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

melting of plagioclase quartz assemblages at 2 kbar water pressure the melting of plagioclase and quartz has been investigated at ph2o 2kb a single crystal of plagioclase was surrounded by quartz powder and water a reaction rim consisting of glass and of an rich plagioclase developed around unchanged starting plagioclase microprobe determinations of melt and coexisting plagioclase compositions reveal a strong fractionation of plagioclase components between melt and new plagioclase for example at 850 c the approximate xan of melt is 0 3 and that of plagioclase is 0 8 the temperature interval between beginning of melting and complete melting of cotectic compositions is 100 c or more for quartz plagioclase mixtures with plagioclases between an 40 and an 75 in comparison to the system ab an h2o the plagioclase melting loop is somewhat wider in the investigated system qz ab an h2o but the temperature interval is much smaller 100 c than in the system ab an h2o 200 c the solidus data indicated by the new plagioclase compositions are practically identical with those observed in beginning of melting experiments the results show that fractionation of plagioclase components between partial melts and restite plagioclase can be more pronounced in multi component rocks than in the pure plagioclase system this finding is important for the development of albite rich rocks from more basic compositions

fluid and sediment dynamics of upper stage plane beds to understand more fully the fluid and sediment dynamics of upper stage plane beds laboratory experiments were conducted using mobile and fixed beds where turbulent motions of fluid and sediment were measured using laser anemometry bed elevation fluctuations on mobile upper stage plane beds reveal millimeter high bed waves vertical profiles of flow velocity mixing length and eddy viscosity diffusivity are represented well by the law of the wall for the mobile bed von karman s 0 33 and equivalent sand roughness to mean bed grain size varies from 9 to 17 because of the presence of bed load and low relief bed waves for fixed beds with no sediment transport 0 41 and equivalent sand roughness is equal to the mean bedgrain size the decrease in for mobile beds is related to the relative motion of grains and fluid mobile bed turbulence intensities are greater than those for sediment free fixed beds because of enhanced wake formation from the lee side of near bed grains and low relief bed waves sediment diffusivities es calculated in a similar way to fluid diffusivities e indicate that es e sediment diffusivities calculated using the equilibrium balance between upward diffusion and downward settling of sediment are similar to e in near bed regions y d 0 3 suggesting that larger more energetic turbulent eddies are responsible for sediment suspension higher in the flow

ductile opening mode fracture by pore growth and coalescence during combustion alteration of siliceous mudstone opening mode fractures with blunt tips and large maximum apertures are characteristic of clinker that formed by combustion alteration of siliceous mudstone these fractures are inferred to result from pore growth and coalescence with initially circular pores inherited from the diatomaceous protolith circular pores grow preferentially in an en echelon arrangement and coalesce to elongate pores and blunt tipped fractures by thinning and rupture of bridges between pores coalescence of overlapping en echelon pores causes fracture propagation in a zig zag path that is considered indicative of significant inelastic deformation outside the immediate vicinity of the fracture tips this process of fracture formation by void growth and coalescence is inferred to result from solution mass transfer and possible bulk melt movement during partial melting of clinker chemical mass transfer provides a mechanism for extensive inelastic deformation in the surrounding host rock concurrent with fracturing that is considered characteristic of ductile fracture the preferred elongation of coalescing pores and local rupture of pore bridges is explained by a tensile sintering stress due to the thermodynamic tendency of the system for energy minimization of solid and liquid surfaces it is suggested that ductile fracture processes that are accompanied by extensive inelastic deformation lead to opening mode fractures with large apertures in a variety of crustal settings including metamorphic and magmatic systems as well as in reactive diagenetic environments thus affecting fracture controlled transfer of heat and mass and the rheology of the earth s crust

the initiation and growth of en echelon veins abstract numerical models are used to understand the evolution of mode i opening fractures from spatially random distributions in a brittle elastic material en echelon arrays commonly develop because mechanical fracture interaction promotes growth for this geometry this provides a new mechanism for en echelon vein formation in rock which is distinctly different than previously proposed mechanisms it is suggested that some macroscopic en echelon vein arrays may have served as zones of weakness that localized later shear zone development in a manner analogous to that observed by experimentalists examining micro cracking and subsequent shear rupture of rocks loaded under compression sigmoidally shaped veins and vein fillings are explicitly modeled showing that they can form in response to the mechanical interaction of neighboring fractures which redirects the propagation path numerical comparison of sigmoidal veins formed by brittle fracture and by ductile shear zones demonstrates some of the pitfalls of failing to correctly identify the mechanism of formation

