how do ocean warm anomalies favor the aggregation of deep convective clouds abstractwe investigate the role of a warm sea surface temperature sst anomaly hot spot of typically 3 k to 5 k on the aggregation of convection using cloud resolving simulations in a non rotati

modelling the generation of gravity waves by a maritime continent thunderstorm observed wind and temperature profiles are used to initialize a model calculation of a maritime continent thunderstorm and the numerical solution is used to explore the effect of tropospheric shear on the gravity wave generation the resultant convective system is qualitatively similar to that observed the modelled gravity waves propagate away from the cloud with wave fronts that are approximately circular implying that the convective clouds do not generate waves which propagate in a preferred direction the gravity wave generation is related to the oscillation of the convective updraughts about their level of neutral buoyancy while the general features of the gravity waves are similar in most respects to a previous study which used an idealized wind profile the frequency of the gravity waves is doppler shifted by the tropospheric wind shear the result is a much broader power spectrum in comparison to the idealized cases

drivers of explosivity and elevated hazard in basaltic fissure eruptions the 1913 eruption of ambrym volcano vanuatu sw pacific abstract fissure eruptions along linear structures can extend for several tens of kilometres with distinct separate manifestations of volcanism along their length they typically involve low viscosity mafic magmas forming long lava flows and cinder cones eruptions in 1894 and 1913 on ambrym volcano vanuatu showed how these mildly explosive eruptions can rapidly transform into violent explosive events producing significant hazard and widespread volcanic ash clouds during the 1913 episode a fissure began in the central caldera and basaltic magmas broke out in a series of locations down the island s western flank in all sites over 100 m in elevation fissure outbreaks produced vigorous lava fountains and highly fluid lava flows that travelled rapidly to the shoreline when the outbreaks propagated along the island s axis into coastal plain areas a climactic series of explosive eruptions occurred producing a 1 2 km long by 600 m wide maar and tephra ring a further small tuff ring was formed later creating a temporary island 400 m offshore the onshore tephra ring destroyed a hospital and associated buildings its last evacuating occupants were close witnesses to the eruption processes deposits exposed in the lower portion of the tephra ring show that this part of the eruption began with a mild phreatomagmatic explosive eruption from a narrow vent followed by a magmatic scoria producing phase subsequently a complex sequence of highly explosive phreatomagmatic eruptions occurred producing pyroclastic surges along with repeated distinctive breccia horizons rich in coral and lava country rock these features tally with eye witness accounts to indicate that the main eruption phase was produced by a periodically shifting locus of phreatomagmatic fragmentation and eruption along a single e w fissure the glassy and vesicle poor pyroclasts produced during this eruption phase were dominantly fragmented in a brittle manner by magma water interaction low volatile content of the magma upon fragmentation is confirmed by ftir analysis showing

formation of tuffisitic kimberlites by phreatomagmatic processes the rock type tuffisitic kimberlite tk occurs in the deeper diatremes of some southern african canadian and siberian kimberlite pipes and has been considered a unique rock type related to a particular emplacement process the key features of this rock type are its fragmental nature its massive well mixed appearance and a specific matrix mineralogy characterized by the presence of serpentine and microlitic clinopyroxene and the absence or scarcity of carbonate historically all these features were thought to be related to a highly specific intrusive extrusive magmatic emplacement process in this process the expansion of magmatic volatiles drives the fluidization of a pre existing shallow crustal vertical magma reservoir the embryonic pipe and its consequent evolution into the final diatreme after breakthrough to the surface the specific matrix mineralization is explained by the dissociation of carbonate into co2 and cao while the expanding co2 drives the fluidization process cao reacts with sio2 released from xenolith and olivine alteration to form microlitic clinopyroxene with the phreatomagmatic process chain we offer an alternative model that can readily explain this specific rock type ongoing explosions in a downward penetrating root zone excavate a pipe consisting of a rather regular cone shaped diatreme underlain by the irregular root zone at this stage the pipe in its majority is infilled by warm to hot pyroclastic tephra during thermohydraulic explosions in the root zone the expansion of water vapor homogenizes the overlying diatreme tephra and mixes it with the adjacent country rock lithologies this result in the massive well mixed volcaniclastic rock type typical for tkbs post emplacement hydrothermal alteration and mineralization under epi to mesothermal conditions are considered responsible for the observed specific matrix mineralogy carbonate is frequently present in coherent root zone rocks but is rare or absent due to its dissolution in hot hydrothermal fluids ascending from the lower diatreme which in turn precipitate their dissolved carbonate higher up in the cooler bedded upper diatreme region clinopyroxene also grows under epi to mesothermal conditions for the crystallization of serpentine a x h2o x co2 ratio of 20 1 is required which suggests both a high availability of groundwater and a low abundance of co2 after and likely during emplacement isotope studies h c o sr of matrix minerals suggest participation of groundwater in the formation of these minerals

reconstruction of a kimberlite eruption using an integrated volcanological geochemical and numerical approach a case study of the fox kimberlite nwt canada abstract an integrated approach involving volcanology geochemistry and numerical modelling has enabled the reconstruction of the volcanic history of the fox kimberlite pipe the observed deposits within the vent include a basal massive poorly sorted matrix supported lithic fragment rich eruption column collapse lapilli tuff extensive vent widening during the climactic magmatic phase of the eruption led to overloading of the eruption column with cold dense country rock lithic fragments dense juvenile pyroclasts and olivine crystals triggering column collapse 40 dilution of the kimberlite by granodiorite country rock lithic fragments is observed both in the physical componentry of the rocks and in the geochemical signature where enrichment in al2o3 and na2o compared to average values for coherent kimberlite is seen the wide deep open vent provided a trap for a significant proportion of the collapsing column material preventing large scale run away in the form of pyroclastic flow onto the ground surface although minor flows probably also occurred a massive to diffusely bedded poorly sorted matrix supported accretionary lapilli bearing lithic fragment rich lapilli tuff overlies the column collapse deposit providing evidence for a late phreatomagmatic eruption stage caused by the explosive interaction of external water with residual magma correlation of pipe morphology and internal stratigraphy indicate that widening of the pipe occurred during this latter stage and a thick granodiorite cobble boulder breccia was deposited ash and accretionary lapilli rich tephra deposited on the crater rim during the late phreatomagmatic stage was subsequently resedimented into the vent incompatible elements such as nb are used as indicators of the proportion of the melt fraction or kimberlite ash retained or removed by eruptive processes when compared to average coherent kimberlite the ash rich deposits exhibit 30 loss of fines whereas the column collapse deposit exhibits 50 loss this shows that despite the poorly sorted nature of the column collapse deposit significant elutriation has occurred during the eruption indicating the existence of a high sustained eruption column the deposits within fox record a complex eruption sequence showing a transition from a probable violent sub plinian style eruption driven by instantaneous exsolution of magmatic volatiles to a late phreatomagmatic eruption phase mass eruption rate and duration of the sub plinian phase of the eruption have been determined based on the dimensions of milled country rock boulders found within the intra vent deposits calculations show a short lived eruption of one to eleven days for the sub plinian magmatic phase which is similar in duration to small volume basaltic eruptions this is in general agreement with durations of kimberlite eruptions calculated using entirely different approaches and parameters such as predictions of magma ascent rates in kimberlite dykes

crustal geometry of the abitibi subprovince in light of three dimensional seismic reflector orientations we provide precise estimates of reflector orientations beneath the archean abitibi subprovince using two distinct approaches based on lithoprobe seismic reflection data in the first we use the dip of reflections observed on intersecting profiles to establish the three dimensional orientation of reflective structures in the second the strikes and dips of reflectors are estimated in the crooked parts of seismic reflection profiles by calculating a measure of coherency along the traveltime trajectories defined by a particular azimuth dip depth and medium velocity mid crustal reflectors define two areas with distinctive geometry reflectors beneath the southern abitibi belt are oppositely dipping and convergent at depth providing a v shape aspect to the greenstone rocks other reflectors beneath the northern abitibi belt are in general subparallel dipping at an average of 30 toward the north these north dipping reflectors are partly disrupted by a low reflectivity zone which is attributed to

evaluating isotopic equilibrium among sulfide mineral pairs in archean ore deposits case study from the kidd creek vms deposit ontario canada recent developments in the measurement of 34s and 33s in sulfide minerals provide a new framework to evaluate isotopic equilibrium among sulfide minerals and a new tool to investigate mineralizing processes or sources of sulfur in ore forming systems isotopic equilibrium requires that values of 34s and 33s are highly correlated between mineral pairs and that deviations quantified as 33s from a reference mass dependent fractionation are the same for both minerals within experimental error in this contribution we apply this framework for the first time to natural samples and evaluate isotopic equilibrium among sulfide minerals from the archean kidd creek volcanogenic massive sulfide deposit in ontario canada two of eight sulfide mineral pairs showed clear isotopic disequilibrium in agreement with interpreted paragenetic relationships six other pairs have isotopic compositions that are consistent with equilibrium we discuss the implications of these results for the interpretation of mineral paragenesis and sulfur isotope geothermometry at kidd creek this technique offers new possibilities for evaluating sulfur isotope systematics in archean 2 45 ga ore deposits in which the range of 34s values is small but non zero values of 33s are common

on the first experimental phreatomagmatic explosion of a kimberlite melt abstract detailed field investigations of kimberlite pipes of the upper cretaceous gibeon kimberlite field in southern namibia revealed geologic features which do neither agree with extensive nor with explosive vesiculation in the pipe hanaus 2 vesicle free magmatic kimberlite intruded the diatreme as a plug here we report on the first experiments in which phreatomagmatic explosions were produced with remolten natural non fragmental magmatic kimberlite from the hanaus 2 central plug as a best fit model kimberlite magma system after water was injected into the melt under controlled conditions the resulting water melt mix was triggered by a low energy shock wave 400 m s

