evidence for multiple metamorphic events in the adirondack mountains n y field evidence consisting of 1 rotated foliated xenoliths 2 country rock foliation truncated by isoclinally folded igneous intrusions bearing granulite facies assemblages document one or more early dynamothermal event s of regional scale and high grade early metamorphism resulted in pronounced linear and planar fabric throughout the adirondacks and preceded the emplacement of the anorthosite mangerite charnockite granite alaskite amca suite which contains xenoliths of the metamorphosed rocks olivine metagabbros believed to be approximately contemporaneous with the amca suite also crosscut and contain xenoliths of strongly foliated metasediments these intrusive rocks caused contact metamorphism in the metasediments which locally exhibit both anatectite and restite assemblages subsequently this already complex framework underwent three phases of folding including an early recumbent isoclinical event and was metamorphosed to granulite facies p t conditions the age of the early metamor

deep crustal ductile extension following thrusting in the southwestern grenville province ontario two lithotectonic assemblages in southern britt domain have different histories of plutonism metamorphism structural development and mafic dyke emplacement these differences are accounted for by postulating that a cryptic grenvillian thrust separates the assemblages amphibolite facies extensional shear along the central britt shear zone cbsz overprinted the thrust obscuring kinematic evidence for its existence the structurally lower bayfield nadeau island assemblage contains orthogneiss suites of disparate age lesser amounts of supracrustal rocks deposited before intrusion of the youngest orthogneiss pre grenvillian and grenvillian metamorphic assemblages and at least three mafic dyke suites the overlying ojibway sand bay assemblage contains only younger orthogneiss with grenvillian metamorphic assemblages volumetrically important supracrustal rocks that are younger than the youngest orthogneiss and lacks cross cutting mafic dykes comparable tectono stratigraphic changes are present acros

numerical analysis of initiation and progressive failure in natural rock slopes the 1991 randa rockslide the 1991 randa rockslide in the swiss alps involved several complex mechanisms relating to geological mechanical and hydrological processes for which no clear trigger can be asserted this paper investigates the concept of progressive failure and the numerical modelling of rock mass strength degradation in natural rock slopes using the randa rockslide as a working example results from continuum i e finite element modelling are presented to illustrate a hypothesis suggesting that initiation of a progressive rock mass degradation process ultimately leading to failure began following deglaciation of the valley below discontinuum distinct element modelling is then applied to investigate the underlying mechanisms contributing to the episodic nature of the rockslide finally the use of a hybrid method that combines both continuum and discontinuum techniques to model fracture propagation are discussed in the context of modelling progressive slide surface development linking initiation and degradation to eventual catastrophic failure

convective mixing in a tropopause fold we present a case study of the passage of a tropopause fold over the uk behind a cold front with emphasis on the mixing caused by convection extending into the fold the event took place on 15 16 january 1999 and was the subject of intensive observations using the met office c130 aircraft and the mesosphere stratosphere troposphere mst radar at aberystwyth here we concentrate on radar and satellite observations during the afternoon of 16 january when the surface cold front had passed over the uk a tongue of moist air moved north eastwards over wales at 700 hpa at this time which because of the very dry air in the fold above resulted in potential instability the resulting convection was clearly observed in noaa satellite images the mst radar depicted the passage of the cold front and tropopause fold as a layer of high echo power and vertical wind shear ascending with time spectral widths showed the fold to be free of turbulence until 1200 utc on 16 january when convection was observed reaching into the frontal zone and generating turbulence eddy dissipation and diffusivity rates of 8 6 mw kg 1 and 8 5 m2s 1 respectively were derived for this event to place these figures in context they are compared with corresponding rates derived for sixteen other passages of tropopause folds over the radar each resulting from shear rather than convective instability the convective event is found to be comparable to the strongest shear events and to correspond to moderate turbulence as experienced by an aircraft this process is of potential importance for atmospheric chemistry because it mixes boundary layer air directly with stratospheric air over a timescale of 1 2 hours copyright 2004 royal meteorological society

docking of the central metasedimentary belt to laurentia in geon 12 evidence from the 1 17 1 16 ga chevreuil intrusive suite and host gneisses quebec the chevreuil intrusive suite 1 17 1 16 ga represents a chronological field marker of regional extent that intruded the central metasedimentary belt in the western grenville province of quebec after peak metamorphism style and site of magma emplacement and extent of deformation of chevreuil plutons and dykes permit unravelling of the early grenvillian evolution of the belt with respect to cratonal north america the suite comprises a series of vertically layered gabbro stocks and monzonite diorite gabbro sheet intrusions and a swarm of microdiorite dykes that cut across gneisses the dykes display systematic variations in extent of deformation across the belt we targeted u pb geochronology on gneisses within the identified strain windows they preserve the record of a ca 1 20 ga high pressure temperature p t metamorphic event the sheet intrusions define magmatic corridors all along and concordant with the western northern and eastern tectonic boundaries of the belt the concordant and elonga

late orogenic plastic to brittle extension along the robertson lake shear zone implications for the style of deep crustal extension in the grenville orogen canada rocks regionally metamorphosed and deformed at middle to lower crustal levels in the contraction dominated mesoproterozoic grenville orogen are exposed in southern ontario canada investigation of the robertson lake shear zone rlsz indicates that extension is a significant component in the late tectonic evolution of this deeply eroded orogen the rlsz is a discrete zone 0 5 1 0 km thick that can be traced for nearly 100 km cross cutting regional structures and lithologic units the zone is composed of a thick mylonite zone and a narrow overlying brecciated zone mylonite foliation shallowly dips east southeast and contains a down dip lineation mylonites are characterized by microstructures indicative of crystal plastic deformation and contain a variety of shear sense indicators including s c and c c composite fabrics sigma porphyroclasts mica fish and other shape fabrics oblique to shear planes consistently indicating a normal down to the east sense of shear a zone of cataclastic structures overlies the mylonite zone and brittle deformation overprints the mylonitic fabrics slickensides in the zone generally strike parallel to foliation in the mylonites but have steeper dips sense of slip indicators and brittle fault orientations also indicate down to the east displacement during east west extension juxtaposition of brittle and crystal plastic structures is resolved with a model of displacement during exhumation whereby localization of cataclasis occurred along a previously active mylonite zone metamorphic facies are upper greenschist in the hanging wall and upper amphibolite in the footwall and metamorphic grade increases from west to east in both the hanging wall and footwall the regional variations in metamorphic grade and the low angle shear zone geometry are a result of isostatic flexural rotations that accompanied extension combined with the crystal plastic to cataclastic nature of the rlsz new 40ar 39ar isotope data from biotite constrain the timing of shear zone displacement until at least 901 1 ma late in the evolution of the grenville orogen