effect of particle concentration on sediment and turbulent diffusion coefficients in open channel turbulent flow to achieve a complete knowledge about the effect of particle concentration on sediment and turbulent diffusion coefficients in open channel turbulent flow is a long standing problem for the community of researchers the effect of particle concentration is investigated on the sediment and turbulent diffusion coefficients through the inverse of turbulent schmidt number or which is defined by the ratio of sediment diffusion coefficient to turbulent diffusion coefficient it is observed that with increasing particle concentration the sediment diffusion coefficient decreases more in comparison with the turbulent diffusion coefficient for both dilute and non dilute sediment laden flows the physical characteristics of observed are expressed mathematically in terms of normalized settling velocity reference level and reference concentration the applicability of the mathematical formulae is confirmed by the agreement analysis between experimental data and particle concentration profile computed from the rouse equations modified through the newly proposed expression of apart from the better agreement between dilute particle concentration data and the developed rouse equation the striking observation is that the modified rouse equation shows reasonable computational accuracy for non dilute particle concentration data also minimum error is obtained from the present model when it is compared with the models proposed by the previous researchers

drift velocity concept for sediment laden flows the vertical distribution of suspended sediment concentration in open channel flows is usually described by the advection diffusion equation hence only the mechanisms of advection and diffusion affect the movement of sediment particles however recent publications show that the equation could not describe fully the sediment concentration distribution in open channels a new model for particle migration in sediment laden turbulent flow is presented the model assumes there is an additional mechanism influencing the particle movement in sediment laden flows a new mechanism which is called the drift acts independently of advection and turbulent diffusion based on all three mechanisms a new equation for the concentration distribution is derived the equation describes the vertical distribution of sediment concentration more accurately than the advection diffusion equation

entablature fracture types and mechanisms entablature is the term used to describe zones or tiers of irregular jointing in basaltic lava flows it is thought to form when water from rivers dammed by the lava inundates the lava flow surface and during lava meltwater interaction in subglacial settings a number of different fracture types are described in entablature outcrops from the burfell lava and older lava flows in jorsardalur southwest iceland these are striae bearing column bounding fractures and pseudopillow fracture systems that themselves consist of two different fracture types master fractures with dimpled surface textures and subsidiary fractures with curved striae the interaction of pseudopillow fracture systems and columnar jointing in the entablature produces the chevron fracture patterns that are commonly observed in entablature cube jointing is a more densely fractured version of entablature which likely forms when more coolant enters the hot lava the entablature tiers display closely spaced striae and dendritic crystal shapes which indicate rapid cooling master fracture surfaces show a thin band with an evolved composition at the fracture surface mineral textures in this band also show evidence of quenching of this material this is interpreted as gas driven filter pressing of late stage residual melt that is drawn into an area of low pressure immediately preceding or during master fracture formation by ductile extensional fracture of hot partially crystallised lava this melt is then quenched by an influx of water and or steam when the master fracture fully opens our findings suggest that master fractures are the main conduit for coolant entering the lava flow during entablature formation

non linear growth kinematics of opening mode fractures abstract fracture aperture is commonly assumed to be a linear function of fracture length stress and elastic material properties under constant stress fracture opening displacement may change without concurrent length or height growth if the material effectively weakens after initial linear elastic fracture growth by changes in elastic properties or by non elastic deformation processes to investigate the kinematics of fracture opening and its dependence on length and height growth we reconstructed the opening history of three opening mode fractures that are bridged by crack seal quartz cement similar crack seal cement bridges had been interpreted to form by repeated incremental fracture opening and subsequent precipitation of quartz cement using scanning electron microscope cathodoluminescence imaging we determined the thickness and number of crack seal cement layers as a function of position along fracture length and height observed trends in crack seal cement layer thickness and number of fracture opening increments are consistent with non linear fracture growth kinematics consisting of an initial stage of fast fracture propagation relative to aperture growth followed by a stage of slow propagation and pronounced aperture growth consistent with earlier fluid inclusion observations indicating fracture opening and propagation occurring over 40 50 m y we interpret the second stage of pronounced aperture growth and slow propagation to result from fracture opening strain accommodated by solution precipitation creep and concurrent slow possibly subcritical fracture propagation

