to scattering birefringence and waves with hybrid polarizations this varied behavior is helping geophysicists explore rock textures within earth9s mantle and crust map present day upper mantle convection and study the formation of lithospheric plates and the accretion of continents in earth history

charnockite and granite formation and influx of co 2 at kabbaldurga the discovery at kabbaldurga quarry karnataka south india1 3 of patches of charnockite apparently in an arrested state of development has refocused attention on the mechanism of charnockite formation in particular the relative roles of co2 and h2o during high grade metamorphism and charnockite development can be studied in such an area during an influx of a co2 rich volatile phase h2o may be liberated as a result of the reduction in the state of hydration by the breakdown of hydrous minerals4 the p t conditions in which this occurs are important because with an increase in xh2o conditions which may allow anatexis could be initiated in advance of such a co2 rich phase indeed weaver5 has suggested that the k rich acid charnockites found at pallavaram madras6 developed due to changes in volatile composition before the onset of charnockite conditions the exposure at kabbaldurga quarry is interpreted here as displaying evidence that such a process has taken place

models of seismic anisotropy in the deep continental lithosphere seismological observations sks wave polarizations systematic p residual variations azimuthal dependence of pn and surface wave velocities or a dispersion of surface waves are not consistent with isotropic if laterally heterogeneous upper mantle structure therefore an anisotropy should be considered as an a priori aspect of future large scale studies of mantle structure most studies of anisotropy however have assumed horizontal or vertical axes of symmetry but such orientations cannot explain bipolar patterns of spatial variations of p residuals which we have observed at many seismological stations on the basis of the petrophysical properties of real upper mantle rocks we consider anisotropy formed either by hexagonal or by orthorhombic aggregates composed of olivine orthopyroxene and clinopyroxene rotations of the aggregates about vertical and horizontal axes allow us to find the three dimensional orientations of symmetry axes that fit combinations of both p and s seismological observations in central europe and in western north america the orientations with plunging symmetry axes velocity extremes seem to be consistent across large spatially uniform tectonic units and change abruptly at important suture zones

shear wave splitting in the upper mantle wedge above the tonga subduction zone summary intermediate period seismograms of shear waves from deep earthquakes are examined for shear wave splitting diagnostic of seismic velocity anisotropy in the upper mantle wedge between the subducting tonga slab and stations in the fiji islands earthquakes with near vertical incidence angles are selected to minimize complications of converted phases and the free surface horizontal component seismograms for 25 deep earthquakes recorded at station lak on the lau ridge show splitting of up to 0 9 s with an average of 0 45 0 25 s between fast and slow s wave components anisotropy in the upper mantle is the most likely cause of the splitting the ese orientation of the fast velocity direction at station lak corresponds to an axis of symmetry of the anisotropic mantle and is approximately perpendicular to magnetic lineations in the lau basin and the northeastern portion of the south iiji basin the observations are consistent with a model of flow alignment of olivine 100 and orthopyroxene 001 axes parallel to the direction of basin extension seismograms for eight deep earthquakes at station sva on viti levu show up to 1 0 s splitting with an average of 0 65 0 29 s and a weak nne preferred orientation of fast velocity polarizations of shear wave particle motions corrected for anisotropy are within 36 5 26 of polarixations predicted for nine moment tensor solutions the magnitude of shearwave splitting and estimates of the thickness of the lithosphere in the fiji region imply that the anisotropy cannot be contained in the lithosphere alone either the anisotropy is distributed in both the lithosphere and asthenosphere or the asthenosphere alone is anisotropic

dehydration melting and the granulite transition in metapelites from southern namaqualand s africa in a prograde amphibolite granulite transition zone in the namaqualand metamorphic complex metapelites show an interbanding of the amphibolite facies association biotite sillimanite quartz with the granulite facies association garnet cordierite k feldspar relict graded bedding shows that compositional banding is of sedimentary origin the garnet cordierite k feldspar gneisses contain quartzofeldspathic segregations surrounding garnets and have more fe rich bulk compositions than the biotite sillimanite schists

the transformation of amphibolite facies gneiss to charnockite in southern karnataka and northern tamil nadu india amphibolite facies metamorphic grade gives way southward to the granulite grade in southern karnataka as acid gneisses develop charnockite patches and streaks and basic enclaves develop pyroxenes petrologic investigations in the transitional zone south of mysore have established the following points 1 the transition is prograde amphibole bearing gneisses intimately associated with charnockite at kabbal and several similar localities are not retrogressive after charnockite as proved by patchy obliteration of their foliation by transgressive very coarse grained charnockite high fluorine content of biotite and amphibole in gneisses and high large ion lithophile element contents in gneisses and charnockites these features are in contrast to very low fluorine in retrogressive amphiboles and biotites very low large ion lithophile element contents and zonal bleaching of charnockite in clearly retrogressive areas as at bhavani sagar tamil nadu 2 metamorphic temperatures in the transitional areas were 700 800 c pressures were 5 7 kbar and h2o pressures were 0 1 0 3 times total pressures based on thermodynamic calculations using mineral analyses dense co2 rich fluid inclusions in the kabbal rocks confirm the low h2o pressures at the first appearance of orthopyroxene farther to the south in the nilgiri hills and adjacent granulite massif areas peak metamorphic temperatures were 800 900 c pressures were 7 9 kbar and water pressures were very low so that primary biotites and amphiboles those with high f contents are rare 3 the incipient granulite grade metamorphism of the transitional areas was introduced by a wave of anatexis and k metasomatism this process was arrested by drying out under heavy co2 influx charnockites so formed are hybrids of anatectic granite and metabasite of metabasite and immediately adjacent gneiss or are virtually isochemical with pre existing gneiss despite gross recrystallization to granulite mineralogy these features show that partial melting and metasomatism are attendant rather than causative in charnockite development copious co2 from a deep crustal or mantle source pushed ahead of it a wave of more aqueous solutions which promoted anatexis granulite metamorphism of both neosome and paleosome followed the process is very similar to that deduced for the madras granulites by weaver 1980 the massif charnockites for the most part extremely depleted in lithophile minor elements show many evidences of having gone through the same process

