early middle paleozoic intraplate orogeny in the ogcheon belt south korea a new insight on the paleozoic buildup of east asia the polycyclic ogcheon belt of south korea represents the boundary between two precambrian blocks investigation on the timing and kinematics of the block agglomeration is of considerable bearing on the understanding of the constitution of the asian continent we report here structural evidence showing that the earlier tectonic event dates back to early middle paleozoic the ogcheon tectonism or orogeny the ductile piling up of nappes of the ogcheon belt corresponds to an intracontinental orogeny involving the opening and the subsequent closure of an aborted rift and not of a wide oceanic area the nappes contain unequivocal evidence for ductile shearing with a northwest over southeast sense of movement field evidence includes the presence of s1 and s1 2 foliations bearing an extensive stretching lineation perpendicular to the belt associated with a strongly noncoaxial deformation regime the f1 isoclinal folds are commonly a type and sheath folds whose axes are parallel to the stretching lineation whereas the southeast facing f2 recumbent folds have subhorizontal axes parallel to the belt all lines of evidence unconformity superposed deformations age of the metamorphism etc imply that the d1 2 ogcheon tectonism is pre middle carboniferous late silurian early devonian the subsequent indosinian middle triassic f3 upright synfolial folds that trend nne ssw and later structures clearly overprint the middle paleozoic ductile thrusts and isoclinal folds the early paleozoic ogcheon supergroup is formed of a thick volcanosedimentary pile deposited above an early platform sequence in a rift basin on the basis of the geochemical characteristics of the metavolcanics it appears that oceanization did not occur and ogcheon rift aborted soon after it was formed we suggest that middle paleozoic ductile nappe structures have been formed by the structural inversion of the rift extensional features during the ductile stacking episode the early formed foliation and isoclinal folds have been folded in a continuum during the same tectonic event middle carboniferous terrigenous formations have been deposited unconformably upon folded and thrust older rocks a tentative correlation with early paleozoic imjingang qinling and cathaysian belts within the asian continent reveals that ogcheon rift was emplaced within the south china plate and that imjingang belt should represent the limit between north and south china blocks in the korean peninsula

importance of depth and intensity of convection on the isotopic composition of water vapor as seen from iasi and tes δd observations abstract we use tropical observations of the water vapor isotopic composition derived from iasi and tes spaceborne measurements to show that the isotopic composition of water vapor in the free troposphere is sensitive to both the depth and the intensity of convection we find that for any given precipitation intensity vapor associated with deep convection is isotopically depleted relative to vapor associated with shallow convection the intensity of precipitation also plays a role as for any given depth of convection the relative enrichment of water vapor decreases as the intensity of precipitation increases shallow convection via the uplifting of enriched boundary layer air into the free troposphere and the convective detrainment enriches the free troposphere in contrast deep convection is associated with processes that deplete the water vapor in the free troposphere such as rain re evaporation the results of this study allow for a better identification of the parameters controlling the isotopic composition of the free troposphere and indicate that the isotopic composition of water vapor can be used to evaluate the relative contributions of shallow and deep convection in global models

impact craters and craterform structures in fennoscandia abstract sixty two nearly circular topographic morphological or geophysical structures in fennoscandia and estonia were studied to discover their origin the structures were divided into five classes fifteen were caused by meteorite impacts class a and a four of them lappajarvi janisjarvi mien and dellen contain large volumes of impact melt the age of the recognized impact craters varies from pre historic 3500 b c to 700 ma and their diameters from 0 04 km to 55 km the majority of the other structures are probable class b or possible class c craters for which there is not yet sufficient proof of impact origin there is increasing evidence that some of the large arcuate morphological or geophysical structures class e represent the deeply eroded scars of very old craters but owing to the lack of identified shock metamorphic features or impact generated rocks e g allogenic breccias they cannot so far be classified as impact craters so far no craterform structure or ejecta layer of archaean or early proterozoic age has been found in fennoscandia although statistically remnants of ancient cratering events should be found in the fennoscandian shield new ways of searching for these structures are proposed the impact cratering rate in fennoscandia is 2 4 10 14 km 2a 1 n 12 the very small craters are omitted which corresponds to about two events per 100 ma for the last 700 ma this is a minimum estimate and is higher than the global cratering rate

geodynamics of the ogcheon belt south korea abstract the ogcheon belt is a stack of synmetamorphic southeastward verging nappes resulting from the deformation of a volcanosedimentary sequence deposited in an early palaeozoic rift which was affected by polyphase tectonics its pre cretaceous structure results from three main orogenic events the ogcheon late silurian devonian indosinian late permian triassic and yenshanian middle jurassic early cretaceous tectonic phases the external autochthonous or para autochthonous units represent platform zones with shallow water deposits whereas rift facies characterize the internal domain the yeongweol sequence forming an outer shelf on the southeastern margin of the rift displays affinities with the jiangnan basin of south china it is in wrench fault contact with the easternmost duwibong sequence correlated with the north china north korea platform in the internal metamorphic units post cambrian early ordovician pelitic and turbiditic rift metasediments overlie a thin cambrian or cambrian ordovician early platform sequence two subdomains are distinguished the upper units chung ju and pibaryeong units representing the deep axial part of the basin contain distal flyshoids and large olistoliths derived from the northwestern narrow margin the lower units turung san poeun and iwharyeong are related to the wide margin located between the deep rift domain and the stable southeastern platform the latter transitional domain is characterized by thick submarine debris flows and volcanics of alkali or transitional tholeiitic affinity the volcanic activity and mass gravity flow sedimentation are closely related to the crustal thinning and subsequent rifting and tilting lithostratigraphic evidence allows the correlation of the unfossiliferous rift metasediments with the cambrian ordovician platform sequence in one single basin the post middle silurian pre mid carboniferous ogcheon ensialic orogeny resulted in the closure of the rift with relatively minor effects in the platform area it gave rise to an intracontinental fold thrust belt without ophiolites this tectonic event may have been contemporaneous with early palaeozoic tectonism in the quinling belt in central china and in the southeast china fold belt the definitive constitution of the sino korean craton took place before the mid carboniferous and molasse sedimentation was initiated on a single platform located on the western edge of the yakuno oceanic basin of southwest japan in late permian and early triassic times the closure of the oceanic area and the subsequent westward collision of the honshu block are marked in korea by the intracratonic indosinian tectonism since late triassic times predominantly transcurrent movements have occurred along nne ssw faults associated with syntectonic intermontane troughs and important granitization

double and single itczs with and without clouds abstracta major bias in tropical precipitation over the pacific in climate simulations stems from the models tendency to produce two strong distinct intertropical convergence zones itczs too often several mechanisms have been proposed that may contribute to the emergence of two itczs but current theories cannot fully explain the bias this problem is tackled by investigating how the interaction between atmospheric cloud radiative effects acres and the large scale circulation influences the itcz position in an atmospheric general circulation model simulations are performed in an idealized aqua planet setup and the longwave and the shortwave acres are turned off individually or jointly the low level moist static energy mse is shown to be a good predictor of the itcz position therefore a mechanism is proposed that explains the changes in mse and thus itcz position due to acres consistently across simulations the mechanism implies that the itcz moves equatorward if the hadley circulation strengt

geographic variability in the export of moist static energy and vertical motion profiles in the tropical pacific 1 column integrated moist static energy mse budgets were calculated using daily data from three reanalyses 1998 2001 and isccp radiative cooling the mse export by the circulation was separated into vertical and horizontal mse advection components in order to examine how observationally derived vertical motion profiles affect gross moist stability calculations in a 2000 km wide region in the central eastern pacific itcz vertical motions import mse implying a negative gross moist stability horizontal mse advection is negative and of similar magnitude to vertical mse advection in rainy regions geographic differences in export of mse by the vertical circulation are primarily due to differences in the shape of the vertical motion profile in the west pacific warm pool rainy regions mean horizontal convergence extends up to 300 mb while in parts of the pacific itcz where meridional sst gradients are strong the vertical motion profile is bottom heavy with convergence below 800 mb and divergence above on daily timescales the mode of vertical motion variability has little dependence on precipitation rate and is consistent with observed regional differences in the vertical structure of trmm pr reflectivities

idealized simulations of the intertropical convergence zone and its multilevel flows abstract a mesoscale numerical model with an idealized tropical channel environment is used to study the dynamics of intertropical convergence zones itczs and the recently identified shallow return flow srf and midlevel inflow mli four idealized sea surface temperature sst distributions are used a meridionally symmetric sst profile with a sharply peaked sst maximum at the equator a similar profile with the maximum sst shifted off the equator a cosine shaped sst profile with zero gradient at the equator and an idealized sst profile modeled after the observed sst of the eastern pacific the simulations show that both the srf and the mli are robust features of the itcz the prior result that the srf is a sea breeze like response to surface temperature gradients is further supported whereas the mli is caused by the upper level maxima in diabatic heating and vertical motion simulations with the sst maximum at the equator generate long lasting convectively coupled kelvin waves when the sst maxi

