formation deformation and chertification of systematic clastic dykes in a differentially lithified carbonate multilayer sw iberia algarve basin lower jurassic this work presents original field evidence for tectonically controlled calciclastic dyke injection and subsequent chertification in shelf carbonates during rifting in the south westernmost part of the eurasian continent in the early pliensbachian it is shown by the detailed description of tectonic structures faults and joints stratigraphic discontinuities and by the distribution orientation and morphology of the injected dykes of calciclastic sands into fine grained carbonates that these soft sediment deformation structures were tectonically controlled extensional tectonics developed vertical tensile joints in semi lithified fine grained limestones permitting upward and downward injection of loose calciclastic sands forming clastic dykes the frequency of injection structures along strata was constrained by the thickness of the layer into which the injection was occurring which implies an elastic behaviour however the curvi planar shapes of the dykes and drop like nodules attests the ductile behaviour of the fractured limestones indicating transition from elastic to ductile response to deformation with time the multi layered system consists of just two lithotypes the two display different mechanical behaviours which evolved in time to more brittle conditions as lithification progressed as shown by the cataclastic faulting and jointing of the previously formed soft sediment deformation structures and strata under an analogous stress regime the injection dykes were disrupted by a short lived episode of compression after which tectonic extension resumed still in the lower pliensbachian sometime before the deposition of the upper pliensbachian a pervasive selective event of chertification occurred only the calciclastic sandy layers dykes and nodules were substituted by silica thus enhancing the mechanical contrast between the primary sedimentary structures and the soft sediment deformation structures all the described events occurred during a time interval of approximately 2 myr as constrained by ammonoid stratigraphy

granite ascent and emplacement during contractional deformation in convergent orogens based on a case study in the central maine belt of west central maine u s a it is proposed that crustal scale shear zone systems provide an eaective focussing mechanism for transfer of granite melt through the crust in convergent orogens during contractional deformation flow of melt in crustal materials at depths below the brittle plastic transition is coupled with plastic deformation of these materials the flow is driven by pressure gradients generated by buoyancy forces and tectonic stresses within the oblique reverse central maine belt shear zone system stromatic migmatite and concordant to weakly discordant irregular granite sheets occur in zones of higher strain which suggests percolative flow of melt to form the migmatite leuco somes and viscous flow of melt channelized in sheet like bodies possibly along fractures cyclic fluctuations of melt pressure may cause instantaneous changes in the eaective permeability of the flow network if self propa gating melt filled tensile and or dilatant shear fractures are produced due to melt enhanced embrittlement in homogeneous migmatite and schlieric granite occur in zones of lower strain which suggests migration of partially molten material through these zones en masse by granular flow and channelized flow of melt carrying entrained residue founded on the central maine belt case study we develop a model of melt extraction and ascent using the driving forces stress conditions and crustal rheologies in convergent especially transpressive orogens ascent of melt becomes inhibited with decreasing depth as the solidus is approached for intermedi ate a h2o muscovite dehydration melting the water saturated solidus occurs between 400 and 200 mpa near the brittle plastic transition during high t low p metamorphism where the balance of forces favors sub horizontal fracture propagation emplacement of melt may be accommodated by ductile flow and or stoping of wall rock and inflation may be accommodated by lifting of the roof at shallower crustal levels and or sink ing of the pluton floor the resultant plutons have sub horizontal tabular geometries with floors that slope down to the ascent conduits although these plutons may have locally discordant relations with country rock structures when viewed at the crustal scale granite ascent and emplacement in convergent orogens are syn tec tonic processes 1998 elsevier science ltd all rights reserved

