late diagenetic redistribution of uranium and disturbance of the u pb whole rock isotope system in a black shale the middle ordovician llandeilo caradoc formation black shales deposited at about 460 ma on the southwest margin of the welsh basin show evidence of a late diagenetic uranium redistribution event evidence for this redistribution event includes a disturbance in the u pb whole rock isotopic system at about 193 ma marked by a large range in the whole rock 238 u 204 pb between 2 and 90 one explanation for this large range is that individual whole rocks have experienced either a gain or a loss of u at the time of the disturbance this hypothesis is supported by the large range in measured u contents from the llandeilo caradoc formation black shales 0 16 to 6 6 ppm upper crust 2 8 ppm post archean average shale paas 3 1 ppm taylor and mclennan 1985 uranium that was likely sequestered during deposition and early diagenesis has been redistributed on a greater than whole rock scale centimeters to meters the u pb isotope systematics identified in this study and other pb isotope data from the welsh basin evans 1996 constrain a reasonable u concentration of 2 91 ppm for an unaltered llandeilo caradoc shale at the time of deposition 460 ma assuming 2 91 ppm to be an average u content at the time of deposition some rocks in the llandeilo caradoc formation have lost as much as 95 of the u likely present at the time of deposition and early diagenesis whereas others exhibit an almost equal enrichment when the u pb isotopic and elemental data from whole rocks and individual u bearing phases i e carbonate apatite monazite organic matter are examined in conjunction with the paragenetic sequence controls on the distribution of u in the llandeilo caradoc whole rocks and a pattern of open system conditions become apparent despite the presence of potential u rich diagenetic phases such as apatite and monazite most of the u in the relatively enriched samples is associated with a non extractable unidentified diagenetic component uranium mass balance considerations and whole rock u pb isotope systematics require that the non extractable component formed at about 193 ma during a late diagenetic uranium redistribution event the redistribution of u within the llandeilo caradoc formation has important implications for the use of u rich sedimentary minerals as geochronometers and for the use of u as a paleo redox indicator specifically a trace element indicator like uranium may in some cases record information about the postdepositional history of a sedimentary rock rather than document the absolute timing and conditions of deposition moreover this unique history can be properly evaluated only through an integrated petrographic geochemical approach that can discern the distribution and source i e depositional versus diagenetic of u in an individual whole rock

surface deformation of westerly granite during creep holographic interferometry has been used to observe the surface deformation of westerly granite during creep experiments at 50 mpa 500 bars confining pressure interference fringes on the holograms produced by this technique constitute contour maps of the sample surface shape change between exposures several distinctive characteristics of the surface shape change have been observed in general surface deformation during primary and secondary creep while highly inhomogeneous is distributed fairly uniformly no large scale pattern is seen in the surface shape changes at approximately the onset of tertiary creep surface displacements become more locallized eventually approx 85 90 of time to failure a distinct ridge is formed which exactly coincides with the location of the final fracture

path dependence of acoustic velocity and attenuation in experimentally deformed westerly granite brittle failure in rocks is generally preceded by a localization of deformation in the region of the final macrofracture this process is associated with a non uniform distribution of subcritical microcracks which should be reflected in the distribution of acoustic velocities and surface deformation the temporal and spatial development of such deformation zones has been studied by monitoring these quantities during the deformation of westerly granite samples

breakage and shear behaviour of intermittent rock joints abstract the breakage and shear behaviour of intermittent rock joints have been investigated in a series of direct shear tests with a new shear device specifically designed for this purpose the tests have been performed on specimens of rock like material or hard rock respectively incorporating idealized non persistent joints made up of a number of short cracks in an en echelon arrangement along the central shear axis the shear behaviour of such a joint constellation has been found to be composed of different phases the first phase of shearing is that of the actual rupture initiated by the formation of wing cracks starting from the existing cracks and growing into the material bridges and concluded by the generation of additional new fractures connecting the initial cracks in the zone between the wing cracks the second phase of shearing is characterized by friction processes and volume increase in the then continuous shear zone finally the third phase of shearing reached after large shear displacements is determined by sliding processes inside the strongly fractured shear zone in a large number of shear tests the geometrical parameters of the discontinuous joints as well as the loading conditions have been found to influence the activated shear resistance in each phase of shearing to a noticeably different extent the orientation of the initial cracks and the normal stress however have been identified as the most influential parameters depending on the test conditions an initially discontinuous rock joint can activate the largest shear resistance not just before rupture but in one of the two subsequent phases of shearing as well the mechanisms which govern the different shear phases could be identified as 1 tensile rupturing 2 rolling and sliding friction of dilatant joint zones and 3 sliding within the joint filling composed of brecciated material

subcritical crack growth and other time and environment dependent behavior in crustal rocks stable crack growth strongly influences both the fracture strength of brittle rocks and some of the phenomena precursory to catastrophic failure quantification of the time and environment dependence of fracture propagation is attempted with the use of a fracture mechanics technique we examine some of the difficulties encountered when applying techniques originally developed for simple synthetic materials to complex materials like rocks a picture of subcritical fracture propagation is developed that embraces the essential ingredients of the microstructure a microcrack process zone and the different roles that the environment plays to do this we examine the results of 1 fracture mechanics experiments on five rock types 2 optical and scanning electron microscopy 3 studies of microstructural aspects of fracture in ceramics and 4 exploratory tests examining the time dependent response of rock to the application of water

development of subcracks between en echelon fractures in rock plates uniaxial compression tests on marble plates containing two prefabricated en echelon fractures were performed in this study photographs showing the typical characteristics of subcrack development were taken under direct scanning electron microscope sem observation during the test from these photographs the effects of the length l of a single fracture the separation distance d and overlapping proportion a l between two prefabricated en echelon fractures on the development of subcracks were analyzed the results show that the interaction between en echelon fractures strengthened with decreasing d for d l there was little interaction and the development of subcracks near one of the en echelon fractures was unaffected by the existence of the other whereas for d l the number of subcracks in the area intermediate between en echelon fractures tended to increase with increasing a and decreasing d in order to ascertain whether the experiment can furnish some implications for research on earthquake prediction and neotectonic activity the test results were compared with the spatial temporal development of foreshocks and ground water anomalies before the 1975 haicheng earthquake and the structural framework of the ganzi pull apart basin the results of the comparison are encouraging according to the similarity between the test results in the laboratory and the natural phenomena in the field the fault on which the haicheng earthquake would occur could be inferred immediately from the relative geometry of the two sub parallel active faults in the area thus it is considered that the test results would advance our understanding of the process of neotectonic activities and give us inspiration for earthquake prediction

nucleation process of an m2 earthquake in a deep gold mine in south africa inferred from on fault foreshock activity using a network of sensitive high frequency acoustic emission sensors we observed foreshock activity of an mw 2 2 earthquake main shock in a deep gold mine in south africa foreshock activity which selectively occurred on a part of the rupture plane of the forthcoming main shock lasted for at least 6 months until the main shock rock samples recovered from the main shock source region showed evidence of ancient hydrothermal alteration on the main shock rupture plane suggesting that the foreshock activity occurred on a preexisting weakness the foreshocks during 3 months before the main shock were concentrated in three clusters f1 f3 which we interpret as representing localized preslip at multiple sites while the location of mining area the source of stress perturbations changed with time the locations of foreshock clusters did not change suggesting that the preslip patches were controlled by strength heterogeneity rather than stress distribution activity over the entire foreshock area was generally constant but the largest cluster f2 showed accelerated activity starting at least 7 days before the main shock while mining stress did not increase in this period the main shock initiated at a point close to f1 away from f2 all the six foreshocks during the final 41 h occurred in f1 and f2 and in between these suggest that in the last stage of the preparation process of the main shock preslip patches interacted with each other through the stress concentration ahead of the expanding preslip patch f2 which should be the only driving force of the preparation process under the constant external load

thunderstorms above frontal surfaces in environments without positive cape part i a climatology abstract the first of two papers describing thunderstorms that occur above frontal surfaces frequently in environments without positive convective available potential energy cape focuses on the climatology of such storms for the conterminous united states the dataset used consists of 1093 observations made over a 4 year period the events were selected using conventional network data and a set of criteria that eliminated thunderstorms rooted in the boundary layer a composite of the dataset shows that the typical elevated thunderstorm occurs northeast of an associated surface low pressure center and north of a surface warm front in a region with northeasterly surface winds the planetary boundary layer is generally very stable as determined by comparisons with both the 50 kpa and 85 kpa air the thunderstorms are usually found in the left exit region of a low level wind maximum an area of horizontal deformation the large scale environment is strongly baroclinic with large vertical wind shear an