responses of tropical deep convection to the qbo cloud resolving simulations abstractobservational studies suggest that the stratospheric quasi biennial oscillation qbo can modulate tropical deep convection the authors use a cloud resolving model with a limited domain representing a convective column in the tropics to study the mechanisms of this modulation the large scale circulation is parameterized using the weak temperature gradient wtg approximation under which the parameterized large scale vertical motion acts to relax the horizontal mean temperature toward a specified reference profile temperature variations typically seen in easterly and westerly phases are imposed in the upper troposphere and lower stratosphere of this reference profile the responses of convection are studied over different sea surface temperatures holding the reference temperature profile fixed this can be thought of as studying the response of convection to the qbo over different relative ssts and also corresponds to different equilibrium precipitation rates in the control simulation the

seasonal and interannual variations of the energy flux equator and itcz part ii zonally varying shifts of the itcz the itcz lies at the ascending branch of the tropical meridional overturning circulation where near surface meridional mass fluxes vanish near the itcz column integrated energy fluxes vanish forming an atmospheric energy flux equator efe this paper extends existing approximations relating the itcz position and efe to the atmospheric energy budget by allowing for zonal variations the resulting relations are tested using reanalysis data for 1979 2014 the zonally varying efe is found as the latitude where the meridional component of the divergent atmospheric energy transport aet vanishes a taylor expansion of the aet around the equator relates the itcz position to derivatives of the aet to a first order the itcz position is proportional to the divergent aet across the equator it is inversely proportional to the local atmospheric net energy input nei that consists of the net energy fluxes at the surface at the top of the atmosphere and zonally across longitudes the first order approximation captures the seasonal migrations of the itcz in the african asian and atlantic sectors in the eastern pacific a third order approximation captures the bifurcation from single to double itcz states that occurs during boreal spring in contrast to linear efe theory during boreal winter in the eastern pacific northward cross equatorial aet goes along with an itcz north of the equator efe and itcz variations driven by enso are characterized by an equatorward poleward shift in the pacific during el nino la nina episodes which are associated with variations in equatorial ocean energy uptake

proportions of convective and stratiform precipitation revealed in water isotope ratios distinguishing convective and stratiform rainfall is key to understanding how the water cycle responds to climate change an analysis of satellite and surface data shows that rain isotope ratios reflect the proportions of these types of rain

magnetic investigations of the west hawk deep bay and clearwater impact structures canada due to the effects of erosion tectonism and burial impact structures are often obscured or destroyed geophysical methods are increasingly being used in detecting the signatures of impact structures while gravity lows associated with impact structures are well understood associated magnetic anomaly lows are not in this study drill cores from three canadian impact structures were analyzed for rock magnetic properties and mineralogy in order to explain the magnetic anomaly lows associated with these structures samples from the drill cores were cut and measured for anisotropy of magnetic susceptibility ams and natural remanent magnetization nrm parameters drill cores from the twin impact craters of the clearwater structure exhibited different nrm characteristics and samples from their respective drill cores were subject to demagnetization by alternating field and thermal techniques the difference noted in their nrm characteristics was attributed to the acquisition of a viscous remanent magnetization vrm at depth in clearwater east at all three structures both magnetic susceptibilities and remanent magnetizations are well below regional values in impact generated breccias melt rocks shocked crystalline rocks and in postimpact sedimentary infill the processes of brecciation alteration shock and infill by nonmagnetic sediments contribute to the development of the magnetic lows however a significant contribution to the observed magnetic anomalies was found by first order forward modelling to arise from basement rocks beneath the impact structures this zone of reduced magnetization may be caused by the partial demagnetization of magnetite by the impact induced transient stress wave traveling away from the point of impact

modeling the isotopic composition of precipitation the physics of the stable isotopes of water is incorporated into a two dimensional kinematic bulk cloud microphysical model the model is run for several idealized classical stratiform and convective storm situations and the resulting isotope ratios of precipitation and water vapor are diagnosed and compared to observations for stratiform snow the model produces low isotope ratios that decrease rapidly poleward of the warm front the lowest isotope ratios occur when the atmosphere is cold and when the vertical velocity attains its maximum value high in the troposphere for stratiform rains the model produces much higher isotope ratios without a significant poleward gradient as a result of isotope exchange between the falling rain and the surrounding vapor isotope ratios of rain are lowest when the melting level is near the ground and isotope exchange is minimized for air mass thunderstorms isotope ratios are uniformly high in warm air no matter what the cloud height unless hail approaches or reaches the ground the model also produces a significant amount effect for rain in which isotope ratios decrease with increasing rainfall totals isotope ratios are particularly low when the rain derives from a recirculation process in which air previously charged by vapor from falling rain subsequently rises under such conditions the model sometimes produces isotope ratios that decrease from the periphery to the core of the precipitation shield it is suggested that this recirculation process is responsible for extraordinarily low isotope ratios observed in some hurricanes and organized thunderstorms the dominant cloud microphysical processes can sometimes be inferred from isotope ratios of precipitation the model produces ice pellets with isotope ratios close to those of rain when the pellets are produced by homogeneous freezing of rain and close to those of snow when the pellets are produced by refreezing of partially melted snow a climatology of isotope values that matches the main features of the observed global data set and of a seven year record of storms at mohonk lake new york is generated by running the model for a wide range of conditions this includes the deuterium excess d d 8 18o for antarctic snows that increases markedly as d falls below 300 and the deuterium deficit observed for rain in warm dry regions

impacts of vertical structure of large scale vertical motion in tropical climate moist static energy framework abstractinteractions between cumulus convection and its large scale environment have been recognized as crucial to the understanding of tropical climate and its variability in this study the moist static energy mse budget is employed to investigate the potential impact of the vertical structure of large scale vertical motion in tropical climate based on results from both reanalysis data and model simulation two domains are selected over the western and eastern pacific with vertical motion profiles that are dominated by top heavy and bottom heavy structures respectively the bottom heavy structure is climatologically associated with more shallow convection while the top heavy structure is related to more deep convection the column integrated vertical mse advection of top heavy vertical motion is positive while that of bottom heavy vertical motion tends to be negative controlling factors responsible for the above vertical mse advection contrast are discussed based on a simple decomposition of the

composite nature of the north china granulite facies belt tectonothermal and geochronological constraints granulite facies rocks are intermittently exposed in a roughly e w trending belt that extends for approximately 2000 km across the north china craton from the helanshan qianlishan wulashan daqingshan guyang and jining complexes in the western block through the huai an hengshan xuanhua and chengde complexes in the trans north china orogen to the jianping western liaoning eastern hebei northern liaoning and southern jilin complexes in the eastern block the belt is generally referred to as the north china granulite facies belt previously interpreted as the lowest part of an obliquely exposed crust of the north china craton recent data indicate that the north china granulite facies belt is not a single terrane instead it represents components of three separate terranes the eastern and western blocks and trans north china orogen each of these units records different metamorphic histories and reflect the complex tectonic evolution of the ncc during the late archean and paleoproterozoic mafic granulites in the eastern block and the yinshan terrane western block underwent medium pressure granulite facies metamorphism at about 2 5 ga with anticlockwise p t paths involving near isobaric cooling following peak metamorphism reflecting an origin related to intrusion and underplating of mantle derived magmas pelitic granulites in the khondalite belt western block underwent medium pressure granulite facies metamorphism at about 2 0 1 9 ga with clockwise p t paths which record the paleoproterozoic amalgamation of the yinshan and ordos terranes to form the western block mafic and pelitic granulites in the trans north china orogen experienced high to medium pressure granulite facies metamorphism at 1 85 ga with clockwise p t paths involving nearly isothermal decompression following peak metamorphism which are in accord with the final collision between the eastern and western blocks to form the north china craton at 1 8 ga the ncgb cannot therefore represent a separate unique terrane instead it reflects the amalgamation of three separate granulite terranes that evolved independently and at different times d 2005 international association for gondwana research published by elsevier b v all rights reserved

late archean sm nd isochron age for mafic ultramafic supracrustal amphibolites from the northeastern sino korean craton china isotopic data published by previous workers document the presence of early archean 3 5 ga metabasaltic enclaves in grey gneisses west of bohai bay eastern hebei province p r c metabasaltic and metakomatiitic amphibolites crop out as lower supracrustal members of the archean section northeast of bohai bay in southern jilin eastern liaoning provinces six analyzed specimens of the latter unit contain more magnesium on analysis averaging approximately 13 vs 7 wt percent mgo but although they exhibit a wider range of ree concentrations they average a similar range of ree abundance patterns la lu 24 7x chondrite as the former 24 9 chondrite new sm nd isotopic and ree analyses combined with available petrologic geologic information indicate that metakomatiitic amphibolites from the sino korean craton northeast of bohai bay are late archean in emplacement age a wholerock sm nd isochron age of 2660 75 2 ma was obtained with nd t 4 4 0 5 as in many parts of the sino korean craton the highly positive nd t value suggests a prior extended history 1 0 ga of depletion of subcontinental mantle sources in some incompatible elements and in lree relative to hree a seventh analyzed sample a mafic inclusion in tonalitic gneiss of unknown age has an unusual chemical composition with strong enrichment in lree sr and zr and severe depletion in ni and cr judging from isotopic data which yield a much younger depleted mantle model age of 1 67 ga it could represent a cognate xenolith in a mid proterozoic tonalitic intrusion shown on present maps as archean in age the protolith might have been an evolved alkali basalt

