past forearc deformation in nicaragua and coupling at the megathrust interface evidence for subduction retreat 1 deformation at the boundary between the cocos and caribbean plates in nicaragua is currently focused in the volcanic arc in the form of arc parallel shearing in the middle miocene however there was widespread subsidence with local uplift in the outer forearc and inner forearc broad uplift to understand this complex deformation we use numerical modeling to investigate the effect of inhomogeneous friction at the megathrust fault on strain localization in the upper plate a good fit is obtained when 1 the interface between the slab and upper plate is not situated at the current wadati benioff zone but rather is moved 50 km landward to where subduction may have been active in the early tertiary and 2 there is high stress accumulation between 15 and 25 km depths and low stress accumulation updip and downdip of the thrust interface the results are consistent with a high velocity block being the remnant of an oceanic plateau that subducted in the eocene and was later incorporated into the upper plate we suggest that this geometry persisted into the middle miocene before the slab broke off and the thrust interface jumped seaward

onset of the summer monsoon over the indochina peninsula climatology and interannual variations abstract the temporal and spatial structures of the atmospheric circulation associated with the climatology and interannual variations of the summer monsoon onset over the indochina peninsula were studied using the observed daily rainfall at 30 stations and the ncep ncar reanalysis from 1951 to 1996 the climatological monsoon onset over indochina is on 9 may with a standard deviation of 12 days the monsoon onset is characterized by the pronounced northeastward progression of the low level southwesterlies over the indian ocean and the intensification and northward extension of the tropical convection from sumatra it coincides with the weakening of the midlatitude westerly over south asia and the westward propagation of the intraseasonal oscillation iso originated in the south china sea scs and the western pacific with a dominant timescale of 12 25 days a close relationship between the interannual variations of the monsoon onset and el nino la nina was identified years with warm cold sea surfac

the structure and annual variation of antisymmetric fluctuations of tropical convection and their association with rossby gravity waves abstract the signature of 4 5 day period rossby gravity waves is searched for in the tropical convection field across the indian pacific oceans the convergence divergence field of these waves in the lower troposphere is anticipated to produce an antisymmetric fluctuation in tropical convection antisymmetric fluctuations of tropical convection are shown to exhibit a pronounced spectral peak at a 4 5 day period only during boreal fall and only within about 30 longitude of the date line the peak amplitude occurs around 7 5 latitude these fluctuations propagate westward at 15 20 m s 1 with zonal wavelength of about 7000 9000 km the fluctuations of convection are coherent and out of phase with the equatorial meridional wind which also exhibits a pronounced spectral peak at a 4 5 day period in the lower troposphere near the date line the antisymmetric zonal wind also is strongly coherent with the antisymmetric convective fluctuations in this region the horizontal distributions of the 4 5 day power an

structural styles of mountain building controls of lithospheric rheologic stratification and extensional inheritance 1 the strength of the crust and the mantle lithosphere are strongly influenced by the temperature the thickness and the composition of the crust and mantle lithosphere and by inherited weaknesses it consequently strongly depends on the geodynamic setting and is then expected to be different from one orogen to another which might explain the diversity of deformation styles here we use 2d thermomechanical models at lithospheric scale to study the effect of the strength of the upper and middle crust on the geometry of contractional systems the role of extensional inheritance is included by forward modeling the formation of rift basin or passive margin and using the resulting extensional structure as initial condition for lithospheric scale inversion our results demonstrate that 1 crustal strength strongly controls the width and the elevation of orogenic wedge 2 relative weak crust facilitates efficient propagating of thick skinned crustal scale thrusts 3 contraction of a strong crust leads to the formation of long thrust sheets and an anti formal stack in the core of the orogen or the accretion of upper crust depending on the depth of the decoupling zone in the mid crust 4 erosion processes favor the localization of shortening in a narrow crustal orogen 5 rifting inheritance can explain the presence at shallower depth of lower crustal or mantle material previously upwelled and facilitates the propagation of the deformation in the external part of the chain the range of predicted behaviors is compared to first order with the zagros the alps and the pyrenees three natural examples for which the crustal structure is well constrained by geophysical data

extensional inheritance and surface processes as controlling factors of mountain belt structure surface processes and inherited structures are widely regarded as factors that strongly influence the evolution of mountain belts the first order effects of these parameters have been studied extensively throughout the last decades but their relative importance remains notoriously difficult to assess and document we use lithospheric scale plane strain thermomechanical model experiments to study the effects of surface processes and extensional inheritance on the internal structure of contractional orogens and their foreland basins extensional inheritance is modeled explicitly by forward modeling the formation of a rift basin before reversing the velocity boundary conditions to model its inversion surface processes are modeled through the combination of a simple sedimentation algorithm where all negative topography is filled up to a prescribed reference level and an elevation dependent erosion model our results show that 1 extensional inheritance facilitates the propagation of basement deformation in the retro wedge and 2 increases the width of the orogen 3 sedimentation increases the length scale of both thin skinned and thick skinned thrust sheets and 4 results in a wider orogen 5 erosion helps to localize deformation resulting in a narrower orogen and a less well developed retro wedge a comparison of the modeled behaviors to the high atlas the pyrenees and the central alps three extensively studied natural examples characterized by different degrees of inversion is presented and confirms the predicted controls of surface processes and extensional inheritance on orogenic structure

zonal and vertical structure of the madden julian oscillation abstract a statistical study of the three dimensional structure of the madden julian oscillation mjo is carried out by projecting dynamical fields from reanalysis and radiosonde data onto space time filtered outgoing longwave radiation olr data mjo convection is generally preceded by low level convergence and upward motion in the lower troposphere while subsidence cooling and drying prevail aloft this leads to moistening of the boundary layer and the development of shallow convection followed by a gradual and then more rapid lofting of moisture into the middle troposphere at the onset of deep convection after the passage of the heaviest rainfall a westerly wind burst region is accompanied by stratiform precipitation where lower tropospheric subsidence and drying coincide with continuing upper tropospheric upward motion the evolution of the heating field leads to a temperature structure that favors the growth of the mjo the analysis also reveals distinct differences in the vertical structure

circulation anomalies associated with tropical convection during northern winter abstract lagged cross correlations between outgoing longwave radiation olr and national meteorological center global analyses are utilized to isolate the preferred upper level and surface circulation anomalies associated with tropical convection during northern winter three intraseasonal time scales are studied 30 70 14 30 and 6 14 days in the 30 70 day band the upper level circulation signals are zonally elongated with zonal wavenumbers 0 2 dominant higher frequency signals are dominated by zonal wavenumbers 5 and 6 in the 14 30 day band convection over the eastern hemisphere is associated with upper level anticyclones in the subtropics and appears to be linked in some cases to midlatitude wave trains the strongest signals are for convection over africa australia and the eastern indian ocean only weak signals are seen for convection over indonesia in these regions of upper level easterlies olr anomalies peak prior to the maximum anomalies in wind suggesting forcing of the circulation b

