shallow seismic reflection profiling over the brevard zone south carolina shallow reflection profiles over crystalline rocks of the brevard zone show events that correlate with projections of mapped lithologic contacts in spite of strong attenuation and statics effects associated with a zone of severe chemical weathering saprolite the profiles which were collected in western south carolina about 5 km from appalachian ultra deep core hole adcoh regional lines 1 and 3 straddle the contact between the brevard fault zone and the wider brevard mylonitic shear zone to the southeast the principal goal of these experiments was to test the feasibility of using a hammer source and 24 channel seismograph to image near surface brittle and ductile structures to a depth of several hundred meters we recorded two dip lines and one strike line with a total common midpoint cmp coverage of 1 km processing focused on eliminating static shifts and separating reflections from overlapping refracted s waves a statics approach based on the alignment of first arrivals on common offset gath

zoisite and clinozoisite segregations in metabasites tauern window austria as evidence for high pressure fluid rock interaction high pressure zoisite and clinozoisite bearing segregations are common in garnet and albite bearing amphibolites of the palaeozoic part of the lower schieferhulle south central tauern window austria the zoisite segregations primary assemblage zo qtz cal formed during an early to pre hercynian high pressure event p 0 6 gpa t 500 550 c by hydrofracturing as a result of protolith dehydration zoisite is growth zoned from fe3 poor cores al 2 fe 9 mol to fe3 rich rims 17 mol and has high sr pb and ga contents and lree enriched ree patterns controlling the trace element budget of the segregations hercynian deformation at c 0 7 gpa 600 c kinked and cracked primary zoisite and enhanced breakdown into secondary zoisite 13 mol al 2 fe clinozoisite 40 55 mol al 2 fe albite an 20 calcite and white mica during an eoalpine high pressure event at 0 9 1 2 gpa 400 500 c the clinozoisite segregations primary assemblage czo qtz omp ttn chl cal are mm to cm wide vein like bodies cross cutting fabric elements of the host garnet amphibolite they formed during the eoalpine high pressure event at 0 9 1 2 gpa 400 500 c during alpine exhumation omphacite was pseudomorphed by amphibole albite quartz and clinozoisite oxygen isotope data suggest equilibrium between host metabasite and zoisite segregations and indicate an internal fluid source and fluid buvering by the protolith mobility of p nb and lree changed the protolith s trace element composition in the vicinity of the zoisite segregations mobilization of lree is evidenced by decreasing modal amounts of lree rich epidote and decreasing lree contents in lree rich epidote towards the segregations chang ing the ree patterns of the host metabasite from lree enriched to lree depleted tectonic discrimi nation diagrams based on the trace element content of metabasites should be applied with extreme caution

the application of reflection seismology to the investigation of the geometry of near surface units and faults in the blake river group abitibi belt quebec reprocessing of part of a lithoprobe high resolution seismic reflection line across the southern part of the abitibi belt has improved the imaging of shallow reflections and allowed correlation of the data with surface geology enhancement of early reflections was accomplished by focusing on the high frequency content of the data this improved resolution of reflections at two way traveltime as early as 0 3 s and attenuated noise such as shear waves the shallow reflections are interpreted as impedance contrasts at the contact between a metadiabase diorite body and metavolcanics rocks offsets of the reflectors correlate with faults mapped at the surface and indicate a downdropped block which may be of interest for mineral exploration

stable isotope geochemistry and phase equilibria of coesite bearing whiteschists dora maira massif western alps peak metamorphic temperatures for the coesite pyrope bearing whiteschists from the dora maira massif western alps were determined with oxygen isotope thermometry the 18o smow values of the quartz after coesite 18o 8 1 to 8 6 n 6 phengite 6 2 to 6 4 n 3 kyanite 6 1 n 2 garnet 5 5 to 5 8 n 9 ellenbergerite 6 3 n 1 and rutile 3 3 to 3 6 n 3 reflect isotopic equilibrium temperature estimates based on quartz garnet rutile fractionation are 700 750 c minimum pressures are 31 32 kb based on the pressure sensitive reaction pyrope coesite kyanite enstatite in order to stabilize pyrope and coesite by the temperature sensitive dehydration reaction talc kyanite pyrope coesite h2o the a h2o must be reduced to 0 4 0 75 at 700 750 c the reduced a h2o cannot be due to dilution by co2 as pyrope is not stable at x co2 0 02 t 750 c p 30 kb in the absence of a more exotic fluid diluent e g ch4 or n2 a melt phase is required granite solidus temperatures are 680 c 30 kb at a h2o 1 0 and are calculated to be 70 c higher at a h2o 0 7 consistent with this hypothesis kyanite jadeite quartz bands may represent a relict melt phase peak p t f h2o estimates for the whiteschist are 34 2 kb 700 750 c and 0 4 0 75 the oxygen isotope fractionation between quartz 18o 11 6 and garnet 18o 8 7 in the surrounding orthognesiss is identical to that in the coesitebearing unit suggesting that the two units shared a common final metamorphic history hydrogen isotope measurements were made on primary talc and phengite d smow 27 to 32 on secondary talc and chlorite rite after pyrope d 39 to 44 and on the surrounding biotite d 64 and phengite d 44 gneiss all phases appear to be in nearequilibrium the very high d values for the primary hydrous phases is consistent with an initial oceanicderived connate fluid source the fluid source for the retrograde talc chlorite after pyrope may be fluids evolved locally during retrograde melt crystallization the similar d but dissimilar 18o values of the coesite bearing whiteschists and hosting orthogneiss suggest that the two were in hydrogen isotope equilibrium but not oxygen isotope equilibrium the unusual hydrogen and oxygen isotope compositions of the coesite bearing unit can be explained as the result of metasomatism from slab derived fluids at depth

brevard fault zone southern appalachians a medium angle dextral alleghanian shear zone the record of alleghanian movements in the rocks of the brevard fault zone has been studied by using modern techniques of kinematic analysis mylonitic and phyllonitic fabric involves 1 strong subhorizontal stretching lineation trending northeast southwest parallel to the fault 2 mylonitic foliation dipping moderately to the southeast and 3 a great variety of kinematic indicators suggesting a dextral sense of shear both palmetto and ben hill granites georgia ca 325 ma were sheared during this event the age of which should be younger probably ca 310 ma the brevard zone is usually regarded as a listric thrust fault along which the piedmont terrains have been displaced toward the northwest onto the blue ridge by contrast the movement picture resulting from our analysis is compatible only with right lateral strike slip faulting along a moderately dipping shear zone several kilometres wide causing southwestward displacement of the piedmont relative to the blue ridge province parallel to the appalachian trend

the pyrope coesite rocks and their country rocks at parigi dora maira massif western alps detailed petrography mineral chemistry and pt path both the coarse and fine grained varieties of the partly coesite bearing pyrope quartzites their interlayered jadeite kyanite rocks and the biotite phengite gneiss country rock common to all of them were subjected to detailed petrographic and textural studies in order to determine the sequence of crystallisation of their mineral constituents which were also studied analytically by microprobe prior to pyrope and coesite growth the mg rich metapelites were talc kyanite chlorite rutile ellenbergerite schists which upon continued prograde metamorphism developed first pyrope megacrysts in silica deficient local environments at the expense of chlorite talc kyanite and subsequently the smaller pyrope crystals with coesite inclusions from reacting talc kyanite based on geobarometrically useful mineral inclusions as well as on experimentally determined phase relations a prograde pt path simplified for water activity 1 is constructed which passes through the approximate pt conditions 16 kbar and 560 c 29 kbar and 720 c and finally up to 37 kbar at about 800 c where the mg rich metapelite was a pyrope coesite rock with phengite kyanite and talc still present during the retrograde path pyrope was altered metasomatically either into phlogopite kyanite quartz or at a later stage to chlorite muscovite quartz both assemblages yield pt constraints the latter about 7 9 kbar 500 600 c the country rock gneisses have also endured high pressures of at least 15 kbar but they provide mostly constraints on the lowest portion of the uplift conditions within the greenschist facies about 5 kbar 450 c microprobe data are presented for the following minerals pyrope ellenbergerite dumortierite unusually mgti rich jadeite vermiculite formed after na phlogopite paragonite and for several generations of phengite chlorite talc phlogopite dravite and glaucophane in the high pressure rocks as well as for biotite chlorite phengites epidote garnet albite and k feldspar in the country rock gneisses an outstanding open problem identified in this study is the preservation of minerals as inclusions within kyanite and pyrope beyond their pt stability limits