quantitative experiments on phreatomagmatic explosions abstract an experimental set up for controlled generation of thermal explosions fuel coolant interactions has been built by an interdisciplinary research group of volcanologists physicists and engineering scientists the tee haus thermal explosion experiment during the last 3 years more than 500 experimental runs were performed aim of the project was not to build a mini volcano but to produce igniteable mixtures of water and hot melt on a small scale in order to investigate basic physical aspects such small scale mixtures however most probably also occur during phreatomagmatic events in nature mixtures of water and magma on a large scale may be modelled by combination of small elements earlier experimental studies of thermal explosions were carried out with the use of metal melts nearly exclusively therefore a carbonate melt approx 0 3 kg of 1 1 mass ratio na2co3 k2co3 was used for initial experimental series because of easy handling and more magma similarity by injection of water into this melt explosive mixtures were produced and ignited self triggering explosive interactions occurred in a wide melt temperature range 720 to 1040 c at melt temperatures exceeding 1000 c co2 increasingly degassed presence of non condensible gas bubbles in the melt strongly reduced the explosivity the intensity of explosions only slightly increased with higher melt temperatures but strongly increased with higher water injection velocities because of more intensive mixing a minimum injection velocity was found to be approx 0 7 m s 1 experiments with silicate melts derived from diverse remelted volcanic rocks were carried out with a slightly modified set up explosions were ignited by an additional trigger a shock wave of approx 8 j in volcanic systems shock waves of this magnitude are abundant volcanic tremor etc strong explosions with silicate melts were produced at melt temperatures between 1350 and 1750 c the produced explosions exerted repulsion forces of up to 24 000 n with a typical pulse duration of 1 ms carbonate melt resp 1 5 ms silicate melts the ejection velocity was found to be in a range between 200 and 400 m s 1 explosion energy values up to 500 j were calculated the maximum explosion pressure was calculated to be in the range between 10 and 100 mpa only part of the melt so called interactive melt reacts explosively with injected water most of the melt is passively ejected by and after the thermal explosion different fragmentation processes result in specific grain size and grain shape populations that can be distinguished from each other particles resulting from the fine fragmentation process that precedes and causes the thermal explosion are characterized by angular to subrounded shapes and grain sizes ranging from 20 m to 180 m the amount of interactive melt is proportional to the intensity of the explosion the maximum amount of water vapourized at the explosive interaction so called interactive water was estimated thus the water melt mass ratio for explosive interactions could be calculated the values range between 1 25 and 1 35 carbonate melt resp between 1 6 and 1 25 silicate melts

structure and evolution of hydrothermal vent complexes in the karoo basin south africa the karoo large igneous province formed at c 183 ma is characterized by the presence of voluminous basaltic intrusive complexes within the karoo basin extrusive lava sequences and hydrothermal vent complexes these last are pipe like structures up to several hundred metres in diameter piercing the horizontally stratified sediments of the basin detailed mapping of two sediment dominated hydrothermal vent complexes shows that they are composed of sediment breccias and sandstone the breccias cut and intrude tilted host rocks and are composed of mudstone and sandstone fragments with rare dolerite boulders sandstone clasts in the breccias are locally cemented by zeolite which represents the only hydrothermal mineral in the vent complexes our data document that the hydrothermal vent complexes were formed by one or a few phreatic events leading to the collapse of the surrounding sedimentary strata we propose a model in which hydrothermal vent complexes originate in contact metamorphic aureoles around sill intrusions heating and expansion of host rock pore fluids resulted in rapid pore pressure build up and phreatic eruptions the hydrothermal vent complexes represent conduits for gases and fluids produced in contact metamorphic aureoles slightly predating the onset of the main phase of flood volcanism

archean continental assembly in the southeastern superior province of canada between 1988 and 1993 seismic reflection and refraction surveys were acquired across the medium to high grade opatica plutonic gneiss belt the low grade abitibi greenstone belt and the pontiac metasedimentary belt all of which form part of the late archean superior province shallowly north dipping reflections define a structural style consistent with the northward underthrusting and accretion over about 30 ma of various exotic terranes against a backstop provided by the opatica belt this rapid southward growth of the archean protocraton was driven by at least one north dipping subduction zone as revealed by north dipping reflections that extend to 65 km depth in the upper mantle below the opatica belt in contrast to the mainly orthogneissic opatica and pontiac belts the midcrust of the abitibi belt comprises metasedimentary and igneous rocks plus imbricated units of unknown affinity relict midcrustal accretionary complexes of substantial size whieh are indicative of primary suture zones are interpreted near the northern and southern limits of the abitibi belt an interpreted basal decollement and significantly older ages in the north suggest that the upper crustal greenstone rocks are allochthonous evidence of large scale extension appears to be confined to the southern volcanic zone of the abitibi which developed into a half graben as the original suture zone was reactivated in extension unusually high seismic p wave velocities 7 5 8 2 kms 1 are present in the lower 8 km of the abitibi crust and they correlate well with a downward reduction in seismic reflectivity attributable to late modification of the deepest part of the crust crustal xenolith studies suggest that this process may be linked to early proterozoic magmatism

intracrustal extension of an archean orogen revealed using single grain u pb zircon geochronology the metaplutonic wawa gneiss domain in the southern superior province is part of an uplifted block that was exhumed during the proterozoic exposing the lower crustal 30 km levels of the late archean 2720 to 2670 ma abitibi wawa orogen previous structural and geophysical studies of the midcrustal wawa gneiss domain reveal subhorizontal shear zones that formed during orogen parallel extension and correspond to gently dipping seismic reflectors present at depth elsewhere in the orogen in order to bracket the age of this extensional deformation the ages of metre scale igneous bodies that crystallized before and after extension have been determined in four outcrops using single grain u pb zircon dating techniques the results constrain the time of extension to the interval between 2660 and 2645 ma and show that deformation was coeval with pegmatite emplacement partial melting of tonalite gneisses as old as 2925 35 ma and regional circa 2660 ma granulite facies metamorphism of adjacent more deeply eroded crust this extension and associated high grade metamorphism in the wawa gneiss domain occurred 10 to 25 m y after the cessation of regional compression major transcurrent fault movement and sediment deposition in the upper structural levels of the orogen the middle and lower crust was therefore mechanically decoupled from the upper crust during this period of orogen parallel extension this intracrustal style of deformation is interpreted to be the consequence of gravitational forces working on a thermally softened middle crust the detachment of lithosphere from the orogen base is the favored mechanism for generating these forces and for introducing the heat that caused synextension metamorphism cyclic construction and delamination of orogen lithosphere could account for even younger metamorphic strain events evident in nearby exposures of lower crust

deep bedded ultramafic diatremes in the missouri river breaks volcanic field montana usa 1 km of syn eruptive subsidence the ultramafic eocene missouri river breaks volcanic field mrbvf montana usa includes over 50 diatremes emplaced in a mostly soft substrate the current erosion level is 1 3 1 5 km below the pre eruptive surface exposing the deep part of the diatreme structures and some dikes five representative diatremes are described here they are 200 375 m across and have sub vertical walls their infill consists mostly of 55 90 bedded pyroclastic rocks fine tuffs to coarse lapilli tuffs with concave upward bedding and 45 10 non bedded pyroclastic rocks medium lapilli tuffs to tuff breccias the latter zones form steep columns 15 135 m in horizontal dimension which cross cut the bedded pyroclastic rocks megablocks of the host sedimentary formations are also present in the diatremes some being found 1 km or more below their sources the diatreme infill contains abundant lithic clasts and ash sized particles indicating efficient fragmentation of magma and country rocks the spherical to sub spherical juvenile clasts are non vesicular they are accompanied by minor accretionary lapilli and armored lapilli the deposits of dilute pyroclastic density currents are locally observed our main interpretations are as follows 1 the observations strongly support phreatomagmatic explosions as the energy source for fragmentation and diatreme excavation 2 the bedded pyroclastic rocks were deposited on the crater floor and subsided by 1 0 1 3 km to their current location with subsidence taking place mostly during the eruption 3 the observed non bedded pyroclastic columns were created by debris jets that punched through the bedded pyroclastic material the debris jets did not empty the mature diatreme occupying only a fraction of its width and some debris jets probably did not reach the crater floor 4 the mature diatreme was nearly always filled and buttressed by pyroclastic debris at depth there was never a 1 3 1 5 km deep empty hole with sub vertical walls otherwise the soft substrate would have collapsed inward which it only did near the surface to create the megablocks we infer that syn eruptive subsidence shifted down bedded pyroclastic material and shallow sedimentary megablocks by 0 8 1 1 km or more after which limited post eruptive subsidence occurred this makes the mrbvf diatremes an extreme end member case of syn eruptive subsidence in the spectrum of possibilities for maar diatreme volcanoes worldwide

