evolution of the south central segment of the archean abitibi belt quebec part iii plutonic and metamorphic evolution and geotectonic model textural criteria permit distinction between the pre kenoran and kenoran phases of plutonism and metamorphism the pre kenoran plutons and pre kenoran metamorphic phases are directly related to the volcanic evolution synvolcanic tonalite trondhjemite plutons and swarms of mafic and felsic dykes core central volcanic complexes the volcanic rocks underwent three types of pre kenoran metamorphism namely a pervasive alteration a thermal contact metamorphism that affected narrow aureoles around synvolcanic plutons and a high intensity hydrothermal alteration that affected cross cutting pipes in central volcanic complexes synkinematic kenoran metamorphism resulted in the growth of minerals chlorite actinolite etc parallel to schistosities synkinematic metamorphic grade ranges form the pumpellyite prehnite facies to the amphibolite facies late to post kinematic metamorphic phases resulted in the growth of minerals across schistosities syn to post kinematic plutons are not voluminous in the part o

the magnetic storms of 3 4 august 2010 and 5 6 august 2011 1 ground and space based observations we have used total electron content tec values from low middle and high latitudes recorded over the american continent and density and ion temperature measured in situ by the dmsp f15 and f17 satellites during the geomagnetic storms of 3 4 august 2010 and 5 6 august 2011 to study the formation and dynamics of plasma density enhancements that developed during these two storms common to both storms are the timing of the main phase that extends between 20 and 24 ut and their seasonality with both storms occurring near the end of the northern hemisphere summer solstice during both storms tec data show incipient equatorial anomalies lacking a poleward expansion beyond 20 magnetic latitude two large scale tec enhancements were observed at middle latitudes showing a complicated pattern of structuring and merging the first tec enhancement corresponds to a storm enhanced density sed seen between 21 and 01 ut on the following day the second tec enhancement was observed over central america located equatorward of the sed and apparently moving northward however careful analysis of the tec values indicates that this second tec enhancement is not transported from lower latitudes through a superfountain effect instead the enhanced plasma has a local origin and is driven by a southward directed meridional wind that moves plasma up the tilted magnetic field lines dmsp flights passing over the second tec enhancement show a reduction of the ion temperature confirming an adiabatic expansion of the plasma as it moves up the field lines it is concluded that the midlatitude tec enhancements do not arise from a low latitude ionospheric fountain effect

storm time signatures of the ionospheric zonal ion drift at middle latitudes 1 using a quantitative identification of convection and particle boundaries at high latitudes the relative extent of auroral convection and auroral precipitation is examined during superstorm events during superstorms the convection reversal boundary normally located near 75 magnetic latitude moves to magnetic latitudes near 60 the edge of the diffuse auroral precipitation that normally terminates near 60 magnetic latitude moves to magnetic latitudes near 40 this limited study shows that during the main phase of the superstorm ion drifts driven by the magnetosphere penetrate to latitudes as low as the dip equator on the dusk side but extend only a few degrees equatorward of the auroral zone on the dawn side evidence for ion drifts driven by a disturbance dynamo may be found during the storm recovery phase when the interplanetary magnetic field is less strongly southward or turns northward but the previously established flows below the auro al region remain

a detachment fold model for fault zones in the late archean abitibi greenstone belt abstract the abitibi belt is one of the largest and most extensively studied late archean greenstone belts the structural geology of the abitibi belt consists of one generation of upright to slightly overturned doubly plunging first order folds with half wavelengths of 20 60 km and e w striking steeply dipping fault zones that are parallel to the fold limbs two of the main fault zones are continuous for hundreds of kilometers previous tectonic models for the abitibi belt interpret the fault zones to have formed as extensional growth faults bounding a volcanic sedimentary basin which were reactivated as thrusts during subsequent crustal shortening other models propose that the fault zones represent tectonic sutures implying that the abitibi belt is a collage of exotic terranes however distinct geological terranes have not been geologically demonstrated we propose a new detachment fold model for the deformational history of the southern abitibi belt in ontario that explains the formation of the fault zones during the single well documented folding event that deformed the entire region the internal structure of the fault zones documented here with emphasis on the porcupine destor fault zone consists of isoclinally folded strongly schistose highly metamorphosed rock cross cut by numerous fault segments we interpret that the upper crust greenstones was folded above a proposed detachment in the lower part of the volcanic stratigraphy the fault zones would be in essence highly evolved detachment anticlines ultramafic metavolcanic rock that crops out within the fault zones would represent material from the detachment horizon that was emplaced in the cores of the detachment anticlines the numerous segments that make up the mapped fault zones would be linked faults that formed within the isoclinal detachment anticlines to accommodate folding of the rheologically complex greenstones the detachment fold model is compared to the results of analogue experiments designed to investigate crustal scale folding using viscous and frictional materials detachment folds are produced in the brittle upper crustal analogue on the limbs of folds formed in the ductile middle and lower crust analogues the experimentally produced structures scale to the structures in the study area and indicate the detachment fold model for the southern abitibi is mechanically viable

simulated degradation of lunar impact craters and a new method for age dating farside mare deposits with the advent of clementine data it is now possible to determine the lithology and extent of geologic materials on the moon particularly the farside mare deposits however traditional crater counting techniques do not provide reliable age estimates of these materials owing to their small surface areas to support such studies we present a model for estimating their age by analyzing the morphometry of degraded craters 1 3 km in diameter a photoclinometric model was adapted for use with monoscopic 0 750 m ultraviolet visible and high resolution images where we extracted the topography of fresh craters a two dimensional computer model simulating linear diffusional creep was applied to fresh craters at a variety of diameters the resulting profiles were then compared to photoclinometric profiles of degraded craters of known ages for calibration application of the resulting model to degraded craters in the mare deposit of the central apollo basin 36 5 latitude 208 0 longitude indicates that this material was emplaced during the early imbrian period 3 85 ga by calculating the amount of material eroded from each of the degraded craters observed in this unit the average erosion rate is estimated to be 2 0 0 1 10 7 mm yr on the moon since the imbrian the estimated amount of material eroded during any given period suggests that the erosion rate has decreased with time implying that the flux of larger impactors has as well