geophysical research on the kardla impact structure hiiumaa island estonia the 4 km wide and 500 m deep circular kardla impact structure in hiiumaa island estonia of middle ordovician age 455 ma is buried under upper ordovician and quaternary sediments to constrain the geophysical models of the structure petrophysical properties such as magnetic susceptibility natural remanent magnetization nrm density electrical conductivity porosity and p wave velocity were measured on samples of crystalline and sedimentary rocks collected from drill cores in different parts of the structure and the surrounding area the results were used to interpret the central gravity anomaly of 3 mgal and the magnetic anomaly of 100 nt and also the surrounding weak positive anomalies revealed by high precision survey data the unshocked granitic rocks outside the structure have a mean density of 2630 kgm 3 their shocked counterparts have densities of 2400 kgm 3 at a depth of 500 m increasing up to 2550 kgm 3 at a depth of 850 m porosity and electrical conductivity decrease but p wave velocity increases as density increases away from the impact point thus the gradual changes in the physical properties of the rocks as a function of radial distance from the crater centre are consistent with an impact origin for kardla as in many other impact structures the magnetization of the shocked rocks are also clearly lower than those of unshocked target rocks a new geophysical and geological model of the kardla structure is presented based on geophysical field measurements and data on gradual changes in petrophysical parameters of the shocked target and overlying rocks together with structural data from numerous boreholes an important feature of this model is the lack of an observable geophysical signature of the central uplift observed in drillcores

anisotropic magnetic susceptibility in the continental lower crust and its implications for the shape of magnetic anomalies magnetic anisotropy measurements on samples from the lower continental crust were made to test the hypothesis that anisotropy may cause deflection of the peak to trough axes of magnetic anomalies caused by mid to lower crustal sources average anisotropy p for these samples is 1 5 but can be as high as 3 4 felsic granulite facies rocks show the highest anisotropy magnetic sources with p equal to the maximum determined in this study can cause azimuthal rotations of magnetic anomalies by up to about 25 but p lower than or equal to 1 5 causes no significant rotation comparison of the model results to the abnormal shape of some magnetic anomalies in southern italy indicate that these anomalies cannot be related easily to a strong and coherent ams of the source rocks the most probable explanation of such shapes is the existence of a strong remanent magnetization in subsequently rotated source bodies

the vertical structure of the eastern pacific itczs and associated circulation using the trmm precipitation radar and in situ data the atmospheric circulation associated with the eastern pacific single and double itczs particularly its vertical structure is little known due to the sparce observations using precipitation profiles from the tropical rainfall measure mission trmm precipitation radar with approximations to the liquid water and energy budget equations we estimated vertical profiles of latent heating and vertical velocity in the far eastern pacific 95 w 85 w itczs in the 800 730 hpa layer we combined this with eastern pacific investigation of climate campaign epic2001 and other in situ data to produce a preliminary characterization of the meridional vertical circulation we found evidence of a double cell structure in boreal fall between the itcz and the equator with both shallow and upper level peaks in vertical velocity in spring the flow poleward of the two itczs has a single cell structure although around the equator it shows some hints of the double cells reanalysis and satellite based data are shown to be unreliable for describing the vertical structure of the circulation

hematite pyroxene and phyllosilicates on mars implications from oxidized impact melt rocks from manicouagan crater quebec canada visible and near ir reflectivity mossbauer and x ray diffraction data were obtained on powders of impact melt rock from the manicouagan impact crater located in quebec canada the iron mineralogy is dominated by pyroxene for the least oxidized samples and by hematite for the most oxidized samples phyllosilicate smectite contents up to 15 wt were found in some heavily oxidized samples nanophase hematite and or paramagnetic ferric iron is observed in all samples no hydrous ferric oxides e g goethite lepidocrocite and ferrihydrite were detected which implies the alteration occurred above 250 c oxidative alteration is thought to have occurred predominantly during late stage crystallization and subsolidus cooling of the impact melt by invasion of oxidizing vapors and or solutions while the impact melt rocks were still hot the near ir band minimum correlated with the extent of aleration fe3 fetot and ranged from 1000 nm high ca pyroxene to 850 nm bulk well crystalline hematite for least and most oxidized samples respectively intermediate band positions 900 920 nm are attributed to low ca pyroxene and or a composite band from hematite pyroxene assemblages manicouagan data are consistent with previous assignments of hematite and pyroxene to the 850 and 1000 nm bands observed in martian reflectivity spectra manicouagan data also show that possible assignments for intermediate band positions 900 920 nm in martian spectra are pyroxene and or hematite pyroxene assemblages by analogy with impact melt sheets and in agreement with observables for mars oxidative alteration of martian impact melt sheets above 250 c and subsequent erosion could produce rocks and soils with variable proportions of hematite both bulk and nanophase pyroxene and phyllosilicates as iron bearing mineralogies if this process is dominant these phases on mars were formed rapidly at relatively high temperatures on a sporadic basis throughout the history of the planet the manicouagan samples also show that this mineralogical diversity can be accomplished at constant chemical composition which is also indicated for mars from analyses of soil at the two viking landing sites

characterizing the north american monsoon in the community atmosphere model sensitivity to resolution and topography abstractthis work examines the effect of horizontal resolution and topography on the north american monsoon nam in experiments with an atmospheric general circulation model observations are used

fast cooling following a late triassic metamorphic and magmatic pulse implications for the tectonic evolution of the korean collision belt we discuss the evolution of korea in the context of a relatively short lived tectonically induced magmatic and metamorphic pulse that affected large portions of the crust of the peninsula s southern part during the late triassic recent 40ar 39ar single grain laser step heating dates imply a prolonged metamorphic recrystallization between 243 and 220 ma which occurred in distinct phases that were not coeval throughout the peninsula we obtained identical plateau ages between 231 4 0 8 and 228 9 0 8 ma 1 85 95 39ar release on single grains of detrital muscovite from jurassic sandstones gimpo group a literature review shows that the ages of detrital muscovites are identical to 1 concordant 40ar 39ar ages of biotite 228 ma and amphibole 230 ma in amphibolites of the deokjeongri gneiss formation and the weolhyeonri complex pointing to very rapid cooling of 100 150 c ma and 2 231 229 ma muscovite from the low grade metamorphic mid paleozoic turbidites of the taean formation the efficiency of cooling is further underlined by the near coincidence of these 40ar 39ar ages with 243 229 ma average 234 6 ma zircon u pb ages in the gyeonggi massif and the hongseong belt in the literature it is argued that the late triassic magmatic and metamorphic pulse is superimposed on an earlier tectono metamorphic event possibly related to collision indicated by 1 243 237 ma muscovite ages or age components in age spectra and 2 two generations of folds and associated tectonic foliations truncated by 229 5 ma old syenites and earlier mafic dykes the late triassic thermal pulse could have been the result of post collisional delamination of the lower crust and uppermost mantle and or oceanic slab break off which is also suggested by almost coeval widespread mantle sourced mg rich potassic magmatism continuing ductile deformation is shown by mylonitization of late triassic magmatic rocks an 220 ma muscovite age may be related to this

demagnetization signatures of lunar impact craters research at the university of california berkeley was supported by nasa through subcontract lri 99 101 from the lunar research institute

structure of the annual mean equatorial planetary waves in the era 40 reanalyses abstract the three dimensional structure of the annual mean equatorial planetary waves in the 40 yr ecmwf re analysis era 40 is documented the features in the free atmosphere are predominantly equatorially symmetric driven by east west heating gradients the geopotential height and wind perturbations are strongest at or just below the 150 hpa level below the level of maximum amplitude the circulations in the waves are thermally direct with latent heat release in deep convective clouds and radiative cooling in the intervening cloud free regions within the overlying capping layer the wave related circulations are thermally indirect with rising of the coldest air and sinking of air that is less cold at the cold point just above the 100 hpa 17 km level the era 40 annual mean vertical velocity in the equatorial belt ranges up to 3 mm s 1 over the equatorial western pacific during the boreal winter implying diabatic heating rates of up to 3 c day 1 an order of magnitude larger than typical clear

the application of zircon cathodoluminescence imaging th u pb chemistry and u pb ages in interpreting discrete magmatic and high grade metamorphic events in the north china craton at the archean proterozoic boundary abstract two distinct late neoarchean to early paleoproterozoic magmatic events are recognized at the northern margin of the eastern block of the north china craton based on interpretation of zircon structure cathodoluminescence signature th u pb chemistry and age the first event is represented by emplacement of the chaihe trondhjemite and xianjinchang tonalite with magmatic zircon core ages ranging from 2559 2553 7 ma respectively these rocks experienced a subsequent high grade metamorphic event constrained by partial recrystallization of primary magmatic zircon and growth of new zircon rims between 2517 and 2490 ma a second magmatic episode generated by partial melting of crustal rocks during this metamorphic event is represented by the hongshilazi syenogranite with an age of 2502 11 ma the short time interval between initial magmatism and high grade regional metamorphism signifies they were related to the same major tectonothermal event when considered along with evidence of a similar sequence of events at this time 800 km to the south in eastern hebei and the previously established 2500 ma age of granulite facies metamorphism in mafic rocks across the craton it is proposed that a large mantle plume developed beneath the north china craton at the close of the archean