experiment and finite element simulation of x type shear fractures from a crack in marble abstract laser holographic interferometry is used to measure the out of plane displacement field in the uniaxial compression testing of a marble plate with an inclined crack the experiment shows that near the tips of the crack tensile fractures first occur they propagate curvilinearly bend toward the axial direction and become stabilized this agrees well with the usual theory however if the crack is an open slot as the compressive load increases x type shear fractures appear near the tips of the slot propagate in the direction opposite to the tensile fractures and lead to final rupture of the specimen this phenomenon can be related to the strain softening behavior near the tip of the slot a mathematical model using finite element analysis for strain softening of elastoplastic material is used to simulate the process and obtain the same result if the crack is a closed cleavage the rupture takes place as x type shear fractures starting from the boundary of the plate

control of the geometric arrangement of material heterogeneities on strain localization at the brittle to ductile transition in experimentally deformed carbonate rocks abstract triaxial high temperature 900 c deformation experiments were conducted at constant strain rate in a paterson type deformation apparatus on cylinders of carrara marble with two right or left stepping non overlapping weak inclusions of solnhofen limestone oriented at 45 to the cylinders longitudinal axes applying different values of confinement 30 50 100 and 300 mpa we induced various amounts of brittle deformation in the marble matrix and investigated the effect of brittle precursors on the initiation and development of heterogeneity induced high temperature shear zones viscosity contrast between the matrix and the inclusions induces local stress concentration at the tips of these latter the initial arrangement of the inclusions results in either an overpressured contractional or underpressured extensional domain in the step over region of the sample at low confinement 30 and 50 mpa abundant brittle deformation is observed but the spatial distribution of microfractures is dependent on the kinematics of the step over region microcracks occur either along the shearing plane between inclusions in extensional bridge samples or broadly distributed outside the step over region contractional bridge samples accordingly ductile deformation localizes along the inclusions plane in the extensional bridge samples as opposed to distributing over large areas of the matrix in the contractional bridge samples if microcracking is suppressed high confinement strain is accommodated by viscous creep and strain progressively de localizes in extensional bridge samples our experiments demonstrate that brittle precursors enhance the degree of localization in the ductile deformation regime but only if the interaction of pre existing heterogeneities induces an extensional mean stress regime in between

first lead isotopic data for cinnabar in the almaden district spain implications for the genesis of the mercury deposits the almaden district constitutes the largest and probably the most intriguing mercury concentration in the world two types of mineralization are recognized 1 stratabound of lower silurian age well represented by the large almaden deposit and 2 fully discordant mercury deposits of minor importance in terms of size and exemplified by the deposit of las cuevas the latter ones can be found at different positions along the almaden stratigraphic column both types of deposits are always associated with the so called frailesca rocks diatremes of alkali basaltic composition this paper reports the first lead isotope compositions of cinnabar in the district whole samples and stepwise leaching cinnabar aliquots display relatively homogeneous isotopic compositions 206pb 204pb 18 112 18 460 207pb 204pb 15 635 15 705 208pb 204pb 38 531 38 826 taken together with jebrak et al s 2002 pyrite lead isotope results the new cinnabar isotopic data define a steep array trend on the 207pb 204pb 206pb 204pb diagram indicating a mixed contribution of lead and probably mercury from different sedimentary sources in the almaden basin the almaden hg deposits are related to a contemporaneous mafic magmatism that might have provided part of the mercury hydrothermal leaching of organic matter from sedimentary rocks and formation of hg organic complexes enhanced metal solubility promoting transport from and within the volcanic units

time dependent cracking and brittle creep in crustal rocks a review abstract rock fracture under upper crustal conditions is driven not only by applied stresses but also by time dependent chemically activated subcritical cracking processes these subcritical processes are of great importance for the understanding of the mechanical behaviour of rocks over geological timescales a macroscopic manifestation of time dependency in the brittle field is the observation that rocks can deform and fail at constant applied stresses a phenomenon known as brittle creep here we review the available experimental evidence for brittle creep in crustal rocks and the various models developed to explain the observations laboratory experiments have shown that brittle creep occurs in all major rock types and that creep strain rates are extremely sensitive to the environmental conditions differential stress confining pressure temperature and pore fluid composition even small changes in any of these parameters produce order of magnitude changes in creep strain rates and times to failure three main classes of brittle creep model have been proposed to explain these observations phenomenological statistical and micromechanical statistical and micromechanical models explain qualitatively how the increasing influence of microcrack interactions and or the increasing accumulated damage produces the observed evolution of macroscopic deformation during brittle creep however no current model can predict quantitatively all of the observed features of brittle creep experimental data are limited by the timescale over which experiments are realistically feasible clearly an extension of the range of available laboratory data to lower strain rates and the development of new modelling approaches are needed to further improve our current understanding of time dependent brittle deformation in rocks

microphysical properties of convectively generated fall streaks within the stratiform comma head region of continental winter cyclones abstractthis paper presents analyses of the microphysical structure of comma head stratiform precipitation in 14 continental cyclones focusing on fall streaks of hydrometeors produced by cloud top convective generating cells data were obtained at temperatures between 4 and 45 c using in situ instrumentation and the w band university of wyoming cloud radar all operated aboard the national science foundation national center for atmospheric research c 130 analyses are presented first for a case study of one cyclone followed by statistical analyses of the full dataset using radar based objective classifications the statistical percentile number concentrations averaged 1 9 times larger within the fall streaks compared to the regions between them and the corresponding ice water content and median mass diameter values averaged 2 2 and 1 1 times larger ice phase conditions were predominant within the stratiform precipitation with deposition and aggregation the primary ice growth mechanisms no distinc

instability characteristics of radar derived mesoscale organization modes within cool season precipitation near portland oregon abstractthe instability characteristics associated with different radar derived mesoscale organization modes are examined using six cool seasons of operational scanning radar data near portland oregon and operational sounding data from salem oregon additionally several years of microwave rain radar ka band vertically pointing radar data from portland and merwin washington are used to characterize the nature and occurrence of generating cells and fall streaks the combination of a new metric convective stratiform intermittency with the classification of radar reflectivity maps into convective and stratiform precipitation types was applied to periods when the freezing level was 1 4 km altitude this method distinguishes periods with embedded convective within stratiform mesoscale organization from those that were mostly convective or mostly stratiform mesoscale organization occurs in a continuum of states with predominantly stratiform structure occurring most frequently generating cells in the s

the north american lightning detection network naldn analysis of flash data 2001 09 abstractcloud to ground cg lightning data have been analyzed for the years 2001 09 for north america which includes alaska canada and the lower 48 u s states flashes recorded within the north american lightning detection network naldn are examined no corrections for detection efficiency variability are made over the 9 yr of the dataset or over the large geographical area comprising north america there were network changes in the naldn during the 9 yr but these changes have not been corrected for nor have the recorded data been altered in any way with the exception that all positive lightning reports with peak currents less than 15 ka have been deleted thus the reader should be aware that secular changes are not just climatological in nature all data were analyzed with a spatial resolution of 20 km the analyses presented in this work provide a synoptic view of the interannual variability of lightning observations in north america including the impacts of physical changes in the network dur

a new approach for the determination of the age of partial or complete homogenization of pb isotopes example anchimetamorphic detrital sediments of the central iberian zone spain abstract pb isotope systematics are discussed for lowermost ordovician to devonian pelites of the almaden area central iberian zone of spain the rocks suffered a carboniferous anchimetamorphic overprint which variably reset the rb sr whole rock systems at 335 ma the weak metamorphic event partially homogenized the pb isotopes of pelites between 330 and 320 ma this time range of partial homogenization was deduced by stepwise age correction t1 206 pb 204 pb vs 207 pb 204 pb are determined for the individual data sets that were stepwise corrected for in situ radiogenic pb the worst correlation monitored by the lowest value of the correlation coefficient is then indicative for the time at which the pb isotopic homogenization reached its maximum degree applying the above described procedures partial or complete homogenization of pb isotopes during any magmatic metamorphic or metasomatic process may readily be detected in any suite of whole rock and or mineral systems thus we strongly recommend determinations of u and pb concentrations on a routine basis even if there exists the possibility of secondary u and or pb mobility

elevated potential instability in the comma head distribution and development abstractthe development of elevated potential instability within the comma head of a continental winter cyclone over the north central united states is examined using a 63 h weather research and forecasting wrf model simulation the simulation is first compared to the observed cyclone the distribution of most unstable convective available potential energy mucape within the comma head is then analyzed the region with positive mucape was based from 2 to 4 km altitude with mucape values up to 93 j kg 1 backward trajectories from five sublayers within the region of elevated convection in the comma head were calculated to investigate how elevated potential instability developed air in the lowest sublayer the source air for convective cells originated 63 h earlier near baja california at elevations between 2 25 and 2 75 km altitude air atop the layer where convection occurred originated at altitudes between 9 25 and 9 75 km in the arctic 5000 km away from the origin of air in the lowest sublayer