structural geology of impact craters abstract the formation of impact craters is a highly dynamic and complex process that subjects the impacted target rocks to numerous types of deformation mechanisms understanding and interpreting these styles of micro meso and macroscale deformation has proved itself challenging for the field of structural geology in this paper we give an overview of the structural inventory found in craters of all size ranges on earth and look into the structures of craters on other planetary bodies structural features are discussed here that are caused by i extremely high pressures and temperatures that occur during the initial passage of the shock wave through the target rock and projectile ii the resulting flow field in the target that excavates and ejects rock materials and iii the gravitationally induced modification of the crater cavity into the final crater form a special focus is put on the effects that low angle impacting bodies have on crater formation we hope that this review will help both planetary scientists and structural geologists understand the deformation processes and resulting structures generated by meteorite impact

response of the equatorial ionosphere at dusk to penetration electric fields during intense magnetic storms 1 we investigate the response of the equatorial ionosphere at dusk to the intense magnetic storms of 30 october 2003 and 20 november 2003 different aspects of which have been widely studied by the community we present here a very complete set of space and ground based diagnostics that provide the vertical and latitudinal structures of the ionosphere within the south atlantic magnetic anomaly sama region and the contiguous parts of south america and africa we show that for both storms the dusk sector corresponding to the universal time ut interval between the fast decrease of the sym h index and minimum sym h value determines uniquely the longitude interval populated by equatorial plasma bubbles and depletions further we find that the ut of these storms is such that the ionospheric density perturbations occur in the sama region which are most extended in latitude and altitude compared with other regions of the globe in the dusk sector the eastward penetration electric field associated with rapid sym h decrease adds to the postsunset eastward e field because of the f region dynamo which may be specially enhanced in this longitude interval because of the increased zonal conductivity gradient caused by energetic particle precipitation this enhanced e field at dusk causes a rapid uplift of the ionosphere and sets off plasma instabilities to form bubbles or bite outs the decreased ion density seen in the defense meteorological satellite program dmsp in situ data at 840 km indicates that the ionospheric plasma has been lifted above the dmsp altitude and transported away from the region by diffusion along magnetic field lines plasma bubbles and bite outs impact satellite communication and navigation systems by introducing scintillations and steep density gradients this paper corroborates that intense magnetic storms follow the framework developed by su basu et al 2001 for moderate storms that specifies the longitude interval in which such disturbances are most likely to occur

super plasma fountain and equatorial ionization anomaly during penetration electric field 1 relative importance of diffusion electric field and neutral wind on equatorial plasma fountain and equatorial ionization anomaly eia during a strong daytime eastward prompt penetration electric field ppef event are evaluated using the sheffield university plasmasphere ionosphere model and the recorded ppef during the super geomagnetic storm of 9 november 2004 the fountain rapidly develops into a super fountain during the ppef event the super fountain becomes strong with less poleward turning of the velocity vectors in the presence of an equatorward wind that reduces or stops the downward velocity component due to diffusion and raises the ionosphere to high altitudes of reduced chemical loss the eia crests in peak electron density and total electron content shift rapidly to higher than normal latitudes during the ppef event however the crests become stronger than normal only in the presence of an equatorward wind the results suggest that the presence of an equatorward neutral wind is required to produce a strong positive ionospheric storm during a daytime eastward ppef event the equatorward neutral wind need not be a storm time wind though stronger wind can lead to stronger ionospheric storms

diapirism during regional compression the structural pattern in the chibougamau region of the archean abitibi belt quebec the major fold pattern of the chibougamau region in the archean abitibi belt quebec is the result of two fold phases the principal f1 synclines first formed as subsiding fault troughs and were shortened in ns direction and molded around volcanic centers that had been consolidated by large synvolcanic plutons they have a general easterly direction but curve locally to the ne or se n trending f1 folds are locally present in the anticlinoria and are thought to have been produced by the upwelling of basement gneiss and or synvolcanic plutons the f2 folds have rather regular easterly trends their axial plane schistosity s2 is the predominant planar fabric element basin margin faults were transformed into east trending thrust faults during the f1 and f2 folding

magnetic declination and zonal wind effects on longitudinal differences of ionospheric electron density at midlatitudes 1 a prominent ionospheric longitudinal variation at midlatitudes in particular over the continental us was found recently this variation is characterized as a higher east side electron density in the evening and a higher west side electron density in the morning and with clear seasonal and solar activity dependencies a combined effect of geomagnetic declination and changing zonal winds was proposed to explain it this paper represents a comprehensive investigation of this effect by examining climatology for both electron density longitudinal differences and the nighttime zonal winds in the eastern us electron density is from incoherent scatter radar extra wide coverage experiments during 1978 2011 over millstone hill for which the spatial separation of the data can be up to 50 in longitude the thermospheric zonal wind is from the on site fabry perot interferometer measurements during 1989 2001 the observed zonal wind climatology is found to be perfectly consistent with the expectation based on the east west electron density differences in terms of local time seasonal and solar cycle dependencies the correlation between the zonal wind and the east west differential ratio is extremely high with an overall correlation coefficient of 0 93 the observed time delay of 3 hours in the response of electron density differences to zonal winds is a marked feature thus these results confirm positively the declination zonal wind mechanism and provide new insight into longitudinal variations at midlatitudes for other geographic sectors