allanite and other ree rich epidote group minerals epidote group minerals rich in rare earth elements ree in particular allanite are common accessory phases in igneous metamorphic metasomatic and sedimentary rocks small amounts of ree are present in most epidote group minerals but in allanite and the related minerals dissakisite ferriallanite dollaseite khristovite and androsite the ree are essential structural constituents an important characteristic of ree rich epidote group minerals is that their octahedrally coordinated m sites contain major amounts of divalent cations this paper summarizes literature data for these minerals and discusses their chemistry occurrence phase relations and petrologic and geologic significance the chapter emphasizes allanite because it is the most common and best studied of the ree rich epidote group minerals epidote group minerals contain isolated silicon tetrahedra and corner sharing groups of two tetrahedra and are thus assigned to the disilicate or sorosilicate structural family for a detailed description of the structure see franz and liebscher 2004 the epidote group structural formula is a2m3 sio4 si2o7 o f oh or in a simplified form a2m3si3o11 o f oh in which a ca sr pb2 mn2 th ree3 and u and m al fe3 fe2 mn3 mn2 mg cr3 and v3 deer et al 1986 there are two structurally different a sites a 1 and a 2 with different coordination numbers and there are three different m sites m 1 m 2 and m 3 which are all octahedrally coordinated ueda 1955 dollase 1971 in epidote group minerals trivalent ree are accommodated in the a sites which in endmember epidote both contain ca the incorporation of ree3 is commonly charge balanced by a divalent cation fe2 mn2 mg substituted for a trivalent one in the m sites table 1

mineral inclusions in zircons of para and orthogneiss from pre pilot drillhole ccsd pp1 chinese continental scientific drilling project the pre pilot drillhole ccsd pp1 chinese continental scientific drilling project ccsd with depth of 432 m is located in the donghai area in the southwestern sulu terrane the core samples are mainly comprised of paragneiss orthogneiss and ultramafic rock with minor intercalated layers of eclogite and phengite bearing kyanite quartzite all analyzed paragneiss and orthogneiss samples were overprinted on amphibolite facies retrograde metamorphism coesite and coesite bearing ultrahigh pressure uhp mineral assemblages were identified by raman spectroscopy and electron microprobe analysis as inclusions in zircons separated from paragneiss eclogite and phengite bearing kyanite quartzite samples in the paragneiss samples uhp mineral inclusion assemblages mainly consist of coe omp grt phe coe jd phe ap preserved in the mantles m and rims r of zircons these uhp mineral inclusion assemblages yield temperatures of 814 852 c and pressures of 28 kbar presenting the p t condition of uhp peak metamorphism of these country rocks according to the mineral inclusions and cathodoluminescence images of zircons the orthogneisses can be divided into two types uhp og1 and non uhp og2 in og1 orthogneisses low pressure mineral inclusion assemblage mainly consisting of qtz phe ab ksp ap were identified in zircon cores c while coesite or coesite bearing uhp mineral inclusions were identified in the mantles m and rims r of the same zircons these features suggest that the og1 orthogneisses together with the paragneisses phengite bearing kyanite quartzite and eclogite experienced widespread uhp metamorphism in the sulu terrane however in the zircons of og2 orthogneiss samples no uhp mineral inclusions were found inclusions mainly comprised qtz phe ap and were identified in cores c mantles m and rims r of og2 zircons the cathdoluminescence images of all analyzed zircons showed clear zonings from cores to rims these features indicate that the og2 orthogneisses in pre pilot drillhole ccsd pp1 did not experience uhp metamorphism therefore we should not rule out the possibility that some orthogneisses in sulu terrane might represent relatively low pressure granitic intrusives emplaced after the uhp event

the origin of reflections beneath the blue ridge piedmont allochthon a view through the grandfather mountain window a perplexing variety of crustal models have been proposed for the region beneath the southern appalachian overthrust determining the true nature of crustal composition and structure in this region carries important scientific and economic implications to evaluate the numerous models compressional wave velocities and densities were measured on 30 rock samples collected within the grandfather mountain window in the blue ridge of north carolina the window breaches high grade crystalline rocks exposing grenville gneisses late proterozoic metasedimentary and metavolcanic rock and cambrian quartzite phyllite and dolostone that are likely to underlie the regional blue ridge piedmont thrust measured velocities range widely from 4 21 km s 1 for cambrian phyllite to 7 47 km s 1 for tectonized cambrian shady dolomite at 200 mpa velocity anisotropy is as great as 47 in the phyllite the samples exhibit a broad range in densities as well from 2500 kg m 3 in fractured quartzite to 3000 kg m 3 in metabasalts synthetic reflection seismograms were generated using these physical properties to consider the origin of subthrust reflection events in addition to the much publicized interpretation of paleozoic sedimentary rocks residing beneath the crystalline sheet modeling reveals that the reflections could also be explained by metamorphosed paleozoic strata by the regional late proterozoic metavolcanic and metaclastic rift sequence and by pervasive shear zones i e mylonites within compositionally homogeneous gneisses we interpret field data to strongly favor mylonitic gneisses as the subthrust reflectors still surface data remain sufficiently ambiguous that the question will not be definitively settled without continental drilling

experimental evidence for diamond facies metamorphism in the dora maira massif the peak metamorphic conditions of subducted continental crust in the dora maira massif western alps have been revised by combining experimental results in the kcmash system with petrologic information from whiteschists textural observations in whiteschists suggest that the peak metamorphic assemblage garnet phengite kyanite coesite talc originates from the reaction kyanite talc garnet coesite liquid in the experimentally determined petrogenetic grid this reaction occurs above 45 kbar at 730 c at lower pressures talc reacts either to orthopyroxene and coesite or together with phengite to biotite coesite and kyanite the liberated liquid contains probably similar amounts of h2o and dissolved granitic components the composition of the liquid in the whiteschists at peak metamorphic conditions a major unknown in earlier studies was probably very similar to the liquid composition produced in the experiments therefore the experimentally determined petrogenetic grid represents a good model for the estimation of the peak metamorphic conditions in whiteschists experimentally determined si isopleths for phengite further constrain peak pressures to 43 kbar for the measured si 3 60 of phengite in the natural whiteschists all these data provide evidence that the whiteschists reached diamond facies conditions the fluid absent equilibrium 4 kyanite 3 celadonite 4 coesite 3 muscovite pyrope has been calibrated on the basis of garnet and phengite compositions in the experiments and serves as a geothermobarometer for ultra high pressure uhp metapelites for graphite bearing metapelites and kyanite phengite eclogites forming the country rocks of the whiteschists peak metamorphic pressures of about 44 3 kbar were calculated from this barometer for temperatures of 750 c estimated from garnet phengite thermometry therefore the whole ultra high pressure unit of the dora maira massif most likely experienced peak metamorphic conditions in the diamond stability field while graphite is common in the metapelites diamond has not been found so far the absence of metamorphic microdiamonds might be explained by the low temperature of metamorphism the absence of a free fluid phase in the metapelites and a short residence time in diamond facies conditions resulting in kinetic problems in the conversion of graphite to diamond

seismic evidence of mylonite reflectivity and deep structure in the kettle dome metamorphic core complex washington the first direct test of the seismic reflectivity of mylonites is provided by a seismic reflection profile over a mylonite zone exposed in the kettle dome metamorphic core complex northeastern washington the mylonite zone developed at mid crustal levels and has undergone a minimum of retrogression and cataclasis during uplift downdip projection of the exposed mylonite zone into the seismic profile indicates that the compositionally layered mylonite zone is reflective and traceable to at least 5 km depth as a complex zone of multicyclic reflections the seismic data also show that the crust beneath the kettle dome is characterized by abundant strongly reflective moderately dipping layers down to at least 18 km these layers can be interpreted in terms of compressional or extensional orogenic events or a combination of both models for the structure and genesis of metamorphic core complexes must take these data into account

shallow high resolution seismic imaging at the ansil mining camp in the abitibi greenstone belt volcanic rocks in and around the ansil mining camp host a large number of massive sulfide mineral deposits a high resolution seismic reflection profile was shot across the camp with the objective of mapping the contacts between the different volcanic units at which most of the ore bodies have been found numerous exploration boreholes define the geology to a depth of 1600 m and allow a precise comparison with the recorded reflections geophysical logs obtained in one deep borehole suggest that reflection coefficients between the andesite rhyolite units of the volcanic stratigraphy are around 0 05 but few corresponding reflections can be identified in the seismic data those reflections in the surface seismic profile that can be correlated with the subsurface geology originate from diorite sills at which reflection coefficients are between 0 05 and 0 11 we suggest that reflections are observed from the diorite sills because the sills were intruded as sheets some along fault planes resulting in inte

imaging of fracture zones in the finnsjon area central sweden using the seismic reflection method in 1987 the swedish nuclear fuel and waste management co skb funded the shooting of a 1 7 km long high resolution seismic profile over the finnsjon study site using a 60 channel acquisition system with a shotpoint and geophone spacing of 10 m the site is located about 140 km north of stockholm and the host rocks are mainly granodioritic the main objective of the profile was to image a known fracture zone with high hydraulic conductivity dipping gently to the west at depths of 100 to 400 m the initial processing of the data failed to image this fracture zone however a steeply dipping reflector was imaged indicating the field data were of adequate quality and that the problem lay in the processing these data have now been reprocessed and a clear image of the gently dipping zone has been obtained in addition several other reflectors were imaged in the reprocessed section both gently and steeply dipping ones correlations with borehole data indicate that the origin of these reflections are also fracture zones the improvement over the previous processing is caused mainly by 1 refraction statics 2 choice of frequency band 3 f k filtering and 4 velocity analyses in addition to reprocessing the data some further analyses were done including simulation of acquisition using only the near offset channels channels 1 30 and the far offset channels channels 31 60 and determining the damping factor q in the upper few hundred meters based upon the amplitude decay of the first arrivals the data acquisition simulation shows the far offset contribution to be significant even for shallow reflectors in this area contrary to what may be expected a q value of 10 determined from observed amplitude decay rates agrees well with theoretical ones assuming plane wave propagation in an attenuating medium

