oxygen isotope trends and anomalies in granitoids of the tibetan plateau abstract extreme transhimalayan oceanic mantle i type and high himalayan anatectic collision s type granitoids in the himalaya kashmir and tibet range in their original oxygen isotope composition from 6 to 7 5 and from 10 to 12 5 18osmow respectively in the sutured paleozoic and mesozoic orogen of the eastern tibetan plateau purely oceanic batholiths seem absent shalouli shan 8 10 but high himalayan values are matched in dongda shan and at luxi towards the east oceanic values are found again only on the margin of the yangtse platform the regional pattern of normal oxygen isotope data provides a background for anomalous feldspars to 1 in lhasa granite and in the active geothermal system at yangbachen recharged by 6000 m a s l meteoric water for lhasa granite oxygen isotope exchange paths and preliminary k ar dating of feldspars suggest exchange with meteoric water from over 3000 m altitude before 20 ma ago

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

geochemistry of a type granites in the huangshaping polymetallic deposit south hunan china implications for granite evolution and associated mineralization abstract the huangshaping granites in hunan province south china were investigated for their geochemical characteristics three types of granites have been petrographically identified quartz porphyry granophyre and granite porphyry whole rock geochemistry suggests that the huangshaping granites especially the granite porphyry exhibit typical a type granite characteristics with their enrichment in si rb u th and nb and significant depletion in ba sr ti eu and p based on the al y and zr contents as well as the ree patterns of the rocks investigated the quartz porphyry and the granophyre are classified as a1 type alkaline granites whereas the granite porphyry is considered as a2 type aluminous granite whole rock and quartz feldspar o isotope data yields a wide range of 18osmow values 11 09 26 32 the granites are characterized by high radiogenic pb isotopic composition the present day whole rock pb isotopic ratios are 206pb 204pb 18 706 19 155 207pb 204pb 15 616 15 711 and 208pb 204pb 38 734 39 296 combining the o pb isotope compositions with major trace and ree geochemistry and regional geology characteristics the huangshaping granites were determined to resemble within plate granites that were mainly derived from a felsic infracrustal source related to continental extension the magma source of the quartz porphyry and the granophyre may have been generated from deeper depths and then ascended rapidly with limited water content and low oxygen fugacity which contributed to cu pb and zn mineralization on the other hand the magma that generated the granite porphyry may have ascended relatively slower and experienced pronounced crystal fractionation upper crustal basement rock contamination assimilation and wall rock interaction producing the sn and w rich granite porphyry this study reveals the crustal extension process and associated magmatic metallogenic activities during 180 150 ma in south hunan

from hydroplastic to brittle deformation controls on fluid flow in fold and thrust belts insights from the lower pedraforca thrust sheet se pyrenees abstract we present a multidisciplinary study to decipher the controls of deformation on fluid flow regime in fold and thrust belts using the lower pedraforca thrust sheet in the se pyrenees as an example we integrate field based and petrographic observations and geochemical and geochronological data to differentiate seven types of fractures eight types of calcite cement cc1 to cc8 and two sets of stylolites during the deformation stretching the studied thrust sheet during syn sedimentary hydroplastic normal faulting affecting poorly consolidated upper cretaceous and eocene syn orogenic sediments calcite cement did not precipitate during burial bed parallel stylolites formed and cc1 and cc2 precipitated from formation waters in a closed palaeohydrological system during the layer parallel shortening cc3 precipitated from formation waters 5 4 vsmow with 87sr 86sr ratios of 0 707922 and at 70 c cc4 precipitated from formation waters recording different burial conditions as the up to 4 dispersion in 18o of this cement suggests contrarily during folding and thrusting cc5 to cc7 precipitated in an open palaeohydrological system cc6 precipitated from formation waters 5 vsmow with 87sr 86sr ratios of 0 707817 and at 75 c these fluids carried hydrocarbons and probably interacted with upper triassic evaporites an 87sr 86sr ratio of 0 708230 for cc5 indicates that formation waters also interacted with clays within continental deposits during this period stylolites formed in relation to faulting and previous hydroplastic normal faults reactivated as reverse and strike slip faults allowing fluid flow cc7 precipitated after cc6 also from fluids in isotopic disequilibrium with their adjacent host rock the fluid system continued open during the oligocene when cc8 precipitated in normal faults affecting syn orogenic conglomerates deposited during the reactivation of the lower pedraforca thrust sheet the influence of deformation on fluid flow observed in the lower pedraforca thrust sheet is similar to that observed in other fractured areas worldwide