subaqueous calderas in the archean abitibi greenstone belt an overview and new ideas abstract the 300 700 km abitibi greenstone belt in canada contains numerous world class archean volcanogenic massive sulfide vms deposits yet documentation of archean subaqueous calderas hosting such deposits is lacking the modern sunrise deposit in the myojin knoll caldera of the izu bonin arc shows that submarine calderas are first order sites for vms the hunter mine and normetal calderas as well as the blake river megacaldera complex are reviewed with respect to geometry regional geology physical volcanology dyke emplacement and hydrothermal carbonate alteration these subaqueous calderas are placed into the geodynamic context of the abitibi greenstone belt the abitibi belt displays a complex history of oceanic arc formation evolution collision and fragmentation with numerous types of volcanic edifices and sedimentary basins controlled by oblique subduction and mantle plumes analogue models yielding new insights into the sequence of caldera forming events and their synvolcanic fault patterns are applied to explain archean examples underpressure experiments develop a consistent sequence of fault patterns that compare favourably with natural caldera events during subsidence two sets of faults propagate from the magma chamber margin 1 early reverse outward dipping faults and 2 normal inward dipping faults the largest massive sulfide deposits form preferentially at the caldera margin based on experimental studies and detailed facies mapping the hunter mine caldera is of the segmented piecemeal variety and the normetal caldera is akin to a piston structure the blake river megacaldera contains 1 the e w trending misema 2 the nw trending new senator and 3 the ne trending noranda calderas with the latter two being nested graben type calderas consistent with oblique archean subduction the poorly documented hydrothermal carbonate alteration is discussed and a new exploration model for calderas is presented the alteration study was evaluated at three levels microprobe analysis geochemical staining of drill core and whole rock analyses there were two principal stages of alteration related to vms formation an early silicification phase developed either at depth and or at the seawater interface whereby silicification was bedding parallel in fine grained volcaniclastic deposits near the water surface a pervasive and extensive semi conformable carbonate alteration zone with a discordant focused root zone along faults evolved subsequently three distinct carbonate pairings were observed 1 proximal siderite sideroplesite fe ankerite next to the vms deposit 2 an intermediate ankerite fe dolomite zone and 3 a distal calcite dolomite zone the carbonate phase is associated with the mineralizing event transitions between carbonate species are subtle and changes are indicated by mineral assemblage overlap the silica carbonate hydrothermal alteration pattern is a replacement product and is far more widespread than has been documented this alteration can be easily confused with primary carbonates and chemical precipitation deposits e g banded iron formation if detailed volcano sedimentary facies mapping with alteration has not been conducted and if outcrop is insufficient

reconnaissance survey of the duolun ring structure in inner mongolia not an impact structure the duolun basin which is located in inner mongolia china has been proposed to be an impact structure with an apparent rim diameter of about 70 or even 170 km the designation as an impact structure was based on its nearly circular topography consisting of an annular moat that surrounds an inner hummocky region and the widespread occurrences of various igneous rocks polymict breccias and deformed crustal rocks critical shock metamorphic evidence is not available to support the impact hypothesis we conducted two independent reconnaissance field surveys to this area and studied the lithology both within and outside of the ring structure we collected samples from all lithologies that might contain evidence of shock metamorphism as suggested by their locations especially those sharing similar appearances with impact breccias suevites impact melt rocks and shatter cones field investigation together with thin section examination discovered that the suspected impact melt rocks are actually early cretaceous and late jurassic lava flows and pyroclastic deposits of rhyolitic to trachytic compositions and the interpreted impact glass is typical volcanic glass petrographic analyses of all the samples reveal no indications for shock metamorphic overprint all these lines of evidence suggest that the duolun basin was not formed through impact cratering the structural deformation and spatial distribution pattern of the igneous rocks suggest that the duolun basin is most likely a jurassic cretaceous complex rhyolite caldera system that has been partly filled with sediments forming an annular basin followed by resurgent doming of the central area

two stage evolution in an archean tonalite suite the taschereau stock abitibi quebec canada the taschereau stock occurs north of timmins and val d or quebec in the abitibi greenstone belt of the superior province this late archean composite pluton is composed mainly of diorite tonalite trondhjemite cut by granitic rocks gold molybdenum occurrences are associated with a zone of albite rich rocks surrounding the granitic rocks diabase dykes and shear zones postdate all rock units field and geochemical evidence suggests that the taschereau stock was emplaced diachronously trace element geochemical modelling shows that trace element abundances rare earth elements ti zr of taschereau granitic rocks are consistent with partial melting of preexisting taschereau tonalitic rocks and implies that these two rock types are not end members of a single magma that evolved through fractional crystallization

multiradar observations of the polar tongue of ionization 1 we present a global view of large scale ionospheric disturbances during the main phase of a major geomagnetic storm we find that the low latitude auroral and polar latitude regions are coupled by processes that redistribute thermal plasma throughout the system for the large geomagnetic storm on 20 november 2003 we examine data from the high latitude incoherent scatter radars at millstone hill sondrestrom and eiscat tromso with superdarn hf radar observations of the high latitude convection pattern and dmsp observations of in situ plasma parameters in the topside ionosphere we combine these with north polar maps of stormtime plumes of enhanced total electron content tec derived from a network of gps receivers the polar tongue of ionization toi is seen to be a continuous stream of dense cold plasma entrained in the global convection pattern the dayside source of the toi is the plume of storm enhanced density sed transported from low latitudes in the postnoon sector by the subauroral disturbance electric field convection carries this material through the dayside cusp and across the polar cap to the nightside where the auroral f region is significantly enhanced by the sed material the three incoherent scatter radars provided full altitude profiles of plasma density temperatures and vertical velocity as the toi plume crossed their different positions under the cusp in the center of the polar cap and at the midnight oval polar cap boundary greatly elevated f peak density 1 5e12 m 3 and low electron and ion temperatures 2500 k at the f peak altitude characterize the sed toi plasma observed at all points along its high latitude trajectory for this event sed toi f region tec 150 1000 km was 50 tecu both in the cusp and in the center of the polar cap large upward directed fluxes of o 1 e14 m 2 s 1 were observed in the topside ionosphere