p wave anisotropy of mylonitic and infrastructural rocks from a cordilleran core complex the ruby east humboldt range nevada abstract in order to evaluate the possibility that compressional wave anisotropy might characterize the upper to middle crust of the north american cordillera we examined the nature and degree of seismic anisotropy of rock samples collected from the mylonite zone and underlying infrastructure of the ruby east humboldt metamorphic core complex in northeastern nevada seismic anisotropy in quartzofeldspathic rocks from all structural levels is low and generally related to the volume of mica relative to feldspar upper amphibolite facies infrastructure quartzite has a stronger and different quartz crystallographic preferred orientation cpo and thus higher seismic anisotropy than the lower grade mylonitic quartzites a feature related to the higher temperature deformation conditions that characterized the infrastructure schistose mylonite zone rocks as a group have the highest anisotropy we infer that this is in large part due to mica cpo although the anisotropy patterns suggest that quartz cpo may be a factor anisotropy is higher in infrastructural quartzites calc silicates and marbles than in mylonite zone equivalents seismic anisotropy data for other cordilleran core complex samples suggest that these patterns are common characteristics of cordilleran core complex rocks

a shallow seismic reflection survey in basalts of the snake river plain idaho the objective of this feasibility study was to determine if the seismic reflection method could help to optimize placement of water quality monitoring wells near a radioactive storage facility seismic reflections from depths less than 30 m were recorded along a 500 m long line over a basalt rhyolite and sedimentary sequence in the snake river plain some shallow reflections at 40 to 50 ms on the field files are of exceptional quality with frequency exceeding 150 hz reflections and refractions from selected seismograms along the line possess vastly different normal moveout nmo and apparent velocities as well as wavelet characteristics extreme variations in quality seismic character and reflector geometries observed on seismograms give the appearance of varying geologic settings and are uncommon for such short distances severe surgical muting was necessary for accurate velocity and statics analyses the seismic reflection data show apparent structural lows in a sedimentary layer sandwiched between

geochemical constraints of the eclogite and granulite facies metamorphism as recognized in the raobazhai complex from north dabie shan china a combined study of major and trace elements fluid inclusions and oxygen isotopes has been carried out on garnet pyroxenite from the raobazhai complex in the north dabie terrane ndt well preserved compositional zoning with na decreasing and ca and mg increasing from the core to rim of pyroxene in the garnet pyroxenite indicates eclogite facies metamorphism at the peak metamorphic stage and subsequent granulite facies metamorphism during uplift a p t path with substantial heating from c 750 to 900 c after the maximum pressure reveals a different uplift history compared with most other eclogites in the south dabie terrane sdt fluid inclusion data can be correlated with the metamorphic grade the fluid regime during the peak metamorphism eclogite facies was dominated by n2 bearing nacl rich solutions whereas it changed into co2 dominated fluids during the granulite facies retrograde metamorphism at a late retrograde metamorphic stage probably after amphibolite facies metamorphism some external low salinity fluids were involved in situ uv laser oxygen isotope analysis was undertaken on a 7 mm garnet and impure pyroxene amphibole and plagioclase the nearly homogeneous oxygen isotopic composition 18ovsmow c 6 7 in the garnet porphyroblast indicates closed fluid system conditions during garnet growth however isotopic fractionations between retrograde phases amphibole and plagioclase and garnet show an oxygen isotopic disequilibrium indicating retrograde fluid rock interactions unusual morb like rare earth element ree patterns for whole rock of the garnet pyroxenite contrast with most ultra high pressure uhp eclogites in the dabie sulu area however the age corrected initial end t is 2 9 which indicates that the protolith of the garnet pyroxenite was derived from an enriched mantle rather than from a morb source combined with the present data of oxygen isotopic compositions and the characteristic n2 content in the fluid inclusions we suggest that the protolith of the garnet pyroxenite from raobazhai formed in an enriched mantle fragment which has been exposed to the surface prior to the triassic metamorphism

multiphase solid inclusions in uhp rocks su lu china remnants of supercritical silicate rich aqueous fluids released during continental subduction abstract primary multiphase solid ms inclusions without preserved fluid are found within peak minerals in kyanite quartzite topaz and kyanite phengite epidote eclogite from donghai area su lu terrane typical mineral association in inclusions is paragonite muscovite anhydrite corundum alunite type sulphate zircon calcite chlorite sio 2 barite pyrite apatite in quartzites and paragonite rutile apatite amphibole zn staurolite magnetite plagioclase zircon pyrite alunite type sulphate zn mg fe al ti spinel in eclogites on the basis of the fluid inclusion textures and of the daughter phase assemblage calculated fluid composition is as follows in quartzites 24 wt sio 2 30 wt al 2 o 3 9 wt cao 5 wt k 2 o 3 wt na 2 o 11 wt so 3 18 wt h 2 o with traces of tio 2 fe 2 o 3 feo mgo bao p 2 o 5 cl f and co 3 2 and in eclogites 26 wt sio 2 21 wt tio 2 20 wt al 2 o 3 2 wt mgo 4 wt feo 6 wt fe 2 o 3 7 wt cao 3 wt na 2 o 3 wt p 2 o 5 7 wt h 2 o 1 wt cl and traces of zno mno k 2 o so 3 and f originally trapped chloride poor aqueous fluids contained very high amounts in the order of tens of wt of si 4 al 3 and ti 4 solute species reflect the chemical composition of the host rocks mg 2 fe 2 ti 4 p 5 and na are abundant in the fluids present in eclogites while fluids are enriched in al 3 k na so 4 2 co 3 2 in quartzites we propose that multiphase solid inclusions represent remnants of high density supercritical silicate rich aqueous fluids that were in equilibrium with peak minerals at uhp conditions these fluids show characters which are transitional between aqueous fluids and silicate melts and were probably produced by dehydration reactions of the host rocks during the latest stages of subduction

slowness adaptive f k filtering of prestack seismic data frequency wavenumber velocity filtering is often applied to prestack seismic data for the attenuation of coherent noise although the process often gives excellent results it can sometimes result in signal smoothing and distortion and poor attenuation of coherent noise a slowness adaptive f k filter reduces signal distortion and improves the attenuation characteristics of the filter the technique uses a time and space variant narrow reject band f k filter optionally coherent noise is compressed before application of the filter the apparent slowness of coherent noise events is estimated using local t x slant stacks weighted by coherence a two dimensional 2 d window is moved across the shot record and at each point on the record slant stacks are taken through the central sample of the window the slowness value that produces the maximum stack is assigned to the central sample of the window in this way an instantaneous slowness image of the shot record is produced a one dimensional 1 d high

interpretation of the reflections in the siljan ring area based on results from the gravberg 1 borehole abstract during 1985 about 80 km of surface multichannel seismic reflection data were collected across the meteorite impact in the siljan ring area in central sweden the area consists mainly of granitic and gneissic rock ranging in age from 1400 to 2000 million years with remnants of palaeozoic sedimentary rocks preserved by downfaulting after a meteorite impact at 360 ma thereby forming a circular ring about 40 km in diameter dolerite intrusions ranging in age from 850 to 1700 million years are also present the seismic data revealed several high amplitude laterally continuous sub horizontal reflections in the northern part of the structure the high amplitude reflections and a possible intermediate low velocity zone were contributing factors in choosing the site for the gravberg 1 deep earth gas test well drilling and vertical seismic profiling vsp found that the reflectors were associated with dolerite sills which had intruded into the granite and which range in thickness from a few metres up to 60 m and with a pre impact area extent of at least 800 km 2 studies of amplitude and frequency versus offset afvo show the observations are compatible with a model of simple granite dolerite granite layering

