on the origin of a wedge crack within the icy crust of europa 1 we suggest that a crack that developed into a wedge shaped band within the icy crust of europa at 28 s 170 w see schenk and mckinnon 1989 figure 4 prockter et al 1999 figure 1 originated under an applied compressive stress through the operation of a frictional sliding mechanism we analyze this suggestion using a scale independent sliding crack model and obtain an estimate of the maximum compressive stress to initiate the crack that compares reasonably well with earlier estimates helfenstein and parmentier 1985 mcewen 1986 greenberg et al 1998 of europan crustal stresses that are based upon elastic deformation of a shell we then show through an application of both power law dislocation creep and diffusion creep that colder near surface europan ice appears to be capable of supporting the deduced stresses with little relaxation over a period from 104 to 105 years

brittle failure of ice ice exhibits either ductile or brittle behavior depending upon the conditions under which it is loaded glaciers for instance are loaded by gravity under deviatoric stresses of 0 1 mpa or lower at temperatures of interest they flow through dislocation creep at strain rates of the order of 10 9 s 1 or lower patterson 1994 sheets of sea ice another terrestrially important ice feature are loaded predominantly by wind under global compressive stresses similar in magnitude to the shear stresses within glaciers richter menge and elder 1998 these bodies deform through a combination of creep and fracture the latter process manifested by oriented leads or open cracks and by compressive shear faults which often form as conjugate sets that traverse a large fraction of the arctic basin kwok 1999 schulson 2002 the cold icy crust of europa an extraterrestrial feature which may shield an ocean beneath within which a form of life may exist or may once have existed reynolds et al 1987 hoppa et al 1999 pappalardo et al 1999 greenberg et al 2000 kargel et al 2000 is loaded by the motion of diurnal tides and by non synchronous rotation greenberg and weidenschilling 1984 the crust deforms in a brittle manner as evident from the networks of cracks that lace through it greeley et al 2000 of the two kinds of ice deformation creep is probably the better known by readers of the geological and geophysical literature and is certainly the more fully explored and better understood the interested reader may wish to consult a number of excellent reviews of the subject e g see duval et al 1983 weertman 1983 durham and stern 2001 fracture in comparison has only recently been systematically examined the motivation with respect to terrestrial mechanics schulson 2001 comes largely

continental crust composition constrained by measurements of crustal poisson s ratio deciphering the geological evolution of the earth s continental crust requires knowledge of its bulk composition and global variability the main uncertainties are associated with the composition of the lower crust seismic measurements probe the elastic properties of the crust at depth from which composition can be inferred of particular note is poisson s ratio this elastic parameter can be determined uniquely from the ratio of p to s wave seismic velocity and provides a better diagnostic of crustal composition than either p or s wave velocity alone1 previous attempts to measure have been limited by difficulties in obtaining coincident p and s wave data sampling the entire crust2 here we report 76 new estimates of crustal spanning all of the continents except antarctica we find that on average increases with the age of the crust our results strongly support the presence of a mafic lower crust beneath cratons and suggest either a uniformitarian craton formation process involving delamination of the lower crust during continental collisions followed by magmatic underplating or a model in which crust formation processes have changed since the precambrian era

intraseasonal oscillations in the global atmosphere part ii southern hemisphere abstract in part ii of this two part article we complete the systematic examination of oscillatory modes in the global atmosphere by studying 12 years of 500 mb geopotential heights in the southern hemisphere as in part i for the tropics and northern hemisphere extratropics the data were band pass filtered to focus on intraseasonal is phenomena and spatial eofs were obtained the leading principal components were subjected to singular spectrum analysis ssa in order to identify nonlinear is oscillations with high statistical confidence in the southern hemisphere the dominant mode has a period of 23 days with spatial patterns carried by the second and third winter eof of the is band it has a zonal wavenumber four structure the 40 day mode is second and dominated by wavenumbers three and four while a 16 day mode is too weak to separate its spatial behavior from the previous two the is dynamics in the southern hemisphere is more complex and dominated by shorter wavenumbers than the northern

evidence for a crustal root beneath the paleoproterozoic collision zone in the northern ordos block north china abstract recent studies have shown that paleoproterozoic rocks called the khondalite belt crop out along a nearly ew trending zone in the northern edge of the ordos block north china this widely distributed khondalite rock suggests that they originated in the margin of a passive continent during the paleoproterozoic era seismic stations deployed in the northern ordos block and its adjacent regions provided abundant information on crustal thicknesses and vp vs ratios which were determined by use of receiver function h stacking the h stacked images show an approximately ew trending anomalous zone with thickened crust 46 5 km and a higher poisson s ratio 0 28 beneath the khondalite belt in the paleoproterozoic collision zone based on the long wavelength information of gravity anomalies a two dimensional crustal density model indicated that a crustal root with a density contrast of 130 kg m3 between the root and surrounding mantle is required for the isostatic gravity compensation the ratio of surface elevation to crustal root thickness was 0 02 suggesting that the exhumation of the crustal root in response to surface erosion continued to occur over time scales of hundreds of millions of years this crustal thickening model is evidence for the crustal root beneath the paleoproterozoic collision zone in the northern ordos block as the density contrast becomes smaller the amount of isostatic rebound in response to erosion also decreases and therefore the crustal root is maintained

the influence of closed brine pockets and permeable brine channels on the thermo elastic properties of saline ice a model of the thermo elastic behaviour of saline ice is formulated and model solutions describing thermo elastic waves tew propagating into a half space of the ice are investigated the model is based on a proposal that saline ice is a matrix which encompasses both closed brine pockets and permeable channels filled with brine experiments on the thermal expansion of saline ice samples and on tew in saline ice have been performed in the cold laboratories of the university centre in svalbard and in university college london the experimental data are compared with theoretical conclusions the experimental data support our hypothesis that the brine in saline ice is divided between closed pockets and open permeable channels this article is part of the themed issue microdynamics of ice

cold surges in tropical and extratropical south america the strong event in june 1994 the authors investigate one case of surges of polar air that occasionally propagate into southeastern brazil during wintertime and are harmful to coffee production because of the freezing conditions associated with them the cooling is also observed in southern and with less intensity in western amazonia the event of 26 june 1994 is studied the frost event caused a sharp drop in coffee production and similarly dramatic increases in coffee prices the event was characterized by the presence of an anticyclonic perturbation off the coast of chile that enters south america and crosses the andes mountains south of 458s hours later the anticyclone center tracks equatorward until it reaches 208 308s and then shifts eastward toward the atlantic based on a diagnostic analysis using the national centers for environmental prediction national center for atmospheric research ncep ncar reanalysis a two stage process can be distinguished in the starting period midtropospheric troughing is established east of the andes over central argentina due to stretching and or vorticity advection this troughing is responsible for cold advection along the eastern flank of the andes which produces substantial temperature drops and deepens the upper trough locally the increase in the magnitude of the zonal temperature gradients east of the cold advection zone at levels near 700 hpa is another result of the cooling process this feature would ensure that the cold advection be extended eastward producing an upper level trough local deepening there this intensification would produce an increase in the cyclonic vorticity advection which would tend to produce sea level pressure drops underneath this near surface low pressure area would eventually contribute to the southeastern brazil sb cooling due to the associated southerly winds another feature is the near surface local anticyclonic generation due to midtropospheric cold advection associated with descending motions low level divergence and anticyclonic growth eventually this high pressure would also contribute to produce southwesterlies to transport cold air to sb the paper is divided in two sections the first part is a study of synoptic and climatic aspects of the cold surge episodes by using daily surface climatic observations the purpose of the second part is to analyze the dynamic aspects of this cold episode and to study the cooling mechanisms by using the four times daily surface and upper air ncep ncar reanalysis as well as to look for possible predictors

