migrating and nonmigrating semidiurnal tides in the upper atmosphere excited by tropospheric latent heat release migrating and nonmigrating semidiurnal tides

an assessment of dynamically recrystallized grainsize as a palaeopiezometer in quartz bearing mylonite zones abstract recrystallized grainsize has been measured in quartz and in some cases feldspar in about 200 samples from 22 mylonite zones from 3 discrete tectonic settings 1 1 supracrustal low temperature shallow dipping thrusts 2 2 deeper level major 100 km thrusts in high grade metamorphic rocks and 3 3 steeply dipping fault zones in granite within a low grade palaeozoic fold belt the concept of a steady state dynamically recrystallized grainsize largely unaffected by post creep processes e g static grain growth receives strong support especially from the microstructures and the independence of finite strain and grainsize in most mylonite zones however the usefulness of this grainsize for predicting stress magnitudes is limited by the apparently independent influence of metamorphic environment on grainsize the available theoretical and empirical stress grainsize relationships do not account for these effects the concept of a steady state grainsize and the ease of accurate measurement offer a potentially powerful method for predicting stress intensity during deformation predicted deviatoric stresses have been computed using twiss theoretical model for comparison with other crustal stress estimates and most samples fall in the range from 20 to 150 mpa in a few cases where discrete high strain mylonite zones occur within contemporaneously regionally deformed rocks the recrystallized grainsize indicates that stresses were 1 5 to 2 times higher within the zones there is a clear need for improved theoretical and or empirical stress grainsize relationships for quartz and other minerals before extensive further application of this technique

cessation of plastic deformation during exhumation of metamorphic tectonites revealed by microboudinage structures abstract microboudinage structures are analysed to examine how quartzose metamorphic tectonites cease plastic deformation during retrograde metamorphism in exhumation the microboudinage of columnar mineral grains embedded in the quartz matrix record whether plastic deformation ceased as result of the relaxation of differential stress or the plastic brittle transition of the matrix the fundamental strain data for microboudinage structures reveal four possible cases in terms of the timing of the plastic brittle transition in the differential stress history the classification is closely related to the degree of post tectonic annealing which obliterates the microstructures produced during earlier plastic deformation the approach is applied to three metamorphic tectonites from high pressure temperature metamorphic belts and all are demonstrated to have ceased plastic deformation due to plastic brittle transition of the matrix suggesting that the rocks experienced increasing differential stress during exhumation in retrograde metamorphism

development of asymmetric shale pull aparts in evaporite shear zones abstract antithetically rotated lozenge shaped shale pull aparts are quite common in the shallow domains of the evaporite shear zones of the jura in northern switzerland the pull aparts are formed by synthetic shear fractures related to a transpressive strain field that also induces the antithetic rotation of the pull aparts immediately after their formation a simple analytical model shows that the acute angle of fracture and the antithetic rotation are determined by structural and rheological parameters and the direction of maximum compressive stress the pull aparts are commonly separated by sulphate veins as the elongation resulting from antithetic rotation does not compensate for the bulk elongation despite this some pull aparts are bordered by synthetic drag zones antithetically rotated pull aparts are indicative either of compression normal to shear zone boundaries in decollement type shear zones or in shear zones deformed by simple shear of an initial obliquity of the dismembered layers relative to the shear zone boundary resemblance of the shale pull aparts to type 1 asymmetric pull aparts and book shelf structures both of which rotate synthetically demonstrates that rotated and asymmetric pull aparts may be ambiguous kinematic indicators

on microboudin paleopiezometers and their applications to constrain stress variations in tectonites abstract accounting for interfacial shear induced near end fracturing we propose a new microboudin piezometer e c 0 25 l w where is the shear flow stress of the ductile matrix e g polycrystalline quartz c is the tensile fracture strength of a columnar mineral e g tourmaline which ranges generally between e 1000 to e 250 e is young s modulus e is a dimensionless parameter named equivalent shear strength and l w is the mean length width ratio of the microboudins formed by near end fracturing this provides a straightforward method of estimating the magnitude of shear flow stresses responsible for ductile deformation of natural rocks the piezometer is easy to apply because it does not require the use of tem sem ebsd or the identification of prevailing deformation mechanism e g dislocation creep diffusion creep hardening or weakening the samples from the gaoligong metamorphic belt yunnan china yield the equivalent shear flow stresses of 0 145 0 424 36 106 mpa if taking c e 800 or 250 mpa for felsic gneisses mylonites and phyllonites the earlier formed microboudins had consistently a larger mean length width ratio than the later formed ones suggesting that the rocks recorded a tectonic history of continuous hardening prior to the cessation of quartz plasticity

strain reversal a windows program to determine extensional strain from rigid brittle layers or inclusions abstract an on line windows based computer program to calculate extensional strain from the separation of rigid brittle layers or inclusions is presented the program uses the incremental strain reversal methodology rather than traditional finite length methodologies this approach not only provides a better estimate of the extensional strain suffered by the system but also allows for fracture and separation histories to be studied an example application is included based on boudinaged belemnites from the french alps to show the facility of the program which can be downloaded direct from the world wide web

exhumation of the schistes lustres complex in situ laser probe 40ar 39ar constraints and implications for the western alps the schistes lustres sl suture zone occupies a key position in the alpine chain between the high pressure hp brianconnais domain and the ultrahigh pressure uhp dora maira massif and reached subduction depths ranging from c 40 65 km cottian alps in order to constrain the timing of hp metamorphism and subsequent exhumation several phengite generations were differentiated on the basis of habit texture paragenesis and chemistry as belonging to the first or second exhumation episode respectively d2 or d3 or to earlier stages of the tectono metamorphic evolution ten carefully selected samples showing d2 d3 d2 d3 or earlier mostly peak temperature phengite population s were subjected to laser probe 40ar 39ar analysis the data support the results of the petrostructural study with two distinct age groups crystallization ages for d2 and d3 phengite at 51 45 and 38 35 ma respectively the data also reveal a coherent age cluster at 62 55 ma for peak temperature phengite associated with chloritoid which were preserved in low strain domains the age of the d3 event in the sl complex appears very similar to ages recently obtained for greenschist facies deformation on the border of most internal crystalline massifs exhumation rates of the order of 1 2 mm yr 1 are obtained for the sl complex which are compatible with velocities documented for accretionary wedge settings similarly cooling velocities are only moderate c 5 c myr 1 which is at variance with recent estimates in the nearby uhp massifs

determination of amphibole fracture strength for quantitative palaeostress analysis using microboudinage structures abstract the fracture strength of amphibole is estimated with the aim of extending the applicability of the microboudin method to palaeostress analysis before estimating the fracture strength of amphibole it is necessary to evaluate the influence of the size effect on fracturing among the three models of size effect effective length effective area and effective volume the effective length model is found to be the most suitable in our microboudinage analysis of a metachert from turkey that contains numerous microboudinaged amphibole grains of variable width embedded within a quartz matrix taking into account the influence of the size effect on fracturing we undertook a comparative microboudinage analysis of the metachert sample which also contains coexisting microboudinage structures of tourmaline and epidote the analysis revealed an instantaneous fracture strength for amphibole of 80 mpa for a 1 mm cube the far field differential stress 0 calculated based on amphibole microboudinage structures and considering the effect of the fatigue limit is 0 8 1 w 1 2 where is the dimensionless stress parameter determined by microboudinage analysis and w is the mean width of amphibole grains

diurnal nonmigrating tides from timed doppler interferometer wind data monthly climatologies and seasonal variations 1 timed doppler interferometer tidi measurements of zonal and meridional winds in the mesosphere lower thermosphere are analyzed for diurnal nonmigrating tides june 2002 to june 2005 climatologies of monthly mean amplitudes and phases for seven tidal components are presented at altitudes between 85 and 105 km and latitudes between 45 s and 45 n westward propagating wave numbers 2 3 and 4 the standing diurnal tide and eastward propagating wave numbers 1 2 and 3 the observed seasonal variations agree well with 1991 1994 uars results at 95 km comparisons between the tidi results and global scale wave model gswm and thermosphere ionosphere mesosphere electrodynamics general circulation model time gcm tidal predictions indicate that the large eastward propagating wave number 3 amplitude is driven by tropical tropospheric latent heat release alone in contrast latent heating and planetary wave migrating tidal interactions are equally important to westward 2 and standing diurnal tidal forcing there is good quantitative agreement between tidi and the model predictions during equinox but the latter tend to underestimate the westward 2 and standing diurnal tide during solstice neither model reproduces the observed seasonal variations of the eastward propagating components