effects of water on rock strength in a brittle regime abstract failure strengths of granite and andesite have been measured under various conditions of strain rate and confining pressure both in the dry and wet states constant stress creep tests for granitic samples have been also conducted all specimens show that the failure strength decreased linearly as the logarithm of the strain rate decreased the strain rate dependence of the failure strength is increased at higher confining pressures the strain rate effect is more apparent on the failure strengths of wet samples than dry samples in lower confining pressure ranges in the constant stress creep experiments the creep failure strength decreased as the logarithm of the time to failure increased time dependent failure strength of all rocks observed in the regime of the present study may be interpreted by subcritical crack growth assisted by the stress corrosion mechanism

the role of moist processes in the formation and evolution of mesoscale snowbands within the comma head of northeast u s cyclones abstract the role of moist processes in regulating mesoscale snowband life cycle within the comma head portion of three northeast u s cyclones is investigated using piecewise potential vorticity pv inversion modeling experiments and potential temperature tendency budgets snowband formation in each case occurred along a mesoscale trough that extended poleward of a 700 hpa low this 700 hpa trough was associated with intense frontogenetical forcing for ascent a variety of pv evolutions among the cases contributed to midlevel trough formation and associated frontogenesis however in each case the induced flow from diabatic pv anomalies accounted for a majority of the midlevel frontogenesis during the band s life cycle highlighting the important role that latent heat release plays in band evolution simulations with varying degrees of latent heating show that diabatic processes associated with the band itself were critical to the development and maintenance of the band however changes in the meso

predicting failure in rocks and its implications for predicting earthquakes abstract in the laboratory rock failure can be predicted both in space and in time the processes leading to failure are well understood and manifest themselves in measurable quantities ultrasonic velocity changes and surface strain anomalies are the most reliable indicators of the impeding failure to a lesser degree and very much dependent on the deformation rate and the moisture content acoustic emissions cluster in the vicinity of the failure shortly before the rupture occurs the precursory failure zone has a linear dimension on the order of 1000 grain diameters if the same physical mechanisms are involved in the earth as failure is approached then detailed velocity and surface strain measurements would greatly enhance our ability to predict earthquakes with the realization of the global positioning system and a possible extension to a satellite holographic system it is now possible to measure surface deformation inexpensively on a densely spaced grid

dynamics of cloud top generating cells in winter cyclones part iii shear and convective organization abstractcloud top generating cells gcs are a common feature atop stratiform clouds within the comma head of winter cyclones the dynamics of cloud top gcs are investigated using very high resolution idealized wrf model simulations to examine the role of shear in modulating the structure and intensity of gcs simulations were run for the same combinations of radiative forcing and instability as in part ii of this series but with six different shear profiles ranging from 0 to 10 m s 1 km 1 within the layer encompassing the gcs the primary role of shear was to modulate the organization of gcs which organized as closed convective cells in simulations with radiative forcing and no shear in simulations with shear and radiative forcing gcs organized in linear streets parallel to the wind no gcs developed in the initially stable simulations with no radiative forcing in the initially unstable and neutral simulations with no radiative forcing or shear gcs were exceptionally weak with no clear organization

crystal growth in cataclasites diagnostic microstructures and implications abstract microstructures in cataclasites from low and high angle fault zones in granitic basement rocks of the scandinavian caledonides indicate the operation of several processes intensive formation of microcracks filled with fine grained mineral fragments indicates brittle deformation the formation of quartz crystals with euhedral terminations suggests crystal growth occurrence of undulose extinction deformation lamellae and grain boundary recrystallization in other quartz grains illustrates plastic deformation crystal growth causes micro cracks to heal and thereby changes the behaviour of rocks undergoing brittle deformation the crystallization products vary in size from c 10 m to c 5 mm narrow banding parallel to rhomb and prism planes is indicative of crystal growth the growth bands show slightly different crystallographic misorientation in some cases the boundaries of these bands could be identified as dislocation arrays decorated by bubbles the occurrence of fibrous chalcedony aggregates and transitions from extremely fine grained quartz to large grains with euhedral terminations suggests the presence of colloidal silica bearing fluids during growth the crystallization process leads to the formation of vein like structures after crystallization plastic deformation may take place followed by brittle deformation and new crystal growth it is concluded that during the formation of the cataclasites a cyclic operation of brittle failure crystal growth followed by plastic deformation took place and this cycle was repeated several times

vertical velocity and physical structure of generating cells and convection in the comma head region of continental winter cyclones abstractthe vertical motion and physical structure of elevated convection and generating cells within the comma heads of three continental winter cyclones are investigated using the wyoming w band cloud radar mounted on the national science foundation national center for atmospheric research nsf ncar c 130 supplemented by analyses from the rapid update cycle model and weather surveillance radar 1988 doppler wsr 88d data the cyclones followed three distinct archetypical tracks and were typical of those producing winter weather in the midwestern united states in two of the cyclones dry air in the middle and upper troposphere behind the pacific cold front intruded over moist gulf of mexico air at lower altitudes within the comma head separating the comma head into two zones elevated convection in the southern zone extended from the cold frontal surface to the tropopause the stronger convective updrafts ranged from 2 to 7 m s 1 and downdrafts ranged from 2 to 6 m s 1 the horizontal scale of the

brittle creep in basalt and its application to time dependent volcano deformation time dependent brittle deformation is a fundamental and pervasive process operating in the earth s upper crust its characterization is a pre requisite to understanding and unraveling the complexities of crustal evolution and dynamics the preferential chemical interaction between pore fluids and strained atomic bonds at crack tips a mechanism known as stress corrosion allows rock to fail under a constant stress that is well below its short term strength over an extended period of time a process known as brittle creep here we present the first experimental measurements of brittle creep in a basic igneous rock a basalt from mt etna volcano under triaxial stress conditions results from conventional creep experiments show that creep strain rates are highly dependent on the level of applied stress and can be equally well fit by a power law or an exponential law with a 20 increase in stress producing close to three orders of magnitude increase in creep strain rate results from stress stepping creep experiments show that creep strain rates are also influenced by the imposed effective confining pressure we show that only part of this change can be attributed to the purely mechanical influence of an increase in effective pressure with the remainder interpreted as due to a reduction in stress corrosion reactions the result of a reduction in crack aperture that restricts the rate of transport of reactive species to crack tips overall our results also suggest that a critical level of crack damage is required before the deformation starts to accelerate to failure regardless of the level of applied stress and the time taken to reach this point the experimental results are discussed in terms of microstructural observations and fits to a macroscopic creep law and compared with the observed deformation history at mt etna volcano c 2011 elsevier b v all rights reserved

an experimental study of the strain field development prior to failure of a marble plate under compression abstract the strain field development prior to failure of a uniaxially compressed marble plate with an inclined central slot is studied experimentally the whole process of failure development of the specimen is directly monitored and photographed by back illumination the displacement fields at different stages are recorded with the moire technique and corresponding strain fields are calculated test results show that when the specimen is loaded to a certain level its strain will become concentrated in relatively narrow belts which are potential locations of the impending ultimate failure the failure process can be roughly divided into five stages 1 initiation and propagation of primary forward tensile cracks also known as wing cracks 2 initiation and propagation of secondary forward tensile cracks 3 initiation and intensification of shear belts starting from the slot tips and extending in an x form toward the four comers of the specimen 4 initiation and propagation of backward tensile cracks and finally 5 the ultimate failure the forward tensile cracks are similar to those previously described in the literature but the backward cracks run in an opposite direction in this study four types of ultimate failure are identified i axial splitting along tensile cracks ii shear fracture along shear belts having a similar trend as the slot iii shear fracture along shear belts sub conjugate to the slot and iv various combinations of splitting and shear fracture specimens with infill mortar material in the slot are also tested where it is found that the mortar suppresses axial splitting the geophysical and seismological implications of the test result are also briefly discussed

high resolution observations of the trowal warm frontal region of two continental winter cyclones abstract this paper compares the structure of the trough of warm air aloft trowal warm frontal region of two continental wintertime cyclones the cyclones were observed over the central great lakes region during the lake induced convection experiment snowband dynamics project field campaign the cyclones had different origins with the first forming east of the colorado rockies and the second forming over the gulf of mexico they were associated with different upper level flow regimes one located just north of a nearly zonal jet and the other located just west of a nearly meridional jet both storms produced heavy swaths of snow across the states of illinois wisconsin and michigan high resolution observations of frontal structure were made during flights of the national center for atmospheric research electra aircraft using dropsondes and the electra doppler radar tail radar system the high resolution observations suggest a different arrangement of air masses in the trowal region compared with the

