iron zinc magnesium and uranium isotopic fractionation during continental crust differentiation the tale from migmatites granitoids and pegmatites abstract the causes of some stable isotopic variations in felsic rocks are not well understood in particular the origin of the heavy fe isotopic compositions i e high 56 fe values deviation in of the 56 fe 54 fe ratio relative to irmm 014 of granites with sio 2 70 wt compared with less silicic rocks is still debated it has been interpreted to reflect isotopic fractionation during late stage aqueous fluid exsolution magma differentiation partial melting or soret thermal diffusion the present study addresses this issue by comparing the fe isotopic compositions of a large range of differentiated crustal rocks whole rocks of migmatites granitoids and pegmatites mineral separates with the isotopic compositions of zn mg and u the samples include granites migmatites and pegmatites from the black hills south dakota usa as well as i s and a type granitoids from lachlan fold belt australia the nature of the protolith i e i or s type does not influence the fe isotopic composition of granitoids leucosomes partial melts in migmatites tend to have higher 56 fe values than melanosomes melt residues indicating that partial melting of continental crust material can possibly fractionate fe isotopes no clear positive correlation is found between the isotopic compositions of mg u and fe which rules out the process of soret diffusion in the systems studied here zinc isotopes were measured to trace fluid exsolution because zn can easily be mobilized by aqueous fluids as chloride complexes pegmatites and some granitic rocks with high 56 fe values also have high 66 zn values in addition high sio 2 granites show a large dispersion in the zn fe ratio that cannot easily be explained by magma differentiation alone these results suggest that fluid exsolution is responsible for some of the fe isotopic fractionation documented in felsic rocks and in particular in pegmatites however some granites with high 56 fe values have unfractionated 66 zn values and were presumably poor in fluids e g a type for these samples iron isotopic fractionation during magma differentiation is a viable interpretation equilibrium fe isotopic fractionation factors between silicic melts and minerals remain to be characterized to quantitatively assess the role of fractional crystallization on iron isotopes in granitoids

a comparison of magnetic character and alteration in three granite drill cores from eastern canada bulk magnetic susceptibility bms measurements have been made on granite drill cores from the st george batholith new brunswick the south mountain batholith nova scotia and the wedgeport pluton nova scotia the primary magnetite concentrations of the two nova scotia cores are statistically indistinguishable thus lending support to the hypothesis that the wedgeport pluton despite being 50 ma younger is a satellite of the south mountain batholith the st george core has a primary magnetite concentration over 30 times greater than the nova scotia cores but low temperature alteration attributable to subsurface weathering has greatly reduced its magnetite content the two nova scotia s type granites are shown to fall into the ilmenite series category whereas the st george granite which is either s or a type is transitional between the magnetite and ilmenite series the general observation of intergranular hematite and reduced bms in the outcrops of some granites is suggested to have importa

secondary fabrics revealed by remanence anisotropy methodological study and examples from plutonic rocks summary the fabric of fine grained magnetite is isolated by partial remanence anisotropy paar measurements that combine cycles of anhysteretic remanence acquisition and alternating frequency af tumbling demagnetization of the low coercivity fraction since remanence intensities of the most coercive fine grained magnetite are very weak rarely exceeding 10 per cent of the measured remanence in the studied samples a methodological study is presented that defines optimal measurement schemes and examines the role of gyroremanence and background remanences due to hard remanence carriers this procedure is applied to some reddened granites from tana corsica and wichita oklahoma and to the tellnes ilmenite rich norite norway the paar fabrics for fine grained magnetites are compared to the fabrics of the coarse grained primary magnetite grains as shown by both the anisotropy of magnetic susceptibility ams and the remanence anisotropy of the low coercivity fraction in tellnes fine grained magnetite is believed to be of secondary origin this is strongly suggested by the correlation between the paar fabrics and the orientations of sets of microfractures decorated by oxides in the reddened granites although no direct correlation between paar fabrics and microstructures could be evidenced the comparison between unaltered specimens and hydrothermally altered ones reveals changes in magnetic properties and fabrics that are ascribable to this secondary geological process

electron microprobe and la icp ms study of monazite hydrothermal alteration implications for u th pb geochronology and nuclear ceramics this paper presents new data that quantify the response of magmatic monazite to three main types of hydrothermal alteration namely sericitization chloritization and greisenization the samples were taken from three paleozoic granites the st nectaire granite massif central france the carnmenellis granite cornwall england and the skiddaw granite lake district england fluid inclusion thermometry and hydrothermal parageneses indicate that alteration took place at temperatures ranging from 260 to 340 c and salinities from 3 to 18 wt nacl equivalent possible monazite alteration mechanisms found in the course of this study using backscattered scanning electron microscopy bse sem electron microprobe laser raman spectroscopy and laser ablation inductively coupled plasma mass spectrometry la icp ms include cationic substitutions monoclinic to hexagonal structure transition accompanied by chemical exchanges selective th removal dissolution reprecipitation and dissolution with replacement by a different mineral these results show that despite the compositional and crystallographic simplicity of monazite it exhibits varied chemical responses to different mineral fluid interactions elemental and isotopic measurements by la icp ms for the unaltered monazites yield statistically significant 232th 208pb and 238u 206pb magmatic crystallization ages in some cases it was possible to give a reasonable estimate of the age of mineral fluid interaction using the altered parts of the monazites or newly precipitated crystals for other minerals 232th 208pb and 238u 206pb systematics were strongly disturbed by variable inputs and or depletions of u th and pb the data suggest that monazite like nuclear waste forms would be good hosts for tetravalent actinides but may release the lanthanides and actinides with lower and higher valencies especially if the fluids are oxidizing or tend to dissolve monazite

quantification of magnetic coercivity components by the analysis of acquisition curves of isothermal remanent magnetisation abstract a new method of analysing isothermal remanent magnetisation irm acquisition curves based on cumulative log gaussian analysis robertson and france phys earth planet inter 82 1994 223 234 is proposed it is based on the curve fitting of the irm acquisition curve versus the logarithm of the applied field with i the acquisition curve on a linear scale ii the acquisition curve expressed as a gradient and iii the acquisition curve on a probability scale even when a sample is not saturated its magnetic properties can be defined although with less certainty the number of magnetic components required for an optimal fit to a measured irm acquisition curve is evaluated statistically the method discriminates on the basis of different mineral coercivity therefore additional rock magnetic tests are still required to separate minerals with similar coercivities

timing and chemistry of igneous events associated with the southern oklahoma aulacogen abstract igneous activity in the southern oklahoma aulacogen of north america was concentrated in the early rifting stages of aulacogen development the time span over which liquids rose may not have exceeded 50 m y and certainly terminated before the upper cambrian igneous activity began with three basaltic liquids stratigraphically identifiable but perhaps not all distinct genetically this was followed by one large rhyolitic granitic episode of a type character one final basaltic event ended the activity all the basaltic types seem to be tholeiitic showing more kinship with either the older proterozoic north american midcontinental rift or the northern part of the cenozoic rio grande rift than the cenozoic east african rift two major uplifts occurred one between the earlier basalts and the rhyolite and one much later after all igneous activity was over in the pennsylvanian

