deciphering the role of solar induced thermal stresses in rock weathering a dearth of direct field observations limits our understanding of individual mechanical weathering processes and how they interact in particular the specific contributions of solar induced thermal stresses to mechanical weathering are poorly characterized here we present an 11 mo data set of cracking using acoustic emissions aes combined with measurements of rock temperature strain and other environmental conditions all recorded continuously for a granite boulder resting on the ground in open sun we also present stresses derived from a numerical model of the temperature and stress fields in the boulder idealized as a uniform elastic sphere experiencing simple solar temperature forcing the thermal model is validated using this study s data most observed cracking coincides with the timing of calculated maximum insolation driven tensile thermal stresses we also observe that most cracking occurs when storms or other weather events strongly perturb the rock surface temperature field at these times we hypothesize that these weather actuated thermal perturbations result in a complex thermal stress distribution that is superimposed on the background stresses arising from simple diurnal forcing these additive stresses ultimately trigger measurable cracking measured locations of observed cracking and surface strain support this hypothesis in that they generally match model predicted locations of maximum solar induced tensile stresses also recorded rock surface strain scales with diurnal temperature cycling and records progressive cumulative extension dilation consistent with ongoing thermal stress driven subcritical crack growth in the boulder our results therefore suggest that 1 insolation related thermal stresses by themselves are of sufficient magnitude to facilitate incremental subcritical crack growth that can subsequently be exploited by other chemical and physical processes and 2 simple insolation can impart an elevated tensile stress field that makes rock more susceptible to cracking triggered by added stress from other weathering mechanisms our observed cracking activity does not correlate simply with environmental conditions including temperature extremes or the often cited 2 c min thermal shock threshold we propose that this lack of correlation is due to both the ever varying ambient stress levels in any rock at earth s surface as well as to the fact that ongoing subcritical crack growth itself will influence a rock s stress field and strength because similar thermal cycling is universally experienced by subaerially exposed rock this study elucidates specific mechanisms by which solar induced thermal stresses may influence virtually all weathering processes

brittle and semibrittle creep of tavel limestone deformed at room temperature deformation and failure mode of carbonate rocks depend on the confining pressure in this study the mechanical behaviour of a limestone with an initial porosity of 14 7 is investigated at constant stress at confining pressures below 55 mpa dilatancy associated with micro fracturing occurs during constant stress steps ultimately leading to failure similar to creep in other brittle media at confining pressures higher than 55 mpa depending on applied differential stress inelastic compaction occurs accommodated by crystal plasticity and characterized by constant ultrasonic wave velocities or dilatancy resulting from nucleation and propagation of cracks due to local stress concentrations associated with dislocation pile ups ultimately causing failure strain rates during secondary creep preceding dilative brittle failure are sensitive to stress while rates during compactive creep exhibit an insensitivity to stress indicative of the operation of crystal plasticity in agreement with elastic wave velocity evolution and microstructural observations

fracture mechanical properties of carbonate and evaporite caprocks in sichuan basin china with implications for reservoir seal integrity abstract fracture development in reservoir rocks enhances the flow and transport properties of the petroleum system yet fracture in caprocks can significantly risk reservoir seal integrity over geological time scales the slow subcritical fracture propagation can contribute considerably to the fracture system development especially under reactive fluid environments during petroleum migration and accumulation using the double torsion technique we measure the subcritical fracture growth properties of carbonate and anhydrite caprock core samples in sichuan basin china under different environments of ambient air de ionized water and formation water experimental results show that the aqueous fluids significantly promote subcritical fracture growth 1 compared to ambient air the presence of aqueous fluids dramatically reduces both the fracture toughness kic and subcritical fracture growth index sci of the caprocks 2 the kic reduction is more prominent for the anhydrite 50 caprock than the carbonate 30 caprock which suggest that mineral solubility is responsible for the fluid weakening effect 3 a systematic change of sci with varying fluid conditions is observed sci is the largest in air smallest in formation water and intermediate in de ionized water microstructural analysis suggests a predominantly opening mode for both the natural fractures and induced fractures during fracture mechanical testing our results further demonstrate that subcritical fracture growth process is also strongly related to rock composition and stress conditions these observations provide insights into the mechanisms controlling fracture growth in carbonate and evaporite caprocks which could extend to brittle failure and fluid rock interactions occurring during the seismic cycle in the upper crust

an experimental study of the mechanical properties of granite after high temperature exposure based on mineral characteristics abstract the macroscopic mechanical properties of granite collected from the fujian province china were measured using uniaxial compression and three point bending after the granite was exposed to high temperatures the stress strain relationship was measured and mechanical properties including the young s modulus uniaxial compressive strength ucs and fracture toughness were calculated in general the uniaxial compressive strength ucs young s modulus and fracture toughness of the granite specimens decrease with an increase in heat treatment temperature up to 800 c above which there is no obvious change the microstructure of the failed specimens was explored using scanning electron microscopy sem and stereomicroscopy after heat treatment the minerals within the granite were analysed using x ray diffraction the failure mechanisms and the mechanical characteristics of the granite are explained in terms of the microstructure and the minerals present the results of the experiments indicate that the crystal structure of the phlogopite in granite transforms to a more stable structure between 400 c and 600 c with an associated increase in volume a dehydroxylation reaction also occurs in the phlogopite at 400 c after which the uniaxial compressive strength ucs decreases and the ductility increases using sem and stereomicroscopy it was observed that the connection between the minerals within the granite becomes weaker with increasing heat treatment temperature intercrystalline cracking is the main failure mode for samples exposed to temperatures below 800 c whereas transcrystalline cracking can be observed in samples exposed to 1000 c

a comparison of seismic and structural measurements of scaling exponents during tensile subcritical crack growth abstract the observed fractal nature of both fault length distributions and earthquake magnitude frequency distributions suggests that there may be a relationship between the structure of active fault systems and the resulting seismicity in previous theoretical work a positive correlation between the exponent d from the fracture length distribution and the seismic or acoustic emission ae b value has been inferred from a simple dislocation model of the seismic source here we present the first experimental evidence for a correlation between d and b from a series of tensile fracture mechanics tests on crystalline rock carried out in different environmental conditions both air dry and water saturated and at ambient temperature and pressure the microseismic acoustic emissions were monitored during subcritical crack growth under controlled conditions of constant stress intensity k i and quantitative analyses of the resulting fracture patterns were carried out on the same specimens it is found that ae b values ranging from 1 0 to 2 3 correlate negatively with the normalized stress intensity k i k ic where k ic is the fracture toughness of the specimen the microcrack length distribution exponent d ranges from 1 0 to 1 7 fluid presence has a first order influence on both the ae and structure produced in these experiments for experiments at low stress intensity or high fluid content the activation of the stress corrosion mechanism for k i k ic leads to a greater relative proportion both of small cracks and of low amplitude acoustic emissions reflected in higher values of d and b the exponent d is found to correlate positively with the ae b value

gravitational slope deformation of a resurgent caldera new insights from the mechanical behaviour of mt nuovo tuffs ischia island italy abstract ischia island italy is an impressive example of the rare phenomenon of caldera resurgence the emplacement and replenishment of magmas at shallow depth resulted in a vertical uplift of about 900 m concentrated in the western portion of mt epomeo 789 m a s l as a consequence of this uplift the island has experienced several slope instabilities at different scales since the holocene from shallow mass movements to large rock and debris avalanches these mass wasting events which mobilised large volumes of greenish alkali trachytic tuff the mt epomeo green tuff megt were strictly related to volcano tectonic activity and the interaction between the volcanic slopes and the hydrothermal system beneath the island deep seated gravitational slope deformation dsgsd at mt nuovo located adjacent to densely populated coastal villages is an ongoing process that covers an area of 1 6 km 2 the mt nuovo dsgsd involves a rock mass volume of 190 mm 3 and is accommodated by a main shear zone and a series of sub vertical fault zones associated with high angle joint sets to improve our understanding of this gravity induced process we performed a physical porosity and permeability and mechanical uniaxial and triaxial deformation experiments characterisation of two ignimbrite deposits both from the megt that form a significant component of the nw sector of mt epomeo the main conclusions drawn from our experiments are twofold first the presence of water dramatically reduces the strength of the tuffs suggesting that the movement of fluids within the hydrothermal system could greatly impact slope stability second the transition from brittle dilatant to ductile compactant behaviour in the tuffs of the megt occurs at a very low effective pressure analogous to a depth of a couple of hundred metres and that this transition is likely moved closer to the surface in the presence of water we hypothesise that compactant porosity decreasing behaviour at the base of the layer could therefore facilitate slope instability although our results show that transient exposure to 300 c does not influence the short term strength of the tuff we speculate that the high in situ temperature could increase the efficiency of brittle and compactant creep and therefore increase the rate of slope deformation taken together our experimental data highlight a potentially important role for the hydrothermal system that reaches a minimum depth of 1 km in dictating the dsgsd at mt nuovo an understanding of this deformation process is not only important for the proximal coastal villages at risk of engulfment by a large debris avalanche but also for the towns and cities along the coast of the gulf of naples that are at risk to a secondary consequence of such an avalanche a tsunami wave