focused subaerial erosion during ridge subduction impact on the geomorphology in south central peru data obtained from low temperature thermochronometers such as apatite fission track and u th he are combined with morphometric information extracted from digital elevation models this combination shows several geomorphological effects that are caused by the migration of the nazca ridge along the peruvian coastal margin offshore the depth of the deep sea trench decreases by 1500 m where the nazca ridge collides with the continental south american plate onshore the ridge causes an uplift of at least 800 m in the coastal cordillera this uplift results in a westward shift of the coastline thereby focusing and increasing erosion in the uplifted areas at the trailing edge the shelf subsides and the coastline retreats eastwards producing at least part of the indentation observed between paita and pisco the ridge acts therefore like a wave uplifting the andean margin as it traverses inland and southwards leaving a clear fingerprint on the topographic evolution of the peruvian coastal margin

rheology and strength of the lithosphere abstract mechanical properties of lithosphere are of primary importance for interpretation of deformation at all spatial and time scales from local scale to large scale geodynamics and from seismic time scale to billions of years depending on loading conditions and time scale lithosphere exhibits elastic brittle plastic or viscous ductile properties as can be inferred from rock mechanics data a large part of the long term lithospheric strength is supported in the ductile or ductile elastic regime while it also maintains important brittle strength yet at short seismic time scale s the entire lithosphere responds in elastic brittle elastic regime even though rock mechanics experiments provide important insights into the rheological properties of the lithosphere their conditions e g time scales strain rates temperature and loading conditions are too far from those of real earth therefore these data cannot be reliably extended to geological time and spatial scales strain rates 10 17 to 10 13 s 1 without additional parameterization or validation based on geological time scale observations of large scale deformation for the oceanic lithosphere the goetze and evan s brittle elastic ductile yield strength envelopes yses were validated by geodynamic scale observations such as the observations of plate flexure however oceanic lithosphere behavior in subduction zones and passive continental margins is strongly conditioned by the properties of the continental counterpart whose rheology is less well understood for continents and continental margins the uncertainties of available data sources are greater due to the complex structure and history of continental plates for example in a common continental rheology model dubbed jelly sandwich the strength mainly resides in crust and mantle while in some alternative models the mantle is weak and the strength is limited to the upper crust we address the problems related to lithosphere rheology and mechanics by first reviewing the rock mechanics data te flexure and ts earthquake data and long term observations such as folding and subsidence data and then by examining the physical plausibility of various rheological models for the latter we review the results of thermo mechanical numerical experiments aimed at testing the possible tectonic implications of different rheology models in particular it appears that irrespective of the actual crustal strength the models implying weak mantle are unable to explain either the persistence of mountain ranges for long periods of time or the integrity of the subducting slabs although there is certainly no single rheology model for continents the jelly sandwich is a useful first order model with which to parameterize the long term strength of the lithosphere it is concluded that dry olivine rheology laws seem to represent well the long term behavior of mantle lithosphere in oceans margins and continents as to the continent and margin crust rheology analysis of the results of thermo mechanical models and of te data based on the most robust variants of flexural models suggests that continental plates with te 30 50 smaller than their theoretical mechanical thickness hm i e te 20 60 km should be characterized by a weak lower or intermediate crustal rheology enabling mechanical decoupling between crust and mantle older plates such as cratons are strong due to crust mantle coupling and specific properties of the cratonic mantle lithosphere

the precipitating cloud population of the madden julian oscillation over the indian and west pacific oceans 1 the variability of the precipitating cloud population of the madden julian oscillation mjo is represented by statistics of echo features seen by the tropical rainfall measuring mission s precipitation radar over the central indian and west pacific oceans echo features include isolated shallow echoes deep convective cores wide convective cores and broad stratiform regions isolated shallow echoes are ever present but most numerous during suppressed mjo phases broad stratiform regions dominate variability in areal coverage and maximize during active phases deep convective and wide convective cores are more common and variable in number than broad stratiform regions the magnitude of variability is similar in both regions in the central indian ocean active mjo phases have synchronous maximization of deep convective entities in the west pacific broad stratiform regions maximize prior to wide convective cores reanalysis indicates that isolated shallow echoes are most numerous in dry mid tropospheric conditions and strong low level 1000 750 hpa shear mid tropospheric moisture increases before deeper convective features increase in number maximizes as deep convective features maximize and decreases as wide convective cores and broad stratiform regions decline in population active stage deep and wide convective cores occur preferentially with moist mid tropospheric conditions and strong low level shear acute shear may favor downdraft momentum transport and consequently more robust gust front convective triggering broad stratiform regions maximize with a moist mid troposphere strong low level shear and moderate upper level 750 500 hpa shear that is not so strong that the stratiform region disconnects from its convective moisture source

new 40ar 39ar ages from the kalatag district in the eastern tianshan nw china constraints on the timing of cu mineralization and stratigraphy abstract the kalatag region in the eastern tianshan of nw china contains a number of economically important cu deposits among these the south meiling vms deposit is hosted in the late ordovician to early silurian dacitic tuff belonging to the daliugou formation the deposit is composed of two domains a concordant massive sulfide ore domain and a discordant vein type ore domain located in the footwall phyllic alteration zone here we report new 40 ar 39 ar ages on sericite from the phyllic alteration zone to better constrain the timing and genesis of mineralization the sericite displays markedly flat 40 ar 39 ar age spectra yielding a 40 ar 39 ar plateau age of 438 8 3 0 ma and an isochron age of 443 0 11 0 ma the 40 ar 39 ar age is in good agreement with the re os age reported earlier placing the timing of cu zn mineralization at 434 2 3 9 ma the age is also consistent with the field relationships that concordant massive sulfide orebody formed later than discordant vein type orebody another deposit at hongshi is hosted in basaltic andesites of the daliugou formation the orebodies cut the phyllic alteration zone indicating that the cu mineralization formed later than the phyllic alteration hydrothermal sericite from the phyllic alteration zone from this deposit yielded flat 40 ar 39 ar age spectra with 40 ar 39 ar plateau ages of 441 5 3 2 ma and 446 4 4 6 ma and isochron ages of 445 8 5 6 ma and 448 8 7 3 ma respectively this age is obviously older than the re os age which is also in conformity with the field relations that quartz sulfide veins cut the sericitization zone our 40 ar 39 ar ages offer precise constraints on early paleozoic cu mineralization in the eastern tianshan orogen these results indicate that the lower unit basaltic andesites of the daliugou formation formed earlier than 446 ma we thus consider that the daliugou formation formed during middle ordovician to early silurian we also propose several metallogenic events in the kalatag area including 1 an early vms cu zn and hydrothermal vein cu mineralization at 440 430 ma 2 porphyry cu and skarn fe mineralization at 390 380 ma and 3 a late cu au mineralization at ca 300 ma these three metallogenic events were coeval with the distinct magmatic pulses in the kalatag area

