fracture and separation histories of stretched belemnites and other rigid brittle inclusions in tectonites abstract apparent extension of a stretched rigid inclusion e r l f l 0 1 where l 0 and l f are original and final lengths respectively is always less than strain reversal extension e f calculated by sequentially closing inter fragment gaps until the inclusion is restored to its original length differences between the two measures are strongly influenced by the number of fractures n and their timing within the total stretch history and weakly influenced by the fracture location several fracture and separation models are established to explore these differences using monte carlo simulations all normalized to e r 0 5 when fragment and gap lengths are random and uncorrelated e f values fall in the range 0 5 e f e f values the expected mean value is still much greater than 0 5 when n is small but approaches e r with increasing n and with decreasing time interval between successive fractures model predictions are compared with stretched belemnites in the external french alps results indicate that belemnite fracturing continued throughout a substantial proportion of total stretch history the proportion varying from locality to locality and correlating strongly with maximum stretch recorded this probably reflects variations in stress and suggests that if constant rheology is assumed stretched belemnites may be used to estimate relative strain rates as well as absolute strains

climatology of mesopause region temperature zonal wind and meridional wind over fort collins colorado 41 n 105 w and comparison with model simulations 1 between may 2002 and april 2006 many continuous observations of mesopause region temperature and horizontal wind each lasting longer than 24 h termed full diurnal cycle observations were completed at the colorado state university na lidar facility in fort collins colorado 41 n 105 w the combined data set consists of 120 full diurnal cycle observations binned on a monthly basis with a minimum of 7 cycles in april and a maximum of 18 cycles in august each monthly data set was analyzed to deduce mean values and tidal period perturbations after removal of tidal signals monthly mean values are used for the study of seasonal variations in mesopause region temperature zonal and meridional winds the results are in qualitative agreement with our current understanding of mean temperature and wind structures in the midlatitude mesopause region with an observed summer mesopause of 167 k at 84 km summer peak eastward zonal wind of 48 m s at 94 km winter zonal wind reversal at 95 km and peak summer pole to winter pole meridional flow of 17 m s at 86 km the observed mean state in temperature zonal and meridional winds are compared with the predictions of three current general circulation models i e the whole atmosphere community climate model version 3 waccm3 with two different simulations of gravity wavefields the hamburg model of the neutral and ionized atmosphere hammonia and the 2003 simulation of the thermosphere ionosphere mesosphere electrodynamics general circulation model time gcm while general agreement is found between observation and model predictions there exist discrepancies between model prediction and observation as well as among predictions from different models specifically the predicted summer mesopause altitude is lower by 3 km 8 km 3 km and 1 km for waccm3 the two waccm runs hammonia and time gcm respectively and the corresponding temperatures are 169 k 170 k 158 k and 161 k the model predicted summer eastward zonal wind peaks to 71 m s at 102 km to 48 m s at 84 km to 75 m s at 93 km and to 29 m s at 94 km in the same order the altitude of the winter zonal wind reversal and seasonal asymmetry of the pole to pole meridional flow are also compared and the importance of full diurnal cycle observations for the determination of mean states is discussed

mobilization of gold as a polymetallic melt during pelite anatexis at the challenger deposit south australia a metamorphosed archean gold deposit the challenger gold deposit in south australia is hosted by pelitic migmatites that underwent peak metamorphism at 7 5 1 5 kbars and at least 800 c nd model ages suggest a protolith age of 2900 ma zircon u pb and garnet sm nd dating indicates that peak metamorphism occurred at 2440 ma and that garnet was present during this event at this time in situ vapor absent melting affected the pelitic host rock indicating that peak metamorphism occurred without introduction of a foreign fluid or melt thus evidence indicating the presence of ore minerals during peak metamorphism implies a prepeak mineralization event this evidence includes the occurrence of invisible gold in lollingite but not in adjacent arsenopyrite and the presence of spherical gold sulfide inclusions in peak metamorphic garnet and other silicates textures developed between pyrrhotite lollingite arsenopyrite and gold the lack of silicate alteration minerals supports this conclusion experimental evidence is presented to support our interpretation that a gold rich polymetallic melt was mobilized into leucosomes synchronously with silicate melt during peak metamorphism visible gold is restricted to migmatitic leucosomes and to a lesser extent melanosomes large inclusions of gold with coexisting arsenopyrite pyrrhotite and bismuth are hosted within peak metamorphic silicate minerals and at grain boundaries trails of micrometric spherical inclusions of predominantly gold and bismuth propagate along annealed fractures from these larger inclusions we show that these gold sulfide inclusions represent the crystallized products of an immiscible polymetallic melt metamorphism of the challenger deposit resulted in partial melting of the pelitic host rock and formation of this gold rich melt these two melts were synchronously redistributed to accumulate as leucosomes during development of a stromatic migmatite formation of a polymetallic melt thereby enabled extensive mobilization of gold producing leucosomes enriched in gold this occurred because the immiscible metallic melt was physically entrained rather than chemically dissolved within the silicate melt gold rich shoots at challenger parallel the plunge of ptygmatically folded leucosomes that are shown to be parasitic to a larger scale fold geometry which appears to be structurally related to the ore shoots it is interpreted that concurrent migration of polymetallic and silicate melt allowed concentration of gold into a series of dilational structures which developed within the larger scale fold geometry challenger represents a new deposit type other deposits around the world such as renco and griffins find are hosted in granulite facies rocks but challenger is the first reported example of leucosome hosted gold in migmatites

the pressure path of solid inclusions in minerals the retention of coesite inclusions during uplift abstract an inclusion model allows calculation of the stresses in and around minerals included in other minerals during changes in pressure and temperature the equations are applied to illustrate cooling and uplift histories of quarzo feldspathic rocks from 500 c and various pressures to ambient conditions even in the absence of pore fluid pressure microfractures may open at external pressures of 200 400 mpa and temperatures of 200 400 c due to differential volume changes of the constituent minerals coesite included in garnet cannot have formed during progressive metamorphism from quartz at lithostatic pressures below the coesite stability field because of differential volume changes coesite inclusions are captured by their host minerals at ultra high pressures and they persist to lower pressures because of the large volume increase occurring at the coesite to quartz transition the pi t path followed by the sio2 inclusion traces the boundary between the stability fields of coesite and quartz until radial fractures develop in the host at low external pressure

observations of the diurnal tide in the mesosphere and lower thermosphere over christmas island atmospheric tidal variations in the mesosphere wind field have been studied extensively using radars most of the climatological studies of the tide have been done at middle and high latitudes previous tropical observations of the diurnal oscillation derived from campaign studies have shown the dominance of the migrating diurnal tide in the mesosphere and lower thermosphere by using wind measurements obtained with a meteor radar system located at christmas island the diurnal tidal behavior in the upper atmosphere of the equatorial pacific has been characterized over several years the data collected from august 1988 to december 1993 are presented in this paper and compared to various model results our observations show the meridional wind component of the diurnal tide maximizes in april near 94 km the observed meridional vertical wavelengths are large and indicate upward propagating energy even though the vertical wavelengths for the zonal diurnal component are also large the phase progression is upward corresponding to downward propagating energy there is considerable interannual and intraseasonal variation in the diurnal amplitudes throughout the year

metamorphic interpretation of high pressure temperature metapelites with preserved growth zoning in garnet eastern grenville province canadian shield high pressure temperature metapelites that occur in close proximity to eclogitized mafic rocks in the southern part of the gagnon terrane parautochthonous belt eastern grenville province were investigated in order to constrain depths of burial and p t paths mineral assemblages and partial melting relationships in these metapelites are consistent with peak temperatures in the range between 700 and 800 c however growth zoning is apparently well preserved in garnets and only narrow rims width 100 500 m are obviously affected by diffusional retrograde resetting despite uncertainties regarding mineral assemblages and compositions of matrix minerals at early stages of garnet growth it can be shown that the observed growth zoning profiles of garnets imply increase of both pressure and temperature up to a common maximum at pressures between 1300 and 1600 mpa and that thermal relaxation did not occur during the initial stages of unloading on the other hand calculated retrograde p t conditions are consistent with steep decompression paths the inferred hair pin shaped p t path is consistent with independent evidence of rapid tectonically driven exhumation resulting in the preservation of growth zoning in garnets from such a high temperature regime

coesite and pure pyrope in high grade blueschists of the western alps a first record and some consequences a pyrope quartzite originally described by vialon 1966 from the dora maira massif was resampled and reinvestigated garnet up to 25 cm in size phengite kyanite talc and rutile are in textural equilibrium in an undeformed matrix of polygonal quartz the garnet is a pyrope almandine solid solution with 90 to 98 mol mg end member it contains inclusions of coesite which has partially inverted to quartz resulting in a typical radial cracking of the host garnet around the inclusions several lines of evidence show that coesite crystallised under nearly static pressure conditions and that the whole matrix has once been coesite

self induced fracture generation in zircon the mineral zircon has particular geological significance because of its use in the age determination of rocks and as a structural analogue phase for radioactive waste forms despite its widespread utility however radiation damage of the crystal structure metamictization causes a volume expansion of the crystal lattice and the generation of internal stresses which can induce fractures in the crystal a model has been developed which describes the state of stress in n concentric spherical shells each of which may have different material properties the case of a three shelled composite sphere has been examined explicitly in combination with fracture mechanics theory the model predicts that spatially distinct radial and or concentric fracture sets can be produced and such fracture patterns accurately describe the distribution and types of self induced fractures observed in natural zircons in general the formation of such microfractures in zircon is a function of the degree of metamictization shell thickness and the confining pressure the model predicts that the maximum depths in the earth s crust at which radial versus concentric fractures can form in a crystal are different suggesting that the types of fractures found in a suite of zircons from a particular rock might be potential indicators of the depth of burial because metamictization induced fractures may serve as potential pathways for the rapid leaching of various elements from the zircon crystal this may also have important implications in interpreting the u pb ages of fractured zircons or in evaluating the suitability of related crystalline phases as hosts for nuclear waste

parting in giant garnets as an indicator of late brittle deformation at gore mountain warren county n y the barton garnet deposit contains large garnet crystals 2 5 25 cm disseminated through a gabbro derived garnet amphibolite these crystals are characterized by two sets of parting planes which intersect at about 80 and have a constant attitude throughout the amphibolite the results of x ray diffraction studies on the parting planes of the garnet crystals by the back reflection laue method and by x ray diffractometry show that the parting plane sets are not crystallographically controlled by the lattice of the garnet and that the garnets are not oriented in the amphibolite each parting set in the giant garnets and its corresponding parallel joint set in the amphibolite matrix are interpreted as brittle fractures which developed in response to a discrete tectonic episode

the effect of fluid and deformation on zoning and inclusion patterns in poly metamorphic garnets within the bergen arcs of w norway caledonian eclogite facies assemblages t 650 c p 15 kbar have formed from grenvillian granulites t 800 900 c p 10 kbar along shear zones and fluid pathways garnets in the granulites grti pyr56 40 alm45 25gro19 14 are unzoned or display a weak ca 1 wt feo over 1000 m zoning the eclogite facies rocks contain garnets inherited from their granulite facies protoliths these relict garnets have certain areas with compositions identical to the garnets in their nearby granulite but can be crosscut by bands of a more almrich composition grtii pyr31 41alm40 47gro17 21 formed during the eclogite facies event these bands orientated preferentially parallel or perpendicular to the eclogite foliation may contain mineral filled veins or trails of eclogite facies minerals omphacite amphibole white mica kyanite quartz and dolomite steep compositional gradients up to 9 wt feo over 40 m separate the two generations of garnets indicating limited volume diffusion the bands are interpreted as fluid rich channels where element mobility must have been infinitely greater than it was for the temperature controlled volume diffusion at mineral interfaces in the granulites the re equilibration of granulite facies garnets during the eclogite facies event must therefore be a function of fracture density deformation and fluid availability the results cast doubts on modern petrological and geochronological methods that assume pure temperature controlled chemical re equilibration of garnets

