structure and evolution of the intracratonic basins of central australia summary the central australian geology is dominated by a series of parallel sedimentary intracratonic basins whose history spans time from the late proterozoic to the carboniferous the basins are separated by regions of exposed proterozoic crust that have a history of significant uplift the formation of the basins cannot be explained by conventional thermal or stretching mechanisms nor does the foreland basin model provide an adequate explanation for their occurrence the geophysical observations particularly gravity point to a structure whereby the crust is considerably thicker under the basins than it is under the regions where basement rock is exposed this suggests that the crust as a whole is involved in the process that led to the basin formation a mechanical model is developed which is based on the supposition that the lithosphere has been in a state of horizontal compression for long time intervals the lithosphere is considered as a viscoelastic plate that has been subjected to a small normal load distributed in an irregular manner under compression initial deflections due to this load are magnified according to the plate s physical properties the magnitude of the compressive force and the wavelength spectrum of the initial perturbations because of the viscosity the deflections grow with time at a rate that is determined by the balance of forces acting on the plate and by the lithosphere s rheology both the elastic and viscous deformations are much enhanced if erosion of the uplifted areas occurs with the sediments deposited into the downwarps these deformations take place at stresses that are much less than the buckling stresses as bending stresses increase with time failure by thrust faulting is predicted to occur within the basins with the exposed basement being thrust over the basins the model has been applied to the central australian region starting at about 900myr ago the first basins to develop are the officer and amadeus basins with uplift occurring in the intervening musgrave block initial rates of uplift and basin subsidence are small but they can increase rapidly with time if the compressive force is main

electrical structure in thunderstorm convective regions 1 mesoscale convective systems electric field e soundings through convective regions of mesoscale convective systems mcss are examined in this paper ten e soundings through updrafts in mcs convective regions and five soundings in mcs convective regions outside updrafts are used to show that a typical electrical structure exists in this region these 15 e soundings plus one other previously published sounding which is included in this analysis comprise all known soundings in the convective region of mcss the basic charge structure identified in mcs updrafts consists of four charge regions alternating in polarity with the lowest region positive and the highest region negative the basic charge structure outside updrafts in mcs convective regions has six charge regions alternating in polarity with a positive charge region lowest maximum e magnitudes of both polarities are larger and are located at lower heights in soundings outside updrafts compared to those within updrafts excepting the upper positive charge inferred charge regions are shallower and have larger charge densities outside updrafts the center of the main negative charge is lower and warmer outside updrafts 5 5 km 6 2 c than within updrafts 6 9 km 15 7 c these features inside and outside updrafts are incorporated in a new conceptual model of mcs convective region charge structure also an expanded conceptual model is developed using previously published data from other parts of mcss this more complete conceptual model shows the typical electrical charge airflow and reflectivity features in the stratiform region the transition zone and the convective region of midlatitude mesoscale convective systems

on isostasy at atlantic type continental margins cross spectral techniques have been used to analyze the relationship between free air gravity anomalies and topography of approximately 100 profiles over the continental margins of eastern north america southwest africa and the coral sea lord howe rise the resulting admittance functions for these margins have been interpreted in terms of the flexure model of isostasy the relatively young coral sea lord howe rise margin is associated with the lowest value of the effective elastic thickness te 5 km while the relatively old eastern north american margin is associated with the highest value 10 te 20 km thus te appears to increase with the age of the margin these results which are consistent with flexure studies from the major ocean basins have been interpreted in terms of a simple model in which the flexural strength of the lithosphere increases with age the model suggests that a low rigidity plate or airy type model is most applicable early in rifting history while a high rigidity plate is more applicable later in margin evolution thus sediments at a margin initially load a relatively thin and hot plate while sediments later in margin evolution load a relatively thick and cold plate the model explains a number of features of the tectonic stratigraphic evolution of margins these include a change in flexural style from a narrow and deep basin to a broad and shallow basin coastal plain onlap and uplift in the continental interior furthermore the flexure model of isostasy contributes in a major way to observed free air gravity anomalies at continental margins in particular the model explains the existance of airy isostatic gravity anomalies at margins the amplitude and wavelength of these anomalies are a function of the sediment supply the type of mechanisms controlling sediment deposition up building and or out building and the thermal structure history and age of the margin

accuracy of the lightning mapping array 1 the location accuracy of the new mexico tech lightning mapping array lma has been investigated experimentally using sounding balloon measurements airplane tracks and observations of distant storms we have also developed simple geometric models for estimating the location uncertainty of sources both over and outside the network the model results are found to be a good estimator of the observed errors and also agree with covariance estimates of the location uncertainties obtained from the least squares solution technique sources over the network are located with an uncertainty of 6 12 m rms in the horizontal and 20 30 m rms in the vertical this corresponds well with the uncertainties of the arrival time measurements determined from the distribution of chi square values to be 40 50 ns rms outside the network the location uncertainties increase with distance the geometric model shows that the range and altitude errors increase as the range squared r2 while the azimuthal error increases linearly with r for the 13 station 70 km diameter network deployed during steps the range and height errors of distant sources were comparable to each other while the azimuthal errors were much smaller the difference in the range and azimuth errors causes distant storms to be elongated radially in plan views of the observations the overall results are shown to agree well with hyperbolic formulations of time of arrival measurements e g proctor 1971 two appendices describe 1 the basic operation of the lma and the detailed manner in which its measurements are processed and 2 the effect of systematic errors on lightning observations the latter provides an alternative explanation for the systematic height errors found by boccippio et al 2001 in distant storm data from the lightning detection and ranging system at kennedy space center