palaeomagnetism of the lappajarvi impact structure western finland abstract palaeomagnetic and rock magnetic results are presented for rocks from the lappajarvi meteorite impact structure western finland oriented samples were collected from the melt rocks in the central island of the crater and of the palaeoproterozoic svecofennian target rocks at the crater rim the natural remanent magnitization nrm of the melt rocks is generally weak and metastable during demagnetization the characteristic nrm component carried by pyrrhotite yields a palaeomagnetic pole latitude 55 4 n longitude 152 6 e dp 6 2 dm 8 7 dg 6 sites suggesting an age of 195 ma for the impact the results from a deep core drilled from the impact melt rock display similar palaeomagnetic directions but with slightly shallower inclinations the palaeomagnetic age differs from the 40ar 3939ar age of 77 ma for the impact assuming that the 40ar 39ar age is correct the discrepancy between palaeomagnetic and radiometric ages is best explained by post impact and post cooling tilting of the melt layer the tilting 15 may be due to isostatic rebounds along joints in the melt impact remanence has also been isolated in a few target rocks at the rim this poorly defined overprint could be a mild shock remanent magnetization superimposed on the 1 9 ga old svecofennian direction this interpretation is supported by the observation that the nrm intensity is enhanced in specimens having the impact overprint the palaeomagnetic results are consistent with earlier interpretations based on geophysical morphological mineralogical and geochemical data that the lappajarvi structure is a complex meteorite impact site less than 200 ma old

crustal development of the hida belt japan evidence from nd sr isotopic and chemical characteristics of igneous and metamorphic rocks abstract chemical and sr nd isotopic investigations were performed on the igneous and metamorphic rocks in the hida belt central japan to clarify their ages and possible sources and to provide constraints on the crustal evolution of the eastern margin of the asian continent igneous rocks of three main stages in the hida belt represented by the late silurian amphibolites first stage the middle carboniferous mafic dikes second stage and the late triassic to early jurassic granite gabbro plutons third stage show alkaline to calc alkaline chemical affinity major and trace element characteristics show that the three stages of igneous rocks were emplaced in a continental margin or continental arc environment the mafic igneous rock suites have relatively high e nd 0 5 to 5 5 and low 87 sr 86 sr 0 7049 to 0 7061 values indicating their origin from upper mantle sources though the third stage granitoids contain recycled crustal components the hida gneisses of clastic origin show two groups of model ages t dm middle proterozoic 1 4 2 2 ga and late proterozoic to early paleozoic 0 55 0 74 ga consistent with geological geochemical and radiometric age data are that the sedimentation and basaltic volcanism occurred in early to middle paleozoic and the protoliths with middle proterozoic model ages are detritals from the proterozoic continental provenances a synthesis of available geological and geochemical data for the basement metamorphic and igneous rocks in eastern asia suggests two possibilities for the paleozoic to early mesozoic history of the hida belt 1 the hida belt was formed at the eastern margin of the north china block with later evolution as a suture zone between the north and south china blocks or 2 it evolved as a part of the east central asian orogenic belt ecaob to the north

the extent of impact induced fracturing from gravity modeling of the granby and tvaren simple craters the granby and tvaren impact structures are two of the 5 recognized early paleozoic impact structures in scandinavia they have a similar diameter of ca 2 km and contain a substantial post impact fill with paleozoic sedimentary rocks the granby structure represents an almost completely preserved impact crater within a paleozoic sedimentary rock sequence only the uppermost part of the raised crater rim might be eroded and was subsequently covered by quaternary sediments the tvaren structure is eroded below the surrounding pre impact paleozoic cover rocks into the underlying crystalline basement and has quaternary sediment fill gravity measurements have been made on land and in the case of the tvaren structure also on sea ice as a basis for modeling of the extent of impact induced fracturing below and around the craters expressed as porosity in the models the contributions to the gravity field from the crater fill materials seawater quaternary sediments and paleozoic sedimentary rocks are accounted for the remaining negative gravity anomaly is related to the volume of impact fractured crystalline rock indicating an outward decreasing intensity of fracturing extending beyond the morphologic crater edge to ca 1 5 km radial distance from the explosion center the remaining porosity in the central part of this sub crater damage zone approaches 15 a model is suggested for the gradual change of density and the equivalent porosity with radial distance within the damage zone around simple craters c 2010 elsevier b v all rights reserved

the global monsoon as seen through the divergent atmospheric circulation a comprehensive description is given of the global monsoon as seen through the large scale overturning in the atmosphere that changes with the seasons and it provides a basis for delimiting the monsoon regions of the world the analysis focuses on the mean annual cycle of the divergent winds and associated vertical motions as given by the monthly mean fields for 1979 93 reanalyses from the national centers for environmental prediction national center for atmospheric research ncep ncar and european centre for medium range weather forecasts ecmwf which are able to reproduce the dominant modes a complex empirical orthogonal function analysis of the divergent circulation brings out two dominant modes with essentially the same vertical structures in all months of the year the first mode which depicts the global monsoon has a simple vertical structure with a maximum in vertical motion at about 400 mb divergence in the upper troposphere that is strongest at 150 mb and decays to zero amplitude above 70 mb and convergence in the lower troposphere with a maximum at 925 mb ecmwf or 850 mb ncep however this mode has a rich three dimensional spatial structure that evolves with the seasons it accounts for 60 of the annual cycle variance of the divergent mass circulation and dominates the hadley circulation as well as three overturning transverse cells these include the pacific walker circulation an americas atlantic walker circulation both of which comprise rising motion in the west and sinking in the east and a transverse cell over asia the middle east north africa and the indian ocean that has rising motion in the east and sinking toward the west these exist year round but migrate and evolve considerably with the seasons and have about a third to half of the mass flux of the peak hadley cell the annual cycle of the two hadley cells reveals peak strength in early february and early august in both reanalyses a second monsoon mode which accounts for 20 of the variance features relatively shallow but vigorous overturning with the maximum vertical velocities near 800 mb outflow from 750 to 350 mb and inflow peaking at 925 mb it is especially strong over africa where the shallow mostly meridional overturning migrates back and forth across the equator with the seasons it influences the middle east has a signature over australia and is also an important component of the overturning in the tropical eastern pacific and atlantic and thus of the convergence zones in these regions the relationship of the global monsoon to the regional monsoons is described over six zonal sectors africa australia asia north america south america and the pacific and atlantic oceans only the two ocean areas do not undergo a seasonal reversal required for monsoons although they have direct overturning cells and they nevertheless participate in the global monsoon through the changes in large scale overturning the regional meridional cross sections highlight the importance of the shallow overturning cell in lower troposphere monsoon activity the steadiness of the overturning circulation is determined by comparing the signal of the seasonal mean vertical motions at 500 mb with the standard deviation of the transient daily variations locations where this signal exceeds 60 of the daily noise correspond closely with the regional centers of the monsoon

reconnaissance study of precambrian metamorphic rocks northeastern sino korean shield people s republic of china in eastern liaoning and southern jilin provinces people9s republic of china rocks of late archean early and middle proterozoic ages consist of interlayered recrystallized banded iron formations pelitic schists maric and ultramafic amphibolites and quartzofeldspathic gneisses the latter two lithologies are volumetrically dominant and include both supracrustals and orthogneisses orthoamphibolites new analytical data document the chemically primitive nature of maficultramafic amphibolite and tonalitic granulites of archean age which are overlain and invaded by more evolved proterozoic supracrustal and plutonic units now metamorphosed late archean 2 8 2 9 b y rock types include metakomatiitic metatholeiitic pyroxene bearing amphibolites and granulitic tonalite gneisses early and middle proterozoic fuping wutai 2 4 2 5 b y 1 9 2 0 b y and zhongtiao 1 6 1 8 b y orogenies produced amphibolite epidote amphibolite and anchimetamorphic to greenschist facies overprints respectively mineral assemblages suggest intermediate crustal levels of recrystallization for the late archean low rank granulite facies metamorphism and olig pargasitic hbl ti rich bio opx cpx garn 700 100 c 6 7 kbar progressively shallower depths for early and middle proterozoic amphibolite olig blue green hbl brown bio 500 100 c 5 1 kbar and greenschist alb chi ep act phengite 400 50 c 2 3 kbar facies events late archean and early proterozoic recrystallizations took place under p t conditions characteristic of moderately thick sialic crust and inferred island arc like heat fluxes crustal growth locally terminated and the sino korean craton was stabilized about 1 7 b y ago much later the complex was affected by a thermal event accompanying mesozoic calc alkaline plutonism volcanism apparently during the terrane assembly of east asia

