the mechanics of intersecting echelon veins and pressure solution seams in limestone abstract many studies that describe the formation of echelon vein arrays relate the causative stresses implicitly to the deformation reliant on simple shear kinematics such that the vein to array angle and the array width are the primary physical quantities in contrast we identify twelve physical quantities to describe echelon veins in two dimensions including coeval vein intersecting pressure solution seams a finite element method is used to reproduce vein shapes in linear elastic and elastic perfectly plastic model limestone model vein geometries are designed using values within the range of geometries measured from echelon veins at raplee anticline and comb monocline utah four physical quantities are significant for describing echelon vein shapes vein spacing vein array angle limestone elastic stiffness and closing of orthogonal pressure solution seams pressure solution seam closing influences the mechanical interaction between adjacent veins and for a range of conditions causes a nearly linear vein opening distribution triangular shapes and encourages straight vein propagation both of which approximate field measurements model results show that small spacing of veins with seams and large vein array angles promote straight vein traces in limestone with stiffness typical of laboratory measurements given the physical geologic conditions inferred from the burial history of the limestone strata

pull aparts shear fractures and pressure solution vein arrays are often composed of pull aparts which are linked by shear fractures good examples of which occur in the lower jurassic limestones of somerset southwestern england such pull apart arrays have displacement distance characteristics which are similar to fault zones with maximum displacement indicated by the largest pull apart widths near the centre of the array and with displacement decreasing towards the tips pull apart arrays usually die out into enechelon or pinnate veins evidence for pressure solution along the shear fractures which connect pull aparts include their dark and braided nature their obliquity to the displacement direction the high dihedral angles often 90 between conjugate shear fractures and the dissolution of earlier structures a range of geometries occurs with varying relative amounts of veins and pressure solution being related to varying amounts of transtension or transpression there is a general trend for an increase in the angle between vein segments and the shear fractures as contraction increases there is therefore a trend for increased pressure solution on the shear fractures in more contractional arrays the concentration of insoluble material along shear fractures has important implications for the mechanics and sealing of faults

the geometry of echelon fractures in rock implications from laboratory and numerical experiments abstract the traces of echelon joints veins and dikes in rock range from curving to straight theoretical analyses using boundary element numerical methods have concluded that straight open fractures imply a significant remote differential stress whereas curving traces imply a more nearly isotropic stress we present a series of laboratory experiments which investigate the two dimensional propagation paths of echelon fractures in pmma plates as a function of the applied biaxial loading and the initial geometry of a simple fracture array the experimental results support the theoretical conclusions and verify the accuracy of the numerical method at the grain scale rock does not rigorously conform to the assumptions of isotropy homogeneity and linear elasticity demanded by the numerical method nevertheless results from crack path stability theory suggest that small scale deviations of a fracture from the ideal path generally do not affect its large scale behavior fracture paths which become kinked or curved by local heterogeneities are found to self correct to a path governed by competition between the fracture tip and remote stress fields provided the remote differential stress is non zero this enhances our confidence that the numerical method can be used to produce and explain realistic fracture geometries in rock an unexpected experimental result is the non perpendicular intersection of fractures grown in the laboratory free surface boundary conditions require that the walls of an open fracture be free of shear stress the principal stresses are therefore aligned with the crack wall and we might consequently expect an approaching opening mode fracture either to intersect the free surface at a right angle or to turn away and follow an asymptotic nonintersecting path experimental and numerical modeling however shows how the near tip stresses generated by an approaching fracture dominate the local stress field and allow it to propagate along an oblique path very close to intersection with the adjacent free surface

measurement of geometry and linkage in vein arrays abstract veins in an array are characterised by measurement of their length l width w and orientation and by the number 0 1 or 2 of adjacent veins to which they are linked data from a single array 129 veins and two regions comprising 146 and 64 arrays have been measured and analysed the system for measurement provides a sound basis for the quantitative assessment of both the array geometry and the effects of interaction of adjacent veins linkage is achieved by a breaching of the bridges of wall rock between veins either by extension fractures shear failure or the development of stylolites it is primarily controlled by the strain as measured by vein width the size and shape of the veins is found to be controlled by two factors the degree of linkage and the kinematics of the array transtension transpression interplay of these produces variability both within and between arrays and this is examined for two large datasets from the lower jurassic of somerset and the lower carboniferous of the gower peninsula south wales

