40ar 39ar evidence for 1 4 ga regional metamorphism in new mexico implications for thermal evolution of lithosphere in the southwestern usa 40ar 39ar dates recorded by hornblende and muscovite from proterozoic rocks in northern new mexico indicate pervasive mesoproterozoic metamorphism followed by a protracted uplift cooling history most hornblendes yield 40ar 39ar dates of 1 43 1 35 ga which record cooling through temperatures of ca 500 c this suggests that a regional amphibolite facies metamorphism 500 550 c 3 5 4 kbars occurred at ca 1 4 ga this metamorphism was superimposed on paleoproterozoic amphibolite grade rocks at mid crustal depths of 10 15 km contact metamorphic aureoles associated with 1 4 ga plutons emplaced at 2 5 4 kbars 8 15 km in the sandia and manzano mountains record nearly concordant hornblende and muscovite 40ar 39ar dates of 1 4 ga these suggest relatively rapid cooling to ambient conditions of 300 c following the 1 4 ga thermal event slightly younger 1 37 1 31 ga dates are recorded by muscovite throughout northern new mexico and by hornblende within high grade tectonic blocks these are interpreted to r

early to mid tertiary inverted metamorphic gradient under the colorado plateau evidence from eclogite xenoliths in ultramafic microbreccias navajo volcanic field mid tertiary ultramafic microbreccia diatremes from the navajo volcanic field in the four corners area of the colorado plateau contain an ultramafic xenoliths suite in which lower temperature prograde metamorphic eclogites are associated with higher temperature garnet clinopyroxenites that were retrograded and hydrated to chlorite eclogite assemblages and directions of metamorphic reactions are consistent with the model of a laramide age inverted metamorphic gradient established as prograde reactions in a dehydrating oceanic slab caused hydration and cooling in the overlying subcontinental upper mantle wedge comparison with an inverse metamorphic gradient in the pelona schists inferred to represent an exposed part of the same subduction system in southwestern arizona and southern california confirms the shallow dip of the laramide subduction zone predicted by the regional distribution of subduction related volcanic rocks

evolution of the early proterozoic colorado province constraints from u pb geochronology the colorado province represents an addition of a belt of rocks more than 500 km wide to the southern margin of the archean wyoming craton during the early proterozoic between about 1790 and 1660 ma correspondence in ages between metamorphism deformation and plutonism association of volcanic rocks with comagmatic calc alkalic plutons and lack of older basement are all consistent with the interpretation that the rocks of the province are products of arc magmatism and cannibalistic sedimentation along a convergent margin at the southern edge of the craton

from granulites to eclogites in the sesia zone italian western alps a record of the opening and closure of the piedmont ocean the sesia zone italian western alps offers one of the best preserved examples of pre alpine basement reactivated under eclogite facies conditions during the alpine orogenesis a detailed mineralogical study of eclogitized acid and basic granulites and related amphibolites is presented in these rare weak to undeformed rocks microstructural investigations allow three main metamorphic stages to be distinguished a a medium to low p granulite stage giving rise to the development of orthopyroxene garnet plagioclase brown amphibole ilmenite biotite in basic granulites and garnet k feldspar plagioclase cordierite sillimanite biotite ilmenite in acid granulites b a post granulite re equilibration associated with the development of shear zones producing discrete amphibolitization of the basic granulites and widespread development of biotite sillimanite cordierite spinel in the acid rocks c an eo alpine eclogite stage giving rise to the crystallization of high p and low t assemblages in an effort to quantify this evolution independent well calibrated thermobarometers were applied to basic and acid rocks for the granulite event p t estimates are 7 9 kbar and 700 800 c and for subsequent retrograde evolution p t was 4 5 kbar and 600 c for the eo alpine eclogite metamorphism pressure and temperature conditions were 14 16 kbar and 550 c the inferred p t path is consistent with an uplift of continental crust produced by crustal thinning prior to the subduction of the continental rocks in the light of the available geochronological constraints we propose to relate the pre alpine granulite and post granulite retrograde evolution to the permo jurassic extensional regime the complex granulite eclogite transition is thus regarded as a record of the opening and of the closure of the piedmont ocean