a sm nd isochron on pelites 1 ga in excess of their depositional age and its possible significance fine grained tremadocian 490 ma sediments of the almaden syncline central iberian zone spain were analyzed for sm nd and rb sr isotopes in order to get information on the crustal evolution of the iberian peninsula these shallow water platform sediments consist mainly of 2 m illite muscovite and quartz plus kaolinite chlorite and minor quantities of plagioclase neodymium contents fall into a range of 11 76 ppm end 490ma range from 1 to 4 and 147sm144nd ratios from 0 1135 0 1504 the rocks yield an isochron age of 1524 173 ma 95 confidence level end init 7 1 more than three times older than the sedimentation age but in line with the mean neodymium crustal residence age of the samples 1 51 ga a comparison of crustal residence ages calculated in a two step model with those calculated in a single step model indicate that syn or post sedimentary fractionation of 147sm144nd ratios is not very likely rather the data seem to indicate an inherited isochron relationship reflecting the composition and mean age of the provenance thus the sm nd system was not disturbed during erosion sedimentation or diagenesis a rb sr errorchron of the tremadocian samples indicates a postdepositional age of 335 15 ma it probably reflects the main folding between lower namurian and upper westphalian or the deepest burial of the strata

holographic in situ stress measurements a new instrument for measuring the in situ level of stress in boreholes has been developed the instrument operates on the principle of locally relieving the stresses acting on a rock mass by drilling a small hole into the borehole surface and recording the resultant displacement field by holographic interferometry because the recording technique is optical the entire displacement field due to stress relief is obtained a description of the stressmeter theory of the interferometric technique data reduction methodology and results of laboratory stress relief calibration tests are presented in addition we present results from a field deployment of the instrument in an underground shale mine in garfield county colorado using a test borehole within a support pillar sufficient data were obtained to constrain five of six stress components at a shallow level of the test borehole thereby demonstrating the viability of the holographic technique the holographic stress relief data yield an approximate ew maximum horizontal stress direction by comparison with previous hydrofracture measurements of bredehoeft et al our results indicate substantial stress relief near the pillar face thus masking any relicts of the far field tectonic stress

a new approach for the determination of age of a partial or complete homogenisation of pb isotopes example anchimetamorphic detrital sediments of the central iberian zone spain comments

artificial generation of pseudotachylyte using friction welding apparatus simulation of melting on a fault plane a 150 m thick fused layer of rock has been produced by rotating two metadolerite core faces against each other at 3000 r p m under an axial load of 330 kg for 11 s using friction welding apparatus scanning electron microscopy and electron microprobe analysis reveal that the melt layer comprises sub angular to rounded porphyroclasts of clinopyroxene feldspar and ilmenite 20 m diameter derived from the host metadolerite set within a silicate glass matrix thermal calculations confirm that melting occurred at the rock interface and that mean surface temperatures in excess of 1400 c were attained the fused layer shows many textural similarities with pseudotachylyte described from fault zones morphologically the fused layer consists of a series of stacks of porphyroclasts welded together by melt to form build ups oriented at right angles to the friction surface there is also evidence of gouging ploughing and plucking as well as transfer and adhesion of material having occurred between the rock faces the mean surface velocity attained by the metadolerite 0 24 m s 1 and duration of the experiment are comparable with velocities and rise times of typical single jerk earthquakes occurring during stick slip seismic faulting within brittle crust i e slip rates of 0 1 0 5 m s 1 for say 1 10s in these respects the experiment successfully simulated frictional fusion on a fault plane in the absence of an intergranular fluid power dissipation during the experiment was about mw m 2 comparable only to very low values for earthquakes e g 1 100 mw m 2 for displacement rates of 0 1 0 5 m s 1 at shear stresses of 100 1000 bars this indicates that melting on fault planes during earthquakes should be commonplace field evidence however does not support this contention either pseudotachylyte is not being recognized in exhumed ancient seismic fault zones or melting only occurs under very special circumstances

the geo dscm system and its application to the deformation measurement of rock materials digital speckle correlation method dscm is a useful tool for whole field deformation measurement and has been applied to analyze the deformation field of rock materials in recent years in this paper a geo dscm system is designed and used to analyse the more complicated problems of rock mechanics such as damage evolution and failure procedure a weighted correlation equation is proposed to improve the accuracy of displacement measurement on a heterogeneous deformation field in addition a data acquisition system is described that can synchronize with the test machine and can capture speckle image at various speeds during experiment for verification of the geo dscm system the failure procedure of a borehole rock structure is inspected and the evolution of the deformation localization is analysed it is shown that the deformation localization generally initializes at the vulnerable area of the rock structure but may develop in a very complicated way

borehole stability analysis accounting for anisotropies in drilling to weak bedding planes abstract borehole instabilities pose significant challenges to drilling and completion operations particularly in regions with weak bedding planes and pre existing fractures where formations have strong anisotropies the bedding planes rock anisotropy and their impacts on horizontal stresses are considered in the proposed model to improve borehole stability modeling this improved model enables to calculate borehole failures and minimum mud weight along borehole trajectories with various drilling orientations versus bedding directions laboratory test data of rock compressive strengths are analyzed and a new correlation is developed to allow for predicting uniaxial compressive strengths in weak rocks from sonic velocities time dependent rock compressive strength is also examined to analyze the wellbore failure evolution with time the slip failure gradient in the weak planes is derived which can be used to model wellbore sliding shear failure in the planes of weakness the mud weight applied to prevent borehole shear failures in both intact rocks and ones with weak bedding planes can be obtained from the proposed model

the mesoscale and microscale structure and organization of clouds and precipitation in midlatitude cyclones part xv a numerical modeling study of frontogenesis and cold frontal rainbands abstract a two dimensional hydrostatic primitive equation model is used to investigate the dynamics of frontogenesis in a moist atmosphere the development of a cold front is simulated through shear deformation associated with the non linear evolution of an eady wave simulations are performed with 5 10 40 and 80 km horizontal resolutions and 14 levels in the vertical four in the boundary layer compared to the dry case the inclusion of moisture in the model produces a stronger low level jet ahead of the front and a stronger upper level jet moisture also produces a stronger ageostrophic circulation across the front and a more concentrated updraft just ahead of the surface front the updraft develops a banded structure above and behind the surface front with a wavelength of about 70 km bands form near the back edge of the cloud shield and move toward the surface front with a relative velocity of 1 m s 1 these characteristics agree with observations of wide cold frontal rainbands the banded str

hypocenter distribution and focal mechanism of ae events during two stress stage creep in yugawara andesite more than a thousand acoustic emission ae hypocenters were determined in a cylindrical andesite specimen under two stage uniaxial creep at stresses of 204 and 214 mpa strains were monitored for 6 peripheral points at the middle part of the cylindrical specimen s wall the strain data indicate gradual increase of nonuniform deformation during steady creep and strong intensification of the nonuniformity during acceleration creep and therefore biased stress distribution within the specimen the correlation between dilatant strain and ae hypocenters was investigated for whether or not tensile cracks emit ae the region with high ae activity shows only a small dilatant strain this negative correlation between ae and the dilatant region may eliminate tensile cracks as possible ae sources a composite focal mechanism solution of local ae events covering a wide solid angle of the focal hemisphere indicates that shear fractures emit ae waves the direction of the compressional axis in this solution shows a significant deviation from that inferred from the applied external force suggesting that the local stress field is governed by preexisting weak zones that are presumably produced by tensile cracks within the specimen ae hypocenters tended to form clusters during steady creep under the constant compressional stress during acceleration creep caused by a small step increase of the external stress the preceding clusters disappeared while a new cluster appeared in an incipient fault plane this suggests that changes in seismicity pattern such as migrations or quiescences of swarm important clues for earthquake predictions may be caused by an instantaneous change in the tectonic stress levels

the propagation of a single shear crack a displacement discontinuity model numerical studies using the displacement discontinuity method show that a single shear crack under compression propagates in its own direction because such propagation results in the maximum release of strain energy the methods of linear elastic fracture mechanics may not be used for such a closed crack and the stress intensity factors are meaningless in that case laboratory observations of propagation by means of kinks at an angle of approximately 70 to the crack may be due to heterogeneities to the effect of a preexisting crack to end effects to microcracking or to some combination of these factors such kinks may thus be local phenomena which cannot release most of the strain energy and are not incompatible with our numerical results which are based on a global energy balance