formation and interpretation of dilatant echelon cracks the relative displacements of the walls of many veins joints and dikes demonstrate that these structures are dilatant cracks we infer that dilatant cracks propagate in a principal stress plane normal to the maximum tensile or least compressive stress arrays of echelon crack segments appear to emerge from the peripheries of some dilatant cracks breakdown of a parent crack into an echelon array may be initiated by a spatial or temporal rotation of the remote principal stresses about an axis parallel to the crack propagation direction near the parent crack tip a rotation of the local principal stresses is induced in the same sense but not necessarily through the same angle incipient echelon cracks form at the parent crack tip normal to the local maximum tensile stress further longitudinal growth along surfaces that twist about axes parallel to the propagation direction realigns each echelon crack into a remote principal stress plane the walls of these twisted cracks may be idealized as helicoidal surfaces an array of helicoidal cracks sweeps out less surface area than one parent crack twisting through the same angle thus many echelon cracks grow from a single parent because the work done in creating the array as measured by its surface area decreases as the number of cracks increases in cross sections perpendicular to the propagation direction echelon cracks grow laterally each crack overlapping its neighbors until the mechanical interaction of adjacent cracks limits this growth dilation of each crack pinches the tips of adjacent cracks into an asymmetrical form and introduces local stresses that can cause lateral growth along a curving sigmoidal path sigmoidal echelon cracks may link at tip to plane intersections leaving a step in the through going crack wall the geometry of dilatant echelon cracks may be used to infer spatial or temporal changes in the orientation of principal stresses in the earth

fracture propagation associated with dike emplacement at the skaergaard intrusion east greenland abstract extensive magmatic activity including several episodes of dike emplacement was associated with early tertiary continental rifting in eastern greenland exposures of the skaergaard intrusion host a large number of these dikes as well as a variety of hydrothermal veins one vein set has a characteristic outcrop expression and is filled with silica rich chlorites relative timing constraints as well as the distribution and geometric relations of these veins relative to the mafic dikes indicate that at least some of these fractures formed during the emplacement of the dikes that cut the intrusion calculations based on elastic theory permit modeling of dike geometries observed in the field and prediction of fracture geometries that could be associated with the inflation of these dikes extensional fractures may form in the area ahead of the dike termination for a variety of loading conditions the predicted width to length ratio for the dike as well as the size of the predicted fracture zone increases with increasing magma pressure within the dike and or increasing differential between the remote principal stresses for dike geometries similar to those observed in the field the predicted fracture zone is roughly elliptical in cross section and is centered in front of the dike termination the size of the zone perpendicular to the dike trend varies from less than one meter to several meters field observations indicate that dike parallel fractures are locally offset normal to the dike margin by distances of a few centimeters to a few meters our analysis implies that at least some of these fractures initiated and propagated ahead of the dikes as they were emplaced the cooling of these dikes initiated localized hydrothermal systems that formed the vein minerals currently filling the fractures

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

the formation and growth of pressure solution seams in clastic rocks a field and analytical study abstract pressure solution seams psss are closing mode structures of localised dissolution resulting in a volume reduction we present new observations constraining the processes by which psss initiate and grow in low porosity clastic rocks from southern ireland psss initiate as inter granular psss at quartz grain to grain contacts as quartz dissolves clay remains as a residue along the contacts as well as filling the adjacent pores to form incipient psss involving multiple grain boundaries further lengthening and thickening occurs by lateral linkage and transverse coalescence of neighbouring segments of incipient psss multiple pss segments are observed to concentrate in thin tabular zones that appear as single macroscopic psss visible in hand samples published numerical modelling results show the ability of both lateral and transverse growth of psss due to the presence of asperities on their flanks which confirm our interpretations based on our laboratory and field observations

fracture initiation development and reactivation in folded sedimentary rocks at raplee ridge ut abstract fractures at raplee ridge a 14 km long monoclinal fold of pennsylvanian sedimentary rock in southeast utah are examined to determine their sequential development and to explain the mechanical relationships among the three primary fracture sets and the fold folding is attributed to slip on an unexposed thrust fault of laramide age fracture orientations were measured at 98 sites and fracture patterns were mapped at 8 sites on two sandstone and two limestone units of the rico formation revealing at least four stages of fracture formation stages 1 and 2 occurred pre folding and are defined respectively by e w trending fracture set i and n s trending fracture set ii set i fractures are characterized by a unimodal distribution of attitudes an intermediately developed outcrop pattern and continuous linear traces on bedding surfaces set ii fractures have a greater dispersion of orientations and are poorly to intermediately developed abutting relationships confirm that set i fractures are older than set ii stage 3 occurred during folding when nw se trending set iii fractures initiated both from isolated flaws and as wing cracks due to right lateral shearing along reactivated set i fractures set iii fractures increase in density as the fore limb dip increases from 10 to 30 later stages of deformation are characterized by the formation of fracture subsets iv and v structurally associated with fore limb dips greater than 30 fracture sets i and ii are found throughout the surrounding monument upwarp and are attributed to that structure whereas set iii is attributed to local stresses induced by monoclinal folding and or thrust faulting

