on the shape and velocity of fluid filled fractures in the earth summary in this paper i present a model to estimate the shape and velocity of slowly ascending buoyancy driven fluid filled fractures the model considers elastic deformation linear fracture mechanics and fluid flow an attempt is made to incorporate the effect ofthe 2 d fluid pattern on the viscous pressure drop most other models assume that the viscous pressure drop can be approximated by flow through a channel with constant width although the form of the fracture is known to deviate from such a simple shape the 2 d flow in my model has an important consequence for the mechanism of buoyancy driven fracture propagation it predicts a large pressure gradient at the tail of the propagating fracture indicating that the tail of the fracture is most important in hindering the fracture propagation a singularity at the tail of the fracture can be avoided when a small amount of fluid trails in the channel left behind the propagating fracture the trailing and decoupling of fluids at the tail seems to be accompanied by small flow and shape instabilities which is indicated by the jerky movement of the tail observed in propagating air filled fractures in solidified gelatine and by numerical boundary element solutions of the coupled flow deformation fracturing problem by comparing the predictions for propagation velocities with laboratory observations of buoyancy driven fracture propagation in gelatine i derive a non dimensional effective thickness at the tail of the fracture for which trailing of fluids may occur the model is applied to earth relevant problems such as oil and water filled fractures in pressurized sediments or magma filled dykes in the lithosphere which are discussed in the paper

magma extraction from the mantle wedge at convergent margins through dikes a parametric sensitivity analysis 1 we address under what conditions a magma generated by partial melting at 100 km depth in the mantle wedge above a subduction zone can reach the crust in dikes before stalling we also address under what conditions primitive basaltic magma mg 60 can be delivered from this depth to the crust we employ linear elastic fracture mechanics with magma solidification theory and perform a parametric sensitivity analysis all dikes are initiated at a depth of 100 km in the thermal core of the wedge and the moho is fixed at 35 km depth we consider a range of melt solidus temperatures 800 1100c viscosities 10 100 pa s and densities 2400 2700 kg m 3 we also consider a range of host rock fracture toughness values 50 300 mpa m 1 2 and dike lengths 2 5 km and two thermal structures for the mantle wedge 1260 and 1400c at 100 km depth and 760 and 900c at 35 km depth for the given parameter space many dikes can reach the moho in less than a few hundred hours well within the time constraints provided by u series isotope disequilibria studies increasing the temperature in the mantle wedge or increasing the dike length allows additional dikes to propagate to the moho we conclude that some dikes with vertical lengths near their critical lengths and relatively high solidus temperatures will stall in the mantle before reaching the moho and these may be returned by corner flow to depths where they can melt under hydrous conditions thus a chemical signature in arc lavas suggesting partial melting of slab basalts may be partly influenced by these recycled dikes alternatively dikes with lengths well above their critical lengths can easily deliver primitive magmas to the crust particularly if the mantle wedge is relatively hot dike transport remains a viable primary mechanism of magma ascent in convergent tectonic settings but the potential for less rapid mechanisms making an important contribution increases as the mantle temperature at the moho approaches the solidus temperature of the magma

seismic and structural characterization of fluid escape pipes using 3d and partial stack seismic from the loyal field uk a multiphase and repeated intrusive mechanism acknowledgements we thank an anonymous reviewer for the several constructive comments the seismic interpretation and image processing was carried out in the seislab facility at the university of aberdeen sponsored by bg bp and chevron seismic imaging analysis was performed using geoteric ffa and analysis of seismic amplitudes was performed in petrel 2016 schlumberger we would like to thank the tuscany phd regional program and the erasmus exchange for funding the aberdeen permanence of one of us d m gazprom for supporting a j phd bp for the release of the loyal field seismic dataset utilized in this research paper and also n vanden beukel bp and m gorling bp and his colleagues for their assistance

the stabilization of self organised leucogranite networks implications for melt segregation and far field melt transfer in the continental crust abstract net structured leucocratic vein systems are ubiquitous in high grade and migmatite terrains and are interpreted to represent interconnected former magma bearing structures that accommodated the migration of granitic magmas out of their anatectic source regions net structured leucogranite vein systems in high grade gneisses from the damara belt in namibia describe a size based hierarchy and spatial distribution that indicates self organisation of the magma transfer system the internal geometry and connectivity of the net structured leucogranites are controlled by variations in melt pressure whereas regional strains control the orientation of the leucogranites foliation parallel stromatic and shear band hosted granite veins accommodate transport at relatively low melt volumes and account for the metre scale segregation of melt close to the anatectic source during times of higher melt supply the formation of extensional fractures that cross cut stromatic and shear band hosted melt sheets at high angles is promoted extensional fracturing and associated dilatancy create gradients in magmastatic head that cause melt to be drained from pervasive stromatic and shear band hosted vein networks into spaced dilatant fractures this has the potential to initiate buoyancy driven far field melt transfer along suitably orientated extensional fractures as vertical dykes or self propagating hydrofractures our results suggest that melt transfer systems are able to adjust to variations in melt supply by forming conduits that accommodate varying melt volumes and will tend to develop towards a state of self organisation

continental scale salt tectonics on mars and the origin of valles marineris and associated outflow channels a synthesis of deformation patterns within and around the thaumasia plateau mars points to a new interpretation for regional deformation and the origin of valles marineris and associated outflow channels the morphology of the thaumasia plateau is typical of thin skinned deformation akin to a mega slide in which extensional deformation in syria planum and noctis labyrinthus connects via lateral zones of transtension and strike slip claritas fossae and valles marineris to a broad zone of compressional uplift and shortening defined by truncated craters and thrust faults along the coprates rise and thaumasia highlands however the low regional slope 1 results in gravitational body forces that are too small to deform the basaltic lava flows conventionally thought to compose the flanks of the tharsis volcanic province instead we conclude that geothermal heating and topographic loading of extensive buried deposits of salts and or mixtures of salts ice and basaltic debris would allow for weak detachments and large scale gravity spreading we propose that the generally linear chasmata of valles marineris reflect extension collapse and excavation along fractures radial to tharsis either forming or reactivated as part of one lateral margin of the thaumasia gravity spreading system the other dextral lateral margin is a massive splay of extensional faults forming the claritas fossae which resembles a trailing extensional imbricate fan the compressional mountain belt defined by the coprates rise and thaumasia highlands forms the toe of the mega slide topographic observations and previous structural analyses reveal evidence for a failed volcanic plume below syria planum that could have provided further thermal energy and topographic potential for initiating regional deformation either intrusively through inflation or extrusively through lava flow and or ash fall emplacement higher heat flow during noachian time or geothermal heating due to burial by tharsis derived volcanic rocks would have contributed to flow of salt deposits as well as formation of groundwater from melting ice and dewatering of hydrous salts we further propose that connection of overpressured groundwater from aquifers near the base of the detachment through the cryosphere to the martian surface created the outflow channels of echus coprates and juventae chasmata at relatively uniform source elevations along the northern margin of the mega slide where regional groundwater flow would have been directed toward the surface our hypothesis provides a unifying framework to explain perplexing relationships between the rise of the tharsis volcanic province deformation of the thaumasia plateau and the formation of valles marineris and associated outflow channels

ductile fractures and magma migration from source mechanisms proposed to explain effi cient melt transport away from hot ductile source regions are problematic brittle elastic fracturing is a well known mechanism that allows fast magma migration as dikes through cold crust ductile fractures have been propo sed as an alternative for ductile environments where brittle elastic diking is inhibited ductile fracturing results from rock creep and growth of microscale voids that become interconnected leading to rock failure in this paper we present observations and numerical models supporting the hypothesis that ductile fracture controls early steps in magma migration we postulate that once developed ductile fracture dikes may reach a critical length where magma stresses at dike tips overcome fracture toughness and lead to brittle elastic diking which subsequently controls magma migration

structural analysis of a sampling of tesserae implications for venus geodynamics abstract understanding the nature and timing of tessera formation is fundamental to venus tectonic and geodynamic models tesserae are commonly considered to exhibit complex deformation histories to represent the oldest global stratigraphic unit to have formed during a global phase of tesserization and to require weak lithosphere for their formation although these characterizations of tesserae are gaining widespread mention in the literature they are essentially unsupported by an assemblage of data yet they figure prominently in the way we frame and address questions about venus tectonics thus they are quickly becoming accepted paradigms despite a lack of scientific foundation open discussion of these hypotheses or assumptions is crucial because of their implications concerning geodynamic models of the surface evolution and planetary dynamics of venus we examined tessera terrain in ishtar terra crustal plateaus and as inliers within the plains using high resolution magellan radar imagery we describe several types of tessera terrain that record a wide range of structural histories fold and s c terrains are found only in ishtar terra ribbon lava flow and basin and dome terrains reside within the interior of crustal plateaus whereas folded ribbon terrain and extended folded terrain comprise margins of crustal plateaus inliers are divisible into fracture dominated and graben dominated tesserae although some inliers host early contractional fabrics the range of deformation histories recorded by the various tessera types indicates that tesserae should not be considered a single map unit tessera deformation records local to regional surface strain patterns and reflects near surface rheology at the time of deformation progressive deformation fabrics in some tessera terrain record changes in shallow crustal rheology through time thus structural analysis of individual tessera types and tessera provinces will allow us to better understand the tectonic processes responsible for tessera formation tesserae likely formed in several tectonic environments including 1 as a result of subsurface flow in ishtar terra 2 as sequences of surface layer extension and contraction in crustal plateaus 3 as flooded crustal plateaus and 4 as densely fractured surface layers fractured as a result of corona and chasma formation