the diurnal cycle of precipitation over the maritime continent in a high resolution atmospheric model climate models can exhibit systematic errors in their mean precipitation over the maritime continent of the indonesian archipelago at the heart of the tropical warm pool these can often be traced back to an erroneous simulation of the diurnal cycle and can lead to errors in global climate through planetary wave propagation here we examine the simulation of the diurnal cycle over the maritime continent in a series of high resolution integrations of the uk met office atmospheric model with horizontal resolutions of 40 and 12 km where the convection is parametrised and 4 km where the convection is explicitly resolved as part of the cascade project in these models the vertical heating profile over the islands changes from a convective profile with a mid tropospheric maximum in the early afternoon to a more stratiform profile with upper tropospheric heating and mid tropospheric cooling later the convective heating profile forces a first internal mode gravity wave that propagates rapidly offshore the deep warm anomalies behind its downwelling wavefront suppress convection offshore during early afternoon the stratiform heating profile forces a gravity wave with a higher order vertical mode that propagates slowly offshore later in the afternoon this mode has a negative destabilising temperature anomaly in the mid troposphere together with the convergence zone between the wave fronts of the two modes favourable conditions are created for offshore convection in the 4 km explicit convection model the offshore convection responds strongly to this gravity wave forcing in agreement with observations supporting a gravity wave convection paradigm for the diurnal cycle over the maritime continent however the convective response in the lower resolution models is much less coherent leading to errors in the diurnal cycle and mean precipitation hence to improve climate model simulations sensitivity to gravity wave forcing should be a factor in future convective parametrisation schemes copyright c 2011 royal meteorological society

thermal and exhumation history of the coastal cordillera arc of northern chile revealed by thermochronological dating abstract the thermal and erosional history of convergent plate boundaries is important for understanding the links between subduction arc magmatism genesis of ore deposits topography and climate of orogenic belts unlike the continent continent collision that formed many of the largest orogenic belts known today the central andes of south america is a unique case where an oceanic continent collision has given rise to the earth s longest and second tallest orogenic belt over the last thirty years a plethora of models have been suggested in an attempt to explain how a plateau type orogen formed at the leading edge of western south america in the central andes most research have focussed attention on the study of the evolution of the arc and backarc since continuous subduction erosion of the forearc has left little trace of the interplate dynamics that initiated the orogenic belt in this article we present a new insight into the thermal and exhumation history of the forearc along the coastal cordillera of northern chile based on biotite k ar apatite fission track and apatite zircon u th he dating we collected diorite samples in a 2 km thick crustal section at the coastal cliff 22 s and a sea level isoelevation profile between 21 and 27 s results from all three dating methods show that the cooling of coastal cordillera took place shortly after emplacement during a period of rifting in jurassic times cooling took place in two episodes mainly in late jurassic early cretaceous 118 152 ma but also during late cretaceous 60 80 ma due to the resumption of compression rift closure arc uplift exhumation eastward migration of magmatic arc activity and thermal relaxation the youngest apatite u th he ages reveal a cooling event never reported previously between 40 and 50 ma eocene this thermal event affected a 500 km long and 1 km thick section of the coastal cordillera in northern chile rock cooling recorded in the eocene cannot be explained by the thermal effect of the magmatic arc we explore two scenarios that can provide an explanation for the observed cooling 1 forearc uplift and exhumation and 2 changes in plate subduction dynamics erosion rates of 0 24 to 0 36 km myr for a period of 10 myr are necessary to explain the cooling event alternatively the subduction of a ridge could explain significant cooling by flat slab dewatering based on recent sea floor spreading reconstruction we suggest the subduction of the farallon phoenix ridge as a possible candidate the subduction of this ridge can account for the cooling event at 40 50 ma but also for some important aspects of the eocene development of the central andes notably the onset of uplift an abrupt decrease in magmatic activity and porphyry copper mineralization

40ar 39ar dating of micas from the east kemptville tin deposit yarmouth county nova scotia the east kemptville tin deposit in nova scotia the largest known tin deposit in north america lies in a greisen zone within the davis lake pluton generally considered to be part of the devonian south mountain batholith our dating of micas from within the deposit suggests that the greisenization process that accompanied mineralization took place about 295 5 ma ago that is ca 60 ma after the emplacement of the batholith hydrothermal alteration mineralization activity in southern nova scotia coincided with extensive shearing and tectonism throughout the hercynian orogen the east kemptville deposit appears to be approximately coeval with similar mineralization in southwest england the iberian peninsula and northwest africa and much younger than the devonian acadian orogeny

a numerical investigation of the organization and interaction of the convective and stratiform regions of tropical squall lines abstract a set of 13 two dimensional numerical simulations based on the 22 and 23 june soundings from the convectionprofonde tropicale in 1981 copt81 experiment in west africa is used to study the organization and interactionof the convective and stratiform regions of squall line type convective systems the initial wind profiles arecharacterized by the african easterly jet aej and the tropical easterly jet tej located at about 3 5 km and14 km respectively the physical processes that generate and maintain the mesoscale inflow at the rear of squall line type mesoscaleconvective systems are thereby examined horizontal potential temperature gradients generated by a combinationof latent heat release in the convective region and unsaturated mesoscale descent both modulated by evaporation cause a horizontal pressure gradient and generate horizontal line parallel vorticity the rear inflow is a consequenceof these processes the convective activity induces a significant upscale influence ahead of the

seasonality and meridional propagation of the mjo abstract the madden julian oscillation mjo is known to have a substantial impact on the variability of the asian australian summer monsoons an important but not well understood aspect of the mjo monsoon connection is the meridional propagation of bands of enhanced or reduced precipitation that are especially pronounced during the northern summer in this study the nature of the seasonality of the mjo is examined with a focus on the meridional propagation using both observations and simulations with an atmospheric general circulation model agcm a key result is that the agcm when forced with idealized eastward propagating equatorial dipole heating anomalies reproduces the salient features of the observed seasonality in the precipitation and wind fields associated with the mjo including meridional propagation into the indian and australian summer monsoon regions an analysis of the simulations and observations shows that the off equatorial precipitation anomalies are initiated by surface fricti

organization of convection within the madden julian oscillation convection associated with the madden julian oscillation mjo is organized over a broad range of time and space scales within a large scale envelope of activity the large scale envelope of convection is characterized as an eastward propagating disturbance predominantly of zonal wavenumber 2 which is confined over the warmest waters of the indian and western pacific oceans large scale convection is shifted toward the disturbance s temperature anomaly which appears to result from frictional effects in the boundary layer synoptic and mesoscale convective activity is enhanced within the wet phase of the large scale envelope the increase at synoptic scales occurs uniformly over all scales but little difference in the character of the variability is observed between dry and wet phases the enhanced mesoscale activity within the wet phases exhibits a marked tendency to be organized into westward propagating clusters with periods near 2 days and half wavelengths of about 1000 km the role of these modes for the evolution of the mjo is discussed