the geometry of echelon fractures in rock implications from laboratory and numerical experiments abstract the traces of echelon joints veins and dikes in rock range from curving to straight theoretical analyses using boundary element numerical methods have concluded that straight open fractures imply a significant remote differential stress whereas curving traces imply a more nearly isotropic stress we present a series of laboratory experiments which investigate the two dimensional propagation paths of echelon fractures in pmma plates as a function of the applied biaxial loading and the initial geometry of a simple fracture array the experimental results support the theoretical conclusions and verify the accuracy of the numerical method at the grain scale rock does not rigorously conform to the assumptions of isotropy homogeneity and linear elasticity demanded by the numerical method nevertheless results from crack path stability theory suggest that small scale deviations of a fracture from the ideal path generally do not affect its large scale behavior fracture paths which become kinked or curved by local heterogeneities are found to self correct to a path governed by competition between the fracture tip and remote stress fields provided the remote differential stress is non zero this enhances our confidence that the numerical method can be used to produce and explain realistic fracture geometries in rock an unexpected experimental result is the non perpendicular intersection of fractures grown in the laboratory free surface boundary conditions require that the walls of an open fracture be free of shear stress the principal stresses are therefore aligned with the crack wall and we might consequently expect an approaching opening mode fracture either to intersect the free surface at a right angle or to turn away and follow an asymptotic nonintersecting path experimental and numerical modeling however shows how the near tip stresses generated by an approaching fracture dominate the local stress field and allow it to propagate along an oblique path very close to intersection with the adjacent free surface

metamorphism during alpine crustal thickening and extension in central anatolia turkey the nigde metamorphic core complex the nig de massif a metamorphic core complex in central anatolia high temperature reactions can obliterate assemblages turkey contains petrologic evidence for the transition from alpine formed during earlier lower temperature phases of meta crustal thickening to extension and exhumation of high grade mid morphism this can hinder use of petrologic information crustal rocks sillimanite potassium feldspar gneiss formed and to interpret tectonic processes if reaction textures are partially melted during barrovian metamorphism and records max preserved however rocks may record a range of pres imum conditions of 5 6 kbar 700 c the two mica uckapili sure temperature conditions from their prograde and granite and a related dike suite intruded the migmatitic metapelitic or retrograde paths and a more complete picture of the rocks forming a contact aureole that contains andalusite and tectonic evolution of a terrane can be reconstructed of cordierite these low pressure minerals indicate that crustal thick particular interest are the mechanisms and timing of ening was followed by exhumation of mid crustal rocks to relatively transition from contraction to extension and the ac shallow depths 10 km at lower temperatures before the em companying exhumation of middle to lower crustal placement of granitic magma formation of andalusite was followed rocks the present study describes a medium pressure by a second prograde episode of sillimanite growth during low high temperature terrane the nig de massif fig 1 in pressure high temperature metamorphism in the central part of the central anatolia turkey that developed as a meta massif where magmatism was most extensive a generalized p t morphic core complex in response to alpine crustal path for the highest grade rocks therefore consists of an initial thickening metamorphic rocks in the central high grade clockwise path with a late thermal spike and characterizes burial part of the massif record in their mineral assemblages and subsequent exhumation accompanied by magmatism reaction textures and structures the processes of burial exhumation and accompanying magmatism

joint development in perturbed stress fields near faults abstract field evidence is presented for complex spatial and temporal perturbations of an otherwise systematic joint pattern around faults from well exposed faulted rock platforms joints propagating in perturbed stress fields will curve to follow the directions of the stress field trajectories a progressive change in joint direction is observed from unperturbed regions away from faults to strongly perturbed zones adjacent to faults this indicates that the joint pattern can reflect perturbations of the regional stress field around faults in the examples the stress field perturbations are probably due to points of high friction on the fault plane which concentrate stress and distort the stress field in the surrounding rock the corresponding joints converge at these points and are sub parallel to the fault along the remainder of the fault plane the possibility that a fault plane acts as a free surface contained within an elastic body is considered in this situation the fault plane induces a rotation of the principal stress axes to become either perpendicular or parallel to the fault the free surface model seems to explain the metre scale curvature of joints in the vicinity of existing joints but at the kilometre scale of a large fault plane the model becomes unrealistic unless the fault is open at the earth s surface two examples are investigated from the lias of great britain at nash point and robin hood s bay both comprise sub horizontal strata of relatively homogeneous lithology and bed thickness which provide striking examples of joints developed near faults

radial cracks around α quartz inclusions in almandine constraints on the metamorphic history of the oman mountains radiating tensional cracks around quartz inclusions in almandine have been observed in metapelite samples from the southeastern saih hatat tectonic window northeastern oman mountains these almandines show an inclusion rich glaucophane epidote and strongly deformed core with inclusions of different mineral phases the rim of the same almandines is inclusion poor and shows only quartz apatite zircon rutile and baal phosphates as inclusions quartz and apatite inclusions in the rim are single crystals often surrounded by radial cracks these radial cracks developed during uplift by the dilation of quartz 4 5 vol without a phase transformation subsequently these cracks were filled with kaolinite phengite si content 3 4 per formula unit p f u chlorite and fe oxides we calculated the appearance of radial cracks without phase transformation using the mathematical procedure of van der molen and van roermund 1 this calculation involves terms for thermal expansion isothermal compressibility and shear modulus for the example of quartz and almandine for the same p and t interval during a retrograde path published geothermobarometric estimates give pressures of between 1 0 and 2 0 gpa and temperatures of between 450 and 600 c for the peak conditions for these rocks of the saih hatat tectonic window on the basis of these p t data we calculated different retrograde p t paths in the quartz domain initiation of garnet fracturing is dependent on the p t starting conditions and the component of isothermal compression of the retrograde path the calculations yield a set of smooth monotonic curves whose exact position on the p t plane between 0 1 and 0 6 gpa and 40 and 500 c depends on the initial p t conditions and the component of isothermal compressibility of the retrograde p t paths this model can be used in general terms to estimate pressure and temperature for the following cases 1 if independent evidence such as petrological data allow the determination of the final pressure at which radial cracks appeared the initial inclusion pressure can be recalculated 2 if the initial inclusion pressure is known e g from petrological data the conditions of radial cracking can be calculated and the pair initial pressure final pressure leads to an estimate of the shape of the retrograde p t path as a function of its component of isothermal decompression in the example from the northeastern saih hatat tectonic window the late syntectonic growth of albite phengite kaolinite suggests that the final pressure for fracturing ranged between 0 4 gpa and 0 5 gpa at temperatures of 300 c these values correspond to high initial pressures of at least 2 0 gpa at a temperature of 550 c the following geodynamic model is suggested a regionally extended metamorphism led to the growth of inclusion rich garnets in the rocks from the northeastern saih hatat tectonic window at depths of about 30 km 0 1 gpa about 450 c continuing prograde metamorphism at a depth of more than 60 km with p 2 0 gpa and t 550 c affected a metapelite unit that is only exposed immediately south of as sifah village in this area clear rims of almandine grew around the older garnets and entrapped mainly quartz and apatite during uplift along a retrograde p t path with a large component of isothermal decompression radial cracks around quartz inclusions developed in the rims of almandines at a depth of about 12 km 0 4 0 5 gpa 300 c

chemical and textural equilibration of garnet during amphibolite facies metamorphism the influence of coupled dissolution reprecipitation metamorphic equilibration requires chemical communication between minerals and may be inhibited through sluggish volume diffusion and or slow rates of dissolution in a fluid phase relatively slow diffusion and the perceived robust nature of chemical growth zoning may preclude garnet porphyroblasts from readily participating in low temperature amphibolite facies metamorphic reactions garnet is widely assumed to be a reactant in staurolite isograd reactions and the evidence for this has been assessed in the late proterozoic dalradian pelitic schists of the scottish highlands the 3d imaging of garnet porphyroblasts in staurolite bearing schists reveals a good crystal shape and little evidence of marginal dissolution however there is also lack of evidence for the involvement of either chlorite or chloritoid in the reaction staurolite forms directly adjacent to the garnet and its nucleation is strongly associated with deformation of the muscovite rich fabrics around the porphyroblasts cloudy fluid inclusion rich garnet forms in both marginal and internal parts of the garnet porphyroblast and is linked both to the production of staurolite and to the introduction of abundant quartz inclusions within the garnet such cloudy garnet typically has a mg rich mn poor composition and is interpreted to have formed during a coupled dissolution reprecipitation process triggered by a local influx of fluid all garnet in the muscovite bearing schists present in this area is potentially reactive irrespective of the garnet composition but very few of the schists contain staurolite the staurolite producing reaction appears to be substantially overstepped during the relatively high pressure barrovian regional metamorphism reflecting the limited permeability of the schists in peak metamorphic conditions fluid influx and hence reaction progress appear to be strongly controlled by subtle differences in deformation history the remaining garnet fails to achieve chemical equilibrium during the reaction creating distinctive patchy compositional zoning such zoning in metamorphic garnet created during coupled dissolution reprecipitation reactions may be difficult to recognize in higher grade pelites due to subsequent diffusive re equilibration fundamental assumptions about metamorphic processes are questioned by the lack of chemical equilibrium during this reaction and the restricted permeability of the regional metamorphic pelitic schists in addition the partial loss of prograde chemical and textural information from the garnet porphyroblasts cautions against their routine use as a reliable monitor of metamorphic history however the partial re equilibration of the porphyroblasts during coupled dissolution reprecipitation opens possibilities of mapping reaction progress in garnet as a means of assessing fluid access during peak metamorphic conditions

radial clastic dykes formed by a salt diapir in the dead sea rift israel fanning structures radiating from a central perturbation are known in various geological environments where different processes have produced similar geometry the present contribution describes and analyses fanning clastic dykes in the dead sea rift a new example of diapir related deformation the dykes are opening mode fractures exposed in lacustrine varved marl of the lisan formation deposited 70 15 ka they are arranged mainly in a radial and tangential geometry the radial traces converge at the black hill structural dome the geometry of the fractures is consistent with stresses exerted by the rise of a salt diapir located underneath the black hill the estimated extension of the radial fractures is in good agreement with the present topographic elevation of the hill the absence of fractures in the overlying holocene alluvium probably indicates that either the rise of the black hill salt diapir paused or is associated now with a different style of deformation