heat flow on the hawaiian swell and lithospheric reheating most oceanic volcanic centres caused by hotspot activity are surrounded by sea floor that is unusually shallow for its age1 2 there is increasing evidence that swells found on older sea floor like the hawaiian swell have formed from a broad scale reheating and thinning of the lithosphere as it passes over a mantle hotspot3 4 if the formation and subsequent disappearance of these swells are controlled by thermal processes they should have an above normal heat flux we present here the results from 95 new heat flow measurements on the hawaiian swell along the older part of the swell the heat flow is 20 25 higher than the normal heat flow for crust of this age this anomaly is consistent with the observed swell uplift the shape and amplitude of the heat flow anomaly require that the reheating be largely confined to the lower half of the lithosphere

effects of charge and electrostatic potential on lightning propagation 1 three dimensional lightning mapping observations are compared to cloud charge structures and electric potential profiles inferred from balloon soundings of electric field in new mexico mountain thunderstorms for six individual intracloud and cloud to ground flashes and for a sequence of 36 flashes in one storm the comparisons consistently show good agreement between the altitudes of horizontal lightning channels and the altitudes of electric potential extrema or wells lightning flashes appear to deposit charge of opposite polarity in relatively localized volumes within the preexisting lower positive midlevel negative and upper positive charge regions associated with the potential wells the net effect of recurring lightning charge deposition at the approximate levels of potential extrema is to increase the complexity in the observed storm charge structure the midlevel breakdown of both normal intracloud flashes and negative cloud to ground flashes is observed to be segregated by flash type into the upper and lower parts of the deep potential well associated with the midlevel negative charge the segregation is consistent with perturbations observed in the bottom of the negative potential well due to embedded positive charge that was probably deposited by earlier flashes it is also consistent with an expected tendency for vertical breakdown to begin branching horizontally before reaching the local potential minimum the joint observations reconcile the apparent dichotomy between the complex charge structures often inferred from balloon soundings through storms and the simpler structures often inferred from lightning measurements

seamount loading and stress in the ocean lithosphere 2 viscoelastic and elastic viscoelastic models analytical solutions are presented for the deformation and stress state of a horizontally stratified earth subject to normal loads of size and wavelength that are characteristic of seamounts the models investigated include layered elastic plates homogeneous viscoelastic plates and elastic over viscoelastic models in all cases the composite models overlie an inviscid half space the principal advantage of the laminated elastic models over homogeneous elastic plates is that they can result in a substantial reduction in the maximum stress differences in those parts of the plate least capable of supporting large stress differences authors

epeirogenic motions of africa as inferred from cretaceous shoreline deposits epeirogenic motions can be quantified if paleodatum planes can be recognized these motions have been determined for the time interval mid cretaceous cenomanian to recent in continental africa using the vertical displacement of ancient shoreline sediments and other nearshore or low elevation deposits that are inferred to represent a cenomanian sea level datum plane the cretaceous trans saharan seaway brought shoreline sediments deep into the african continental interior and additional data suggest that most of the remaining terrestrial regions were also low lying during the cenomanian the present elevations of the sea level sediments indicate that large portions of africa have undergone epeirogenic uplift since the cenomanian the character of the motions in north africa can be described as small scale high amplitude 3 km doming separated by relatively flat lying areas of deposition the largest single epeirogenic feature is the uplift associated with red sea and east african rift development the timing of post cenomanian epeirogenic motions can be further constrained using paleontologic k ar faulting paleodrainage and sedimentary data the most likely causal mechanism for african epeirogeny may be hot spot activity reflective of the mantle convection regime underlying the african lithosphere

a gps based three dimensional lightning mapping system initial observations in central new mexico a gps based system has been developed that accurately locates the sources of vhf radiation from lightning discharges in three spatial dimensions and time the observations are found to reflect the basic charge structure of electrified storms observations have also been obtained of a distinct type of energetic discharge referred to as positive bipolar breakdown recently identified as the source of trans ionospheric pulse pairs tipps observed by satellites from space the bipolar breakdown has been confirmed to occur between the main negative and upper positive charge regions of a storm and found to be the initial event of otherwise normal intracloud discharges the latter is contrary to previous findings that the breakdown appeared to be temporally isolated from other lightning in a storm peak vhf radiation from the energetic discharges is observed to be typically 30 db stronger than that from other lightning processes and to correspond to source power in excess of 100 kw over a 6 mhz bandwidth centered at 63 mhz