influence of surrounding environment on subcritical crack growth in marble abstract understanding subcritical crack growth in rock is essential for determining appropriate measures to ensure the long term integrity of rock masses surrounding structures and for construction from rock material in this study subcritical crack growth in marble was investigated experimentally focusing on the influence of the surrounding environment on the relationship between the crack velocity and stress intensity factor the crack velocity increased with increasing temperature and or relative humidity in all cases the crack velocity increased with increasing stress intensity factor however for carrara marble cm in air we observed a region in which the crack velocity still increased with temperature but the increase in the crack velocity with increasing stress intensity factor was not significant this is similar to region ii of subcritical crack growth observed in glass in air region ii in glass is controlled by mass transport to the crack tip in the case of rock the transport of water to the crack tip is important in general region ii is not observed for subcritical crack growth in rock materials because rocks contain water because the porosity of cm is very low the amount of water contained in the marble is also very small therefore our results imply that we observed region ii in cm because the crack velocity increased in both water and air with increasing temperature and humidity we concluded that dry conditions at low temperature are desirable for the long term integrity of a carbonate rock mass additionally mass transport to the crack tip is an important process for subcritical crack growth in rock with low porosity

subcritical crack growth and other time and environment dependent behavior in crustal rocks stable crack growth strongly influences both the fracture strength of brittle rocks and some of the phenomena precursory to catastrophic failure quantification of the time and environment dependence of fracture propagation is attempted with the use of a fracture mechanics technique we examine some of the difficulties encountered when applying techniques originally developed for simple synthetic materials to complex materials like rocks a picture of subcritical fracture propagation is developed that embraces the essential ingredients of the microstructure a microcrack process zone and the different roles that the environment plays to do this we examine the results of 1 fracture mechanics experiments on five rock types 2 optical and scanning electron microscopy 3 studies of microstructural aspects of fracture in ceramics and 4 exploratory tests examining the time dependent response of rock to the application of water

thermomechanical milling of accessory lithics in volcanic conduits accessory lithic clasts recovered from pyroclastic deposits commonly result from the failure of conduit wall rocks and represent an underutilized resource for constraining conduit processes during explosive volcanic eruptions the morphological features of lithic clasts provide distinctive textural fingerprints of processes that have reshaped them during transport in the conduit here we present the first study focused on accessory lithic clast morphology and show how the shapes and surfaces of these accessory pyroclasts can inform on conduit processes we use two main types of accessory lithic clasts from pyroclastic fallout deposits of the 2360 b p subplinian eruption of mount meager british columbia as a case study i rough and subangular dacite clasts and ii variably rounded and smoothed monzogranite clasts the quantitative morphological data collected on these lithics include mass volume density 2 d image analysis of convexity c and 3 d laser scans for sphericity and smoothness s shaping and comminution i e milling of clasts within the conduit are ascribed to three processes 1 disruptive fragmentation due to high energy impacts between clasts or between clasts and conduit walls 2 ash blasting of clasts suspended within the volcanic flux and 3 thermal effects we use a simplified conduit eruption model to predict ash blasting velocities and lithic residence times as a function of clast size and source depth thereby constraining the lithic milling processes the extent of shape and surface modification i e rounding and honing is directly proportional to clast residence times within the conduit prior to evacuation we postulate that the shallow seated dacite clasts remain subangular and rough due to short 2 min residence times whereas monzogranite clasts are much more rounded and smoothed due to deeper source depths and consequently longer residence times up to 1 h larger monzogranite clasts are smoother than smaller clasts due to longer residence times and to greater differential velocities within the ash laden jet lastly our model residence times and mass loss estimates for rounded clasts are used to estimate minimum attrition rates due to volcanic ash blasting within the conduit e g 12 cm3 s 1 for 25 cm clasts sourced at 2500 m depth

earthquake source nucleation a physical model for short term precursors abstract ohnaka m 1992 earthquake source nucleation a physical model for short term precursors in t mikumo k aki m ohnaka l j ruff and p k p spudich editors earthquake source physics and earthquake precursors tectonophysics 211 149 178 this paper deals with a quasistatic leading to a quasidynamic at a later time nucleation process that precedes the earthquake dynamic rupture and models of the earthquake source nucleation are put forward based on physical principles a quasistatic to quasidynamic rupture nucleation process is an intrinsic part of the ensuing earthquake dynamic rupture in other words the nucleation process itself is an short term or immediate precursor that occurs in a localized zone during the earthquake nucleation premonitory slip proceeds in the localized nucleation zone and shear stress also decreases gradually in the zone since slip weakening occurs during the nucleation immediate foreshock activity is a part of the nucleation process leading to the mainshock dynamic rupture therefore hypocentral locations of foreshocks are necessarily restricted to lie near the mainshock hypocentre onset of the mainshock rupture whether or not foreshocks occur during the mainshock nucleation depends on how the rupture growth resistance varies on a local to small scale since patches of greater rupture growth resistance are considered to prevail on a local to small scale in the fault zone carrying immediate foreshocks would be one of the major characteristics of earthquakes that nucleate within the brittle seismogenic layer when a mainshock earthquake nucleates within the brittle seismogenic layer and its hypocentre is located near the base of the seismogenic layer immediate foreshocks for this mainshock are necessarily restricted to lie within a localized region shallower than hypocentral depth of the mainshock by contrast the nucleation process below the base of the seismogenic layer is aseismic in nature so that carrying no conspicuous foreshocks would be a common characteristic of interplate earthquakes that nucleate below the base of the brittle seismogenic layer the breakdown strength p the breakdown stress drop b and the critical slip displacement dc are indicative of the rupture growth resistance to estimate depth variations of these parameters the effects of the normal stress n and temperature t on those parameters are examined using available data so far published the combined effects of n and t on p in the brittle to semibrittle regime are found to be represented empirically by p n t po n 1 cosh 50 t 40 sinh 50 t exp 2000 t where po n 135 7 0 750 n and t is measured in k and n in mpa it is found that dc increases sharply with t but is insensitive to n above 300 c while dc depends on n but is insensitive to t below 300 c on the basis of these results variations in p dc and b at mid crustal depths are estimated for quartzo feldspathic rocks for a given geothermal gradient

thermal elastic stresses and the criticality of the continental crust 1 heating or cooling can lead to high stresses in rocks due to the different thermal elastic properties of minerals in the upper 4 km of the crust such internal stresses might cause fracturing yet it is unclear if thermal elasticity contributes significantly to critical stresses and failure deeper in earth s continental crust where ductile creep causes stress relaxation we combined a heating experiment conducted in a synchrotron microtomograph advanced photon source usa with numerical simulations to calculate the grain scale stress field in granite generated by slow burial we find that deviatoric stresses 100 mpa can be stored during burial with relaxation times from 100 s to 1000 s ka even in the ductile crust hence grain scale thermal elastic stresses may serve as nuclei for instabilities thus rendering the continental crust close to criticality

loess stratigraphy in central china abstract loess sequences in central china provide a detailed record of climate changes during the last 2 5 million years at least 44 major shifts from glacial to interglacial conditions occurred during this time in china prolonged loess deposition lasting more than 40 millennia occurred eleven times with only four of these intervals in brunhes chron the climate oscillations took place with increased frequency between 2 4 and 0 5 ma compared to the last half million years major shifts toward cooler and drier climates occurred at about 2 4 ma 1 2 ma and 0 5 ma while less pronounced shifts are dated to 1 65 ma 0 8 ma and 0 2 ma three of the last four reversals of the earth s magnetic field whose lithostratigraphic position has been detail are recorded in a glacial deposit the low field magnetic susceptibility of loess and soils which reflects the concentration of ultrafine magnetite is related to the degree of pedogenesis and serves as a proxy measure of climate the accumulation rate of the magnetite was fairly constant throughout the last 1 2 million years and can serve as a measure of time susceptibility records from localities near xifeng and luochuan located 160 km apart are compared and used to define informal lithostratigraphic units in the loess plateau of china

the processes controlling damage zone propagation induced by wellbore fluid injection summary induced seismicity by wellbore fluid injection is an important tool for enhancing permeability in hydrocarbon and geothermal reservoirs we model nucleation and propagation of damage zones and seismicity patterns for two dimensional plane strain configuration at a depth of 5 km using novel numerical software developed in the course of this study simulations include the coupling of poro elastic deformation and groundwater flow with damage evolution weakening and healing and its effect on the elastic and hydrologic parameters results show that the process occurring during fluid injection can be divided into four stages the duration of each stage depends on the hydrological and mechanical parameters initially fluid flows into the rock with no seismic events 5 to 20 hr at this stage damage increases from 0 to 1 creating two sets of conjugate zones four narrow damage zones thereafter the occurrence of seismic events and faulting begins and accelerates for the next 20 to 70 hr at the initial part of this stage two of the damage zones create stress shadows on the other two damage zones that stop progressing the velocity of the advancing damage is limited only by the rock parameters controlling damage evolution at the third stage which lasts for the following 20 30 hr damage acceleration decreases because fluid transport becomes a limiting factor as the damage zones are too long to efficiently transfer the pressure from the well to the tip of the damage zones finally the damage decelerates and even stops in some cases the propagation of damage is controlled and limited by fluid transport from the injection well to the tip of the damage zones becausefluid transport does not keep up with the dilatancy of the damage zones the time and distance of propagation depend on the damage permeability coupling and the remote shear stress higher remote shear stress causes shorter initial periods of no seismicity strong damage permeability coupling causes longer acceleration stage