foredunes and blowouts initiation geomorphology and dynamics this paper reviews the initiation dynamics geomorphology and evolution of incipient foredunes established foredunes and blowouts incipient foredunes may be initiated in a variety of ways leading to the formation of ramps terraces or ridges depending on progradation rates vegetation type and cover sediment transport rates and scale of erosional processes vegetation cover morphology growth rates and types combine with aerodynamic processes to determine dune and swale morphology established foredunes are classified into five morpho ecological types and the flow behaviour over two types is detailed a new model of foredune evolutionary paths at various possible time scales is presented blowout initiation dynamics and sediment transport and evolution is examined throughout the review the gaps in present understanding of dune processes and a range of ideas for new research possibilities are provided

the carancas meteorite impact crater peru geologic surveying and modeling of crater formation and atmospheric passage the recent carancas meteorite impact event caused a worldwide sensation an h4 5 chondrite struck the earth south of lake titicaca in peru on september 15 2007 and formed a crater 14 2 m across it is the smallest youngest and one of two eye witnessed impact crater events on earth the impact violated the hitherto existing view that stony meteorites below a size of 100 m undergo major disruption and deceleration during their passage through the atmosphere and are not capable of producing craters fragmentation occurs if the strength of the meteoroid is less than the aerodynamic stresses that occur in flight the small fragments that result from a breakup rain down at terminal velocity and are not capable of producing impact craters the carancas cratering event however demonstrates that meter sized stony meteoroids indeed can survive the atmospheric passage under specific circumstances we present results of a detailed geologic survey of the crater and its ejecta to constrain the possible range of impact parameters we carried out numerical models of crater formation with the isale hydrocode in two and three dimensions depending on the strength properties of the target the impact energies range between approximately 100 1000 mj 0 024 0 24 t tnt by modeling the atmospheric traverse we demonstrate that low cosmic velocities 12 1 4 kms 1 and shallow entry angles 20 are prerequisites to keep aerodynamic stresses low 10 mpa and thus to prevent fragmentation of stony meteoroids with standard strength properties this scenario results in a strong meteoroid deceleration a deflection of the trajectory to a steeper impact angle 40 60 and an impact velocity of 350 600 ms 1 which is insufficient to produce a shock wave and significant shock effects in target minerals aerodynamic and crater modeling are consistent with field data and our microscopic inspection however these data are in conflict with trajectories inferred from the analysis of infrasound signals

partial least squares discriminant analysis of trace element compositions of magnetite from various vms deposit subtypes application to mineral exploration abstract the petrography and mineral chemistry of magnetite from fifteen volcanogenic massive sulfide vms deposits in canada and the lasail vms deposit in oman as well as from two vms associated banded iron formations bif austin brook new brunswick canada and izok lake nunavut canada were investigated using optical microscopy electron probe micro analyzer epma and laser ablation inductively coupled plasma mass spectrometry la icp ms the method of robust estimation for compositional data robcompositions was applied to investigate geochemical censored data among thirty seven elements analyzed by epma and or la icp ms in magnetite from the studied deposits bedrock lithologies only the results for si ca zr al mg ti zn co and ni contain pls da shows that variable compositions of magnetite slightly discriminate different studied deposits bedrock lithologies the geochemical observations suggest that the variation in magnetite chemistry from different vms settings might be sourced from differences in 1 the composition and temperature of parental magmas or hydrothermal fluids 2 the composition of host bedrocks 3 the composition of co forming minerals and 4 oxygen fugacity pls da distinguishes magnetite compositions from the studied vms deposits and bifs from that of the other ore deposit types including ni cu porphyry cu mo au iron oxide copper gold iron oxide apatite and the bayan obo ree fe nb deposit magnetite from the vms settings on average contains lower concentrations of si zr al mg ti zn co and ni relative to that from the other mineral deposit types pls da of magnetite data from vms deposits and bifs of the bathurst mining camp as well as pls da of magnetite compositions from various mineral deposit types yield discrimination models for application to mineral exploration for vms deposits using indicator minerals in quaternary lithified sedimentary rocks

moho reflectivity patterns a comparison of canadian lithoprobe transects abstract the complex nature of the continental moho may be strongly influenced by the tectonic and magmatic history of the lithosphere observations of moho variability from near vertical incidence seismic reflection surveys suggest that it may be possible to correlate moho reflectivity and lithospheric evolution as an initial step in attempting to determine such correlations we qualitatively compare seismic reflection data from canadian lithoprobe transects that sample regions of exceptionally diverse age and tectonic history the seven selected datasets were collected using consistent acquisition parameters facilitating effective and meaningful comparisons among transects the moho reflections exhibit considerable variability however in virtually all recent transects a distinct moho is observed even in regions where lower crust is considered to have stabilized in the early proterozoic or archean this suggests not only that relationships between moho reflectivity and tectonics can be extended into the archean but that tectonic age is poorly correlated with moho reflectivity the qualitative comparisons do not clearly link a specific tectonic process with a distinctive reflection pattern however well defined moho reflections are associated with lower crustal deformational processes including compression transpression and extension this indicates that ductile shear and the rheologies of the lower crust moho transition and upper mantle play important roles in influencing reflective character indistinct moho reflectivity may be associated with regions that have experienced little ductile deformation or with processes that have disrupted reflective laminations however the majority of incoherent moho reflections appear related to signal degradation associated with data acquisition the study demonstrates that the qualitative characteristics of moho reflectivity are difficult to correlate with tectonic history and thus use as an interpretive tool isolating quantitative reflection characteristics of the moho from distortions imposed by the overlying crust is a challenging problem but appears necessary to further understanding of the crust mantle transition