break up of a stratospheric streamer observed by mst radar the break up of a stratospheric streamer over the british isles is described using satellite mesosphere stratosphere troposphere mst radar and in situ observations together with european centre for medium range weather forecasts analyses the streamer started as a thin ribbon of high potential vorticity to the west of the british isles as this was strained by the background flow it broke up into a series of mesoscale vortices mst radar and ozonesonde observations delineated a tropopause fold to the west of the initial streamer taking the form of a thin layer of stratospheric air embedded in a deep layer of descending upper tropospheric air the fold appears to have been mixed out by small scale turbulence as the streamer broke up this break up also generated strong inertia gravity waves which broke just above the tropopause introducing extensive mixing to the lowermost stratosphere one of the mesoscale vortices returned over the mst radar showing a very indistinct tropopause typical of a synoptic scale cut off low this vortex was also responsible for the development of a rain band on its eastern flank which brought significant rainfall to the british isles at a time when the synoptic flow pattern was strongly anticyclonic

climatology of tropopause folds at midlatitudes a statistical analysis of a 25 years time series of routine ozone soundings at uccle 51 48 n 4 21 e reveals that the frequency of tropopause folds is within the limits of uncertainty similar for all seasons on the average 4 8 of the ozone profiles show the presence of a fold whereas a shorter time series of soundings 1985 1993 obtained at the observatoire de haute provence 43 54 n 5 45 e yields only a 2 fraction of folds the difference between both stations is however riot entirely statistically significant the average wet and dry artificial radioactivity deposition in the sixties and early seventies was at a maximum at uccle around june in good agreement with global sr 90 wet deposition measurements volchok and kleinman 1971 suggesting that stratosphere troposphere exchange is more important during spring compared to the rest of the year this leads to the hypothesis that if tropopause folds are the major agent of stratosphere troposphere exchange ste they may be more efficient in spring than in autumn

the influence of upper tropospheric potential vorticity on convective morphology abstract extensive research has illuminated the roles that buoyancy and vertical wind shear have in determining convective mode the goal of this study is to examine synoptic forcing as measured by the form of the waves on the dynamic tropopause and the strength of the resultant temperature advection together with the environmental variables that are currently used to differentiate storm type logistic regression is used to make this discrimination and provides the preliminary results first multicellular lines and isolated rotating cells are associated with weaker synoptic forcing second where an upper level feature is present multicellular convection lines and clusters combined is favored relative to isolated storms when there is stronger synoptic forcing the shape of the upper level potential vorticity pv feature is elongated and there is a more southerly component in the deep layer 0 6 km mean wind third when an upper level feature is present and the convection is multicellular cluste

baroclinic development within zonally varying flows previous idealized modelling studies have shown the importance of across jet barotropic shear to the resulting evolution of cyclones anticyclones surface based fronts and upper level fronts meanwhile many observational studies of cyclones have shown the importance of along jet variations in the horizontal wind speed i e confluence and diffluence this study investigates the importance of these along jet zonal for zonally oriented jets variations in the horizontal wind speed to the resulting structures and evolutions of baroclinic waves using idealized models of growing baroclinic waves an idealized primitive equation channel model is configured with growing baroclinic perturbations embedded within confluent and diffluent background flows when the baroclinic perturbations are placed in background confluence the lower tropospheric frontal structure and evolution initially resemble the shapiro keyser cyclone model with a zonally oriented cyclone strong warm front and bent back warm front later as the baroclinic wave is amplified in the stronger downstream baroclinicity the warm sector of the cyclone narrows becoming more reminiscent of the norwegian cyclone model the upper level frontal structure develops with a southwest northeast orientation and becomes strongest at the base of the trough where geostrophic cold advection is occurring in contrast when the baroclinic perturbations are placed in background diffluence the lower tropospheric frontal structure and evolution resemble the norwegian cyclone model with a meridionally oriented cyclone strong cold front and occluded front the upper level frontal structure is initially oriented northwest southeast on the western side of the trough before becoming zonally oriented weak geostrophic temperature advection occurs along its length these results are compared to those from previous observational and idealized modelling studies copyright 2007 royal meteorological society

the thermal history of the new york basement determined from 40ar 39ar k feldspar studies constraints on the thermochronologic evolution of the new york crystalline basement are mainly restricted to the high temperature early cooling history following 1 1 ga grenville metamorphism u pb methods and the late cooling history fission track methods the thermal history for the one billion years between these intervals is herein assessed using 40ar 39ar hornblende muscovite biotite and k feldspar thermochronology hornblende preferred ages suggest cooling below 450 500 c between ca 900 and 950 ma total gas ages of biotites range from 746 1060 ma and their age spectra complexity is probably related to younger thermal events and or excess argon a single muscovite analysis yields a preferred age of 854 ma argon isotope analyses of k feldspar provide a quantitative evaluation of the thermal history between 175 and 350 c multi diffusion domain thermochronology on highly variable k feldspar results yield internally consistent thermal histories and along with geologic constraints suggest thermal maxima of 275 350 c at ca 700 470 450 and 300 ma combining all of the k feldspar analyses provides an internally consistent thermal history for the region and allows for the following inferences reheating at 700 ma associated with late precambrian rifting during the formation of the iapetus ocean apparently affected the entire adirondack region with the highest temperatures occurring in the eastern part of new york local reheating in the ordovician is inferred to have affected the eastern adirondack mountains and probably resulted from the combined effects of burial during the early paleozoic emplacement of taconic thrust sheets and migration of hot fluids along normal faults during the taconic orogeny high paleotemperatures for the devonian section of eastern new york are related to maximum burial of the basement during the carboniferous at ca 300 ma

the 40ar 39ar laser analysis of k feldspar constraints on the uplift history of the grenville province in ontario and new york 1 a comprehensive geochronologic database describes a period of late extension across the metasedimentary belt mb of the grenville province in northeastern north america because extension continued through much of the young unroofing history of the region these data do not bracket the timing of final extension across all shear zones and do not constrain the timing of final juxtaposition of terranes analysis of k feldspar in the mb by the 40ar 39ar method can give information on the very latest perturbations in the area since k feldspars have multiple low closure temperatures ranging from about 150 to 300 c these multiple diffusion domains require temperature time modeling to fully interpret argon release spectra this modeling requires precise knowledge of the temperature of degassing of each step which usually requires the use of a resistance furnace for sample analysis we establish a first order relationship between laser power and temperature which can be used for multidiffusion domain modeling of k feldspars comparison of samples analyzed by both methods reveals virtually no difference between the systems supporting the validity of k feldspar analyses by laser step heating our data suggest that using the laser for k feldspars can give results that are geologically reasonable precise and easier to collect in this case these data are then applied to the cooling history of the north american grenville province our k feldspar analyses show that the latest extensional motion along shear zones in the mb is after 900 ma and the region is uplifting as a uniform block by 780 ma