accuracy of north american human skeleton ages accelerator mass spectrometry ams radiocarbon dates fail to provide conclusive evidence that all new world human fossils are younger than approximately 11 000 yr because fossil bones vary widely in preservation their radiocarbon dates are not equally accurate molecular level radiocarbon dating which used individual amino acids to assess fossil diagenesis revealed that dates on known age noncollagenous bone were underestimated by at least 2000 to 9000 yr the significance is that 11 000 yr old fossil bones with poor preservation would yield holocene and not pleistocene radiocarbon ages regardless of what chemical pretreatment or 14c counting method was used irreplaceable evidence for pleistocene age fossils in the new world could be lost if the diagenesis of fossil bones is not evaluated before the bones are radiocarbon dated in contrast radiocarbon ages for collagenous fossils can be determined more accurately if 14c is measured in several individual amino acids that are isolated from collagenous bone protein molecular level radiocarbon dating will greatly improve not only the accuracy of chronologies for human migrations and animal extinctions but of all late quaternary chronologies that are based upon the 14c dating of fossil proteins

evidence of thermally induced borehole elongation a case study at soultz france abstract when they occur borehole breakouts are considered strong markers of principal stress directions at depth an innovative processing method for automatically identifying breakouts from ultrasonic borehole wall images has been developed it has been applied to data sets from two deep sub vertical wells gpk1 and gpk2 at the soultz geothermal site in eastern france in well gpk1 below 3 km depth compression breakouts with a 95 7 azimuth increasingly occur with depth they result from time dependent compression failure at sub critical stress levels and are indicators of the minimum horizontal principal stress orientation however in the uppermost logged section of well gpk2 1 6 2 9 km depth continuous borehole elongations share roughly the same azimuth with so called drilling induced fractures 164 18 and 175 17 azimuth respectively both features concomitantly vanish with depth together with the amplitude of the thermal perturbation induced by drilling it is proposed that these latter borehole elongations result from a pervasive cooling induced tensile micro cracking process prior to macroscopic failure localization they are termed thermal elongations and are indicators of the maximum horizontal principal stress orientation had a simple logging caliper tool been used for this work these thermal elongations might have been confused with classical compression breakouts a simple criterion for differentiating compression breakouts from thermal elongation is proposed

comparison of fluid inclusion decrepitation and acoustic emission profiles of westerly granite and sioux quartzite abstract acoustic emissions recorded during slow heating of rocks at 1 atm have generally been attributed to intergranular stresses generated by differences in thermal expansion coefficients and elastic moduli of neighboring grains these experiments have considered only the solid phases in analysis of the results and little attention has been given to the role of intragranular or grain boundary fluid inclusions in initiating or facilitating fracturing in principle fluid inclusions may be thought of as a special case in which the neighboring grains are fluids rather than the usual solid mineral phases the major differences are that 1 the number of inclusions per unit volume is usually 10 2 to 10 6 times greater than the number of mineral grains in that same volume and that 2 the volume increase with temperature for most fluids is 1 2 orders of magnitude greater than for minerals thus when fluid inclusion grains are considered the absolute number of grain to grain contacts in a given sample volume is several orders of magnitude greater than if the inclusions are not considered further the magnitude of the stresses between any two adjacent grains may be much higher if one of the grains is a fluid inclusion owing to the larger coefficient of thermal expansion of the fluid relative to a solid phase as a result substantial overpressures may be generated at the fluid inclusion grain mineral grain contact during heating experiments these overpressures result in brittle failure when the stress around the inclusion exceeds the local strength of the host crystal in fluid inclusion terminology this fracturing event is referred to as decrepitation samples of westerly granite and sioux quartzite were heated at 2 c min in a gas flow stage in order to determine the decrepitation profiles number of decrepitations as a function of temperature of these rocks decrepitation was monitored visually during heating experiments and was evidenced by the explosive loss of fluids from the inclusion during a fracturing event decrepitation produces microfractures which originate at decrepitated fluid inclusions and propagate outward into the surrounding mineral four different types of inclusions based on room temperature phase ratios and distribution within the host phase were noted in westerly granite type 1 inclusions contain a single aqueous liquid type 2 are two phase liquid rich inclusions type 3 are mostly one phase fluid inclusions that occur in dense clusters in plagioclase and type 4 are two phase moderate density h 2 o co 2 salt inclusions type 5 inclusions are one phase aqueous liquid inclusions observed in sioux quartzite fluid inclusions in westerly granite decrepitate between 75 and 573 c with peaks at 275 300 c and 400 450 c type 4 inclusions in westerly granite tend to decrepitate at lower temperatures 75 300 c than other inclusion types initial fracturing of the quartz host surrounding type 1 inclusions primary decrepitation generally occurs over the range 100 400 c and the fractures continue to propogate through the quartz secondary decrepitation during further heating from 400 to 573 c fluid inclusions in sioux quartzite decrepitate between 188 and 573 c with a broad maximum at 250 450 c primary decrepitation generally occurs at 200 450 c while secondary decrepitation occurs at 350 573 c in sioux quartzite some small inclusions in both samples do not decrepitate until the quartz transition temperature 573 c is reached decrepitation profiles determined in this study display many similarities and differences when compared to previously published acoustic emission profiles of westerly granite and sioux quartzite owing to fundamental differences in the techniques used to measure acoustic emission and decrepitation and the potentially different energies associated with the two events it is unclear whether decrepitating fluid inclusions are recorded during acoustic emission studies however if decrepitating fluid inclusions are being recorded during acoustic emission measurements then our results suggest that 1 decrepitation contributes more to low temperature portions of acoustic emission profiles than to high temperature portions for both sioux quartzite and westerly granite 2 compared to westerly granite a larger percentage of acoustic emissions in the low temperature decrepitation profile of sioux quartzite is due to fluid inclusions 3 decrepitating fluid inclusions not stresses generated by solid solid interactions cause the majority of intragranular fracturing in quartz below the temperature of the transition in both these rocks

time dependent brittle creep in darley dale sandstone 1 the characterization of time dependent brittle rock deformation is fundamental to understanding the long term evolution and dynamics of the earth s crust the chemical influence of pore water promotes time dependent deformation through stress corrosion cracking that allows rocks to deform at stresses far below their short term failure strength here we report results from a study of time dependent brittle creep in water saturated samples of darley dale sandstone initial porosity 13 under triaxial stress conditions results from conventional creep experiments show that axial strain rate is heavily dependent on the applied differential stress a reduction of only 10 in differential stress results in a decrease in strain rate of more than two orders of magnitude however natural sample variability means that multiple experiments must be performed to yield consistent results hence we also demonstrate that the use of stress stepping creep experiments can successfully overcome this issue we have used the stress stepping technique to investigate the influence of confining pressure at effective confining pressures of 10 30 and 50 mpa while maintaining a constant 20 mpa pore fluid pressure our results demonstrate that the stress corrosion process appears to be significantly inhibited at higher effective pressures with the creep strain rate reduced by multiple orders of magnitude the influence of doubling the pore fluid pressure however while maintaining a constant effective confining pressure is shown to influence the rate of stress corrosion within the range expected from sample variability we discuss these results in the context of microstructural analysis acoustic emission hypocenter locations and fits to proposed macroscopic creep laws

influences of electrolyte concentration on subcritical crack growth in sandstone in water abstract information on subcritical crack growth in rock is essential to ensure the long term stability of structures in a rock mass subcritical crack growth in rock is known to be affected by its surrounding environment in most cases rock found underground is saturated by water the underground water can be fresh water or salt water with variable electrolyte concentrations however the influence of electrolyte concentration on subcritical crack growth in rock has not been fully elucidated in this study subcritical crack growth in berea sandstone and shirahama sandstone was measured when saturated with distilled water and with sodium chloride solutions of differing concentrations crack velocity in berea sandstone changed very little with different electrolyte concentrations by contrast in the shirahama sandstone crack velocity became lower with increased electrolyte concentrations up to 1 0 mol l with increasing electrolyte concentration the width of the electric double layer on the surface of mineral grains decreased which caused a decrease in the repulsive force acting on the crack surface leading to a decrease in crack velocity up to concentrations of 1 0 mol l however when the electrolyte concentration was higher than 1 0 mol l crack velocity increased due to the nucleation of microscopic defects on the boundary between clays and stiff mineral grains thus electrolyte concentration affects subcritical crack growth by decreasing the width of the electric double layer and the condensation of clay minerals