hailar crater a possible impact structure in inner mongolia china abstract hailar crater a probable impact structure is a circular depression about 300 m diameter in inner mongolia northeast china with broad elevated rims the present rim to floor depth is 8 20 m regional geological background and geomorphological comparison suggest that this feature is likely not formed by surface processes such as salt diapir karst aeolian glacial or volcanic activity its unique occurrence in this region and well preserved morphology are most consistent with it being a cenozoic impact crater two field expeditions in 2016 and 2017 investigated the origin of this structure recognizing that 1 no additional craters were identified around hailar crater in the centimeter scale digital topography models that were constructed using a drone imaging system and stereo photogrammetry 2 no bedrock exposures are visible within or adjacent to the crater because of thick regolith coverage and only small pieces of angular unconsolidated rocks are present on the crater wall and the gently sloped crater rim suggesting recent energetic formation of the crater 3 most samples collected from the crater have identical lithology and petrographic characteristics with the background terrain but some crater samples contain more abundant clasts and silicate hydrothermal veins indicating that rocks from depths have been exposed by the crater 4 no shock metamorphic features were found in the samples after thin section examinations and 5 a systematic sample survey and iron detector scan within and outside of the crater found no iron rich meteorites larger than 2 cm in size in a depth of 30 cm although no conclusive evidence for an impact origin is found yet hailar crater was most likely formed by an impact based on its unique occurrence and comparative geomorphologic study we suggest that drilling in the crater center is required to verify the impact origin where hypothesized melt bearing impactites may be encountered

the number of impact craters on earth any room for further discoveries abstract only 128 impact craters exposed at earth s surface have been found so far while new craters are discovered occasionally taking into account the permanent consumption of craters by erosion we present the first estimate on the number of impact craters that should be present at earth s surface our study yields no evidence for any systematic incompleteness in the available inventory of the craters larger than about 6 km in diameter exposed at the surface in contrast more than 90 craters in the diameter range from 1 km to 6 km should still be waiting to be discovered and even more than 250 between 0 25 km and 1 km diameter the transition from a probably complete inventory above 6 km to a strongly incomplete record at smaller sizes may be related to the difference between simple and complex craters beyond these results on the terrestrial crater record our findings tentatively suggest that erosion rates on the 10 to 100 million year scale may be closer to present day erosion rates than previously assumed

u pb geochronology of archean metasedimentary rocks in the pontiac and abitibi subprovinces quebec constraints on timing provenance and regional tectonics abstract u pb isotopic ages of single detrital zircons have been obtained from metasandstones in the northern part of the pontiac subprovince and from the cadillac kewagama and lac caste groups within the southern abitibi subprovince youngest detrital zircon ages from these sequences are 2685 2686 2687 and 2693 ma respectively the lac fourniere pluton which intrudes pontiac group metasedimentary rocks is dated at 2682 1 ma a similar age was previously measured from a pluton that cuts the kewagama group these data bracket deposition of greywacke in the pontiac and abitibi subprovinces to 2685 3 ma in contrast the youngest zircon found in a volcaniclastic unit interbedded with timiskaming group metawacke is 2673 2 ma while the age of a pluton that intrudes the timiskaming is 2672 1 ma thus the timiskaming group is distinctly younger than the other metasedimentary rocks the normalized probability distributions of detrital zircon ages from the pontiac and the abitibi metasedimentary rocks as well as of dated igneous rocks from the southern abitibi subprovince all show that their major zircon populations formed in the age range 2685 2710 ma the distributions for the abitibi metaturbidites and dated igneous rocks are very similar suggesting that metasedimentary rocks in the abitibi belt were mostly derived from the surrounding granite greenstone terrain the largest peak of the pontiac distribution is a few million years younger than that from the abitibi metasedimentary rocks but there is a correlation between major peaks and both groups show similar age ranges for older pre abitibi components 2750 2850 and 3020 3030 ma deposition of the ca 2685 ma sediments was virtually syn deformational based on previous field geochemical and isotopic studies as well as the detrital zircon age data the pontiac group may represent a foreland basin that may have developed from an accretionary prism the source of much of the sediment may have been an uplifted extinct continental arc to the north the arc was mostly eroded into the sediment basin and perhaps tectonically overridden by a southward advancing nappe complex comprised of its back arc basin now partly preserved in the southern volcanic zone of the abitibi subprovince metaturbidites exposed between metavolcanic assemblages within the greenstone belt may represent overturned unconformities at the structural base of the greenstones these were brought up by high angle faults which also localized deposition of non marine timiskaming type sediments it is tentatively suggested that pontiac type metasedimentary rocks extend most of the way beneath the abitibi subprovince the oldest part of the basin emerging as the quetico group in the western superior province

on a possible parent crater for australasian tektites geochemical isotopic geographical and other constraints abstract tektites are natural glasses formed by the melting of earth s surface materials during the impact of a large extraterrestrial body at present they are found in four main strewn fields associated with separate impacts central european moldavites australasian australites indochinites philippinites javaites etc north american georgiaites and bediasites and west african tektites from the ivory coast ivorites the location of the impact site and parent crater for australasian tektites aat has remained an unanswered question for decades the current review discusses possible locations of the aat parent crater on the basis of published geochemical mineralogical petrographic and isotopic data on both aat and potential source materials this was aided by new geochemical data for a representative set of aat covering the main morphological types and parts of the aat strewn field and also assumes some ballistic palaeogeographic palaeoclimatic and palaeoecological constraints the review focuses particularly on three parallel tasks 1 collecting comprehensive data geochemical composition sr nd rb li and b isotopes and cosmogenic 10 be for revisiting chinese loess or its precursors as the most suitable source materials for aat 2 criticisms of the generally accepted location of the impact in southeast asia indochina based mainly on a lack of suitable targets and a mismatch between the geochemical and isotopic compositions of aat and prevailing sedimentary targets in indochina 3 supporting evidence in addition to 1 for a hypothesis locating the aat parent crater to the arid area of northwest china which could provide a sufficient supply of suitable source materials as well as ideal conditions for burial of the crater under the tallest megadunes on earth