magnetic fabric and microstructural evidence for a tectono thermal overprint of the early proterozoic murray pluton central ontario canada abstract the 2 38 ga old murray granite pluton intruded the southern footwall of the sudbury igneous complex and was subjected to repeated ductile deformation its nw and se domains display differences in strain state ams signature and the nature and distribution of fe oxide minerals the magnetic foliation is parallel to the visible foliation in the nw domain but discordant to it in the se domain microstructural data show that the magnetic fabric in the nw domain coincides with a foliation formed at high temperature probably in response to thermal softening during the emplacement of the adjacent sudbury igneous complex this tectono thermal overprint affected pre existing shape fabrics and was facilitated by incipient melting of the granite near the intrusive contact with the sudbury igneous complex rocks of the se domain are only partially overprinted and two mineral subfabrics are preserved prolate susceptibility ellipsoids and clusters of maximum susceptibility directions at the intersections of the two planar fabrics suggest oblique superposition of strain increments increases in bulk magnetic susceptibility and magnetic anisotropy degree towards the intrusive contact with the sudbury igneous complex can be explained by respective increases in the proportion of magnetite over ilmenite and strain intensity in the murray pluton

first wsr 88d documentation of an anticyclonic supercell with anticyclonic tornadoes the sunnyvale los altos california tornadoes of 4 may 1998 abstract on 4 may 1998 a pair of tornadoes occurred in the san francisco bay area in the cities of sunnyvale f2 on the fujita scale and los altos f1 the parent thunderstorm was anticyclonically rotating and produced tornadoes that were documented photographically to be anticyclonic as well making for an extremely rare event the tornadic thunderstorm was one of several pulse type thunderstorms that developed on outflow boundaries on the left flank of an earlier occurring thunderstorm east of san jose satellite imagery showed that the tomadic storm moved northwestward along a sea breeze boundary and ahead of the outflow boundary associated with the prior thunderstorms the shear environment into which the storm propagated was characterized by a straight hodograph with some cyclonic curvature and by shear and buoyancy profiles that were favorable for anticyclonically rotating updrafts mesoanticyclones were detected in the monterey kmux radar data in association with each tornado by the nationa

wintertime supercell thunderstorms in a subtropical environment a diagnostic study abstract the present study documents the environment initiation and evolution of three isolated supercell storms on 19 december 2002 as the first case near taiwan reported in the literature mainly using radar data and manual and gridded analyses in a subtropical environment the supercells occurred behind a winter cold front that provided a large west southwesterly vertical wind shear of 6 4 10 3 s 1 at 0 3 km this combined with weak to moderate instability cape 887 j kg 1 above the shallow surface cold air to yield a favorable environment for supercells an approaching upper level jet ulj at 200 hpa also provided strong shear through deep layers farther aloft prior to storm initiation significant daytime solar heating occurred over the mountain slopes along the coast of southeastern china leading to development of local circulation and onshore upslope winds resulting in convergence and uplifting three storms were initiated about 80 km inland around 1400 lst near the peaks of local terr

the structure and classification of numerically simulated convective storms in directionally varying wind shears abstract using a three dimensional numerical cloud model we investigate the effects of directionally varying wind shear on convective storm structure and evolution over a wide range of shear magnitudes as with a previous series of experiments using unidirectional wind shear profiles weisman and klemp the current results evince a spectrum of storm types ranging from short lived single cells at low shears multicells at intermediate shears to supercells at high shears with a clockwise curved hodograph the supercellular growth is confined to the right flank of the storm system while multicellular growth is favored on the left flank an analysis of the dynamic structure of the various cells reveals that the quasi steady supercell updrafts are strongly enhanced by dynamically induced pressure gradients on the right flank of the storm system we use this feature along with other related storm characteristics such as updraft rotation to propose a dynamically based storm classification scheme following

characterization of the magnetic fabric of rocks abstract an inextensive system of magnetic susceptibility anisotropy factors is suggested which is sufficient in the majority of practical applications for characterizing the magnetic fabric of rocks the system involves three newly devised factors the corrected anisotropy degree the shape factor and the difference shape factor the first of these characterizes the quantity of anisotropy somewhat better than the commonly used anisotropy degree the shape factor characterizes the quality of anisotropy i e the shape of the susceptibility ellipsoid in a convenient way its definition is based on a certain analogy between the anisotropy ellipsoid and the strain ellipsoid the difference shape factor can replace the shape factor when only the differences between the principal susceptibilities are given while the mean susceptibility is unknown

a tornadic supercell over elevated complex terrain the divide colorado storm of 12 july 1996 abstract a supercell storm that produced a tornado and large hail over the rocky mountains is documented with wsr 88d data a damage survey and visual observations this case study demonstrates how well the wsr 88d doppler radar was able to detect this storm and used to issue a timely tornado warning it also alerts us that supercell tornadoes can form over elevated complex terrain

multiple magmatic fabrics in the sazava pluton bohemian massif czech republic a result of superposition of wrench dominated regional transpression on final emplacement abstract the sazava pluton 354 ma of the central bohemian plutonic complex was emplaced syntectonically into the upper crustal neo proterozoic and lower paleozoic host rock along the se margin of the tepla barrandian zone bohemian massif czech republic the host rock is characterized by transpressional fabric dominated by steep se nw dipping foliations and subhorizontal ne sw stretching lineations associated with flattening to a plane strain ellipsoid the strain intensity degree of oblateness and temperature conditions of deformation increase in the host rock towards the pluton margin in the pluton quartz microfabric indicates progressive cooling of the syntectonically emplaced magma the ams method revealed multiple magmatic fabrics and lateral variations in the shapes of the ams ellipsoid and the magnetic fabric intensity in the pluton the central part of the pluton yields prolate shapes of the ams ellipsoid and a low degree of anisotropy with preserved steep magnetic lineations whereas along the pluton margins oblate ams ellipsoids are associated with a high degree of anisotropy we interpret the multiple fabrics and the partially decoupled fabric pattern between the host rock and the pluton as being a result of superposition of regional tectonic strain during highly oblique transpression on emplacement related intrusive strain where outer solidified and more rigid edges of the pluton enabled preservation of older steep fabrics in its interior