outcrops as guides to subsurface natural fractures example from the nikanassin formation tight gas sandstone grande cache alberta foothills canada abstract outcrop studies are essential for understanding subsurface fractures because some key attributes such as length and height distribution cannot be measured using wellbores discrepancies between outcrop and subsurface fracture attributes arise because loading paths due to burial exhumation and thermal paths differ consequently even where rock type and structural setting are close matches differences may exist in mechanical properties and fracture growth here we investigate petrologic criteria and disseminated arrays of quartz filled microfractures as tools for comparing and contrasting fractures in outcrop with those as sampled by core for tight gas sandstones fracture cements are key diagnostic criteria for proving correspondence between outcrop and core we offer guidelines on selecting suitable outcrop analogs that differ markedly from current practice our illustration uses outcrops and core samples of late jurassic to early cretaceous nikanassin formation a tight gas sandstone but aspects of the outcrop vetting procedure can be applied to other rock types in some core and outcrops nikanassin formation fractures contain quartz and ankerite cement assemblages crack seal and rind textures and aperture size distributions that show that the outcrop and subsurface localities we examined share the same fracture stratigraphy and similar diagenesis these outcrops can thus be used to measure attributes including those that are unlikely to be captured in core such as fracture height and length distributions relative and absolute fracture intensity with structural position and degree of fill by synkinematic cement in our example absolute fracture intensity and distribution of postkinematic carbonate cements are site specific and differ between outcrop and core but in ways that can be quantified

tropical cyclones in rotating radiative convective equilibrium with coupled sst abstracttropical cyclones are studied under the idealized framework of rotating radiative convective equilibrium achieved in a large doubly periodic f plane by coupling the column physics of a global atmospheric model to rotating hydrostatic dynamics unlike previous studies that prescribe uniform sea surface temperature sst over the domain ssts are now predicted by coupling the atmosphere to a simple slab ocean model with coupling ssts under the eyewall region of tropical cyclones tcs become cooler than the environment however the domain still fills up with multiple long lived tcs in all cases examined including at the limit of the very small depth of the slab the cooling of ssts under the eyewall increases as the depth of the slab ocean layer decreases but levels off at roughly 6 5 k as the depth approaches zero at the eyewall the storm interior is decoupled from the cooler surface and moist entropy is no longer well mixed along the angular momentum surface in the boundary layer tc intens

facies architecture and high resolution sequence stratigraphy of an upper cretaceous platform margin succession southern central pyrenees spain abstract excellent exposures of upper cretaceous santonian carbonate platforms on the northern flank of sant corneli anticline southern central pyrenees provide detailed information of facies architecture in both depositional strike and dip directions basic accretional units are differentiated by facies contrast across mappable surfaces these surfaces do not show clear evidence of subaerial erosion and are correlated basinward with bedding planes across which there are subtle changes in skeletal composition two types of basic accretional units have been identified based on bedding patterns internal facies architecture and skeletal composition 1 rudist buildups consist of a rudist and coral belt at the platform margin passing landward into a slender hippuritid lithosome locally overlain by a bioclastic blanket that passes basinward into bioclastic apron like clinobeds and into fine grained packstone wackestone 2 calcarenite wedges consist of yellow brown benthic foraminifer rich grainstones to grain dominated packstones with scattered rudist shells and small coral colonies passing basinward into mud dominated packstones to wackestones with variable siliciclastic content quartz sand to silt and clay rudist buildups and calcarenite wedges alternate although not rhythmically these changes in platform skeletal composition reflect changes in the dominant type of carbonate producing biota independently of the changes in accommodation both types of basic accretional units rudist buildups and calcarenite wedges form simple sequences and parasequences according to internal lithofacies arrangement and inferred sea level cyclicity cycles or paracycles high frequency sea level cyclicity fits in the milankovitch frequency band long term changes in accommodation governing aggradation progradation and backstepping of basic sequences and parasequences reflect tectonic influence rather than long term changes in eustatic sea level