magnetic fields of lunar multi ring impact basins we survey the magnetic fields of lunar multi ring impact basins using data from the electron reflectometer instrument on the lunar prospector spacecraft as for smaller lunar craters the primary signature is a magnetic low that extends to 1 5 2 basin radii suggesting shock demagnetization of relatively soft crustal magnetization a secondary signature as for large terrestrial basins is the presence of central magnetic anomalies which may be due to thermal remanence in impact melt rocks and or shock remanence in the central uplift the radial extent of the anomalies may argue for the former possibility but the latter or a combination of the two are also possible central anomaly fields are absent for the oldest pre nectarian basins increase to a peak in early nectarian basins and decrease to a low level for imbrian basins if basin associated anomalies provide a good indication of ambient magnetic fields when the basins formed this suggests the existence of a magnetic era possibly due to a lunar core dynamo similar to that implied by paleointensity results from returned lunar samples however the central basin anomalies suggest that the fields peaked in early nectarian times and were low in imbrian times while samples provide evidence for high fields in nectarian and early imbrian times

mutually constrained geophysical data for the evaluation of a proposed impact structure lake hummeln sweden abstract lake hummeln covers a 1 2 km wide near circular depression in the precambrian basement of the baltic shield in the depression more than 150 m thick cambrian and ordovician marine sediments are underlain by non volcanic breccia the only explanation that does not meet contradictions is that the structure was caused by impact but evidence of shock metamorphism is until now missing this study focuses on the geophysical characterisation and includes magnetic and gravimetric modelling constrained by information from resistivity measurements geological mapping and a drill core the hummeln structure shows both a gravimetric and a magnetic low the magnetic low is not broken by any short wavelength anomalies which would indicate the presence of remanent melt occurrences the diameter of the magnetic low is about twice that of the proposed crater depression and probably indicates fracturing both the magnetic and the gravimetric models show a structure that is consistent with the impact hypothesis resistivity measurements were carried out in the surroundings of the proposed crater the fractured region indicated by the magnetometry does not produce a detectable resistivity anomaly however resistivity measurements proved useful in investigating the tectonic fracture zones crossing the area in the vicinity of the structure following the impact model a reconstruction of the pre erosional structure resulted in an approximately 50 wider original crater than seen today

rb sr dating of the impact melt from east clearwater quebec the rb sr composition of eight melt rock and three basement samples from the east clearwater impact structure quebec and two basement samples from the west clearwater structure has been determined the whole rock 87sr 86sr ratios of the melt samples 0 7167 0 7253 are within the range of the basement samples 0 7054 0 7322 and provide further evidence that the melt rocks represent shock melted basement a mineral isochron obtained from a relatively coarse grained melt rock gives an age of 287 26 ma for the crystallization age of the melt this is equivalent to k ar whole rock ages of 285 30 ma and 300 30 ma and a rb sr age of 266 15 ma obtained on melt rocks from west clearwater and confirms the previously generally held assumption that the east and west clearwater structures resulted from the simultaneous impact of two bodies at 285 300 ma ago

neoarchean to paleoproterozoic continental evolution and tectonic history of the north china craton a review rocks of archean age are widely distributed in the north china craton ncc and can be divided into high grade regions and granite greenstone belts even though most underwent amphibolite to granulite facies metamorphism quartzo feldspathic gneisses occupy 70 to 85 of the total exposure of archean rocks in the craton supracrustal rocks commonly consist of intercalated volcano sedimentary rocks including metabasites meta intermediate to acid rocks banded iron formations metapelites and minor marbles the oldest rocks outcrop over a small area of 4 km2 and are a quartzite amphibolite sequence that yield 3 8 3 5 ga isotopic ages chemical and geochronological data constrain the earliest main crust forming episode at ca 2 9 2 7 ga when most rocks were metamorphosed to upper amphibolite or granulite facies at moderate pressures mafic granulites can be classified into two textural types referred to as near isobaric cooling ibc and near isothermal decompression itd types high pressure granulites and retrograded eclogites occur mainly near the junction of the provinces of hebei shanxi and inner mongolia in the central ncc and in eastern shandong province of the eastern ncc as well as locally in henan province in the southern ncc these high pressure rocks show clockwise p t paths indicating decompression from 14 kb to 5 6 kb an exposed section interpreted as representing precambrian continental lower crust has also been identified in the northern part of the craton although some mafic ultramafic metamorphic rocks have been interpreted as remnants of ancient oceanic crust and mantle by a number of geologists geochemical and petrological evidence demonstrates that many supracrustal orthogneiss associations have island arc affinities therefore several tectonic models have been proposed including continent continent collision and island arc terrain accretion achieved by arc arc or arc microcontinent collision the ncc underwent multiple high grade metamorphic events of which the two most important took place during the intervals 2600 2450 ma and 1950 1750 ma which are considered to be related to early precambrian supercontinental cycles

lake st martin impact structure manitoba canada magnetic anomalies and magnetizations a ground magnetic survey has further delineated the previously known intense 1000 nt negative anomaly at the centre of the lake st martin impact structure manitoba shocked gneiss in the central uplift has a strong reverse remanent magnetization whose direction is similar to that found after alternating field demagnetization of melt rock specimens from within the impact structure shocked gneiss is interpreted as the source of the central negative anomaly the direction of the remanence is consistent with a permo triassic age for the impact event the magnetization in the shocked gneiss is interpreted as a chemical remanent magnetization carried by hematite formed during an extensive alteration of mafic silicates occurring immediately after the impact

shallow meridional circulation in the tropical eastern pacific abstract based on four independent datasets this study presents observational evidence of a shallow meridional circulation cell in the eastern tropical pacific in this shallow meridional circulation cell the northerly cross equatorial return flow from the itcz into the southern hemisphere is found immediately above the atmospheric boundary layer in contrast to the classic concept of the hadley type deep meridional circulation whose northerly return flow resides in the upper troposphere the strength and depth of this shallow meridional circulation undergo a distinct annual cycle possible causes and climatic implications of this shallow meridional circulation are discussed

modeling tropical convergence based on the moist static energy budget abstract the vertically integrated moist static energy equation provides a convenient starting point for the construction of simple models of the time mean low level convergence in the tropics a vertically integrated measure of the moist static stability the gross moist stability proves to be of central importance minima in this quantity mark the positions of the tropical convergence zones we argue that the positions of these minima are determined by the time mean moisture field which is in turn closely tied to the time mean surface temperature

weaker links between zonal convective clustering and itcz width in climate models than in observations strong links are seen in observations between convective clustering and several properties of the intertropical convergence zone itcz these links suggest that biases in how climate models simulate the itcz may be related to model biases in convective clustering or that there may be biases in how models represent the relationship between clustering and the itcz we investigate these issues by analyzing convective clustering and the link between clustering and itcz properties in 18 climate models we find that the links between variability in convective clustering and variability of itcz properties are generally weaker and less robust in models than in observations by contrast model biases in the climatological convective clustering explain a substantial fraction of the climatological double itcz bias though they do not explain biases in the climatological itcz width

regional energy budget control of the intertropical convergence zone and application to mid holocene rainfall shifts in the latitude of the intertropical convergence zone a region of intense tropical rainfall have often been explained through changes in the atmospheric energy budget specifically through theories that tie rainfall to meridional energy fluxes these quantitative theories can explain shifts in the zonal mean but often have limited relevance for regional climate shifts such as a period of enhanced precipitation over saharan africa during the mid holocene here we present a theory for regional tropical rainfall shifts that utilizes both zonal and meridional energy fluxes we first identify a qualitative link between zonal and meridional energy fluxes and rainfall variations associated with the seasonal cycle and the el nino southern oscillation we then develop a quantitative theory based on these fluxes that relates atmospheric energy transport to tropical rainfall when applied to the orbital configuration of the mid holocene our theory predicts continental rainfall shifts over africa and southeast asia that are consistent with complex model simulations however the predicted rainfall over the sahara is not sufficient to sustain vegetation at a level seen in the palaeo record which instead requires an additional large energy source such as that due to reductions in saharan surface albedo we thus conclude that additional feedbacks such as those involving changes in vegetation or soil type are required to explain changes in rainfall over africa during the mid holocene shifts in the latitude of the tropical rainfall band are constrained by meridional energy fluxes calculations show that combining zonal and meridional energy fluxes can explain past regional rainfall variations like the african humid period

a new mechanism of the slow eastward propagation of unstable disturbances with convection in the tropics implications for the mjo abstractto understand the slow eastward propagation of the madden julian oscillation mjo the authors propose a new mechanism of slow eastward propagation of an unstable disturbance slow eastward propagating disturbance sed in the tropics assuming a simplified instability source two conditions are necessary over the convective region associated with the sed for this mechanism i moderately negative gross moist stability gms and ii strong stretching in the lower layer the instability due to i needs to be moderate for causing a propagating convective disturbance under the structure of twin cyclonic vortices to the west and convection to the east stretching of planetary vorticity associated with the convection produces cyclonic vorticity which results in eastward propagation of the vortices equatorial plane dynamics explain the positional relation between the twin vortices and convection coherently strong equatorial low level westerly winds accompanying the vortices induce convergence