influence of ice crystal anisotropy on seismic velocity analysis in 2010 a reflection seismic survey was carried out on the alpine glacier colle gnifetti the processed and depth converted data could be compared to a nearby ice core drilled almost to the bed comparisons showed that the depth of the p wave bed reflection was too shallow while the depth of the sh wave bed reflection fitted the ice core length well we are now able to explain the major part of these differences using the existing crystal orientations of the ice at colle gnifetti we calculate anisotropic velocities for p and sh waves that are usually picked for stacking and compare them with zero offset velocities needed for the depth conversion here we take the firn pack at colle gnifetti into account for p and s wave analysis to incorporate the s wave analysis we first derive a new equation for the relationship between density and s wave velocity from diving waves we show that anisotropic fabrics observed at colle gnifetti introduce a difference of only 1 between stacking and depth conversion velocities for the sh wave but 7 for the p wave we suggest that this difference in stacking and depth conversion velocity for the p wave can be used to derive information about the existing anisotropy by combining our seismic data with for example radar data

east west faults due to planetary contraction abstract contraction expansion and despinning have been common in the past evolution of solar system bodies these processes deform the lithosphere until it breaks along faults their characteristic tectonic patterns have thus been sought for on all planets and large satellites with an ancient surface while the search for despinning tectonics has not been conclusive there is good observational evidence on several bodies for the global faulting pattern associated with contraction or expansion though the pattern is seldom isotropic as predicted the cause of the non random orientation of the faults has been attributed either to regional stresses or to the combined action of contraction expansion with another deformation despinning tidal deformation reorientation another cause of the mismatch may be the neglect of the lithospheric thinning at the equator or at the poles due either to latitudinal variation in solar insolation or to localized tidal dissipation using thin elastic shells with variable thickness i show that the equatorial thinning of the lithosphere transforms the homogeneous and isotropic fault pattern caused by contraction expansion into a pattern of faults striking east west preferably formed in the equatorial region by contrast lithospheric thickness variations only weakly affect the despinning faulting pattern consisting of equatorial strike slip faults and polar normal faults if contraction is added to despinning the despinning pattern first shifts to thrust faults striking north south and then to thrust faults striking east west if the lithosphere is thinner at the poles the tectonic pattern caused by contraction expansion consists of faults striking north south i start by predicting the main characteristics of the stress pattern with symmetry arguments i further prove that the solutions for contraction and despinning are dual if the inverse elastic thickness is limited to harmonic degree two making it easy to determine fault orientation for combined contraction and despinning i give two methods for solving the equations of elasticity one numerical and the other semi analytical the latter method yields explicit formulas for stresses as expansions in legendre polynomials about the solution for constant shell thickness though i only discuss the cases of a lithosphere thinner at the equator or at the poles the method is applicable for any latitudinal variation of the lithospheric thickness on iapetus contraction or expansion on a lithosphere thinner at the equator explains the location and orientation of the equatorial ridge on mercury the combination of contraction and despinning makes possible the existence of zonal provinces of thrust faults differing in orientation north south or east west which may be relevant to the orientation of lobate scarps

seismic multiplet response triggered by melt at blood falls taylor glacier antarctica 1 meltwater input often triggers a seismic response from glaciers and ice sheets it is difficult however to measure melt production on glaciers directly while subglacial water storage is not directly observable therefore we document temporal changes in seismicity from a dry based polar glacier taylor glacier antarctica during a melt season using a synthesis of seismic observation and melt modeling we record icequakes using a dense six receiver network of three component geophones and compare this with melt input generated from a calibrated surface energy balance model in the absence of modeled surface melt we find that seismicity is well described by a diurnal signal composed of microseismic events in lake and glacial ice during melt events the diurnal signal is suppressed and seismicity is instead characterized by large glacial icequakes we perform network based correlation and clustering analyses of seismic record sections and determine that 18 of melt season icequakes are repetitive multiplets the epicentral locations for these multiplets suggest that they are triggered by meltwater produced near a brine seep known as blood falls our observations of the correspondingp wave first motions are consistent with volumetric source mechanisms we suggest that surface melt enables a persistent pathway through this cold ice to an englacial fracture system that is responsible for brine release episodes from the blood falls seep the scalar moments for these events suggest that the volumetric increase at the source region can be explained by melt input

a novel method for predicting fracture in floating ice basal crevasses may play an important precursory role in determining both the location and propagation of rifts and iceberg dimensions for example icebergs calved recently from thwaites glacier antarctica have the same width as surface undulations strengthening the connection between basal crevasses rifting and calving we explore a novel method for estimating the heights of basal crevasses formed at the grounding lines of ice shelves and ice streams we employ a thin elastic beam teb formulation and tensional yielding criterion to capture the physics of flexed ice at grounding lines observations of basal crevasse heights compare well with model predictions in the siple coast region of the ross ice shelf we find that the teb method is most accurate in areas of low strain rate we also test the method in other areas of antarctica to produce order of magnitude maps of grounding line basal crevasses and find general agreement with reported observations assuming basal crevasses develop in spatio temporal sequence and are advected downstream this method is computationally cheap and could be relatively easy to implement into damage oriented large scale ice models which aim at physically simulating calving and fracture processes

waning buoyancy in the crustal roots of old mountains when mountains form through the collision of lithospheric plates uplift of the earth s surface is accompanied by thickening of the crust and the buoyancy of these deep crustal roots relative to the surrounding mantle is thought to contribute to the support of mountain topography once active tectonism ceases continuing erosion will progressively wear away surface relief here i provide new constraints on how crustal roots respond to erosional unloading over very long timescales in old collisional mountain belts ratios of surface relief to the thickness of the underlying crustal root are observed to be smaller than in young mountains on the basis of gravity data this trend is best explained by a decrease in the buoyancy of the crustal root with greater age since the most recent mountain building episode which is consistent with metamorphic reactions1 2 produced by long term cooling an approximate balance between mountain and root mass anomalies suggests that the continental lithosphere remains weak enough to permit exhumation of crustal roots in response to surface erosion for hundreds of millions of years the amount of such uplift however appears to be significantly reduced by progressive loss of root buoyancy

the crystal fabric of ice from full waveform borehole sonic logging 1 in an ice sheet a preferred crystal orientation fabric affects deformation rates because ice crystals are strongly anisotropic shear along the basal plane is significantly easier than shear perpendicular to the basal plane the effect of fabric can be as important as temperature in defining deformation rates fabric is typically measured using analysis of thin sections under the microscope with co polarized light due to the time consuming and destructive nature of these measurements however it is difficult to capture the spatial variation in fabric necessary for evincing ice sheet flow patterns because an ice crystal is similarly elastically anisotropic the speed of elastic waves through ice can be used as a proxy for quantify anisotropy we use borehole sonic logging measurements and thin section data from dome c east antarctica to define the relations between apparent fabric and borehole measured elastic speeds compressionalvp and vertically polarized shear vsv these relations valid for single maximum fabrics allow in situ depth continuous fabric estimates of unimodal fabric strength from borehole sonic logging we describe the single maximum fabric usinga1 the largest eigenvalue of the second order orientation tensor for ice at 16 c anda1in the 0 7 1 range the relations arevp 248 a13 7 3755 m s 1 and vsv 210a17 3 1968 m s 1