seasonality in the madden julian oscillation abstract this study using observational data and global reanalysis products synthesizes existing knowledge on the seasonality in the madden julian oscillation mjo and discusses its possible dependence on the mean background state the seasonality in the mjo is documented in terms of its components in low level zonal wind and precipitation a single peak season boreal winter exists near the equator in the indian and western pacific oceans for the broad tropical region however the seasonality in the mjo is featured by a latitudinal migration across the equator between two peak seasons the primary peak season is in boreal winter december march during which mjo signals are mainly confined to the indian and western pacific ocean and reach their maxima in the south pacific convergence zone the secondary peak season is boreal summer june september during which the strongest mjo occurs north of the equator from the bay of bengal to the south china sea and another separated region of strong mjo

the role of environmental shear and thermodynamic conditions in determining the structure and evolution of mesoscale convective systems during toga coare abstract a collection of case studies is used to elucidate the influence of environmental soundings on the structure and evolution of the convection in the mesoscale convective systems sampled by the turboprop aircraft in the tropical ocean global atmosphere toga coupled ocean atmosphere response experiment coare the soundings were constructed primarily from aircraft data below 5 6 km and primarily from radiosonde data at higher altitudes the well documented role of the vertical shear of the horizontal wind in determining the mesoscale structure of tropical convection is confirmed and extended as noted by earlier investigators nearly all convective bands occurring in environments with appreciable shear below a low level wind maximum are oriented nearly normal to the shear beneath the wind maximum and propagate in the direction of the low level shear at a speed close to the wind maximum when there is appreciable shear at middle levels 800 400 mb convective bands form parallel to the shear with

a unified moisture mode framework for seasonality of the madden julian oscillation abstractthe madden julian oscillation mjo exhibits pronounced seasonality while it is largely characterized by equatorially eastward propagation during the boreal winter mjo convection undergoes marked poleward movement over the asian monsoon region during summer producing a significant modulation of monsoon rainfall in classical mjo theories that seek to interpret the distinct seasonality in mjo propagation features the role of equatorial wave dynamics has been emphasized for its eastward propagation whereas coupling between mjo convection and the mean monsoon flow is considered essential for its northward propagation in this study a unified physical framework based on the moisture mode theory is offered to explain the seasonality in mjo propagation moistening and drying caused by horizontal advection of the lower tropospheric mean moisture by mjo winds which was recently found to be critical for the eastward propagation of the winter mjo is also shown to play a dominant role in operating t

thermal history of the southwestern meguma zone nova scotia from an 40ar 39ar and fission track dating study of intrusive rocks two geologically distinct regions within the meguma terrane have been studied in detail and are shown to have contrasting thermal histories this conclusion is based on 31 new 40ar 39ar age spectra on micas k feldspars and hornblendes from several of the plutons in southern nova scotia which are satellites to the much larger south mountain batholith smb in addition we report four k feldspar age spectra from a locality in the northeastern part of the smb and seven fission track ages from both the southern and northern areas the complex age spectrum of hornblende from a mafic phase associated with the bamngton passage pluton suggests a minimum intrusive age of 385 ma a result consistent with geologic evidence that this pluton represents an early less evolved magmatic pulse later intrusive activity appears to have been coeval with the intrusion of the smb at ca 370 ma ago in the southern region argon clocks in feldspars were completely reset and mica clocks variably reset by a later thermal event the cumulative geochronologic and geologic evidence constrains this event to 300 320 ma ago in the smb this alleghanianl hercynian thermal disturbance is much less pronounced and appears to be localized to small areas that are often associated with economic mineralization k feldspars from the southern plutons record an episode of argon loss 220 230 ma ago which is less evident in the smb to the north upper triassic dike injection in the southern region associated with the initial rifting phase of the canadian atlantic margin accompanied this milder thermal pulse apatites in the two regions record a mean fission track age of ca 180 ma which we attribute to the final cooling of the terrane below about 100 c the timing of this event coincides with regional uplift recorded in sedimentary sequences along much of the maritime continental shelf deux kgions gtologiques distinctes inttgkes au terrane de meguma ont ctt ctudites en dctai1 et leurs histoires thermales

continental plate collision p t t z conditions and unstable vs stable plate dynamics insights from thermo mechanical modelling there was much debate recently on the mechanisms of continental convergence and related pressure temperature p t conditions both in modeling and petrologic community depending on the mechanisms of convergence subduction collision folding or rt instability one can argue about the possibility of large scale deviations of pressure and temperature in the accretion prism and below it from reference i e lithostatic conditions commonly used for petrologic reconstructions of evolution of exhumed metamorphic rocks these deviations can be caused for example by tectonic overpressure that as suggested in some studies may be responsible for formation of uhp ultra high pressure rocks however overpressure in exhumation zones can be built only in specific collision scenarios associated with plate coupling pure shear folding in this study we analyze conditions that define various mechanisms of convergence and consequently of exhumation these mechanisms can be represented as a superposition of 1 simple shear subduction 2 pure shear collision 3 folding and 4 rayleigh taylor instability we study these scenarios using a thermo mechanical model that accounts for brittle elastic ductile rheology surface processes and metamorphic changes it appears that stable oceanic type subduction may occur in the case of cold lithospheres tmohob550 c and relatively high convergence rates n3 5 cm yr depending on the lower crustal rheology strong or weak either the whole upper and lower crust or only the lower crust can be involved in subduction in case of weak metamorphic rheologies phase changes improve chances for stable subduction pure shear becomes a dominant mechanism when tmohon550 c or convergence rates are lower than 3 cm yr large scale folding is favored in case of tmoho 500 650 c and is more effective in the case of mechanical coupling between crust and mantle e g strong lower crust gravitational rayleigh taylor instabilities overcome other mechanisms for very high values of tmoho n800 c and may lead to development of subvertical cold spots however it is reasonable to assume that in most cases continental collision is initiated at oceanic subduction rate which is rarely slower than 5 cm yr this rate is sufficient to drive continental subduction during the first several myr of collision in this case the subduction channel is characterized by nearly lithostatic pressure conditions large scale zones of tectonic overpressure may be built outside the channel but do not affect the exhumed rocks overpressure may be built inside the channel in the short moment of its closure we suggest that most continental orogenic belts could have started their formation from continental subduction this evokes small tectonic overpressures and thus deep origin of the uhp rocks that may be brought to the surface via the suggested multi level mechanism of exhumation

thermodynamic and circulation characteristics of winter monsoon tropical mesoscale convection abstract during the december 1978 field phase of the international winter monsoon experiment winter monex a regular diurnal cycle of deep convective activity occurred over the south china sea immediately to the north of borneo the convection was initiated during the late night hours by convergence of the low level northeast monsoon flow with the land breeze along borneo s north coast the precipitation feature that developed was characterized during its mature stage by a nearly continuous stratiform cloud referred to here as a mesoscale anvil of 100 500 km dimension extending from the mid troposphere 5 km to near the tropopause 17 km using rawinsonde data from the sounding network thermodynamic and circulation characteristics of the convection are documented mesoscale upward motion is found to within the anvils with a mcsomc downdraft below producing a warm dry region over a lame area just above the boundary layer air flow relative to the anvil on both its leading and trailing edges i