double vein ore bearing vs ore free structures in the xitian ore field south china implications for fluid evolution and mineral exploration abstract two types of spatially and temporally associated hydrothermal veins ore bearing and ore free occur as double vein structures in the xitian ore field south china here we investigated the genetic relationship material provenance fluid properties and prospecting significance of the co developed veins in the granite related fault controlled vein type xiangdong w goudanlan w chaling pb zn and xinggao fluorite deposits of the xitian ore field focusing on two common minerals quartz and zircon petrographic cl microthermometric and h o isotopic analyses of stage i and stage ii quartz in both the ore bearing and ore free veins of these deposits document two episodes of hydrothermal activity the first episode involved a high temperature up to 370 high salinity up to 28 nacl equiv high 18o 2 1 to 7 2 and high 2h 82 to 54 mean 68 n 13 fluid that corresponds to the main magmatic hydrothermal stage of the deposits and the second episode involved a low temperature down to 90 low salinity down to 1 nacl equiv low 18o 8 8 to 5 3 and low 2h 83 to 60 mean 73 n 6 fluid that represents an accessary mineralization stage variations in fluid inclusion types and compositions indicate that the processes of fluid mixing boiling cooling and degassing played important roles in mineralization and that magmatic fluid became less important than meteoric water along during the formation of ore free veins la icpms u pb dating reveals totally different age populations for these veins zircons from the ore bearing quartz veins yield older and more variable ages pre devonian 400 ma n 68 that are similar to that of the local basement sedimentary rocks whereas zircons from the ore free quartz veins yield younger and less variable ages triassic 250 200 ma n 31 and jurassic 170 140 ma n 13 that are identical to those of the host granite zircons in addition the ore free veins show lower temperatures salinities and total rees as recorded by quartz but greater alteration as reflected by cl and trace element features of hydrothermally altered zircons and wider vein size than the ore bearing veins we propose a mineral water separation model to explain the formation of the double vein structures in this model ore bearing veins and ore free veins formed in succession during faulting events ore bearing veins formed first in the smaller fractures and were the dominant sites of mineralization whereas the ore free veins served as dissipating passageways for residual water and silica in larger fractures we suggest that the mineral water separation model has good application prospects in mineral exploration and that zircons can be applied as an indicative mineral to differentiate the mineralization potential of different types of quartz veins

wrinkle ridges stress domains and kinematics of venusian plains wrinkle ridges are nearly ubiquitous landforms on the plains of venus by analogy with similar structures on other planets venusian wrinkle ridges are inferred to trend normal to the direction of maximum principal compression in the crust an inference that is verified by geometrical relationships with positive and negative relief features on venus because plains are the dominant terrain on venus wrinkle ridges provide an excellent opportunity to determine the orientations of shallow crustal principal stress trajectories over most of the planet in most places there are two or more sets of wrinkle ridges and commonly one of these persists over a very large area defining a regional stress domain intersection relationships indicate that these domains differ in age

an experimental deformation study of partially molten amphibolite application to low melt fraction segregation an amphibolite has been experimentally deformed under subsolidus and partially molten conditions to evaluate the influence of melt on the mechanical behavior of a natural mafic rock and to assess low melt fraction segregation processes experiments were performed under fluid absent conditions at 1 8 gpa between temperatures of 650 c and 1000 c these conditions are similar to those of thickened lower continental crust or intermediate depths in subducting oceanic lithosphere at 850 c melt is granitoid in composition and its viscosity is that of wet granite 103 105 pa s the results can be summarized as follows 1 under subsolidus conditions tests at 650 c and 750 c the amphibolite is macroscopically ductile and deformation is homogeneously distributed throughout the sample 2 at near solidus conditions 800 c 0 5 vol melt fractures 1 10 m in width displace hornblende and plagioclase grains and melt formed in situ is found in some of these cracks the formation of a ductile shear zone in one sample is attributed to the presence of very fine grained reaction products from combined dehydration hydration reactions that involve plagioclase the dehydration reaction products have apparently changed the deformation mechanism and even overrode the melt embrittlement process trapping melt in pockets of lower strain with higher melt fractions 10 15 vol broad melt bearing shear zones form and grains within these zones are brittlely deformed 3 at 20 vol melt 1000 c additional weakening occurs but fractures are not observed and the overall deformation is by viscous flow the results show that at low melt fractions 15 vol fluid absent melting reactions can induce fracture in previously ductile rocks this suggests that the percentage at which melt may escape by fracture is lower than the theoretical critical melt fraction or cmf of 26 40 vol in general estimating the fraction of melt at the onset of segregation cannot be predicted by the cmf melt segregation models have to be adapted to variations in such major factors as pressure temperature type of melting reaction rate of melting and strain rate such models will greatly assist our understanding of continental growth and evolution during orogenesis

sites on the landscape paleoenvironmental context of late pleistocene archaeological sites from the lake victoria basin equatorial east africa open air archaeological sites record only a small fraction of the behavioral traces of mobile forager populations whereas caves and rockshelters were often occupied at least in part for protection from the elements the reasons why human foragers occupied other places on the landscape however briefly are varied and not always readily recoverable we develop a framework for interpreting human use of the landscape and modeling occupation of open air sites using the archaeological and paleoenvironmental record of middle stone age msa sites from rusinga and mfangano islands located near the eastern margin of lake victoria paleoenvironmental reconstructions using fossil faunas suggest an arid grassland setting unlike the present paleoecological modeling of the habitats of extant and extinct bovids combined with gis based reconstructions of lake level change indicate that human occupation of these sites coincided with substantial declines in the level of lake victoria during this time both rusinga and mfangano would have been connected to the mainland and represented local topographic highs within an extensive grassland geological ecological and ethnobotanical observations suggest that these topographic high points would likely have been important sources of stone raw material fresh water and a variety of plant resources for food fuel and other purposes in contrast the grassy lowland plains were probably exploited primarily as a source of large game which included numerous species of large gregarious grazers several of which may have followed now extinct migration routes

microseismicity linked to gas migration and leakage on the western svalbard shelf the continental margin off prins karls forland western svalbard is characterized by widespread natural gas seepage into the water column at and upslope of the gas hydrate stability zone we deployed an ocean bottom seismometer integrated into the masox monitoring arctic seafloor ocean exchange automated seabed observatory at the pinch out of this zone at 389 m water depth to investigate passive seismicity over a continuous 297 day period from 13 october 2010 an automated triggering algorithm was applied to detect over 220 000 short duration events sdes defined as having a duration of less than 1 s the analysis reveals two different types of sdes each with a distinctive characteristic seismic signature we infer that the first type consists of vocal signals generated by moving mammals likely finback whales the second type corresponds to signals with a source within a few hundred meters of the seismometer either due east or west that vary on short tens of days and seasonal time scales based on evidence of prevalent seafloor seepage and subseafloor gas accumulations we hypothesize that the second type of sdes is related to subseafloor fluid migration and gas seepage furthermore we postulate that the observed temporal variations in microseismicity are driven by transient fluid release and due to the dynamics of thermally forced seasonal gas hydrate decomposition our analysis presents a novel technique for monitoring the duration intensity and periodicity of fluid migration and seepage at the seabed and can help elucidate the environmental controls on gas hydrate decomposition and release

fluid rock interaction in crustal shear zones a directed percolation approach the interplay among mechnical chemical hydrological and thermal processes in the evolution of crustal shear zones makes an analytical approach to their study difficult as an alternative a stochastic description using percolation theory is used to gain insight into reaction softening and volume loss in ductfle deformation zones directed percolation is preferred to ordinary percolation as a model because in common with natural shear zones directed percolation clusters have high length width ratios and anisotropic permeability in addition transport along the cluster length for p p c is linear with time modeling fluid advection the process of strain softening is modeled by subjecting a critical cluster to variable amounts of simple shear resulting in a geometry similar to that of natural shear zones a stochastic model for the evolution of porosity with time on a directed lattice displays fixed point behavior for a range of pore collapse rates the observation that natural shear zones from a variety of tectonic settings display mean volume losses of 60 70 suggests that the system naturally evolves toward a critical state this can be explained by assuming that pore collapse is regulated by the tendency for fluid pressure to remain close to lithostatic volume loss in crustal shear zones appears to be an example of hydromechanical chemical self organized critical behavior

a global survey of possible subduction sites on venus abstract about 10 000 km of trenches in chasmata and coronae have been identified as possible sites of retrograde subduction on venus all the sites have narrow deep trenches elongate along strike with arcuate planforms ridge trench outer rise topographic profiles typical of terrestrial subduction zones large outer rise curvatures 10 7 m 1 fractures parallel to the strike of the trench on the outer trench wall and outer rise and no cross strike fractures across the trench both the northern and southern margins of latona corona are possible subduction sites identification of a major graben between the two principal outer ridges in southern latona corona is evidence of back arc extension in the corona the amount of extension is estimated to be more than 2 11 km the moment exerted by the ridges of southern latona corona is insufficient to bend the lithosphere into the observed outer rise shape a negatively buoyant subducted or underthrust slab is needed depending on the unknown trench migration rate lithospheric subduction can make a significant contribution to mantle cooling on venus venusian chasmata could have a dual character they may be propagating rifts near major volcanic rises and subduction trenches far from the rises in the lowlands subduction and rifting may occur in close proximity on venus unlike on earth rifting induced by hotspots on venus may be necessary to break the lithosphere and allow subduction to occur such a process could result in gradual lithospheric subduction or global episodic overturn of the lithosphere

magma accumulation and segregation during regional scale folding the holland s dome granite injection complex damara belt namibia abstract the regional scale upright fold of the holland s dome in the damara belt of central namibia contains a kilometre scale network of intrusive highly fractionated uraniferous leucogranites three broadly orthogonal and intersecting sets of leucogranite sheets that intruded parallel and at right angles to the axial plane of the first order fold can be distinguished the granites are internally sheeted and illustrate the growth of the injection complex through the successive addition of thousands of smaller magma batches spatial and timing relationships point to a stepwise evolution of the injection complex early dilatancy driven segregation and accumulation of granitic magmas in the core of the fold above a basal detachment was followed by compaction driven segregation of a melt phase during fold tightening the intersecting leucogranite sets provide a suitably organized permeability structure for melt segregation while the successive injection of magma batches ensures compatibility between regional strain rates during folding and the rates of magma segregation the three dimensional network of melt bearing structures further assisted regional shortening past the lock up of the fold the holland s dome injection complex illustrates the geometric complexity of magma transfer pathways and the significance of regional scale folding for the accumulation segregation and fractionation of granitic magmas in suprasolidus crust