strain fabric analysis applied to hydrothermal ore deposits emplaced during changing geodynamical conditions infiernillo and las picazas san rafael massif argentina abstract infiernillo and las picazas are two small hydrothermal ore deposits in the northern san rafael massif argentina they are genetically linked to the permian choiyoi volcanic province which reflects the transition from a convergent plate margin to an extensional regime calc alkaline early permian lower choiyoi magmatism was syntectonic with transpressional deformation of the san rafael orogeny whereas transitional late permian upper choiyoi sequences were emplaced under a transtensional regime during the post san rafael extension infiernillo is a cu mo porphyry type deposit hosted by pyroclastic rocks of the lower choiyoi it consists of a central quartz plug surrounded by a potassic and a phyllic halo and a set of peripheral polymetallic cu pb zn veins cropping out close to the alteration zone the las picazas deposit consists of a group of low sulfidation epithermal galena bearing veins hosted by early palaeozoic meta sediments strain fabric and available 2 d and 3 d kinematic analyses were performed in order to define the relationships between both deposits and the geodynamic scenario these data reveal that a porphyry emplacement at infiernillo occurred during the declination of the san rafael orogeny shortly before the change in stress regime and b las picazas low sulfidation epithermal veins were emplaced during the subsequent transtensional post san rafael regime these results prove that each ore deposit is linked to a different stress regime reflecting the change in geodynamical conditions that prevailed during the emplacement of the choiyoi volcanism thus strain fabric analysis at both ore deposits allows us to confirm that the tectonic regime is a major factor in controlling the mineralization style of hydrothermal ore deposits

formation of bedding parallel fibrous calcite veins in laminated source rocks of the eocene dongying depression a growth model based on petrographic observations abstract bedding parallel fibrous calcite veins commonly referred to as beefs are widely developed within eocene lacustrine laminated organic rich source rocks in the dongying depression of bohai bay basin east china understanding the formation of this type vein is essential to elucidate the diagenetic evolution and fluid migration in muddy source rocks this study investigates the mechanism s of growth of bedding parallel veins in hydrocarbon source rocks of the es4 upper and the es3 lower units in the dongying depression based on systematic observations of macro and micro scale features both in veins and their wall rocks combined with x ray diffraction and analyses of geochemical indicators the source rocks are heterogeneous in both composition and structure the beefs mainly occur in laminated rocks consisting of organic rich laminae which occur together with horizontally continuous banded lamalginites and rare carbonate rich laminae overall corresponding to high toc low carbonate contents and low maturity average ro 0 57 most beefs are embedded in the neighboring lamalginites of organic rich laminae with low clay mineral contents the beefs are products of diagenesis accompanying burial and both the initial fracture opening and subsequent dilation represent early stages of hydrocarbon generation and expulsion the presence of bedding parallel bituminite veins and the median plane or parting is evidence for horizontally oriented hydrofracturing due to fluid overpressure in turn fluid overpressure reflects the generation and primary migration of hydrocarbon bearing fluids hydrocarbon generation organic matter evolution and clay mineral dehydration illitization of smectite were responsible for the fluid overpressure crystal growth within the beefs was a continuous process with an antitaxial growth direction and there was no growth competition between adjacent fibres these observations indicate that the crystal growth and fracture dilation were synchronous and growth space was limited the force exerted by crystallizing vein fill contributed to the high fluid pressure suggesting that fibrous vein minerals actively promoted further dilation as the veins grew vein dilation accompanied the migration of hydrocarbon bearing fluids evinced by the occurrence of primary hydrocarbon inclusions found in the fibrous calcites therefore the hydrocarbon bearing fluids were present at the time of horizontal fluid flow from the start of fracture opening to the end of the dilation

the relationship between joint aperture spacing distribution vertical dimension and carbonate stratification an example from the kimmeridgian limestones of pointe du chay france joint aperture and joint development have been studied in the kimmeridgian limestones of the pointe du chay at the northern boundary of the jurassic biscay basin france at belette outcrop in some layers the mean joint spacing of the n120 joint set is close to mean layer thickness there the classical spacing to thickness relationship appears to be valid in the competent carbonate layers that are included in a more argillaceous matrix at pillar outcrop the n10 joint set is characterised by a high level of joint density and a non saturated spacing distribution as indicated by the mode mean ratio values and the cv values cv is the ratio of standard deviation to mean fracture spacing the classical relationship between layer thickness and fracture spacing has not been observed at the pillar outcrop joint aperture reaches larger values at the pillar outcrop than at the belette outcrop where aperture is more homogeneously distributed almost all the joints are opened with moderate aperture values at belette outcrop whereas most joints with large vertical dimension have large apertures at pillar outcrop and smaller fractures are closed or poorly opened from two outcrops that have been subjected to the same geological conditions apertures of non stratabound joints appear to be controlled by the vertical dimension of the joints whereas stratabound joints are more regularly spaced and opened