paleo redox boundaries in fractured granite at the earth s surface fe ii often oxidises and forms insoluble fe iii oxyhydr oxides whose particle size and structure depend on solution composition and temperature during formation and afterwards bacterial processes and exposure to reducing environments reduces them again releasing dissolved iron to the groundwater during such cycling the fe isotopes fractionate to an extent that is expected to depend on temperature in this study we report on the use of fe oxides as paleo redox indicators using their structure morphology and fe composition as a clue for formation conditions in samples taken from 120 m drill cores in granite from se sweden x ray amorphous superparamagnetic nanometre sized fe oxides are confined to fractures of the upper 50 m whereas well crystalline fe oxides with particle sizes typical for soils occur down to 110 m we also identified hematite with a particle size of 100 nm similar to hematite of hydrothermal origin the fe isotope composition of the fine grained fe oxides 1 56fe 1 irmm 14 referenced scatter significantly compared to the distribution previously observed for hydrothermal material 0 26 56fe 0 12 and they are dominantly heavier than fe bearing silicates from fractures 0 56 56fe 0 35 this is consistent with formation by low temperature weathering where fe silicates dissolve fe ii oxidises and fe iii oxides precipitate the x ray amorphous nanometre sized nature of near surface fe oxides suggests recent formation the deeper situated well crystalline fe oxides are more mature and we interpret that they record earlier oxidising events they exist in fractures that are not significantly altered indicating formation during periods of oxidation our results show that oxygenated water may reach depths of 110 m in fractured granite the absence of natural low temperature fe oxides from deeper drill cores suggests that oxygenated waters do not readily penetrate beyond about 100 m and suggests that radioactive waste repositories located at a depth of 500 m should be well protected from oxygenated waters

partial anhysteretic remanence and its anisotropy applications and grainsize dependence the distribution of remanence coercivities in a rock sample can be obtained by differentiating alternating field af demagnetization curves of anhysteretic remanent magnetization arm however significant errors can be introduced by grain interactions and by the noise inherent in differentiation we have found that 1 this error can be significantly reduced by measuring partial arm s parm s and 2 parm patterns show a strong empirical correlation with grain size parm s are imparted by exposing a sample to a small direct magnetic field in the presence of a large decreasing af the direct field is switched on over a specified range of af s thus magnetizing a fraction of the magnetite grains in the corresponding coercivity range we have further found in both synthetic and natural sediments that measurement of the anisotropy of parm enables us to detect the preferred orientations of distinct subpopulations of magnetite particles related to the size and shape of the grains

convective inhibition as a predictor of convection during ave sesame ii abstract the ave sesame 11 data set of 19 april 1979 is examined to determine the thermodynamic conditions prior to the onset of deep convection in western kansas the observations indicate that the convective region was characterized locally by substantial potential convective energy and low convective inhibition above the boundary layer one dimensional modeling shows that the convection occurred where the convective inhibition was a minimum but not where the potential convective energy was a maximum the model results suggest that the convective inhibition above the boundary layer was almost zero when the first echoes were observed

linear and nonlinear propagation of supercell storms abstract a nonlinear formula for updraft motion in supercell storms is derived from petterssen s formula for the motion of systems and the vertical equation of motion and tested on form preserving disturbances at each level continuous propagation of an updraft maximum is determined largely by the horizontal gradient of the nonhydrostatic vertical pressure gradient force nhvpgf at the updraft center the nhvpgf is deduced from the formal solution of the poisson equation for nonhydrostatic pressure in anelastic flow subject to homogeneous neumann boundary conditions at the ground and top boundary recourse also is made to published fields of partitioned vertical pressure gradient force updraft motion is partitioned into parts forced by horizontal gradients of hydrostatic pressure linear interaction between the environmental shear and updraft and nonlinear dynamical effects the dynamics of supercell storms for nearly straight and highly curved hodographs are found to be different nonlinear rotation

mineral fabrics in high level intrusions recording crustal strain and volcano tectonic interactions the shellenbarger pluton sierra nevada california the minarets caldera is a volcano plutonic complex in the sierra nevada california that exemplifies complex interactions between volcanism and tectonic deformation in continental margin arcs caldera evolution commenced with emplacement of pre collapse rhyolitic ash flow tuff followed by collapse and deposition of volcanic breccia and rhyodacitic ash flow tuff subsequently the volcanic rocks were deformed along the regional bench canyon shear zone the caldera centre was then intruded by the resurgent c 100 ma steep sided shellenbarger granite pluton which steepened the shear zone foliation the pluton was overprinted by syn to post magmatic nne ssw horizontal shortening the same shortening was documented in several other late cretaceous syntectonic plutons in the sierra nevada and interpreted to record dextral transpression during convergence of the farallon and north american plates to explain the unusual tectonic fabric in the shallow level shellenbarger pluton we develop a general model for strain partitioning in syntectonic magma bodies emplaced at various crustal levels we propose that shallow intrusions isolated within stiff crust may tend to accommodate minor pure shear strain whereas simple shear dominates along weak faults and shear zones by contrast a rheological reversal is crossed deeper in the crust and magma bodies become the weakest simple shear dominated parts of the system supplementary material analytical methods and anisotropy of magnetic susceptibility and u th pb isotopic data are available at https doi org 10 6084 m9 figshare c 3582749

wintertime supercell thunderstorms in a subtropical environment numerical simulation abstract following an earlier diagnostic study the present paper performs numerical simulations of the rare wintertime supercell storms during 19 20 december 2002 in a subtropical environment near taiwan using japan meteorology agency jma 20 km analyses and horizontal grid spacing of 1 5 and 0 5 km the cloud resolving storm simulator cress of nagoya university successfully reproduced the three major storms at the correct time and location but the southern storm decayed too early over the taiwan strait the two experiments produce similar overall results suggesting that the 1 5 km grid spacing is sufficient even for storm dynamics model results are further used to examine the storm structure kinematics splitting process and the variation in the mesoscale environment over the taiwan strait the strong surface northeasterly flow enhanced low level vertical shear and helped the storms evolve into isolated supercells consistent with previous studies the vorticity budget analysis indicates that

general characteristics of squall lines observed in tamex abstract six cases of prefrontal squall lines were observed over the taiwan strait and western taiwan during the 1987 taiwan area mesoscale experiment tamex the mean propagation speed was 10 m s 1 and the mean life span was 11 4 h for the six squall lines all the lines occurred ahead of the mei yu front and moved away from the front with time the mean environmental conditions associated with the squall lines were analyzed by compositing the six cases the environmental conditions observed during the tamex squall lines were found with characteristics between tropical and midlatitude squall lines the steering level was near 7 km during the mature stage a low level jet at 3 4 km was present with strong vertical shear in the presquall environment below 700 hpa the squall lines were oriented 45 to the shear in the 1 3 km layer like midlatitude cases the cape however is similar to the tropical squall lines the inflow ahead of the squall lines was deeper and stronger below 400 hpa and the cape w

