propagation characteristics of lg across the tibetan plateau abstract we present the results of a study on the propagation of lg within the tibetan plateau we analyze over 1200 seismograms for lg propagation from 185 local and regional earthquakes recorded at distances from 150 to 2000 km 52 of which are located within the tibetan plateau although the propagation of lg has been observed across most of asia lg has not been observed for paths crossing the tibetan plateau we analyze paths that cross the plateau boundaries and paths contained entirely within the tibetan plateau lg is generated within the tibetan plateau and can propagate to epicentral distances of at least 600 km for events located outside of the tibetan plateau lg is either absent or severely diminished for paths that cross both the himalayan and kunlun ranges confirming that the margins of the plateau weaken lg transmission based on the observations from our qualitative analysis we invert lg amplitudes for event station paths that are confined to the tibetan plateau to estimate the quality factor q the inversion yields a frequency dependent q function q f 366 37 f 0 45 0 06 0 5 f 16 h z similar observations in other areas indicate that frequency dependent apparent q within the tibetan plateau is well below that expected for a typical continental interior instead it has a frequency dependence that is characteristic of a tectonically active region

s wave residuals from earthquakes in the tibetan region and lateral variations in the upper mantle abstract mean s wave residuals from 46 earthquakes within and on the margins of the tibetan plateau exhibit systematic lateral variations that do not correlate well with elevation or with simple aspects of the geologic history the earliest s waves come from earthquakes in western tibet the karakorum and the western himalaya and the latest come from earthquakes in north central tibet although s waves from earthquakes in the himalaya tend to be early the east west variation in residuals across tibet is at least as large as the north south difference between the himalaya and northern tibet if the variations in residuals are a reflection of temperature variations in the upper mantle associated with convection then upwelling beneath north central tibet seems to be flanked by downwelling in western eastern and probably southern tibet this convective flow might reflect the detachment and removal of thickened mantle lithosphere beneath tibet

evidence from earthquake data for a partially molten crustal layer in southern tibet earthquake data collected by the indepth ii passive source experiment show that there is a substantial south to north variation in the velocity structure of the crust beneath southern tibet north of the zangbo suture beneath the southern lhasa block a midcrustal low velocity zone is revealed by inversion of receiver functions rayleigh wave phase velocities and modeling of the radial component of teleseismic p waveforms conversely to the south beneath the tethyan himalaya no low velocity zone was observed the presence of the midcrustal low velocity zone in the north implies that a partially molten layer is in the middle crust beneath the northern yadong gulu rift and possibly much of southern tibet

theoretical study of the excitation spectral characteristics and geometrical attenuation of regional seismic phases abstract we present a theoretical study of the generation and geometrical attenuation of regional crustal phases we do this through the computation of seismograms in the epicentral distance range from 60 to 500 km the geometrical attenuation of lg waves with epicentral distance is of the form r 0 83 pg wave amplitudes display a much stronger decay of the form r 1 5 the spectral density of the crustal transfer function for pg waves is relatively flat for frequencies between 0 1 and 5 hz while lg wave spectra strongly fall off beyond 2 to 3 hz the excitation of pg wave is insensitive to the depth of the source within the crust while the lg amplitude is about 50 per cent higher for a source in the upper and middle crust than in the lower crust the amplitudes of these two phases drastically decrease when the source is below the moho these results illustrate the important role of wave guide played by the crust for the propagation of lg and pg we find that the geometrical attenuation of pg and lg waves is independent of source mechanisms in the case of an explosion the excitation of pg is insensitive to the source depth the lg wave amplitude is small in comparison to pg and rayleigh waves and depends on the closeness of the source to an interface or to the free surface

high velocity zone beneath the southern tibetan plateau from p wave differential travel time data we analyzed p wave travel time data of earthquakes along a north south vertical cross section in the central tibetan plateau to search for high velocity zones related to downwellings in the upper mantle differential travel time residuals of an event in southern tibet vs north central and himalayan events show relatively fast arrivals at stations in the northwest southeast direction results of numerical simulation with simple velocity anomaly models suggest that a narrow nearly vertical high velocity zone extending to at least 400 km beneath the southern plateau can account for the observed fast arrivals this high velocity zone may result from cold downwellings predicted by the delamination model for continental convergence

mineralogy age and fluid geochemistry of the rila emerald deposit bulgaria gem quality emerald is found within granitic pegmatites cutting talc schists in the western rhodopa massif in southwestern bulgaria the 40 ar 39 ar data for phlogopite from emerald bearing phlogopite schist yield a plateau age of 34 2 0 4 ma which indicates the emplacement of the pegmatite during late alpine orogenesis emerald crystals form within the metasomatic zone near the contacts between altered pegmatite and talc schists oxygen and hydrogen isotope analyses of emerald l8 o 9 4 d 29 8 to 32 7 combined with fluid inclusion study indicate that a single stage of a homogeneous fluid in the system h 2 o co 2 nacl of metamorphic origin produced the pegmatite talc schists hydrothermal alteration at temperatures between 300 and 400 c

emerald deposits and occurrences a review abstract emerald the green gem variety of beryl is the third most valuable gemstone after diamond and ruby although it is difficult to obtain accurate statistics colombia supplies most an estimated 60 worth more than 500 000 000 per year of the world s emeralds however there is speculation that the emerald mines in colombia are becoming depleted brazil currently accounts for approximately 10 of world emerald production emeralds have also been mined in afghanistan australia austria bulgaria china india madagascar namibia nigeria pakistan south africa spain tanzania the united states and zimbabwe because it is difficult to obtain accurate analyses of beryllium most published analyses of beryl are renormalized on the basis of 18 oxygen and 3 be atoms per formula unit the color of emerald is due to trace amounts of chromium and or vanadium replacing aluminum at the y site in most cases the cr content is much greater than that of v to achieve charge balance the substitution of divalent cations at the y site is coupled with the substitution of a monovalent cation for a vacancy at a channel site beryl is relatively rare because there is very little be in the upper continental crust unusual geologic and geochemical conditions are required for be and cr and or v to meet in the classic model be bearing pegmatites interact with cr bearing ultrama c or ma c rocks however in the colombian deposits there is no evidence of magmatic activity and it has been demonstrated that circulation processes within the host black shales were sufficient to form emerald in addition researchers are recognizing that regional metamorphism and tectonometamorphic processes such as shear zone formation may play a signi cant role in certain emerald deposits a number of genetic classification schemes have been proposed for emerald deposits most are ambiguous when it comes to understanding the mechanisms and conditions that lead to the formation of an emerald deposit studies of individual emerald deposits show that in most cases a combination of mechanisms magmatic hydrothermal and metamorphic were needed to bring be into contact with the chromophores this suggests the need for a more flexible classification scheme based on mode of formation stable isotopes can be used to estimate the contribution of each mechanism in the formation of a particular deposit such estimates could perhaps be more precisely defined using trace element data which should reflect the mode of formation emerald may be identified in the field by color hardness and form it will tend to show up in stream sediment samples but because its specific gravity is relatively low it will not concentrate in the heavy mineral fraction in colombia structural geology the sodium content of stream sediment samples and the lithium sodium and lead contents of soil samples have all been used to find emerald occurrences exploration for gem beryl could result in the discovery of new occurrences of non gem beryl or other be minerals that could become new sources of be and be oxide future efforts should go towards creating a comprehensive data base of emerald compositions including trace elements determination of the role of metamorphism in the formation of some emerald deposits improved classification schemes and more effective exploration guidelines

timing of tibetan uplift constrained by analysis of volcanic rocks the tibetan plateau has had a central role in the development of recent models for the mechanics of mountain belts1 2 and cenozoic global climate change3 the present elevation and extensional deformation of the plateau probably result from uplift owing to convective thinning of the underlying lithospheric mantle1 2 an age for the uplift would provide a valuable constraint on these models but because recently proposed indicators of uplift are all climate dependent they are equivocal possibly reflecting global cooling rather than regional uplift4 here we present new geochemical data on post mid miocene lavas from the plateau which show that the lavas were derived from the lithospheric mantle simple thermal arguments indicate that the generation of such magmas also necessitates thinning of the lithospheric mantle thus volcanism is coincident with uplift providing a climate independent means of dating the onset of uplift dating by the laser 40ar 39ar technique places the beginning of this volcanism and therefore the time of uplift of the tibetan plateau at 13 myr ago