α β phase transition in quartz monitored using acoustic emissions summary it is usually suggested that thermal cracking in a quartz bearing rock results from the anomalously high volumetric expansion coefficients of quartz e g simmons cooper 1978 it has also been recognized that thermal expansion mismatch and mineral anisotropy contribute to thermal cracking in materials that consist of a polycrystalline aggregate composed of several anisotropic minerals even in the absence of a temperature gradient friedrich wong 1986 experiments investigating thermal cracking in rocks commonly involve imaging and quantitative stereology of crack populations induced in rocks treated to various peak temperatures e g friedrich johnson 1978 homand etienne troalan 1984 atkinson mcdonald meredith 1984 meredith atkinson 1985 here we report on acoustic emission experiments that monitor the process of thermal cracking as it occurs during heating supported by measurements of crack surface area pore fluid permeability porosity and surface conductivity carried out on rock samples treated to various peak temperatures the acoustic emission measurements show a strong peak of microcracking at the phase transition temperature for quartz 573 c superimposed upon a background of microcracking due to thermal expansion there is also a clear peak of microcracking at higher temperatures 800 c that can be attributed to oxidation dehydroxylation reactions of hornblende and chlorite measurements of fluid permeability pore surface area porosity and electrical conductivity made on samples that have been heat treated to various maximum temperatures show increases associated with a major episode of cracking in the 500 600 c temperature range indicating that the new cracks form a well interconnected network this has been confirmed by sem and optical microscopy these results have implications for the electrical conductivity of the continental crust providing a mechanism enabling the high pore fluid connectivity needed to explain zones of high electrical conductivity at depth providing that cracks opened in this way remain open at the high pressures existing at depth it should be recognized however that th se measurements are limited in their direct application since they were obtained under initially dry conditions at laboratory pressures

regional aridity in north america during the middle holocene increased aridity throughout the great plains and rocky mountain region during the middle holo cene has been documented from pollen records aeolian proxy variables in lake cores and active sand dune migration varve calibration provided by a continuously varved record of the holocene from a core from elk lake northwestern minnesota shows that the influx of aeolian clastic material increased beginning about 8 ka and ended about 3 8 ka with peak aeolian activity at about 6 ka if aeolian influx to elk lake corresponds in time to aeolian influx in other lakes and to maximum dune activity in minnesota dune fields then the varve calibration in elk lake provides precise time calibration of periods of peak aeolian activity in minnesota palaeowind studies from the minnesota dune fields show that the dominant wind direction when the dunes were active was from the northwest the same as the dominant wind direction in dune fields throughout the great plains and rocky mountains if the mid holocene aeolian act

fracture toughness and fracture roughness interrelationship in thermally treated westerly granite this paper presents an experimental work aimed at assessing the correlation between fracture toughness k ic and fracture roughness for a series of westerly granite specimens thermally treated up to 850 c mode i fracture toughness as a function of thermal treatment is determined using cracked chevron notched brazilian disc specimens the degree of roughness of the resultant fracture surfaces is analyzed with the aid of a high accuracy high precision stereo topometric measurement system roughness and toughness values display a negative correlation as a function of temperature fracture toughness decreases with increasing temperature due to the gradual opening of grain grain boundaries in response to thermal stresses mode i fractures preferentially follow these weakened grain grain boundaries which in addition to the thermal expansion of individual grains result in rougher failure profiles with increasing temperature at low temperature a distinct anisotropy in roughness was observed in all fracture surfaces with higher roughness value perpendicular to the direction of fracture propagation however higher treatment temperatures resulted in the homogenization of fracture roughness in all directions these results confirm the important link among petrofabric analysis fracture toughness and fracture roughness in response to thermal treatment

thermal stress weathering and the spalling of antarctic rocks using in situ field measurements laboratory analyses and numerical modeling we test the potential efficacy of thermal stress weathering in the flaking of millimeter thick alteration rinds observed on cobbles and boulders of ferrar dolerite on mullins glacier mcmurdo dry valleys mdv in particular we examine whether low magnitude stresses arising from temperature variations over time result in thermal fatigue weathering yielding slow crack propagation along existing cracks and ultimate flake detachment our field results show that during summer months clasts of ferrar dolerite experience large temperature gradients across partially detached alteration rinds 4 7 c mm 1 and abrupt fluctuations in surface temperature up to 12 c min 1 the latter are likely due to the combined effects of changing solar irradiation and cooling from episodic winds the results of our thermal stress model coupled with subcritical crack growth theory suggest that thermal stresses induced at the base of thin alteration rinds 2 mm thick common on rocks exposed for 105 years may be sufficient to cause existing cracks to propagate under present day meteorological forcing eventually leading to rind detachment the increase in porosity observed within alteration rinds relative to unaltered rock interiors as well as predicted decreases in rind strength based on allied weathering studies likely facilitates thermal stress crack propagation through a reduction of fracture toughness we conclude that thermal stress weathering may be an active though undervalued weathering process in hyperarid terrestrial polar deserts such as the stable upland region of the mdv

pleistocene climates in china dated by magnetic susceptibility the magnetic susceptibility of loess and interbedded soils in central china varies with the degree of pedogenesis and serves as a proxy measure of climate the concentration of magnetic minerals in the sediment is inversely proportional to sedimentation rates throughout the brunhes epoch susceptibility measurements combined with the reversal stratigraphy provide a time scale that is independent of astronomic chronology on this scale the susceptibility record closely parallels the oxygen isotope fluctuations in deep sea sediments suggesting a close interdependence of the chinese dust falls the volume of land based ice and global climate

comparing dry and wet friction of plagioclase implication to the mechanism of frictional evolution effect at hydrothermal conditions to understand mechanism for unstable velocity weakening slip behavior of plagioclase at hydrothermal conditions in the lower crust we employed nominally dry experiments on plagioclase gouge to isolate fluid assisted processes that may be activated at hydrothermal conditions and obtained constitutive parameters fitted to rate and state friction laws with temperatures of 99 617 c and confining pressure of 150 mpa allowing comparison to the hydrothermal counterpart while the direct effect a value has relatively high values of 0 007 0 01 in the high temperature range above 300 400 c the evolution effect b value shows a diminishing trend from peak values of 0 0075 and 0 0057 respectively at 305 c and 400 c down to 0 at 617 c in contrast to the increasing trend in both effects at hydrothermal conditions the evolution effect in the dry experiments may be controlled by structural changes related to degrees of shear localization and evolution of porosity change rate in response to a change in loading rate the evolution effect in dry friction compared to the thermally enhanced much stronger one at hydrothermal conditions suggests activation of fluid assisted time dependent mechanism operating in the wet experiments

fracturing of volcanic systems experimental insights into pre eruptive conditions conditions for fracturing are a primary control on the behaviour of volcanic systems especially during the approach to eruption we here present the results of deformation experiments under simulated volcanic conditions on a porhyritic andesite from ancestral mount shasta andesite was chosen as a representative material because it is common at subduction zone volcanoes among both erupted products and country rock we deformed the lava in tension and triaxial compression tests at strain rates of 10 5 s 1 confining pressures from 0 to 50 mpa and temperatures up to 900 degrees c we also concurrently recorded acoustic emissions ae in order to monitor cracking activity the results show that deformation behaviour changes significantly in the temperature range 600 750 degrees c thus as temperatures increased across this interval the tensile fracture toughness increased from 2 5 0 5 mpa m 1 2 to 3 5 1 mpa m 1 2 the compressive strength decreased from 110 30 mpa to 55 35 mpa at 900 degrees c and the corresponding young s modulus decreased from 20 4 gpa to 6 4 gpa the changes occur when the deformation of the sample changes from elastic brittle to brittle ductile behaviour which we attribute to the blunting of crack tips due to melting of the glass phase and enhanced crystal plasticity at high temperature ae activity was observed in all experiments indicating that earthquakes can be generated not only in country rock but also in hot magma such as may be found in lava domes and at the margins of magma conduits in addition the trends in accelerating ae event rates before sample failure were comparable to those seen in earthquakes before some volcanic eruptions and a minimum in the seismic b value coincided with sample failure applied to volcanic systems the results suggest that 1 andesite strength and elasticity will not be affected by temperature or pressure beyond similar to 10 100 m from active magma 2 before eruptions fractures propagate preferentially through weaker horizons in a mechanically heterogeneous volcano and 3 volcanic rocks have characteristic seismic b values that are perturbed during the approach to bulk failure each of these conditions provides quantitative constraints on models for seismic precursors to eruption or intrusion c 2009 elsevier b v all rights reserved

brittle creep in basalt and its application to time dependent volcano deformation time dependent brittle deformation is a fundamental and pervasive process operating in the earth s upper crust its characterization is a pre requisite to understanding and unraveling the complexities of crustal evolution and dynamics the preferential chemical interaction between pore fluids and strained atomic bonds at crack tips a mechanism known as stress corrosion allows rock to fail under a constant stress that is well below its short term strength over an extended period of time a process known as brittle creep here we present the first experimental measurements of brittle creep in a basic igneous rock a basalt from mt etna volcano under triaxial stress conditions results from conventional creep experiments show that creep strain rates are highly dependent on the level of applied stress and can be equally well fit by a power law or an exponential law with a 20 increase in stress producing close to three orders of magnitude increase in creep strain rate results from stress stepping creep experiments show that creep strain rates are also influenced by the imposed effective confining pressure we show that only part of this change can be attributed to the purely mechanical influence of an increase in effective pressure with the remainder interpreted as due to a reduction in stress corrosion reactions the result of a reduction in crack aperture that restricts the rate of transport of reactive species to crack tips overall our results also suggest that a critical level of crack damage is required before the deformation starts to accelerate to failure regardless of the level of applied stress and the time taken to reach this point the experimental results are discussed in terms of microstructural observations and fits to a macroscopic creep law and compared with the observed deformation history at mt etna volcano c 2011 elsevier b v all rights reserved