tropopause folds in era interim global climatology and relation to extreme weather events tropopause folds are intimately linked to upper level frontogenesis and jet stream dynamics they play an important role for stratosphere troposphere exchange the dynamical coupling of upper and lower tropospheric levels and for generating severe weather events this study presents a global climatology of tropopause folds using era interim reanalysis data from 1979 to 2012 and a refined version of a previously developed 3 d labeling and fold identification algorithm this algorithm objectively separates stratospheric and tropospheric air in complex situations e g in regions with strong low level inversions and in extratropical cyclones where diabatically generated potential vorticity anomalies typically occur three classes of tropopause folds are defined shallow medium and deep and their geographical distribution vertical extent and seasonal cycle are investigated most shallow folds occur along the subtropical jet stream in agreement with previous studies hot spots of medium and deep tropopause folds are found west of australia and along the coast of antarctica in the southern hemisphere and around the east coast of north america in the northern hemisphere seasonal cycles show maxima in winter for all fold classes medium and deep folds are frequently associated with surface wind gust and precipitation extremes as quantified for folds over the southern indian ocean wind gust extremes occur mainly in an elongated band upstream and equatorward of folds whereas precipitation extremes occur mainly east and poleward of folds overall in the considered region about 20 of medium folds and 33 of deep folds are associated with surface wind or precipitation extremes in the vicinity of the fold

diagnostic study of a narrow cold frontal rainband and severe winds associated with a stratospheric intrusion a diagnostic study of a severe wind event associated with a cold front is presented based on output from an operational numerical weather prediction model together with radar and satellite imagery and detailed surface observations an intrusion of stratospheric air initially with a potential vorticity of 2 pv units is shown as part of a backward tilted filament to have plunged down to the lower troposphere where it was associated with a narrow cold frontal rainband at its leading edge and produced severe wind gusts up to 70 knots a close correspondence in time and space is revealed between these surface events and the appearance in the model of a pocket of diluted stratospheric air with pv 1 at the 2 km level the stratospheric intrusion produced two distinguishable wind surges one behind the narrow cold frontal rainband and another mainly dry about 200 km farther behind the strength of the winds was attributable to the downward transport of momentum as part of the large scale dynamics local moist processes played only a minor role at coarse resolution the radar echo from the narrow cold frontal rainband resembled line convection but it is shown that there were significant differences between this event and the archetypal form of line convection often seen in the british isles

differential unroofing within the central metasedimentary belt of the grenville orogen constraints from 40ar 39ar thermochronology an 40ar 39ar thermochronological investigation of upper greenschist to granulite facies gneiss amphibolite and marble was conducted in the central metasedimentary belt cmb ontario to constrain its cooling history incremental 40ar 39ar release spectra indicate that substantial differential unroofing occurred in the cmb between 1000 and 600 ma a consistent pattern of significantly older hornblende and phlogopite 40ar 3ar cooling ages on the southeast sides of major northeast striking shear zones is interpreted to reflect late displacement due to extensional deformation variations in hornblende 40ar 39ar age plateaus exceeding 200 ma occur over distances less than 50 km with major age discontinuities occurring across the robertson lake shear zone and the sharbot lake mylonite zone which separate the sharbot lake terrane from the elzevir and frontenac terranes extensional displacements of up to 14 km are inferred between the frontenac and elzevir terranes of the cmb no evidence for significant post argon closure vertical displacement is indicated in the vicinity of the perth road mylonite within the frontenac terrane variations of nearly 100 ma in phlogopite 40ar 39ar plateau ages occur in undeformed marble on either side of the bancroft shear zone phlogopites from sheared and mylonitized marble within the shear zone yield 40ar 39ar diffusional loss profiles but have older geologically meaningless ages thought to reflect incorporation of excess argon by 900 ma southeast directed extension was occurring throughout the cmb possibly initiated along previous zones of compressional shearing an easterly migration of active zones of extension is inferred possibly related to an earlier overall easterly migration of active zones of regional thrusting and easterly migration of an ancient subduction zone the duration of extensional shearing is not well constrained but must have ceased before 600 ma as required by the deposition of overlying undeformed cambrian and or ordovician sedimentary rocks

exhumation of a collisional orogen a perspective from the north american grenville province combined structural and geochronologic research in the southernmost portion of the contiguous grenville province of north america ontario and new york state show protracted periods of extension after the last episode of contraction the grenville province in this area is characterized by synorogenic extension at ca 1040 ma supported by u pb data on titanites and 40 ar 39 ar data on hornblendes followed by regional extension occurring along crustal scale shear zones between 945 and 780 ma as recorded by 40 ar 39 ar analysis of hornblende biotite and k feldspar by ca 780 ma the southern portion of the grenville province from ontario to the adirondack highlands underwent uplift as a uniform block tectonic hypotheses have invoked various driving mechanisms to explain the transition from compression to extension however such explanations are thus far geodynamically unconstrained numerical models indicate that mechanisms such as gravitational collapse and mantle delamination act over timescales that cannot explain a protracted 300 m y extensional history that is contemporaneous with ongoing uplift of the grenville province rather the presence of a plume upwelling underneath the laurentian margin combined with changes in regional stress directions permitted the observed uplift and extension in the grenville province during this time the uplift history while on a slightly different timescale from those of most plume models is similar to that seen in models of uplift and extension caused by the interaction of a plume with the base of the lithosphere some of the protracted extension likely reflects the contribution of farfield effects possibly caused by tectonic activity in other cratons within the rodinian supercontinent effectively changing the stress distributions in the grenville province of northeastern north america

climatology of tropopause folds over a european arctic station esrange 1 eleven years september 1996 to august 2007 of continuous measurements of three dimensional wind and backscattered signal strength observed with esrange radar esrad have been utilized to study the annual and interannual variation of tropopause folds over an arctic station two typical tropopause fold events one is associated with a streamer type of system and the other with a cutoff low are selected and are characterized with the help of synoptic charts and potential vorticity pv analysis typical characteristics of radar parameters during the passage of folds are identified such as the sudden rise in the tropopause altitude high reflectivity layer sloping downward from the tropopause beneath the jet stream and intensification of the jet stream these characteristics are utilized to discern the tropopause fold in the radar data the climatology of tropopause folds exhibits a pronounced annual cycle with a large number of folds in winter and fewer in summer the annual cycle of folds is more or less similar in all the years however significant interannual variation is observed with winter periods exhibiting maximum interannual variability the climatology of folds and its annual cycle are compared and contrasted with similar climatological studies available in the literature the differences in the climatologies are discussed in light of differences in the algorithms and the spatial variability of fold frequency