some comments on two layer extensional models for the evolution of sedimentary basins simple extensional models that involve listric faulting in the brittle upper crust and plastic flow in the lower lithosphere have been shown to account for the subsidence history of various continental sedimentary basins and continental shelves in this paper we present structural evidence that extension in the continental lithosphere during rifting is not uniform we review evidence that the rifting process in major intracratonic rift basins is spasmodic rather than continuous that extension and subsidence within the rift during a tectonic phase is generally accompanied by uplift and erosion of the rift flanks that subcrustal heat input during rifting occurs over an area much broader than the rift proper and that there may be broad regional uplift before and or during rifting to explain the uplift of the flanks during rifting and their postrift subsidence we invoke two mechanisms subcrustal thinning by extension below the flanks and tilted major normal faults in the crust between graben and flanks the uplift and erosion of the flanks due to tilted faults probably do not significantly affect estimates of extension from subsidence analysis but uplift due to both sources can in certain cases be incorrectly attributed to regional uplift or doming we present a simplified analysis of the two layer extensional model for the elementary case in which extension is instantaneous and the crust is thinned by a different amount from the subcrustal lithosphere we provide a simple graphical method of data analysis to obtain the parameters that describe thinning of the crust and lithosphere during extension from the subsidence history application of this method to the central part of the pattani trough in the intracratonic gulf of thailand shows that extension and thinning of the lithosphere of a factor of 2 since late eocene can account for much of the subsidence history and the present day heat flow our analysis suggests that thinning of the crust in this area is 20 greater than thinning of the whole lithosphere assuming that none of the lithosphere is removed from the region of extension during rifting we show by simple geometric examples that a process of nonuniform extension implies that much greater thicknesses of sediment can be deposited in the center of a young basin than in the case of uniform extension of both crust and subcrustal lithosphere such an extensional process produces significant uplift of the flanks of the rift and as a result of erosion of the uplifted areas the effective cross sectional area of the basin can be increased by as much as 10 20 depending on the rate of erosion compared to the area that would have been created by a process of uniform extension finally we show that it may ultimately be possible to compute erosion rates from seismic cross sections of basins

effects of lithospheric in plane stress on sedimentary basin stratigraphy in plane lithospheric stress though known to exist has generally been ignored in the quantitative modeling of basin stratigraphy however low levels of intraplate lateral stress can induce observable plate deformations 10 100 m of vertical motion if the lithosphere contains a preexisting deformation such as a sedimentary basin the importance of in plane stress in modifying the stratigraphy of a sedimentary basin has been assessed using an elastic plate model for the lithosphere a compressive in plane stress generally induces basin subsidence with peripheral uplift shoreline regression while a tensile in plane stress induces basin uplift and peripheral subsidence shoreline transgression these simple results are complicated by variations in crustal thickness as now two interfaces are involved that is the sediment basement interface of the sedimentary basin and the moho topography the isostatic state of a sedimentary basin therefore ultimately controls the resultant deformation induced by in plane stress the implications of in plane stress modification of basin stratigraphy are profound regionally correlatable transgressions and regressions of basin interiors e g cyclothems and passive continental margins the third order variations of the vail et al 1977a coastal onlap curve may be tectonically produced by the interaction of stress induced base level changes and a basin tectonic driving subsidence in plane stress its generation magnitude and variation is considered a consequence of plate boundary reconfigurations during continental collisions rifting and subduction obduction

seamounts as recorders of hot spot epeirogeny observations in the central pacific suggest that older seamounts are uplifted if plate motion carries them over mantle hot spots a depth anomaly map of the area reveals several volcanic chains now located on younger hot spot swells uplift is estimated by comparing the observed depth of a seamount summit or the observed stratigraphic section in an atoll with that predicted assuming standard subsidence of the sea floor after the seamount formed cretaceous seamounts on the margin of the hawaiian swell have standard summit depths whereas those on the crest of the swell are at least 1 100 m shallower summit depth correlates with sea floor depth anomaly implying that the hawaiian swell has caused the uplift eocene fossils are exposed at the surface in the tuamotu atolls whereas according to the standard subsidence relation eocene deposits should be covered by 1 000 to 2 000 m of more recent material the inferred uplift is consistent with the atoll9s present position atop the society swell possible evidence for 600 m of uplift in the southern cook islands however is unexplained by swell height the observed uplifts of the cretaceous seamounts near hawaii and of the tuamotu atolls put a limit on when the hawaiian and society swells formed at their present locations of maximum elevation the swells must have developed within the past several million years and probably formed at the same time as did the younger volcanic edifices on their summits

late cenozoic uplift of stable continents in a reference frame fixed to south america a reference frame fixed to south america may approximate to an absolute reference frame for the past 100 myr in this frame otherwise stable continents that have moved onto the former positions of oceanic ridges have experienced rapid plateau uplift probably caused by phase changes in the continental mantle and occasional volcanism with time lags of up to 70 myr

radar and lightning observations of normal and inverted polarity multicellular storms from steps abstract this study discusses radar and lightning observations of two multicellular storms observed during the severe thunderstorm electrification and precipitation study the lightning mapping array data indicated that the charge structure of the 19 june 2000 storm was consistent with a normal polarity tripole while the 22 june 2000 storm exhibited an overall inverted tripolar charge structure the 19 june storm consisted of weaker convection and produced little to no hail and moderate total flash rates peaking between 80 and 120 min 1 the cells in the 22 june 2000 storm were much more vigorous exhibited strong broad updrafts and produced large quantities of hail as well as extraordinary total flash rates as high as 500 min 1 the national lightning detection network nldn indicated that the 19 june storm produced mostly negative cloud to ground cg lightning while the 22 june storm produced predominantly positive cg lightning peaking at 10 min 1 just after two cells merged however the los al