oxygen isotope composition as a tracer for the origins of rubies and sapphires oxygen isotopic compositions of rubies and sapphires from 106 deposits worldwide as well as heated natural corundum have been measured in this study artificially heated corundums have the same oxygen isotopic composition as unheated material the 18o 16o ratio of natural corundum is a good indicator of its geological environment of formation the consistently restricted 18o range found for each type of deposit is explained by host rock buffering during fluid rock interaction the 18o constrains the geological source of the major type of gem quality rubies sold on the market and brings new insight to gems found in placers high quality blue sapphires from kashmir andranondambo and sri lanka have specific oxygen isotopic ranges but they overlap those of mogok in myanmar combined with traditional gemology techniques oxygen isotope analysis will contribute toward defining the origin of some commercial high value blue sapphires especially from kashmir

observations of regional phase propagation across the tibetan plateau we present observations of regional phase velocity and propagation characteristics using data recorded during a 1 year deployment of broadband digital seismic stations across the central tibetan plateau along the qinghai tibet highway from golmud to lhasa previous seismological studies within this region have had to rely on earthquakes recorded almost exclusively at stations outside of the plateau we have the opportunity to study numerous source receiver paths confined entirely within the tibetan plateau our analysis concentrates on travel time amplitude and frequency content measurements of the pg pn and sn phases pn can be clearly picked for all observed paths and propagates at an average velocity of 8 16 0 07 km s within the tibetan plateau sn however shows dramatic variations in propagation efficiency across the tibetan plateau that is strongly dependent on frequency we observe that sn rapidly decreases in frequency and amplitude as it passes through the northern portion of the plateau we show that in general sn propagation efficiency decreases with increasing frequency content we use 122 events from outside of the plateau and 61 from within to refine the boundaries of a region of inefficient high frequency sn propagation specifically we show that a larger portion of the northern tibetan plateau attenuates sn energy than was previously suggested in the southern plateau where high frequency sn is observed we computed an average velocity of 4 59 0 18 km s we also observed that the pn velocity within this region of inefficient high frequency sn propagation is nearly 4 slower than the pn velocity computed for paths restricted to the southern plateau and that the crust is about 10 km thinner than in the south the coincident locations of inefficient sn propagation and slow pn velocity is commonly observed in regions of active tectonics our results add constraints to the velocity structure of the lithosphere beneath the tibetan plateau and require first order lateral variations in the uppermost mantle structure despite the relatively uniform topography of the plateau

oxygen isotope systematics of gem corundum deposits in madagascar relevance for their geological origin the oxygen isotopic composition of gem corundum was measured from 22 deposits and occurrences in madagascar to provide a gemstone geological identification and characterization primary corundum deposits in madagascar are hosted in magmatic syenite and alkali basalt and metamorphic rocks gneiss cordieritite mafic and ultramafic rocks marble and calc silicate rocks in both domains the circulation of fluids especially along shear zones for metamorphic deposits provoked in situ transformation of the corundum host rocks with the formation of metasomatites such as phlogopite sakenite and corundumite secondary deposits placers are the most important economically and are contained in detrital basins and karsts the oxygen isotopic ratios 18o 16o of ruby and sapphire from primary deposits are a good indicator of their geological origin and reveal a wide range of 18o vienna standard mean ocean water between 1 3 and 15 6 metamorphic rubies are defined by two groups of 18o values in the range of 1 7 to 2 9 cordieritite and 3 8 to 6 1 amphibolite magmatic rubies from pyroxenitic xenoliths contained in the alkali basalt of soamiakatra have 18o values ranging between 1 3 and 4 7 sapphires are classified into two main groups with 18o in the range of 4 7 to 9 0 pyroxenite and feldspathic gneiss and 10 7 to 15 6 skarn in marble from andranondambo the 18o values for gem corundum from secondary deposits have a wide spread between 0 3 and 16 5 the ruby and sapphire found in placers linked to alkali basalt environments in the northern and central regions of madagascar have consistent 18o values between 3 5 and 6 9 ruby from the placers of vatomandry and andilamena has 18o values of 5 9 and between 0 5 and 4 0 respectively the placers of the ilakaka area are characterized by a huge variety of colored sapphires and rubies with 18o values between 0 3 and 16 5 and their origin is debated a comparison with oxygen isotope data obtained on gem corundum from eastern africa india and sri lanka is presented giant placer deposits from sri lanka madagascar and tanzania have a large variety of colored sapphires and rubies with a large variation in 18o due to mingling of corundum of different origin mafic and ultramafic rocks for ruby desilicated pegmatites for blue sapphire syenite for yellow green and blue sapphire and skarn in marbles for blue sapphire

a novel technique for measuring lg attenuation results from eastern canada between 1 to 10 hz spectral amplitudes of regionally recorded lg waves are studied in detail between 0 6 and 10 hz using vertical component velocity seismograms of the eastern canada telemetered network stations and a supplementary seismic research observatory type station at glen almond gac quebec we find that the site responses vary among these stations by more than a factor of 3 within the frequency range of interest furthermore they are found to be strongly frequency dependent consequently it is essential that they be taken into consideration in studies of lg wave attenuation and lg source spectra of regionally recorded seismic events we present a new method of measuring interstation surface wave attenuation which is closely related to the conventional two station method while retaining all the desirable features of the conventional two station method the new technique which we will call the reversed two station method allows simple direct one parameter determination of the lg wave attenuation from sparse spectral data in a manner unaffected by station site effects and associated instrument error the reversed two station method is successfully tested over weakly attenuating short 53 to 210 km interstation paths in eastern canada a difficult experimental condition by normal standards the lg attenuation coefficient 0 6 to 10 hz in eastern canada is found to be frequency dependent and of the form f 0 0008 f 0 81 km 1 at higher frequencies the lg attenuation appears to be essentially frequency independent this latter finding is preliminarily interpreted as evidence that regionally recorded lg waves in the canadian shield are as in the case of lg waves propagating through the structurally complex appalachian province contaminated by the high frequency coda of sn waves the lg contamination over the shield paths becomes severe starting at 14 hz twice the frequency above which the lg signal propagating over the appalachian province becomes completely dominated by non lg arrivals

strong lg wave attenuation in the northern and eastern tibetan plateau measured by a two station two event stacking method 1 the regional seismic phase lg which involves shear wave reverberations in the crust is strongly attenuated along propagation paths in northern and eastern tibet robust estimates of the attenuation quality factor qlg in these regions are obtained using a two station two event stacking method for broadband vertical component lg data in the passband 0 2 1 0 hz we find average 1 hz qlg values qo of 79 4 in northern central tibet and 119 17 in eastern tibet for the 0 2 0 5 hz passband localized regions of eastern tibet have qo estimates ranging from 66 to 121 these results confirm and augment the spatial coverage of recent estimates of low qo values in north central and south central tibet and support the notion of widespread partial melting of the crust throughout the plateau

implications of crustal property variations for models of tibetan plateau evolution shear coupled teleseismic p waves sampling the interior of the tibetan plateau provide evidence of systematic variations in crustal structure the crust thins by up to 20 km from south to north with a concomitant increase in poisson s ratio from normal values in the south to unusually high values in the north this suggests that the crust of the northern plateau is partially melted due to high temperatures these changes imply spatial and perhaps temporal variations in the way the elevation of the high plateau is created and maintained