anisotropy of the indian continental lithospheric mantle summary due to the paucity of seismological data available in the public domain the structure of the indian lithosphere is still little known we investigate the lithospheric structure and potential mechanical coupling between the crust and upper mantle along the himalayan arc and underneath peninsular india using seismic anisotropy shear wave splitting measurements are performed on core refracted phases for each event recorded at a given seismological station we measured the orientation of the polarization plane of the fast s wave phi assumed to be a proxy for the orientation of the a axis of olivine and the delay dt between the arrival time of the fast and slow s waves we present a very comprehensive data set recorded at 86 seismological stations deployed from the himalayas to the southern tip of the indian peninsula in a joint effort by the national geophysical research institute hyderabad india the university of cambridge and the indian institute of astrophysics the unprecedented data set we present sheds light on the mechanisms involved in the india eurasia continental collision in a region along the himalayan arc south of the indus tsangpo suture zone at the scale of the indian plate the majority of the stations show a nne ssw orientation of phi over hundreds of kilometres from sri lanka to the northern part of the dharwar craton this direction closely parallels the trend of the indian plate motion with respect to a fixed eurasian plate as defined through the nuvel1a plate model along the himalayan arc from ladakh in the northwest to bhutan and the shillong plateau in the east the orientation of phi rotates to become ew perpendicular to the plate motion as defined through nuvel1a unlike previous studies we do find strong evidence for seismic anisotropy south of the indus tsangpo suture zone a large number of null results have been computed with consistent orientation of the two fast polarization directions phi across the subcontinent we demonstrate the potential value of the too often neglected null measurements in the interpretation of seismic anisotropy from these results we infer the dominance beneath the indian lithosphere of the asthenospheric flow in aligning minerals in the sheared lithosphere asthenosphere boundary layer masking any compression induced anisotropy expected to be normal to this direction closer to the collision front in northern india the anisotropy may in part be due to the foliation planes of the himalayan fold and thrust belt aligning the a axis of olivine perpendicular to the compression axis but more likely to the turning of the relative asthenospheric flow along the strike caused by the downthrusting indian lithosphere acting as a barrier the continent wide consistency of results strengthens the understanding that the indian lithosphere has distinct anisotropic signatures contrary to the hitherto assumed isotropy and allows one to interpret the results in a coherent framework of indo eurasian convergence

the heat flow heat generation relationship implications for the nature of continental crust abstract the apparent linear relationship between surface heat flow q and heat generation a is usually interpreted in terms of heat flow provinces i e terrains that have a common tectonothermal history and a uniform mantle component of heat flow the slope d of the linear regression of heat flow on heat generation is an indication of the thickness of the upper heat producing layer for many terrains d is numerically similar to the depth of a major mid crustal transition zone these observations are modelled by a two layer crust consisting of an upper layer approximately 10 15 km thick of variable heat generation underlain by a layer about 20 km thick in which heat generation is lower and more uniform than that in the upper layer the model is well able to simulate q a regression parameters for different sets of real data it is also consistent with other geological and geophysical observations the higher and more variable heat generation of the upper layer is consistent with ideas concerning removal of heat producing elements from the lower layer during amphibolite to granulite metamorphism and their redistribution into the upper crust further redistribution of heat producing elements may occur in zones of hydrothermal circulation whose depth is a function of stress pressure and temperature and which may migrate vertically in response to stress changes during uplift and erosion large scale metamorphism to granulite facies has been postulated as one mechanism for stabilisation of continental crust in the late archaean it is shown however that redistribution of heat producing elements in such a process would have only a small effect on crustal geotherms so that partitioning of heat production was not a major factor in the crustal stabilisation

the pikwitonei granulite domain manitoba a giant neoarchean high grade terrane in the northwest superior provincethis article is one of a series of papers published in this special issue on the theme of geochronology in honour of tom krogh n machado t e krogh and w weber are deceased the pikwitonei granulite domain located at the northwestern margin of the superior province is one of the largest neoarchean high grade terranes in the world with well preserved granulite metamorphic assemblages preserved in a variety of lithologies including enderbite opdalite charnockite and mafic granulite u pb geochronology has been attempted to unravel the protolith ages and metamorphic history of numerous lithologies at three main localities natawahunan lake sipiwesk lake and cauchon lake the u pb age results indicate that some of the layered enderbite gneisses are mesoarchean 3 4 3 0 ga and the more massive enderbites are neoarchean the high grade metamorphic history of the pikwitonei granulite domain is complex and multistage with at least four episodes of metamorphic zircon growth identified 1 2716 1 3 8 ma 2 2694 6 0 6 ma 3 2679 6 0 9 ma and 4 2642 5 0 9 ma metamorphic zircon growth during episodes 2 and 3 are interpreted to be regional in extent corresponding