incomplete blueschist re crytallization in high grade metamorphics from the sesia lanzo unit vasario sparone subunit western alps a case history of metastability abstract the vasario sparone vs subunit is part of the sesia lanzo unit western alps and represents a thin and discontinuous slice of pre alpine deep continental crust it is tectonically interposed between the eclogitic mica schists and the gneiss minuti subunits metastable relics and a sequence of incomplete transformations controlled by a local equilibrium reaction mechanism are preserved in the vs subunit the vs rocks record a pre alpine high grade of metamorphism t 780 50 c p 6 kbar which is comparable to the metamorphism dated around 500 ma in the ivrea zone during the alpine evolution the vs rocks sustained an early blueschist stage t 300 400 c p 8 10 kbar of probable cretaceous age and a later low t low p stage of tertiary age the blueschist transformations are governed by slow reaction kinetics by the heterogeneous distribution of pervasive deformations and by syn metamorphic fluid phase in particular transient kyanite formed in the early stages of the blueschist re crystallization in response to a process controlled by fluid deficient conditions and by internal buffering of a h 2 o at low values

tidal variability in the lower thermosphere comparison of whole atmosphere model wam simulations with observations from timed 1 the upper atmosphere and ionosphere exhibit variability on spatial and temporal scales characteristic of tides and planetary waves originating in the lower atmosphere to study their generation vertical propagation possible nonlinear interactions and effects a new whole atmosphere model wam has been developed as part of the integrated dynamics through earth s atmosphere idea project wam is a 150 layer general circulation model based on the us national weather service s operational global forecast system gfs model extended upward to cover the atmosphere from the ground to about 600 km first simulations reveal the presence of migrating and nonmigrating tides modulated at planetary wave periods in the upper atmosphere comparisons with observations from the timed satellite in the lower thermosphere show that wam reproduces the seasonal variability of tides remarkably well including the diurnal eastward harmonic with zonal wavenumber 3 de3 recently implicated in the observed spatial morphology of the ionosphere

calcite twins their geometry appearance and significance as stress strain markers and indicators of tectonic regime a review twin lamellae are a widespread feature in calcites of any type and origin twinning in calcite is an important intracrystalline deformation mechanism with low critical resolved shear stress and virtually no temperature dependence in contrast to most slip systems in rock forming minerals twinning therefore permits intracrystalline deformation at very low temperature and very little confining pressure twinning alone however cannot lead to large strains because only one independent slip system is effectively available twins lead to discontinuous shearing on the grain scale which gives rise to considerable strain incompatibilities at grain boundaries twinning consequently leads to strain hardening detailed observations of the geometry of twin lamellae show that incompatibilities at grain boundaries are eliminated by pressure solution on the grain scale at low temperature and dynamic recovery processes at higher temperatures twins are found in various tectonic environments and record very small strains in rocks deformed under very little cover different twinning strain and twinning stress methods and palaeopiezometers are critically reviewed all methods are limited to strains of less than 15 shortening or extension by twinning twinning nevertheless is an important although not directly measurable contribution to the total strain in highly strained calcite rocks at least up to middle greenschist facies under which conditions it is the most effective contributor to the formation of strong lattice preferred orientations in calcite tectonites the appearance of calcite twin lamellae changes systematically as a function of deformation temperature micro twins and straight narrow 1 5 m but fewer twins are developed above approximately 200 c curved twins twinned twins and completely twinned grains indicate the progressive importance of other slip systems and larger intracrystalline strains are possible at ca 250 c and above ancient straight twin lamellae are modified into irregular geometries by recrystallization and grain boundary migration the appearance of calcite twins is proposed as an approximate but rapid and easy to use geothermometer in low temperature environments

fracture strength of tourmaline and epidote by three point bending test application to microboudin method for estimating absolute magnitude of palaeodifferential stress abstract three point bending tests are conducted on millimetre scale tourmaline and epidote samples at room temperature and pressure to determine fracture strengths suitable for use with the microboudin method of palaeostress analysis the fracture strength of the samples is obtained from the weibull scale and shape parameters using a size effect model three different size effect models are evaluated and it is found that the effective length model is the most appropriate for describing the fracture strength of tourmaline and epidote based on geological constraints the palaeodifferential stresses determined by this method for samples from wadi tayin sultanate of oman greenbushes australia and four high pressure metamorphic belts in japan are in the range of 10 300 mpa

deformation mode switches in the penninic units of the urtier valley western alps evidence for a dynamic orogen abstract multiple deformation mode switches from shortening to extension have been detected in the penninic units of the urtier valley to the north of the gran paradiso massif an early shortening phase whose traces are poorly preserved led to the tectonic burial of rock units to high pressure conditions subsequent extensional shearing sz gr 1 led to the formation of mylonitic fabrics during this event the eclogitic piemonte unit was exhumed to greenschist facies conditions p 0 2 0 3 gpa in the footwall of the blueschist piemonte unit exotic slices of continental basement and mesozoic continental shelf carbonates are found along their contact implying the existence of older thrust stacks dissected by the younger extensional shear zones the extensional structures were later affected by upright folding fu indicating yet another deformation mode switch which led to re burial to p 0 6 0 8 gpa later multi stage deformation culminated in an episode of extensional shearing with top to the west kinematics sz gr 2 therefore at least two shortening extension cycles affected the piemonte units of the urtier valley existing geochronological data allow less than 20 myr for the complex deformation history discussed here shortening extension cycles can be inferred also from published studies from other sections of the western alps however while the first shortening extension cycle was widespread in the penninic units the lateral extent and relative timing of the second cycle in different areas are still unclear

deformation mechanisms and flow regimes in limestones from the helvetic zone of the swiss alps abstract this paper is based on a combined field transmission electron tem and transmission optical tom microscope study of limestones from the helvetic zone swiss alps and discusses the deformation mechanisms and flow regimes that governed the deformation of these rocks during pre metamorphic regional ductile deformations the limestones deformed by power law dislocation creep with differential stresses probably not exceeding 1 kbar dynamic recrystallization with grain boundary sliding and grain boundary migration allowed the grains to be less elliptical than the strain ellipse a characteristic of the structure is the existence of dislocation free subgrains in the footwall of and approaching the lochseiten calc mylonite along the glarus overthrust grain boundary sliding becomes more important shift to diffusional creep or superplastic flow during a syn and post metamorphic deformation dynamic recovery seems to have become less competitive no dislocation free sub grains and along thrust faults twinning indicates a shift to higher differential stresses at the close of the deformation it was not possible to separate these deformation phases on the basis of the dislocation debris sub grain sizes as observed in tem and tom were identical in limestones that underwent cataclastic deformation the rocks seem to have started breaking up along the grain boundaries the new grain fragments are very small 0 1 0 3 m and are heavily twinned in tem the old large grains show very long straight glide dislocations cleavage and when shattered ring patterns in diffraction

the quasi 2 day wave and spatial temporal variability of the oh emission and ionosphere 1 the quasi 2 day wave qtdw during january 2006 is investigated using sounding of the atmosphere using broadband emission radiometry saber observations of oh airglow and temperature in the mesosphere and lower thermosphere mlt and champ in situ electron density observations at an altitude of 360 km the zonal structure of the qtdw during early 2006 in the southern hemisphere mlt and low latitude ionosphere is dominated by westward propagating zonal wave number 3 w3 and eastward propagating zonal wave number 2 e2 components however the observed longitude variability is not fully described by the e2 and w3 components of the qtdw the remaining longitude variability is attributed to the presence of nonmigrating tides and secondary waves generated by the nonlinear interaction of the qtdw with migrating tides the nonmigrating tides are a combination of those that are generated in situ by qtdw tide nonlinear interactions and those of tropospheric origin that propagate upward into the mlt the results presented demonstrate that nonmigrating tides and secondary waves are of considerable importance for determining the total longitude variability during periods of enhanced qtdw activity the general similarity between the qtdw in oh airglow and temperature indicates that variations in atomic oxygen concentrations due to vertical winds during the qtdw are the primary driver of the qtdw in the oh airglow emission however there are some differences in the oh airglow and temperature that we consider to be connected to changes in eddy diffusion due to changes in gravity waves during the qtdw last although similar zonal structure of the qtdw is observed in the ionosphere the observed longitude variations are not well represented by the qtdw components indicating that nonmigrating tides play a critical role in determining longitudinal variability in the ionosphere during the qtdw the importance of nonmigrating tides in the ionosphere may be related to the relative efficiency of the qtdw and different nonmigrating tides in the generation of electric fields