granitic magma transport by fracture propagation granitic magmas commonly ascend tens of kilometres from their source terranes to upper crustal emplacement levels or to the earth s surface apart from its obvious bearing on the interpretation of the geology and geochemistry of felsic igneous rocks the magma ascent mechanism critically affects any modelling of the metamorphic evolution of the upper lithosphere as well as its rheology we propose that in general granitic magmas ascend via propagating fractures as dykes in extremely short time periods long distance diapiric transport of granitoid magmas through crustal sections is not viable on thermal or mechanical grounds and there is an apparent total absence of field evidence for diapiric rise of such magmas even in supposed type localities neither the shapes nor internal or external characteristics of granitic plutons necessarily reveal anything about the transport of their precursor magmas these are purely arrival phenomena dictated by local structure kinematics and stress states based on existing numerical treatments of the problem we show that granitic magmas are apparently sufficiently inviscid to travel through fractures to high crustal levels without suffering thermal death by freezing for example a 2000 km3 granitic batholith can be inflated by a 1 km 3m 20 km deep dyke system in less than 900 years the model proposed has numerous far reaching petrological and rheological consequences some of which are outlined

the paragenetic history of a marinoan cap carbonate abstract carbonates that cap late neoproterozoic glacial diamictites contain large atypical tepee like structures that have been interpreted as evidence for seafloor dolomite cementation the nuccaleena formation in the adelaide fold thrust belt is the stratotype marinoan age cap dolostone at parachilna gorge the nuccaleena formation contains paired cogenetic tepees and growth faults that demonstrate these structures grew well below the sediment water interface these tepees and their associated sheet veins were the result of pore fluid overpressure driven by expansive early diagenetic dolomite cementation fluid overpressure resulted in two forces 1 macroscale lateral compression that overcame the lithostatic weight to buckle sediments upward forming the tepee structure and 2 macro to microscale extension that overcame the lithologic tensile strength to form sheet veins the physical processes of fluid overpressure within the parachilna gorge cap carbonate are in many ways analogous to the fluid overpressure dynamics in mud volcanoes and were sufficiently high to generate veins extending 17 m below the cap carbonate overpressure driven dewatering removed pore fluid clay and organic matter from the dolomicrospar matrix into dewatering structures and fabrics the latter are hierarchically arranged from small linear clay fabrics to clay lined dendritic arrays to clay choked curvilinear veins to large sheet veins packets of red clay laminae were deposited when pore fluids vented to the sea floor the early diagenetic chemical evolution of the parachilna gorge cap carbonate was equally dynamic in which an initial reducing trend was supplanted by an oxidation trend brought about by tepee sheet vein deformation once established the sheet vein network also reinvigorated diffusional processes that likely enhanced early diagenetic cementation of the cap dolostone the detailed paragenetic history also suggests that calcite not dolomite was the initial carbonate precipitated from the post glacial adelaidean sea the nuccaleena formation exhibits the dolomicrospar isotopic signatures and tepee like structures that distinguish marinoan age cap carbonates at parachilna gorge these typical features are a product of early diagenesis suggesting that other marinoan age cap carbonates may also be early diagenetic in origin

late stage anhydrite gypsum siderite dolomite calcite assemblages record the transition from a deep to a shallow hydrothermal system in the schwarzwald mining district sw germany abstract the majority of hydrothermal vein systems of economic interest occur at relatively shallow crustal levels although many of them formed at significantly greater depths their present position is a consequence of uplift and erosion although many aspects of their formation are well constrained the temporal chemical evolution of such systems during uplift and erosion is still poorly understood these vein minerals comprise calcite dolomite ankerite siderite magnesite anhydrite and gypsum forming the last gangue assemblages in jurassic and tertiary sulphide fluorite quartz barite veins of the schwarzwald mining district sw germany mineral textures of samples from nine localities reveal that in these sequences mineral precipitation follows a recurring pattern early calcite is followed by anhydrite or gypsum siderite and or dolomite this succession may repeat up to three times in situ la icp ms u pb age dating of 15 carbonates from three subsequent generations of the late stage vein assemblage yield robust ages between 20 and 0 6 ma each mineral sequence forms in a distinctive period of about 2 5 ma these ages clearly relate these late stage mineral phases to the youngest geological episode of the schwarzwald which is associated with the cenozoic rhine graben rifting and basement uplift based on thermodynamic modelling the formation of the observed mineral assemblages required an deeply sourced mg fe and so4 rich fluid b which was episodically mixed with a shallow crustal hco3 rich fluid a as a consequence of fluid mixing concentrations of mg fe and so4 temporarily increased and initiated the formation of the observed sulphate carbonate mineral sequences this discontinuous large scale vertical fluid mixing was presumably directly related to episodes of active tectonics associated with the cenozoic strike slip regime of the upper rhine graben analogously episodic fluid mixing is a major key in the formation of older jurassic to early tertiary pb zn fluorite quartz barite assemblages in the same specific vein systems albeit involving different fluid compositions late stage hydrothermal 20 70 c vein assemblages reported in this study record the transition from deep 2 km to very shallow 0 1 km crustal conditions as a consequence of successive uplift increasing proportions of shallower and cooler 50 70 c fluids could take part in such mixing processes associated changes in the fluid composition caused the vein mineralogy to change from sulphide quartz fluorite barite to calcite anhydrite gypsum siderite dolomite as the system passively ascended closer to the surface

microfracturing and microporosity in shales shales are ubiquitous rocks in sedimentary basins where their low permeability makes them efficient seals for conventional oil and gas reservoirs and underground waste storage repositories waste waters co2 nuclear fuels moreover when they contain organic matter they form source rocks for hydrocarbons that may escape towards a more porous reservoir during burial a process referred to as primary migration and when the hydrocarbons cannot escape these rocks can be exploited as oil or shale gas reservoirs while the presence of fractures at the outcrop scale has been described the existence of fractures at smaller scales their link with microporosity the mechanisms that created them their persistence over geological times and their effect on the petrophysical properties of shales represent scientific challenges for which drillings in various sedimentary basins over the past decades may hold timely key data here we review and synthetize the current knowledge on how microfractures and micropores in shales can be imaged and characterized and how they control their anisotropic mechanical properties and permeability one question is whether such microfractures when observed in outcrops or in drilled core samples extracted from boreholes are related to decompaction and do not exist at depth another question is whether veins observed in shales represent microfractures that were open long enough to have acted as flow paths across the formation the mechanisms of microfracture development are described some have an internal origin fracturing by maturation of organic matter dehydration of clays while others are caused by external factors tectonic loading importantly the amount of microfracturing in shales is shown to depend strongly on the content in 1 organic matter and 2 strong minerals the nucleation of microfractures depends on the existence of mechanical heterogeneities down to the nanometer scale their propagation and linkage to create a percolating network will depend on the presence of heterogeneities at the meso to macro scales such percolating microfracture networks could control both the long term sealing capabilities of cap rocks and the further propagation of hydraulic fracturing cracks finally possible areas of research for describing the mechanism of microfracture formation in greater detail and how this impacts the transport and mechanical properties of shales are also discussed

regional topographic rises on venus geology of western eistla regio and comparison to beta regio and atla regio regional topographic rises in the equatorial region of venus western eistla regio beta regio and atla regio form a unique class of structures which on the basis of geologic mapping and geophysical data are interpreted to be sites of mantle upwelling or plumes the geologically least complex of these regions is western eistla regio an elongated domical rise that contains the volcanoes sif and gula montes sif mons is a large volcano diameter 100 km whose local stratigraphy indicates that it has undergone a history marked by initial radial fracturing widespread outpouring of lava additional fracturing along its northern flank and extrusion of lava that is either extremely fluid or was erupted at a high rate of effusion sif contains a 40 km diameter summit caldera within which are nested smaller calderas and pit structures theoretical considerations suggest multiple levels of magma emplacement within the volcano with the primary reservoir residing in the crust below the edifice gula mons is made up of multiple volcanic source regions and is associated with guor linea a zone of lithospheric stretching faulting and rifting the geologically complex highland of beta regio is a tectonic junction with three arms of devana chasma intersecting at theia mons magellan data reveal 1 the details of rifting and volcanism concentrated at theia mons 2 that rhea mons is highly deformed and possesses characteristics of tesserae and whose origin as a volcano is placed in question 3 that tesserae are a major unit arrayed in a pattern around theia from east to west starting on the eastern flank of beta and continuing westward to the northern part of asteria regio 4 that the northern periphery of the highland contains coronae connected by fractures forming chains and 5 the presence of a deformed crater within devana chasma split and extended in an east west direction indicating that part of the rift has undergone as much as 20 30 extension like beta atla regio is a complex area of converging rifts five separate arms that are centered on the volcano ozza mons the magellan images show that volcanism and rifting have occurred contemporaneously at ozza with lava flows both covering and being crosscut by faults a second volcano maat mons contributes to the infilling of the rift dali chasma and does not appear to be deformed by faulting two of the rifts at atla contain chains of coronae suggesting that upwelling along linear zones of extension has occurred beta atla and western eistla are similar in that they are broad regional rises are associated with major rift zones have large apparent depths of isostatic compensation and contain large volcanoes their associations with features mapped as coronae and tesserae are quite variable suggesting that the detailed characteristics of the individual highlands are most likely linked to each areas local geologic environment