mineralogical and microtextural parameters in metals ores traceability studies abstract the demand in mineral resources is increasing rapidly but there is a lack of transparency in the trade of concentrated raw mineral materials because of speculation and involvement in the finance of armed conflicts because of the distance between primary extraction and the final production sites it is difficult to check the origin of these products an identity card is required for mineral commodities so that trading in the industry can be verified and the traceability of concentrates ensured this problem may be considered as an inversion process studying the products sold to identify the original ore the discriminant parameters are mineralogical composition identification of textural microfacies of the target minerals pseudo paragenetic sequence and the contents and distribution of minor elements of target minerals for base metal the selected target minerals are pyrite for its ubiquity sphalerite for its ability to host numerous discriminant and potentially valuable minor elements in its lattice and chalcopyrite for its proximity with the two other minerals the kolmogorov smirnov distance and the colin white test are used to compare the chemical composition of the three target minerals the application to volcanic massive sulfide ore deposits shows that it is possible to distinguish pyrite sphalerite and chalcopyrite between two ore deposits in the iberian pyrite belt province and seven ore deposits from the urals province using the selected characteristics ore deposits from different provinces may be discriminated using the identity cards as well as different deposits in the same province

fluid inclusions and stable isotope study of quartz tourmaline veins associated with beryl and emerald mineralization kafubu area zambia abstract the kafubu emerald area in zambia is an important producer of gemstone quality emeralds these are hosted mostly by metasomatic phlogopite rich zones max 2 m wide that rim numerous pegmatite dykes and epigenetic quartz tourmaline veins in mg rich metabasites where these dykes veins occur in other rock types the phlogopite rich zones are either missing or are very narrow and free of emeralds three types of hydrothermal quartz tourmaline mineralization were identified banded fine grained rock discordant to semiconcordant veins and late veins a fluid inclusion study of the discordant type of veins revealed the presence of aqueous carbonic and aqueous fluids the earliest fluids mostly trapped in the quartz rarely in beryl represent complex h 2 o co 2 n 2 ch 4 nacl cacl 2 mixtures x co2 0 25 to 0 04 with traces of n 2 and ch 4 only generally as for their salinity low medium and high salinity fluids were distinguished 0 10 12 25 and 34 wt nacl eq respectively each salinity class usually comprises both aqueous carbonic and aqueous fluids solid phases present in inclusions were studied by raman and or by sem edx daughter phases include halite sylvite and fe mn carbonate accidental phases are quartz beryl carbonate nahcolite pyrophyllite and muscovite the oxygen isotope composition 18 o of quartz and tourmaline is quite homogeneous 10 9 1 and 8 4 0 7 smow respectively and yield vein formation temperatures of 350 450 c and parent fluid composition 5 to 7 smow three evolution stages of quartz tourmaline beryl mineralization are inferred from fluid isochores and isotope thermometry early 620 400 mpa and 450 350 c main 400 150 mpa and 450 350 c and late 250 200 mpa and 350 280 c the early stage includes formation of banded quartz tourmaline rocks and of some quartz tourmaline veins most of the studied quartz tourmaline veins and all studied beryls were formed within the main stage late stage corresponds to the formation of late quartz tourmaline veins with lamellar tourmaline quartz intergrowths

regional wave propagation in china and its surrounding regions a comprehensive study of the gross characteristics of high frequency seismic wave propagation in china and its surrounding regions was accomplished to learn more about how regional geology affects high frequency phases amplitudes of the regional seismic waves sn and lg relative to p coda were used to map lateral variations of shear wave attenuation in the crust and upper mantle over 7000 digital seismograms from 13 broadband stations of the chinese digital seismic network and global seismic network in the frequency range 0 5 to 5 0 hz were visually examined propagation efficiencies of sn and lg were qualitatively analyzed by ranking their amplitudes relative to the p wave coda efficient sn propagation is observed in the tien shan tarim platform southern tibet yangzi paraplatform and sino korean platform we have confirmed results of previous studies by finding a lack of sn transmission in north central tibet the ryukyu and japan arcs burma and the baikal rift an important observation which has not been previously reported is that sn does not propagate across mongolia the elimination of sn and the occurrence of recent volcanism in north central tibet and mongolia suggest that there is partial melt in the uppermost mantle beneath those regions high frequency lg waves propagate efficiently for most of china indochina and mongolia lg signals are attenuated within central tibet as well as along its southern boundary crustal structure variations along with an anomalously low q crust are primarily responsible for weakened lg amplitudes in tibet propagation paths across oceanic crust in the sea of japan and the andaman sea show blockage of the lg phase as well this investigation has relevance to comprehensive test ban treaty monitoring by showing where certain regional phases will or will not propagate efficiently

strong lg attenuation in the tibetan plateau the widespread existence of strong lg attenuation in the tibetan plateau is further demonstrated by analysis of lg spectra on many paths within the plateau and quantitative estimates of the lg attenuation from low frequency lg signals for new localized path geometries strong path length dependent shifts of the lg spectra to lower corner frequencies with increasing distance are observed across the plateau consistent with a low regional average 1 hz lg attenuation value q 0 of about 125 there are clearly lateral variations within the plateau found in this and other recent studies with localized areas having q 0 values of 60 90 low enough to eliminate high frequency lg energy over path lengths of just several hundred kilometers while some localized areas may have higher q 0 values of up to 147 or higher a q 0 value of 103 is found in south central tibet compatible with recent work by others for higher frequencies on very localized scales and values from 83 to 147 are found in eastern tibet the lowest q 0 estimates found in tibet tend to be in areas for which there is evidence of volcanism and or partial melting within the crust however the strong regional attenuation may have a contribution from scattering by small scale crustal heterogeneity the strong lg attenuation in tibet gives a new constraint for understanding the tectonic development of the plateau and presents challenges to seismic monitoring of the region for possible clandestine nuclear tests

lateral variation of pn and lg attenuation at the cdsn station lsa abstract the propagation efficiencies of the regional phases lg and pn are indicative of how active and recent tectonics influence crust and uppermost mantle properties respectively in this study regional scale lateral heterogeneity of lg and pn attenuation for the region in and around the southern tibetan plateau is investigated the data set is comprised of seismograms recorded at the chinese digital seismogram network cdsn station lsa from regional events with epicentral distances within 11 attenuation of lg and pn is calculated using spectral methods and assuming constant q lg and q pn models for the frequency bands 0 3 to 3 0 hz and 0 5 to 4 0 hz respectively lateral variation in attenuation is estimated by analyzing data on an event by event basis significant lateral variation is observed with q lg decreasing from about 520 for events south of lsa to about 340 for events north of lsa and q pn 670 for southern backazimuths while q pn 240 for northern events for pn this north south variation is consistent with other observations indicating partially melted upper mantle beneath north central tibet for lg the azimuthal variation in attenuation indicates that lg propagation as observed at lsa is efficient for paths crossing the southern boundary of the tibetan plateau relative to paths within the plateau itself

geology and geochemistry of d o c isotope systematics of the qolqoleh gold deposit northwestern iran implications for ore genesis abstract the qolqoleh gold deposit is located in northwestern part of the sanandaj sirjan metamorphic belt northwestern iran igneous and sedimentary units exposed in the area have undergone greenschist metamorphism the area was affected by a ne sw trending shear zone and subsequent deformation two different types of mineralization are distinguished in the qolqoleh gold deposit based on geological structural conditions indicated by microtextural analysis ductile and then brittle ore forming processes are divided into three stages early i middle ii and late iii which include quartz pyrite i sulfides and gold ii and carbonate veinlets iii respectively the stage i fluids are characterized by 18o 15 5 at 440 oc and are thought to be deep sourced metamorphic waters the stage iii fluids with 18o 1 6 are shallow sourced meteoric waters whereas the stage ii fluids with 18o 13 1 are a mixture of deep sourced metamorphic and shallow sourced meteoric fluids based on comparisons of the d o c isotopic systematics the ore forming fluids with characteristic high 18o and 13c and low d originated from metamorphic devolatilization of cretaceous volcano sedimentary felsic to mafic metavolcanic rocks shale carbonate carbonaceous chert sequences locally rich in organic matter during late cretaceous continental collision of the afro arabian continent and the iranian microcontinent a crustal slab consisting of felsic to mafic metavolcanic rocks carbonate shale and carbonaceous chert was underthrust northwards beneath the central iranian microcontinent along the zagros fault during further contraction deformation was localized in reverse oblique slip structures with vergence toward south shear zones generally follow contacts between more competent and less competent rock units metamorphic devolatilization of this underthrust slab is the source of the ore forming fluids that generated the au ore belt which includes the qolqoleh gold deposit