the negative gravity field over the 85 e ridge an isopach map made from seismic reflection and refraction data in the bay of bengal shows two prominent n s trending features in the basement topography one is the northernmost portion of the ninetyeast ridge which is totally buried by sediments north of 10on the other buried ridge trends roughly n s for 1400 km at 85oe to the latitude of sri lanka and then curves toward the west it has basement relief up to 6 km two free air gravity anomaly profiles across the region show a strong gravity low 60 mgal over the 85oe ridge while the ninetyeast ridge shows a gravity high to understand the negative free air gravity anomaly over the 85oe ridge we model the lithosphere as a thin elastic plate and calculate its flexural and gravitational response to an uneven sediment load a plausible formation history for a buried ridge consists of at least two major episodes the first is the formation of the ridge on a lithosphere with a flexural rigidity of d i at some later time the ridge is buried by an influx of sediments the lithosphere is cooler and the flexural rigidity has increased to d 2 the character of the gravity field depends primarily upon the initial and final values of flexural rigidity these d1 and d2 values are varied to obtain good agreement between the model and observed gravity anomalies best fitting models have a 180 times increase in flexural rigidity between ridge formation and sediment burial an approximate relationship between flexural rigidity and crustal age shows that the 85oe ridge was formed on relatively young lithosphere 5 15 m y old and that it was buried when the lithosphere was 40 80 m y old

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

reliability of self boring pressuremeter in sand the cone penetration test cpt is viewed by a majority in the geotechnical engineering profession as a preferred in situ testing tool while the self boring pressuremeter test sbpmt is sometimes

radar observations of the kinematic microphysical and precipitation characteristics of two mcss in trmm lba 1 dual doppler and polarimetric radar observations are used to analyze two mesoscale convective systems mcss that occurred during the tropical rainfall measuring mission large scale biosphere atmosphere field campaign the mcss formed in different meteorological regimes based on profiles of atmospheric wind and thermodynamic data the first mcs event 26 january 1999 was a squall line that formed in low level easterly flow and had an intense leading line of convection in contrast the 25 february 1999 mcs formed in low level westerly flow and was best characterized by stratiform precipitation with embedded convective elements the radar analyses suggest that the mcss were distinct in terms of overall vertical structure characteristics in particular polarimetric radar cross sections indicated the presence of an active mixed phase zone in the easterly mcs that was largely absent in the westerly case the easterly mcs had considerably more precipitation ice in the middle to upper troposphere compared to the westerly mcs composite analyses showed that the easterly mcs had higher peak reflectivities and a smaller reflectivity gradient above the 08c level in convective regions of the storm compared to the westerly mcs event moreover mean profiles of both vertical air motion and vertical mass transport in the convective portion of the easterly mcs were larger over a factor of 2 at some heights below the 08 c level than those in the westerly event these observations suggest that the easterly and westerly wind regimes in the southwest amazon region produce convection with different vertical structure characteristics similar to regimes elsewhere in the global tropics e g maritime continent index terms 3314 meteorology and atmospheric dynamics convective processes 3329 meteorology and atmospheric dynamics mesoscale meteorology 3354 meteorology and atmospheric dynamics precipitation 1854 3374 meteorology and atmospheric dynamics tropical meteorology keywords microphysics tropical precipitation polarimetric radar mesoscale convective system

calibration chamber tests of a cone penetrometer in sand the results of a series of tests using a cone penetrometer in sand in a large calibration chamber are reported sand at three different densities and a range of stress states was studied the principal finding from the tests is that cone resistance in sand depends primarily on the horizontal stress and the angle of friction but is relatively unaffected by the vertical stress a relationship between qc h and tc is proposed and compared with previous work in calibration chambers l article presente les resultats d une serie d essais effectues avec un penetrometre a cone dans du sable dans une chambre d etalonnage le sable a ete etudie avec trois densites differentes dans un intervalle d etats de contrainte les essais ont demontre surtout que la resistance au cone dans le sable depend d abord de la contrainte horizontale et de l angle de friction mais reste relativement peu influencee par la contrainte verticale on propose une relation entre qc h et tc et on la compare avec des recherches anteri

interpretation of moduli from self boring pressuremeter tests in sand the pressuremeter is a unique method for assessing directly the in situ shear stiffness of soils however the correct interpretation and application of the measured modulus must account for the relevant stress and strain level acting around the pressuremeter during the test a method to correct the measured unload reload shear modulus from self bored pressuremeter tests in sands is proposed the method has been evaluated using extensive data obtained from 47 tests performed in a large calibration chamber using pluvially deposited silica sand and from 25 tests performed in situ in a natural deposit of relatively clean silica sand at the river po italy a consistent relationship was obtained between the corrected unload reload shear modulus and the small strain shear modulus determined from resonant column tests and field cross hole tests suggestions are given to link the measured moduli with moduli values required for geotechnical design problems the importance of strain level stress strain model yie

three dimensional kinematic and microphysical evolution of florida cumulonimbus part ii frequency distributions of vertical velocity reflectivity and differential reflectivity abstract high resolution radar data collected in florida during the convection and precipitation electrification experiment are used to elucidate the microphysical and kinematic processes occurring during the transition of a multicellular storm from convective to stratiform stages a statistical technique is employed to examine the evolving properties of the ensemble small scale variability of radar reflectivity vertical velocity and differential reflectivity over the entire storm differential radar reflectivity data indicate that the precipitation at upper levels was nearly glaciated early in the storm s lifetime dual doppler radar data show that throughout the storm s lifetime both updrafts and down drafts were present at all altitudes and that most of the volume of the radar echo contained vertical velocities incapable of supporting precipitation size particles thus the ensemble microphysical properties of the storm were increasingly dominated by particles falling in an environment of weak vertic