large amplitude short scale stationary rossby waves in the southern hemisphere observations and mechanistic experiments to determine their origin abstract studies by van loon and jenne van loon et al trenberth and others indicate that stationary waves in the southern hemisphere are dominated by planetary scales kalnay and halem reported the presence of large amplitude short scale stationary waves during the month of january 1979 in the lee of south america and their disappearance in february 1979 in this paper we present further observational evidence of the january waves we also perform two 15 day forecast experiments with the glas fourth order general circulation model and initial conditions corresponding to 5 january and 4 february 1979 these factors reproduce reasonably well the presence of the january waves and their absence in february several mechanistic experiments to determine the origin of the waves are then performed the principal conclusions are a large amplitude stationary rossby waves with zonal wavenumber 7 were present between 20 and 40 s both in the south pacific and east of south america during january 1979 they app

interannual and interdecadal variability of the south atlantic convergence zone interannual variations of the summertime january march atmospheric circulation over subtropical south america are examined during the period 1958 97 using the national centers for environmental prediction national center for atmospheric research reanalysis data it is found from an empirical orthogonal function analysis that an anomalous upper tropospheric large scale stationary eddy in the lee of the andes tends to accompany a dipole in anomalous vertical motion an anomalous cyclonic anticyclonic eddy accompanies an intensified diffuse south atlantic convergence zone sacz with anomalous descent ascent to the southwest the cold core equivalent barotropic vertical structure of the anomalous cyclonic eddy and the 200 hpa vorticity balance are both characteristic of a stationary rossby wave the tendency for the eddy to be advected downstream by the mean westerlies is compensated by meridional advection of planetary vorticity and stretching associated with vertical motion the anomalous cyclonic flow at low levels reinforces the thermally direct circulation associated with the sacz a weak funneling of submonthly rossby wave activity into this descent region is also identified the interannual time series of the eddy is significantly correlated with north south dipolar sea surface temperature sst anomalies over the southwest atlantic one standard deviation 200 hpa wind speed anomalies of up to 5 m s21 are accompanied by sst anomalies of up to 0 38c a near cyclic 15 yr component is identified which the authors corroborate from independent analyses of southwest atlantic ssts and river flows both are

seismicity within a propagating ice shelf rift the relationship between icequake locations and ice shelf structure iceberg calving is a dominant mass loss mechanism for antarctic ice shelves second only to basal melting an important process involved in calving is the initiation and propagation of through penetrating fractures called rifts however the mechanisms controlling rift propagation remain poorly understood to investigate the mechanics of ice shelf rifting we analyzed seismicity associated with a propagating rift tip on the amery ice shelf using data collected during the austral summers of 2004 2007 we apply a suite of passive seismological techniques including icequake locations back projection and moment tensor inversion we confirm previous results that show ice shelf rifting is characterized by periods of relative quiescence punctuated by swarms of intense seismicity of 1 to 3 h even during periods of quiescence we find significant deformation around the rift tip moment tensors calculated for a subset of the largest icequakes mw 2 0 located near the rift tip show steeply dipping fault planes horizontal or shallowly plunging stress orientations and often have a significant volumetric component they also reveal that much of the observed seismicity is limited to the upper 50 m of the ice shelf this suggests a complex system of deformation that involves the propagating rift the region behind the rift tip and a system of rift transverse crevasses small scale variations in the mechanical structure of the ice shelf especially rift transverse crevasses and accreted marine ice play an important role in modulating the rate and location of seismicity associated with the propagating ice shelf rifts

approximations to seismic ava responses validity and potential in glaciological applications amplitude variation with angle ava methods establish the seismic properties of material either side of a reflective interface and their use is growing in glaciology the ava response of an interface is defined by the complex knott zoeppritz k z equations numerous approximations to which we typically assume weak interface contrasts and isotropic propagation inconsistent with the strong contrasts at glacier beds and the vertically transverse isotropic vti fabrics were associated with englacial reflectivity we considered the validity of a suite of approximate k z equations for the exact p wave reflectivity rp of ice overlying bedrock sediment and water and englacial interfaces between isotropic and vti ice we found that the approximations of aki richards shuey and fatti match exact glacier bed reflectivity to within rp 0 05 smaller than the uncertainty in typical glaciological ava analyses but only for maximum incident angle i limited to 30 a stricter limit of i 20 o ffered comparable accuracy to a hydrocarbon benchmark case of shale overlying gas chargedsand the vti compliant rugerapproximation accurately described englacial reflectivity to within rp 0 01 and it can be modified to give a quadratic expression in sin 2 i suitable for curve matching operations having shown the circumstances under which ava approximations were valid for glaciological applications we have suggested that their interpretative advantages can be exploited in the future ava interpretations

waterfall ice mechanical stability of vertical structures we present a study of the mechanical in stability of the ephemeral waterfall ice structures that form from the freezing of liquid water seeping on steep rock three vertical structures were studied two near glacier d argentiere france and one in the valsavarenche valley northern italy the generation of internal stresses in the ice structure in relation to air and ice temperature conditions is analyzed from pressure sensor records their role in the mechanical instability of the structures is discussed from a photographic survey of these structures the main result is that dramatic air cooling several 8 8c h 1 over several hours and low temperatures 108c generating tensile stresses and brittleness can trigger a spontaneous or climber induced mechanical collapse leading to unfavorable climbing conditions ice internal pressure fluctuations are also associated with episodes of marked diurnal air temperature cycle with mild days few 8 8c above 08 8c and cool nights few 8 8c below 08 8c through the occurrence of water ice phase transitions within the structure these ice internal stress fluctuations seem however to have a local influence are associated with warm near 08 8c wet and therefore particularly soft ice and do not trigger a collapse of the structure

subseasonal variations of rainfall in south america in the vicinity of the low level jet east of the andes and comparison to those in the south atlantic convergence zone abstract regional and large scale circulation anomalies associated with variations in rainfall downstream of the south american low level jet are identified and compared to those in the south atlantic convergence zone sacz composites of precipitation associated with strong jets reveal an approximate doubling of the quantities one would expect from climatology with an evolution of the rainfall pattern from south to north the occurrence of extreme precipitation events follows a similar pattern meridional cross sections of composite wind reveal a distinct low level jet near 20 s and a baroclinic development farther south that appears to force the jet geopotential height temperature and large scale wind composites suggest that this developing disturbance is tied to a wave train that originates in the midlatitude pacific and turns equatorward as it crosses the andes mountains similar composites based on sacz rainfall reveal similar features but of opposite sign suggesting that the phase of the wave

does a monsoon climate exist over south america the climatology and the basic state of the summertime circulation and rainfall over south america are studied using assimilation products from the data assimilation system of goddard earth observing system 1 geos1 and satellite derived rainfall results indicate the existence of a regional summer monsoon circulation regime induced by strong diabatic heating over the subtropical south american highland centered at the altiplano plateau sensitivity of the results to the assimilation scheme is tested by comparing that with the national centers for environmental prediction ncep reanalysis and with satellite rainfall estimates results show general agreement between the model produced rainfall anomaly and the satellite estimates as well as consistency between the basic circulation features in the geos 1 and the ncep reanalyses a case study of 1989 90 south american summer monsoon sasm reveals the following characteristics 1 in late spring the onset of sasm is signaled by an abrupt merging of the upper tropospheric double westerly jets one in the subtropics and the other in the subpolar region into a single jet in the midlatitudes this is followed by the establishment of a vortex to the southeast of altiplano and occurrence of heavy precipitation over subtropical eastern brazil 2 during the mature phase of sasm the heavy rainfall zone moves over the altiplano plateau and the southernmost brazilian highland the fully established sasm features are the following a an enhancement of equatorial north atlantic trade wind which emanates from the sahara high and crosses the equator over the south american continent b a buildup of strong northwesterlies along the eastern side of the tropical andes and c development of the south atlantic convergence zone in the southernmost position with strong convective activity meanwhile the upper tropospheric return flow emerges from an anticyclone formed over the altiplano plateau crosses the equator and sinks over northwestern africa 3 the withdrawal of sasm in late summer is signaled by the resplitting of the midlatitude westerly jet at the same time the low level northwest monsoon flow diminishes reducing the moisture supply and leading to the termination of heavy precipitation over the subtropical highland results also show that the above mentioned characteristics of sasm are clearly linked to the tropospheric temperature changes over the central south american highland sensible versus latent heating over the highland are bound to play an important role in the evolution of sasm to provide further support of presence of a monsoon climate over south america sasm is compared and contrasted to the classic east asian summer monsoon easm many similar features including evolution characteristics between the two systems have been identified contrasting aspects of the sasm from the easm are also discussed it is pointed out that a number of monsoonal characteristics of the climate of south america such as the seasonal reversal of the low level wind become apparent only when the strong annual mean wind is removed based on the characteristic features and their evolution the authors conclude that a monsoon climate does exist over south america