a no lid zone in the central chang thang platform of tibet evidence from pure path phase velocity measurements of long period rayleigh waves pure path phase velocities of fundamental mode rayleigh waves in the period range 30 80s are obtained for different paths across the tibet plateau using a two event method with events located in tibet and aligned with world wide standard seismograph network stations at teleseismic distance evidence is found for significant lateral variations of structure of the crust and uppermost mantle in tibet in particular phase velocities obtained on paths contained in the chang thang platform are higher than average in tibet at periods shorter than 60s while remaining very low at longer periods when interpreting these phase velocities in terms of simple layered s velocity models it is found that contrary to other areas in tibet they are incompatible with the existence of a high velocity lid s velocities of 4 4 4 5 km s are found immediately beneath the moho and the crust is somewhat thinner 50 60 km than average in tibet this indicates a possible upwelling of hot asthenospheric material compatible with landsat evidence for subrecent basaltic volcanism and independent seismic evidence such as inefficient transmission of sn waves on the other hand a 70 km thick crust and very high upper mantle s velocities are found on the western border of tibet karakorum hindu kush compatible with previous finer scale results concerning the associated zone of intermediate depth seismicity

characteristics of lg attenuation in the tibetan plateau 1 lg a regional seismic wave comprised of multiple shear wave reverberations trapped in the crustal waveguide is important for magnitude estimation and source discrimination for monitoring nuclear testing treaties in stable continental regions lg propagates with a group velocity of about 3 5 km s and can often be observed at distances up to 4000 km to better understand the absence of high frequency lg arrivals for paths traversing the northern boundary of the tibetan plateau we investigate spatial variations of broadband 0 15 5 0 hz energy in the lg group velocity window 3 6 3 0 km s using regional waveforms recorded at the chinese digital seismic network station wmq vertical component seismograms are analyzed for 90 events with magnitudes of 4 4 mb 6 4 that occurred between 1987 and 1999 in the tibetan plateau and around its margins the lg amplitude spectra for events located near the northern margin of the plateau have apparent corner frequencies of 1 2 hz nearly identical to those for comparable size events at similar distances outside the plateau high frequency 1 hz lg energy recorded at wmq decreases rapidly as a function of source distance into the plateau a path length of 300 400 km within the northern plateau suffices to eliminate 2 5 hz lg energy for events in southern tibet with paths crossing the central portion of the tibetan plateau almost total lg extinction occurs even for energy in the low frequency band of 0 2 1 hz corresponding apparent corner frequencies of the lg amplitude spectra range between 0 2 and 0 4 hz the corner frequency shift is found to vary systematically with path length across the plateau linear regressions demonstrate that the shift in apparent corner frequency of lg amplitude spectra is negatively correlated with features of the tibetan plateau such as mean elevation along the paths or travel distance within the plateau above specified elevation thresholds the systematic variations in the amplitude and frequency content of energy in the lg window as a function of path length within the plateau indicate that strong crustal attenuation plays an important role in lg extinction for paths traversing central and northern tibet superimposed on any structural blockage effects associated with abrupt thinning of the crust near the northern boundary of the plateau spectral ratios of many event pairs along great circle paths give estimates of frequency dependent lg attenuation for paths crossing western central and eastern sectors of tibet the region of strong sn attenuation in northern central tibet also has strong lg attenuation with qlg for 1 hz on the order of qo 80 90 while in southern central tibet qo increases to about 316 and in eastern and western tibet qo is on the order of 120 200 for paths traversing the entire plateau the strong lg attenuation in northern central tibet is responsible for the so called lg blockage and may be associated with partial melting in the crustal low velocity layer in northern tibet our qlg values for tibet are significantly lower than most earlier estimates primarily as a result of not excluding blocked observations along with allowing for lateral variations within tibet index terms 7203 seismology body wave propagation 7255 seismology surface waves and free oscillations 7205 seismology continental crust 1242 keywords lg tibetan plateau nuclear event discrimination crustal attenuation lg blockage crustal partial melting

upper mantle velocity structure beneath the tibetan plateau from pn travel time tomography we inverted 1510 p arrival times from regional distances 333 1600 km in and around the tibetan plateau to map the lateral velocity variation within the uppermost mantle previous studies have placed first order constraints on upper mantle velocities but relied on data recorded almost exclusively at stations outside of the plateau we improve resolution by using 40 events recorded at stations within the tibetan plateau we combine these data with observations obtained from the international seismological centre isc to extend our coverage by including pn arrivals from 85 additional plateau events relocated in previous studies and recorded at stations in and around the tibetan plateau we use synthetic travel time data to evaluate the resolution of our data set the observations provide good resolution to about 1 over most of the plateau and surrounding regions our results show average pn velocities that are about 3 lower in the northern plateau relative to the southern plateau these variations correlate well with major tectonic features and previous geophysical observations in the qiangtang terrane of the northern plateau an area known to be inefficient for sn propagation pn is slow relative to both the plateau south of the banggong nujiang suture and the tectonically stable tarim basin north of the plateau this is strong evidence for the existence of partial melt within the uppermost mantle beneath the northern tibetan plateau however when laboratory estimates of relationships between temperature velocity and attenuation are applied a relatively small temperature variation 240 to 370 c is required to explain our pn velocity observations when combined with geochemical constraints from volcanics in the northern plateau our results strongly suggest that the mantle lid is intact beneath the northern plateau this result would preclude tectonic models involving wholesale delamination of the mantle lithosphere in the northern tibetan plateau

oblique stepwise rise and growth of the tibet plateau two end member models of how the high elevations in tibet formed are i continuous thickening and widespread viscous flow of the crust and mantle of the entire plateau and ii time dependent localized shear between coherent lithospheric blocks recent studies of cenozoic deformation magmatism and seismic structure lend support to the latter since india collided with asia 55 million years ago the rise of the high tibetan plateau likely occurred in three main steps by successive growth and uplift of 300 to 500 kilometer wide crustal thrust wedges the crust thickened while the mantle decoupled beneath gently dipping shear zones did not sediment infilling bathtub like of dammed intermontane basins formed flat high plains at each step the existence of magmatic belts younging northward implies that slabs of asian mantle subducted one after another under ranges north of the himalayas subduction was oblique and accompanied by extrusion along the left lateral strike slip faults that slice tibet s east side these mechanisms akin to plate tectonics hidden by thickening crust with slip partitioning account for the dominant growth of the tibet plateau toward the east and northeast

attenuation of multiphase surface waves in the basin and range province iii inversion for crustal anelasticity summary a combined inversion forward modelling procedure in which the frequency dependence of shear wave internal friction q 1 is allowed to vary with depth was developed and applied to selected rayleigh wave and lg attenuation data in the basin and range province of the south western united states both q and the frequency dependence of lg waves were used to constrain the models many models can explain rayleigh wave and lg data sets within their uncertainties but at 1 hz most have low values of q 50 80 in the upper 8 km of the crust rapidly increasing values to about 1000 at mid crustal depths and decreasing values at greater depths models which include a layer of higher q values 80 150 in the upper few kilometres of the crust overlying a region of lower q values cannot be precluded by the attenuation data of this study assuming that q varies with frequency as f models for which the frequency dependence is low 0 0 0 1 in the upper crust best explain the data of this study in the lower crust that frequency dependence is not well determined but the models which best explain both the fundamental mode and lg data and produce realistic models of q are characterized at 1 hz by high values of both q and its frequency dependence because of that frequency dependence q may be an order of magnitude lower at a period of 100 s 100 than it is at 1 s 1000 investigations of the effects of changing crustal velocity on values for lg q and its frequency dependence indicate that realistic velocity changes cause only small changes in those values and thus are inconsequential to our results the low q values and their constancy with frequency in the uppermost crust can be explained by fluid flow in a network of cracks in brittle rock increasing q with depth to 10 15 km can be explained by the closing of those cracks due to pressure and their enhanced healing with increasing temperature plastic flow at greater depths may contribute further to the dissipation of cracks and to further increases in q decreasing q values at greater depths can be explained as being the result of increasing temperature increasing content of partial melt enhanced dislocation motion or some combination of these effects

crust and upper mantle tomography in tibet using surface waves we have obtained maps of lateral variations of structure in the crust and lithosphere in tibet and surrounding regions by inversion of single station measurements of fundamental mode rayleigh wave phase velocities in the period range 25 100 sec for earthquakes located in central asia and observed at teleseismic digital stations in eurasia phase velocity maps are obtained using a tomographic method without a priori constraints on region boundaries the lateral variations of s velocity reveal a symmetric pattern on each side of crustal low velocity zone centered on the chang tang region north central tibet this zone is bounded by rather steep velocity gradients to the south and east about 250 and 400 km north and west of the frontal thrusts of the himalayas and lungmen shan respectively the northern boundary of the zone crosses the northern limit of the tibet plateau altyn tagh and kunlun extending into the southwest tarim and qaidam basins the low crustal velocities are consistent with a thick hot crust in the central part of tibet consistent with quaternary volcanism in chang tang the steep gradients towards higher velocities in the south and in the north are compatible with the documented continental subduction in the south and suggest that a similar situation might exist in the north