indian monsoon precipitation isotopes linked with high level cloud cover at local and regional scales abstract precipitation stable isotopes preserve historic changes of evaporation in the source regions and precipitation processes therefore they can be used to reveal regional hydrological cycle dynamics and paleoclimate reconstructions in monsoon regions strong inverse impacts of convection on precipitation isotope ratios have created a debate regarding the interpretation of isotope records as local climate proxies the proportions of stratiform to convective precipitation on water isotopes together with the influence mechanisms on seasonal and interannual scales remain highly uncertain to further address the influence of precipitation patterns on water isotopes we used 10 yrs of precipitation isotope data from the southern tibetan plateau tp to explore the effects of large scale cloud cover and local climate on precipitation isotopes correlation analysis performed between local precipitation 18 o values and different level cloud data indicated significant negative correlations between precipitation isotopes and high level cloud cover on both seasonal and interannual time scales this result suggests that high level convection in the upper moisture transport stream is a main control on precipitation isotopes in the southern tp the clear and coherent variations of precipitation isotopes with the southern oscillation index and outgoing longwave radiation confirmed that strong convection activity in the moisture source region and during transport significantly depleted heavy isotopes in vapor producing substantially decreased precipitation 18 o in the study region these results agree with earlier findings of tree ring cellulose isotope records that correlate with cloud cover but we emphasized the important role of larger scale regional cloud cover we also delineated different maximum correlation zones for seasonal and interannual time scales likely due to different mechanisms these findings further improve the interpretation of paleoisotope records from the indian summer monsoon region

geophysical aspects of meteorite impact craters in eroded shield environment with special emphasis on electric resistivity abstract the traditional geophysical methods of magnetics and gravity as applied to meteorite impact crater studies are shortly reviewed their optimal resolution power with respect to crater size and pre existing structures is discussed the morphology of some typical impact craters is illustrated to show the character of the post impact deformation a new geophysical method in impact crater studies the in situ measurement of electrical resistivity is introduced and illustrated with case studies from simple and complex craters it is concluded that the resistivity method represents a useful tool for mapping the extent of authigenic brecciation in crater structures and their surroundings the method constitutes in combination with detailed elevation data gravity and magnetic measurements a powerful tool for constraining the classification of impact craters in crystalline shield environments the resistivity measurements have revealed a complex brecciation pattern in large central uplift structures consisting of a central less brecciated core surrounded by a ring of more intense brecciation typical impact brecciated crystalline rocks have resistivities between 2000 and 8000 ohm m the crater structures treated in this paper are bjorko dellen siljan and uppland all located in sweden

on large scale circulations in convecting atmospheres the dominant thinking about the interaction between large scale atmospheric circulations and moist convection holds that convection acts as a heat source for the large scale circulations while the latter supply water vapour to the convection we show that this idea has led to fundamental misconceptions about this interaction and offer an alternative paradigm based on the idea that convection is nearly in statistical equilibrium with its environment according to the alternative paradigm the vertical temperature profile itself rather than the heating is controlled by the convection which ties the temperature directly to the subcloud layer entropy the understanding of large scale circulations in convecting atmospheres can therefore be regarded as a problem of understanding the distribution in space and time of the subcloud layer entropy we show that the subcloud layer entropy is controlled by the sea surface temperature the surface wind speed and the large scale vertical velocity in the convecting layer and demonstrate how the recognition of this control leads to a simple physically consistent view of large scale flows ranging from the hadley and walker circulations to the 30 50 day oscillation in particular we argue that the direct effect of convection on large scale circulations is to reduce by roughly an order of magnitude the effective static stability felt by such circulations and to damp all of them

shock metamorphism of quartz in nature and experiment ii significance in geoscience the occurrence of shock metamorphosed quartz is the most common petrographic criterion for the identification of terrestrial impact structures and lithologies its utility is due to its almost ubiquitous occurrence in terrestrial rocks its overall stability and the fact that a variety of shock metamorphic effects occurring over a range of shock pressures have been well documented these shock effects have been generally duplicated in shock recovery experiments and thus serve as shock pressure barometers after reviewing the general character of shock effects in quartz the differences between experimental and natural shock events and their potential effects on the shock metamorphism of quartz are explored the short pulse lengths in experiments may account for the difficulty in synthesizing the high pressure polymorphs coesite and stishovite compared to natural occurrences in addition post shock thermal effects are possible in natural events which can affect shock altered physical properties such as refractive index and cause annealing of shock damage and recrystallization the orientations of planar microstructures however are unaffected by post impact thermal events except if quartz is recrystallized and provide the best natural shock barometer in terms of utility and occurrence the nature of planar microstructures particularly planar deformation features pdfs is discussed in some detail and a scheme of variations in orientations with shock pressure is provided the effect of post impact events on pdfs is generally limited to annealing of the original glass lamellae to produce decorated pdfs resulting from the exsolution of dissolved water during recrystallization basal 0001 pdfs differ from other pdf orientations in that they are multiple mechanical brazil twins which are difficult to detect if not partially annealed and decorated the occurrence and significance of shock metamorphosed quartz and its other phases namely coesite stishovite diaplectic glass and lechatelierite are discussed for terrestrial impact structures in both crystalline non porous and sedimentary porous targets the bulk of past studies have dealt with crystalline targets where variations in recorded shock pressure in quartz have been used to constrain aspects of the cratering process and to estimate crater dimensions at eroded structures in sedimentary targets the effect of pore space results in an inhomogeneous distribution in recorded shock pressure and temperature which requires a different classification scheme for the variation of recorded shock compared to that in crystalline targets this is discussed along with examples of variations in the relative abundances of planar microstructures and their orientations which are attributed to textural variations in sedimentary target rocks examples of the shock metamorphism of quartz in distal ejecta such as at the k t boundary and from nuclear explosions are illustrated and are equivalent to that of known impact structures except with respect to characteristics that are due to long term post shock thermal effects finally the differences between the deformation and phase transformation of quartz by shock and by endogenic tectonic and volcanic processes are discussed we confirm previous conclusions that they are completely dissimilar in character due to the vastly different physical conditions and time scales typical for shock events compared to tectonic and volcanic events well characterized and documented shock effects in quartz are unequivocal indicators of impact in the natural environment

on the role of sea surface temperature gradients in forcing low level winds and convergence in the tropics abstract we examine the importance of pressure gradients due to surface temperature gradients to low level p 700 mb flow and convergence in the tropics over time scales 1 month the latter plays a crucial role in determining the distribution of cumulonimbus convection and rainfall our approach is to consider a simple one layer model of the trade cumulus boundary layer wherein surface temperature gradients are mixed vertically consistent with ecmwf analyzed data the top of the layer is taken at 700 mb the influence from higher levels is intentionally suppressed by setting horizontal pressure gradients and frictional stresses to zero at the top of the layer horizontal convergence within the layer is taken up by cumulonimbus mass flux however the development of the cumulonimbus mass flux is associated with a short relaxation time o hr roughly the development time for such convection during this short time horizontal convergence acts to redistribute mass so as to reduce horizontal pressur

the terrestrial impact cratering record approximately 130 terrestrial hypervelocity impact craters are currently known due to variations in preservation and in geologic knowledge this sample is biased towards young x 20 km craters on the cratons of australia europe including the former u s s r and north america the rate of discovery of new craters is 3 5 craters per year although modified by erosion terrestrial impact craters exhibit the range of morphologies observed for craters on other terrestrial planetary bodies such as the moon terrestrial craters provide essential ground truth data on the geologic effects of impact and the subsurface structure of impact craters which can be used to constrain interpretations of lunar samples and models of crater formation due to erosion and its effects on form terrestrial craters are recognized primarily by the occurrence of shock metamorphic effects these include shatter cones microscopic planar deformation features solid state and fusion glasses high pressure polymorphs and whole rock melting and vaporization shock recovery experiments indicate that these features occur over shock pressures of 5 gpa to x 100 gpa terrestrial craters have a set of geophysical characteristics which are largely the result of the passage of a shock wave and impact induced fracturing they include gravity and magnetic lows and reductions in seismic velocity the gravity anomalies are seldom greater than 30 mgal due to the limiting effects of lithostatic pressure on fracturing at large complex craters the gravity signature may include a central relative gravity high due to uplift and short wavelength central magnetic anomalies due to a variety of processes much current work is focused on the effects of impact on earth evolution previous work on shock metamorphism and the contamination of impact melt rocks by meteoritic siderophile elements provides a basis for the interpretation of the physical and chemical evidence from cretaceous tertiary boundary sites as resulting from a major impact suggestions that other biological boundaries in the stratigraphie record are due to periodic impacts are not supported by time series analysis of the terrestrial cratering record by analogy with the lunar record and modelling of the effects of very large impacts it has been proposed that biological and atmospheric evolution of the earth could not stabilize before the end of the late heavy bombardment 3 8 ga ago the present terrestrial cratering rate is 5 4 2 7 10 15 km 2a 1 for a diameter 20 km this represents a local threat on historic time scales on a global scale a major impact sufficient to cripple human civilization severely will occur on time scales of 106 a