dynamical aspects of the life cycle of the winter storm lothar 24 26 december 1999 dynamical aspects of the life cycle of the winter storm lothar 24 26 december 1999 are investigated with the aid of the european centre for medium range weather forecasts analysis data and mesoscale model simulations neither of these datasets capture the full amplitude of the observed extreme pressure fall and surface wind speeds but they do help identify a range of key dynamical and physical features that characterize the development of this unusual event the analysis and interpretation is primarily based upon the evolution of the lower and upper level potential vorticity pv field complemented by three dimensional trajectory calculations lothar originated in the western atlantic and travelled as a shallow low level cyclone of moderate intensity towards europe this translation took place below and slightly to the south of a very intense upper level jet and was accompanied by continuous and intense condensational heating that sustained a pronounced positive low level pv anomaly not unlike the concept of a diabatic rossby wave no significant pv anomalies were evident at the tropopause level during this early phase of the life cycle the surface cyclone intensified rapidly when the shallow cyclone approached the jet stream axis the circulation induced by the diabatically produced low tropospheric pv anomaly on steeply sloping isentropic surfaces that transect the intense upper level jet contributed significantly to the rapid formation of a narrow and deep tropopause fold this stratospheric pv anomaly virtually merged with the diabatically produced ephemeral pv feature to form a vertically aligned tower of positive pv at the time of maximum storm intensity a sensitivity study with a dry adiabatic hindcast simulation shows no pv tower configuration and only a very weak surface development and confirms the primary importance of the cloud diabatic heating for the tropopause fold formation and the rapid bottom up intensification of lothar a comparison of the anomalously high atlantic sea surface temperatures in december 1999 with the water vapour source regions for the latent heat release that accompanied the rapid intensification phase of lothar shows a close relationship this is of importance when discussing the possible implications of climate variability and change on the development of north atlantic winter storms copyright 2002 royal meteorological society

the dependence of numerically simulated convective storms on vertical wind shear and buoyancy abstract the effects of vertical wind shear and buoyancy on convective storm structure and evolution are investigated with the use of a three dimensional numerical cloud model by varying the magnitude of buoyant energy and one directional vertical shear over a wide range of environmental conditions associated with severe storms the model is able to produce a spectrum of storm types qualitatively similar to those observed in nature these include short lived single cells certain types of multicells and rotating supercells the relationship between wind shear and buoyancy is expressed in terms of a nondimensional convective parameter which delineates various regimes of storm structure and in particular suggests optimal conditions for the development of supercell type storms applications of this parameter to well documented severe storm cases agree favorably with the model results suggesting both the value of the model in studying these modes of convection as well as the value of this representation i

the carthage colton mylonite zone adirondack mountains new york the site of a cryptic suture in the grenville orogen u pb ages were determined on metamorphic sphenes and monazites from the late proterozoic adirondack highlands and lowlands in the vicinity of the carthage colton mylonite zone monazites were extracted from metapelites and sphenes were separated from marbles calc silicate gneisses and granite gneisses in order to determine the timing and the duration of metamorphism as well as the cooling histories for rocks on either side of the mylonite zone monazite ages from the lowlands range from 1171 1137 ma sphene ages in the lowlands range from 1156 1103 ma those from the highlands immediately to the east of the mylonite zone range from 1050 982 ma the ages indicate that the last high grade metamorphism in the highlands is ca 100 m y younger than in the lowlands and that both terranes had separate cooling histories at least until ca 1000 ma sphenes from within the carthage colton mylonite zone yield ages of about 1098 ma which are distinct from sphene ages on either side of the shear zone the mineral

thermal regime and post orogenic extension in collision belts in order to assess the conditions leading to post orogenic extension the temperature and strength of the lithosphere during continental collision are calculated the calculations assume that heat is transported by conduction and either that the lithosphere is homogeneously thickened or that only the crust is thickened during the collision despite increased crustal thickness and heat production the temperature does not increase if the rate of shortening is moderate 5 10 16s 1 leading to 100 increase in crustal thickness in 40 myears the temperature distribution is used to determine a rheological profile that is compared with the stress induced by compensated topography the calculations show that for homogeneous lithospheric thickening the strength of the lithosphere increases and is higher than the tensile stress except in the shallow crust the total strength of stable continental lithosphere at the end of shortening is on the order of 1013 n m 1 it is larger by one order than the force induced by compensated topography 1012 n m 1 it appears that homogeneous lithospheric thickening would not lead to post orogenic extension unless the initial conditions are very special with the lithosphere hotter than normal for crustal thickening only the strength of the lithosphere decreases slightly and is on the same order as the tensile stress extension does not necessarily follow from thickening of the crust only but it could take place for a relatively wide range of initial conditions with initial surface heat flow 60 mw m 2 or higher alternatively an event such as rapid removal of the mantle lithosphere by delamination or small scale convection would increase the tensile stress heat rapidly the lithosphere reduce its strength and always trigger extension