response of the equatorial ionosphere in the south atlantic region to the great magnetic storm of july 15 2000 the effects of the great magnetic storm of july 15 2000 on the equatorial ionosphere have been studied by ground based and satellite in situ measurements a large westward plasma drift in the evening equatorial ionosphere was observed as a result of the ionospheric disturbance dynamo in that environment the imf bz turned southward and presumably caused penetration of e fields to low latitudes this e field initiated the onset of 250 mhz and l band scintillations at ascension island 15 w and precipitous tec decrease at fortaleza brazil 38 w bounding the narrow longitude region in the south atlantic these impulsive ionospheric effects were extremely well correlated with abrupt decreases of sym h 1 min resolution dst the dmsp in situ measurements showed the presence of severe ion density bite outs extending over 30 latitude in the south atlantic magnetic anomaly region the rocsat 1 satellite measured upward and large southward ion drifts in the same sector

effects of greenschist facies metamorphism and related deformation on the mobrun massive sulfide deposit quebec canada the mobrun zn cu ag au deposit in the noranda mining camp is hosted by archean mafic and felsic submarine volcanic rocks the deposit comprises three massive sulfide complexes the main and satellite lenses near surface and the 1100 orebody at depth the rocks have been subjected to lower greenschist facies metamorphism and related deformation which resulted in changes in ore textures development of shear zones and veins systems remobilization of gold and formation of a new mineral electrum within the orebodies both mechanical and chemical processes operated to produce secondary textures and structures resulting from brittle deformation ductile deformation and annealing the specific deformation mechanisms include brittle failure and cataclastic flow dislocation glide dislocation creep and solution precipitation creep the main and satellite lenses are characterized by excellent preservation of primary sulfides deposited from and reworked by synvolcanic hydrothermal fluids these orebodies were affected to a limited degree by mechanical processes of deformation in contrast the 1100 orebody is characterized by a higher degree of development of textures and structures related to metamorphism and deformation especially those formed by chemical processes the differences may be due to the greater depth of the 1100 orebody relative to the other lenses as regional metamorphic isograds are subhorizontal and more extensive interaction between metamorphic fluids and the 1100 lens

a statistical study of the response of the dayside equatorial f2 layer to the main phase of intense geomagnetic storms as an indicator of penetration electric field 1 the response of the dayside equatorial f2 layer to the main phases of the 22 intense geomagnetic storms dst 150 nt in 1998 2008 is investigated using the digital ionosonde data from the equatorial stations in brazilian indian and australian longitudes together with equatorial electrojet strength and imf bz the storms include 15 superstorms dst 200 nt the observations show that there is a period during all mps when the f2 layer peak rises and falls rapidly with large peak electron density nmax reduction the rise velocity strongly correlates with the intensity dst of the storms and the duration of the nmax reduction corresponds to that of strong eastward electrojet when imf bz remains highly negative the observations indicate the occurrence of strong eastward prompt penetration electric fields ppef during the rapid f2 layer response the ppef drives the f2 layer peak rapidly upward which reduces nmax due to vertical expansion and diffusion the results therefore suggest that the rapid f2 layer response rapid rise and fall of peak height hmax with large nmax reduction observed by ionosondes can be used to detect the occurrence of the daytime eastward ppef during intense geomagnetic storms irrespective of season and level of solar activity the data also show two rare events of strong daytime westward electric fields due to disturbance dynamo and or prompt penetration the results are important when radars are not available to monitor the occurrence of the ppef

a physical mechanism of positive ionospheric storms at low latitudes and midlatitudes 1 a physical mechanism of the positive ionospheric storms at low latitudes and midlatitudes is presented through multi instrument observations theoretical modeling and basic principles according to the mechanism an equatorward neutral wind is required to produce positive ionospheric storms the mechanical effects of the wind 1 reduce or stop the downward diffusion of plasma along the geomagnetic field lines 2 raise the ionosphere to high altitudes of reduced chemical loss and hence 3 accumulate the plasma at altitudes near and above the ionospheric peak centered at around 30 magnetic latitudes daytime eastward prompt penetration electric field ppef if it occurs also shifts the equatorial ionization anomaly crests to higher than normal latitudes up to approximately 30 latitudes the positive ionospheric storms are most likely in the longitudes where the onset of the geomagnetic storms falls in the ionization production dominated morning noon local time sector when the plasma accumulation due to the mechanical effects of the wind largely exceeds the plasma loss due to the chemical effect of the wind the mechanism agrees with the multi instrument observations made during the supergeomagnetic storm of 7 8 november 2004 with 18 h long initial phase ip and 10 h long main phase mp the observations which are mainly in the japanese australian longitudes where the mp onset was in the morning 0600 lt 2100 ut show 1 strong positive ionospheric storms in ne nmax hmax global positioning system total electron content gps tec and 630 nm airglow intensity in both northern and southern hemispheres started at the morning 0600 lt mp onset and lasted for a day 2 repeated occurrence of strong eastward ppef events penetrated after the mp onset and superposed with westward electric field started before the mp onset and 3 storm time equatorward neutral winds inferred from 1 and 2 repeated occurrence of an unusually strong f3 layer with large density depletions around the equator was also observed during the morning noon mp