temporal modulations of the longitudinal structure in f2 peak height in the equatorial ionosphere as observed by cosmic 1 this study investigates the temporal variability of longitudinal structure in f2 layer peak height hmf2 in the equatorial ionosphere for this study electron density profiles retrieved from the radio occultation measurements by the constellation observing system for meteorology ionosphere and climate cosmic are fitted with a two layer chapman function to determine hmf2 the hmf2 values in the magnetic equatorial region display a four peaked longitudinal structure that has been identified in other ionospheric observations the four peaked structure is likely the signature of the forcing by the eastward propagating nonmigrating diurnal tide with zonal wave number 3 de3 further analysis for the period of prominent four peaked structure over august september 2008 finds that this structure exhibits a 5 day periodicity coincidentally the all sky interferometric meteor radar s wind measurements at thumba 8 5 n 77 e in india for the mesosphere and lower thermosphere indicate strong 5 day planetary waves within that time period the observed temporal periodicity of the four peaked longitudinal structure in hmf2 could thus be attributed to the interaction of de3 with a 5 day planetary wave

temporal modulation of the four peaked longitudinal structure of the equatorial ionosphere by the 2 day planetary wave 1 observations of electron densities by the constellation observing system for meteorology ionosphere and climate in august to october 2008 have shown a prominent four peaked longitudinal structure in the height of the f2 layer hmf2 in the equatorial ionosphere the development of this ionospheric structure in daytime is found to be consistent with the forcing by the eastward propagating nonmigrating diurnal tide with zonal wave number 3 de3 it is believed that tidal winds can modify the e region electric fields and subsequently produce variations in the ionosphere through the dynamo effect this study reveals that the amplitude of the hmf2 four peaked longitudinal structure is subject to a 2 day periodic modulation on certain intervals in the two month time period simultaneously wind measurements from the skiymet meteor radar at thumba 8 5 n 77 e india indicate corresponding 2 day planetary wave activity in the mesosphere and lower thermosphere mlt the 2 day planetary wave has both zonal and meridional wind components and it is the variability in the zonal component that most closely corresponds to f2 layer changes the zonal wind observations by the radar also show that the amplitude of the diurnal tide is modulated by the 2 day wave this study suggests that the identified 2 day variation of the hmf2 four peaked longitudinal structure in the equatorial ionosphere is caused by the interaction between the de3 tide and the 2 day planetary wave

a case study of the mesospheric 6 5 day wave observed by radar systems 1 in this paper analysis of wind data detected by six ground based radar systems located in equatorial and midlatitude belts shows that a strong mesospheric 6 5 day wave event occurred during april may 2003 we compared the global distribution of the observed 6 5 day wave event with the theoretical wave structure rossby normal mode s n 1 2 additionally we investigated several important wave characteristics to understand the mesospheric 6 5 day wave event i e wave period vertical structure relationship with background wind propagating direction and the zonal wave number our results are summarized into three points 1 the latitudinal structure of the mesospheric 6 5 day wave during april may 2003 is basically in agreement with the theoretical rossby mode s n 1 2 although the wave amplitude of zonal wind peaked at the subequatorial latitude of northern hemisphere but not at the theoretical place equatorial region 2 the main wave periods and the altitude distribution of large amplitude of this wave event varied with latitude 3 the downward propagating wave phases indicated that this wave event originated in the lower atmosphere and propagated upward to the upper region

exhumation of ultrahigh pressure metamorphic oceanic crust from lago di cignana piemontese zone western alps the structural record in metabasites the metamorphic and deformational history of coesite bearing ultrahigh pressure metamorphic oceanic crust from lago di cignana valtoumanche western alps as recorded along the decompression and cooling path has been resolved to derive constraints on the physical state of the crust during exhumation the following history is proposed eclogites derived from pillow basalts were deformed by dislocation creep of pyroxene at a depth of more than 80 km in the stability field of coesite the deformation was progressively localised into shear bands over the first 40 km of subsequent exhumation the rocks remained essentially unmodified any deformation must have been localised beyond the limits of the present outcrop retrogression first in a closed system started without concomitant deformation at a depth of about 40 km and temperatures of 550 to 500 c subsequently the system size increased fluid infiltration focussed along tensile fractures is documented by the formation of veins vein shapes and orientations indicate high pore fluid pressures and low differential stress after cooling to about 350 450 c at depths of 12 to 18 km localised ductile deformation transformed the retrogressed eclogites into greenschists for that stage the microstructure of the quartz veins indicates deformation by dislocation creep at falling temperatures and increasing differential stress final cooling to below ca 300 c took place at depths of 7 to 12 km as indicated by the density of fluid inclusions later deformation in the brittle field cannot be correlated with the p t path

tidal variability in the ionospheric dynamo region 1 the seasonal and interannual variability of migrating sun synchronous and nonmigrating solar atmospheric tides at altitudes between 100 and 116 km are investigated using temperature measurements made with the saber instrument on the timed spacecraft during 2002 2006 quasi biennial variations of order 10 15 in migrating diurnal and semidiurnal tidal amplitudes are found presumably due to modulation by the quasi biennial oscillation qbo as the tides propagate from their troposphere and stratospheric sources to the lower thermosphere a number of nonmigrating tidal components are found that have the potential to produce significant longitudinal variability of the total tidal fields the most prominent of these i e those that appear at amplitudes of order 5 10 k in a 5 year mean climatology include the zonally symmetric s 0 diurnal tide d0 the eastward propagating diurnal and semidiurnal tides with zonal wave numbers s 2 de2 and se2 and s 3 de3 and se3 and the following westward propagating waves diurnal s 2 dw2 semidiurnal s 1 sw1 s 3 sw3 and s 4 sw4 and terdiurnal s 5 tw5 these waves can be plausibly accounted for by nonlinear interaction between migrating tidal components and stationary planetary waves with s 1 or s 2 or by longitudinal variations of tropospheric thermal forcing additional waves that occur during some years or undergo phase cancellation within construction of a 5 year climatology include dw5 se1 se4 sw6 te1 tw1 and tw7 it is anticipated that the winds that accompany all of these waves in the 100 170 km region will impose longitudinal variability in the electric fields produced through the ionospheric dynamo mechanism thereby modulating vertical motion of the equatorial ionosphere and the concomitant plasma densities in addition to the wave 4 modulation of the equatorial ionosphere that has recently been discovered and replicated in modeling studies the waves revealed here will generate wave 1 sw1 sw3 d0 dw2 wave 2 sw4 tw1 wave 3 de2 se1 wave 4 de3 se2 dw5 sw6 te1 tw7 wave 5 se3 and wave 6 se4 components of this ionospheric variability depending on year and time of year however the absolute and relative efficiencies with which these waves produce electric fields remains to be determined

dating microstructures by the ar 40 ar 39 step heating technique deformation pressure temperature time history of the penninic units of the western alps abstract white micas from the penninic units of the urtier valley in the western alps have been dated by the 40 ar 39 ar step heating method a strong correlation is observed between measured ages mica composition and deformation fabrics samples characterized by a single mineral reservoir yielded relatively flat age spectra allowing to estimate an age of 44 4 0 4 ma for the formation of phengite bearing fabrics in eclogite facies conditions s hp and an age of 42 0 0 4 ma for the microstructurally younger muscovite bearing shear fabrics which formed in greenschist facies conditions sz gr1 several micaschist samples are characterized by the overprint of the phengite bearing s hp fabric by a younger muscovite bearing fabric f u that is axial planar to tight upright folds that post date the sz gr1 shearing event bse images reveal that the different generations of syn kinematic micas are not exclusively tied to their respective fabric but muscovite is commonly found also in the s hp microlithons while relict phengite is often transposed into the new f u axial planar fabric all the samples containing these two generations of white micas yielded convex upward spectra which following considerations based on the 36 ar 40 ar vs 40 ar 39 ar plots have been interpreted to indicate mixing of two argon reservoirs of different ages related to the older phengite and the younger muscovite visible with optical and electron microscopy this method allowed to estimate a maximum age of ca 36 5 0 5 ma for the folding of the shear related structures the ages estimated for s hp and sz gr1 in the study area are consistent with those obtained in other parts of the penninic units with other geochronological techniques therefore the post crystallization loss of argon from the phengite associated with s hp and the muscovite defining sz gr1 is negligible despite evidence indicating that conditions of t 480 510 c were reached during the later upright folding event

