plane stress and plane strain fracture in eocene chalks around beer sheva abstract a distinction has been made between plane strain and plane stress fracture conditions in metals in these materials flat fracture normal to the tensile stress develops in plane strain however the fracture is slanted when conditions change to plane stress analogies with rock mechanics suggest a simple model of en echelon cracking along fringes of vertical joints in thin horizontal sedimentary layers according to this model the en echelon cracking is a result of fracture under plane stress conditions when failure is explained by the coulomb criterion and subject to limited overburden

development of strike slip faults from dikes sequoia national park california abstract faults several kilometers long near pear lake in sequoia national park california nucleated on preexisting dikes weakened by closely spaced echelon joints these joints essentially parallel much more widely spaced regional joints fault nucleation by dike fracturing can explain how a nearly planar fault can grow to lengths of several kilometers or more we suggest that opening mode fractures including dikes might serve more commonly as nuclei for faults than is generally appreciated the longest fault in the study area the pear lake fault has a trace length of 7 km and a maximum measured lateral separation of 86 98 m a sharp decrease in slip occurs near the center of the trace of the pear lake fault where another fault branches from it mechanical analyses demonstrate that a sharp decrease in slip is expected where a fault branches all the faults near pear lake show non elliptical slip distributions based on our mechanical analyses we infer that the observed slip distributions reflect a mechanical interaction among the faults the geometry and distribution of structures that precede faulting i e dikes and joints control to a large extent the structure of a fault network and hence the ultimate slip distribution on faults in the network

imaging 3 d fracture networks around boreholes a new method has been developed for building and visualizing three dimensional 3 d subsurface natural fracture networks in rocks surrounding boreholes using image logs such as cast emi fmi fms rosi and others the method correlates fracture patterns with different stages of fracture network development for individual sets of fractures and extrapolates fracture density and connectivity from one dimension 1 d in boreholes to 3 d in the surrounding rocks the application of this work is 3 d visualizations of fracture distributions in volumes close to the boreholes for well planning reservoir scale fracture model building reservoir flow simulation and hydraulic fracture control

synmetamorphic vein spacing distributions characterisation and origin of a distribution of veins from nw sardinia italy abstract the spatial distribution of a single generation of axial planar synmetamorphic veins exposed in greenschist facies rocks on nw sardinia was investigated with the aim of gaining insight into the processes controlling the fracture spacing the vein spacing distribution measured along linear profiles is non uniform and approximately log normal although the vein abundance and absolute vein spacing is strongly influenced by rock type the vein spacing distribution is independent of rock type comparison of the observed data with synthetically generated fracture distributions indicates that the observed distribution can be reproduced by kolmogorov fragmentation but cannot be reproduced by a random even bisection or a fractal cantor dust process these results are interpreted to indicate that the positions of fractures that become veins are interdependent in an attempt to explain the interdependency of fracture positions the perturbations of stress and fluid pressure associated with an isolated underpressured axial planar fracture were investigated simple calculations show that the fluid pressure drop inside a fracture coinciding with failure locally perturbs the stress and fluid pressure fields such that the surrounding rock extending over a lateral extent equivalent to approximately two fracture lengths is instantaneously driven into a critical stress state that satisfies the failure condition numerical simulations demonstrate that this process tends to localise the nucleation of subsequent fractures close to existing fractures this mechanism can explain the existence of non random or clustered fracture or vein spacing distributions forming by hydrofracture

interaction between normal faults and fractures and fault scarp morphology fault slip and geomorphic surface processes create and modify bedrock normal fault scarps our field studies and numerical models show that mechanical interaction between near surface fractures and active faults may diminish scarp slopes and broaden deformation near the surface in our models increasing fracture density and depth reduces and complicates the scarp s topographic expression fracture depth and density and orientation of the structures control the location and magnitude of zones of positive and negative shear displacements along the fractures our field studies of the interaction of active graben bounding normal faults and fractures in northern arizona illustrate the mechanical analysis reduction in scarp slopes and broadening of the deformation field may cause incorrect interpretations of shallow bedrock scarp morphologies as old rather than as the result of slip along fractures around the fault