the generation of magnetic anomalies by combustion metamorphism of sedimentary rock and its significance to hydrocarbon exploration low temperature exothermic oxidation of fine pyrite in carbonaceous marine rocks can lead to spontaneous combustion as a result the organic material in the rocks is consumed as fuel but unlike burnt coal seams and lignite beds the host rock remains after combustion thermal metamorphism and in some cases partial to complete melting of the rock will result zones of combustion metamorphism are frequently associated with oil fields these occurrences can be related to either surface outcrops of source rock or leakage from reservoirs a study of the magnetic properties of a suite of combustion metamorphosed specimens of the monterey and sisquoc formations in southern california shows orders of magnitude increases in magnetic intensity and susceptibility for both thermally metamorphosed and fused samples these changes reflect both the production of new magnetic minerals derived mainly from oxidized iron sulfide and their acquisition of thermal remanence ground magnetic surveys across combustion metamorphism zones in three southern california oil fields reveal anomalies of up to 2 000 gammas associated with the metamorphosed and melted rock large scale aero magnetic anomalies are also associated with extensive combustion metamorphism zones in the marcelina formation of venezuela and the ghareb and taquiya formations the mottled zone in israel the recognition of magnetic anomalies caused by combustion metamorphism in areas of oil production suggests that closely spaced low altitude aeromagnetic surveying can be a useful petroleum exploration tool

resistance in flat bed sediment laden flows the effect on flow resistance in uniform flat bed open channel flows due to suspended sediment is investigated flows with and without an equilibrium sand bed are studied in a laboratory channel three well sorted natural sands are used point velocity measurements as well as estimates of local wall shear are obtained with a laser doppler anemomentry system and are used to evaluate friction factors the results indicate that contrary to recent models the presence of suspended sediment does not necessarily lead to a reduction in flow resistance but will generally result in an increase the relation between changes in the velocity profile and changes in friction factors is discussed

the role of strain localization in magma injection into a transtensional shear zone variscan belt sw iberia this study deals with the interaction between deformation and magmatism in mid to deep crustal domains the relation is analysed between migmatites and shear zones and the spatial distribution of leucogranitoid veins and dykes running through a footwall migmatite system and reaching a transtensional shear zone operated under amphibolite to greenschist facies metamorphic conditions boa fe shear zone variscan belt sw iberia statistical results show that the frequency of width and spacing of the leucogranitoid dykes conform to power law distributions comparable with observations in volcanic systems the fractal geometry of the distribution of leucogranitoid dykes highlights the development of a dense framework of thinner weakly or non mineralized veins and dykes formed at higher nucleation growth ratios in the footwall migmatite system that contrasts with the emplacement of thicker dykes associated with strongly mineralized thinner veins within the shear zone the volume of injected leucogranitoid dykes in the shear zone is lower as compared with the footwall and is comparable with an expanding footwall shear zone with non coaxial flow and volume increase the boa fe shear zone seems to form a physical barrier to the transport of magma to the hanging wall

origin of calcite in syntectonic veins as determined from carbon isotope ratios a particular feature of many alpine tectonic thrust sheets composed of carbonates is the development of distinct generations of calcite filled veins in the doldenhorn nappe one of the helvetic nappes of switzerland we were able to distinguish five different generations of calcite filled veins determination of carbon isotope ratios in these calcite veins and their adjacent limestone matrix indicates that the earlier vein systems contain carbonate material not in isotopic equilibrium with the adjacent matrix in the case of the later vein systems the carbon 13 content approaches that of the nearby matrix the isotope data indicate a transition from a chemically open system to a closed system this trend is related to an overall modification of the deformation mechanism of the nappe resulting in the increasing importance of pressure solution with time