convective initiation near the andes in subtropical south america abstractsatellite radar and radiometer data indicate that subtropical south america has some of the deepest and most extreme convective storms on earth this study uses the full 15 yr trmm precipitation radar dataset in conjunction with high resolution simulations from the weather research and forecasting model to better understand the physical factors that control the climatology of high impact weather in subtropical south america the occurrence of intense storms with an extreme horizontal dimension is generally associated with lee cyclogenesis and a strengthening south american low level jet sallj in the la plata basin the orography of the andes is critical and model sensitivity calculations removing and or reducing various topographic features indicate the orographic control on the initiation of convection and its upscale growth into mesoscale convective systems mcss reduced andes experiments show more widespread convective initiation weaker average storm intensity and more rapid propagation

formation of charge structures in a supercell abstract lightning mapping electric field and radar data from the 26 may 2004 supercell in central oklahoma are used to examine the storm s charge structure an initial arc shaped maximum in lightning activity on the right flank of the storm s bounded weak echo region was composed of an elevated normal polarity tripole in the region of precipitation lofted above the storm s weak echo region later in the storm two charge structures were associated with precipitation that reached the ground to the left of the weak echo region six charge regions were inferred with positive charge carried on hail at the bottom of the stack farther forward in the storm s precipitation region four charge regions were inferred with negative charge at the bottom of the stack there were different charge structures in adjacent regions of the storm at the same time and regions of opposite polarity charge were horizontally adjacent at the same altitude flashes occasionally lowered positive charge to ground from the forwa

uplift and strength evolution of passive margins inferred from 2 d conductive modelling summary post breakup vertical motions of passive margins are seen here as a result of the post rift 2 d thermal evolution a 2 d finite element numerical model is performed to evaluate both the vertical and horizontal conduction that drive the thermal evolution of continental passive margins from breakup to post breakup states initial temperature configurations corresponding to non volcanic and volcanic margins are tested and lead to different thermal evolution of the lithosphere for both margins a thermal thickening is observed in the stretched lithosphere whereas the unstretched lithosphere undergoes first 0 80 ma a thermal thinning and secondly after 80 ma a thermal thickening in comparison with non volcanic margins volcanic margins show a slower thermal thickening and a greater thermal thinning in stretched and unstretched lithosphere respectively the variations with time of lithosphere thickness are then translated into isostatic vertical movements and reveal seaward thermal induced subsidence and landward thermal induced uplift the estimated uplift reaches up to 250 m in volcanic margins and 120 m in non volcanic margins the modelled timing and amount of uplift in both margins are consistent with present day topography of volcanic passive margins that stand two to three times higher than non volcanic margins using these thermal models we finally show that the 2 d strength of the margins drastically evolves with time from a seeward dominant strength 0 80 ma toward a landward dominant strength time larger than 80 ma these lateral strength evolution could have strong effect on the flexural response of the margin through time

erosion isostatic rebound models for uplift an application to south eastern australia summary a mountain s history includes two distinctive phases one of active tectonism and construction followed by one of erosion and passive isostatic rebound in the first phase uplift is driven by tectonic mechanisms while in the second phase base levels of the terrain are regionally uplifted it is this latter phase that is modelled here the starting model is a mountain range initially in isostatic equilibrium on a viscoelastic plate defined by the effective flexural rigidity d and relaxation time 7 the rate of erosion at any time t is assumed to be proportional to the elevation at that time with an erosional time constant 7 for a given present day topography the uplift erosion gravity and stress can be computed through time as functions of d 7 7 and the time to at which the erosional rebound mechanism became the dominant landscaping process the model has been applied to the highlands of south eastern australia which we assume to be an erosional residue of the palaeozoic lachlan fold belt observations of rivers cutting through cainozoic basalts and other geomorphological indicators of uplift can then be interpreted in terms of the isostatic rebound and there is no need to invoke active tectonic uplift mechanisms the model parameters that fit the observations are t 150 3 50myr dt 2 5 x ioz4nmmyr and to 180 200myr the predicted late palaeozoic fold belt topography is about 75 per cent greater than the present day values topographic depressions are predicted to have occurred along the margins of the formerly elevated areas and the model is relevant to understanding the evolution of basins flanking the highlands within the highland area erosion generally exceeds uplift but in some regions the two are approximately equal in keeping with recent geomorphological observations the flexural stresses associated with the erosion initially increase with time and maximum values in south eastern australia are predicted to occur in late cainozoic time with the stress state near the surface being mainly tensional