influence of cross andes flow on the south american low level jet abstract by analyzing the 15 yr 1979 93 reanalysis data of the european centre for medium range weather forecasts ecmwf it has been found that the seasonal and synoptic time scale variations of the south american low level jets lljs are largely controlled by an upper level trough and associated low level zonal flow rather than by horizontal temperature gradients along the slope of the andes the northerly lljs are maintained by strong zonal pressure gradients caused by the upstream trough and westerly flow crossing the andes through lee cyclogenesis the process involves both baroclinic development of the upper level trough and mechanical deflection of the westerly flow by the andes when an anticyclonic circulation replaces the trough and westerly flow over the eastern south pacific the northerly lljs tend to diminish or reverse into southerly lljs the dependence of the lljs upon the upstream wind pattern helps to explain how the seasonal variation of the south american lljs is related to the s

implications of crustal property variations for models of tibetan plateau evolution shear coupled teleseismic p waves sampling the interior of the tibetan plateau provide evidence of systematic variations in crustal structure the crust thins by up to 20 km from south to north with a concomitant increase in poisson s ratio from normal values in the south to unusually high values in the north this suggests that the crust of the northern plateau is partially melted due to high temperatures these changes imply spatial and perhaps temporal variations in the way the elevation of the high plateau is created and maintained

climatology of the low level jet east of the andes as derived from the ncep ncar reanalyses characteristics and temporal variability abstract a climatology of the south american low level jet east of the andes sallj is developed using the 1950 2000 circulation and moisture fields from the ncep ncar reanalyses and available upper air observations made in bolivia and paraguay since 1998 upper and low level circulation fields were derived for seasonal means and sallj composites during the warm and cold seasons the bonner criterion 1 was applied for sites in central bolivia and downstream near northern paraguay to determine the spatial and temporal characteristics of the sallj on the circulation characteristics sallj composites during the warm season show the enhanced low level meridional moisture transport coming from equatorial south america as well as an upper level wave train emanating from the west pacific propagating toward south america the intensification of the warm season sallj follows the establishment of an upper level ridge over southern brazil and a trough over most of argentina the circulation anomalies at upper a

chaco low level jet events characterization during the austral summer season 1 previous studies showed a relationship between convection maxima and the convergence of vapor flux over southeastern south america nicolini and saulo 2000 explored the hypothesis of an intensification of this mutual dependency during the chaco jet events they defined these events as extreme cases of low level jets east of the andes that penetrate southernmost to 25 s using eta operative products during the 1997 1998 austral warm season the interest of the present paper centers on the climatic characterization of the chaco jet events cjes and the possibility of researching this hypothesis using the european centre for medium range weather forecasts reanalyses era which cover a 15 year period 1979 1993 the cjes represent a subensemble of low level jet events east of the andes that are infrequent in the era data set their duration varies from 1 to 10 days more frequently extending from 1 to 5 days the outstanding features of the circulation and the thermodynamic field that represent this ensemble are a maximum contrast of air masses in a latitude close to 39 s the presence of a trough centered on 70 w within a baroclinic wave train penetrating from the pacific ocean and a maximum of heat and moisture over northen argentina and paraguay during the cjes there is an important flux of moisture and convergence at low and mid levels that is about 10 times more intense than the summer mean the intensity found in the water vapor flux anomaly reinforces the importance of studying these episodes for the purpose of determining the water balance over southeastern south america the statement that the cjes represent an important characteristic of the southeastern south american climate is founded on the fact that although the cjes only represent 17 of the austral summer days they account for a significant fraction of the precipitation a maximun of 55 over northeastern argentina however it is indispensable to contrast these results with future field experiments to test the hypothesis addressed in this paper

surface displacements resulting from magma chamber roof subsidence with application to the 2014 2015 bardarbunga holuhraun volcanotectonic episode in iceland abstract the conditions which lead to caldera collapse are still poorly constrained as there have only been four possibly five well documented caldera forming events in the past century the geodetic signals produced during chamber roof subsidence or chamber volume reduction shrinkage in general are not well documented or understood in particular when two or more geodetic sources are operating and providing signals at the same time it is important to be able to estimate the likely contribution of each simultaneous activities of different geodetic sources are common and include pressure changes in magma chambers reservoirs occurring at the same time as dyke emplacement here we present results from numerical models designed to simulate the subsidence of a magma chamber roof either directly chamber shrinkage or through ring fault displacement and the induced surface deformation and crustal stresses we consider chamber depths at 3 km 5 km and 7 km below the crustal surface using both non layered isotropic and layered anisotropic crustal models we also model the effects of a caldera lake and of a thick ice cover ice sheet on top of the caldera the results suggest that magma chamber roof subsidences between 20 m and 100 m generate large tens of centimetres vertical and in particular horizontal displacements at the surfaces of the ice and the crust out to distances of up to tens of kilometres from the caldera chamber centre crustal layering tends to reduce but increasing chamber depth to enlarge the horizontal and vertical surface displacements applying the results to the ice subsidence in the bardarbunga caldera during the 2014 2015 bardarbunga holuhraun volcanotectonic episode indicates that the modelled ice displacements are less than those geodetically measured also the geodetically measured crustal displacements are less than expected for a 60 m chamber roof subsidence the modelling results thus suggest that only part of the ice subsidence is due to chamber roof subsidence the other part being related to flow in and down bending of the ice we show that such a flow is likely within the caldera as a result of the stress induced by the 45 km long regional dyke emplaced primarily in vertical magma flow during the episode this conclusion is further supported by the model results suggesting that the ring fault piston like displacements must have been much less than the total 60 m ice subsidence or else faults with tens of metres displacements would have cut through the ice these are not observed we suggest that the ring fault subsidence was triggered by small doming of the volcanic field and system hosting the bardarbunga caldera and that this doming occurred as a result of magma inflow and pressure increase in a deep seated reservoir the doming is confirmed by gps measurements and supported by the seismicity results the magmatic pressure increase in the reservoir was in terms of the present model responsible for the regional dyke emplacement the holuhraun eruption and part of the stress concentration around and displacement of the bardarbunga caldera

cohesion versus friction in controlling the long term strength of a self healing experimental fault the coulomb s failure criterion which postulates that failure occurs along a fault plane when the applied shear stress overcomes a resistance made of two parts of different nature cohesion and friction remains the standard conceptual framework of faulting mechanics more recently rate and state friction laws became the main modelling tool of the seismic cycle these laws implicitly assume that only frictional resistance sets the fault strength and its evolution we therefore raise the question of the role of cohesion and related healing sealing mechanisms on fault mechanics we designed an original laboratory experiment based on a model material an ice thin layer sitting on top of a water tank and mechanically deformed at various rates with a circular couette like geometry this allowed sliding along the fault plane over arbitrarily large slip distances and analyzing the competition between faulting and cohesion healing rates whereas frictional resistance resulting from the fault roughness is limited in magnitude we show that cohesion healing plays an essential role in the time evolution of the interface strength under constant loading rate in particular the magnitudes of shear stress fluctuations are observed to be temperature and velocity weakening the present experiment shares several properties similar to that of active faults during seismic cycles suggesting that cohesive healing could control some of these features scale invariance properties gutenberg richter law of rupture events size distribution omori s law of energy dissipation at least after major ruptures time asymmetry in energy dissipation and periods of creep under high shear stress alternating with major earthquake like ruptures