a rock mechanical assessment of mercury s global tectonic fabric abstract mercury s global tectonic history is thought to have been dominated by two major processes tidal despinning and global contraction each process is expected to have produced a distinctive global stress field and resultant fault pattern thousands of thrust fault related landforms documented on mercury can be attributed to global contraction but no global signature of tidal despinning has been conclusively documented because global contraction operated throughout an extended portion of mercury s geologic history any tidal despinning pattern either would have formed together with global contraction or would have been modified by global contraction after despinning was complete here we reassess global fracture patterns predicted to result from tidal despinning and from a combination of tidal despinning and global contraction we specifically make use of rock strength and deformability parameters appropriate for mercury s fractured lithosphere results indicate that a tidal despinning pattern would consist only of a global set of opening mode fractures joints in the upper part of the lithosphere whereas the combination of tidal despinning and global contraction would have produced a global population of thrust faults with no preferred orientations in the polar regions but with an increasing preference for north south orientations toward the equator if an equatorial bulge from an early state of rapid spin was supported by mercury s lithosphere two end member scenarios for the timing and duration of these two processes may be considered in one tidal despinning predated global contraction in the other tidal despinning and global contraction overlapped in time we test the predictions of both scenarios against the distribution and orientations of mercury s tectonic landforms the global pattern of thrust faults is generally consistent with predictions for the scenario under which tidal despinning and global contraction temporally overlapped

estimating natural fracture producibility in tight gas sandstones coupling diagenesis with geomechanical modeling natural fracture attributes important for tight gas reservoir characterization include hydraulic aperture length spacing and connectivity all of these parameters contribute to the permeability of the fractured reservoir but many are difficult to observe and or quantify with core logs or seismic our approach to estimating permeability for fractured layers is to augment direct fracture observation with geomechanical modeling and diagenetic characterization

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

characteristics and origins of coal cleat a review abstract cleats are natural opening mode fractures in coal beds they account for most of the permeability and much of the porosity of coalbed gas reservoirs and can have a significant effect on the success of engineering procedures such as cavity stimulations because permeability and stimulation success are commonly limiting factors in gas well performance knowledge of cleat characteristics and origins is essential for successful exploration and production although the coal cleat literature spans at least 160 years mining issues have been the principal focus and quantitative data are almost exclusively limited to orientation and spacing information few data are available on apertures heights lengths connectivity and the relation of cleat formation to diagenesis characteristics that are critical to permeability moreover recent studies of cleat orientation patterns and fracture style suggest that new investigations of even these well studied parameters can yield insight into coal permeability more effective predictions of cleat patterns will come from advances in understanding cleat origins although cleat formation has been speculatively attributed to diagenetic and or tectonic processes a viable mechanical process for creating cleats has yet to be demonstrated progress in this area may come from recent developments in fracture mechanics and in coal geochemistry

the role of the interbed thickness on the step over fracture under overburden pressure multilayer jointing in tight oil and gas reservoirs is a useful indicator for the prediction of fluid mobility since pore pressure balances the principal minor horizontal stress at depth joint nucleations considered as sub vertical are mainly driven by the overburden pressure we demonstrate with a mixed criterion combining necessary conditions of stress and energy how heterogeneity such as an interbed promotes initiation of a fracture by a step over mechanism referring to critical characteristics such as the interbed thickness a specific initiation criterion emerges giving rise to a size effect due to the interbed thickness and is compared to a typical homogeneous rock failure criterion jointing over an interbed is enhanced by large interbed thicknesses even for moderate effective confining stresses size effect is reversed according to the relative importance of singular effects effective minor stress normal to the fracture plane and structural effect parallel loading effective conditions

experimental determination of subcritical crack growth parameters in sedimentary rock mounting evidence suggests that subcritical crack growth is an important mechanism for the development of natural fractures numerical simulations of fracture patterns are sensitive to the subcritical crack growth index the exponent used to describe the power law dependence of crack velocity on stress intensity few measured values of subcritical indices in sedimentary rocks have been published however and no systematic studies of diagenetic controls of subcritical crack index behavior are available subcritical crack index measurements are especially difficult in weak porous rock primarily because non elastic material response can obscure fracture related behavior we have carried out numerous subcritical crack index measurements with the widely used dual torsion beam test configuration and developed a modified test methodology for subcritical crack index measurements on porous rocks the principal element of the methodology is a specimen pre loading procedure to minimize transient material response measurement reliability is documented in the study by documentation of reproducibility in three sets of sandstone and carbonate specimens