looping p t paths and high t low p middle crustal metamorphism proterozoic evolution of the southwestern united states proterozoic rocks of the southwestern united states exhibit a variety of tectonometamorphic histories that can all be explained by a characteristic looping style of pressure temperature p t path the path is fundamentally clockwise however because the rocks were cooled isobarically at 3 kbar after tectonism the cooling path crosses the prograde path leading to a geometry with a counterclockwise character the absolute size of the loop in the p t path varies across the region because of different amounts of overthickening and resulting exhumation during the contractional event combined with variable amounts of heating from syntectonic plutons this style of looping p t path may be characteristic of high t low p metamorphic terranes and middle crustal tectonism particularly where syntectonic plutons contribute to elevated geotherms and polymetamorphism and where rocks become stabilized and reside for long periods at middle crustal depths before being exhumed during a later tectonic event

flexural waves in a floating ice sheet modeling and comparison with data ice vibrations caused by a vertical impulse force are calculated in this paper using fluid loaded thin elastic plate theory the plate response is calculated in the frequency domain multiplied with the source spectrum and inverse fourier transformed to obtain the time domain solution the time domain solution is compared with the vertical axis geophone array data collected in the arctic on a smooth ice floe using a vertical hammer strike as the source the model assuming a linear frequency dependence in ice attenuation provides a reasonably good fit to the spectral densities as a function of distance for three bands of frequencies the modeled waveforms along the array of receivers matches reasonably well with the data frequency wave number analysis is applied to the modeled time series as was done for the data overall good agreement was found between the modeled and measured frequency dispersion curves but the model could not account for the splitting of the frequency dispersion seen in the data th

seismic exploration on a floating ice sheet using a point source viscoelastic seismic wave modeling program we simulated the seismograms that would be recorded for a system of an ice sheet floating on a water layer the latter underlain by a solid containing a single isolated reflector ice and water layer thicknesses were 1 52 6 10 m sources were placed successively on the surface of the ice in the water and in the bottom detectors vertical geophones or hydrophones were placed successively on the ice in the water and in the bottom a source on the ice generated such strong antisymmetric modal energy that no reflections could be detected a source buried 15 30 m below the bottom of the water resulted in clear reflections whether the reflections were detected with surface geophones or buried hydrophones geophysicists have often observed data very similar to that which we modeled but not universally in practice vibrators on floating ice produce usable reflections for a fixed but year to year variable water depth to explain this observation we are forced to assume that the floating annual ice of the arctic is a strongly attenuative material

tectonic implications of the age composition and orientation of lamprophyre dikes navajo volcanic field arizona abstract fifty one lamprophyre dikes of the navajo volcanic field are randomly oriented in contrast to dikes of similar age from adjacent tectonic provinces k ar ages obtained on nine of these dikes indicate that they were emplaced between 28 and 19 m y ago initiation of dike emplacement was approximately coeval with the initial demarcation of the colorado plateau in the mid tertiary chemical analyses obtained on nine dike samples show a wide range in composition silica contents range from 33 6 in a phlogopite katungite to 55 9 in a felsic minette two of the samples have na 2 o k 2 o indicating that sodic volcanism was occurring concurrently with the dominant potassic volcanism published results of high pressure experimental studies of katungite suggest that this magma was derived from k metasomatised mantle because this potassium metasomatism produces a positive volume change we propose a model that relates uplift of the colorado plateau to metasomatism magma genesis and dike emplacement