the inception and growth of leucosomes microstructure at the start of melt segregation in migmatites the beginning stages of melt segregation and the formation of leucosomes are rarely preserved in migmatites most arrays of leucosomes record a more advanced stage where flow dominates over segregation however the early stages in the formation of leucosomes and the segregation of melt are preserved in a partially melted meta argillite from the metatexite zone 800 c of the contact aureole around the duluth complex minnesota the rock contains 2 4 modal leucosome in a matrix consisting of 40 5 in situ neosome and 57 1 cordierite plagioclase framework the domainal microstructure in the matrix is a pre anatectic feature resulting from the bulk composition terminal chlorite reactions produced a large volume of cordierite which with plagioclase formed a framework that enclosed patches of biotite quartz plagioclase k feldspar upon melting these fertile domains became patches of in situ neosome plagioclase in the neosome is less sodic than in the leucosome hence segregation of melt occurred during crystallization not melting segregation was delayed because the cordierite plagioclase framework was strong enough to resist dilatation and compaction until after crystallization started the leucosomes are small i e they are microleucosomes and display a systematic progression in morphology as length and aspect ratio increase from 1 to 19 mm and from 2 5 to 30 respectively small equant micropores form first and in places these coalesce into small 1 mm aspect ratio 2 5 isolated blunt ended elliptical microleucosomes in the next stage micropores develop ahead of and at 45 to the left and right of the blunt tip of a microleucosome one of these develops into an elliptical leucosome and an en echelon array of either a left or right stepping elliptical microleucosome forms each elliptical microleucosome in the en echelon arrays is separated by a bridge of matrix next microleucosomes of greater length 4 mm and aspect ratio 5 form when the bridges of cordierite plagioclase matrix rupture and the elliptical microleucosomes link together to form a zigzag shaped microleucosome finally still longer microleucosomes with greater aspect ratios 30 are formed by the joining of zigzag arrays such a progression is characteristic of the way ductile fractures grow the segregation of melt was driven by the pressure gradient between the dilatant fracture and an adjacent in situ neosome which drew melt to the growing fracture thereby creating a microleucosome the microleucosomes are filled arrays of ductile fractures melt was contiguous only between microleucosomes and adjacent patches of in situ neosome the length scale of segregation was 5 mm the size of a typical patch of in situ neosome and restricted by the surrounding impermeable cordierite plagioclase framework the melt in the microleucosome was the most fractionated and the last to crystallize all microleucosomes contain entrained minerals as a consequence of their mechanism of growth rupture of the bridges resulted in the entrainment of pre anatectic phases however microleucosomes that cross patches of in situ neosome are also contaminated with peritectic phases that were transported with the melt

opening mode fracture in siliceous mudstone at high homologous temperature effect of surface forces in analogy to high temperature sintering of ceramics and metal powder compacts the formation of opening mode fractures in siliceous mudstone during natural in situ combustion of hydrocarbons is attributed to contractile surface forces between mineral grains and an interstitial melt phase a comparison between bulk density increase during sintering and created fracture space indicates that fracturing resulted from contraction of the rock matrix due to porosity reduction grain scale mass transfer and high temperature mineral formation it is suggested that contractile surface forces between mineral grains and between mineral grains and pore fluid contribute to subcritical fracture formation under a wide range of subsurface conditions

a similarity approach to turbulent sediment laden flows in open channels a basis is developed for a similarity treatment of the coupled mean fields in turbulent open channel flows over a flat sand bed in equilibrium with a suspension of sand hypotheses involving multiple scales asymptotic similarity and matching over an overlapping region are made instead of the traditional dependence on eddy diffusion and reynolds analogies velocity measurements using the laser doppler technique in experiments with well sorted natural sands showed that contrary to the implications of previous models the effect of suspended sediment on the mean velocity profile may over a variety of laboratory conditions be confined to a layer adjacent to the bed this is interpreted within the similarity framework in terms of the existence of an inner lengthscale differing from either the viscous or the grain scale associated with the presence of sediment the suggested lengthscales and concentration scales are examined in light of the experimental results