upper level frontogenesis associated with the birth of mobile troughs in northwesterly flow previous studies have shown that 500 hpa mobile trough births or genesis occur preferentially in northwesterly flow during upper level frontogenesis and that cold advection assists in and is a product of mobile trough intensification this study focuses on the synoptic environments and thermal advection patterns of upperlevel fronts associated with mobile trough births in northwesterly flow a climatology of 186 such events derived from an earlier study by sanders shows that most births tend to occur within uniform or diffluent flow and that most tend to be associated with relatively weaker 500 hpa thermal advection most mobile trough births in diffluence however tend to be associated with increasing 500 hpa cold advection typically indicated by a cyclonic rotation of isentropes whereas most mobile trough births in confluence tend to be associated with weaker 500 hpa thermal advection two cases of upper level frontogenesis associated with mobile trough genesis one in diffluence and one in confluence are compared to determine the processes acting to produce the differing thermal advection patterns at 500 hpa a thermal advection tendency equation is developed and shows that the magnitude of the temperature advection can be changed by accelerating the advecting wind speed or by changing the temperature gradient i e vector frontogenesis the latter can be accomplished either by changing the magnitude of the temperature gradient the frontogenetical component fn also known as scalar frontogenesis or by rotating the direction of the temperature gradient relative to the flow the rotational component fs the dominant processes acting on fn for the diffluence and confluence cases are tilting and deformation frontogenesis respectively the dominant process acting on fs for the diffluence case is rotation of the isentropes due to the vorticity term whereas rotation of the isentropes due to the vorticity and tilting terms are both important for the confluence case the rotational component of frontogenesis is cyclonic downstream of the vorticity maximum for both cases favoring increasing cold advection downstream of the vorticity maximum for both cases the rate of rotation of the isentropes at a point due to horizontal advection is large and that due to vertical advection is negligible since advection can only transport the existing isentrope angle and cannot change the isentrope angle the rotational component of frontogenesis normalized by the temperature gradient is the only term that can increase the isentrope angle following the flow this term dominates in the diffluence case but is small in the confluence case this diagnosis suggests the following reasoning in diffluent flow the vorticity associated with the incipient trough is compacted into a more circular shape and intensifies the potent vorticity maximum leads to robust isentrope rotation in confluent flow however the vorticity is deformed into an elongated maximum inhibiting both strong isentrope rotation and increasing cold advection thus the rotational frontogenesis component explains the rotation of the isentropes that is responsible for the differing thermal advection patterns diagnosis of these cases supports the results from the climatology indicating a strong relationship between the synoptic environment and the upper tropospheric thermal advection pattern nevertheless current conceptual models of upper level frontogenesis do not fully explain the variety of these features in the real atmosphere in particular mobile trough genesis and its associated upper level frontogenesis can occur in weak 500 hpa thermaladvection patterns in contrast to the confluence and cold advection that have been previously identified as important to upper level frontal intensification this result provides further support for the possibility that generation and intensification of mobile troughs can occur by barotropic processes

current controversies in the caledonides this set of papers arose from a discussion meeting held in the department of geology applied geology university of glasgow on 9 10 september 1997 the aim of the meeting was to review the progress which had been made in resolving some of the more controversial aspects of the pre silurian caledonide geology of the british isles and scandinavia it was also held to mark the retirements of two well known caledonide geologists professors bernard leake and donald ramsay there were 34 talks and poster contributions and the meeting was attended by over 100 people we would like to thank our colleague tim dempster for helping to convene and organize the meeting it was fitting that the meeting should be held in the gregory building which was named after the glasgow professor who not only pioneered work on rift tectonics and regional uplift in africa but also wrote a perceptive book on dalradian geology gregory 1931 in this work gregory pointed out that his observations were made over a period of 25 years being made on recreational weekends and mainly in winter when the ground is less obscured by vegetation and the highland mountains most attractive an approach motivated by a spirit of enquiry and very productive but one which would be out of step with current research strategies we would like to dedicate this set of papers to mark piasecki who died on 13 june 1999 mark was the first person to draw attention to major problems concerning the relationship

convective destabilization by a tropopause fold diagnosed using potential vorticity inversion there is observational evidence to suggest that at the time tropopause folds in extra tropical cyclones are descending into the mid troposphere there is frequently a destablization of the lower troposphere to moist convection here we consider the dynamical reasons for such a linkage using so called potential vorticity pv attribution concepts in particular pv inversion is used to find the wind shear attributable to the tropopause fold itself then this shear is used to quantify the tendency to generate regions of potential instability for an observed case of a modest tropopause fold it is found that the pv anomaly contained in the tropopause fold contributed substantially to the instantaneous convective destabilization in the sense of increasing the potential instability indeed at some locations it overcame a tendency to stabilize by the atmosphere without the fold on the other hand only a fraction of the region with increased potential instability experienced sufficient large scale ascent to lead to convection and it remains to be seen whether folds in general make a significant contribution to the generation of potential instability that can actually be realized the importance of tropopause folds or indeed of tropopause depressions in general in the context of convection is that their small horizontal scale has the potential to focus and amplify any convective destabilization locally this may account for the localization of outbreaks of severe convective weather which it is important to forecast accurately currently numerical weather prediction models often do not handle these small scales very accurately

models of isostatic and dynamic topography geoid anomalies and their uncertainties knowledge of the dynamic topography at earth s surface caused by sublithospheric density contrasts would provide crucial constraints on models of mantle dynamics we calculate global models of this dynamic topography by subtracting from the observed topography estimates of the topography caused by assumed isostatically compensated density variations in the crust the oceanic lithosphere and the continental tectosphere we also calculate the isostatic geoid anomaly that would result from these compensated near surface density anomalies because there is controversy about the thermal structure of old oceanic lithosphere we investigate two models of the cooling of the oceanic lithosphere half space and plate and calculate the effect of each on the inferred dynamic topography peak amplitudes of dynamic topography from sublithospheric sources expanded to spherical harmonic degree 6 are approximately 1 km we estimate the uncertainties in the calculated dynamic topography and geoid by combining the errors in the data and systematic errors to obtain the spectral coefficients of the fields we use weighted least squares inversions the calculated dynamic topography arising from sublithospheric sources residual geoid and errors associated with these fields can be used in studies of mantle dynamics

a five year radar based climatology of tropopause folds and deep convection over wales united kingdom abstracta five year 2006 10 radar based climatology of tropopause folds and convective storms was constructed for wales united kingdom to determine how deep moist convection is modulated by tropopause folds based on the continuous high resolution data from a very high frequency vhf wind profiling radar located at capel dewi wales 183 tropopause folds were identified tropopause folds were most frequent in january with a secondary maximum in july based on data from the u k weather radar network a climatology of 685 convective storms was developed the occurrence of convective storms was relatively high year round except for an abrupt minimum in february april multicellular lines 43 5 were the most common morphology with a maximum in october followed by isolated cells 33 1 with a maximum in may september and nonlinear clusters 23 4 with a maximum in november january convective storms were associated with 104 56 8 of the tropopause folds identified in this study with the associ

potential vorticity diagnosis of the severe convective regime part iii the hesston tornado outbreak nonlinear balance potential vorticity pv inversion is used to diagnose the sensitivity of the severe convective parameter space to the amplitude of a subsynoptic scale pv anomaly on 13 march 1990 a day on which a significant tornado outbreak impacted the great plains pv surgery is used to both amplify and remove the pv anomaly and the contemporaneous impact on various convective parameters is subsequently quantified by using piecewise pv inversion to compute the changes in those parameters attributable to each pv alteration it is found that amplifying the anomaly increases the cape by amounts typically ranging from 20 to 30 within the atmospheric columns experiencing the maximum pv increase ascent is increased slightly downshear of the pv anomaly consistent with extant conceptual models governing synoptic scale forcing for vertical motion amplifying the pv anomaly increases deep layer shear over the southern half of the outbreak region and reduces storm relative helicity over the northern half primarily through changes in the estimated storm motion vector removing the anomaly produces complementary changes of the opposite sign thresholds of several commonly used convective parameters are chosen on the basis of prior empirical studies and the horizontal displacement of these threshold contours produced by the pv alterations reveals that relatively modest subsynoptic scale pv changes would not likely change the predominant convective mode during the hesston outbreak