remagnetization in bituminous limestones of the neoproterozoic araras group amazon craton hydrocarbon maturation burial diagenesis or both 1 neoproterozoic carbonates of the araras group exhibit two distinct magnetic components across the same carbonate succession in a cross section between the amazon craton and the paraguay fold belt pink dolostones of the mirassol d oeste formation carry a dual polarity primary component whereas black bituminous limestones of the guia formation yield a secondary postfolding component magnetic signatures of the guia limestones such as high anhysteretic remanence magnetization saturation isothermal remanence magnetization ratios high frequency dependent magnetic susceptibility and contradictory lowrie fuller and cisowski tests are typical of remagnetized carbonates unblocking temperatures suggest that the stable high temperature remanence is carried by both pyrrhotite and magnetite for which an authigenic origin is suggested by scanning electron microscope observations the different magnetic properties noted between dolostones with or without bitumen and between dolostones and limestones in the same metamorphic conditions lead to the hypothesis that the amount of hydrocarbon as well as the lithology influence nucleation of authigenic magnetic minerals in these rocks presence of magnetite pseudoframboids and euhedral iron sulphide crystals occurring in fracture and voids are in favor of a chemical remanence crm the presence of pyrrhotite as one of the main carriers of crm in these rocks and its association with bitumen in fractures is probably related to an epigenetic enrichment of sulfur due to hydrocarbon seepage however hydrocarbon maturation solely could not explain the differences of the magnetic mineralogy observed in the craton and the fold belt enhanced magnetite formation in the thrust and fold belt is interpreted to be the result of higher temperatures leading to stronger diagenesis of clay minerals

aluminous subsolvus anorogenic granite genesis in the light of nd isotopic heterogeneity the problem of the genesis of anorogenic granites is far from being resolved and this may result from their petrogenetic variability several kinds of a type granites are indeed present in many anorogenic provinces a peralkaline and an aluminous trend commonly occur and the latter often comprises hypersolvus and subsolvus granites as an example of the aluminous subsolvus group we have studied major trace elements and nd sr isotopes of the tana granite sw corsica france along with the coeval mafic and intermediate rocks petrological and trace element constraints suggest that the associated mafic rocks are not representative of the magmatic precursor of the granite rather the intermediate rocks monzonites are more akin to the parental magma that yielded the granite through fractional crystallization the aluminous subsolvus anorogenic granite and related monzonite are characterized by variable nd isotopic compositions ndi from 4 7 to 1 3 and from 2 to 0 1 respectively while sr isotopes give good isochrons it is argued that the nd isotopic heterogeneity was not acquired at a late stage in the upper crust by contamination from partial crustal melts or f rich late to post magmatic fluids similarly the lack of correlation between trace element concentrations and nd isotopes in a single granitic body precludes a process of crustal contamination of mantle melts instead it is inferred that these low and very heterogeneous ndi s were probably inherited from the magma source region in the lower crust the preservation of this heterogeneity during magma ascent suggests that only limited mechanical and chemical mixing occurred in magma chambers it is more likely that separate batches of low viscosity a type magmas ascended individually mainly through fracture propagation in an extensional regime the possible derivation of granites from monzonites through fractional crystallization together with the examination of the corsican lower crust composition and already published dehydration melting experiments suggest that the aluminous subsolvus trend of a type granites may be generated through the partial melting of metamorphosed amphibole bearing and k2o rich mafic cumulates that have not undergone a previous partial melting event

retrieved pressure field and its influence on the propagation of a supercell thunderstorm an analysis of the observed propagation of the garden city supercell storm based on data collected by an airborne doppler radar is presented the environmental wind profile was characterized by a straight hodograph wind syntheses were used to retrieve the perturbation pressure fields in order to isolate the important dynamical processes controlling storm movement the pressure perturbations produced by the linear and nonlinear effects were comparable in magnitude the rightward bias in storm motion was primarily a result of the forcing produced by the vertical gradients of perturbation pressure this finding is consistent with the results based on numerical simulations the vertical gradients of the linear perturbation pressure were important in determining storm motion early in its life cycle subsequently the nonlinear effects dominated a decomposition of the nonlinear forcing into contributions from cyclostrophically balanced flow and nonlinear shear was also presented this partitioning revealed that both the forcing produced by the mesocyclone and the horizontal shear that was baroclinically produced by the gradient of buoyancy along the flanks of the updraft were important effects to consider

a three dimensional numerical simulation of splitting severe storms on 3 april 1964 abstract a three dimensional numerical storm model is used to investigate the observed splitting of several reflectivity echoes on 3 april 1964 in oklahoma representative soundings from this day exhibit a nearly one directional environmental wind shear vector and the presence of strong low level wind shear in the numerical simulation an initial cloud splits into two long lived rotating storms one that moves to the left of the mean winds and the other to the right the left moving storm develops more slowly than the right moving one due to the deviation of the environmental wind hodograph from a straight line below 1 km further the left mover eventually splits convergence induced by the cold low level storm outflow plays a major role in the development of both the first and second splits however the second split appears to be dynamically different than the first as the left moving updraft remains essentially unchanged while a new updraft forms immediately adjacent to it because of the different p

the timing and nature of late quaternary vegetation changes in the northern great plains usa and canada a re assessment of the spruce phase abstract this paper revises the chronology for the northward migration of picea glauca white spruce across the northern great plains following the recession of the laurentide ice sheet and reinterprets the species composition and structure of the late glacial vegetation on the basis of pollen and plant macrofossil analysis the timing of spruce migration is based on 26 14 c ages obtained from picea macrofossils the date for the appearance of white spruce in southern south dakota usa remains unchanged 12 600 14 c yr bp ca 15 000 cal yr bp but its arrival in southern saskatchewan canada by 10 300 14 c yr bp ca 12 100 cal yr bp is about 1500 years later than previously estimated based on an organic sediment date picea glauca thus migrated northwards at an average rate of 0 38 km 14 c year 0 30 km calendar year significantly slower than the previously published rate of 2 km 14 c year white spruce trees probably inhabited lake shorelines whereas prairie parkland and boreal plants occupied both lowlands and uplands forming an open white spruce parkland this interpretation differs from a previous reconstruction of a boreal type spruce forest and thus offers another paleoclimatic interpretation precipitation was probably low and summer temperatures relatively mild averaging about 19 c

the evolution of a severe mesoscale convective system cloud to ground lightning location and storm structure abstract cloud to ground lightning location data are correlated with the doppler radar observed structure of the evolution of a severe mesoscale convective system in oklahoma on 23 may 1981 while many of the results are not new this study is unique in that the evolution of electrical activity in a severe storm is compared to storm structure for most of the storm s life the first storm formed ahead of a mesoscale low pressure area located at the intersection of a front and a dryline and developed into a tornadic supercell in an environment of locally enhanced vertical shear ordinary cells subsequently formed both to the southwest and northeast of the supercell along the dryline and the front respectively and merged with the supercell to form a squall line having a trailing stratiform precipitation region with midlevel rear inflow it is shown that the cloud to ground flash rate in the convective region is related to the apparent strength of the updraft before the storm became a supercell the ligh