eclogite from the ultrahigh pressure metamorphic unit at lago di cignana western alps a process oriented transmission electron microscope study abstract in the western alps the ocean derived lago di cignana unit of the piemonte zone has experienced ultrahigh pressure metamorphism we have studied the minerals of two eclogite samples from this unit using transmission electron microscopy tem in order to characterise their microstructures these microstructures are the result of deformation phase transformation and reactions and allow conclusions on the processes that had generated them the goal of this process oriented paper is to contribute to the knowledge of the formation and exhumation of ultrahigh pressure eclogites in our tem study we found omphacite amphibole barroisite glaucophane hornblende clinozoisite garnet albite and thin layers of chlorite and mica in amphibole the omphacite is well ordered and has the space group p2 n we observed antiphase domains apds dislocations that are organised in small angle grain boundaries and very rare crystallographic shear planes parallel to 010 and 110 deformation twin lamellae on 100 have not been observed most interesting is the first observation of faults parallel to 110 in a natural omphacite they are due to deformation chain multiplicity faults are common in the amphibole grains in one case the orientation of crystallographic shear planes was not only parallel to 010 but also parallel to 130 and 110 clinozoisite showed deformation twin lamellae on 100 with widths of a few nm up to about 50 nm dislocations organised into a small angle grain boundary have been found which have not been described before the garnet is almost free of dislocations while there are differences regarding the deformation microstructures from the uhp lago di cignana eclogite and other hp occurrences it is unlikely that these are due to higher pressures it is much more likely that the whole formation and exhumation history of the different geological units and the individual positions of the collected rocks within them are reflected in their microstructures a tentative correlation of the omphacite microstructures with the p t path of the lago di cignana unit is presented an important point is the early crystallisation of c2 c omphacite at a temperature below 500 c perhaps around 400 c the low temperature prevents rapid diffusion controlled ordering to p2 nomphacite and thus enables deformation of the metastable c2 c omphacite since c2 c omphacite is more easily to deform than p2 n omphacite this may influence the interplate coupling the onset of the transition to p2 n omphacite however will increase the shear strength of eclogite and the coupling with the overriding plate which may slow down the subduction and finally stop it

flow stresses from microstructures in mylonitic quartzites of the moine thrust zone assynt area scotland recrystallized grain sizes subgrain sizes and dislocation densities of quartz grains in quartzose mylonitic rocks have been examined using optical and transmission electron microscopy tem the samples come from the moine thrust zone in the assynt district scotland they had been studied previously and described in detail with respect to their structural position in relation to the various thrusts in the region and to their preferred orientation stresses were derived from these samples using empirical and theoretical equations relating flow stress to the scale of the microstructures the stresses determined 43 244 mpa from recrystallized grain size 9 13 mpa from etched subgrain size 50 95 mpa from tem scale subgrain size and 75 147 mpa from dislocation density are not constant in individual samples simultaneous formation of the quartz fabrics and of the dynamically recrystallized grains during the period of mylonitization may have occurred under flow stresses ranging from 43 to 244 mpa a later dynamic recovery event reset subgrain sizes and dislocation densities to a constant value for each microstructure throughout the area there is insufficient empirical information available on the flow stress subgrain size relationship and on the effects of annealing recovery and annealing recrystallization to allow for a more detailed interpretation recrystallized grain size is still the most easily measured microstructural feature and the relationship of recrystallized grain size to flow stress has a sounder experimental basis than subgrain size or dislocation density the effect of chemical environment on the behaviour of all the microstructures is still unknown

comparative study of short term diurnal tidal variability examination of the simultaneous temperature measurement from the sounding of the atmosphere using broadband emission radiometry saber instrument on board the thermosphere ionosphere mesosphere energetics and dynamics timed satellite not only confirms the existence of the inversion layer but also reveals the global nature of the inversion suggesting the presence of a transient planetary wave in the mesosphere the large tidal variability therefore is probably a consequence of the interaction between the transient planetary wave and tides this possibility is investigated by using the ncar thermosphere ionosphere mesosphere electrodynamics general circulation model time gcm and by comparing model results with the lidar saber and timed doppler interferometer tidi measurements with a large transient planetary wave specified at the model lower boundary the model is able to produce strong diurnal tidal variability comparable to that from the lidar observation and the modeled temperature inversion is similar to that from the saber measurement the model results suggest that the planetary tidal wave interaction excites nonmigrating tides and modulates the gravity modes and or the rotational modes of the diurnal migrating tide among the nonmigrating tides the diurnal zonally symmetric s 0 component is the strongest and its interaction with the planetary wave leads to a strong diurnal eastward wave number 1 component

progress in microboudin method for palaeo stress analysis of metamorphic tectonites application of mathematically refined expression abstract the microboudin method for palaeo stress analysis is mathematically refined by incorporating the recently proposed shear lag model which describes the relationship between stress distribution along a fibre embedded within an elastic matrix and the far field differential stress applied to the matrix the refined probability density function of fractured fibres g with respect to the aspect ratio of the fibre r and the stress parameter is given by g r 1 exp m 1 m r m e f e q m 1 1 e q e f 1 cosh ar m where eq and ef are the elastic constants of the matrix and fibre m is the weibull modulus of the fibre material and a is a constant the stress parameter is defined as 0 s where 0 is the far field differential stress applied to the matrix and s is the modal fracture strength of the fibre material at r 1 this method is applied to tourmaline and piemontite boudins in quartzose rocks from six areas and it is revealed that the probability density function adequately models the proportion of boudinaged tourmaline and piemontite grains the value of is determined for each sample allowing 0 to be determined by this method when s is known

connections between deep tropical clouds and the earth s ionosphere 1 we report on a series of simulations with the national center for atmospheric research ncar thermosphere ionosphere mesosphere electrodynamics general circulation model time gcm which were designed to replicate and facilitate the interpretation of the longitudinal structure discovered in image satellite airglow observations of the equatorial ionization anomaly eia at the far ultraviolet fuv 135 6 nm wavelength during march april 2002 equinox our time gcm results indicate that the four peaked longitudinal variation in the eia observed by image fuv near 20 00 local solar time can be explained by the effects of an eastward propagating zonal wavenumber 3 diurnal tide de3 that is excited by latent heat release associated with raindrop formation in the tropical troposphere

observations of the 6 5 day wave in the mesosphere and lower thermosphere previous observations of atmospheric oscillations with zonal wave number 1 have consistently located waves of periods between 5 and 7 days this study presents a global and seasonal analysis of the 6 5 day wave in high resolution doppler imager daytime mesosphere and lower thermosphere horizontal winds temperatures and nighttime atomic oxygen at 95 km the horizontal structures of all these atmospheric variables are similar to the gravest symmetric wave number 1 rossby wave i e the 1 1 mode the seasonal and spatial analysis displays possible modification by the zonal mean wind finally the observed vertical structures of the 6 5 day wave indicate that it is an internal rossby wave not an external or lamb wave

development of microboudinage an estimate of changing differential stress with increasing strain microboudin structure has so far been used in structural analysis as a reliable measure of plastic strain and recently it has been proposed to use it as a measure of relative differential stress this paper demonstrates that stress and strain can be simultaneously deduced if the stress and strain analyses are carried out in combination the analysed samples are quartz schists from four localities of different high p low t type metamorphic belts in japan the samples include microboudinaged competent piedmontite grains within a ductile quartz matrix the analyses strongly suggest that the estimated differential stresses tend to increase with increasing strain and that at the final stage of the microboudinage they reach a maximum

september 2019 antarctic sudden stratospheric warming quasi 6 day wave burst and ionospheric effects an exceptionally strong stationary planetary wave with zonal wavenumber 1 led to a sudden stratospheric warming ssw in the southern hemisphere in september 2019 ionospheric data from esa s swarm