initiation of convective storms at radar observed boundary layer convergence lines abstract the origin of 653 convective storms occurring over a 5000 km2 area immediately east of the colorado rocky mountains from 18 may to 15 august 1984 was examined seventy nine percent of the 418 storms that initiated within the study area occurred in close proximity to radar observed boundary layer convergence lines this percentage increased to 95 when only the more intense storms 60 dbze were considered colliding convergence lines initiated new storms or intensified existing storms in 71 of the cases a new storm took a median time of 24 min to grow to 30 dbz following line collision the convergence lines ranged in length between ten and several hundred kilometers both radar and mesonet stations indicated that the primary convergence was concentrated in a zone 0 5 to 5 km in width these lines were characterized on doppler radar as thin lines of enhanced reflectivity between 0 and 20 dbze and as a line of strong radial or azimuthal gradient in doppler velocity these lines were observed ev

conceptual models of upper level frontogenesis in south westerly and north westerly flow the conceptual model proposed by shapiro as it is applied to the evolution of an upper level frontal zone within a baroclinic wave is reviewed and its limitations are investigated through previous literature and two casestudies presented in this paper the early stages in the evolutions of these two cases are used to examine specific limitations of this conceptual model 1 upper level frontogenesis in south westerly flow that evolves from a state of equivalent barotropy to a state of cold advection along the front and 2 upper level frontogenesis in northwesterly flow with along front variation in the sign of the thermal advection such that warm advection occurs upstream of cold advection in the thermal trough vector frontogenesis diagnostics for the lagrangian rate of change of the magnitude and direction of the horizontal potential temperature gradient including tilting due to vertical motion are derived these diagnostics are applied to the two cases to examine the maintenance of the potential temperature gradient and the development of cold advection along each upper level front the upper level front in south westerly north westerly flow was maintained primarily by deformation tilting frontogenesis in agreement with previous research the increasing cold advection along the upper level front in both cases was related to an upstream vorticity maximum for the case in south westerly flow the pre existing vorticity maximum approached a downstream equivalent barotropic upper level front in a manner similar to an instant occlusion resulting in cold advection along the length of the upper level front for the case in north westerly flow an intensifying vorticity maximum concentrated the cold advection in the base of the thermal trough as warm advection developed upstream these two cases are compared to upper level fronts in previous literature and a climatology of upper level fronts associated with landfalling cyclones over the eastern north pacific ocean the results indicate that these two cases are typical of early evolutions of upper level fronts that can occur in south westerly and north westerly flow therefore a revised version of the shapiro conceptual model is presented that represents more accurately the early evolutions exhibited in the present and previous studies

on the linkage between the asian summer monsoon and tropopause fold activity over the eastern mediterranean and the middle east a climatology of tropopause folds occurring over the eastern mediterranean and the middle east emme has been established using the era interim reanalyses for the years 1979 2012 the methodology employs an algorithm that detects folds at grid points where the vertical profile features multiple crossings of the dynamical tropopause and allows their classification according to their vertical extent our results confirm the findings of an earlier 1 year climatology that recognized a global hot spot of summertime fold activity between the eastern mediterranean and central asia in the vicinity of the subtropical jet two distinct maxima of activity are identified over turkey and iran afghanistan where fold frequency exceeds 25 occasionally medium and deep folds form over the two regions at surprisingly low latitudes this summertime peak in fold activity diverges from the zonal mean seasonal cycle over the subtropics and is driven by the south asian monsoon starting in late spring the emme is gradually brought under the influence of the zonally asymmetric background state induced by the monsoon as areas of sharply sloping isentropes develop especially over the eastern mediterranean and iran afghanistan subsidence and fold formation are favored further investigation of the reanalysis data provided empirical evidence that the monsoon also drives the interannual variability of emme fold activity an upward trend in fold activity is identified especially in may attributed to the recent advanced monsoon onset and the deepening convective activity throughout summer which promotes upper level baroclinicity over the emme and favors folding

tropopause folds and cross tropopause exchange a global investigation based upon ecmwf analyses for the time period march 2000 to february 2001 1 a new methodology is proposed to identify folds of the dynamical tropopause taken as the 2 potential vorticity pv units pvu isosurface from global analysis data sets from the european centre for medium range weather forecasts ecmwf it consists of a three dimensional subdivision of the atmosphere into stratospheric and tropospheric parts and a subsequent examination of multiple tropopause crossings in vertical profiles of the analyses the method is applied to a 1 year period starting in march 2000 seasonal mean fold frequency distributions show that folds occur preferentially in the subtropics with maximum frequencies of about 30 pronounced maxima are located in the subtropical bands that extend from 20 e to 120 e on both hemispheres during summer generally subtropical folds are comparatively shallow deep folds occur most frequently in midlatitudes during winter over eastern north america and the western north atlantic with maximum frequencies of about 1 detailed investigation of the relationship between individual folds and cross tropopause exchange events reveals that on the average individual folds in the extratropics are associated with larger exchange mass fluxes compared to the subtropics however due to the much higher frequency of subtropical folds the percentage of exchange events that are associated with tropopause folds is larger near 20 40 latitude 50 70 than further poleward 20 30 during winter and 10 20 during summer this indicates that tropopause folds are the key feature for cross tropopause exchange in the subtropics whereas in the extratropics other tropopause structures are at least of equal importance

age of xenolith bearing olivine metagabbro eastern adirondack mountains new york a u pb zircon age of 1143 6 pm 7 ma has been obtained on a xenolith bearing olivine metagabbro in the eastern adirondack highlands thus fixing a minimum age for metamorphism represented by truncated foliation in the xenolith and the igneous crystallization age of the rock the igneous age predates commonly accepted metamorphic ages of 1000 1100 ma for the adirondacks and documents that more than one high grade metamorphic event affected the region

sharbot lake terrane and its relationships to frontenac terrane central metasedimentary belt grenville province new insights from u pb geochronology the sharbot lake terrane of the central metasedimentary belt in ontario comprises marbles minor tholeiitic basalts and siliciclastic rocks metamorphosed at low to moderate degrees and is intruded by several suites of mafic to felsic plutons gabbroic and associated monzogranitic phases of the lavant complex in northwestern parts of the terrane yield ages of 1224 2 ma for zircon and monazite and 1221 1211 ma for titanite the wolf grove structure a gneissic domain in the northeastern part of the area was migmatized and intruded by granites at 1168 ma prior to its tectonic juxtaposition on amphibolites and marbles of the sharbot lake terrane correlative events recorded by gneiss quartzite and marble from carleton place farther east stress the strong affinity of the wolf grove structure with rocks of the frontenac terrane the 1156 2 ma posttectonic maberly stock intrudes rocks near the boundary zone between the frontenac and sharbot lake terranes suggesting that the two terranes were linked by