1 hz lg q tomography in central asia we have applied tomographic techniques to a data set of over 1700 regional distance lg amplitudes from 12 stations for paths inside a 30 by 40 region of central asia our purpose is to create high resolution path correction maps for use in regional distance source discrimination and magnitude determination as well as to study lg propagation characteristics in one of the most heterogeneous portions of the earth for 1 0 hz data tomography reduced variance 33 relative to a best fit uniform q model lgq varied with geologic region with low values in tibet the tien shan and the pamir range q 200 to 400 intermediate values in basins such as the tarim q 500 and high values for platforms and older crust q 800 the relatively efficient propagation across the tarim differs from the poor propagation that is generally observed across sedimentary basins

non pegmatitic deposits of beryllium mineralogy geology phase equilibria and origin non pegmatitic occurrences of be minerals constitute a diverse set of geologic environments of considerable mineralogical and petrological interest they currently provide the majority of the world s be ore and emeralds and they contain the greatest resource of these commodities of the approximately 100 be minerals known see chapter 1 by grew appendix a most occur in hydrothermal deposits or non pegmatitic igneous rocks where their distribution varies systematically with the setting and origin table 1 fig 1 view this table table 1 beryllium minerals in non pegmatitic deposits formulas use and occurrence 1 figure 1 chemography of the principal solid phases in the beo al2o3 sio2 h2o f2o 1 bash system with the projected positions of helvite group and alkali be silicates also shown are generalized fields for some of the major types natural of occurrences cf table 1 fig 4 see text for discussion beryllium minerals are best known from geologic systems associated with felsic magmatism they also occur in a variety of settings that lack evident igneous associations environments range from the surface to the deep crust and host rocks range from feldspathic to carbonate to ultramafic in composition genetically related igneous rocks are felsic and share low calcium and high f contents but are diverse in composition setting and origin compositions range from strongly peraluminous to peralkaline and can be silica undersaturated beryllium minerals also occur in metamorphic and basinal environments and are redistributed by surface processes table 2 summarizes the types and significance of major groups of occurrences by their lithologic associations figure 2 shows the global distribution of some important examples and regional belts for most types at least one example has been described in some detail and can be used to help evaluate general patterns however

attenuation of 1 6 s lg waves in eurasia we report our investigation of the lg attenuation characteristics in eurasia using digital recordings from the incorporated research institutions for seismology chinese digital seismic network broadband stations to maximize the utilization of the current station and source distribution in the region we have derived a new technique known as the source pair receiver pair sprp method for lg attenuation measurement the procedure eliminates seismic source and site effects leaving behind decontaminated amplitude data from which regional wave attenuation is estimated for lg attenuation studies the sprp method is more flexible and of greater general applicability than its predecessor the reversed two station method chun et al 1987 the application of the sprp method to our time domain analysis of 33 regional and far regional seismic events has provided stable regional average for lg attenuation in periods of 1 6 s our time domain analysis aided by a damped least squares inversion algorithm has led to robust measurements of lg attenuation q and its frequency dependence in nine subregions of tectonic significance regions of active tectonics and seismicity generally have lower q0 than that in the stable continental interior subregional q and are found compatible with the published results in areas of relatively uniform geology values are comparatively poorly determined suggesting that they are susceptible to the presence of crustal heterogeneities along the wave paths

oxygen isotope systematics of emerald relevance for its origin and geological significance oxygen isotopic composition of emerald from 62 occurrences and deposits in the world reveals a wide range in 18o smow between 6 2 and 24 7 the 18o values for each deposit are restricted and can be used to determine the origin of emerald from the world s most important producers the 18o value of emerald appears to be a fingerprint of its origin especially for gems of exceptional quality from colombia eastern emerald zone 18o 16 8 0 1 western emerald zone 18o 21 2 0 5 afghanistan 18o 13 5 0 1 pakistan swat mingora districts 18o 15 7 0 1 brazil santa terezinha de goias 18o 12 2 0 1 quadrilatero ferrifero 18o 6 9 0 4 and zimbabwe sandawana 18o 7 5 0 5 furthermore the 18o composition of emerald appears to be a good marker of its geological environment because the data suggest that host rock buffering of fluid 18o is considerable during fluid rock interaction

determination of the crustal structure in southern tibet by dispersion and amplitude analysis of rayleigh waves summary surface waves recorded by the nine broad band stations of the indepth ii experiment are analysed to study the crustal structure of southern tibet their frequency range is between approximately 0 015 and 0 050 hz i e between 20 and 60 s period phase velocity dispersion curves are calculated for the regions north and south of the tsangpo suture using wiener filtering of fundamental mode rayleigh waves incident upon the array with different backazimuths the two dispersion curves are inverted to obtain the s wave crustal models north and south of the suture they show that a low velocity layer is present in the lower crust north but not south of the tsangpo suture to confirm these findings variations in the amplitudes of the rayleigh waves across the suture are interpreted by numerical simulation the strong amplitude variations at a 30 s period are reproduced using a model with a low velocity layer in the lower crust north of the suture a normal high velocity layer to the south and a sharp transition between these layers

lithospheric structure of the qiangtang terrane northern tibetan plateau from complete regional waveform modeling evidence for partial melt we report models of p and s wave velocity and attenuation for the the crust and uppermost mantle of the qiangtang terrane northern tibetan plateau inferred by fitting reflectivity synthetic seismograms to observed complete regional waveforms the data are three component broadband seismograms recorded by the 1991 1992 iris passcal incorporated research institutions for seismology program for array seismic studies of the continental lithosphere tibetan plateau experiment and global seismic network stations in the region the qiangtang terrane has thick crust 65 5 km with p and s wave velocities of 6 1 6 3 and 3 34 3 43 km s respectively yielding an anomalously high crustal poisson s ratio of 0 29 0 02 seismic velocities of the upper mantle of the qiangtang terrane are normal for p waves and slow for s waves 8 10 and 4 35 4 41 km s respectively with a high mantle poisson s ratio of 0 29 0 01 attenuation in the crust and upper mantle is high qp 100 200 and qs 44 89 modeling of the broadband p waveforms suggests that a decrease in mantle velocity occurs at about 160 km depth in the mantle however this is not unambiguously supported by the data and modeling the crust and uppermost mantle of the qiangtang terrane probably contains partial melt based on the high poisson s ratio low shear wave velocities and low q the absence of high frequency sn and the presence of volcanism of mantle lithospheric origin support the presence of partial melt crustal and uppermost mantle structure in the qiangtang terrane is different from that for the lhasa terrane immediately to the south based on results of our previous studies the average crustal p and s wave velocities are 4 faster and 2 slower respectively in the qiangtang terrane relative to the lhasa terrane this yields a significant difference in the crustal poisson s ratio with values of 0 29 for the qiangtang terrane and 0 25 for the lhasa terrane differences in the uppermost mantle p and s wave velocities and poisson s ratios of these two adjacent terranes cannot be explained by temperature differences alone using the mantle temperature estimates of mcnamara et al 1997 we suggest that partial melt of an ultramafic composition beneath the qiangtang terrane fits the velocity and poisson s ratio estimates