transient permeability and reseal hardening in fault zones evidence from dilation breccia textures t is well established that earthquake faulting can create permeability along a fault zone in high competence rocks by mismatch of the fault walls and by secondary fracture in a surrounding damage zone and that this permeability is created repeatedly during successive earthquake cycles less well proven is that such permeability is transient being episodically reduced by precipitation of cements in the fracture porosity the textures of carbonate dilation breccias formed at around 1 7 km depth on the dent fault zone nw england lend support to this economically important concept of transient fracture permeability the key observation is that many breccias reflect only a single episode of brecciation and reseal a generally applicable explanation of such single phase breccias is that they were resealed in the interval between major earthquakes that this reseal made the breccia stronger that the intact rock and that subsequent brecciation in the same rock volume was inhibited this reseal hardening model implies that transient permeability in fault zones may last no longer than the recurrence times of large earthquakes and that the permeability conduits will change position in the damage zone with time unless focused at a major fault jog or termination

integrated structural geology and engineering rock mechanics approach to site characterization abstract in line with the symposium theme of linking science to rock engineering this paper presents an integrated structural geology and engineering rock mechanics approach to site characterization structural geology interpretation has much to contribute to the overall understanding of rock mass structure and site characterization for engineering design this applies not only to rock mass geometry rock properties and geological mechanisms but also to the process of modelling and identifying the critical mechanisms for engineering project design the approach is illustrated by three examples relating to rock properties gold mine stability and restoration of 6th century buddhist monolithic cave temples in india

sediment deformation in miocene and post miocene strata northeastern brazil evidence for paleoseismicity in a passive margin abstract stratigraphic intervals characterized by varied and complex styles of soft sediment deformation structures are well preserved in miocene and late pleistocene to holocene deposits of a sedimentary basin located in northeastern brazil the miocene strata represented by the barreiras formation record only brittle structures including numerous faults and fractures with straight and high angle dipping planes that are often filled with sands derived from overlying beds folds consisting of broad anticlines and synclines are also present in this unit the late pleistocene to holocene deposits named post barreiras sediments contain an indurated sandy package with a large variety of ductile and brittle deformation structures i e massive sandstones with isolated sand fragments and breccias undulatory strata sand dykes and diapirs sinks and bowls pebbly pockets plunged sediment mixtures fitted sand masses cone shaped cracks fault grading and sedimentary enclaves these features confined to sharp based stratigraphic horizons that progressively grade downward into undisturbed deposits are related to seismic shocks of high surface wave magnitude i e ms 5 or 6 amalgamated seismites suggest that previously formed seismites were affected by subsequent seismic wave propagation seismic waves caused by activity along one or most likely several tectonic structures would have propagated throughout the depositional environment producing laterally extensive seismites the close proximity to earthquake epicenters would have promoted pervasive re sedimentation due to pore overpressure resulting high volumes of massive sandstones and breccia the similarity between deposits with correlatable strata from many other areas along the brazilian coast allows raise the hypothesis that the seismic episodes might have affected sedimentation patterns in a large i e extension of several hundreds of kilometers geographic area thus the modern seismicity recorded along northeastern brazil was recurrent during the quaternary and perhaps also in the pliocene the estimated high magnitude of the seismic events and the great regional extent of the affected area demonstrate that the brazilian coast experienced tectonic stress through the last geological episodes of its evolution which would have favored sediment accumulation and penecontemporaneous re sedimentation this geological context is unexpected in a passive margin inducing to revisit the debate on how active is a passive margin

regional sand injectite architecture as a record of pore pressure evolution and sand redistribution in the shallow crust insights from the panoche giant injection complex california abstract observations on outcrop of a regionally developed sand injectite are used to infer and estimate the pore pressure conditions in the shallow crust that caused the fluidization and injection of tens of cubic kilometres of sand the estimated pore fluid pressures at the base of the injection complex at 1500 m burial depth below a regionally developed shale dominated seal are from 22 26 to 25 08 mpa which respectively correspond to 0 81 and 0 95 lithostatic pressure a theoretical basis for prediction of sand injection is defined and applied to the prediction of pore pressure at the time of sand injection the depth at which seal failure occurred and the density and granular content of the fluidized flow lateral variations in the style and abundance of sandstone intrusions are described and these all fit into a remarkably uniform tripartite division of parent units an intrusive complex and an extrusive complex a sill zone intrusions are dominated by sills occurs in a restricted stratigraphic interval 200 270 m thick location of the base of the sill zone is directly related to the thickness of the overburden and an isobaric surface at the time of sand injection the lithostatic equilibrium surface is defined at the base of the sill zone when the sills formed an extended period of supra lithostatic pressure occurred within the sill zone