the relationship between precipitation and lightning in tropical island convection a c band polarimetric radar study one of the primary scientific objectives of the maritime continent thunderstorm experiment was to study cloud electrification processes in tropical island convection in particular the coupling between ice phase precipitation and lightning production to accomplish this goal a c band polarimetric radar was deployed in the tropics 11 68s 130 88e for the first time accompanied by a suite of lightning measurements using observations of the propagation corrected horizontal reflectivity and differential reflectivity along with specific differential phase rain and ice masses were estimated during the entire life cycle of an electrically active tropical convective complex known locally as hector over the tiwi islands on 28 november 1995 hector s precipitation structure as inferred from these raw and derived radar fields was then compared in time and space to the measured surface electric field cloud to ground cg and total lightning flash rates and ground strike locations during hector s developing stage precipitating convective cells along island sea breezes were dominated by warm rain processes no significant electric fields or lightning were associated with this stage of hector despite substantial rainfall rates aided by gust front forcing a cumulus merger process resulted in larger taller and more intense convective complexes that were dominated by mixed phase precipitation processes during the mature phase of hector lightning and the surface electric field were strongly correlated to the mixed phase ice mass and rainfall merged convective complexes produced 97 of the rainfall and mixed phase ice mass and 100 of the cg lightning as hector dissipated lightning activity rapidly ceased as evidenced from the multiparameter radar observations the multicell nature of hector resulted in the continuous lofting of supercooled drops to temperatures between 2108 and 2208c in discrete updraft cores during both the early and mature phases the freezing of these drops provided instantaneous precipitation sized ice particles that may have subsequently rimed and participated in thunderstorm electrification via the noninductive charging mechanism

electrical and multiparameter radar observations of a severe hailstorm using data from the 11 cm csu chill multiparameter radar the simultaneous evolution of the radar inferred precipitation structure and electrical characteristics of a severe hailstorm is investigated we compare the substorm point discharge current intracloud ic lightning flash rate cloud to ground cg lightning flash rate ground strike location and flash polarity to the progression of precipitation types and amounts this study is unique in that it presents multiparameter radar observations of a thunderstorm complex which exhibited an extremely high ic to cg ratio ic cg 20 70 and predominantly positive cg lightning over 74 after it became severe producing large hail and weak tornadoes in particular we investigate the reported relationship between large hail and positive cg lightning although a direct correlation is found between a rapid increase in ic cg the first positive cg lightning flashes and the initial production of hail aloft the temporal and spatial behavior of large hail and positive cg lightning appear to be anticorrelated as broad peaks in the positive cg flash rate lag relative maxima in the fallout of large hail by up to 30 min the majority of positive ground strikes were adjacent to the main precipitation core in a region of light rain and little or no hail at the surface aloft radar data indicated that ice crystals were aligned vertically in a strong electric field corona point observations at the surface indicate that these regions adjacent to the convective core were characterized by net positive charge aloft possible mechanisms to explain these observations are discussed

effect of disturbance on parameters derived from self boring pressuremeter tests in sand a series of field tests has been performed to investigate the effects of drilling disturbance on the values of the parameters derived from self boring pressuremeter tests in sand the approach adopted consisted of using different drilling shoe and cutter geometries thereby deliberately causing varying amounts of drilling disturbance the results show that the values of friction angle and dilation angle are sensitive to drilling disturbance a procedure for correcting for the effects of drilling disturbance is proposed however it is demonstrated that the best results are obtained when drilling disturbance is kept to a minimum une serie d essais in situ a ete executee pour examiner les effets du remaniement par le forage sur les valeurs des parametres derives des essais d autoforage pressiometrique dans le sable la methode adoptee comporte l emploi de differentes geometries du sabot de forage et de l outil de coupe afin de provoquer expres des valeurs variees de remaniement par le forage les res

a new isostatic model for the east pacific rise crest we have developed a new isostatic model for the east pacific rise epr that explains the origin of both the axial topographic high and the rise crest gravity anomaly our model involves a thin elastic plate with a constant thickness crust that is broken at the ridge crest we assume that a buoyant force beneath the plate bends the free edge of the plate upward at the rise axis forming the topographic high and the rise crest gravity anomaly we have used this model and two conventional isostatic models to examine the isostasy of three segments of the epr at 9 l4 n 6 11 s and 16 21 s rather than modeling individual profiles we have stacked summed the gravity and bathymetry data in each area and computed a single 200 km long mean mirrored profile for each different ridge segment these stacked profiles are a remarkable improvement over even the best individual profiles and clearly show both the axial topographic high and its associated free air gravity anomaly in all three areas the best fitting model parameters are similar and give average compensation depths of about 6 7 km below the seafloor near the base of the oceanic crust and an upper limit on the flexural rigidity of the plate that increases from about 1018 n m at the ridge axis to about 3 1019 n m 5 km from the axis and 1020 n m 25 km or more from the rise crest these results indicate that the epr crest is compensated at much shallower depths than suggested by previous investigators our results are consistent with the presence of a small low density magma chamber in the lower crust although at least part of the compensating root for the axial high must extend into the upper mantle we interpret the low upper mantle densities below the rise crest as a zone of partial melt which accumulates at the base of the oceanic crust between episodic rifting events when the overlying crustal magma chamber is replenished