evidence for a local crustal root beneath the santa catalina metamorphic core complex arizona we used data from the tucson arizona broad band seismic station to analyze crustal and mantle structure under and adjacent to the catalina metamorphic core complex the data can be modeled with a simple two layer crust having an average moho depth of 30 km and seismic velocities consistent with a felsic composition the lower crust thickens under the catalina mountains depressing the moho by 4 2 km the crustal root can isostatically compensate the average topography of the catalina mountains indicating that the depth of isostatic compensation is at or below the moho and that the catatina mountains act as a coherent block bounded by high angle normal faults the presence of the crustal root is not consistent with models that predict upward bowing of the moho under the catalina metamorphic core complex the presence of the root cannot rule out models of lower crustal flow during the formation of the core complex however the presence of a root favors a model of metamorphic core complex development described by crustal thickening in shortening episodes crustal welt model followed by collapse of the crustal welt through extension

stresses generated in cooling viscoelastic ice shells application to europa 1 a cooling viscoelastic ice shell overlying an ocean develops stresses due to two effects thermal contraction of the ice due to cooling and the expansion of the shell due to the ice water volume change the former effect generates near surface compression and deeper extension the second effect generates extension only in both cases stresses are smaller at depth due to viscous creep the resulting combined stresses are extensional except at shallow 1 km depths in thin ice shells for ice shells thicker than 45 km stresses are extensional throughout the extensional stresses exceed 10 mpa for shells thicker than 20 km and thus dominate all other likely sources of stress as long as shell cooling occurs the dominantly extensional nature of the stresses may help to explain the puzzling lack of compression observed on europa and other large icy satellites however after 100 myr of conductive cooling the maximum theoretical elastic strains for europa are 0 35 which are probably insufficient to explain the total amount of observed extension

elastic dynamics and tidal migration of grounding lines modify subglacial lubrication and melting 1 we combine the dynamics of ice bed and ocean in a new elastic model for the tidal timescale migration of grounding lines on deformable foundations previous interpretations of tidal flexure using models of elastic ice shelves with fixed grounding lines were found to be inconsistent suggesting an elasticity of ice that varies spatially and temporally and that is significantly smaller than measured experimentally we argue here that with our model a consistent purely elastic interpretation can be made combining this new approach with remote sensing measurements we show that the grounding line migrates several kilometers during a tidal cycle that we can infer the effective elastic properties of the bed and that the elastic pressure of the ice leads to a hydrological barrier near the grounding line that controls subglacial hydrology our findings imply that subglacial lubrication and melting induced by the ocean thermal forcing can increase substantially during high tide

eruptions arising from tidally controlled periodic openings of rifts on enceladus the cassini flyby of 14 july 2005 revealed plumes of water vapour and ice associated with the tiger stripe features on the surface of saturn s icy moon enceladus since then the challenge has been to explain the nature of the plumes and the forces driving them two papers this week offer an explanation that accounts for both the plume characteristics and the presence of hot spots without the need to assume the existence of near surface liquid water a requirement of some previous models nimmo et al identify tidally driven lateral fault motions near the tiger stripes as the most likely drivers of heat and vapour production and hurford et al show that as enceladus orbits saturn the parent planet s tides make the satellite s ice flex this may cause the tiger stripes to open and close periodically exposing volatile gases and allowing them to be released plumes of gas have been seen near the south polar region of enceladus a small icy satellite of saturn and observations revealed large rifts in the crust informally called tiger stripes hurford et al report that during each orbit every portion of each tiger stripe rift spends about half the time in tension which allows the rift to open exposing volatiles and allowing eruptions plume activity is expected to vary periodically in 2005 plumes were detected near the south polar region of enceladus1 a small icy satellite of saturn observations of the south pole revealed large rifts in the crust informally called tiger stripes which exhibit higher temperatures than the surrounding terrain and are probably sources of the observed eruptions2 models of the ultimate interior source for the eruptions are under consideration1 3 4 5 other models of an expanding plume6 require eruptions from discrete sources as well as less voluminous eruptions from a more extended source to match the observations no physical mechanism that matches the observations has been identified to control these eruptions here we report a mechanism in which temporal variations in tidal stress open and close the tiger stripe rifts governing the timing of eruptions during each orbit every portion of each tiger stripe rift spends about half the time in tension which allows the rift to open exposing volatiles and allowing eruptions in a complementary process periodic shear stress along the rifts also generates heat along their lengths7 8 9 which has the capacity to enhance eruptions plume activity is expected to vary periodically affecting the injection of material into saturn s e ring10 and its formation evolution and structure moreover the stresses controlling eruptions imply that enceladus icy shell behaves as a thin elastic layer perhaps only a few tens of kilometres thick

reversal of ice motion during the outburst of a glacier dammed lake on gornergletscher switzerland during the outburst flood of a glacier dammed lake on gornergletscher switzerland in july 2004 the drained lake water triggered anomalous glacier motion at the onset of the outburst the ice flow direction in the vicinity of the lake became closer to the central flowline when the lake discharge magnitude decreased the flow direction altered such that the ice moved back to the azimuth of the initial motion at one of the survey points where the ice flows parallel to the central flowline the ice accelerated along the pre event flow direction followed by a 1808 backward motion that lasted over 2 days these observations indicate the impact of the lake outburst on the subglacial and englacial stress conditions however the reversal in the flow direction is difficult to explain by drawing on our current understanding of glacier mechanics the timing and the timescale of the flow direction changes suggest that the elastic glacier motion and its rebound played a role under the rapidly changing stress conditions but the young s modulus of ice is too large to cause the observed ice motion other processes including basal separation and subglacial sediment deformation are discussed as possible mechanisms for the reversal of the ice motion

atmospheric circulation anomalies during episodes of enhanced and reduced convective cloudiness over uruguay abstract regional and large scale circulation anomalies associated with periods of enhanced and reduced convective cloudiness over uruguay are studied for austral spring and summer when rainfall associated with deep convection is more frequent in this region the analysis was performed at a submonthly timescale considering that the essential nature of the mechanisms producing rainfall is not well captured by anomalies calculated on a monthly or seasonal basis in regions where precipitation is highly episodic periods of enhanced and reduced convective cloudiness over uruguay are characterized by a marked dipolar structure in the outgoing longwave radiation anomaly field along eastern south america from 10 to 40 s with the centers of the dipole located over the south atlantic convergence zone sacz and over a broad region including uruguay southern brazil and northeastern argentina this dipole which corresponds to one of the key factors of climate dynamics in south america during spring and summer