mylitta fluctus venus rift related centralized volcanism and the emplacement of large volume flow units mylitta fluctus is a volcanic flow field that covers approximately 300 000 km2 in southern lavinia planitia the flows are typically radar bright with uniform surface textures central channels are common maximum flow lengths range from 400 to 1000 km flow widths range 30 100 km in the medial and distal portions of the flow field the total volume of the flow field based on estimates of flow thickness is of the order of 2 104 km3 the flow field is composed of six smaller flow fields that are interpreted to represent major eruption events in the evolution of mylitta an asymmetric shield volcano 200 km in radius with a central caldera has been identified as the single major source it is located along a possible rift zone at the northern edge of lada terra no evidence for fissure fed eruptions is observed although eruptions may have occurred along fissures within the rift before the main vent centralized mylitta is similar in scale to many terrestrial flood basalt provinces although it is lower in total estimated volume and is distinguished by the presence of a central source edifice the origin of mylitta is proposed to be linked to regional extension and possible hotspot activity in a manner similar to that suggested for the origin of terrestrial flood basalts the lower apparent volume of mylitta relative to terrestrial flood basalts suggests that the amount of material upwelling from the mantle may have been lower in this case or that the formation of the flow field has not yet finished detailed studies of the morphology and distribution of flow fields similar to mylitta should yield insight into the variation and evolution of hotspot related volcanism and the formation of possible flood basalts on venus

dating volcanism and rifting on venus using impact crater densities although the distribution of impact craters on venus is indistinguishable from a completely spatially random population on the basis of crater data alone the addition of geologic information indicates that the craters are not random with respect to geology areas of low crater density correlate with concentrations of tectonic and volcanic features as well as high proportions of faulted and embayed craters high resolution mapping of large volcanoes flood type lava flow fields rifts and coronae shows that these features have low crater densities that are unlikely to have occurred merely by chance because these features also appear young based on stratigraphic data difference in age is the most likely reason for differences in crater density between geologically defined terrains crater densities may be used to determine reliable relative ages for sufficiently large geologic terrains containing at least eight to nine craters as long as the mapping criteria are independent of impact crater locations this result provides a tool to begin identifying and dating major geologic provinces and to begin developing a geologic history of venus using a simple data model and calculating crater ages relative to an assumed global mean age of 300 ma we show that the ages of large volcanoes 72 45 ma flow fields 128 91 ma rifts 130 145 ma and coronae 120 115 ma are all substantially younger than the mean plains age and probably represent ongoing volcanic and tectonic activity rather than the end of a brief global resurfacing event with a mean age of 300 ma

a new view of the stratigraphic history of venus abstract studies of venus using magellan data have so far generated two views about the way venus has evolved on the one hand venus has been suggested to have had a history in which there was a series of epochs each represented by a different volcanic or tectonic process on a global scale basilevsky and head 1995 j w head et al 1996 lunar planet sci 27th 525 526 p t basilevsky et al 1997 in venus ii univ of arizona press tucson this we define as a directional history on the other hand there is evidence to suggest that coronae rifts wrinkle ridges small and large edifices and large flow fields have each formed throughout the portion of venus history revealed by presently exposed rock units we propose that the plains have been built up by lavas erupted in a number of different styles each occurring throughout the history represented by the exposed stratigraphy of the planet dates derived from crater counts accumulated from the combined area of specific types of feature such as coronae e g m h price et al 1996 j geophys res 101 4657 4672 must be interpreted with care as the method is based upon the assumption that features of like morphology have the same age detailed studies from full resolution magellan data indicate that venus has had a complex history in which most geologic processes have operated in a nondirectional fashion to a greater or lesser extent throughout the planet s history

hillslope evolution by nonlinear creep and landsliding an experimental study landscape evolution models are widely used to explore links between tectonics climate and hillslope morphology yet mecha nisms of hillslope erosion remain poorly understood here we use a laboratory hillslope of granular material to experimentally test how creep and landsliding contribute to hillslope erosion in our experimental hillslope disturbance driven sediment transport rates increase nonlinearly with slope due to dilation driven gran ular creep and become increasingly episodic at steep slope angles as creep gives way to periodic landsliding we use spectral analysis to quantify the variability of sediment flux and estimate the slope dependent transition from creep to landsliding the power spec trum of sediment flux steepens with hillslope gradient exhibiting fractal 1 f scaling just below the creep landsliding transition by evolving the experimental hillslope under fixed base level boundary conditions we demonstrate how disturbance driven transport gen erates hillslope convexity the transient evolution is consistent with numerical predictions derived from a recently proposed nonlinear transport model as initially steep hillslopes are lowered rapidly by landsliding before slopes decay slowly by creep dominated transport

new experiment to model self organized critical transport and accumulation of melt and hydrocarbons from their source rocks a new simple and easily reproducible experiment was designed to simulate the production accumulation and transport of melt within rock the transport was found to be of the self organized critical type the emergence of self organized criticality is explained by the availability of hydrofracture propagation as a rapid or ballistic transport mechanism this mechanism also serves as a mechanism for stepwise accumulation these findings are confirmed by a numerical model which shows the emergence of self organized critical behavior when darcian transport cannot accommodate transport and the dormant transport mechanism of hydrofracture propagation is activated ballistic and self organized critical transport may play a significant role in the transport and accumulation of geological fluids such as melt and hydrocarbons this conclusion has a profound impact on the modeling of many transport processes in geology e g accumulation of melt oil and gas

the geomorphology and evolution of hecate chasma venus abstract hecate chasma is a discontinuous trough and fracture system extending from atla regio through asteria regio a detailed examination of hecate chasma using magellan image and altimetry data reveals morphologic features such as normal faults including graben compressional ridges a variety of volcanic edifices and coronae there is no evidence for a systematic age progression among coronae in chains thus a single stationary thermal anomaly underlying a moving lithosphere is an unlikely origin for the chains a comparison of the observed features with two possible models of evolution subduction delamination or lithospheric extension with limited rifting shows some similarities between the predicted morphologies of each model and hecate chasma the dominance of features related to upwelling and extensional tectonism strongly favor an extensional origin for this zone the corona chain may have been formed by upwellings related to rayleigh taylor instabilities similar to those observed at terrestrial mid ocean ridges but in an environment of limited spreading

the generation and shape of feeder dykes from mantle sources the criterion for fracture propagation suggests that the primary magma generated in the mantle ascends in a stepwise fashion initially a pulse of magma forms a dyke above the mantle source and ascends for some distance whereafter it stops a subsequent pulse of magma joins the first one and the ascent of the accumulating body of magma continues for some distance each time a new pulse of magma reaches the body of magma situated in the top of the dyke in this manner a dyke is formed that may extend either partly or the whole way from the mantle source to the surface this model may explain the large pressures up to 2000 bars that prevail during basaltic eruptions the calculated shape of the ascending pulses of magma suggests that their heights and widths are similar

trace element geochemistry of sphalerite in contrasting hydrothermal fluid systems of the freiberg district germany insights from la icp ms analysis near infrared light microthermometry of sphalerite hosted fluid inclusions and sulfur isotope geochemistry the historic silver mining district of freiberg germany comprises hydrothermal vein style mineralization of permian and cretaceous age we compare sphalerite compositions with associated ore forming fluids and constrain the behavior of critical metals such as in ge and ga in contrasting hydrothermal environments fluid inclusion studies reveal that the permian veins formed due to boiling and cooling of a low salinity 0 to 6 eq w nacl magmatic hydrothermal fluid at 350 to 230 c in contrast cretaceous veins formed by mixing of highly saline 17 to 24 eq w nacl cacl2 and variable na na ca ratios brines at low temperatures 120 c sulfides of the permian ore stage have a narrow range of 34svcdt from 2 3 to 0 9 while the sulfides of the cretaceous stage have a large scatter and significantly more negative 34svcdt values 30 9 to 5 5 supporting the different nature of the hydrothermal systems contrasting fluid systems and ore forming mechanisms correspond to markedly different trace element systematics in sphalerite permian sphalerite is significantly enriched in in up to 2500 g g in relative to two sphalerite generations of cretaceous veins the latter have higher ge up to 2700 g g ge and ga up to 1000 g g ga concentrations the observed trace element systematics of different sphalerite generations imply that in is enriched in high temperature low to intermediate salinity fluids with a significant magmatic hydrothermal fluid component while ge and ga are more concentrated in low temperature high salinity crustal fluids with no obvious magmatic hydrothermal affiliation

the thermodynamics of deformed metamorphic rocks a review abstract the deformation of rocks is a disequilibrium and strongly non linear phenomenon with a number of interacting chemical thermal and microstructural processes operating simultaneously we review progress in this area over the past 30 years deforming chemically reacting systems are dissipative systems and hence are characterised by highly ordered structures that develop through cooperative processes once parameters such as critical strains strain rates fluid infiltration rates damage densities or temperatures are attained such criticality is the hallmark of deformed rocks at all length scales and is the basis for a diverse range of structures such as foliations and lineations produced by metamorphic differentiation rotation recrystallisation folding boudinage and micro to regional scale fracture systems criticality is identified with classical criticality and not self organised criticality the first and second laws of thermodynamics are used to show that such structural diversity arises from reaction diffusion deformation equations criticality of the system is associated with the stored energy becoming non convex and structures arise in order to minimise this non convex energy these structures are scale invariant and hence are characterised by fractal and minimal surface geometries thermodynamics is a powerful discipline to integrate seemingly unrelated processes in structural geology and produce an integrated approach to the subject that crosses all length scales

constraints on the thermal evolution of venus inferred from magellan data one interpretation of the magellan data suggests that the cratering record on venus was erased by a global resurfacing event or events the latest ending about 500 m y ago in this global resurfacing model the resurfacing was followed by minor volcanism and tectonism that has been concentrated primarily in the equatorial highland regions characterized by extensive fracture belts and rifts a thermal evolution model of venus that can explain these observations is one in which a deformable lithosphere capable of being incorporated in mantle circulations provides an almost stress free condition at the surface mantle convection with an almost stress free boundary at the surface cools the interior more efficiently rapid cooling decreases the rayleigh number of mantle convection below a transition value required for oscillatory convection and the vigor of convection diminishes as the mantle changes to a quasi steady circulation after about 500 m y ago