xenolithic evidence for proterozoic crustal evolution beneath the colorado plateau tertiary diatremes of the navajo volcanic field brought a wide variety of proterozoic xenoliths to the surface of the colorado plateau examination of crustal xenoliths from the navajo volcanic field diatremes permits reconstruction of proterozoic pressure temperature p t histories beneath the colorado plateau diatremes from the northwest part of the navajo volcanic field carry the greatest variety of xenoliths including metasedimentary rocks amphibolites felsic gneisses mafic granulites and crustally derived eclogites these rock types show variable degrees of hydrous alteration and evidence for complex reaction histories in contrast diatremes from the southeast part of the navajo volcanic field contain primarily mafic and felsic granulites that show fewer reaction textures and less alteration metasedimentary and eclogitic xenoliths are absent in these diatremes the p t paths from the northwest xenoliths are counterclockwise and reach temperatures as high as 850 c and pressures equal to or greater than 10 kbar later hydrous alteration occurred around 500 c and 8 12 kbar xenoliths from the southeast diatremes preserve little evidence of their p t evolution but paths involve heating and or decompression the differing rock types p t paths and alteration histories of the northwest and southeast populations suggest that proterozoic tectonism juxtaposed two distinct crustal blocks beneath the colorado plateau previous workers have postulated that the boundary between the proterozoic yavapai and mazatzal provinces occurs in the region our data support the existence of a northeast trending boundary beneath the four corners area if the eclogitic metamorphism and hydrous alteration are proterozoic in age their restricted occurrence beneath the northwest part of the plateau suggests that subduction was northwest dipping during proterozoic accretion of the mazatzal province onto north america

origin of coexisting minette and ultramafic breccia navajo volcanic field trace element evidence indicates that at the buell park diatreme navajo volcanic field the felsic minette can be best explained by crystal fractionation from a potassic magma similar in composition to the mafic minettes compatible trace element cr ni sc abundances decrease while concentrations of most incompatible elements ce yb rb ba sr remain constant or increase from mafic to felsic minette in particular the nearly constant ce yb ratio of the minettes combined with the decrease in cr ni and sc abundances from mafic to felsic minette is inconsistent with a model of varying amounts of partial melting as the process to explain minette compositions the uniformity of rare earth element ree abundances in all the minettes requires that an accessory mineral apatite dominated the geochemistry of the ree during fractionation a decrease in p2o5 from mafic to felsic minette and the presence of apatite in cognate inclusions are also consistent with apatite fractionation higher initial87sr 86sr ratios in the felsic minettes relative to the proposed parental mafic minettes however is inconsistent with a simple fractionation model also a separated phlogopite has a higher initial87sr 86sr ratio than host minette these anomalous isotopic features probably reflect interaction of minette magma with crust the associated ultramafic breccia at buell park is one of the navajo kimberlites but ree concentrations of the matrix do not support the kimberlite classification although the matrix of the breccia is enriched in the light ree relative to chondrites and has high la rb ba and sr concentrations relative to peridotites the concentrations of these elements are significantly lower than in south african kimberlites a high initial87sr 86sr ratio combined with petrographic evidence of ubiquitous crustal xenoliths in the navajo kimberlites suggests that the relatively high incompatible element concentrations are due to a crustal component apparently navajo kimberlites are most likely a mixture of comminuted mantle wall rock and crustal material there is no evidence for an incompatible element rich magma which is characteristic of south african kimberlites if the mafic minettes are primary magmas derived from a garnet peridotite source with chondritic ree abundances then ree geochemistry requires very small less than 1 degrees of melting to explain the minettes alternatively the minettes could have formed by a larger degree of melting of a metasomatized relatively light ree enriched garnet peridotite the important role of phlogopite and apatite in the differentiation of the minettes supports this latter hypothesis

mud aprons in front of svalbard surge moraines evidence of subglacial deforming layers or proglacial glaciotectonics abstract large debris flow units commonly occur on the distal sides of subaqueous end moraines deposited by surges of svalbard tidewater glaciers but have rarely been described in terrestrial settings some researchers have argued that these kinds of debris flows reflect processes unique to the subaqueous environment such as the extrusion of subglacial deforming layers or extensive failure of oversteepened moraine fronts in this paper we describe terrestrial and subaqueous parts of a single late holocene moraine system deposited by a major surge of the tidewater glacier paulabreen in west spitsbergen the ice marginal landforms on land closely resemble the corresponding landforms on the seabed as evidenced by geomorphic mapping and geophysical profiles from both environments both onland and offshore extensive areas of hummocky moraine occur on the proximal side of the maximum glacier position and large mud aprons interpreted as debris flows occur on the distal side we show that the debris flow sediments were pushed in front of the advancing glacier as a continuously failing mobile push moraine we propose that the mud aprons are end members of a proglacial landforms continuum that has thrust block moraines as the opposite end member two clusters of dates 8000 ybp and 700 ybp have previously been interpreted to indicate two separate surges responsible for the moraine formation new dates suggest that the early cluster indicates a local extinction of the abounded species chlamys islandica other changes corresponding to the widespread 8 2 ka event within the fjord may suggest that the extinction of the c islandica corresponds to that time