mineral and fluid inclusions in zircon of uhp metamorphic rocks from the ccsd main drill hole a record of metamorphism and fluid activity abstract the chinese continental scientific drilling ccsd main drill hole 0 3000 m in donghai southern sulu orogen consists of eclogite paragneiss orthogneiss schist and garnet peridotite detailed investigations of raman cathodoluminescence and microprobe analyses show that zircons from most eclogites gneisses and schists have oscillatory zoned magmatic cores with low pressure mineral inclusions of qtz pl kf and ap and a metamorphic rim with relatively uniform luminescence and eclogite facies mineral inclusions of grt omp phn coe and rt the chemical compositions of the uhp metamorphic mineral inclusions in zircon are similar to those from the matrix of the host rocks similar uhp metamorphic p t conditions of about 770 c and 32 kbar were estimated from coexisting minerals in zircon and in the matrix these observations suggest that all investigated lithologies experienced a joint in situ uhp metamorphism during continental deep subduction in rare cases magmatic cores of zircon contain coesite and omphacite inclusions and show patchy and irregular luminescence implying that the cores have been largely altered possibly by fluid mineral interaction during uhp metamorphism abundant h 2 o co 2 h 2 o or co 2 dominated fluid inclusions with low to medium salinities occur isolated or clustered in the magmatic cores of some zircons coexisting with low p mineral inclusions these fluid inclusions should have been trapped during magmatic crystallization and thus as primary only few h 2 o and or co 2 dominated fluid inclusions were found to occur together with uhp mineral inclusions in zircons of metamorphic origin indicating that uhp metamorphism occurred under relatively dry conditions the diversity in fluid inclusion populations in uhp rocks from different depths suggests a closed fluid system without large scale fluid migration during subduction and exhumation

trace element rich brines in eclogitic veins implications for fluid composition and transport during subduction primary and pseudosecondary fluid inclusions occur in oscillatory and sector zoned omphacite in eclogitic veins from the monviso ophiolitic complex in the western alps the inclusions contain aqueous brines and daughter crystals of halite sylvite calcite dolomite albite anhydrite and or gypsum barite baddeleyite rutile sphene fe oxides pyrite and monazite this daughter mineral suite indicates high solubilites of na k ca mg fe zr ti p ba ce la th and s species and provides direct evidence for transport of high fieldstrenght large ion lithophile and light rare earth elements as dissolved species during subduction fluid inclusion heterogeneities preserved within and between adjacent grains in the veins however suggest that the scale of fluid equilibration was small a crack seal geometry in some of the veins implies that fluid release in pulses rather than steady flow controlled mineral deposition and growth in the veins from these observations we develop a model of fluid release and entrapment in which pulses of fluid are associated in time with increments of shear and tensile failure the rate of fluid release and the reduction in porosity both depend on the rate of plastic flow vein fluids may initially be derived from decreptitation of early fluid inclusions in the host eclogites small scale fluid heterogeneities implied by the fluid inclusions in the veins are best interpreted in terms of limited fluid flow and hence limited metasomatism we conclude that element recycling into the mantle wedge during subduction will depend at least as strongly on fluid transport mechanisms as on element solubilities in the fluid phase at monviso despite evidence for high trace element solubilities in saline brines the elements were not removed from the downgoing slab prior to teaching depths of 40 km

shear wave properties and seismic imaging of mylonite zones major crustal fault zones have displayed high reflectivity on observed compressional reflection profiles and synthetic models of laboratory measurements this high compressional wave reflectivity suggests that shear waves may also produce similar reflectivity the shear wave properties and reflectivity of mylonites from the brevard fault zone have been determined using measured shear velocities as a function of confining pressure the measured shear wave velocities are strongly influenced by mineralogy especially quartz and oriented micas the strong oriented fabric of brevard mylonites creates seismic anisotropy which for nonnormal incident propagation produces significant shear wave splitting a wide range of vp vs ratios within the brevard fault zone originates from a complex combination of composition and anisotropy making lithology determination difficult the shear wave reflectivity of the brevard fault zone was derived from one dimensional normal incidence modeling of detailed geologic fault zone models the shear wave reflectivity of the brevard fault zone which originates from fine compositional layering is moderate and can be directly correlated with individual geologic formations within the fault zone variable shear wave reflectivity both horizontally and vertically originates from geologic heterogeneity the modeled shear wave reflectivity also depends on frequency frequencies below 20 hz will resolve boundary structure while higher frequencies image detailed geologic features within the fault zone

shallow architecture of the wadi araba fault dead sea transform from high resolution seismic investigations in a high resolution small scale seismic experiment we investigated the shallow structure of the wadi araba fault waf the principal fault strand of the dead sea transform system between the gulf of aqaba eilat and the dead sea the experiment consisted of 8 sub parallel 1 km long seismic lines crossing the waf the recording station spacing was 5 m and the source point distance was 20 m the first break tomography yields insight into the fault structure down to a depth of about 200 m the velocity structure varies from one section to the other which were 1 to 2 km apart but destinct velocity variations along the fault are visible between several profiles the reflection seismic images show positive flower structures and indications for different sedimentary layers at the two sides of the main fault often the superficial sedimentary layers are bent upward close to the waf our results indicate that this section of the fault at shallow depths is characterized by a transpressional regime we detected a 100 to 300 m wide heterogeneous zone of deformed and displaced material which however is not characterized by low seismic velocities at a larger scale at greater depth the geophysical images indicate a blocked cross fault structure the structure revealed fault cores not wider than 10 m are consistent with scaling from wear mechanics and with the low loading to healing ratio anticipated for the fault

wide angle seismic imaging of the elberton granite georgia a pilot study using instantaneous blasts at dimension stone quarries abstractwide angle recordings of small multiple hole blasts at dimension stone quarries in the elberton granite show coherent reflections at offsets between 7 and 15 km in contrast with much larger blasts at crushed stone quarries this type of blast is fired without delays between holes the data were recorded with an array of twenty portable seismographs using three component 4 5 hz geophones migration of source gathers for two of the blasts shows a number of subhorizontal interfaces at depths between 2 and 4 km that may represent a layered complex near the base of the granite other reflectors include southeast dipping interfaces at depths between 6 and 9 3 km and a more gently southeast dipping complex at 9 5 11 km that may represent the master decollement the results of the pilot study show that this type of quarry blast can be useful for imaging structure within the upper crust although preliminary the wide angle results support the interpretation that the elberton granite is a tabular body l

stable isotopic and trace element evidence for restricted fluid migration in 2 gpa eclogites mineral stable isotopic and trace element studies in 2 gpa banded eclogites of the tauern window eastern alps record mm to cm scale heterogeneities that reflect compositional variations in the accompanying metamorphic fluids a close correlation between dolomite mode and dolomite 18o is consistent with equilibrium partitioning among coexisting minerals and fluids small variations in dolomite 13c values correspond with 18o variations but an overall decrease in dolomite 13c by c 1 o across a 12 cm sample is a relict feature that pre dates eclogite equilibration garnet omphacite and clinozoisite rims show little systematic mineral mineral partitioning behaviour for ti v cr y sr or zr major elements however are well equilibrated among these same minerals despite the apparent lack of mineral mineral trace element equilibration most of the trace elements vary systematically with water activity calculated in each layer trace element behaviour during the eclogite metamorphism thus appears to have been controlled largely by mineral fluid interactions along grain boundaries shallow structural levels in other subduction complexes c 10 45 km typically exhibit fracture controlled permeability and extensive metasomatism but there is no field or geochemical evidence for extensive fluid advection during high pressure metamorphism in the tauern eclogites because most dewatering and devolatilization during tectonic burial occurs prior to eclogite conditions the volumetric fluid rock ratio in eclogites should generally be low low fluid rock ratios coupled with the possible non wetting nature of the fluids permits the production and preservation of fine scale chemical heterogeneities in deeply subducted eclogites and associated fluids however the eventual breakdown at greater depth of volatile bearing dolomite phengite clinozoisite zoisite or amphibole could lead to renewed fracture controlled fluid release from the subducted rocks to regions appropriate for arc magma generation

silica mobility and fluid movement during metamorphism of the connemara schists ireland quartz veins are developed in a wide range of metasediment types in the upper amphibolite facies rocks of connemara and attest to considerable migration of silica contrary to common assumptions there is clear evidence that these veins do not primarily result from movement of fluid to regions of lower p t down the regional geothermal gradient under amphibolite facies conditions a dilute chloride fluid moving down temperature has the potential to alter 60g of plagioclase to muscovite for each gram of vein quartz precipitated while cooling over the temperature interval from 650 to 500 c the absence of significant metasomatic effects in the vein walls effectively precludes a simple origin from such through flowing externally derived fluids the oxygen isotopic composition of matrix quartz shows considerable differences between different rock types quartzite pelite and marble with a range of 18osmow from c 11 5 quartzite to 18 5 marble in each rock type vein quartz compositions closely match those of the matrix quartz these results demonstrate the importance of local segregation processes in the formation of veins and suggest that fluid convection cells were not developed during metamorphism on a scale larger than the individual sedimentary formations if at all both oxygen isotope data and the absence of metasomatism indicate that veins form primarily by segregation of quartz from the host lithologies with only a relatively minor component of through flow of externally derived fluid veins are clearly not the major pathways of metamorphic dewatering it is proposed that abundant veins in the predominantly pelitic ballynakill formation formed during peak metamorphic d3 folding because the formation was embrittled by high fluid pressures but was capped by impermeable marble hence the pelitic formation fractured repeatedly and the pore fluid drained through the fractures to form veins while irreversible loss through the rest of the succession was a much less important process in the central mountains of connemara rather pure unreactive quartzites are cut by widely spaced laterally extensive quartz veins that are axial planar to d3 folds these veins may mark pathways whereby metamorphic fluid made its way through the massive impermeable quartzite from lower parts of the nappe pile but here too oxygen isotope data indicate considerable segregation of locally derived quartz reflecting the importance of pumping of fluid between wail rocks and fractures relative to the component of through flow