mesoscale variation in diabatic heating around sumatra and its modulation with the madden julian oscillation abstractdiabatic heating in the maritime continent region is controlled by a unique blend of mesoscale variability associated with steep topography and complex coastlines and intraseasonal variabil

late paleozoic to cenozoic history of the offshore sydney basin atlantic canada the sydney basin covers a large offshore area south of newfoundland with a well exposed outcrop belt on cape breton island the geological history of the poorly known offshore area is interpreted using an industry seismic grid and lithoprobe line 86 5 tied to outcrops and two wells the mid devonian to upper carboniferous permian basin fill is 6 7 km thick and represents three extensional phases with intervening and succeeding compressive phases the mid devonian mcadams lake formation was deposited in a local half graben during early post acadian extension following deformation a suite of early carboniferous extensional basins filled mainly with horton group conglomerates developed on northeast trending and southeast dipping master faults some faults developed along acadian terrane boundaries the windsor group extends over the master faults to onlap basement as a result of thermal sag and visean eustatic rise mid carboniferous deformation linked to the alleghanian orogeny reactivated faults an

k ar and 40ar 39ar geochronology of granitoid rocks from southern nova scotia its bearing on the geological evolution of the meguma zone of the appalachians k ar and or 40ar 39ar apparent ages are reported for 28 samples of biotite and muscovite from nova scotia s south mountain batholith and associated northern satellite stocks in addition 14 mica samples from the southern satellite stocks were dated previous workers using primarily k ar and rb sr techniques have reported a spread of apparent ages ranging from 240 420 ma for these intrusive rocks the present data set is internally consistent and has been calibrated against accepted interlaboratory standards a mean age of 367 ma was obtained for the south mountain batholith and the northern stocks a value consistent with results recently obtained in an rb sr study by clarke and halliday no differences in apparent age among the various phases or clear geographic age distribution patterns were evident satellite stocks in the southern part of the province have in general younger apparent ages clustered in the range 300 320 ma we suggest that in southern areas mica argon systems record the effect

dating mineralization using several isotopic methods an example from the south mountain batholith nova scotia canada abstract granitic plutons in the meguma terrane may be broadly divided into two suites 1 an areally predominant cogenetic suite of peraluminous granitoid rocks tonalite leucogranite and 2 an areally subordinate paraintrusive suite specialized in sn w mo cu f u p zn as which appears to have been produced by fluid interaction with either residual magma or crystallized rock granite related mineralization is generally considered to have developed during terminal stages of magmatic crystallization that lead to metasomatism and hydrothermal alteration previous geochronological data have suggested that the main cogenetic suite was emplaced at 350 375 ma whereas some of the paraintrusive suite is markedly younger 250 350 ma the apparently large time interval between the intrusion of the cogenetic suite and some of the mineralization suggests that they are not coeval and has led to the conclusions that the younger mineralization may be related to later magmatism and or tectonothermal events rocks associated with two mineral occurrences in the south mountain batholith that had previously yielded anomalously young isotopic ages millet brook and westfield have been analyzed for u pb and ar isotopes in an attempt to resolve this dilemma new geochronological data from the millet brook area indicate that intrusion of granodiorite at 377 5 3 ma nearly concordant monazite age was closely followed by intrusion of most other phases all of which had cooled through 375 c closure temperature for ar in muscovite by 377 373 6 ma 40 ar 39 ar plateau ages new data from westfield indicate that a paraintrusive porphyry was intruded at 385 2 ma upper intercept of a chord through four nearly concordant zircon fractions and that greisen muscovite cooled through the closure temperature by 368 2 6 ma 40 ar 39 ar plateau age these data indicate that a previously reported rb sr whole rock isochron age of 270 ma at westfield reflect isotopic resetting that occurred below the blocking temperature for 40 ar 39 ar in muscovite 375 c clearly further rb sr analysis is required to document this process other 250 300 ma ages in the meguma terrane are recorded along the tobeatic fault zone away from the fault ages increase upto 370 ma suggesting that the fault may have acted as an important pathway for fluids and heat that facilitated isotopic resetting rb sr whole rock and mineral systems may be affected after formation of an alteration assemblage associated with mineralization by the migration of rb and its concentration in secondary minerals

thermochronometer record of central andean plateau growth bolivia 19 5 s 1 quantifying the timing magnitude and rates of exhumation and deformation across the central andes is a prerequisite for understanding the history of plateau rise we present 23 new apatite and zircon fission track thermochronometer samples to chronicle the exhumation and deformation across the entire 500 km andean fold thrust belt at 19 5 s in bolivia exhumation and deformation are constrained with inverse thermal modeling of the thermochronometer data regional stratigraphy geothermal gradients and mass deficits inferred from a balanced section results suggest the following 1 initial exhumation of the eastern cordillera ec fore thrust and back thrust belts began in the late eocene to early oligocene 27 36 ma and continued in a distributed manner in the late oligocene to early miocene 19 25 ma interandean zone ia exhumation began 19 22 ma followed by a third pulse of exhumation 11 16 ma in the ec back thrust belt and initial cooling in the westernmost subandes sa 8 20 ma finally exhumation propagated eastward across the sa during the late mio pliocene 2 8 ma 2 exhumation magnitudes are spatially variable and range from maximums of 8 km in the ec fore thrust belt to average values of 4 7 km across the ec 2 5 3 km in the altiplano 4 6 km in the ia and 3 km in the sa 3 exhumation rates range from 0 1 to 0 2 mm a in the ec from 0 1 to 0 6 mm a in the ia and from 0 1 to 0 4 mm a to locally 1 4 mm a or more in the eastern sa we synthesize similar constraints with sediments throughout bolivia and characterize plateau development by 1 distributed deformation throughout the altiplano and ec regions from 20 to 40 ma with minor deformation continuing until 10 ma 2 contemporaneous cessation of most ec deformation and exhumation of the ia 20 ma implying establishment of the modern plateau width with significant but unknown crustal thickness and elevation shortly thereafter by 15 20 ma and 3 dominantly eastward propagation of deformation from the ia since 20 ma with minor out of sequence deformation in the central to eastern sa

numerical modeling of gravity wave generation by deep tropical convection although convective clouds are known to generate internal gravity waves the mechanisms responsible are not well understood the present study seeks to clarify the dynamics of wave generation using a high resolution numerical model of deep convection over the tiwi islands australia the numerical calculations presented explicitly resolve both the mesoscale convective cloud cluster and the gravity waves generated as the convective clouds evolve they excite gravity waves which are prominent features of the model solutions in both the troposphere and stratosphere the source location is variable in time and space but is related to the development of individual convective cells the largest amplitude gravity waves are generated when the cloud tops reach the upper troposphere a new analysis technique is introduced in which the nonlinear terms in the governing equations are taken as the forcing for linear gravity waves the analysis shows that in the present calculation neither the shear nor the diabatic heating are the dominant forcing terms instead the wave source is most easily understood when viewed in a frame of reference moving with the wind at the level of neutral buoyancy whereupon the source may be described as a vertically oriented oscillating convective updraft this description is consistent with the properties of the modeled stratospheric waves