revisiting wintertime cold air intrusions at the east of the andes propagating features from subtropical argentina to peruvian amazon and relationship with large scale circulation patterns this study investigates the spatial and temporal characteristics of cold surges that propagates northward along the eastern flank of the andes from subtropical to tropical south america analysing wintertime in situ daily minimum temperature observations from argentina bolivia and peru and era 40 reanalysis over the 1975 2001 period cold surges usually last 2 or 3 days but are generally less persistent in the southern la plata basin compared to tropical regions on average three to four cold surges are reported each year our analysis reveals that 52 of cold episodes registered in the south of la plata basin propagate northward to the northern peruvian amazon at a speed of around 20 m s 1 in comparison to cold surges that do not reach the tropical region we demonstrate that these cold surges are characterized before they reach the tropical region by a higher occurrence of a specific circulation pattern associated to southern low level winds progression toward low latitudes combined with subsidence and dry condition in the middle and low troposphere that reinforce the cold episode through a radiative effect finally the relationship between cold surges and atmosphere dynamics is illustrated for the two most severe cold intrusions that reached the peruvian and bolivian amazon in the last 20 years

the microstructure of polar ice part ii state of the art an important feature of natural ice in addition to the obvious relevance of glaciers and ice sheets for climate related issues is its ability to creep on geological time scales and low deviatoric stresses at temperatures very close to its melting point without losing its polycrystalline character this fact together with its strong mechanical anisotropy and other notable properties makes natural ice an interesting model material for studying the high temperature creep and recrystallization of rocks in earth s interior after having reviewed the major contributions of deep ice coring to the research on natural ice microstructures in part i of this work faria et al 2014 here in part ii we present an up todate view of the modern understanding of natural ice microstructures and the deformation processes that may produce them in particular we analyze a large body of evidence that reveals fundamental flaws in the widely accepted tripartite paradigm of polar ice microstructure also known as the three stage model cf part i these results prove that grain growth in ice sheets is dynamic in the sense that it occurs during deformation and is markedly affected by the stored strain energy as well as by air inclusions and other impurities the strong plastic anisotropy of the ice lattice gives rise to high internal stresses and concentrated strain heterogeneities in the polycrystal which demand large amounts of strain accommodation from the microstructural analyses of ice cores we conclude that the formation of many and diverse subgrain boundaries and the splitting of grains by rotation recrystallization are the most fundamental mechanisms of dynamic recovery and strain accommodation in polar ice additionally in fine grained high impurity ice layers e g cloudy bands strain may sometimes be accommodated by diffusional flow at low temperatures and stresses or microscopic grain boundary sliding via microshear in anisotropic ice sheared at high temperatures grain boundaries bulged by migration recrystallization and subgrain boundaries are endemic and very frequent at almost all depths in ice sheets evidence of nucleation of new grains is also observed at various depths provided that the local concentration of strain energy is high enough which is not seldom the case as a substitute for the tripartite paradigm we propose a novel dynamic recrystallization diagram in the three dimensional state space of strain rate temperature and mean grain size which summarizes the various competing recrystallization processes that contribute to the evolution of the polar ice microstructure dedicated to sepp kipfstuhl on occasion of his 60th birthday

the effects of laterally varying icy shell structure on the tidal response of ganymede and europa we use a finite element model to solve for the response of ganymede and europa to tidal forcing from jupiter using various icy shell models with laterally variable 3 d structure in all cases the shell is assumed to be underlain by a liquid water ocean icy shells with laterally varying thickness are derived from a thermal conduction model three dimensional shear modulus profiles for the shell are built either from a conduction model or for europa by assuming a hemispherical difference in composition icy shell structures with a nonglobal ocean are built for ganymede using these shell structures to calculate the tidal response of ganymede and europa we conclude the following 1 the presence of lateral variations in thickness or in shear modulus would not degrade future attempts to use tidal observations to decide on the existence or absence of a liquid ocean and to determine the mean icy shell thickness 2 given accurate enough observations the presence of lateral variations in thickness or in shear modulus could be determined by searching for nondegree 2 components in the tidal response 3 in the absence of significant viscous convective flow in the shell the effects of a laterally varying shear modulus on the tidal response would be smaller than those of a laterally varying shell thickness 4 if the shell is partially grounded tidal observations of either gravity or uplift would be able to roughly differentiate regions where the ice is grounded from those where it is floating

tidal dissipation in creeping ice and the thermal evolution of europa abstract the thermal and mechanical evolution of europa and comparable icy satellites the physics behind creating and sustaining a subsurface water ocean depends almost entirely on the mechanical dissipation of tidal energy in ice to produce heat the mechanism s of which remain poorly understood in deformation experiments we combine steady state creep and low frequency small strain periodic loading similar conditions in which tectonics and tidal flexing are occurring simultaneously the data reveal that the relevant power law attenuation in ice i is non linear depending on strain amplitude ii is independent of grain size and iii exceeds in absorption the prediction of the maxwell solid model by an order of magnitude the maxwell solid model is widely used to model the dynamics of planetary ice shells so this discrepancy is important the prevalent understanding of damping in the geophysical context is that it is controlled by chemical diffusion on grain boundaries which renders attenuation strongly dependent on grain size in sharp contrast our results indicate instead the importance of intracrystalline dislocations and their spatial interactions as the critical structural variable affecting dissipation these dislocation structures are controlled by stress and realized by accumulated plastic strain thus tectonics and attenuation are coupled which beyond the icy satellite subsurface ocean problem has implications also for understanding the attenuation of seismic waves in deforming regions of the earth s upper mantle

deep convection east of the andes cordillera a test case analysis of airmass origin warm and moist air masses required to generate deep convection east of the andes cordillera are generally the result of humidity transport by the so called low level jet llj in this paper it is shown from detailed test cases that the eastern part of the continent and the adjacent atlantic ocean may constitute another source of moist warm air which could be of potential importance even in the presence of a southerly llj the position of the anticyclones crossing south america from the pacific ocean to the atlantic ocean appears to be a key factor affecting the origin of moisture over the continent in particular the llj may weaken and even change its direction when the eastern side of the south pacific anticyclone crosses the mountains this wind reversal is generally associated with deep convection suppression thus as a south pacific anticyclone crosses the continent more to the east and its western side reaches the east coast of south america deep convection can reappear east of the andes over the mendoza region although the llj is frequently suppressed this is associated with a transport of warm and moist air from uruguay southeast brazil or even directly from the atlantic ocean

archean blocks and their boundaries in the north china craton lithological geochemical structural and p t path constraints and tectonic evolution an examination of lithological geochemical geochronological structural and metamorphic p t path data suggests that the basement of the north china craton can be divided into eastern and western blocks separated by major crustal boundaries that roughly correspond with the limits of a 300 km wide zone called the trans north china orogen the eastern block consists predominantly of late archean domiform tonalitic trondhjemitic granodioritic ttg batholiths surrounded by anastomosing networks and linear belts of open to tight synforms of minor volcanic and sedimentary rocks metamorphosed from greenschist to granulite facies at 2 5 ga with anticlockwise p t paths some early to middle archean rocks are locally present in the eastern block but their tectonic history is unclear due to reworking by the 2 5 ga tectonothermal event the western block has a late archean assemblage structural style and metamorphic history similar to that of the eastern block but it differs in the absence of early to middle archean assemblages and in being overlain by and interleaved with paleoproterozoic khondalites which were affected by a 1 8 ga metamorphic event involving clockwise p t paths a mantle plume model is proposed for the formation and evolution of late archean basement rocks in the eastern and western blocks based on a combination of extensive exposure of ttg gneisses affinities of mafic rocks to continental tholeiitic basalts presence of voluminous komatiitic rocks dominant diaprism related domiform structures anticlockwise p t paths and a short time span from the primary emplacement of ttg and ultramafic to mafic rocks until the onset of regional metamorphism between the two blocks is the trans north china orogen which is bounded by two major fault systems and is composed of late archean to paleoproterozoic ttg gneisses and granitoids interleaved with abundant sedimentary and volcanic rocks that are geochemically interpreted as having developed in magmatic arc and intra arc basin environments these rocks underwent multiple phases of compressional deformation and peak high pressure metamorphism followed by rapid exhumation during the late paleoproterozoic at 1 8 ga as a result of collision between the eastern and western blocks resulting in the amalgamation of the north china craton