the effect of experimental and microstructural parameters on the transition from brittle failure to cataclastic flow of carbonate rocks abstract triaxial compression tests were conducted on cold pressed calcite aragonite and limestone aggregates and on solnhofen limestone specimens to study the effect of experimental and microstructural parameters on the transition from brittle failure to cataclastic flow the tests were performed at confining pressures up to 195 mpa and at strain rates between 5 10 4 s 1 and 5 10 6 s 1 axial as well as volumetric strain were measured samples were produced by cold pressing powders of crushed calcite and aragonite crystals and of crushed solnhofen limestone sample porosity ranged between 5 and 25 and the average grain size varied between 5 and 400 m for both the cold pressed aggregates and the intact limestone specimens the confining pressure at the transition from localized brittle failure to non localized cataclastic flow decreases with increasing porosity and grain size the transition is characterized by a zero work hardening coefficient by dilation for low porosity and compaction for high porosity rocks by a constant ratio between axial stress and confining pressure and by decreasing yield strength for increasing confining pressure the experimental results disagree with the critical state concept over most of the porosity range investigated and indicate non associated material behaviour these properties of the brittle ductile transition are addressed on the basis of continuum mechanics or by models suggested for granular materials the problems discussed and the results obtained are of fundamental interest to rock deformation and structural geology

extreme aridification since the beginning of the pliocene in the tarim basin western china abstract mid latitude central asia is characterized by an extreme arid landscape among the central asian deserts the taklimakan desert is the largest shifting sand desert which is located in the rain shadow of the tibetan plateau and of the other central asian high mountains the formation of this desert is important for placing widespread aridification into a regional tectonic context at the western end of the himalayan tibetan orogen however there still exists considerable controversy regarding the timing of desert formation thus impeding our understanding of the climatic effects of the tibetan uplift and regional environmental changes here we report new biostratigraphic age control and multiple high resolution climatic records from the center of the taklimakan desert our results reveal dramatic environmental changes at 5 ma suggesting that an extremely dry climate has prevailed since the beginning of the pliocene which is consistent with findings from a high resolution borehole record about 670 km to the east in the same basin the two records combined demonstrate that an extremely dry environment prevailed in the entire tarim basin from approximately 5 ma this was related to the reduced transport of water vapor by westerlies in response to the retreat of the paratethys ocean driven by global climatic cooling and the closed oceanic water vapor pathway between the pamir and the tian shan ranges driven by the ongoing india eurasia collision

fluid filled fractures in earth s lithosphere gravitational loading interpenetration and stable height of dikes and veins abstract we present two analytical solutions for the boundary value problem of a homogeneous isotropic and linear elastic solid with a vertical fluid filled crack and use these to investigate the effects of gravitationally induced stress and pressure gradients on the opening distribution fracture tip stress intensity and stable heights of veins and dikes stable fractures are not propagating or closing anywhere along their tipline so the vein or dike is in its final arrested configuration as preserved in the rock record the solutions do not preclude interpenetration of crack surfaces so we propose special constraints to avoid such ill posed problems veins and dikes both grow as fluid filled opening fractures in earth s brittle lithosphere but attain markedly different heights we show how gravity induced pressure gradients in water and magma and stress gradients in the host rock lead to different stable heights dikes but not veins can attain stable heights near the level of neutral buoyancy in a density stratified rock mass even if the host rock fracture toughness is zero depth to center and opening distributions for dikes are different if host rock density is stratified or continuously variable these solutions offer a new set of analytical tools for interpreting field data from veins and dikes