relationship between joint spacing and bed thickness in sedimentary rocks effects of interbed slip this paper consists of three parts the first part presents a critical review of previous mechanical models on the relationship between joint spacing s and bed thickness t in sedimentary rocks the second part describes a new mechanical model dealing with the effects of interbed slip on joint spacing the third part presents the comparison between theoretical results of the model and joint data measured from cambrian flysch sediments at plage victor in the saint jean port joli area of the quebec appalachians this study demonstrates two formation mechanisms of tensile joints near end fracturing takes place in the layers with smaller tensile fracture strength c 0 30 mpa and smaller interbed shear strength 0 20 mpa while mid point fracturing occurs in those layers with larger c 0 50 mpa and 0 30 mpa values for the rocks with moderate tensile fracture strength 30 c 0 50 mpa and interbed shear strength 20 0 30 mpa the near end fracturing and mid point fracturing take place preferentially in the thicker and thinner brittle layers respectively in the regime of near end fracturing the correlation between s and t is linear and independent of the thickness of the bounding non jointing layers d in the regime of mid point fracturing however the s t relationship can be either linear or non linear depending on the variation of d value the present study also suggests that the near end fracturing is probably the prevailing process for the formation of tensile joints in bedded sedimentary rocks and that the coefficient of joint spacing k defined by the ratio of s to t is considered as an indicator of c 0 2 0 for the jointed layer

factors affecting the kinematic interpretation of asymmetric boudinage in shear zones abstract based on work along a major mylonite zone in the northern appalachians and scale model studies a new mechanism for the origin of asymmetric boudins in shear zones is proposed along the honey hill fault in southern connecticut granitic sills intruded into calc silicate gneisses and schists were oblique to the boundaries of the mylonite zone and experienced the following sequence in the production of sigmoidal boudins 1 as the calc silicate schists experienced mylonitization and flow the more competent coarse grained granitic sills deformed by extensional fracturing and quartz veining 2 continued extension of granitic sills was accommodated by normal shear on early formed quartz veins 3 continuing extension of sills and ductile modification of the corners of boudins resulted in granitic fish with tails which stream from the top of the boudin in the down dip direction and from the bottom in the up dip direction based on a variety of kinematic indicators the sense of asymmetry of the tails is identical to that expected for recrystallization tails on sheared augen structure models composed of silicone putty and plasticine were created to investigate the effect of pre shearing geometry on boudin evolution and were deformed in a simple shear device the models reproduce the kinematics deduced from field relations and suggest that one of the primary factors in controlling the amount of extension is the angle which early veins make with the shear zone boundary varying the angular relationships in the models suggests that other pre shearing geometric factors may affect the geometry of boudins formed in this way boudins formed through this mechanism appear very similar to hanmer s type ii asymmetric boudins because the pre shearing geometry can exert a control on boudin asymmetry caution should be used when attempting to deduce shear sense or shear strain values from asymmetric boudinage

short term variability in the migrating diurnal tide caused by interactions with the quasi 2 day wave 1 the migrating diurnal tide is one of the dominant dynamical features in the low latitudes of the earth s mesosphere and lower thermosphere mlt region representing the atmospheric response to the largest component of solar forcing ground based observations of the tide have resolved short term variations attributed to nonlinear interactions between the tide and planetary waves that are also in the region using the ncar thermosphere ionosphere mesosphere electrodynamics general circulation model time gcm we simulate a quasi 2 day wave qtdw event under late january conditions in this case sideband sum and difference child waves are resolved indicating that a nonlinear interaction is occurring between the qtdw and the tide the migrating diurnal tide in the mlt displays local amplitude decreases of 20 40 as well as a shortening of vertical wavelength by roughly 4 km examining the physical mechanisms driving the interaction nonlinear advection is found to result in amplification of the tide in some regions and damping in others manifesting as increased smoothing of the tidal structure when the qtdw is present in the mlt additionally the qtdw also enhances the easterly summer mean wind jet that can also account for changes in tidal amplitude and vertical wavelength we find that qtdw induced background atmosphere changes in time gcm can drive tidal variability at levels greater than nonlinear advection a possibility not previously considered

s c mylonites two types of foliations are commonly developed in mylonites and mylonitic rocks a s surfaces related to the accumulation of finite strain and b c surfaces related to displacement discontinuities or zones of relatively high shear strain there are two types of s c mylonites type i s c mylonites described by berthe et al typically occur in deformed granitoids they involve narrow zones of intense shear strain which cut across mylonitic foliation type ii s c mylonites described here have widespread occurrence in quartz mica rocks involved in zones of intense non coaxial laminar flow the c surfaces are defined by trails of mica fish formed as the result of microscopic displacement discontinuities or zones of very high shear strain the s surfaces are defined by oblique foliations in the adjacent quartz aggregates formed as the result of dynamic recrystallization which periodically resets the finite strain clock these oblique foliations are characterized by grain elongations alignments of segments of the grain boundary enveloping surfaces and by trails of grains with similar c axis orientations examples of this aspect of foliation development in mylonitic rocks are so widespread that we suggest the creation of a broad class of s c tectonites and a deviation from the general tradition of purely geometric analysis of foliation and time relationships kinematic indicators such as those discussed here allow the recognition of kilometre scale zones of intense non coaxial laminar flow in crustal rocks and unambiguous determination of the sense of shear

on estimating the strength of calcite rocks under natural conditions abstract field studies of calcite mylonites often document microstructures produced by dislocation creep in contrast flow laws derived from experiments predict that calcite rocks should deform mostly by diffusion creep during tectonic processes to investigate this apparent discrepancy we compare stresses estimated by microstructural piezometers to those obtained by extrapolation of experimentally derived flow laws considering shear zones from different geological settings a clear trend is observed of increasing recrystallized grain size with increasing temperature however there is a large spread in grain size and associated stress because separate flow laws have been defined for various different marbles and limestones the strengths predicted for a given set of conditions differ significantly the stress estimates based on the piezometers and strength extrapolated from the various experimentally derived dislocation creep flow laws agree qualitatively but no single flow law predicts all the palaeostress estimates even if experimental data are disregarded the field observations are not consistent with a hypothetical law for coble creep they are consistent with a power law for dislocation creep but only if the material constants are different from those currently determined in laboratory experiments

the 6 5 day wave and its seasonal variability in the middle and upper atmosphere 1 the zonal wave number 1 planetary wave of period near 6 5 days is a robust feature in the mesosphere and lower thermosphere mlt region with prominent seasonal variability as revealed by ground based and satellite observations this wave and its seasonal variability are well reproduced in a recent one model year run of the national center for atmospheric research thermosphere ionosphere mesosphere electrodynamics general circulation model time gcm with its lower boundary specified according to the national centers for environmental prediction analysis year 1993 wavelet analysis of the model output shows that in the mlt region the wave maximizes before and after the equinoxes and minimizes at solstices the wave amplitudes at the equinoxes are smaller than the peaks before and after but are still larger than the wave amplitudes at solstices however at the lower boundary near 30 km the wave peaks are predominantly between fall and the following spring by examining the episodes of maximum and minimum wave amplitude and by conducting additional control experiments using the time gcm the structure of this planetary wave and the factors determining the wave characteristics and seasonal variability are studied in detail it is found that the wave source mean wind structure instability and the critical layers of the wave can all affect the wave response in the mlt region and can have a strong seasonal dependence before and after equinox the wave follows the waveguide and propagates from the stratosphere to the summer mesosphere mesopause where it may amplify due to baroclinic barotropic instability such instability is usually absent from the equinoctial atmosphere so that there is no wave amplification at equinox at solstice the wave decays significantly when propagating away from its winter source due to the strong eastward winter stratospheric jet in the summer side the westward jet is also strong and the meridional and vertical extension of the critical layer of the wave is large enough to enclose the instability in the summer mesosphere mesopause at middle to high latitudes the wave is thus reflected away and prevented from reaching and amplifying at the unstable region the seasonal variation of the quasi two day wave which has zonal phase speed similar to the 6 5 day wave is also studied using similar diagnostics it is further shown that within certain seasonal window periods the variability in the mlt especially the summer mlt may closely track the lower atmospheric variability associated with these waves