scapolite phase equilibria and carbon isotopes constraints on the nature and distribution of co2 in the lower continental crust abstract recent applications of scapolite phase equilibria and carbon isotopes are reviewed in order to evaluate the role played by co2 in the genesis of granulites in the lowermost crust scapolite occurs mainly in mafic granulites from exposed high grade terranes most of which have had a prior upper crustal history before equilibration at 9 10 kbar and in high pressure 11 15 kbar xenoliths and orthogneisses which likely represent mantle derived material that has not interacted with supracrustal lithologies calculated equilibrium co2 activities are sufficiently high 0 7 to be consistent with the presence of a co2 rich fluid phase accounting for the low h2o activities in scapolite bearing granulites and some xenoliths co2 activities less than ca 0 5 or the absence of scapolite are consistent with fluid absent metamorphism the measured isotopic composition of carbon 13cpdb in scapolite is only a tight constraint on the source of carbon in scapolite for mafic xenoliths and high pressure orthogneisses carbon isotope values for these lithologies 7 to 2 overlap the range of values for co2 dissolved in or exsolved from basalts and are consistent with a magmatic origin for the co2 the carbon isotope composition of scapolite in granulites from exposed terranes 10 to 4 is more equivocal and may instead be related to pregranulite facies carbonate present in the rock minimum values of volumetric fluid rock calculated to account for the modal abundance and composition of scapolite in granulites on the order of 0 001 to 0 1 are much lower than those required for the desiccation of amphibolites to granulites 0 1 to 0 5 thus scapolite should form in response to infiltration of such large volumes of co2 if p t and bulk compositional constraints are met however only seven of twenty granulite occurrences contain primary carbonate bearing scapolite in mafic granulites it appears that co2 occurs in the lower crust only locally in a co2 rich fluid phase or dissolved in mafic melts and not on a pervasive scale or at the high volumetric fluid rock required the model of carbonic metamorphism

late archaean early proterozoic co2 streaming through the lower crust and geochemical segregation considerable evidence exists for large scale migration of co2 rich fluids through the lower continental crust in the late archaean and early proterozoic with transfer of heat producing elements and h2o to higher levels the evidence manifest in the large charnockitic terrains of this age includes deformation related charnockitic alteration of gneisses nearly ubiquitous co2 rich fluid inclusions in minerals depletion of large ion lithophile elements lile particularly rb relative to typical upper crustal rocks similar depletion of 18o in paragneisses and petrographic and geochemical evidence of open system replacement of amphibole gneiss by charnockite a conveyor belt upper mantle source appears to have been necessary for prolonged co2 supply in high grade crustal metamorphism most plausibly by deep subduction of marine carbonate under continental interiors this plate tectonic mode seems consonant with crustal thickening and orogenic patterns inferred for most charnockitic terrains and with co2 transport mechanisms constrained by experimental petrology

seismic anisotropy beneath the indian continent from splitting of direct s waves 1 in this study we investigate the shear wave splitting beneath 39 broadband seismic stations located over various tectonic units of the indian shield using direct s waves from deep to intermediate depth earthquakes the delay times between fast and slow axes of anisotropy do not reveal anomalous values that reflect contributions from midmantle or the mantle transition zone the fast polarization azimuths are generally consistent with the results from sks splitting that indicate a predominance of plate motion related strain in the indian continent splitting results at station hyb which was hitherto considered isotropic indicate significant anisotropy with a ne oriented fast axis although a general trend of sh leading sv is indicated by most of the measurements invoking radial anisotropy to explain this phenomenon appears farfetched since the near vertical incidence of the s waves in the usable distance range of 40 80 makes them more sensitive to azimuthal anisotropy underneath the station forward modeling of the dependence of the splitting parameters on incoming polarization assuming horizontal axis of symmetry brings out two layers of anisotropy beneath the indian shield with fast axis azimuths oriented n15 e in the bottom layer and n60 e in the top layer while stronger anisotropy t 1 s in the bottom layer is related to asthenospheric flow a weaker one t 0 4 s in the upper layer could represent anisotropy frozen in the lithosphere

shear wave splitting continental keels and patterns of mantle flow in this study we investigated the origin of seismic anisotropy in the mantle beneath north america in particular we evaluated whether shear wave splitting patterns in eastern north america are better explained by anisotropy caused by lithospheric deformation anisotropy due to mantle flow beneath the lithosphere or a combination of both we examined new measurements of shear wave splitting from the missouri to massachusetts broadband seismometer array moma the north american mantle anisotropy and discontinuity experiment nomad as well as splitting parameters from several previous studies we developed a simple finite difference model that approximates mantle flow around a complex three dimensional continental lithospheric keel to evaluate potential anisotropy from mantle flow beneath the lithosphere in eastern north america we compared shear wave splitting observations to predicted splitting parameters calculated using this mantle flow model our results indicate that a significant portion of observed shear wave splitting in eastern north america can be explained by mantle flow around the continental keel however shear wave splitting patterns in a few regions of eastern north america indicate that a component of lithospheric anisotropy must exist particularly in regions containing the largest keel thicknesses for eastern north america as well as for splitting observations in australia europe and south america we favor a model in which anisotropy is controlled by a combination of both lithospheric deformation and subcontinental mantle flow

upper mantle seismic anisotropy in the intra continental kachchh rift zone gujarat india abstract shear wave splitting study of 411 sks skks phases covering backazimuth range of 13 to 305 recorded by 12 broadband stations in the kachchh rift has led to estimates of fast axis orientations and splitting times for 118 good measurements the average vector mean of fast axis orientation 86 14 corresponds to the e w axis of the kachchh rift and the delay time 1 6 s is attributed to the 184 km thick upper mantle layer with 4 anisotropy the anisotropic character observed for the kachchh rift kr is comparable to other continental rifts and these are related to the high temperature lattice preferred orientation fabric of olivine inherited from the mantle flows the source of the rift axis parallel anisotropy is traced to the rift parallel flows within the 76 6 km thick lithosphere additionally the rift parallel pockets of partial melts also induce anisotropy within the asthenosphere both these are inherited from the plume lithosphere interaction during the deccan reunion plume episode 65 ma