the evolution seasonality and impacts of western disturbances western disturbances wds are upper level synoptic scale systems embedded in the subtropical westerly jet stream stwj often associated with extreme rainfall events in north india and pakistan during boreal winter here a tracking algorithm is applied to the upper tropospheric vorticity field in 37 years of era interim reanalysis data giving a catalogue of over 3000 events these events are analysed in a composite framework the vertical structure is explored across a large number of dynamic and thermodynamic fields revealing a significant northwestward tilt with height strong ascent ahead of the centre which sits above the maximum surface precipitation and a warm over cold dry over moist structure among other signatures of strong baroclinicity evolution of the structures of cloud cover and vertical wind speed are investigated as the composite wd passes across northern india cloud cover in particular is found to be particularly sensitive to the presence of the himalayan foothills with a significant maximum at 300 hpa approximately one day after the wd reaches peak intensity k means clustering is used to classify wds both according to dynamical structure and precipitation footprint and the relationship between the two sets is explored finally the statistical relationship between the stwj position and wds on interannual time scales is explored showing that wd frequency in north india is highly sensitive to the jet location over eurasia years with a greater number of wds feature a stwj shifted to the south a pattern that is substantially more coherent and reaches as far west as north america during boreal winter this suggests that it may be possible to predict the statistics of western disturbance events on seasonal time scales if suitable indicators of jet position can also be predicted

sedimentological and deformational criteria for discriminating subglaciofluvial deposits from subaqueous ice contact fan deposits a pleistocene example ireland a pit located near ballyhorsey 28 km south of dublin eastern ireland displays subglacially deposited glaciofluvial sediments passing upwards into proglacial subaqueous ice contact fan deposits the coexistence of these two different depositional environments at the same location will help with differentiation between two very similar and easily confused glacial lithofacies the lowermost sediments show aggrading subglacial deposits indicating a constrained accommodation space mainly controlled by the position of an overlying ice roof during ice bed decoupling these sediments are characterized by vertically stacked tills with large lenses of tabular to channelized sorted sediments the sorted sediments consist of fine grained laminated facies cross laminated sand and channelized gravels and are interpreted as subglaciofluvial sediments deposited within a subglacial de coupled space the subglaciofluvial sequence is characterized by glaciotectonic deformation structures within discrete beds triggered by fluid overpressure and shear stress during episodes of ice bed recoupling clastic dykes and folds the upper deposits correspond to the deposition of successive hyperpycnal flows in a proximal proglacial lake forming a thick sedimentary wedge erosively overlying the subglacial deposits gravel facies and large scale trough bedding sand are observed within this proximal wedge while normally graded sand beds with developed bedforms are observed further downflow the building of the prograding ice contact subaqueous fan implies an unrestricted accommodation space and is associated with deformation structures related to gravity destabilization during fan spreading normal faults this study facilitates the recognition of subglacial submarginal depositional environments formed in part during localized ice bed coupling episodes in the sedimentary record the sedimentary sequence exposed in ballyhorsey permits characterization of the temporal framework of meltwater production during deglaciation the impact on the subglacial drainage system and the consequences on the irish sea ice stream flow mechanisms