evolution of mesoscale precipitation band environments within the comma head of northeast u s cyclones abstract this paper explores the mesoscale forcing and stability evolution of intense precipitation bands in the comma head sector of extratropical cyclones using the 32 km north american regional reanalysis hourly 20 km rapid update cycle analyses and 2 km composite radar reflectivity data a statistical and composite analysis of 36 banded events occurring during the 2002 08 cool seasons reveals a common cyclone evolution and associated band life cycle a majority 61 of banded events develop along the northern portion of a hook shaped upper level potential vorticity pv anomaly during the 6 h leading up to band formation lower tropospheric frontogenesis nearly doubles and the conditional stability above the frontal zone is reduced the frontogenesis increase is primarily due to changes in the kinematic flow associated with the development of a mesoscale geopotential height trough this trough extends poleward of the 700 hpa low and is the vertical extension of the surface warm front and surface

fracturing process of granite inferred from measurements of spatial and temporal variations in velocity during triaxial deformations fracturing in fine grained homogeneous granite during triaxial deformation was investigated by measuring the spatial and temporal variations of elastic wave velocities velocities along 20 ray paths were measured with 12 piezoelectric transducers attached to a sample under confining pressures up to 400 mpa systematic velocity differences observed among the 20 paths apparently indicate the formation of a dilatant region near the surface of a sample the damaged region formed near the surface due to asymmetric development of dilatancy seemed to play an important role in controlling ultimate failure no evidence of the formation of a crack concentration zone adjacent to the ultimate fracture plane could be found before failure insofar as could be estimated from the velocity variations of elastic waves propagating along its plane however a highly concentrated region of cracks in which the fracture nucleated was generated in the damaged region at stresses of more than 97 99 of the ultimate strength in all the specimens based on tomographic images computed using an algebraic reconstruction technique the elastic waves were strongly affected by the damaged region even under the high confining pressures at which cracks are considered to close

seismic and strain studies on large laboratory rock samples being stressed to failure abstract large granite 700 700 700 mm and basalt 580 580 915mm samples were cyclicly loaded to failure strain and ultrasonic velocity measurements were made and acoustic emissions recorded during deformation all measurements were correlated in space and time several zones of acoustic emission sources were identified the acoustic patterns of emission activity within these zones varied greatly with increasing stress 1 a initially active then quiet 2 b initially active then quiet and active again before failure and 3 c activity increasing continuously and at accelerating rate toward failure at constant applied stress the seismic character of the acoustic emission signals was quite different in terms of frequency content and amplitude for the different zones the signals emanating from within a particular zone were very reproducible as recorded on a particular transducer but quite different from transducer to transducer the kaiser effect seems to hold up to about one half of the stress of failure i e very few acoustic emissions were detected when the sample was cycled below previously attained stress as long as was less than about one half of the eventual stress at failure as failure was approached the individual acoustic emission events became more energetic and relatively more power was recorded at lower frequencies strain and ultrasonic velocity measurements indicate large variations in dilatancy in the sample similar measurements on much smaller samples are consistent with these results

sidewall ripouts in strike slip faults abstract distinctive splay fault configurations in wallrocks associated with pseudotachylyte bearing strike slip fault structures at three different localities are recognized as sidewall ripouts sidewall ripout structures occur as asymmetric doubly tapered fault bounded lenses or slabs adjacent to a dominant planar fault surface scallop shaped lens ripouts develop in isotropic or weakly anisotropic host rocks and elongate slab ripouts develop in strongly anisotropic host rocks the tapered ends of these structures consist of leading contractional p type ramp structures at a low angle to the dominant planar fault surface and trailing extensional r or x type ramp structures at higher angles to the dominant fault surface the formation of ripout structures is attributed to transient variations in the coefficient of friction that cause adhesion between fault surfaces leading to rupture of the sidewalls ripouts are interpreted as large outcrop scale examples of adhesive wear during rupture of the sidewalls developed along planar fault surfaces during slip the asymmetry of the ripout structures in these exposures allows their use as a kinematic indicator where the narrow tapered leading end points in the direction of movement of the ripout lens or slab during adhesion to the opposing fault block wall

stability and charging characteristics of the comma head region of continental winter cyclones abstractthis paper presents analyses of the finescale structure of convection in the comma head of two continental winter cyclones and a 16 storm climatology analyzing the distribution of lightning within the comma head a case study of a deep cyclone is presented illustrating how upper tropospheric dry air associated with the dry slot can intrude over moist gulf air creating two zones of precipitation within the comma head a northern zone characterized by deep stratiform clouds topped by generating cells and a southern zone marked by elevated convection lightning when it occurred originated from the elevated convection a second case study of a cutoff low is presented to examine the relationship between lightning flashes and wintertime convection updrafts within convective cells in both storms approached 6 8 m s 1 and convective available potential energy in the cell environment reached approximately 50 250 j kg 1 radar measurements obtained in convective updraft regions showed enhanced spectral

acoustic emission study of microfracturing during the cyclic loading of westerly granite a rectangular prism of westerly granite was deformed in a uniaxial cycling experiment acoustic emission ae activity was recorded using eight transducers in real time hypocenters of 116 events were determined using a standard least squares approach event clustering was definitely evident and could easily be assessed using a stereographic projection of hypocenters this clustering was observed at volumetric strains well below 0 03 the distribution of hypocenters changed from cycle to cycle suggestive of a spatial migration the cumulative distribution of hypocenters documents the strongly anisotropic development of dilatancy in westerly granite once the sample underwent the initial deformation and a zone of weakness was established subsequent events clustered in and about the initial zone volumetric strain differential stress acoustic emission event rate and cumulative numbers were continously measured during the experiment the cumulative number of events per cycle generally decreased with the number of cycles the majority of events occurred upon loading those that occured during unloading were few in number and not as systematically related to stress as were those for loading in general waveform quality deteriorated with increased cycling and in later cycles waveforms typically displayed low frequency components in initial onsets followed by a more typical higher frequency arrival

rheology of the lithosphere selected topics we review recent results concerning the rheology of the lithosphere with special attention to the following topics 1 the flexure of the oceanic lithosphere 2 deformation of the continental lithosphere resulting from vertical surface loads and forces applied at plate margins 3 the rheological stratification of the continents 4 strain localization and shear zone development and 5 strain induced crystallographic preferred orientations and anisotropies in body wave velocities we conclude with a section citing the 1983 1986 rock mechanics literature by category

a study of lightning flashes attending periods of banded snowfall 1 lightning flashes n 1088 associated with 24 thundersnow events in the central united states were analyzed to document flash polarity signal strength and multiplicity negative lightning flashes n 872 80 dominated positive flashes n 216 20 with wintry precipitation in this study which stands in contrast to the majority of the research done on winter thunderstorms primarily in japan otherwise limited work has been done although thundersnow has been documented in the mid latitudes of north america europe and asia statistics on peak amplitude were determined for negative positive flashes yielding mean and standard deviation values of 24 ka 22 ka 38 ka 34 ka a subset of winter lightning events n 16 were then sought that occurred with banded single or multiple snowfall as banding often denotes greater organization in the atmosphere e g a jet streak aloft to aid in ascent or a low level jet streak to aid with moisture and thermal transport and thus the potential for deeper snow totals radar reflectivity values were recorded at the location of each lightning flash as well as the maximum radar reflectivity within the associated snow band the location of the lightning activity within the snow band was also noted as being either leading edge le trailing edge te core c or not correlated nc with respect to the motion of the parent band the majority of lightning flashes were found downstream of areas of highest radar reflectivity with respect to the motion of the snow bands and not with the highest reflectivity values if one uses the highest reflectivity values in a snowband as a proxy for the greatest surface snowfall intensity then the ground terminus of a cloud to ground lightning cg flash is often not co located with the heaviest snowfall rates however the work completed here does place the location of the typical cg flash 15 km downstream of the snowband location so one could use the occurrence of lightning as a nowcasting tool for impending snow intensification at the site of the cg flash

the control of deformation intensity around a fault natural and experimental examples abstract the deformation intensity around a wrench fault is studied through a comparison between a well known natural fault and experimental models each model consists of a horizontal paraffin wax layer containing a precut plane which was submitted to uniaxial compression the meyrueis fault in the jurassic formations of the causses massif southern french massif central gives the opportunity of estimating strain around a wrench fault the comparison of this fault with the experimental models shows a good similarity in relation to the following aspects variation of the principal strain axes around the fault rotation of the strain axes with time which confirms a non coaxial strain history heterogeneity of strain with some domains strongly deformed and others protected symmetry of these domains with respect to the fault and the strong influence of kinematic boundary conditions in the direction perpendicular to the compression these features provide new criteria for the interpretation of natural wrench faults