statistics and dynamics of persistent anomalies abstract persistent anomalies with recurrent spatial patterns play an important role in the atmosphere s low frequency variability we establish a connection between statistical and dynamical methods of description and prediction of persistent anomalies this is done by computing and analyzing the empirical orthogonal functions eofs in a simple deterministic model on the one hand and in southern hemisphere geopotential heights on the other the dynamical model is governed by the fully nonlinear equivalent barotropic vorticity equation on the sphere with simplified forcing dissipation and topography model solutions exhibit persistent anomalies identifiable with blocked zonal and wave train anomalies in northern hemisphere atmospheric data flow structures similar to the patterns above occur as high variance eofs of this nonlinear model the southern hemisphere data we analyze consist in gridded daily maps of 500 mb heights from june 1972 to july 1983 two types of persistent anomalies appear in t

tidal dissipation in enceladus uneven fractured ice shell abstract analysis of enceladus gravity topography and libration data suggests that the thickness of the ice crust significantly varies which may have important consequences for the heat transfer across and the tidal dissipation within the ice shell understanding these processes is a critical prerequisite for analyzing enceladus thermal evolution and assessing the long term stability of its subsurface ocean here we investigate the impact of ice shell thickness variations on the tidal deformation of the moon and the associated heat production using a finite element model that includes faults tiger stripes in the south polar region spr since the tidal deformation and the thermal structure of the ice shell are coupled through temperature and deviatoric stress we simultaneously solve the equations governing the anelastic deformation of ice and also equations which model conductive heat transfer in the ice shell we find that tidal heating is concentrated in a narrow low viscosity zone near the base of the ice shell and along faults outside the spr the thickness of this zone is about 1 10 of the local ice thickness and the associated volumetric heating is 1 0 6 w m 3 corresponding to less than 1 gw of dissipated power in the spr the tidal effects are enhanced by the combined action of faults and ice shell thinning although the volumetric heating in this relatively small region may be larger than 10 4 w m 3 the total heat production in this region does not exceed 1 1 gw our computations show that tidal heating in the ice shell can explain only a small fraction of enceladus heat production derived from astrometric observations implying that enceladus heat engine is powered by dissipation in the core or in the ocean

transient behavior of the meridional moisture transport across south america and its relation to atmospheric circulation patterns the transient behavior of meridional moisture transport across the south american continent is examined with the reanalysis data provided by the european centre for medium range weather forecasts ecmwf the results show clearly the effects of the valley between the andes and the brazilian plateau in canalizing the southward moisture transport an intense jet is episodically formed in this valley the synoptic variability of this low level jet and its relation to the large scale atmospheric circulation are studied

anisotropy and crystalline fabric of whillans ice stream west antarctica inferred from multicomponent seismic data crystal orientation fabric cof describes the intrinsic anisotropic nature of ice and is an important parameter for modeling glacier flow we present the results of three component active source seismic observations from the whillans ice stream wis a fast flowing ice stream in west antarctica surface wave dispersion analysis ray tracing and traveltime inversion of compressional p and shear s waves reveal the presence of transversely isotropic ice with a vertical axis of symmetry vti beneath approximately 65 m of isotropic firn the ice stream is characterized by weak anisotropy involving an average ice thickness of approximately 780 m the analysis indicates that about 95 of the ice mass is anisotropic and the crystalline c axes span within an average broad cone angle of 73 10 with respect to the vertical axis moreover the mean temperature t below the firn estimated from seismic data is 15 5 c these data do not show evidence of englacial seismic reflectivity which indicates the lack of abrupt changes in the cof the presence of azimuthal anisotropy due to transversely compressive flow or fractures aligned along a preferential direction is also excluded we suggest that the observed vti ice structure is typical of large ice streams in regions where basal sliding and bed deformation dominate over internal glacial deformation

tide deflection of campbell glacier tongue antarctica analyzed by double differential sar interferometry and finite element method abstract a glacier tongue the terminus of a glacier projecting out from the coastline and floating over the ocean experiences vertical deflection due to the fluctuation of ocean tide accurate analysis of the tidal deflection of a glacier tongue is necessary to understand the elastic properties and dynamics of glacial ice we extracted the tidal deflection of campbell glacier tongue cgt one of the fast flowing glacier tongues in east antarctica by using 120 double differential interferometric sar ddinsar images from the 16 cosmo skymed one day tandem interferometric sar image pairs obtained during 2010 and 2011 a map of tide deflection ratio map defined as the ratio of the tidal deflection over tide height was generated by pixel based linear regression between the ddinsar derived tidal deflection and the tidal variation predicted by various tide models such as tpxo7 1 fes2004 cats2008a and ross inv after the inverse barometer effect correction we also generated the maps of r 2 root mean square error rmse and intercept of the linear regression model we confirmed that the horizontal flow of the campbell glacier is steady over the whole data acquisition period due to the fact that values were close to zero over the grounded part of the glacier ross inv provided the most reliable map with the highest r 2 0 97 and the smallest rmse 0 3 cm beyond the hinge zone than other tide models the spatial variation of the tidal response of the whole cgt was clearly defined from the map the grounding line retreated by 0 3 1 5 km in comparison with that in 1996 defined by rignot et al 2011a the ice thickness of cgt was estimated to be 397 71 m from icesat surface elevation data the young s modulus of cgt was found to be 0 5 1 3 gpa 0 8 gpa at an ice thickness of 397 m by simulating the in the hinge zone using a 2 dimensional finite element method

u pb and hf isotopic study of zircons of the helanshan complex constrains on the evolution of the khondalite belt in the western block of the north china craton abstract the helanshan complex is located at the westernmost segment of the khondalite belt a continent continent collisional belt along which the yinshan block in the north collided with the ordos block in the south to form the western block of the north china craton the complex consists of s type granites and high grade pelitic granulite gneiss felsic paragneiss quartzite calc silicate rock and marble together called the khondalite series rocks la q icp ms u pb geochronology and la mc icp ms lu hf isotopic data of zircons combined with cathodoluminescence cl imaging enable the resolution of magmatic and metamorphic events that can be directed towards understanding the formation and evolution of the khondalite belt in the western block of the north china craton cl images reveal the coexistence of magmatic type detrital zircons and metamorphic zircons in most of the khondalite series rocks of which the metamorphic zircons occur as either single grains or overgrowth rims surrounding and truncating magmatic type detrital zircon cores la q icp ms u pb analyses on magmatic type detrital zircons reveal two distinct age populations with one in the archaean 2 85 2 53 ga and the other in the palaeoproterozoic 2 2 2 0 ga suggesting that the sedimentary protoliths of the khondalite series rocks in the helanshan complex must have been deposited at some time after 2 0 ga the hf analyses show that the archaean detrital zircons possess negative ehf t values from 7 64 to 0 15 with depleted mantle model ages ranging from 3 34 to 3 10 ga this implies the existence of a paleo mesoarchaean 3 34 3 10 ga crust in the western block which underwent a recycling event in the period 2 85 2 53 ga nearly all palaeoproterozoic 2 2 2 0 ga detrital zircons possess positive ehf t values 0 86 9 38 with depleted mantle model ages of 2 45 2 15 ga suggesting a significant crustal growth event in the western block in the palaeoproterozoic metamorphic zircons yield two groups of age with one at 1 95 ga and the other at 1 87 ga of which the former is considered as the timing of the collision between the yinshan and ordos blocks to form the western block whereas the latter is consistent with the age of 1 86 ga from two s type granites which are considered to have formed by the partial melting of pelitic granulites gneisses at the stage of the exhumation of the helanshan complex