constraints on bed scale fracture chronology with a fem mechanical model of folding the case of split mountain utah usa abstract a technique is presented for improving the structural analysis of natural fractures development in large scale fold structures a 3d restoration of a fold provides the external displacement loading conditions to solve by the finite element method the forward mechanical problem of an idealized rock material with a stress strain relationship based on the activation of pervasive fracture sets in this elasto plasticity constitutive law any activated fracture set contributes to the total plastic strain by either an opening or a sliding mode of rock failure inherited versus syn folding fracture sets development can be studied using this mechanical model the workflow of this methodology was applied to the weber sandstone formation deformed by forced folding at split mountain anticline utah for which the different fracture sets were created and developed successively during the sevier and the syn folding laramide orogenic phases the field observations at the top stratigraphic surface of the weber sandstone lead to classify the fracture sets into a pre fold wnw ese fracture set and a ne sw fracture set post dating the former the development and relative chronology of the fracture sets are discussed based on the geomechanical modeling results starting with a 3d restoration of the split mountain anticline three fold fracture development models were generated alternately assuming that the wnw ese fracture set is either present or absent prior to folding process depending on the initial fracture configuration the calculated fracture patterns are markedly different showing that assuming a wnw ese joint set to predate the fold best correlates with field observations this study is a first step addressing the complex problem of identification of fold related fracturing events using an elementary concept of rock mechanics when tight to complementary field observations including petrography diagenesis and burial history the approach can be used to better constrain fractured reservoir characterization

microtectonic and geochemical characterization of thrusting in a foreland basin example of the south pyrenean orogenic wedge spain abstract in orogenic systems thrust faults play a major role in stacking different tectonic units and may act as conduits for the expulsion of large amounts of fluid of different origins metamorphic diagenetic meteoric this study focuses on the monte perdido thrust unit emplaced in the paleogene jaca thrust sheet top basin in the sw central pyrenees we aim to decipher the mechanisms and p t conditions of deformation in fault zones and characterize the related fluid involvement through combined microstructural geochemical and microthermometry analyses two thrust faults cutting platform limestones marls and siliciclastic turbidites of the lower part of the basin fill paleocene lower eocene have been studied the fault zones are characterized by metre thick shear zones with highly deformed foliated clay rich sediments foliation is underlined by preferentially oriented phyllosilicates several generations of shear and extension calcite quartz and chlorite bearing veins attest to fluid rock interactions during a multi stage deformation microstructural observations and stable isotope analyses on calcite from veins and host sediments suggest that deformation was aseismic and dominated by diffusive mass transfer from pressure solution sites along cleavage and stylolites to the precipitation sites in veins with mineralizing fluids in equilibrium with the host sediments our results suggest an essentially closed hydrologic system and imply the absence of significant fluid flow along the studied fault zones microthermometric study on fluid inclusions present in calcite and quartz veins and calcite quartz oxygen isotopic fractionation determined for the first generation shear veins allow a geothermal gradient of 34 c km to be estimated analytical results demonstrate an evolution of the fault zones in three stages the first stage was related to the emplacement of the monte perdido thrust unit during the middle eocene at a temperature of 208 c and a burial depth of 5 7 km the second stage corresponds to a fault reactivation at a temperature of 240 c and a burial depth of 6 5 km the latter deformation may have been related to folding of the monte perdido thrust unit during the emplacement of the underlying gavarnie thrust unit during the late eocene early oligocene with deeper burial resulting from aggradation of the thrust sheet top basin fill the last event corresponds to the formation of a dilatant vein system likely related to the exhumation of the massif

information from folds a review abstract folds are spectacular geological structures that are seen in layered rock on many different scales to mark 30 years of the journal of structural geology we review the information that can be gained from studies of folds in theory experiment and nature we first review theoretical considerations and modeling from classical approaches to current developments the subject is dominated by single layer fold theory with the assumption of perfect layer parallel shortening but we also review multilayer fold theory and modeling and folding of layers that are oblique to principal stresses and strains this work demonstrates that viscosity ratio degree of non linearity of the flow law anisotropy and the thickness and spacing distribution of layers of different competence are all important in determining the nature and strength of the folding instability theory and modeling provide the basis for obtaining rheological information from natural folds through analysis of wavelength thickness ratios of single layer folds and fold shapes they also provide a basis for estimating the bulk strain from folded layers information about folding mechanisms can be obtained by analysis of cleavage and fabric patterns in folded rocks and the history of deformation can be revealed by understanding how asymmetry can develop in folds by how folds develop in shear zones and how folds develop in more complex three dimensional deformations

multi scale fracture network characterisation on carbonate platforms abstract characterisation of fracture networks at different scales is challenging and important to many fields of geoscience especially when access to multiple resolution datasets is limited here we develop an integrated analysis of fracture networks on carbonate platforms using three scales of observation small outcrop intermediate airborne lidar and large 3d seismic statistical analyses and ternary diagrams of geometrical and topological data from cariatiz south east spain and pernambuco east brazil are used to understand the relationships and distribution of fracture networks between multi scale datasets a variety of fracture types at each scale of observation reveal how complex fracture networks are on carbonate platforms our results demonstrate that fracture network properties behave differently depending on the fracture size and that transitional scale gaps between datasets constrain fracture characterisation airborne lidar maps show that intermediate sized fractures appear to have a better controlled orientation and a lower connectivity than smaller fractures from the same area in cariatiz fracture branch length distributions fit a negative exponential or log normal distribution for massive non stratabound units this work is important as it demonstrates that the use of outcrop data is a good approach to understand fracture complexity of carbonate platforms understanding sub seismic fracture networks is therefore critical in quantifying fluid flow and permeability in carbonate reservoirs