geochemistry nd and sr isotopes and u pb zircon ages of granitoid and metasedimentary xenoliths from the navajo volcanic field four corners area southwestern united states abstract proterozoic granitoid xenoliths from the navajo volcanic field nvf in the southwestern united states range in composition from tonalite diorite to granite with the latter greatly dominating there is no apparent relationship between chemical or isotopic composition and age geographic location or degree of alteration or deformation of the xenoliths two of the major ages of proterozoic plutonism recognized in the southwest 1 75 1 70 yavapai and 1 45 1 41 ga anorogenic are represented by the nvf xenoliths in some instances within a single diatreme we did not find any xenoliths in the 1 65 1 62 ma mazatzal range the xenoliths are dominantly i types and both nd and sr isotopes and trace element distributions indicate juvenile arc sources approximately 1 8 ga the granite magmas may have been produced by either vapor absent partial melting or fractional crystallization of a tonalite parent juvenile sources must have been available for melting in each of the three periods of magma production and hence some juvenile 1 8 ga crust in this region must have escaped melting in the first two episodes of magmatism and deformation metasedimentary xenoliths are chiefly biotite sillimanite schists garnet biotite schists and garnet paragneisses these three groups are strikingly similar in chemical and isotopic composition suggesting similar graywacke pelite protoliths the schists appear to be limited to the far northern part of the volcanic field consistent with a possible crustal boundary in the basement compositional and thermobarometric results suggest that the biotite schist xenoliths are not restites but that the paragneisses may be mixtures of melt and restite sedimentary protoliths may have been deposited in a forearc basin and later tectonically transported to the deep crust during one or more arc continent collisions in the late paleoproterozoic

lower crustal evolution under central arizona sr nd and pb isotopic and geochemical evidence from the mafic xenoliths of camp creek abstract oligocene potassic volcanism at the camp creek area arizona has sampled the lower crust by bringing mafic granulite amphibolite and garnet bearing pyroxenite eclogite xenoliths to the surface xenolith geobarometry records metamorphic pressures of approximately 10 kbar and temperatures of 650 900 c whole rock sr and nd isotopic compositions correlate roughly with xenolith metamorphic mineral assemblages higher temperature eclogites have 87sr 86sr ranging from 0 7066 to 0 7080 and end of 2 to 5 8 while lower temperature amphibolites have 87sr 86sr of 0 7074 to 0 7082 and end of 6 to 9 sr and nd isotopic compositions of mineral separates reflect in some xenoliths intermineral diffusion up to the time of latite eruption and limited exchange with the host latite pb isotopic compositions of the xenoliths show little mineralogical dependence and overlap the field for normal morb these data suggest that lower crustal metamorphism has significantly affected the sr and nd isotope evolution of the eclogites but has had little effect on their pb isotope systematics for xenoliths closer to igneous bulk compositions i e not enriched in metamorphic garnet and pyroxene nd depleted mantle model ages range from 1 2 to 1 9 ga in agreement with the pb pb array age of 1 1 0 6ga this is interpreted as dating the separation of the lower crust from the mantle during the proterozoic crust building event in this area similar nd model ages for nearby cenozoic and mesozoic granitoids suggest that the type of mafic lower crust represented by the camp creek xenoliths may be a suitable source for younger plutonics in the area