large areal extent of ultrahigh pressure metamorphism in the sulu ultrahigh pressure terrane of east china new implications from coesite and omphacite inclusions in zircon of granitic gneiss abstract coesite and omphacite inclusions have been identified for the first time as minute inclusions in zircon from amphibolite facies granitic orthogneiss in the sulu ultrahigh pressure uhp metamorphic terrane of eastern china by raman spectroscopy and microprobe analyses the occurrences of these minerals in the voluminous granitic gneiss of sulu support a regional and pervasive uhp metamorphic event that predated regional amphibolitic retrogression taking into account the widespread discoveries of coesite in other lithologies we thus conclude that a substantial crustal component in the sulu uhp metamorphic terrane appears to have shared a common history of triassic subduction to mantle depths and later exhumation

the nature of seismic reflections from deep crustal fault zones deep seismic reflection profiles have recorded reflections from ductile shear zones within the crystalline basement perhaps the best example is the consortium for continental reflection profiling cocorp wind river line in wyoming which shows the wind river thrust fault to be a strong reflector from the surface to depths of about 30 km to identify the physical properties responsible for the seismic reflections from fault zones we report measurements of compressional wave velocity and velocity anisotropy in mylonites recovered from exhumed ductile shear zones these rocks are characterized by extensive ductile deformation of plastic minerals brittle deformation of the more rigid minerals grain size reduction and development of a strong fabric average velocity and density were not found to change significantly and systematically between the mylonites and the adjacent undeformed rock seismic anisotropy of 7 or greater is present in several mylonites dependent upon their composition and fabric using our data on mylonite properties and models for the crustal structure near the wind river line we compute true amplitude synthetic reflection seismograms and compare them to true amplitude plots of the cocorp wind river reflection data when anisotropy and the finely laminated structure of the fault zone are considered our modeling indicates fairly strong reflections from mylonite zones in most cases we also show that elevated pore pressure in shear zones may produce strong reflections however a permeability of the order of 10 16 darcy is required to maintain sufficient pore pressure to produce a velocity anomaly in a fault zone which has long been inactive

seismic reflectivity of mylonite zones in the crust deep fault zones are characterized by mylonitic rocks with strong preferred orientation of constituent minerals and retrograde mineral assemblages sonic velocities are consequently lower for propagation directions normal to mylonite foliation than in surrounding unmylonitized rocks synthetic seismograms computed for low velocity near horizontal thick mylonite zones of complex laminated geometry show multicyclic reflections with amplitudes up to twice those generated by a single interface seismic characteristics of mylonite zones are sufficient to produce reflections in crystalline rocks provided noise levels are relatively low

the effect of oriented microcracks on seismic velocities in an ultramylonite the influence of oriented microcracks on the directional dependence of p and s wave velocities has been investigated in an epidote muscovite ultramylonite optical and sem studies revealed that the fabric of the mylonite is characterized by oriented intragranular and grain boundary cracks comparison of orientation patterns of cracks with calculated elastic properties voigt average of a model rock isotropic matrix ultramylonite crack geometry and laboratory seismic data indicates that wave propagation is particularly affected by oriented grain boundary cracks microcracks are most effective at low pressure and largely closed above about 200 300 mpa the residual part of velocity anisotropy observed above this crack closing pressure is mainly due to preferred mineral orientation in particular of mica maximum concentration of poles to 001 mica planes corresponds to the maximum of poles to oriented grain boundary cracks so that the effects of texture and oriented cracks on wave velocities line up giving rise to extreme values of velocity anisotropy and shear wave splitting at low pressure conditions

origin of reflections from the brevard fault zone the brevard fault zone is a belt of mylonitic metamorphic rocks that extends northeast over 700 km from alabama into virginia seismic reflection studies show that the fault zone produces excellent reflections which dip gently to the southeast and sole at depth the origin of these reflections has been investigated by measuring compressional wave velocities to 800 mpa confining pressure in 127 samples obtained from drill core which continuously sampled the fault zone to a depth of over 300 m densities are reported as well as chemical analyses at selected depths anisotropy at elevated pressures ranges within the fault zone from less than 5 to over 26 similarly the fault zone rocks show significant chemical variability with sio2 ranging from 58 0 to 70 4 by weight synthetic seismograms generated from the velocity and density measurements show that the reflections originate within the brevard fault zone from the complex interaction of compositional variations and seismic anisotropy resulting from ductile strain essential to this study has been the continuous coring of a major section of the brevard fault zone which has provided the necessary stratigraphic control as well as unweathered samples for laboratory studies

imaging of reflection seismic energy for mapping shallow fracture zones in crystalline rocks the high resolution reflection seismic technique is being used increasingly to address geologic exploration and engineering problems there are however a number of problems in applying reflection seismic techniques in a crystalline rock environment the reflection seismic data collected over a fractured crystalline rock environment are often characterized by low signal to noise ratios s n and inconsistent reflection events thus it is important to develop data processing strategies and correlation schemes for the imaging of fracture zones in crystalline rocks two sets of very low s n high resolution seismic data previously collected by two different contractors in pinawa canada and the island of aspo sweden were reprocessed and analyzed with special emphasis on the shallow reflection events occurring at depths as shallow as 60 100 m the processing strategy included enhancing the signals hidden behind large amplitude noise including clipped ground roll the pre and poststack processing inclu

very low temperature record of the subduction process a review of worldwide lawsonite eclogites abstract lawsonite eclogites preserve a record of very low temperature conditions in subduction zones all occur at active margin settings typically characterized by accretionary complexes lithologies and as tectonic blocks within serpentinite matrix melange peak lawsonite eclogite facies mineral assemblages garnet omphacite lawsonite rutile typically occur in prograde zoned garnet porphyroblasts their matrix is commonly overprinted by higher temperature epidote bearing assemblages greenschist or amphibolite facies conditions erase former lawsonite eclogite relics various pseudomorphs after lawsonite occur particularly in some blueschist eclogite transitional facies rocks coesite bearing lawsonite eclogite xenoliths in kimberlitic pipes and lawsonite pseudomorphs in some relatively low temperature ultrahigh pressure eclogites are known using inclusion assemblages in garnet lawsonite eclogites can be classified into two types l type such as those from guatemala and british columbia contain garnet porphyroblasts that grew only within the lawsonite stability field and e type such as from the dominican republic record maximum temperature in the epidote stability field formation and preservation of lawsonite eclogites requires cold subduction to mantle depths and rapid exhumation the earliest occurrences of lawsonite eclogite facies mineral assemblages are early paleozoic in spitsbergen and the new england fold belt of australia this suggests that since the phanerozoic secular cooling of earth and subduction zone thermal structures evolved the necessary high pressure temperature conditions buoyancy of serpentinite and oblique convergence with a major strike slip component may facilitate the exhumation of lawsonite eclogites from mantle depths

fluid inclusions in coesite bearing eclogites and jadeite quartzite at shuanghe dabie shan china fluid inclusions in coesite bearing eclogites and jadeite quartzite at shuanghe in dabie shan east central china have preserved remnants of early prograde and or peak metamorphic fluids reset during post uhp ultrahigh pressure metamorphic uplift inclusions occur in several minerals e g omphacite epidote notably as isolated primary inclusions in quartz included in various host minerals two major fluid types have been identified non polar fluid species n2 or co2 and aqueous the latter is by far the most predominant aqueous fluids cover a wide range of salinity from halite bearing brines to low salinity fluids for non polar fluids few n2 inclusions occur in undeformed eclogite whereas a number of co2 rich inclusions have been found in microshear zones of eclogite or jadeite quartzite in close proximity to marble occurrences the primary character of n2 and high salinity aqueous inclusions indicates that they are remnants from uhp metamorphic fluids and for some there is the distinct possibility that they are ultimately derived from pre metamorphic fluids this conclusion is supported by the preservation in some samples of microdomains containing synchronous inclusions of variable salinities which tend to relate to the chemical composition of the host crystal carbonic fluids may be derived from neighbouring rocks notably marble and carbonate bearing metasediments during post metamorphic uplift during post uhp exhumation a limited decrease of the fluid density has occurred with formation of new sets of fluid inclusions fluid movements however remained exceedingly limited at the scale of the enclosing crystal