impact of fluid pressure on failure mode in shear zones numerical simulation of en echelon tensile fracturing and transition to shear abstract en echelon veins are a set of tensile fractures that develop during the initiation of shear zones under certain conditions but factors that control fracture patterns remain unclear in this study we performed numerical simulations using the two dimensional distinct element method to understand the influence of effective normal stress on the development of fractures during shear deformation at low effective normal stress 10 mpa dilation takes place at an angle of 40 45 to the shear boundary and en echelon arrays of tensile fractures develop in contrast at high effective normal stress 150 mpa en echelon veins are not developed but a distinct shear plane forms a transient behavior characterizes intermediate effective normal stress 50 mpa early tensile cracks are generated and then propagate to form larger brittle faults in this case the larger strain energy released in the later stage is due to the greater shear strength under crustal conditions effective normal stress varies due to the change in fluid pressure our results suggest that en echelon veins are indicators of high fluid pressure close to lithostatic conditions i e effective normal stress close to zero and that a transition of the failure mode from tensile to shear could occur in response to a dynamic change of fluid pressure during fracturing

linkage and evolution of conjugate strike slip fault zones in limestones of somerset and northumbria conjugate strike slip fault zones that cover metre scale areas at beadnell northumbria and kilve somerset were initiated as conjugate vein arrays early conjugate faults are linked by the propagation of one fault that eventually by passes the other fault a model for the development of strike slip faults is presented using fault and vein geometries and the position of damage zones with respect to the master faults as an indication of the propagation direction this model includes the evolution of networks from 1 the initial random development of vein arrays to 2 the isolated development of several unconnected conjugate fault segments that pass into vein arrays through 3 the intersection of a conjugate set of master faults and linkage with minor antithetic faults and the formation of new vein arrays with extensional geometries after a linked network of faults is established to 4 breaching of intersection points by dominant faults and finally 5 the propagation towards oversteps that are breached to form a through going fault the geometry of the active structures simplifies with time as strain is localised along the master fault but the complexities are preserved in the fault walls

inferring paleostresses from natural fracture patterns a new method we introduce a method to infer the remote differential stress magnitude from the curvature of overlapping echelon fracture traces the curving paths of overlapping echelon cracks imply the predominance of local crack induced stresses over remote stresses during propagation nearly straight crack paths imply the controlling influence of a remote compressive crack parallel differential stress this method is used to interpret complex joint patterns mapped in sedimentary rock it is also applied to the problem of fracture pattern generation using computer models

conditions during syntectonic vein formation in the footwall of the absaroka thrust fault idaho wyoming utah fold and thrust belt abstract the twin creek limestone in the footwall of the absaroka thrust sheet contains three sets of bed normal syntectonic calcite veins vein formation occurred during cretaceous motion along the absaroka thrust fault as indicated by 1 crosscutting relationships among these vein sets 2 a previously dated solution cleavage and 3 calcite twin analysis fluid inclusions in the veins and overburden estimates constrain inclusion entrapment temperatures to be between 175 c and 328 c results from stable oxygen isotopes indicate that the host and vein fluid compositions were in near isotopic equilibrium applying both reasonable geothermal gradients and constraints on overburden temperature yields fluid pressures during vein precipitation that are near hydrostatic all data taken together suggest both that vein formation within the twin creek formation occurred in a relatively closed system and that the veins filled near hydrostatic fluid pressure because the veins fill precursory cracks vein filling might not reflect the maximum fluid pressure that existed during the complete vein forming process

inter array and intra array kinematics of en echelon sigmoidal veins in cross bedded sandstone merimbula southeastern australia abstract the kinematics of en echelon arrays of quartz veins hosted by sandstone of the upper devonian worange point formation are strongly influenced by bedding and cross bedding bulk deformation recorded by the vein arrays inter array kinematics was influenced by the anisotropy of the host rock such that three types of arrays are observed 1 east verging arrays parallel to bedding 2 east verging arrays oblique to bedding with veins parallel to cross bedding and 3 west verging arrays parallel to cross bedding with veins parallel to bedding these three array types correspond to flats ramps and backthrusts respectively and represent the earliest stages of thrust fault formation flat and ramp arrays form a staircase geometry and conjugate ramp and backthrust arrays form pop up structures the interpretation of the morphology of veins and wall rock intra array kinematics indicates rotation of bridges was the primary kinematic process in the arrays the opening of vein space by rotation of bridges which maintain constant thickness is predicted from measurements of the orientations of vein wall rock interfaces at vein tips and at the array centre comparison of the predictions with direct measurement of the proportion of vein to wall rock along the centreline of arrays demonstrates the applicability of the bridge rotation model which has been generalised to include the thickening or thinning of bridges during rotation