common evolution of mechanical and transport properties in thermally cracked westerly granite at elevated hydrostatic pressure increasing the damage and crack porosity in crustal rocks can result in significant changes to various key physical properties including mechanical strength elastic and mechanical anisotropy and the enhancement of transport properties using a non interactive crack effective medium nic theory as a fundamental tool we show that elastic wave dispersion can be inverted to evaluate crack density as a function of temperature and is compared with optically determined crack density further we show how the existence of embedded microcrack fabrics in rocks also significantly influences the fracture toughness kic of rocks as measured via a suite of tensile failure experiments chevron cracked notch brazilian disk finally we include fluid flow in our analysis via the gueguen and dienes crack porosity permeability model using the crack density and aspect ratio recovered from the elastic wave velocity inversion we successfully compare permeability evolution with pressure with the laboratory measurements of permeability

frictional properties of the south china sea oceanic basalt and implications for strength of the manila subduction seismogenic zone abstract we conduct friction experiments on a basalt sample no u1431e41r6w from international oceanic discovery program iodp expedition 349 in the temperature range of 100 600 c and effective normal stress of 50 mpa with 100 mpa pore fluid pressure loading rates are stepped up and down in the range of 0 04 1 0 m s to obtain the velocity dependence of friction based on the rate and state friction law we numerically fit the experimental curves to acquire constitutive parameters that cannot be obtained by direct measurement experimental results show that basalt friction coefficient varies from 0 70 to 0 74 similar to the range of gabbro documented in a previous study he et al 2007 stable velocity strengthening behavior is observed at temperatures of 100 c and 200 c and unstable velocity weakening at temperatures above 300 c with occurrences of periodic stick slips both a and b values inferred by numerical fittings show positive temperature dependence in the temperature range of 100 600 c the stick slip motions above 300 c seem to be due to a dramatic decrease in the characteristic slip distance dc from 11 to 15 m to 0 4 2 5 m based on our results and plastic flow laws of mafic rocks from a previous study fault strength profile and slip stability of manila subduction zone are discussed with calculations in the context of a unified friction flow model the inferred lower boundary of seismogenic zone of manila subduction fault is around 40 km which is in a reasonable range compared with seismicity data

quasi static modelling of stress histories during the earthquake cycle precursory seismic and aseismic stress release summary a functional form describing non linearity in the stress history during the quasistatic phase of anelasticity in the final stages of semi brittle failure is presented semi brittle failure is known to occur in laboratory rock samples and the existence of earthquake precursors indicates that the earth s upper crust also exhibits an anelastic response in the final stages of the buildup to dynamic failure the functional form of the stress history depends on the amount of localized aseism stress relaxation on a fault nucleation patch and the amount of relatively more distributed seismic damage around the fault measured by a catalogue of earthquakes or acoustic emissions aseismic stress relaxation on the nucleation patch is modelled assuming slip occurs behind a crack front which is growing due to subcritical crack growth by the mechanism of stress corrosion under these conditions it is impossible to model correctly the observed duration of intermediate term earthquake precursors given realistic estimates of the size of the nucleation patch 0 1 1 km and the stress corrosion index n where 10 n 50 therefore intermediate term earthquake precursors must result from more distributed seismic or aseismic damage coupled with fluid transport in a volume much larger than that of the nucleation patch in terms of seismic moment or seismic energy release the precursory seismic damage in the quasi static anelastic phase is dominated by the largest magnitudes for the usual case of b 2c 3 the smaller events dominate the remote stress drop in terms of the inferred power law exponent d 3b c of the fault length distribution d 2 corresponds to distributed damage and d 2 and strain softening and shear localization occurs when d 2

influence of temperature on brittle creep in sandstones the characterization of time dependent brittle creep promoted by chemically active pore fluids is fundamental to our understanding of the long term evolution and dynamics of the earth s crust here we report results from a study of the influence of temperature on both short term strength and time dependent brittle creep in three sandstones under triaxial stress conditions we show that an increase in temperature from 20 degrees to 75 degrees c significantly enhances stress corrosion cracking in all three sandstones leading to 1 a systematic reduction in strength during constant strain rate experiments and 2 an increase by several orders of magnitude in brittle creep strain rates during stress stepping creep experiments we also show that a conventional creep experiment performed at 75 degrees c exhibits a qualitatively similar three stage brittle creep curve as that observed at ambient temperature extrapolation of our results suggests that temperature is likely to be the dominant influence on the evolution of creep strain rate with depth in the shallow crust citation heap m j p baud and p g meredith 2009 influence of temperature on brittle creep in sandstones geophys res lett 36 l19305 doi 10 1029 2009gl039373

depositional and post depositional processes in human modified cave contexts of west central patagonia southernmost south america abstract this contribution aims to analyse depositional and post depositional processes from two volcanic caves bano nuevo 1 and cueva de la vieja from west central patagonia in order to evaluate if natural and anthropogenic processes recorded therein are representative of basin scale processes for this purpose geomorphological sedimentological micromorphological petrographic magnetic paleontological and archaeological data are integrated results allow correlating the bano nuevo 1 and cueva de la vieja sequences comprising a record of ca 19 000 years in the first place the distinction between endogenous and exogenous detritus input mainly recorded by the mineralogy and lithotypes provided environmental data related to landscape stability i e vegetation cover the latter in turn could be interpreted as part of a complex feedback associated with major climatic changes on the other hand cave weathering rates observed through vertical variations of the gravel fraction along with bioactivity recorded by percentages of total organic carbon and micromorphological analysis provided constraints on temperature and or humidity being in agreement with available regional information main regional palaeoenvironmental changes observed in the sequences were the deglaciation processes the pleistocene holocene transition the holocene amelioration and the anthropocene although no distinctive variations were observed within holocene deposits biological processes mainly resultant of the human occupations could have obliterated such finer scale distinctions

experimental evidence for chemo mechanical coupling during carbon mineralization in ultramafic rocks abstract storing carbon dioxide in the subsurface as carbonate minerals has the benefit of long term stability and immobility ultramafic rock formations have been suggested as a potential reservoir for this type of storage due to the availability of cations to react with dissolved carbon dioxide and the fast reaction rates associated with minerals common in ultramafic formations however the rapid reactions have the potential to couple with the mechanical and hydraulic behavior of the rocks and little is known about the extent and mechanisms of this coupling in this study we argue that the dissolution of primary minerals and the precipitation of secondary minerals along pre existing fractures in samples lead to reductions in both the apparent young s modulus and shear strength of aggregates accompanied by reduction in permeability hydrostatic and triaxial deformation experiments were run on dunite samples saturated with de ionized water and carbon dioxide rich solutions while stress strain permeability and pore fluid chemistry were monitored sample microstructures were examined after reaction and deformation using scanning electron microscopy sem the results show that channelized dissolution and carbonate mineral precipitation in the samples saturated with carbon dioxide rich solutions modify the structure of grain boundaries leading to the observed reductions in stiffness strength and permeability a geochemical model was run to help interpret fluid chemical data and we find that the apparent reaction rates in our experiments are faster than rates calculated from powder reactors suggesting mechanically enhanced reaction rates in conclusion we find that chemo mechanical coupling during carbon mineralization in dunites leads to substantial modification of mechanical and hydraulic behavior that needs to be accounted for in future modeling efforts of in situ carbon mineralization projects

early human remains from bano nuevo 1 cave central patagonian andes chile abstract the notable sparseness of human skeletal remains is a characteristic trait of early american prehistory and therefore this aspect of the archaeological record is seldom considered in its discussion in this context the finding of remains from five individuals dated to the 9th millenium bp on the re excavations at bano nuevo cave andean central patagonia chile is particularly interesting they may not appear excessively old but several radiocarbon dates two of them done through ams directly on the bone of one of the individuals place them among the very few well dated early human skeletons throughout the americas the small sample size militates against any interpretation of biological affiliation to say nothing of population movements and we can only affirm that these people belonged to a generalized mongoloid stock different from historically known patagonian populations after a brief presentation of the skeletal evidence and the bone dating methods we evaluate the relevance of this context with respect to the sparse osteological record of humans in the americas on the pleistocene holocene transition

rheology of the lithosphere selected topics we review recent results concerning the rheology of the lithosphere with special attention to the following topics 1 the flexure of the oceanic lithosphere 2 deformation of the continental lithosphere resulting from vertical surface loads and forces applied at plate margins 3 the rheological stratification of the continents 4 strain localization and shear zone development and 5 strain induced crystallographic preferred orientations and anisotropies in body wave velocities we conclude with a section citing the 1983 1986 rock mechanics literature by category

the instantaneous rate dependence in low temperature laboratory rock friction and rock deformation experiments 1 earthquake occurrence probabilities that account for stress transfer and time dependent failure depend on the product of the effective normal stress and a lab derived dimensionless coefficient a this coefficient describes the instantaneous dependence of fault strength on deformation rate and determines the duration of precursory slip although an instantaneous rate dependence is observed for fracture friction crack growth and low temperature plasticity in laboratory experiments the physical origin of this effect during earthquake faulting is obscure we examine this rate dependence in laboratory experiments on different rock types using a normalization scheme modified from one proposed by tullis and weeks 1987 we compare the instantaneous rate dependence in rock friction with rate dependence measurements from higher temperature dislocation glide experiments the same normalization scheme is used to compare rate dependence in friction to rock fracture and to low temperature crack growth tests for particular weak phyllosilicate minerals the instantaneous friction rate dependence is consistent with dislocation glide in intact rock failure tests for each rock type considered the instantaneous rate dependence is the same size as for friction suggesting a common physical origin during subcritical crack growth in strong quartzofeldspathic and carbonate rock where glide is not possible the instantaneous rate dependence measured during failure or creep tests at high stress has long been thought to be due to crack growth however direct comparison between crack growth and friction tests shows poor agreement the crack growth rate dependence appears to be higher than the rate dependence of friction and fracture by a factor of two to three for all rock types considered