3d reconstruction of volcanic and ore forming environments of a giant vms system a case study from the kidd creek mine canada abstract the archean north rhyolite is a bimodal suite of rocks located directly northwest of but structurally separate from the kidd creek volcanogenic massive sulfide deposit which has produced 140 4 million tonnes of ore grading 2 29 wt cu 6 15 wt zn 0 22 pb and 86 2 g t ag the stratigraphy of the north rhyolite is identical to that of the kidd creek mine consisting from oldest to youngest of ultramafic flows aphyric rhyolite quartz porphyritic rhyolite and basalt flows divisible into a low tio2 basalt unit 1 55 wt with minor interflow graphitic argillites the north rhyolite is interpreted to be the folded continuation of the kidd creek mine stratigraphy similar alteration patterns in the north rhyolite and the kidd creek mine stratigraphy as well as significant copper and zinc values within the thickest portions of rhyolite domes and ridges of the north rhyolite suggests the potential for economically significant mineralization within the north rhyolite stratigraphy correlation between alteration mineralization and the rhyolite domes and ridges demonstrate the volcanic structures through which the rhyolite was extruded also acted as hydrothermal fluid pathways and were controlling structures in ore formation a 3d volcanic reconstruction showing the paleo seafloor at the time of ore formation illustrates that the rhyolite flows of the north rhyolite were emplaced into the same subsidence structure as the strata of the kidd creek mine area this 3d model generated by translating complex geological data acquired through core logging and underground mapping indicates the volcanic setting of and ore forming environment at kidd creek is much larger than previously thought and provides new spatial data that can be used in exploration targeting

upscale evolution of mcss doppler radar analysis and analytical investigation abstract the development of two small mesoscale convective systems mcss in northeastern colorado is investigated via dual doppler radar analysis the first system developed from several initially isolated cumulonimbi which gradually coalesced into a minimal mcs with relatively little stratiform precipitation the second system developed more rapidly along an axis of convection and generated a more extensive and persistent stratiform echo and mcs cloud shield in both systems the volumetric precipitation rate exhibited an early meso scale convective cycle a maximum and subsequent minimum followed by reintensification into a modest mature stage this sequence is similar to that noted previously in the developing stage of larger mcss by mcanelly and cotton they speculated that the early meso convective cycle is a characteristic feature of development in many mcss that is dynamically linked to a rather abrupt transition toward mature stage structure this study presents kinematic evidence in suppo

impacts of lapse rates on low level rotation in idealized storms abstractadiabatic lapse rates appear to be a common feature in the lower troposphere on tornado days this article reviews physical reasons why lapse rates may influence surface vortex intensification and reports on numerical simulations designed to study the key processes in the idealized numerical model an initial mesocyclone like vortex and nonvarying convection like heat source are used in different environmental stability profiles the scales of interest in these simulations typify those of a parent supercell and the developing circulations constitute direct responses to the imposed heating downward parcel displacements are needed for surface vortex development in environments with no preexisting surface vorticity in the simulations under neutral stratification there is strong heating induced subsidence anchored near the storm edge whereas under stable stratification there are instead gravity waves that propagate away to the far field in addition under weak or neutral low level stratification

rapid ascent conditions of diamond bearing kimberlitic magmas findings from high pressure temperature experiments and finite element modeling abstract this paper deals with the problem of rapid ascent mechanism of kimberlite magmas with a multi directional approach 1 the kinetics of diamond graphite transition 2 settling velocity of diamond phenocrysts in magmas and 3 formation of ruptures required for magma ascent with a high speed based on the diamond graphite transition we present an estimate of the ascent rates from high pressure and temperature experiments using walker type multi anvil apparatus the experiments were conducted with diamond placed within a synthetic kimberlitic assemblage keeping an initial pressure and temperature of 6 gpa and 1350 c respectively it was observed that the volume fraction of diamond to graphite conversion strongly depended on the ascent rates using electrical resistivity and x ray diffraction studies we measured the degree of graphitization as a function of the ascent rate u for u 90 whereas it remained nearly intact conversion u 10 m s our theoretical calculations of the settling velocity of mantle xenoliths again confirm that diamond can exist when u 3 m s we performed numerical experiments with finite element visco elastic models to analyze the dynamics of tensile failure at the tip of magma pools leading to dilatational vertical fractures for magma transport considering the tensile strength of mantle in the order of 0 5 kb our models show this failure process as a function of the critical shape a r ratio of vertical and horizontal dimensions and density contrast of magma pools the critical is estimated nearly 200 kg m 3 when a r is very large 4

secondary initiation of multiple bands of cumulonimbus over southern britain ii dynamics of secondary initiation summary the convective storm initiation project csip pilot field programme took place in the summer of 2004 in southern england during this field campaign a case of a cold pool outflow initiating an arc of convection downwind of the generating storm the primary storm was observed three further arcs were initiated further downwind of this first arc these arcs all later gave significant rainfall over south east england results from the modelling studies described in this paper show that gravity waves generated by the primary storm may have been responsible for initiating the further three arcs of convective showers that were observed the modelled primary storm generates waves with a range of vertical wavelengths these are separated by dispersion with the higher order modes with the largest vertical wavelengths travelling fastest approximately 30 m s 1 the fastest two modes suppressed convection and later modes increased boundary layer depth and so initiated convection the multiple peaks from non hydrostatic effects in the third mode may have been responsible for the two main arcs observed and the fourth mode may have been responsible for the third arc the amplitude of the modelled waves at low levels was much larger downwind of the primary storm than upwind and only initiated convection there which agrees with the observations of the arcs forming downwind of the primary storm this asymmetry appears to be caused by the moving wave source and asymmetric wave trapping finally we discuss the implications for numerical weather prediction nwp reducing the resolution of the model and varying the magnitude of the wind speed used shows that a non hydrostatic high resolution 1k m nwp model should be able to capture such wave processes if the generating storm is forecast or analysed correctly

organization of tropical convection in low vertical wind shears the role of water vapor abstract a modeling study is conducted to gain insight into the factors that control the intensity and organization of tropical convection and in particular to examine if organization occurs in the absence of factors such as vertical wind shear or underlying sea surface temperature sst gradient the control experiment integrates a cloud resolving model for 15 days using a 3d domain exceeding 1000 km in length with no imposed winds and horizontally uniform sst and forcing for convection after 2 days of random activity the convection organizes into clusters with dimensions of approximately 200 km convective systems propagate through the clusters at speeds of 2 3 m s 1 while the clusters themselves propagate at minimal speeds of around 0 5 m s 1 examining the thermodynamic structure of the model domain it is found that the convective free bands separating the clusters are very dry throughout the troposphere and due to virtual temperature effects are correspondingly warmer in the lower tropospher

upstream influence of numerically simulated squall line storms summary the squall line s impact on its upstream environment is examined using traditional cloud and simplieed parametrized moisture models the study was motivated by the need to explain signie cant differences between the two dynamical frameworks in both the erst environmental response to the convection consists of a rapidly propagating gravity wave characterized by deep tropospheric subsidence this gravity wave accelerates the lowlevel storm ine ow in its wake with the largest effect seen near the surface in the more sophisticated model however this wave is eventually followed by a shorter vertical wavelength feature one possessing lower tropospheric ascent this second gravity wave absent from the simplieed model runs substantially alters the upstream environment yet again the gentle low level uplift establishes the cool moist tongue ofairthat has been found to stretch well ahead of the storm insimulations between thesecond wave and the main storm updraught the accelerated ineow is shifted from the ground to the middle troposphere where it helps to push dry air into the convecting region this subsequent environmental adjustment responds to the establishment of a small yet persistent area of weak cooling during the early mature phase located in the middle troposphere at the upstream edge of the main cloud mass this cooling is the combined effect of cloud water evaporation and the ascent of subsaturated air though the cooling that excites it is of relatively small magnitude 2 k this wave s effect is dramatic and signie cant a crude ex to the moisture parametrization served to establish a qualitatively similar tongue like feature in the simplie ed model simulations bringing the results from the two dynamical frameworks more into line

characteristics of mid level clouds over west africa mid level clouds located between 2 and 9 km height are ubiquitous in the tropical belt however few studies have documented their characteristics and tried to identify the associated thermodynamic properties particularly in west africa this region is characterized by a strong seasonality with precipitation occurring in the sahel from june to september monsoon season this period also coincides with the annual maximum of the cloud cover here we document the macro and microphysical properties of mid level clouds the environment in which such clouds occur as well as their radiative properties across west africa to do so we combined high resolution observations from two ground based sites including lidar and cloud radar in contrasted environments one in the sahel niamey amma campaign 2006 and the other in the sahara bordj badji mokhtar fennec campaign june 2011 along with the merged cloudsat calipso satellite products the results show that mid level clouds are found throughout the year with a predominance around the monsoon season early in the morning they also are preferentially observed in the southern and western parts of west africa they are usually thin most of them are less than 1000 m deep and as observed in niamey mainly composed of liquid water a clustering method applied to niamey data allows us to distinguish three different types of cloud one with low bases one with high bases and another with large thicknesses the two first cloud families are capped by an inversion the last family is associated with a large vertical moisture transport and likely has the highest radiative effect at the earth s surface among the three cloud types

convective self aggregation and tropical cyclogenesis under the hypohydrostatic rescaling abstractthe behavior of rotating and nonrotating aggregated convection is examined at various horizontal resolutions using the hypohydrostatic or reduced acceleration in the vertical rave rescaling this modification of the equations of motion reduces the scale separation between convective and larger scale motions enabling the simultaneous and explicit representation of both types of flow in a single model without convective parameterization without the rave rescaling a dry bias develops when simulations of nonrotating radiative convective equilibrium are integrated at coarse resolution in domains large enough to permit convective self aggregation the rescaling reduces this dry bias and here it is suggested that the rescaling moistens the troposphere by weakening the amplitude and slowing the group velocity of gravity waves thus reducing the subsidence drying around aggregated convection separate simulations of rotating radiative convective equilibrium exhibit tropical cyclogenesis as horizo