possible effects of pre existing basement topography on thrust fault ramping finite element models show that one way in which thrust ramps may arise is through the mechanical interaction between basement and overlying sediments in the simplest case shear coupling between a planar basement sediment causes the differential stresses in the sediments to die out with depth and distance from the applied load for such cases curved thrust faults may result if the strength of the rock is exceeded basement topography may also affect the location and shape of ramps by acting as a stress concentrator by producing a stress shadow and by changing principal stress orientations modeling suggests that whether or not these basement topographic features cause ramping will depend on the height and angularity of the feature as well as the rock types that overlie it under the assumption of linear elasticity and for given boundary conditions the poisson s ratio plays an important role in determining the orientation and magnitude of the principal stresses calculations using experimentally measured poisson s ratios predict that the earliest maximum compressive stress directions should be nearly vertical in the more cratonward portions of thrust belts however the stress directions which are inferred to have occurred earliest in this part of thrust belts are nearly horizontal this suggests that non elastic or ductile processes have an effect on the propagation of thrust faults

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

tectonics fracturing of rock and erosion 1 we argue that by fracturing rock not by raising it relative to base level tectonics plays its most important role in causing rapid incision of valleys and rapid erosion of hillslopes tectonic deformation riddles the upper crust with fractures which not only provide avenues for water flow and thus promote weathering and further disintegration of rock but also fragment bedrock into debris that is readily extracted and transported by surface processes bends in active faults require straining of adjacent rock masses aftershocks that occur subsequent to slip on primary faults reflect penetrative brittle deformation of the upper crust at least some aftershocks must nucleate or lengthen cracks which contribute to the comminution of these rock masses scaling rules suggest that dimensions of ruptures for very small m 2 earthquakes can be meters or less the gutenberg richter recurrence relationship implies that such earthquakes are common as high magnification seismographs in low noise environments confirm moreover large differences among fault plane solutions for aftershocks show that the small faults on which they occur are not parallel to one another some faults must intersect thus the upper crust in tectonically active regions should be fragmented into blocks down to the scale of boulders or smaller dismembered rock arrives at the earth s surface already prepared to be transported away as a corollary both deeply exhumed lower crust and posttectonic igneous rock never deformed under brittle conditions and not deformed recently should be less susceptible to detachment and subsequent transport than fractured rock

dispersive tsunami of the 2010 chile earthquake recorded by the high sampling rate ocean bottom pressure gauges 1 cabled ocean bottom pressure gauges deployed offshore japan recorded the pressure change associated with the 2010 chile earthquake tsunami over a wider frequency range than that of coastal tide gauges or hydrophones although it was difficult to recognize the dispersive features in the original records the spectrograms clearly showed wave dispersion a low frequency tsunami 1 mhz arrived after an elapsed time of 24 hr from the earthquake origin time while a high frequency tsunami 9 mhz arrived after an elapsed time of 48 hr the arrival times can be explained by assuming a constant water depth of 4 km however the calculated waveform does not correctly reproduce the number of wave packets appearing in the observations cabled ocean bottom pressure gauges deployed offshore can record broadband tsunami signals reflecting the wide wavenumber range spatial spectrum for sea bottom deformation caused by the earthquake