influence of thermal treatment on mode i fracture toughness of certain indian rocks abstract an experiment was carried out to measure the static mode i fracture toughness of manoharpur sandstone bellary dolerite and dholpur sandstone after thermal treatment at different temperatures that ranged from ambient conditions to 600 c the three point bending method was applied using cracks straight through semi circular bending cstscb specimens that were fabricated according to international society for rock mechanics isrm standards three different rocks were used to measure the degree of influence of thermal treatment on fracture toughness in different rocks in addition petrographic and x ray diffraction xrd analyses were carried out to identify the compositions of these rocks finally scanning electron microscope sem analysis was performed in order to measure the micro cracks that were induced within these rocks as a result of thermal treatment the experimental results demonstrated that up to a temperature range of 100 c the fracture toughness of manoharpur sandstone bellary dolerite and dholpur sandstone increased by 40 25 and 65 respectively when compared to the ambient condition and thereafter decreased with a gradual increase of temperature at 600 c when compared to the ambient condition the fracture toughness of these rock types decreased by 59 36 and 30 for manoharpur sandstone bellary dolerite and dholpur sandstone respectively

accelerated seismic release and related aspects of seismicity patterns on earthquake faults observational studies indicate that large earthquakes are sometimes preceded by phases of accelerated seismic release asr characterized by cumulative benioff strain following a power law timeto failure relation with a term tf t m where tr is the failure time of the large event and observed values of m are close to 0 3 we discuss properties of asr and related aspects of seismicity patterns associated with several theoretical frameworks the subcritical crack growth approach developed to describe deformation on a crack prior to the occurrence of dynamic rupture predicts great variability and low asymptotic values of the exponent m that are not compatible with observed asr phases statistical physics studies assuming that system size failures in a deforming region correspond to critical phase transitions predict establishment of long range correlations of dynamic variables and power law statistics before large events using stress and earthquake histories simulated by the model of ben zion 1996 for a discrete fault with quenched heterogeneities in a 3 d elastic half space we show that large model earthquakes are associated with nonrepeating cyclical establishment and destruction of long range stress correlations accompanied by nonstationary cumulative benioff strain release we then analyze results associated with a regional lithospheric model consisting of a seismogenic upper crust governed by the damage rheology of lyakhovsky et al 39 over a viscoelastic substrate we demonstrate analytically for a simplified 1 d case that the employed damage rheology leads to a singular power law equation for strain proportional to t f t 1 3 and a nonsingular power law relation for cumulative benioff strain proportional to t f t 1 3 a simple approximate generalization of the latter for regional cumulative benioff strain is obtained by adding to the result a linear function of time representing a stationary background release to go beyond the analytical expectations we examine results generated by various realizations of the regional lithospheric model producing seismicity following the characteristic frequency size statistics gutenberg richter power law distribution and mode switching activity we find that phases of asr exist only when the seismicity preceding a given large event has broad frequency size statistics in such cases the simulated asr phases can be fitted well by the singular analytical relation with m 1 3 the nonsingular equation with m 0 2 and the generalized version of the latter including a linear term with m 1 3 the obtained good fits with all three relations highlight the difficulty of deriving reliable information on functional forms and parameter values from such data sets the activation process in the simulated asr phases is found to be accommodated both by increasing rates of moderate events and increasing average event size with the former starting a few years earlier than the latter the lack of asr in portions of the seismicity not having broad frequency size statistics may explain why some large earthquakes are preceded by asr and other are not the results suggest that observations of moderate and large events contain two complementary end member predictive signals on the time of future large earthquakes in portions of seismicity following the characteristic earthquake distribution such information exists directly in the associated quasi periodic temporal distribution of large events in portions of seismicity having broad frequency size statistics withrandom or clustered temporal distribution of large events the asr phases have predictive information the extent to which natural seismicity may be understood in terms of these end member cases remains to be clarified continuing studies of evolving stress and other dynamic variables in model calculations combined with advanced analyses of simulated and observed seismicity patterns may lead to improvements in existing forecasting strategies

influence of macro fractures and micro fractures on permeability and elastic wave velocities in basalt at elevated pressure fractures are ubiquitous on all scales in crustal rocks the investigation of fractures and their influence on physical and transport properties of rocks is therefore essential for understanding of many key problems in seismology volcanology and rock engineering in crystalline rocks pore water is primarily stored in and migrates through networks of cracks and fractures at all scales it is therefore essential to know how fluid flow in such fracture networks responds to the elevated pressures found at depth here we report results from an investigation of changes in fluid permeability and associated changes in p wave and s wave velocities at elevated effective pressure for intact macro fractured and micro fractured samples of seljadur basalt in all cases permeability decreases and both wave velocities increase with increasing effective pressure permeability decreases were smallest in the intact material from approximately 10 19 m 2 to 3 x 10 20 m 2 intermediate in the micro fractured material from approximately 5 x 10 17 m 2 to 1 x 10 17 m 2 and largest in the macro fractured material from approximately 3 x 10 15 m 2 to 9 x 10 19 m 2 for a material containing both micro fractures and macro fractures the closure of macro fractures dominated the permeability reduction at low pressure with the closure of micro fractures exerting an increasing influence at higher pressure c 2010 elsevier b v all rights reserved

synchronized terrestrial atmospheric deglacial records around the north atlantic on the basis of synchronization of three carbon 14 c 14 dated lacustrine sequences from sweden with tree ring and ice core records the absolute age of the younger dryas preboreal climatic shift was determined to be 11 450 to 11 390 80 years before the present a 150 year long cooling in the early preboreal associated with rising delta 14 c values is evident in all records and indicates an ocean ventilation change this cooling is similar to earlier deglacial coolings and box model calculations suggest that they all may have been the result of increased freshwater forcing that inhibited the strength of the north atlantic heat conveyor although the younger dryas may have begun as an anomalous meltwater event

fracture toughness and subcritical crack growth during high temperature tensile deformation of westerly granite and black gabbro abstract the double torsion testing method has been used to determine catastrophic and subcritical crack propagation parameters for pre cracked specimens of westerly granite and black gabbro under a number of environmental conditions the critical stress intensity factor for catastrophic crack propagation fracture toughness of granite and gabbro has been measured at temperatures from 20 to 400 c in a vacuum at 20 c the fracture toughness of westerly granite was 1 79 0 02 mpa m 1 2 and for two blocks of black gabbro it was 3 03 0 08 mpa m 1 2 and 2 71 0 15 mpa m 1 2 respectively these values are very close to those reported by other investigators for tests conducted in air of ambient humidity at room temperature for both rocks fracture toughness at first increased slightly and then decreased steadily on raising the temperature above ambient conditions this behaviour is explained in terms of the density and distribution of thermally induced microcracks as determined by quantitative optical microscopy subcritical crack growth behaviour has been studied at temperatures up to 300 c and under water vapour at pressures of 0 6 to 15 kpa both the load relaxation and incremental constant displacement rate forms of the double torsion testing method were utilised to generate stress intensity factor crack velocity diagrams crack growth was measured over the velocity range 5 10 3 to 10 7 m s 1 increasing both temperature and water vapour pressure resulted in substantially higher crack growth rates the overall effect of raising the temperature over the range studied here 20 300 c was to increase the crack growth rate in granite and gabbro by 5 and 7 orders of magnitude respectively at constant stress intensity factor and vapour pressure of water for both rocks the slopes of stress intensity factor crack velocity curves were sensitive to changes in both temperature and water vapour pressure at low values of the latter parameter slopes fell substantially on raising the water vapour pressure but were relatively insensitive to changes in temperature at these higher pressures no subcritical crack growth limit was encountered estimates of the uncertainty in our experimental data are given from the results of multiple load relaxation experiments on westerly granite specimens we estimate the uncertainty in position of stress intensity factor crack velocity curves along the stress intensity axis to be c 10 of the fracture toughness and the uncertainty in slope of such curves to be c 12 problems associated with the extrapolation of our experimental data to regions of higher effective confining pressure in the earth s crust are discussed

late quaternary dust flow on the chinese loess plateau abstract the dust flux of over 25 g cm2 103yr was reached in the central part of the chinese loess plateau during the last glacial maximum this is more than three times higher than during the last interglacial and early glacial time the grain size of the pleniglacial dust reached a peak shortly after 20 000 years ago and testifies to the occurrence of strong winds and frequent dust storms

differentiation of pedogenic and lithogenic carbonate forms in texas abstract sixteen pedons representing soils developed from limestone and fluvial deltaic sediments were sampled in central and western texas 300 800 mm annual precipitation to differentiate pedogenic secondary and lithogenic inherited carbonate forms by field and laboratory techniques lithogenic carbonate forms identified in the field included indurated limestone bedrock and coarse limestone fragments lithorelicts field identified pedogenic forms included petrocalcic horizons thin laminar cappings on indurated limestone carbonate joint seams pendants on pebbles and carbonate films and threads on ped faces small