timing and kinematics of post timiskaming deformation within the larder lake cadillac deformation zone southwest abitibi greenstone belt ontario canada the larder lake cadillac deformation zone is one of several anastomosing zones of high strain within the abitibi greenstone belt in the kirkland lake area ontario the larder lake cadillac de

a model for stress controlled pipe growth abstract the rock mechanics theory for deformation of underground mining excavations under high stress conditions can be used to explain the growth and geometry of volcanic pipes in an underground excavation stress concentrates greatest on the sides of an excavation perpendicular to the principal vector of compression if the stress is high enough fractures will develop causing scaling of the tunnel sidewalls and tunnel growth perpendicular to the principal vector of compression pre existing structures aid the physical mechanisms of pipe growth such as gravitational collapse explosive fragmentation and turbulent erosion and by reducing the strength of the rock mass should also aid stress induced scaling universal distinct element code numerical modelling demonstrated in this study reproduces the stress conditions around a circular pipe under uniaxial compression and simulates pipe growth as wedges bounded by failed pre existing joints form around the pipe and are assimilated into the pipe the results show how a volcanic pipe will tend to grow perpendicular to the principal vector of compression if the internal magma pressure is low or absent the orientations of the pre existing joints affect the exact direction of pipe growth in a predictable manner examples from other publications demonstrate that the model is consistent even in extensional tectonic environments case studies from kimberlite occurrences in the limpopo belt at finsch mine and the gross brukkaros volcanic complex demonstrate that dykes and magmatic bodies of kimberlite with high overpressures during emplacement normally have geometries trending near parallel to the principal vector of compression yet pipes or parts of pipes that underwent strong underpressures during emplacement often ending up comprising fragmental volcaniclastic infill have an elongation near perpendicular to the same vector thus the stress induced pipe growth model is demonstrated to be important for some pipes the study of volcanic pipe and dyke shapes can therefore be used to determine the stress regime at the time of emplacement and to distinguish between kimberlite occurrences formed at different times within different stress tensors

physical parameters governing the formation of pele s hair and tears the physical parameters that affect the formation of pele s hair and pele s tears during lava fountaining are discussed experiments on ink jets produced from a nozzle under different weber number we and reynolds number re show the following results if re is relatively large compared with we an ink droplet is produced however if re is relatively small and we is large the spurting ink becomes thread like i define the pele number pe as we re which is expressed as v 0 where v is the spuring velocity from an erupting vent and are viscosity and interfacial tension of the erupting magma and and 0 are density of air and magma the experimental results from ink jets suggest that pele s hair will be produced for larger pe while pele s tears are very likely produced for relatively small pe i conclude that pele s hair could be produced when the spurting velocity of erupting magmas is high and pele s tears when it is relatively low as an additional point of interest the similarity of sem photographs of the characteristic shape of pele s hair to those of the failed products of commercial glass fibre are shown

archean magmatism and deformation were not products of plate tectonics abstract the granite and greenstone terrains that dominate upper crust formed from about 3 6 to about 2 6 ga and record magmatic and tectonic processes very different from those of a younger time they indicate heat loss by the archean earth primarily by voluminous magmatism from a mantle much hotter than that of the present plate tectonic processes were not then operating the distinctive array of petrologic structural and stratigraphic features that characterize phanerozoic convergent plate systems ophiolites magmatic arcs accretionary wedges fore arc basins etc have no viable analogues in archean terrains purported archean plate tectonic indicators consist merely of rock types that even superficially resemble actual phanerozoic indicators only when considered in isolation from their association and structure archean ultramafic and mafic volcanic rocks neither resemble ophiolitic rocks in petrology nor occur in ophiolite type successions they often depositionally overlie felsic basement rocks and often overlie and are intercalated with sedimentary and felsic volcanic rocks and they require a mantle about 200 c hotter than now archean graywackes are coherent strata derived from nearby volcanic rocks late in the histories of their regions and they lack the setting and disruption that characterize modern accretionary wedges the lithologic structural and stratigraphic assemblages that typify proterozoic and phanerozoic rifted and reassembled margins also lack archean analogues and no evidence has been found for archean rifting rotation and reassembly of continental plates conversely characteristic archean assemblages lack modern equivalents in any tectonic setting ultramafic lavas erupted at liquidus temperatures are voluminous granite and greenstone terrains have no modern analogues greenstone belts are typically anastomosing networks of upright synforms formed by crowding aside by and sinking between large rising diapiric elliptical composite batholiths the batholiths include both the products of new crustal melts and variably remobilized mid crustal gneisses the greenstone belts are defined by late deformation of regionally semiconcordant volcanic and sedimentary successions and are not relics of successively formed volcanic chains little deformation generally preceded the diapirism and metamorphism was primarily of contact type the regionally uniform areal density and accordant crustal level of the diapiric batholiths their contacts primarily against the oldest strata of the synforms their general age 10 20 million years younger than most of the flanking stratiform rocks and considerations of high archean radiogenic heat productivity all fit the explanation that the batholiths were mobilized by partial melting of hydrous lower crust by radiogenic heating diapirism was accompanied by modest regional orthogonal shortening and extension of the hot upper crust producing the orientations of the batholiths rise of the batholiths greatly increased the petrologic fractionation of the crust and the concentration of radionuclides high in it resulting in cooling of the deeper crust and subjacent mantle and thus cratonization the upper crust containing the granite and greenstone aggregates was decoupled from the gneissic middle crust which underwent flattening and extension subparallel to the elongation of the shallow batholiths this deep deformation may have been driven by flow of dense restites toward delamination loci from which they sank into the mantle the early earth was probably wholly molten the surface of the earth like that of the moon must have been wholly recycled by impacts before 3 9 ga and heavily modified by them until 3 8 ga zircons as old as 4 2 ga have been found as clastic grains in much younger archean quartzites and polycyclic migmatites last partly melted and reconstituted under hydrous conditions only after 3 6 ga contain relict zircons as old as 4 0 ga the lithologies of the early earth protoliths in which these zircons formed have not been established but impact melts and breccias must be represented and magma ocean fractionates may be the nature of the transition in tectonic style into the granite and greenstone mode is unknown plate tectonic rifting and convergence were operating by about 2 0 ga and were in an essentially modern mode by about 0 8 ga the nature of the transition from the granite and greenstone mode at about 2 6 ga to plate mode by about 2 0 ga has yet to be defined the change may have been facilitated by the increasing content of water and carbon dioxide in the mantle as dense but hydrated delaminated archean crust sank into it

volcanology of tuzo pipe gahcho kue cluster root diatreme processes re interpreted abstract the middle cambrian 540 ma gahcho kue kimberlite field is situated about 275 km ene of yellowknife nwt canada the kimberlites were emplaced into 2 6 ga archean granitic rocks of the yellowknife supergroup four larger kimberlite bodies 5034 tesla tuzo and hearne as well as a number of smaller pipes and associated sheets occur in the field in plan view the tuzo pipe has a circular outline at the surface and it widens towards deeper levels the pipe infill consists of several types of coherent and fragmental kimberlite facies coherent or apparent coherent possibly welded kimberlite facies dominate at depth but also occur at shallow levels as dikes intruded late in the eruptive sequence or individual coherent kimberlite clasts the central and shallower portions of the pipe consist of several fragmental kimberlite varieties that are texturally classified as tuffisitic kimberlites the definition geometry and extent of the geological units are complex and zones controlled by vertical elements are most significant the fluidal outlines of some of the coherent kimberlite clasts suggest that at least some are the product of disruption of magma that was in a semi plastic state or even of welded material ragged clasts at low levels are inferred to form part of a complex peperite like system that intrudes the base of the root zone a variable often high abundance of local wall rock xenoliths between and within the kimberlite phases is observed varying in size from sub millimeter to several tens of meters wall rock fragments are common at all locations within the pipe but are especially frequent in a domain with a belt like geometry between 120 and 200 m depth in the pipe steeply outward dipping bedded deposits made up of wall rock fragments occur in deep levels of the pipe and are especially common under the downward widening roof segments the gradational contact relationships of these deposits with the surrounding kimberlite bearing rocks as well as their location suggest that they formed more or less in situ different breccia facies inside the pipe suggest an origin by slumping grain flows rock fall or pyroclastic deposition the shape and facies architecture of the tuzo pipe suggests that the studied section of the pipe lies at a root zone diatreme transitional structural level composite coherent kimberlite clasts imply that recycling processes were active over time while reworked wall rock rich deposits and ductily deformed clasts of welded kimberlite point to the presence of temporary cavities in the root zone the emplacement of the tuzo pipe did not occur in a single violent explosion but involved repetitive volcanic explosions alternating with periods of relative quiescence the observed features are typical of phreatomagmatic processes which may include phases of less explosive magmatic activity

transition zone between the upper diatreme and lower diatreme origin and significance at round butte hopi buttes volcanic field navajo nation arizona round butte is a small but complex miocene diatreme that crops out 190 m below the pre eruptive surface in the southeastern part of the hopi buttes volcanic field erosional remnants consist of a diatreme 170 190 m in diameter of which the central 130 150 m is well exposed in a massif featuring 20 30 m high sub vertical cliffs and a 50 cm thick basanite dike field mapping allowed us to define three main groups of pyroclastic rocks in the diatreme undisturbed beds disturbed beds and non bedded rocks pyroclastic rocks range in grain size from coarse tuff to tuff breccia and in componentry from juvenile rich to lithic rich with a dominance of heterolithic lapilli tuffs rocks from the undisturbed bedded pyroclastic group are present above an unconformity found all around the massif whereas the disturbed bedded and the non bedded pyroclastic groups are always found below it this unconformity was previously understood as the contact between the upper and the lower diatremes the undisturbed beds above the unconformity indeed compose the upper diatreme but the assemblage of non bedded rocks invasive columns and disturbed beds residual columns below it is not typical of the lower diatreme instead they represent a transition zone between the upper and lower diatremes such a transition zone also occurs in other diatremes it is important genetically and we propose to add it to the general model of maar diatreme volcanoes