shear wave velocities anisotropy and splitting in high grade mylonites shear wave velocities have been measured for 10 high grade granulite and upper amphibolite facies mylonites from the snowbird tectonic zone canada at various pressures up to 600 mpa in order to explore the kinematic significance of shear wave splitting in the lower crust at 600 mpa vs anisotropy ranges from 11 in amphibolitic mylonites to 2 in granulite facies pyroxene bearing and tonalitic mylonites hornblende bearing granitic mylonites display intermediate vs anisotropy 6 7 vs anisotropy is lower than vp anisotropy in hornblende rich mylonites whereas vs anisotropy is higher than vp anisotropy in feldspar rich mafic tonalitic granitic and diatexitic mylonites shear wave splitting is pronounced in rocks displaying strong s wave anisotropy in most of the high grade mylonites studied the polarization direction of the first arrival vs1 corresponds to the extension lineation x when propagation is parallel to y perpendicular to lineation and parallel to foliation or to the y direction when propagation is parallel to x for all other propagation directions the polarization direction of vs1 is oblique to the x y and z directions therefore it may be difficult to infer the structural implications of shear wave splitting in lower continental crustal shear zones if one does not know 1 the orientation of the propagation and fast vibration directions with respect to in situ lineation and foliation and 2 the lattice preferred orientation of in situ rock forming minerals

how large are departures from lithostatic pressure constraints from host inclusion elasticity minerals trapped as inclusions within other host minerals will develop non lithostatic pressures during both prograde and retrograde metamorphism because of the differences between the thermoelastic properties of the host and inclusion phases there is only a single possible path in p t space the entrapment isomeke along which no residual pressure would be developed in a host inclusion system non lithostatic pressures are developed in inclusions as a result of the external pressure and temperature deviating from the isomeke that passes through the entrapment conditions with modern equation of state and elasticity data for minerals now available it is possible to perform precise calculations of the isomekes for mineral pairs these show that isomeke lines are not straight lines in p t space at metamorphic conditions we show that silicate inclusions in silicate hosts tend to have flat isomekes with small values of because of the small range of thermal expansion coefficients of silicate minerals as a consequence the general behaviour under decompression is for soft silicate inclusions in stiffer hosts to develop excess pressures whereas a stiff silicate inclusion in a softer matrix will experience lower pressures than lithostatic pressure the opposite effects occur for compression after entrapment on the prograde path the excess pressures in inclusions including allowance for mutual elastic relaxation of the host and inclusion are most easily calculated by using the isomeke as a basis analysis of the simplest possible model of a host inclusion system indicates that deviations from lithostatic pressure in excess of 1 gpa can be readily produced in quartz inclusions within garnet in metamorphic rocks for softer host minerals such as feldspar the pressure deviations are smaller because of greater elastic relaxation of the host the maximum pressure deviation from lithostatic pressure in the host phase around the inclusion is one third of the pressure deviation in the inclusion routines for performing these calculations have been added to the eosfit7c software package

crack growth during unroofing of crustal rocks effects on thermoelastic behavior and near surface stresses overcoring and hydrofracturing measurements show that the horizontal normal stresses at or near the earth s surface are nearly always compressive with magnitudes which occasionally reach tens of megapascals this is surprising because simple theoretical arguments suggest that the crustal extension and cooling which occur during unroofing exposure of rocks by uplift and erosion should frequently produce large tensile stresses near the surface one likely cause of this discrepancy is the growth of microcracks driven by the grain scale internal stresses produced by cooling and depressurization during unroofing the unroofing process has been simulated for a simple two dimensional model of a rock in which internal stresses arise only from cooling these calculations show that crack growth leads to compressive changes in macroscopic stress by increasing the elastic compliance of the rock and by decreasing its linear thermal expansion coefficient the growth and opening of cracks also produces an expansion which must be canceled by increased horizontal compression in order to satisfy the lateral constraints imposed by the presence of adjacent rocks the resulting stress changes relative to the stresses expected in a crack free rock could easily reach tens of megapascals at the surface in some rocks especially granites and marbles this effect tends to make the near surface stresses relatively insensitive to the mean geothermal gradient in marked contrast to the behavior of uncracked rocks crack growth driven by internal stresses is also likely to be triggered by drilling tunneling or quarrying in rocks subjected to substantial in situ compressive stresses and could lead to macroscopic deformation quite different from that expected on the basis of a simple elastic analysis the importance of such effects in a given rock depends upon whether the internal stresses are sufficient to produce significant crack growth which in turn depends upon the relative values of two key parameters that are poorly known at present these quantities are the temperature at which ductile deformation effectively ceases during unroofing and the minimum stress intensity factor capable of producing significant crack growth during unroofing better estimates of these parameters would permit more quantitative conclusions regarding the effects of crack growth in particular rock types or geologic settings and would facilitate the development of constitutive laws describing the macroscopic deformation resulting from crack growth

cataclastic deformation of garnet a record of synseismic loading and postseismic creep abstract microstructural evidence for episodic deformation of the uppermost plastosphere related to failure of the schizosphere in major seismic events has been previously reported from the sesia zone western alps with inferred extraordinarily high stresses and strain rates for the early stages of creep during stress relaxation at temperatures of about 300 350 c in some of these rocks crystals of the high strength mineral garnet embedded in a quartz matrix are intensely fragmented detailed optical electron microprobe and scanning electron microscopic study combined with electron backscatter diffraction techniques suggests that fragmentation of garnet took place quasi instantaneously at very high peak stresses subsequent deformation during stress relaxation caused displacement of the garnet fragments by frictional sliding controlled by dislocation creep in the surrounding quartz matrix in a final stage after restoration of a near lithostatic pore fluid pressure the garnet fragments were pulled apart and the opening cracks were filled by quartz and feldspar precipitated from the pore fluid this record appears to be consistent with the proposed model of synseismic loading and postseismic creep and the related pore pressure changes

calcium depletion halos and fe mn mg zoning around faceted plagioclase inclusions in garnet from a high grade pelitic gneiss

u pb zircon dates of morin anorthosite suite rocks grenville province quebec u pb zircon ages of samples of anorthosite pyroxene monzodiorite jotunite and pyroxene quartz monzonite quartz mangerite of the morin anorthosite complex grenville province quebec are 1155 plus minus 3 1146 plus minus 4 and 1135 plus minus 3 ma respectively these dates are very similar to available dates for equivalent units of the lac st jean and adirondack anorthosite suite occurrences and slightly predate estimates of 1075 1100 ma for peak metamorphic conditions during the grenville orogeny in this region sup 87 sr sup 86 sr initial ratios of 0 7048 0 7051 for the three morin units sampled permit a comagmatic origin if the different emplacement or cooling times can be reconciled the mangerite sampled cannot have been formed by fusion of the upper crust but may have been derived from a relatively juvenile crust at depth all three units have interacted with continental crust given the likely depleted nature of the mantle in this region

the partial equilibration of garnet porphyroblasts in pelitic schists and its control on prograde metamorphism glen roy scotland garnet porphyroblasts in sillimanite bearing pelitic schists contain complex textural and compositional zoning with considerable variation both within and between adjacent samples the sillimanite bearing schists locally occur in regional barrovian garnet zone assemblages and are indicative of a persistent lack of equilibrium during prograde metamorphism garnet in these dalradian rocks from the scottish highlands preserves evidence of a range of metamorphic responses including initial growth and patchy coupled dissolution reprecipitation followed by partial dissolution individual porphyroblasts each have a unique and variable response to prograde metamorphism and garnet with mainly flat compositional profiles co exists with those containing largely unmodified characteristic bell shaped mn profiles this highlights the need for caution in applying traditional interpretations of effective volume diffusion eliminating compositional variation cloudy garnet with abundant fluid inclusions is produced during incomplete modification of the initial porphyroblasts and these porous garnet are then particularly prone to partial replacement in sillimanite producing reactions the modification of garnet via a dissolution reprecipitation process releases ca into the effective whole rock composition displacing the pressure temperature positions of subsequent isograd reactions this represents the first report of internal metasomatism controlling reaction pathways the behaviour of garnet highlights the importance of kinetic factors especially deformation and fluids in controlling reaction progress and how the resulting variability influences subsequent prograde history the lack of a consistent metamorphic response within and between adjacent schists suggests that on both local and regional scales these rocks have largely not equilibrated at peak metamorphic conditions

study of mesosphere lower thermosphere mlt mean winds over yamagawa 31 2 n 130 6 e during 1996 1998 the mean motions in the mesosphere and lower thermosphere from 70 to 98 km were observed with the recently installed mf radar at yamagawa 31 2 n 130 6 e we have utilized the continuous wind data collected during the 3 year period 1996 1998 seasonal variations of mean winds over the site are constructed and the climatologies are then compared with observations from similar latitudes and the latest empirical model results the mean zonal circulation in the mesosphere is characterized by a clear annual variation with summer westward easterly flow and winter eastward westerly flow the summer westward jet closes at a height in between 80 and 85 km southward motion exhibits its dominance at 90 98 km during all times of the year and at lower heights 70 90 km southward northward motions predominate during summer winter season the annual mean zonal wind has an eastward motion in the 70 98 km it maximizes at around 90 km with a strength of 15 17 m s the annual mean meridional circulation shows both poleward and equatorward motions harmonic analysis of mean zonal winds clearly demonstrates the dominance of annual oscillation over the other periods comparison of mean winds over yamagawa japan and london ontario canada demonstrates that the zonal circulation over these two sites has some similar features similarly the results compared for yamagawa and urbana show some consistency in mean wind circulation the paper also presents the results of the comparison of mf radar derived winds and the hwm93 model outputs there is reasonable agreement between the observation and the model values

sub critical fracture and associated retrogression of garnet during mylonitic deformation garnets from the alpine fault mylonites are fractured most fractures have a retrogressive fracture fill a small number have garnet fill a detailed sem study has been carried out to elucidate the mechanisms of fracture and associated retrogression wide fracture zones contain free floating islands of garnet in a retrogressive matrix electron channelling patterns show that segments of garnet across such fracture zones rarely show any relative rotations where rotations are observed they are small and systematic some large fractures show clearly that fracture and retrogression occurs by a replacement process fracture surface morphologies are indicative of surface dissolution a model is established where crack tips advance by a process of dissolution at a high stress crack tip whilst precipitation occurs as the tip and its associated stress field advance precipitation of retrogressive fracture fill occurs if fracture occurs out of the field of stability of garnet

three dimensional quantitative textural analysis of metamorphic rocks using high resolution computed x ray tomography part i methods and techniques macroscopic textures resulting from different atomic scale mechanisms for metamorphic crystallization display different degrees of order clustering intergrowth and relative isolation of porphyroblasts data on the sizes and locations of thousands of crystals in a three dimensional volume are required to identify reliably the mechanisms governing nucleation and growth of porphyroblasts from these textural features these data can now be acquired by means of high resolution computed x ray tomography numerical models that simulate porphyroblast formation governed by either interface controlled or diffusion controlled reaction mechanisms indicate that quantitative textural analysis can discriminate between these possibilities these numerical models also allow a comparison between textures predicted for different crystallization mechanisms and textures measured in natural samples from which inferences can be drawn concerning the relative importance of these mechanisms in nature an independent test of the validity of such inferences is possible for porphyroblasts such as garnet that may preserve prograde growth zoning and allow the examination of normalized radius rate relations