thermal evolution of two textural types of mafic granulites in the north china craton evidence for both mantle plume and collisional tectonics mafic granulites from the north china craton can be divided into two textural types referred to as a and b types a type mafic granulites display garnet quartz symplectic coronas and outcrop in the eastern and western zones of the craton whereas b type mafic granulites exhibit orthopyroxene plagioclase clinopyroxene symplectites or coronas and are mainly exposed in the central zone of the craton most a type mafic granulites preserve the prograde m 1 peak m 2 and post peak near isobaric cooling m 3 assemblages which are represented respectively by inclusions of hornblende plagioclase quartz a peak mineralogy of orthopyroxene clinopyroxene plagio clase quartz garnet and overprinted by garnet quartz symplectic coronas these mineral assem blages and their p t pressure temperature estimates define anticlockwise p t evolutionary paths the b type mafic granulites preserve the peak m 1 post peak near isothermal decompression m 2 and cooling m 3 assemblages which are represented by the peak assemblage of orthopyroxene clinopyroxene plagioclase quartz garnet hornblende post peak orthopyroxene plagioclase clinopyroxene symplectites or coronas and later hornblende plagioclase magnetite symplectites respectively these mineral assemblages and their p t estimates define clockwise p t paths the anticlockwise p t paths of the a type mafic granulites in the eastern and western zones of the north china craton are consistent with a model of underplating and intrusion of mantle derived magmas in combination with lithological structural and geochronological data the eastern and west ern zones of the north china craton are considered to represent two continental blocks that devel oped through the interaction of mantle plumes with the lithosphere from the palaeoarchaean to the neoarchaean era the b type mafic granulites and associated rocks in the central zone represent a magmatic arc that was metamorphosed and deformed during amalgamation of the eastern and west ern continental blocks in the late palaeoproterozoic era the mineral reaction relations and clockwise p t paths of the b type mafic granulites from the central zone record the tectonothermal history of the collision that resulted in the final assembly of the north china craton at c 1800 ma

new 40ar 39ar dating of the clearwater lake impact structures quebec canada not the binary asteroid impact it seems abstract the two clearwater lake impact structures quebec canada are generally interpreted as a crater doublet formed by the impact of a binary asteroid here arguments are presented that raise important questions about the proposed double impact scenario new 40 ar 39 ar dating of two virtually fresh impact melt rock samples from the 36 km west clearwater lake impact structure yielded two statistically robust early permian plateau ages with a weighted mean of 286 2 2 2 2 6 ma 2 mswd 0 33 p 0 57 in contrast 40 ar 39 ar results for two chloritized melt rocks from the 26 km east clearwater lake impact structure produced disturbed age spectra suggestive of a distinct extraneous argon component although individually weakly robust age spectra corrected for the trapped argon component and inverse isochron plots for the east clearwater melt rocks consistently yielded apparent ages around 460 470 ma no permian signal was found in either of these melt aliquots our new 40 ar 39 ar results reproduce earlier 40 ar 39 ar plateau ages 283 ma and 465 ma respectively for the two impact structures by bottomley et al 1990 and are in conflict with a previous statistically non robust rb sr age of 287 293 26 ma for east clearwater the combined cluster of apparent ages of 460 470 ma derived from four different samples across the impact melt sheet is very unlikely to represent a false age effect due to the incorporation of extraneous argon into the melt instead it strongly favors a middle ordovician age for the east clearwater impact and impact induced hydrothermal chloritization moreover the clearwater impact structures are characterized by different natural remanent magnetizations testifying to separate geologic histories an effect unexpected in the case of a permian double impact whereas the west clearwater impact affected ordovician carbonates incorporated into the impact breccia drill core reports from the 1960s concluded that clasts of ordovician sedimentary rocks are seemingly absent in the impact breccia lens of the east clearwater lake impact structure which is overlain by 100 m of post impact sandstones shales and carbonates no resolvable impactor contamination has so far been detected in the west clearwater impact melt rocks whereas east clearwater carries a distinct ordinary possibly l chondritic impactor signature in its melt rocks east clearwater lake might thus represent one among a long list of ordovician impact structures in north america and northern europe that were presumably formed in response to the l chondrite asteroid breakup event 470 ma ago paleogeographic reconstructions show that the ordovician east clearwater impact probably occurred in a near coastal to shallow marine setting while the permian west clearwater impact hit continental pangaea along with the new 40 ar 39 ar data the paleomagnetic sedimentologic and geochemical findings suggest that the close spatial arrangement of the two clearwater lakes is most likely pure coincidence the two impact structures seem to represent a false doublet struck by impacts separated by 180 million years in time the new results for the clearwater lake impact structures have major implications for the reliable identification of doublet impact craters and the rate of binary asteroid impacts on earth and on other planetary bodies in the inner solar system

petrophysical properties density and magnetization of rocks from the northern part of the baltic shield abstract density magnetic susceptibility and remanent magnetization of more than 30 000 samples of crystalline rocks from the northern part of the baltic shield have been measured by the national geological surveys of the nordic countries these data were compiled into a database as part of a project involving finland norway and sweden the nordkalott project in addition more than 20 000 in situ susceptibility measurements are available for a large part of the baltic shield from all these measurements a number of general conclusions can be drawn regarding the distribution of magnetic minerals in the different lithologies 1 1 the precambrian metasupracrustal rocks show a rather typical bimodal distribution in susceptibility the spatial distribution of these susceptibility modes gives rise to the banded patterns observed in high resolution low altitude aeromagnetic measurements 2 2 the amount of remanence in the various rock types is generally low as indicated by q values konigsberger ratios of around 0 2 on average thus magnetization is parallel to the earth s magnetic field simplifying the interpretation ol the total field magnetic survey results 3 3 plutonic rocks as a whole have the same bimodal susceptibility distribution but individual plutons have usually unimodal susceptibility distributions and thereby create characteristic local anomaly patterns 4 4 metamorphic processes significantly affect the distribution of magnetite well documented examples include the amphibolite granulite facies transition tectonophysics vol 192 the serpentinization of ultramafic rocks pure appl geophys vol 114 and oxidation in fracture zones geoexploration 15 the metamorphic development of magnetite appears to reflect in many cases the variation in primary composition 5 5 the range in density variation and magnetic susceptibility in crystalline rocks is now well defined in local structures it is however always necessary to determine the actual petrophysical contrasts in order to minimize the ambiguity in quantitative interpretation of potential field anomalies

stable water isotopes and large scale vertical motions in the tropics a complete understanding of the interaction between convection and the large scale circulation in the tropics remains an outstanding problem although there is evidence that the vertical structure of convective heating has great influence in the large scale response and that this structure also presents considerable geographical variations more need to be done one of the questions that are still unanswered is how the vertical structure of the convective heating or similarly of vertical velocity varies across the tropical pacific here it is suggested that some light can be shed on this debate by considering stable water isotopes because these tend to be progressively less abundant with increasing height precipitation associated with top heavy profiles should be expected to be more depleted than that associated with bottom heavy profiles this claim is verified with a variety of data first using observations from iaea wmo global network of isotopes in precipitation stations then using a simple model based on the budget of water isotopes in precipitation finally using a more complex isotope enabled general circulation model evidence provided by these sources confirms that different structures of vertical velocities are associated with different isotopic abundances with top heavy profiles giving rise to more depleted rainfall finally the data from over the pacific although scarce seem to suggest that precipitation in the eastern part is more enriched than in the western thus hinting at velocity profiles over the east being more bottom heavy than over the west pacific

geophysical profile of the roter kamm impact crater namibia new gravity and magnetic data were obtained along ground profiles across the roter kamm impact crater in the southern namib desert of namibia as the traverses of previous studies did not extend sufficiently beyond the crater rim it had not been possible to adequately determine the regional background values the gravity results of this study are similar to those obtained by fudali in 1973 in that a negative near symmetrical anomaly was obtained over the crater center this anomaly conforms to the results expected for a sediment and impact breccia filled simple bowl shaped crater the magnetic results of this study however are different to those previously reported which is most probably as a result of the longer profiles used in this new study a slight positive magnetic anomaly was obtained over the crater interior short wavelength high amplitude anomalies observed in the vicinity of the crater rim reflect magnetization contrasts that are probably related to brecciation and block rotation modelling of the positive magnetic anomaly indicates the possibility of a small magnetic body or lining at the crater floor breccia interface in the interior of the crater also presented is a 10 m contour digital elevation model of the crater and its environs

atmospheric dynamic and thermodynamic processes driving the western north pacific anomalous anticyclone during el nino part ii formation processes abstractin part i the authors showed that northerly anomalies associated with the rossby wave response to el nino heating anomalies in the equatorial central pacific lead to the southward advection of low moist enthalpy air forming the western north pacific anomalous anticyclone wnpac why does such a remote forcing not cause the formation of the anomalous anticyclone in el nino developing summer the physical mechanism responsible for the timing of the wnpac formation is investigated in part ii through both an observational analysis and idealized numerical model experiments the authors find that the onset timing of the wnpac relies on the following three factors the first is a sign change from positive to negative of the meridional gradient of background low level specific humidity over the key tropical western north pacific wnp region in november the second is a sign change from positive to negative of the meridional gradient of background relative vorticity which efficiently reduces the w