results from regional vibroseis profiling appalachian ultra deep core hole site study summary in 1985 180 km of regional vibroseis profiles were acquired in the carolinas and georgia southeastern united states as part of the appalachian ultra deep core hole adcoh site study the data quality is excellent with large amplitude reflections from faults and crystalline rocks lower palaeozoic shelf strata and from within autochthonous grenville basement the profiles image the subsurface more clearly than other available data and allow the possibility of alternative interpretations of important elements of the tectonic framework of the southern appalachians the major points in the interpretation are 1 the blue ridge master decollement is at a depth of 2 3 km beneath the blue ridge this thrust increases in dip just nw of the brevard fault zone 2 the brevard fault zone appears to splay from the master decollement at 6 km 2 2 s near westminster s c and defines the base of the crystalline inner piedmont allochthon 3 below the blue ridge thrust sheet are images of duplex and imbricate structures duplex tuning wedges connected by other thrust faults that duplicate shelf strata to a thickness of 4 5 km 4 subhorizontal reflections from depths of 6 to 9 km may be from relatively undisturbed lower palaeozoic strata as suggested by others 5 eocambrian cambrian rift basins in the grenville basement are also imaged the adcoh data were originally recorded with 14 56 hz bandwidth and 8 s length but an extended vibroseis correlation was used to produce 17 s data length revealing reflections from within the upper crust below 8 s reflections from within the grenville basement become weak but are observable as late as 13 s however these moho reflections are generally short segments

an in hole shotgun source for engineering seismic surveys we have conducted several source comparisons involving 12 gauge and 8 gauge buffalo guns a 7 3 kg sledgehammer and a 75 kg weight drop the results are strongly site dependent we found that when the near surface consisted of fine grained water saturated sediments the 12 gauge buffalo gun produced up to two orders of magnitude more energy than the conventional hammer across a broad frequency range under such conditions the gun produced the greatest improvement in energy between 200 and 400 hz where it yielded up to ten times more energy than the 75 kg weight drop this indicates that the buffalo gun may be particularly useful as a shallow reflection seismic source however at sites where the near surface materials were coarse grained and the water table was well below the ground surface the advantages of using an in hole shotgun source as opposed to a hammer or weight drop were minimal nevertheless in many areas we believe that the buffalo gun is an excellent source for engineering seismic surv

fluids in deeply subducted continental crust petrology mineral chemistry and fluid inclusion of uhp metamorphic veins from the sulu orogen eastern china abstract the complex vein associations hosted in southern sulu ultrahigh pressure uhp eclogites contain quartz omphacite or jadeite kyanite allanite zoisite rutile garnet these minerals have chemical compositions similar to those of host eclogites inclusions of polycrystalline quartz pseudomorphs after coesite were identified in vein allanite and garnet and coesite inclusions were found in vein zircon these facts suggest that the veins together with host eclogites have been subjected to synchronous uhp metamorphism the vein minerals contain relatively high concentrations of rare earth elements ree high field strength elements hfse and transition metal elements tme a kyanite quartz vein has a whole rock composition similar to adjacent uhp metamorphic granitic gneisses abundant primary multi solid fluid inclusions trapped within uhp vein minerals contain complex daughter minerals of muscovite calcite anhydrite magnetite pyrite apatite celestite and liquid and gas phase of h 2 o with solids up to 30 70 of the inclusion volume the presence of daughter minerals anhydrite and magnetite indicates the subduction fluids were oxidizing and provides a possible interpretation for the high oxygen fugacity of subduction zone magmas these characteristics imply that the uhp vein minerals were crystallized from supercritical silicate rich aqueous fluids that were in equilibrium with peak uhp minerals and that the fluids in deeply subducted continental crust may contain very high concentrations of silicate as well as hree hfse and tme such fluids might have resulted in major fractionation between nb and ta i e the uhp fluids have subchondritic nb ta values whereas the host eclogites after extraction of the fluids have suprachondritic nb ta values therefore voluminous residual eclogites with high nb ta ratios may be the complementary suprachondritic reservoir capable of balancing the subchondritic depleted mantle and continental crust reservoirs

migration why doesn t it work for deep continental data summary conventional migration of deep seismic reflection data often produces disappointingly poor results even when the original unmigrated data are of high quality and are relatively noise free surprisingly deep data often subjectively appear to be best migrated at velocities which are up to 50 less than appropriate interval velocities derived from crustal refraction experiments or directly from stacking velocities the explanation for this behaviour is that near surface features distort and attenuate the seismic wave field and produce apparent discontinuities in deep reflections since these discontinuities are spurious they are not associated with the appropriate large diffractions which real discontinuities at depth would produce during the process of migration reflections are invented in order to cancel out the missing diffractions thereby producing a smiley section which appears overmigrated since the lateral extent of individual smiles increases with increasing migration velocity increasing two way time and increasing seismic wavelength the effect is almost unnoticeable on conventional shallow seismic data but is overwhelming for deep crustal data

the brevard zone in south carolina and adjacent areas an alleghanian orogen scale dextral shear zone reactivated as a thrust fault the displacement history of the brevard zone a linear orogen scale structure in the metamorphic core of the southern appalachians has been controversial geological and seismic data indicate that the brevard zone and the taconic acadian metamorphic rocks in which it resides are contained in a huge crystalline thrust sheet that overlies relatively unmetamorphosed platform sedimentary rocks along a major alleghanian thrust fault along a 100 km segment in south carolina and adjacent states the brevard zone contains three separate structural features all of which dip moderately se 1 the narrow rosman fault which contain tectonic breccia and slices of the buried platform sedimentary rocks 2 the brevard mylonite zone which contains mylonitic rocks that extend about 3 km on either side of the rosman fault and exhibits shear sense criteria which consistently indicate dextral strike slip 3 the brevard fault zone se of the rosman fault which contains stratified metamorphic rocks that are repeated

migration of noisy crustal seismic data a slowness weighted diffraction stack algorithm is used to migrate crustal seismic reflection data slowness information is extracted from the data using a slant stack beam forming process the main purpose of beam forming is to improve the signal to noise ratio over a given range of slowness values localized slant stacks are used to design a coherency weighted slowness filter in the time distance domain through migration the subsurface structure and the observed seismic data are tied via slownessselective diffraction curves a combination of nonlinear slowness filtering with migration gives excellent noise rejection and leads to an improved image in the depth domain the method which is particularly effective for migrating moderate to steep dipping events with low signal to noise ratio is applied to various poststack and prestack synthetic data examples and to a deep crustal data set collected on vancouver island as part of the canadian lithoprobe project

first arrival picking on common offset trace collections for automatic estimation of static corrections the increase in the number of geophone groups in production records during recent years and the requirement for accurate basic static corrections for high resolution records have made it necessary to develop sufficiently accurate automatic techniques for the determination of static corrections a fully automatic method is presented which makes use of the delay time method in order to compute static corrections at each shot position delay times weathering and subweathering velocities are determined from automatic picks of the first arrivals on common offset trace collections it is assumed that the weathering is a single layer and that the dip of the subweathering layer under the geophone groups is small the picking routine is fully automatic and successful in most cases provided the signal to noise ratio is sufficiently high the subsequent filtering of erroneous values for picked times is performed by means of statistical techniques using curves of picked times on common offset trace collections if the distance between receivers and shot points on the profile is sufficiently short one can expect only little change in the picked times of two contiguous traces the method is well adapted to end on spreads with a great number of traces where distances between geophone groups are short examples are presented showing the possibilities of the method for the determination of long wavelength as well as short wavelength components of static corrections

primary fluids entrapped at blueschist to eclogite transition evidence from the tianshan meta subduction complex in northwestern china the tianshan is the first locality worldwide where primary fluids at blueschist to eclogite transition have been documented veins containing high pressure minerals in massive host eclogites transitional to blueschists or eclogite boudins surrounded by blueschists indicate that a free fluid phase was present at the time of eclogitization the high pressure veins are predominantly composed of omphacite fibers with minor quartz or calcite the transition from blueschist to eclogite facies parageneses occurs as dehydration halos around these veins clinozoisite paragonite glaucophane and omphacite inclusions preserved in garnet porphyroblasts in wall eclogites document the transformation of blueschist to eclogite c axis parallel non planar populations of fluid inclusions were trapped during the growth of omphacite in high pressure veins and dehydrated wall rocks low salinity h2o nacl solid bearing inclusions are preserved in omphacite fibers in veins and matrix omphacite of wall rocks none of the isochores of these low salinity aqueous fluid inclusions intersect peak eclogite facies metamorphic conditions suggesting that although the textural evidence constrains the entrapment of fluid inclusions to peak metamorphic conditions their densities must have been modified during exhumation the fluids are interpreted to have been derived from the host blueschist as a result of dehydration reactions such as 13 gln 5 czo 9 prp 26 jd 12 di 19 qtz 15 h2o and gln pg prp 3 jd 2 otz 2 h2o the similarity of vein and wall rock mineral compositions fluid inclusion characteristics and o isotope data also favor an internal source for the fluids the major element composition of veins indicate that si na and ca rich aqueous fluids were released during dehydration at a depth of 50 10 km within a paleozoic subduction zone