strong crustal heterogeneity in the bolivian altiplano as suggested by attenuation of lg waves using the seismological data of four temporary networks installed across the andes of bolivia and chile between 1990 and 1995 we study the amplitudes of the lg phase for 45 regional events located at crustal depths this analysis reveals that lg is strongly attenuated for most of the ray paths crossing the altiplano independent of the azimuth of propagation to map the anomalies and evaluate their frequency dependence we conduct a regionalization of the crustal average qs factor using a damped least squares inversion technique in the frequency range 0 6 5 hz the resulting maps delineate regions with contrasting values of apparent attenuation the altiplano is characterized by strong attenuation with an average qs of 100 at 1 hz whereas the cordilleras are regions of weaker attenuation the frequency dependence of qs is investigated in an attempt to separate the source of the apparent attenuation into scattering and anelastic absorption within the hypothesis of a weak frequency dependence of the intrinsic q we show that at 1 hz scattering plays a major part in the attenuation of lg waves whereas at 4 hz both mechanisms are of equal importance these results indicate that the attenuation of lg waves in the altiplano is due more to scattering by small scale heterogeneities than to a high percentage 1 of partial melt involving a large part of the crust

lateral variations in compressional velocities beneath the tibetan plateau from pn traveltime tomography summary a back projection algorithm is applied to 353 p traveltime measurements to image the lateral variations in compressional velocity in the uppermost mantle in and around tibet prior to the tomographic inversion we refined the apparent p slowness measurements based on a recently developed relocation procedure zhao helmberger 1991 and mathematical formulations of zhao 1993 these refinements are crucial in obtaining a high quality velocity image we obtained an average p velocity value for the uppermost mantle of 7 93 kms l and an average mantle p velocity gradient of 3 1 x lo s for tibet the final 2 d p velocity image in the uppermost mantle is characterized by a large 7 to 9 in dimension low velocity down to about 7 78 km s region in north central tibet and by large high velocity up to 8 11 km s i regions in the western and eastern flanks of tibet the crustal thickness is greater than 70 km inside the tibetan plateau it decreases toward the southern eastern and western boundaries of the plateau at and beyond these boundaries the crustal thickness is similar to those beneath the eurasian shield rigorous analyses of resolution and error suggest that 1 there is a pronounced difference between the tomographic velocity image obtained with and without the refinements to the p slowness measurements indicating a significant improvement in the quality of our inversion by these refinements 2 random errors in measuring earthquake origin times and locations and in the p arrival times should not cause errors of more than 0 1 km s in the velocity image 3 the resolution is generally within 6 to 10 the velocity image obtained in this study is consistent with previous s p traveltime studies and geothermal petrological studies and is consistent with a mantle convection model in which a convective upwelling is occurring in north central tibet and downwellings occurring in the surrounding regions molnar 1988 1990

interpretation of temperature measurements from the kaiko nankai cruise modeling of fluid flow in clam colonies during the kaiko nankai detailed submersible survey numerous measurements of the temperature gradient inside the sediment were performed on the deepest active zone of fluid venting which is situated on the anticline related to the frontal thrust using the ifremer t naut temperature probe operated from the submersiblenautile we thus obtained the temperature structure below different types of clam colonies associated with fluid venting we used the finite element method to model the thermal structure and fluid flow pattern of these vents and to determine the velocity of upward fluid flow through the colonies on a biological basis four types of clam colonies are defined each biological type has distinctive thermal characteristics and corresponds to a particular fluid flow pattern darcian flow velocity in the most active type of colony type a is of the order of 100 m a the total amount of fluid flowing through colonies in the studied area is estimated to be 200 m3 a 1 per metre width of subduction zone most of the flow is vented through type a colonies this value is more than one order of magnitude too high to be compatible with the amount of water available from steady state compaction of sediments in the whole wedge thermal arguments suggest that downwelling of seawater occurs around type a colonies and that seawater is then mixed with upcoming fluids at a depth of 1 or 2 metres furthermore finite element modeling shows that a salinity difference of a few parts per mil between the upcoming fluids and seawater is sufficient to drive convection around the colonies as water samples from a few vents indicate that the fluid source should actually be significantly less saline than seawater we propose that the very high fluid flows measured are a consequence of the dilution of the fluid of deep origin with seawater by a factor of 5 to 10

investigation of teleseismic lg blockage and scattering using regional arrays abstract incoherent beamforming and continuous frequency wavenumber polar scan analysis have been applied to study the characteristics of p coda and lg waves recorded at the regional arrays noress and arcess from presumed explosions on the northwestern russian platform and at test sites of novaya zemlya and eastern kazakh also recordings at the graefenburg array near the new geress regional array of novaya zemlya events and an eastern european platform white sea event have been compared with the norwegian array recordings of the same events the white sea explosion at distances of 14 and 22 from noress and graefenburg respectively exhibits lg rms amplitudes on incoherent beams that exceed those of sn however for explosions at novaya zemlya at 20 from noress and eastern kazakh at 30 from arcess the lg waves have much smaller amplitudes than sn in the case of novaya zemlya the lg wave is almost completely blocked at both noress and arcess but is well recorded at graefenburg this reduction of the lg amplitudes relative to those from events at longer distances cannot be accounted for by anelastic attenuation and spreading comparison of propagation paths suggests that the reduction in amplitudes may be due to blockage and scattering of lg in the barents sea analysis of sn coda waves on incoherent beams and continuous frequency wavenumber analysis polar scans suggests that they can result from lg to sn scattering from the coastline between the barents sea and scandinavia recent geologic studies have revealed significant variations in crustal structure in the barents sea basin which may cause the blockage of lg waves from novaya zemlya in the case of eastern kazakh explosions arcess incoherent beams show much reduced lg amplitudes relative to the p coda waves compared with noress even though the distance to arcess 31 is less than that to noress 38 this reduction in lg amplitude at both noress and arcess is accompanied by enhancement of p and sn coda the timing of the coda waves and the propagation paths suggest that lg waves may be partially blocked and scattered by the ural mountains and that the p and sn coda waves may result from lg to pn pn to sn sn to lg and lg to sn conversions from lateral heterogeneities in the crust beneath the urals the blockage is greater for the eastern kazakh to arcess path because it crosses the middle part of the urals whereas the eastern kazakh to noress path crosses the southern part of the urals chain these blockage effects seem to result from sudden lateral variations in shallow crustal structure such as variations in sediment thicknesses in the sedimentary basins of the russian platform and adjacent to the urals these blockage effects can be predicted from geologic studies and need to be accounted for when using lg amplitudes to estimate absolute yields of explosions and for event identification

s p wave travel time residuals and lateral inhomogeneity in the mantle beneath tibet and the himalaya the average s wave travel time residuals from 11 earthquakes each in tibet and the himalaya imply that there are marked lateral variations in structure beneath tibet and the himalaya average s p wave residuals measured with respect to the jeffreys bullen j b tables for each of 11 earthquakes in the himalaya are less than 1 s average j b s p residuals from 10 of 11 earthquakes in tibet however are greater than 1 s even when corrected for the large crustal thickness of tibet the largest values 2 5 3 2 3 8 4 5 and 4 9 s are for five events in central and northern tibet and they imply that the average velocities in the crust and upper mantle of these portions of tibet are 4 10 lower than those beneath the himalaya therefore it is unlikely that a shield structure underlies these parts of tibet unless these s p residuals are due to variations in structure deeper than 250 km s p residuals from two other events each in western and eastern tibet are 1 2 s late and hence suggest that velocities beneath these areas are also somewhat lower than beneath shields the mean s wave velocity seems not only to be different beneath tibet and the himalaya but also seems to increase from a minimum beneath northern tibet to values close to those of the average earth toward the east south and west appendix a is available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington dc 20009 document b84 010 2 50 payment must accompany order