temperatures in zones of steady state underthrusting of young oceanic lithosphere abstract rapid conduction of heat through the top surface of young oceanic lithosphere can contribute at least some of the heat necessary for high temperature low pressure metamorphism of rock buried in subduction zones and perhaps for melting in such environments we show that the steady state temperature on a major underthrust fault underlain by a slab of downgoing lithosphere equals the quotient of the temperature that would exist due to the cooling of a slab below the wedge in the absence of advection of heat by the downgoing slab divided by a dimensionless divisor this divisor depends only on the speed and dip of subduction the depth of the fault the coefficient of thermal diffusivity and the ratio of coefficients of thermal conductivity in the downgoing slab and in the overlying wedge this result extends an earlier analysis that did not take into account either the cooling of lithosphere as it is subducted or different coefficients of thermal conductivity in the slab and overlying wedge for a wide range of geologically common parameters numerically calculated steady state temperature fields for two dimensional heat transfer differ by less than a few percent and by at most 10 from those given by this simple analytical approximation this approximation also concurs with numerical calculations of james et al 1 that show that temperatures as high as 400 c at a depth of only 10 km in a subduction zone would require the subducted lithosphere to be younger than roughly 1 ma if no other source of heat were available in addition the formalism presented here permits a simple assessment of the relative importance of the youth of the subducted lithosphere and dissipative heating in the enhancement of temperatures near the fault surface for example modest shear stresses of about 50 mpa on faults with high slip rates of 100 mm yr or more at depths as small as 10 km can contribute as much as 200 c to temperatures on the fault making temperatures of 400 c at 10 km possible even when lithosphere as old as 20 30 ma is subducted

early proterozoic assembly of tectonostratigraphic terranes in southwestern north america a 500 km wide early proterozoic orogenic belt in arizona and adjacent areas is divided into lithotectonic blocks by northeast and north trending shear zones structural and u pb zircon studies suggest that these blocks may have experienced different tectonic histories prior to their juxtaposition by thrust and strike slip movements on shear zones a northwestern province here called the yavapai province is composed of at least five tectonic blocks and was assembled at about 1700 ma a southeastern province here called the mazatzal province is composed of at least three blocks and was assembled and juxtaposed with the northwestern province during the mazatzal orogeny between 1695 and 1630 ma the diversity of seemingly incompatible 1700 ma tectonic regimes now in close proximity to each other and the absence of systematic cross strike changes in age and character of deformation plutonism and metamorphism suggest a model whereby the orogenic belt developed and north america grew by assembly of d

the role of deformation in the formation of metamorphic gradients ridge subduction beneath the oman ophiolite two tectonic scenarios have been proposed for genesis and emplacement of the oman ophiolite one suggests that the ophiolite was generated at a spreading center the other suggests generation within an intraoceanic arc an integrated thermal and kinematic model of the temperature stress rock type and displacement fields during early stages of the emplacement of the oman ophiolite was developed to test these two possibilities the thermal evolution was calculated by a finite difference algorithm for heat conduction considering heats of metamorphic reactions deformational heating heat advection by flowing rock mantle heat flow and radioactive heating the stress and displacement fields were calculated by an analytical model using a velocity boundary condition power law constitutive relations and a brittle frictional sliding relationship field observations in oman can be satisfied with the spreading center model but not with the arc model moreover simulations indicate that the ophiolite was probably 2 m y old at the time of intraoceanic thrusting and that shear stresses of 100 mpa were attained during thrusting in addition to satisfying currently available field constraints these simulations also indicate specific further work that will help resolve the controversy over thevolution of this unique orogen future work should be directed toward 1 using thermobarometry and thermochronology to constrain the spatial variation in ptt paths of the metamorphic sole 2 dating the igneous genesis of the ophiolite and 3 using quartzite and dunite recrystallized grain size paleopiezometers to infer stresses during thrusting these simulations point out the important effect synmetamorphic deformation can have on the formation of metamorphic field gradients in contractional fault zones metamorphic field gradients may be normal or inverted and contracted or extended relative to the initial thermobarometric gradient and adjacent fault zone rocks may display crossing pt paths unusually contracted or extended metamorphic field gradients may also develop in the footwalls of normal fault zones where portions of the heating hangingwall accrete to the uplifting cooling footwall