microphysical and kinematic processes associated with anomalous charge structures in isolated convection microphysical and kinematic characteristics of two storm populations based on their macroscale charge structures are investigated in an effort to increase our understanding of the processes that lead to anomalous or inverted charge structures nine normal polarity cases mid level negative charge with dual doppler and polarimetric coverage that occurred in northern alabama and six anomalous polarity cases mid level positive charge that occurred in northeastern colorado are included in this study the results show that even though anomalous polarity storms formed in environments with similar instability they had significantly larger and stronger updrafts moreover the anomalous polarity storms evidently have more robust mixed phase microphysics based on a variety of metrics anomalous polarity storms in colorado have much higher cloud base heights and shallower warm cloud depths in this study leading us to hypothesize that anomalous polarity storms have lower amounts of dilution and entrainment we infer positively charged graupel and therefore high supercooled water contents in the mid levels of the anomalous storms based on the relationship between colocations of graupel and inferred positive charge from lightning mapping array data using representative updraft speeds and warm cloud depths the time required for a parcel to traverse from cloud base to the freezing level was estimated for each storm observation we suggest this metric is the key discriminator between the two storm populations and leads us to hypothesize that it strongly influences the amount of supercooled water and the probability of positive charge in the midlevels leading to an anomalous charge structure

thermally induced residual topography within oceanic lithosphere the lithosphere following a thermal event should subside isostatically to its pre event configuration however if the thermal event is sufficient to reset the rigidity of the lithosphere then the subsidence is forced to follow a different path to the initial uplift lithospheric rigidity decrease during uplift allows flexural deformation resetting while during subsidence an increasing rigidity with time results in flexural interference this thermo mechanical hysteresis effect may be responsible for anomalous bathymetry and free air gravity anomalies associated with hotspot traces extinct spreading centres and the ocean continent boundary of rifted passive continental margins

flexural models of continental lithosphere based on the long term erosional decay of topography summary the most easily observed geophysical characteristic of continents is their surface topography topographic power in the spectral range 1000 100 km has been computed for north american geological provinces and is found to decrease systematically as the tectonic age of the underlying crust increases these data have been quantitatively modelled in terms of topographic erosion and stress relaxation within the continental lithosphere through geological time spans the lithosphere is treated as a thin maxwell viscoelastic plate which deforms flexurally in response to the vertical surface loads of topography and its erosion effective rheological model parameters are flexural rigidity d a measure of the lithosphere s elastic strength and viscous relaxation time constant a measure of its viscosity erosion is assumed to occur at a rate linearly proportional to the height of topography at any given time the proportionality being determined by the erosion time constant the models are constructed in space and time frequency domains using simple linear systems theory two loading schemes are considered model 1 assumes that a flexurally competent lithosphere is suddenly loaded by topography which subsequently erodes whereas model 2 ignores the emplacement and erosion of topography during orogenesis when the lithosphere is thermally activated and considers only the load effected by erosion beginning some time later when the lithosphere has cooled and strengthened both models predict that the remnant topography of very old regions such as the canadian shield can be maintained in fact is favoured by a viscoelastic lithosphere with as small as 1 10ma the continued existence of such topography does not necessarily imply significant long term lithospheric strength model 1 results indicate low values of the flexural rigidity d 1021 1022nm which are interpreted to characterize not so much presentday d but that which was effective when topography was formed model 2 more successfully reproduces the data and constrains the product d to values 1024 1025nm ma which compare very favourably to those found from studies of individual loads on continental lithosphere the model results show that the erosion time constant is probably wavenumber dependent and falls in the range 200 400ma for the topographic wavelengths under consideration

seafloor spreading history of the western north atlantic basin derived from the keathley sequence and computer graphics abstract an analysis of sea surface magnetic profiles and an aeromagnetic contour chart west of bermuda yields a new tectonic framework for the keathley m series seafloor spreading magnetic anomaly sequence changes in spreading direction derived from the western keathley sequence occur at times corresponding to m24 m21 and m11 of which m11 is a time of continental breakup the models facilitate new identifications of the anomalies between m16 and m4 obviating the need for anomalously slow spreading between m11 and m4 previously suggested by other workers and by us and placing the location of the well defined rough smooth basement boundary at m11 the models show that spreading rates contemporaneously vary among profiles collected between flowlines as determined by a recently presented kinematic history this strongly suggests that individual ridge axis segments act independently of another in detail our reconstruction technique records in map form where the ridge crest has made significant departures from perfect symmetry and a predictable spreading rate during its evolutionary history among eight profiles collected parallel to seafloor spreading flowlines in the western north atlantic encompassing only 2700 km of normal oceanic crust we can identify more than fourteen ridge jump events there is evidence for a small 30 km ridge jump to the east of many ridge axis segments at m24 this is supported in the eastern north atlantic conjugate crust of the canary basin where m25 and m24 are poorly represented a series of four ridge jumps to the east along one paleoridge axis segment now located at 36 n 65 w is related to the passage of the mesozoic mid atlantic ridge crest over the verde hotspot at 137 m y b p the project magnet magnetic anomaly contour chart for the area between 29 n and 40 n has been used to determine stage poles of motion for processes occurring at the ridge crest related to its segmentation which have a component oblique to spreading between chrons m21 and m11 these trends may relate to the motion of the ridge crest over the mantle