accelerated subglacial erosion in response to stick slip motion subglacial stick slip motion speeds erosion by hydrofracturing and in other ways as determined from analysis of the growing body of field data microearthquake monitoring commonly detects subglacial earthquakes likely mostly from stick slip motion of debris laden ice over bedrock source parameters show that many quakes cause enough motion to greatly lower water pressure in cavities on the lee sides of bedrock steps we calculate that the resulting expansion of higher pressure water in nearby cracks promotes hydrofracturing with even relatively small cracks growing unstably under thick glaciers and all cracks growing faster than for aseismic behavior this mechanism also helps generate the step like topography favoring block plucking this stick slip glacier erosion hypothesis suggests that the erosion rate will increase with ice thickness as well as basal shear stress ice flow velocity and water supply

the initial peopling of central western patagonia southernmost south america late pleistocene through holocene site context and archaeological assemblages from cueva de la vieja site this article discusses new data on the initial peopling of central western patagonia based on research conducted at the cueva de la vieja site 45 16 27 s 71 32 24 w 718 masl contextualizing this event it in the broader pleistocene human dispersal of southernmost south america archaeological excavations and analyses at this cave site were undertaken to address the chronology of the initial settlement of the region characteristics of the first human presence and subsequent site redundancy this paper includes a description of the site context based on macro and microscopic stratigraphy of the excavated section the characterization of anthropogenic features and a comprehensive radiocarbon dating program archaeological assemblages i e lithics bones and charred seeds are used to explain the variations in the human occupation of this venue with particular emphasis on the earliest materials and occupational surfaces as well as post depositional processes the occupation events at cueva de la vieja span the last 12 000 calibrated years and indicate at this point the earliest securely dated human presence in central western patagonia at least one millennium after other thoroughly dated sites toward the east this suggests that the initial settlers must have occupied the eastern flank of the andes at this latitude only after glaciers and other glaciogenetic features retreated and viable ecosystems emerged

when patagonia was colonized people mobility at high latitudes during pleistocene holocene transition abstract the pleistocene holocene transition was a critical time for the dispersal of human societies all over south america people looking for places to settle had to accept high environmental variability during the colonizing process the case studied for this paper is patagonia southern south america where the oldest dates ca 13 10 5 ka bp have been obtained for the peopling in the deseado river basin and magellan basin as well however two archaeological sites yielded similar dates on the western side of the andes monte verde and tagua tagua following archaeological and palaeoenvironmental data palynological faunal sedimentological and glaciological information as well as radiocarbon datings a hypothesis about the colonization of patagonia is presented analysing the ways and time of colonization for this region it is remarkable the coincidence of these ages in the centre of the steppe and close to the magellan strait even in the present tierra del fuego island though the eastern andean foothills seems to be occupied at least two millennia later it is proposed that independent peopling entries would have occurred both through the atlantic and pacific facades and that the andean foothills were colonized much later only when the available spaces allowed it

climatic implications of alternating clay and carbonate formation in semiarid soils of south central montana abstract evidence for climatic change is found in petrographic thin sections from soils formed on glaciofluvial deposits of rock creek and the lower clarks fork montana these soils presently in a semiarid climate range from late pliocene to holocene in age and have undergone periodic fluctuations in soil moisture caused by climatic changes in the lower parts of soil b horizons accretion of illuvial layers of clay argillans occurs mainly during wet glacial climatic periods whereas carbonate precipitates mainly during dry interglacial climatic periods thin section studies of the argillan and carbonate layers show that 1 post pinedale soils that have formed only in the present interglacial climate contain areas of secondary carbonate unrelated to argillans 2 soils formed on outwash of successively older glaciations contain proportionately more alternating layers of argillans and carbonate and 3 the maximum number and sequence of layers in a soil correspond to the number of local cycles of glacial outwash deposition and subsequent stream incision that followed the beginning of soil formation these cycles are inferred to correspond to local glacial interglacial fluctuations the correspondence between the microscopic record and the glacial outwash record for rock creek suggests that some of the climatic changes seen in the marine oxygen isotope record did not strongly affect the study area

effects of relative humidity and temperature on subcritical crack growth in igneous rock abstract effects of relative humidity and temperature on subcritical crack growth in igneous rock have been investigated experimentally on samples of kumamoto andesite and oshima granite stress intensity factors and crack velocities were measured using the double torsion technique and all experiments were conducted in moist air our results show that in experiments conducted under the same relative humidity crack velocity increased with increasing temperature in agreement with previous studies our results also show that in experiments conducted at the same temperature crack velocity increased dramatically with increasing relative humidity a three to fourfold increase in relative humidity resulted in an increase in crack velocity of between one and four orders of magnitude such an increase is larger than that predicted by classical stress corrosion theory it is suggested that capillary condensation of water vapour close to crack tips of small aperture influences the rate of crack growth it is concluded that relative humidity needs to be controlled to avoid time dependent weakening and extend the lifetime of structures in a rock mass

the effect of fracture density and stress state on the static and dynamic bulk moduli of westerly granite elastic properties are key parameters during the deformation of rocks they can be measured statically or dynamically but the two measurements are often different in this study the static and dynamic bulk moduli kstatic and kdynamic were measured at varying effective stress for dry and fluid saturated westerly granite with controlled fracture densities under isotropic and differential stress states isotropic fracturing of different densities was induced in samples by thermal treatment to 250 450 650 and 850 c results show that fluid saturation does not greatly affect static moduli but increases dynamic moduli under isotropic loading high fracture density and or low effective pressure results in a low kstatic kdynamic ratio for dry conditions kstatic kdynamic approaches 1 at low fracture densities when the effective pressure is high consistent with previous studies stress induced anisotropy exists under differential stress state that greatly affects kstatic compared to kdynamic as a result the kstatic kdynamic ratio is higher than that for the isotropic stress state and approaches 1 with increasing axial loading the effect of stress induced anisotropy increases with increasing fracture density a key omission in previous studies comparing static and dynamic properties is that anisotropy has not been considered the standard methods for measuring static elastic properties such as poisson s ratio young s and shear modulus involve subjecting the sample to a differential stress state that promotes anisotropy our results show that stress induced anisotropy resulting from differential stress state is a major contributor to the difference between static and dynamic elasticity and is dominant with high fracture density

mineral magnetic record of the chinese loess and paleosols variations in the magnetic susceptiblity of the chinese loess and its interbedded paleosols correlate strikingly well with the deep sea oxygen isotope record low susceptibility values are found for the loess layers and high values are found for the soil horizons two interpretative models have been advanced to account for these magnetic variations both of these models discount any significant in situ formation of magnetite during soil forming periods instead they infer relative concentration of detrital or atmospheric magnetite at these times we critically examine the assumptions made in these models and on the basis of some new mineral magnetic data and fromalized magnetic flux calculations offer an alternative explanation of the loess magnetic record our model identifies pedogenic formation of magnetite as the major contributor to the high magnetic susceptibility of the paleosols the formation and preservation of this pedogenic magnetite is dependent on soil froming conditions and hence reflects the regional climate

dynamic rupture and seismic radiation in a damage breakage rheology model we present simulations of dynamic ruptures in a continuum damage breakage rheological model and waves radiated by the ruptures and observed in the far field the propagating rupture produces rock damage and granulation in the process zone ahead of the rupture front an expansion compaction process in the process zone leads to an isotropic source term while shear motion that accumulates behind the propagating front produces a deviatoric source term and shear heating behind the rupture front the process zone dissipation due to the damage breakage mechanism and the isotropic source component significantly affect the s p energy partitioning and the radiation patterns of the waves the calculated s p seismic energy ratio can be significantly lower than results of standard models with no damage breakage mechanism and no source volume components the p radiation pattern becomes more isotropic compared with the classical deviatoric solution with increased lobes at 45 to the direction of rupture propagation the s radiation pattern is affected more strongly by the damage breakage process in the source volume mainly within the process zone and is significantly different from classical deviatoric results the s waves propagate from the rupture front through the process zone unlike the p waves experiencing stronger dissipation so the s radiation pattern is more affected than the p radiation pattern hence analysis of p waves can provide more reliable results on rupture directivity than s waves

time lapse reconstruction of the fracture front from diffracted waves arrivals in laboratory hydraulic fracture experiments 4d acoustic imaging via an array of 32 sources 32 receivers is used to monitor hydraulic fracture propagating in a 250 mm cubic specimen under a true triaxial state of stress we present a method based on the arrivals of diffracted waves to reconstruct the fracture geometry and fluid front when distinct from the fracture front using bayesian model selection we rank different possible fracture geometries radial elliptical tilted or not and estimate model error the imaging is repeated every 4 seconds and provide a quantitative measurement of the growth of these low velocity fractures we test the proposed method on two experiments performed in two different rocks marble and gabbro under experimental conditions characteristic respectively of the fluid lag viscosity marble and toughness gabbro dominated hydraulic fracture propagation regimes in both experiments about 150 to 200 source receiver combinations exhibit clear diffracted wave arrivals the results of the inversion indicate a radial geometry evolving slightly into an ellipse towards the end of the experiment when the fractures feel the specimen boundaries the estimated modelling error with all models is of the order of the wave arrival picking error posterior estimates indicate an uncertainty of the order of a millimeter on the fracture front location for a given acquisition sequence the reconstructed fracture evolution from diffracted waves is shown to be consistent with the analysis of 90 circ incidence transmitted waves across the growing fracture