a geometrically based method for predicting stress induced fracture aperture and flow in discrete fracture networks modeling of fluid flow in naturally fractured reservoirs is often done through modeling and upscaling of discrete fracture networks dfns the two dimensional fracture geometry required for dfns is obtained from subsurface and outcropping analog data however these data provide little information on subsurface fracture aperture which is essential for quantifying porosity and permeability apertures are difficult to obtain from either outcropping or subsurface data and are therefore often based on fracture size or scaling relationships but these do not consider the orientation and spatial distribution of fractures with respect to the in situ stress field using finite element simulations mechanical aperture can be modeled explicitly but because changes in fracture geometry require renewed meshing and simulating this approach is not easily integrated into subsurface dfn modeling workflows we present a geometrically based method for calculating the shear induced hydraulic aperture that is an aperture of up to 0 5 mm 0 02 in that can result from shear displacement along irregular fracture walls the geometrically based method does not require numerical simulations but it can instead be directly applied to dfns using the fracture orientation and spacing distributions in combination with an estimate of the regional stress tensor and orientation the frequency distribution of hydraulic aperture from the geometrically based method is compared with finite element models constructed from five real fracture networks digitized from outcropping pavements these networks cover a wide range of possible geometries and spatial distributions the geometrically based method predicts the average hydraulic aperture and equivalent permeability of fractured porous media with error margins of less than 5

a classification of dyke fracture geometry with examples from precambrian dyke swarms in the vestfold hills antarctica a classification of dilational fracture geometry is proposed it applies to structures that can be characterized by the combination of a fracture system and a dilation vector field the classification is illustrated with examples of igneous dykes pegmatites and pseudotachylites segmentation in the form of offsets jogs or bifurcations is common to most fracture systems four basic types of dilational fracture systems are distinguished on the basis of the geometry of segmentation these are irregular braided en echelon and zigzag zigzag fracture systems are further differentiated they consist of newly formed obliquely dilated fractures or alternatively of pre existing planes in the host rock that are reactivated by oblique dilation this paper is the first to actually report an igneous zigzag dyke involving newly formed fractures rotation of the regional stress field in the direction of propagation leads to en echelon segmentation braided fracture systems reflect high local stress intensities probably related to propagation rate two possibilities exist for the formation of zigzag dykes consisting of newly formed fractures that are obliquely dilated they may form by extreme interaction of tensile fracture segments in a regional stress field with a low differential stress alternatively they may form in a regional stress field with high differential stress through the propagation of shear fractures segmentation of dykes characterized by offsets is common such segmentation can be the result of protrusions of the fracture termination offsets also occur where a dyke cuts an older planar structure such offsets are the result of the local inhibition of fracture propagation the resulting apparent offset can lead to a misinterpretation of relative age apophyses form as a result of the dilation of a segmented fracture system

fracture accommodated strain in the diamond joe stock arizona fractures filled with hydrothermal minerals form a dominantly radial pattern in the diamond joe stock these fractures record a portion of the deformation experienced by the pluton from the time that it was emplaced until it cooled to ambient conditions in individual outcrops fracture orientations are very consistant to within 10 where fractures cross k feldspar phenocrysts no shear offset is observed indicating that the fractures have opened by dilation we have measured the apertures of these veins and define a two dimensional deformation gradient matrix to characterize the uniaxial strain accommodated by each fracture we assume that each fracture represents one increment of strain and superimpose all increments in an individual outcrop to compute the fracture accommodated strain for that outcrop we calculate the orientation and magnitude of the principal strains for each outcrop as well as the area change accommodated by the fractures all of the outcrops have experienced a decrease in area that ranges from 5 a portion of this area change is interpreted as thermal contraction however in the central part of the pluton additional contraction is present in this portion of the pluton the amount of strain and area change correlates well with fracture abundance we interpret this additional contraction to result from volume changes associated with crystallization and or vapor exsolution during crystallization of the pluton

natural fractures in shale a review and new observations natural fractures have long been suspected as a factor in production from shale reservoirs because gas and oil production commonly exceeds the rates expected from low porosity and low permeability shale host rock many shale outcrops cores and image logs contain fractures or fracture traces and microseismic event patterns associated with hydraulic fracture stimulation have been ascribed to natural fracture reactivation here we review previous work and present new core and outcrop data from 18 shale plays that reveal common types of shale fractures and their mineralization orientation and size patterns a wide range of shales have a common suite of types and configurations of fractures those at high angle to bedding faults bed parallel fractures early compacted fractures and fractures associated with concretions these fractures differ markedly in their prevalence and arrangement within each shale play however constituting different fracture stratigraphies differences that depend on interface and mechanical properties governed by depositional diagenetic and structural setting several mechanisms may act independently or in combination to cause fracture growth including differential compaction local and regional stress changes associated with tectonic events strain accommodation around large structures catagenesis and uplift fracture systems in shales are heterogeneous they can enhance or detract from producibility augment or reduce rock strength and the propensity to interact with hydraulic fracture stimulation burial history and fracture diagenesis influence fracture attributes and may provide more information for fracture prediction than is commonly appreciated the role of microfractures in production from shale is currently poorly understood yet potentially critical we identify a need for further work in this field and on the role of natural fractures generally