petrography and isotopic geochemistry of bedding parallel fibrous gypsum veins in the neogene qingshuiying formation of the ningnan basin north china insights for growth model of antitaxial fibrous veins abstract bedding parallel fibrous gypsum veins which are widely developed in the neogene qingshuiying formation of the ningnan basin a tertiary basin located in the northeastern margin of the tibetan plateau provide valuable information on stress field fluid origin and host rock deformation during their formation the systematic petrographic observation and isotope analysis of both of veins and wall rocks were carried out to investigate the formation mechanism of bedding parallel fibrous gypsum veins the distribution of fibrous gypsum veins shows vertical heterogeneity and mainly occurs in the middle and lower part of the qingshuiying formation with relatively high gypsum content single fibrous gypsum veins consist of a median zone and two fibrous zones the median zone is characterized by blocky gypsum crystals and multiple host rock fragments formed by crack seal events the fibrous zone is composed of parallel gypsum fibres a high angle to the vein walls indicating shear component vein opening is considered to be the result of fluid overpressure caused by horizontal compression in low permeability mudstone during basin inversion fibrous gypsum crystals grow continuously without obvious growth competition between adjacent crystals this indicates that growth of gypsum fibres occurs in limited space even without fractures the main driving force for vein dilation is force of crystallization of gypsum fibre growth and fluid pressure also has a certain contribution the sulfur and strontium isotopes of gypsum veins exhibit a high degree of consistency with wall rocks which indicates the material of the fibrous gypsum veins is derived from the wall rock a suitable growth model of bedding parallel fibrous gypsum veins in the ningnan basin is established which explains the process of vein interaction and the formation of v shaped or u shaped solid inclusions

fluid flow history from damage zone cements near the dent and rawthey faults nw england fault related breccias in carboniferous limestone have been well documented in the footwall damage zone to the reverse oblique dent fault this study describes breccias in the more complicated hanging wall hosted in pre deformed ordovician and silurian lithologies along the dent fault or one of its splays such as the rawthey fault brecciation style is shown to be influenced by lithology with the most extensive brecciation in mechanically strong units siliceous or calcareous mudstone cemented sandstone and fine tuff most brecciation involves dilational strain in the fault damage zones attritional breccias are rare even in fault cores dilation breccias are cemented sporadically by quartz or barite but mostly by carbonate minerals transition matrix analysis shows that the sequence of hanging wall carbonate cements matches that in the footwall first dolomite or calcite then ferroan dolomite then ferroan calcite this similarity suggests hydrological connectivity from hanging wall to footwall and through tens of cubic kilometres across the dent rawthey fault system such connectivity is predicted across upper crustal faults that cut strong mechanical stratigraphy it is proposed that fluid flow though the fault system was driven mainly by syntectonic topography above the hanging wall and possibly also by thermal convection caused by a buried granite pluton in the footwall

the influence of mechanical anisotropy on the behaviour of the lower crust abstract the aim of this paper is to determine what can be deduced about the rheology of the lower crust undergoing extension the state of stress existing there and how the finite strain pattern is developed from a consideration of the theoretical analyses of layer normal compression analogue models and outcrops of rocks from the lower crust it is concluded that the extended lower crust contains units with a significant competence contrast has a high mechanical anisotropy and experiences bulk rheological changes during deformation based on a consideration of hydraulic fracturing in anisotropic materials it is argued that depending on the magnitude differential stress fluids will move through and out of the lower crust by fluid fracturing along the planar horizontal stretching fabric and gently inclined normal kink bands extensional shears or by the formation of vertical fractures

emplacement and deformation features of syntectonic leucocratic veins from rainy lake zone western superior province canada abstract networks of leucocratic veins are widespread in the rice bay redgut bay domain of the rainy lake zone at the wabigoon quetico subprovince boundary archean superior province canada vein geometries and their cross cutting relationships with host rocks are indicative of syntectonic emplacement within a regional late archean transpressional deformation event kenoran orogeny the relative folding and boudinage of the veins compared to their host rocks indicate that they were emplaced syntectonically with the d2 deformation semi quantitative strain and vorticity analyses confirm the syn d2 timing of the veins the observed structural features are interpreted as being formed during magma emplacement along dilatant fractures and subsequent solid state deformation under overall ductile deformation conditions the structures related to emplacement are better preserved in domains of low post emplacement strain and veins intruded into competent host rocks whereas the folds and boudins related to post emplacement deformation are well developed in veins intruded into relatively incompetent host rocks in domains of high strain the primary intrusive structures are modified or even locally obliterated due to increasing deformation it is envisaged that regional kinematics and the rheological state of host rocks are the main controls on the style of vein emplacement and deformation the variety of observed structures is considered the result of deformation and lithological heterogeneity rather than several distinct magmatic events