shear strain and volume change associated with sigmoidal vein arrays in the boda claystone abstract the boda claystone formation a potential host rock for high level nuclear waste in hungary contains numerous sigmoidal vein arrays with angular vein terminations these veins are excellent strain markers and can be used in the interpretation of palaeo environments sigmoidal vein arrays were investigated in the baf 2 well which penetrates the formation to depths above 900 m with nearly 100 core recovery the veins have antitaxial growth morphologies and are filled by calcite anhydrite and barite celestine phases to estimate the volume change in shear zones we used geometric mass balance and density change calculations these methods exhibit different volume loss rates the geometric approach indicates a large amount of volume loss whereas the isocon method and density change approach suggest that volume loss in the shear zones was less than 10 we propose that the volume loss in the shear zone occurred because of tectonic compaction vein shapes indicate formation under compressional stress fields based on their intersections with other vein generations the sigmoidal en echelon vein generation is determined to be one of the youngest structures in the well which probably formed during a late cretaceous compression event

extensional veins induced by self similar dissolution at stylolites analytical modeling abstract stylolites are surfaces of localized dissolution observed in rocks many stylolites share a common morphological feature the amount of dissolved rock in the center of the seam is proportional to the stylolite s length field observations demonstrate that stylolites can locally influence formation of veins the developed analytical model of the indirect solution of eshelby s transformation problem calculates the stress field within and around the stylolite the results suggest that growth of a stylolite with an elliptic dissolution pattern generates a local non uniform stress field in the host rock which is in turn accommodated by brittle mode i fracturing in the stylolite midsection further resulting in veins formation parametric analysis and qualitative comparison with anticrack model describing stress field within and around stylolite are conducted

shear zone deformation determined from sigmoidal tension gashes the potential of using sigmoidal tension gashes as strain markers for assessing strain localisation in shear zones is discussed the appropriate analytical methods for this purpose depend on the assumed mechanism of tension gash formation two such models are considered the first is one in which the curvature of the gash is produced by passive rotation of different segments of the gash in response shear strain gradients across the shear zone the other model is one in which the curvature of the gashes is governed by the folding of the competent rock bridges between adjacent gashes in the latter case the tension gashes progressively grow within spaces created by the buckling bridges and therefore lead to a bulk dilatation of the shear zone however for the folded bridge mechanism to continue to operate beyond shear strains greater than unity requires a significant volume loss which in turn may signal the increase of the shear strength of the zone the geometrical characteristics of gash arrays resulting from these two mechanisms are described and criteria given for the recognition of the two types of gash arrays a new graphical method is proposed for the analysis of deformation in shear zones containing folded bridge tension gashes tension gash arrays from marloes west wales are used as examples of the procedures for shear zone analysis

a late pleistocene coarse grained spit platform sequence in northern jylland denmark several well preserved late pleistocene spit systems occur uplifted in northern jylland denmark their present day morphological expression allows detailed study of spit growth patterns while the internal sedimentological organisation can be examined in a series of pits distributed along the length of the spits two characteristic vertical sequences are recognized in the systems the first sequence i consists of a giant scale cross bedded foreset unit overlain by topset and beach units while the second sequence ii consists of the foreset unit overlain by bar trough and beach units the two sequence types pass laterally into each other with a short overlap zone they can be interpreted in terms of meistrell s 1966 1972 model for spit platform growth based on scaled wave tank experiments the giant scale cross bedded unit corresponds to prograding of a coarse grained subaqueous spit platform while the topset bar trough and beach units reflect the growth of the subaerial spit the alternation between sequence i and ii reflects the inversely related growth of the spit and platform structures when the rate of subaqueous platform progradation declines the subaerial spit grows uniformly and when the platform progrades uniformly spit growth declines the model is probably only valid for relatively coarse grained systems because only these deposits would have a relatively steep front the water depth in which the spit system progrades and thus bottom topography determines the thickness of the giant scale cross bedded foreset unit because the water depth over the top of the platform is relatively constant if the water is less than a few metres deep the spit platform is not developed as seen where the late pleistocene spit systems prograded over elevations of the sea bottom conversely the correct recognition of spit platform sequences allows precise determination of sea level and water depth at the time of formation finally the model adds one further mode of formation of giant scale cross bedding to those already known from fluvial transverse lateral and point bars subtidal sand waves and gilbert deltas