deformation of upper carboniferous coal measures in the sydney basin evidence for late alleghanian tectonism in atlantic canada upper carboniferous stephanian coal measures of the sydney basin are deformed by reverse faults and folds with dips of up to 50 on the limbs these structures represent compressional or transpressional events donkin episode of unknown sequence and timing offshore swath bathymetry data indicate that northwest striking reverse faults are folded by prominent northeast trending folds vitrinite reflectance patterns of oriented coal samples from the fold limbs are uniaxial negative with minimum reflectance axes rmin near normal to bedding and fanning across fold axes this pattern implies that the bulk of coalification took place through burial compaction without directed horizontal stress prior to the deformation that produced the folds slight divergence between rmin axes and poles to bedding may reflect slight tilting associated with differential subsidence or early reverse faulting before or during coalification or may lie within analytical error one moderately biaxial sample suggests that some deformation accompanied coalification locally available apatite fission track data for the sydney basin indicate that cooling of the coal measures to below 100 20 c was underway by the middle to late triassic or earlier because cooling implies that coalification was largely complete the fission track results are compatible with latest paleozoic or early mesozoic deformation we infer that the donkin episode represents late alleghanian permian compression within the appalachian orogen if so the sydney structures are manifestations of a little known tectonic episode in atlantic canada however mesozoic cenozoic deformation cannot be ruled out resume les series houilleres du bassin de sydney carbonifere superieur stephanien sont deformees par des failles inverses et des plis deverses dont les flancs atteignent des pendages de 50o ces structures representent des evenements de compression ou de transpression episode donkin dont la sequence et la synchronisation sont inconnues les donnees de bathymetrie par secteurs au large indiquent que les failles inverses a direction nord ouest sont pliees par des plis proeminents a tendance nord est les patrons du facteur de reflexion de la vitrinite provenant d echantillons de charbon orientes preleves sur les flancs des plis sont uniaxes negatifs avec des axes de reflectance minimum rmin presque perpendiculaires au litage et se dispersant en travers des axes de plis ce patron implique que la plus grande part de la carbonisation a eu lieu par compaction d enfouissement sans contrainte horizontale directe avant la deformation qui a produit les plis la legere divergence entre les axes rmin et les poles du litage pourrait refleter un leger basculement associe a une subsidence differentielle ou a des failles inverses precoces avant ou durant la carbonisation ou encore cette divergence pourrait etre a l interieur de la fourchette de l erreur analytique notre echantillon moderement biaxe suggere qu une certaine deformation ait localement accompagne la carbonisation pour le bassin de sydney les donnees disponibles de datation par traces de fission sur l apatite indiquent que le refroidissement des series houilleres a des temperatures inferieures a 100 20 oc etait deja en cours au trias moyen a tardif ou plus tot puisque le refroidissement implique que la carbonisation etait presque terminee les resultats de datation par traces de fission concordent avec la deformation au paleozoique terminal ou au mesozoique precoce nous concluons que l episode donkin represente une compression a l alleghanien tardif permien a l interieur de l orogene des appalaches si tel est le cas les structures de sydney sont des evidences d un episode tectonique peu connu au canada atlantique cependant une deformation au mesozoique cenozoique ne peut etre rejetee d emblee

quantifying canyon incision and andean plateau surface uplift southwest peru a thermochronometer and numerical modeling approach incision we quantify the timing and magnitude of incision by integrating previously published ages from the valley bottom with 19 new sample ages from four valley wall transects interpretation of the incision history from cooling ages is complicated by a southwest to northeast increase in temperatures at the base of the crust due to subduction and volcanism furthermore the large magnitude of incision leads to additional threedimensional variations in the thermal field we address these complications with finite element thermal and thermochronometer age prediction models to quantify the range of topographic evolution scenarios consistent with observed cooling ages comparison of 275 model simulations to observed cooling ages and regional heat flow determinations identify a best fit history with 0 2 km of incision in the forearc region prior to 14 ma and up to 3 0 km of incision starting between 7 and 11 ma incision starting at 7 ma requires incision to end by 5 5 to 6 ma however a 2 2 ma age on a volcanic flow on the current valley floor and 5 ma gravels on the uplifted piedmont surface together suggest that incision ended during the time span between 2 2 and 5 ma these additional constraints for incision end time lead to a range of best fit incision onset times between 8 and 11 ma which must coincide with or postdate surface uplift

collision of continental corner from 3 d numerical modeling abstract continental collision has been extensively investigated with 2 d numerical models assuming infinitely wide plates or insignificant along strike deformation in the third dimension however the corners of natural collision zones normally have structural characteristics that differ from linear parts of mountain belt we conducted 3 d high resolution numerical simulations to study the dynamics of a continental corner lateral continental oceanic transition zone during subduction collision the results demonstrate different modes between the oceanic subduction side continuous subduction and retreating trench and the continental collision side slab break off and topography uplift slab break off occurs at a depth 100 km to 300 km that depends on the convergence velocity the numerical models produce lateral extrusion of the overriding crust from the collisional side to the subduction side which is also a phenomenon recognized around natural collision of continental corners for instance around the western corner of the arabia asia collision zone and around the eastern corner of the india asia collision zone modeling results also indicate that extrusion tectonics may be driven both from above by the topography and gravitational potentials and from below by the trench retreat and asthenospheric mantle return flow which supports the link between deep mantle dynamics and shallower crustal deformation

plutonic and hydrothermal events in the ackley granite southeast newfoundland as indicated by total fusion 40ar 39ar geochronology results of total fusion 40ar 39ar dating eight biotites four muscovites and one hornblende of magmatic and hydrothermal stages of the high silica chemically zoned ackley granite indicate three distinct episodes of magmatic activity viz 410 378 374 and 355 ma and that the metallogenic system w sn mo evolved synchronously with only one of the magmatic phases the oldest suite is indicated by two biotite dates at 410 4 4 4 and 392 5 7 6 ma which are themselves considered as representing partial resetting related to a tectonothermal event of acadian age thus the 410 ma age offers the best estimate of intrusion although it must be considered a minimum the main magmatic pulse for the ackley granite is indicated by dates between 378 and 374 ma with most of these phases occurring in the southern part of the complex concordant hornblende biotite ages 374 8 3 8 and 372 3 4 8 ma respectively for one of the southern phases rencontre lake granite suggests rapid cooling for this part o