palaeostress estimation using calcite twinning experimental calibration and application to nature abstract by means of high temperature deformation experiments on calcite rocks of different grain sizes we have established the relationships between differential stress and twinning incidence twin density and volume fraction and the way that these relationships depend on grain size both twinning incidence and volume fraction increase with grain size at a constant level of differential stress log twin density is proportional to stress but is independent of grain size at a given grain size volume fraction of twinning increases non linearly with differential stress twinning activity measured by twinning incidence increases linearly with stress the behaviour of each of these parameters is independent of temperature strain and strain rate these relationships can be used as a basis for the estimation of palaeostress values in naturally deformed calcite rocks this is illustrated by a study of twinning activity in limestones close to thrust faults in the cantabrian zone of the variscan orogenic belt in northwestern spain high peak differential stresses were measured ranging from 150 to 250 mpa using the turner dynamic analysis technique however it is clear that shear stresses resolved parallel to the fault planes were very low the attainment of high stress differences when the resistance to faulting was low is attributed either to bending forces induced during the motion of the non planar surfaces relative to each other or to the stress peak being attained prior to the localization of deformation onto the fault surfaces

paleoearthquakes and eolian dominated fault sedimentation along the hubbell spring fault zone near albuquerque new mexico the hubbell spring fault zone forms the modern eastern margin of the rio grande rift in the albuquerque basin of north central new mexico knowledge of its seismic potential is important because the fault zone transects kirtland air force base sandia national laboratories and underlies the southern albuquerque metropolitan area no earthquakes larger than m l 5 5 have been reported in the last 150 years in this region so we excavated the first trench across this fault zone to determine its late quaternary paleoseismic history our trench excavations revealed a complex 16 m wide fault zone overlain by four tapered blankets of mixed eolian sand and minor colluvium that we infer were deposited after four large magnitude surface rupturing earthquakes although the first oldest rupture event is undated we used luminescence thermoluminescence and infrared stimulated luminescence ages to determine that the subsequent three rupture events occurred about 56 6 29 3 and 12 1 ka these ages yield recurrence intervals of 27 and 17 k y between events and an elapsed time of 12 k y since the latest surface rupturing paleoearthquake slip rates are not well constrained but our preferred average slip rate since rupture event 2 post 56 ka is 0 05 mm yr and interval slip rates between the last three events are 0 06 and 0 09 mm yr respectively vertical displacements of 1 2 m per event and probable rupture lengths of 34 43 km indicate probable paleoearthquake magnitudes m s or m w of 6 8 7 1 future earthquakes of this size likely would cause strong ground motions in the albuquerque metropolitan area

monthly tidal temperatures 20 120 km from timed saber 1 the saber instrument on the timed satellite provides unprecedented geographical coverage for the determination and study of atmospheric tides however the slow local time precession rate of timed can cause longer term temperature variations to alias into the tidal signals a new method of analyzing satellite data for tides has been developed to circumvent this difficulty but at the expense of temporal resolution of the tidal fields i e 120 day mean tidal structures are obtained in this work we apply this method to saber temperature data to derive a series of 120 day mean tidal structures extending between 20 and 120 km altitude 50 s 50 n latitude and centered on each month from september 2003 to september 2004 in addition to the migrating sun synchronous diurnal and semidiurnal tides a number of nonmigrating tides are revealed in the saber measurements some of these waves are thought to originate via nonlinear coupling between the migrating tides and the stationary planetary wave with zonal wave number s 1 other nonmigrating tidal components appear to be forced by latent heating due to deep tropical convection of the latter the eastward propagating diurnal tide with s 3 is dominant and is as large as the migrating diurnal tide during some months of particular interest is the wave 4 structure with respect to longitude that characterizes both the diurnal and semidiurnal total tidal fields this feature is a result of the predominant wave 4 topography land sea longitude dependence at the surface which is reflected in the diurnal and semidiurnal components of the latent heating rates due to deep tropical convection the ability of the global scale wave model gswm to approximate the observed tidal fields including the wave 4 total tidal structures is also assessed

climatology of upward propagating diurnal and semidiurnal tides in the thermosphere 1 a major challenge in delineating and understanding the space weather of the ionosphere thermosphere system is the lack of global tidal observations in the 120 400 km thermospheric gap between satellite remote sensing and in situ tidal diagnostics this paper aims to close this gap by presenting an observation based climatological tidal model of the thermosphere ctmt of self consistent upward propagating migrating and nonmigrating diurnal and semidiurnal tides from 80 400 km and pole to pole for moderate f10 7 110 sfu solar flux conditions ctmt includes the 6 8 most important diurnal semidiurnal tidal components for temperature density and zonal meridional and vertical winds and is based on hough mode extension fits to 2002 2008 mean timed satellite tidal diagnostics in the mesosphere lower thermosphere mlt validation with independent 2002 2008 champ tidal diagnostics f10 7 105 sfu around 400 km proves that the approach captures nonmigrating tides well indicating that these waves propagate directly upward without significant tidal forcing occurring in the thermosphere notable exceptions are the dw2 and d0 components that are most likely generated in situ by nonlinear interaction forcing in the upper thermosphere ctmt is suitable for driving upper atmosphere models that require self consistent tidal fields in the mlt region as a lower boundary condition it does not include migrating tides due to in situ euv absorption in the upper thermosphere but allows us to quantitatively assess thermospheric variability due to tides from the lower atmosphere this is done by discussing longitude latitude maps of reconstructed diurnal and semidiurnal tidal variations as function of altitude and local time

on the evolution of orogens pressure cycles and deformation mode switches abstract evidence of two burial exhumation cycles that took place during a single orogeny has been found in rocks belonging to the piemonte unit of the western alps an early high pressure event which resulted from tectonic burial down to pressures of 1 5 gpa was followed by exhumation to ca 0 20 0 35 gpa as a result of extensional deformation renewed shortening culminated in a second burial episode down to pressures of 0 65 0 80 gpa before the final exhumation took place existing geochronological data allow only ca 13 19 ma for the completion of both burial exhumation cycles therefore we suggest that the evolution of orogens is characterized by multiple short lived burial exhumation cycles related to orogen scale alternance between shortening and extensional deformation

two day wave coupling of the low latitude atmosphere ionosphere system 1 vertical coupling in the low latitude atmosphere ionosphere system driven by the 2 day wave in the tropical mlt region has been investigated the problem is studied from an observational point of view three different types of data were analyzed in order to detect and extract the 2 day wave signals the 2 day wave event during the period from 1 december 2002 to 28 february 2003 was identified in the neutral winds by radar measurements located at four tropical stations the 2 day variations in the ionospheric electric currents registered by perturbations in the geomagnetic field and in the f region electron densities were detected in the data from 23 magnetometer and seven ionosonde stations situated at low latitudes two features for each kind of wave were investigated in detail the variation with time of the wave amplitude and the zonal wave number the results show that the westward propagating global 2 day wave with zonal wave number 2 seen in the ionospheric currents and in f region plasma is forced by the simultaneous 2 day wave activity in the mlt region the main forcing agent in this atmosphere ionosphere coupling seems to be the modulated tides particularly the semidiurnal tide this tide has a larger vertical wavelength than the diurnal tide and propagates well into the thermosphere the parameter that appears to be affected and thus drives the observed 2 day wave response of the ionosphere is the dynamo electric field

observations of the 5 day wave in the mesosphere and lower thermosphere the 5 day planetary wave has been detected in the winds measured by the high resolution doppler imager hrdi on the upper atmosphere research satellite uars in the mesosphere and lower thermosphere 50 110 kin the appearances of the 5 day wave are transient with a lifetime of 10 20 days in the two year data set the structures of selected 5 day wave events are in generally good agreement with the 1 1 rossby normal mode for both zonal and meridional components a climatology of the 5 day wave is presented for an altitude of 95 km and latitudes mainly between 40os and 40on