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

a review of deformation pattern templates in foreland basin systems and fold and thrust belts implications for the state of stress in the frontal regions of thrust wedges aesthetically appealing thrust systems and related large scale anticlines in both active and fossil foreland fold and thrust belts and the economic potential associated with the m have captured the interest of structural geologists for many decades as a consequence a large amount of data on sub seismic deformation patterns from thrust related an ticlines is available in the literature we provide a review of deformation pattern templates from fi eld data in foreland fold and thrust belts and show that the most frequent tre nds of sub seismic syn orogenic deformation structures hosted in km scale thrust related folds frequently and pa radoxically indicate a syn th rusting strike slip stress fi eld con fi guration with a near vertical 2 and a sub horizontal 3 rather than a contractional one where the latter is expected to be the vertical principal axis of the stress ellipsoid this a pparent inconsistency between sub seismic syn orogenic deformation structures and stress fi eld orientation is here named the 2 paradox field data support a possible expla nation of the paradox provided by the major role played by i nherited early orogenic extensional deformation struc tures on thrust fault nucleation nucleation of major thru sts and their propagation is facilitated and driven by the positive inversion and linkage of the early orogenic sub s eismic extensional inheritance developed in the foreland basin this process eventually leads to the development of l argereversefaultzonesandcanoccurbothincontractional and strike slip stress fi eld con fi gurations

production characteristics of the fractured reservoirs of the la paz field maracaibo basin venezuela la paz oil field is one of the most famous well documented fractured reservoirs in the world since 1945 it has produced more than 830 million bbl of oil from both low porosity cretaceous limestones and underlying granitic basement the field is situated on a large inversion structure partially uplifted in the late eocene having the majority of the inversion occurring in the miocene pliocene fracture distributions initial rate and cumulative production distributions and trends in formation water chemistry all suggest that reservoir quality and reserves are controlled by natural fractures associated with miocene pliocene and younger strike slip faults and possibly by secondary microporosity related to the geometry of the earlier eocene block uplift production levels within the area of eocene uplift are anomalously high with one well producing 59 million bbl of oil typical wells in other parts of the field produce 1 million bbl of oil analysis of the distributions cited previously suggests that production has occurred from highly elliptical to linear drainage areas surrounding faults these restricted zones of fault related fractures allow for communication of hydrocarbons stored in the low porosity rock carbonate matrix to the wellbore in addition diagenetic microporosity may be an important component in matrix storage in the carbonates and may be due to local subaerial exposure during the eocene uplift this article shows that analysis of the natural fracture system with respect to the production characteristics in even old fields can give rise to new reservoir models leading to new infill locations within the field limits or additional exploration opportunities in the area

karst dissolution along fracture corridors in an anticline hinge jandaira formation brazil implications for reservoir quality abstract folded fractured and karstified structures have been recognized in several carbonate reservoirs however they are rarely fully identified from seismic or well data and have been poorly described the present study investigates the geological and structural controls that regional anticlines may exert on solution enlarged fracture corridors and epigenic karst concentrations in folded fractured carbonate units we performed regional mapping based on 106 2d seismic lines 51 well logs unmanned aerial vehicle uav imagery of four major outcrop sites i ii iii and iv and field investigations to parameterize fracture and karstic features density spacing length and aperture the study area is potiguar basin brazil where we identified the gentle ne trending apodi fold 10 km wide and 20 km long the fold formed along an inverted extensional fault during the latest miocene quaternary stress field and the envelope of the meandering trend of the apodi river coincides with the ne trending fold axis the fracture pattern in the fold hinge zone consists of two orthogonal syn folding fracture sets a ne sw striking hinge parallel set and an orthogonal nw se striking set these fractures and superposed karstification are mostly concentrated in the hinge zone along fracture corridors in an area 1 5 km wide and 20 km long we conclude that fracture corridors in anticline hinges are likely sites to have high fracture densities and wide apertures these corridors provide reservoir spaces that are favorable for the formation of karst cavities with significant increases in permeability and porosity along the fracture sets these features act as important fluid pathways and storage areas where matrix porosity is low the findings can be used for first order prediction in karstified carbonate reservoirs