seismic anisotropy beneath the southern himalayas tibet collision zone shear wave phases scs sks and skks collected from the project international deep seismic sounding of tibet and himalayas ii indepth ii array in the eastern himalayas and tibet are analyzed for the orientation and extent of polarization seismic anisotropy beneath the southern himalayas tibet collision zone no evidence is seen for any shear wave splitting for stations in this part of the collision zone except for two northern stations located about 100 km north of the indus tsangpo suture the lack of shear wave splitting beneath the stations in the vicinity of suture and the tethyan himalayas indicates that there is no appreciable large scale mantle polarization seismic anisotropy beneath this part of the collision zone one explanation for the lack of polarization seismic anisotropy would be a subvertical mantle shear strain field resulting from a downwelling mantle flow of a thickened indian lower continental lithosphere north of the gandese belt the fast directions of the split shear waves are oriented in a northeast southwest direction approximately parallel to the trend of a major left lateral strike slip fault zone that bounds the gulu rift the coincidence of fast polarization directions parallel to the surficial geologic shear zones and the direction of shear planes within the upper crustal strain rate field suggests lithospheric wide coherent deformation

continental scale shear wave splitting analysis investigation of seismic anisotropy underneath the australian continent the structure of the upper mantle beneath the australian continent is investigated using teleseismic shear wave splitting to extract seismic anisotropy measurements have been performed on data recorded at 190 sites with portable broadband seismic recorders spanning almost the entire surface of the continent since 1992 the average time span of the various deployments primarily designed for surface wave tomography is 6 months which is rather limited for shear wave splitting analysis however the data set provides a full continental scale survey using the reasonably favourable distribution of seismicity to australia seismic anisotropy has the potential to provide insights into the lithospheric structure and the possible mechanical coupling between the crust and the upper mantle but prior results for australia have indicated relatively small levels of splitting and a complex pattern these results are confirmed with our new and far more extensive measurements across the whole continent the pattern of seismic anisotropy from shear wave splitting beneath australia is rather complex and is not correlated with the almost north south absolute plate motion apm from recent models deviation of the asthenospheric mantle flow around the lithospheric roots associated with the extensive archaean and proterozoic zones of central and western australia could be occurring and so mantle flow related anisotropy cannot be completely ruled out despite the limited geological outcrop especially in phanerozoic eastern australia that is almost entirely covered by sedimentary basins some relationships can be highlighted between the orientation of the polarization plane of the fast s waves and structural trends along for instance the halls creek orogen bordering the eastern edge of the kimberley basin or along the new england and lachlan fold belts in the southeastern part of the continent such relationships might account for anisotropy frozen in the lithosphere during post tectonic thermal relaxation along the poorly constrained and controversial tasman line tl the geological boundary between precambrian and phanerozoic australia directions of anisotropy measured at some stations located in the close vicinity of the line appear to exhibit a curvilinear trend somewhat similar to that of the tl suggesting that fossil deformation associated with the tl might be recorded in the pattern of seismic anisotropy

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

banded iron formations through much of precambrian time similarities between proterozoic 1 8 2 5 gyr and archean 2 5 gyr banded iron formations are probably more significant than their differences the contrasts largely reflect differences in the tectonic settings of proterozoic and archean terrains archean banded iron formations are not as thick nor laterally as extensive as the major proterozoic iron formations nevertheless some archean iron formations have strike lengths of over 150 200 km and may have been quite extensive prior to the deformation that has affected most archean terrains stratigraphic sequences in which iron formations occur are highly variable and indicate that iron formations formed in many depositional environments sedimentary textures in the iron formations are dominated either by granules and oolites or laminations including microbanding reflecting differences in their physical conditions of deposition granular and oolitic textures are abundant in only three proterozoic depositional basins and most precambrian iron forma

lithospheric mantle deformation beneath the indian cratons abstract the nature of deformation of the deep continental roots beneath the archean early proterozoic terrains opens the question whether these ancient terrains have had stable roots since the precambrian or whether recent plate motions have deformed them in view of this we make an attempt to study the thermal structure beneath the cratonic regions of the indian shield which vary in lithospheric thickness from 65 km in the singhbhum craton to 148 km in the archean dharwars the average depth of 104 km to the top of the underlying asthenosphere is consistent with other termination methods and is in fact less than half the 200 400 km depth found in other stable areas of the earth similarly the average reduced heat flow of about 35 mw m2 and moho temperature of about 550 c range 400 730 c for the indian cratons are also much higher than their counterparts elsewhere our study indicates a large scale deformation of the cratonic mantle lithosphere beneath the indian shield since the mesoproterozoic c

the identification of archaeological obsidian sources on kamchatka peninsula russian far east using geochemical and geological data current progress abstract the current state of the art of provenance studies for archaeological obsidian on kamchatka peninsula is presented prehistoric people widely used obsidian as a raw material for making tools and acquired it from several primary sources the exact position of seven sources allowed us to understand the general features of obsidian geochemistry and tectonic position of obsidian bearing volcanic formations this also made it possible to suggest the localization of seven still unknown sources for archaeological obsidian on kamchatka verification of our preliminary conclusions can be made by fieldwork in selected areas

water activity changes across an amphibolite granulite facies transition broken hill australia phase compositions in pelitic and mafic gneisses place tight constraints on pressure ranging from 3 up to 6 kb and to a lesser extent on temperature 500 up to 800 c during prograde regional metamorphism of the willyama complex broken hill se australia these limits allow an evaluation of water activity across the terrain using various equilibria in pelitic and mafic gneisses the stability of cummingtonite and biotite over much of the terrain places upper limits on temperature and the presence of syn metamorphic partial melts in the metasediments places lower limits on a h2o garnet biotitesillimanite k feldspar quartz relations combined with the partial melting data suggest a decrease in water activity from near 1 0 in the lower grade zones to 0 5 0 2 in the broken hill little broken hill part of the two pyroxene zone this result is compatible with less precise hornblende orthopyroxene clinopyroxene quartz relations