linking structures with the genesis and activity of mud volcanoes examples from emilia and marche northern apennines italy mud volcanism is known to be strictly linked to tectonic structures since they have the ability to trap hydrocarbon and other fluids and eventually induce them to migrate from a deep reservoir e g sited into an anticline core where large overpressures may be generated to the surface a central theme is how fluids can migrate upward and which is the role in this process of near structures i e structures directly controlling the mud volcano system and far structures i e faults located far away from the mud volcano system in this study we investigate the role of both types of structures in the genesis and evolution of mud volcanoes in particular we investigate six mud volcano case studies from the emilia romagna and marche pede apennine margin in italy through integration of fieldwork interpretation of available seismic reflection profiles and aerial photos the results of these analyses support an intimate link of the investigated mud volcanoes with anticline structures we discuss two different fluid migration settings particularly i mud volcanoes emplaced on outcropping anticlines and ii mud volcanoes located on top of buried structures discerning when fluids are likely to exploit anticline related fracture sets or secondary structures and porosity finally we speculate on how far structures via the seismic triggering may play a role in the occurrence of historical eruption of some of the investigated mud volcanoes

finding what is now not there anymore recognizing missing fluid and magma volumes although large volumes of magma and fluid move through the crust the sites where these volumes once resided are difficult to recognize we show that some foliation boudinage and shear band structures which are classically interpreted as resulting from externally imposed extension or shearing are better explained by the ductile collapse of magma or fluid filled voids experiments show that the shape of the collapse structures depends on the original shape of the void and the direction of confluence of its walls when its contents are drained this can lead to both symmetrical boudin like and asymmetrical shear band like structures the recognition of the true nature of these structures necessitates a reevaluation of strain measurements reported in the literature and the tectonic models based on them identifying the missing volumes clarifies the way magma makes granite plutons many cubic kilometers in volume in the upper crust

great lava flow fields on venus abstract we have analyzed the morphology dimensions radar backscatter topography sources and tectonic settings of lava flow fields and assemblages of fields on venus with total areas greater than 50 000 km 2 and we term these great flow fields great flow fields are typically several hundred thousand square kilometers in extent with lengths of several hundred kilometers and estimated volumes of 10 3 to 10 4 km 3 we identify five morphological types of flow field and draw a basic distinction between sheet like and digitate morphologies the sheet flow fields represent single massive eruptions from laterally extensive stretches of fissure onto near horizontal terrain while the digitate flows were formed by repeated eruptions we divide digitate fields into aprons fans and subparallel types and identify fields transitional between sheet and digitate morphologies an analysis of the backscatter coefficient of flow units within several of the great flow fields compared to airsar data of terrestrial lava flows suggests that most units have the surface roughness of smooth pavements to pahoehoe however many channel fed flow units with higher backscatter in the digitate fields have a surface roughness comparable to terrestrial aa the great flow fields may have been erupted with effusion rates of between 10 5 and 10 6 m 3 sec 1 most of the great flows were erupted from sources within zones of lithospheric extension and thinning although the set of great flows are younger than the background plains large regions of plains may have been resurfaced by sheet type great flow fields which are no longer visible due to homogenization of surface properties in terms of their dimensions estimated eruption rates possible surface textures and tectonic settings we consider the great flow fields to be venusian analogues of terrestrial flood basalt provinces

mean age of rifting and volcanism on venus deduced from impact crater densities unlike the extensively cratered highlands of the moon and mars the surface of venus does not preserve a record of heavy bombard ment from the early history of the solar system1 3 those craters that are found on venus appear to be statistically indistinguishable from a random spatial population and rarely show modification by folds faults and lava flows1 3 although the volcanic and tec tonic history of venus is still much debated2 5 there is mounting evidence for near global resurfacing 300 500 myr ago1 2 6 moreover it has recently been noted that the density of impact craters on large volcanic structures is less than the average crater density of the planet suggestive of significant activity after the resurfacing event7 it is not clear however whether these features represent late remnants of the global event or continuing volcanism and tectonism of a still active planet to address this question we have used the regional variations in crater density to date volcan oes rifts and coronae which based on stratigraphic evidence clearly post date the main resurfacing event8 11 the calculated mean ages of 70 125 myr exclude the possibility that the majority of these features represent the final stages of the global event

migmatite structures in the central gneiss complex boca de quadra alaska migmatite structures in the coast plutonic metamorphic complex are well exposed in the inlet of boca de quadra southeast alaska two types of anatectic migmatites are present patch migmatites formed by in situ melting and subsequent crystallization of melt diktyonitic migmatites comprise a discontinuous veined network of leucocratic material in which leucosomes enclose boudins of host rock the margins of these boudins show the development of both melanosomes and shear band fabrics strain analysis of diktyonitic melanosomes indicates that these regions have undergone volume decreases of 20 27 this volume decrease is attributed to melt extraction into the adjacent fracture filling leucosomes thus diktyonitic migmatites formed by shear induced segregation of partial melt whereas in patch migmatites the lack of shear stresses inhibited melt segregation the variable structural style of anatectic migmatites in boca de quadra is not related to host rock composition but may be due to differences in the amount of differential stress during migmatization these in turn may be controlled by host rock strength and or diachroneity of migmatization and deformation determination of volume changes during migmatization using strain analysis is potentially capable of discriminating intrusive and anatectic migmatites and consequently of documenting melt segregation and subsequent migration across crustal levels

small volcanic edifices and volcanism in the plains of venus the most widespread terrain type on venus consists of volcanic lowland plains several styles of volcanism are represented in the plains the most extensive volcanic units consist of flood lavas the largest of which have volumes of the order of thousands of cubic kilometers as with terrestrial flood lavas they are inferred to have erupted at high effusion rates they show a range of radar backscatter characteristics indicating different surface textures and ages small edifices on the plains occur mainly in clusters associated with fracture belts the majority are shield volcanoes that may be up to a few tens of kilometers across but are generally 10 km or less in diameter volcanic cones have the same size range volcanic domes have diameters up to several tens of kilometers and volumes of the order of 100 km3 these are interpreted as being constructed of lava erupted with a relatively high effective viscosity and thus possibly composed of more silicic lava for many domes the flanks were unstable during and afte eruption and suffered gravity sliding that produced steep scalloped outer margins because of the high atmospheric pressures on venus explosive activity is less likely to occur than on earth however n a few plains areas there is evidence of pyroclastic deposits surrounding craters indicating that volatile contents in some of the magmas may be high in comparison to earth the clusters of small volcanic edifices are considered to be analogous to plains volcanism similar to that of the snake river plain of idaho there may also be analogues with terrestrial volcanic clusters associated with mid oceanic ridges

the evolution of crack seal vein and fracture networks in an evolving stress field insights from discrete element models of fracture sealing veins are ubiquitous in upper and middle crustal rocks due to strength and stiffness contrast to the host rock veins can influence crack propagation here we present discrete element models to investigate crack vein interactions by simulating cycles of fracturing of a rock mass sealing the cracks to form veins and refracturing the rock mass after rotating the stress field we observe different styles of interaction between new fractures and existing veins depending on the strength ratio between vein and host rock and on the changes in the stress field between the different deformation stages if the orientation of stress field does not change between deformation stages ataxial crack seal veins are produced if the veins are weak and a bundle of subparallel microveins if the veins are strong if the stress field is rotated between deformation stages the interactions include reactivation fracture deflection and crosscutting reactivation of weak veins occurs even if the vein orientation is highly unfavorable relative to the stress field relays of fractures between reactivated veins form at a higher angle to the veins than expected this demonstrates that the orientation of secondary veins does not reflect the regional stress field in a simple manner and that veins can strongly influence fracture connectivity with implications for paleostress analysis and basin modeling simulation results compare well with field examples of multiphase vein networks in carbonates from jebel akhdar oman

antimoniferous vein type mineralization of the berga antiform eastern thuringia germany a fluid inclusion study abstract the antimoniferous vein mineralization in the schleiz and greiz area eastern thuringia are bound to ne sw striking variscan fault zones along the margins of the berga antiform the sb pb zn ores form massive sulfide sulfosalt quartz carbonate veins breccias or stockworks within ordovician and silurian metasedimentary units devonian metabasalts and metagranitoids mineralogical investigations from several sb mines and hard rock quarries near schleiz and greiz germany show that the sb pb zn sulfides are texturally in close association with quartz and carbonates stibnite is the major sb bearing mineral minerals of the sb pb zn sulfide assemblage replace an earlier zn fe as sulfide quartz assemblage quartz i named the ore stage i mineralization stibnite and quartz ii overgrow these early sulfides accounting for the ore stage ii lead dominated sb sulfides e g zinkenite plagionite semseyite represent the ore stage iii phase followed by sphalerite ii galena and quartz iii accounting for ore stage iv fluid inclusion studies on ore and gangue minerals of the different ore stages give insights into the composition and temperature of the mineralizing fluid s primary fluid inclusion assemblages in ore stage ii stibnite and co genetic quartz ii show variable vapor fractions within fluid inclusion assemblages heterogeneous trapping of co2 at constant salinities of fluid inclusions of the same assemblage indicates the influx of a buoyant pure co2 fluid into an aqueous fluid phase the presented data demonstrates the first example of co2 influx into an aqueous fluid identified by fluid inclusion analyses during this ore stage ii a h2o co2 nacl fluid with eq w nacl of 1 to 2 7 at 220 226 c prevails in contrast fluid inclusions in ore stage iii minerals boulangerite contain a homogeneously trapped h2o nacl cacl2 fluid with eq w nacl up to 8 1 at temperatures 146 c ore stage iv minerals sphalerite ii quartz iii contain a homogeneously trapped h2o nacl cacl2 fluid with eq w nacl of 20 and homogenization into the liquid phase at around 120 c fluid inclusion data suggest p t conditions at the onset of mineralization at around 220 c at 0 70 to 0 75 kbar fluid pressure the mechanisms of sb ore precipitation were induced by the influx of co2 fluid cooling quartz ii stibnite boulangerite and late stage mixing with a high salinity fluid resulting in the precipitation of sphalerite ii galena and quartz iii the unique opportunity to analyze fluid inclusions in many different subsequently precipitating ore and gangue minerals enables the reconstruction of the temporal evolution of the sb veins the distinct change of the ore forming processes from the influx of co2 into an aqueous fluid and cooling ore stage i ii to mixing ore stage iii iv as the dominant precipitation mechanisms are also accompanied by a drastic change in the mineralogy from sb sulfides to pb zn sulfides late orogenic granitic intrusions served as a potential heat metal and sulfur source variscan wrench and thrust tectonics and associated metamorphism account for the ore stage i mineralization hydrothermal sb pb zn assemblages ore stages ii iii and iv precipitated during the latest part of the variscan orogeny most likely around the carboniferous permian boundary although fluid composition and formation temperatures of the greiz and schleiz mineralization are on the first view quite similar to those of other variscan sb mineralization the observation of co2 influx and fluid mixing with a high salinity fluid is unique for european sb occurrences