day to day migrating and nonmigrating tidal variability due to the six day planetary wave 1 to investigate day to day variability in the mesosphere and lower thermosphere mlt an idealized simulation of a six day westward propagating zonal wave number 1 planetary wave is performed using the national center for atmospheric research ncar thermosphere ionosphere mesosphere electrodynamics general circulation model time gcm the six day planetary wave introduces a six day periodicity in the zonal mean atmosphere migrating and nonmigrating tides as well as in secondary waves that are produced by nonlinear planetary wave tide interactions we have further used the linear global scale wave model gswm to isolate the effect of how the day to day changes in zonal mean zonal winds may influence tides in the mlt the most significant changes are observed in the migrating diurnal tide dw1 eastward propagating nonmigrating tides with zonal wave numbers 2 and 3 de2 and de3 and a 20 hr eastward propagating wave with zonal wave number 2 20e2 because we have included the lower atmospheric source of nonmigrating tides de2 and de3 are present with relatively large amplitudes in the mlt even in the absence of planetary wave forcing the 20e2 wave is produced by the nonlinear interaction between the de3 and the six day planetary wave and its large amplitude indicates the importance of including the realistic spectra of nonmigrating tides in numerical simulations of planetary waves the gswm simulations reveal that the dw1 is not significantly influenced by the changes in the zonal mean winds we thus conclude that the dw1 changes are driven by a combination of changes due to nonlinear interaction with the six day planetary wave as well as changes due to zonal asymmetries that result from the six day planetary wave the six day planetary wave induced changes in zonal mean zonal winds lead to a general reduction in the amplitude of de2 and de3 and introduce a slight periodic behavior in these tides the effect of changing zonal mean zonal winds appears to be the primary driver of the changes in the de2 however for de3 although the changes that can be attributed to zonal mean zonal wind variability are not insignificant the primary driver of the de3 perturbations appears to be the nonlinear interaction with the six day planetary wave last we demonstrate that the day to day changes in the de3 introduce similar day to day changes in the daytime wave number 4 longitude structure in the low latitude ionosphere these results indicate that short term variability in the low latitude ionosphere is likely to be driven by similar short term variability in nonmigrating tides in the mlt

kinematic reworking and exhumation within the convergent alpine orogen abstract kinematic data from the internal zones of the western alps indicate both top to se and top to nw shearing during synkinematic greenschist facies recrystallisation rb sr data from white micas from different kinematic domains record a range of ages that does not represent closure through a single thermal event but reflects the variable timing of synkinematic mica recrystallisation at temperatures between 300 and 450 c the data indicate an initial phase of accretion and foreland directed thrusting at ca 60 ma followed by almost complete reworking of thrust related deformation by se directed shearing this deformation is localised within oceanic units of the combin zone and the base of the overlying austroalpine basement and forms a regional scale shear zone that can be traced for almost 50 km perpendicular to strike the timing of deformation in this shear zone spans 9 ma from 45 to 36 ma the se directed shear leads to local structures that cut upwards in the transport direction with respect to tectonic stratigraphy and such structures have been interpreted in the past as backthrusts in response to ongoing alpine convergence however on a regional scale the top to se deformation is related to crustal extension not shortening and is coincident with exhumation of eclogites in its footwall during this extension phase deformation within the shear zone migrated both spatially and temporally giving rise to domains of older shear zone fabrics intercalated with zones of localised reworking top nw kinematics preserved within the combin zone show a range of ages the oldest 48 ma may reflect the final stages of emplacement of austroalpine units above piemonte oceanic rocks prior to the onset of extension however much of the top to nw deformation took place over the period of extension and may reflect either continuing or episodic convergence or tectonic thinning of the shear zone 40ar 39ar data from the region are complicated due to the widespread occurrence of excess 40ar in eclogite facies micas and partial ar loss during alpine heating reliable ages from both eclogite and greenschist facies micas indicate cooling ages in different tectonic units of between 32 and 40 ma these ages are slightly younger than rb sr deformation ages and suggest that cooling below ca 350 c occurred after juxtaposition of the units by se directed extensional deformation our data indicate a complex kinematic history involving both crustal shortening and extension within the internal zones of the alpine orogen to constrain the palaeogeographic and geodynamic evolution of the alps requires that these data be integrated with data from the more external zones of the orogen complexity such as that described is unlikely to be restricted to the western alps and spatially and temporally variable kinematic data are probably the norm in convergent orogens recognising such features is fundamental to the correct tectonic interpretation of both modern and ancient orogens

rapid exhumation of the zermatt saas ophiolite deduced from high precision smnd and rbsr geochronology sm nd isotope data from garnets in ultrahigh pressure coesite bearing eclogite from the western alps were obtained to determine the age of peak metamorphism and exhumation rates of deeply buried oceanic crust in the zermatt saas ophiolite complex we report an exceptionally well constrained sm nd isochron age of 40 62 6 ma from the lago di cignana eclogite difficulties in dating alpine eclogites using this method were overcome by using an hf acid leaching procedure on garnet to remove mineral inclusions the zermatt saas rocks reached greenschist facies conditions around 38 2m a on the basis of rb sr whole rock phengite isochrons these data combined with existing estimates of the pressure and temperature conditions of the eclogite give an estimate for the initial exhumation rate of 10 to 26 km m y depending on the inferred pressure for the recrystallization of phengite followed by slow exhumation of 0 3 km m y from 34 to 14 ma the initial exhumation rates at lago di cignana are among the highest yet determined for high pressure terranes and are well constrained because the age has been determined directly on the high pressure assemblage the rapid exhumation rates presented here place critical constraints on collisional tectonic models 1999 elsevier science b v all rights reserved

a statistical approach to determination of a mineral lineation abstract a statistical model of the bipolar von mises type was applied to the analysis of mineral lineation defined by the preferred dimensional orientation of inequant grains on a foliation surface orientation data of 203 258 grains from each of four rock specimens fit the model well suggesting a useful analytical technique with three advantages 1 confidence in whether a metamorphic tectonite has a mineral lineation can be determined using a 2 test 2 the orientation of the mineral lineation can be represented by the mean direction with a specified confidence interval and 3 the degree of preferred dimensional orientation of grains can be quantified by the concentration parameter

a strain reversal method for estimating extension from fragmented rigid inclusions abstract in the standard method of strain analysis using fragmented rigid inclusions such as stretched belemnites extension is estimated by comparing the initial length of the inclusion with its final stretched length this approach fails to recognise that not all of the total extension is recorded by separation of the fragments even when cross fractures are present at the onset of deformation this paper presents an alternative approach in which the extensional strain is reversed the inter fragment gaps being incrementally closed until the inclusion is restored to its original state this allows an estimate of the unrecorded extension accommodated by inhomogeneous matrix flow near the inclusion as well as that recorded by fragment separation model experiments in which the fragmented inclusion is represented by wooden blocks set in a ductile matrix allow the strain reversal method to be compared with the standard method the mean percentage relative error is much smaller in the former 10 2 than in the latter 37 2 the main disadvantage of the strain reversal method is that it requires the collection of more data fragment lengths and gap lengths in correct sequence which must be processed by computer a short basic program is provided although there is no one to one correspondence between strain estimates derived from the two methods an approximate numerical transformation from one to the other may be possible one such transformation based on a study of natural boudinage has been applied to the experimental data and reduces the mean percentage relative error in the standard method estimates to 11 5

critical re evaluation of differential stress estimates from calcite twins in coarse grained limestone abstract comparison of two techniques to estimate differential stress magnitude based on calcite twinning reveals significantly different calculated values of differential stress for limestone experimentally and naturally deformed at low temperature this inequality may be due to fundamental differences between the rock type marble versus limestone and deformation temperatures above and below 200 c for samples used in technique calibration versus samples used in testing and application a technique that was empirically derived from marble experimentally deformed at 200 800 c yields values of differential stress that are factors of 2 to 9 times larger than maximum differential stress measured during experimental deformation of limestone at low temperature

intra annual variability of the low latitude ionosphere due to nonmigrating tides 1 in situ plasma density measurements from the champ satellite between about 350 420 km altitude are used to delineate intra annual variations in the longitudinal structure of the f region ionosphere it is shown that the longitude structures during mid day local times are dominated by the space based longitudinal wavenumbers ks 2 4 and 3 in january july and december respectively these conform to the same dominating ks values characterizing solar thermal tide zonal winds in the dynamo region namely the westward propagating semidiurnal tide with planetary fixed zonal wavenumber s 4 sw4 and the eastward propagating diurnal tides with s 3 de3 and s 2 de2 respectively de3 is the dominating tide during the other months the results presented indicate that nonmigrating tides other than de3 may significantly impact the longitudinal structure of the low latitude ionosphere during certain months