variations in sks splitting across western canada measurements of sks splitting parameters have been made at stations of the canadian national seismograph network for events over the period 1993 mid1994 here we report on results obtained from western stations in the northern cordillera the alignment of polarization directions with major transcurrent fault systems suggests that the crust and upper mantle are being deformed coherently splitting parameters in the southern cordillera however are oriented approximately perpendicular to the strike of major tectonic features notably the cascadia subduction zone this contrast in the character of fast polarization directions between the southern and northern cordillera is thought to reflect the change in the tectonic style of the adjacent plate margin from convergent along the southern b c coast to transform north of the triple junction one plausible explanation is that upper mantle anisotropy in the southern region is due primarily to mantle flow perpendicular to the margin involving upwelling at the juan de fuca ridge subduction along the coast and associated back arc asthenospheric flow beneath the b c interior

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

archean granulite gneisses from eastern hebei province china rare earth geochemistry and tectonic implications the granulite gneisses and their retrograded products of the qianxi group from eastern hebei province china have been investigated for their isotope and trace element geochemistry a consistent age of about 2 5 ae has been obtained by the rb sr and sm nd whole rock isochron methods in agreement with the zircon u pb data pidgeon 1980 d y liu unpubl geochemical arguments from initial isotopic ratios isr and ind and elemental distribution patterns have led us to conclude that this age of about 2 5 ae represents the time of granulite facies metamorphism which must have followed closely the primary emplacement of their protoliths previous claims for early archean ages 3 5 ae of these granulites are not substantiated the mineral isotope systematics register an important thermal event at about 1 7 ae roughly corresponding to the time of the widespread luliang orogeny ma and wu 1981 or chungtiao movement huang 1978

upper and lower mantle anisotropy inferred from comprehensive sks and skks splitting measurements from india abstract in this study we investigate the upper mantle anisotropy beneath india using high quality sks and skks waveforms from 382 teleseismic earthquakes recorded at 119 broadband seismic stations in addition we present evidence for anisotropy in the d layer beneath southeast asia using sks and skks splitting discrepancies on the same seismogram during this exercise we obtain 200 new splitting measurements from 35 stations recently deployed in the indo gangetic plains igp central india and northeast india while the delay times between the fast and slow axes of anisotropy t range from 0 3 to 1 7 s the fast polarization azimuths at a majority of stations in the igp and central india coincide with the absolute plate motion of india implying shear at the base of the lithosphere as the dominant mechanism for forging anisotropy however stations in ne india reveal fast polarization azimuths mainly in the ene wsw direction suggestive of lithospheric strain induced by the ongoing indo eurasian collision our analysis for d anisotropy yielded a total of 100 sks skks pairs which can be categorized into those exhibiting i null measurements for one phase and significant splitting for the other phase ii null measurement for both the phases iii significant splitting for both the phases a pair is considered to be anomalous if the splitting difference between sks and skks is 0 5 s and the individual split time is 0 5 s using this criterion we obtain 12 measurements under category iii and 9 under category i that show a null measurement for sks and large splitting for the skks phase further we quantify the strength of the lower mantle anisotropy by correcting the skks measurement for the upper mantle anisotropy obtained by the sks phase on the same seismogram the sks delay times are found to be consistently less than skks times suggesting that the sks phases do not capture the lower mantle anisotropy in comparison to their skks counterparts seven coherent measurements thus obtained reveal measurable d anisotropy with fast polarization azimuths oriented mainly in the ene wsw direction these results suggest presence of a large region of deformation in the lowermost mantle beneath southeast asia a possible model for anisotropy in these regions could be the presence of slab material that pounded upon the core mantle boundary cmb and is experiencing large shear deformation resulting in lattice preferred orientation lpo of the lower mantle van der hilst and karason 1999 long 2009 the other possibility is the phase transformation from mgsio3 perovskite to a more stable post perovskite phase under favorable conditions which results in lpo of the lower mantle

shear wave splitting measurements problems and solutions abstract we compare sensitivity and reliability of three basic techniques of shear wave splitting analysis cross correlation of wave components minimization of covariance matrix eigenvalues and minimizing energy on the transverse component of both synthetic and real data point out some problems and propose solutions we discuss major difficulties of evaluating shear wave splitting parameters which are not always obvious in routine processing and can sometimes lead to false results and wrong interpretations we describe effects of pre processing of digital records of shear waves e g filtering and setting lengths of analyzed signals and discuss reliability of results to minimize outliers in evaluated splitting parameters for reliable evaluation of splitting of medium quality sks wave forms which are valuable mainly in temporary deployments of stations we recommend a careful band pass filtering of the signal using a window length not shorter than the dominant period inspecting the signal noise ratio on t component and the particle motion the transverse minimization method provides usually more robust and stable solutions than the other two methods in the conclusion we suggest how to proceed in evaluation of splitting parameters to ensure reliable solutions even for medium or low quality shear waveforms recorded frequently during temporary field measurements