the morphology and evolution of coronae on venus coronae on venus are large circular to ovoidal surface features that have distinctive tectonic volcanic and topographic expressions they range in diameter from less than 200 km to at least 1000 km new data from the magellan spacecraft have shown coronae to be among the dominant tectonic forms on the planet and have revealed their morphology in unprecedented detail typical coronae are dominated by concentric tectonic features and have a raised rim a central region higher than the surounding plains but in many instances lower than the rim and commonly a peripheral depression or moat some coronae also show significant amounts of radial tectonic structure and in most cases this predates the concentric features in addition there are other features on venus recognized for the first time in magellan data that consist of domical rises with intense radial tectonic patterns and little or no concentric structure all of these features commonly are associated with moderate to large quantities of volcanism in fact some radially fractured domes have undergone so much volcanism that volcanic construction appears to have played a significant role in establishing their topography we explore a model of corona formation that links these forms into a genetic sequence the model begins with the ascent of a mantle diapir upward mantle flow driven by its ascent forces the lithosphere above the diapir upward producing a gentle dome with a radiating pattern of extensional fractures as the diapir impinges on the underside of the lithosphere it flattens and spreads transforming the uplift to a more flat topped shape in this flattened near surface configuration the diapir can cool rapidly with the resultant loss of buoyancy the raised plateau can relax to form a central sag a raised rim and a depressed moat concentric tectonic features develop primarily during the latter stages of corona formation and hence are best preserved on mature coronae volcanism takes place during all phases of the uplift and may diminish as the relaxation occurs our analyses to date suggest that this scenario is broadly consistent with many of the coronae on venus however there is enormous diversity in corona morphology and features are present that require substantial deviations from this simple model in particular some circular depressions appear corona like in synthetic aperature radar images but may in fact be large calderas some of the variations observed in corona morphology may ultimately be interpretable in terms of variations in the behavior of individual diapirs and in the local properties of the venusian lithosphere

scale invariance and self organized criticality in migmatites of the southern hualapai mountains arizona whether or not melt flow networks are scale invariant may have broad implications for the understanding of the development of magmatic systems leucosome distributions in natural migmatite samples from the hualapai mountains arizona are used to investigate the scaling characteristics of an inferred syndeformational melt flow network one dimensional line and two dimensional box counting analyses yield distributions that are consistent with a scale invariant relationship between the leucosome size and leucosome frequency we infer from these results that the interactions of anatexis and deformation gave rise to a self organized critical system in which melt distribution and melt movement were linked over a range of scales at least millimeter to meter scale we conclude that even in metatexite migmatites volumetrically dominated by solid phases melts can play an active role in determining the architecture of the melt flow network one implication of the envisioned self organized critical system is the potential for rapid large scale melt coalescence and escape without long term maintenance of a fully interconnected melt flow network

microfracturing during primary migration in shales abstract in several geological environments chemical reactions are coupled to rock deformation and the associated stresses induced locally interact with the far field loading this is the case in immature shales that undergo burial and diagenesis where the organic matter evolves with temperature into hydrocarbons which induces local volume expansion at large scale this mechanism is responsible for the transport of hydrocarbons from source to reservoir rocks a process referred to as primary migration however how the interactions between local fluid production microfracturing and transport are coupled remain to be understood here we analyze this coupling phenomenon by developing a discrete element model where the generation of local overpressures occurring in kerogen patches is simulated while the surrounding rock is subjected to external loading it is shown that due to local fluid overpressure microfracturing occurs and brings the fluids to migrate through the medium the numerical results are confirmed by laboratory experiments where the network of microfractures induced in an immature green river shale sample heated under small differential stress was imaged in three dimensions using x ray microtomography moreover the numerical simulations identify that the state of differential stress and the initial kerogen distribution constitute two key parameters that control the formation of the three dimensional percolating microfracture network and could thus explain primary migration in shale rocks

venus crater distribution and plains resurfacing models detailed analysis of the distribution of craters on venus using mth nearest neighbor analysis coupled with models based upon surface morphology constraints indicates that the hypothesis of complete spatial randomness csr cannot be rejected but is not a unique model of the observed crater distribution based on morphologic mapping the extensive volcanic plains can be divided into four units that have a spread in age of the order of 0 5t the mean surface age of the planet this four unit plains model along with its derivatives produce test statistics that indicate such models also cannot be rejected further the probability of obtaining a result at least as extreme as the observed test statistic given that the null hypothesis model corresponds to venus is true is lowest for the csr model there is no particular reason to pick a csr model along with its implications for catastrophic resurfacing as a constraint on the evolution of venus and there are geological reasons to choose the multiage models we find that we cannot distinguish statistically among models that have two three or four distinct production ages within the plains however the hypothesis that the variation in crater density within all of the plains is due to a single random process can be rejected for two reasons first the binomial probability that such a process could exist within each of the plains units is 0 05 except the smallest and youngest unit pl1 second using a chi squared statistic to test the hypothesis that four plains units have the same age gives a p value of 10 4 indicating confident rejection of the hypothesis thus csr cannot be used as a constraint on models of resurfacing or planetary evolution of venus because of the non uniqueness in matching such a model to the observed crater distribution and the strong indication of distinct ages within the plains with a significant spread in age geological and geophysical constraints provide our best clues for understanding venus

tectonics and volcanism of eastern aphrodite terra venus no subduction no spreading eastern aphrodite terra a deformed region with high topographic relief on venus has been interpreted as analogous to a terrestrial extensional or convergent plate boundary however analysis of geological and structural relations indicates that the tectonics of eastern aphrodite terra is dominated by blistering of the crust by magma diapirs the findings imply that within this region vertical tectonism dominates over horizontal tectonism and consequently that this region is neither a divergent nor a convergent plate boundary

impact crater densities on volcanoes and coronae on venus implications for volcanic resurfacing the density of impact craters on large volcanoes on venus is half the average crater density for the planet the crater density on some classes of coronae is not significantly different from the global average density but coronae with extensive associated volcanic deposits have lower crater densities these results are inconsistent with both single age and steady state models for global resurfacing and suggest that volcanoes and coronae with associated volcanism have been active on venus over the last 500 million years

in situ imaging of fracture development during maturation of an organic rich shale effects of heating rate and confinement abstract the statistics and dynamics of fractures formed during the accelerated maturation of kerogen rich shale was investigated by heating green river shale r 8 unit piceance basin northwestern colorado core samples while 3d x ray microtomographic images were acquired previous studies have shown that when there was no confining stress fractures formed while the kerogen contained in the shale matured and the produced hydrocarbon was expelled through these fractures in the present study x ray tomographic scans at multiple voxel sizes were conducted on similar samples during heating in one experiment the shale sample was tightly fitted in a non porous ceramic tube to confine it while it was heated three unconfined samples were heated and then held at different final temperatures to investigate the effects of the gas production rate on fracturing 3d image processing was used to survey fracture network development and time lapse 2d digital image correlation analysis was used to monitor the development of the displacement and strain fields the results revealed that fracturing is strongly dependent on the heating rate and the final heating temperature while most of the fractures were oriented more or less parallel to the bedding plane some were strongly inclined relative to the bedding plane the formation of inclined fractures is attributed primarily to the shape and orientation of a minority of the flake like kerogen patches and also to the effective stress field and fractured zones of weakness a conceptual model is proposed to explain the dynamics of fluid expulsion and the associate fracturing behavior

melt extraction and accumulation from partially molten rocks current models for melt segregation and ascent are not adequate to accurately describe transport and accumulation in combination we propose that transport is discontinuous and in batches and that accumulation occurs by stepwise merging of batches a simple numerical model of jostling spheres that merge when they touch was used to represent stepwise accumulation and transport of batches by propagation of hydrofractures results of the numerical model indicate that such a system may quickly develop into a self organised critical soc state in this state the distribution of melt batch volumes can be described by a power law with an exponent m that lies between 2 3 and 1 once a self organised critical state is established the system is capable of discharging any additional melt without further change to itself deformation aids melt extraction efficiency as it increases the mobility of hydrofractures enhances accumulation and hence lowers the exponent m full connectivity of melt needs never to be reached in the system and melt transport and extraction can occur at very low melt fractions the chemical evolution of melt from source to emplacement level will be governed by the discontinuous mixing and mingling of batches each with different histories and possibly different sources if no subsequent homogenisation occurs in a magma chamber or the final emplacement structure the process can be identified by chemical heterogeneity of plutons and volcanic rocks