lithological control on fracture cementation in the keuper marl triassic north somerset uk the spatial arrangement of gypsum veins as preserved natural hydraulic fractures have been characterized in the triassic keuper marl formation uk a caprock for hydrocarbon reservoirs and co 2 sequestration the marls cropping out are subdivided into five discrete fracture units based on the presence and abundance of gypsum veins the nodular gypsum in evaporite horizons provides excess gypsum for nodule rooted horizontal gypsum veins our petrographic observations demonstrate that the development of gypsum veins in beds lacking macroscopic evaporites is closely associated with disseminated gypsum cement in the marls we interpret that the gypsum veins in marl are sourced from disseminated gypsum cements in the host rocks based on stratigraphic correlations and much lower sr concentrations than gypsum nodules gypsum was transported to adjacent veins mainly through diffusion in the low permeability marls the localization of gypsum veins and varied sr concentrations of veins and nodules indicate that the diagenetic fluids are a mix of connate water with meteoric water rather than brines transported from evaporite beds along faults to non evaporite beds this results in the absence of gypsum fillings in fractures in rocks without primary gypsum cements the study implies that the cementation of natural fractures in low permeability rocks can highly depend on the presence of cement minerals in the host rock

stress state evolution of the brittle upper crust during compressional tectonic inversion as defined by successive quartz vein types high ardenne slate belt germany abstract in the frontal part of the rhenohercynian fold and thrust belt high ardenne slate belt germany two successive types of quartz veins oriented normal and parallel to bedding respectively are interpreted to reflect the early variscan compressional tectonic inversion of the ardenne eifel sedimentary basin fracturing and sealing occurred in lower devonian siliciclastic multilayers under very low grade metamorphic conditions in a brittle upper crust a geometrical and microthermometric analysis of these veins has helped to constrain the kinematic and pressure temperature conditions of both vein types allowing the reconstruction of the stress state evolution in a basin during tectonic inversion it is demonstrated that bedding normal extension veins which developed under low differential stresses and repeatedly opened and sealed crack seal under near lithostatic fluid pressures reflect the latest stage of an extensional stress regime bedding parallel veins which developed at differential stresses that were still low enough to allow the formation of extension veins cross cut the bedding normal veins and preceded the regional fold and cleavage development these veins show a pronounced bedding parallel fabric reflecting bedding normal uplift and bedding parallel shearing under lithostatic to supra lithostatic fluid pressures during the early stages of a compressional stress regime this kinematic history corroborates that fluid overpressures are easy to maintain during compressional tectonic inversion at the onset of orogeny

pancake joints in utica gas shale mechanisms for lifting overburden during exhumation abstract the flat creek member of the utica formation along canajoharie creek south of canajoharie ny is populated with unmineralized pancake joints a term for horizontal bed parallel partings each pancake joint propagated outward from a central initiation point with radial growth in excess of 0 5 m in some instances propagation arrests with out of plane growth displaying either a tilt characteristic of an arrest line or bifurcation into a complex set of cylindrical splays or hackles a rough surface morphology relative to vertical fractures i e joints and veins in the flat creek coupled with bifurcation at arrest are the manifestation of an accelerating crack tip reaching critical growth at the shear wave velocity of the shale the lack of mineralization and containment between vertical fractures are consistent with propagation during exhumation one driving mechanism for a late propagation event could be axial splitting which places pancake joints in a class of brittle fracture that includes sheeting joints the high speed propagation is a manifestation of the rapid release of fracture energy characteristic of sheeting joints however we also consider a second driving mechanism natural hydraulic fracturing in the appalachian basin high pore pressure is maintained in gas shale at depths less than 50 of the original overburden if abnormal pore pressure is maintained for 300 my or more it is present when exhumation carries the shale upward through the crossover depth where the least stress becomes vertical if pore pressure exceeds the vertical stress then pore pressure will lift overburden a possible mechanism for maintaining pore pressure during exhumation to shallow depths is a poroelastic phenomenon known as skemption s behavior we posit that pancake joints differ from common sheeting joints because of the additional contribution from abnormal pore pressure as a driving mechanism that allows for deeper propagation depth