fractures in a trachyandesitic lava at oraefajokull iceland used to infer subglacial emplacement in 1727 8 eruption abstract we present detailed field observations of cooling fractures in a small volume trachyandesitic lava informally named the slaga lava on the south west flank of oraefajokull volcano south east iceland columnar joints pseudopillow fracture systems and curved platy jointing occur in the lava whose exposed section is approximately 600 m in length and generally 2 3 m in thickness columnar jointing may occur at the base of flow lobes whereas pseudopillow fracture systems occur throughout the lava in an outer glassy fractured carapace and curved platy fractures occur in the centres of larger flow lobes pseudopillow fracture systems composed of a single master fracture and multiple subsidiary fractures formed normal to the master fracture are of two types g type pseudopillow fracture systems have very narrow striae chisel marks on their master fractures indicative of rapid cooling and brittle fracture sr type pseudopillow fracture systems display alternating smooth and rough master fracture surface textures evidence of alternating brittle and ductile fracture propagation mechanisms subsidiary fractures in both types show curved striae on their fracture surfaces which enable the determination of fracture propagation directions pseudopillow fracture systems are thought only to form in the presence of water including water caused by the melting of ice and snow the curved platy fractures display prominent river lines and may have resulted from cooling contraction post emplacement degassing flow deflation or shearing in the flow against the outer solid crust of the flow during inflation due to recent advances in the understanding of the formation mechanisms of pseudopillow fracture systems they and the other fractures present in the flow can be used to reconstruct the cooling environment the lava is inferred to have been emplaced within subglacial drainage channels incised into or beneath a thin alpine type glacier with coolant infiltrating the lava radially from all sides and ponding or draining of water at or along the lava base we link this flow to the 1727 8 eruption of oraefajokull volcano on the basis of historical records which describe lava effusion in this approximate location and striking chemical compositional similarity between 1727 tephra and the slaga lava

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

combustion metamorphism of bituminous sediments and the formation of melts of granitic and sedimentary composition during combustion metamorphism i e the heating of sediments rich in organic matter by spontaneous subsurface combustion temperatures obtained are frequently high enough to cause partial or total melting of the original rocks these melts are of particular geochemical interest because at lower temperatures their composition closely simulates that of granitic and at higher temperatures that of common sedimentary rocks at the california localities studied here the parent rocks are mainly bituminous mudstones with smaller amounts of diatomites and phosphorites and lesser dolomites limestones shales and cherts it is estimated that rock melting started below 1000 c the first partial melts correspond to a melt fraction of about 5 have a constant composition which is controlled by the eutectic of the dominant mudstones and are undistinguishable in major and trace element composition from common s type granites compared with the original rocks these melts are enriched in sio2 al2o3 k2o na2o as well as in ta hf zr th and ree and are depleted in all other elements studied the only difference between these low temperature melts and granitic ones is their oxygen isotope composition which is that of the sedimentary parent rocks these melts intruded the country rocks as sills and dikes and cooled to glassy rocks simulating obsidians as temperatures rose the melts changed composition and left the magmatic field at the highest temperatures in excess of 1650 c their composition equals that of the original sediments from which they are derived except for volatile components such as h2o and co2 these melts formed intrusive bodies mainly stocks up to several kilometers across on cooling these melts formed fine grained but holocrystalline rocks the dominant minerals of which are cristobalite and tridymite calcic plagioclase cordierite wollastonite gehlenite andradite and apatite where the parent rocks included phosphorites two immiscible melts formed an emulsion in which droplets of apatitic composition are surrounded by a silicate melt almost devoid of p2o5

melt instabilities in an intraplate lithosphere and implications for volcanism in the harrat ash shaam volcanic field nw arabia we investigate melt generation in a slowly extending lithosphere with the aim of understanding the spatial and temporal relationships between magmatism and preexisting rift systems we present numerical models that consider feedback between melt generation and lithospheric deformation and we incorporate three different damage mechanisms brittle damage creep damage and melt damage melt conditions are calculated with a helmholtz free energy minimization method and the energy equation is solved self consistently for latent heat and shear heating effects using a case of a slowly extending 1 1 5mm yr continental lithosphere with a relatively low surface heat flow similar to 50mw m 2 we show that melt rich shear bands are nucleated at the bottom of the lithosphere as a result of shear heating and damage mechanisms upon further deformation melt zones intersect creep damage zones thus forming channels that may be used for the melt to migrate upward if a preexisting structure resides only in the brittle crust it does not control the path of melt migration to the surface and melt filled channels propagate from the bottom upwards independently of upper crustal structures in contrast a preexisting weak structure that reaches a critical depth of 20km allows fast similar to 2ma propagation of melt filled channels that link melt damage from the bottom of the lithosphere to near surface structures our model results may explain the short time scale volume and magma extraction from the asthenosphere through a low surface heat flow lithosphere such as observed for example in the harrat ash shaam volcanic field northwestern arabia which developed in the arabian plate and is spatially linked to the azraq sirhan graben