the pacific south american modes and tropical convection during the southern hemisphere winter abstract atmospheric circulation features and convection patterns associated with two leading low frequency modes in the southern hemisphere sh are examined in multiyear global reanalyses produced by ncep ncar and nasa dao the two leading modes referred to as the pacific south american psa modes are represented by the first two eof patterns the two patterns are in quadrature with each other and are dominated by wavenumber 3 in midlatitudes with large amplitudes in the pacific south american sector in the pacific anomalies in the subtropics and in the midlatitudes are opposite in phase taken together the two psa modes represent the intraseasonal oscillation in the sh with periods of roughly 40 days the evolution of the psa modes shows a coherent eastward propagation a composite analysis was conducted to study the evolution of tropical convection and the corresponding circulation changes associated with the psa modes outgoing longwave radiation olr anomaly composites during the mature phase

brittle compressive failure of ice proportional straining vs proportional loading proportional straining experiments have been performed on columnar grained s2 freshwater ice biaxially compressed across the columns at 10 c at a strain rate of 4 5 1 5 x 10 3 s 1 the results are compared with those obtained earlier iliescu and schulson 2004 from the same kind of material deformed to terminal failure under the same conditions but through proportional loading the exercise shows that the biaxial strength is practically independent of the path taken at least under low confinement where coulombic shear faulting limits terminal failure first year sea ice is expected to exhibit the same behavior

analysis of tropical extratropical interactions with influence functions of a barotropic model abstract influence functions ifs of a linear barotropic vorticity equation model are computed in order to determine the regions in which anomalous divergence at upper levels related to tropical heating anomalies has the largest impact on certain prominent low frequency anomalies the present computation differs from that of branstator in two aspects a the model includes the effects of the basic flow divergence and the advection by anomalous divergent wind and b the influence functions directly assess the influence of upper level divergence anomalies rather than sources of vorticity the ifs are applied to the study of low frequency tropical extratropical interactions at the interannual enso and intraseasonal 30 60 day oscillation timescales the origin of well known teleconnection patterns is explored through the identification of comma influence regions in the tropics and subtropics for their main action centers the subtropical west and central east pacific north of the equator is an impo

does titan s long wavelength topography contain information about subsurface ocean dynamics abstract the long wavelength topography of titan is characterized by relatively small amplitudes about 1 km peak to peak an anomalous equatorial bulge the poles are about 300 m lower than the equator and small gravity anomalies indicating a high degree of compensation in the past years the nature of titan s non hydrostatic topography has been addressed in several studies the topography has been interpreted in terms of isostatic or viscous models and discussed in connection with tidal heating in the ice shell and surface erosion here we present a model of the shape evolution of titan s ice shell driven by tidal heating in the shell and spatial variations of the heat flux from a subsurface ocean the model is obtained by solving a general set of equations coupling the viscoelastic flow of ice with the thermal evolution of the ice shell and phase transitions at the ice water interface the equations are solved in a domain with radially varying material properties and moving boundaries the motion of the boundaries is a consequence of ice flow within the shell melting and crystallization at the bottom boundary and erosion and deposition at the surface our model suggests that titan s anomalous topographic bulge can be explained by lateral variations of ocean heat flux of the order of 0 1 1 mw m 2 provided that the heat flux is stable over a period of at least 10 myr and the ice shell has a sufficiently high viscosity exceeding 1016 pa s at the base of the shell such a high value of viscosity implies that either the ice grains are coarse 10 mm or the temperature of the ocean is significantly by more than 40 k lower than the melting temperature of pure water ice the heat flux pattern predicted on top of the ocean is consistent with a flow characterized by upwelling of warm water in polar regions and downwelling of cold water at low latitudes the negative correlation between the topography and geoid at degree 3 reported in a previous study but not confirmed at higher degrees yet is shown to be compatible with erosion and deposition occurring at a rate of 0 01 0 1 mm yr 1 our results underline the importance of gravity and topography measurements for understanding titan s surface and deep interior processes

seismic evidence for an ancient rift beneath the cumberland plateau tennessee a detailed analysis of broadband teleseismic p waveforms broadband receiver functions developed from teleseismic p waveforms recorded on the midperiod passband of regional seismic test network station rscp are inverted for vertical velocity structure beneath the cumberland plateau tennessee the detailed broadband receiver functions are obtained by stacking source equalizd horizontal components of teleseismic p waveforms the resulting receiver functions are most sensitive to the shear velocity structure near the station a time domain inversion routine utilizes the radial receiver function to determine this structure assuming a crustal model parameterized by many thin flat lying homogeneous layers lateral changes in structure are identified by examining azimuthal variations in the vertical structure the results reveal significant rapid lateral changes in the midcrustal structure beneath the station that are interpreted in relation to the origin of the east continent gravity high located northeast of rscp the results from events arriving from the northeast show a high velocity midcrustal layer not present in results from the southeast azimuth this velocity structure can be shown to support the idea that this feature is part of a keweenawan rift system another interesting feature of the derived velocity models is the indication that the crust mantle boundary beneath the cumberland plateau is a thick probably laminated transition zonemore between the depths of 40 and 55 km a result consistent with interpretations of early refraction work in the area less

crustal structure beneath the florida to edmonton broadband seismometer array 1 ps receiver functions were stacked to determine crustal thickness and vp vs ratios for the 28 stations of the florida to edmonton broadband seismometer array and 6 adjacent permanent stations roots of thickened crust exist beneath the paleozoic appalachian orogen and the proterozoic trans hudson orogen the ratios of surface elevation to root thickness are lower than observed in young active orogens as are average crustal root buoyancies determined from gravity data these results are consistent with global trends and may be explained by increased crustal root density due to metamorphism crustal thickening is also observed beneath the mid continent rift in iowa consistent with compressional inversion of the rift during the grenville orogeny

the influence of obliquity on europan cycloid formation tectonic patterns on europa are influenced by tidal stress an important well recognized component is associated with the orbital eccentricity which produces a diurnally varying stress as jupiter s apparent position in europa s sky oscillates in longitude cycloidal lineaments seem to have formed as cracks propagated in this diurnally varying stress field maps of theoretical cycloid patterns capture many of the characteristics of the observed distribution on europa however a few details of the observed cycloid distribution have not been reproduced by previous models recently it has been shown that europa has a finite forced obliquity so jupiter s apparent position in europa s sky will also oscillate in latitude we explore this new type of diurnal effect on cycloid formation we find that stress from obliquity may be the key to explaining several characteristics of observed cycloids such as the shape of equator crossing cycloids and the shift in the crack patterns in the argadnel regio region all of these improvements of the fit between observation and theory seem to require jupiter crossing europa s equatorial plane 45 to 180 after perijove passage suggestive of complex orbital dynamics that locks the direction of europa s pericenter with the direction of the ascending node at the time these cycloids were formed

summer precipitation variability over south america on long and short intraseasonal timescales a dipole pattern in convection between the south atlantic convergence zone and the subtropical plains of southeastern south america characterizes summer intraseasonal variability over the region the dipole pattern presents two main bands of temporal variability with periods between 10 and 30 days and 30 and 90 days each influenced by different large scale dynamical forcings the dipole activity on the 30 90 day band is related to an eastward traveling wavenumber 1 structure in both olr and circulation anomalies in the tropics similar to that associated with the madden julian oscillation the dipole is also related to a teleconnection pattern extended along the south pacific between australia and south america conversely the dipole activity on the 10 30 day band does not seem to be associated with tropical convection anomalies the corresponding circulation anomalies exhibit in the extratropics the structure of rossby like wave trains although their sources are not completely clear