shallow water sediment properties derived from high frequency shear and interface waves low frequency sound propagation in shallow water environments is not restricted to the water column but also involves the subbottom thus as well as being important for geophysical description of the seabed subbottom velocity attenuation structure is essential input for predictive propagation models to estimate this structure bottom mounted sources and receivers were used to make measurements of shear and compressional wave propagation in shallow water sediments of the continental shelf usually where boreholes and high resolution reflection profiles give substantial supporting geologic information about the subsurface this colocation provides an opportunity to compare seismically determined estimates of physical properties of the seabed with the ground truth properties measurements were made in 1986 with source detector offsets up to 200 m producing shear wave velocity versus depth profiles of the upper 30 50 m of the seabed and p wave profiles to lesser depths measurements in 1988 were made with smaller source devices designed to emphasize higher frequencies and recorded by an array of 30 sensors spaced at 1 m intervals to improve spatial sampling and resolution of shallow structure these investigations with shear waves have shown that significant lateral and vertical variations in the physical properties of the shallow seabed are common and are principally created by erosional and depositional processes associated with glacial cycles and sea level oscillations during the quaternary when the seabed structure is relatively uniform over the length of the profiles the shear wave fields are well ordered and the matching of the data with full waveform synthetics has been successful producing velocity attenuation models consistent with the subsurface lithology indicated by coring results both body waves and interface waves have been modeled for velocity attenuation as a function of depth with the aid of synthetic seismograms and other analytical techniques some results give strong evidence of anisotropy and lateral heterogeneity in shear velocity of the upper 5 10 m of sediments and of extremely high velocity gradients in the topmost 1 2 m possibly exceeding 30 s 1

constraints on crustal hydration below the colorado plateau from vp measurements on crustal xenoliths abstract seismic velocities have been measured as a function of confining pressure to 8 kbar for crustal xenoliths from the moses rock dike and mule ear diatreme two kimberlite pipes on the colorado plateau rock types measured include rhyolite granite diorite metasedimentary schists and gneisses mafic amphibolites and granulites many of our samples have been hydrothermally altered to greenschist facies mineral assemblages during transport to the earth s surface the velocity of compressional waves measured on altered amphibolites and granulites are too low by 0 1 0 3 km s for such rock types to be characteristic of deep crustal levels a direct correlation exists between progressive alteration and the presence of microcracks extending into the xenoliths from the kimberlitic host rock velocities of pristine samples are compatible with existing velocity profiles for the colorado plateau and we conclude that the crust at depths greater than 15 km has probably not undergone a greenschist facies metamorphic event the xenolith suite reflects a crustal profile similar to that exposed in the ivrea verbano and strona ceneri zones in northern italy

thermobarometry in granulites pitfalls and new approaches abstract computer models of cooling granulite facies rocks with the assemblage garnet biotite sillimanite k feldspar plagioclase quartz are presented that incorporate simultaneous reaction net transfer and exchange and multicomponent diffusion in garnet in models where cooling is accompanied by exchange reactions plus diffusion only the calculated garnet core matrix biotite temperature is always lower than the metamorphic peak by an amount that depends on the cooling rate the size of the garnet and the volumetric ratio of garnet to biotite maximum calculated core temperatures of approximately 700 750 c are to be expected from typical granulite facies garnet biotite assemblages and calculated core temperatures can easily be as low as 600 c in models where both net transfer and exchange reactions are possible calculated garnet core matrix biotite temperatures are typically higher than the metamorphic peak temperatures by more than 150 c for large garnets r 2 mm but may be lower than the metamorphic peak for small garnets r the composition of matrix plagioclase in equilibrium with garnet is very sensitive to the amount of modal plagioclase that reacts and hence calculated p t trajectories are very sensitive to the petrologic interpretation of this recrystallization history for models in which all the modal plagioclase reacts the core and rim pressures may be faithfully recovered provided that the temperature estimate for the core is reliable for models in which only a small percentage of the modal plagioclase reacts very small modal amounts of plagioclase may be in equilibrium with garnet rim which if incorrectly interpreted may lead to grossly erroneous p t path interpretations a method to test an inferred p t path model is presented that involves attempting to match the observed garnet and plagioclase zoning information with a forward model that involves simultaneous reaction and diffusion an example from western new hampshire usa is presented that closely matches the measured profile