geophysical models for the formation and evolution of coronae on venus coronae are large circular features on venus characterized by an annulus of concentric tectonic features interior fracturing volcanism and generally upraised topography they are suggested to form over sites of mantle upwelling and modified by subsequent gravitational relaxation we examine this proposition using two geophysical models to determine whether and under what conditions these mechanisms can produce the topography and tectonics exhibited by coronae in the magellan altimetry data and radar images our results show that mantle diapirism can produce the domical topography of novae which may be coronae in the earliest stage of formation the model stresses induced at the surface by a mantle diapir imply the formation of radially oriented extensional fracturing as observed in novae the dimensions of novae indicate that the diapirs responsible for them are smaller than about 100 km in radius and that the elastic lithosphere is less than 32 km thick diapirs that have reached the top of the mantle are expected to spread and flatten producing plateaulike rather than domical topography we model a flattened diapir at the top of the mantle and show that it will result in plateaulike uplift the volume of the flattened model diapir is similar to that of the spherical diapirs derived for novae we model gravitational relaxation of isostatically uncompensated plateaus and show that they relax to the topographic forms associated with coronae and that the model stresses are consistent with the development of the annulus of tectonic features around coronae

reactivity of dolomitizing fluids and mg source evaluation of fault controlled dolomitization at the benicassim outcrop analogue maestrat basin e spain abstract the mechanisms responsible for the formation of huge volumes of dolomitized rocks associated with faults are not well understood we present a case study for high temperature dolomitization of an early cretaceous aptian albian ramp in benicassim maestrat basin e spain in this area seismic scale fault controlled stratabound dolostone bodies extend over several kilometres away from large scale faults this work aims at evaluating different mg sources for dolomitization estimating the reactivity of dolomitizing fluids at variable temperature and quantifying the required versus available fluid volumes to account for the benicassim dolostones field relationships stable 13 c and 18 o isotopes as well as radiogenic 87 sr 86 sr isotopes indicate that dolomitization at benicassim was produced by a high temperature fluid 80 c 13 c and 18 o isotopic compositions for dolomite vary from 0 5 to 2 9 v pdb and from 21 1 to 24 3 v smow respectively a mg source analysis reveals that the most likely dolomitizing fluid was seawater derived brine that interacted with underlying triassic red beds and the paleozoic basement geochemical models suggest that evolved seawater can be considerably more reactive than high salinity brines and the maximum reactivity occurs at about 100 c mass balance calculations indicate that interstitial fluids with high pressure and or high temperature relative to the normal geothermal gradient cannot account for the volume of dolomite at benicassim instead a pervasive fluid circulation mechanism like thermal convection is required to provide a sufficient volume of dolomitizing fluid which most likely occurred during the late cretaceous post rift stage of the maestrat basin this study illustrates the importance of fluid budget quantification to critically evaluate genetic models for dolomitization and other diagenetic processes

thermal evolution of venus abstract the theory of three dimensional and finite amplitude convection in a viscous spherical shell with temperature and pressure dependent physical parameters is developed on the basis of a modified boussinesq fluid assumption the lateral dependences of the variables are resolved through their spherical harmonic representations whereas their radial and time dependences are determined by numerical procedures the theory is then applied to produce thermal evolution models for venus the emphasis is on illustrating the effects of certain physical parameters on the thermal evolution rather than proposing a specific thermal history for the planet the main conclusions achieved in this paper are 1 a significant portion of the present temperature in the mantle and heat flux at the surface of venus is probably owing to the decay of a high temperature established in the planet at the completion of its core formation 2 the effective rayleigh number of the mantle is so high that even the lower order modes of convection cool the planet sufficiently and maintain an almost adiabatic temperature gradient in the convecting region and high temperature gradients in the thermal boundary layers 3 the convection is oscillatory with avalanche type properties which induces oscillatory features to the surface heat flux and the thickness of the crustal layer and 4 a planetary model with a recycling crust cools much faster than those with a permanently buoyant crust the models presented in this paper suggest that venus has been highly convective during its history until 0 5 ga ago the vigorous convection was bringing hot and fresh material from the deep interior to the surface and dragging down the crustal slags floating on the surface in to the mantle the rate of cooling of the planet was so high that its core has solidified in the last 0 5 ga the vigour of convection diminished considerably and the crustal slags developed into a global and permanently buoyant crustal layer the tectonic style on venus has consequently changed from the recycling of crustal plates to hot spot volcanics at the present time the planet is completely solid except in the upper part of its mantle where partial melting may occur

implications of a global survey of venusian impact craters abstract we present a global survey of the areal distribution size frequency distribution and morphometric properties of the venusian impact cratering record we explore the resurfacing history of venus crater degradation ejecta emplacement and cratering mechanics the number of volcanically embayed and tectonically deformed craters from 0 5 to 1 0 km above mean planetary radius is disproportionately high for an otherwise crater deficient elevation range more resurfacing occurred in this range an elevation range dominated by volcanic rises rifts and coronae than elsewhere on venus although the majority of craters appear to be relatively undisturbed and have intact ejecta blankets some craters appear particularly fresh because they have radar bright floors a radar dark halo surrounding the ejecta blanket and a west facing parabola of low radar return 20 35 and 8 respectively of craters with diameters 22 6 km have these features statistical correlations support the idea that such craters are younger than the remaining crater population characteristics of ejecta deposits for venusian craters change substantially with size particularly at 20 km crater diameter which marks the transition at which the boundaries of ejecta blankets go from ragged to lobate and the slope of the ejecta distance vs diameter curve steepens we suggest that this transition may be similar to the transition from inertial granular flow to fluidized granular flow for terrestrial debris avalanches a diameter independent variation in extent of the ejecta deposits is not related to impact angle or target properties but may be related to the shape distribution and dispersion of impacting fragments secondary craters are a ubiquitous part of the ejecta blanket for craters over 50 km but occur infrequently as isolated rays about smaller craters the large distances secondary craters extend from the crater rim imply that the secondary forming projectiles traveled in an atmosphere significantly altered by the impact process comparison of complex craters found on venus with those of other planets gave results that were consistent with the idea that inter planetary differences in complex crater shape are controlled by interplanetary differences in gravity and crustal strength the interplanetary comparison indicates that venus the moon and mercury appear to have stronger crusts than do mars and ganymede callisto

an organodiagenetic model for marinoan age cap carbonates abstract the nuccaleena formation cap carbonate is the global stratotype for the marinoan glaciation and ediacaran gssp a designation that emphasises the importance of cap carbonates to our current understanding of global neoproterozoic biogeochemistry sedimentology and stratigraphy however to date there is no agreed depositional model for cap carbonates and there remains minimal detailed paragenetic data for developing process based depositional models here an early diagenetic organodiagenetic dolomite cementation model for marinoan cap carbonate formation is hypothesised and explored it is demonstrated how this process based model can explain the three main atypical sedimentary structures that are the basis for correlation of marinoan age cap carbonates giant tepee like structures sheet veins and tubestones these features are interpreted to be products of fluid overpressure deformation induced by organodiagenetic expansive cementation giant tepee like structures the hallmark of marinoan cap carbonates conform to a simple structural analysis that is consistent with the predicted stress field induced by expansive cementation driven fluid overpressure the undeformed to highly deformed gradient of deformation features in cap carbonates is consistent with the range of rheologies in a shallowly buried cementing carbonate modification of matrix textures and bedding surfaces also conforms to the range of expected textures within this model overall it is hypothesised that the aggradational condensed section architecture of cap carbonates was the primary control over the generation of these atypical sedimentary features the available paragenetic analysis suggests that the geochemical data particularly the c isotopic data can be reinterpreted as supportive of the organodiagenetic model and that the neoproterozoic ocean was perhaps similar to that of today the proposed model indicates that the current criteria for the placement of the ediacaran gssp are non unique and potentially non isochronous perhaps the most important aspect of this model is that it is testable and is a call for focused research on the much overlooked paragenesis of cap carbonates

long lived crustal scale fluid flow the hydrothermal mega breccia of hidden valley mt painter inlier south australia the palaeozoic hidden valley breccia in the northern flinders ranges of south australia is exceptional for 1 its size of about 10 km2 2 the large continuous range in clast sizes from tens of microns to hundreds of metres and 3 thorough mixing of lithologies of different provenance some originally kilometres apart stratigraphically the size distribution follows a single fractal dimension of about unity over at least 6 orders of magnitude implying that a single process was responsible for diminution from the 100 m scale down to 12 km exhumation which lasted about 200 myrs starting during the 500 ma delamerian orogeny and continuing during the alice springs orogeny fluids released during exhumation were structurally focussed towards hidden valley where an estimated 20 5 30 km3 total fluid volume caused the extensive brecciation brecciation initiated in neoproterozoic cover metasedimentary rocks at a level that is now fully exhumed as hydrothermal fluid ascent continued with ongoing exhumation the level of brecciation shifted down into the underlying mesoproterozoic basement rocks taking with it clasts of cover rocks in this model rocks pass through the zone of brecciation which can thus incorporate a variety of lithologies that were originally kilometres apart

apparent boudinage in dykes abstract intrusive rocks may be arranged in the form of strings of lenses or beads as found on the cap de creus peninsula ne spain and in the south finland migmatite granite belt these structures first appear to be the result of stretching and boudinage of intrusive sheets or dykes however closer examination reveals that they are not boudins but are instead primary intrusive structures a detailed study was performed on a swarm of pegmatite intrusions at cap de creus layering is often continuous between beads and in some cases individual beads exhibit a very irregular shape these observations are shown to be incompatible with an origin by boudinage analogue experiments were used to test the effect of the two models boudinage and emplacement on the structures around beads and show that an emplacement model of local expansion and collapse of magmatic sheets is the most compatible with field observations such structures can only form when dykes intrude hot rocks when magma solidifies slowly enough to allow enough ductile flow of the wall rock to accommodate the formation of the beads the fact that the pegmatite bead strings are not boudins has repercussions for the interpretation of the deformation at cap de creus and necessitates caution in the interpretation of deformation based on apparent boudins in intrusive rocks in other areas