scaling relationships of joint and vein arrays from the burren co clare ireland abstract we present a study of the systematics of veins and joints in carboniferous limestones of the burren ireland scaling relationships were established for fracture arrays mapped from low elevation aerial photographs that image fractures on numerous limestone pavements for areas up to ca 1 km2 the veins and joints occur in the same sequence but have contrasting scaling properties the veins strike north south and cut many beds to form vertically persistent non stratabound arrays they are strongly clustered and have scale invariant geometric properties vein geometries suggest they grew sub critically under relatively high differential stresses during north south directed variscan compression the joints form stratabound arrays with regular spacings that scale with bed thickness they show greater strike variation than the veins and have lognormal length distributions the joints formed during uplift under low differential stress conditions the contrasting scaling properties of the joints and veins are attributed to different overburden stresses at the time of formation the veins formed at greater depths than the joints in conditions that favoured fracture propagation across mechanical discontinuities resulting in the development of non stratabound scaling properties

pseudopillow fracture systems in lavas insights into cooling mechanisms and environments from lava flow fractures abstract detailed field observations of structures within the flow front of a holocene trachyandesite lava from snaefellsnes iceland are presented the lava provides exceptional three dimensional exposure of complex brittle and ductile deformation textures that record processes of lava fracture and quenching driven by external water the flow front interior is characterised by structures consisting of a large metre scale curviplanar master fracture with many smaller centimetre scale subsidiary fractures perpendicular to the master fracture such structures have previously been recognised in a range of lava compositions from basalt to dacite and called pseudopillows or pseudopillow fractures we propose the term pseudopillow fracture systems to emphasise the consistent package of different fracture types occurring together all documented occurrences of pseudopillow fracture systems are in lavas that have been inferred to interact with an aqueous coolant i e liquid water ice or snow we use fracture surface textures and their orientation in relation to flow banding to identify three distinct types of master fracture and two types of subsidiary fractures master fracture surface textures used to identify fracture mechanisms include chisel marks striae cavitation dimples river lines and rough smooth fracture surface textures these indicate both brittle and ductile fracture happening on different types of master fracture chisel marks on subsidiary fractures indicate comparative cooling rates cooling directions and isotherm orientations at the time of fracture we propose a model for pseudopillow fracture system formation taking into account all the various fracture types textures and fracture propagation mechanisms and discuss their implications for interaction mechanisms between lava flows and external coolants

reappraising columnar joints in different rock types and settings abstract columnar joints comprise a network of opening mode fractures that divide material into prismatic columns they are known in several materials and rock types including basalt mud sandstone and starch in all types the resultant prismatic columns have a similar geometry facilitating a comparison of columnar joints in different materials and processes such as cooling and drying in this study we provide a unique example of columnar joints in low grade meta chalks that are the product of spontaneous combustion metamorphism of organic rich chalks in the mottled zone complex israel although the geometry of the prismatic columns observed in these rocks resembles those seen in basalts our approximate calculations based on thermal expansion and temperature difference show that contraction due to cooling is two orders of magnitude less than the measured contraction moreover the cooling effect should be much more significant in high grade ca 550 900 c rather than in low grade ca 200 c rocks but only the latter contain prismatic columns the prismatic columns are restricted to a discrete stratigraphic level between 1 and 10 meters above a chert and phosphorite unit we suggest that burning of organic matter in the protolith results in mass and volume losses and is the principal process that leads to contraction changes in texture and pore structure recrystallization accompany the contraction resulting in the rearrangement of the protolith into columnar meta chalk this intriguing example of columnar joints in meta chalk indicates that the geometric similarity of prismatic columns should not serve as the sole argument for their formation during cooling