plate tectonics model for proterozoic continental accretion in the southwestern united states proterozoic crustal provinces in the southwestern united states decrease in age south from the archean wyoming province supracrustal rocks define three major age provinces from north to south 1 72 to 1 80 1 65 to 1 72 and 1 1 to 1 2 b y successions within each province are similar and define two assemblages a bimodal volcanic assemblage commonly overlain by a quartzite shale assemblage precambrian granitic plutons intrude all three provinces geochemical and sr isotope data indicate a variably depleted upper mantle source for basalts and a short lived these observations can be explained by a plate tectonics model involving successive marginal basin closures and andean type orogenies associated with southward migrating arcs the net result is to accrete 1 300 km of continental crust to the southwestern margin of north america between 1 8 and 1 1 b y ago

ice saw an incisive solution to seismic noise anyone could take the wrong turn driving on the featureless arctic plains it happened to michael corrigan no novice on winter roads en route to meet the transport delivering a new piece of equipment to his camp near deadhorse alaska he ended up at a well site instead he asked a group of roughnecks if they had spotted a large trailer within the last hour or so a yes or no would have done 250 miles north of the arctic circle chances are better for polar bear than for motor vehicle sightings nonetheless they wanted specifics

the stratigraphy of mass extinctions and recoveries interpretations of the tempo of mass extinctions and recoveries often rely on the distribution of fossils in a stratigraphic column these interpretations are generally compromised when they are no

dispersive noise removal in t x space application to arctic data strongly dispersive noise from surface waves can be attenuated on seismic records by flexfil a new prestack process which uses wavelet spreading rather than velocity as the criterion for noise discrimination the process comprises three steps trace by trace compression to collapse the noise to a narrow fan in time offset t x space muting of the noise in this narrow fan and inverse compression to recompress the reflection signals the process will work on spatially undersampled data the compression is accomplished by a frequency domain linear operator which is independent of trace offset this operator is the basis of a robust method of dispersion estimation a flexural ice wave occurs on data recorded on floating ice in the near offshore of the north slope of alaska it is both highly dispersed and of broad frequency bandwidth application of flexfil to these data can increase the signal to noise ratio up to 20 db a noise analysis obtained from a microspread record is ideal to use for dispersion

nd and sr isotope chronostratigraphy of colorado plateau lithosphere implications for magmatic and tectonic underplating of the continental crust nd sr isotopes and major and trace element abundances were measured in lower and upper crustal xenoliths carried in tertiary minettes and kimberlitic diatremes of the colorado plateau the objective was to establish the vertical distribution of mantle extraction ages for an undisturbed precambrian crustal province characterized by normal thickness and low cratonic heat flow and to thereby evaluate the importance of magmatic or tectonic underplating in crustal growth mid proterozoic age 1 7 2 0 ga material is found at all levels of the crust although mafic granulites and amphibolites have major and trace element compositions compatible with an origin as magmatically underplated material nd model ages do not indicate that there has been significant mantle contribution to the lower crust by igneous processes such as magmatic underplating since the main crust forming event ca 1 85 ga ago the meta igneous lower crustal xenoliths have relatively radiogenic nd isotopic compositions and unradiogenic sr compositions there is little variability in nd model ages in spite of large variability in sm nd high and variable sm nd ratios were therefore established early in the history of the protolith during igneous or metamorphic processes coincident with or shortly after extraction from the mantle metapelitic xenoliths tectonically emplaced into the lower crust have the same proterozoic model ages as meta igneous parts of the lower crust these paragneisses are probably arc associated sediments that were underthrust during terrane accretion in the proterozoic the isotopic data indicate that eclogite xenoliths are likely to represent proterozoic lithosphere rather than subducted mesozoic morb as previously proposed a number of the eclogite xenoliths have experienced a metasomatic event which introduced lree sio2 and na2o and shifted the nd and sr isotopic compositions the least altered eclogite xenoliths have major and trace element and radiogenic nd isotopic compositions similar to those of other proterozoic mafic lower crustal xenoliths a more plausible protolith for the eclogites is proterozoic lithosphere that was metamorphosed to eclogite facies conditions either during precambrian continental assembly or in more recent times