the mulgandinnah shear zone an archean crustal scale strike slip zone eastern pilbara western australia abstract a large part of the deformation in the archean pilbara granitoid greenstone terrain is localized in relatively narrow shear zones the mulgandinnah shear zone msz is a major one of these with a width up to 8 km that can be followed for over 70 km along strike in the shaw batholith in the eastern pilbara it forms part of the mulgandinnah lineament that can be traced to the lalla rookh basin and the carlindi batholith in the north giving it a total length of over 150 km the msz contains both mylonites and ultramylonites both of which have foliations that are subvertical to steeply dipping with the ultramylonitic foliation overprinting the mylonitic foliation to form more localized zones of high strain stretching lineations are mostly subhorizontal to shallow south plunging for both the mylonites and ultramylonites but can vary between 50 n and 50 s both foliations formed under upper greenschist to amphibolite grade conditions the msz displays a consistent sinistral sense of shear as indicated by and type porphyroclasts s c fabrics and rotated pegmatite veins the mylonites and ultramylonites are interpreted to have developed as part of a progressive deformation event leading to strain localization in the ultramylonites timing constraints indicate that the msz has been active at ca 2930 ma but it is likely to have been a long lived structure that was reactivated at that stage when it acted as a conjugate shear during a nw se crustal shortening in the eastern pilbara the final juxtaposition of the various domains in the pilbara granitoid greenstone terrain occurred during this deformation phase

pluton emplacement within an extensional transfer zone during dextral strike slip faulting an example from the late archaean abitibi greenstone belt abstract the lake abitibi area within the late archaean abitibi greenstone belt exhibits an interlinked plutonic structural and metamorphic evolution that may characterize segmented strike slip faults at upper to mid crustal levels along the major southeastward propagating macamic d 2 dextral strike slip fault theological and preexisting d 1 structural heterogeneities induced the development of nnw trending dextral oblique splays which evolved into an extensional trailing fan and created an extensional nnw dipping stepover magma flowing upwards from deeper parts of the macamic fault spread towards the southeast at upper crustal levels along both the oblique slip and extensional d 2 splays and built several plutons in a pull apart domain between 2696 and 2690 ma different emplacement and material transfer mechanisms operated simultaneously in different parts of the system including fault dilation and wedging lateral expansion wall rock ductile flow and stoping transfer of movement between d 2 splays occurred under ductile conditions during syn emplacement amphibolite grade metamorphism 500 700 c during cooling d 2 faults appear to have been closely linked during the superior province wide late transpressional event

formation of an archean tectonic melange in the schreiber hemlo greenstone belt superior province canada implications for archean subduction accretion process the late archean circa 2750 2670 ma schreiber hemlo greenstone belt superior province canada is composed of tectonically juxtaposed fragments of oceanic plateaus circa 2750 2700 ma oceanic island arcs circa 2720 2695 ma and siliciclastic trench turbidites circa 2705 2697 ma following juxtaposition these lithotectonic assemblages were collectively intruded by synkinematic tonalite trondhjemite granodiorite ttg plutons circa 2720 2690 ma and ultramafic to felsic dikes and sills circa 2690 2680 ma with subduction zone geochemical signatures overprinting relations between different sequences of structures suggest that the belt underwent at least three phases of deformation during d1 circa 2695 2685 ma oceanic plateau basalts and associated komatiites arc derived trench turbidites and oceanic island arc sequences were all tectonically juxtaposed as they were incorporated into an accretionary complex fragmentation of these sequences resulted in broken formations and a tectonic melange in the schreiber assemblage of the belt d2 circa 2685 2680 ma is consistent with an intra arc right lateral transpressional deformation fragmentation and mixing of d2 synkinematic dikes and sills suggest that melange formation continued during d2 the d1 to d2 transition is interpreted in terms of a trenchward migration of the magmatic arc axis due to continued accretion and underplating the d2 intra arc strike slip faults may have provided conduits for uprising melts from the descending slab and they may have induced decompressional partial melting in the subarc mantle wedge to yield synkinematic ultramafic to felsic intrusions a similar close relationship between orogen parallel strike slip faulting and magmatism has recently been recognized in several phanerozoic transpressional orogenic belts suggesting that as in phanerozoic counterparts orogen parallel strike slip faulting in the schreiber hemlo greenstone belt played an important role in magma emplacement

towards a new hybrid cumulus parametrization scheme for use in non hydrostatic weather prediction models classical mass flux parametrization schemes for cumulus convection generally transport heat and moisture only but do not include a net mass transport this is well justified for large grid spacings comprising the whole convective circulation in the local grid column such that all convective mass fluxes locally cancel out a conceptual problem arises for finer grid spacings as used in contemporary numerical weather prediction nwp models when convection becomes partially resolvable this problem can be overcome by the hybrid approach presented here only updraft and downdraft are parametrized with a net mass transport the environmental subsidence is treated by the grid scale equations the total mass flux in the continuity equation is split into a grid scale and a subgrid scale contribution this parametrization scheme is designed for use in any nonlinear non hydrostatic and fully compressible nwp model we here have chosen the lokal modell lm of deutscher wetterdienst idealized dry mass lifting experiments without convective heat transport demonstrate the feasibility of the hybrid approach entrainment causes grid scale convergence and the detrained air if set to the environmental temperature spreads mainly horizontally on the grid gravity waves are generated when convection starts and ends whereas their amplitude depends on the details of the switching on and off of convection the stationary state after about 30 minutes does not four model runs with different grid spacings 3 5 km to 28 km confirm that the mass exchange between the model grid and the parametrization scheme is independent of the chosen grid spacing total mass in a convective circulation cell is conserved to better than 0 1 but only if the damping layer at the upper boundary of the lm is shifted to above 20 km for moist convection with convective heat transport a simple cloud model for an updraft has been set up as the detrained air at the cloud top is colder than the environment it moves down by about 1 km but then mainly spreads horizontally again over several tens of kilometres as in the dry case without convective heat transport the hybrid mass flux approach with both a grid scale and a subgrid scale contribution may fill the gap between coarse grid models grid spacing 50 km with classical parametrization schemes and very highly resolved explicit convection modelling with a grid spacing of the order of 100 m copyright 2007 royal meteorological society

kimberlite wall rock fragmentation processes venetia k08 pipe development current kimberlite pipe development models strongly advocate a downward growth process with the pipe cutting down onto its feeder dyke by means of volcanic explosions evidence is presented from the k08 kimberlite pipe in venetia mine south africa which suggests that some pipes or sub components of pipes develop upwards the k08 pipe in pit exposure comprises 90 vol chaotic mega breccia of country rock clasts gneiss and schist and 10 vol coherent kimberlite sub horizontal breccia layers tens of metres thick are defined by lithic clast size variations and contain zones of shearing and secondary fragmentation textural studies of the breccias and fractal statistics on clast size distributions are used to characterize sheared and non sheared breccia zones and to deduce a fragmentation mechanism breccia statistics are compared directly with the statistics of fragmented rock produced from mining processes in order to support interpretations results are consistent with an initial stage of brecciation formed by upward moving collapse of an explosively pre conditioned hanging wall into a sub terranean volcanic excavation our analysis suggests that the pre conditioning is most likely to have been caused by explosions either phreatic or phreatomagmatic in nature with a total energy output of 2 7 109 kj 656 t of tnt a second stage of fragmentation is interpreted as shearing of the breccia caused by multiple late kimberlite intrusions and possible bulk movement of material in the pipe conduit related to adjacent volcanism in the k02 pipe

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

numerical modeling of gravity wave generation by deep tropical convection although convective clouds are known to generate internal gravity waves the mechanisms responsible are not well understood the present study seeks to clarify the dynamics of wave generation using a high resolution numerical model of deep convection over the tiwi islands australia the numerical calculations presented explicitly resolve both the mesoscale convective cloud cluster and the gravity waves generated as the convective clouds evolve they excite gravity waves which are prominent features of the model solutions in both the troposphere and stratosphere the source location is variable in time and space but is related to the development of individual convective cells the largest amplitude gravity waves are generated when the cloud tops reach the upper troposphere a new analysis technique is introduced in which the nonlinear terms in the governing equations are taken as the forcing for linear gravity waves the analysis shows that in the present calculation neither the shear nor the diabatic heating are the dominant forcing terms instead the wave source is most easily understood when viewed in a frame of reference moving with the wind at the level of neutral buoyancy whereupon the source may be described as a vertically oriented oscillating convective updraft this description is consistent with the properties of the modeled stratospheric waves