experimentally based water budgets for dehydrating slabs and consequences for arc magma generation phase diagrams of hydrous mid ocean ridge mor basalts to 330 km depth and of hydrous peridotites to 250 km depth are compiled for conditions characteristic for subduction zones a synthesis of our experimentally determined phase relations of chlorite lawsonite epidote zoisite amphibole paragonite chloritoid talc and phengite in basalts and of phase relations from the literature of serpentine talc chlorite amphibole and phase a in ultramafics permits calculation of h2o contents in hydrous phase assemblages that occur in natural compositions this yields the information necessary to calculate water budgets for descending slabs starting from low grade blueschist conditions 10 20 km depth with h2o contents between 5 and 6 wt for hydrated oceanic crust complete dehydration is achieved between 70 and 300 km depth as a function of individual slab geotherms hydrous phases which decompose at depth below volcanic arcs are lawsonite zoisite chloritoid and talc phengite in mafic compositions and chlorite and serpentine in peridotite approximately 15 35 of the initially subducted h2o are released below volcanic arcs the contribution of amphibole dehydration to the water budget is small 5 20 and occurs at relatively shallow depth 65 90 km in any predicted thermal structure dehydration is a combination of a stepwise and a continuous process through many different reactions which occur simultaneously in the different portions of the descending slab such a dehydration characteristic is incompatible with single phase dehydration models which focus fluid flow through a unique major dehydration event in order to explain volcanic fronts as a consequence of continuously progressing dehydration water ascending from the slab will be generally available to depth of ca 150 200 km the fluid rising from the subducting lithosphere will cause partial melting in the hot portion of the mantle wedge we propose that the volcanic front simply forms above the mantle wedge isotherm where the extent of melting is sufficient to allow for the mechanical extraction of parental arc magmas thermal models show that such an isotherm ca 1300oc locates below volcanic fronts slab surface depths below such an isotherm are compatible with the observed depths of the slab surface below volcanic fronts 1998 elsevier science b v all rights reserved

paleozoic and grenvillian structures in the southern appalachians extended interpretation of seismic reflection data interpretive reprocessing of seismic reflection data has elucidated paleozoic and grenvillian structures in the southern appalachians the seismic data include a 7500 km grid of adcoh seisdata and cocorp reflection profiles that traverse the blue ridge and inner picdmont geologic provinces of north carolina south carolina and georgia surface geology and potential field data were used to constrain the interpretation the reprocessed seismic reflection data have delineated the internal and external geometry of the crystalline blue ridge inner piedmont allochthon including the locations of the blue ridge master decollement haycsville fault and brevard fault zone on the basis of the reprocessed data all of the major faults within the allochthonous upper crust sole in the blue ridge master decollement reflections extending to the southeast from beneath the surface location of the hayesville fault to the blue ridge thrust might be the seismic signature of a high strain zone this implies that internal deformation of the blue ridge allochthon associated with the alleghanian orogeny might have occurred farther to the west than has been previously documented from field studies relative amplitude seismic data enabled the discrimination between blue ridge inner piedmont crystalline rocks and underlying lower paleozoic shelf strata thereby delineating the blue ridge thrust the interpreted geometry constrains the top of the shelf sequence beneath the blue ridge to depths of less than 3 km this relatively shallow depth of the shelf strata together with the presence of duplex structures and bright spots that are imaged within the sequence might imply favorable conditions for hydrocarbon exploration beneath the blue ridge midcrustal reflections from within the upper to iower crust are interpreted to originate from preserved grenvillian structures that were reactivated at the basement surface during late proterozoic early cambrian extension reflection continuity is occasionally disrupted by interpreted post grenvillian pre early cambrian low density intrusions topography at the basement surface possibly caused by the intrusions is interpreted to have controlled the formation of some of the structures within the overlying allochthon including blue ridge and brevard fault zone ramps correlation of seismic time structure contour maps with available gravity data and two dimensional gravity modeling suggest that anomalies in the gravity field can be attributed to low density sources within the autochthonous crust discontinuous reflection packages from depths of 36 42 km are interpreted to originate from the mohorovi i discontinuity the reflectors trend about n15 e with a true dip of approximately 15 nw

remnants of premetamorphic fluid and oxygen isotopic signatures in eclogites and garnet clinopyroxenite from the dabie sulu terranes eastern china a combined oxygen isotope and fluid inclusion study has been carried out on high and ultrahigh pressure metamorphic hp uhpm eclogites and garnet clinopyroxenite from the dabie sulu terranes in eastern china coesite bearing eclogites garnet clinopyroxenite and quartz eclogites have a wide range in whole rock 18ovsmow from 0 to 11 the high t oxygen isotope fractionations preserved between quartz and garnet preclude significant retrograde isotope exchange during exhumation and the wide range in whole rock oxygen isotope composition is thought to be a presubduction signature of the precursors aqueous fluids with variable salinities and gas species n2 co2 or ch4 rich are trapped as primary inclusions in garnet omphacite and epidote and in quartz blebs enclosed within eclogitic minerals in high 18o hp uhpm rocks from hujialin and shima high salinity brine and or n2 inclusions occur in garnet porphyroblasts which also contain inclusions of coesite cl rich blue amphibole and dolomite in contrast in low 18o eclogites from qinglongshan and huangzhen the cl concentrations in amphibole are very low 0 2 wt and low salinity aqueous inclusions occur in quartz inclusions in epidote porphyroblasts and in epidote cores these low salinity fluid inclusions are believed to be remnants of meteoric water although the fluid composition was modified during pre and syn peak hp uhpm eclogites at houshuichegou and hetang contain ch4 rich fluid inclusions coexisting with high salinity brine inclusions methane was probably formed under the influence of co2 rich aqueous fluids during serpentinisation of mantle derived peridotites prior to or during plate subduction remnants of premetamorphic low to high salinity aqueous fluid with minor n2 and or other gas species preserved in the dabie sulu hp uhpm eclogites and garnet clinopyroxenite indicate a great diversity of initial fluid composition in the precursors implying very limited fluid rock interaction during syn and post peak hp uhpm

improved resolution of reflections from the crystalline upper crust summary use of higher frequencies and a careful selection of the other data acquisition parameters can significantly enhance the resolution obtained in reflection seismic cdp profiling in hard rock terranes experiments with a 20 80 hz non linear vibroseis sweep near the projected adcoh site reveal that the wave field reflected from the upper crust contains frequencies spanning the entire source spectrum even in the presence of a weathered layer of saprolite attenuation of higher frequencies and the severe distortion produced by the notch filter for line frequency necessitate some pre processing spectral shaping the operations are linear and yield as a by product an estimate of the average q for the section about 250 in the present case

seismic reflectivity of the whipple mountain shear zone in southern california the whipple mountains shear zone of southeastern california comprises part of the whipple mountains metamorphic core complex of the north american cordillera the 3 9 km thick shear zone displays excellent exposures of ductilely deformed mid tertiary mylonitic gneisses as well as kinematically related but younger cataclasites which underlie a major low angle normal fault the whipple detachment fault prominent crustal reflections beginning at 3 4 s were recorded on calcrust seismic profiles southwest of the whipple mountains in order to interpret these seismic reflections and to understand why detachment faults are imaged on some profiles but not on others we collected oriented rock specimens from all major structural units in the whipple mountains and determined p wave velocities for these samples parallel to three principal fabric directions under laboratory induced confining pressures up to 500 mpa using a geologic section through the mylonitic gneisses from a previous investigation we then constructed a detailed acoustic impedance section for the whipple mountain shear zone based upon laboratory results this section was in turn used to compose two dimensional synthetic reflection seismograms for comparison with the calcrust records significant impedance contrasts are enhanced by the opposing fabric orientations between the interlayered but structurally isolated relict domains of nonmylonitized rocks and the surrounding mylonitic gneisses the finite thickness and lateral extent of these rock units cause constructive interference in seismic modeling to create the seismic fabric of strong laterally discontinuous but subparallel reflections as seen in the calcrust profiles southwest of the whipple mountains chloritic breccias beneath the whipple detachment fault wdf have much lower seismic impedance than that of rocks occurring both structurally above and below however the significant thinning of the chloritic breccia zone to the southwest of the whipple mountains may have caused the poor image of the wdf on the calcrust profiles the absence of a detachment reflection is thus not necessarily indicative of the absence of a subsurface fault zone

basement beneath the blue ridge and inner piedmont in northeastern georgia and the carolinas a preserved late proterozoic rifted continental margin seismic reflection magnetic and gravity data from the blue ridge and inner piedmont of georgia north carolina and south carolina indicate that southern appalachian basement contains two sets of steeply dipping normal faults that bound rotated and tilted blocks the predominant faults strike about n35 e subparallel to the axis of the orogen whereas the oblique set strikes about n10 w vertical offsets of as much as 1 3 km across individual n10 w striking faults have been observed on seismic profiles regionally the pattern is probably similar to normal faults in precambrian rocks exposed in the adiron dacks or along other rifted margins the initial age of faulting was late proterozoic 570 m y or older inferred from comparison of seismic reflectors in the eastern blue ridge and inner piedmont with stratigraphy along the great smoky thrust basement beneath the western portion of the inner piedmont in south carolina and northeastern georgia has been stable at least since the middle triassic and did not reactivate during the mesozoic breakup of pangea based on the absence of crosscutting relationships between basement faults and the brevard zone reflection basement structure related to rifting may partially explain the focal mechanisms of regional earthquakes the oblique fault set is consistent with well constrained north south striking subvertical nodal planes obtained for many basement earthquakes in the valley and ridge of eastern tennessee valley and ridge earthquakes in southwestern virginia are consistent with reactivation of the faults that strike parallel to the orogen we suggest that the basement structures survived phanerozoic orogeny because alleghanian continental collision was interpreted in the southern appalachians so that convergence between laurentia and gondwana stopped about the time their respective lithospheres came into contact