amplitude ratio tomography for regional phase q 1 we image regional seismic phase attenuation q by extending amplitude ratio techniques which are widely used to determine average q within a sampled region to obtain laterally varying q structure using tomographic techniques the method assumes that source radiation is isotropic and that the amplitude ratios can be modeled by a two dimensional q map and relative site terms application to 2286 1 hz lg amplitudes from 816 events recorded at 35 central and east asia stations yields a misfit of 0 09 log10 units a variance reduction of 73 relative to the uniform q model that best fits the data and laterally varying q from 100 to 1000 with patterns closely following regional geology the image is resolved on average to 2 5 with resolution peaking at 1 5 in the best covered areas which is remarkable given the sparse station geometry employed the high resolution results from the dense distribution of earthquake sources and is aided by the increased confidence in source locations provided by merging catalogs from many different agencies

the excitation of lg waves by explosions a finite difference investigation several mechanisms have been proposed to explain the excitation of the regional lg phase by explosions we examine some of these mechanisms with finite difference simulations an energy flux technique is used to analyze the synthetic signals the method is adapted from conventional array analysis techniques and gives a clear characterization of wave field intensity wave slow ness and propagation direction the vector energy flux in the model is studied instead of the scalar distribution of energy in the space time domain the cal culations show that for an explosive source in a high p wave velocity crust p wave energy cannot be effectively trapped in the crust and the nongeomet rical phase s may be the primary contributor to thelg phase for an explosive source buried in a low velocity upper crust i e where the p wave velocity is lower than the upper mantle s wave velocity part of the ps wave can be trapped in the crustal wave guide to form the lg phase the relative lg excitation de pends on the ratio between the p wave velocity in the source region and the s wave velocity in the uppermost mantle scattering effects can transfer p wave or surface wave energy into lg but the efficiency strongly depends on both the wave frequency and the characteristic size of the scatterers any source process that directly excites s waves is more efficient for generating lg than the afore mentioned mechanisms spall directly generates s wave energy and conse quently is an efficient source for lg for actual explosions all of these factors may simultaneously contribute to lg excitation determining their relative im portance and interactions requires additional observations

conditions for early cretaceous emerald formation at dyakou china fluid inclusion ar ar and stable isotope studies the dyakou emerald occurrence is located in malipo county in the province of yunnan southern china the occurrence lies in the northern part of the laojunshan song chay metamorphic core complex which is exposed in an area of approximately 2 000 km2 and extends across the border between china and vietnam emerald mineralization is hosted by pegmatite and associated quartz veins that intrude deformed proterozoic biotite muscovite granofels and schist hydrogen and oxygen isotope results from the emerald channel waters and emerald respectively are consistent with an igneous fluid source the 18o fractionation between emerald and quartz yields vein temperatures of 365 to 420 c fluid inclusions indicate that the emerald precipitated from saline brines ranging from almost pure water to 10 5 mass percent nacl equiv fluid inclusion isochores intersected with 18o data yield pressures changing along the geothermal gradient from 1 500 to 3 300 bars ar ar geochronology of biotite and muscovite from the emerald veins yields consistent ages of 124 1 ma these constraints combined with field observations indicate that the dyakou emerald deposit is consistent with the igneous related model for emerald formation

phase and spectral ratio discrimination between nts earthquakes and explosions part i empirical observations seismic discrimination of small magnitude mb 0 5 hz regional measurements in this study we evaluate the most promising short period discriminants p wave to s wave phase amplitude ratios and low to high frequency spectral amplitude ratios for a data set from the nevada test site the data consist of 130 underground nuclear explosions one very large chemical explosion and 50 earthquakes ranging from magnitude 2 to 6 recorded at two broadband digital seismic stations operated by lawrence livermore national laboratory in contrast with much previous discrimination work in the western united states the common stations and paths for all the events in this study allow the discriminants to be evaluated without applying propagation corrections and differences between events can be ascribed to very near source effects the source effects found to have the most influence on the discriminants in this study are depth for the earthquakes and the source medium properties of gas porosity density and velocity for the explosions the phase amplitude ratio discriminants tested are pn lg and pg lg both show little dependence on magnitude and work better at higher frequencies and when averaged over the two stations at 6 to 8 hz pn lg discriminates all except the shallowest earthquakes while pg lg discriminates all but the high gas porosity explosions low to high frequency spectral amplitude ratio discriminants were tested in each of the phases pn pg lg and lg coda the explosions show significant dependence on source material properties such as gas porosity all the spectral ratios show dependence on magnitude and for 1 2 hz 6 8 hz none performs well below magnitude 3 5 of the spectral ratios lg coda performs the best at least in part because of its extremely small interstation variability discriminating all but the low gasporosity explosions the best discriminant performance is given by combining phase and spectral ratios a sum of the logarithmic values of pn lg pg lg and the lg coda discriminates all but two of the shallowest earthquakes the source medium dependence of the discriminants suggests that discrimination performance differences between the western united states and other regions of the world may be due more to source property differences than differences in the regional path simply put western united states discriminant studies differ from those in the rest of the world because of the lack of explosions in very high gas porosity low velocity materials outside of the nevada test site

velocities and propagation characteristics of pn and sn beneath the himalayan arc and tibetan plateau possible evidence for underthrusting of indian continental lithosphere beneath tibet new seismological observations on velocities and propagation characteristics of pn and sn waves beneath tibet can be interpreted although not uniquely to indicate the shallow angle underthrusting of the greater indian continental lithosphere beneath the tibetan plateau this is inferred from available geological and geophysical data and is now supported by new seismological observations the most significant observation is that high frequency sn waves shear waves that travel in the mantle lithosphere propagate efficiently in the uppermost mantle beneath the tibetan plateau except beneath the north central part of tibet the chang thang terrane furthermore sn propagates efficiently across the stable block of the tarim basin along the tien shan and himalayan mountains and across the indian shield apparent velocities of pn and sn waves that traverse the uppermost mantle beneath the tibetan plateau are 8 42 and 4 73 km s respectively these velocities are very similar to those beneath the himalayan mountains and the indian shield the efficient propagation and the velocities of pn and sn waves beneath tibet are also similar to those commonly observed beneath shield and stable continental regions these results are consistent with the underthrusting model and do not favor the class of alternative models in which the tibetan plateau is formed by shortening and thickening of hot crustal and uppermost mantle material in response to the convergence of india and asia

the high k2o volcanism of northwestern tibet geochemistry and tectonic implications cenozoic volcanic rocks sampled during the 1989 sino french kunlun geotraverse have been analyzed to constrain the origin of widespread tertiary to neogene tibetan basic magmatism samples ranging from phonotephrites to trachyandesites are high k lavas k na always greater than 1 which exhibit typical calc alkaline characteristics although they are highly enriched in mobile trace elements and ree 600 times chondrites for la isotopic compositions exhibit radiogenic sr and pb 87sr 86sr 0 708 208pb 204pb 39 39 4 207pb 204pb 15 6 15 7 206pb 204pb 18 7 and unradiogenic nd 143nd 144nd 0 5123 0 5124 constancy of sr and nd isotopic values contrasts with variations in pb isotopes and highlights the problem of crustal contamination afc and simple mixing models are compatible with isotopic variations but were probably buffered by elemental concentration in sr and nd while pb isotopic values for the most primitive magmas are probably uncontaminated isotopic features are therefore proposed to be representative of a mantle source in which heterogeneity could be highlighted in the pb isotopic values such a source highly enriched in trace elements and radiogenic sr and pb is proposed to lay in the subcontinental mantle isolated from the overall convection movements the calc alkaline characteristics can date from the succession of enrichment phases linked to the numerous subduction events that have affected this part of asia while the isotopic characteristics are proposed to be the result of the aging of the concomitant lile enrichments the broad geographical geochemical and timing aspects of recent basic cenozoic magmatism are discussed in the light of our new data and we suggest that two mechanisms are acting together 1 a limited delamination of the base of the lithosphere or a thermal reequilibration of isotherms subsequent to thickening and 2 intracontinental subduction under the kunlun such mechanisms appear the most probable as they respect at the same time recent geophysical data and other geochemical studies