precollision tilt of crustal blocks in rifted island arcs structural evidence from the kohistan arc 1 we describe results from field and microstructural investigations in the unusually large chilas plutonic complex which was emplaced within the mesozoic kohistan island arc northern pakistan magmatic and deformational fabrics of the chilas gabbronorite are concordant with magmatic fabrics of associated gabbros diorites and tonalites and show that this reportedly huge intrusion is composed of juxtaposed plutonic bodies with predominantly ssw dipping flow structures over the northeastern part of the complex which turn to sw ne striking subvertical attitudes to the west evidence for multiple intrusions includes concordant and discordant intrusive and or mylonitic contacts between gabbronorites and associated rocks owing to structural and kinematic consistency we attribute transitions from magmatic to more localized mylonitic fabrics to higher finite strain and or strain rate produced from drag of magma against its margin we discuss this structural pattern and conclude that the chilas gabbronorite emplaced incrementally into generally south dipping listric faults that dominated arc extension about 85 ma in addition several sets of hornblende pegmatite indicate that brittle deformation occurred in the presence of residual melt during consolidation of the main body the latest pegmatite set is composed of variably striking vertical extensional veins that show no relationship to the magmatic fabric we argue that the original attitude of these veins along with that of the enclosing chilas complex has been mostly preserved we hypothesize that the northward dip of the southern side of the kohistan is inherited from late cretaceous rotation of crustal blocks in the hanging wall of the listric faults

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

testing a thermal stress origin of fractures in a pluton abstract several factors suggest that mineralized joints in the lake edison granodiorite of the sierra nevada formed as the pluton cooled biotites and hornblendes in the granodiorite yield radiometric ages of 80 m y and 85 m y respectively whereas minerals in fault zones that developed from the joints are dated at 79 m y the age of the joints thus is consistent with cooling of the pluton mineral fillings in the joints consist principally of epidote and chlorite an assemblage consistent with the joints having formed as the pluton cooled the joints approach the boundary of the elongate pluton at nearly right angles the joints usually terminate at or near the pluton boundary rather than extending well into an older adjacent pluton thermoelastic stress analyses suggest conductive cooling of the pluton could have caused the observed joint patterns and the strains accommodated by joints if joints in plutons characteristically form as a result of thermal stresses during cooling then the large scale pattern of joints should be predictable based on pluton geometry practical applications of this work with regard to large scale fluid flow in the crust include petroleum recovery and nuclear waste repository siting

hydrothermal breccias in vein type ore deposits a review of mechanisms morphology and size distribution abstract breccias are among the most widely distributed rock textures found in hydrothermal vein type deposits previous studies have mainly been interested in developing qualitative descriptive approaches leading to a confusing profusion of terms brecciation originates in numerous ways resulting in highly complex classification systems and frequent misinterpretations of facies field observations are difficult to reconcile with physical theories of fragmentation partly due to the fact that few satisfactory quantitative tools have been developed a review of the main brecciation processes occurring in hydrothermal vein type deposits allows for the discrimination between chemical and physical mechanisms including tectonic comminution wear abrasion two types of fluid assisted brecciation hydraulic and critical volume expansion or reduction impact and collapse each of these mechanisms can be distinguished using nonscalar parameters that describe breccia geometry including fragment morphology size distribution of the fragments fabric and dilation ratio the first two parameters are especially important because 1 the morphology of the fragments allows chemical and physical mechanical breccias to be distinguished and 2 the particle size distribution psd is a function of the energy input during breccia formation the slope of the cumulative psd fractal dimension ranges from high values for high energy brecciation processes to low values for low energy processes indicated by an isometric distribution the evolution of a vein system can be divided into three stages propagation wear and dilation these stages are separated by one threshold of mechanical discontinuity and one of hydraulic continuity these two thresholds also mark the transition between different types of brecciation mineralization occurs during all three stages and may display different textures due to pressure variations the use of quantitative parameters in fault related hydrothermal breccias allows a better understanding of the physical parameters related to a vein environment including structural setting and crustal level as well as fluid rock interactions recognition of the different breccia types could also be important during the early stages of mineral exploration