genesis of natural hydraulic fractures as an indicator of basin inversion abstract satin spar fibrous gypsum veins which occur in evaporite basins worldwide provide significant insights into host rock deformation and fluid flow although the genetic mechanism remains obscure satin spar veins in the red marls of the triassic mercia mudstone of the bristol channel basin were characterized in the context of regional and local setting the vein network in the keuper marl lower mercia exhibits a lack of systematic cross cutting between three distinct vein sets two sets of veins are observed in the overlying tea green marl with one set clearly crossing the other the gypsum veins commonly contain a blocky median zone of multiple thin bands of host rock inclusions and alabastrine gypsum crystals exhibiting crack seal patterns fibrous zones on either side of the median zone consist of pure parallel aligned gypsum fibres that are oblique to vein walls indicating a hybrid shear extensional mode of vein widening veins developed within reverse reactivated faults contain fibre lineations in the median zones and also on vein surfaces suggesting a minimum of two phases of fault slip the veins are interpreted to have formed as a result of overpressure in the low permeability mudstones by tectonic compression during basin inversion giving rise to the median zone growth of gypsum fibres commenced when the initial fractures were completely sealed the timing of vein formation is suggested to be miocene based on cross cutting relationships with folds formed during the latest phase of basin inversion because the fractures initiated in response to fluid overpressures they provide a useful analog to subsurface hydraulic fracture systems in low permeability rocks subjected to tectonic compression the fibrous widening of the veins post dates their initiation and led to continued propagation and increased connectivity

metasomatic vein formation by stationary fluids in carbonate xenoliths at the eastern margin of the bergell intrusion val sissone italy fluid flow is an important mechanism associated with heat and mass transport within the earth s crust the study of veins which represent channelling of fluids can thus be key in understanding these fluid movements unravelling fluid composition and origin paleo stress regimes and the history of the host rock new stable isotope data on carbonates and silicates have been combined with phase petrology mass balance and field observations to evaluate the formation mechanism of metasomatic reaction veins in dolomitic xenoliths in the bergell tonalites val sissone italy multiple generations of extensional veins can be followed from the contact zone between the dolomites and the intrusion to a few meters within either the tonalites with the epidote quartz veins or within the dolomites where they terminate each type of vein contains a central zone which is formed by open fracture crystallization this central fracture is framed by relatively symmetric replacement zones where the original dolomite reacted to form either forsterite diopside tremolite or talc all accompanied by calcite in either a succession of reaction zones or in simpler bi mineral silicate calcite veins the 18o and 13c values across the veins allow temperatures to be estimated from different mineral pairs silicate calcite and which confirm vein formation along a retrograde cooling path of the intrusion at least four different fluid infiltration events are required the first one around 555 c to form the forsterite calcite veins followed by the epidote quartz veins at temperatures around 430 c then the tremolite calcite veins at around 390 c and finally the talc calcite veins at around 140 c the shape of the 18o and 13c profiles which are flat across the central part and the replacement zones of the veins buffered by the intrusion change substantially over short distances both of these isotope profiles overlap with the equally sharp mineralogical front between the veins and the unreacted dolomites these profiles are interpreted to be the result of an isotopic exchange mechanism driven by dissolution and re precipitation reactions all veins are oriented perpendicular to the contact with the intrusive body except for the late talc veins elevated fluid pressures above the confining pressures caused by the regional and intrusion emplacement stress field are suggested to be responsible for the initial fracturing of the carbonates and intrusive rocks the contact zones between the tonalites and carbonates likely served as fluid conduits where fluids accumulated and the pressure built up until hydrofracturing occurred we propose that the veins formed through episodic pulses of highly reactive fluids that remained stationary during reaction rather than a system where fluids flushed through the veins based on the xco2 constrained mass balance the formation of the veins would only require a relatively small amount of fluid which could potentially originate from the intrusive rocks in vicinity of the xenoliths veining is not ubiquitous around the bergell intrusion suggesting that it only may have been a localized event and thus there is no need to involve a larger convective hydrothermal system for their formation

the interplay between fluids folds and thrusts during the deformation of a sedimentary succession abstract in this paper field evidence is presented to support the idea that fluid pressures in excess of lithostatic pressure were operating during the initiation and reactivation of large thrusts and folds in the wenlock shales of llangollen north wales it is also argued that as major thrusts are reactivated and as major folds amplify into finite structures they in turn exert an influence on the subsequent migration and concentration of fluids within the deforming sedimentary succession