physical distribution trends in darwin glass darwin glass formed by impact melting probably during excavation of the 1 2 km diameter darwin crater tasmania australia the glass was ejected up to 20 km from the source crater and forms a strewn field of 400 km 2 there is at least 11 250 m 3 of glass in the strewn field and relative to the size of the crater this is the most abundant ejected impact glass on earth the glass population can be subdivided on the basis of shape 74 irregular 20 ropy 0 5 spheroid 6 droplet and 0 7 elongate and color 53 dark green 31 light green 11 black and 5 white the white glasses contain up to 92 wt sio2 and are formed from melting of quartzite black glasses contain a minimum of 76 wt sio2 and formed from melting of shale systematic variations in the proportion of glasses falling into each of the color and shape classes relative to distance from the crater show 1 a decrease in glass abundance away from the crater 2 the largest fragments of glass are found closest to the crater 3 small fragments 2 g dominate finds close to the crater 4 the proportion of white glass is greatest closest to the crater 5 the proportion of black glass increases with distance from the crater and 6 the proportion of splashform glasses increases with distance from the crater these distribution trends can only be explained by the molten glass having been ballistically ejected from darwin crater during impact and are related to 1 the depth of excavation from the target rock stratigraphy and or 2 viscosity contrasts between the high and low sio2 melt the high abundance and wide distribution of ejected melt is attributed to a volatile charged target stratigraphy produced by surface swamps that are indicated by the paleoclimate record

petrogenetic and geodynamic origin of the neoarchean dore lake complex abitibi subprovince superior province canada the neoarchean ca 2728 ma anorthosite bearing dore lake complex in the northeastern abitibi subprovince quebec was emplaced into an association of intra oceanic tholeiitic basalts and gabbros known as the obatogamau formation the obatogamau formation constitutes the lower part of the roy group which is composed of two cycles of tholeiitic to calc alkaline volcanic and volcaniclastic rocks siliciclastic and chemical sedimentary rocks and layered mafic to ultramafic sills in this study we report major and trace element results and nd sr pb and o isotope data for anorthosites leucogabbros gabbros and mafic dykes from the dore lake complex and spatially associated basalts and gabbros of the obatogamau formation to assess their petrogenetic origin and geodynamic setting field and petrographic observations indicate that the dore lake complex and associated volcanic rocks underwent extensive metamorphic alteration under greenschist facies conditions resulting in widespread epidotization 20 40 and chloritization 10 40 of many rock types plagioclase recrystallized mainly to anorthite and albite endmembers erasing intermediate compositions metamorphic alteration also led to the mobilization of many elements e g lile and transition metals and to significant disturbance of the rb sr and u pb isotope systems resulting in 1935 150 and 3326 270 ma errorchron ages respectively the sm nd isotope system was less disturbed yielding an errorchron age of 2624 160 ma on many binary major and trace element diagrams the least altered anorthosites and leucogabbros and the gabbros and mafic dykes of the dore lake complex plot in separate fields signifying the presence of two distinct magma types in the complex the gabbros and mafic dykes in the dore lake complex share the geochemical characteristics of tholeiitic basalts and gabbros in the obatogamau formation suggesting a possible genetic link between the two rock associations initial end 2 6 to 5 0 and 18o 6 1 to 7 9 values for the dore lake complex and gabbros of the obatogamau formation end 2 8 to 4 0 18o 7 3 to 8 0 are consistent with depleted mantle sources all rock types in the dore lake complex and the roy group share the trace element characteristics of modern arc magmas suggesting a suprasubduction zone setting for these two lithological associations on the basis of regional geology and geochemical data we suggest that the dore lake complex and the obatogamau formation represent a dismembered fragment of a suture zone like many phanerozoic ophiolites resulting from closure of a back arc basin between 2703 and 2690 ma

a rare occurrence of a crater filling clastogenic extrusive coherent kimberlite victor northwest ontario canada kimberlite pipes can contain significant proportions of dark and dense kimberlite that have mostly been interpreted as intrusive coherent hypabyssal in origin this study reports a well documented occurrence of a fresh intra crater clastogenic extrusive coherent kimberlite that is concluded to have formed as a result of lava fountaining this paper focuses on a dark dense competent generally crystal rich massive kimberlite unit within the victor northwest kimberlite pipe ontario canada using a comprehensive volcanological and petrographic analysis of all available drill cores it is shown that this unit has a fresh well crystallised coherent groundmass and is extrusive and pyroclastic in origin the proposed clastogenic coherent extrusive origin is based on deposit morphology gradational contacts to enveloping pyroclastic units as well as the presence of remnant pyroclast outlines and angular broken olivines this paper and an increasing number of other studies suggest that fragmental extrusive coherent kimberlite in intra crater settings may be more common than previously thought the emplacement history and volcanology of these pipes need to be reconsidered based on the emerging importance of this particular kimberlite facies

comment on development of spheroid composite bombs by welding of juvenile spinning and isotropic droplets inside a mafic eruption column by carracedo sanchez et al 2009 abstract pelletal and spinning lapilli as well as concentric shelled lapilli bombs are present in ultra alkaline rocks worldwide there are well established models for generation of those pyroclasts in diatreme conduit leading to the formation of subvolcanic pyroclastic rocks i e tuffisites by diatremic fluidisation in contrast carracedo sanchez et al 2009 introduced a completely different model for all the above pyroclasts and composite concentric shelled bombs in particular in the strombolian deposits of cabezo segura ii cs ii calatrava spain in their model the pyroclasts are formed in a strombolian eruptive column on the basis of our observations and physical chemical constraints we contend that this model is not feasible the authors aa model requires agglutination accretion by sintering in the eruption column keeping the particles in a partially molten state we find that composite bombs from cs ii show evidence for very rapid quenching of the melt preventing sintering which in any case is impossible in a convective region where the temperature will be reduced below the solidus of the melt analysis of terminal velocity indicates that it would be possible to transport pelletal lapilli in a strombolian thrust region but dense between 2 25 and 2 85 g cm 3 lapilli and bombs cannot reach a convective region stage 3 of the aa s model which in any case is unlikely to exist at cs ii by contrast pelletal spinning and concentric shelled lapilli and composite bombs can be erupted but their formation does not depend on the surficial extrusion mechanism the presence of carbonatite at cs ii indicates juvenile co 2 to be fundamental and indicative of diatremic fluidisation concentric shelled bombs composed of agglutinated pelletal and spinning lapilli are interpreted as a development in the upper diatreme

growth of granite greenstone terranes at convergent margins and stabilization of archean cratons abstract archean granite greenstone terranes represent juvenile continental crust formed in a variety of plate tectonic settings and metamorphosed through a complex series of structural and magmatic events most archean granite greenstone terranes appear to have acquired their first order structural and metamorphic characteristics at convergent plate margins where large accretionary wedges similar in aspect to the chugach makran and altaids grew through offscraping and accretion of oceanic plateaux oceanic crustal fragments juvenile island arcs rifted continental margins and pelagic and terrigenous sediments buoyant slabs of parts of archean oceanic lithosphere may have been underplated beneath these orogens forming thick crustal roots characterized by interleaving between the depleted slabs and undepleted asthenosphere back stepping of the subduction zones after accretion of plateaux and arcs caused the arcs magmatic fronts to migrate trenchward through the accretionary wedges dehydration of the subducting slabs hydrated the mantle wedges below the new arcs and generated magmas sanukitoid suite in the mantle wedge whereas other magmas tonalite trondhjemite granodiorite or ttg suite appear to have been generated by melting of hot young subducted slabs eventual collision of these juvenile orogens with other continental blocks formed anatectic granites then thickened the crust beyond its ability to support its own mass which initiated gravitational collapse and decompressional release of syn to late tectonic granitoids from wedges of fertile mantle trapped between underplated oceanic lithospheric slabs and aided in the cratonization of the granite greenstone terranes deeply penetrating structural discontinuities such as shear zones and sutures provided pathways for fluids and granitoids to migrate into the mid and upper crust forming ore deposits and plutons most preserved granite greenstone terranes have been tectonically stable since the archean and form the cratonic interiors of many continents

emplacement temperatures of pyroclastic and volcaniclastic deposits in kimberlite pipes in southern africa palaeomagnetic techniques for estimating the emplacement temperatures of volcanic deposits have been applied to pyroclastic and volcaniclastic deposits in kimberlite pipes in southern africa lithic clasts were sampled from a variety of lithofacies from three pipes for which the internal geology is well constrained the cretaceous a k1 pipe orapa mine botswana and the cambrian k1 and k2 pipes venetia mine south africa the sampled deposits included massive and layered vent filling breccias with varying abundances of lithic inclusions layered crater filling pyroclastic deposits talus breccias and volcaniclastic breccias basalt lithic clasts in the layered and massive vent filling pyroclastic deposits in the a k1 pipe at orapa were emplaced at 570 c in the pyroclastic crater filling deposits at 200 440 c and in crater filling talus breccias and volcaniclastic breccias at 560 c although the interpretation of these results is hampered by the presence of mesozoic magnetic overprints these temperatures are comparable to the estimated emplacement temperatures of other kimberlite deposits and fall within the proposed stability field for common interstitial matrix mineral assemblages within vent filling volcaniclastic kimberlites the temperatures are also comparable to those obtained for pyroclastic deposits in other silicic volcanic systems because the lithic content of the studied deposits is 10 30 the initial bulk temperature of the pyroclastic mixture of cold lithic clasts and juvenile kimberlite magma could have been 300 400 c hotter than the palaeomagnetic estimates together with the discovery of welded and agglutinated juvenile pyroclasts in some pyroclastic kimberlites the palaeomagnetic results indicate that there are examples of kimberlites where phreatomagmatism did not play a major role in the generation of the pyroclastic deposits this study indicates that palaeomagnetic methods can successfully distinguish differences in the emplacement temperatures of different kimberlite facies