low energy phase boundary pairs and preferred crystallographic orientations of olivines in nanometer sized ultrapotassic fluid inclusions of aykhal diamond abstract the healed internal conjugated cleavages at the core of aykhal octahedral diamond sample ah2 were decorated with 111 dia facetted ultrapotassic fluid melt inclusion pockets containing nanosized graphite phlogopite and olivine fo92 inclusions these olivines are either rounded in pockets with ample fluid or facetted by the 111 dia mold in the pockets with a fluid film transmission electron microscopy revealed two distinct crystallographic characteristics of olivine inclusions 1 pronounced crystallographic texture of olivines grouped in specific diamond domain and 2 frequent parallelism or sub parallelism of specific low energy faces of the two phases mainly 010 ol 120 ol 001 ol and 111 dia 110 dia 100 dia in the order of decreasing preference forming prominent 010 ol 111 dia 010 ol 110 dia 001 ol 110 dia 120 ol 111 dia and 120 ol 110 dia low energy phase boundaries with thin liquid film of 1 2 nm in between these findings not only testify to the extremely low adhesion energies of olivine diamond boundary pairs but also imply that in the presence of a fluid phase the interfacial energetics and the energetically favored crystallographic orientations of olivine inclusions in diamond can be controlled simply by the settlement attachment of low energy facets of olivine crystals precipitating from the parental fluid upon the low energy 111 dia or 110 dia surfaces of diamond such interfacial energetics control and the resultant low energy boundary pairs are characteristically distinct from the common topotaxy or epitaxy between oxide silicate mineral pairs but are in a sense like the van der waals heteroepitaxy in artificial systems

on the origin of oriented rutile needles in garnet from uhp eclogites although oriented rutile needles in garnet have been reported from several ultrahigh pressure uhp rocks and considered to be important uhp indicators their crystallographic features including growth habit and lattice correspondences with garnet host have never been properly characterized this paper presents a detailed analytical electron microscopic aem study on evenly distributed oriented rutile needles in garnet of two eclogitic rocks from sulu some garnet in one uhp diamondiferous quartzofeldspathic rock from the saxonian erzgebirge and in one high pressure hp felsic granulite from bohemia also contain a few unevenly distributed oriented rutile needles they have also been studied for the purpose of comparison despite different distribution patterns aem revealed that all rutile needles are oriented along the 111 directions of garnet with their lateral sides surrounded by the 110 planes of garnet and that the growth directions of most needles are close to the normal of the 101 planes of rutile no other specific crystallographic orientation relationships between rutile and garnet host were observed and there is no pyroxene associated with rutile as necessitated by the precipitation reaction of rutile in garnet as previously proposed a simple solid state precipitation scenario for the formation of the rutile needles in garnet in these two eclogitic rocks is not justified three alternative mechanisms are considered for the formation of oriented rutile needles i the rutile needles may be inherited from precursor minerals ii the rutile needles may be formed by a dissolution reprecipitation mechanism and iii the rutile needles may be formed by cleaving and healing of garnet with rutile deposition none of these mechanisms can fully explain the observations although the first one is less likely and the third one is preferred this study presents an example where the presence of oriented aligned inclusions in minerals does not necessarily imply a precipitation origin

petrofabric p wave anisotropy and seismic reflectivity of high grade tectonites abstract ji s salisbury m h and hanmer s 1993 petrofabric p wave anisotropy and seismic reflectivity of high grade tectonites tectonophysics 222 195 226 in order to understand seismic anisotropy and its influence on the reflectivity of lower crustal fault zones we have undertaken experimental measurements of p wave velocities up to 600 mpa for 20 granulite and upper amphibolite facies mylonites from the snowbird tectonic zone canada the rocks whose composition ranges from ultramafic to felsic have experienced extremely large ductile strain it is found that the amphibolitic mylonites exhibit significant vp anisotropy 6 13 at 600 mpa whereas the pyroxene bearing granulite facies mylonites and the quartz or feldspathic tonalitic granitic and diatexitic mylonites are quasi isotropic in order to constrain interpretation of the measured vp properties microstructural analyses have been systematically performed in the samples studied it is shown that the interaction between feldspar and quartz or pyroxene and the absence of amphibole cause anisotropy in the granulite facies mylonites to be low the amphibolitic mylonites are strongly anisotropic because of a large volume fraction of hornblende with strong lpo 001 lineation and 100 foliation the anisotropy pattern in the amphibolitic mylonites is consistently orthorhombic vp x vp y vp z where x is parallel to stretching lineation y is normal to the lineation in the foliation plane and z is normal to the foliation plane but the anisotropy pattern in the mica rich mylonites is transversely isotropic vp x vp y vp z if sufficiently large volumes of the crust display such patterns this difference may be an important indicator for kinematic analysis in the middle to lower crust using modern seismic techniques the study demonstrates that while seismic reflectivity is strongly lithology controlled fabric induced anisotropy can both enhance and decrease seismic reflectivity hence the discontinuity of seismic reflections along ductile shear zones in the lower crust is not necessarily indicative of the discontinuity of the shear zones but may indicate changes in composition or metamorphic grade

decrepitated uhp fluid inclusions about diverse phase assemblages and extreme decompression rates erzgebirge germany minute polyphase inclusions in garnet of quartzofeldspathic rocks saidenbachite from the saxonian erzgebirge germany contain microdiamond or graphite phlogopite quartz paragonite phengite and other minerals in minor amounts these inclusions are interpreted to represent an original dense coh silicate fluid trapped in crystallizing garnet at depths of 150 km inspection of the inclusion population in a single garnet by sem reveals two characteristic features i the shape of most inclusions indicates healed radial cracks in the host garnet and thus the buildup of a significant differential pressure p i e a contrast in pressure between the inclusion pi and the host mineral pe the mineral assemblages sealing the cracks and showing an equilibrated microstructure indicate temperatures of 750 50 c and pressures below 2 5 gpa ii the diverse types of inclusions appear to be randomly distributed in the garnet host thus the variable phase assemblages do not reflect a compositional evolution of the fluid trapped in the garnet combining observations i and ii we propose that the diversity of the phase assemblage in the inclusions is the result of decrepitation at different times and thus of distinct histories of pi as p at decrepitation is primarily controlled by inclusion size and shape applying a flow law for dislocation creep of garnet a low strength of garnet at 750 50 c for low geological strain rates is predicted thus differential pressure should have been kept low i e pi pe by continuous stretching of the inclusion for typical exhumation rates of metamorphic rocks to attain the differential pressure pi pe required for catastrophic brittle failure of the garnet host the decompression rate must have been extremely high as a robust lower bound a minimum exhumation rate on the order of 100 m year 1 is suggested which corresponds to ascent rates of magma

pressure temperature reaction history of metapelitic rocks from the maryland piedmont on the basis of correlated garnet zoning and plagioclase inclusion composition regional structural interpretation thermobarometry and gibbs method calculations were combined to determine an approximate p t path for kyanite grade metapelitic rocks of the baltimore gneiss terrane garnet growth along the estimated p t path was modeled using the computer program diffgibbs models were evaluated by comparing calculated profiles of garnet components with measured profiles of a large garnet rich in plagioclase inclusions in a kyanite bearing sample from the area modifications in the p t path constrained by changes in garnet components along critical portions of the profile resulted in a very satisfactory match between measured and model garnet profiles in the most successful model garnet growth began at 550 c and 6500 bars in the assemblage chlorite biotite garnet muscovite plagioclase quartz with increasing temperature and with decreasing and then constant pressure at 570 c and 5724 bars staurolite was added to the assemblage and some garnet was consumed in the model garnet growth resumed after chlorite was consumed along a path having a substantial increase in pressure the region near the rim with low measured grossular content was reproduced by cessation of garnet growth as temperature increased at constant pressure to a maximum of 615 c at 6474 bars after kyanite was added to the assemblage grossular increased to the constant rim value only if temperature decreased and pressure increased cooling and decompression affected garnet zoning only very near the rim measured compositions of the rims of plagioclase inclusions in the garnet are consistent with plagioclase compositions predicted by the model this study demonstrates that complex garnet zoning can be modeled successfully even when the reaction history includes episodes of garnet consumption over a path that is consistent with geologic evidence details of garnet zoning provide surprisingly tight constraints on the p t path

deformation induced reconstitution and local resetting of mineral equilibria in polymetamorphic gneisses tectonic and metamorphic implications a sequence of at least three al2sio5 bearing mineral assemblages are preserved in successively overprinted ductile shear zones in the willimantic window connecticut the ductile deformation localized at and near the boundary between the putnam nashoba terrane and underlying avalon terrane is characterized by a network of anastomozing shear zones that outline metre scale tectonic blocks of migmatitic kfs sil gt bi pg qtz ilm ru gneiss these assemblages record acadian or older metamorphic conditions of 6 kbar 700d c mylonitic gneisses in shear zones that define block margins were formed by reconstitution and recrystallization of the migmatitic gneiss the reconstituted rocks exhibit relict ky st grt pl bt qtz rt ilm assemblages and require a minimum pressure for the ky str grade metamorphism of 8 5 kbar kyanite in block margins is widely replaced by sillimanite and locally by andalusite during a period of post alleghanian ductile deformation the interiors of blocks do not record this sequence of polymorphs the pattern of reconstitution is accounted for by localization of strain along block margins within a regionally extensive terrane bounding fault zone strain provided the activation energy for recrystallization and retrograde mineral reactions the p t conditions of post alleghanian ductile deformation evolved from 600d c and 6 kbar to 550d c and 3 kbar the occurrence of ky str bearing assemblages overprinting acadian kfs sil bearing assemblages and subsequently overprinted by alleghanian sillimanite and andalusite bearing assemblages along with reset hornblende 40ar 39 ar mineral ages from mississippian to permian requires a prograde alleghanian metamorphism of rocks previously metamorphosed during the acadian thus mineral assemblages from gneisses in the willimantic fault zone retain evidence of a protracted tectonothermal evolution that included high grade acadian orogenesis tectonic loading resulting from alleghanian collision of avalon with north america and tectonic exhumation in permo triassic time the c 3 kbar pressure decrease between prograde and retrograde alleghanian metamorphic conditions corresponds to 10 km of crust that must have been tectonically excised from the base of the putnam nashoba terrane cover sequence following alleghanian orogenesis in south eastern new england

mean winds and tides in the upper middle atmosphere at urbana 40 n 88 w during 1991 1992 a summary of the first 18 months of continuous wind observations using the urbana medium frequency mf radar is presented emphasis is placed on height time contours of monthly mean winds and on amplitudes and phases of 24 hour and 12 hour tides results are compared with data from other midlatitude stations and models below 85 km monthly mean winds are shown to agree closely with the zonal mean geostrophic wind model cira86 significant discrepancies above 85 km have been noted in comparisons between cira86 and the other mesosphere lower thermosphere mlt radar stations including mf meteor and low frequency lf radars and are also sustained in the observations reported here tidal characteristics are generally consistent with other midlatitude observations although significant differences are noted between urbana and its closest mlt network neighbor station durham 43 n 71 w the mean wind observations provide a context for the gravity wave climatology derived from na lidar observations at urbana and promise to contribute toward improvements of middle atmosphere wind and tidal models