the role of cloud top generating cells and boundary layer circulations in the finescale radar structure of a winter cyclone over the great lakes abstractdata from airborne w band radar are used in conjunction with thermodynamic fields from the weather research and forecasting model and air parcel back trajectories from the hysplit model to investigate the finescale reflectivity vertical motion and airmass structure of the comma head of a winter cyclone in the vicinity of the great lakes cloud top generating cells formed along an upper level frontal boundary vertically separating dry air which 48 h earlier was located in the upper troposphere over south central canada from moist air which was located in the lower troposphere over the southeast united states the stronger updrafts within the generating cells had vertical velocities ranging from 1 to 3 m s 1 the generating cells were important to precipitation production within the comma head precipitation trails formed within the generating cells could sometimes be followed to the boundary layer before merging boundary layer air beneath the cyclone s comma head exhibited convective circulati

laser holographic measurements of tidal deformation of a tunnel summary a new technique based on holographic interferometry has been developed for measuring crustal deformations the holographic recording system consisting of an he ne gas laser and associated optical elements was first installed in a tunnel at the amagase crustal movement observatory in 1984 tunnel deformations caused by tidal and tectonic forces have been precisely determined using the real time technique of holographic interferometry in this procedure a hologram of the tunnel wall within a section 1 2 m in diameter is directly recorded on a photographic plate and then the plate is carefully reset in the position at which the hologram was taken when the reconstructed image of the hologram is superimposed on the current image of the tunnel wall many interference fringes can be seen through the hologram the fringe displacement formed by the deformation of the tunnel is continuously recorded on a video cassette tape using a video camera and a time lapse video recorder the change in the fringe patterns is analysed using the image processing system tidal deformations obtained from the holographic method are consistent with the strain changes observed with laser extensometers in the same tunnel these observational results substantiate the tunnel deformation estimated by finite element calculations

the origin of s c mylonites and a new fault zone model abstract this paper presents a synthesis of experimental data on mechanical behaviour and deformation textures of simulated halite shear zones with special regard to the internal structures of s c mylonites and their mechanical implications halite is the only mineral so far for which a complete transition between brittle shearing deformation to fully ductile shearing flow in the dislocation glide regime has been studied experimentally under large shear strains that may be encountered along faults or plate boundaries moreover deformation textures in the shear zones are similar to those of mylonites and cataclasites thus the experimental results provide an insight into the petrogenesis of fault rocks mylonites in particular and the establishment of a realistic fault model between the brittle regime and the fully ductile regime i e pressure insensitive inelastic regime there exists a wide and distinct regime called semiductile in which deformation textures are nearly identical to those developed in the ductile regime yet the shear resistance is pressure dependent and potentially unstable fault motion can occur in the low pressure part in cooking salt shear zone experiments the characteristic texture in the semiductile and ductile regimes is foliation resembling mylonitic foliation which formed in the direction of maximum elongation in the semiductile regime nearly homogeneous shearing deformation characterized by the formation of uniform and pervasive foliation changes into heterogeneous deformation after a critical shear strain and this strain localization induces potentially unstable fault motion the primary mode of internal slip after the strain localization is slip along y riedel shears parallel to the shear zone boundary interconnected with r 1 and some p riedel shears these shear surfaces of concentrated deformation are very similar to the internal structures of so called s c mylonites hence the semiductile regime is the primary candidate for the formation of s c mylonites and at least some s c mylonites must have formed at seismogenic depths this interpretation is consistent with recently reported interlaced pseudotachylytes and mylonites it is also shown that the size of recrystallized halite grains presumably mostly postdeformational decreases with increasing shear strains but not with the shear stress and this is consistent with the grain size reduction with strain concentration which has been confirmed for many mylonites in the world thus estimation of flow stress from recrystallized grains needs to be done with great care and might often be erroneous existing fault models are critically examined from the viewpoint of the halite experiments and a new fault model is proposed whereby fault zones are divided into brittle semibrittle semiductile and ductile regimes with increasing depth the shear resistance of faults is highest in the semiductile regime and the seismogenic depth is likely to extend down to the upper part of this regime the semiductile regime thus emerges as an area of primary interest with regard to the petrogenesis of many s c mylonites modelling of large and great shallow earthquakes along pre existing faults and plate boundaries and quantitative evaluation of the mechanical interactions of plates across their boundaries

insights into the evolving microphysical and kinematic structure of northeastern u s winter storms from dual polarization doppler radar abstractthe recent weather surveillance radar 1988 doppler wsr 88d network upgrade to dual polarization capabilities allows for bulk characterization of microphysical processes in northeastern u s winter storms for the first time in this study the quasi vertical profile qvp technique wherein data from a given elevation angle scan are azimuthally averaged and the range coordinate is converted to height is extended and applied to polarimetric wsr 88d observations of six northeast winter storms to survey their evolving bulk vertical microphysical and kinematic structures these analyses are supplemented using hourly analyses from the rapid refresh rap model regions of ascent inferred from qvps were consistently associated with notable polarimetric signatures implying planar crystal growth when near 15 c and riming and secondary ice production at higher temperatures the heaviest snowfall occurred most often when ascent and enhanced propagation differential phase shift occurred near 15 c

the lagrangian parcel dynamics of moist symmetric instability abstract a simple model describing the slantwise ascent of a two dimensional horizontal air tube subject to moist symmetric instability is developed under the assumptions that the froude number is small and that mixing is absent it is shown that in general the horizontal velocities attained by the tube are comparable to those of the mean flow and that vertical velocities of up to a few meters per second are possible the tube ascends slantwise in such a way that its buoyancy remains nearly zero unless the environment is my very nearly moist adiabatic in which case ascent at an angle of 45 to the vertical is preferred results of the analysis support the contentions of bennetts and hoskins and emanual that moist symmetric instability is the cause of some mesoscale rainbands in a companion paper it is demonstrated that the stability of the moist baroclinic atmosphere to two dimensional slantwise displacements of arbitrary magnitude can be approximately assessed by reversibly lifting parcel along sur

mechanical problems in geodynamics and work done in china the subject of geodynamics concerns the dynamics of the global motion of the earth of the motion in the earth s interior and its interaction with surface features together with mechanical processes in the deformation and rupture of geological structures a brief historical review is given from sir i newton maclauwin jacobi poincare poisson lame darwin rayleigh and love who dealt with the homogeneous sphere to leibenson takeuchi and meinesz concentrating on layered spheres and after the advent of plate tectonics on the use of numerical simulation to the analyses of tectonic features earthquake mechanism the application of nonlinear dynamics by discussing the forward and inverse mechanical problems and the questions facing the inverse problems of searching for the structural parameters driving forces etc are raised in more detail thereafter some works accomplished in china on the global and regional stress fields analyses tectonic features analyses mantle flow studies experimental studies of rocks and their constitutive relations are presented finally the interdisciplinary nature of the subject is emphasized and the main mechanical problems that need special attention are then proposed

influence of terrain resolution on banded convection in the lee of the rocky mountains abstracton 16 17 february 2007 snowbands formed in the lee of the rocky mountains in wyoming colorado and nebraska on the anticyclonic shear side of a midlevel jet streak two types of bands were prevalent a longer wider band associated with frontogenesis along an equatorward moving cold front major band and multiple shorter narrower bands farther poleward minor bands to understand how the upstream terrain affected the occurrence and intensity of the bands multiple mesoscale model simulations were performed in which the terrain was incrementally smoothed the evolutions of the synoptic patterns were similar in all simulations that included topography but the synoptic pattern differed and no bands developed in a simulation with a flat land surface these results allowed a focus on the changes to the banded precipitation due to the terrain resolution remarkably although the exact location of the bands differed from run to run the bands in all simulations with topography were in roughly the s

pseudotachylyte bearing strike slip duplex structures in the fort foster brittle zone s maine abstract detailed 1 60 scale mapping of the fort foster brittle zone in the mylonitic rye formation of southernmost maine has revealed the intricate internal duplex structure of a system of probable paleozoic age dextral strike slip faults that have produced abundant pseudotachylyte and minor breccia the internal configuration of this brittle zone consists of a mosaic of individual pseudotachylyte generation zones as slab duplex structures individual duplex zones are up to 100 m in length and 1 m or less in width and are defined by pairs of layer parallel slip surfaces along which frictional melts were produced these slab duplex structures are interpreted as zones of displacement transfer between long overlapping layer parallel en echelon strike slip fault surfaces contractional duplexes develop layer parallel compressional structures that tend to shorten and thicken the fault bounded slabs by the formation of lateral ramps and conjugate faults kinks and asymmetric folds extensional duplexes develop layer parallel stretching and thinning by the formation of oblique dextral shears high angle conjugate pairs and localized fault breccias the production of pseudotachylyte by friction melting along layer parallel fault surfaces in these exposures is attributed to rapid slip during paleoseismic events the rupture structures developed during these events may be characteristic of fault structure and mechanics at near focal depths in a strike slip seismogenic zone