mechanisms controlling rupture shape during subcritical growth of joints in layered rocks during joint propagation the orientation of plume lines on joint faces allows for mapping the path taken by the crack tip line during rupture growth rupture shapes in layered clastic sediments of the devonian catskill delta finger lakes district new york indicate joint growth through three stages where velocity of the tip line v tl varies as a function of the crack tip stress intensity k i the initial stage of growth is characterized by a rupture of approximately circular or elliptical shape that expands from an initiation fl aw with a velocity v tl in some cases primary growth involves self correction where an elliptical rupture redistributes the crack tip stress so that the rupture returns to a more stable circular shape the crack tip line eventually propagates to a bedding interface where properties of either a plastic or a noncohesive bedding interface cause v tl 0 along a portion of the tip line the onset of secondary growth occurs when the rupture tip line intersects both bedding interfaces and splits into two discontinuous segments that propagate synchronously as a single coherent rupture nonuniform nonsystematic v tl eventually interferes with the rupture s ability to grow coherently and thus leads to the transition to tertiary growth characterized by the detachment of the coherent rupture into one or more independently propagating noncoherent tip lines throughout all three stages of rupture growth the k i dependent v tl points to subcritical propagation variation from a smooth to rough plume morphology is consistent with propagation through region i of the subcritical regime with a transition at v tl 10 5 m sec rupture velocity may enter region ii of the subcritical regime but such a broad range of k i must be crossed to reach region iii and quasistatic behavior at the critical stress intensity k ic that region ii acts as a barrier through which few joints pass thus greatly limiting the number of postcritical joints in the catskill delta if not in the crust of the earth

kinematic implications of joint zones and isolated joints in the navajo sandstone at zion national park utah evidence for cordilleran relaxation 1 at zion national park znp the landscape is a consequence of differential weathering of the navajo sandstone where closely spaced vertical joints constitute joint zones that erode to form regularly spaced half kilometer slot canyons striking 351 between these joint zones is a set of isolated joints striking 339 fracture interaction and horsetail wing crack development indicate that the 339 striking joint set is younger than the 351 striking joint zones despite the lateral extent of stress reduction shadows in the vicinity of the large scale joint zones in addition to an older less pervasive 020 joint set this sequence of jointing records a counterclockwise rotation of the regional extension directed from wnw to wsw in the navajo sandstone at znp znp is located at the western margin of the colorado plateau 100 km east of the major normal faults of the northeastern central basin and range subprovince extension within the eastern central basin and range initiated during the miocene and exhibited a wsw extension direction anderson 1971 wernicke et al 1988 snow and wernicke 2000 the correlation between nearby basin and range extension and the extension direction for the 351 tending joint zones of znp is so close that the jointing at znp is interpreted as evidence for modest yet pervasive basin and range extension in the western margin of the colorado plateau

magma driven hydraulic fracturing and infiltration of fluids into the damaged host rock an example from dronning maud land antarctica abstract excellent outcrops in dronning maud land antarctica provide unique insight into the mode and extent of fluid infiltration into metamorphic and plutonic rocks in the middle crust the fluids are liberated from pegmatitic veins and give rise to alteration halos in the alteration halos the conspicuous change in colour is correlated with 1 hydration mineral reactions and 2 high density of microcracks in quartz and feldspar exceeding that observed in the unaltered host rock by an order of magnitude the field relations indicate that the veins originated as melt driven hydraulic fractures sealed by pegmatite and aplite crystallising from volatile rich melts with the alteration halo being the wake of the process zone formed at the tip of the propagating fractures it is proposed that 1 the size of the alteration zone and the width of the vein are correlated resulting in higher values of both these quantities for cracks propagating at higher velocities and consequently higher crack propagation toughnesses 2 the damage zone is characterised by a transient state of high permeability which was short lived due to rapid healing and sealing of microcracks 3 the infiltration and retrogression of the high grade rocks can be considered as a quasi instantaneous process on geologic time scales with a duration of hours to weeks