hardness toughness and modulus of some common metamorphic minerals studies of the hardness and toughness of minerals have historically focused on minerals of the mohs scale although with the exceptions of quartz orthoclase and calcite mohs phases are not common rock forming minerals we report new hardness h toughness resistance to fracture k ic and indentation modulus e data obtained by microhardness and depth sensing indentation dsi or nanoindentation experiments for common metamorphic minerals sillimanite kyanite andalusite garnet quartz and orthoclase feldspar because the experimental techniques involve indentation induced cracking as well as depth sensing indentation the new data set can be used to investigate a range of plastic behavior for minerals in the crust and mantle the three al2sio5 polymorphs have similar h values 10 12 gpa kyanite has the largest values and andalusite the smallest these values are similar to that of quartz 12 gpa and greater than that of orthoclase 7 gpa garnet h values vary with composition for grossular h 13 gpa and for almandine pyrope h 15 gpa although h values for the minerals we analyzed span a range of 10 gpa most fracture toughness values are between 1 1 8 mpa m garnet is much harder than al2sio5 but has a similar to slightly lower k ic grossular 1 2 mpa m almandine pyrope 1 4 mpa m andalusite 1 8 mpa m sillimanite 1 6 mpa m kyanite k ic is difficult to measure owing to the ease with which kyanite cleaves garnet has properties similar to those of cubic zirconia zro2 which we measured as a reference another reference mineral periclase mgo has the lowest h 5 gpa and the highest k ic 4 mpa m of minerals we measured among the silicates e varies significantly from orthoclase 89 gpa to quartz 117 gpa to garnet 245 260 gpa and al2sio5 has intermediate values kyanite 186 253 gpa sillimanite 207 gpa andalusite 232 gpa

climatology and modeling of quasi monochromatic atmospheric gravity waves observed over urbana illinois from analyzing nine months of airglow imaging observations of atmospheric gravity waves agws over adelaide australia 35 s walterscheid et al 1999 have proposed that many of the quasi monochromatic waves seen in the images were primarily thermally ducted here are presented 15 months of observations from february 1996 to may 1997 for agw frequency and propagation direction from a northern latitude site urbana illinois 40 n as adelaide urbana is geographically distant from large orographic features similar to what was found in adelaide the agws seem to originate from a preferred location during the time period around summer solstice in conjunction with these airglow data there exists mf radar data to provide winds in the 90 km region and near simultaneous lidar data which provide a temperature climatology the temperature data have previously been analyzed by states and gardner 2000 the temperature and wind data are used here in a full wave model analysis to determine the characteristics of the wave ducting and wave reflection during the 15 month observation period this model analysis is applied to this and another existing data set recently described by nakamura et al 1999 it is shown that the existence of a thermal duct around summer solstice can plausibly account for our observations however the characteristics of the thermal duct and the ability of waves to be ducted is also greatly dependent on the characteristics of the background wind a simple model is constructed to simulate the trapping of these waves by such a duct it is suggested that the waves seen over urbana originate no more than a few thousand kilometers from the observation site

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

modeling of radial microcracks at corners of inclusions in garnet using fracture mechanics radial microcracks around inclusions in garnet are important indicators of ultrahigh pressure metamorphism and have been used to interpret changes in pressure as a function of temperature during decompression of metamorphic terrains we investigate the growth of microcracks from inclusion corners using a two dimensional fracture mechanics based boundary element method code and propose that microcracks emanating from inclusions are primarily controlled by inclusion shape and size and do not require high confining lithostatic pressure or significant decompression to initiate results suggest that inclusions 100 m diameter develop microcracks during modest decompression the magnitude of decompression required to initiate microcracks decreases with increased degree of faceting and increased inclusion size the potential for crack growth at each corner depends on the corner sharpness and the overall shape of the inclusion we integrate model results with petrographic observations to understand mechanisms and conditions of garnet microcracking during regional metamorphism

quartz deformation in a very fine grained quartzo feldspathic mylonite a lack of evidence for dominant grain boundary sliding deformation abstract microstructural and microtextural investigations by transmission electron microscopy tem have been performed on very fine grained quartzo feldspathic mylonites from the red bank deformed zone central australia the study of the quartz reveals a mean grain size 2 3 0 8 m and microstructural features indicative of dislocation creep processes within the very fine grained quartz analysis of local crystallographic preferred orientation cpo by tem showed a strong cpo for both the c and a axes the crystallographic relationships between adjacent grains and subgrains are consistent with dominant c a and a slip microstructural and microtextural studies also show that the recrystallization mechanism included local grain boundary migration and the formation of high angle grain boundaries from low angle sub grain boundaries by progressive misorientation we found no abnormal grain rotations as would be expected to result from grain boundary sliding processes this suggests that grain boundary sliding was not significant and the dominant deformation mechanism was dislocation creep

garnets as open systems during regional metamorphism if a garnet fractures during metamorphism its interior will be in chemical communication with matrix phases solids and fluids and may participate in late prograde or retrograde metamorphic reactions the core of a growth zoned garnet is unlikely to be in equilibrium with matrix phases and may react locally in the vicinity of fractures or extensively resulting in dissolution and reprecipitation of the garnet interior and or production of new phases the results of petrologic techniques such as thermobarometry pressure temperature path calculations and geochronometry may be misinterpreted if sites of garnet and mineral inclusion compositional modification are not identified brittle fracturing of garnet during regional metamorphism may be facilitated by the presence of mineral inclusions the conditions at which differential expansion of an inclusion relative to its host garnet are sufficient to cause radial cracking around the inclusion vary as a function of peak pressure retrograde pressure temperature path and thermoelastic properties of the inclusion and garnet inclusion induced fracturing is favored by nearly isothermal decompression a common metamorphic pressure temperature path other fractures may form as a result of propagation of an inclusion host boundary or by mechanisms external to the garnet

garnet polycrystals and the significance of clustered crystallization polycrystalline garnets are common in metamorphic rocks and may form as a result of close spacing of nuclei if clustering is early or impingement of larger grains if clustering occurs later in the growth history the timing of clustering relative to garnet growth is relevant to understanding the formation and evolution of porphyroblasts and evaluating the significance if any of clustering electron backscattered diffraction ebsd analysis of garnet bearing metamorphic rocks reveals the presence of polycrystalline garnet in nine localities examined in this study the northern appalachians vermont maine new york usa north american cordillera north cascades range washington snake range nevada usa western rocky mountains british columbia canada southern menderes massif turkey santander massif colombia and the sanandaj sirjan zone hamadan iran in some samples polycrystals comprise 20 30 of garnets analyzed and chemical and textural evidence suggests that early coalescence of garnet polycrystals is common some early coalescing polycrystals exhibit growth zoning that is concentric about the geometric center of the polycrystal in thin section these garnets may be undetectable as polycrystals based on morphology or zoning in some polycrystals zoning is unrelated to the location of internal grain boundaries in others fe mn mg zoning has a different pattern than that of ca zoning patterns may vary on the scale of a single thin section in addition some polycrystals are characterized by high angle misorientation boundaries that may be in special non random orientations an observation that indicates that these polycrystals are not random clusters of grains the presence of internal grain boundaries may affect diffusion pathways and length scales and may facilitate communication of porphyroblast interiors with matrix phases thereby influencing reaction history of the rock and the composition zoning of garnet

thrust related mudstone fabrics from the barbados forearc a backscattered scanning electron microscope study backscattered electron imaging on the scanning electron microscope has been used to examine the nature of microfabrics in mudstones from the barbados forearc samples from two important tectonic junctions in hole 671b of ocean drilling program leg 110 have been examined in detail at thrust a 128 m depth clays and clasts are pervasively aligned at about 50 to the thrust surface and a macroscopic scaly fabric is defined by anastamosing fractures in the decollement zone 500 550 m depth there is no preferred alignment of clays or clasts even though an irregular macroscopic scaly fabric is present scaly fabric in hole 671b is in part an artifact of specimen desiccation but may also reflect the release of anisotropic elastic strain relating to in situ stress conditions the disclosure that scaly fabric does not relate to grain alignment has profound implications for the nature of the barbados decollement seismic reflector the reflector does not correspond to a fabric anisotropy and is most likely to relate to the documented fluid flow anomaly in the decollement in contrast to scaly fabrics the microfabrics are the product of sediment deformation history the differences between the microfabrics at thrust a and those in the decollement probably relate to contrasts in deformation conditions in the decollement deformation probably occurred by paniculate flow under high pore fluid pressures where the individual panicles were aggregates of phyllosilicates at thrust a high pore fluid pressures were not maintained due to surface proximity enabling preconsolidation stresses to be exceeded as a result a pervasive phyllosilicate alignment has developed

new tectonic divisions of the grenville province southeast canadian shield on the basis of geological geophysical and geochronological data the grenville province has been divided into three first order longitudinal belts the parautochthonous belt pb allochthonous polycyclic belt apb and allochthonous monocyclic belt amb these are set apart by three first order tectonic boundaries the grenville front gf allochthon boundary thrust abt and monocyclic belt boundary zone mbbz the belts are subdivided into terranes based on internal lithological character the gf separates the archean to proterozoic foreland northwest of the orogen from reworked equivalents to the southeast continuous at the scale of the orogen its main characteristic is that of a crustal scale contraction fault the pb although less clearly identified along the length of the orogen in most places represents upgraded and tectonically reworked rocks of the adjacent foreland the boundary between the pb and the apb to the southeast the abt is most clearly delineated in the eastern half of the province it is the locus of major crustal delamination along which high grade mostly middle proterozoic polycyclic terranes were tectonically transported northwest toward and onto the pb the amb comprises two separate areas underlain by the wakeham supergroup and what is currently known as the grenville supergroup respectively its basal contact the mbbz is a decollement zone of variable kinematic significance between older polycyclic rocks and tectonically overlying monocyclic rocks this first order zonation implies a tectonic polarity to the grenville province superimposed on which are second order features evident from contrasting tectonic styles and radiometric ages these characteristics are consistent with a diachronous or oblique collisional model for the grenville orogen

mineralogy bulk and rare earth element geochemistry of massive sulphide associated hydrothermal sediments of the brunswick horizon bathurst mining camp new brunswick massive sulphides are spatially and temporally associated with iron formation if and other hydrothermal sedimentary rocks in the vicinity of the brunswick no 12 brunswick no 6 and austin brook deposits bathurst mining camp sulphide carbonate oxide and silicate predominant if is present carbonate predominant if is best developed in and around the brunswick no 12 deposit whereas hematite bearing if is absent here but prominent in the austin brook brunswick no 6 area the if is composed dominantly of si co2 fe mn and ca minor constituents include mg p ti al and s statistically significant interelement correlations between eu fe mn pb zn cd au ca sr ba p co2 and s indicate that these elements were precipitated from hydrothermal fluids vented onto the seafloor positive interelement correlations between si ti al mg k zr rare earth elements ree s except eu se v y yb co ni and cr reflect the presence of detrital clastic mafic and aluminosilicate minerals and