the a type mount scott granite sheet importance of crystal magma traps the presence of rapakivi feldspar and of distinctive porphyritic texture of mount scott granite indicates a period of crystallization prior to final emplacement beneath an extensive penecontemporaneous rhyolite volcanic pile final crystallization conditions are interpreted to have been 50 mpa at depths 1 4 km based on stratigraphic constraints however geobarometry based on the al content of amphibole phenocrysts and comparison of granite compositions with phase relations in the sio2 naalsi3o8 kalsi3o8 ternary system both yield pressure estimates of 200 mpa these pressure estimates are interpreted as plumbing the depth of a temporary storage chamber at 7 8 km this depth coincides in this case both with the probable proterozoic basement cover contact and with the calculated brittle ductile transition at time of ascent of mount scott magma although rising magma that fed the preceeding voluminous carlton rhyolite apparently rose unimpeded past these horizontal anisotropies rising magma that formed mount scott granite temporarily paused at this depth based on magmastatic calculations we suggest that horizontal anisotropies e g brittle ductile transition become crustal magma traps where the magma driving pressure exceeds the lithostatic load when the anisotropy is encountered during rifting initial large influxes of magma may proceed passed crustal anisotropies but have the effect of increasing the relative magma driving pressure through reducing horizontal stress thus magma driving pressure may eventually exceed the lithostatic load at the depth of the brittle ductile transition thereby activating this crustal magma trap ponding of magma at the brittle ductile transition chokes the eruption such a pause in magma supply rate may permit a return to initial stress conditions and deactivate the crustal magma trap once again magma will rise to the surface initiating a new magmatic cycle

magnetite grain shape fabric and distribution anisotropy vs rock magnetic fabric a three dimensional case study abstract shape fabric and distribution anisotropy of magnetite grains in a highly ferromagnetic s l syenite are determined for three mutually perpendicular planes on about 600 magnetite grains the grains are lenticular with an average aspect ratio of about 2 0 their preferred orientation determined by the inertia tensor method is consistent with the principal directions of the magnetic susceptibility ellipsoid and the axial ratios of the magnetic fabric ellipses are very close to those of the shape fabric ellipses e g 1 41 and 1 39 in the section plane perpendicular to the magnetic foliation and parallel to the magnetic lineation the anisotropic wavelet transform can detect and quantify the anisotropic distribution of the magnetite grains which are located interstitially along the feldspar grain boundaries this anisotropic distribution has no noticeable effect on the rock s anisotropy of magnetic susceptibility ams since only a few percent of grains are close enough to interact magnetically therefore it is realistic to consider that the ams of ferromagnetic granitic rocks originates mainly from the shape fabric of anisotropic magnetite grains and a close correlation between the magnetic fabric and the magnetite grain shape fabric is expected

hematite in porous red clouded feldspars evidence of large scale crustal fluid rock interaction abstract transmission electron microscopy shows that red clouded alkali feldspars from granitic rocks contain numerous open pores up to several hundred nanometres in cross section and that almost every pore contains rosettes or needles of crystalline hematite in the samples described here the hematite forms at the reaction interfaces at which plagioclase feldspar is replaced by alkali feldspar these observations together with the fact that the origin of the porosity can be explained by the mechanism of sub solidus mineral fluid replacement processes indicate that hematite is a direct product of fluid rock interaction and is not a solid state exsolution product from the feldspar

the 29 june 2000 supercell observed during steps part i kinematics and microphysics this is a two part study that addresses the kinematic microphysical and electrical aspects of a severe storm that occurred in western kansas on 29 june 2000 observed during the severe thunderstorm electrification and precipitation study steps field campaign in this first part polarimetric and doppler radar data are used along with a simple particle growth model to examine the evolution of the kinematic and microphysical properties of the storm from its earliest developing phase through its mature and dissipating phases during its severe stage the storm exhibited frequent positive cloud to ground lightning strikes very large 5 cm hail and a tornado doppler derived winds radar reflectivity and hydrometeor classifications from the polarimetric data over a nearly 4 h period are presented it is shown that updraft velocity and vertical vorticity had to reach magnitudes of at least 10 m s 1 and 10 2 s 1 and occupy major portions of the storm before it could produce most of the observed severe storm characteristics furthermore the establishment of cyclonic horizontal flow around the right flank of the updraft core was essential for hail production most of the largest hail grew from near millimeter sized particles that originated in the mid to upper level stagnation region that resulted from obstacle like flow of environmental air around the divergent outflow from the upper part of the updraft these recycling embryonic particles descended around the right flank of the updraft core and reentered the updraft intermingling with other smaller particles that had grown from cloud base along the main low level updraft stream

a numerical study of a nontornadic supercell over france abstract a case of a nontornadic supercell over france is simulated with a three dimensional nonhydrostatic cloud model the simulation starts from an operational data analysis without any superimposed perturbation the initial convective cell is triggered by a mesoscale convergence line associated with a preexisting thermal boundary the model succeeds in simulating right and left moving storms arising from a splitting process of the initial convective cell the right moving storm exhibits the characteristics of a supercell with a hooklike structure in the precipitation field a midlevel rotating updraft and a low level cyclonic vortex a vorticity budget analysis is performed along backward parcel trajectories for the initial storm and for the supercell phases with emphasis on the impacts of the preexisting thermal boundary and the associated low level variations of shear similar mechanisms as those found for the cases over homogeneous environment operate globally indeed the simulation exhibits a

orographic effects on a squall line system over taiwan abstract a north south oriented multicellular squall line during the taiwan area mesoscale experiment s tamex intensive observation period 2 was studied using dual doppler radar data as it moved over the island of taiwan over the open ocean it moved eastward at a constant speed 15 5 m s 1 as the squall line encountered taiwan island s complex mountainous areas the leading edge moved slower in the mountain ridge areas than over the valleys as a result its orientation became approximately parallel to the terrain contours in the upper levels the eastward movement of the squall line aloft was less affected by the terrain compared to the low levels as a result the westward tilt of the rising motion at the front became less significant part of the system relative rear to front flow entered the rising branch of the storm s circulation as a result of orographic lifting as illustrated by the trajectory analysis and was enhanced by latent heat release the system relative rear to front flow did

magnetic fabric methods and applications fabric is a ubiquitous and significant feature of geological materials the processes involved in the formation and deformation of rocks and sediments leave their mark on the orientations of the constituent mineral grains petrofabrics thus provide essential keys to understanding the history of geological materials magnetic anisotropy is directly related to petrofabric and has become one of the most rapid sensitive and widely used tools for its characterization the relationship between magnetic fabric and petrofabric is complex and depends on various factors including the composition concentration and grain size of mineral grains ongoing research in geological applications is paralleled by studies of the fundamental mineral magnetic phenomena involved the papers in this book represent the current state of investigations in magnetic anisotropy studies as a discipline that integrates geological interpretations mineral fabric development technical advances and rock magnetic properties