single zircon age constraints on the tectonic juxtaposition of the archean abitibi greenstone belt and pontiac subprovince quebec canada abstract zircons from metasediments and granitoids in the high grade lacorne block within the low grade archean abitibi greenstone belt have been dated by the single zircon pb evaporation technique yielding 207pb 206pb minimum ages detrital zircons in the mature clastic metasediments of the lacorne block display a range of ages from 2691 8 ma to 3042 6 ma the younger zircon ages 2691 8 2695 4 2715 16 and 2719 4 ma thus impose an upper limit for deposition and indicate that the high grade lacorne block is not basement to the abitibi supracrustal sequence 2747 2680 ma existence of abundant 69 older detrital zircons 2750 ma suggests in turn that the abitibi supracrustal rocks are not the source of the lacorne sediments two generations of granitoids occur in the lacorne block an early monzodiorite monzonite granodiorite syenite series mmgs 2671 4 to 2675 24 ma and a younger s type garnet muscovite granite series gmg 2630 20 to 2655 8 ma this contrasts with granitoid magmatism in the abitibi greenstone belt which ended at 2675 ma the pontiac subprovince to the south of the abitibi greenstone belt shares all of the above features of the lacorne block including detrital zircon ages as well as the composition and timing of granitoid magmatism this is interpreted to indicate that the lacorne block was originally part of the same tectonic terrane as the pontiac subprovince after development of the mmgs magmatism 21670 2680 ma the pontiac subprovince locally underthrust the abitibi greenstone belt and crustal thickening promoted partial melting of underthrust pontiac metasediments to form the 2644 13 ma s type granites later differential uplift exposed the lacorne block as a high grade tectonic window in the abitibi greenstone belt

storm time plasma transport at middle and high latitudes associated with the large scale enhancement of the ionospheric convection electric field during disturbed geomagnetic conditions solar produced f region plasma is transported to and through the noontime cleft from a source region at middle and low latitudes in the afternoon sector as a result of the offset between the geomagnetic and geographic poles the afternoon sector region of strong sunward convection is shifted to increasingly lower geographic latitude throughout the interval between 12 ut and 24 ut a snowplow effect occurs in which the convection cell continually encounters fresh corotating ionospheric plasma along its equatorward edge producing a latitudinally narrow region of storm enhanced plasma density sed and increased total electron content which is advected toward higher latitudes in the noon sector the millstone hill incoherent scatter radar regularly observes sed as a spatially continuous large scale feature spanning local times between noon and midnight and at latitudes between the polar cap and its mid or low latitude source region for local times away from noon the latitude of most probable sed occurrence moves equatorward by 6 for an increase of 2 in the kp index during strong disturbances the topside sed is observed to be convecting sunward at 750 m s 1 with a flux of 1014 m 2 s 1 this feature accounts for the pronounced enhancement of ionospheric density near dusk at middle latitudes observed during the early stages of magnetic storms called the dusk effect and constitutes a source for the enhanced f region plasma observed in the polar cap during disturbed conditions

east west coast differences in total electron content over the continental us 1 total electron content tec measurements made by a network of dense gps receivers over the continental us are used to investigate ionospheric longitudinal differences we find that the evening tec is substantially higher on the us east coast than on the west and vice versa for the morning tec the longitudinal difference displays a clear diurnal variation through an analysis of morning evening variability in the east west tec difference minimum variability is found to coincide with the longitudes of zero magnetic declination over the continental us we suggest that these new findings of longitudinal differences in ionospheric tec at midlatitudes are caused by the longitudinal difference in magnetic declination combined with the effects of thermospheric zonal winds which are subject to directional reversal over the course of a day this study indicates that longitudinal variations in tec measurements contain critical information on thermospheric zonal winds the proposed declination zonal wind mechanism may also provide a new insight into longitude ut changes at midlatitudes on a global scale as well as into some geospace disturbances

prompt midlatitude electric field effects during severe geomagnetic storms meridian plane elevation scans with the millstone hill incoherent scatter radar provide evidence of a strong perturbation of the coupled mid latitude magnetosphere ionosphere system during the early phases of the november 4 1993 magnetic storm a narrow ionospheric trough formed at l 3 5 in the pre midnight sector immediately poleward of the millstone hill site the most pronounced radar signature of the developing activity was a brief 20 min uplifting of the f region plasma equatorward of the trough such that the peak altitude increased with distance away from the trough a similar signature had been observed during storm onset on march 20 1990 and in that event a pronounced topside ionospheric depletion developed in the region far equatorward of the mid latitude trough and was observed by the radar and the dmsp f9 satellite during the november 4 1993 event the dmsp f10 satellite observed narrow magnetically conjugate regions of plasma density depletion and strong horizontal and upward plasma velocity 1500 m s at l 1 5 at the time of the uplifting of the mid latitude f region observed by the radar these observations were confined to longitudes near the south atlantic magnetic anomaly and in the nov 1993 case the perturbation was coincident with the peak of the precipitating particle fluxes associated with innerbelt losses at the anomaly both the uplifting of the ionospheric f layer and the triggering of topside density perturbations can be explained in terms of an eastward electric field imposed on the mid and low latitude ionosphere during the initial stages of the geomagnetic storm the low latitude ionospheric perturbations in these events were similar to supersonic equatorial bubbles triggered by the destabilizing effects of the upward e b drift associated with the eastward electric field

estimates of yet to find impact crater population on earth abstract one hundred and seventy six craters on earth are currently accepted as resulting from bolide impact this list is growing at a rate of approximately two craters per year because of new discoveries and revised interpretations here we take the subset of craters most relevant to sedimentary basin exploration and utilization phanerozoic kilometre scale craters and tackle the question of how many remain to be found in phanerozoic strata on earth a surprising range in predictions of crater numbers is obtained from published crater population functions reaching two orders of magnitude variation for craters in the range 1 5 km diameter this range in predictions is largely due to variations in treatment of impactor breakup in the atmosphere also required is quantification of the total area and timespan of phanerozoic sediment currently preserved on earth an estimate of 8 87 10 15 km 2 a is derived here together with a relatively conservative crater population function this yields a prediction of 845 craters subtracting the 131 known phanerozoic craters larger than 1 km leaves a predicted population yet to find of 714 of which 228 are larger than 2 5 km uncertainties subsets of this population and spatial distribution are also considered