aperture size scaling variations in a low strain opening mode fracture set cozzette sandstone colorado the aperture size distribution of a set of opening mode fractures in the cozzette sandstone is complex and cannot be described uniquely by a single equation this study of horizontal core includes aperture size data of macrofractures measured using a hand lens and microfractures measured using scanning electron microscope based cathodoluminescence sem cl macrofractures are poorly cemented and total fracture strain is an order of magnitude lower than the smallest strain values of most previously described vein sets macrofractures in this set therefore resemble typical joints as opposed to veins in most respects although intragranular microfractures compose about 98 of the microfracture population orientation fracture fill and fracture trace characteristics suggest that only transgranular microfractures can be shown to be genetically related to macrofractures aperture size data below 0 012 mm and above 1 mm follow a log normal distribution intermediate size data though relatively sparse suggest a power law aperture size distribution these results support the notion that this fracture set is an intermediate case between typical veins and joints the results also suggest that processes leading to size distributions commonly associated with veins i e power laws may be present in joint sets but over a more limited range bound at small size scales by grain scale effects and above by mechanical layering effects

three dimensional fracture distribution in relation to local cooling in a granitic body an example from the toki granitic pluton central japan this paper describes a spatial distribution of fracture frequency fracture number per unit interval in the toki granitic pluton central japan and their evaluation with respect to cooling behavior after solidification of the pluton three dimensional distribution of fracture frequency of all fractures all angle fractures based on borehole television data collected from 19 boreholes shows that 1 the central region of the granite has a high fracture frequency and 2 fracture frequency decreases with decreasing elevation measured from the mean sea level with the exception of the west side of the body the development of sub solidus reaction textures reflects the annealing duration of the pluton after solidification the extent of sub solidus reaction textures was measured at two neighboring positions and the difference was divided by the distance between the two positions the resulting quantity is here defined as a local cooling indicator lci we observed a related pattern between the distribution of fracture frequency and lci the high lci region corresponds to the region with a high frequency of all fractures and vice versa this relation indicates that fracture genesis has been mostly controlled by local cooling after solidification which caused thermal stress thus the three dimensional pattern of lci is a potential diagnostic tool for obtaining the spatial distribution of all fractures

regional fractures i a mechanism for the formation of regional fractures at depth in flat lying reservoirs 1 regional fractures may form slowly at depth in extension and parallel to regional horizontal tectonic compression such fracture sets can be common in reservoirs despite the absence of flexure of the strata and are commonly important contributors to reservoir permeability such fractures make good targets for deviated wells high formation pore pressure is crucial to creating low effective confining stresses and brittle rock properties in the reservoirs conditions conducive to fracturing despite deep burial but pore pressure does not exceed the least principal stress and does not cause tensile fracturing the mechanism of natural hydraulic fracturing is incompatible with the characteristics of most regional fracture sets rather many regional fractures form duri g far field compression initiate at locally induced tensile stresses caused by flaws in the rock and propagate in the plane of the maximum and intermediate compressive stresses the differential stress required for initiation and propagation of regional fractures is well below that necessary for shear failure in the presence of high pore pressure fractures can be held open at depth by a tectonic basinwide dilatancy of the strata and the open void space commonly becomes mineralized