fracture analysis of outcrop analogues to support modelling of the subseismic domain in carbonate reservoirs south central pyrenees the productivity of wells in fractured reservoirs depends in terms of rate and sustainability on the heterogeneity and variable connectivity of the open fracture network outcrop studies in cretaceous carbonates from the catalan pyrenees illuminate this issue and reveal the degree of uncertainty associated with the interpretation of fracture data from wells and seismic three examples are chosen to provide verifiable data parameters and concepts which can be applied to the workflow of fractured reservoir characterization we discuss fracture properties and distributions in the subseismic volume the coupled behaviour between litho mechanical properties in situ stress and fracturing and the permeability properties of fault damage zones the outcrops also highlight some of the difficulties involved in constructing static reservoir models and evaluating fracture interpretations derived from software based techniques such as surface curvature

pb isotopes during crustal melting and magma mingling a cautionary tale from the miki fjord macrodike central east greenland abstract pb isotopic data are presented for hybrid rocks formed by mingling between mantle derived tholeiitic magma of the eocene miki fjord macrodike east greenland and melt derived from the adjacent precambrian basement bulk mixing and afc processes between end members readily identified in the field fail to model the pb isotope systematics selective contamination during diffusional exchange which can explain the complex sr and nd isotope compositions of the hybrid rocks blichert toft et al 1992 cannot fully account for the variability of the pb isotopic data using the identified crustal end members the crustal anatectic end member although similar in sr and nd isotope composition has a markedly different pb isotopic composition than its source gneiss the differences are consistent with preferential incorporation of radiogenic pb from accessory phases such as metamict zircon or loosely bound pb from grain boundaries during disequilibrium melting of the host rock by the mafic magma the crustal melt involved in magma interactions was therefore heterogeneous with respect to pb isotopes on a metre scale these results illustrate the difficulties inherent in interpreting isotopic variations in contaminated mafic magmas even when the end members are well constrained by field relations we show that the pb isotopic composition of the crustal contaminants and contamination trajectories for the miki fjord hybrid magmatic lithologies are markedly different from regional basement gneisses and contaminated lavas of the associated flood basalt province that were contaminated by characteristically unradiogenic pb and sr lithologies resembling lewisian basement not exposed locally

mechanical and fracture stratigraphy using examples from core studies this article shows that separate identification of mechanical stratigraphy and fracture stratigraphy leads to a clearer understanding of fracture patterns and more accurate prediction of fracture attributes away from the wellbore mechanical stratigraphy subdivides stratified rock into discrete mechanical units defined by properties such as tensile strength elastic stiffness brittleness and fracture mechanics properties fracture stratigraphy subdivides rock into fracture units according to extent intensity or some other observed fracture attribute mechanical stratigraphy is the by product of depositional composition and structure and chemical and mechanical changes superimposed on rock composition texture and interfaces after deposition fracture stratigraphy reflects a specific loading history and mechanical stratigraphy during failure because mechanical property changes reflect diagenesis and fractures evolve with loading history mechanical stratigraphy and fracture stratigraphy need not coincide in subsurface studies current mechanical stratigraphy is generally measurable but because of inherent limitations of sampling fracture stratigraphy is commonly incompletely known to accurately predict fractures in diagenetically and structurally complex settings we need to use evidence of loading and mechanical property history as well as current mechanical states

evidence for temporally changing mechanical stratigraphy and effects on joint network architecture previous studies of jointing in sedimentary rocks have shown that joint network archi tecture is controlled by mechanical stratigraphy which is described by 1 the thickness and rigidity of stratigraphic units and 2 the nature of interfaces between beds using joint pattern to infer rigidity contrast between beds is complicated by the possibility that material properties of the beds may change between fracturing events observation of both an early bed contained joint set and a differently oriented late throughgoing joint set in the same beds of growth strata in the oliana anticline in the spanish pyrenees may reflect changing mechanical properties i e via diagenesis during the folding process using a schmidt hammer we assess the rigidity contrast between the individual units and show that the present day contrast is not great enough to terminate joints at interfaces this result is consistent with an interpretation that the late stage throughgoing joints formed in strata with conditions similar to those of the present day and that the early bed contained joints formed when the rigidity contrast between beds was significantly greater than that of the present day for example differential diagenesis rates between layers of differing grain size would produce temporally changing mechanical stratigraphy we propose that changes in mechanical stratigraphy may have previously been unrec ognized in other stratigraphic sections and that these changes affect joint network archi tecture and subsequent fluid flow pathways