depositional gaps in abitibi greenstone belt stratigraphy a key to exploration for syngenetic mineralization models of greenstone belt development are crucial for exploration allochthonous models predict belts to be a collage of unrelated fragments whereas autochthonous models allow for prediction of syngenetic mineral deposits within specific stratigraphic intervals superior province greenstone belts consist of mainly volcanic units unconformably overlain by largely sedimentary timiskaming style assemblages and field and geochronological data indicate that the abitibi greenstone belt developed autochthonously we describe major revisions to stratigraphy of the abitibi greenstone belt and the implications of an autochthonous development of the volcanic stratigraphy for exploration for syngenetic mineralization the abitibi greenstone belt is subdivided into seven discrete volcanic stratigraphic episodes on the basis of groupings of numerous u pb zircon ages of pre 2750 2750 to 2735 2734 to 2724 2723 to 2720 2719 to 2711 2710 to 2704 and 2704 to 2695 ma we present revised lithotectonic and or stratigraphic nomenclature using these time intervals including 1 isotopic inheritance in younger episodes which indicates that the older episodes 2750 2735 and 2734 2724 ma had greater extent than is presently seen 2 dikes feeding younger volcanic episodes 2706 ma cutting older volcanic units 2734 2724 ma and 3 2710 to 2704 ma mafic to ultramafic sills intruding the 2719 to 2711 ma episode changes to the nomenclature include the identification of pre 2750 ma volcanic episode supracrustal fragments in the northern and southern abitibi greenstone belt and subdivision of the 2719 to 2711 ma 2710 to 2704 ma and 2704 to 2695 ma episodes into lower and upper parts we present the results of this lithostratigraphic subdivision as the first geochronologically constrained stratigraphic and or lithotectonic map of the abitibi greenstone belt many of the volcanic episodes are intercalated with and capped by a relatively thin sedimentary interface zone dominated by chemical sedimentary rocks stratigraphic and geochronological analysis of these zones indicates discontinuous deposition with localized gaps of 2 to 27 m y between volcanic episodes the zones consist of up to 200 m of iron formation chert breccia heterolithic debris flows of volcanic provenance sandstone and or argillite and conglomerate modeling of the time required for deposition of the volcanic units based on rates of magma production in modern arc and plume environments is on the order of 103 to 104 years whereas the time interval between basalt rhyolite cycles is 106 years the sedimentary interface zones are therefore interpreted as condensed sections zones with very low rates of sedimentation in a basinal setting or zones with negligible rates of sedimentation marked by silicification of existing rock types the sedimentary interface zones are therefore considered submarine correlative conformities disconformities or unconformities separating the equivalent of group level stratigraphic and lithotectonic units the unconformity bounded stratigraphic model provides a new regional to deposit scale interpretive model for use in exploration for syngenetic mineralization

lithofacies associations and structural evolution of the archean rio das velhas greenstone belt quadrilatero ferrifero brazil a review of the setting of gold deposits abstract the rio das velhas greenstone belt is located in the quadrilatero ferrifero region in the southern extremity of the sao francisco craton central southern part of the state of minas gerais se brazil the metavolcano sedimentary rocks of the rio das velhas supergroup in this region are subdivided into the nova lima and maquine groups the former occurs at the base of the sequence and contains the major au deposits of the region new geochronological data along with a review of geochemical data for volcanic and sedimentary rocks suggest at least two generations of greenstone belts dated at 2900 and 2780 ma seven lithofacies associations are identified from bottom to top encompassing 1 mafic ultramafic volcanic 2 volcano chemical sedimentary 3 clastic chemical sedimentary 4 volcaniclastic association with four lithofacies monomictic and polymictic breccias conglomerate graywacke graywacke sandstone graywacke argillite 5 resedimented association including three sequences of graywacke argillite in the north and eastern at greenschist facies and in the south at amphibolite metamorphic facies 6 coastal association with four lithofacies sandstone with medium to large scale cross bedding sandstone with ripple marks sandstone with herringbone cross bedding sandstone siltstone 7 non marine association with the lithofacies conglomerate sandstone coarse grained sandstone fine to medium grained sandstone four generations of structures are recognized the first and second are archean and compressional driven from nne to ssw the third is extensional and attributed to the paleoproterozoic transamazonian orogenic cycle and the fourth is compressional driven from e to w is related to the neoproterozoic brasiliano orogenic cycle gold deposits in the rio das velhas greenstone belt are structurally controlled and occur associated with hydrothermal alterations along archean thrust shear zones of the second generation of structures sedimentation occurred during four episodes cycle 1 is interpreted to have occurred between 2800 and 2780 ma based on the ages of the mafic and felsic volcanism and comprises predominantly chemical sedimentary rocks intercalated with mafic ultramafic volcanic flows it includes the volcano chemical sedimentary lithofacies association and part of the mafic ultramafic volcanic association the cycle is related to the initial extensional stage of the greenstone belt formation with the deposition of sediments contemporaneous with volcanic flows that formed the submarine mafic plains cycle 2 encompasses the clastic chemical sedimentary association and distal turbidites of the resedimented association in the eastern sector of the quadrilatero ferrifero it was deposited in the initial stages of the felsic volcanism cycle 2 includes the coastal and resedimented associations in the southern sector in advanced stages of subduction in this southern sedimentary cycle it is also possible to recognize a stable shelf environment following the felsic volcanism cycle 3 comprises sedimentary rocks of the volcaniclastic and resedimented lithofacies associations largely in the northern sector of the area the characteristics of both associations indicate a submarine fan environment transitional to non marine successions related to felsic volcanic edifices and related to the formation of island arcs cycle 4 is made up of clastic sedimentary rocks belonging to the non marine lithofacies association they are interpreted as braided plain and alluvial fan deposits in a retroarc foreland basin with the supply of debris from the previous cycles