rock magnetism of remagnetized paleozoic carbonates low temperature behavior and susceptibility characteristics we have conducted a new set of rock magnetic experiments on samples of remagnetized paleozoic carbonates of eastern north america these experiments were designed to investigate the origin of the unusual hysteresis behavior of these rocks by evaluating 1 the importance of ferrimagnetic pyrrhotite as a remanence carrier and 2 the sources of low field susceptibility low temperature measurements of saturation isothermal remanent magnetization sirm indicate that the pyrrhotite magnetic transition at 32 k is absent in the trenton and onondaga limestones of new york this transition is observed but poorly expressed in the knox group of east tennessee room temperature sirms cooled to 10 k in zero field lose a small fraction of their intensity between 30 and 35 k samples from all three formations show a broad peak in irm intensity at about 200 k which is typical of pyrrhotite in the trenton and onondaga samples the ferrimagnetic component of low field susceptibility is significantly larger than the ratio mrs hcr and is thus probably due dominantly to magnetite in some of the knox samples the reverse is true suggesting an important pyrrhotite contribution for all of the samples the ferrimagnetic susceptibility normalized by the saturation magnetization is anomalously high about a factor of 5 or 10 higher than the typical value for magnetite we believe that this indicates a very substantial contribution from superparamagnetic particles strong frequency dependence of susceptibility and very high ratios of anhysteretic to saturation remanence confirm the importance of ultrafine particles spanning the superparamagnetic single domain boundary all three of these chemically remagnetized carbonates units exhibit the following properties which have not previously been found together for any rock or synthetic analog and which therefore appear to constitute a diagnostic set of rockmagnetic criteria for recognizing chemically remagnetized rocks mrs ms 0 89 hcr hc 0 6 k ms 50 m a arm sirm 20 ka mrs hcr 50

iron and lithium isotope systematics of the hekla volcano iceland evidence for fe isotope fractionation during magma differentiation abstract in this study potential iron isotope fractionation by magmatic processes in the earth s crust was systematically investigated high precision iron isotope analyses by mc icp ms were performed on a suite of rock samples representative for the volcanic evolution of the hekla volcano iceland the whole series of hekla s rocks results from several processes i basaltic magmas rise and induce partial melting of meta basalts in the lower part of the icelandic crust the resulting dacitic magma evolves to rhyolitic composition through crystal fractionation during this differentiation the 56 54feirmm 014 values increase successively from 0 051 0 021 for the primitive dacites to 0 168 0 021 for the rhyolites this increase can be described by a rayleigh fractionation model using a constant bulk fractionation factor between all mineral phases m and the silicate liquid l of 56 54fem l 0 1 ii the basaltic magma itself differentiates by crystal fractionation to basaltic andesite composition no fe isotope fractionation was found in this series all basalts and basaltic andesites have an average 56 54feirmm 014 value of 0 062 0 042 2sd n 9 identical to mean terrestrial basaltic values reported in previous studies this observation is consistent with the limited removal of iron from the remaining silicate melt through crystal fractionation and small mineral melt fe isotope fractionation factors expected at temperatures in excess of 1050 c iii andesites are produced by mixing of basaltic andesite with dacitic melts the iron isotope composition of the andesites is matching that of the basaltic andesites and the less evolved dacites in agreement with a mixing process in the hekla volcanic suite li concentrations are positively correlated with indicators of magma differentiation all hekla rocks have 7li values typical for the upper mantle and demonstrate the absence of resolvable li isotope fractionation during crystal fractionation as a fluid mobile trace element li concentrations and isotopes are a potential tracer of magma fluid interaction at hekla li concentrations and isotope compositions do not indicate any extensive fluid exsolution hence the heavy fe isotope composition of the dacites and rhyolites can be predominately attributed to fractional crystallisation iron isotope analyses on single samples from other icelandic volcanoes torfajokull vestmannaeyjar confirm heavy fe isotope enrichment in evolving magmas our results suggest that the iron isotope composition of highly evolved crust can be slightly modified by magmatic processes

effect of hot desert weathering on the bulk rock iron isotope composition of l6 and h5 ordinary chondrites abstract although iron isotopes are increasingly used for meteorites studies no attempt has been made to evaluate the effect of terrestrial weathering on this isotopic tracer we have thus conducted a petrographic chemical and iron isotopic study of equilibrated ordinary chondrites oc recovered from hot moroccan and algerian saharan deserts environment as previously noticed we observe that terrestrial desertic weathering is characterized by the oxidation of fe ni metal fe0 sulfide and fe2 occurring in olivine and pyroxene it produces fe oxides and oxyhydroxides that partially replace metal sulfide grains and also fill fractures the bulk chemical compositions of the ordinary chondrites studied show a strong sr and ba enrichment and a s depletion during weathering bulk meteoritic iron isotope compositions are well correlated with the degree of weathering and s sr and ba contents most weathered chondrites display the heaviest isotopic composition by up to 0 1 which is of similar magnitude to the isotopic variations resulting from meteorite parent bodies formation and evolution this is probably due to the release of isotopically light fe2 to waters on the earth s surface hence when subtle fe isotopic effects have to be studied in chondrites meteorites with weathering grade above w2 should be avoided

hydrothermally induced changes in mineralogy and magnetic properties of oxidized a type granites abstract the changes in magnetic mineralogy due to the hydrothermal alteration of a type granitic rocks have been thoroughly investigated in samples from the granite of tana corsica france and compared with other a type granites meruoca ne brazil bushveld south africa mount scott wichita mountains oklahoma usa and the stratoid hypersolvus granites of madagascar the altered red colored samples and their non altered equivalents were magnetically characterized by means of magnetic susceptibility measurements hysteresis loops remanent coercivity spectra and lowrie test it is shown that hydrothermalization in magnetite bearing granites is related to the formation of fine grained magnetite and hematite and to coeval depletion in the content of primary low coercive coarse grained magnetite these mineralogical changes give typical rock magnetic signatures namely lower susceptibility magnitudes and anisotropy degrees prolate ams anisotropy of magnetic susceptibility fabrics and increased coercivities optical microscopy and sem scanning electronic microscopy images suggest that the orientation of the secondary magnetic minerals is related to fluid pathways and micro fractures formed during the hydrothermal event and therefore may be unrelated to magma emplacement and crystallization fabrics changes in magnetic mineralogy and grain size distribution have also to be considered for any paleomagnetic and iron isotope studies in granites

photographic documentation and environmental analysis of an intense anticyclonic supercell on the colorado plains abstract anticyclonic left moving supercells are observed each year in the united states emanating both discretely and from storm splitting processes such thunderstorms often produce severe hail and wind gusts and on rare occasion tornadoes the body of documentary literature on this subset of supercells is relatively scant compared with right moving storms and this is especially true regarding visual characteristics and conceptual models here a characteristic example of the anticyclonic supercell is presented using an intense and well defined specimen that passed over aroya colorado on 15 june 2002 photographic and radar documentation is provided in original and mirrored forms for aid in conceptualizing the left moving supercell and associated structures and processes a summary overview is presented of the environment development evolution and effects of this remotely located but noteworthy event