formation and growth of extensional fracture sets naturally occurring extension fractures including some joints and veins form sets consisting of numerous subparallel fractures each with well defined ends in order to correctly interpret these fractures in the field it is necessary to determine which factors control the geometries of fracture sets this requires consideration of the conditions leading to both the initiation and the cessation of fracture propagation a fracture begins to propagate when the crack extension force reaches a critical value which is a property of the rock and of environmental conditions after propagation initiates the crack extension force varies owing to 1 increase in crack length 2 change in remote strain 3 elastic interaction with nearby cracks 4 change in internal fluid pressure and 5 stress relaxation due to growth of cracks in the surrounding rock each fracture continues to propagate as long as its crack extension force exceeds the critical value and terminates when this condition is no longer met neglecting fluid effects the final geometry of the fracture set is a function of the remote strain history and the initial crack geometry which can be characterized by the initial crack density for low initial densities large increases in crack length occur with little or no increase in remote strain however for large initial densities crack lengths increase only if the applied strain increases this analysis is used to estimate the variation in remote strain during formation of a fracture set in granitic rock from observations of the final fracture geometry

a model of fracture nucleation growth and arrest and consequences for fracture density and scaling 1 in order to improve discrete fracture network dfn models which are increasingly required into groundwater and rock mechanics applications we propose a new dfn modeling based on the evolution of fracture network formation nucleation growth and arrest with simplified mechanical rules the central idea of the model relies on the mechanical role played by large fractures in stopping the growth of smaller ones the modeling framework combines in a time wise approach fracture nucleation growth and arrest it yields two main regimes below a certain critical scale the density distribution of fracture sizes is a power law with a scaling exponent directly derived from the growth law and nuclei properties above the critical scale a quasi universal self similar regime establishes with a self similar scaling the density term of the dense regime is related to the details of arrest rule and to the orientation distribution of the fractures the dfn model so defined is fully consistent with field cases former studied unlike more usual stochastic dfn models ours is based on a simplified description of fracture interactions which eventually reproduces the multiscale self similar fracture size distribution often observed and reported in the literature the model is a potential significant step forward for further applications to groundwater flow and rock mechanical issues

formation of joints in cooling plutons abstract the geometry age and mineralogy of steeply dipping joints in the lake edison granodiorite of california indicate that thermal stresses played a key role in the formation of the joints joint traces curve to approach the pluton boundary at high angles at both large and small scales and they generally terminate at or near the contact with an older pluton radiometric dates and the epidote and chlorite fillings of the joints tie jointing to the initial cooling of the pluton a thermo mechanical stress analysis assuming two dimensional conductive cooling predicts thermal stresses of several tens of mpa the same order of magnitude as plausible fluid pressures regional stresses and lateral normal stresses associated with the overburden the orientation of the joints can be accounted for rather well by the stress field formed by superposing a uniform regional stress field on the predicted thermal stresses the large scale pattern of the early joints in many plutons should be predictable based on a pluton s geometry its age relative to the adjacent rock and knowledge of the regional stress at the time of initial cooling these findings bear on issues pertinent to mining petroleum recovery nuclear waste repository siting and ground water flow

growth of faults by accumulation of seismic slip the accommodation of large strains in the upper crust is largely achieved by the accumulation of displacement on faults observation shows that as a fault accumulates displacement it grows in size i e its surface area and its length increase here we address the question for an increase in the amount of displacement on a fault by how much would the length of the fault change it is argued by cowie and scholz 1992a that the displacement on a fault is linearly related to the length of the fault this simple result is expanded upon in this paper by assuming that a fault accumulates displacement by repeated earthquakes two different approaches are presented the first approach considers the balance that must be achieved between the energy available for deformation and the work involved in creating new fault surface area as the fault grows the available energy is provided by changes in strain energy when the fault slips the second approach is to construct a geometrical model for fault growth using the scaling relationship between the slip during a single earthquake and the length of the rupture the total displacement on a fault is the sum of the slips contributed by many earthquakes the usefulness of these two approaches is that the growth of a fault over geologic time can be described by parameters that can be obtained from earthquake and fault data the models presented here predict that 1 the maximum amount a fault can grow in a single earthquake that ruptures the entire fault is of the order of 1 of its previous length and 2 the work of faulting may account for 10 of the total energy available during an earthquake the energy lost from the system is accounted for by work done against friction and seismic radiation consequences of fault growth for the segmentation and thus seismogenic potential of a fault over geologic time are discussed using predictions of the theory