a preliminary study of regional seismic discrimination in central asia with emphasis on western china abstract in support of a comprehensive test ban treaty we have started to evaluate regional seismic event discrimination capabilities for central asia emphasizing western china we have measured noise and signal levels of over 260 earthquakes and 26 underground nuclear explosions recorded at the broadband digital station wmq in western china and over 120 earthquakes and five nuclear explosions at station aak in kyrgyzstan the explosions are from the kazakh test site kts and lop nor china the earthquakes are mostly from northwest china event magnitudes measured as either m b or m l range between 2 5 and 6 5 and maximum event station distance is about 1700 km we formed phase spectral cross spectral short period long period and long period ratios to test many possible event discriminants all ratios were corrected for distance effects before forming ratio versus magnitude discrimination plots the most consistent discriminants for separating earthquakes from explosions are p s phase ratios for frequencies above 3 to 4 hz p wave f 1 hz to rayleigh wave 0 03 hz f p s ratios separation between earthquakes and explosions increases with frequency and for the p r ratios separation increases with increasing p frequency the p 3 to 6 hz s 0 75 to 1 5 hz cross spectral ratios also separate earthquakes and explosions these cross spectral ratios may prove quite useful because due to weak high frequency l g signals from explosions the high frequency p l g ratio sometimes cannot be evaluated but the p 3 to 6 hz l g 0 75 to 1 5 hz can be evaluated successfully p spectral ratios 3 to 6 hz 0 75 to 1 5 hz also separate earthquakes and explosions but not quite as consistently as the p s ratios however p spectral ratios may prove useful in cases where path effects block or highly attenuate s n and l g waves rendering the p s ratios unavailable in contrast to nevada test site nts events the l g 3 to 6 hz 0 75 to 1 5 hz spectral ratio does not separate earthquakes and nuclear explosions in central asia this observation may be a source effect as asian explosions are thought to be detonated in highly lithified rocks below the water table while most of the smaller m b for station aak the short period spectral and cross spectral ratios identify a few earthquakes not recorded at wmq from the north pamir region that plot with the lop nor explosions the waveforms from these earthquakes lack surface waves and a distinct l g phase these events which we interpret as having occurred in a subducted slab do separate from the explosions on high frequency f 4 hz p s discrimination plots for station wmq long period r l rayleigh wave love wave ratios separate kts explosions and asian earthquakes however r l ratios fail to identify the large lop nor explosions recorded at station aak because the love waves are as large as the rayleigh waves suggesting that strong tectonic release occurs with these explosions because we have examined only one path at a single station for these explosions source radiation pattern may be influencing this observation

yield estimates of nevada test site explosions obtained from seismic lg waves a methodology is presented for determining the yield of underground nuclear explosions from lg wave amplitudes the methodology is applied to nevada test site nts explosions for which the data from short period vertical component analog seismographs at three stations are used to develop calibration curves for unsaturated material and water saturated rock source conditions the latter curves are found to provide reasonably accurate estimates of the yields of explosions in other areas of the united states and in the french sahara suggesting that they may be applicable to all continental areas if so they also can provide an estimate of the bias of mb p magnitudes between different continental sites for example the lg data from nts explosions indicate a 0 31 0 02 magnitude unit bias between nts and eastern north america similar to the approximately 0 33 unit bias found between western and eastern north america previously by use of earthquake data

seismic attenuation characteristics along the hi climb profile in tibet from lg q inversion we investigate the q of crustal lg waves along the himalayan tibetan continental lithosphere during mountain building hi climb profile in tibet using a reliable two station method four earthquakes of m 5 5 recorded along a 800 km long densely spaced seismic profile are used to extract lg wave spectra one hundred seven high quality interstation paths that allow us to use the standard two station method for lg q measurement are selected from 2369 possible pairs by using those 107 interstation q0 1 hz lg q values as input lateral variations in attenuation char acteristics are derived through inversion the region has q0 estimates ranging from 88 5 to 165 15 with the lowest value in the qiangtang terrane and the highest across some part of the lower himalaya the lowest value is attributed to the high poisson s ratio higher temperatures as well as partial melts in the crust the results do not contain any detectable variation in q0 either across the himalaya tibet collision zone i e the indus tsangpo suture zone or along the major bangong nujiang suture zone bnsz consistent low q0 values along the hi climb profile manifest the presence of omnipresent partial melting of the crust throughout the plateau

frequency dependent attenuation in eastern kazakhstan and implications for seismic detection thresholds in the soviet union abstract the frequency dependent attenuation of regional seismic phases recorded by three stations near the nuclear explosion test site in eastern kazakhstan is estimated by inverting spectra from 21 events with magnitudes between 2 3 and 4 6 at distances between 200 and 1300 km the pn spectra are inverted between 1 and 10 hz and the lg spectra are inverted between 0 5 and 2 5 hz the motivation for this study is that previous estimates of detection capability in the soviet union are based on data recorded in other regions eastern north america and scandinavia and therefore have large uncertainty the data recently recorded in eastern kazakhstan provide an excellent opportunity to compare regional wave propagation and noise characteristics at these sites to conditions assumed in previous detection capability simulations it is found that attenuation in eastern kazakhstan is not much different from attenuation in scandinavia but it is greater than attenuation in eastern north america this implies that estimates of detection thresholds that assume attenuation like that observed in eastern north america will be lower than estimates of detection thresholds that assume attenuation like that observed in eastern kazakhstan or scandinavia however it is not known how well data recorded in eastern kazakhstan represent conditions in other areas of the soviet union

high frequency seismic wave propagation beneath the indian shield himalayan arc tibetan plateau and surrounding regions high uppermost mantle velocities and efficient sn propagation beneath tibet summary this paper reports the regional propagation characteristics of high frequhcy p p s and l waves 0 5 to 2hz and p pg and s velocities for several regions in central asia tibet and india an important result is that s waves propagate efficiently in the uppermost mantle beneath most of the tibetan plateau also s transmission in tibet is not affected by crossing many physiographic and tectonic boundaries such as the indus tsangpo suture these observations are similar to observations of s propagation in stable continental and shield regions efficient s propagation is also observed across the stable tarim basin along the tien shan and himalayan mountains and across the indian shield s is strongly attenuated across the northern tibetan plateau chang thang terrane such strong attenuation suggests the existence of a low q zone in the uppermost mantle beneath that region this interpretation is supported by recent geological observations of widespread quaternary to recent basaltic volcanism in the chang thang terrane l propagates efficiently across the tarim basin the indian shield and along the strike of the himalayan mountains l is also observed although less pronounced for paths crossing large portions of the complicated structures of tien shan karakoram and pamir l is not observed for paths crossing the tibetan plateau the southern boundary of the region of inefficient l propagation in tibet lies in the vicinity of the indus tsangpo suture zone the lack of l can be attributed to attenuation within the tibetan crust however other possibilities including scattering by structural discontinuities at the margins of tibet are equally plausible pg and s are the most prominent features on most short period wwssn seismograms at epicentral distances less than about 4 apparent pg velocity determined for the area of the himalayan mountains is 6 0 0 2 km s l apparent velocities of p and s waves that have at least 50 per cent of their paths in the uppermost mantle beneath the tibetan plateau are 8 42 it 0 10 and 4 73 f 0 06 km s respectively these velocities are quite similar to those determined for the himalayan mountains p 8 45 f 0 08 km s i s 4 74 0 05 km s and the indian shield p 8 40 0 08 km s s 4 69 k 0 04 km s p

ionospheric disturbances induced by a missile launched from north korea on 12 december 2012 1 ionospheric disturbances caused by a missile launched from north korea on 12 december 2012 were investigated by using the gps total electron content tec the spatial characteristic of the front edge of v shaped disturbances produced by missiles and rockets was first determined considering the launch direction and the height of estimated ionospheric points at which gps radio signal pierces the ionosphere the missile passed through the ionosphere at heights of 391 425 and 435 km at 0056 30 0057 00 and 0057 30 ut respectively the observed velocities of the missile were 2 8 and 3 2 km s at that time which was estimated from the traveling speed of the front edge of v shaped disturbances westward and eastward v shaped disturbances propagated at 1 8 2 6 km s the phase velocities of the westward and eastward v shaped disturbances were much faster than the speed of acoustic waves reported in previous studies suggesting that sources other than acoustic waves may have played an important role furthermore the plasma density depletion that is often observed following missile and rocket launches was not found this suggests that the depletion resulting from the missile s exhaust was not strong enough to be observed in the tec distribution in the topside ionosphere