phreatomagmatic eruptions through unconsolidated coastal plain sequences maungataketake auckland volcanic field new zealand abstract maungataketake is a monogenetic basaltic volcano formed at 85 89 ka in the southern part of the auckland volcanic field avf new zealand it comprises a basal 1100 m diameter tuff ring with a central scoria spatter cone and lava flows the tuff ring was formed under hydrogeological and geographic conditions very similar to the present the tuff records numerous density stratified wet base surges that radiated outward up to 1 km decelerating rapidly and becoming less turbulent with distance the pyroclastic units dominantly comprise fine grained expelled grains from various sedimentary deposits beneath the volcano mixed with a minor component of juvenile pyroclasts 35 vol subtle lateral changes relate to deceleration with distance and vertical transformations are minor pointing to stable explosion depths and conditions with gradual transitions between units and no evidence for eruptive pauses this volcano formed within and on 60 m thick plio pleistocene poorly consolidated highly permeable shelly sands and silts kaawa formation capped by near impermeable water saturated muds tauranga group these sediments rest on moderately consolidated miocene aged permeable turbiditic sandstones and siltstones waitemata group magma water fuelled thermohydraulic explosions remained in the shallow sedimentary layers excavating fine grained sediments without brittle fragmentation required on the whole the resulting cool wet pyroclastic density currents were of low energy the unconsolidated shallow sediments deformed to accommodate rapidly rising magma leading to development of complex sill like bodies and a range of magma water contact conditions at any time the weak saturated sediments were also readily liquefied to provide an enduring supply of water and fine sediment to the explosion loci changes in magma flux and or subsequent stabilisation of the conduit area by a lava ring barrier led to ensuing strombolian and fire fountaining eruption phases future eruptions in littoral environments around auckland are likely to be of this type producing base surges that rapidly decrease in energy over short runout distances 1 km

tectonic implications of new appalachian ultradeep core hole adcoh seismic reflection data from the crystalline southern appalachians summary some 180 km of new vibroseis profiles have been acquired in the southern appalachian inner piedmont brevard fault zone and eastern blue ridge as part of the adcoh project site investigation these data are of the highest quality yet obtained in a crystalline terrane in the us perhaps in the world and reveal several conclusions that should have a direct bearing upon the world wide nature of composite crystalline thrust sheets and their modes of interaction with the platform rocks beneath strong reflections previously interpreted as the base of the crystalline sheet are clearly part of the platform sedimentary clastic rocks sequence resting upon the autochthonous basement and early palaeozoic rift basins this reflection package and related transparent zones are clearly repeated beneath the crystalline sheet indicating a complex of thrusts repeating units within the platform succession reflectors granitoid amplibolite contacts in the crystalline sheet in the inner piedmont represent recumbent folds of similar wavelengths and amplitudes to folds mappable on the surface duplexing of platform rocks beneath the crystalline sheet appears to have resulted in doming of the crystalline sheet similarly duplex formation in the platform was probably controlled by both the thickness of the crystalline sheet and the rheological properties of the platform succession

fluid inclusion and mineral isotopic compositions hco in eclogitic rocks as tracers of local fluid migration during high pressure metamorphism eclogite facies metagabbros from the monviso ophiolitic complex italian western alps provide a unique opportunity to trace fluid migration processes in a portion of the oceanic crust that has undergone subduction at a depth of 40 km we have determined the omphacites 18o and the abundance d and 13c of hydrous and carbonaceous compounds present in whole rocks which are believed to trace the residual phases of what was mobilized in the original rocks during subduction prograde dehydration reactions and eclogitization of hydrothermally altered oceanic metagabbros was accompanied by approximately 90 fluid loss the remaining fluid was trapped as primary water rich fluid inclusions in omphacite megacrysts that developed at the expense of the precursor magmatic pyroxene deformation of the eclogitic rocks resulted in continuous recycling of fluid between mylonites and omphacite veins without further fluid loss from the host ductile shear zone oxygen isotopes of omphacite omp and hydrogen isotopes of water in the fluid inclusions fi analyzed in low strain rocks mylonites and undeformed deformed veins show marked variations 18oomp and dfi values ranging from 3 0 to 5 3 and from 31 to 93 respectively detailed isotopic analysis of several individual vein wallrock pairs show that the scale of isotopic equilibration is of the order of one centimeter therefore the eclogitic minerals and fluids filling the veins are concluded to be locally derived our results argue against recent models which suggest large scale mass flushing of isotopically homogeneous fluids during subduction zone metamorphism on average the h2o contents and dfi value are within the upper mantle range the carbon has been inherited from the metamorphic transformation of the original carbon present in the oceanic crust and has a mean 13c value of 24 2 1 2 two carbonaceous components can be recognized condensed carbon which represents the major carbon species and carbonate daughter crystals present in fluid inclusions 18oomp values are significantly lower than those reported for mantle minerals and are similar to pyroxene and whole rock values from hydrothermally altered oceanic crust it is suggested that the isotopic imprint of the monviso eclogitic minerals and fluids represents the signature of mid ocean ridge hydrothermal alteration and subduction zone eclogitization processes the rocks studied could be the missing link between altered oceanic gabbros and eclogitic xenoliths in addition the range in 18oomp values recorded in the different microstructural domains covers most of the 18o values reported in type a b and c eclogitic xenoliths implying that caution must be exercised when using oxygen isotopes as an indicator of the origin of eclogitic xenoliths

local origin of high pressure vein material in eclogite facies rocks of the zermatt saas zone switzerland prograde dehydration reactions within metabasalts at the blueschist eclogite facies transition produce kyanite veins usually with quartz sometimes with omphacite or chloritoid of up to several meters length and 20 cm width bulk rock analyses show clear depletion haloes for al2o3 and sio2 near the veins mass balance calculations support the suggestion that the vein material must have been derived from the immediate surroundings these veins are interpreted as local segregation phenom ena and not as fluid dehydration channels of the eo alpine subduction zone calcula tions have been performed for a stagnant vein formation model that considers the rate of the dehydration reactions k responsible for providing the h2o for al and si transport and the rate of diffusion d of these from wall rock to growing vein the model is satisfied for quite a wide range of values for k d and upper limits on the duration of diffusional vein formation of about 1 ma are indicated

petrology of ultrahigh pressure rocks from the southern su lu region eastern china abstract in the su lu ultrahigh p terrane eastern china many coesite bearing eclogite pods and layers within biotite gneiss occur together with interlayered metasediments now represented by garnet quartz jadeite rock and kyanite quartzite in addition to garnet omphacite rutile coesite other peak stage minerals in some eclogites include kyanite phengite epidote zoisite talc nyboite and high al titanite the garnet quartz jadeite rock and kyanite quartzite contain jadeite quartz garnet rutile zoisite apatite and quartz kyanite garnet epidote phengite rutile omphacite assemblages respectively coesite and quartz pseudomorphs after coesite occur as inclusions in garnet omphacite jadeite kyanite and epidote from both eclogites and metasediments study of major elements indicates that the protolith of the garnet quartz jadeite rock and the kyanite quartzite was supracrustal sediments most eclogites have basaltic composition some have experienced variable crustal contamination or metasomatism and others may have had a basaltic tuff or pyroclastic rock protolith the su lu ultrahigh p rocks have been subjected to multi stage recrystallization and exhibit a clockwise p t path inclusion assemblages within garnet record a pre eclogite epidote amphibolite facies metamorphic event ultrahigh p peak metamorphism took place at 700 890 c and p 28 kbar at c 210 230 ma the symplectitic assemblage plagioclase hornblende epidote biotite titanite implies amphibolite facies retrogressive metamorphism during exhumation at c 180 200 ma metasedimentary and metamafic lithologies have similar p t paths several lines of evidence indicate that the supracrustal rocks were subducted to mantle depths and experienced in situ ultrahigh p metamorphism during the triassic collision between the sino korean and yangtze cratons

migration of shallow seismic reflection data we present an analysis of migration effects on seismic reflection images of very shallow targets such as those that are common objectives of engineering groundwater and environmental investigations we use an example of seismic reflection data from depths of 5 to 15 m that show negligible effect from migration despite the apparent steep dip on the seismic section our analysis of the question of when to migrate shallow reflection data indicates it is critical to take into account the highly variable near surface velocities and the vertical exaggeration on the seismic section a simple set of calculations is developed as well as a flow chart based on the migrator s equation that can predict whether migration of an arbitrary shallow seismic section is advisable because shallow reflection data are often processed on personal computers unnecessary migration of a large data set can be prohibitively time consuming and wasteful