mesoscale structure in the megalopolitan snowstorm of 11 12 february 1983 part iii a large amplitude gravity wave abstract a large amplitude singular gravity wave is studied through the analysis of hourly observations radar and satellite data and observations of cloud to ground lightning flashes the wave activity was initiated just ahead of and to the left of the track of the major atlantic coast cyclone of 11 12 february 1983 the initial wave of depression was followed by a wave of elevation and finally a wake depression pressure amplitudes were several millibars the system propagated northeastward at 15 m s 1 accelerating to more than 25 m s 1 against increasing northeasterly flow in the lower troposphere surface northeasterlies increased and became gusty with the approach of the first pressure falls reaching a maximum at the pressure trough then abruptly weakening and backing as strong pressure increases were observed strong fluctuations in column total condensation rate and radar reflectivity occurred with the passage of the wave clustered lightning activity accompanied wave passage principally near

the structure and evolution of a continental winter cyclone part ii frontal forcing of an extreme snow event abstract the production of a narrow heavy occasionally convective snowband that fell within a modest surface cyclone on 19 january 1995 is examined using gridded model output from a successful numerical simulation performed using the university of wisconsin nonhydrostatic modeling system it is found that the snowband was produced by a thermally direct vertical circulation forced by significant lower tropospheric warm frontogenesis in the presence of across front effective static stability differences as measured in terms of the equivalent potential vorticity pve the sometimes convective nature of the snowband resulted from the development of freely convective motions forced by frontal lifting of the environmental stratification model trajectories demonstrate that a stream of warm moist air ascended through the trowal portion of the warm occluded structure that developed during the cyclone life cycle the lifting of air in the trowal was in this case forced by lower tropospheric frontogenesis occ

convective snowbands downstream of the rocky mountains in an environment with conditional dry symmetric and inertial instabilities abstract convective snowbands moved slowly over wyoming and northern colorado on 16 17 february 2007 and produced up to 71 mm 2 8 in of snow that was unpredicted by operational numerical weather prediction models and human forecasters the northwest southeast oriented bands lasted for over 6 h comprising both a single major band more than 30 km wide and multiple minor bands about 10 km wide the convective bands initiated within the ascending branch of a secondary circulation associated with both near surface and elevated frontogenesis but the bands remained nearly stationary while the near surface frontogenesis moved quickly equatorward the bands occurred downstream of complex terrain on the anticyclonic shear side of a midlevel jet streak where conditional dry symmetric negative potential vorticity and inertial negative absolute vorticity instabilities were present to determine the mechanisms responsible for the development and organization of these bands simulations using a convectio

fluid infiltration into fault zones chemical isotopic and mechanical effects fluid infiltration into fault zones and their deeper level counterparts brittle ductile shear zones is examined in diverse tectonic environments in the 2 7 ga abitibi greenstone belt major tectonic discontinuities with lateral extents of hundreds of kilometres initiated as listric normal faults accommodating rift extension and acted as sites for komatiite extrusion and locally intense metasomatism during reverse motion on the structures accommodating shortening of the belt these transcrustal faults were utilised as a conduit for the ascent of trondhjemitic magmas from the base of the crust and of alkaline magmas from the asthenosphere and for the discharge of thousands of cubic kilometres of hydrothermal fluids such fluids were characterised by 18o 6 2 d 50 20 13c 4 4 and temperatures of 270 to 450 c probably derived from devolatilisation of crustal rocks undergoing prograde metamorphism hydrothermal fluids were more radiogenic 87sr 86sr 0 7010 to 0 7040 and possessed higher than did contemporaneous mantle komatiites or tholeiites and thus carried a contribution from older sialic basement a provinciality of87sr 86sr and 13c is evident signifying that fault plumbing sampled lower crust which was heterogeneous at the scale of tens of kilometres mineralised faults possess enrichments of large ion lithophile lil lil elements including k rb ba cs b and co2 and rare elements such as au ag as sb se te bi and w fluids were characterised by xco2 0 1 neutral to slightly acidic ph low salinity 3 wt k na 0 1 they carried minor ch4 co and n2 and they underwent transient effervescence of co2 during decompression clastic sediments occupy graben developed at fault flexures the40ar 39ar release spectra indicate that fault rocks experienced episodic disturbance on time scales of hundreds of millions of years

winter lightning along the east coast more than 150 000 cloud to earth lightning flashes were recorded along the east coast during the period 15 january 15 march 1986 in the region bounded by 25 n 40 n and 90 w 70 w the majority of the flashes for the two month period occurred over the atlantic ocean in the vicinity of the gulf stream where a peak flash density of 0 7 flashes km2 was observed approximately 270 km from the carolina coast this relatively sharp peak in the spatial distribution of lightning flashes to earth over the gulf stream occurs with a width at half maximum of approximately 130 km in the february 1986 data a close correlation is observed of the maximum lightning flash density contours with the maximum sea surface temperature contours

antimony quartz and antimony gold quartz veins from northern portugal abstract antimony and pb sb quartz veins from the braganca district portugal are mainly hosted by silurian phyllites antimony au quartz veins from the durico beira region are mainly hosted by a cambrian schist metagraywacke complex as well as ordovician phyllites and quartzites the deposits were mostly exploited in the late 19th century mineralogical characteristics and chemical compositions of individual ore minerals are similar in the two areas first and second generations of arsenopyrite precipitated at 390 and 300 c respectively berthierite and stibnite are the most abundant sb bearing minerals and precipitated between 225 and 128 c native antimony at

multiscale structure and evolution of an oklahoma winter precipitation event abstract a significant winter precipitation event occurred on 8 9 march 1994 in oklahoma snow accumulations greater than 30 cm 12 in were measured within a narrow corridor in northern oklahoma on the synoptic scale and mesoscale a correspondence between large snow accumulations and 600 hpa frontogenesis was revealed the precipitation was formed above the cold frontal surface owing to midtropospheric ascent associated with the cross frontal circulation in a region of elevated conditional instability the location of such a narrow corridor of large accumulations was not however disclosed by any patterns in the radar reflectivity data indeed during this event an elongated maximum of snow accumulation was not associated with a persistent band of enhanced reflectivity and vice versa dual polarization and dual doppler radar data allowed for a novel analysis of winter precipitation processes and structures within the context of the larger scale diagnosis it was possible to identify in order of

fault structure wear mechanisms and rupture processes in pseudotachylyte generation abstract fault generated pseudotachylyte is found within both cataclastic and mylonitic host rocks suggesting that rapid catastrophic displacements have occurred at a variety of depths within paleoseismogenic zones pseudotachylyte bearing fault zones represent a composite of structural features associated with the process of earthquake rupture propagation and coseismic slip the development of multiple pseudotachylyte veins in fault linkages duplexes sidewall ripouts en echelon arrays and brittle zones suggests repeated rupturing during a series of characteristic earthquakes each earthquake as a coseismic slip event can be subdivided into initial rupture acceleration stable sliding and final deceleration stages these evolve through distinctive sequences of wear and deformation mechanisms that vary with sliding velocity duration of slip total displacement and the hydrodynamics of the developing fault zone slip is thought to proceed toward surface refinement and possible frictional melting following the propagation of leading shear fracture process zones the passage of the initial process zone of oblique fracturing would be followed by linkage to a throughgoing structure with asperity reduction through brecciation comminution and refined cataclastic flow for frictional melting in an abrasive wear dominant model at greater depths in the presence of mylonitic anisotropy slip would proceed through initial layer parallel surfaces and duplex linkages with rapid surface refinement through plastic smearing and laminar flow for frictional melting in an adhesive wear dominant model

monitoring fracture propagation in a soft rock neapolitan tuff using acoustic emissions and digital images sudden and unexpected collapses of underground cavities below the city of naples italy represent a serious safety hazard the collapses occur due to the detachment of large blocks from the cavity roofs walls and pillars often a long time after the original quarry excavation has been completed it is recognised that existing discontinuities e g fractures play an important role in the failure process by inducing local stress concentrations and reducing the overall material strength the larger fractures which ultimately lead to collapse occur through interaction propagation and coalescence of these discontinuities this paper presents recent results of experiments carried out on natural dry specimens of neapolitan finegrained tuff tuff to investigate the mechanisms involved in sample failure a better understanding of fracture development and rock bridge behaviour is gained through a combination of ae and photographic monitoring in an experimental program considering samples with artificial pre existing heterogeneities which simulate the in situ discontinuities for a range of rock bridge geometries the mechanisms and timing of different stages of the failure process are identified and characterised the results show that in general a classical description of failure for samples without artificial flaws or with only a single flaw is followed 1 crack closure 2 linear stress strain response and crack initiation with stable crack growth 3 crack damage and unstable crack growth leading to failure for samples with two artificial pre existing flaws the third phase is split into two parts and failure of the sample occurs only after both the unstable propagation of external wing cracks and coalescence of the internal cracks in the bridge in terms of the timing and duration of each phase it is seen that phases 1 and 2 have little dependence on the flaw configuration but phase 3 seems to depend directly on this in particular the angle in rock bridge between the inner tips of the pre existing flaws plays a key role phase 3 is shorter for 120 than for 105 these differences by bridge rotation which appears to take longer than the simpler mode of failure for 120 it has only been possible to determine the time ranges of interest using the ae signatures whilst the photographs allow the fracture geometry evolution to be described additionally the frequency character of ae events is investigated and shown to have significant potential for characterisation of ae source types and thus failure processes