gravity waves compensating subsidence and detrainment around cumulus clouds abstract gravity waves play an important role in the redistribution of heat and moisture in a deep convecting cloud field we explore this role in a two dimensional numerical experiment on a simple moist convecting system consisting of an isolated long lasting nonprecipitating cloud in a calm atmosphere with no surface forcing the cloud develops a horizontally averaged density variation with height which is neutrally buoyant with respect to a moist adiabatic the buoyancy difference between the cloud and the undisturbed sounding produces circulations that can be understood as spreading gravity waves which adjust the environmental buoyancy to be equal to the cloud buoyancy by compensating subsidence unlike the circulations inside clouds this adjustment takes place without turbulent mixing hence the buoyancy adjustment time t1 during which the environment comes into rough buoyant equilibrium with the clouds is much shorter than the mixing time t2 which it takes a tracer initially concentrated at s

root zone processes in the phreatomagmatic pipe emplacement model and consequences for the evolution of maar diatreme volcanoes abstract the understanding of processes within the root zone of maar diatreme volcanoes is important for the interpretation of the geology volcanology and even hazard assessment of these volcanoes in the phreatomagmatic model of pipe formation the irregularly shaped root zone is the site of the phreatomagmatic explosions and thus functions as the engine for pipe formation in this model the root zone grows over a period of time in a series of many single thermohydraulic i e phreatomagmatic explosions the explosions initially occur close to the surface and with ongoing explosive activity penetrate towards deeper levels the ejection of country rock clasts from the root zone results in a mass deficiency in the root zone that causes the overlying tephra and the adjacent country rocks to subside passively in a sinkhole like fashion into the root zone many phreatomagmatic eruptions consequently result in the formation of a cone shaped diatreme thus with ongoing eruptions the cone shaped diatreme has to grow systematically both in depth and diameter during its growth processes in the lower diatreme levels successively destroy the upper levels of the evolving root zone at the surface the maar crater in turn reacts to the underlying subsidence processes and also grows both in depth and diameter thermohydraulic explosions which fragment both magma and the surrounding country rocks mostly occur within the bottom part of the root zone violent explosions in small pipes may clear the overlying diatreme for a short period of time before tephra fall and collapse of the walls of the new crater refill the small initial diatreme in larger pipes via expansion of the mixture of highly pressurized water vapor juvenile gas phases and explosively produced tephra the confined and expanding eruption cloud has to pierce through the diatreme fill in a feeder conduit in order to erupt diatreme clearing events in large pipes are difficult or impossible to maintain since the explosive force in the root zone is only in exceptional instances strong enough to lift or entrain the entire diatreme tephra knowledge of the genetic relationships between root zones and diatremes is critical to understand pipe growth processes the combination of such processes can lead to substantial variation in volcanic behavior and thus produce fundamentally different volcano and rock types it is the purpose of this paper to outline important features of root zones and suggest their significance for the genesis and evolution of maar diatreme and related volcanoes

mesoscale convective systems 1 mesoscale convective systems mcss have regions of both convective and stratiform precipitation and they develop mesoscale circulations as they mature the upward motion takes the form of a deep layer ascent drawn into the mcs in response to the latent heating and cooling in the convective region the ascending layer overturns as it rises but overall retains a coherent layer structure a middle level layer of inflow enters the stratiform region of the mcs from a direction determined by the large scale flow and descends in response to diabatic cooling at middle to low levels a middle level mesoscale convective vortex mcv develops in the stratiform region prolongs the mcs and may contribute to tropical cyclone development the propagation of an mcs may have a discrete component but may further be influenced by waves and disturbances generated both in response to the mcs and external to the mcs waves of a larger scale may affect the propagation velocity by phase locking with the mcs in a cooperative mode the horizontal scale of an mcs may be limited either by a balance between the formation rate of convective precipitation and dissipation of stratiform precipitation or by the rossby radius of the mcv the vertical redistribution of momentum by an mcs depends on the size of the stratiform region while the net vertical profile of heating of the large scale environment depends on the amount of stratiform rain regional variability of the stratiform rain from mcss affects the large scale circulation s response to mcs heating

syn and posteruptive hazards of maar diatreme volcanoes abstract maar diatreme volcanoes represent the second most common volcano type on continents and islands this study presents a first review of syn and posteruptive volcanic and related hazards and intends to stimulate future research in this field maar diatreme volcanoes are phreatomagmatic monogenetic volcanoes they may erupt explosively for days to 15 years above the preeruptive surface a relatively flat tephra ring forms below the preeruptive surface the maar crater is incised because of formation and downward penetration of a cone shaped diatreme and its root zone during activity both the maar crater and the diatreme grow in depth and diameter inside the diatreme which may penetrate downwards for up to 2 5 km fragmented country rocks and juvenile pyroclasts accumulate in primary pyroclastic deposits but to a large extent also as reworked deposits ejection of large volumes of country rocks results in a mass deficiency in the root zone of the diatreme and causes the diatreme fill to subside thus the diatreme represents a kind of growing sinkhole due to the subsidence of the diatreme underneath the maar crater is a subsidence crater and also grows in depth and diameter with ongoing activity as long as phreatomagmatic eruptions continue the tephra ring grows in thickness and outer slope angle syneruptive hazards of maar diatreme volcanoes are earthquakes eruption clouds tephra fall base surges ballistic blocks and bombs lahars volcanic gases cutting of the growing maar crater into the preeruptive ground formation of a tephra ring fragmentation of country rocks thus destruction of area and ground changes in groundwater table and potential renewal of eruptions the main hazards mostly affect an area 3 to possibly 5 km in radius distal effects are comparable to those of small eruption clouds from polygenetic volcanoes syneruptive effects on infrastructure people animals vegetation agricultural land and drainage are pointed out posteruptive hazards concern erosion and formation of lahars inside the crater a lake usually forms and diverse types of sediments accumulate in the crater volcanic gases may be released in the crater compaction and other diagenetic processes within the diatreme fill result in its subsidence this posteruptive subsidence of the diatreme fill and thus crater floor is relatively large initially but will decrease with time it may last millions of years various studies and monitoring are suggested for syn and posteruptive activities of maar diatreme volcanoes erupting in the future the recently formed maar diatreme volcanoes should be investigated repeatedly to understand more about their syneruptive behaviour and hazards and also their posteruptive topographic limnic and biologic evolution and potential posteruptive hazards for future maar diatreme eruptions a hazard map with four principal hazard zones is suggested with the two innermost ones having a joint radius of up to 5 km areas that are potentially endangered by maar diatreme eruptions in the future are pointed out

gravity waves in shear and implications for organized convection abstract it is known that gravity waves in the troposphere which are often excited by preexisting convection can favor or suppress the formation of new convection here it is shown that in the presence of wind shear or barotropic wind the gravity waves can create a more favorable environment on one side of preexisting convection than the other side both the nonlinear and linear analytic models developed here show that the greatest difference in favorability between the two sides is created by jet shears and little or no difference in favorability is created by wind profiles with shear at low levels and no shear in the upper troposphere a nonzero barotropic wind or equivalently a propagating heat source is shown to also affect the favorability on each side of the preexisting convection it is shown that these main features are captured by linear theory and advection by the background wind is the main physical mechanism at work these processes should play an important role in the organization of

formation of the abitibi greenstone belt by arc trench migration volcanic and plutonic rocks of archean greenstone belts in the abitibi subprovince become younger southward plutonic rocks are 10 30 m y younger than volcanic rocks the abitibi greenstone belt has an upper crust of 2 75 2 70 ga oceanic supracrustal assemblages and a middle crust of 2 70 2 68 ga granitic and gneissic rocks the greenstone belt upper crust is greenschist facies with upright structures and low reflectivity and the middle crust is amphibolite facies with moderate to shallowly dipping structures and strong reflectivity we propose that the age progression and crustal layering resulted from accretion of oceanic assemblages followed by extensive mid crustal magmatism linked to southward migration of an arc trench system thermal softening of the middle crust during magmatism caused ductile flow now represented by strong mid crustal reflectivity upward heat flow following magmatism caused late tectonic 2 67 2 65 ga low pressure metamorphism observed in the upper crust

some major problems with existing models and terminology associated with kimberlite pipes from a volcanological perspective and some suggestions five significant problems hinder advances in understanding of the volcanology of kimberlites 1 kimberlite geology is very model driven 2 a highly genetic terminology drives deposit or facies interpretation 3 the effects of alteration on preserved depositional textures have been grossly underestimated 4 the level of understanding of the physical process significance of preserved textures is limited and 5 some inferred processes and deposits are not based on actual modern volcanological processes these issues need to be addressed in order to advance understanding of kimberlite volcanological pipe forming processes and deposits the traditional steep sided southern african pipe model class i consists of a steep tapering pipe with a deep root zone a middle diatreme zone and an upper crater zone if preserved each zone is thought to be dominated by distinctive facies respectively hypabyssal kimberlite hk descriptively called here massive coherent porphyritic kimberlite tuffisitic kimberlite breccia tkb descriptively here called massive poorly sorted lapilli tuff and crater zone facies which include variably bedded pyroclastic kimberlite and resedimented and reworked volcaniclastic kimberlite rvk porphyritic coherent kimberlite may however also be emplaced at different levels in the pipe as later stage intrusions as well as dykes in the surrounding country rock the relationship between hk and tkb is not always clear sub terranean fluidisation as an emplacement process is a largely unsubstantiated hypothesis modern in vent volcanological processes should initially be considered to explain observed deposits crater zone volcaniclastic deposits can occur within the diatreme zone of some pipes indicating that the pipe was largely empty at the end of the eruption and subsequently began to fill in largely through resedimentation and sourcing of pyroclastic deposits from nearby vents classes ii and iii canadian kimberlite models have a more factual descriptive basis but are still inadequately documented given the recency of their discovery the diversity amongst kimberlite bodies suggests that a three model classification is an over simplification every kimberlite is altered to varying degrees which is an intrinsic consequence of the ultrabasic composition of kimberlite and the in vent context few preserve original textures the effects of syn to post emplacement alteration on original textures have not been adequately considered to date and should be back stripped to identify original textural elements and configurations applying sedimentological textural configurations as a guide to emplacement processes would be useful the traditional terminology has many connotations about spatial position in pipe and of process perhaps the traditional terminology can be retained in the industrial situation as a general lithofacies mining terminological scheme because it is so entrenched however for research purposes a more descriptive lithofacies terminology should be adopted to facilitate detailed understanding of deposit characteristics important variations in these and the process origins for example every deposit of tkb is different in componentry texture or depositional structure however because so many deposits in many different pipes are called tkb there is an implication that they are all similar and that similar processes were involved which is far from clear