an episodic hypothesis for venusian tectonics it is suggested that episodic plate tectonics occurs on venus episodes of rapid plate tectonics are separated by periods of surface quiescence for the last 500 200 m y it is postulated that the surface of venus has been a single rigid plate that has been thickening due to conductive cooling a near uniform surface age is consistent with observed crater densities and the relatively small number of craters modified by surface tectonics or embayed by lava flows a lithosphere that has conductively thickened for some 500 m y has a thickness of about 300 km nearly an order of magnitude greater than the thickness associated with steady state conductive heat loss such a thick lithosphere can support the high topography and associated gravity anomalies on venus as well as the unrelaxed craters studies of lithospheric flexure at coronae are also consistent with a thick elastic lithosphere incipient subduction associated with large coronae may represent the onset of a new episode of rapid plate tectonics on the earth 75 90 of mantle heat transport is attributed to the creation of new oceanic lithosphere at ocean ridges this process is not operative on venus this paper suggests that episodic plate tectonics on venus constitutes the primary mechanism for mantle heat transport on that planet

venus tectonics initial analysis from magellan radar imaging and altimetry data from the magellan mission have revealed a diversity of deformational features at a variety of spatial scales on the venus surface the plains record a superposition of different episodes of deformation and volcanism strain is both areally distributed and concentrated into zones of extension and shortening the common coherence of strain patterns over hundreds of kilometers implies that many features in the plains reflect a crustal response to mantle dynamic processes ridge belts and mountain belts represent successive degrees of lithospheric shortening and crustal thickening the mountain belts also show widespread evidence for extension and collapse both during and following crustal compression venus displays two geometrical patterns of concentrated lithospheric extension quasi circular coronae and broad rises with linear rift zones both are sites of significant volcanism no long large offset strike slip faults have been observed although limited local horizontal shear is accommodated across many zones of crustal shortening in general tectonic features on venus are unlike those in earth s oceanic regions in that strain typically is distributed across broad zones that are one to a few hundred kilometers wide and separated by stronger and less deformed blocks hundreds of kilometers in width as in actively deforming continental regions on earth

the global resurfacing of venus the impact cratering record on venus is unique among the terrestrial planets fully 84 of the craters are in pristine condition and only 12 are fractured remarkably only 2 5 of the craters and crater related features are embayed by lava although intense volcanism and tectonism have affected the entire planet furthermore the spatial and hypsometric distribution of the craters is consistent with a completely random one including stochastic variations monte carlo simulations of equilibrium resurfacing models result in a minimum of 17 times more embayed craters than observed or unobserved nonrandom crater distributions for resurfacing areas between 0 03 and 100 of the planet s surface these models also are not consistent with the number and nonrandom distribution of volcanoes and the nonrandom distribution of embayed and heavily fractured craters the constraints imposed by the cratering record strongly indicate that venus experienced a global resurfacing event about 300 m y ago followed by a dramatic reduction of volcanism and tectonism this global resurfacing event ended abruptly 10 m y the present crater population has accumulated since then and remains largely intact thermal history models suggest that similar global resurfacing events probably occurred episodically in the past the tesserae statistically have the same average surface age crater retention age as the rest of the planet but they probably represent older rock units deformed by earlier episodes of global resurfacing although they largely survived the latest global resurfacing event their surfaces were severely deformed by it monte carlo simulations indicate that only about 4 6 of the planet has been volcanically resurfaced since the global event and that the lava production rate has been no more than 0 01 0 15 km3 yr during this time this rate is significantly less than the current rate of intraplate volcanism on earth 0 33 0 5 km3 yr most of venus recent volcanism occurs in the beta atla themis region and most of the recent tectonism is associated with the major global scale tectonic disruption zones that lie within and connect the equatorial highlands the approximately 33 of the planet s surface bounded by latitudes 30 n and 30 s longitudes 60 and 300 e contains twice as many heavily fractured craters and 1 4 times more lava embayed craters as the planetary average this region includes most of the major tectonic belts in the equatorial region because the craters are indistinguishable from a statistically random distribution both spatially and hypsometrically this concentration of strongly fractured and embayed craters is considered indicative of a continuing low level of limited extension and volcanic activity in this region over the past 300 m y however these craters are simply fractured and or embayed and very few have been subjected to complete tectonic disruption complete burial and subsequent removal from the surface as was the case during the global resurfacing event we show that neither the present level and style of geologic activity nor anything less than global resurfacing could have produced the observed cratering record the effects of recent geologic activity are much less than those of the earlier global resurfacing event when the record of all the early heavy bombardment and much of the later light bombardment was erased from the surface by massive volcanism and tectonic activity episodic regional resurfacing events that had global effects also occurred on earth e g the mid cretaceous superplume and probably on mars on mars they may have triggered the catastrophic releases of water that formed the outflow channels

initial rise of bubbles in cohesive sediments by a process of viscoelastic fracture 1 an understanding of the mechanics of bubble rise in sediments is essential because of the role of bubbles in releasing methane to the atmosphere and the formation and melting of gas hydrates past models to describe and predict the rise of other buoyant geological bodies through a surrounding solid e g magmas and hydrofractures appear not to be applicable to bubbles in soft sediments and this paper presents a new model for gas bubble rise in soft fine grained cohesive sediments bubbles in such sediments are essentially dry little if any free water and grow through a process of elastic expansion and fracture that can be described using the principles of linear elastic fracture mechanics which assume the existence of a spectrum of flaws within the sediment fabric by extending this theory we predict that bubbles initially rise by preferential propagation of a fracture in a sub vertical direction we present a criterion for initial bubble rise once rise is initiated the speed of rise is controlled by the viscoelastic response of the sediments to stress using this new bubble rise model we estimate rise velocities to be of the order of centimeters per second we again show that capillary pressure plays no substantive role in controlling bubble growth or rise

from migmatites to plutons power law relationships in the evolution of magmatic bodies magma is generated by partial melting from micrometre scale droplets at the source and may accumulate to form 100 km scale plutons magma accumulation thus spans well over ten orders of magnitude in scale here we provide measurements of migmatitic leucosomes and granitic veins in drill cores from the estonian proterozoic basement and outcrops at masku in sw finland and montemor o novo central portugal despite the differences in size and number of measured leucosomes and magmatic veins differences in host rock types and metamorphic grades the cumulative width distribution of the studied magmatic leucosomes veins follows a power law with exponents usually between 0 7 and 1 8 published maps of the se australian lachlan fold belt were used to investigate the distribution of granitoid pluton sizes the granites occupy ca 22 of the 2 6 105 km2 area the cumulative pluton area distributions show good power law distributions with exponents between 0 6 and 0 8 depending on pluton area group using the self affine nature of pluton shapes it is possible to estimate the total volume of magma that was expelled from the source in the 2 6 105 km2 map area giving an estimated 0 8 km3 of magma per km2 it has been suggested in the literature that magma batches in the source merge to form ever bigger batches in a self organized way this leads to a power law for the cumulative distribution of magma volumes with an exponent m v between 1 for inefficient melt extraction and 2 3 for maximum accumulation efficiency as most of the volume resides in the largest batches that can escape from the source if m v 1 the mass of the magma is dominated by small batches in case m 2 3 about 50 of all magma in the system is placed in a single largest batch our observations support the model that the crust develops a self organized critical state during magma generation in this state magma batches accumulate in a non continuous step wise manner to form ever larger accumulations there is no characteristic length or time scale in the partial melting process or its products smallest melt segregations and km scale plotuns form the end members of a continuous chain of mergers of magma batches

formation of martian outflow channels by catastrophic dewatering of evaporite deposits geological mapping based on topographic analysis of mars orbiter laser altimeter mola data together with photointerpretation of mars orbiter camera moc images and thermodynamic and heat flow considerations frame a new hypothesis for the formation of martian outflow channels through catastrophic dewatering of evaporite deposits mola transects across valles marineris show that the valley is located at the crest of a 3 km high topographic bulge on the flank of the much larger tharsis rise interpretation of moc images showing layered deposits within valles marineris as unconformably underlying hesperian age lava flows means that these thick deposits thought to contain hydrous sulfates were heated by an increased geothermal gradient due to development of tharis increased temperatures adequate to dehydrate hydrous evaporites would trigger significant volumetric expansion and catastrophically release tremendous amounts of overpressured water

experimental deformation of partially melted granitic aggregates abstract the effects of varying amounts of partial melt on the deformation of granitic aggregates have been tested experimentally at conditions 900 c 1500 mpa 10 4 to 10 6 s where melt free samples deform by dislocation creep with microstructures approximately equivalent to those of upper greenschist facies experiments were performed on samples of various grain sizes including an aplite 150 m and sintered aggregates of quartz albitemicrocline 10 50 and 2 10 m water was added to the samples to obtain various amounts of melt 1 15 in the aplite 1 5 in the sintered aggregates optical and tem observations of the melt distribution in hydrostatically annealed samples show that the melt in the sintered aggregates is homogeneously distributed along an interconnected network of triple junction channels while the melt in the aplites is inhomogeneously distributed the effect of partial melt on deformation depends an melt amount and distribution grain size and strain rate for samples deformed with 1 melt all grain sizes exhibit microstructures indicative of dislocation creep for samples deformed with 3 5 melt the 150 m and 10 50 m grain size samples also exhibit dislocation creep microstructures but the 2 10 m grain size samples exhibit abundant tem scale evidence of dissolution precipitation and little evidence of dislocation activity suggesting a switch in deformation mechanism to predominantly melt enhanced diffusion creep at natural strain rates melt enhanced diffusion creep would predominate at larger grain sizes although probably not for most coarse grained granites the effects of melt percentage and strain rate have been studied for the 150 m aplites for samples with 5 and 10 melt deformation at 10 6 s squeezes excess melt out of the central compressed region allowing predominantly dislocation creep conversely deformation at 10 5 s produces considerable cataclasis presumably because the excess melt cannot flow laterally fast enough and a high pore fluid pressure results for samples with 15 melt deformation at both strain rates produces cataclasis presumably because the inhomogeneous melt distribution resulted in regions of decoupled grains which would produce high stress concentrations at point contacts at natural strain rates there should be little or no cataclasis if an equilibrium melt texture exists and if the melt can flow as fast as the imposed strain rate however if the melt is confined and cannot migrate a high pore fluid pressure should promote brittle deformation