asymmetrical pull aparts and foliation fish as kinematic indicators abstract from field observations asymmetrical boudins pinch and swell structures and ellipsoidal volumes of oblique foliation segments in anisotropic rocks lacking competent layers are potentially useful kinematic indicators in shear zones they are derived by the modification of initially symmetrical pull aparts and or layering by superposed layer parallel shearing the response of boudins to such shear is a synthetic flow of those corner volumes lying within the extensional quadrants of the kinematic framework of the deformation into the inter boudin gaps type 1 asymmetrical pull aparts the response of pinch and swell structure is a back rotation of the swells to form type 2 asymmetrical pull aparts discrete extensional shears in the pinches may be either absent type 2a or present type 2b analogues of types 2a and 2b asymmetrical pull aparts occur in anisotropic materials with no compositionally distinct competent members where ellipsoidal volumes contain back rotated foliation segments foliation fish the field based kinematic interpretation was successfully tested by laboratory experiment wherein initially symmetrical trains of pre formed competent internally anisotropic boudins and pinch and swell structures were deformed in bulk simple shear a mechanical model is presented in terms of two material properties of the inclusions competence viscosity and internal rheological anisotropy in an attempt to account for the observed flow in the experiments and the deduced flow and kinematic significance of the natural examples in the presence of angular corners and associated geometrical stress concentrations competence viscosity controls the response to shear in the boudins in the absence of angular corners internal anisotropy controls the response to shear in the pinch and swell structures and in the foliation fish

contrast in stress strain history during exhumation between high and ultrahigh pressure metamorphic units in the western alps microboudinage analysis of piemontite in metacherts abstract our analyses of microboudinage structures of piemontite grains embedded within six samples of metachert one collected from an ultrahigh pressure uhp metamorphic unit at lago di cignana in italy of the western alps and the other five from surrounding high pressure hp metamorphic units in italy and france have revealed that the structures are all symmetrical in type and were presumably produced in coaxial strain fields stress strain analyses of the microboudinaged grains revealed significant contrasts in the stress and strain histories of the uhp and hp metamorphic units with the differential stress recorded by the uhp sample being unequivocally lower than that recorded by the five hp samples in addition our analyses showed that the uhp sample underwent stress relaxation during microboudinage whereas the five hp samples did not on the basis of these observations and analyses we discuss the mechanical decoupling of the uhp and hp units that led to different histories in differential stress between the units during exhumation of the western alps

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

triaxial stress state deep in orogenic belts an example from turkey abstract a rare metachert pebble containing amphibole grains with microboudin structures in a wide range of orientations provides an opportunity to perform stress analysis in two orthogonal orientations on the foliation surface the sample was analysed by the microboudin method to infer the triaxial stress state during microboudinage stress parameters proportional to the far field differential stress were determined for sodic amphibole grains in the two orientations the ratio of the stresses in the two orthogonal orientations 1 2 1 3 was calculated to be 0 64 indicating that 2 lies closer to the midpoint between 1 and 3 than to 3

microboudin structure of piedmontite in quartz schists a proposal for a new indicator of relative palaeodifferential stress abstract this paper proposes a new method to infer the relative differential stress for the rock body bulk differential stress from microboundinage the method is applied to four piedmontite bearing quartz schists collected from the high pressure low temperature metamorphic belts in japan nuporomaporo yamagami asemi and matsunosako in the samples brittle piedmontite grains are broken and subsequently pulled apart within the flowing quartz matrix the method consists of 1 statistical measurement of the fraction of microboudinaged grains 2 theoretical prediction of this fraction as a function of bulk differential stress and 3 inference of bulk differential stress by comparing the measured with the theoretical fraction the theoretical prediction has two aspects one is the model connecting bulk differential stress acting on a whole rock quartz schist with the tensile stress generated in the competent material piedmontite and the other is the model for the fracture strength of the competent material we use a fibre loading theory for the stress and a weakest link theory for the fracture strength the theoretical analysis is found to explain successfully the measured data by a chi squared test and we can obtain relative estimates of bulk differential stress the b value they are 1 0 0 7 0 4 and 0 2 for nuporomaporo yamagami matsunosako and asemi respectively

effect of atmospheric tides on the morphology of the quiet time postsunset equatorial ionospheric anomaly longitudinal wave number four pattern in the magnetic latitude and concentration of the f region peak ion density when measured at a fixed local time in a new comparison of two data sets with observations made by the ogo 4 satellite this pattern is seen to be persistent over many days around equinox during magnetically quiet conditions close to solar maximum but can be dominated by other processes such as cross equator winds during other periods it is found that the longitudinal variability is created by a processes occurring in the dayside ionosphere a longitudinal modulation of the dayside equatorial fountainisthemostlikelydrivingmechanism throughcomparisonwithgwsm 02model it isshownthatthepredictedmodulationofthedaysidethermosphericwindsandtemperaturesat e region altitudes created by non migrating diurnal tides can explain the modulation in the dayside equatorial fountain this result highlights the importance of understanding the temporal variability of tropospheric weather systems on our understanding and possible predictability of the development of the f region ionosphere it may also provide a possible further means of testing our understanding of atmospheric tides on a global scale

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

magnitude of σ1 σ2 and σ3 at mid crustal levels in an orogenic belt microboudin method applied to an impure metachert from turkey abstract we revised an equation for estimating palaeostress magnitude using the microboudin technique by incorporating the influence of time on the fracture strength of minerals the equation was used to estimate triaxial palaeostresses from a rare sample of metachert from turkey that contains microboudinaged columnar tourmaline grains in a wide range of orientations within the foliation plane the estimated principal palaeostresses are 1 605 mpa 2 598 mpa and 3 597 mpa as the microboudinage is considered to have occurred immediately before the rock encountered the brittle plastic transition during exhumation these stress values correspond to conditions at approximately 18 km depth and 300 c within a cretaceous orogenic belt

determining deviatoric stress tensors from calcite twins applications to monophased synthetic and natural polycrystals theoretically calcite twins analysis makes possible the full determination of the deviatoric stress tensor responsible for the formation of the twin lamellae that is the orientations of the principal stress axes and the three differential stress values i j may be calculated assuming conditions of constant resolved shear stress for twinning inverse method using nonlinear regression and robust estimation is proposed applications to monophased synthetic polycrystals that are generated numerically by solving the direct problem and to a natural sample from the quercy southwest of france have given good results comparison with the first method proposed by turner 1953 modified by nissen 1964 for principal stress orientations and by jamison and spang 1976 for differential stress values clearly emphasizes the considerable interest of this new method

the 6 5 day wave in the tropical stratosphere and mesosphere 1 the 6 5 day wave is a westward propagating zonal wave number 1 s 1 planetary wave that is prominent near equinox the spatial temporal structure of the 6 5 day wave seen in the united kingdom meteorological office ukmo data in the stratosphere is found to be consistent with that obtained from upper atmosphere research satellite high resolution doppler imager hrdi measurements of horizontal winds and temperatures in the mesosphere lower thermosphere mlt two wave events are commonly observed before and after equinox at low latitudes in both ukmo and hrdi data this paper focuses on a pair of such events in late 1994 the amplitude growth and phase decrease with altitude of the zonal wind 6 5 day wave appear to be continuous between the two data sets for the period analyzed both wave events appear to grow from the stratosphere to the upper atmosphere at a rate consistent with the suggested vertical phase velocity such consistency between the two data sets establishes that the wave previously observed in the mlt is propagating from the stratosphere further analyses of the ukmo data shows the presence of the 10 day s 1 and 4 day s 2 waves that appear to be either nonlinearly coupled with the 6 5 day wave or related through a common sourcing mechanism

tropospheric tidal effects on the middle and upper atmosphere 1 numerical experiments that explore the effects of tides of tropospheric origin on the upper and middle atmosphere reveal strong signatures of an eastward propagating zonal wave number 3 diurnal tide de3 which peaks near 110 km and penetrates into the upper thermosphere we demonstrate that nonmigrating tidal dissipation dominated by de3 effects in the upper mesosphere and lower thermosphere mlt strongly accelerates the zonal mean wind field affecting both the altitude and magnitude of the low latitude jets we also quantify for the first time a stationary planetary wave 4 oscillation spw4 in the mlt which is excited by nonlinear interaction between the de3 and the migrating diurnal tide dw1 our results suggest that the spw4 modulates the de3 mlt response and may impact the e region dynamo as well as ionospheric and thermospheric signatures at much higher altitudes

determination of principal stress magnitudes using calcite twins and rock mechanics data abstract this paper proposes a new method for determining principal stress magnitudes based on the use of the deviatoric stress tensor determined from computer inversion of calcite twin data additional evaluation of the vertical stress weight of overburden or of the mean stress is enough to calculate the complete stress tensor we emphasize that the best reasoning consists of combining calcite twin analysis with rock mechanics data failure friction criteria because the determination of the six parameters of the absolute stress tensor is overconstrained