internal vs external forcing in shallow marine diatreme formation a case study from the iblean mountains se sicily central mediterranean a model of diatreme evolution in a shallow marine setting is based on a multi disciplinary analysis of diatremes in the iblean mountains sicily the approach includes stratigraphic volcanological structural petrologic and compositional data we invoke a complex interplay of internal rapid ascent and pyroclastic fragmentation of a volatile co2 rich nephelinitic magma at depth and external factors these comprise hydroclastic explosions due to near surface interaction of the rising particle volatile mixture with seawater and water saturated lime mud other external factors contributing to diatreme formation include regional and local tectonics graben formation in pull apart motion combined with lateral pipe enlargement by bedrock spalling and radial block subsidence into the diatreme pipe we suggest that fragmentation of volatile rich magma due to internal eruption forcing was fundamental in the formation of the iblean shallow marine diatremes internal and external factors may act to a variable degree however during diatreme evolution in general

geology of a complex kimberlite pipe k2 pipe venetia mine south africa insights into conduit processes during explosive ultrabasic eruptions k2 is a steep sided kimberlite pipe with a complex internal geology geological mapping logging of drillcore and petrographic studies indicate that it comprises layered breccias and pyroclastic rocks of various grain sizes lithic contents and internal structures the pipe comprises two geologically distinct parts k2 west is a layered sequence of juvenile and lithic rich breccias which dip 20 45 inwards and k2 east consists of a steep sided pipe like body filled with massive volcaniclastic kimberlite nested within the k2 pipe the layered sequence in k2 west is present to 900 m below present surface and is interpreted as a sequence of pyroclastic rocks generated by explosive eruptions and mass wasting breccias generated by rock fall and sector collapse of the pipe walls both processes occurred in tandem during the infill of the pipe several breccia lobes extend across the pipe and are truncated by the steep contact with k2 east dense pyroclastic rocks within the layered sequence are interpreted as welded deposits k2 east represents a conduit that was blasted through the layered breccia sequence at a late stage in the eruption this phase may have involved fluidisation of trapped pyroclasts with loss of fine particles and comminution of coarse clasts we conclude that the k2 kimberlite pipe was emplaced in several distinct stages that consisted of an initial explosive enlargement followed by alternating phases of accumulation and ejection

on the volcanology of the gibeon kimberlite field namibia kimberlite volcanism in the upper cretaceous gibeon kimberlite field southern namibia consisting of at least 42 diatremes and a number of associated dykes is closely related to carbonatitic and ultrabasic volcanic and intrusive activity which occurred at the margin of the field the volcanology of the diatremes and dykes as well as their structural setting is reported here because of the paleohydrogeological setting and since juvenile kimberlite occurring in dykes intrusive plugs and spherical lapilli is devoid of vesicles a phreatomagmatic eruption mechanism is proposed for the genesis of the kimberlite diatremes karoo dolerite basalt and sediment xenoliths in the diatremes provide evidence for the former extent of karoo strata at the time of eruption

the role of fluidisation in the formation of volcaniclastic kimberlite grain size observations and experimental investigation abstract deep steep sided kimberlite pipes in southern africa are normally partially filled with multiple units of a characteristically massive structureless volcaniclastic rock here termed massive volcaniclastic kimberlite mvk these units can be arranged in a pseudo concentric nested pipes within pipes structure several key observations on typical mvk from the venetia kimberlite pipes south africa indicate that fluidisation may have been an important process during its emplacement these include 1 the thorough mixing together of juvenile components and of lithic components derived from all stratigraphic levels of the country rock and 2 a matrix composed of void filling minerals related to hydrothermal metamorphism mainly diopside and serpentine which indicates that the primary deposit had a high porosity 50 and 3 particle size distributions poor in fine ash 10 cm analogue experiments utilising a sand bed laced with marker horizons were conducted in order to investigate the role of fluidisation in the formation of mvk high pressure gas fed through the bed from a point source resulted in the formation of an upwardly diverging pipe structure of thoroughly mixed fluidised material bounded by undisturbed sand strong upward gas flow along the centre of the pipe is characteristically bubbly and results in internal circulation with downward moving regions at the pipe margins analysis of the experimental results suggests that the gas velocity decreases linearly with height and has a gaussian distribution of vertical velocity in the cross flow direction a decrease in the gas flow rate resulted in the development of nested pipes reminiscent of the pipes within pipes structures in kimberlite pipes the results suggest that fluidisation could be a major control on the structure and sorting of deposits in kimberlite pipes we propose that the fluidisation of particles in a kimberlite pipe occurs during the final stages of the eruption when erupting pyroclasts and incorporated fragmented lithic material can no longer be ejected out of the deep wide conduit this bed is subject to quasi sustained fluidisation by the erupting gas particle flow which imparts on it the distinctive characteristics of a massive well mixed poorly sorted deposit

models of convergent boundary tectonics implications for the interpretation of lithoprobe data physical models of convergent boundary processes can provide insights into compressional orogens such as those studied by lithoprobe we summarize the qualitative tectonic style of a series of mode

late kinematic timing of orogenic gold deposits and significance for computer based exploration techniques with emphasis on the yilgarn block western australia abstract orogenic gold deposits are a widespread coherent group of epigenetic ore deposits that are sited in accretionary or collisional orogens they formed over a large crustal depth range from deep seated low salinity h2o co2 ch4 n2 ore fluids and with au transported as thio complexes regional structures provide the main control on deposit distribution in many terranes first order faults or shear zones appear to have controlled regional fluid flow with greatest ore fluid fluxes in and adjacent to lower order faults shear zones and or large folds highly competent and or chemically reactive rocks are the most common hosts to the larger deposits focusing of supralithostatic ore fluids into dilatant zones appears to occur late during the evolutionary history of the host terranes normally within d3 or d4 in a d1 d4 deformation sequence reactivation of suitably oriented pre existing structures during a change in far field stress orientation is a factor common to many deposits and repeated reactivation may account for multiple mineralization episodes in some larger deposits absolute robust ages of mineralization support their late kinematic timing and in general suggest that deposits formed diachronously towards the end of the 100 to 200 m y long evolutionary history of hosting orogens for example in the yilgarn block a region specifically emphasised in this study orogenic gold deposits formed in the time interval between 40 and 90 m y with most about 60 to 70 m y after the youngest widespread basic ultrabasic volcanism and towards the end of felsic magmatism the late timing of orogenic gold deposits is pivotal to geologically based exploration methodologies this is because the present structural geometries of i the deposits ii the hosting goldfields and iii the enclosing terranes are all essentially similar to those during gold mineralization at least in their relative position to each other thus interpretation of geological maps and cross sections and three dimensional models can be used to accurately simulate the physical conditions that existed at the time of ore deposition it is particularly significant that the deposits are commonly related to repetitive and predictable geometries such as structural heterogeneities within or adjacent to first order structures around rigid granitoid bodies or in specific locked up fold thrust structures importantly the two giant greenstone hosted goldfields kalgoorlie and timmins show a remarkably similar geometry at the regional scale computer based stress mapping and gis based prospectivity mapping are two computer based quantitative methodologies that can utilize and take advantage of the late timing aspect of this deposit type to provide important geological aids in exploration both in broad regions and more localized goldfields both require an accurate and consistent solid geology map stress mapping requires knowledge of the far field stresses during mineralization and the empirical prospectivity mapping requires data from a significant number of known deposits in the terrane the kalgoorlie terrane in the yilgarn block meets these criteria and illustrates the potential of these methodologies in the exploration for orogenic gold deposits low minimum stress anomalies interpreted to represent dilational zones during gold related deformation coincide well with the positions of known goldfields rather than individual gold deposits in the terrane and there are additional as yet unexplained anomalies the prospectivity analysis confirms that predictable and repetitive factors controlling the siting of deposits are i proximity to and orientation and curvature of granitoid greenstone contacts ii proximity to segments of crustal faults which strike in a preferred direction iii proximity to specific lithological contacts which have similar preferred strike iv proximity to anticlinal structures and v the presence of preferred reactive host rocks e g dolerite the prospectivity map defines a series of anomalous areas which broadly conform to those of the stress map 78 correspondence the most prospective category on this map covers less than 0 3 of the greenstone belts and yet hosts 16 of the known deposits which have produced 80 of known gold thus it discriminates in favour of the larger economically more attractive deposits in the terrane the successful application of stress mapping and prospectivity mapping to geology based exploration for orogenic gold deposits indicates that more quantitative analysis of geological map data is a profitable line of research the computer based nature of these methodologies is ideal for the production of an ultimate integrated deposit target map which can be compared to other more conventional targeting parameters such as geophysical and geochemical anomalies such an integrated strategy appears the way forward in the increasingly difficult task of cost effective global exploration for orogenic gold deposits in poorly exposed terranes