correlation between seismic anisotropy and bouguer gravity anomalies in tibet and its implications for lithospheric structures summary as a baseline measurement for understanding the himalayan tibetan orogen a product of continent continent collision between india and eurasia we analyse digital seismic data in order to constrain the seismic anisotropy of the indian shield based on spatially sparse data that are currently available in the public domain there is little shear wave birefringence for sks phases under the indian shield even though it is part of a fast moving plate in the hotspot frame of reference if most of the northern indian mantle has little transverse anisotropy the onset of significant anisotropy under tibet marks the northern terminus of intact indian lithosphere that is thrusting under the himalayan tibetan orogen beyond this terminus tectonic fabric such as that associated with the deforming lithospheric mantle of eurasia must be present in the upper mantle along the profile from yadong to golmud the only profile in tibet where a number of shear wave birefringence data are available the amount of birefringence shows two marked increases near 30 and 33 n between which a local high in bouguer gravity anomaly is observed such a correlation between patterns of shear wave birefringence and gravity anomalies is explained by the juxtaposition of indian lithosphere against the overlying eurasian lithosphere while the eurasian lithospheric mantle appears only to the north of 30 n the indian lithospheric mantle extends northwards to near 33 n

fluid inclusions in rocks from the amphibolite facies gneiss to charnockite progression in southern karnataka india direct evidence concerning the fluids of granulite metamorphism fluid inclusion studies of rocks from the late archaean amphibolite facies to granulite facies transition zone of southern india provide support for the hypothesis that co2 rich h2o poor fluids were a major factor in the origin of the high grade terrain charnockites closely associated leucogranites and quartzo feldspathic veins contain vast numbers of large co2 rich inclusions in planar arrays in quartz and feldspar whereas amphibole bearing gray gneisses of essentially the same compositions as adjacent charnockites in mixed facies quarries contain no large fluid inclusions inclusions in the northernmost incipient charnockites as at kabbal karnataka occasionally contain about 25 mol of immiscible h2o lining cavity walls whereas inclusions from the charnockite massif terrane farther south do not have visibile h2o microthermometry of co2 inclusions shows that miscible ch4 and n2 must be small probably less than 10mol combined densities of co2 increase steadily from north to south across the transitional terrane entrapment pressures calculated from the co2 equation of state range from 5 kbar in the north to 7 5 kbar in the south at the mineralogically inferred average metamorphic temperature of 750 c in quantitative agreement with mineralogic geobarometry this agreement leads to the inference that the fluid inclusions were trapped at or near peak metamorphic conditions calculations on the stability of the charnockite assemblage biotite orthopyroxene k feldspar quartz show that an associated fluid phase must have less than 0 35 h2o activity at the inferred p and t conditions which agrees with the petrographic observations high tio2 content of biotite stabilizes it to lower h2o activities and the steady increase of biotite tio2 southward in the area suggests progressive decrease of ah2o with increasing grade oxygen fugacities calculated from orthopyroxene magnetite quartz are considerably higher than the graphite co2 o2 buffer which explains the absence of graphite in the charnockites the present study quantifies the nature of the vapours in the southern india granulite metamorphism it remains to be determined whether co2 flushing of the crust can by itself create large terranes of largeion lithophile depleted granulites or whether removal of h2o bearing anatectic melts is essential

shear wave splitting in the rajasthan craton india abstract the shear wave splitting parameters i e fast axes orientation and delay time t are measured at four broadband stations in rajasthan using sks skks core phases from three component broadband waveforms of 30 events recorded during 2014 16 the thicknesses of lithosphere at these four stations are also inferred from the estimated delays the average splitting parameters t at the northern most jai and southern most ban stations are modeled to be 36 44 08 36 and 1 48 0 32 s and 35 72 10 47 and 1 86 0 34 s respectively while these parameters at udi and bun stations are found to be 41 08 08 54 and 1 61 0 26 s and 43 86 08 96 and 1 68 0 27 s respectively thus our modeling reveals a mean fast axes orientation of 39 9 with a mean delay of 1 7 0 3 s for rajasthan which could be attributed to the frozen lithospheric anisotropy in the direction of current absolute plate motion of the indian plate a slight regional variation of observed orientation of fast axes ranging from 35 7 at ban to 43 9 at bun could be attributed to the tectonic control rather than current plate motion in addition the mean orientations of fast axes are measured to be 36 4o at udi and 41 1o at jai respectively thus this observed coherent variation of fast axes orientation across the rajasthan suggests a significant contribution from the asthenospheric flow further estimated mean delay time 1 7 0 3 s does also suggest a contribution from the anisotropy associated with asthenospheric flows the maximum delay 1 86 s is estimated at southernmost ban station suggesting a thick anisotropic layer of 214 km while minimum delay 1 48 s is modeled at northernmost jai station suggesting a 170 km thick anisotropic layer while asthenospheric layer thicknesses at udi and bun are found to be 185 and 193 km respectively this observation suggests a relatively more anisotropic and thick lithosphere in the south in comparison to that in the north the measured deviation of fast axis orientation from the absolute indian plate motion varies from 1 to 4 3 which could be related to the changes in asthenosphric flow pattern due to the presence of topography at the base of lithosphere

continental thermal and tectonic regimes during the archaean metamorphic pressures estimated from mineral assemblages in archaean high grade terrains may be used to constrain geothermal regimes in the archaean continental crust in two ways first they may be combined with temperature estimates to yield bounds on the heat flux during the thickening events that produced the metamorphism the pressure temperature data do not require any greater supply of heat to the base of the continental crust in the archaean than is supplied at the present day second the pressures alone may be used to infer the height of archaean mountains these mountains appear to have had elevations comparable with those of present day mountain belts it seems probable that the elevation of present day belts is limited by the creep strength of the lithosphere and in view of the strong temperature dependence of creep in earth materials this second observation also implies that archaean continental thermal regimes were similar to present day ones these observations together with the common