p and s wave velocity model along crustal scale refraction and wide angle reflection profile in the southern korean peninsula abstract the onshore seismic experiment kcrt2004 explored the crustal velocity structure of the southern korean peninsula we present an interpretation of seismic data along this 340 km long nnw sse trending profile the crust was found to consist of three layers the upper middle and lower crust with p and s wave velocities ranging 5 50 to 6 95 km s and 2 82 to 3 91 km s respectively the average p wave velocity 6 26 km s and pn velocity 7 82 7 88 km s are lower than the worldwide average of continental crust moho depths are 29 0 34 9 km gradually thickening toward south the vp vs ratio of crustal material is estimated to be 1 73 0 249 for the upper and middle crust and the ratio increases with the depth in the lower crust the gyeonggi massif in the north of the profile has a lower vp vs ratio than other tectonic units the average crustal vp vs ratio of 1 74 0 253 is remarkably lower than the average value 1 78 0 27 for the bulk continental crust the low average crustal vp vs ratio is similar to that measured in eastern china the empirical analysis using both p wave velocity and vp vs ratio shows that the upper and middle crust is dominantly felsic and the lower crust is intermediate in composition the absence of the mafic material in the lower crust that is also found in eastern china contrasts with the generally accepted global model of the mafic lower crust

diachroneity of the clearwater west and clearwater east impact structures indicated by the u th he dating method the u th he method has been applied to constrain the formation ages of the clearwater west and east impact structures of quebec canada zircons were separated from impact melt samples derived from a surface exposure at clearwater west 32 km diameter and from a drill core at clearwater east 26 km diameter the u th he results indicate ages of 280 27 ma 2 n 7 for clearwater west and 450 56 ma 2 n 8 for clearwater east our u th he date for clearwater west supports the findings of previous rb sr 266 15 ma 2 and 40ar 39ar 280 4 ma and 283 8 2 2 ma 2 impact melt studies our u th he date for clearwater east also overlaps with previously published 40ar 39ar dating results which yielded u shaped spectra with maximum and best estimate dates of 460 470 ma our results support the contention previously based solely on 40ar 39ar data that the clearwater west and east impact structures do not comprise an impact doublet that formed coevally from a binary asteroid pair

paleomagnetism and rock magnetism of east and west clearwater lake impact structures the east and west cleawater lake impact structures wiy sh kimi lake quebec 26 and 32 km in diameter respectively have been proposed to represent an impact doublet we investigated their pale

contrasting meteoritic signatures within the clearwater east and clearwater west impact structures the view from osmium isotopes abstract osmium isotopes provide a powerful tool for identifying meteoritic signatures in impactites we apply the osmium isotope method to impact melt and country rocks from the clearwater east and clearwater west craters located in quebec canada impact melts from clearwater east have 187 os 188 os ratios of 0 1281 0 1285 these values indicate a significant meteoritic component which exceeds that in impact melts at all terrestrial craters studied to date except morokweng such findings align with earlier results from chromium isotopes and platinum group elements in contrast impact melts from clearwater west have 187 os 188 os ratios between 6 604 and 59 12 these highly radiogenic ratios are indistinguishable from the 187 os 188 os ratios in country rocks hence osmium isotopes provide no evidence for a meteoritic component in impact melts at clearwater west the clearwater craters formed in almost identical targets therefore target effects cannot readily explain the stark difference between the two clearwater craters if melt sheet heterogeneity is similar at the two craters the probability that melts at clearwater west host an undetected chondritic component is 1 may best explain the unusually strong meteoritic signature although the signature or its nondetection at each crater may be related to asymmetric preservation of the impactor component the results presented here provide further evidence that clearwater east and clearwater west were temporally separate impact events

transient and disruption cavity dimensions of complex terrestrial impact structures derived from magnetic data 1 accurate transient and disruption cavity dimensions are critical for estimating the energy release associated with impact transient and disruption cavity size can in principle be inferred from morphometric relationships based on crater diameter however locating the crater rim can be difficult for eroded terrestrial craters and existing morphometric relationships are mostly based on observations of extraterrestrial craters where morphologic features at best provide imprecise constraints on the collapsed disruption cavity margin fortunately magnetic survey data collected over terrestrial impact structures demonstrate that collapsed disruption cavity size can be estimated directly from changes in the magnetic anomaly character a lower bound on this parameter can be defined by the outer limit of short wavelength intense magnetic anomalies produced by impact melt and or suevite deposits an upper bound is given by the inner limit of magnetic anomaly trends associated with the pre impact target rock configuration using published values of crater diameters d and values of collapsed disruption cavity diameters dcdc derived from magnetic data for 19 complex terrestrial impact structures we derive the relationship dcdc 0 49d these data and the possibility of geometrical similarity in crater collapse suggest that this relationship is independent of complex crater size over more than a decade of size variation

constraints on the depth of magnetized crust on mars from impact craters large diameter greater than 500 km martian impact basins are associated with observed magnetic fields which are statistically distinct from and smaller than fields associated with smaller craters we suggest that this effect arises because impacts cause shock heating and excavation reducing the magnetization of previously magnetized crust for a simple uniformly magnetized model the magnetic field at 100 km altitude is reduced by 50 when a crater shaped demagnetization zone reaches the base of the magnetized layer by analogy with terrestrial data we assume that in martian craters the zone of demagnetization extends to a depth of 0 04 0 15 crater diameters on the basis of this assumption the data suggest that the depth to the base of the magnetized layer on mars if uniform is 35 km with lower and upper bounds of 10 and 100 km respectively these bounds imply magnetizations of 5 40 a m 1 and are consistent with likely mars geotherms at 4 gyr b p

magnetic anomaly near the center of the vredefort structure implications for impact related magnetic signatures a strong magnetic anomaly near the center of the ancient and deeply eroded vredefort structure is attributed to remanent magnetization caused by a large meteorite impact at 2 0 ga the rocks underlying the anomaly are archean gneisses thought to represent mid crustal depths that were uplifted to the surface during the postulated impact event measurements of the remanent magnetization of the basement rocks yielded consistent vectors of declination 25 inclination 56 k 18 16 that correspond to the paleomagnetic pole position at time of impact petrologic studies indicate that during impact large volumes of rock were heated enough to cause thermal remagnetization in the ambient field thermal effects of all large impacts on cratons may induce a remanent magnetization of sufficient intensity to cause anomalies in the geomagnetic field that are detectable even by satellites

comparison of atmospheric hydrology over convective continental regions using water vapor isotope measurements from space 1 global measurements of the 500 825 hpa layer mean hdo h2o ratio from the tropospheric emission spectrometer tes are used to expose differences in the dominant hydrologic processes in the amazon north australian and asian monsoon regions the data show high regional isotopic variability and numerous values unexpected from classical rayleigh theory correlation analysis shows that mixing with boundary layer air enhanced isotopic fractionation during precipitation and subsiding air parcels contribute to intraseasonal isotopic variability these local controls explain only 8 30 of total regional variance which suggests that the isotopes are primarily indicators of moist processes that occur upstream seasonal trajectory analysis demonstrates that rayleigh distillation in a lagrangrian framework underestimates the observed isotopic depletion during the monsoons and suggests substantial recycling of water within or below clouds the trajectory results for the dry seasons reveal that subsiding air parcels periodically introduce isotopically depleted air into the north australia and asian monsoon regions whereas vigorous low level convection over the amazon basin acts to quickly enrich and moisten dry subsiding air the analysis indicates variations in the strength of convective detrainment into the lower to middle troposphere over all regions which during the dry seasons of the north australian and asian monsoon regions correlate with increases in relative humidity this study shows that isotopic measurements provide unique diagnostics of mechanisms that control the seasonal sources of water and that these provide a refined understanding of the differences in the characteristics of hydrologic budgets in these monsoonal regions

the bosumtwi meteorite impact structure ghana a magnetic model abstract a magnetic model is proposed for the bosumtwi meteorite impact structure in ghana africa this relatively young 1 07 ma structure with a diameter of 10 5 km is exposed within early proterozoic birimian tarkwaian rocks the central part of the structure is buried under postimpact lake sediments and because of lack of drill cores geophysics is the only way to reveal its internal structure to study the structure below and beyond the lake a high resolution low altitude 70 m airborne geophysical survey across the structure was conducted which included measurements of the total magnetic field electromagnetic data and gamma radiation the magnetic data show a circumferential magnetic halo outside the lakeshore 12 km in diameter the central north part of the lake reveals a central negative magnetic anomaly with smaller positive side anomalies north and south of it which is typical for magnetized bodies at shallow latitudes a few weaker negative magnetic anomalies exist in the eastern and western part of the lake together with the northern one they seem to encircle a central uplift our model shows that the magnetic anomaly of the structure is presumably produced by one or several relatively strongly remanently magnetized impact melt rock or melt rich suevite bodies petrophysical measurements show a clear difference between the physical properties of preimpact target rocks and impactites suevites have a higher magnetization and have low densities and high porosities compared to the target rocks in suevites the remanent magnetization dominates over induced magnetization koenigsberger ratio 3 preliminary palaeomagnetic results reveal that the normally magnetized remanence component in suevites was acquired during the jaramillo normal polarity epoch this interpretation is consistent with the modelling results that also require a normal polarity magnetization for the magnetic body beneath the lake the reverse polarity remanence component superimposed on the normal component is probably acquired during subsequent reverse polarity events