from rock to magma and back again the evolution of temperature and deformation mechanism in conduit margin zones abstract explosive silicic volcanism is driven by gas overpressure in systems that are inefficient at outgassing the zone at the margin of a volcanic conduit thought to play an important role in the outgassing of magma and therefore pore pressure changes and explosivity is the boundary through which heat is exchanged from the hot magma to the colder country rock using a simple heat transfer model we first show that the isotherm for the glass transition temperature whereat the glass within the groundmass transitions from a glass to an undercooled liquid moves into the country rock when the magma within the conduit can stay hot or into the conduit when the magma is quasi stagnant and cools on the centimetric scale over days to months we then explore the influence of a migrating viscous boundary on compactive deformation micromechanisms in the conduit margin zone using high pressure effective pressure of 40 mpa high temperature up to 800 c triaxial deformation experiments on porous andesite our experiments show that the micromechanism facilitating compaction in andesite is localised cataclastic pore collapse at all temperatures below the glass transition of the amorphous groundmass glass t g i e rock in this regime porosity is only reduced within the bands of crushed pores the porosity outside the bands remains unchanged further the strength of andesite is a positive function of temperature below the threshold t g due to thermal expansion driven microcrack closure the micromechanism driving compaction above t g i e magma is the distributed viscous flow of the melt phase in this regime porosity loss is distributed and is accommodated by the widespread flattening and closure of pores we find that viscous flow is much more efficient at reducing porosity than cataclastic pore collapse and that it requires stresses much lower than those required to form bands of crushed pores our study therefore highlights that temperature excursions can result in a change in deformation micromechanism that drastically alters the mechanical and hydraulic properties of the material within the conduit margin zone with possible implications for pore pressure augmentation and explosive behaviour

thermoluminescence dating of sediments abstract the potential for using the thermoluminescence behaviour of sediments for dating them was first recognized by soviet scientists g v morozov and v n shelkoplyas and for over a decade their tl dates obtained from a variety of sediments have appeared in the soviet literature since 1977 tl sediment dates have been published by six additional groups using a variety of methods in this review we describe the principles of tl dating the various methods used and contrast tl dating of sediments with the now well accepted tl dating of pottery we conclude that while tl dating has the potential to solve many sedimentary problems more fundamental research needs to be carried out before such dates should be accepted a set of criteria for acceptable dates is proposed

accelerator radiocarbon dating at the molecular level molecular level 14c dating is the isolation of specific classes of molecules for their 14c dating by accelerator mass spectrometry ams complex matrices such as fossil bone are difficult to date due to their extreme chemical heterogeneity by isolating individual amino acids contaminants humates are removed and crystalline amino acids result bones with 0 1 0 2 n and collagenous compositions can be dated accurately because structural collagen is present contaminants are removable with xad resin bones with 0 1 n and non collagenous compositions yield dates hundreds to thousands of years too young because most of the preserved organic matter is exogenous accelerator 14c dates on collagenous and non collagenous bones are not comparable due to intrinsic dating inaccuracies ams 14c dating of amino acids demonstrated that 1 post 10 800 year ages for north american megafauna are due to sample contamination not holocene ages on extinct fauna 2 a clovis age 10 900 years was established for a human fossil from the anzick site montana 3 holocene ages cannot be established absolutely for many north american human fossils because the bones were non collagenous 4 accurate ages are attainable on vertebrate fossils as small as passerine birds from pacific island localities 5 well preserved bones are datable without their destruction by extracting protein with water at high temperatures and 6 stratigraphic anomalies to 45 000 years in european upper paleolithic rock shelters are recognizable by dating bone directly

chronologic evidence for multiple periods of loess deposition during the late pleistocene in the missouri and mississippi river valley united states implications for the activity of the laurentide ice sheet abstract the loess stratigraphy of the mid continental u s is an important proxy record for the activity of the laurentide ice sheet in north america one of the most outstanding problems is deciphering the age of loess deposits in this area during the late pleistocene radiocarbon dating of snails and thermoluminescence dating of the fine silt fraction 4 11 m from loess at the loveland loess type section loveland iowa and a recent excavation at the pleasant grove school section madison county illinois provide new chronologic control on loess deposition in the mississippi missouri river valley chronology indicates that the loveland loess is illinoian in age 135 20 ka but is not correlative with the teneriffe silt which is dated to 77 8 ka concordant radiocarbon and thermoluminescence age estimates demonstrate that the roxana silt and a correlative loess in iowa the pisgah formation is probably 40 30 ka old these age estimates in conjunction with previous results indicate that there were four periods of loess deposition during the last 150 ka at 25 12 ka 45 30 ka 85 70 ka and at ca 135 20 ka this chronology of loess deposition supports the presence of both a late illinoian and early wisconsinan loess and associated soils thus there may be more than one soil in the loess stratigraphy of the mid continental u s with morphologies similar to the sangamon soil the last three periods of loess deposition may be correlative with periods of elevated dust concentrations recorded in the dye 3 ice core from southern greenland this is particularly significant because both areas possibly had the same source for eolian particles reconstructions of atmospheric circulation for glacial periods show a southerly deflected jet stream that could have transported dust from the mid continental usa to southern greenland lastly the inferred record of loess deposition is parallel to a chronology for deglaciation of the laurentide ice sheet deciphered from chronologic and stratigraphic studies of raised glacial and marine sediments in the hudson bay lowlands canada these chronologies indicate that the laurentide ice sheet was quite dynamic during the late pleistocene advancing and retreating across north america at least four times during the last 150 ka

climatic signals from loess soil sequences in the central great plains usa abstract late quaternary eolian deposits and buried soils in the central great plains are interpreted to reflect the fluctuation of climatic conditions under which they formed the loveland loess was deposited approximately between 500 and 100 ka within the loess are four zones of pedogenic carbonate accumulation after 416 35 ka before 260 25 ka around 193 22 ka and before 92 7 0 ka the pedogenic carbonate zones were probably formed under warm and dry conditions the moderately weathered barton sand at the barton section and the equivalent sandy silt ii at other sections deposited between 92 7 0 and 69 6 4 ka suggest windy probably warm and fluctuating moisture consitions a reddish pedocomplex was formed from 70 to 35 ka under relatively warm and moist climatic conditions with a very low rate of loess deposition overlying the reddish pedocomplex is the sandy silt i probably a stratigraphic equivalent to the roxana silt dated at 40 30 ka in the central lowlands the gilman canyon formation 35 20 ka a thick cumulic loessial and organic matter rich pedocomplex developed under relatively moist and probably cool conditions the peoria loess was deposited at a very rapid rate under dry and cold conditions the brady soil 10 5 8 5 ka was formed under relatively warm and moist conditions the poorly weathered bignell loess was deposited during the warm and dry altithermal period 9 5 ka

temporal variations in seismicity during quasi static and dynamic rock failure abstract a comprehensive model is presented which can explain temporal fluctuations in seismic b values in the period leading to mechanical failure in terms of the underlying physical processes of time varying applied stress and stress corrosion enhanced crack growth under conditions of constant strain rate the form of the b value anomaly in the period leading to failure depends on the form of the stress time relationship for the case where dynamic failure occurs at peak stress after a period of strain hardening the model predicts a single cusp like b value anomaly reaching a critically low value of 0 5 at failure for the physically most realistic case where dynamic failure is preceded by a period of precursory strain energy release during strain softening the model predicts two minima in the b value separated by a temporary maximum or inflection point these fluctuations in the b value are consistent with reported intermediate term and short term earthquake precursors separated by a period of seismic quiescence for the case of quasi static cataclastic flow the b value mirrors the stress and never falls to the critical value because there is no critical rupture new results from a series of controlled laboratory experiments are presented in which microseismic event rates and b value were monitored contemporaneously with stress time data for all variants of the stress time relationship recent field observations of temporal changes in seismicity rates and b value preceding major earthquakes are also reported both data sets exhibit b value anomalies which are consistent with the model predictions

radiocarbon ages of soils and charcoal in late wisconsinan loess south central nebraska abstract the farmdale soil occurs below late wisconsinan loess throughout the u s midwest at the la sena site in the central great plains humates in the farmdale interstadial soil have a corrected age of 21 000 yr b p humates in a buried bt horizon and a bulk sample of overlying loess 2 5 m above the farmdale interstadial soil have ages of 17 000 and 19 000 yr b p respectively in the republican river valley picea spruce charcoal is common in the lower meter of peoria loess near bloomington nebraska humates from burned organic matter only 60 cm above the base of peoria loess have a corrected age of ca 19 000 yr b p

temperature dependence of the electrical resistivity of water saturated rocks the electrical resistivities of several dacitic tuffs sandstone andesite granite and crystalline limestone samples saturated with a 0 001 m aqueous solution of kc x1d4c1 were measured in the range from room temperature to 250 c the experiments were made using a cell technique with platinum electrodes of particular interest are the data collected at temperatures above 200 c under high pressure a region scarcely documented in the literature basically the samples used in the present study show a quasi exponential decrease of resistivity with temperature up to 200 c the same temperature dependence is found for the resistivity of the saturating solution thus confirming that conduction in water saturated rocks is essentially electrolytic at temperatures above 200 c some specimens of highly porous dacitic tuff still closely follow the saturating solution in the pattern of resistivity variation with temperature exhibiting a minimum around 220 c however the behavior of low porosity crystalline rocks n