the complexity of 3d stress state changes during compressional tectonic inversion at the onset of orogeny compressional tectonic inversions are classically represented in 2d brittle failure mode bfm plots that illustrate the change in differential stress s1 2 s3 versus the pore fluid pressure during orogenic shortening in these bfm plots the tectonic switch between extension and compression occurs at a differential stress state of zero however mostly anisotropic conditions are present in the earth s crust making isotropic stress conditions highly questionable in this study theoretical 3d stress state reconstructions are proposed to illustrate the complexity of triaxial stress transitions during compressional inversion of andersonian stress regimes these reconstructions are based on successive late burial and early tectonic quartz veins which reflect early variscan tectonic inversion in the rhenohercynian foreland fold and thrust belt high ardenne slate belt belgium germany this theoretical exercise predicts that no matter the geometry of the basin or the orientation of shortening a transitional wrench tectonic regime should always occur between extension and compression to date this intermediate regime has never been observed in structures in a shortened basin affected by tectonic inversion our study implies that stress transitions are therefore more complex than classically represented in 2d ideally a transitional wrench regime should be implemented in bfm plots at the switch between the extensional and compressional regimes extension mode i fractures are classically interpreted to form at low differential stresses less than four times the tensile strength t of rock e g secor 1965 they open perpendicular to the minimum principal stress s3 and propagate in the plane of the maximum s1 and intermediate s2 principal stresses the alignment of fractures has therefore often been used in orogenic belts to determine the orientation of the palaeostress field at the time of fracturing cox et al 2001 gillespie et al 2001 laubach et al 2004 if veins which initiated as extension fractures are uniform in trend over a large area then they can also be used as such a palaeostress indicator e g boullier robert 1992 cosgrove 2001 gillespie et al 2001 mazzarini et al 2010 van noten sintubin 2010 different cross cutting or successive vein generations are even more interesting because they have the potential 1 to serve as a tool to decipher changes in the overall stress regime during multiple phases of fracturing and 2 to define the p2t conditions at which these stress changes occurred on the one hand different crosscutting fracture and or vein generations have been used in many studies to deduce local stress field rotation in a consistent remote stress regime jackson 1991 crespi chan 1996 stowell et al 1999 laubach diaz tushman 2009 wiltschko et al 2009 on the other hand successive cross cutting veins i e vein sets that are oriented differently with respect to bedding have often been used to determine a transition in regional stress regime from extension to compression or reversely manning bird 1991 boullier robert 1992 teixell et al 2000 hilgers et al 2006b van noten et al 2008 2011 laubach diaz tushman 2009 alternatively cross cutting vein sets are observed in close relationship to faults in which they represent fluctuating fluid pressures during fault valve activity and are related to short lived local stress transitions in a consistent remote stress regime boullier robert 1992 sibson 1995 nielsen et al 1998 muchez et al 2000 collettini et al 2006 local mutually cross cutting vein sets may also develop in the presence of mechanical anisotropy around fault planes in a consistent remote stress field healy 2009 fagereng et al 2010 in all these studies transitions between regional stress regimes are defined as tectonic inversions a positive compressional tectonic inversion corresponds to a transition from an extensional to a compressional stress regime while a negative extensional tectonic inversion reflects the transition from compression to extension from healy d butler r w h shipton z k sibson r h eds 2012 faulting fracturing and igneous intrusion in the earth s crust geological society london special publications 367 51 69 http dx doi org 10 1144 sp367 5 the geological society of london 2012 publishing disclaimer www geolsoc org uk pub ethics by guest on august 9 2012 http sp lyellcollection org downloaded from

hydraulic fractures how far can they go the maximum reported height of an upward propagating hydraulic fracture from several thousand fracturing operations in the marcellus barnett woodford eagle ford and niobrara shale usa is 588 m of the 1170 natural hydraulic fracture pipes imaged with three dimensional seismic data offshore of west africa and mid norway it is 1106 m based on these empirical data the probability of a stimulated and natural hydraulic fracture extending vertically 350 m is 1 and 33 respectively constraining the probability of stimulating unusually tall hydraulic fractures in sedimentary rocks is extremely important as an evidence base for decisions on the safe vertical separation between the depth of stimulation and rock strata not intended for penetration