global patterns of azimuthal anisotropy and deformations in the continental mantle summary we present a summary of measurements of azimuthal anisotropy in the continental mantle based on the sks technique and performed mostly with the active participation of the authors the directions of polarization of the fast quasi shear wave and the time delays between the quasi shear waves are obtained at nearly 70 locations in all continents except antarctica these data are interpreted in terms of lattice preferred orientation of olivine which is caused by deformations in the mantle the depth interval responsible for anisotropy is unknown but the data suggest that it may reach at least 300 km the fast directions in sks do not show clear correlation with the fast directions of the teleseismic p at the same seismograph stations in the regions of present day convergence the fast direction of anisotropy usually aligns with the plate boundary this correlation implies that the direction of shortening is the same in the crust and the upper mantle in the regions of rifting the inferred direction of mantle flow usually aligns with the direction of extension in the crust outside the regions of recent tectonic activity we most likely observe a combined effect of frozen anisotropy in the subcrustal lithosphere and of recently formed anisotropy in the asthenosphere on a global scale in these regions there is a positive correlation between the absolute plate velocity directions and the fast directions of anisotropy the correlation is especially strong in central and eastern parts of north america a clear absence of any evidence of large scale azimuthal anisotropy in the data of long range refraction profiling for the upper 100 km of the mantle of that region implies that the effect in sks is generated mainly at greater depths in the asthenosphere orientation of olivine at these depths reflects recent and present day flow in the mantle rather than processes of a distant geologic past

nature and timing of anatexis in the eastern and southern adirondack highlands metapelitic rocks in the eastern and southern adirondack highlands contain two feldspar quartz leuco somes that pre date most deformational fabric as well as granulite facies recrystallization whole rock xrf analyses of these leucosomes yield cipw norms which correspond closely with experimental melts in the system ab or qt an h 2 o thus indicating that the leucosomes are anatectic in origin derived p t values for anatexis indicate high temperatures and low pressures suggestive of widespread contact metamorphism the heat source is believed to have been provided by anorthositic gabbroic magmas postulated to have been anorogenically emplaced prior to the grenville orogeny

u pb zircon geochronology of the selbaie cu zn ag au mine abitibi subprovince canada the selbaie mine is a large tonnage low grade cu zn ag au deposit with both stratiform and epithermal characteristics mineralization comprises lower grade disseminated zn ag ore and pyritic siliceous sinter of the a1 zone at the surface and higher grade cu zn ag au quartz sulfide lenses of the a2 zone below and transitional with the a1 zone and of the b zone 1 km to the west the copper rich quartz sulfide lenses comprise networks of subvertical quartz sulfide veins vein arrays and breccia they have irregular locally gradational boundaries and variable attitudes with dips from 25 o to 90os the mineralization is within a volcanic stratigraphic succession of heterolithic breccias and felsic and intermediate tuffs and flows which are cut by mafic and felsic intrusions and felsite dikes the felsic volcanic rocks and a tonalitic marginal phase from the adjacent brouillan pluton are calc alkalic and have geochemical signatures that support a cogenetic relationship the mineralization and mine stratigraphic succession are truncated by the brouillan faults a series of subparallel west directed predominantly brittle thrusts with the brouillan pluton on the upper plate above the mine stratigraphic succession galenas from stratiform and vein ore have uniform lead isotope compositions of 2o6pb 2o4pb 13 184 to 13 211 2o7pb 2o4pb 14s351 to 14 369 and 2ospb 2o4pb 33s039 to 33 100 with model values of 8 1 8 2 and model k values of 4 1 to 4 2 n 5 among the most primitive of abitibi volcanic associated massive sulfide depositss they have model ages that range from 2 71 to 2 68 ga a u pb zircon age of 2729 ma for a sericite altered calc alkalic quartz porphyry flow 150 m up section from stratiform mineralization closely approximates the age for this mineralization type a u pb zircon age for a felsite dike that cuts postore east west foliation is 2726 3 ma slightly younger but overlapping at the 20 level with the age for the quartz feldspar porphyry flow indicating that the copper rich quartz sulfide lenses are essentially coeval with the lower grade stratiform mineralization a sample of sheared chloritic granodiorite taken from one of the brouillan faults near the margin of the brouillan pluton has a complex zircon opulation six concordant anal ses of abraded multigrain fractions individual grains or grain tips 20 p 206 s

spatial and temporal constraints on sources of seismic anisotropy evidence from the scottish highlands 1 routine sks splitting analyses of seismic anisotropy usually suffer from limitations in station density that hinder attempts to place spatial constraints on anisotropic fabrics data from a 20 km station spacing network in scotland show that splitting parameters vary considerably t 0 45 1 5 s 49 128 over short 10 20 km length scales improved spatial constraints then lead to tighter temporal constraints on the anisotropic sources asthenospheric fabrics due to tertiary rifting and present day plate motions do not strongly influence our results that instead correlate with lithospheric scale trends inferred from surface geology splitting observations track scotland s tectonic history from the precambrian emplacement of crustal basement and through the activation of large scale faulting and thrusting during the caledonian orogeny the shallow lithosphere beneath scotland has preserved a fossil anisotropic signature up to hundreds of millions of years after it was formed

shear stress at the base of shield lithosphere 1 one of the basic unresolved issues regarding plate tectonics is the degree of coupling between surface plates and convecting mantle below are the plates effectively decoupled from the mantle flow field by a low viscosity asthenosphere or are they strongly coupled to mantle flow while these two views are essentially incompatible they both do a reasonably good job of predicting the motions of the surface plates and cannot therefore be distinguished on this basis the significant distinguishing feature for these models is the magnitude of basal shear stress that is applied to the base of the plate while it is difficult to measure this stress directly it is possible in principle to measure the corresponding deformation of the plate through observations of seismic anisotropy and to infer stress here we focus on the canadian shield for which we expect strong plate mantle interaction we show that seismic anisotropy can be used to constrain the magnitude of the stress level applied to the base of the plate and to document the level of interaction between tectonic plates and the mantle below