large scale crustal deformation of the tibetan plateau the topography velocity and strain fields calculated from a three dimensional newtonian viscous model for large scale crustal deformation are generally in good agreement with results from geological geodetic and earthquake studies in and around the tibetan plateau provided that the model rheology incorporates a weak zone within the deep crust beneath the plateau equivalent to a viscosity of 1012 pa s within a 250 m thick channel or 1018 pa s within a 15 km thick channel model studies and observations show a plateau at 5 km elevation with steep topographic gradients across the southern and northern plateau margins and more gentle gradients across the southeastern and northeastern margins rapid shortening strain is concentrated along the lower portions of the northern and southern plateau margins at rates 20 mm yr model results show north south shortening 10 mm yr in reasonable agreement with gps data 5 8 mm yr of north south shortening across the northern two thirds of the plateau and east west stretching 10 15 mm yr across the eastern half of the high plateau in reasonable agreement with seismic geologic and gps data upper crustal material moves eastward from the plateau proper into a lobe of elevated topography that extends to the south and east clockwise rotation of material around the east himalayan syntaxis at rates up to 10 mm yr occurs partly as a result of dextral shear between indian and asian mantle at depth and partly as a result of gravitational spreading from the high plateau to the south and east there is little difference in model surface deformation for assumptions of moderately weak or extremely weak lower crust except in southern and northern tibet where margin perpendicular extension occurs only for the case of an extremely weak lower crust our results suggest that the tibetan plateau is likely to have gone through a two stage development the first stage produced a long narrow high orogen whose height may have been comparable to the modern plateau the second stage produced a plateau that grew progressively to the north and east east west stretching eastward plateau growth and dextral rotation around the east himalayan syntaxis probably did not begin until well into the second stage of plateau growth perhaps becoming significant after 20 m y of convergence

u pb geochronology of the raglan gabbro belt central metasedimentary belt ontario implications for an ensialic marginal basin in the grenville orogen the raglan gabbro belt of the ontario grenville orogen is coincident with the top of the central metasedimentary belt boundary thrust zone a major mid crustal shear zone separating the central gneiss belt in the footwall from the central metasedimentary belt in the hanging wall it has been suggested that the gabbros making up the belt are coeval that they formed in a marginal basin within the central metasedimentary belt and that they formed a horizon of theologically stiff material that controlled the localization of the top of the boundary thrust zone during its initiation as the marginal basin closed at ca 1190 ma u pb zircon dating of plutons within the raglan gabbro belt was undertaken to test the coeval nature of intrusions in the belt magmatic crystallization ages for three of the gabbros fall in the range 1246 1227 ma and a fourth yields a minimum age of ca 1175 ma the results are permissive of a common origin for the gabbros and allow that the raglan gabbro belt may have been related to

invigoration and capping of a convective rainband ahead of a potential vorticity anomaly abstractdeep convection frequently occurs on the eastern side of upper level troughs or potential vorticity pv anomalies this is consistent with uplift ahead of a cyclonic pv anomaly and consequent reduction in static stability and increase of convective available potential energy cape nevertheless the causal link between upper level pv and deep convection has not been proven and given that lift moisture and instability must all be present for deep convection to occur it is not clear that upper level forcing is sufficient in this paper a convective rainband that intensified ahead of a cyclonic pv anomaly in an environment with little cape 10 j kg 1 is examined to determine the factors responsible for its intensification the key feature was a low level convergence line arising from the remnants of an occluded front embedded in the low level cyclonic flow the rainband s intensity and morphology was influenced by the remnants of a tropopause fold that capped convection at midlevels in the

delamination in collisional orogens during continental collision the weight of the downgoing slab may lead to extensional shearing in the descending plate permitting separation of the downdip dense oceanic lithosphere from the adjacent lighter continental lithospher separation may occur by means of a lithosphere penetrating normal sense shear zone by means of pure shear necking of the lithosphere or by some combination of these mechanisms during separation the continental crust attached to the lower plate may undergo an episode of extensional tectonics the proximity of asthenosphere to crust may result in syntectonic regional metamorphism and plutonism if the continents continue to converge following delamination a compressive tectonic regime will be reestablished in the crust the subsequent cooling of asthenosphere at the base of the crust may lead to the establishment of a continuous flat moho the likely imbrication of crustal and mantle lithologies during delamination and during the subsequent compressive tectonic regime as well as underplating associated with syntectonic plutonism may account for the seismic complexities observed at moho depths beneath some mountain belts

a midlatitude climatology and interannual variability of 200 and 500 hpa cut off lows abstracta climatology of midlatitude 200 and 500 hpa cut off low systems in the northern and southern hemispheres is constructed from the ncep ncar reanalysis by detecting and tracking under one

constraints on pb closure temperature in titanite based on rocks from the ungava orogen canada implications for u pb geochronology and p t t path determinations on the basis of multiequilibrium metamorphic pressure temperature p t estimates we have determined that 660 700 c is a reasonable minimum estimate of the closure t of pb diffusion in titanite a temperature significantly higher than previously suggested activity corrected reactions that produce titanite define narrow t stability fields 40 c that correspond closely to metamorphic p t determinations derived for coronitic garnet clinopyroxene hornblende samples growth of titanite occurring between 1789 and 1814 ma in the underthrust archean basement of the paleoproterozoic ungava orogen northern quebec is associated with retrograde metamorphism during underthrusting of the archean basement approx 1 b y after formation of the original granulitic assemblages

attribution and its application to mesoscale structure associated with tropopause folds it is important in meteorology to be able to establish a link between various features this link in its strongest form would be referred to as cause and effect but this is extremely difficult to establish for atmospheric flows a weaker but nonetheless powerful link is attribution which attempts to provide a way to associate various fundamental features such as potential vorticity anomalies to weather events involving regions of ascent attribution is not new and it is implicit in the way that atmospheric evolution is usually described in this paper an attempt is made to define more precisely what are the steps and assumptions being made in such a method these ideas become more concrete by applying them to a simple model of a two dimensional front at which there is a tropopause fold the aim is to find the vertical motion attributable to the descended tropopause fold in an extra tropical cyclone this is the first step in a longer programme of research designed to provide a test of the hypothesis that such a descended tropopause can trigger mesoscale phenomena such as convection and cyclones