importance of late magmatic and hydrothermal fluids on the sm nd isotope mineral systematics of hypersolvus granites abstract despite the significant impact of the sm nd radiometric system in various fields of isotope geochemistry very few attempts have been made to use it at the mineral scale in granite studies this approach has been used on two granites from the anorogenic province of corsica se france two statistically acceptable alignments were obtained in an isochron diagram for the hypersolvus fayalite bearing granite of mantelluccio alkali feldspar amphibole inclusion free zircon fluorite and fergusonite give an age of 331 10 ma whereas allanite whole rock amphibole and inclusion free zircon yield a significantly younger age at 291 13 ma the latter is validated by a zircon u pb age of 283 1 ma as the magmatic emplacement date of the granite the other alignment giving a 50 ma older age can be interpreted either as being inherited or more likely as the result of hydrothermal convection interacting with the country rocks at the end of granite crystallization the sm nd systematics of epidote records a second fluid rock event which probably occurred at least 200 ma after the emplacement of the granite extremely low 143 nd 144 nd ratio indicates that the epidote crystallized from a fluid that carried lanthanides in the crust over a distance of at least 1 km zircons with inclusions have a lower initial 143 nd 144 nd ratio than inclusion free zircons this feature is attributed to apatite and or zircon core inheritance through contamination during the ascent of the granitic melt in the crust two isochrons have been also obtained for the hypersolvus peralkaline granite of evisa amphibole fluorite sphene and fergusonite yield an age of 259 6 ma whereas the whole rock alkali feldspar and metamict zircon give an age at 209 14 ma in this case the former value is roughly in agreement with a possibly slightly rejuvenated whole rock rb sr date of 249 3 ma suggesting that it documents the magmatic crystallization age of the granite sm nd isotope systematics of the alkali feldspar metamict zircon and possibly allanite were re equilibrated during an important hydrothermal event that affected western europe in early jurassic times in both examples however it appears that the whole rock nd isotopic signatures were not strongly affected by the various fluid rock events indicating that they can still be used for petrogenetic investigations in contrast these results show that like k ar and rb sr the sm nd systematics of minerals with weak resistance to post magmatic processes can be used to trace and date hydrothermal events

the sensitivity of simulated supercell structure and intensity to variations in the shapes of environmental buoyancy and shear profiles abstract convective storm simulations are conducted using varying thermal and wind profile shapes subject to the constraints of strict conservation of convective available potential energy cape and hodograph trace small and large cape regimes and straight and curved hodographs are studied each with a matrix of systematically varying thermal and wind profile shapes having identical levels of free convection and bulk richardson numbers favorable to supercell development differences in storm intensity and morphology resulting from changes in the profile shapes can be profound especially in the small cape regime where for the moderate shears studied here storms are generally weak except when the buoyancy is concentrated at low levels in stronger cape regimes less dramatic relative enhancements of storm updraft intensity are found when both the buoyancy and shear are concentrated at low levels peak midlevel vertical vorticity correlates roughly with peak updraft speed in the small cape regime but

the influence of the shear induced pressure gradient on thunderstorm motion abstract in the present investigation we propose a simple theory to explain how a veering environmental wind shear vector can cause an initially symmetric updraft to grow preferentially to the right of the shear vector and acquire cyclonic rotation the explanation offered is based on linear theory which predicts that interaction of the mean shear with the updraft produces favorable vertical pressure gradients along its right flank to asses the validity of linear theory for large amplitude updrafts the three dimensional shallow anelastic equations are numerically integrated using a simple parameterization for latent heating within a cloud and the linear and nonlinear forcing terms are separately analyzed these results suggest that although the nonlinear effects strongly promote splitting of the updraft the linear forcing remains the dominant factor in preferentially enhancing updraft growth on the right flank we believe this differential forcing is a major contributor to the observed predominance o

the structure and evolution of a numerically simulated high precipitation supercell thunderstorm the structure and evolution of a high precipitation hp supercell thunderstorm is investigated using a threedimensional nonhydrostatic cloud scale numerical model tass the model is initialized with a sounding taken from a mesoscale modeling study of the environment that produced the 28 november 1988 raleigh tornadic thunderstorm tass produces a long lived convective system that compares favorably with the observed raleigh tornadic thunderstorm the simulated storm evolves from a multicell type storm to a multiple updraft supercell storm the storm complex resembles a hybrid multicell supercell thunderstorm and is consistent with the conceptual model of cool season strong dynamic hp supercells that are characterized by shallow mesocyclones the origin of rotation in this type of storm is often in the lowest levels interactions between various cells in the simulated convective system are responsible for the transition to a supercellular structure an intense low level updraft core forms on the southwest flank of the simulated storm and moves over a region that is rich in vertical vorticity the stretching of this preexisting vertical vorticity in the storm s lowest levels is the most important vertical vorticity production mechanism during the initial stages of the main updraft s development interactions with an extensive cold pool created by the storm complex are also important in producing vertical vorticity as the main updraft grows overall the development of vorticity associated with the main updraft appears similar to nonsupercellular tornadic storms however classic supercell signatures are seen early in the simulation associated with other updrafts e g formation of vortex couplet due to tilting of ambient horizontal vorticity storm splitting etc and are deemed important in the storm s supercell stage rotation is sustained in the lowest levels of the storm despite large amounts of precipitation located near and within the main mesocyclone pulsating downdrafts periodically invigorate the storm and the gust front never occludes thus allowing the main updraft to persist for a prolonged period of time the storm s intensity is also maintained by frequent updraft mergers

the influence of preexisting boundaries on supercell evolution abstract a three dimensional nonhydrostatic cloud model is used to study the evolution of supercell thunderstorms with emphasis on the low level mesocyclone interacting with preexisting boundaries the impacts of low level environmental shear storm motion relative to boundary orientation and boundary strength are assessed in the low level shear experiments significant low level rotation is consistently observed earlier tends to be stronger and is longer lived in storms interacting with a boundary than in storms initiated in a homogeneous environment low level rotation is weaker in storms crossing the boundary and moving into the colder air in contrast all storms moving along or into the warm air ahead of the boundary develop significant low level rotation increasing the temperature gradient and shear across the boundary has little impact on the low level mesocyclone evolution storms interacting with a boundary characterized by only horizontal shear produce weaker mesocyclones than those creat

calibration of radiocarbon ages and the interpretation of paleoenvironmental records abstract calibration of the radiocarbon timescale of paleoecological records is essential if they are to be explained correctly in terms of their governing ecological or climatological controls the differences between calendar ages and radiocarbon ages that arise from variations in 14c production through time can distort the chronologies of individual records and the interpretations based on them misleading impressions of synchrony or diachrony of events among multiple records can result and estimates of the apparent duration of episodes and rates of sedimentation and local population changes can be biased displays of the temporal patterns of migration or extinction may also be affected spurious correlations may arise between records with radiocarbon controlled chronologies and time series of potential controls that are expressed on a calendar time scale support for particular explanations of features in a paleoecological record may vary depending on whether radiocarbon ages are calibrated or not this situation is illustrated using the eastern beringian populus subzone as an example when the radiocarbon ages that control the timing of the populus subzone are calibrated the contemporaneous decrease in ice volume and increase in summer insolation are implicated as the ultimate controls of the occurrence of the subzone