evolution of the population of precipitating convective systems over the equatorial indian ocean in active phases of the madden julian oscillation three dimensional radar reflectivity fields from a dual wavelength doppler polarimetric radar s polka deployed in the equatorialindian ocean are used to evaluate the composition of the populationof convective cloud elements during active phases of the mjo rainfall in active periods was intermittent occurring in 11 episodesofabout 2 4days separated byseveralnonrainydays dataforthese 2 dayperiodswerecomposited relative to the time of maximum rainfall analysis of the s polka data shows the makeup of the convective population during the rainfall episodes four types of echo structures were analyzed statistically for the 11 rainfallepisodes shallowconvectiveechoes sce deepconvectivecores dcc wideconvectiveechocores wcc and broad stratiform bsr echo regions sce and dcc events were most frequent before the maximum rainfall with the peak frequency of sce leading that of dccs wccs were most frequent during therainfallmaximum and bsr regionswere most frequentinthe later part oftherainfallepisode composited ecmwf interim re analysis era interim data and 3 hourly atmospheric soundings indicate that the 2 4 day episodes were related to the passage of equatorial waves in the early part of a rainfall episode the wave passageconditionswereunstable favoringdeeppenetratingconvectiveelements whileinthelaterperiod the wave divergence profile was commensurate with convective systems in late anvil producing stages these results support the stretched building block notion of the life cycle of tropical convection and confirm satellitebased interpretations of sce dcc wcc and bsr statistics in the composition of the convective population

why is terrestrial subduction one sided subduction of the lithosphere at convergent plate boundaries takes place asymmetrically the subducted slab sinks downward while the overriding plate moves horizontally one sided subduction in contrast global mantle convection models generally predict downwelling of both plates at convergent margins two sided subduction we carried out two dimensional 2 d numerical experiments with a mineralogical thermomechanical viscoelastic plastic model to elucidate the cause of one sided subduction our experiments show that the stability intensity and mode of subduction depend mainly on slab strength and the amount of weak hydrated rocks present above the slab two sided subduction occurs at low slab strength sin 0 15 the weak interface is maintained by the release of fluids from the subducted oceanic crust as a consequence of metamorphism the resulting weak interplate zone localizes deformation at the interface and decouples the strong plates facilitating asymmetric plate movement our work suggests that high plate strength and the presence of water are major factors controlling the style of plate tectonics driven by self sustaining one sided subduction processes

simulation of the madden julian oscillation in the ncar ccm3 using a revised zhang mcfarlane convection parameterization scheme abstract this study presents the simulation of the madden julian oscillation mjo in the ncar ccm3 using a modified zhang mcfarlane convection parameterization scheme it is shown that with the modified scheme the intraseasonal 20 80 day variability in precipitation zonal wind and outgoing longwave radiation olr is enhanced substantially compared to the standard ccm3 simulation using a composite technique based on the empirical orthogonal function eof analysis the paper demonstrates that the simulated mjos are in better agreement with the observations than the standard model in many important aspects the amplitudes of the mjos in 850 mb zonal wind precipitation and olr are comparable to those of the observations and the mjos show clearly eastward propagation from the indian ocean to the pacific in contrast the simulated mjos in the standard ccm3 simulation are weak and have a tendency to propagate westward in the indian ocean nevertheless there remain several deficiencies that are yet

simulation modeling and dynamically based parameterization of organized tropical convection for global climate models abstracta new approach for treating organized convection in global climate models gcms referred to as multiscale coherent structure parameterization mcsp introduces physical and dynamical effects of organized convection that are missing from contemporary parameterizations the effects of vertical shear are approximated by a nonlinear slantwise overturning model based on lagrangian conservation principles simulation of the april 2009 madden julian oscillation event during the year of tropical convection yotc over the indian ocean using the weather research and forecasting wrf model at 1 3 km grid spacing identifies self similar properties for squall lines mcss and superclusters embedded in equatorial waves the slantwise overturning model approximates this observed self similarity the large scale effects of mcsp are examined in two categories of gcm first large scale convective systems simulated in an aquaplanet model are approximated by slantwise overturning with attention to convective mome

feeding the aneurysm orogen parallel mass transport into nanga parbat and the western himalayan syntaxis the nanga parbat haramosh massif nphm western himalayan syntaxis requires an influx of mass exceeding that in the adjacent himalayan arc to sustain high topography and rapid erosional exhumation rates what supplies this mass flux and feeds this tectonic aneurysm we show using a simple 3 d model of oblique orogen convergence that velocity strain partitioning results in horizontal orogen parallel op crustal transport and the same behavior is inferred for the himalaya with op transport diverting converging crust toward the syntaxis model results also show that the op flow rate decreases in the syntaxis thereby thickening the crust and forming a structure like the nphm the additional crustal thickening over and above that elsewhere in the himalayan arc sustains the rapid exhumation of this aneurysm normally velocity strain partitioning would be minimal for the himalayan arc where the convergence obliquity is no greater than 40 however we show analytically that the himalayan system can act both as a critical wedge and exhibit strain partitioning if both the detachment beneath the wedge and the bounding rear shear zone which accommodates op transport are very weak corresponding numerical results confirm this requirement and demonstrate that a nanga parbat type shortening structure can develop spontaneously if the orogenic wedge and bounding rear shear zone can strain rate soften while active these results lead us to question whether the position of nphm aneurysm is localized by river incision as previously suggested or by a priori focused tectonic shortening of the crust in the syntaxis region as demonstrated by our models

on the gravitational potential of the earth s lithosphere the mean potential energy of the lithosphere is useful for defining the tectonic reference state trs of the earth and can be used to constrain the ambient state of stress in the plates in the absence of external forces applied at the base or along plate boundaries a lithospheric column with the potential energy of the trs would remain undeformed thus the difference between the potential energy of a lithospheric column and the trs determines whether the column is in an extensional joeutral or compressional state of stress we evaluate and intraplate variations about this mean using a simple first order lithospheric density model this model assumed that the continental geotherm is linear and density variations below a depth of 125 km have negligible influence on and is consistent with observed geoid anomalies across continental margins is estimated to be 2 379 1014 n m 1 which is equivalent to the potential energy of both near sea level continental lithosphere 160 to 220 m for an assumed crustal density c in the range 2800 2700 kg m 3 and cooling oceanic lithosphere at a depth of 4 3 km with the exception of eurasia which has anomalously high mean potential energy 2 383 1014 n m 1 the mean potential energies of the continental plates are nearly identical to the global mean the mean potential of the oceanic plates was found to be a strong function of the mean age of the oceanic lithosphere both the global and plate mean potential energies are relatively insensitive to a wide range in c the potential of the mid ocean ridges 2 391 1014 n m 1 is greater than the global mean which is consistent with the divergent nature of the ridges elevated continental lithosphere with a height of about 70 m has an equivalent potential energy to suggesting that in the absence of external forces continental regions will be in a slightly extensional state of stress the importance of our potential energy formulation is substantiated by the strong correlation between the torque poles associated with the potential energy distributions and the observed plate velocity poles for the south american nazca indo australian and pacific plates