40ar 39ar thermochronology along a western quebec transect of the grenville province canada ar ar thermochronology on 24 hornblendes 3 biotites 2 muscovites and 2 k feldspars collected along a 400 km long nw se geotraverse through the grenville province in western quebec is employed to provide time constraints on the intermediate and low temperature stages of cooling of part of the grenville orogen in the grenville front zone the c 1000 ma time of exhumation previously established from thermobarometric and isotopic studies is supported by the hornblende age data presented here from 60 km to 160 km se of the front reworked archaean migmatites of the parautochthonous reservoir dozois terrane rdt 1004 ma old metamorphic monazites contain hornblendes with 972 950 ma cooling ages assuming metamorphic geotherms between 25 and 30 c km 1 calculated cooling and unroofing rates are about 6 c ma 1 and 0 33 km ma 1 in the p t range 725 c 800 mpa and 450 c 400 mpa hornblendes from monocyclic rocks of the mont laurier and morin terranes mlt and mt monazite ages c 1165 ma give ages of about 1040 and 1010 ma respectively calculation of cooling unroofing rates from peak metamorphic conditions in this area is hampered by thermal perturbations associated with the still poorly dated grenville collision which took place approximately between 1060 and 1020 ma cooling ages of c 900 ma for muscovite and biotite and 860 810 ma for k feldspar show that cooling rates decreased to around 1 5 c ma 1 under retrograde greenschist facies conditions in the mlt on a time vs distance diagram the hornblende data define several distinct age ranges suggesting that each terrane had a characteristic thermal history thus cooling was diachronous and probably non homogeneous throughout this segment of the grenville orogen the time lag between the cooling history of the parautochthon 972 950 ma and the allochthons 1040 1010 ma is compatible with an earlier pre 1040 ma peak of metamorphism in the allochthons the reservoir cabonga allochthon was transported toward the nnw from its probable root zone in the mlt during the 1060 1020 ma grenvillian collision as a partially cooled slab the remobilization of the archaean parautochthon is attributed to this collision in the grenville front zone slightly older cooling ages and cooling rates initially faster than in the remaining part of the parautochthon are probably as a result of rapid tectonic exhumation shortly after collision the minor delay 20 30 ma in unroofing of the mt compared to the adjacent mlt is most likely related to post 1040 ma extensional displacement along the labelle shear zone in terranes like those described above where metamorphism is diachronous determination of cooling rates and the history of exhumation may be meaningless without a firm control on the regional structure however identification of contrasting cooling histories contributes to unravelling the independent movement of terranes

the presidents day cyclone of 18 19 february 1979 influence of upstream trough amplification and associated tropopause folding on rapid cyclogenesis abstract a diagnostic analysis of an amplifying polar jet trough system and associated tropopause fold which preceded the 19 february 1979 presidents day cyclone is presented the analysis is based on conventional radiosonde data infrared and visible satellite imagery 6 7 m water vapor measurements from the temperature humidity infrared radiometer thir and ozone measurements from the total ozone mapping spectrometer toms the case study indicates that dynamically forced meso scale vertical circulations played an important role in the extrusion of stratospheric air along the axis of a polar jet and the subsequent development of the storm system specific findings include 1 tropopause folding accompanying an amplifying polar jet trough system occurred along the axis of the intensifying polar jet in response to subsidence forced by geostrophic deformation patterns associated with the jet streak 2 the folding process extruded dry stratosphere air marked by high values of potential vorticity dow

onset of seafloor spreading in the iapetus ocean at 608 ma precise age of the sarek dyke swarm northern swedish caledonides abstract the sarek dyke swarm sds crops out in the sarektjakka nappe sn of the seve kalak superterrane in the northern swedish caledonides the sn has two main components 1 a 4 5 km thick succession of rift related sedimentary rocks which is intruded by 2 a suite of tholeiitic dykes the sds constituting 70 80 of the nappe the nappe was deformed during caledonian thrusting but dykes and sedimentary rocks in the interior of the eastern parts of the sn are preserved in a pristine state the tholeiitic dykes of the sds commonly occur in sheeted dyke complexes and up to 11 successive generations can be identified from crosscutting relations the sn represents the fossil continent ocean transition between the baltic craton and the iapetus ocean marking the initiation of seafloor spreading bubble shaped pods and veinlets of diorite are present in the sds sheeted dyke complexes the pods are absent in the oldest dykes but the younger a dyke the more frequent the pods the diorite pods are the equivalent of gabbro pegmatites and both cogenetic and coeval with the dykes the rapid successive emplacement of tholeiitic magma raised the ambient temperature in the dyke complex so that crystallization in the youngest dykes mimicked similar processes in gabbro plutons six zircon fractions from the diorite pods including two single grains were analysed geochronologically by the u pb thermal ionization mass spectrometry method the data yield a linear array of points that are 0 4 0 8 normally discordant indicating a crystallization age of 608 1 ma 207pb 206pb 607 9 0 7 ma mswd 0 33 this age is inferred to date the onset of seafloor spreading in the iapetus ocean along the baltoscandian margin

a review of the structure and dynamics of upper level frontal zones abstract this article presents a review of upper level fronts with the intent of synthesizing observational and modeling studies into a conceptual and dynamical description of these fronts and their evolution relative to the life cycle of midlatitude baroclinic waves the discussion begins by tracing present day concepts concerning the structure of upper level frontal systems which are based on composite analyses of radiosonde and aircraft data from their origins in the pioneering analyses of upper air data in the 1930s perspectives from scales both smaller and larger than upper level frontal systems are provided respectively by considering the effects of turbulent processes on frontal structure and dynamics and by relating variations in frontal structure to the evolution of the baroclinic waves that provide the dynamical environment for upper level frontogenesis the dynamics of upper level fronts are shown to comprise the interactions between the primary geostrophic and secondary ageostrophic cir

synorogenic collapse a perspective from the middle crust the proterozoic grenville orogen structural petrological and geochronological studies of the middle to late proterozoic grenville orogen in ontario canada indicate that a major extensional fault developed synchronously with late thrusting this fault zone was initiated during peak metamorphism and extended into the crust to depths of at least 25 kilometers the temporal and spatial relations among faulting metamorphism and regional compression indicate that synorogenic collapse initiated because the crust exceeded the maximum physiographic height and thickness that could be supported by its rheology comparison of grenville with recent himalayan orogenic activity suggests that during proterozoic times physiographic height crustal thickness and crustal strength were similar to modern conditions in orogenic belts