apparent isobaric cooling paths from granulites two counterexamples from british columbia and new hampshire textural evidence for the reaction histories of pelitic granulites from british columbia and new hampshire shows that pressure temperature paths obtained from thermobarometry on these samples are an artifact of the calculation technique samples from both localities show breakdown of garnet to form cordierite a reaction that occurs during decompression additional reaction textures and fluid inclusion data also imply decompression at high temperatures for both localities however core rim thermobarometry yields nearly isobaric cooling paths similar cooling paths have been used by other authors to argue for a magmatic underplating model for regional mafic granulites we show that mineral homogenization in pelitic granulites can lead to erroneous isobaric cooling paths and hence to incorrect conclusions concerning tectonic similarities between different types of granulites

pressure temperature time paths of regional metamorphism i heat transfer during the evolution of regions of thickened continental crust the development of regional metamorphism in areas of thickened continental crust is investigated in terms of the major controls on regional scale thermal regimes these are the total radiogenic heat supply within the thickened crust the supply of heat from the mantle the thermal conductivity of the medium and the length and time scales of erosion of the continental crust the orogenic episode is regarded as consisting of a relatively rapid phase of crustal thickening during which little temperature change occurs in individual rocks followed by a lengthier phase of erosion at the end of which the crust is at its original thickness the principal features of pressure temperature time ptt paths followed by rocks in this environment are a period of thermal relaxation during which the temperature rises towards the higher geotherm that would be supported by the thickened crust followed by a period of cooling as the rock approaches the cold land surface the temperature increase that occurs is governed by the degree of thickening of the crust its conductivity and the time that elapses before the rock is exhumed sufficiently to be affected by the proximity of the cold upper boundary for much of the parameter range considered the heating phase encompasses a considerable portion of the exhumation decompression part of the ptt path in addition to the detailed calculation of ptt paths we present an idealized model of the thickening and exhumation process which may be used to make simple calculations of the amount of heating to be expected during a given thickening and exhumation episode and of the depth at which a rock will start to cool on its ascent path an important feature of these ptt paths is that most of them lie within 50 c of the maximum temperature attained for one third or more of the total duration of their burial and uplift and for a geologically plausible range of erosion rates the rocks do not begin to cool until they have completed 20 to 40 per cent of the total uplift they experience considerable melting of the continental crust is a likely consequence of thickening of crust with an average continental geotherm a companion paper discusses these results in the context of attempts to use metamorphic petrology data to give information on tectonic processes 1984 oxford university press

large scale hydration of the lithosphere above subducting slabs during subduction oceanic lithosphere capped by variably hydrated uppermost mantle oceanic crust and sediments undergoes progressive metamorphism and devolatilization the petrologic evolution of subducting oceanic lithosphere and the overriding continental oceanic lithosphere can be predicted by integrating thermal models with petrologic phase equilibria in mature subduction zones that have achieved thermal steady state the subducting slab lies at subsolidus conditions for depths 100 150 km most subducting oceanic crust passes through the blueschist to eclogite facies transition where large amounts of h2o are released by the breakdown of na amphibole lawsonite or clinozoisite and chlorite the large amounts of h2o released by continuous blueschist eclogite reactions can cause large scale hydration of the overriding lithosphere and can trigger partial melting of the mantle wedge lowermost lithosphere and asthenosphere upward migration of slab derived fluids add h2o co2 sio2 and incompatible elements to the overlying lithospheric plate that result in the formation of new phases e g serpentine amphibole and mica and chemical modification of existing phases e g trace element enrichment of clinopyroxene evidence for large scale lithospheric hydration above subducting slabs includes serpentinite diapirs observed in the marianas forearc low p wave velocities observed in the hanging wall of the alaskan and japanese subduction zones and studies of high pressure metamorphic terrains