the accommodation of large strains in the upper lithosphere of the earth and other solids by self similar fault systems the geometrical origin of b value plate tectonics has provided a method of visualizing the geometry of the deformation of the earth s lithosphere on a large scale the description is so concise that for many purposes it provides an explanation of geological processes that overshadows the need to understand the driving processes the mechanics of the zones between the plates are less well understood particularly in continental regions where large areas are subject to deformation both continuous and discontinuous models have been tried but both have obvious drawbacks

melt origin of fault generated pseudotachylytes demonstrated by textures a petrographic and electron microprobe study of fault generated pseudotachylytes from the outer hebrides thrust zone scotland demonstrates that the textures have resulted from the primary crystallization of a clast laden melt rather than the devitrification of a glass or by crushing and cataclasis the observations are consistent with these pseudotachylytes having formed by frictional fusion followed by rapid quenching implying faulting at shallow crustal depths

formation and evolution of frontal rainbands and geostrophic potential vorticity anomalies abstract a viscous semigeostrophic model is developed and used to study the formation and evolution of frontal rainbands in association with the dry and moist geostrophic potential vorticity gpv anomalies the numerical results show that when moist gpv mgpv becomes negative in the saturated region but the flow is still stable to viscous symmetric perturbations banded substructures can be generated internally by a positive feedback between the moist circulation bands and geostrophic forcing anomalies in association with the generation of mesoscale gpv anomalies in addition to the previous diagnostic results for idealized forcings the new aspect here is that the large scale moist ascent evolves into much finer multiple moist bands as soon as the positive feedback begins to generate banded substructures in the forcing and gpv fields when mgpv is positive multiple rainbands can only be generated externally by preexisting gpv or mgpv anomalies these rainbands can be self maintained by a weak feedba

proximity soundings of thundersnow in the central united states 1 proximity balloon soundings for snow events with lightning and thunder during the period 1961 through 1990 reveal a less statically stable environment than similar nonthundering snow events when thundersnow is present a less stable environment and in some cases subsequent upright convection is found aloft in all of the thundering cases examined here all of the events feature their most unstable parcel originating above a frontal inversion in fact only events in the cold air north of an extratropical cyclone are included in this study events with a lake effect or orographic enhancement are eliminated from the sample the basic composite derived by averaging temperatures at an established interval reveals a nearly saturated lower atmosphere below 0 c throughout its depth with the frontal inversion present and its most unstable parcel occurring just above the top of the inversion the feature preserving composite approach of r a brown 1993 better defines the frontal inversion bottom and top as well as the level and temperature of the most unstable parcel these are the features in need of preservation and a less statically stable environment emerges by doing so other salient features include the most unstable parcel originating some 30 50 mbar above the top of the frontal inversion and significant drying 100 mbar above the level of the most unstable parcel the bulk sounding characteristics also favor the existence of lightning the composite temperature at the level of the most unstable parcel is 8 7 c which allows for enhanced amounts of supercooled water to enter any updraft that may form the temperature of the most unstable parcel at its origin is also warmer than the charge reversal temperature therefore convection of any appreciable depth will span that level moreover the height of the composited 10 c level is 2959 m above ground level which previous investigators have shown is sufficiently high to favor lightning production yet no convective available potential energy cape appears with either composite approach which concurs with previous studies while several of the composite members feature cape for elevated layers the majority do not suggesting that other processes e g the release of symmetric instability which are difficult to assess from a single sounding tend to be at work

climatology of banded precipitation over the contiguous united states abstracta climatology of banded precipitation features over the contiguous united states from 2003 to 2014 is constructed a band is defined as a precipitation feature with a major axis of 100 km or greater and a ratio of major axis length to minor axis length hereafter aspect ratio of 3 1 or greater by applying an automated feature based detection algorithm to composite radar imagery a database of 48 916 844 precipitation features is created of which 7 213 505 14 8 are bands this algorithm produces the first climatology of precipitation bands over the contiguous united states banded precipitation occurrence is broadly similar to total precipitation occurrence with a maximum of 175 h of banded precipitation annually over the ohio river valley in the warm season there is a strong diurnal signature associated with convective storm development for both the precipitation feature area and total area covered by precipitation but little diurnal signature in aspect ratio strong west east gradients

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

plumbotectonic evolution of the ossa morena zone iberian peninsula tracing the influence of mantle crust interaction in ore forming processes the ossa morena zone in the variscan belt of the iberian peninsula is characterized by a complex geotec tonic evolution it hosts a neoproterozoic paleozoic sequence that records the existence of two orogenic events cadomian and variscan separated by rift and stable platform stages middle cambrian to late devonian volcanogenic and sediment hosted massive sulfides skarns magmatic ni cu deposits synmetamorphic and intragranitic and perigranitic veins and replacement bodies are related to different magmatic hydrothermal systems ranging in age from 650 ma to about 250 ma the lead isotope signatures from 26 different mineral deposits show variable ratios of 206pb 204pb 17 49 18 44 207pb 204pb 15 47 15 67 and 208pb 204pb 37 37 38 78 that have low average values 9 7 but high values 38 3 as a whole the lead isotope data define an irregular cluster that plots around the stacey and kramers 1975 growth curve but clearly marks the existence of two main ore forming events during cadomian and variscan times the most radiogenic samples are from some minor zn pb stratiform sedimentary exhalative mineralization they are interpreted as having a dominantly crustal derivation 10 1 in which most of the lead was leached from a pre cadomian continental basement with a long crustal history this basement has strong geochemical affinities with the oldest rocks of the iberian terrane however most of the ore samples investigated plot below the stacey and kramers 1975 reference curve indicating polyphase but variable mixing of lead from upper crustal and mantle lead reservoirs at the terrane scale clear mixing lines between the two end member reser voirs are evident only in the oldest volcanic rock and sedimentary rock hosted massive sulfide deposits related to the cadomian late neoproterozoic early cambrian magmatic arc in the other deposit types lead isotopes have more restricted values the crustal lead is interpreted as having been derived from ancient crust 2 1 2 5 ga and the derived shales montemolin formation that predated the cadomian orogeny the mantle derived lead may have been derived from three different sources neoproterozoic tholeiitic orthoamphibolite syn cadomian calc alkaline andesite or calc alkaline metaluminous plutonic rocks intruded during the variscan orogeny the first two sources probably controlled the lead isotope signatures of the cadomian massive sulfide deposits and the variscan synmetamorphic veins the input of primitive lead is especially significant during variscan times 350 300 ma and is interpreted as having been related to the existence of a large primitive reservoir that controlled the variscan magmatic metallogenic and hydrothermal evolution of the ossa morena zone this primitive lead is recognized in most of the magmatic hydrothermal deposits including the typically crust derived w and pb bearing perigranitic hydrothermal systems likewise associated plutonic rocks have more primitive lead isotope signatures than their counterparts in other iberian terranes the lead isotope evolution of the region is consistent with recent geophysical data that suggest that a deep large mafic sill was intruded below the ossa morena zone during variscan times this sill was a likely key factor in the different pb isotope evolution of the ossa morena zone compared with the nearby iberian terranes and the entire western mediterranean province which are char acterized by lengthy crustal histories with very little input of primitive lead the extensive mantle crust inter action probably explains the wide variety and the unusual metallogenic features of the ossa morena zone

radar kinematic information as a surrogate for isentropes in stratiform precipitation systems abstractthis study illustrates that dual doppler derived wind shear vertical gradient of the horizontal wind in stratiform nonturbulent flow is structured in long thin striations the reason this has not been documented before is that scanning ground based radars have inadequate vertical resolution deteriorating with range here data from an airborne radar with a fine range independent vertical resolution are used a comparison of the radar derived wind shear with model output of isentropes in vertical transects in the comma head of two frontal disturbances suggests that the wind shear layers describe material surfaces model output itself further confirms the alignment of isentropes with wind shear in vertical transects thus doppler radar derived wind shear a kinematic conserved variable may serve as a suitable proxy for thermodynamic conserved variables such as equivalent potential temperature in stratiform precipitation furthermore the presence of shear striations in vertical transects can