emplacement modes of the ladinian plutonic rocks of the dolomites insights from anisotropy of magnetic susceptibility abstract in the dolomites eastern southern alps italy a diffuse middle triassic igneous activity is now present mostly as lava flow and pyroclastic successions with rare shallow depth intrusive bodies cropping out at predazzo monzoni and cima pape areas in this work the emplacement modes of the predazzo and monzoni bodies were investigated by means of petrographic and anisotropy of magnetic susceptibility ams data coupled with a field geological study the presence of intrusive rocks in between these two bodies and the continuity of the metamorphic aureola from west of the predazzo to east of the monzoni body suggest that they are parts of a 20 km wide sw ne oriented continuous pluton sub parallel to ladinian strike slip faults ams and petrographic data from the predazzo body are consistent with a multistage ring dyke emplacement mode with areas of upward flow of the magma located in the nw and ese part of the intrusion whereas in the southern sector of the pluton our data suggest prevalently horizontal flows in general the predazzo sheets indicate either upward magma flow or along strike lateral magma transport and the round shape suggests no influence of ladinian tectonic structures on the contrary the ene wsw elongated shape of the monzoni body was controlled by the occurrence of strike slip faults associated with ladinian tectonics the feeder being likely located at the ne edge of the body however the absence of deformation at the field and micro scale is consistent with a post ladinian tectonics timing of emplacement as for the predazzo pluton

emplacement and deformation of the a type madeira granite amazonian craton brazil abstract the madeira granite is one of the paleoproterozoic 1 82 ga a type granite intrusions in the amazonian craton it is elongated in the ne sw direction and is composed of four facies classical structural techniques and the anisotropy of magnetic susceptibility ams method were applied to the study of its internal fabric magnetic susceptibility measurements thermomagnetic curves remanent coercivity spectra optical microscopy and sem scanning electron microscopy analyses were carried out on the earlier and later facies of the madeira granite the rapakivi granite rg and the albite granite ag respectively the last one is subdivided into the border albite granite bag and the core albite granite cag subfacies ams fabric pattern is controlled by pure magnetite in all facies despite significant amounts of hematite in the bag subfacies microstructural observations show that in almost all sites magnetic fabric correlates to magmatic state fabrics that are defined by a weak ne sw orientation of mafic and felsic silicates however strain mechanisms in both subfacies of ag also exhibit evidence for solid state deformation at high to moderate temperatures pegmatite dyke strike slip fault sf a b c hydrothermal vein normal fault f 1 2 and joint j structures were observed and their orientation and kinematics is consistent with the magmatic and solid state structures dykes sf a c and f 1 are usually orientated along the n70 e 40 n plane which is nearly parallel to the strike of ams and magmatic foliations in contrast veins sf b f 2 and some j are oriented perpendicular to the n70 e trend kinematic analysis in these structures shows evidence for a dextral sense of movement in the system in the brittle regime the coherent structural pattern for the three facies of madeira granite suggests that the different facies form a nested pluton the coherence in orientation and kinematics from magmatic to high temperature solid state and into the brittle regime indicates the continuity in the stress regime from the last magmatic stages until the complete cooling of the pluton likely along a ne sw dextral corridor related to the regional deformation in the uatuma anaua domain of the amazonian craton

elevated mixed layers in the regional severe storm environment conceptual model and case studies abstract a conceptual model of the evolution of the severe local storm environment is discussed in conjunction with three case studies drawn from data obtained during the 1979 sesame field program the conceptual model describes how a particular configuration of topography and air flow can produce a low level restraining inversion or lid which focuses the location and even enhances the intensity of severe local storms while capping inversions have often been attributed to subsidence in this model it is demonstrated that the lid frequently originates from differential advection of a hot dry mixed layer from an elevated plateau over a cooler moister layer advected northward ahead of a trough in the westerlies for either the elevated mixed layer or subsidence inversion this type of vertical stratification suppresses release of convective instability while nevertheless allowing the latent instability of the boundary layer to increase with time however for elevated mixed layer lids the differing s

interpretation of magnetic susceptibility a new approach to geophysical evaluation of the degree of rock alteration magnetic susceptibility measurements on cores from boreholes in the eye dashwa lakes pluton an archean granitic pluton near atikokan ontario give a quantitative evaluation of the alteration leve

late quaternary paleoclimate of western alaska inferred from fossil chironomids and its relation to vegetation histories abstract fossil chironomidae assemblages with a few chaoboridae and ceratopogonidae from zagoskin and burial lakes in western alaska provide quantitative reconstructions of mean july air temperatures for periods of the late middle wisconsin 39 000 34 000 cal yr b p to the present inferred temperatures are compared with previously analyzed pollen data from each site summarized here by indirect ordination paleotemperature trends reveal substantial differences in the timing of climatic warming following the late wisconsin at each site although chronological uncertainty exists zagoskin lake shows early warming beginning at about 21 000 cal yr b p whereas warming at burial lake begins 4000 years later summer climates during the last glacial maximum lgm were on average 3 5 c below the modern temperatures at each site major shifts in vegetation occurred from 19 000 to 10 000 cal yr b p at zagoskin lake and from 17 000 to 10 000 cal yr b p at burial lake vegetation shifts followed climatic warming when temperatures neared modern values both sites provide evidence of an early postglacial thermal maximum at 12 300 cal yr b p these chironomid records combined with other insect based climatic reconstructions from beringia indicate that during the lgm 1 greater continentality likely influenced regions adjacent to the bering land bridge and 2 summer climates were at times not dominated by severe cold

magnetic susceptibility and ams of the bushveld alkaline granites south africa the bushveld complex in south africa includes one of the world s largest anorogenic alkaline granite intrusions 66 000 km2 the granite forms a composite laccolith of 350 250 km in area and about 2 km in thickness which was emplaced at about 5 km depth into sediments overlying the kaapvaal craton at 2054 ma the bushveld granite and its roof rocks have long been mined for sn w and f the bushveld granites have high magnetic susceptibilities km from 1000 to 4000 si and a quantitative model is presented suggesting that susceptibility fabrics are primarily carried by ferromagnetic minerals the measured ams foliations coincide with observed subhorizontal mineral lineations and compositional layering magnetic lineation trends vary considerably within the horizontal plane the existence of a weak planar fabric and an almost absent linear component may reflect a laccolithic emplacement by roof uplift causing flattening magmatic fabrics or b emplacement of largely crystal free magma crystallizing in situ and developing horizontal compositional layering from thermal chemical diffusion fronts and gravity driven mechanisms weak magnetic fabrics like those identified in the bushveld granites require specific sampling schemes and procedures in addition to rigorous constraint of magnetic mineralogy and crystallization sequence