using shear wave splitting measurements to investigate the upper mantle anisotropy beneath the north china craton distinct variation from east to west 1 a compilation of shear wave splitting measurements in the north china craton ncc is presented to investigate the upper mantle anisotropy beneath the region by applying the method of silver and chan 1991 the splitting parameters of fast polarization direction and delay time are determined from sks waveforms for 38 broadband stations the results reveal the presence of small to large seismic anisotropy in the mantle beneath the ncc the most striking result is the distinct difference of fast polarization directions between the eastern block eb and the central zone cz of the ncc the fast polarization directions trend se beneath the eb which implies that northwestward mantle flow has played a significant role in the reactivating of the eb during late mesozoic to early cenozoic and the boundary between the cz and the eb was possibly a west limit where the mantle flow was deflected however a collision event occurred in early proterozoic was also a possible cause for the anisotropy of the lithosphere beneath the cz depending on the craton lithosphere beneath the region had not been completely reformed by later tectonic activities

shear wave splitting observations in the archean craton of western superior shear wave splitting observations in the archean superior province of the canadian shield show moderate to large delay times 1 1 2 1 s with azimuths suggesting upper mantle anisotropy is subparallel to the structural grain of the craton two regions with uniform anisotropy azimuth 62 7 and 87 3 are separated by a transitional zone showing strong dependence of observed azimuth on source direction suggesting that lateral structure is being observed no splitting was observed at stations near the coast of hudson bay continuing a pattern of weak splitting in surrounding proterozoic orogenic belts current absolute plate motion is consistent with the direction of anisotropy but would not explain the regional contrasts instead the anisotropy appears to be related to archean structure and tectonic history

metamorphism of reduced granulites in low co 2 vapour free environment pervasive flooding of co2 has been proposed as the cause of granulite facies metamorphism that is capable of producing many distinctive characteristics of the deep continental crust reduced water activity orthopyroxene bearing assemblages depletion of large ion lithophile lil elements and dehydration1 6 the calculations presented here of c o h fluid composition for conditions of granulite facies metamorphism show that oxygen fugacity f o2 estimates from many terranes are sufficiently low that the addition of co2 rich fluid causes graphite to precipitate for values of pressure p and temperature t common to granulites and with f o2 slightly below the quartz fayalite magnetite buffer qfm the addition of co2 sufficient to grow 10 vol orthopyroxene requires the precipitation of 1 5 vol graphite as 0 1 vol graphite is readily recognizable but is not reported in most low f o2 granulites these rocks have not been flooded by co2 and low f h2o is probably due to extraction of a magma or recrystallization of an already dry rock7 8 granulite terranes may thus result from a combination of these three processes and the dominance of any one cannot now be demonstrated on a regional basis

insights into rifting from shear wave splitting and receiver functions an example from ethiopia summary seismic anisotropy beneath broad band stations in the vicinity of the east african rift are compared with those on stable cratonic parts of africa and arabia such measurements offer potential constraints on rift processes absolute plate motions apm and tectonic structure new sks shear wave splitting parameters are analysed beneath the broad band stations of furi and aae ethiopia bgca central african republic and rayn saudi arabia the number of events considered at the four stations varies from 13 to 32 and provides good azimuthal coverage stations on or near the rift show the polarization of the fast shear wave aligned parallel to the rift axis the magnitude of the splitting delay t increases northward along the east african rift previously published measurements in kenya show the smallest splitting value 1 0 s whilst the djibouti station atd shows the largest splitting 1 6 s the ethiopian results t 1 38 0 03 s 36 1 show constancy in t and with respect to backazimuth thus suggesting a single anisotropic layer beneath the stations there is no observed correlation of with apm direction less splitting t is observed beneath cratonic parts of africa bgca in central africa shows splitting parallel to the inferred direction of transpression not the apm direction receiver function analysis at furi and aae supports evidence from refraction experiments of thick crust ca 40 km in the region of continental rifting however the analysis shows a deeper interface at a depth of 90 km also this interface may mark the base of the lithosphere in this region one interpretation of the splitting results is that the anisotropy at the ethiopian stations is the result of aligned melt in this upper 90 km of lithosphere a 1 per cent volume fraction of melt aligned in thin aspect ratio 0 03 vertical ellipsoidal pockets generates sufficient splitting to explain the data higher splitting magnitudes in the north correlates with higher melt production observed in the ethiopian part of the rift alternatively anisotropy may be the result of the alignment of olivine in the asthenosphere parallel to the ridge axis as material flows laterally to fill the gap caused by lithospheric extension this would either suggest a northward thickening anisotropic layer beneath the rift or enhanced olivine alignment as a result of changes in strain whatever the mechanism it appears that the anisotropy is sensitive to and provides insight into the transition from continental to oceanic rifting in the northern ethiopian rift

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

fluid heterogeneity during granulite facies metamorphism in the adirondacks stable isotope evidence the preservation of premetamorphic whole rock oxygen isotope ratios in adirondack metasediments shows that neither these rocks nor adjacent anorthosites and gneisses have been penetrated by large amounts of externally derived hot co2 h2o fluids during granulite facies metamorphism this conclusion is supported by calculations of the effect of fluid volatilization and exchange and is also independently supported by petrologic and phase equilibria considerations the data suggest that these rocks were not an open system during metamorphism that fluid rock ratios were in many instances between 0 0 and 0 1 that externally derived fluids as well as fluids derived by metamorphic volatilization rose along localized channels and were not pervasive and thus that no single generalization can be applied to metamorphic fluid conditions in the adirondacks