a stress transfer model for the development of extension fracture boudinage abstract the term boudinage is used to describe a wide variety of extensional structures in deformed rocks this paper is mainly concerned with boudinage resulting from through layer extension fractures followed by separation of the layer segments thus forming boudins with more or less rectangular cross sections in principle this process is similar to the break up of fibres in fibre reinforced composite materials extended parallel to the fibre direction both processes are controlled by the transfer of stress from the matrix to the fibre or layer and a mathematical model for fibre matrix stress transfer the fibre loading model is well established we have used this as a basis for developing a stress transfer model for boudinage the only difference in the basic mathematical formulation results from geometric differences between the two systems the geometric expressions in the fibre loading model have therefore been rederived for the layer matrix case stress transfer theory predicts that the tensile stress in a layer segment rises from a minimum at the end of a segment to a maximum at the centre this behaviour which is clearly shown by finite element models of boudinage structure suggests that extension fracture boudinage develops by successive mid point fracturing according to stress transfer theory the process will continue until a layer is reduced to segments boudins all of which are shorter than some critical length for which the tensile fracture strength of the layer is equal to the tensile stress at the mid point in practice successive fracturing will be influenced by two other factors 1 in nature the controlling material properties tensile fracture strength elastic moduli will not be single valued but will have a distribution reflecting local variations in lithology and microstructure and 2 major pre deformation flaws may be present in a layer which will control the starting length of layer segments these factors are incorporated with the stress transfer theory into a statistical monte carlo model for extension fracture boudinage which results in a prediction of boudin aspect ratios the predicted distribution compares very closely with the observed distribution of 91 quartzite boudins within lower carboniferous slates at tintagel cornwall the stress transfer model implies that boudin defining fractures occur sequentially so that inter boudin gap lengths will be unequal strain estimates based on boudinage structure will vary according to which part and how much of a layer is sampled a much improved strain estimate is possible based on sequentially closing the inter boudin gaps the stress transfer theory also leads to the possibility of estimating palaeostress from boudinage structure and is the only model available which predicts an aspect ratio distribution of boudins formed by extension fracture other than our own we know of no published data on boudin aspect ratio distributions hence further elaboration of the model is not possible until more field data is available we hope that our work will encourage the systematic measurement of boudinage as well as the development of alternative models

why blind thrust faults do not propagate to the earth s surface numerical modeling of coseismic deformation associated with thrust related anticlines high fault tip stress concentrations are associated with coseismic slip on blind thrust faults and suggest that these structures should readily propagate to the earth s surface seismic profiles of blind thrust related earthquakes reveal diffuse zones of aftershocks surrounding the fault tip which are attributed to inelastic deformation such as flexural slip or extensional fracturing the complex interaction between blind thrust faults and secondary structures may control the evolution of blind thrust systems the influence of bedding plane slip on fault propagation is simulated with numerical models using the boundary element method we use two parameters to estimate the tendency for thrust fault propagation 1 the mode ii stress intensity factor and 2 the maximum coulomb stress near the fault tip calculations from both analyses suggest that shallow thrust faults may exhibit an increased tendency to propagate as a result of interaction with the earth s surface and slip along bedding planes above the fault tip and a decreased tendency to propagate due to slip along bedding planes at or below the fault tip our results demonstrate that the magnitude and style of inelastic deformation in active fault systems control fault propagation

reaction induced fracturing in a hot pressed calcite periclase aggregate abstract the chemo mechanical feedbacks associated with hydration of periclase immersed in a calcite matrix were investigated experimentally dense calcite periclase aggregates with

metamorphism of high grade gneisses from the moldanubian zone austria with particular reference to the garnets gneisses from the southern part of the bunte series have been studied in order to determine their metamorphic evolution they all belong to the paragenetic facies type bt sil kfs qtz grt the garnets of these rocks generally exhibit typical features of garnets from this high temperature facies type diffusional homogenization throughout the bulk of the garnet and retrograde zoning patterns at the outer rim ca and mn richer relict cores in some garnets are evidence of the early metamorphism these cores are enveloped by the high temperature homogenized garnet the element distribution curves of the relict cores show evidence of partial element specific modification by volume diffusion paragenetic and textural relations as well as temperature and pressure calculations suggest the following conditions of the main metamorphism 700 770 c at 7 9 kbar and ph2o ptot despite the uncertainty due to the incomplete equilibration among garnet rims and other matrix minerals an attempt is made to model the cooling and uplift history of the rocks using textural paragenetic chemical and literature data zusammenfassung gneise aus dem sudlichen teil der bunten serie wurden im hinblick auf ihre metamorphoseentwicklung untersucht sie gehoren allgemein dem faziestyp bt sil kfs qtz grt an die granate weisen alle fur diesen faziestyp charakteristischen merkmale auf weitgeheride homogenisierung durch diffusion und abkuhlungserscheinungen am au eren rand ca und mn reiche reliktische kerne mancher granate weisen auf ein alteres ereignis hin diese relikte werden im homogenisierten granat hoherer bildungstem peratur eingeschlossen ihre element verteilungskurven sind teilweise durch die intrakristalline diffusion dem kationenradius entsprechend abgeflacht paragenetische beziehungen und t und p berechnungen ergeben folgende bedingungen fur die hauptmetamorphose 700 770 c bei 7 9 kbar und ph2optot trotz der unsicherheit die auf eine unvoll standige einstellung des gleichgewichtes zwischen granatrandern und den ubrigen matrixmineralen zuruckzufuhren ist wurde ver sucht ein modell uber die abkuhlungs und hebungsgeschichte der untersuchten gesteine zu erstelleu

the shift of the α β transition temperature of quartz associated with the thermal expansion of granite at high pressure abstract measurements are presented of volume changes in granite during room temperature compression to 100 200 and 300 mpa confining pressure followed by temperature increase to 900 c comparison with thermal expansion and compressibility data for the constituent minerals allows changes in porosity to be estimated under confining pressure porosity is found to decrease with heating to 200 c through expansion of the minerals into cracks which are thought to be related to the geological cooling history of the rock between 200 c and 840 c porosity increases as a result of differential thermal expansion of the constituent minerals but crack opening is increasingly suppressed at higher confining pressures extrapolation of the results indicates that differential thermal expansion can no longer cause crack opening in dry granite at confining pressures in excess of 450 mpa the quartz transition temperature in granite is marked by a kink in the thermal expansion curve of the rock and it is found to increase by 60 c 70 c per 100 mpa confining pressure as opposed to the published value of 26 c per 100 mpa for single crystals of quartz equations are presented which allow calculation of the effects of confining pressure and temperature on the stresses and displacements in and around a spherical inclusion embedded in a matrix of different elasticity and thermal expansion the theory together with a simple self consistent model for granite accounts semiquantitatively for the observations of thermal expansion and the effect of confining pressure thereon and for the observed transition temperatures for quartz in granite

marine tectonics processes at mid ocean ridges the use of multibeam echo sounders and various side looking sonar systems has greatly increased our knowledge and understanding of mid ocean ridge mor processes since the last iugg review on mid ocean ridge research macdonald 1983 the extensive use of swath mapping tools such as sea beam deep tow seamarc i and ii the application of multichannel seismic reflection techniques to the detection of axial magma chambers and the use of satellite data in the study of oceanic lithosphere have rejuvenated marine tectonics and have generated and stimulated a large number of important contributions this review will focus on what we feel were the major developments in marine tectonics with an emphasis on mid ocean ridges in the last four years certain topics such as marine seismicity and satellite studies are only briefly mentionned for reference and will be covered in greater detail in other reviews because of the exponential growth in the number of publications related to mors during 1982 1986 only a few references pertinent to each of the major research topics will be quoted in the text as examples but the reader will find an extensive bibliography at the end of the text references published prior to 1982 were added only where appropriate

observations of tidal temperature and wind perturbations in the mesopause region above urbana il 40 n 88 w na lidar and mf radar measurements of temperature and horizontal winds were conducted simultaneously during several continuous 24 h periods in march and april 1996 at the urbana atmospheric observatory the data are used to determine the amplitude and phase profiles of the 24 and 12 h tides in the mesopause region between about 82 and 100 km the measured amplitudes and phases of the tidal components of the horizontal winds are generally consistent with the predictions of the global scale wave model gswm the measured 24 h temperature amplitude is comparable to the gswm predictions between 85 and 96 km but is significantly larger than gswm above 96 km the measured phase progression is downward below 94 km with an inferred vertical wavelength of 14 1 km above 94 km the temperature phase progression is upward with a vertical wavelength of 12 1 km the measurements suggest the presence of a diurnal heat source in the lower thermosphere which is generating a strong downwardly propagating 24 h wave

coesite in subducted continental crust p t history deduced from an elastic model abstract coesite has been found in a metasedimentary rock in a continental unit of the western alps dora maira massif the rock preserves the high pressure low temperature assemblage of nearly pure pyrope talc phengite kyanite in a quartz matrix coesite exclusively occurs as inclusions in pyrope and is partly inverted to quartz textural observations and mechanical considerations suggest that the rock crystallised in the coesite stability field a simple elastic model for a coesite inclusion in pyrope explains why coesite has been exclusively preserved in the inclusions and not in the matrix combined with petrological data the model constrains the crystallisation conditions of the pyrope coesite rock p 28kbar 650 this finding has an important bearing on the significance of p t conditions estimated from minerals occurring as inclusions and on the actual behaviour of continental crust in collision zones

fracturing of garnet crystals in anisotropic metamorphic rocks during uplift garnet crystals in granulite facies mylonites and gneisses from the morin shear zone in the grenville province quebec are characterized by pervasive closely spaced relatively straight tensile fractures aligned systematically normal to the mylonitic foliation and lineation the fractures developed preferentially in coarse grains 0 2 mm or in grains with a large aspect ratio 2 fractured segments of each garnet crystal have not been separated or filled with matrix minerals quartz and feldspar or with retrograde minerals biotite muscovite and chlorite this suggests that the garnets were fractured at shallow crustal depths ti300 c or 15 km for a thermal gradient zo c km we propose that the garnet fractures were formed by the response of anisotropic metamorphic rocks to a horizontal extension during uplift and cooling of the metamorphic terrane within the upper crust using a modified shear lag theory we can explain why tensile fracturing took place preferentially in stiff garnet rather than in felsic material and why the fractures are unequally spaced in a garnet grain our data suggest that the seouential tensile fractures of garnet grains record a progressive uplift process of metamorphic rocks within the upper crust 0 1997 elsevier science ltd

radial cracks around chromite inclusions in olivine from a variscan peridotite shear zone in the ivrea zone northern italy abstract radiating tensional cracks around chromite inclusions in olivine fo 91 have been observed in the protomylonitic rim of a variscan shear zone in the finero peridotite complex in the ivrea zone of northern italy the first assumptions used to develop a thermomechanical model based on the investigation of e g wendt et al are discussed and a first application of a chromite olivine geothermobarometer is shown for the example of the finero peridotite

tio2nanoparticle trails in garnet implications of inclusion pressure induced microcracks and spontaneous metamorphic reaction healing during exhumation a new mechanism of cleaving healing of garnet in metamorphic rocks during exhumation is reported almandine garnet with submicron multiple phase inclusions characteristic of the grail reaction garnet rutile kyanite ilmenite low quartz was found by analytical electron microscopy to have non epitaxial rutile nanoparticles distributed along internal 110 microcrack trails at the acute corner of submicron multiple phase inclusions defined by the intersecting 110 and 011 mould surfaces such garnet microcracks were formed during lithostatic decompression due to a relatively high compressibility of quartz in the inclusion pockets and stress concentration at the acute corner spontaneous healing with accompanied formation of non epitaxial rutile nanoparticles was activated predominantly via the grail reaction due to high surface tension at the tip of microcracks and to a lesser extent decreasing lithostatic pressure or increasing inclusion pressure upon exhumation this new mechanism of microcracking and healing involves stress build up fracture propagation mineral reaction transport of elements to the reaction site and lowering of reaction boundaries by capillarity effect all under the influence of thermodynamic and kinetic factors at the submicron scale thus tio2 nanoparticle trails in garnet provide additional information on the p t path and may shed light on exhumation rates mechanisms and metamorphic reactions processes careful scrutiny of host minerals on the submicron scale is required to tell whether there are other metamorphic reaction facilitated healing processes being overlooked by inappropriate techniques or being obliterated by the predominant healing processes of fluid infiltration and resorption zoning