how vein sealing boosts fracture widening rates the buckling enhanced aperture growth mechanism for syn tectonic veins abstract the paper introduces the mechanism of buckling enhanced aperture growth for syn tectonic veins that formed in simple shear dominated kinematic frameworks in the middle or lower crust apart from the well understood concepts of fracture widening driven by effective tensile stresses buckling enhanced fracture aperture growth relates widening to active outward buckling of more viscous incipient cement layers precipitated as hydrothermal minerals for the pore fluid on the walls of juvenile syn tectonic veins driven by fracture parallel compressive creep strain in the host rocks thus the mechanism proposed here follows similar principles as tectonic folding although important differences exist inspired by the structural record of low aspect ratio veins exposed in hp lt metamorphic rocks cropping out on south evia island greece generic numerical models are calculated to study development of buckling instabilities in such incompletely cemented veins and their impact on aperture growth rates the models indicate 1 that aperture growth rates increase with increasing viscosity contrast between the host rocks and the cement layers 2 an increase in the thickness of the cement layers cause acceleration of aperture growth 3 that support of restraining forces at the vein tips offered by the host rocks against buckling of the cement layers cause fully compressive states of stress ahead the fracture tips and 4 that fracture aperture growth is possible against fully compressive fracture normal stresses the buckling enhanced vein aperture growth mechanism yields important implications for the maintenance and decay of fracture bound permeability and for the mechanical state of the middle and lower crust in seismically active regions

chert spheroids of the monterey formation california usa early diagenetic structures of bedded siliceous deposits chert spheroids are distinctive early diagenetic features that occur in bedded siliceous deposits spanning the phanerozoic these features are distinct in structure and genesis from similar concentrically banded wood grain or onion skin chert nodules from carbonate successions in the miocene monterey formation of california usa chert spheroids are irregular concentrically banded nodules which formed by a unique version of brittle differential compaction that results from the contrasting physical properties of chert and diatomite during shortening there is brittle fracture of diatomite around and horizontally away from the convex surface of strain resistant chert nodules unlike most older siliceous deposits the monterey formation still preserves all stages of silica diagenesis thus retaining textural mineralogical and geochemical features key to unravelling the origin of chert spheroids and other enigmatic chert structures chert spheroids found in opal a diatomite form individual nodules composed of alternating bands of impure opal ct chert and pure opal ct or chalcedony with increased burial diagenesis surrounding diatomite transforms to bedded porcelanite or chert and spheroids no longer form discrete nodules yet still display characteristic concentric bands of pure and impure microcrystalline quartz and chalcedony petrographic observations show that the purer silica bands are composed of void filling cement that precipitated in curved dilational fractures and do not reflect geochemical growth banding in the manner of liesegang phenomena invoked to explain concentrically banded chert nodules in limestone chertification of bedded siliceous sediment can occur more shallowly 100 m and rapidly 1 myr than the bulk silica phase transitions forming porcelanite or siliceous shale in the monterey formation and other hemipelagic pelagic siliceous deposits early diagenesis is indicated by physical properties deformational style and oxygen isotopic composition of chert spheroids early formed cherts formed by pore filling impregnation of the purest primary diatomaceous beds along permeable fractures and in calcareous siliceous strata

impact of land cover change on the climate of southwest western australia 1 a sudden reduction in rainfall occurred in the southwest of western australia in the mid 20th century this reduced inflows to the perth water supply by about 120 gl 42 and led to an acceleration of projects to develop new water sources at a cost of about 300 million the reduction in rainfall was coincident with warmer temperatures a major analysis of these changes indicated that the changes in temperature were likely caused by the enhanced greenhouse effect and that the changes in rainfall were likely caused by a large scale reorganization of the atmospheric circulation we explore an alternative hypothesis that large scale land cover change explains the observed changes in rainfall and temperature we use three high resolution mesoscale model configurations forced at the boundaries to simulate for each model five july climates for each of natural and current land cover we find that land cover change explains up to 50 of the observed warming following land cover change we also find in every simulation a reduction in rainfall over southwest western australia and an increase in rainfall inland that matches the observations well we show that the reduced surface roughness following land cover change largely explains the simulated changes in rainfall by increasing moisture divergence over southwest western australia and increasing moisture convergence inland increased horizontal wind magnitudes and suppressed vertical velocities over southwest western australia reduce the likelihood of precipitation inland moisture convergence and increased vertical velocities lead to an increase in rainfall our results indicate that rainfall over southwest western australia may be returned to the long term average through large scale reforestation a policy option within the control of local government such a program would also provide a century scale carbon sink to ameliorate australia s very high per capita greenhouse gas emissions index terms 1655 global change water cycles 1836 3322 meteorology and atmospheric dynamics land atmosphere interactions 3329 meteorology and atmospheric dynamics mesoscale meteorology keywords land cover change mesoscale modeling regional climate change