lithospheric flexure and thermal anomalies plots of effective elastic thickness te of the oceanic lithosphere as a function of age when elastically deformed show a general increase in te with the square root of age although there is much scatter some of the scatter can be eliminated by correcting for the effect of finite yield strength in rocks which makes te for more sharply bent plates underestimate the true depth tm to the rheological boundary at the base of the high strength mechanical lithosphere after converting effective values te to true thicknesses tm it appears that the mechanical lithosphere is always thinner beneath islands and seamounts compared to lithosphere of similar age flexed at subduction zones from several other lines of evidence such as depth anomalies seamount subsidence and heat flow data it has been suggested that the thermal structure in plates subjected to hot spot type volcanism is reset to values appropriate for much younger lithosphere the reduction in mechanical plate thickness beneath hot spot volcanoes is also consistent with this concept of thermal rejuvenation since the strength of rocks decreases exponentially with rising temperature this dependence implies that variations in mechanical plate thickness can be used to map temperature anomalies in the lithosphere since the mechanical plate thickness depends principally upon the temperature at one depth specifically the depth at which the temperature reaches 550 600 c depending on strain rate flexural data are particularly sensitive indicators of the vertical structure of lithospheric reheating compared to quantities dependent on depth integrals of temperature such as swell height and subsidence the thermal rejuvenation hypothesis predicts that the flexural profile should mimic viscoelastic relaxation in the first 10 ma after loading if initially reheating is confined to depths close to and below the 550 c isotherm the flexural data combined with depth anomaly information for hawaii point to extremely rapid reheating localized in the lower lithosphere rejuvenation in the society islands on the other hand is compatible with slower methods of heat injection involving the entire lithospheric column

observations of two sprite producing storms in colorado two sprite producing thunderstorms were observed on 8 and 25 june 2012 in northeastern colorado by a combination of low light cameras a lightning mapping array polarimetric and doppler radars the national lightning detection network and charge moment change measurements the 8 june event evolved from a tornadic hailstorm to a larger multicellular system that produced 21 observed positive sprites in 2 h the majority of sprites occurred during a lull in convective strength as measured by total flash rate flash energy and radar echo volume mean flash area spiked multiple times during this period however total flash rates still exceeded 60 min sup 1 and portions of the storm featured a complex anomalous charge structure with midlevel positive charge near 20degc the storm produced predominantly positive cloud to ground lightning all sprite parent flashes occurred on the northeastern flank of the storm where strong westerly upper level flow was consistent with advection of charged precipitation away from convection providing a pathway for stratiform lightning the 25 june event was another multicellular hailstorm with an anomalous charge structure that produced 26 positive sprites in less than 1 h the sprites again occurred during a convective lull with relatively weaker reflectivity and lower total flash rate but relatively larger mean flash area however all sprite parents occurred in or near convection and tapped charge layers in adjacent anvil cloud the results demonstrate the sprite production by convective ground strokes in anomalously charged storms and also indicate that sprite production and convective vigor are inversely related in mature storms

volcanism around k t boundary time its role in an impact scenario for k t extinction events abstract a widespread debate on the cretaceous tertiary k t boundary 65 1 ma extinctions has largely revolved around an impact or a volcanic cause evidence for both events is strengthened by recent work impact triggered deccan volcanism or independent impact and mantle plume events have been suggested this review examines evidence for both events and suggests that they overlapped in time but with globally uneven effects the combined effects produced generally inhospitable k t environments leading to mass extinctions although some life forms flourished afterwards volcanic contributions to the k t boundary environment probably include increased platinum group element pge and acid rain fall out particularly from large scale mantle upwellings e g deccan cameroon and coral sea events a previously proposed post k t greenland starting plume apparently already existed in the cretaceous k t features such as the iridium ir anomaly most likely include longer term volcanic as well as sudden meteoritic impact contributions volcanic contributions were greatest from southern hemisphere sources whereas meteorite impacts featured in the northern hemisphere

the vertical profile of radar reflectivity of convective cells a strong indicator of storm intensity and lightning probability abstract reflectivity data from doppler radars are used to construct vertical profiles of radar reflectivity vprr of convective cells in mesoscale convective systems mcss in three different environmental regimes the national center for atmospheric research cp 3 and cp 4 radars are used to calculate median vprr for mcss in the oklahoma kansas preliminary regional experiment for storm central in 1985 the national oceanic and atmospheric administration tropical ocean global atmosphere radar in darwin australia is used to calculate vprr for mcss observed both in oceanic monsoon regimes and in continental break period regimes during the wet seasons of 1987 88 and 1988 89 the midlatitude and tropical continental vprrs both exhibit maximum reflectivity somewhat above the surface and have a gradual decrease in reflectivity with height above the freezing level in sharp contrast the tropical oceanic profile has a maximum reflectivity at the lowest level and a very rapid decrease in reflectivity with he

evolution of lightning activity and storm charge relative to dual doppler analysis of a high precipitation supercell storm abstracta high precipitation tornadic supercell storm was observed on 29 30 may 2004 during the thunderstorm electrification and lightning experiment observational systems included the oklahoma lightning mapping array mobile balloon borne soundings and two mobile c band radars the spatial distribution and evolution of lightning are related to storm kinematics and microphysics specifically through regions of microphysical charging and the location and geometry of those charge regions lightning flashes near the core of this storm were extraordinarily frequent but tended to be of shorter duration and smaller horizontal extent than typical flashes elsewhere this is hypothesized to be due to the charge being in many small pockets with opposite polarities of charge close together in adjoining pockets thus each polarity of lightning leader could propagate only a relatively short distance before reaching regions of unfavorable electric potential in the anvil however lightning extended tens of kilome