chemistry of waters sampled from oceanic basement boreholes 1979 1988 waters have been sampled from the basement section of four deep sea drilling project holes 395a 396b 418a and 504b since 1979 in an attempt to determine the composition of oceanic basement formation waters samples have been obtained by two methods active in which water is drawn under negative pressure from the interstices of the wall rocks and passive in which it is collected from the open borehole the most successful sampling has been done on re entering the holes months to years after they were last disturbed by drilling and or pumping of surface seawater downhole most of the samples show large changes in composition from seawater that are consistent with alteration of basalts at the in situ temperatures of 10 to 146 c the samples are typically depleted in mg2 k na sulfate and 87sr and enriched in ca2 and si however all of the samples are contaminated to some extent mainly by seawater which is either introduced during drilling or sampling flows downhole due to underpressure in the upper basement or mixes downhole due to convective overturn in the hole itself the latter process is apparently common and dilutes any formation water which may enter the hole via diffusion and horizontal convection in hole 504b the tritium content of the waters indicates that 24 to 35 of the surface seawater which originally filled the hole is still present after periods of 470 to 1233 days the more general problem of what has displaced the surface seawater and produced the large chemical changes observed in the borehole waters whether exchange with formation water or simply reaction of seawater with the wall rocks of the hole remains unsolved as a result no component of true basement formation water has been identified unambiguously in any of the holes this problem may be solved by 1 compartmentalizing a hole with a series of plugs to limit downhole mixing or 2 using a straddle packer to draw a series of samples from the interstices of the wall rocks of a hole either shortly after drilling or later following re entry in either case an array of chemical tracers will have to be measured either in situ or in the laboratory to infer the nature of the waters obtained

the geochemistry of carbonate diagenesis the past present and future stable carbon and oxygen isotopes 18o and 13c values and trace elements have been applied to the study of diagenesis of carbonate rocks for over 50 years as valuable as these insights have been many problems regarding the interpretation of geochemical signals within mature rocks remain for example while the 18o values of carbonate rocks are dependent both upon the temperature and the 18o value of the fluid and additional information including trace element composition aids in interpreting such signals direct evidence of either the temperature or the composition of the fluids is required such information can be obtained by analysing the 18o value of any fluid inclusions or by measuring the temperature using a method such as the clumped isotope technique such data speak directly to a large number of problems in interpreting the oxygen isotope record including the well known tendency for 18o values of carbonate rocks to decrease with increasing age unlike the 18o 13c values of carbonates are considered to be less influenced by diagenesis and more a reflection of primary changes in the global carbon cycle through time however many studies have not sufficiently emphasized the effects of diagenesis and other post depositional influences on the eventual carbon isotopic composition of the rock with the classic paradigm that the present is the key to the past being frequently ignored finally many additional proxies are poised to contribute to the interpretation of carbonate diagenesis although the study of carbonate diagenesis is at an exciting point with an explosion of new proxies and methods care should be taken to understand both old and new proxies before applying them to the ancient record

tidal signatures in temperature data from crista 1 mission temperature measurements in the stratosphere and mesosphere were taken during the first cryogenic infrared spectrometers and telescopes for the atmosphere crista 1 mission using co2 emissions these measurements range from 13 to close to 100 km and individual temperature measurements have a precision of 1 k the crista orbit was circular at an inclination of 57 so local time variations during the 7 day mission were small for a given latitude and orbit leg zonal averages of the data show significant structure in the vertical and as a function of latitude temperature differences between the zonal mean data from the ascending and descending portions of the orbit are of the form expected from the diurnal tide the maximum zonal mean difference is 20 k and occurs over the equator at an altitude of 75 km zonal variations in the temperature difference indicate that the tidal amplitude is not uniform at all longitudes at the equator the maximum amplitude 30 k appears over the african sector and the minimum 10 k over the pacific sector this variation is most likely a nonmigrating wavenumber 2 diurnal tide although other less plausible possibilities exist to facilitate comparisons with model results the temperature variations are converted to equivalent vertical displacements assuming the tidal motions are adiabatic such an approach is appropriate in the mesosphere where diurnal variations due to radiative effects are small and reduces the dependence of the measured temperature amplitude on the background temperature profile equivalent vertical displacements are also calculated using results from the global scale wave model for the most part the model results and observations are in excellent qualitative and good quantitative agreement from the tropopause to the mesopause however the observed vertical wavelength is smaller and its amplitude in terms of equivalent displacement is smaller in the upper mesosphere than that in the model the decrease in amplitude is consistent with that expected from the transition from equinox to solstice conditions

v pull apart microstructures a new shear sense indicator abstract granitoids deformed in a low grade shear zone show conspicuous development of asymmetric pull apart microstructures in feldspars characterized by v shaped gaps between the separated fragments v pull aparts they occur beside active shear planes and result from rigid body rotation of one fragment relative to the other the shear sense in the adjacent flow surface can be easily determined through the asymmetry normally shown by v pull aparts where the fracture wall at a lower angle to the shear plane indicates which fragment has rotated synthetically to the adjacent shearing another diagnostic feature is the foliation deflection that occurs adjacent to the rotated fragment the v shaped gaps are infilled by recrystallized quartz mica aggregates showing quartz c axis patterns indicative of strong participation of crystal plastic processes in these domains evidence of crack seal by precipitation c axes parallel to the direction of separation have also been found the rigid body rotation indicated by these microstructures worked as a mechanism of removal of protruding porphyroclast edges enabling the smoothing of the shear planes rigid body rotation should operate in circumstances where other mechanisms such as dissolution or crystal plasticity did not succeed in eliminating undulations along the shear planes

the temperatures and oxygen isotope composition of early devonian oceans the oxygen isotope composition of pre carboniferous 360 myr old marine carbonates cherts and phosphates has been reported1 8 to be lower than that of later post devonian samples according to one explanation1 3 this is a reflection of warmer generally 40 c pre carboniferous oceans relative to modern oceans alternatively it has been proposed5 7 that the pre carboniferous oceans were depleted in 18o by 2 smow relative to modern oceans here i report high 18o values 1 9 to 2 9 pdb in normal shallow marine limestones and three brachiopod shells from the lower devonian haragan and bois d arc formations in south central oklahoma i interpret these values as near primary and therefore constraining the temperature and oxygen isotope composition of early devonian sea water to 25 7 c and 0 1 smow respectively in conjunction with similarly high 18o values obtained from older ordovician and silurian samples8 these results imply that the temperature and oxygen isotope composition of pre carboniferous oceans may at least during some time intervals have been similar to those of modern oceans

the role of intragranular fracturing on grain size reduction in feldspar during mylonitization abstract measurements of the size shape and orientation of k feldspar and plagioclase grains within a granitic mylonite in the kashio shear zone central honshu japan revealed that grain size was the most effective parameter controlling development of intragranular fracturing during mylonitization intragranular fractures are dominantly located near the centre of each grain and form preferentially along the 001 cleavage the grain size distribution of both feldspars is log normal with an average of 760 m the proportion of fractured grains expands dramatically to 100 as the grain size increases whereas few feldspar grains smaller than 300 m have been fractured during the mylonitization indicating that the fractures have preferentially developed in coarser grains furthermore the shape and shape preferred orientation of feldspar grains also affects the development of fractures the proportion of fractured grains not only increases up to 20 towards the extension direction but also rises slightly as the aspect ratio of feldspar grains increases it is therefore suggested that feldspar grains lying perpendicular to the extension direction may resist fracturing regardless of grain size unless their shape preferred orientations became sub parallel to the extension direction due to rotation or modification of their grain shapes during mylonitization the development of intragranular fractures could be statistically explained in terms of a fibre loading model although individual fractures may result from various fracture mechanisms

climatology of mesospheric gravity wave activity over urbana illinois 40 n 88 w a 5 year 1991 1995 climatology of mesospheric gravity wave activity observed with the urbana medium frequency mf radar is presented we focus on the monthly mean horizontal wind variances associated with gravity waves with observed periods between 10 min and 8 hours these results are compared with the seasonal characteristics from other gravity wave observations the total wave variance displays a dominant semiannual variation maxima summer winter minima spring fall below 87 km with little seasonal variation above 87 km the zonal wave variance exceeds the meridional wave variance except around regions of zonal wind reversal in all seasons the observed variance scale height is greater than that of a nondissipating wave field a simple zero mean wind model based on linear saturation theory is found to be inadequate in reproducing the seasonal characteristics of the observed wave variances an alternative model based on critical layer filtering by a height dependent wind field can reproduce the observed semiannual oscillation in the wave variance given a specific source input

location of tensile fracture within rigid brittle inclusions in a ductile flowing matrix abstract the fundamental assumption in the fibre loading theory proposed by cox was used to develop a new theoretical model that predicts the location of tensile fracture within rigid brittle inclusions embedded in a continuous ductile flowing matrix both theoretical prediction and microstructural observations show that fracture location depends strongly on inclusion shape successive mid point fracturing occurs within cylindrical grains while the circular truncated conical grains sequentially fragment into two segments of unequal length the longer of which has the greater diameter

mapping and modeling of radial fracture patterns on venus synthetic aperture radar images indicate numerous radial fracture systems on venus the formation of these systems has been attributed either to surface deformation related to subsurface dike emplacement or to fracturing caused by domical uplift above an ascending magma plume we evaluate the dike emplacement hypothesis by investigating the relationship between the process of subsurface dike emplacement and the geometric attributes of radial fracture patterns assuming the surface fracture patterns correspond to the patterns of dikes about a magma chamber below the volcanic summit we select a radial fracture system for mapping and model its formation using a two dimensional analysis which idealizes the chamber as a pressurized hole in an elastic plate subject to a remote biaxial stress field radial fractures correspond to maximum principal stress trajectories and the pattern of these trajectories is controlled primarily by four variables the chamber radius the magma pressure the remote differential stress and the remote principal stress direction we find that stress trajectories also depend on the remote mean stress and that the location of isotropic points in the trajectory pattern can be used to interpret the mean stress state these models are used to estimate the remote loading conditions under which dikes were emplaced the mapped fracture pattern is consistent with dikes emplaced under remote differential stress conditions ranging from 2 to 8 mpa calculated over a range of possible emplacement depths from 1 to 9 km this is consistent with regional stress states on earth

passive off axis convection through the southern flank of the costa rica rift pore fluids are passively convecting through young oceanic sediments and crust around deep sea drilling project dsdp site 504 on the southern flank of the costa rica rift as inferred from a variety of geological geochemical and geothermal observations the presence of a fluid circulation system is supported by new data collected on ocean drilling program odp leg 111 and a predrilling survey cruise over the heavily sedimented 5 9 ma site during the latter elongated heat flow anomalies were mapped subparallel to structural strike with individual measurements of twice the regional mean value and strong lateral and vertical geochemical gradients were detected in pore waters squeezed from sediment cores also there is a strong correlation between heat flow bathymetry sediment thickness and inferred fluid velocities up through the sediments on an earlier dsdp leg an 8 bar underpressure was measured in the upper 200 m of basement beneath thick sediment cover although the forces which drive passive circulation are not well understood it has generally been thought that the length scale of heat flow variations provides a good indication of the depth of hydrothermal circulation within the oceanic crust this assumption was based on analytical and numerical analyses of relatively simple porous media models deep crustal convection had been inferred near site 504 based on the geometry of surface heat flow anomalies with a wavelength of 4 7 km but appears to be precluded by low crustal permeability as measured in dsdp hole 504b the widely varied geothermal and hydrogeological observations near site 504 are readily explained by a model which combines 1 basement relief 2 irregular sediment drape 3 largely conductive heat transfer through the sediments overlying the crust and 4 thermal and geochemical homogenization of pore fluids at the sediment basement interface which results from 5 topographically induced passive hydrothermal circulation with large aspect ratio convection cells this convection involves mainly the permeable upper 200 300 m of crust the deeper crust is not involved this convection is induced through a combination of buoyancy fluxes due to heating from below and topographic variations on the seafloor and at the basement sediment interface this model was designed to incorporate data from both near the sediment surface and deep within boreholes it is successful in duplicating numerous field observations