convectively coupled equatorial waves within the mjo during cindy dynamo slow kelvin waves as building blocks this study examines the relationship between the mjo and convectively coupled equatorial waves ccews during the cindy2011 dynamo field campaign using satellite borne infrared radiation data in order to better understand the interaction between convection and the large scale circulation the spatio temporal wavelet transform stwt enables us to document the convective signals within the mjo envelope in terms of ccews in great detail through localization of space time spectra at any given location and time three mjo events that occurred in october november and december 2011 are examined it is in general difficult to find universal relationships between the mjo and ccews implying that mjos are diverse in terms of the types of disturbances that make up its convective envelope however it is found in all mjo events that the major convective body of the mjo is made up mainly by slow convectively coupled kelvin waves these kelvin waves have relatively fast phase speeds of 10 13 m s 1 outside of and slow phase speeds of 8 9 m s 1 within the mjo sometimes even slower eastward propagating signals with 3 5 m s 1 phase speed show up within the mjo which as well as the slow kelvin waves appear to comprise major building blocks of the mjo it is also suggested that these eastward propagating waves often occur coincident with n 1 wig waves which is consistent with the schematic model from nakazawa in 1988 some practical aspects that facilitate use of the stwt are also elaborated upon and discussed

late paleozoic collision delamination short lived magmatism and rapid denudation in the meguma terrane nova scotia canada constraints from 40ar 39ar isotopic data the meguma terrane consists of 8 15 km of early paleozoic stratified rocks deposited on a continental basement that were deformed and metamorphosed during the late paleozoic as a result of lithospheric plate collision the oldest cleavage previously published whole rock 40ar 39ar plateau ages of 415 395 ma dates the onset of crustal thickening which was followed by voluminous but short lived late devonian granitic and minor mafic magmatism 380 370 ma this magmatism may have been the product of delamination of the lower lithosphere and upwelling of asthenosphere which effected melting above the new moho and resulted in intrusion at depths of 5 12 km 40ar 39ar plateau ages of hornblende muscovite and biotite indicate that at the present erosion level most of the meguma terrane cooled through 300 c by 368 360 ma slightly earlier than for the southwestern meguma terrane ca 345 ma the present erosion level was exhumed by the latest devonian early carboniferous visean the age of the

geochronological evidence for multiple tectono thermal overprinting events in the east kemptville muscovite topaz leucogranite yarmouth county nova scotia canada the east kemptville muscovite topaz leucogranite located in yarmouth county nova scotia canada is host rock to the only producing primary tin deposit in north america 56 mt 0 165 sn previous geochronological studies include i rb sr whole rock analyses of the quartz topaz greisens which indicated a date of 337 5 ma and ii 40ar 39ar analyses of greisen muscovite which indicated apparent plateau dates of ca 300 ma however the pervasive development of deformational fabrics at east kemptville suggests that both the rb sr whole rock and 40ar 39ar muscovite ages are at best minimum estimates for the inferred time of mineralization in the present study rb sr whole rock and mineral muscovite plagioclase k feldspar analyses and 40ar 39ar incremental step heating of a muscovite separate indicate the following i diffusion of sr on the whole rock scale terminated at 344 5 ma 11 point isochron date coincident with closure of muscovite to intracrystalline diffusion of ar apparent plat

the boreal summer madden julian oscillation and moist convective morphology over the maritime continent abstractwhile the boreal summer madden julian oscillation mjo is commonly defined as a planetary scale disturbance the convective elements that constitute its cloud dipole exhibit pronounced var

clouds associated with the madden julian oscillation a new perspective from cloudsat abstractthe evolution of total cloud cover and cloud types is composited across the madden julian oscillation mjo using cloudsat data for june 2006 may 2010 two approaches are used to define mjo phases 1 the local phase is determined at each longitude and time from filtered outgoing longwave radiation and 2 the global phase is defined using a popular real time multivariate mjo rmm index which assigns the tropics to an mjo phase each day in terms of local phase cloudsat results show a familiar evolution of cloud type predominance in the growing stages shallow clouds coexist with deep intense but narrow convective systems widespread cloud coverage and rainfall appear during the active phases becoming more anvil dominated with time and finally suppressed conditions return results are compared to the convectively coupled kelvin wave which has a similar life cycle to the mjo convection in the mjo tends to be modulated more by moisture variations compared to the kelvin wave in terms of globa

a model for the boreal summer intraseasonal oscillation abstract the tropical intraseasonal oscillation iso exhibits pronounced seasonality the boreal summer iso is more complex than its winter counterpart due to the coexistence of equatorial eastward off equatorial westward and northward propagating low frequency modes and their interactions based on observational evidence and results obtained from numerical experiments a mechanism is proposed for the boreal summer iso in which the northern hemisphere summer monsoon nhsm circulation and moist static energy distribution play essential roles with a climatological july mean basic state the life cycle of model low frequency waves consists of four processes an equatorial eastward propagation of a coupled kelvin rossby wave packet an emanation of moist rossby waves in the western pacific a westward propagation and amplification of the rossby waves in south asian monsoon regions and a reinitiation of the equatorial disturbances over the central indian ocean the life cycle spans about one month and p

upscale impact of mesoscale convective systems and its parameterization in an idealized gcm for an mjo analog above the equator abstractthe madden julian oscillation mjo typically contains several superclusters and numerous embedded mesoscale convective systems mcss it is hypothesized here that the poorly simulated mjo

extreme variations in strontium initial ratios in ore related fluids magmato hydrothermal cassiterite topaz ore at the carboniferous east kemptville ek greisenhosted tin deposit nova scotia canada is cross cut by veins containing apatite triplite vivianite and fluorite initial 87sr 86sr ratios of these minerals have an extreme range 0 7135 to 0 8284 the initial ratios of the host rocks ek quartz topaz rock and davis lake biotite monzogranite 0 729 0 001 0 727 0 004 are also high the adjacent meguma group metasedimentary rocks are more typical of crustal material 0 712 0 719 at 330 ma rb and sr contents of ek fluorite max 13 0 1420 ppm and apatite max 88 1 6660 ppm are unusually high and variable unexpectedly high sr contents correlate positively with the high initial 87sr 86sr ratios fluorite and phosphate minerals from the first set of post greisen veins at east kemptville reflect the unusual chemistry of a high 87sr 86sr fluid present in the deposit after ore formation the most extreme composition of this fluid was characterized by 87sr 86sr 0 8284 high rb sr high p rb cu zn and fe contents but low abundances of ca pb and sn with respect to the davis lake monzogranite such a fluid could have been derived from the greisen fluid and modified by reaction with the overlying meguma metawacke a second alternative which cannot be well constrained at present is that an extremely radiogenic fluid entered the deposit after ore formation and mixed with the postore fluid in either case the modified fluid subsequently mixed with meteoric water and precipitated the minerals with much lower 87sr 86sr ratios and rb and sr contents