doppler imager wind measurements with simulations from the ncar thermosphere ionosphere mesosphere electrodynamics circulation model in this paper a comparison is presented between simulations of the diurnal and semidiurnal tides in the middle and upper atmosphere from the thermosphere ionosphere mesosphere electrodynamics general circulation model timegcm and satellite observations from the upper atmosphere research satellite uars high resolution doppler imager hrdi reasonable agreement is found between the model and observations for a march condition when a strong diurnal tide was observed the model is able to reproduce many tidal features at altitudes between 65 and 110 km as observed in the hrdi wind measurements of interest in these features is the shortening vertical wavelength of the 1 1 tide in the upper mesosphere and lower thermosphere which appears in both model simulations and the wind observations the hrdi data also show large longitudinal variations in the diurnal tide of which the amplitude can be as high as 50 of the migrating tide the wavelike structures in latitude and altitude dimensions suggest that the longitudinal variations are likely due to the presence of propagating nonmigrating tides in the upper mesosphere and lower thermosphere for further investigations of the observed tidal variability the model demonstrates great potential with the use of more realistic heating cooling and wave forcing schemes

strength of two phase rocks a model based on fiber loading theory abstract in order to predict the strength of two phase rocks in terms of the component volume fraction the aspect ratio of the strong phase and the strength contrast between the strong and weak phases a simple model is developed according to a fiber loading theory modified for short fiber composites the analytical solution derived from elastic theory is found to hold in good approximation for rocks consisting of power law viscous minerals there is a good agreement between the predicted results and the experiments for two phase aggregates anhydrite halite calcite halite fe ag and clinopyroxene plagioclase the model is compared with several previous different models

refinements of shear lag model and its applications abstract since it was developed by cox cox h l 1952 the elasticity and strength of paper and other fibrous materials br j appl phys 3 72 79 the shear lag model slm or fiber loading theory has been widely used by material scientists and structural geologists as a powerful analytical method for analyzing mechanical interactions between hard and soft phases layers in composites and two phase rocks we found that several approximations in previous mechanical derivations of the model can be improved in terms of mechanical equilibrium and boundary conditions our refinements make the model more rigorous and elaborate applications of the improved model include 1 prediction of the axial tensile stress in strong inclusions embedded in a continuous weak matrix and consequently interpretation of extension fracture boundinage 2 modeling of the distribution of shear stresses in the weak matrix surrounding the strong inclusions and accordingly interpretation of the variations of dislocation density and of recrystallized grain size in composites and polyphase rocks and 3 prediction of mechanical properties of industrial composites and two phase rocks the analytical results of the improved model agree approximately with those obtained from both experimental investigations and numerical calculations

carbonate recrystallization in basal sediments evidence for convective fluid flow on a ridge flank in many deep sea drilling project dsdp drill holes chalks and or limestones and even dolomites overlie oceanic basement rocks these occurrences have often been interpreted as the result of increased calcium carbonate dissolution and reprecipitation as cement and overgrowths recrystallization at higher prevailing temperatures either near an oceanic ridge system or at greater burial depths burial diagnesis we report here detailed chemical and isotopic analyses of carbonate sediments recovered from a drill hole on the western flank of the east pacific rise during leg 92 of dsdp this hole was drilled on 4 6 myr old oceanic crust and recovered chalks directly overlying oceanic basement at a depth as shallow as 8 m below the sediment water interface oceanic basement was not reached in this hole because of the hardness of deeper chalks or more likely limestones the chemical and isotopical evidence detailed below shows that considerable calcite recrystallization did occur in an aqueous medium similar to normal seawater and at relatively low temperature this suggests that the observed recrystallization of calcareous ooze to chalk and limestone took place not as a result of burial diagenesis at higher temperatures but as a result of extensive advective pore water flow through the sediments which would allow recrystallization of calcium carbonate to take place this process was and apparently still is widespread in the south east pacific ocean

seasonal variation of diurnal perturbations in mesopause region temperature zonal and meridional winds above fort collins colorado 40 6 n 105 w 1 on the basis of lidar observations from may 2002 through april 2003 covering both day and night we performed a harmonic analysis to extract the diurnal perturbations in mesopause region temperature zonal and meridional winds over fort collins colorado 40 6 n 105 w binned every 2 months the results were compared to predictions of the 2000 and 2002 versions of global scale wave model gswm00 and gswm02 the diurnal tidal period oscillations showed a mixture of propagating and evanescent trapped modes but the propagating modes dominated for most of the year the agreement in temperature diurnal phases between observation and gswm prediction is marginal on the other hand other than july august meridional winds the observed diurnal phases in both wind components are in good agreement with gswm predictions for most of the altitude range reported the diurnal amplitude predictions of gswm00 were reasonably close to lidar observations while other than january february the gswm02 amplitude prediction overestimated the observations typically by a factor of two we also conducted comparisons on tidal perturbations in zonal wind between radar campaigns and our lidar observations the lidar data agreed reasonably well with the mf radar data from 2000 to 2001 at nearby platteville colorado 40 2 n 104 7 w but showed considerable differences with the data from other midlatitude stations from 1992 to 1993 the dominance of the evanescent mode in the temperature diurnal tidal oscillation during the early winter november and december which reached a peak value at midnight was interesting and anomalous by invoking the more recent data november and december in 2003 as well as the diurnal temperature observations from december 1998 we report that the evanescent trapped diurnal tidal perturbations were robust and persisted from one year to the next

successive mid point fracturing during microboudinage an estimate of the stress strain relation during a natural deformation abstract this paper describes a piedmontite microboudinage during plastic flow of surrounding quartz in a quartz schist collected from the high pressure low temperature sambagawa metamorphic belt in southwest japan nine microboudins originated from a single grain by successive mid point fracturing and subsequent pull apart such features strongly indicate a fibre loading mechanism for the microboudinage the strain history inferred by successive use of the strain reversal method and stress data quantitatively calculated by a formula of the fibre loading theory provide us with an estimate of the stress strain relation during the microboudinage it apparently proceeded with increasing bulk stress during the last stage of metamorphism

latitude and seasonal dependence of the semidiurnal tide observed by the high resolution doppler imager the high resolution doppler imager hrdi on the upper atmosphere research satellite uars has provided measurements of the horizontal wind field in the mesosphere and lower thermosphere mlt since november 1991 the observed dynamical patterns are dominated by the 1 1 diurnal tide but semidiurnal perturbations are also very prominent in the hrdi data particularly at latitudes greater than 40 this paper presents a preliminary study of the semidiurnal tide and discusses latitudinal and seasonal variations in the phenomenon regular seasonal changes are clearly detected and significant interhemispheric asymmetries have been identified in general the observed features are consistent with previous measurements and model results but some discrepancies have been found this new technique which brings a global perspective to the observation of mlt dynamics is complementary to the very detailed but localized radar data sets currently available and should provide important constraints for numerical models

comparisons between satellite derived gradient winds and radar derived winds from the cira 86 abstract satellite radiance data nimbus 5 6 80 km and the msis 83 model have been used to prepare global zonal mean gradient winds 30 120 km for the new cira 1986 here these are supplemented by planetary wave morphology from the same nimbus data to provide local gradient winds the zonal wind and the eddy portion of the meridional wind are calculated by this method these data are then compared with radar derived wind contours 60 110 km extending the comparisons done earlier manson et al for heights below 80 km overall the agreement for the zonal winds is good especially below 80 km differences are shown so the user can evaluate each product the comparison of meridional winds is particularly valuable and unique as it reveals considerable ageostrophy particularly in summer months near the height of the zonal wind s reversal from west to eastward flow coriolis torques due to this meridional flow are available from saskatoon 52 poker flat 65 and tromso 70 in the northern hemis

seasonal variations of semidiurnal tidal perturbations in mesopause region temperature and zonal and meridional winds above fort collins colorado 40 6 n 105 1 w 1 on the basis of colorado state university csu na lidar observations over full diurnal cycles from may 2002 to april 2006 a harmonic analysis was performed to extract semidiurnal perturbations in mesopause region temperature and zonal and meridional winds over fort collins colorado 40 6 n 105 1 w the observed monthly results are in good agreement with mf radar tidal climatology for urbana illinois and with predictions of the hamburg model of the neutral and ionized atmosphere hammonia sampled at the csu na lidar coordinates the observed semidiurnal tidal period perturbation within the mesopause region is found to be dominated by propagating modes in winter and equinoctial months with a combined vertical wavelength varying from 50 km to almost 90 km and by a mode with evanescent behavior and longer vertical wavelength 100 150 km in summer months most likely due to dominance of 2 2 and 2 3 tidal hough modes the observed semidiurnal tidal amplitude shows strong seasonal variation with a large amplitude during the winter months with a higher growth rate above 85 90 km and minimal amplitudes during the summer months maximum tidal amplitudes over 50 m s for wind and 12 k for temperature occur during fall equinox a detailed comparison with hammonia predictions shows excellent agreement in semidiurnal phases hammonia predicted semidiurnal amplitudes generally agree well with observations however hommonia underestimates temperature amplitudes in some of the nonsummer months as well as zonal wind and meridional wind amplitudes in april and september but overestimates them in february to reveal the effects of the atmospheric background on vertical propagation of tidal modes and their relative importance in the composite semidiurnal tide during different seasons we use the lidar observed monthly mean temperature and zonal wind from the same data set as well as hammonia output to calculate the vertical wave number seasonal variations of the major tidal modes of the migrating semidiurnal tide this leads to a qualitative understanding of the lidar observed and hammonia predicted seasonal variation of the semidiurnal tidal perturbation

tides in the mesopause region over fort collins colorado 41 n 105 w based on lidar temperature observations covering full diurnal cycles 1 eighteen sets of 24 hour continuous lidar temperature observation well distributed throughout the year have been analyzed to reveal the mean amplitude and phase of oscillations with 24 12 8 and 6 hour periods for four seasons the amplitude and phase of both 24 and 12 hour components are compared to the prediction of the most recent global scale wave model gswm 00 the variability of diurnal and semidiurnal oscillations is examined by comparing the amplitude and phases of each observed day in autumn winter and spring even with the limited data sets reported here surprisingly good general agreement between the observed and model tidal phases can be established implicating the prevalence of migrating diurnal and semidiurnal tides compared to the diurnal and semidiurnal phases which typically show clear downward phase progression the 8 and 6 hour components exhibit small amplitude and show unorganized phase propagation as a function of altitude the advantage of having coverage over complete diurnal cycle and entire mesopause altitudes as demonstrated here would hopefully stimulate future tidal studies with lidar temperature and zonal and meridional wind observations